Power tranformer controlled witching Variou trategie to cope with detrimental inruh current and enhance power quality F. Aït Abdelmalek, J-P. De Santi GE Grid Solution GIS R&D Aix-le-Bain, France farid.ait-abdelmalek@ge.com A. Fanget, J. Borde GE Grid Solution ARC reearch center Villeurbanne, France alain.fanget@ge.com Abtract The target of thi paper i to focu on power tranformer controlled witching. The variou applicable trategie to limit inruh current and conequent tranient, tree and power quality iue, are introduced, and ome ite experience feedback decribed. Keyword Controlled witching, power tranformer, inruh current, reidual flux, voltage tranformer, power quality I. INTRODUCTION The increae of electrical power demand, energy market deregulation, the introduction of new operator and producer, and the variety of new power ource AC/DC converter, wind farm, olar farm, etc. require high-voltage tranmiion grid to withtand ever more witching operation and higher tree during witching operation. In order to mitigate uch tree and improve power quality, controlled witching i the ound alternative to circuitbreaker cloing reitor, the over-rating of electrical equipment, and the intallation of additional protective device, a it carrie huge aving on aet ranging from power tranformer to overhead line and through to circuit-breaker and additional protective device. Energizing of power tranformer i of particular interet not only becaue of electromechanical contraint generated due to trong inruh current. On top of that it ha a direct influence on power quality and the overall electrical tranmiion ytem reliability. II. TRANSFORMER SWITCHING BACKGROUND A. Ideal witching Ideal witching equence of power tranformer correpond to a cloing operation at the time of flux matching, where propective flux equal magnetic core reidual flux, for firt winding to cloe. For remaining winding to cloe, propective flux hould be equal to dynamic flux impoed by winding already cloed. From thi tatement, one can think that taking into account the tranformer magnetic core reidual flux, for each energizing equence, i the bet olution to control level of inruh current and it conequence. However, due to flux haring, the above equence i a compromie: in order to reduce the maximum flux error for all three phae, the winding with highet level of flux i commonly energized firt, which impoe a compromie for the two remaining one. Furthermore, ome challenging tak (and operational contraint, availability of voltage tranformer) are required to guarantee good performance. B. Reidual flux computation Firt challenge conit in the availability of the reidual flux value, either by direct meaurement or integration of the voltage at the tranformer terminal, which require accurate ignal toward low frequencie (around 1Hz) and advanced ignal proceing algorithm, along with ufficient embedded proceing power. C. Reidual flux memorization Second challenge i the long term tability of the reidual flux inide the tranformer core under variou external influence and epecially in cae of de-energizing by the mean of multi-break circuit-breaker with grading capacitor in parallel of chamber. Variou approache, when dealing with power tranformer energizing, are dicued in the following, among which conideration of reidual flux and alternative trategie coniting in controlling reidual flux level by core demagnetization or controlled opening. III. ENERGIZING STRATEGIES When performing tranformer controlled energizing operation, particularly when back-feeding from the tranmiion grid, the main method uually conidered are: Energizing at fixed angle without any conideration of reidual flux level. The witching equence depend on winding arrangement, neutral poition and preence or abence of load current during previou opening. Energizing at fixed angle with conideration of a known reidual flux level fixed by a previou controlled de-energizing. Energizing at variable angle taking into conideration reidual flux level computed by the aid of the controlled witching device (CSD), whatever the previou deenergizing condition.
Phae (deg) Magnitude (db) Weber Volt Volt Amp A. Energizing at fixed angle Energizing a power tranformer at fixed witching angle when tranformer core reidual flux value i not appraied i typically achieved at relevant peak voltage, when propective flux for firt cloed phae i null and equal to dynamic flux for remaining one. To avoid witching under the wort condition, i.e. cloing the firt phae at the time where reidual and propective flux gap i high and with an oppoite polarity, thi trategy i adopted a the minimum requirement. However, performance i limited. B. Energizing following controlled opening Controlled opening of circuit-breaker feeding power tranformer erve a a good upport to prepare ubequent cloing operation. Depending on the load current, reidual flux level can be fixed, on recurrent bai during de-energizing, to a certain predictable amount. Particularly, demagnetization of iron core can be achieved through a proper equence. When the tranformer i under no-load condition, the ability of ga circuit-breaker to chop tranformer magnetizing current without any re-ignition nor retrike iue (thank to moderate TRV, Fig. 1) give the opportunity to control the reidual flux level inide the tranformer magnetic core. Interruption of magnetizing current can be controlled with a good accuracy. Subequent energizing equence can be achieved at fixed angle correponding to the expected reidual flux level. C. Reidual flux computation Each winding flux i practically obtained from integration of the voltage meaured at tranformer terminal, either LV or HV ide, taking into account the winding coupling and vector group. Voltage i integrated on the fly, the de-energizing i detected, and the flux i ampled and memorized at the time of flux tabilization. A the reidual flux i a DC value, thi proce require advanced ignal proceing technique, like parametric method, to cope with weaknee in DC limited bandwidth of voltage tranformer and neceary limited bandwidth of the integration filter. Pure integrator are not uitable becaue of the unavoidable offet of acquiition channel. Purely inductive voltage tranformer (IVT, Fig. 2) are favorable for that tak ince their accuracy toward very low frequencie i better. The near DC accuracy of IVT i only limited by the izing of their magnetic core; a a conequence there i no explicit corner frequency. 5 I -5 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2 2 x 15 V 2 x 15 1-1 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2 -.18 -.16 -.14 -.12 -.1 -.8 -.6 -.4 -.2 Fig. 1. Interruption of no load current (current/voltage/cb voltage/flux) The angular target upon cloing after controlled opening are ummarized in TABLE I.. TRV Flux Fig. 2. GIS inductive voltage tranformer On the contrary, for capacitive voltage tranformer (CVT), the combination of capacitor divider and tuning inductance raie explicit corner frequencie. Typically, firt one occur at around 1Hz a repreented on Fig. 3. Therefore, integration i not traightforward becaue of their limited bandwidth and tranient when de-energized. However, if the voltage tranformer tranfer function i known, one can compenate thi weakne by applying the invere tranfer function, a decribed in [1]. TABLE I. CONTROLLED CLOSING TARGETED ANGLES Bode Diagram Cae (CB ide) Re. flux Ref* Ref -12 Ref 4 Star Low 9 18 +nπ 18 +nπ Neutral grounded Star / Delta Low 9 +nπ Neutral iolated Yy Low 9 +kπ 3 +mπ 15 +nπ Independent core Star High 36 +nπ 36 +nπ 18 Neutral grounded (>) Star / Delta High 45 +nπ 27 27 Neutral iolated (>) Yy Independent core High (>) 18 +k2π 3 +m2π 6 +n2π * Ref : zero croing of reference voltage, with poitive dv/dt -1-3 -4-5 18 9-9 -18 1-1 1 1 1 1 2 1 3 1 4 Frequency (Hz) Fig. 3. CVT bode diagram
Weber In all cae, the ignal proceing ha to decide preciely and automatically when ampling the reidual flux for memorization: jut after flux tabilization and before the flux return to zero. Thi i illutrated in Fig. 4. 5 4 3 2 1-1 -3-4 -5 fluxa fluxb fluxc Sum Flux.25.3.35.4.45.5.55.6 Fig. 4. Inductive voltage tranformer ignal integration (random opening) IV. EXPERIENCE FEED-BACK Thi chapter focue on the reult obtained during field tet campaign. A. 5 kva tranformer mock-up Etimation of controlled witching ytem overall performance baed on tatitical operational field feedback, including a repreentative number of energizing equence record, i motly difficult to obtain. However, demontration of the ytem performance under variou energizing circumtance i aeed uing a miniature tet et-up arranged to be a repreentative a an operational intallation. The tet bench developed for thi tet campaign purpoe i made of a 3*4 Vac 5 kva 3 limb tranformer energized from a 3*4 Vac/5Hz ource through a et of 3 independent electromechanical latching relay with an operating time catter lower than.1m. Thee relay are operated by a CSD, which contitute the tet object, including algorithm able to apply any of the witching trategy decribed above. Even though the tet campaign i performed at reduced ize, reduced voltage level, and the type of relay ued to energize the tranformer doen t have the dielectric propertie of a GCB, the configuration i accurate enough to provide repreentative reult. Of coure, to have a good field expectation projection, the reult have to be correlated with any HV witching apparatu operating time and dielectric behavior catter. The 5 equence decribed in TABLE II. are defined to cover the mot common field operational condition. To produce tatitically admiible reult, one hundred (1) energizing and de-energizing operation are run for each equence. The reult are diplayed in Fig. 5 depicting the inruh current peak value cumulative ditribution for each equence. They are ummarized in TABLE III., with the introduction of the I 98% and P (I=α) indicator: I 98% : define the peak inruh current maximum value that can be guaranteed/achieved for 98% of tranformer energizing equence P (I=α) : define the probability to reduce inruh current to a value lower than α Thee reult demontrate that the combination of controlled de-energizing and controlled energizing (C) at fixed pre-determined angle tatitically produce the bet reult in term of inruh current limitation and the mot determinitic manner to perform tranformer controlled energizing. However, ue of «reidual flux» method to energize the tranformer i favorable in cae the preceding de-energizing i uncontrolled (D). Controlled cloing at fixed targeted angle (B) remain of interet in the event that operational contraint are retrictive (voltage divider with inadequate bandwidth at tranformer terminal, inability to perform controlled opening). Sequence TABLE II. TRANSFORMER ENERGIZING SCENARII Tranformer energizing Decription Tranformer de-energizing A Random Random B Fixed angle Random C Fixed angle Fixed angle D E Variable angle taking into account reidual flux etimation Variable angle taking into account reidual flux etimation TABLE III. Random Fixed angle PEAK INRUSH CURRENT INDICATORS Energizing trategy A B C D E I 98% [p.u.] 27.4 1.4.5 11.5 5. P (I=1p.u.).1.25 1.55.9 Fig. 5. 5 kva tranformer inruh current cumulative ditribution
A V A V Weber B. Field tet reult Performance obtained from two 23kV generating ubtation where following equence have been experimented 1) Controlled cloing after controlled no-load opening Fig. 6, Fig. 7 and Fig. 8 illutrate the efficiency of opening aided controlled cloing. Both cloing operation have been performed under zero reidual flux hypothei, targeting peak voltage. can be achieved, at fixed angle. Same reaoning may be applied to other load type (capacitive ). 5 4 3 2 1 fluxa fluxb fluxc Sum Flux Fig. 9. Flux computation during inductive load current interruption : tranformer almot demagnetized -1-3 -4-5.35.4.45.5.55.6.65.7.75 Fig. 6. Inductive voltage tranformer ignal integration (controlled opening) : tranformer almot demagnetized 3) Controlled cloing after de-energizing from LV ide with low decreae of the voltage Regarding generator tranformer (GSUT), another option i to progreively reduce the voltage at generator ide (while high voltage ide i diconnected): the dynamic flux i progreively reduced and generator circuit breaker i open when the potential reidual flux i conidered acceptable (low enough), a illutrated in Fig. 1. Once again, ubequent cloing operation at fixed angle lead to very good reult. 4 x 15 2 VAload VBload VCload -4.44.45.46.47.48.49.5.51 5 Ia Ib Ic -5-1.44.45.46.47.48.49.5.51 Fig. 7. Controlled cloing attempt #1 after controlled opening (lat phae making time too early by 1.5m) Fig. 1. Flux computation during progreive voltage reduction: tranformer almot demagnetized 2 4 2-4 x 1 5.42.44.46.48.5.52.42.44.46.48.5.52 Fig. 8. Controlled cloing attempt #2 after controlled opening and pretrike time fine tuning / RDDS adjutment (narrower equence). 2) Controlled cloing after inductive current interruption Interrupting a load current by SF6 circuit breaker lead to traditional arcing time and current interruption at the time of natural zero croing. Therefore, the mall magnetizing current i accompanied to a conitent point of interruption by the load current. Furthermore, if the load current if of inductive type, the current zero i the intant at which the flux i null and the tranformer i demagnetized, a illutrated in Fig. 9. Thi i a favourable ituation, and ubequent perfect cloing operation VAload VBload VCload Ia Ib Ic C. Power lab tet reult In thi tet eion, 6 cloing operation over 3 month were performed in the following condition: Outdoor AIS 4kV / 4 chamber per pole hydraulic breaker feeding 3 ingle phae 25 MVA tranformer (4/ 3kV 225/ 3kV). The inruh current, upon wort no load cloing operation, can rie up to 12kA peak. Controlled cloing ytematically baed on reidual flux computation from previou opening operation. Inductive voltage tranformer feeding the CSD. Controlled and random opening operation. The tatitical repartition of peak inruh current, phae by phae i repreented on Fig. 11.
Ditribution 25 pha phb phc 2 15 1 5 1 2 3 4 5 A Power quality invetigation ha been conducted during one IPP integration project on a enitive indutrial location (Fig. 12). Connection to the point of interconnection (POI) i ubject to regulatory contraint. For availability and reliability motivation, particularly to guarantee the quality of the power delivered to enitive indutrial cutomer, r.m.. voltage dip ha to be contained within a maximum value of 5% on the mot affected phae during the 525 MVA GSUT energizing. To demontrate the efficiency of controlled witching olution adopted by the IPP and confirm uitability of the olution, multiple controlled energizing equence, decribed in TABLE IV., have been held to etimate the wort cae condition. Tranformer inruh current and bu voltage are meaured uing a DFR at a 25.6 khz ampling rate. Fig. 11. Hitogram of peak inruh current Phae C reult are rather good, ince more than 8% of peak inruh current i below 5 Amp. Phae A and B reult are le good and can be explained by exceive mechanical cattering (aged hydraulic drive breaker), ince the CSD algorithm i the ame for each phae. V. POWER QUALITY INVESTIGATION Energizing of large power tranformer from the grid i likely to produce a high diturbance level from a ytem operator perpective, particularly when connected to a weak tranmiion ytem or during retoration phae. Major effect on tranmiion ytem and conequence for enitive cutomer are decribed in [2]. To prevent from any power quality iue, tranmiion ytem regulator are gradually introducing grid code defining criteria to oberve in term of witching tranient uch a voltage dip level upon power tranformer energizing. Thee regulatory confine, applicable to any generating plant, are uually accompanied with witching retriction. To remove witching retriction, utilitie have to take corrective meaure in order to reduce potential diturbance level, and prove their correct field implementation. Controlled witching ytem i one of the olution now regularly conidered. Sequence TABLE IV. Tranformer energizing TRANSFORMER ENERGIZING SCENARII Decription Tranformer de-energizing A Fixed angle Core demagnetization B Fixed angle Controlled opening C Variable angle Random opening D Fixed angle De-energizing from LV ide with gradual voltage reduction E Fixed angle Inductive current interruption Voltage dip i derived from the IPP ubtation bu voltage meaurement elected a the POE. During the energizing equence of the GSUT, line-to-neutral and line-to-line r.m.. voltage are continuouly computed on a one cycle liding window bai, and refrehed each half cycle U rm(1/2) a per the meaurement method decribed by [3] and [4]. Correlation between inruh current and voltage dip i etablihed and reult are ummarized in TABLE V. Note: line-to-neutral and line-to-line voltage dip are lightly different. While the wort witching cae produce a 4% line-toneutral r.m.. voltage dip, the line-to-line dip i limited to 3%. Fig. 13. Bubar r.m.. voltage dip (line-to-neutral and line-to-line) Fig. 12. IPP integration Note that, for POE imilar hort circuit level, r.m.. voltage dip can be correlated with inruh current through a linear relation a ummarized in TABLE V.
Cae TABLE V. Inruh current peak value [A] POWER QUALITY INDICATORS Power quality indicator Voltage dip (ph-n) [%] Voltage dip (ph-ph) [%] A 116.1.1 B 36.8.6 C 84 1.6 1.1 D 145 2.7 1.8 E 23 4.7 2.5 Note: power quality analyi may include voltage harmonic content to check uitability with protection etting and prevent from any tripping during tranformer energizing equence. VI. CONCLUSION: COMBINED APPROACH FOR OPTIMIZED ENERGIZING SEQUENCE How to chooe the mot uitable trategy to energize a power tranformer? Comparing the different method it obviou that controlled cloing after controlled opening method tatitically lead to rather better reult than controlled cloing according to computed reidual flux. Rationally, thi method alleviate the CSD from the challenge of etimating the reidual flux and aociated uncertainty. Moreover, it i eaier to implement, and, in all cae, prevent from extreme inruh current (cloing at oppoite flux polarity). Depending on the application, the frequency of operation, one can chooe the appropriate mode to implement controlled energizing of a power tranformer: Intentional and frequent operation, for which reidual flux etimation i not required Protection operation for which reidual flux etimation i advied Optimized energizing equence combine the advantage of thee method. The global trategy, decribed by Fig. 14, conit in prioritizing controlled cloing at fixed target when the previou opening i identified a controlled (therefore, fixing the level of reidual flux to a known value). In the event of previou opening operation i identified a uncontrolled, cae of protection tripping, the fallback cheme conit in cloing with conideration of the computed reidual flux at the time of de-energizing. Regarding the reult and uncertaintie of each option, thi global trategy i the mot efficient. However, in both mode, return of experience how that the required making time accuracy, to reach almot perfect operation, hall be le than 1m, which i challenging. Thi ha been oberved on virtually all project, either targeting peak voltage or near zero voltage: fine tuning wa neceary to reach nearly zero inruh current, due to light difference between practical ite condition and theoretical factory condition. To give an order of magnitude, an error of 2m for making time typically lead to [.5p.u. 1p.u.] of inruh current. On a long term bai, in order to keep making time conitent, and to guarantee inruh current limitation, the coupling between the CSD and the breaker i eential. Relevant breaker information hall be known and CSD hall able to take it into account. Such controlled witching i even better achieved when aociated with modern low catter pure-pring CB drive. etting Flux computation & memorization External demag.? no IDLE POS=OPEN Cloe order Deenergization detected Cloing according to reidual flux IDLE POS=CLOSE ye Open order Controlled opening IDLE POS=OPEN Cloe order Cloing at fixed angle according to reidual flux hypothei Fig. 14. Algorithm deciion tree REFERENCES [1] T. Liu, H. Siguerdidjane, M. Petit, T. Jung, J.P. Dupraz, Recontitution of Power Tranformer Reidual Flux with CVT Meaurement during it de-energization, 21 IEEE International Conference on Control Application (CCA). [2] Cigré WG C4.37, Tranformer Energization in Power Sytem: A Study Guide, Cigré Technical Brochure 568, 213. [3] IEC 61-8, Voltage dip and hort interruption on public electric power upply ytem with tatitical meaurement reult, IEC Technical Report, 22. [4] IEC 61-4-3, Power quality meaurment method, IEC tandard, 215. @ GE Grid Solution, all right reerved, Photo credit: GE Grid Solution etting