Coupling Capacitor Voltage Transformers As Harmonics .

Coupling Capacitor Voltage Transformers asHarmonics Distortion Monitoring Devices inTransmission SystemsMiloje Tanaskovic, nonmember, Abdul Nabi, nonmember, Slobodan Misur , nonmember ,Paolo Diamanti, member IEEE, Ross McTaggart, member IEEEAbstract—The proliferation of HVDC links in high voltagetransmission systems and the growth of large industrial customers(with large non-linear loads) are increasing the need for harmonicdistortion voltage measurements at transmission voltages.This paper addresses the use of a Coupling Capacitor VoltageTransformer with built-in harmonic monitoring device(CCVTHM) to provide frequency response and accuratemeasurement of harmonic voltage distortions in a high voltagetransmission system in an economical and effective way.Keywords: Coupling Capacitor Voltage transformers (CCVT),Coupling Capacitor Voltage transformers with harmonicmonitoring device (CCVTHM) , Harmonic distortion voltagemeasurement, frequency response, THD (Total harmonicdistortion)I. INTRODUCTIONHarmonic measurements are typically performed at the lowvoltage level at industrial customers using wire wound voltagetransformers (VT's) and current transformers (CT's). Theincrease of large industrial customers directly connected to thehigh voltage transmission system has increased the need toaccurately measure and monitor the harmonic distortion attransmission voltage levels. It is a very important to have anaccurate and reliable measurement of the harmonics distortion(THD), in order to obtain parameters for functional filterdesign and for resolving eventual disputes between supplierand consumer of the power regarding THD. Though existingcommon design of CCVT provide many benefits, theseCCVT's do not provide the linear, flat frequency responserequired to accurately measure harmonic voltage distortionsacross the most critical range of the harmonic frequencyspectrums, thus requiring in the field calibration [1] that mayhave an adverse effect on the accuracy of the measurement.Trench Limited has developed a "special" CCVT design and /or "field installation kit" to solve this problem.II. CCVT BASIC PRINCIPLE AND DESIGNCoupling Capacitor Voltage Transformers (CCVT's) aredesigned to be applied on high voltage transmission systems toprovide lower more manageable (approximately 57-115V)The authors are with Trench Ltd. CanadaPresented at the International Conference on Power SystemsTransients (IPST’05) in Montreal, Canada on June 19-23, 2005Paper No. IPST05 - 031output voltage signals proportional to and in the phase with theprimary line-to-ground voltage, power line carriercommunication and transient recovery voltage (TRV)mitigation for circuit breakers during short line short circuitfaults. The low voltage output signals are most often used forthe following applications:metering/instrumentation,protection schemes and to act as a low-voltage and low-powersupply. CCVT's are designed with an inherent margin of safetyfor the various in-service operating conditions, thus providingthe customer with a highly reliable and economical instrumentfor the supervision and control of their power transmissionlines.The main components in the CCVT are:Capacitor Divider Step Down or Intermediate Transformer Series Compensation Reactor Ferroresonancedamping circuit Carrier accessories The capacitor divider is made up of many series connectedcapacitor elements, connected line to ground. A tap is broughtout at an appropriate voltage level carefully coordinated withthe intermediate transformer to provide the required outputvoltages. The capacitor elements on the high voltage side ofthe tap are called C1 and the capacitor elements on the lowvoltage side of the tap are called C2. To provide the reducedlevel tap voltage there are many more C1 capacitor elementsthan C2 capacitor elements. The capacitor elements are housedin hollow porcelain or composite insulators filled with animpregnating fluid.The series reactor and primary windings of the step-downtransformer are manufactured with taps to enable voltage ratioand phase angle adjustments. The step-down intermediatetransformer, series compensating reactor and ferroresonancedamping circuit are housed in the electromagnetic unit (EMU)and immersed in mineral oil.Typical schematic diagram of common design CouplingCapacitor Voltage Transformer (CCVT) is shown in Fig.11while Coupling Capacitor Voltage Transformer with HarmonicMonitoring terminals (CCVTHM) is shown in Fig.12.

III. CCVT TEST AND MEASUREMENTSMany papers, field and laboratory tests, digital simulationsand ATP- EMTP models have been developed and performedon common design CCVT and other voltage transducers inorder to evaluate, errors [1], transient response, [2] andfrequency response characteristics [3]. To the author’sknowledge, most of the existing research, measurements andsimulations are focused on common design CCVT.Using a modified Trench 145kV, 60Hz nominal voltageTEMP138 CCVT model built with tapped (115Vrms)secondary voltage terminals and special harmonic monitoringterminals (200Vrms), a frequency scan test was performedusing the test circuit shown in Fig. 1. The objective was to takeadvantage of the capacitor divider as an integral part of CCVTas an "ideal" monitoring device for harmonics [4].disconnected from the capacitor divider of the CCVT.Since total harmonics distortion (THD) is the most relevantinformation to be obtained from the power systems for furtheranalysis, the accuracy of the monitoring device (VT/PT, CT,CCVT etc.) in the frequency spectrum of the interest is of theutmost importance. For this reason the test and measurementshad been performed at relatively low voltage level (up to1kVrms at 60 Hz), while accuracy of the transfer of THD fromprimary side to the secondary side (harmonic monitoringthterminals) had been measured for harmonics up to 50 (3000Hz).Fig. 1 Test circuit for frequency scan of CCVT withbuilt-in harmonic monitoring terminalsMeasurements were performed by using multi-inputoscilloscope (PHILIPS-PM3384) to obtain and store data andwaveforms for post-processing using ATP Analyzer. It is avery important to use proper cables (i.e. double-shielded) forcabling in order to eliminate any back ground noise. Thepremise of the test was to inject power frequency signal withharmonic content (measured as % of THD) (Input 1- Fig. 1.)and simultaneously measure % of THD across harmonicmonitoring terminals (Input 2- Fig. 1.) and secondary terminals(Input 3- Fig. 1.). The same test was performed with CCVTbeing loaded with 200VA and without secondary load as wellas with tap point (C1/C2) of electromagnetic unit (EMU) beingIV. TEST RESULTSThe results of the laboratory frequency scan tests are shownin Tables I-III and corresponding Figures 2-4. It should benoticed how in agreement are THD values of CCVT inputand what was measured across the harmonic monitoringterminals. At the same time THD values measured acrosssecondary of the CCVT yields the typical frequency responsefor a common CCVT design. Note the large error around 500800Hz.Figures 5 to 10 show waveforms of different harmonicscaptured during frequency scan. Comparing waveforms(CCVT input vs. Harmonic terminals) there was no apparentphase shift noticed, while (CCVT input vs. sec. terminals)there is a phase shift.

Table 4020003000No secondary loadCCVTHarm onicsCCVT inputsec.term . m on. term .Osc. Input1 Osc. Input3 Osc. .906.805.606.7018[%]16CCVT inputHarmonic 00025003000(file Noload.ADF; x-var Freqeucy)% InHi % H1H2 % X1X3Fig.2. THD % at no secondary loadTable IISecondary load - 200VACCVTHarmonicsFnCCVT inputsec.term . mon. term.(Hz)Osc. Input1 Osc. Input3 Osc. 30002.061.501.90Percentage of Total Harmonics Distortion (THD) at no-load40[%]35Percentage of Total Harmonics Distortion (THD) at Full-loadCCVT input30Secondary terminals2520Harmonic terminals151050050010001500200025003000(file Fullload.ADF; x-var Frqucy) % InHi % H1H2 % X1X3Fig.3. THD % at secondary load-200 VATable IIIE le c .m a g . u n it ( E M U ) d is c o n n e c t e dH a r m o n ic sFnC C V T in p u tm o n . te r m .(H z)O s c . In p u t1 O s c . In p u t2T H D (% )T H D (% )1806 8 .9 66 9 .2 83003 8 .9 93 9 .4 34202 3 .0 12 2 .8 15601 4 .7 41 4 .6 36601 0 .7 51 0 .3 87806 .8 96 .8 79004 .9 34 .9 110209 .0 88 .8 111406 .7 56 .5 712605 .4 05 .0 315003 .2 03 .1 016203 .1 83 .0 817403 .1 73 .0 720003 .1 53 .1 030002 .2 82 .3 570[%]60Percentage of Total Harmonics Distortion (THD) E.M. DisconnectedCCVT input50Harmonic terminals40302010005001000150020002500(file EMdisconnected.ADF; x-var Freqeucy)% InHi % H1H2Fig.4. THD % Tap point (C1/C2) of (EMU) disconnected3000

Fundamental frequency with 3rd harmonics (No-load)3[V]CCVT input2Fundamental frequency with 3rd harmonics (Full-load)2.0[V]1.51.0Harmonic terminals100.50.0Sec. terminals-0.5-1-1.0-2-1.5-3051015(file 3rd.ADF; x-var tn) v TpDvfactors:14offsets:0020v HarT1.602535 [ms] 4030-2.00th101520253035 [ms] 40thFig.6. 3 (180Hz) Harm. waveform- sec.load-200 VAFig.5. 3 (180Hz) Harmonic waveform-no loadFundamental frequency with 50th harmonics (No-load)35(file 3rd.ADF; x-var tn) v TpDv v HarT v Secfactors:1411offsets:0000v Sec10[V]Fundamental frequency with 50th harmonics (Full-load)1.5[V]CCVT input1.02Harmonic terminals0.510.0Sec. terminals0-0.5-1-1.0-2051015(file 3kHz.ADF; x-var tn) v TpDv v HarTfactors:151.6offsets:0002030 [ms] 3525v Sec100510152030 [ms] 3525(file 3kHz.ADF; x-var tn) v TpDv v HarT v Secfactors:1411offsets:0000ththFig.7. 50 (3000Hz) Harmonic waveform-no load4[V]3-1.5Fig.8. 50 (3000Hz)Harm. waveform- sec. load-200 VAFundamental frequency with 3rd harmonics (E.M. Disconnected)Fundamental frequency with 15th harmonics (E.M. Disconnected)3CCVT input[V]22Harmonic terminals1100-1-1-2-2-3-3-4510152025303540 [ms] 45(file 3rd.ADF; x-var tn) v TpDv v HarTfactors:141offsets:000thFig.9. 3 (180Hz) Harm. waveform- Tap point (C1/C2) of(EMU) disconnected05101520253035 [ms] 40(file 15th.ADF; x-var tn) v TpDv v HarTfactors:141offsets:000thFig.10. 15 (900Hz) Harm. waveform- Tap point (C1/C2)of (EMU) disconnected

Fig.11 Common design CCVTFig.12 Coupling Capacitor Voltage Transformer with Harmonic Monitoring terminals