SERIES K: PROTECTION AGAINST INTERFERENCE

Transcription

1 I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n ITU-T K.104 TELECOMMUNICATION STANDARDIATION SECTOR OF ITU (03/2015) SERIES K: PROTECTION AGAINST INTERFERENCE Method for identifying the tranfer potential of the earth potential rie from high or medium voltage network to the earthing ytem or neutral of low voltage network Recommendation ITU-T K.104

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3 Recommendation ITU-T K.104 Method for identifying the tranfer potential of the earth potential rie from high or medium voltage network to the earthing ytem or neutral of low voltage network Summary In the cae of earth fault in high or medium voltage AC network, ignificant earth potential rie (EPR) can occur in the earthing tructure where the current i dicharged to the earth; typically thi i in the earthing grid of the ubtation involved in the fault. When the earthing grid i connected metallically to long conductor uch a earth wire, neutral conductor, counterpoie, cable heath, pipe and rail, the EPR can be tranferred over far ditance well beyond the zone of influence. Recommendation ITU-T K.104 decribe the mechanim of potential tranfer to a cutomer' premie with a pecial view of the tranfer through the neutral conductor of a low-voltage network and the heath of a telecommunication cable. Calculation technique are given for the determination of the magnitude of EPR and tranferred potential. Mitigation technique for preventing the tranfer of EPR are propoed. Different iolation technique are propoed a poible mitigation technique applicable in a telecommunication plant. Hitory Edition Recommendation Approval Study Group Unique ID * 1.0 ITU-T K /1000/12424 Keyword Double earth fault, earth electrode effect, earth fault, earthing, earth potential rie, EPR, impedance to earth, metallic tranfer, multi earthed, creening factor, tranferred potential. * To acce the Recommendation, type the URL in the addre field of your web brower, followed by the Recommendation' unique ID. For example, Rec. ITU-T K.104 (03/2015) i

4 FOREWORD The International Telecommunication Union (ITU) i the United Nation pecialized agency in the field of telecommunication, information and communication technologie (ICT). The ITU Telecommunication Standardization Sector (ITU-T) i a permanent organ of ITU. ITU-T i reponible for tudying technical, operating and tariff quetion and iuing Recommendation on them with a view to tandardizing telecommunication on a worldwide bai. The World Telecommunication Standardization Aembly (WTSA), which meet every four year, etablihe the topic for tudy by the ITU-T tudy group which, in turn, produce Recommendation on thee topic. The approval of ITU-T Recommendation i covered by the procedure laid down in WTSA Reolution 1. In ome area of information technology which fall within ITU-T' purview, the neceary tandard are prepared on a collaborative bai with ISO and IEC. NOTE In thi Recommendation, the expreion "Adminitration" i ued for conciene to indicate both a telecommunication adminitration and a recognized operating agency. Compliance with thi Recommendation i voluntary. However, the Recommendation may contain certain mandatory proviion (to enure, e.g., interoperability or applicability) and compliance with the Recommendation i achieved when all of thee mandatory proviion are met. The word "hall" or ome other obligatory language uch a "mut" and the negative equivalent are ued to expre requirement. The ue of uch word doe not ugget that compliance with the Recommendation i required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draw attention to the poibility that the practice or implementation of thi Recommendation may involve the ue of a claimed Intellectual Property Right. ITU take no poition concerning the evidence, validity or applicability of claimed Intellectual Property Right, whether aerted by ITU member or other outide of the Recommendation development proce. A of the date of approval of thi Recommendation, ITU had not received notice of intellectual property, protected by patent, which may be required to implement thi Recommendation. However, implementer are cautioned that thi may not repreent the latet information and are therefore trongly urged to conult the TSB patent databae at ITU 2015 All right reerved. No part of thi publication may be reproduced, by any mean whatoever, without the prior written permiion of ITU. ii Rec. ITU-T K.104 (03/2015)

5 Table of Content Page 1 Scope Reference Definition Term defined elewhere Term defined in thi Recommendation Abbreviation and acronym Convention Earth potential rie in electric power ytem Metallic tranfer of EPR Decription of metallic tranfer and influence on telecommunication Calculation of metallic tranfer Tranfer of the EPR by power line Tranfer of the EPR due to an HV fault Tranfer of the EPR due to an MV fault and influence on cutomer premie Mitigation technique Protecting telecommunication line erving LV intallation (MV fault) Annex A Technique for calculating the EPR in electric power ytem A.1 Network parameter affecting the EPR A.2 Technique for calculating the EPR Appendix I Calculation of fault current ditribution Appendix II Through tower earthing during power line fault II.1 Equivalent circuit of the earth wire with earth return II.2 Solution of the circuit II.3 Example of application Appendix III Impedance to earth of MV/LV tranformer tation III.1 Type of meaured tranformer tation III.2 Meaurement method III.3 Reult of the meaurement III.4 Concluion Appendix IV Tranferred voltage and current by mean of LV neutral conductor IV.1 Sytem modelling, option and parameter IV.2 Feeding of the neutral-to-earth loop IV.3 Voltage and current profile v. length of the neutral Appendix V Input impedance of the LV neutral-to-earth loop VI.1 Problem identification Rec. ITU-T K.104 (03/2015) iii

6 VI.2 Page Study of the relative importance of the network parameter and condition VI.3 Main concluion Appendix VII Screening factor of a power cable with an imperfectly earthed heath VII.1 Criterion for long cable VII.2 Short (finite length) cable heath with continuou earthing VII.3 Screening factor of a power cable with an inulating cover VII.4 Screening factor for non-uniform line Appendix VIII Screening factor of telecommunication cable with imperfectly earthed heath VIII.1 Telecommunication cable affected by longitudinal induction VIII.2 Telecommunication cable affected by EPR Bibliography iv Rec. ITU-T K.104 (03/2015)

7 Recommendation ITU-T K.104 Method for identifying the tranfer potential of the earth potential rie from high or medium voltage network to the earthing ytem or neutral of low voltage network 1 Scope Thi Recommendation pecifically addree the magnitude of tranferred earth potential rie (EPR) to telecommunication ytem due to fault in AC power ytem, and how to mitigate thee effect. The intent of thi Recommendation i to provide a good ource of information for engineer who need guidance on how to ae the magnitude of tranferred EPR to telecommunication ytem due to fault in AC power ytem, and how they can mitigate uch effect. The main objective of thi Recommendation i to identify ituation where the tranfer of EPR to telecommunication ytem may caue problem. 2 Reference The following ITU-T Recommendation and other reference contain proviion which, through reference in thi text, contitute proviion of thi Recommendation. At the time of publication, the edition indicated were valid. All Recommendation and other reference are ubject to reviion; uer of thi Recommendation are therefore encouraged to invetigate the poibility of applying the mot recent edition of the Recommendation and other reference lited below. A lit of the currently valid ITU-T Recommendation i regularly publihed. The reference to a document within thi Recommendation doe not give it, a a tand-alone document, the tatu of a Recommendation. [ITU-T K.21] [ITU-T K.26] [ITU-T K.35] [ITU-T K.45] [ITU-T K.57] [ITU-T K.66] [ITU-T K.68] [EN ] [EN 50310] Recommendation ITU-T K.21 (2003), Reitibility of telecommunication equipment intalled in cutomer premie to overvoltage and overcurrent. Recommendation ITU-T K.26 (2008), Protection of telecommunication line againt harmful effect from electric power and electrified railway line. Recommendation ITU-T K.35 (1996), Bonding configuration and earthing at remote electronic ite. Recommendation ITU-T K.45 (2011), Reitibility of telecommunication equipment intalled in the acce and trunk network to overvoltage and overcurrent. Recommendation ITU-T K.57 (2003), Protection meaure for radio bae tation ited on power line tower. Recommendation ITU-T K.66 (2011), Protection of cutomer premie from overvoltage. Recommendation ITU-T K.68 (2008), Operator reponibilitie in the management of electromagnetic interference by power ytem on telecommunication ytem. CENELEC EN (2011), Railway application Fixed intallation Electrical afety, earthing and the return circuit Part 1: Protective proviion againt electric hock. CENELEC EN (2006), Application of equipotential bonding and earthing in building with information technology equipment. Rec. ITU-T K.104 (03/2015) 1

8 [EN 50522] CENELEC EN (2010), Earthing of power intallation exceeding 1 kv a.c. [IEC ] IEC (2010), Power intallation exceeding 1 kv a.c. Part 1: Common rule. [IEEE ] [IEEE ] IEEE Std (2000), IEEE Guide for Safety in AC Subtation Grounding. IEEE Std (2012), IEEE Guide for Meauring Earth Reitivity, Ground Impedance, and Earth Surface Potential of a Grounding Sytem. 3 Definition 3.1 Term defined elewhere Thi Recommendation ue the following term defined in [EN 50522], [IEC ] and [b-electropedia]: cable with earth electrode effect: Cable whoe heath, creen or armouring have the ame effect a a trip earth electrode circulating tranformer neutral current: Portion of fault current which flow back to the tranformer neutral point via the metallic part and/or the earthing ytem without ever dicharging into oil (local) earth [b-electropedia]: (definition , modified) Part of the earth, which i in electric contact with an earth electrode and the electric potential of which i not necearily equal to zero. NOTE The conductive ma of the earth, whoe electric potential at any point i conventionally taken a equal to zero earthing conductor [b-electropedia]: (definition ) Conductor which provide a conductive path, or part of the conductive path, between a given point in a ytem or in an intallation or in equipment and an earth electrode. NOTE Where the connection between part of the intallation and the earth electrode i made via a diconnecting link, diconnecting witch, urge arreter counter, urge arreter control gap etc., then only that part of the connection permanently attached to the earth electrode i an earthing conductor earth electrode [b-electropedia]: (definition ) Conductive part, which may be embedded in a pecific conductive medium, e.g., in concrete or coke, in electric contact with the earth earth fault [b-electropedia]: (definition ) Fault caued by a conductor being connected to earth or by the inulation reitance to earth becoming le than a pecified value earth fault current, IF: Current which flow from the main circuit to earth or earthed part at the fault location. NOTE 1 For ingle earth fault, thi i: in ytem with iolated neutral, the capacitive earth fault current; in ytem with high reitive earthing, the RC compoed earth fault current; in ytem with reonant earthing, the earth fault reidual current; in ytem with olid or low impedance neutral earthing, the line-to-earth hort-circuit current. NOTE 2 Further earth fault current may reult from double earth fault and line to line to earth earth potential rie, EPR UE: Voltage between an earthing ytem and reference earth electric reitivity of oil, ρe: Reitivity of a typical ample of oil. 2 Rec. ITU-T K.104 (03/2015)

9 earthing ytem [b-electropedia]: (definition ) Arrangement of connection and device neceary to earth equipment or a ytem eparately or jointly foundation earth electrode [b-electropedia]: (definition , modified) Conductive tructural embedded in concrete which i in conductive contact with the earth via a large urface global earthing ytem: Equivalent earthing ytem created by the interconnection of local earthing ytem that enure, by the proximity of the earthing ytem, that there are no dangerou touch voltage. NOTE 1 Such ytem permit the diviion of the earth fault current in a way that reult in a reduction of the earth potential rie at the local earthing ytem. Such a ytem could be aid to form a quai equipotential urface. NOTE 2 The exitence of a global earthing ytem may be determined by ample meaurement or calculation for typical ytem. Typical example of global earthing ytem are in city centre; urban or indutrial area with ditributed low- and high-voltage earthing high voltage (HV) [b-electropedia]: (definition ) Voltage having a value above a conventionally adopted limit. NOTE 1 An example i the et of upper voltage value ued in bulk power ytem. NOTE 2 In the cae of a three-phae ytem the voltage refer to the line-to-line voltage impedance to earth, e: Impedance at a given frequency between a pecified point in a ytem or in an intallation or in equipment and reference earth. NOTE The impedance to earth i determined by the directly connected earth electrode and alo by connected overhead earth wire and wire buried in earth of overhead line, by connected cable with earth electrode effect and by other earthing ytem which are conductively connected to the relevant earthing ytem by conductive cable heath, hield, PEN conductor or in another way low voltage (LV) [b-electropedia]: (definition ) Voltage having a value below a conventionally adopted limit. NOTE 1 For the ditribution of AC electric power, the upper limit i generally accepted to be 1000 V. NOTE 2 In the cae of a three-phae ytem the voltage refer to the line-to-line voltage medium voltage (MV) [b-electropedia]: (definition ) (not ued in the UK or in Autralia) Any et of voltage level lying between low and high voltage. NOTE 1 The boundarie between medium and high voltage level overlap and depend on local circumtance and hitory or common uage. Neverthele, the band 30 kv to 100 kv frequently contain the accepted boundary. NOTE 2 Medium voltage i not a tandardized term. It i pecified a a ytem, voltage cla by IEEE [b-term]. NOTE 3 The preferred nominal (line-to-line) medium voltage in North America are: 4.16 kv, kv, 13.8 kv, 34.5 kv and 69 kv [b-term]. Typical MV ytem voltage for public ditribution: in Europe 10 kv (mainly underground) 20 kv and 35 kv (mainly overhead) [b-cahier173], in Japan 6.6 kv multi earthed HV neutral conductor: Neutral conductor of a ditribution line connected to the earthing ytem of the ource tranformer and regularly earthed nominal voltage of a ytem [b-electropedia]: (definition ) Suitable approximate value of voltage ued to deignate or identify a ytem PEN conductor [b-electropedia]: (definition ) Conductor combining the function of both protective earthing conductor and neutral conductor potential: Voltage between an obervation point and reference earth potential grading earth electrode: Conductor which due to hape and arrangement i principally ued for potential grading rather than for etablihing a certain reitance to earth. Rec. ITU-T K.104 (03/2015) 3

10 power tation [b-electropedia]: (definition ) Intallation whoe purpoe i to generate electricity and which include civil engineering work, energy converion equipment and all neceary ancillary equipment protective earthing conductor: Protective conductor for enuring equipotential bonding reference earth [b-electropedia]: (definition , modified): (remote earth) Part of the earth conidered a conductive, the electric potential of which i conventionally taken a zero, being outide the zone of influence of the relevant earthing arrangement. NOTE The term "earth" mean the planet and all it phyical matter reitance to earth, Re: Real part of the impedance to earth creening factor k (alo called reduction factor, r: Factor k of a three phae line i the ratio of the current flowing in the earth, IE over the um of the zero equence current in the phae conductor of the main circuit (k = IE /3I0) at a point remote from the hort-circuit location and the earthing ytem of an intallation, (alo referred to a reduction factor, r [EN 50522]) olidly earthed neutral ytem [b-electropedia]: (definition ) Sytem whoe neutral point() i(are) earthed directly tre voltage: Voltage appearing during earth fault condition between an earthed part or encloure of equipment or device and any other of it part and which could affect it normal operation or afety tructural earth electrode: Metal part, which i in conductive contact with the earth or with water directly or via concrete, whoe original purpoe i not earthing, but which fulfil all requirement of an earth electrode without impairment of the original purpoe. NOTE Example of tructural earth electrode are: pipeline, heet piling, concrete reinforcement bar in foundation and the teel tructure of building, etc ubtation [b-electropedia]: (definition ) Part of a power ytem, concentrated in a given place, including mainly the termination of tranmiion or ditribution line, witchgear and houing which may alo include tranformer. It generally include facilitie neceary for ytem ecurity and control (e.g., protective device). NOTE According to the nature of the ytem within which the ubtation i included, a prefix may qualify the ubtation type. Example include: tranmiion ubtation (of a tranmiion ytem), ditribution ubtation, 400 kv ubtation, 20 kv ubtation ytem with iolated neutral [b-electropedia]: (definition , modified) Sytem in which the neutral of tranformer and generator are not intentionally connected to earth, except for high impedance connection for ignalling, meauring or protection purpoe ytem with low-impedance neutral earthing [b-electropedia]: (definition , ) Sytem in which at leat one neutral of a tranformer, earthing tranformer or generator i earthed via an impedance deigned uch that due to an earth fault at any location the magnitude of the fault current lead to a reliable automatic tripping due to the magnitude of the fault current ytem with reonant earthing: Sytem in which at leat one neutral of a tranformer or earthing tranformer i earthed via an arc uppreion coil and the combined inductance of all arc uppreion coil i eentially tuned to the earth capacitance of the ytem for the operating frequency. NOTE 1 In cae of no elf-extinguihing arc fault there are two different operation method ued: automatic diconnection; continuou operation during fault localization proce. In order to facilitate the fault localization and operation there are different upporting procedure: hort term earthing for detection; 4 Rec. ITU-T K.104 (03/2015)

11 hort term earthing for tripping; operation meaure, uch a diconnection of coupled bu bar; phae earthing. NOTE 2 Arc uppreion coil may have high ohmic reitor in parallel to facilitate fault detection tranferred earth potential: Potential rie of an earthing ytem caued by a current to earth tranferred by mean of a connected conductor (for example a metallic cable heath, PEN conductor, pipeline, rail) into area with low or no potential rie relative to reference earth, reulting in a potential difference occurring between the conductor and it urrounding. NOTE The definition alo applie where a conductor, which i connected to reference earth, lead into the area of the potential rie. 3.2 Term defined in thi Recommendation Thi Recommendation define the following term: effective tation impedance to earth, e,t: I compoed of the reitance to earth, Re of the earthing grid and the parallel equivalent of the impedance to earth of the connected paive (not in-feeding on the fault) line with earth electrode effect equivalent current to earth: I the um of the earth current induced by the zero equence component of fault current of the in-feeding power line. It can be expreed a um of the product of the creening factor ki and the zero equence current 3I0,i relevant to the i-th line equivalent impedance to earth: I the parallel equivalent of the reitance Re to earth of the meh earth electrode and input (earth wire/tower footing chain, or cable heath) impedance to earth of the connected paive line with earth electrode effect and impedance to earth of the return conductor of the in-feeding line having earth electrode effect. 4 Abbreviation and acronym Thi Recommendation ue the following abbreviation and acronym: ADSL Aymmetric Digital Subcriber Line CPU Combination Protection Unit EMF Electromotive Force EPR Earth Potential Rie GMR Geometric Mean Radiu HV High Voltage LV Low Voltage MET Main Earth Terminal MV Medium Voltage OHL Overhead (power) Line PEN Protective Earth and Neutral PSTN Public Service Telecommunication Network SPD Surge Protective Device OI one of Influence Rec. ITU-T K.104 (03/2015) 5

12 5 Convention None. 6 Earth potential rie in electric power ytem Earth potential rie (EPR), occurring due to earth fault in electric power ytem, can caue damage to telecommunication plant and endanger people working in the plant, when EPR i tranferred by metallic tranfer to the telecommunication plant. The International Electrotechnical Commiion (IEC) define EPR a the voltage between an earthing ytem and the reference earth [EN 50522], [IEC ]. The "reference earth" (remote earth) i a point ditant enough from the earthing ytem that the electric potential at thi point can be conveniently taken a zero. Roughly peaking, EPR i the product of current to the earth and impedance to earth of the intallation. Thi EPR can be tranferred totally or partially by mean of a connected conductor (e.g., metallic cable heath, protective earth and neutral (PEN) conductor, pipeline, rail) into area with low or no potential rie relative to reference earth, reulting in a potential difference occurring between the conductor and it urrounding, or converely (ee Note in claue ), by a conductor which i connected to reference earth and lead into the area of the potential rie. Thi ituation occur often in telecommunication line. In any cae, the EPR i the tarting point when conidering the metallic tranfer of EPR. The key network parameter affecting EPR are (ee claue A.1): 1) The magnitude of the hort-circuit current, i.e., the phae-to-earth fault current for network with olidly-earthed neutral (high voltage (HV) ytem and medium voltage (MV) ditribution network in North America) and double earth fault current on iolated or reonant earthing neutral MV ytem. The current 3I0 in-feeding by the line() to the fault i not cauing the EPR, but the fraction of the current which return through the earth and thu on the effective tation impedance to earth. In the cae of long homogenou line, the effective tation earth current can be expreed by the creening factor k of the return conductor with the following imple equation: Ie,t = k 3I0. In the cae of a more complex in-feeding line arrangement (hort, or non-homogenou or multiple coupled line ytem), the current ditribution for the in-feeding line ytem hould be determined by an appropriate multi-conductor line olution technique, thu obtaining the tation earth current cauing the EPR. 2) The tation impedance to earth e,t of a ubtation reult from the contribution of the grid reitance, Re in parallel with the outgoing line carrying paive conductor connected to the grid with earth electrode effect characterized by e.pl. It hould be noted that the tation impedance to earth e,t of an MV/low voltage (LV) tranformer tation can be highly affected (decreaed) by the earth electrode effect of the outgoing LV neutral conductor. A an example, auming a mall MV/LV ubtation grid with reitance to earth of Re = 5 and five LV line neutral conductor with input impedance of 0.9, phae 14 o each (ee Appendix V), the parallel-connected reultant impedance to earth i: e,t= phae 13.5 o. (See the impedance to earth value given in Appendix III.) Note in the cae of a large ubtation (in hundred meter order of ize) there i ignificant voltage drop from the point of current injection (e.g., fault) to the edge of the grid. For example, the EPR could leen to 1/3rd of the maximum EPR magnitude. In uch large ubtation, the EPR relevant to the point of the grid where the line cauing the metallic tranfer i connected hall be taken into account. Technique for calculating the EPR caued by earth fault in electric power ytem are given for three calculation level in Annex A. 6 Rec. ITU-T K.104 (03/2015)

13 7 Metallic tranfer of EPR 7.1 Decription of metallic tranfer and influence on telecommunication When the earth grid i connected metallically to long conductor uch a earth wire, neutral conductor, counterpoie, cable heath, pipe and rail, the potential aumed by the earth grid can be tranferred to ditance well beyond the zone of influence (OI). Figure 1 how an example of tranferred voltage. Aume that a fault occur on the MV bu of an HV/MV ubtation A. The earthing ytem of the intallation include the ubtation grid and the earth electrode on line and the cable connection. The current i ditributed between the different earth electrode of the ytem. The neutral of the MV overhead line and the cable heath tranfer the ubtation EPR outide the OI of the ubtation. Part of the EPR, depending on the impedance involved, at the HV/MV ubtation A i tranferred to MV/LV ubtation B and C. Figure 1 Illutration of tranferred voltage outide a ubtation When a metal-heathed telecom cable enter an HV ubtation, it heath can tranfer the EPR in a imilar manner. Of coure, the earthing at the remote end of the cable i the one actually applied, e.g., the earthing of a telecommunication centre. Another apect to be conidered for telecommunication line entering large ubtation i that the EPR relevant to the point of the grid where the cable heath i connected hall be taken into account (additional explanation i given in claue 6). For calculating the creening action of the cable heath the technique given in Appendix VII applie. 7.2 Calculation of metallic tranfer Simple equation are given below to roughly etimate the typical ditance that a particular potential i tranferred along a metallic tructure. Eentially, two poibilitie have to be conidered: 1) ditributed earthing (e.g., pipe, rail and old power cable heath which are not inulated from the oil by mean of a polyethylene covering); 2) earthing at dicrete point (e.g., earthed location along overhead line with hield wire, neutral conductor and power or telecommunication cable heath with inulating jacket). In both cae, the equation to etimate the tranferred voltage V(x) at a certain ditance x from the grid edge i: Rec. ITU-T K.104 (03/2015) 7

14 where i the propagation contant of the tranmiion line that model the long metallic tructure connected to the grid and Ve i the ubtation EPR. Equation (1) can be applied only when the tructure connected to the grid i very long (L>3τ where τ i the line contant a defined in equation (8)) and homogeneou. It can be een that what i needed in order to apply the equation i the knowledge of the propagation contant ; different equation are given to calculate according to the two previouly mentioned cae. 1) Ditributed earthing In thi cae, it i convenient to introduce the per unit length impedance z of the tructure connected to the grid and the admittance per unit length y aociated to the tructure connected to the grid; in thi cae the propagation contant i given by: The per unit length impedance of a conductor with earth return z i given by (ITU-T Directive Vol. II claue , [ITU-T K.26]): z r where rc i the per unit length conductor reitance, geometric mean radiu (GMR) i the geometric mean radiu of the conductor and De, the equivalent depth of the hypothetical return path of the earth current: The per unit length conductance-to-earth: jx V r c x V e e j D e y 3 f For a continuouly or frequently earthed conductor auming that g>>ωc thu y can be approximated by g in equation (2), the attenuation () and phae () contant are given by: x z y j ( m) f g jc 4 De ln GMR ( / km) (1) (2) (3) (4) (5) r g 1 1 ( x 2 / r ) 2 (6) x g 2 The = 1/ i the "length contant" which correpond to the time contant aociated with a time dependent phenomenon; at a ditance of x =, the voltage i reduced to 37 per cent of the value at x = 0. The length contant i given by: (7) ( x / r ) Thi expreion can be implified in two tep. Firt, if (x/r) 2 >> 1, then 1 r g 2 2 (8) 8 Rec. ITU-T K.104 (03/2015)

15 Furthermore, if x/r >> 1, then 1 2 r g 1 x / r (9) Auming a conductor, horizontally buried, of diameter d, length >> L (defined in equation (4)) and lying in oil of reitivity ρ at a depth h, when h<<l, the admittance per unit length y can be etimated by (ITU-T Directive Vol. II claue 5.6, [ITU-T K.26]): Thi equation reveal that with increaing length the conductance per unit length drop toward zero. The numerical tudie verify that equation (11) reult in a proper value for y when ubtituting L= /5. For thi condition, the length L i etimated by: 2) Earthing in dicrete point g L If Rg i the value of the earthing (uppoed contant at each point), L i the length between two conecutive earthing point and z i the per unit length impedance of the tructure connected to the grid, uch that [IEC ]: Equation (13) i valid when Rg>>zL. A for the ditributed line, the line contant i given by = 1/=real(). Figure 2 give an example of the tranfer of EPR for two typical ituation encountered in MV ytem. The graph how the ditance at which the voltage on the earth wire i reduced to 37 per cent (x = ) of the ubtation EPR a a function of the oil reitivity. The reitance of the conductor (neutral or cable heath) i either 0.1 or 1 in the example. The GMR and the oil reitivity are ued to compute the reactance of the conductor at 50 Hz. The line are aumed to be of infinite length. The example for dicrete earthing i a mult earthed neutral intalled on ditribution line in North America. The value for Rg (/10 for L=100 m) i typical on rural line and include the contribution of both the earth electrode along the line and thoe of cutomer. For reitivitie compried between 100 and 1'000 m, ditance exceeding 1 km are required for the voltage on the neutral to reduce below 37 per cent of the ubtation EPR. The econd example refer to ditributed earthing and decribe a power cable heath in direct contact with earth. For reitivitie compried between 100 and 1'000 m, ditance ranging between 0.5 and more than 1 km are required for the voltage on the cable heath to reduce below 37 per cent of the ubtation EPR. 2 x g L ln 1.36 hd 1 10 ( m) z ( / m) 1 L z R L g ( S / m) ( m) (10) (11) (12) (13) Rec. ITU-T K.104 (03/2015) 9

16 A een in Figure 2, the earth potential outide the ubtation fall off at a higher rate. The ditance required for the earth potential to reduce to 37 per cent of the EPR varie between 10 and 200 m depending on the ubtation grid ize. Figure 2 Ditance at which the voltage on the earth wire i reduced to 37 % (x = ) of the ubtation EPR a a function of the oil reitivity 7.3 Tranfer of the EPR by power line The EPR reulting from fault on an HV or MV ytem can be tranferred by power line. Thee tranferred voltage can affect the telephone ytem in different way. Figure 3 illutrate typical configuration leading to the tranfer of EPR outide ubtation. Figure 3 Configuration leading to the tranfer of EPR in LV intallation The following connection or link can tranfer the EPR outide ubtation: 1) the neutral of an overhead line, if applied, or the heath of a cable can tranfer the EPR of the HV ubtation to the MV network; 2) the connection of the tranformer cae to the LV neutral can tranfer the EPR reulting from an HV or MV fault to the LV network; 3) the connection of the LV neutral to the local earth electrode (TN ytem) can tranfer the EPR to expoed conductive part in LV intallation. 7.4 Tranfer of the EPR due to an HV fault In the cae of an HV fault, mot occurrence of metallic tranfer of the EPR to telecommunication ytem will occur in HV ubtation. Typical configuration are preented in claue In ome intance, HV tower can alo tranfer the EPR metallically. The example of radio bae tation antenna located on power line tower i given in the next claue. 10 Rec. ITU-T K.104 (03/2015)

17 7.4.1 Tranfer of the EPR from an HV tower and influence on radio tation Radio bae tation antenna located on power line tower are mainly found in rural area where there are no tall building on which they may be intalled. Precaution have to be taken to make the intallation afe and not to caue damage to equipment in cae of an HV fault. Figure 4 Typical intallation of a radio bae tation antenna located on an HV tower [EN ] Figure 4 preent a typical intallation. A cabinet i located near the tower or between the leg of the tower and i ometime elevated. The cabinet hot tranmitting and receiving equipment and ha cable connection for power feeding and ignal tranmiion. The cabinet and the HV tower earthing are bonded. The intallation can be fed by either an LV or an MV line. If the EPR of the tower exceed acceptable limit, the heath of the telecommunication cable i inulated by a jacket. Thi inulation extend to the limit of the OI of the tower. The inulation method are imilar to thoe ued for the protection of telecommunication cable entering HV ubtation. CIGRÉ [b-cigre Protection] in collaboration with [ITU-T K.57] propoed method for the protection of the feeding power line (LV or MV) againt the EPR of the HV tower. It i recommended to iolate the neutral (or cable creen) of the feeding power line from the HV tower earthing. Neverthele, in ome countrie, the neutral of the power line i connected to the tower. In thoe cae, the power line neutral tranfer the EPR of the HV tower. The impact on the telecommunication ytem of the voltage tranferred by the neutral i the ame whether the EPR originate from an HV tower or a ubtation. The next claue give ome example on how the telecommunication ytem can be affected by the EPR tranferred outide of ubtation Tranfer of the EPR from an HV ubtation A ubtation experience an EPR due to an HV fault. Different metallic connection to the ubtation grid can tranfer the EPR outide the ubtation and have an effect on telephone line. A few example are given Tranfer through a cable heath A fraction of the EPR at the faulted ubtation i tranferred to an MV/LV ubtation through the metallic heath of an MV cable (ee Figure 5). A telephone cable i erving the MV/LV ubtation (for a SCADA ytem for example). Appropriate meaure may be neceary to protect the telephone cable if the voltage exceed acceptable limit [ITU-T K.68]. Rec. ITU-T K.104 (03/2015) 11

18 Figure 5 Tranfer of the EPR in MV/LV ubtation through a cable heath Furthermore, the EPR at the MV/LV ubtation can be tranferred to LV intallation and main port of the telecommunication equipment a well, if a TN ytem i ued (ee Figure 5). Telephone cable erving the intallation can be affected by the EPR. If a telephone cable i erving an HV ubtation, the metallic heath i typically iolated from earth within the OI. If the cable i alo erving intallation experiencing tranferred voltage, the latter can be higher than the earth potential at the limit of the OI. In uch cae, the protection of the telephone cable entering the ubtation hould be baed on the metallic tranferred voltage intead of the OI of the ubtation Tranfer through the multi earthed neutral of an overhead line The EPR i tranferred on the multi earthed neutral of a ditribution overhead line (North American ytem). Telecommunication cable hare the ame tructure a the ditribution line (ee Figure 6). Figure 6 Tranfer of the EPR through the multi earthed neutral of an overhead line The metallic heath of a telecommunication cable i regularly connected to the neutral (typically every 300 m). Although the cable heath i iolated from the ubtation grid, a ignificant fraction of the ubtation EPR i tranferred on the heath due to it connection to the neutral of the ditribution line. In uch cae, the protection of the telecommunication cable entering the ubtation hould be baed on the metallic tranferred voltage intead of the OI of the ubtation (ee Figure 1). Even if the telecommunication cable doe not hare the ame tructure a the ditribution line, it can be affected by the tranferred EPR by the MV neutral if the cable erve cutomer fed by the ubtation, a decribed in the previou claue. A hown in Figure 2, a ignificant fraction of the ubtation EPR i tranferred by the neutral on ditance ranging from everal hundred meter to a few kilometre in rural area Tranfer through rail or metallic pipe Rail or pipe entering an HV ubtation can create a hazard by tranferring a portion of the EPR to a remote point in the ubtation. If required, thee hazard can be eliminated by inulating a ection of pipe or railway. In cae of rail, thee hazard can be eliminated by moving the track ection into the ubtation after initial ue or by uing removable track ection where the rail exit the ubtation. 12 Rec. ITU-T K.104 (03/2015)

19 A telecommunication plant can be affected by the potential tranferred through railway or metallic pipe indirectly, i.e., through an earthing which i connected to the rail or pipe erving the telecommunication plant (e.g., for ubcriber) a well. The tranferred potential appear between the earth port and the line port of the apparatu which are on the tranferred potential and on the remote (zero) potential, repectively. 7.5 Tranfer of the EPR due to an MV fault and influence on cutomer premie If the neutral HV/MV tranformer i olidly earthed (or earthed through a low impedance) on the MV ide, all earth fault lead to the circulation of zero equence current and EPR. On iolated (or high impedance) neutral ytem only double earth fault caue EPR (at both fault location) MV network with olidly (or low impedance) earthed neutral MV line of ytem with olidly-earthed neutral carry a neutral conductor (cable heath or multi earthed neutral). A fault on the line produce an EPR that i tranferred to the MV/LV ubtation and to LV intallation (ee Figure 7). A telecommunication cable erving the intallation can be affected by the EPR if acceptable limit are exceeded. Figure 7 MV earth fault in ytem with directly earthed neutral and tranfer of the EPR in LV intallation MV network with iolated (or high impedance) neutral On iolated (or high impedance) neutral ytem, ingle-phae-to-earth fault produce low value of current and EPR. During a fault, the voltage on healthy phae i increaed by a factor cloe to 3. Thee overvoltage can caue a econd fault elewhere on the line in one of the healthy phae. The double earth fault i a phae-to-phae fault with an earth return current circulating between the two fault location (ee Figure 8) On ytem with iolated neutral or with reonant or high-impedance earthing, overhead line are uually not equipped with a metallic return path (neutral conductor or multi earthed earth wire) and the total current of a double earth fault on the network i circulating through the earth and produce EPR at each fault point. When earth fault occur in an MV/LV ubtation the EPR i tranferred from the faulty ubtation through the LV neutral conductor to the LV intallation (ee Figure 8). Rec. ITU-T K.104 (03/2015) 13

20 Figure 8 MV fault at the MV/LV ubtation and tranfer of the EPR in LV intallation (double earth fault on an overhead line network) If an MV network i compoed of cable line, only that fraction of the double earth fault current, which doe not return through the cable heath/creen, circulate through the tation earth and produce EPR. Thi EPR i tranferred from the faulty MV/LV ubtation through the LV neutral conductor to the LV intallation (ee Figure 9). Figure 9 MV fault at the MV/LV ubtation and tranfer of the EPR in LV intallation (double earth fault on a cable network) Condition affecting the potential tranfer on LV neutral ytem The potential tranfer on the neutral conductor i affected by the following two condition: 1) the earthing arrangement at the MV/LV tranformer tation; 2) the LV ytem earthing. Figure 10 Earthing option of the MV/LV tranformer tation 14 Rec. ITU-T K.104 (03/2015)

21 For the earthing arrangement at the MV/LV tranformer tation one of the two option hown in Figure 10 can be applied. Generally the cheme hown in Figure 10 a) i realized, i.e., the MV frame are combined with the N bu to which the tranformer neutral and the neutral conductor of the outgoing line are connected. Thi jointed terminal i earthed commonly through the effective impedance to earth e,t of the MV/LV tranformer tation. The EPR Ue i given by the product of thi impedance and the current to the earth, Ie,t, i.e., Ue = e,t Ie,t. Note that the e,t value i the parallel equivalent of the reitance Re to the earth of the tation earth and the parallel reultant of the earthing impedance N of the neutral conductor. The N could reduce, in a great extent, the e,t (ee Appendix III) and thu reduce the EPR a well, epecially when the LV ytem earthing i TN (ee Appendix V). In thi exceptional cae, when the tranfer potential i beyond the limit given in [ITU-T K.68], the earthing for the MV frame repreented by Re and the earthing of the neutral N hould be eparated a hown in Figure 10 b). Thi i a mitigation technique that prevent the tranfer of EPR through neutral, to the cutomer premie (ee the mitigation technique in claue 8.1.1). The following can be tated for the different LV ytem earthing from the point of view of tranfer of the MV/LV EPR through the neutral: the EPR i tranferred in TN and TT ytem through the neutral conductor due to it connection to MV/LV tation earth; the EPR i not tranferred in IT ytem through the neutral conductor becaue it i not connected to MV/LV tation earth. The following ignificant difference between the TN and TT ytem earthing are: In TN ytem earthing the neutral conductor i frequently earthed, i.e., at the conumer' premie and at regular ditance along the line (every 250 to 400 m) a pecified by the utilitie. A a reult: the input impedance of the neutral-to-earth loop i mall (ee Appendix V) and the effective impedance to earth of the MV/LV tranformer tation i alo mall (ee Appendix III), and reult in low tation EPR; the tranfer potential decreae rapidly with the ditance from the tation due to the attenuation effect reulted by the frequent earthing of the LV neutral conductor (ee Appendix IV). Conequently, the total EPR i tranferred to only thoe conumer' premie, which are in the tation neighborhood; the potential tranfer from the MV network to the neutral of the LV network i blocked at the MV/LV tranformer when the LV neutral i not connected to the tation earthing, but the neutral conductor are earthed at the conumer' premie. In principle thi correpond to the cheme hown in the Figure 10 b); however, the LV neutral earthing point repreenting RLV are applied at the conumer premie. (See alo the mitigation technique given in claue ). In the TT ytem earthing the neutral conductor i earthed only at a ingle point i.e., to the MV/LV tation earth. Conequently: the neutral conductor do not leen the impedance to earth of the MV/LV tranformer tation and do not lower the tation EPR; the tation EPR i practically tranferred without any attenuation to cutomer premie. 8 Mitigation technique 8.1 Protecting telecommunication line erving LV intallation (MV fault) MV network with a olidly-earthed neutral, carry line with a multi earthed neutral. The neutral conductor of the MV line i connected to the neutral of the LV line. The EPR caued by ingle-phae fault on the MV ytem i tranferred to LV conumer through the LV neutral. In general, the earth Rec. ITU-T K.104 (03/2015) 15

22 impedance of LV line i low (typically below 1 ) due to the contribution of earth electrode along the line in parallel with thoe of the conumer. A a conequence, in the vat majority of cae, EPR reulting from MV fault produce voltage on telecommunication circuit that are below the limit of tandard protection ytem intalled in LV intallation. In ome cae, uch a rural line located in high oil reitivity area, pecial protection may be required; claue give an example of the type of equipment that can be ued. In open wire line MV network with iolated (or compenated) neutral, the EPR, due to double earth fault, can be ignificantly higher than in cable line network, due to the following: no return path (creen or earth wire) for the fault current are available; the earthing reitance of the MV/LV tranformer tation can be higher due to the lack of an earthing grid and the reduction effect of a leaky heathed cable; the frequency of an occurrence of a double earth fault i bigger in an open wire line network. On thee network, two way of mitigation are a follow: 1) preventing the tranfer of EPR on the LV network; 2) protecting againt the potential tranferred to the LV conumer (apply to both iolated and olidly-earthed ytem) Preventing the tranfer of EPR on iolated neutral from MV ytem Forbidding earthing of the tar point at the MV/LV tranformer tation A mitigation technique ued to avoid the tranfer of the higher EPR to the LV network i the forbidding earthing of the tar point, and thu the neutral of the LV network at the location of the MV/LV tranformer. Thi rule, applied to the neutral earthing in rural overhead public ditribution line (e.g., in France) i hown in Figure 11. Figure 11 Rule on the neutral earthing in rural overhead public ditribution in France [b-cahier173 ] Appropriate bonding of the junction cable In rural and uburban area newly intalled MV/LV tranformer tation are often connected to exiting aerial MV line by the inertion of a hort junction cable (ee Figure 12 a). In thi cae the creen earthed, both at the pole and at the tranformer, can tranfer the EPR due to the earth fault at the junction pole. If the equivalent earth reitance of the tranformer tation i low enough (e,t < 0.2 Ω), the tranferred potential remain below the limit value. Thu, the creen can be directly earthed at the junction pole end (ee Figure 12 b). In contrat, the creen hall be iolated at the junction pole when the earth reitance of the tranformer tation i not low enough (e,t >> 0.2 Ω). For the protection of the inulation jacket of the junction cable, the lightning-originated overvoltage hall be 16 Rec. ITU-T K.104 (03/2015)

23 limited by an MV type overvoltage protector applied between the creen and pole earthing (ee Figure 12 c). Figure 12 Rule on earthing of the creen of hort MV junction cable Protecting againt potential tranferred to LV conumer The increaing ue and interconnection of complex electronic telecommunication equipment, uch a ISDN terminal, modem and computer, at cutomer' building require pecial care for protecting againt overvoltage and overcurrent. Such overvoltage and overcurrent include expoure of erving telecommunication cable and power line to lightning, and the coupling of AC voltage onto the telecommunication cable due to fault on the external power ytem. Two technique are reviewed for the protection againt thi imultaneou overvoltage effect a follow: Rec. ITU-T K.104 (03/2015) 17

24 Iolation technique Equipment at the ubcriber' premie, which i powered from the LV upply network and connected to the telecommunication network, hould be protected againt the potential tranferred through the neutral of the LV ytem by a unit providing appropriate iolation between the equipment port. Such unit, for the protection of different telecommunication facilitie, are already available in the market and are reviewed in the following example: 1) The iolation unit, LIU 3C, hown in Figure 13 can be ued for the protection of public ervice telecommunication network (PSTN) voice circuit. Thi iolation unit i powered by line current for the line ide and local power at the cutomer' ide. Information i coded a 64 kbit/ tream and tranmitted via hort fibre link. The left-hand ide i the line ide and the right-hand ide i the cutomer' ide. Two fibre link and the internal caing provide the required iolation. It would be very eay to extend thi to a greater ditance. The iolation unit i teted to 25 kvrm. Figure 13 Iolation unit for PSTN (voice) circuit (LIU 3C) 2) A unit imilar in operation to LIU 3C i ued for ISDN2e circuit, but ha four fibre link. The unit i teted to 25 kvrm (Figure 14). Analogue private circuit ue a imple tranformer iolator and it attendant circuit, teted to 20 kvrm. Figure 14 Iolation unit for ISDN2e circuit (LIU 3C) 3) A tranformer baed olution can be ued which i teted to 25 kvrm (minimum). 4) The iolation unit hown in Figure 15 can be ued for the protection of aymmetric digital ubcriber line (ADSL) circuit. Thi iolation unit ue a tranformer iolation olution, but with a digital ubcriber line (DSL) plitter filter, uch that PSTN voice iolator can till be connected. The tranformer i a plit winding type, o it i coupled acro the winding on the line ide with a capacitor (with no DC current flow to upet the PSTN circuit). The left- 18 Rec. ITU-T K.104 (03/2015)

25 hand ide i the line ide and the right-hand ide i the cutomer' ide. The unit i teted to 25 kvrm. Figure 15 Iolation unit for ADSL circuit Coordinated protection at cutomer' premie Overvoltage protection may be required for the afety of peronnel and for protection of equipment. To provide thi protection, it i neceary to bond metallic ervice and creen to the building earth and intall urge protective device (SPD) at the building entry point on power and telecommunication conductor. Properly configured equipotential bonding within the building help to achieve the neceary protection, while alo helping to enure the afety of thoe uing terminal equipment. Such bonding configuration are detailed in [ITU-T K.21], [ITU-T K.35], [ITU-T K.45] and [ITU-T K.66]. Equipment and peronnel in a building are expoed to externally produced energy becaue conductive ervice uch a telecommunication line, power line, antenna lead, waveguide, earthing conductor and metallic pipe penetrate the hell of the building. The penetration of conducted energy i mitigated by interconnecting all of thee with low-impedance bonding conductor to the main earth terminal (MET) (Figure 16), or to the meh-bonding network or common bonding network (CBN) (Figure 17). Thi low impedance i achieved by keeping the length of bonding conductor hort (< 1.5 m). The ue of low impedance bonding conductor i particularly important when there i a ignificant rik of a direct lightning trike to the tructure or to the line immediately adjacent to the building. A combined utilitie encloure (CUE) can be ued to houe the primary protector, both for the main electricity upply and the telecommunication upply, to achieve hort bond wire. It alo ha the advantage that all metallic ervice can enter at the ame point and be bonded together. Thi i the bet method to protect all ervice in a cutomer' premie. Where it i not poible to achieve the requirement for hort bond wire or additional protection i required, combination protection unit (CPU) may be ued (Figure 18). CPU contain the SPD for all port. They are intalled near the equipment and thu, alo protect againt overvoltage occurring in internal wiring. CPU hall be coordinated with the primary protector, a hown in Figure 18. The propoed earthing and bonding method are eay to implement in a new building. Therefore, in new intallation where practical, thee recommendation hould be followed. In exiting intallation, it may be very difficult to modify the intallation to comply with thee bonding requirement. It i therefore uggeted that in older intallation, an upgrade to comply with thee Rec. ITU-T K.104 (03/2015) 19

26 claue hould be conidered only when a major wiring upgrade i being undertaken or there are exceptional afety iue that require an upgrade. Figure 16 Co-location ervice next to a MET Figure 17 Common bonding network (CBN) 20 Rec. ITU-T K.104 (03/2015)

27 NOTE SPD 1), 2) and 3) need to be uitable for ue on the main a pecified in [ITU-T K.66]. Figure 18 Equipment protected by the ame combination protection unit (multiervice urge protection device (MSPD)) Rec. ITU-T K.104 (03/2015) 21

28 Annex A Technique for calculating the EPR in electric power ytem (Thi annex form an integral part of thi Recommendation.) A.1 Network parameter affecting the EPR The IEC define the EPR a the voltage between an earthing ytem and reference earth [EN 50522], [IEC ]. The "reference earth" (remote earth) i a point far enough away from the earthing ytem that the electric potential of thi point can be conveniently taken a zero. The EPR i the product of the current to earth and impedance to earth of the intallation. The current path to the earth can be identified in different way and therefore, the current to earth and impedance to earth of the intallation hould be pecified accordingly. In the mot practical cae the highet EPR in a ubtation i een when an earth fault occur in the ubtation itelf. The relevant current and the impedance to earth are hown for thi fault ituation in Figure A.1. Thee current and impedance, and therefore the EPR, are affected by the following network parameter. Figure A.1 Current and impedance to earth relevant to the faulty ubtation 1) The hort-circuit current, Ic = 3I0,c depend on the impedance of the in-feed network including the tranformer and the line feeding the fault. On HV ytem, ingle-phae earth fault current level typically range between 5 and 30 ka. In an actual cae it value hall be provided by the power network operator. On olidly-earthed MV ytem (a practice in North America), they typically range between 1 and 10 ka. On iolated or reonant earthing neutral MV ytem, imilar value can be expected during double earth fault. The hort-circuit current i the um of the current 3I0 of the line, feeding to the fault and the circulating tranformer current, IN = 3I0N: Ic = 3I0,c = 3I0 + IN (A.1) 22 Rec. ITU-T K.104 (03/2015)

29 Note that the circulating tranformer current contribute neither to the current to earth nor to the EPR. It only caue grid current between the faulty point and the earth connection point of the tranformer neutral, which caue internal voltage difference, uch a tep voltage. 2) The tation impedance to earth e,t of a ubtation reult from the contribution of the grid reitance, Re in parallel with the outgoing line carrying paive conductor, with the earth electrode effect characterized by e.pl connected to the grid. If the reitance to earth Re of the grid i not available from meaurement, it can be etimated uing the following equation: R e 4 A (A.2) where i the oil reitivity, A i the area of the grid. The earth reitance of the grid i approximately inverely proportional to the quare root of it area (ee equation (A.2)). The earth electrode effect of the outgoing line carrying paive conductor can be claified according to the type of paive conductor. In the cae of a line, the earthing impedance repreented by it earth wire i proportional to the tower footing reitance Rt and thu to the quare root of the oil reitivity a well (ee equation (I.4)). Value for input impedance of LV neutral-to-earth loop are preented for different option in Appendix V. Figure A.2 compare the earth reitance of grid of different dimenion to different type of line uing the parameter given in Table A.1 and A.2 [b-cigre Guide]: a cable with a metallic heath in direct contact with oil; a rural MV line carrying a multi earthed neutral typical of North American ditribution ytem; an HV line carrying one earth wire. In a 100 Ωm oil for example, a 100 m 2 grid ha a reitance of 4.4 Ω and thi value i reduced to 0.44 Ω for a m 2 grid; the reitance increae linearly with the oil reitivity. The earth impedance of line i the input impedance of the earth conductor (earth wire or heath/creen) to be connected to the grid. An overhead HV line with reitance of 1 Ω/km earth wire, 300 m pan and tower footing with a reitance of /30 ha an earth impedance of approximately 1.5 Ω. Thi earth impedance i increaed to 4.2 Ω for a 1'000 Ωm oil. Rec. ITU-T K.104 (03/2015) 23

30 Figure A.2 Earth impedance Re of grid and different line type v. oil reitivity [b-cigre Guide] Table A.1 Parameter of overhead line ued in Figure A.1 R c (/km) GMR (m) Span (m) R e () MV line /10 HV line /30 R c: conductor reitance, GMR: geometric mean radiu R e: tower footing reitance Re: tower footing earth reitance Table A.2 Parameter of cable ued in Figure A.1 R c (/km) GMR (m) g (S/km)) g: heath-to-earth conductance 24 Rec. ITU-T K.104 (03/2015) Cable / The propoed imple expreion can be ued for the etimation of the earth reitance of the grid and the earth impedance of line. The reitance of grid i proportional to the oil reitivity wherea the impedance of line i proportional to the quare root of the oil reitivity. A a conequence, the contribution of line to the earth impedance of the intallation increae with the oil reitivity, i.e., the relative importance of the line with earth electrode effect i more ignificant. The paive conductor (earth wire, cable heath or counterpoie) aociated with the in-feeding power line are referred to a return conductor. The difference between a return conductor and a paive conductor of the outgoing line with earth electrode effect i that, the return conductor of an in-feeding line i doubly affected. In principle the current in the return conductor can be compoed of current fraction caued by the following two effect. The firt effect i the conductive coupling reulted by the connection to the grid and thu affected by the EPR. A a reult, a conductive current, Ie,cond occur. It magnitude i given by the ratio of the EPR, Ue to the input impedance to earth, e,rt of the return conductor. Thi current of conductive

31 origin enter form the grid to the earth wire and gradually leave it through the tower footing (or continuou leakage of heath). It become practically zero at a ditance of three length contant (ee equation (8) to (10) for ). In fact, thi effect i imilar to the one of the paive conductor of the outgoing line with earth electrode effect. In another view point, the e,rt tend to reduce the overall reitance to earth of the earthing ytem of the ubtation. The econd effect i the inductive coupling reulted by the mutual impedance between the phae conductor group and the return conductor both with earth return. The electromotive force (emf) induced by the 3I0 through thi coupling caue an induced current which circulate in the earth wire to earth loop. The induced current reditribute the 3I0 current and reulted current in the teady zone are thoe given for the earth wire by equation (A.3) and for the earth by equation (A.4). Thee current are expreed by the creening factor k (equation (A.5)). The creening factor (k) correpond to the fraction of the earth fault current of a line that contribute to the EPR (ee Appendix VII). On overhead line without an earth wire, it i equal to 1. On HV line with one or two teel earth wire(), the creening factor typically range between 0.8 and If the HV line i equipped with a low reitance (in the order of 0.1 Ω/km) earth wire, the creening factor can decreae to approximately 0.5. On North American ditribution line carrying a multi earthed neutral, the creening factor typically range between 0.5 and 0.7. Cable have low creening factor due to the high level of inductive coupling between the phae conductor and the creen. The creening factor of MV or HV cable typically varie between 0.05 and 0.5; the lower value correpond to the low reitance (le than 0.1 Ω/km) of the creen/heath of thee cable. The current hare between the earth and earth wire expreed by the creening factor i, trictly peaking, valid only in the teady zone of the line. Cloe to the ubtation (at ditance le than 3) the induced current tend to leave the earth and enter to the earth wire. The rate of thi current exchange depend on the value of the grid impedance, e,t to earth of the tation. At the ubtation end the induced earth current Ie,ind would remain contant, i.e., k3i0 if e,t 0 (very low), while Ie,ind would decreae to zero if e,t (very high). The conductive current, alo change with ditance. It value goe down from it tarting value of Ie,cond =Ue/e,rt to practically zero at a ditance of 3 from the ubtation. Conequently, the change in both component of the return current tend to decreae the current through the impedance to earth and thu the EPR a well. The rate of decreae in the EPR i maller than the rate of a poible decreae (improvement) in the impedance to earth of the grid. The effect of the above mentioned current exchange i referred to a end effect and i approximated by modified (end effect) value of the creening factor. Due to the above-mentioned end-effect, the value of the equivalent earth impedance ha a contrary effect on the EPR. On the one hand, the increae of the equivalent earth impedance tend to increae the EPR, but on the other hand, decreae the ubtation earth current which oppoe the increae of the EPR. In practical cae the final reult i the decreae in the EPR, due to the decreae (improvement) in the end-effect creening factor. A.2 Technique for calculating the EPR In cae of an actual EPR invetigation the following three level of calculation can be followed depending on: the complexity of the tak, the accuracy required, the availability of the input data and the available calculation technique: 1) ue of the complex imulation for the actual network and fault condition; 2) calculation for the in-feeding line terminated by tation impedance to earth; 3) calculation by the creening factor of the in-feeding line terminated by equivalent impedance to earth. Rec. ITU-T K.104 (03/2015) 25

32 A.2.1 Ue of complex imulation for the actual network ituation and fault condition In the cae of a complex ituation, the calculation of the EPR of a ubtation need a comprehenive procedure (e.g., ITU-T Directive Vol. II and Vol. III, [ITU-T K.26], [b-paul]) and require dedicated computer oftware, e.g., [b-sollerkvit]. In fact, the complete network with the element hown in Figure A.1 i imulated in the calculation. The following lit give a few condition and ytem parameter that have to be taken into account for accurate imulation: 1. The fault contributing to the maximum value of the EPR may be located outide the ubtation. Thi ituation can occur in the ubtation where high power Y- tranformer() i (are) intalled with olidly-earthed neutral(). In thi cae, the fault ditance at which the maximum value of the EPR occur ha to be identified by hort-circuit analye. The current injected to the grid by the tranformer neutral are the primary ource of the EPR. 2. If tranformer with earthed Y-Y connection are ued, a fault at one voltage level caue the circulation of earth fault current at different voltage level. Thee contribution mut all be taken into account. 3. If tranformer with earthed Y-Y- connection are ued, circulating tranformer neutral current i flowing, i.e., a portion of fault current flow back to the tranformer neutral point via the metallic part of the earthing ytem without ever dicharging into oil. NOTE The current mentioned in point 1 to 3 are obtained from the hort-circuit analye of the power ytem and hould be made available by the power ytem operator. 4. If two or more line are in cloe proximity, a ignificant error can be introduced if the calculation of the earth impedance of the line i done eparately. For example, if two cable are located in the ame treet only a few metre apart, the coupling between them (in the cae of metallic heath in direct contact with the oil the conductive coupling may be ignificant a well) hould be taken into account when etimating the contribution of thee two line to the reduction of the earth impedance of the ubtation. 5. Simple equation uch a (I.4) can be ued for the calculation of the earth impedance of line if they are long. However, if they are horter than a few time the length contant (ee equation (8) to (10) for ) or if their parameter change within thi ditance, more elaborate calculation are required. 6. If the creening factor of a line() that contribute() to the fault current change() within a ditance horter than a few time the length contant, inductive (creening factor) and conductive (impedance repreenting the line earth electrode effect) coupling cannot be treated eparately. The next claue give an example on how thi problem can be olved. A.2.2 Calculation for the in-feeding line terminated by tation impedance to earth In the mot practical cae, calculation of the EPR of a ubtation grid can be implified to a great extent with the circuit repreentation contituted by the following two key element: 1) The line() in-feeding to the fault, which i (are) compoed of the live (phae) conductor and the paive return conductor with earth electrode effect. Both kind of conductor are individually repreented in the imulation calculation, thu the voltage and current are obtained from the calculation for the paive conductor a well. The paive conductor of the in-feeding line are connected to the earthing ytem of the ubtation. For the in-feeding line the zero equence current flowing in the active conductor to the fault are given a follow: in an HV/MV ubtation the total zero equence current, 3I0 in the in-feeding line() a the difference of the hort-circuit current and the circulating-tranformer neutral-current: 3I0 = Ic IN=3I0c IN; 26 Rec. ITU-T K.104 (03/2015)

33 in an MV/LV ubtation it i equal to the phae-to-earth hort-circuit current 3I0c in a ytem with olidly-earthed neutral, while it i equal to the double earth fault current 3I0 = Ic,2Ff = 3I0c,2Ff in a ytem with iolated or reonant earthing. For deign purpoe (e.g., etting of the relay protection) the above mentioned current are known from the hort-circuit analye for the power ytem. 2) The impedance to earth of the earthing ytem i repreented by the effective tation impedance to earth, e,t of the ubtation, which i compoed of the reitance to earth, Re of the earthing grid and the parallel equivalent of the impedance to earth of the connected paive (not in-feeding on the fault) line with earth electrode effect. The procedure for calculating the ubtation EPR i compoed of the following tep: 1. deign of the earth grid and evaluation of the earth reitance Re; 2. evaluating the total earth impedance e,pl provided by the paive conductor-to-earth loop connected to the grid; 3. calculating the effective tation impedance to earth e,t of the ubtation (parallel reultant of the above two value). Thi will be the terminating impedance of the return conductor to earth loop() at the ubtation end; 4. identifying the equivalent earth current, a the difference between the earth fault current (Ic = 3I0,c) and the circulating-tranformer neutral-current IN (both are given a input data), which hould be injected zero equence way through the active conductor into the grid; 5. olving the in-feeding line ytem by an appropriate multi-conductor line olution technique (e.g., ITU-T Directive Vol. II and Vol. III, [ITU-T K.26], [b-paul]). From thi olution the voltage() and current() of the paive return conductor will be obtained a a function of the ditance from the ubtation, i.e., the voltage and current length profile. The voltage of the paive conductor at the ubtation end i equal to the tation EPR itelf. The voltage length profile of the return conductor give the voltage tranferred by the paive conductor of the in-feeding line. The effective tation impedance to earth of an HV/MV ubtation i demontrated in Figure A.3. It i compoed of the reitance to earth, Re of the earthing grid and the parallel equivalent of the impedance to earth of the connected paive conductor with earth electrode effect, e.g., the cable heath/creen of cable with earth electrode effect or multipoint-earthed neutral of the MV line according to North-American practice (ee in the pink dotted line quare). The paive conductor are the earthed conductor() of thoe line of the active conductor that do not carry any fault current or do not have any active conductor at all (e.g., pipe or rail). In fact, thee paive earth conductor are functioning either a horizontal buried wire, or trip or multipoint earthed conductor. The input impedance of each paive conductor-to-earth loop i electrically parallel connected with each other and repreent reultant impedance to earth of the paive conductor of the MV, e,mv. The paive earth conductor create current path from the grid to the earth in addition to the reitance to earth, Re of the earth grid. Therefore, the equivalent earth impedance e,t (inide the blue dotted line quare) of the ubtation i the parallel equivalent of the reitance Re to the earth of the grid and the reultant impedance e,mv of the connected paive earth conductor. Rec. ITU-T K.104 (03/2015) 27

34 Figure A.3 Scheme howing the in-feeding HV power line and the MV line with earth electrode effect contributing to the equivalent impedance to earth [b-cigre Guide] The calculation of the EPR of MV/LV tranformer tation due to earth fault i identical to the one decribed above for the HV/MV ubtation. However, the compoition of both the paive earthing element and the return conductor are thoe hown in Figure A.4. The return conductor of the MV network are compoed of: the heath of MV cable having continuou contact (leakage) to the earth, i.e., older type lead-heathed cable with teel-tape armoring; the creen of MV cable (with inulating jacket) which are locally earthed at the MV/LV tranformer tation and at the HV/MV ubtation; North American ditribution line carrying a multi earthed neutral (ee the neutral of the aerial line marked by dotted line in Figure A.4). The paive earthing conductor of the LV network reulting in the hunting impedance e.lv are compoed of (ee in the pink dotted line quare in Figure A.4): 28 Rec. ITU-T K.104 (03/2015)

35 Figure A.4 Scheme of an MV/LV ubtation howing the in-feeding MV power line and the paive line with earth electrode effect contributing to the equivalent impedance to earth [b-cigre Guide] In the cae of a TN ytem the neutral conductor of the LV feeder (either overhead or cable line), which are locally earthed at the conumer' premie and certain point of the line correpondingly to the utility requirement. The impedance to earth of the neutral i highly reduced when the neutral i connected to the pipe ytem of the metallic public utility network (e.g., water, ga) at the cutomer applying the TN ytem (ee example for numerical value in Appendix V). In the cae of a TT ytem, the LV neutral i earthed only at the MV/LV tranformer tation, but the neutral conductor of the LV feeder are not earthed locally at the conumer' premie. Thi condition ha the following conequence: 1) The neutral conductor are not multiple-earthed, thu they have no earth electrode effect any more. Such a neutral conductor ytem doe not reduce tation impedance to earth. Conequently, a double earth fault caue much higher EPR in the MV/LV tranformer tation compared to a TN ytem (ee Appendice III and VI). 2) The tation EPR i tranferred by the neutral conductor of the TT ytem to the conumer' premie (main port) without any attenuation in contrat to the TN ytem (ee Appendix IV). 3) The potential tranferred by the neutral conductor appear acro the main port and the locally earthed frame of the conumer' appliance. NOTE In the cae of the North American practice the multi earthed MV neutral conductor i croconnected to the LV neutral at a tranformer feeding an individual conumer, thu it ha the analogy with the TN ytem. The heath of LV cable having continuou contact (leakage) to the earth, i.e., older type lead-heathed cable with teel-tape armoring. Finally, it hould be noted that the effective tation impedance to earth, e,t a defined in thi claue i different from the equivalent impedance to earth applied in the next claue, becaue the effective tation earth impedance doe not involve the additional earthing effect (reduction) due to the return conductor of thoe line which are in-feeding the fault current to the ubtation. Rec. ITU-T K.104 (03/2015) 29

36 A.2.3 Calculation by the creening factor of the in-feeding line terminated by equivalent impedance to earth When the paive conductor of the in-feeing power line are uniform along the line and the line are long (at leat ix time the length contant) then the EPR calculation can be originated from the creening factor of the return conductor of the in-feeding line. The two criteria mentioned involve that the current induced in the return conductor() to earth loop reache it teady value at leat along the middle zone of the line. When zero equence current 3I0 are preent on a three-phae tranmiion line, they induce longitudinal emf: E = m 3I0 per unit length, where m i the mutual impedance between the group of the phae conductor and the return conductor(), per unit length (ee in Figure I.3). Auming a perfectly earthed return conductor (e.g., earth wire) thi emf caue an induced current which i circulating in the earth wire to earth loop. It value, which i equal to the earth wire current Iew, i given by: The induced current in the earth: zm Iew 3I 3 I (1 k )3I z I e,ind = 3I 0 I EW = k 3I 0 (A.3) (A.4) where µ i the earth wire/return conductor factor and k i the creening factor of the earth wire/return conductor. The creening factor defined a the ratio of the earth current and the 3I0 i given by: k (A.5) For a cable connected to the ubtation, intead of the earth wire creening factor, the cable heath creening factor ha to be ued in the equation above for Ie,ind. For cable with inulated metal heath, which lead fault current to the ubtation, the cable heath creening factor i the primary effect. In addition, the chain impedance (cable heath/neighbouring earth grid) can be conidered if the cable i ignificantly longer than the ection forming the chain impedance. For the determination of the creening factor further information i given in Appendix VII. The EPR of an HV/MV tranformer tation can be calculated on the bai of the network cheme and the equivalent circuit hown in Figure A.5. For the determination of the equivalent current to earth Ieq the following current component are conidered: Ic=3I0c=3I0 + IN Three time zero equence current of the earth fault current 3I0 Three time zero equence current of the line IN The circulating tranformer neutral current Ie 3 I0 - Iew = 1 3 I 3 I Ie Current to earth (cannot be meaured directly) I eq 0 k (A.6) For an earth fault in a three-phae ytem and for a imilar return conductor creening factor of n in-feeding line connecting to the ubtation, the current to earth can be determined by equation (A.6) the current hall be Σ3I0 the phaor um of the zero equence current of the in-feeding line. 0 3I 0c I N k 3I0 m 30 Rec. ITU-T K.104 (03/2015)

37 If the earth wire reduction factor of the line A, B, C... leaving the ubtation are different, the current to earth i given by: where: Ieq = ka 3I0A + kb 3I0B + kc 3I0C + I0A i the zero equence current of a phae conductor (for example phae L1) of the line A, I0B of the line B, etc. ka i the earth wire creening factor of the line A, kb of the line B, etc. For the determination of the equivalent impedance to earth eq the following current component: Re ehv emv reitance to earth of the meh earth electrode (earthing grid) (Calculated by ophiticated technique [b-cdegs] or meaured according to [EN 50522], [IEEE ] input (earth wire/tower footing chain) impedance of the return conductor of the in-feeding line. In cae of n in-feeding line their parallel equivalent (ee equation (A.8)) input (earth wire/tower footing chain) impedance of the paive (not in-feeding) line. In cae m not in-feeding line their parallel equivalent (ee equation (A.9)) The equivalent impedance to earth i given by the following expreion: eq 1 emv (A.7) When there are n in-feeding line having identical chain impedance of HV each with their parallel equivalent: Similarly for m not in-feeding line with MV chain impedance each their equivalent impedance i: ehv emv 1 1 R (A.8) (A.9) Such a imple calculation of the parallel equivalent impedance i valid only in the cae where the line are not coupled with each other. (For conideration of cable group in the ame trench (ee ITU- T Directive Vol. II claue 4.3.7, [ITU-T K.26].) Finally, the EPR in cae of earth fault i given by the product of the equivalent impedance to earth and the overall current to earth: U e e n HV m eq MV I 1 eq ehv (A.10) Rec. ITU-T K.104 (03/2015) 31

38 Figure A.5 Network cheme and equivalent circuit demontrating the current to earth Ieq and equivalent impedance eq to earth for an earth fault in an HV/MV tranformer tation 32 Rec. ITU-T K.104 (03/2015)

39 Appendix I Calculation of fault current ditribution (Thi appendix doe not form an integral part of thi Recommendation.) NOTE Thi appendix i baed on [b-cigre Guide]. On low impedance or olidly-earthed ytem, both ingle-phae and phae-to-phae to earth fault caue high zero equence current to circulate between the ubtation and the fault location (ee Figure I.1). EPR appear both at the fault location and at the ubtation. On iolated or compenated neutral ytem, only double earth fault lead to high zero equence current circulating between the two fault (ee next figure). EPR appear at both fault location. Double earth fault are much le frequent than ingle-phae fault. A good guide to the calculation of the fault current ditribution can be found in the ITU Directive Vol. V claue 5, [ITU-T K.26]. The calculation of the zero equence current in the earth wire and the earth, involve a two tage proce. Firt the current are calculated auming there i no voltage difference between the earth wire and the earth (i.e., auming a perfect earthing). At the fault location and at the tranformer or generator neutral feed point there will be an EPR which will lead to a voltage difference between the earth wire and the earth. There will alo be a voltage difference generated at point where the earth wire type or layout of the conductor change. The ditribution of earth wire current i then recalculated allowing for the voltage gradient produced at thee zero equence ource and ink point. The calculation can be performed eparately if all impedance are linear. If the light nonlinear reitance and internal inductance of the teel wire have to be conidered then an iterative approach ha to be adopted. Figure I.1 Fault leading to the circulation of high zero equence current depending on tranformer neutral connection When zero equence current I0 are preent on a three-phae tranmiion line they induce return current in the earth wire if it i earthed a hown in Figure I.3. Auming a perfectly earthed earth wire, only the induced current i circulating in the earthed wire and the earth wire current Iew i given by: Rec. ITU-T K.104 (03/2015) 33

40 Figure I.2 Tranmiion line zero equence current in phae and earth wire zm Iew 3I 3 I (1 k )3I z where zm i the mutual impedance per unit length between the earth wire or wire bundle and the phae conductor or conductor bundle, z i the earth-return impedance per unit length of the earth wire or wire bundle, μ i the coupling factor between the earth wire and phae conductor and k i the creening factor of the earth wire ( k 1 ). The return current through the earth Ie i: Thee current will not caue a voltage between the earth wire and the earth. I 3I I k 3I e 0 ew 0 The ource and ink point which are cauing the circulating zero equence current (e.g., a fault) will create a voltage difference between the earth wire and the earth due to the EPR and thi will lead to a reditribution of the current flowing in the earth wire and the earth. The reditribution of thi current depend on the impedance per unit length of the earth wire and the tower footing reitance Rt. The equivalent circuit i a hown in Figure I.3. The current in each earth wire pan decreae with ditance from the ource or ink point (fault location, neutral earth point) and i a function of the number of pan n from that point, where n=0 i the point conidered (e.g., fault location). For the uniform cae with non-inulated earth wire the pan earth current Id between tower n-1 and n i given by: I ( n 1, n) d k 3I R R 0 t t 2Rt c Rt c where c i the impedance of the infinite long ditributed chain of earth wire and tower footing reitance a een from the tart and neglecting the overlapping of the tower footing reitance, and i given by: where z i the impedance of the earth wire with earth return, per pan, and Rt i the tower footing reitance [b-endrenyi]. n1 0.5 z z 4R z 0.5z R z c t t (I.1) (I.2) (I.3) (I.4) 34 Rec. ITU-T K.104 (03/2015)

41 Figure I.3 Earth and earth wire current due to a phae-to-earth fault a) induced earth and earth wire current b) equivalent circuit for the ditributed earth wire current The current to earth It at tower n, which give rie to a footing EPR, i: I ( n) t k 3I R 0 c t 2Rt c Rt c n1 (I.5) Near a ubtation all the connecting line to the ubtation and the zero equence current fed from the ubtation itelf need to be taken into conideration. The earth electrode of the ubtation can be treated like a tower footing. Figure I.4 how two poible ituation of an internal ubtation fault and a remote fault to the ubtation. For an internal fault (Figure I.4 a) the earth current at the ubtation Ie i the um of all the line earth current flowing into the tation: n I k 3I e l 0l l 1 where n i the number of line, kl i the creen factor and I0l i the zero equence current in line l. For a remote fault (Figure I.4 b) the earth current i given by the current from the feed tranformer and in general, if other connected line have a different creen factor then: n I k 3 I ( k k )3I e l 0t l i 0i i1 The induced earth wire current can be approximately expreed by the difference of the two involved creening factor (ee Figure I.6). If the earth wire i inulated, then uually the inulator or park gap will flahover at the tower containing the fault and at ome adjacent tower, depending of the reitance of the tower footing. (I.6) (I.7) Rec. ITU-T K.104 (03/2015) 35

42 Figure I.4 Earth fault current at a ubtation G for a) internal fault b) remote fault on a connected line For a double circuit line feeding a fault on one circuit, a hown in Figure I.5, then the induced current on the earth wire are k(3i0a+3i0ab) between ubtation A and the fault and k(3i0b-3i0ab) between the fault and ubtation B. The current ource for the ditributed earth wire current at the fault location, however, i k(3i0a+3i0b). If the fault involve both circuit then the induced current i k time the total zero equence current on both line ection. Figure I.5 ero equence current for double circuit line For non-uniform line, where all the line parameter and footing reitance are known, a olution can be found by referring to the equivalent circuit given in Figure I Rec. ITU-T K.104 (03/2015)

43 Figure I.6 Earth and earth wire current due to a phae-to-earth fault howing a reditribution of the earth wire current at the point of connection to a cable due to change in k for a) induced earth and earth wire current b) equivalent circuit for the reditributed earth wire current If the type or number of earth wire change, uch that the earth wire creening factor change, then the point of change repreent another earth current ource point that can lead to an EPR and the equivalent circuit i a hown in Figure I.6, where there i an additional ource/ink point of magnitude (k2-k1)3i0 located at the tranition point between the cable and the overhead line. Rec. ITU-T K.104 (03/2015) 37

44 Appendix II Through tower earthing during power line fault (Thi appendix doe not form an integral part of thi Recommendation.) NOTE Thi appendix i baed on [b-cigre Guide]. Thi appendix preent an algorithm for the calculation of the current ditribution along a hield wire in the cae of a fault occurring on a relevant power line. In particular, attention i focued on the calculation of the current injected into the oil at all the tower location. The EPR produced into the earth i trongly influenced by thee current. Moreover, ome calculation example are given by varying the main parameter influencing the phenomenon within typical range. II.1 Equivalent circuit of the earth wire with earth return The circuit repreenting the earth wire with earth return can be modelled by mean of a cacade of cell a hown in Figure II.1. Figure II.1 Lumped element repreentation of earth wire with earth return circuit In Figure II.1, the lumped parameter have the following meaning: Rt1 and Rtn+1 are the ubtation earth grid reitance; Rtk are the tower earthing reitance (k=2,3,,n); k are impedance with earth return aociated with the k-th cell that ha to be identified with the k-th pan between tower k and k+1 (k=1,2,,n). When a ingle fault 1 to earth occur at the location k: a current J(k)=JA(k)+JB(k) injected on the earth wire at the fault location k (JA(k) and JB(k) being the fault current flowing on the faulted phae and coming from the two feeding ubtation); the fault current coming from the two feeding ubtation JA(k) and JB(k) induce, due to inductive coupling, an electromotive force (emf) on the earth wire, earth circuit. The induced emf in the generic i-th cell i given by the expreion: zmi JA k Fi zmi JB k L L i i i k i k (II.1) 1 Conider the cae of low impedance or olidly-earthed ytem. 38 Rec. ITU-T K.104 (03/2015)

45 zmi being the per unit length mutual impedance between the faulty phae-earth circuit and hield wire-earth circuit and Li being the length of the i-th pan. The inducing current JA(k) or JB(k) mut be known 2 and have oppoite ign. Therefore, in the equivalent circuit in Figure II.1 it i neceary to add uitable current and emf generator a repreented in Figure II.2: Figure II.2 Earth wire earth equivalent circuit with a faulty phae In the cae of two earth wire, the equivalent circuit repreented in Figure II.2 can till be ued provided that the longitudinal element of each cell repreent the the venin equivalent of the parallel of the two earth wire (ee Figure II.3). Figure II.3 The venin equivalent circuit of two coupled earth wire In Figure II.3 and with reference to the k-th cell, 1k and 2k are the impedance with earth return of the two hield wire, 12k i the mutual impedance between the two circuit and F1k and F2k are the induced emf by inductive coupling with the faulted phae. Explicit expreion for eqk and Feqk are: II.2 Feq Solution of the circuit k eq F2 k k 2 1k 2k 12k k (II.2) (II.3) The above two-conductor line circuit can be olved by an appropriate calculation technique uch a given in ITU-T Directive Vol. II and Vol. III, [ITU-T K.26]. k 1 12 F k 1 k k 2 12 k k k 2 k k k 2 They are a function of fault location and have to be previouly calculated (generally by the power ytem operator). Rec. ITU-T K.104 (03/2015) 39

46 II.3 Example of application In thi example, conider a ingle circuit power line, 30 km long, equipped with one earth wire and a pan length of 300 m; uppoe that a fault occur at km 15 and that the fault current i 10 ka (5 ka coming from ubtation A and 5 ka coming from ubtation B). Calculation were made by conidering different type of earth wire (ee Table II.1) and different value of tower earthing reitance related to different value of oil reitivity. (Rt=10 Ω with oil reitivity =300 Ωm; Rt=50 Ω with oil reitivity =1500 Ωm, Rt=100 Ω with oil reitivity =3000 Ωm). At both end of the line, two ubtation earth grid having an area Agrid of 400 m 2 ; their reitance ha been etimated by mean of thi implified equation: R grid 4 A grid (II.4) (thu: with =300 Ωm Rgrid=6.65 Ω; with =1500 Ωm Rgrid=33.23 Ω; with =3000 Ωm, Rgrid=66.47 Ω). Table II.1 Earth wire characteritic Type of earth wire Diameter [mm] Reitance [Ω/km] Steel Alumoweld Copperweld Figure II.4 to Figure II.6 repreent the current It injected into the oil at tower location along the route of the power line. Looking at Figure II.4 to Figure II.6, how that by increaing the value of the earthing reitance a decreae of the magnitude of the injected current at the tower near the fault location, however, at the ame time, the curve tend to become flater howing a kind of "balance effect". The final reult how many more tower along the power line that are ignificantly involved in the earth wire current diipation to oil. The ame conideration hould be taken into account with repect to the earth wire characteritic. For given value of the tower earthing reitance, the curve tend to be more balanced by decreaing the per unit length reitance of the earth wire. To conclude, in the cae of oil with low reitivity and a teel earth wire, a high current diipation i hown (eentially concentrated near the faulty location). In comparion, in the cae of oil with high reitivity and a copperweld earth wire, a larger number of tower contribute to diipate the current in a more uniform way along the line 40 Rec. ITU-T K.104 (03/2015)

47 Figure II.4 Current injected into the oil at the earthing point for different kind of earth wire: Rt=10 Ω, Ic = 10 ka Figure II.5 Current injected into the oil at the earthing point for different kind of earth wire: Rt=50 Ω, Ic = 10 ka Rec. ITU-T K.104 (03/2015) 41

48 Figure II.6 Current injected into the oil at the earthing point for different kind of earth wire: Rt=100 Ω, Ic = 10 ka 42 Rec. ITU-T K.104 (03/2015)

49 Appendix III Impedance to earth of MV/LV tranformer tation (Thi appendix doe not form an integral part of thi Recommendation.) NOTE Thi appendix i baed on [b-cigre Guide]. III.1 Type of meaured tranformer tation Thi claue preent the reult of ite meaurement of the impedance to earth of 20 kv / 0.4 kv and 10 kv / 0.4 kv tranformer tation that feed rural overhead or cable LV line. The meaurement were made in the 40 Hz to 8000 Hz frequency range, with the aim of determining the global impedance to earth and it component, i.e., the input impedance of the LV neutral and the reitance of the local earthing. Note that the global impedance of the tation i equal to the equivalent impedance to earth only in the cae when the MV feeding line ha no return earthed conductor (heath or earth wire). In other cae the meaured global impedance i maller than the equivalent earth impedance of the tation. In thee cae, the meaured value could be modified with the cable heathto-earth or earth wire-to-earth loop input impedance of the MV line(). The meaured tranformer tation can be claified into the categorie identified in Table III.1. III.2 Meaurement method The cheme of the applied meaurement method i hown in Figure III.1. The meauring current wa injected between the neutral bu and the current probe. Meaurement were taken at dicrete injected frequencie avoiding the harmonic frequencie. Both the current and voltage probe were located m away from the tranformer tation in a perpendicular direction to the MV feeder and alo a far a poible from the earthing of the LV line. The meauring current flowing into the neutral conductor, local earth and the neutral of the tranformer (if poible) were eparately meaured by Rogowki coil. The applied meauring technique i hown in Figure III.2. The injected and the part current and alo the EPR of the neutral bar were meaured by a elective meter. The global impedance to earth and the impedance to earth of the neutral conductor have been calculated a the ratio of the EPR and the appropriate current. The meaurement made by ome commercially available earth reitance and reitivity meauring intrument apply the ame principle, but the meaured value are proceed and thu the impedance reult themelve are monitored. Rec. ITU-T K.104 (03/2015) 43

50 Table III.1 The characterization of the meaured tranformer tation Station Type of MV line Type of LV line Pipeline Environment Station 1 Station 2 Station 3 overhead line overhead line overhead line 3 line with platic inulated cable 3 line with platic inulated cable, 1 overhead line 4 line with platic inulated cable platic teel, connected platic new, uburban rural new, rural Station 4 lead heathed arm. cable with cont. leakage to the earth 7 line, mixture of overhead line and cable teel, connected rural Station 5 Station 6 Station 7 Station 8 Station 9 Station 10 Station 1 Station 12 (Note) Station 13 (Note) lead heathed arm. cable with cont. leakage to the earth lead heathed arm. cable with cont. leakage to the earth platic inulated cable platic inulated cable platic inulated cable overhead line overhead line platic inulated cable mixed inulated cable line 5 overhead line 4 overhead line, 2 line with platic inulated cable 7 line with platic inulated cable 5 line with platic inulated cable 8 line with platic inulated cable 4 line with platic inulated cable 2 line with platic inulated cable 6 line with platic inulated cable 4 line with platic inulated cable NOTE 10 kv/0.4 kv tation, otherwie 20 kv/0.4 kv tation. teel, connected teel, connected platic platic mixed platic teel, connected platic platic week-end place week-end place new, uburban new, uburban rural new, uburban week-end place city centre city centre 44 Rec. ITU-T K.104 (03/2015)

51 Figure III.1 Circuit cheme of earthing meaurement of an MV/LV tranformer tation a) Current meaurement of the current in the LV neutral of the MV/LV tranformer and in the neutral conductor of the outgoing LV line Rec. ITU-T K.104 (03/2015) 45

52 III.3 b) Current meaurement in the neutral conductor of the outgoing LN line Figure III.2 Meaurement of the current in an MV/LV tranformer tation by flexible clamp-on current tranformer (Rogowki coil) Reult of the meaurement The global impedance of the meaured thirteen tranformer tation are plotted v. the frequency in Figure III.3 and are given for characteritic frequencie in Table III.2. The global impedance of the tranformer tation and it component meaured by Chauvin-Arnoux intrument are plotted in Figure III.4. III.4 Concluion The global impedance to earth of the tranformer tation i very mall at the main frequency. Thi eentially reult from the mall input impedance of the LV neutral conductor, which are in line with the value obtained from the imulation calculation auming frequent cutomer (Lt = 25 m), low reitance of the dicrete earthing tructure (Rf = 3 ) and metallic connection to teel pipe (ee Appendix V). 46 Rec. ITU-T K.104 (03/2015)

53 a) Frequency range: 50 Hz to 8'000 Hz b) Frequency range: 50 Hz to 200 Hz Figure III.3 Global impedance to earth of the 10/0.4 kv and 20/0.4 kv tranformer tation characterized in Table III.1 Rec. ITU-T K.104 (03/2015) 47

54 Table III.2 Value of the impedance to earth of MV/LV tranformer tation in three frequencie Station No. Frequency [Hz] 50 1'900 8' Station 6 Station 4 global impedance of the tation global [Ohm] Iterative impedance 4pole meaurement Obj:01 Tet Frequency [Hz] global [Ohm Iterative impedance 4pole meaurement Obj:02 Tet Frequency [Hz] elective Impedance into Ground Obj:01 Tet 02 Selective impedance into ground Obj:02 Tet local earthing electrode local [Ohm] Frequency [Hz] local [Ohm] Frequency [Hz] 48 Rec. ITU-T K.104 (03/2015)

55 elective impedance into PEN wire Obj:01 Tet 03 elective impedance into PEN wire Obj:02 Tet 05 input of the combined neutral in [Ohm] Frequency [Hz] in [Ohm] Frequency [Hz] Figure III.4 The meaured global impedance of the MV/HV tranformer tation and their component Rec. ITU-T K.104 (03/2015) 49

56 Appendix IV Tranferred voltage and current by mean of LV neutral conductor (Thi appendix doe not form an integral part of thi Recommendation.) NOTE Thi appendix i baed on [b-cigre Guide]. IV.1 Sytem modelling, option and parameter The neutral line connecting to an MV/LV tranformer tation affect both the global impedance, a well a the EPR of the tranformer tation and the potential tranferred to the LV cutomer. The effect of the different parameter of the neutral line ytem are invetigated by a imulation tudy. The preence of a metallic pipe laid on the treet parallel to the LV line contitute an additional path for the earth current. Therefore, it i affecting the reultant characteritic of the neutral ytem epecially in TN ytem when the neutral and the pipe are metallically cro-connected at regular interval, i.e., at the cutomer' premie. The neutral line ytem together with the coupled pipeline, if preent, ha been imulated a a multiconductor line ytem with multipoint boundary condition, applying the MULTS oftware [b-sollerkvit]. In thi method, the line ection are imulated a ditributed parameter line and the lumped element (pole earthing reitance, connection between the neutral and the pipe and the termination) a boundary condition. The applied technique olve the differential equation ytem (et of the telegraph equation) with the conideration of the boundary condition. The arrangement of the invetigated LV line and pipeline are hown in Figure IV.1. The imulation calculation have been performed for the following three ituation: 1) only the neutral of an overhead LV line i conidered; 2) the neutral of the LV line and a buried teel pipe laid in parallel, not connected with the neutral; 3) the neutral of the LV line and a buried teel pipe laid in parallel, connected with the neutral. The characteritic of the neutral of an overhead LV line are: aluminium trand of 95 mm 2, GMR=5.76 mm, Rac = /km; length: 1 km; terminated by: RE = 0.1, 0.5, 1.0 or infinite reitance; Note that, the terminating reitance RE repreent the input reitance of the neutral of the neighbouring feeding area. It i infinite when the neutral conductor i dirupted at the feeding boundary. the ditance between the dicrete earthing location: Lt = 25, 100 or 250 m; the value of the concentrated earthing reitance: Rf = 1, 3, 10, 30, 100 or 300. Rf i the concentrated reitance, which repreent the local earthing at the cutomer' premie. 50 Rec. ITU-T K.104 (03/2015)

57 Figure IV.1 Arrangement of the invetigated LV line and pipeline Note that the neutral-to-earth leakage ha been imulated in the following two way: 1) with concentrated earthing reitance Rf according to the above defined option; or 2) with a uniform leakage conductance: S/km n where n i the number the concentrated earthing connection along the 1 km long neutral line. The feature of the aumed teel pipe are: the lateral ditance between the LV and the pipeline: LP = 2 or 12 m; the pipe ha 500 m extenion at both end of the LV line and i terminated by the characteritic impedance of the pipe-to-earth loop (relevant to the invetigated frequency); the pipe diameter: 80 mm and wall-thickne: 3 mm; G f R 1 / f the impedance of the pipe (external urface impedance with external current return) wa conidered according to Table IV.1. Rec. ITU-T K.104 (03/2015) 51

58 Table IV.1 Meaured external urface impedance of teel pipe f [Hz] kk [Ω/km] [degree] R kk [Ω/km] X kk [Ω/km] The calculation have been performed for all of the above option (6 4 = 24 for each of the three Lt option, i.e., total of 24 3 = 72) for the following frequencie: Frequencie conidered in the imulation calculation and ite meaurement IV.2 Feeding of the neutral-to-earth loop The imulation calculation have been performed for the condition of 100 A current being injected into the neutral-to-earth loop at the MV/LV tranformer tation end. Thi type of injection ha the advantage that the neutral voltage obtained can eaily be recalculated to any other (fault) current value. Furthermore, the input impedance of the neutral i imply given a the input voltage divided by 100 A. IV.3 Voltage and current profile v. length of the neutral The voltage and current profile v. length figure have been drawn for all calculated option. Thee are reproduced for a few repreentative cae which reflect the main tendencie. The profile curve drawn with olid line how the reult obtained by imulation with dicrete earthing reitance, while thoe drawn with dotted line how the reult obtained by the imulation by the uniform leakage Gf. 52 Rec. ITU-T K.104 (03/2015)

59 The curve marked with different colour are related to the following Lt with dicrete earthing ditance: The firt group of figure how the profile for f = 50 Hz, Lp = 2 m with not-connected pipe (Figure IV.2 and IV.3) and connected pipe (Figure IV.4 and IV.5) conidering termination reitance of RE = 0.5 i infinite, repectively. The voltage and current profile v. length are plotted for a higher frequency (1'980 Hz) and alo for cae with not-connected pipe and connected pipe, hown in Figure IV.6 and IV.7, repectively. The claification of thee figure i given in Table IV.2. Table IV.2 Claification of the figure howing the voltage and current profile of the LV neutral conductor Condition for the connection with pipeline Frequency 9 Hz Not connected Terminating reitance Ω Connected Terminating reitance Ω Figure IV.2 Figure IV.3 Figure IV.4 Figure IV.5 1'980 Figure IV.6 Figure IV.7 The following main tendencie can be oberved from the voltage profile of the neutral of the neutral conductor: the tranferred voltage decreae with decreaing earthing reitance Rf and with horter pan between earthing electrode; the connection to the pipeline reduce the tranferred voltage, roughly peaking, by 50 per cent; the connection of the neutral at the feeding boundary reduce the tranferred voltage, epecially when Rf i high; the difference between voltage calculated by dicrete Rf (real condition) and by ditributed leakage Gf i higher for maller Rf and lower for horter pan between earthing electrode. The attenuation with the length of the tranferred voltage i much quicker for thi higher frequency epecially for maller Rf value and pipe connected cae. Thi i due to the change (decreae) in the length contant of the neutral-to-earth loop. Rec. ITU-T K.104 (03/2015) 53

60 a) R f = 3 b) R f = 30 c) R f = 300 Figure IV.2 Voltage and current profile of the neutral v. length, not connected to pipeline, f = 50 Hz, Re = 0.5, LP = 2 m 54 Rec. ITU-T K.104 (03/2015)

61 a) R f = 3 b) R f = 30 c) R f = 300 Figure IV.3 Voltage and current profile of the neutral v. length, not connected to pipeline, f = 50 Hz, Re = inf., LP = 2 m Rec. ITU-T K.104 (03/2015) 55

62 a) R f = 3 b) R f = 30 c) R f = 300 Figure IV.4 Voltage and current profile of the neutral v. length, connected to pipeline, f = 50 Hz, Re = Rec. ITU-T K.104 (03/2015)

63 a) R f = 3 b) R f = 30 c) R f = 300 Figure IV.5 Voltage and current profile of the neutral v. length, connected to pipeline, f = 50 Hz, Re = inf. Rec. ITU-T K.104 (03/2015) 57

64 a) R f = 3 b) R f = 30 c) R f = 300 Figure IV.6 Voltage and current profile of the neutral v. length, not connected to pipeline, f = 1'979 Hz, Re = inf., LP = 2 m 58 Rec. ITU-T K.104 (03/2015)

65 a) R f = 3 b) R f = 30 c) R f = 300 Figure IV.7 Voltage and current profile of the neutral v. length, connected to pipeline, f = 1'979 Hz, Re = inf., LP = 2 m Rec. ITU-T K.104 (03/2015) 59

66 Appendix V Input impedance of the LV neutral-to-earth loop (Thi appendix doe not form an integral part of thi Recommendation.) NOTE Thi appendix i baed on [b-cigre Guide]. Input impedance of the LV neutral-to-earth loop ha been calculated according to the modelling, option and parameter decribed in claue 4. A urvey of the value (modulu and phae) of the power frequency (50 Hz) input impedance of the neutral-to-earth loop i given in Table V.1. Thee value are given for the option of the two key parameter, i.e., the ditance Lt between the ubequent dicrete earthing location and the reitance Rf of dicrete earthing tructure. The effect of the terminating reitance Re repreenting the poible continuation of the neutral i alo demontrated. The relative importance of the different parameter can be more eaily identified on the bai of the bar diagram hown in Figure V.1. For the value of the input impedance the following main concluion can be tated: input impedance value can vary in a wide range, i.e., between 0.25 to 74 ; the increae of Rf by an order of magnitude caue an increae of the input impedance by a factor of two; the input impedance increae nearly proportionally with the ditance Lt between the dicrete earthing if the neutral ha no extenion; the preence or abence of the terminating reitance Re i important in the cae of a poor earthing condition of the neutral ection under tudy; the teel pipe effectively reduce (by factor two or even greater extent) the input impedance only in the cae where the neutral and the pipe are metallically connected (TN ytem). When the neutral conductor i terminated at the feeding boundary (i.e., Re i infinite) and the neutral ha no metallic connection to the pipeline, the input impedance increae in the following extent, depending on the value of the earthing reitance Rf and frequency: for mall reitance (Rf = 3 Ω) 2-3 time increae, depending on the frequency; for medium reitance (Rf = 30 Ω) 3-15 depending on the frequency; for high reitance (Rf = 300 Ω) time increae, increae depending on the frequency. Therefore, the neutral termination ha a pecial importance in the cae of high reitance earthing and/or it rare application. 60 Rec. ITU-T K.104 (03/2015)

67 Table V.1 Input impedance of the neutral-to-earth loop for f = 50 Hz, Re = 0.5 Ω Ditance between dicrete earthling Reitance of dicrete earthing Terminating reitance Connected (c) Condition for the teel pipe connection Separated () Without pipe (wo) L t [m] (R f) [Ω] (R e) [Ω] in [Ω] Phae degree in [Ω] Phae degree in [Ω] Phae degree inf inf inf inf inf inf inf inf inf a) Neutral continued at ection boundary, repreented by R e = 0.5 Ω Rec. ITU-T K.104 (03/2015) 61

68 b) Neutral continued at ection boundary, repreented by the characteritic impedance of the neutral conductor-to-earth circuit c) Neutral eparated at ection boundary Figure V.1 Input impedance in of the neutral-to-earth loop at 50 Hz for different earthing reitance, ditance between the earthing electrode location and different condition regarding the pipe: no bonding (be_ö), neutral and pipe iolated (be_z) or no pipe at all (be_n) 62 Rec. ITU-T K.104 (03/2015)

69 Appendix VI EPR due to a double earth fault in an MV ditribution network without a olidly-earthed neutral (Thi appendix doe not form an integral part of thi Recommendation.) NOTE Thi appendix i baed on [b-cigre Guide]. Thi appendix i focued on the EPR occurring in MV/LV tranformer tation when double earth fault occur in an MV ditribution network with a non-olidly-earthed neutral (i.e., network with earthing through an arc uppreion (Peteren) coil or thoe earthed through a high reitance (25 to 100 ), both of which are common in Europe). The EPR of the MV/LV tranformer tation could be tranferred to the LV network through the neutral of TN or TT ytem earthing of the LV network. VI.1 Problem identification 1) Network condition In the cae of an MV ditribution ytem an HV/MV tranformer tation feed a group of MV line (typically 10 to 30 kv). The line are aumed to be in a ring-type configuration, however the ring are open at one point, o the network i operated a a radial ytem. The network are typically compoed of underground cable in urban area, a mixture of cable and overhead line in uburban area and overhead line in rural area. The MV cable network of an urban area i illutrated in Figure VI.1. Figure VI.1 Illutration of an MV cable network fed from an HV/MV tranformer tation The network i a mixture of: older, lead-heathed, teel-armoured cable, directly laid in the oil (with type code ended by "VB") where they reduce the overall earthing reitance of both the HV/MV and MV/LV tranformer tation; Rec. ITU-T K.104 (03/2015) 63

70 newer, platic-jacketed cable the heath/creen of which are earthed only in the ubtation. Older MV cable tend to decreae over time, a thee cable are replaced with newer cable, cauing an increae in the overall earthing reitance of the tation. Similar tendency apply to the LV cable. 2) Frequency of the occurrence of double earth fault In the cae of an HV power ytem with olidly-earthed neutral the phae-to-earth hort-circuit level hould be conidered when examining the induction to telecom line, becaue thi fault i conidered a a high probability event. The "high probability" event i correct in the ene that more than 95 per cent of the fault are phae-to-earth fault in network with a directly earthed neutral. In the cae of an MV ditribution ytem with no olidly-earthed neutral, the frequency per km and year, of phae-to-earth fault i higher by about two order of magnitude than that of an HV ytem with olidly-earthed neutral. However, the magnitude of the fault current caued by a ingle earth fault i low, thu it hort-term induction effect uually doe need not to be conidered. On the other hand, about 5 per cent of ingle earth fault create double earth fault, i.e., one phae-to-earth fault at a given location and imultaneouly another phae-to-earth fault in another phae at another location (ee Figure VI.2) 3. However, due to the large number of MV network compared to HV network, the number of double earth fault on MV network could, in practice, be up to 5 to 10 time higher than the number of ingle earth fault occurring on HV network. The double earth fault i a hortcircuit fault reulting in earth-return (zero equence) current along the line between the two faulty point. The double earth fault current can caue ignificant induction effect along the faulty ection and EPR at the fault location (generally in tranformer tation). VI.2 Study of the relative importance of the network parameter and condition For the identification of the relative importance of the different parameter, imulation tudie have been carried out uing the following parameter et: 1) hort-circuit power of the network. Eentially determined by the HV/MV (120/10.5 kv) tranformer, rated power (31.5 MVA) and hort-circuit impedance (18 per cent); 2) earthing reitance of the grid of the HV/MV tranformer tation (Rf120 = 0.3 ); 3) earthing reitance of the earth grid of the MV/LV tranformer tation (Rf10 = 2 or 0.2 ); 4) heath reitance of the MV cable (Rk10 = /km); 5) reitance at the fault point (e.g., arc reitance) (Rh = 1 or 10 ); 6) ditance of the faulty point from the feeding point: (Li = km) from one another: (L1-2 = 0.6 km); 7) heath-to-earth leakage conductance 3 A German utility (PESAG) note that 30 per cent of fault are double earth fault on an 80 per cent underground MV ytem with compenated neutral (NMT95). 4 Rounded value of the actual 0.75 /km. 64 Rec. ITU-T K.104 (03/2015)

71 (G = S/km). NOTE The value in bracket have been ued in the parametric tudy. The invetigated network arrangement and the ome of the key parameter are hown in Figure VI.2. Figure VI.2 Double earth fault, one in the HV/MV and the other in the MV/LV tation The fault current and heath current due to the double earth fault are plotted in Figure VI.3. The EPR of the HV and MV tation are plotted for the above et of parameter in Figure VI.4. VI.3 Main concluion The main concluion drawn from the parameter tudy are a follow: the EPR occurring in the MV tation i ignificantly higher than the EPR in the HV tation; the EPR ignificantly increae with an increae of the earthing reitance of the MV/LV tation. Increae in the earthing reitance of the MV/LV tation can occur due to: maller leakage to earth of the cable (fewer older lead-heathed cable); bigger earthing reitance at the cutomer premie (fewer metallic tube ued for water, ga upply ytem); the EPR increae with the increae of the reitance of the heath (i.e., due to the ue of a thinner creen); the EPR increae with the length of the cable to the faulty point up to a certain ditance (about 2 km) and then it tend to decreae; the EPR could decreae ignificantly with the increae of a fault reitance (e.g., reitance of the arc), which i not a controllable parameter. Thi change i caued by the change in the magnitude of the fault current (ee Figure VI.4). Rec. ITU-T K.104 (03/2015) 65

72 66 Rec. ITU-T K.104 (03/2015) Figure VI.3 EPR of the HV and MV ubtation grid due to double earth fault

73 67 Rec. ITU-T K.104 (03/2015) Figure VI.4 Fault and heath current due to double earth fault

74 VII.1 Appendix VII Screening factor of a power cable with an imperfectly earthed heath (Thi appendix doe not form an integral part of thi Recommendation.) Criterion for long cable For cable with continuouly earthed heath, the criterion for long cable i: L >> 6 (VII.1) where L i the length of the cable and i the length contant of the heath-to-earth loop (ee equation (4) to (6)). Under thi condition, i.e., in the cae of long (in principle infinite) cable line the current induced in the heath-to-earth loop i uniform; the current in the creening conductor and earth can be expreed by equation (2) and equation (3) repectively with the creening factor k given by equation (4). In real line, the value calculated by the creening factor are relevant only to the teady middle ection of the line. For cable with inulating cover, the criterion for long cable i: zl >> a + b (VII.2) where z i the impedance of the heath-to-earth loop (ee equation (3)), per unit length, a and b are the impedance/reitance to earth at the end of the cable ection (ee Figure VII.1). VII.2 Short (finite length) cable heath with continuou earthing When the criterion (VII.1) i not fulfilled for a cable heath with continuou earthing, i.e., the cable i hort then the creening current continuouly change along the length, and it i affected by the dicrete earthing impedance at the line end, thu the creening action cannot be expreed by the creening factor any more. A a conequence, the EPR calculation baed on the ue of the creening factor of the in-feeding power line (ee claue A.2.3) cannot be applied. In hi cae the heath-to-earth loop circuit hould be olved, by conidering it actual condition. The current and voltage reult v. length, provide the relevant information on the creening effect and potential of the creening conductor and the EPR of earthing connected to it. Analytical expreion are contained for the particular cae when the heath/creening conductor i affected by uniform induced emf. See ITU-T Directive Vol. II claue 4.3.7, [ITU-T K.26]. VII.3 Screening factor of a power cable with an inulating cover When a cable heath/creen ha an inulating cover it can be earthed with dicrete earthing applied at different location. In the following, the creening factor i pecified for hort cable ection earthed only at the end of the ection. In thi cae, the creening factor i not an intrinic characteritic of the cable becaue it i affected by the impedance of the applied earthing and ditinction hould be made between the creening factor related to the heath or to the earth. The cheme and circuit repreentation of a hort power cable heath covered by an inulating jacket and terminated with dicrete earthing i hown in Figure VII.1 (ee ITU-T Directive Vol. II claue , [ITU-T K.26]). The figure how the poible earth fault option, i.e., fault to the heath (to the heath connected earthing ytem) at the a end (a) or b end (b) or to the earth at the earth in the overhead line in the ide a (ea) or in that of in the ide b (eb). The witche in the equivalent circuit hould be poitioned according to the fault location. Thu, the equivalent circuit can be adjuted according to the poible four earth fault option. 68 Rec. ITU-T K.104 (03/2015)

75 Figure VII.1 Scheme and circuit repreentation of a power cable heath covered by inulating jacket and terminated with dicrete earthing The poition of the witche hown by olid line correpond to the cae when both fault (Sa and Sb) of a double earth fault are to the heath. In thi cae, the creening factor i the one related to the heath. The poition of the witche hown by broken line correpond to the cae when both fault of a double earth fault occur on the overhead line. In thi cae, the creening factor i the one related to the earth. Mixed fault location are alo poible, i.e., when one fault i on the overhead line (OHL) and thu i earth related, while the other fault occur in the cable ection and thu i heath related. The following circuit element are hown in Figure VII.1b: R E A L reitance of the heath, per unit length external impedance of the heath-to-earth loop obtained according to equation (3), per unit length additional impedance due to the teel armoring, if applied, per unit length the length of the in the ame unit a ued in the per unit impedance Ra (a) reitance (impedance) to earth at the a end of the cable ection Rb (b) reitance (impedance) to earth at the b end of the cable ection 3I0 IE 3IS k = IE/3IS zero equence component of the fault current earth current along the cable ection the heath current i the creening factor of the heath. The earth current and thu the creening factor k of the heath can be expreed by the current hare rule applied to the circuit repreentation hown in Figure VII.1b. In fact, thi i the fraction of the earth current given a the ratio of the impedance of the upper branch of the circuit to the total impedance of the heath-to-earth loop. The impedance of the upper branch hould be varied according to the fault location repreented by the poition of the witche. Rec. ITU-T K.104 (03/2015) 69

76 The creening factor for the poible four fault poition location are given by the following expreion: heath related at both ide: earth related at both ide: earth related at the ide a and heath related at the ide b: heath related at the ide a and heath related at the ide b: (VII.3) (VII.4) (VII.5) (VII.6) It i recognized that the impedance to earth improve the creening factor related to the heath and woren the creening factor related to the earth. When any of the earthing impedance become infinitive (unearthing) the creening factor related to the heath become zero, i.e., perfect creening, while the creening factor related to the earth become one, i.e., not creening at all. VII.4, SS = ( Screening factor for non-uniform line k k k k, EE, ES, SE I E 3 I 0 I E 3 I 0 I E 3 I 0 I E 3 I 0 = ( = ( = ( E E E E R In power ytem with olidly-earthed neutral one phae-to-earth, hort-circuit caue high fault current. In thi cae the 3I0 current i circulating between the fault location and the feeding point(). The OHL of a power ytem with a olidly-earthed neutral ha earth wire or multi earthed neutral which i connected to the cable heath and to the earthing of the tower at the OHL to cable tranition. Such a ituation i hown and an equivalent circuit upporting an approximate olution can be found by referring to Figure I.6. Due to the difference in the creening factor k,ohl of the earth wire and the creening factor k,c of the cable heath there i a difference in the creening current of magnitude (k,ohl k,c) 3I0. Thi i an additional current ource/ink point at the tranition point between the cable and the overhead line. The majority of thi current i paing through the tower earthing at the tranition point that can lead to a ignificant EPR which may exceed the EPR caued by the ame 3I0 in the ubtation. For non-uniform line, where the type or number of earth wire change or the relative poition between the earth wire and phae conductor' change (at the location of phae tranpoition) uch that the earth wire creening factor change alo repreent another earth current ource point that can lead to an EPR. A A A R L R ) L R L Ra Rb R ) L R R L Ra R ) L R A R L Rb R ) L R a a a a R b R b R b R b 70 Rec. ITU-T K.104 (03/2015)

77 Appendix VIII Screening factor of telecommunication cable with imperfectly earthed heath (Thi appendix doe not form an integral part of thi Recommendation.) To ditinguih the telecommunication cable of long and hort length, the ame criterion applie a with power cable i.e., cable with continuouly earthed heath equation (VII.1) and cable with inulating cover equation (VII.2). In the following the creening factor are conidered for telecommunication cable with inulating cover for two induction option. VIII.1 Telecommunication cable affected by longitudinal induction The creening factor of a telecommunication cable i given for the following condition: the cable heath ha an inulating cover; the heath i earthed at the A and B end by dicrete impedance; the length of the cable i l km; the heath-to-earth loop i affected by longitudinal induction cauing total longitudinal emf of: E l l E (VIII.1) Under thee condition four longitudinal voltage can be ditinguihed a hown in Figure VIII.1: k E k Figure VIII.1 Longitudinal voltage with repect to different reference Uing the coaxial hypothei, the longitudinal voltage, by conideration of the creening action of the heath, can be expreed with the following creening current: I S l R a e A B (VIII.2) where: R i the heath reitance, a i additional impedance due to teel armor; e i the external impedance of the heath with earth return, all in /km, A and B are the earthing impedance at the end of the cable, in. The longitudinal voltage meaured to the heath at the end A on the conductor (blue) connected to the heath at the end B (ee Figure VIII.2) i given by: k E k Rec. ITU-T K.104 (03/2015) 71

78 72 Rec. ITU-T K.104 (03/2015) (VIII.3) Figure VIII.2 Longitudinal voltage related to the heath at both end The creening factor kss related to the heath at both end i obtained by applying the definition expreion and ubtituting USS a follow: (VIII.4) Similarly, the longitudinal voltage meaured to the earth at the end A on the conductor (green) connected to the earth at the end B (ee Figure VIII.3) i given by: (VIII.5) Figure VIII.3 Longitudinal voltage related to the heath at both end The creening factor kee related to the earth at both end i obtained by applying the definition expreion and ubtituting UEE a follow: (VIII.6) The creening factor kes related to the earth at the A end and to the heath at the B end can be derived from the UES voltage (purple conductor) a follow: k k B A e a SS E R l R l I R l U B A E a S S k k SS SS R l R l E U k k k B A e a B A S B A EE E R l R l I R l U ) ( ) ( B A E a S B A S k k EE EE R l R l E U k

79 (VIII.7) Finally the creening factor kse related to the heath at the A end and to the earth at the B end can be derived from the USE voltage (red conductor) a follow: VIII.2 Telecommunication cable affected by EPR k k ES SE U SS E k U SE E k k k l l RS a E A B (VIII.8) Thi claue dicue the particular creening effect of a telecommunication cable creen connected to earthing which i affected by EPR (Figure VIII.4). The telecommunication cable enter the EPR zone and it heath/creen i connected to the earthing ytem affected by the EPR. The EPR zone can be e.g., an MV/LV tranformer tation including the earthing ytem of thoe nearby conumer to which the EPR i totally or partly tranferred through the neutral conductor of a TN ytem. It i aumed that the creened telecom cable ha an inulating covering. In the telecom ite the heath i bonded to the earthing ytem of the ite (to the MET, ee Figure 16). Therefore, the longitudinal voltage, Ut tranferred to thi ite i related to the heath. However, if there i an uncreened extenion of the telecom line, the longitudinal voltage Uet in the ite of thi extenion i earth related. l R S l R S RS a E A B A B Figure VIII.4 Arrangement of telecommunication cable affected by EPR The creening factor relevant to the decribed ituation can be determined in the ame tep a decribed in the previou claue, however the affecting ource i a dicrete voltage ource repreenting the EPR. NOTE The reaction of the telecom creen to the EPR magnitude i neglected. The current IS in the telecom heath-to-earth loop i given by the following equation: I S l U R a e T (VIII.9) where: R i the heath reitance, a i additional impedance due to teel armor (if applied); e i the external impedance of the heath with earth return, all in /km; RT (may be T) i the earthing reitance (impedance) at the telecommunication ite, in. EPR Rec. ITU-T K.104 (03/2015) 73