NEW METHOD FOR THE STATE EVALUATION OF THE ZERO-SEQUENCE SYSTEM

TX it NEW METHOD FOR THE STATE EVALUATION OF THE ZERO-SEQUENCE SYSTEM Gernot DRUML A. Eberle GmbH Germny g.druml@ieee.org Olf SEIFERT Dresden University of Technology Germny seifert@ieeh.et.tu-dresden.de Abstrct In this pper we present new method for the evlution of the zero-sequence network prmeters by injecting two frequencies in the zero-sequence system. These prmeters cn be used for the decision to move Petersen-Coil nd lso for the high ohmic erth fult detection in resonnt grounded networks. The mjor problems for the correct clcultion of the line-to-ground cpcity,respectively of the resonnt-point re the missing or very low zero-sequence voltge nd the non negligible crosstlk of the vrying lod currents to the zero-sequence-voltge. As consequence, the number of tuning opertions nd non correct tuning opertions increses in tody's networks. The new method uses the injection of two currents with frequencies unequl to 5 Hz into the zero-sequence system for the clcultion of the network prmeters. Therefore it is possible to supervise complete symmetricl networks nd to suppress the 5 Hz crosstlk of the lod current. In consequence, the number of coil movements nd lso the number of wrong tuning positions re drsticlly reduced. This new method cn be extended for the estimtion of the prmeters of ech feeder, to detect lso high ohmic erth fults. Keywords: resonnt grounded system, erth fult, high ohmic erthfult detection, current injection, Petersen-Coil INTRODUCTION The tuning of the Petersen-Coil is preventive opertion lredy done in the helthy network. With the existing methods it is not possible to determine the network prmeters during solid erthfult. The fult loction nd the resistnce t the fult loction re unknown nd re not ccessible for mesurement. In cse of solid erthfult, the zero-sequence voltge is impressed nd the mesurement of the zero-sequence current t the fult loction is impossible. The zerosequence current cn only be mesured t the substtion or in some cses t some dedicted switching-sttions. In the pst, different control lgorithms were developed. Most of these lgorithms re bsed on the necessity to move the Petersen-Coil. The development of tody's distribution networks is chrcterized on one side by n increse of symmetricl cbles, which results in smller usble zero-sequence-voltges nd, on the other side, in n increse of the crosstlk of the positive sequence of the lod current to the zero-sequence system. With the decresing zero-sequence voltge the controller must be set much more sensitive. Due to the crosstlk of the lod current to the zero-sequence voltge, ech chnge of the lod current cn relese tuning opertion, which is, in most of the ctul lgorithms, combined with physicl movement of the Petersen- Coil. Due to the disturbnces the stte nd prmeter estimtion of the network is much more difficult nd results in necessry movement of the Petersen-Coil over longer distnce. Nevertheless, sometimes correct tuning is impossible. One problem rises becuse the motor-drive of the Petersen-Coil is only designed for few tuning opertions per dy. The other problem rises becuse of the longer detuning time. This is cused by the increse of tuning cycles, respectively by wrong tuning positions. Therefore it is necessry to find methods, which re ble to find the correct tuning position, even if the nturl zero-sequence voltge is zero, respectively if the disturbnces in the zero-sequence voltge re not negligible. Additionlly, the number of necessry moving opertions should be reduced. 2 DISTURBANCES OF THE CONTROL OPERATION Using the stndrd simplified equivlent circuit for resonnt grounded system X[3]X,X[]X s it is shown in XTFig. seems to be very esy to find the resonnce point of the sound network, even for very smll neutrl-to-erth voltges. TFig. : Stndrd equivlent circuit for resonnt grounded system The problem becomes more difficult becuse severl disturbnces generte n dditionl non-zero neutrl-toerth voltge UUUBNEB. Thus, it is very difficult for the control lgorithm to distinguish between rel resonnce points nd fictitious resonnce points, cused by the disturbnces. 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge

influence. 2. Description of the network The network under considertion consists of trnsformer, the Petersen-Coil, trnsmission line nd lod s depicted in XTFig. 2TX N Trnsformer U 3N U 2N U N U NE Y P du I P Erth Z L3 Z L2 Z L Z M23 Z M2 Line Z M3 Y Y 2 Y 3 I I 2 I 3 U 3 U 2 U Lod Z Lod3 Z Lod2 Z Lod TFig. 2: Equivlent circuit for the investigtion of disturbnces on UUUBNEB For the ske of clrity, we further ssume, without restriction of generlity, tht unblnces of the trnsmission line only occur in phse. Furthermore, the mutul coupling of the trnsmission lines is neglected, becuse if the network is symmetricl this only hs minor influence on the results. The cse of symmetricl mutul coupling cn be treted in similr wy s n unblnce in the series rectnce of one phse. It is worth mentioning tht the equtions for complete description of the different coupling effects of networks with symmetricl components re very complex, nd cnnot be simplified by using the clssicl symmetricl component concept. The disturbnces cn be summrized to the following three min coupling effects X[3]X bsed on XTFig. 2TX:. Unblnce of the voltge 2. Unblnce of the line-to-erth cpcitnces 3. Coupling of the lod current over the normlly negligible series line resistnces nd rectnces The second item results in nturl unsymmetry, which depends more or less on the nturl cpcitive unblnce of the network. Due to the voltge drop long the line, the unblnce cn chnge in smll rnge with the chnge of the lod current (see XTFig. 3TX). Also chnges of the lod current in other feeders, not shown in XTFig. 2TX the residul voltge UUUBNEB.. N 2 chnges of the residul voltge, due to the crosstlk, become very lrge nd re not more negligible. bs(u EN / U N ) TFig. 3: bs(u NE / U N ) TFig. 4:.45.4.35.3.25.2.5..5 5 5 2 25 3 lod current /A.35.3.25.2.5..5 TNeutrl-to-erth voltge due to the voltge drop long the linet 5 5 2 25 3 lod current /A TNeutrl-to-erth voltge due to n unblnce of the seril impednces in the linet The symmetry of line my be cused for exmple by the kind of cble lying, s shown in XTFig. 5TX (for further detils the reder is referred to X[5]X, X[6]X, X[7]X, X[9]X). If the cbles re lid in tringle like in XTFig. 5TXb the mutul coupling of the three phses is obviously the sme. A similr sitution cn be found for overhed lines where n improvement cn be mde by trnsposing the phses. Z M2 Z M3 Z M23 2 3 Z M2 2 Z M23 The tp-chnge of the trnsformer lso influences the zero-sequence voltge, minly by chnging the size of UUUBNB, but lso by n dditionl unsymmetry. 2r Z M3 3 The most importnt result describes item 3. Due to this behviour, n dditionl phsor is dded to the nturl UUUBNEB nd the size of this phsor depends on the size of the lod current (see XTFig. 4TX). This phsor is dded in the complex plne to the nturl unblnce nd cn increse or decrese the residul voltge UUUBNEB. Especilly in symmetricl networks the resulting reltive TFig. 5: ) b) ) Single conductor cbles in prllel. b) Single conductor cbles in tringle 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge 2

nd this one 3 CONTROL ALGORITHMS 3. Existing Algorithms Up to now, minly the following lgorithms re used to determine the network prmeters respectively to tune the Petersen-Coil. The reltive chnge of the zerosequence voltge is normlly used s the criterion for the detection of switching opertion in the network.. Artificil Erthfult By mesuring the current over the rtificil erthfult loction nd serching for the minimum of the current by tuning the Petersen-Coil, the tuning point nd the prmeters of the equivlent network cn be determined. This method is ctully only used to check the qulity of control lgorithm. 2. Serch of mx UUUBNEB This lgorithm serches the mximum of the residul voltge. Improved versions of this lgorithm determine dditionlly the network prmeters by using the 2 method X[3]X. Alterntive lgorithms re using lestsqure techniques to estimte the network prmeters lredy from prt of the resonnce curve. 3. Lest squre bsed on /UUUBNEB A lower sensitivity ginst disturbnces cn be reched by using n lgorithm bsed on the inverse of the resonnce curve X[]XX[3]X. 4. Locus Digrm of UUUBB This method is bsed on the fct tht circle cn be constructed with only three points. This method ssumes tht the third point of the circle is the origin of the complex plne. A short detuning cn be chieved for exmple by switching cpcity in prllel to the Petersen-Coil. This switching results in second point of the locus digrm of UUUBNEB. Mesuring the voltge with mplitude nd ngle it is possible to construct the locus digrm. 5. 5 Hz Current Injection This lgorithm is bsed on the ide to inject n rtificil current into the neutrl point of the system if there is no unsymmetricl current from the nturl unsymmetry. The influence of the nturl unblnce cn be prtly compensted by using differentil mesurement from two time points. Eq. X()X in combintion with the coil position enbles to determine the network prmeters. dici YCI = YW + j( BC B L ) () du NE 3.2 New Algorithm Principle All the existing lgorithms re bsed on the fct, tht the residul voltge is generted either by the nturl unblnce of the network or by n rtificil 5 Hz current injection. These methods re ssuming, tht there is no chnge in the network respectively no chnge of the crosstlk of the lod current during the clcultion period. Plese py ttention tht the clcultion period cn lst from severl seconds up to severl minutes. In relity there re lot of situtions where these ssumptions re not vlid, for exmple in the sphere of hevy industry with symmetricl networks but hevy chnges of lod. The new CIF-lgorithm (UCUontrol by UIUnjecting UFUrequencies) suppresses the 5 Hz crosstlk from the lod current by using frequencies unequl to 5 Hz for the mesuring nd for the prmeter estimtion. The simplified equivlent circuit with current injection ccording to XTFig. 6TX TFig. 6: Simple equivlent circuit with current injection results for the frequencies unequl to 5 Hz to XTFig. 7TX TFig. 7: Simple equivlent circuit with current injection unequl to 5 Hz For the frequency fn the dmittnce, seen from the current injection, cn be described s: I Y = = Y + YW + j( ωnc ) (2) ω L CI _ fn CI _ fn U U NE _ fn re- For symmetricl networks with smll UYUBUB sults in NE _ fn n I CI _ fn YCI _ fn = YW + j( ωnc ) (3) U ω L Using two different frequencies fbb fb2b gets two complex equtions with three vribles, which leds to the following solution: n 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge 3

, I f YW = rel U NE _ f img( Y CI _ f ) ω img( Y CI _ f 2) ω2 C = 2 2 ω ω2 L = ω ( img( Y ) +ωc) CI _ f (4) (5) (6) Assuming liner system enbles the current injection of two frequencies nd evlution of the corresponding UYUBCI_fnB t the sme time. This results in very fst mesurement possibilities nd depends more or less on the used frequencies nd filter lgorithms X[8]X. The durtion of the mesurement is usully in the rnge of 24 ms. The following items list the min dvntges of this new CIF-lgorithm: Very fst mesurement Suitble lso for symmetricl networks Determintion of the sum of ll Petersen-Coils including distributed fixed-coils in the compensted re Insensitive to the 5 Hz open-delt VT error Suppression of 5 Hz crosstlk ccurcy of the prmeter estimtion is incresed, especilly for systems with lrge stndrd detuning. Opertion philosophy Depending on the opertion philosophy the current injection cn be ctivted only for short time fter the detection of n essentil reltive chnge of the zerosequence voltge, to check if new tuning of the Petersen-Coil is necessry. In symmetricl networks the current injection cn be switched on continuously, to detect ny switching opertion in the network immeditely. Combintions of these two philosophies re possible, for exmple to check every min the ctul network prmeters in symmetricl networks. More Precise Models In XTFig. 8TX connection of the Petersen-Coil to the neutrl point of the trnsformer is shown. For more ccurte clcultion of the network including Petersen coil, s shown in XTFig. 8TX it is necessry to use more precise equivlent circuit s depicted in XTFig. 9TX. Additionl requirements Depending on the resonnce curve nd the norml opertion philosophy of the network, there rise some dditionl requirements for the current injection. ) The injected current should be vrible in the mplitude to enble dpttion to the losses of different switching sttes of the network. One of the most used criteri for the erthfult detection is the zero-sequence voltge. In smll networks the losses in the network re smller, so tht only reduced current should be injected, not to exceed the threshold level of the erthfult detection system, especilly in the resonnce point. On the other side, in cse of situtions with lrge detuning smll injected current will not deliver relible mesurement of the residul voltge UUUBNE_fn B.In this cse higher injected current is recommended. 2) The injected frequencies should not include 5 Hz components. TFig. 8: TFig. 9: Petersen-Coil with current injection (CI) nd wttmetric increse GBWB Simple equivlent circuit with current injection (CI) nd wttmetric increse GBWB 3) Using current injection with vrible frequencies, it is possible to select the injected frequencies in such wy, tht these frequencies re ner to the resonnce of the network. In this cse smll injected currents result in lrge vlues of the residul voltge. The Using frequencies unequl to 5 Hz enbles now n ccurte mesurement of the following components during norml opertion of the network 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge 4

Zero-sequence cpcity of the network Externl Petersen Coils existing in the network (distributed Petersen-Coils) Zero-Sequence Impednce of the Trnsformer Vlues of the fixed-coils in the substtion Detuning Vlue of dditionl dmping resistors Clcultion of the unsymmetry of the network The most simple wy is to use stndrd frequency converter (FC) in the mode of current source s shown in XTFig. TX. To reduce the disturbnces on the 4 V side, frequency converter with power fctor correction module (PFC) is recommended X[4]X. The coil L respectively the prllel circuit L//L2 is used to convert the pulsed voltge to n impressed current. The size of L//L2 defines the mximum vilble injected current. The uxiliry winding of the Petersen-Coil is usully designed for 5 V, which mkes necessry, in these cses, n dditionl trnsformer for the dpttion. With this type of current injection two currents with individul mplitude, frequency nd phse cn be injected very esy. On the other side the physicl relistion is not the chepest one. 3.3 High Ohmic Erthfult Detection with the DIFlgorithm (UDUetection by UIUnjecting UFUrequencies ) The prmeter estimtion of the network cn be extended for ech feeder by mesuring the injected currents in ech feeder of interest either with the Holmgreen-Circuit (summtion CT) or with the core-blnce trnsformer. B C2 I Y2 B L2 Y W2 TFig. : Current injection using Frequency-Converter(FC) with Power-Fctor-Correction (PFC) If the requirement for vrible frequencies is cncelled, much cheper version to generte current with more frequencies is vilble, s shown in XTFig. 2TX Z Tr I Y I CI _ f,f2 U NE U 2 U L I CI Y W B L U B C U 3 L2 U ne L P I P TFig. TT: Prmeter estimtion for ech feeder TT As the crosstlk from 5 Hz is suppressed, the mesurement of UUUBDB cn be used for the clcultion of the essentil prmeters for ech line. It is possible to clculte the cpcitive prt BBCxB, the losses YBWxB nd the size of distributed coils BBLxB seen on feeder x, with the sme method s explined bove. By using dditionlly the 5 Hz components t the sme time, the ctul unblnce of the network cn be determined nd supervised. The dvntge of this lgorithm is, tht ll mesurements re mde t the sme time. The usul problem to check for switching opertion is removed. The determintion of the network prmeter is included in the lgorithm directly. 3.4 Types of multi-frequency Current Injections (CI) TFig. 2: Current injection with AC-switch for three frequencies (AC-) The following figure shows one possible pttern of pulses for the current injection..8.6.4.2 -.2 -.4 -.6 -.8-5 5 2 25 t / ms TFig. T3T: Smple pulse pttern for AC-TT The corresponding frequency spectrum is shown in XTFig. 4TX 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge 5

.6.5 4.67 :.34724 5. :.5 58.33 :.29383.6.5 25. :.28292 4.67 :.69448 5. :. 58.33 :.58767.4.4.3.3.2.2.. 5 5 2 25 f / Hz TFig. 4: Frequency spectrum for AC- 5 5 2 25 f / Hz TFig. T7T: Frequency spectrum for AC-2 TT The mjor disdvntge of this type of current injection is tht the min spectrum of the injected current is 5 Hz. This cn be voided by the following type of thyristor-switch, where it is possible to invert the direction of the current during the previous puse time..8.6.4.2 -.2 -.4 -.6 -.8-5 5 2 25 t / ms TFig. T5T: TTCurrent injection with AC-switch for two frequencies (AC-2) TT The resulting pulse pttern is shown in XTFig. T6X.8.6.4.2 -.2 -.4 -.6 -.8 TFig. T8T: Smple pulse pttern with AC-2 with phse-firingtt TT 4 RESULTS OF FIELD TESTS In the mentime, the new lgorithms hve been implemented in rel hrdwre nd they hve shown their dvntges in rel network configurtions. The following picture shows for exmple 9 rck with two controllers using the CIF-Algorithm for two Petersen-Coils nd one erthfult monitoring nd detection system for up to 4 feeders using the DIF- Algorithm. - TFig. T6T: 5 5 2 25 t / ms Smple pulse pttern with AC-2 TT with the corresponding frequency spectrum shown in XTFig. T7X Depending on the pulse pttern nd the number of periods different frequencies re vilble. The previous figures show % phse-firing. The mplitude cn be reduced by reduced phse-firing, s for exmple depicted in XTFig. T8X. This AC-switch (AC-2) cn lso be used to generte the pttern for three frequencies like shown in XTFig. T3X. TFig. T9T: EDCSys (UEUrthfult UDUetection nd UCUontrol USysUtem)TT TT 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge 6

The Current Injection, type AC-Switch, hs been included directly in the enclosure of the motor-drive of the Petersen-Coil. This results in tuning of the Petersen-Coil with n essentil reduced number of coil movements. Additionl functions, like the mesurement of the zero-sequence impednce of the trnsformer under norml opertion nd the use of it for the correction of the mesured cpcitive erth-current, cn be chieved with the new CIF lgorithm. With this lgorithm it is possible for the first time to mesure lso the vlue of distributed Petersen-Coils. With the new UDUetection by UIUnjecting UFUrequencies (DIF) lgorithm, which is bsed on the sme mesurement principles s the CIF lgorithm, now fst nd high sensitive erthfult detection system is vilble. The field tests nd the first prcticl experiences hve shown the effectiveness of this new concept for the control of Petersen-Coils nd detection of high ohmic erthfults. REFERENCES TFig. T2T: 375A Petersen Coil with AC-Switch Current Injection nd Resistor for Wttmetric-IncreseTT TT This combintion hs been used for tests in rel network with different rtificil high ohmic erthfults of 2kOhms in different phses. The estimtion of the 2 kohm unblnce ws detected with n ccurcy of %. The clcultion of the resonnt-point under the worse condition of 25 A overcompenstion hd n ccurcy of bout 2%. This system ws tested nd pproved by ENEL. 5 CONCLUSION In this contribution we hve discussed the effects of the crosstlk of the positive sequence lod current to the zero-sequence system nd the consequences to existing control lgorithms. With the new UCUontrol by UIUnjecting UFUrequencies (CIF) lgorithm the crosstlk cn be suppressed. With the CIF fster nd more ccurte stte estimtion of the zero-sequence system cn be chieved. [] Druml G., "Resonnzregler REG-DP, Betriebsnleitung", A-Eberle GmbH&CoKG, 22, Nürnberg, Germny [2] Druml G., "EDCSys Opertion Mnul - UEUrthfult UDUetection nd UCUontrol USysUtem", A-Eberle GmbH&CoKG, 24, Nürnberg, Germny [3] Druml G., Kugi A., Prr B., "Control of Petersen Coils", XI. Interntionl Symposium on Theoreticl Electricl Engineering, 2, Linz [4] Hgmnn G, "Leistungselektronik - Systemtische Drstellung und Anwendungen in der elektrischen Antriebstechnik", 2.Aufl, AULA-Verlg Wiesbden, 998, Germny [5] Heinhold L., Stubbe R., "Kbel und Leitungen für Strkstrom Teil 2", 4.Aufl., Siemens, 987, Berlin- München, [6] Heinhold L., Stubbe R., "Kbel und Leitungen für Strkstrom Teil ", 5.Aufl, Publicis MCD Verlg, Erlngen, 999, Germny [7] Herold Gerhrd, "Elektrische Energieversorgung II", J.Schlembch Fchverlg, Weil der Stdt, 22, Germny [8] Sterns S.D., Hush D.R., "Digitle Verrbeitung nloger Signle", 7. Aufl, Oldenbourg, Wien, 999, Austri [9] Weßnigk K., "Krftwerkselektrotechnik", VDE Verlg, Berlin-Offenbch, 993, Germny 5th PSCC, Liege, 22-26 August 25 Session 39, Pper 2, Pge 7