R3 - Fast Reponse SS on Load Transformer Tap-Changers

8/6/2019 R3 - Fast Reponse SS on Load Transformer Tap-Changers

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FAST RESPONSE SOLID-STATE ON LOADTRANSFORl$lERSTAP-CHANGER

J Faiz

University of Tehra n. IranABSTRACT

This paper presents a new method for on-load ta pchanging of power transformers using solid state andvacuum - switches The tapcha nging system iscomprised of solid-state devices (for example GTOs).fast re$onse circuit breaker (for example vacuumintcrrupters) and a number of transformer taps. In thismethod GTOs and vacuum switches with suitable on-off timing makes t possible to cha nge taps Over a shortperiod. Simulation resuits show that changing twoadjacent taps takes less than 100m.i which is veryshorter that 5 s required for the corresponding changeby the mechanical tap-changers.

INTRODUCTION

Tapchanging is used extensively in a wide variety ofelectrical inductive apparatus. In order to supply highqualih elcctrical energy to the users. electrical powerutilities apply voltage regulation techniques. Generally,loltage. regulation objects are the voltage regulationover the required limits. control of active and reactive1 (%5-%lO), daily (%3-%5) and shortadjustment versus load variations (1).

This s done by adjk ting the turns-ratio or phase anglele the device is serving load. Most ofmethods in use utilize a witchingtely connect various sections of onecal inductive apparatus circuit. Atpresent mechanical contact witch is extensively used.

or secondary winding or both to increase the tap rangeof tlic clcctrid inductive apparatus at the lowest cost.Depending on rated power and voltage of transformerand voltage re gulation level. various circuits are used intap changers; some of them have been introduced in (2)and (3) and their switching patterns are given. Toprevent-the shon circuit of different taps a pro*rresistanq: or.reactanc e is used which enters into f3ecircuit when tap is changing. Thus,'the load cumntpasses through the resistor or reactor (2). Fig. 1shows

H avidnia

University of Tabriz. Ir ana typical mechanical tap changer in which tapchanging procedure from tap 2 lo tap 3 has been shownin live stages. Disadvantage of the mechanical tapchangers is long ta p changing time; therefore they maybe only used for steady-state voltage control of powersystem.

4Y -iii T.oa21nd 3

Fig. 1. A typical mechanical tap changer 1)Another type of tap changers are solid-state tapchangers (4-6). In this type of tap changcn gaiccontrolled devices, such as conventional thynstors orgate turn-off thyristors (GTO) re used. A controldevice' triggers predetermined groups of the thyristorpairs to connect or bypass certain ones of the tapwinding sections and thereby provide a wide range ofoutput voltage. One of the drawbacks of thesc tapchangers is the harmonic contents of the output voltagcin continuous change of the volmge. In (6) umber ofrequired solid-state switches reduced into 50%.However use of this method needs accurate witchingtime, which lea ds to a complicated control system.In general, the major drawbacks of these tap changersare high copper losses, low reliability and e.xpensivesystem due to several circuits required for protectionand commutation of the thyristors. Therefore, these tapcbngersarenot in common use. and only for someparticular cases re applicable.In this paper a new scheme first introduced in (7) isanalyzed and its performancc is described. In thisscheme. GTO switches are used to change the taps and

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- -- .-_Power ElectronicsMd Variable Speed Drives, 18-19 September21WaConference PublicationNo. 475Q IEE 2000

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by switching on and off the vacuum switches at theinstant of zero current. the load current is transferredfrom one tap to other taps and the output voltage isregulated over a short period.

NEW TAP-CHANGER SCHEME AND ITSPERFORMANCEHigh voltage (HV) winding of transformer and requiredswitches for tap changing of the proposed scheme hasbeen shown in Fig. 2. The leakage inductance andresistance of secondary winding are divided betweentaps based on the number of tum ratio. SA. SB an d Scare A C solid-slate switches. The reason for laking intoaccount tliree taps is the limitations of circuitsimulation software and also for simplicity. Of course,iiicrease of tap number is possible and inay not changethe principle. The details of the proposed tap changerare described in the following parts.Selector

Circuit topology of the selector is similar to theconventional tap changers except that this selector'sresponse is faster. For this purpose, contacts in the oil-tank are replaced by vacuum intempters incorporatedwith a two-position electromagnetic actuator. Thevacuum interrupters have high level power transfercapability and long life and are very convenient for thisapplication. By applylng a current pulse to two coils,the contacts of the vacuum switch will be on and off.depending on the corresponding coils. After positionchange of the switch, it is possible to keep switch innew position by a permanent magnet and holdingtorque is also applied on it. Standards indicate that thevacuutp switches must be able to switch off the faultcurrent in two cycles. In the present work, the vacuumswitches are switched on and off when the currentpasses zero. Thus, there is no electric arc within theswitch and switch on and off imes depend only on thetime constant of the mechanical drive qstem of thecontacts. This position change takes about 20milliseconds. In the simulation, the vacuuminterrupters' off ime has been taken to be 20ms, whichisequivalent to one period at freq uency of 5OHz.

DiverterFig. 3 shows the structure of the proposed diverter,consisting of two semiconductor switches A and B,which are esplained, in next part. Each switch isconnected between t he selector output leads and neutralor middle point of the star connection. VA nd VB arevacuum switches connected in parallel \ k i t h switches Arind B.Therefore: th e vacuum switches do not break the

line current. After tap changing. the current transfersfrom the semiconductor switch to its parallel vacuumswitch. In order to protect the semiconductor switchesfrom possible faults occurring on the transformer, thesciiiiconduaor switches are disconnected fromtransformcr minding by the selector after tap-changingtakes place.

V2=nV

R O A D

LLOAD

Fig. 2. Circuit diagram of tap changer

AC Semiconductor Switches

Generally in power systems hg h efficiencytransmission line has almost unip power factorAlthough. if a fast tap changer is used to damp theoscillation occurred due to a fault, it must be ableoperate over all power factors and pass the current intwo directions. One of the first attempts to designelectronic tap changers is the use of anti-parallelthyristors as proper switches. In such designs, thecurrent may be not in-phase with the voltage; ths maylead to the switching off the forward bias thyristor,which can da mage the thyristor. To prevent such eventthe load power factor is measured and the instants ofthe thyristor switching is determined corresponding tothe measured power factor. The principle of this hasbeen described in details in (6). A proper AC switcliesstrudure, recommended for tapchanger, is shown inFig. 4. One of the configurations is anti-parallel and theother is bridge configuration.In order to improve the reliability of the switches. twoGTOs in each current passing path may be employed.GTO witches with capacity of 2kA are now available(8) and there is no need to use two parallel switches ineach path. However, it is recommended that thenominal current of the semiconductor switches arechosen twice the actual cumnt for reliability.

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Obviously, if on e semiconductor switch can bc used ineach current path. there will not be difficulty ofparalleling process of two elemcnts.from fromselector Seleclor

semiconductor switch. and a IkV varistor. Duringsteadystate operation. vacuum switch V A andsemiconductor switch C conduct. Therefore. phasecurrent passes through the primary of the ausiliarytransformer The transformer reduces the currentthrough switch C and voltage drop on the s~ i t c l lo1.20. Assume 5V voltage drop on GTO, hus thetransformer prinlary drop u i l l be 0.2SV.

Fig. 3. Diverter (7)

bFig. 4. AC semiconductor switches (a) Anti-parallelconfiguration (b)Bridge configurationBased on the data given by manufacturer, switch-offtime of a GTO switch with average current of 55QA s1Sps (9). In the present work. switch-off time for GTOIS considered equal to 3Qps. which is a reasonable time.Voltagc drop duringaon time of GT O is between 3V andjV , which is neglig ble compared to the high voltage oftransformer.

Auxiliary Current Diverter

An mmiliary current diverter is used to transfer currentof a vacuum switch to its parallel semiconductorswitch. This amdiary divener has been shown forswitches A and V in Fig. 5 . I t consists of a low-powerausi l iaq transformer with tum ratio of 1:20, a

Fig. 5 . Auxiliary current diverter (7)At the beginning of the tap changing process, phasecurrent must be transfer from switch VA to switch A.To do this, switch C is off and switch A is on.secondary current of transformer passes throughvaristor and 1kV induces on the secondary and 50V onthe primary. This Joltage is in the path of switch VAand changes the current path and passes through thelow resistance path. Transfer of the current fromvacuum switch to semiconductor switch takes feuseconds. M e r completing current transfer, whencurrent does not pass vacuum switch, it is switched off.In Fig. 2. SC indicates the semiconductor switch C inSPICE ofluare.In order to transfer each tap to the adjacent tap. asimilar snitching pattern can be used. For instance,Fig. 6shows the transfer from tap S2 to tap S . As thisfigure indicates, first S1 is switched on, since %.B an dSB are off, the current does not pass through SI. t thefirst instant of zero current, SA on and Sc off whichleads to transferring current from %A and SA. In sucha case, current of switch %.A is zero and this switch iseasily off.At the zero current instant and after completeswitch-off of SVA , switch S A is off and SB is on. T hiscauses transferring transformer current from SI andS Then, S vg is switched-off and S c is svitched-on,this transfers output current of transformer from S1 toSw. he last stage is the switching-off S2. whichcompletes the tap changing process.B.

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. . . . . . . . . . . . . . . . . . . . . .oxom:.. .............. : . .snmlllr. . . . . . . . . . . . . . . . . . . . . . . .

ow' , * . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . ..owO R - . . . . . . . . . . . . . . . . L .......................................... ................ ,,

L

ow............0.I-

Fig. 6. Switching time p t te m for tap changing from S2to s 1

SIMULATION RESULTS

Circuit shown in Fig. 2 is simula ted by Pspice.Parameters of the proposed transformer are given inTable 1. Transformer supplies 4OkVA with laggingpoucr factor 0.8 load. n is assumed 20%. In such acase.output voltages of transformer are 2000 .2400 and1600 V. It is assumed that the transformer used in theproposed current diverter is ideal; in fact two linearinductances habring approdmatety unit couplingcoefficient has been employed. Also a resistance hasbeen replaced instead of varistor used in (7). Th eGmulation re sults show that by a proper selection of theresistance, tap c hang ing may be properly done.

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4onu 501u 6Oms(b) T i m eFig. 7. Output voltage of transformerLoad current wavefonn has been sh om in Fig. 8. Asseen ther e in no transient on it an d user does not realizethe tap changing process. Current of semiconductorswitch Sc has been represented in Fig. 9.Before andafter tap changing, tap has a small current and duringtwo taps changing. this switch is switch-off and curr entdoes not pass through it. Fig. 10 exhibits current ofsemiconductor witch Sg. The current passing thissemiconductor switch depends upon the auxiliarytransformer characteristics and in the tap changerdesign process, care must be made in the selection ofthis low-power transformer.

Gc=2mSThe simulation results based on the switching pattemof Fig. 6, n tapchanging from S2 to S1 has beenshown in Fig. 7. Fig. 7a shows the output voltage oftransformer; its smaller time-intend ' has beenrepresented in Fig. 7b. As seen, ap changing is done att=52ms which leads to a sudden change of the outputvoltage.

I

I RI=0.02R I , R2=0.5R I CONCLUSIONSA new scheme has been studied and simulated for tapchanging of transformers. The simulation resultsindicate that tap changing using this method is 50times faster that the corresponding on-load mechanicaltap changers. In the worst case, changin g one tap to theother tap takes 100ms; the corresponding time inmechanical tap changer is about 5s. The transientcurrents of the switches have no effect on the loadcurrent and its peak value is standed by the switches.When an actual auxiliary transformer is proposed,current of GTO and vacuum switch is considerablylarger than the deal case. This may be assigned to thesaturation of the core.

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7 - - - - - - - - - - -i20ms 40111s 8 m s 120ms-2

OA -L - - - - _I _- - - - - - - - - -T i m e

Fig. 9. Current of semiconductor switch S c

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Say. M G. . 1983. "Alternating CurrentMachines". Longm an Scientific and TechnicnlPublishcr. UKFranklin, A.C.. Franklin, D.P.. 1983. "JBPTransformer Book'. Buttcnvonh. UK1964. "Electrical Transmission andDistribution Rcfercnce Book. WestinglrouscElectric Corporation, East Pittsburg.Pennsylvania, USARoberts. M.E.. Ashman. W.G., I Y 6 Y .Confercnce Publication 53. Power thvristorsand the ir applications. 185-192Cook. G.H..Williams. K.T.. 1992. IEE Proc..You scf-hi. F.Q.. OKelly. D.. 996. IEE Proc.,Shttlewonh. R.. Tian. X., an. C., Power. A. ,1996. IEE Proc.. PI B. 143-1, 108-112Mohan, N., Undeland, Y.M., obbins. W. P .1995. "Power Electronics: Converters.Applications and Design", John W iley iy:Sons,USARashid. M.H., 1993, "Power Electronics:Circuits, Devices and Applications". Prenticc-Hall International Inc.. USA

PI. B. 139- 6,507-5 13Pt.B. 143- 6 , 4 8 1 4 9 1

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R3 - Fast Reponse SS on Load Transformer Tap-Changers