High Voltage Dry-Type Air-Core Shunt Reactors

Transcription

1 1, rue d Artoi, F PARIS A3-101 CIGRE 014 http : // High Voltage Dry-Type Air-Core Shunt Reactor K. PAPP* M. R. SHARP D.F. PEELO Trench Autria GmbH Trench Limited Conultant Autria Canada Canada SUMMARY Dry-type air-core hunt reactor are now being ued more frequently on high voltage power tranmiion ytem to limit overvoltage. Recently, high voltage dry-type air-core hunt reactor have been deigned, manufactured and intalled directly connected to the tranmiion ytem at voltage up to and including 345 kv. Application at 500 kv are preently being conidered. Thee rating require appropriate analyi in term of witching tranient overvoltage and electrical and magnetic clearance ince dry-type air-core reactor have ome aliently unique characteritic a compared to traditionally applied liquid-immered unit. Dry-type air-core reactor have a tray magnetic field that extend beyond the periphery of the equipment. Hence the magnetic clearance to urrounding metallic object and for peronnel mut be etablihed. High voltage dry-type air-core reactor are typically made with a modular winding deign that allow ue of a lower cot partial phae pare unit. Thi deign technique alo allow ue of hunt connected urge arreter protection of each erie connected winding module. High voltage direct connected liquid-immered iron-core reactor have a higher inherent capacitance value to ground than dry-type air-core device. Hence the magnitude and/or frequency of the witching tranient overvoltage can be ignificantly higher where dry-type air-core unit are employed. The tranient overvoltage to be conidered are the tranient recovery voltage (TRV) at current interruption which tree the circuit breaker and the reignition overvoltage which tree the reactor. The TRV i the ignificant quantity for the circuit breaker or other load break device. Thi paper dicue thee important apect of dry-type air-core reactor and their ramification with repect to the application of thee device for hunt connection on high voltage power tranmiion ytem. Further, information concerning reactor protection, calculation method for magnetic field level around reactor, guidance regarding pecification and type teting and appropriate method to mitigate TRV frequency and magnitude, where neceary, are provided. KEYWORDS Air-Core Dry-Type Shunt Reactor - Magnetic Field - Shunt Reactor Switching Overvoltage * klau.papp@trench-group.com

2 INTRODUCTION Shunt reactor are ued in power ytem for voltage control. They may be connected either to the tertiary winding of a power tranformer, or directly to the tation bubar, or anywhere along a tranmiion line. In the two latter cae thee reactor were uually of liquid-immered, iron-core deign. Reactive hunt compenation i one of the mot common application of dry-type, air-core reactor. Dry-type air-core hunt reactor have been employed for over 50 year at voltage up to 69 kv. Figure 1 illutrate the deign concept of a dry-type, air-core reactor. 1) winding ) winding conductor 3) pider 4) terminal 5) duct tick 6) protective roof/hed 7) bae inulator 8) mounting bracket Figure 1: Air-core dry-type reactor A dry-type air-core reactor conit of a cylindrical winding (1) made of one or everal concentric layer of film/gla tape inulated aluminum conductor (). Aluminum i preferred veru copper becaue of it lower pecific weight and the lower invetment cot of the winding at a comparable level of loe. All layer are electrically connected in parallel by welding their top and bottom end to metallic cro arm, commonly referred to a pider (3). Each pider carrie a terminal (4) for electrical connection of the reactor. The individual layer are configured uch that radial voltage tre i virtually nil and the remaining axial voltage tre reult in urface tre value and turn-to-turn teady tate operating voltage that are well below pecific limit derived from field experience. All layer are radially paced by everal gla fiber tick (5) which form air duct neceary for the cooling of the winding. Cooling i provided by natural convection of ambient air, which enter at the bottom end of the winding and exit at it top end. The winding i impregnated/encapulated by epoxy rein reulting (after curing) in a mechanically trong and compact unit. Finally, protective coating i applied to the winding to improve track- and weather reitance. In cae of advere pollution condition at the ite of intallation, hunt reactor for ub-tranmiion and tranmiion ytem may be equipped with a protective roof/hed (6). The reactor i mounted on everal bae inulator (7) and mounting bracket (8). The rating of the inulator depend on the pecific ytem requirement at the ite of intallation. Dry-type hunt reactor are uually of ingle phae deign and three wye-connected unit are mounted ide-by-ide forming a three-phae hunt reactor bank with the neutral earthed or unearthed depending on ytem requirement. 1

3 A dry-type hunt reactor i not encloed in a grounded teel tank and all part of the reactor mut be conidered to be live. Therefore, dry-type hunt reactor mut be intalled uch that accidental contact by tation peronnel i not poible. Two method that can be ued to achieve thi are fencing, and elevating the reactor at a afe ditance above ground by electing the length of the mounting bracket accordingly. One of the major advantage of air v. liquid-inulated hunt reactor i the implicity of the inulation between the phae reactor and to ground which i provided by the urrounding air and by the upport inulator on which the equipment i reting. Auming adequate triking ditance, fault external to the reactor are very unlikely to occur. Therefore the widely ued differential relay protection for liquid-inulated hunt reactor i not very meaningful for dry-type unit. A for all hunt reactor the detection of turn-to-turn failure i very difficult and early failure detection i ometime nearly impoible. Bet practice of turn-to-turn failure protection i to monitor the neutral through a voltage tranformer if the neutral i ungrounded or a current tranformer for hunt reactor bank with the neutral grounded. Guidance for hunt reactor relay protection i found for example in [1]. MAGNETIC FIELD CONSIDERATIONS A the name implie, air-core reactor do not have an iron core and therefore the magnetic field i not contrained and will fringe out from the winding end and will occupy the pace around the reactor winding. The trength of thi tray field depend on the unit power rating of the reactor, the higher the rating, the higher the magnetic field level. Figure how the magnetic field plot of an air-core reactor. The elected example i one phae of a 69 kv, 50 Mvar (50 Hz) wye connected 3-phae reactor bank. Figure : Magnetic Field Plot Reactor data: reactance (50 Hz) Ohm inductance mh winding length....7 m mean winding diameter m turn number current A(rm) The winding i cylindrically haped o that the magnetic field generated by the winding poee rotational ymmetry and i, in addition, ymmetrical to the winding midplane. The field trength quickly drop off with increaing ditance from the reactor, ince a olenoid ha a magnetic dipole characteritic, o that the field decay with the third power of ditance. The preence of the magnetic field mut be taken into conideration in dry-type hunt reactor intallation for two apect: a) To enure that the magnetic field of the reactor doe not induce undue current in nearby metallic geometrie (tation earthing grid or rebar of the concrete platform on which the reactor i mounted, etc.) to avoid overheating and undue loe. There are ome imple rule of thumb that can be employed. Clearance to mall metallic part not forming cloed loop hould be at leat one-half the coil diameter radially from the edge of the reactor. Larger geometrie or cloed loop hould be located at leat one coil diameter from all the urface of the reactor. The reactor manufacturer can provide

4 guideline about magnetic clearance a well a recommendation for the deign of foundation and nearby tation tructure. b) Maintaining the limit for expoure of the tation peronnel to magnetic field. Guideline on human-expoure limit to magnetic field are propoed by everal national and international organization. The ICNIRP guideline [] for human expoure limit to electric and magnetic field have gained widepread upport in the global electric power indutry. The latet verion of thee guideline recommend a reference level for 50 Hz or 60 Hz magnetic field of 1 mt, for occupational expoure. The reference level are expreed in term of the effective field trength, that i the root of the um of the quare of the rm field component along three orthogonal axe. Biot-Savart' law may be applied to calculate the magnetic field of a cylindrical winding [3]. The external magnetic field of a dry-type air-core reactor winding at a greater ditance from the winding may be approximated by the field of a current loop a hown in Figure 3. Thi approximation hold for coil having a winding length horter than about three time the winding diameter. The field produced by a current carrying winding loop in a ditance r of more than around three time the loop diameter may be approximated according to [3] by the equation (1) and (). B... n I D 3 8. r 0 f ( ) (1) co ( ) () f ( ) in ( ) 4 B... magnitude of the magnetic field µ 0... permeability in air (µ 0 = 4.E-7 H/m) n... turn number I... current D... loop diameter (= mean winding diameter) r,... coordinate a per Figure 3 f()... directivity function a per equation () Figure 3: loop, equivalent to a reactor winding Uing (1) and () in lateral direction ( = 0, f() = 0.5) the magnitude of the magnetic field at greater ditance may be etimated by For the hunt reactor winding a per Figure, the required minimum lateral ditance from the coil axi etimated by equation (3) i 4.6 m. Thi reult i le than 1% off from a full analytical calculation by [3]. For hunt reactor bank coniting of three phae reactor the magnetic clearance gained by ingle phae calculation i even higher than that found by the uperpoition of the magnetic field of all three reactor auming that all three coil do have the ame ene of magnetization. HV APPLICATION n I D B 4. r In the pat dry-type air-core hunt reactor have been applied at ditribution cla voltage level. Typically they have been connected to the tertiary winding of a power tranformer. Common rating have been in the range of 10 to 50 Mvar per phae with reactor current greater than 1000 A and reactance in the 3 to 50 ohm range. Recently, however, due to the advance made in material, and application of novel deign technique, high voltage dry-type air-core hunt reactor have been deigned, manufactured and intalled, directly connected to the tranmiion ytem at voltage up to and including 345 kv. One factor driving thi demand increae i the integration of renewable generation uch a wind park to the grid. The con- (3) 3

5 necting overhead line or cable require hunt compenation with rating for reactive compenation in the range of ome ten of Mvar. Uually uch wind park are located in environmentally enitive area thereby favoring dry-type air-core technology. Becaue thee reactor have a low invetment cot, require minimal maintenance, there i no oil pill or fire hazard, intallation i fat and protection i imple, they are well uited for remote intallation at any point along the power tranmiion line. Becaue dry-type air-core reactor are uually cutom deigned for each pecification, the ize, hape and layout of reactor module can be modified to uit the particular requirement of each particular intallation For uch tranmiion cla hunt reactor, current rating are often le than 100 A and reactance value can be in exce of 5000 ohm. Thee rating require appropriate analyi in term of teady tate voltage and witching tranient overvoltage. The main deign parameter for dry-type hunt reactor employed at ub-tranmiion or tranmiion ytem i the teady tate voltage drop along the urface of the reactor. The length of the winding i choen to provide abundant creepage ditance along the winding urface to cope with the continuou voltage tre, conidering the pollution condition at the ite of intallation. Shunt reactor connected to 110 kv ytem are deigned for a winding length of about 3.5 m which tranlate into a reactor height (over pider) of about 4 m. Thi dimenion correpond with a typical maximum height for road tranportation on a truck. Dry-type hunt reactor winding of thi length will have a high afety margin to withtand the tranient and dynamic overvoltage aociated with thi voltage cla. Thi feature i of particular importance for hunt reactor ubjected to frequent witching. For ytem voltage of around 30 kv two ingle unit, tacked one above the other and connected in erie are required to keep the winding voltage tree within acceptable limit. For even higher voltage more than two erie connected winding module are required which i achieved by a eparately ide-by-ide mounted reactor tack connected electrically via cable or buwork. Standard phae to phae and phae to ground electrical clearance ued by the utility are applicable for thee reactor. Figure 4 how a 345 kv, 0 Mvar, 60 Hz, hunt reactor bank. Each phae conit of totally 4 erie connected winding module each having a winding height of 3.55 m. The pace requirement for thi 345 kv dry-type air core hunt reactor bank i around 17 m x 1 m. However, becaue the reactor are typically cutom deigned, the coil dimenion and coil orientation can be adjuted to uit ite condition. Figure 4: 345 kv, 0 Mvar (60 Hz) 3-phae hunt reactor. 4

6 The tranient voltage ditribution along a chain of erie connected reactor i non-linear and it i governed by the capacitance of the individual winding module between turn and to ground. Figure 5 how a implified electrical chematic of one phae of the 345 kv example reactor. Figure 5: implified electrical chematic. Both, the capacitance between turn -and to ground are repreented by lumped capacitor, the ratio of which i crucial for the tranient voltage along the reactor. The voltage nonlinearity factor i expreed by the quare root of the capacitance to ground v. the capacitance between terminal. The maximum peak tranient overvoltage acro reactor 1 in thi example wa found to be around 75 % of the total peak voltage of 1550 kv applied at the HV terminal. Thi i about three time the voltage at linear ditribution, i.e. the nonlinearity factor i around three which i typical for a HV hunt reactor made up by four ingle winding module. A high voltage dry-type air-core reactor made with a modular winding deign allow ue of hunt connected urge arreter protection of each erie connected winding module. The arreter may be mounted inide the winding. Care mut be taken to elect arreter having metallic fitting of limited ize made of non-ferrou teel or aluminum to avoid undue heating by the magnetic field of the winding. The arreter will clamp the tranient voltage of the winding module effectively protecting the hunt reactor and will alo limit the magnitude of reignition overvoltage. Further, the capacitance of the urge arreter will add to the capacitance between the terminal of each winding module and thu will lower the nonlinearity of the tranient voltage ditribution of the HV hunt reactor. TESTING OF HIGH VOLTAGE SHUNT REACTORS Recommendation for acceptance teting of dry-type air core hunt reactor i provided in the international tandard, IEC , 007 Power tranformer- Part6: Reactor, and IEEE C57.1, 008 IEEE Standard Requirement, Terminology, and Tet Code for Shunt Reactor Rated Over 500 kva a well a many national tandard. Both IEC and IEEE provide pecific teting recommendation for both oil immered and dry-type air-core hunt reactor. For impule teting of high voltage dry-type hunt reactor coniting of more than one reactor module it i recommended to fully aemble the reactor with all module connected in erie but with the urge arreter removed. The full tet voltage hall be applied to the HV terminal while the neutral terminal i earthed. SHUNT REACTOR SWITCHING: GENERAL CONSIDERATIONS Shunt reactor witching repreent a unique interactive and evere duty for circuit breaker. The duty i not tandardized becaue hunt rating are application dependent and uer hould elect circuit breaker according to the application. Guidance in thi regard i dicued later. 5

7 The general circuit for the firt-pole-to-clear i hown in Figure 6. The tranient recovery voltage (TRV) impoed on the circuit breaker i the difference between the ource voltage and the load ide ocillation. The load ide circuit and ocillation i characterized a a erie RLC circuit with a precharged capacitor [4]. The capacitance C L i initially charged to the voltage at the hunt reactor at current interruption, which then ring down to zero at a frequency given by L and C L. The peak of the TRV occur when the load ide ocillation peak i of oppoite polarity to the ource voltage. The TRV trv v trv V V : Source voltage peak value k pp : Firt-pole-to-clear factor L : Shunt reactor phae inductance K : Neutral hift equal to k pp 1 C : Load ide capacitance L Figure 6: v ha a coine k pp k Shunt reactor witching general cae: circuit for firt-pole-to-clear. 1 wavehape and i given by: a K e d t g co 1 d 1 d 1 d where k a i the uppreion peak overvoltage, d i the degree of damping in the circuit and t g i generic time baed on period of the undamped ocillation [4]. The application dependency and interactive nature of witching can be undertood by conidering the variable other than the ource voltage: k pp = 1+K i dependent on the hunt reactor neutral grounding arrangement, being 1 pu, 1.5 pu or 1. to 1.35 pu when the neutral i directly grounded, ungrounded (iolated) or grounded through a neutral reactor, repectively. k a i dependent on the circuit breaker arc voltage and it ability to chop current. For SF6 circuit breaker, k a increae with arcing time and the TRV become more and more onerou at ucceive current zero croing. d i the degree of damping in the load and i given by d = R/R c where R i the actual reitance in the load circuit and R c i the reitance value that would give critical damping. For hunt reactor circuit, R i generally very low and d << 1. t g i given by 1 KLCL and i thu, not only dependent on L and C L, but alo the neutral grounding arrangement. While not indicated by the equation (4), k a alo ha a dependency on the magnitude of the load current. For hunt reactor applied at 100 kv and above, the load current a hown by urvey i le than 500 A and both arc voltage and current chopping need to be conidered. However, a further iue i that k a increae with decreaing load current and the tandard thu require type teting at a minimum current value [5]. For hunt reactor applied below 100 kv, more uually below 5 kv, the load current tend to be greater than 1000 A and only the arc voltage need to be conidered in calculating the k a value. t g d in t g (4) 6

8 To illutrate the ue of the above equation, conider a dry-type hunt reactor rated at 30 kv, 100 Mvar, 50 Hz in the three configuration below: 1 pu t g Ocillation frequency Configuration k pp d khz Neutral grounded Neutral ungrounded Neutral grounded and RC damper R 0 and C 0.15 F Auming an ideal circuit breaker (k a = 1), the calculated TRV are hown in Figure 7. With the reactor neutral grounded, the TRV ha a peak value of 45 kv at a frequency of 59.6 khz a hown in Figure 7(a). The effect of having the neutral ungrounded i hown in the ame figure, i.e. the peak value increae by 1 pu voltage with a decreae in the frequency. The reaon for the large increae in the peak value i due to axi of ocillation of the load tranient now being the hifted neutral [4]. The addition of the RC damper line to ground with the neutral grounded ha a major effect on the frequency with the peak value virtually unchanged (Figure 7(b)). The large decreae in the rate-of-rie of the recovery voltage (RRRV) i beneficial for the circuit breaker regardle of type (low probability of reignition) and for the reactor (lower turn-to-turn tree). Figure 7: TRV for witching of 30 kv, 100 Mvar, 50 Hz dry-type reactor. An actual meaurement for the neutral grounded cae with the RC damper i hown in Figure 8. The trace how that the load ide ocillation i very underdamped with an amplitude factor of 1.9 or greater. The peak value of the TRV for the circuit breaker will therefore approach pu voltage a indicated in Figure 7(a). Figure 8: Load ide ocillation on witching out 30 kv, 100 Mvar hunt reactor with RC damper. 7

9 The election of circuit breaker for hunt reactor witching require pecial attention. For application at 100 kv and above, where the load current i uually le than 500 A, type teting i carried out at a minimum current of 100 A [5]. The intent of the tet i not to demontrate interrupting capability but rather to derive the chopping characteritic of the circuit breaker which can then be ued to calculate performance in the field a decribed in [6]. If the actual load current i le than 100 A, then a type tet hould be performed at that current. For application at load current greater than 1000 A, current interrupting capability for the expected TRV i the iue [5, 6]. For thi reaon circuit breaker hould not necearily be elected on the bai of the hunt reactor rated voltage, i.e. a circuit breaker with a higher rated voltage may be required to meet the TRV requirement. At current interruption the circuit breaker i treed by the TRV and the hunt reactor by the load ide tranient both to ground by k a pu and acro the winding by (k a + K) pu. The frequency of both tranient depend on the load circuit inductance and capacitance (Figure 6). For dry-type reactor, the value of C L i five or more time lower than that for oil-filled reactor and the above frequency i two or more time higher. The remedy for thi, if neceary for the election of the circuit breaker, i to add capacitance a wa done for the 30 kv, 100 Mvar reactor dicued earlier. In the event of a reignition in the circuit breaker, a perfectly normal event, the load ide voltage recover to the ource voltage but overhoot producing a reignition overvoltage. Thi overvoltage i a characteritic of the circuit involving the ource and load ide capacitance and the inductance of the aociated circuit loop [6]. The expoure of the hunt reactor to thi overvoltage i limited in magnitude by the reactor urge arreter and it frequency, uually hundred of khz, i not conidered a deign iue. BIBLIOGRAPHY [1] IEEE Std C : IEEE Guide for the Protection of Shunt Reactor [] International Commiion on non-ionizing Radiation Protection ICNIRP: Guideline for Limiting Expoure to Time-Varying Electric and Magnetic Field (1 Hz khz). Health Phyic 99(6): ; 010. [3] W.R. Smythe, Static and Dynamic Electricity, McGraw-Hill, 1968 [4] D.F. Peelo, Current Interruption Tranient Calculation. John Wiley & Son Ltd., 014. [5] IEC High-voltage witchgear and controlgear Part 110: Inductive load witching. [6] IEC High-voltage witchgear and controlgear Part 306: Guide to IEC , IEC and other IEC tandard related to alternating current circuit-breaker. 8