TUTORIAL B1.23 TB 559

Transcription

1 TUTORIAL B1.23 TB 559 IMPACT OF ELECTROMAGNETIC FIELDS ON CURRENT RATINGS AND CABLE SYSTEMS Convener: Harry Orton Secretary: Paolo Maioli Page 1

2 IMPACT OF EMF ON CURRENT RATINGS AND CABLE SYSTEMS Members H. Orton, Convener (CA), P. Maioli, Secretary (IT), T. Barnes (AU), H. Brakelmann (DE), J. Bremnes (NO), F. Lesur (FR), J. Lopes (BR), J. Saenz Orella (ES), J. Smit (NL) Corresponding Members S. Cherukupalli (CA), J. Hoeffelman (BE), Contributions also received from F. Cochet (CH), J. Stammen (DE), J. Brüggmann (DE) Page 2

3 IMPACT OF EMF ON CURRENT RATINGS AND CABLE SYSTEMS This tutorial is the result of the work performed by CIGRE WG B1.23 and published in Technical Brochure 559 (December 2013). Scope of work: The work shall focus on single conductor, high voltage, AC, land cable systems, excluding pipe type and GIL (Gas Insulated Lines) cables, considering: 1. Extruded dielectric insulation, 2. Laminar dielectric insulation, 3. Single-point bonding or balanced cross-bonding, so that there is no ground return current. Page 3

4 IMPACT OF EMF ON CURRENT RATINGS AND CABLE SYSTEMS Terms of Reference 1/2: To define correct terminology for magnetic field management techniques. To review practical magnetic field management methods that are currently used for underground power cable systems. To quantify the shielding effectiveness of practical methods. To review practical engineering design and construction considerations. Page 4

5 IMPACT OF EMF ON CURRENT RATINGS AND CABLE SYSTEMS Terms of Reference 2/2: To review the effectiveness of field management methods. To quantify the cable current de-rating aspects of the various field management methods. This tutorial does not cover any environmental or biological effects of EMF or discusses any specific levels of EMF. Note: this tutorial can be applied to other voltage classes, such as EHV and MV, provided that Item 3 of the Scope of Work is fulfilled, which is there is no ground return current (see slide 3). Page 5

6 INTRODUCTION TO THE TUTORIAL Numerous MF mitigations methods are available as presented in CIGRE TB 373 developed WG C4.204 [ref. 2] Note: this tutorial prefers the use of MF instead of EMF, since only the magnetic fields are mitigated and not the electrical field; there is no electrical field external to an underground power cable. There is a need to assess impact on cable rating, losses, installation and operational costs due to application of mitigation techniques Mitigation disadvantages: 1. Current rating may decrease 2. Installation cost may increase 3. Operation cost may increase Page 6

7 INTRODUCTION TO THE TUTORIAL Where is mitigation required? 1. Directly above cable circuit 2. At a specified distance from the circuit 3. To minimise the corridor width Selection of the mitigation device is dependent upon the above three questions. This tutorial deals with 3 phase balanced currents only. Zero sequence currents may have an undefined return path. There is no electrical field external to underground power cables due to their inherent design. Page 7

8 GENERAL PRINCIPLES OF MF MITIGATION Thermal design is computed for a continuous load (100% load factor) as given in IEC For cyclic loads and for emergency situations IEC is used. The mitigation device design will be based upon the above load conditions plus the magnetic field limit required by the project. Present versions of IEC and IEC do not take into account the presence of mitigation devices. Numerical Methods are required for plates and ferromagnetic constructions; passive loops can be studied by analytical methods. Page 8

9 GENERAL PRINCIPLES (CONTINUED) Most mitigation devices will be installed in urban areas, with variations in laying configurations. The possible reduction of the current rating depends upon the shielding method and localized factors that elevate the ambient temperature of the cables. Additional heat sources induced by shielding devices, different burial depth, parallel circuits and other heat sources can locally modify the current rating. The minimum of all the local current carrying capacities is selected as the rating of the circuit.. Page 9

10 SHIELDING FACTOR Shielding Factor (SF) is defined as the ratio of the magnetic flux density at a given point (P) in the absence of mitigation (B 0 ) and in the presence of shielding (B s ). SF(P)=B 0 (P)/B s (P). The SF can be the starting point of the shielding design, as reported in the Summary Table. The approach followed in the TB is the Shielding Factor approach, which consists of determining the method to be used according to the ratio between the computed value and the specified value at the sensitive location. Page 10

11 CURRENT (DE)RATING PRINCIPLES Installation of mitigation devices in close proximity to power cables inevitably generates losses that may increase cable temperatures and may de-rate the cable circuit. The closer the mitigation device is to the cables, the greater the mitigation, but also the greater the de-rating may be. The ideal solution maximizes the mitigation and rating and minimizes the cost. De-rating is relative to a conductor temperature of 90 C and unperturbed soil temperature of 20 C. De-rating is computed on the hottest cable in the section where mitigation is applied. It could result in a de-rating of the whole link, if it is or becomes the limiting section. De-rating, or current rating reduction, is not always present. Page 11

12 MF MITIGATION METHODS For a single current flowing in a straight infinitely long conductor, the magnetic field far from the conductor (far field) can be computed according to the following equation 6.1 (refer e.g. to TB 373, page 17) where: B = m0 2 p I r I is the conductor current [A], r is the distance of the computation point from circuit centre [m]. Page 12

13 MF MITIGATION METHODS For balanced three phase circuits, again in far field approximation and after an algebraic arrangement, eq. 6.1 becomes: m 0 k d B = 2 4 p r where: d is the interaxial distance between the conductors [m], k is the coefficient for the geometrical arrangement of the cables (refer e.g. to TB 373, page 27) and where the other symbols have the same meaning of eq The important difference is the presence of exponent 2 at the distance from circuit centre, which determines a rapid decay of the magnetic field with distance and in every direction. I Page 13

14 MF MITIGATION METHODS Principal methods used to mitigate the magnetic field in HV circuits are: Cable management Passive loops Metallic plates Ferromagnetic raceways Steel pipes Special cable design Page 14

15 MITIGATION METHOD SUMMARY TABLE Mitigation method Solid bonding (conventional screen) Solid bonding (enlarged screen) Solid bonding (enlarged screen and magnetic foil) Trefoil from touching flat (same depth) Derating [%] Shielding Factor (SF) [times] Increasing initial cost Increasing operational cost $ $$$ $$ $$ $$$ $ Triangular from flat $ 0 Split phase $$ 0 Passive loops $ $ Metallic plates $$ $$ Ferromagnetic raceway $$ $$ Page 15

16 MITIGATION METHOD SUMMARY TABLE Cost comparisons in the Summary Table are provided in multiple of $ s, $$$ being the most expensive. Increasing initial cost refers to increased cost of the cable project, of the shielding components and additional installation costs. Increasing operational cost comprises the capitalization of the extra losses due to the mitigation technique and maintenance and repair of the installed components. Cost increase are referred to the mitigated section length and not to the all circuit. The reason for taking this approach is the vast cost differences between countries worldwide. Final cost comparison has to be carried out on project specific basis. Page 16

17 CABLE MANAGEMENT Cable management is the design of the cable installation with respect to a reduced magnetic field, but without modifying the cable construction itself. Relevant parameters are: the positions of the cables (laying arrangement), the bonding of sheath or screens, the phase-sequences rearrangement, phase splitting to subdivide the current. Cable management is the easiest way to reduce the MF directly at the source, acting on the position of the conductors that generate the electromagnetic field. Page 17

18 CABLE MANAGEMENT Trefoil formation is often used as an alternative to flat formation, giving a reduction of the magnetic field of 2, for the same interaxial distance. Solid bonding may give a significant reduction of the magnetic field, but the derating in this case is so relevant, that this technique is rarely applied to underground cables to mitigate MF. For any laying geometry and for any electrical connection of the sheaths, IEC and IEC standards contain the information required for the computation of the conductor current, temperatures and losses. Page 18

19 CABLE MANAGEMENT For single circuits, straight-forward methods to reduce the magnetic field are: to increase the cable laying depth, to minimise the cable interaxial distance, and to prefer triangle arrangements of cables instead of flat formations. For double circuits, the choice of the phase sequences in the systems does (normally) not influence the distribution of the conductor current, but definitely influences the sheath/screen currents as well as the MF. For double circuits, the interaxial distance inside the systems, and also the separation between the systems can be optimised for any given trench width. In addition phase sequence may have to revisited to optimise mitigation. Page 19

20 CABLE MANAGEMENT: phase splitting Utilizing two single-core cables per electrical phase introduces the possibility of active field compensation. Each trefoil will generate a rotating magnetic field rotating 180 relative to each other and their fields will be opposing, nearly cancelling each other out. Trefoil (left) and split-phase in hexagonal configurations (right). Page 20

21 CABLE MANAGEMENT: split-phase Typical example of Shielding Factor for split-phase configuration SF at ground surface Distance from central axis [m] Page 21

22 CABLE MANAGEMENT Cable realignment to reduce the magnetic field at selected locations Advantages: Disadvantages: Reduced field at selected locations Increased field at the opposite side Page 22

23 CABLE MANAGEMENT Triangular installation Preparing for trefoil installation Advantages: Disadvantages: Reduced field at source Increased installation costs, small derating Page 23

24 PASSIVE LOOPS Passive loop techniques make use of cables which are laid parallel to the power cables insulated and not connected to them, but they are electrically short circuited at both ends, creating loops. Reverse currents are induced into them, resulting in a cancellation of the total magnetic field. Passive loops are easily installed into the trench together with HV cables, with negligible impact on the laying operations of the main cables. Standard practice is to limit the increase of local ambient temperature of the power cables to a few degrees Celsius, due to the losses generated in passive loops. Page 24

25 PASSIVE LOOPS Low voltage cables are normally used. The loops are arranged in the trench, joint bay or manhole, either on the surface of the compacted backfill or at the same level of the HV cables. Installation on top of a buried joint bay Connection details Page 25

26 PASSIVE LOOPS Currents tend to circulate mainly in the loops placed closer to the power cables contributing also to higher losses and thermal influence on the main cables due to their proximity. Balancing is required at the design stage to select the section and the position of the passive cables. The approach is to compute the increase of the ambient temperature of the power cables, due to the presence of passive cable. The influence increases with the proximity of the passive cables and their losses, giving rise to mutual heating between adjacent cables. Page 26

27 PASSIVE LOOPS Let Δθ kp be the thermal increment, above ambient temperature, of the p cables, HV cables, due to the presence of the cable k (passive cable). The overall increment of ambient temperature, Δθ p, for each HV cable p is given by the sum of the q contributions Δθ kp computed for all the passive cables, as shown in the following simplified diagram: q D q = Dq = Dq + Dq +... Dq +... Dq p 1 kp 1p 2 p kp qp where: Dq kp = r t 2p W k ln Ł d d ' pk pk ł Page 27

28 PASSIVE LOOPS In the previous equation the losses W k [W/m] of the passive cable k are given by W = R I 2 k k k where: R k is the electrical resistivity of the passive cable k, at operating temperature (Ohm/m), I k is the current flowing into the passive cable k (A). The increment Δθ p has to be computed for each power cable p and then the new rating for each p cable is computed. The new rating can be now computed for that section and compared with the previous rating of the whole circuit. Page 28

29 PASSIVE LOOPS Loops above the cable to reduce magnetic field Advantages: Disadvantages: Simple to install, minimal costs Large currents can flow in loop(s) Page 29

30 PASSIVE LOOPS Soil surface Outer loop Inner loop Installation scheme in a joint bay Installation in a joint bay Page 30

31 METALLIC PLATES Conductive plates can have a significant shielding effect far from the source. Ferromagnetic plates are only effective near the source (unless they fully encompass the source as for a raceway). Conductive plates generally have lower losses and are more robust with respect to installation imperfections. Installation requires welding between longitudinally adjacent plates. Calculation of derating can be done using two dimensional finite element simulation. During installation, care must be taken to avoid air pockets and possible effects on moisture migration, leading to soil thermal resistivity changes and unwanted derating. Plates may be affected by corrosion. Page 31

32 METALLIC PLATES Open conductive shielding Page 32

33 METALLIC PLATES Welding is necessary to obtain the shielding factor reported below Advantages: Disadvantages: Simple to install High installation and maintenance costs Page 33

34 METALLIC PLATES On top of the trench there is the maximum shielding factor (around 20) that decays rapidly to approximately 10 within a few meters from the central line. The asymmetry in B R is caused by the geometry of the shielding device in this case plates in an H-shape. The field mitigation achieved by conductive plates is achieved indirectly from induced current flow in the shielding plates. SF at ground surface Shielding Factor (SF) obtained by H-shielding Distance from central cable [m] Page 34

35 FERROMAGNETIC RACEWAYS A ferromagnetic raceway is a steel box consisting of sides, a bottom part and a cover, that can be bolted and used to contain the three phase cables (see next slide). In ferromagnetic raceways the cables are normally arranged in trefoil and their configuration can be controlled and adjusted. The raceway is initially open, allowing the cable to be laid from the top: cement mortar is then used to lock the cables in position, before the cover is fixed. Experience confirmed that the raceway loss factor derived from IEC used for pipe type cables, is still valid, provided that the power cables are in closed trefoil or flat touching arrangement, as described later. Page 35

36 FERROMAGNETIC RACEWAYS Ferromagnetic raceways shield the field effectively Advantages: Disadvantages: Simple to install High initial cost Page 36

37 FERROMAGNETIC RACEWAYS Cable are installed in polymer pipes for easier installation Natural convection cooling Installation in a pedestrian tunnel using ferromagnetic material Page 37

38 FERROMAGNETIC RACEWAYS For 3 cables equally loaded, in trefoil formation, the following formula for the raceway losses λ 2rt can be applied: where: l = Ł s R e -5 2rt 10 D ł s is the cable interaxial distance [mm], D e is the raceway equivalent diameter [mm], R is the conductor resistance at maximum operating temperature [Ohm/m]. The formula is valid up to cable diameter of about 200 mm, due to unknown accuracy above this physical size. For other configurations (e.g. centred cables in large raceway) and larger diameters, FEM computation is advised. The factor of 0.76 in the formula is applicable for 50 Hz and must be replaced with 1 for 60 Hz. Page 38

39 FERROMAGNETIC RACEWAYS For 3 cables equally loaded, in flat formation, the following formula for the raceway losses λ 2rf can be applied: l = e 2rf Ł s R D ł -5 where: s is the cable interaxial distance [mm], D e is the raceway equivalent diameter [mm], R is the conductor resistance at maximum operating temperature [Ohm/m], The formula is valid up to cable diameter of about 200 mm. The factor of 0.76 in the formula is applicable for 50 Hz and must be replaced with 1 for 60 Hz. Page 39

40 FERROMAGNETIC RACEWAYS As the losses are generated in a thin layer, uniformly in the perimeter, temperature rise of the area inside is uniform and can be computed according to: Dq r = W r ln Ł where W r is the power dissipated in the raceway [W/m], T 4 is the external thermal resistance of the raceway [K*m/W], ρ t is the thermal resistivity of the soil [K*m/W], L is the laying depth at the centre of the raceway [mm], D e is the equivalent diameter of the raceway [mm]. T 4 = W r r t 2p 4 L D e ł In case of large raceways, FEM computation is advised, either for losses or for temperature rise. Page 40

41 FERROMAGNETIC RACEWAYS The equivalent radius Re can be computed according to the formula derived from IEC , valid for X/Y< 3: 2 1 x 4 x y ln( Re ) = - ln y Ł p y ł Ł x It is straightforward to define the equivalent diameter as For 3 cables equally loaded, in trefoil formation in special steel raceway, the following formula for the total losses Wr [W/m] can be used: 3 l 2 W = R I r p 2 where: I is the cable current [A], λ2r is the raceway losses parameter (either in trefoil or flat configuration), R is the conductor resistance at maximum operating temperature [Ohm/m]. ł ln D = 2 x 2 e R e Page 41

42 STEEL PIPES It is a well-known mitigation technique to encapsulate the three cables of a three-phase system within a closed ferromagnetic structure like a steel pipe. Available steel pipes normally have a high relative permeability in a range of 500 up to 1300 and offer a low reluctance magnetic path to the magnetic flux, thus decreasing the magnetic field outside the steel pipe. Steel permeability may be affected by bending and welding during manufacturing. Shielding factors of more than 100 can be achieved. It is essential for the shielding to be effective that the three-phase system of currents must be inside the steel pipe, not necessarily symmetrical, but must add to zero: any residual zero-sequence current is not shielded and may induce large losses. Page 42

43 STEEL PIPES Installation of three phase cables in polymer pipes inside a steel pipe. Typical field installation is shown on the right. Page 43

44 STEEL PIPES Steel pipes greatly reduce the magnetic field Steel pipe Filler to be removed after concrete injection PE duct Cable Concrete Advantages: Disadvantages: Highest shielding factor Not simple to install and high derating Page 44

45 Influence of wall thickness Page 45

46 SPECIAL CABLE DESIGN For lowest magnetic fields outside of single and three ac cables, a high permeability shielding integrated into the cable construction with an enlarged copper screen allows extremely high shielding factors. The copper screens of the three cables are bonded to each other at both ends. The essential presence of a surrounding high-permeability tape forces the current sum to zero inside of it, i.e. the conductor current is flowing back in the copper screen. This cable design enables very high shielding factors for single-core cables, independent of their laying distances. Single core ac cable with integrated electromagnetic shielding Page 46

47 SPECIAL CABLE DESIGN Three core ac cable within a steel pipe (left) and with integrated electromagnetic shielding (right) Page 47

48 SPECIAL CABLE DESIGN Shielding factor (SF) for conventional and shielded cables with enlarged copper screens as a function of screen cross-section A S Page 48

49 OTHER INSTALLATIONS The reinforcement within concrete structures reduces field Advantages: Disadvantages: Shared purpose e.g. Protection of cable Increased costs to install Page 49

50 INTERACTION WITH NON THERMAL CABLE SYSTEM DESIGN Mitigation technique Change of cable route Increase of laying depth Cable management Phase splitting Metallic components Design parameter ( 7.1) ( 7.2) ( 7.3) ( 7.4) ( 7.5) Induced voltages / / Impedances / = Electrodynamic stresses and fault current Fault containment Installation / / Mechanical protection Stray currents Operation Fault location / Repair Maintenance of mitigation device Page 50

51 INTERACTION WITH NON THERMAL CABLE SYSTEM DESIGN Comments: The use of MF mitigation techniques may affect system design parameters not linked to thermal impact. These interactions are presented in the above table. The influence of the design parameters is assessed with a positive impact ( = advantage), a negative impact ( = drawback). Page 51

52 MITIGATION METHOD SUMMARY TABLE Mitigation method Derating [%] Shielding Factor (SF) [times] Increasing initial cost Increasing operational cost Solid bonding (conventional screen) Solid bonding (enlarged screen) Solid bonding (enlarged screen and magnetic foil) Trefoil from touching flat (same depth) $ 1 $$$ $$ 1 $$ $$$ 1 $ Triangular from flat $ 0 Split phase $$ 0 Passive loops $ $ Metallic plates $$ $$ Ferromagnetic raceway $$ $$ Steel pipe $$$ $$$ Page 52

53 SUMMARY TABLE NOTES Notes to the previous Summary Table slide 52: 1. The table provides cost comparisons only and higher costs due to cable increased complexity. 2. This is a calculated value only and not confirmed by measurements. 3. The negative value means a higher cable rating. 4. In this case the Shielding Factor is sensitive to the distance from the circuit. The Shielding Factor in the Table provides a range to cover cases from centreline to the edge of the right-of-way. Page 53

54 CONCLUSIONS 1/2 This tutorial presents guidelines for managing (de)rating of a HV underground power cable electric system with magnetic field mitigation techniques. The presence of mitigation devices may modify the ambient conditions surrounding the cables. The examples provided give good technical solutions leading to practical installations. Page 54

55 CONCLUSIONS 2/2 A Summary Table is provided to assist with the most judicious selection of the shielding method. Economical evaluation of investment costs, cost of additional losses and maintenance of the mitigation device is presented. The overall conclusion is that magnetic fields from cable systems can be effectively shielded, with minimal influence on the rating of the whole link. Page 55

56 SALIENT REFERENCES [1] Cigrè TB 559, Impact of EMF on current ratings and cable systems, December [2] Cigrè TB 373, Mitigation Techniques of Power Frequency Magnetic Fields Originating from Electric Power Systems, February [3] Cigrè TB 375, Technical Guide for Measurement of Low Frequency Electric and Magnetic Fields near Overhead Power Lines, April [4] Cigrè TB 320, Characterization of ELF Magnetic Fields, April [5] IEC International Standard: Electric cables Calculation of the current rating, [6] IEC International Standard: Calculation of the cyclic and emergency current rating of cables, [7] IEC TR Technical Report: Electric cables Calculation for current rating Finite element method, [8] IEC International Standard: Measurement of low-frequency magnetic and electric fields with regard to exposure of human beings Special requirements instruments and guidance for measurements, Page 56

57 FURTHER READING [1] A. Bolza, F. Donazzi, P. Maioli: "Campi elettrici e magnetici: possibilità offerte dagli elettrodotti in cavo". AEI Conference Elettrodotti e territorio, Padova 22/11/2000. [2] H. Orton: "Progress report of WGB1.23: Impact of EMF on current ratings and cable systems, communication presented at 2008 CIGRE Conference, Paris. [3] J. Vavra, M. Wanda, Vienna 400 kv North input, 2006 CIGRE Conference, paper B [4] J. Vavra, M. Wanda, 400 kv Vienna: the Vienna 400 kv north input, 2006 CIGRE Conference, paper B [5] R. Benato, M. Del Brenna, C. Di Mario, A. Lorenzoni, E. Zaccone, A new procedure to compare the social costs of EHV-HV overhead lines and underground XLPE cables, 2006 CIGRE Conf., paper B [6] P. Maioli: "Environmental constraints: electromagnetic shielding with ferromagnetic raceway, communication presented at 2004 CIGRE Conference. [7] P. Maioli, E. Zaccone: Passive loops technique for electromagnetic field mitigation: applications and theoretical considerations 7 th International Conference on Power Insulated Cables, JICABLE 07, [8] P. Maioli, E. Zaccone: Thermal design of HV electric systems with EMF mitigation devices Cigré International Colloquium on Power frequency Electromagnetic Fields ELF EMF, 3-4 June 2009 Sarajevo, Bosni [9] H. Orton, P. Maioli et al: Impact of EMF on current rating and cable systems 2 nd International Conference on EMF-ELF, March 2011 Paris. Page 57

58 IMPACT OF EMF ON CURRENT RATINGS AND CABLE SYSTEMS Page 58