OPTIMISED TOOL FOR THE MEASUREMENT OF WINDING RESISTANCES ON POWER TRANSFORMERS

Transcription

1 OPTMSED TOOL FOR THE MEASREMENT OF WNDNG RESSTANCES ON POWER TRANSFORMERS Mrc Müller 1 1 Hefely Test AG, Bsel, Switzerlnd *Emil: mueller.mrc@hefely.com Astrct: The topic of this pper is the nlysis of the fundmentls of the winding resistnce mesurement nd their ppliction for the development of n optimized tool. The ultimte gol is to crete portle device, which mesures complete trnsformer the fstest nd esiest wy possile. The influence of the chrging voltge nd the mesuring current on the mesuring time hs to e nlyzed. Another importnt issue is the stiliztion time on low ohmic delt windings. Further, testing time cn e reduced y introducing demgnetiztion feture, which elimintes the need of pplying high voltge AC fter DC resistnce test. Finlly, the implementtion of the optimized tool is presented with its efficient connection scheme nd the multichnnel rchitecture. 1 NTRODCTON Fst, efficient nd ccurte mesurement of winding resistnces on lrge power trnsformers cretes severl difficulties. Long chrging nd dischrging times, unstle vlues on closed delt winding systems due to long stiliztion times, inccurte temperture mesurements for resistnce correction, residul mgnetism nd its unwnted effects, inefficient connection nd disconnection of the mesuring equipment re just some of the difficulties to del with. The pper descries n integrted, moile instrument developed to speed up the stilistion time when supplying DC to trnsformer winding y n intelligent mgneticflux optimised chrging. With exmple digrms of vrious trnsformers the chrging nd stilistion effects re shown. After pplying DC to trnsformer, the core remins mgnetized. This cn cuse prolems for further mesurements or reconnecting the trnsformer to the grid. Thus, n integrted, low voltge demgnetiztion function to ring power trnsformer into defined, demgnetised stte will e shown. Typicl prolems rising when connecting mesuring system to power trnsformer hve een solved. n prticulr these prolems re time consumption nd fulty connections, which hve een solved y fully integrted connection set nd multiplexing circuit. Error possiilities in setting up the mesurement equipment hve to e fced nd eliminted y offering stteofthert grphicl, selfexplntory user interfces with online informtion out ll relted vlues nd conditions in n esy nd wellrrnged wy. 2 SPPLY VOLTAGE AND CRRENT 2.1 Generl mesuring principle Figure 1 shows schemtic of common winding resistnce mesurement technique. nstrument R L M DT Figure 1: Winding resistnce mesuring principle using DC power supply The instrument consists of progrmmle power supply, which is normlly operted in constnt current mode. t is used to supply userdefined current into the DT. Additionlly, there is voltge nd current mesurement unit. Thus, the resistnce is clculted ccording eqution (1): R LM R d R = = = LM (1) dt 2.2 Mgnetizing inductivity L M Also, Eqution 1 shows the min difference etween norml nd winding resistnce mesurement. There is lrge inductnce in series with the resistnce of interest. This inductnce is typiclly in the rnge of L 0 = 0.1H to

2 5000H. n figure 2, the current dependency of the mgnetizing inductivity is depicted. L M current of the inductnce, there is no relevnt difference nymore. Bsiclly we cn stte the following rules: L 0 The higher the supply voltge, the fster the current cn e chrged or dischrged. L St 0A St Figure 2: nductnce of trnsformer core By rule of thum the sturtion current cn e expressed in terms of the no lod current 0 : St 2 0, RMS (2) Becuse of the lrge mgnetizing inductivity of the trnsformer, the mesuring current cn not e pplied instntneously. The current cn only chnge ccording to eqution (3): L d = LM (3) dt M For given trnsformer, the rte of chnge of the mesuring current depends only on the voltge pplied to the mgnetizing inductivity L M. Current chrging nd dischrging time depends minly on the mximlly ville supply voltge. Figure 3 compres two chrging processes using voltge of either 50V or 100V. 100V 50V The higher the selected current, the longer it tkes for the current to e chrged or dischrged. 2.3 Power Supply stility t is common prctice to use off the shelf power supplies to supply the mesuring current to the trnsformer. The feedck current control loop of such devices is not designed for high inductive lods. Therefore the output voltge of these devices oscilltes depending on the lod. Figure 4 shows cse with dmped oscilltion, where the supply tkes long time to stilize. t is lso possile, tht the oscilltion is not dmped nd no stle mesurement vlue is reched. Mx R Set 0s 10s 20s Set Figure 4: Trnsient oscilltion of power supply in constnt current mode with high inductive lod t St 0A 0s 10s 20s t ncresing the mesurement current over the sturtion level leds to significntly lower mgnetizing inductivity. This lower inductivity helps the stndrd power supply to stilize fster. This is one reson why people often elieve, tht high mesurement current is necessry to get fst stilistion. Figure 3: Trnsformer chrging nd dischrging using constnt voltge Also, Figure 3 illustrtes, tht it tkes longer to rech higher current, which is inherently cler. But if the current is higher thn the sturtion

3 2.4 Delt winding considertions Figure 5 shows common trnsformer configurtion with delt connected winding on the low voltge side. A B C N R A R B R C L A L B L C R N c R R R c L L L c Figure 5: Exmple trnsformer YNd11 The common method for mesuring the winding resistnce of delt connected winding is illustrted in figure 6. The exmple shows mesurement t the trnsformer terminls to. But in stedy stte the current distriution will e ccording eqution 6 (for R R R c ): = R Rc R 2.0 (6) sing the method depicted in figure 6, the mesured resistnce vlue will only e correct when reching stedy stte. By introducing virtully circulting current, eqution (4) cn e rewritten s follows (where nd re the stedy stte currents): ' ' ( t) = ( ( t)) ( ( t)) (7) Circle Circle Stedy stte is reched when Circle decyed to zero. The decy is n exponentil process chrcterized y the pssive components involved nd the initil circulting current Cirlce0. t cn e descried s follows: nstrument R L R L R c L c DT Figure 6: Winding resistnce mesurement on delt winding When we pply voltge etween terminls nd, current strts to flow. This current splits etween the two depicted rnches. ( t) = ( t) ( t) (4) During chrging, the current distriution etween the two rnches is dominted y the rtio of the mgnetizing inductnces nd not y the rtio of the resistnces. During nd directly fter chrging, the current distriution is s follows (ecuse the three inductivities re not equl): Circle = Circle0 t τ ( t) e (8) L L L c τ = (9) R R Rc This leds to nother prolem for winding resistnce mesurement. When the delt winding is on the low voltge side of lrge genertor trnsformer, the winding resistnce is typiclly very smll (<10mΩ). According to eqution (9), this leds to lrge time constnt for the circulting current to go to zero. Mesurement stiliztion times from severl minutes up to one hour cn e oserved on lrge trnsformers. One wy to speed up this stiliztion time is to decrese the mgnetizing inductivity of the trnsformer. This cn only e done y sturting the trnsformer core. On low voltge windings of lrge power trnsformers, the sturtion current cn esily exceed 10A to 100A. Which is the reson why people often wnt to hve high current (>50A) mesuring devices. = L L L c 2.0 (5)

4 3 FLX OPTMZED YNDELTA METHOD 3.1 Bckground This section descries nother method to decrese the stiliztion time on low ohmic delt winding. The gol of this method is to reduce the mgnetizing inductivity y sturting the core, too. But this method uses the high voltge winding to sturte the core, ecuse the sturtion current on the high voltge side cn e significntly lower thn on the low voltge side (depending on the turns rtio): 3.2 Experimentl Results Tests with the descried method hve een performed on n 1100MV genertor trnsformer (YNd5, 27kV to 420kV stepup) to compre the performnce of the different methods. The nominl resistnces of the phses on the low voltge side re out 1.12mΩ. The sturtion currents re St1 0.9A nd St2 12A Figure 8 illustrtes the stiliztion time of the resistnce reding, when supplying 8A to the low voltge winding only. HV St N LV 2 = St (10) N1 Figure 7 shows the reltive stedy stte flux distriution in the trnsformer core, when pplying current 2 from to on the trnsformer depicted in figure 5. Since the rtio of the currents nd is out 2, lso the rtio of the fluxes in the corresponding core legs is out 2. Θ A =N 1 1/2 1 Θ B =N Θ C =N 1 1/ Θ =N 2 1/3 2 Θ =N 2 2/3 2 Θ c =N 2 1/3 2 Figure 7: Flux distriution when mesuring LV As lredy mentioned, the core gets sturted y injecting nother current into the high voltge winding, which hs much more turns (N 1 ) thn the low voltge winding (N 2 ). The key of the descried method is tht the flux distriution generted y the high voltge winding mtches the flux distriution generted y the low voltge side. Figure 7 lso show the electromotive forces Θ generted y ech winding, which indicte the current distriution in the windings. Figure 8: Resistnce mesurement on lrge genertor trnsformer (YNd5, 1100MVA) using the common method with 2 = 8A (Only LV)

5 4 DEMAGNETZATON 4.1 Bckground After disconnecting trnsformer from the grid or performing winding resistnce mesurement with direct current, the trnsformer core will e mgnetized. Figure 10 shows trnsformer core hysteresis curve with possile mgnetiztion M 0. M 0 cn e nywhere on the yxis within the hysteresis loop. B=μ (HM) Φ R ΔΦ Check Demg M 0 M Demg Demg H=N ΔΦ Check Φ R Figure 9: Resistnce mesurement on lrge genertor trnsformer (YNd5, 1100MVA) using the flux optimized YNDelt method with 1 = 8A (HV) nd 2 = 8A (LV) And figure 9 illustrtes the stiliztion time of the resistnce reding, when the flux optimized YN Delt method is used with 8A on the high voltge side nd 8A on the low voltge side. n oth cses the mesurement current injected on low voltge side is 8A, which is smller thn the sturtion current St2 12A. But the stiliztion time with the trditionl method is much longer (25 to 30 minutes) thn with the optimized method (6 to 14 minutes). By incresing the mesuring current of the trditionl method to 100A, stiliztion times etween 7 nd 23 minutes hve een chieved. Figure 10: Hysteresis loop of the mgnetizing inductnce of trnsformer The mgnetiztion M 0 cn influence vrious mesurements like turns rtio or frequency response. For these mesurements the mgnetiztion should e M 0 0Am 1, otherwise the results cn e wrong or not comprle. Further, connecting mgnetized trnsformer to the grid cn cuse high inrush currents. The common method to demgnetize trnsformer core is to pply nominl AC voltge to the trnsformer nd slowly decrese its mplitude to zero. But this method requires very lrge nd not portle controllle AC voltge source. On this prticulr trnsformer the flux optimised method performed even etter thn the clssicl method using high current. The conclusion is tht even the lrgest trnsformers cn e mesured within n cceptle time using non sturting current on the low voltge side.

6 4.2 Demgnetiztion Procedure The invented demgnetiztion procedure consists of two su procedures nd requires only low voltge power supply (<100V): Anlyzing the trnsformer: The hysteresis loop of the trnsformer is clculted using specil lgorithm. The lgorithm comprises the injection of current Demg to the trnsformer nd the mesurement of the response of the trnsformer. tertive reduction of the remining mgnetiztion: The current core flux is continuously clculted nd n lgorithm is used to determine how to regulte the voltge supply connected to the trnsformer. Since it is n itertive lgorithm, multiple cycles re necessry to rech sufficiently low residul mgnetiztion. 4.3 Experimentl Results To verify the demgnetiztion procedure ΔΦ Check nd ΔΦ Check hve een mesured fter demgnetizing the DT. f the trnsformer is demgnetized properly, then M Demg 0, which leds to ΔΦ Check ΔΦ Check (refer to figure 10). DT : Yy0 12.5MVA 49kV 16.9kV Anlyze with Demg = 5A Φ R = 160.2Vs Check fter demgnetiztion ΔΦ Check = 318.8Vs ΔΦ Check = 317.6Vs Check when mgnetized ΔΦ Check = 150.5Vs ΔΦ Check = 469.3Vs DT : Dyn5 160kVA 4.25kV 1kV Anlyze with Demg = 5A Φ R = 9.44Vs Check fter demgnetiztion ΔΦ Check = 19.8Vs ΔΦ Check = 19.3Vs On oth exmple DTs the trnsformer core is demgnetised properly. The voltseconds pplied to rech positive or negtive current Demg is the sme in oth cses fter demgnetiztion. On the DT we cn clerly see the mislnce of the pplied voltseconds when the core is mgnetized. 5 DT CONNECTVTY This chpter dels with the prolems rising when connecting mesuring system to power trnsformer. Prolems like time consumption nd fulty connections re discussed. The connectivity of trditionl high current winding resistnce mesurement device with two chnnels nd one current supply is depicted in figure 11. Stndrd Resistnce Mesurement Device N A B C N A B C R 1 R DT 2 HV LV Figure 11: Trditionl winding resistnce mesurement with two chnnels A totl mount of 7 cles is needed to perform this mesurement: Two supply cles, one jumper cle nd four sense cles to mesure the voltge drop t the DT. With the trditionl method mximum of two resistnces cn e mesured, efore one hs to reconnect the DT. One on the low voltge side nd one on the high voltge side. The user of the device hs to figure out the right wy to connect y him self. This increses the proility of fulty connection. With multichnnel topology the user hs to connect the DT only once. As illustrted in figure 12, for ech ushing on the trnsformer there is exctly one clmp. Thus, no reconnection is required t ll. This reduces the proility of fulty connection to minimum. To reduce the mount of cles nd clmps required to perform full connection, Kelvin clmps re eing used. Kelvin clmps hve two electriclly isolted jws, ech connected to seprte wire. This mkes it possile to connect the complete trnsformer with only 8 Kelvin clmp cle sets. The results re reduced connection nd disconnection times.

7 Multichnnel Resistnce Mesurement Device Temp. Mes. Control System TpCh. Control N A B C N A B C R 1 R DT 2 HV LV 1 2 Power Multiplexer Signl Multiplexer 1 2 Figure 12: Multichnnel winding resistnce mesurement device 6 OPTMZED TOOL The key to portle nd optimized winding resistnce mesurement device is to reduce the mximlly needed supply current. Becuse high supply current results in lrger nd hevier power supplies, connectors, cles nd switching mtrices. The switching mtrix is needed to relize the multichnnel concept of the device. The two key elements necessry to reduce the mximlly needed mesuring current hve lredy een introduced: A power supply with specilly designed control loop for high inductive lods. A method to del with the long stiliztion times when mesuring low ohmic delt winding (e.g. lrge genertor trnsformers). This leds to n instrument which comines ll the fetures presented in this pper. Figure 13 shows lock schemtic of the proposed instrument. The system consists of two voltge nd current controlled power supplies, which cn e connected in ny configurtion to the DT. This is relized using power rely mtrix. Additionlly there re two voltge mesurement units, which cn e connected in ny configurtion to the DT. This is relized using smll signl rely mtrix. Further there is lso tp chnger control circuit nd temperture mesurement unit. N A B C N A B C HV DT LV Figure 13: Block Schemtic of the nstrument With this instrument complete trnsformer cn e mesured fully utomtic. This includes ll three phses on the high voltge side nd low voltge side. Additionlly, the device cn cycle trough ll tps utomticlly nd correct ll resistnce vlues to reference temperture without user intervention. Figure 14 shows comprison etween the mesurement procedures of three phse trnsformer with remote controlled tp chnger. The clssicl procedure requires repetitive user interctions. The user hs to write down mesurement vlues, operte the tp chnger nd reconnect the mesuring cles. sing the optimized tool, user interction is reduced to the initil connection nd setup of the device. This helps to reduce the overll testing time nd does not llocte user resources during the mesurement.

8 Events Clssicl procedure Optimized procedure Time Setup or Reconnection Record Dt Chnge Tp Automted Tsks Figure 14: Comprison of the mesurement procedure for three phse trnsformer with 5 remote controlled tps 5 CONCLSONS This pper demystifies effects oserved when mesuring winding resistnces on vrious types of trnsformers. The sics of winding resistnce mesurement re commonly not well known. Also, this pper introduces theoreticl concepts such s the flux optimized mesuring method or the demgnetiztion procedure. These concepts hve een verified y experimentl results. Bsed on the reserch presented n optimized nd flexile hrdwre with complex control system hs een developed.