GPS-synchronized harmonic measurements performed on a 400kV transmission network

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1 GPS-synchrnized harmnic measurements perfrmed n a 4kV transmissin netwrk Wjciech Wiechwski, Jan Lykkegaard Birgitte Bak-Jensen, laus Leth Bak Jacek Wasilewski Planning and Transmissin dept. nst. f Energy Technlgy nst. f Electrical Pwer Engineering Energinet.dk (TSO) Aalbrg University Warsaw University f Technlgy Fredericia, Denmark Aalbrg, Denmark Warsaw, Pland wwi@energinet.dk, jly@energinet.dk bbj@iet.aau.dk, clb@iet.aau.dk jacek.wasilewski@ien.pw.edu.pl Abstract n this paper, GPS-synchrnized harmnic measurements perfrmed n the transmissin netwrk f the Danish TSO Energinet.dk are presented. The purpse f the measurements was t verify a cmputer mdel f the netwrk. The selected verificatin methd was based n determinatin f transfer harmnic impedance by switching a series cmpnent that cntains a shunt branch [1], [2]. Harmnic currents and vltages were simultaneusly measured at three 4kV substatins with ORON 26 units with EnerLyzer functin and GPS synchrnizatin units. Time dmain snapsht measurements f three-phase vltages and currents were synchrnusly taken, while a pwer line was switched ut and in, and the results were pst-prcessed in atlab. Harmnic vltages were measured using capacitive taps f the 4kV/kV auttransfrmer bushings. Harmnic currents were measured as vltage drp n lw-inductance resistrs inserted in series with the metering equipment. Due t largely unbalanced harmnic currents, the cmputer netwrk mdel was verified by a mdified verificatin methd, based n injectin f measured harmnic currents and cmparisn f resultant vltage increments. The methd requires precise synchrnizatin f the units and sufficient level f harmnic distrtin in the netwrk. Keywrds- bus impedance matrix; glbal psitining system; harmnic analysis; impedance matrix; impedance measurement; time dmain measurements; pwer system harmnics.. NTRODUTON mputer simulatin mdel f the Danish transmissin netwrk created in the DgSLENT PwerFactry sftware was adapted t be used fr harmnic analysis fr frequencies up t 2,kHz [1]. Befre the usage, the mdel shall have had t be validated by real-wrld harmnic measurements. n rder t eliminate the effects f backgrund harmnic distrtin, an incremental methd was chsen. Preliminary simulatins have shwn that the highest harmnic increments can be btained by switching ne f the transmissin lines. N such methd f verificatin f a harmnic mdel by switching a linear series element was fund in the literature. Therefre, tw such verificatin methds based n calculatin f transfer harmnic impedance were develped and are described in [2]. One f them allws that series element cntains a shunt impedance, such as it is in case f a transmissin line. When such a line is discnnected at ne end, the transfer harmnic impedance f the netwrk can be then calculated frm harmnic currents and vltages that are measured befre and after line discnnectin. n this paper, the required by the methd harmnic measurements are described. The measured harmnic currents ccurred t be largely unbalanced, and therefre, an alternative methd has been als develped. This methd and the verificatin f the cmputer mdel are presented in this paper.. LOATON OF THE EASUREENTS Due t mdificatins f the 4kV netwrk, tw pwer lines lcated between 4kV substatins NVV and FER were temprarily perated in parallel. One f the lines culd be discnnected and recnnected at ne end. At bth ends f the line harmnic vltages and harmnic currents were synchrnusly measured using GPS-synchrnized ORON 26 units with the EnerLyzer functin. n additin, measuring equipment was als installed at a third substatin, Tjele. Lcatin f the three substatins is shwn in Fig. 1 and a schematic diagram f the netwrk sectin is shwn in Fig. 2.

2 Figure 3. Typical HV transfrmer bushings with a capacitive tap []. High vltage behind transfrmer bushing capacitance 1 can be represented as a current surce. mpedance f capacitance 2 is in range f Ohm, s virtually entire current flws thrugh 2 Ohm resistr R 3. Figure 1. Three selected substatins TJELE (TJE), FERSLEV (FER) and VENDSYSSELVÆRKET (NVV), shwn in the picture with arrws. Figure 2. Lcatin f the measuring pints at substatins NVV, FER and TJE. Pwer line FER-NVV- is discnnected at NVV substatin.. (2) FER-NVV- (1) FER-NVV-x ETHODS AND EQUPENT A. Harmnic vltage measurement At the 4kV Danish netwrk the capacitive vltage transfrmers (VT) are installed. VT's, due t the presence f capacitances and inductances, are tuned t measure precisely the fundamental frequency cmpnent. They experience severe respnse fluctuatin at frequencies as lw as several hundred hertz, therefre they are practically nt suitable fr measurement f higher frequencies [4], [], [6], [12] and anther way f measuring harmnic vltage has t be used. 1) apacitive taps in the transfrmer bushings (TB). nsulatin layers in a HV transfrmer bushings frm 1 (main) and 2 (tap) capacitances, see Fig. 3 []. apacitances 1 and 2 perfrm like a vltage divider, which is linear in frequency, thus it can be used fr measurement f the higher harmnic cmpnents f the distrted vltage []. The value f 1 can be expected t be in the range 3 1pF and value f 2 usually varies in the range frm 4% t 9% f 1 [], [4]. These values f capacitance crrespnd t impedance values in the range f mega Ohms. The vltage at the capacitive tap is high and cannt be used directly fr measurement. Therefre, vltage drp n an additinal capacitr r a resistr R 3 must be measured, as shwn in Fig. 3. Vltage rati f the created setup is nt knwn in advance, s firstly a simultaneus measurement f the secndary vltage f TB and secndary vltage f a standard VT must be perfrmed. High vltage behind the very high impedance f capacitance 1 will be seen as a current surce. Practically, the entire current will flw thrugh the additinal impedance Z 3. The impedance f capacitance 1 lwers linearly with the frequency, therefre the amplitude f the capacitive current will increase linearly with the frequency. f the additinal impedance Z will be a resistance, the resultant vltage drp wuld increase with the frequency effecting in a different rati at each harmnic frequency, see Fig. 4, increasing the signalt-nise rati f the measurement. At all three substatins these TB's f auttransfrmer bushings are used fr harmnic vltage measurement, as shwn in Fig.. axial cables were led frm the TB's t the cntrl rms, where the resistances R 3 are installed. T prevent against high vltage in case f accidental pening f the measuring circuit vervltage prtectin was installed at the transfrmer, and in the cntrl rm. 2) Wiring The distances frm the TB's t the cntrl rms were up t 2 meters. Therefre, the electrically and magnetically Figure 4. Amplificatin f higher rder harmnics. Uc vltage drp measured n a capacitr, Ur vltage drp measured n a resistr.

3 current prbes, permissin was btained t intrduce lwinductance resistances in series with the metering equipment and measure vltage drp n these. The values f the intrduced resistances were 2Ω.. easuring equipment An example measuring setup installed at the TJE substatin is shwn in Fig.. The measurements were perfrmed using three Omicrn 26 [9] units installed at the three substatins. The units were equipped with test mdule sftware EnerLyzer that allws fr capturing synchrnusly time dmain vltages with 1 inputs f the Omicrn S26 units, with a sampling rate f 28.44kHz and vertical reslutin f 12bit. The recrdings were triggered by external GPS units GPS [9]. Figure. Vltage measurement using the TB. cupled nise had t be minimized by selectin f prper cables and circuit arrangement. A typical caxial cable RG-8 has been used. Varius circuit arrangements were tested and the fllwing arrangement prvided the lwest level f induced nise. The inner wire was cnnected t the capacitive tap, and the shield was used as a return path, grunded at a single pint at the TB. n the cntrl rm vltage drp was measured n the resistance R 3. As t minimize magnetic cupling the area f the lp must be minimized, therefre the shield must be grunded at a single pint, and the earth cannt be used as a return path [8]. apacitively cupled nise is flwing t the grund thrugh the shield and nt thrugh the resistr, n which the actual measurement is perfrmed see Fig. 6. The measurement was perfrmed n the measuring range f 1V, where the quantizatin nise is 2,44mV. As can be seen in Fig. 6, the induced nise had similar amplitude t the quantizatin nise, therefre culd be ignred. Fr sme ther arrangements the induced nise was reaching levels 2 higher [1]. signal wire shield Un 2hm Un [V],4,2 -,2 -,4,4,8,12,16,2 [s] Figure 6. Applied caxial cable cnfiguratin f the length f 2m (left) and induced nise measured as a vltage drp n a 2 Ohm resistr (right). B. Harmnic current measurement urrent is measured thrugh tridally-wund current transfrmers with ferrmagnetic cre (T). The frequency respnse f these transfrmers in the frequency range up t 2,kHz can be assumed linear []. Under nrmal perating cnditins, the primary current f bth the prtective and metering cres f these current transfrmers is lwer than that necessary t saturate them, therefre they perate at the linear sectin f magnetisatin characteristic [3]. The metering and prtective cres in the 4kV lines in Denmark are rarely laded abve 4%. n rder t avid the usage f clamp-n Figure. easuring setup installed at the TJE substatin. One measuring channel f each Omicrn unit had t be used fr sensing the triggering signal sent by the GPS. Additinally a Brüel & Kjær Pulse 36B acquisitin unit (sampling rate 6kHz, vertical reslutin 24bit) [1] was installed at the TJE substatin, t cnfirm the measurement perfrmed using the Omicrn unit. Omicrn units installed at At FER and NVV substatins were remtely cntrlled frm the TJE substatin. List f the equipment is shwn in Table 1. V. ONDTONS FOR ORRET ALULATON OF HARON PEDANE (ADTTANE) ATRX Using the methd presented in [2], the harmnic impedance matrix shall be calculated frm the measurements, and in the next step, analgus impedance matrix f the cmputer mdel shall be determined using frequency sweep calculatin and bth impedance matrices shall be cmpared, as demnstrated in Fig. 8. n three-phase systems, the transfer harmnic impedance has the frm f an unsymmetrical matrix. The unbalanced impedance matrix can be determined either by TABLE. LST OF EQUPENT USED N THE HARON EASUREENTS. Omicrn GPS receiver 3 B&K Pulse 36B / Tektrnix TDS 314B / mputers 1/1/4 Lw-ind resistrs R / TB resistrs R U / vervltage prtectin 18/9/18 axial cable RG-8 1,2km

4 a) b) Y 12= U Physical netwrk easured values Y 12 = Y 12 Y 12= m puter m del Frequency sweep Fs U 12 Fs 12 Figure 8. Determinatin f the harmnic admittance matrix (a) frm the measurements, and (b) f the cmputer mdel. injecting harmnic current int ne phase at a time and measuring resultant vltage respnse at all three phases, r by injecting purely sequential current and measuring resultant vltages f all sequences. f the injected current is nt purely sequential, the calculated impedance matrix will be errneus. The measured harmnic currents f the discnnected line FER-NVV- have been inspected if they are balanced. Fr example, value f measured th and th harmnic current are: a A b = A c A a A b = A 1 c A A, = A A A = A A Bth harmnic currents cntain up t 1% f ther sequences. V. ALTERNATVE ETHOD OF VERFATON Unbalanced harmnic currents are injected int the physical netwrk, see Fig. 9. They result in sme value f unbalanced harmnic vltage. The unbalanced admittance matrix cannt be determined withut errrs. Hwever, if the cmputer mdel is crrect, bth admittance matrices f the physical netwrk and f the cmputer mdel must be the same. f the injected harmnic current is measured, and then the same value f the current is injected int the cmputer mdel using a current surce, the resultant harmnic vltage respnse btained frm cmputer mdel shall be the same as btained frm the actual measurement in the physical netwrk, regardless f the unbalance f the admittance matrix. A. Applicatin f the alternative verificatin methd Pwer line FER-NVV- has been discnnected and recnnected at ne end, and harmnic vltages and currents. flwing int the discnnected line were measured. Assuming that backgrund harmnic distrtin and netwrk cnfiguratin was cnstant during the measurement, the nly peratin that affects the level f harmnic vltages at the three substatins is switching f the pwer line FER-NVV-. nitially harmnic currents have the values NB, FB and TB and harmnic vltages at the busbars f NVV, FER and TJE have the values U N, U F and, see Fig. 1 (a). After the line is discnnected at the NVV substatin at instant t, current N becmes N =, and current F becmes F. urrents NB, FB and TB will change and becme NB, FB and TB and the harmnic vltages will becme U N, U F and U T. The nly reasn fr the change in the values f bus vltages and line currents was the change in injected currents frm the line FER-NVV-, frm N t N = and frm F t F. f these currents were knwn, then the line culd be mdelled by tw ideal current surces, see Fig. 1 (b). B. The mdelling apprach Harmnic currents N, N, F and F shuld be measured tgether with the resultant changes in harmnic vltages U N, U N, U F, U F and, U T. The increments ΔU N =U N -U' N, ΔU F =U F -U' F and Δ = -U' T are a cmbined result f current increments Δ N = N -' N and Δ F = F -' F. n the cmputer mdel, the pwer line FER-NVV- shuld be replaced with tw harmnic current surces. Harmnic spectra that were measured at the instant t, N and F shall be assigned t these harmnic surces. After perfrming harmnic lad flw calculatin, sme value f harmnic distrtin will appear at substatins NVV, FER and TJE: U N, U F and, nly as a cnsequence f the harmnic currents N and F. n the next step, harmnic spectra that were measured at instant t, N and F (when the line was discnnected) shuld be assigned t the current surces and harmnic lad flw calculatin shall be repeated. New values f harmnic distrtin U N, U F and U T will be btained. f the harmnic mdel ideally reflects the behavir f the real netwrk, then the calculated increments frm the measured values must have exactly the same values as the increments calculated frm the results f harmnic lad flw simulatins. Fig. (a) shws vltage measured at TJE substatin at a particular harmnic rder h, at the instant t NVV N ' N = F 'F t NB 'NB t' U N U' N Z NF NVV N ' N = F ' F NB 'NB U N U'N Z NF a) Physical netwrk b) mputer mdel FER FB U F U'F FER FB U F U' F 'FB 'FB Y 12 Y 12 Z FT Z TN Z FT Z TN abc = abc Figure 9. Alternative methd f verificatin. (a) Physical measurements, and (b) injectin f measured harmnic current int the cmputer mdel. TJE TB 'TB U'T TJE Figure 1. nceptual diagram f the netwrk arund substatins NVV, FER and TJE. (left) Switching f the pwer line FER-NVV-, (right) delling f the pwer line as tw harmnic current surces. TB 'TB U' T

5 This measured value f vltage can be decmpsed int the backgrund harmnic vltage cmpnent U B = cnst., vltage cmpnent U( F ), which is the effect f injected current frm the pwer line FER-NVV- at substatin FER and vltage cmpnent U( N ), which is the effect f injected current frm the pwer line FER-NVV- at substatin NVV. Fig. (b) shws vltage U T measured at TJE substatin at the instant t. At this time line FER-NVV- is discnnected frm the substatin NVV cmpletely, therefre the vltage cmpnent U( N ) =. Vltage cmpnent frm current injected at FER has different value U( F ) than befre, and the backgrund harmnic vltage cmpnent U B is cnstant. f vltage measured at instant t is subtracted frm the value measured at instant t then the resultant incremental value Δ = -U' T is like shwn in Fig. (c). n The next step, the measured harmnic currents N, N, F and F shall be assigned t current surces in the cmputer mdel. Assuming that the mdel is ideally accurate, the vltage cmpnents U( F ) and U( N ) f the vltage riginating frm the harmnic currents F and N will be exactly the same as in reality, as was shwn in Fig. (a). Since there is n backgrund harmnic distrtin present in the harmnic mdel, U B = and the resultant vltage calculated at substatin TJE is a result f the injected currents N and F nly, as shwn in Fig. 12 (a). At instant t vltage U T will be a result f the current injected at FER substatin F, as shwn in Fig. 12 (b). U( N )= since N =. When calculating the vltage increment by subtracting U T frm it can be seen that the btained vectr frm the simulatin Δ = -U' T in Fig. 12 is exactly the same as the vectr btained frm measurements, shwn in Fig.. This is nly pssible if the harmnic mdel f the netwrk is exactly reflecting the behavir f the real netwrk. The abve reasning is actually true fr any measured vltage r current value in the netwrk. V. SYNHRONZATON ERROR The time dmain snapshts are taken each time synchrnizatin pulse is sent frm the three GPS units. The pulses are sensed by inputs f Omicrn 26 units, sampled with the rate 28,44 khz, therefre a maximal U B (a) U( F) U( N) U B U'T (b) U(') F U T U -U' T T (c) -U 'T Figure. Vltage measured at TJE substatin at a particular harmnic rder h. (a) measured at the instant t, (b) measured at the instant t, (c) resultant incremental value Δ=-U' T. U( F) (a) U( N) (b) U(') F = U' T -U' T (c) -U' T Figure 12. Vltage that shuld be btained as a result f harmnic lad flw simulatins at TJE substatin, at a particular harmnic rder h. (a) calculated at instant t, (b) calculated at instant t, (c) resultant incremental value Δ = -U' T. synchrnizatin errr δ f up t +3μs can appear. The errr can be present between the three units and als between tw cnsecutive snapshts f a single unit. This synchrnizatin errr δ results in magnitude errr ε when calculating vltage increments ΔU = U U, as shwn in Fig. 13 (a) and (b). The magnitude f the vltage increments depends n the synchrnizatin errr δ as well as n the phase shift α between bth vectrs U and U. Fr values f α clse t º r 18 º, the magnitude errr ε is small, and it is larger if α is clse t 9 º r 2 º, as can be seen in Fig. 13 (b). The dependence f magnitude f vltage increments ΔU n angles δ and α is: ΔU = U U ' cs ( α ± δ ) Example results f harmnic vltage measurements perfrmed at tw instants at substatin NVV are shwn in Fig. 14. Harmnic vltage increments calculated frm the measured vltage values at tw instants at substatin NVV are shwn in Fig.. The errr bars shw the maximal range, in which the increments culd vary, due t synchrnizatin errr. Harmnic currents measured at tw lcatins are shwn in Fig. 16 and Fig. 1. These harmnic currents measured at tw instances have t be cmbined resulting in 8 cmbinatins, which injected int the cmputer mdel will prduce 8 sets f harmnic vltage incremental values. These vltage increments have t be cmpared with the vltage increments btained frm the measurements, see Fig. 18. ε (a) δ (b) ε U -U' Δ U α δ -U' Δ U U Figure 13. Figure V.1 Synchrnizatin errr δ resulted in the magnitude errr ε f the vltage increments (a) fr α = º, (b) fr larger α ε is higher. Vltage [%] Phase angle [deg] 1,9,8,,6,,4,3,2, Vltage at NVV substatin Harmnic rder Ua U'a Ub U'b Uc U'c. Ua U'a Ub U'b Uc U'c Harmnic rder Figure 14. Harmnic vltages U and U measured at substatin NVV. Upper plt - amplitudes, and lwer plt - phase angles. Vltage [%],2,2,,1, Ua Ub Uc ΔU at NVV substatin Harmnic rder Figure. alculated vltage increments ΔU=U-U at NVV substatin. ()

6 urrent [A] Phase angle [deg] urrent in the line FER-NVV- at FER substatin a 'a b 'b c 'c Harmnic rder a 'a b 'b c 'c Harmnic rder Figure 16. Harmnic currents measured in the line at FER substatin befre and after line switching. agnitude (upper plt) and phase angle (lwer plt) urrent [A] Phase angle [deg] urrent in the line FER-NVV- at NVV substatin a b c Harmnic rder a b c Harmnic rder Figure 1. Harmnic currents measured in the line at NVV substatin befre line switching at NVV. agnitude (upper plt) and phase angle (lwer plt). As can be seen in Fig. 18, nt all incremental values are within the desired limits. Hwever, the differences are still small, and in general the agreement is relatively gd. The reasns f nt ideal agreement can be a result f errrs in the simulatin mdel r, mre likely in the unstable level f backgrund harmnic distrtin. V. ONLUSONS The aim f this wrk was t verify a harmnic mdel f the Danish transmissin netwrk. Harmnic vltages were measured using the capacitive taps in the transfrmer bushings. Harmnic currents were measured using lwinductance resistrs inserted in series with the secndary circuits f current transfrmers. The nise induced in caxial cables was minimized by selecting prper circuit cnfiguratin. Three GPS-synchrnized Omicrn 26 units with Enerlyzer ptin were used fr the measurements. The synchrnizatin errr between cnsecutive measurements was taken int accunt. Due t the large values f unbalance f measured harmnic currents, an alternative verificatin methd had t be develped. The methd presented in this paper requires sme engineering effrt, but in sme cnditins may be preferable [2]. Precise synchrnizatin between the units is required. The best way f perfrming the measurement wuld be t make a single lng time dmain measurement. Additinally, backgrund harmnic distrtin shuld be mnitred fr sme time befrehand. REFERENES Vltage [%] Vltage [%] Vltage [%],2,2,,1,,2,16,12,8,4,2,16,12,8,4 Harmnic rder easured ase 1 ase 2 ase 3 ase 4 ase ase 6 ase ase 8 Figure 18. Vltage increments ΔU=U-U at NVV substatin. Results btained frm simulatins cmpared t the vltage increments btained frm measurements. Upper plt phase a, middle plt phase b and lwer plt phase c. The bars frm the left: ΔU btained frm the measurements, including the effect f synchrnizatin errr shwn by the errr bars (ΔU±ε). Next are the bars shwing vltage increments ΔU btained frm mdelling cases 1 t 8. [1] W. Wiechwski, "Harmnics in transmissin pwer systems", PhD dissertatin, Aalbrg University, Denmark, 26. [2] W. Wiechwski, B. Bak-Jensen,. Leth Bak and J. Lykkegaard, "Validatin techniques f netwrk harmnic mdels based n switching f a series linear cmpnent and measuring resultant harmnic increments", 9th nternatinal nference n Electrical Pwer Quality and Utilisatin, 9- Octber 2, Barcelna, Spain (in press). [3] J. Arrillaga and N. Watsn, "Pwer System Harmnics", Jhn Wiley and Sns, 23. [4] Y. Xia, J. Fu, B. Hu, X. Li and h. Deng, "Prblems f vltage transducer in harmnic measurement", Pwer Delivery, EEE Transactins n, Vlume 19, ssue 3, July 24 Page(s): [] F., P. F. Ghassemi, B. Gale, T. legg and. utts, "ethd t easure VT Transfer Functin", EEE Trans. Pw Deliv. Vl. 1, N. 4, pp. 9-92, Oct. 22 [6] A. P. S. elipuls, F. Zhang and S. Zelingher, "Hardware and Sftware Requirements fr a Transmissin System Harmnic easurement System", Harmnics in Pwer Systems, HPS V nternatinal nference, pp , Sept [] S. Pritpal, "2 Pwer Factr and apacitance f ABB Type O plus, AB, and Type T ndenser Bushings", ABB nc. publ. USA, 2 [8] H.,W., Ott, Nise reductin techniques in electrnic systems, secnd ed., AT&T Bell Lab., Jhn Willey & Sns, 1988 [9] Omicrn User anual, article nr ESD21, V26.AE.3 [1] Brüel &Kjær Hardware manual, PULSE X. Type 36 B [] D.A., Duglass, "urrent Transfrmer Accuracy with Asymmetric and High Frequency Fault urrents", EEE Transactins n Pwer Apparatus and Systems, Vl. PAS-1 N. 3, arch [12] R. A. inkner, "Universal R vltage transfrmer fr High Vltage Netwrks, ETZ vl 22/22, pp 22 29, VDE Verlag