Frequency response and harmonic distortion testing of inductive voltage transformer used for power quality measurements

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Procedia Engineering 202 (2017) 159–167

4th International Colloquium "Transformer Research and Asset Management” 4th International Colloquium "Transformer Research and Asset Management”

Frequency Frequency response response and and harmonic harmonic distortion distortion testing testing of of inductive inductive voltage voltage transformer used for power quality measurements transformer used for power quality measurements Dalibor Filipović-Grčićaa, Božidar Filipović-Grčićbb*, Danijel Krajtnercc Dalibor Filipović-Grčić , Božidar Filipović-Grčić *, Danijel Krajtner a Končar – Electrical Engineering Institute, Fallerovo šetalište 22, 10000 Zagreb, Croatia a Končar – Electrical Engineering Institute, Fallerovo šetalište 22, Unska 10000 3, Zagreb, *University of Zagreb, Faculty of Electrical Engineering and Computing, 10000Croatia Zagreb, Croatia b c of Zagreb, Faculty of Electrical Engineering and Computing, Unska 3, 10000 Zagreb, Croatia *University Končar – Instrument Transformers Inc., Josipa Mokrovića 10, 10000 Zagreb, Croatia c Končar – Instrument Transformers Inc., Josipa Mokrovića 10, 10000 Zagreb, Croatia b

Abstract Abstract International standards related to power quality measurements define methods and accuracies for the measuring instruments, but do not specify International related to power quality measurements define methods and for the for measuring instruments, but do not voltage specify the accuracy standards of instrument transformers. Therefore, it is not possible to specify anaccuracies overall accuracy such measurements. Inductive the accuracy of instrument it is measurements not possible toshould specifybeantested overall such measurements. Inductive voltage transformers (IVTs) which transformers. are used for Therefore, power quality to accuracy determineforthe ratio correction factors (RCFs) and transformers (IVTs) are used for In power qualityharmonic measurements should be tested to determine correction factors (RCFs) phase angle errors at which higher frequencies. this paper, distortion and frequency response teststhe areratio performed to determine the leveland of phase anglegenerated errors at higher frequencies. In thistopaper, harmonic distortion frequency performed determine thevoltages level of harmonics by IVT and its ability transform harmonics fromand high voltageresponse (HV) to tests low are voltage (LV) to side. Complex harmonics generated by IVT and and its ability transform harmonics from high voltage to low voltage (LV) side. Complex voltages consisting of fundamental voltage certaintoamount of superimposed harmonic are used(HV) to check the frequency response of IVT. RCFs and consisting fundamental voltage certain amount superimposed harmonic are used to check the frequency of response of IVT. RCFs and phase angleoferrors of the IVT areand determined and canofbe applied to power quality monitors for compensation errors that occur at high phase angle errors of the IVT are can beforapplied to power monitors for compensation that occur at high harmonic frequencies. Different testdetermined circuits areand proposed generation of HVquality consisting of fundamental voltage of anderrors harmonic voltages with harmonic frequencies. Different test circuits are proposed for generation in of HV consisting of fundamental voltage and harmonic voltagesrange with amplitudes amplitudes 5-15 % of the applied fundamental voltage. In order to improve theintesting capabilities at higher voltage levels (for equipment range with 5-15kV≤U % of ≤420 the applied fundamental for voltage. In order to and improve the voltages testing capabilities higher voltage levels through (for equipment kV), a compensation both fundamental harmonic is proposed at with a special connection blockingwith and 123 m 123 kV≤U pass filters.m≤420 kV), a compensation for both fundamental and harmonic voltages is proposed with a special connection through blocking and pass filters. © 2017 The Authors. Published by Elsevier Ltd. 2017 Published by Ltd. Ltd. © 2017The TheAuthors. Authors. Published by Peer-review under responsibility of Elsevier the Elsevier organizing committee of ICTRAM 2017. Peer-review under of the committee of ICTRAM 2017. 2017. Peer-review underresponsibility responsibility of organizing the organizing committee of ICTRAM Keywords: Inductive voltage transformer; frequency response; harmonic distortion; power quality; high voltage testing. Keywords: Inductive voltage transformer; frequency response; harmonic distortion; power quality; high voltage testing.

1. Introduction 1. Introduction Voltage harmonic distortion level is one of the significant parameters of power quality in the power system. Higher harmonics Voltage harmoniceffects distortion level is one of the significant power the power system. Higher harmonics cause the following in electrical networks: additional parameters heating andoflosses onquality power in system elements (such as transmission cause the following effects in electrical networks: additional heating and losses on power system elements (such as transmission lines, transformers, compensation devices, etc.), unwanted voltage distortion, increased flow of circulating currents through lines, transformers, compensation devices,rated etc.), unwanted voltage distortion, increased flow of circulating currents through grounding wire, decrease of transformer power and interference with conventional telecommunication lines. Numerous grounding wire, decrease of transformer rated power and interference with conventional telecommunication lines. Numerous problems related to voltage and current harmonic effects in power systems are commonly observed nowadays due to growth of problems relatedconsumers to voltagewith and current harmonic effects in power systems are commonly observed nowadays to growth of large industrial non-linear loads and power electronic equipment [1]. Voltage harmonics may due disturb sensitive large non-linearthey loads and power electronic [1].content Voltageofharmonics may disturb sensitive loads industrial connectedconsumers to the grid with and therefore should be limited. Levelsequipment and spectral voltage distortions injected into loads connected the grid therefore should limited. levels Levelsare anddetermined spectral content of voltage distortionsIEC injected into electric grids aretotending to and increase eventhey though the be acceptable by numerous regulations. standards electric grids are tending to increase even though the acceptable levels are determined by numerous regulations. IEC standards

* Corresponding author. Tel.: +38516129714; fax: +38516129890. * Corresponding Tel.: +38516129714; fax: +38516129890. E-mail address:author. [email protected] E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. 1877-7058 ©under 2017responsibility The Authors. of Published by Elsevier Ltd. of ICTRAM 2017. Peer-review the organizing committee Peer-review under responsibility of the organizing committee of ICTRAM 2017.

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of ICTRAM 2017. 10.1016/j.proeng.2017.09.703

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[2]-[4] define compatibility levels or planning levels for voltage harmonics in LV, MV and HV networks. Furthermore, European standard [5] defines, describes and specifies the main characteristics of the voltage at a network user's supply terminals in public LV, MV and HV networks under normal operating conditions. According to [6], power quality and voltage distortion measurements are carried out by high accuracy measuring equipment of class A. In MV and HV grids this measuring equipment is commonly connected to secondary side of IVTs installed within the substations. So, an overall accuracy of the measurement depends on the accuracy of IVTs. The accuracy requirements in [6] are defined only for the measuring equipment but not for IVTs. The standard for IVTs [7] defines the accuracy limits at rated frequency, but it doesn’t define the accuracy limits at higher frequencies. The accuracy of IVTs for frequencies higher than the nominal frequency is usually not known. It is therefore difficult to specify an overall accuracy not only for harmonic voltage measurements but also for measurement of any kind of transient containing a wide harmonic spectrum. Literature survey [8]-[17] showed that conventional IVTs may have significant difference between the accuracy at rated frequency and the accuracies at higher frequencies. Therefore, a frequency response of IVTs in the concerned range of frequency should be known to use them for harmonic measurements in power system. Different measuring setups and calculation procedures for frequency response of IVTs are presented in [18]-[21]. Significant differences between IVT ratio at rated frequency and at higher frequencies can be observed, caused by resonances within the IVT. Also, a little research has been conducted concerning the phase angle characteristics of IVTs operating with non-sinusoidal waveforms. To determine the behaviour of the IVT’s ratio and phase angle at higher frequencies two separate tests should be performed: frequency response test and harmonic distortion test. Although different circuit setups for testing the harmonic responses of IVTs are described in literature, only a few of them could achieve the voltage level above 200 kV. Besides, the equipment used in the test circuit setups and test methods are also largely unspecified and differ from each other. In [22], a test circuit is proposed for determining the harmonic responses of IVTs. The frequency response assessment of a single phase 400 kV IVT and a single phase 275 kV capacitor voltage transformer in a range of frequency from 50 Hz up to 5 kHz is presented. However, at 400 kV level the test circuit has a limitation regarding the harmonic injection ability. The maximum amplitudes of generated voltage harmonics are 1 % with respect to the amplitude of the fundamental voltage up to 1 kHz, while the harmonic amplitudes from 1 kHz up to 5 k Hz are lower than 0.2 %. The harmonic injection ability of the test circuit is limited by maximum power capacity of the amplifier which was used as a harmonic power source. In this paper, three different test circuits are proposed for generation of HV which consists of fundamental voltage and harmonic voltages with amplitudes in range 5-15 % of the applied fundamental voltage. This is of great importance since high signal to noise ratios reduce measurement uncertainty. The selection of appropriate test circuit depends on highest voltage Um for equipment under test and its required active and reactive power. In order to improve the testing capabilities at voltage levels 123 kV≤Um≤420 kV, a compensation for both fundamental and harmonic voltages is proposed with a special connection through blocking and pass filters. A test method is presented for frequency response and harmonic distortion testing of IVTs used for power quality measurements. Experimental verification is demonstrated in case of medium voltage IVT. RCFs and phase angle errors of the IVT are determined at fundamental frequency and at each harmonic frequency from 2nd to 50th harmonic. These correction factors can be applied to power quality monitors connected to the secondary terminals of the IVT. 2. Test circuits for generation of high voltages containing higher harmonics 2.1. Test circuit with a single source and without compensation The first test setup shown in Fig. 1 is in most cases suitable for testing of medium voltage equipment. In this test circuit, both fundamental and harmonic voltages are generated from the same source which consists of arbitrary waveform generator (AWG), low frequency amplifier and test transformer. In this case a single source is generating both active and reactive power required by test object and capacitor voltage divider which is used as a reference measuring system.

Fig. 1. Test circuit with single source without compensation.



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Combined voltages are generated with AWG from a file which contains time-voltage data and are defined as: U AWG = U 1 sin( ω t ) + U i sin( iω t ) ,

(1)

where U1 is amplitude of fundamental harmonic and Ui is amplitude of i-th harmonic. AWG signal is amplified by low frequency amplifier and afterwards by the test transformer. However, test transformer ratio is frequency dependent and therefore it is necessary to adjust Ui for each harmonic. This can be done by performing a separate automated test. Software controls AWG to produce sine signals with frequencies from fundamental up to 50th harmonic, reads voltages on LV and HV side of test transformer and calculates frequency dependent ratio pi of test transformer. Ratios for different frequencies shown in Fig. 2 are expressed in p.u. of the fundamental frequency ratio p1. 8 7

pi/p1 (p.u.)

6 5 4 3 2 1 0

0

5

10

15

20 25 30 Harmonic

35

40

45

50

Fig. 2. Frequency dependency of test transformer ratio.

In order to obtain X % amplitude of higher harmonics with respect to fundamental the following expression is used:

U AWG = U1 sin(ωt ) +

X U1 ⋅ p1 sin(iωt ) . ⋅ 100 pi

(2)

Test voltage waveforms are generated using (2) and imported to AWG. 2.2. Test circuit with a single source and with compensation Test circuit with a single source and with compensation is shown in Fig. 3. The same harmonic source is used as in the first test circuit.

Blocking filter Cib for fundamental L1b harmonic

Blocking filter Lib for ith harmonic AWG

Amplifier

Compensation of fundamental harmonic

C1

Compensation of ith harmonic

Measuring cable

Test object

C1b

Measuring equipment Li

Ci Test transformer

Capacitor voltage divider HV measuring system

Fig. 3. Test circuit with a single source and with compensation.

The reactive power required by test object can be in some cases higher than the power capacity of the source. To overcome this limitation, a compensation for both fundamental (C1) and harmonic voltages (Li or Ci) is proposed. Since the reactive power at the fundamental harmonic is often an additional burden for the higher harmonics and vice versa, it is necessary to connect the compensation for fundamental and harmonic voltages through suitable blocking filters for fundamental harmonic (L1b, C1b) and

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for higher harmonics (Lib, Cib). Capacitances and inductances used for design of blocking filters and compensation of higher harmonics are variable elements which should be adjusted according to reactive power required by the test object for each harmonic. Compensation of fundamental harmonic requires capacitive reactive power since the test circuit has an inductive character (for example when test object is IVT). Compensation for higher harmonics can be inductive for lower frequencies, but normally is capacitive. The test circuit shown in Fig. 3 is in most cases suitable for testing of HV equipment with Um up to 123 kV. 2.3. Test circuit with two sources and with compensation Test circuit with two sources and with compensation is shown in Fig. 4.

Blocking filter for ith harmonic

Lib

Cib

Regulating transformer

Pass filter for ith harmonic

Blocking filter C1b for fundamental harmonic

L1b

Pass filter for fundamental harmonic

C1p

Measuring cable

Test object

Lip

Measuring equipment

LV network 220 V

C1

Li

AWG

Amplifier

Matching transformer Branch 1: generation of fundamental voltage harmonic (50 Hz)

Ci

Matching transformer

Test transformer

Capacitor voltage divider

Branch 2: generation of higher voltage harmonics

HV measuring system

Fig. 4. Test circuit with two sources and with compensation.

In case when harmonic power source is unable to supply all active power required by the test circuit, a fundamental harmonic is generated from the separate source. Voltage of fundamental frequency (50 Hz) is generated directly from low voltage network (220 V) through regulating transformer and matching transformer (branch 1). In this branch, a blocking filter (Lib, Cib) for higher harmonics and a pass filter (Lib, Cib, C1p) for fundamental harmonic are connected. Compensation capacitance C1 is connected in parallel with matching transformer. Higher harmonics are generated from AWG signal which is amplified by low frequency amplifier and afterwards stepped up by the matching transformer (branch 2). Matching transformer in this branch can be omitted for higher frequencies. In this branch, a blocking filter (L1b, C1b) for fundamental harmonic and a pass filter (L1b, C1b, Lip) for higher harmonics are connected. Compensation inductance Li or capacitance Ci is connected in parallel with matching transformer. The test circuit shown in Fig. 4 is suitable for testing of HV equipment with Um up to 420 kV. 3. Frequency response and harmonic distortion testing of medium voltage inductive voltage transformers 3.1. Capacitor divider as a reference measuring system A capacitor voltage divider for AC voltages up to 50 kV is used as a reference measuring system for measurement of primary voltage on IVT, since it has sufficient accuracy and stability to be applied for the approval of other measuring systems. Scale factor (ratio) and phase displacement of capacitor divider are determined according to [23] for frequencies 50 Hz – 2.5 kHz, in 50 Hz steps. Measurement results are shown in Fig. 5. 245.5 245

-20

244.5

-30

244

-40

243.5

-50

243

-60

Phase displacement

-70 -80

Ratio 50

400

750

1100

f (Hz)

1450

1800

2150

Fig. 5. Divider ratio and phase displacement versus frequency.

242.5 242 241.5 2500

Ratio

Phase displacement δ (minutes)

0 -10



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3.2. Frequency response test The purpose of the test is to evaluate the ratio and phase angle errors of the IVT from 2nd to 50th harmonic. Determination of errors at the fundamental frequency is a routine testing and therefore it is not discussed in this paper. Each test is carried out with 50 Hz component of the applied primary voltage equal to the nominal primary phase-to-ground voltage of IVT, at 3 secondary terminal burdens (0 %, 50 % and 100 % of rated burden, cosφ=1). The magnitudes of the applied harmonic voltages are 10 % of the applied fundamental voltage to produce high signal to noise ratios at each harmonic. Each harmonic is individually superimposed onto fundamental 50 Hz voltage and the combined voltage is applied to the primary of IVT. For each of the 3 specified burdens measurements of the magnitude and phase angle of the harmonic voltages are performed at both the primary and the secondary terminals of the IVT. Test circuit for performing both frequency response and harmonic distortion test is shown in Fig. 6 a) and photograph of the test setup is shown in Fig. 6 b).

IVT

Measuring system

PC controlled

a) b) Fig. 6. (a) Test circuit for frequency response and harmonic distortion testing; (b) photograph of the test setup.

RCF is defined as ratio of voltage on HV side measured with reference measuring system and with IVT under test:

RCFi =

pdi ⋅ U di , p ⋅ U LVi

(3)

where p is rated ratio of IVT under test, ULVi voltage on LV side of IVT, pdi ratio of capacitor divider and Udi voltage on LV side of capacitor divider. The phase angle error δi of IVT is defined as angle difference between i-th voltage harmonics at HV and LV side of IVT. δi is determined by using the following expression:

δ i = δ di + δ d − LVi ,

(4)

where δdi is angle difference between harmonics measured at HV and LV side of capacitor voltage divider, δd-LVi angle difference between harmonics measured at LV side of capacitor voltage divider and LV side of IVT. Figs. 7 and 8 show frequency dependency of RCF and δi for two secondary windings of IVT. 50 VA

25 VA

250

0 VA

1.002

200

0.999

150

δ (minutes)

RCF

1.005

0.996 0.993 0.99 0.987

50 VA

25 VA

0 VA

100 50 0 -50

0

500

1000 1500 Frequency (Hz)

2000

2500

-100

0

500

1000 1500 Frequency (Hz)

2000

a) b) Fig. 7. (a) RCF versus frequency - secondary winding 1a-1n; (b) δ versus frequency - secondary winding 1a-1n.

2500

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1.02

50 VA

25 VA

200

0 VA

25 VA

0 VA

100

δ (minutes)

RCF

50 VA

150

1.015 1.01 1.005

50 0

1

-50

0.995

-100

0

500

1000 1500 Frequency (Hz)

2000

2500

0

500

1000 1500 Frequency (Hz)

2000

2500

a) b) Fig. 8. (a) RCF versus frequency - secondary winding 2a-2n; (b) δ versus frequency - secondary winding 2a-2n.

The results of these tests provide an information regarding frequency dependent correction factors for both ratio and phase angle. These correction factors can be applied to power quality monitors connected to the secondary terminals of the IVT. Harmonic distortion (generation of higher harmonics on LV side by IVT when pure sine voltage of fundamental frequency is applied on HV side) can be neglected during frequency response test because applied harmonic amplitudes are much higher than those originating from distortion. Expanded measurement uncertainty of RCF can be determined using the following expression: 2

2

2

 ∂RCF   ∂RCF   ∂RCF   ∂RCF u RCFi =  ⋅ u pd  +  ⋅ uUd  +  ⋅ u p  +  ⋅ uULV   ∂U LV  ∂pdi   ∂U di   ∂p

2

  , 

(5)

where upd is expanded measurement uncertainty of capacitor voltage divider ratio (0.50 %), uUd is expanded uncertainty of voltage measurement at LV side of capacitor divider (0.12 %), up is expanded uncertainty of IVT ratio (0.23 %), uULV is expanded uncertainty of voltage measurement at LV side of IVT (0.12 %). Maximum value of RCF expanded uncertainty is 0.57 %. Expanded uncertainty of phase angle error δi is 0.82 minutes. 3.3. Harmonic distortion test The purpose of test is to determine the level of harmonic voltages (up to 50th harmonic) at the secondary terminals generated by the IVT itself when a harmonic free 50 Hz voltage is applied to the primary. The tests are carried out at 2 applied 50 Hz (primary) voltage levels being 1.0 and 1.1 times the nominal IVT primary phase-to-ground voltage and at 3 secondary terminal burdens (0 %, 50 % and 100 % of rated burden, cosφ=1). Since the applied 50 Hz primary voltage had a low content of higher harmonics, no filter was applied on HV side. Test results show that IVT introduces very low harmonic content on LV side voltage. Since the applied primary is not completely free of higher harmonics, harmonic magnitudes introduced by IVT are calculated using following expression:

ΔU i =

(U i )

2

 p di +  U di ⋅ p ⋅ RCFi 

2

 p di 100  − 2 ⋅ U i ⋅ U di ⋅ ⋅ cos ϕ i ⋅ %, p ⋅ RCFi U LV 1 

(6)

where ΔUi is calculated level of i-th voltage harmonic generated by the IVT (Fig. 9) expressed as percentage of the fundamental voltage, Ui is voltage of i-th harmonic measured at IVT secondary terminals, ULV1 is voltage of fundamental harmonic measured at secondary terminals, p is ratio of the IVT, φi is the angle between Ui and Udi which is reduced to secondary side of IVT with appropriate RCFi determined from frequency response test.

Ui φi U di ⋅

pdi p ⋅ RCFi

ΔUi

Fig. 9. Vector diagram showing ΔUi generated by the IVT

Figs. 10 and 11 show frequency dependency of ΔUi for two secondary windings of IVT.



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0.007

50 VA

0.006

25 VA

0.03

0 VA

ΔUi (%)

ΔUi (%)

0.005 0.004

25 VA

0 VA

0.02

0.015

0.003 0.002

0.01

0.005

0.001 0

50 VA

0.025

7

0

500

1000 1500 Frequency (Hz)

2000

0

2500

0

500

1000 1500 Frequency (Hz)

2000

2500

a) b) Fig. 10. (a) ΔUi versus frequency - secondary winding 1a-1n, voltage Un; (b) ΔUi versus frequency - secondary winding 1a-1n, voltage 1.1·Un. 0.006

50 VA

0.005

25 VA

0.012

0 VA

0.004

25 VA

0 VA

ΔUi (%)

ΔUi (%)

0.008

0.003

0.006

0.002

0.004

0.001 0

50 VA

0.01

0.002 0

500

1000 1500 Frequency (Hz)

2000

0

2500

0

500

1000 1500 Frequency (Hz)

2000

2500

a) b) Fig. 11. (a) ΔUi versus frequency - secondary winding 2a-2n, voltage Un; (b) ΔUi versus frequency - secondary winding 2a-2n, voltage 1.1· Un.

The results of these tests can be used to determine appropriate offset correction factors (for each harmonic) that should be applied to power quality monitors connected to the secondary terminals of the IVT to compensate for the small fixed levels of harmonic voltages expected to be generated by the non-linearity of the IVT. In that case measurement uncertainty of ΔUi can be determined from (6), while angle φi with the corresponding measurement uncertainty should be also considered. In the case of IVT considered in this paper, ΔUi is much lower than harmonic amplitudes applied in the frequency response test, so it can be excluded from further analysis. 4. Testing of equipment Um=420 kV with fundamental voltage and superimposed higher harmonic This example shows a circuit for generation of fundamental and superimposed higher harmonic from two sources as described in section 2.3. In particular case only generation of third harmonic is described. For other harmonics, filter components and compensation need to be adjusted. The test circuit generates a HV on test object composed of the fundamental (50 Hz) harmonic with RMS value 420/√3 kV and superimposed 15 % of third harmonic (150 Hz) corresponding to RMS value 36.4 kV. Test circuit is shown in Fig. 12. RMS values which correspond to fundamental frequency are marked blue, while red ones correspond to 150 Hz. 66.2 mA 25.6 A C3b

Blocking filter L3b for 3rd harmonic

56.6 A

230.7 A C1p

Regulating transformer

Pass filter for fundamental harmonic

17 A

173.2 V

230.1 A C1 AWG

41.5 A

L1b

36.8 A

Blocking filter C1b for fundamental harmonic Test object

L3p

Measuring equipment

23.4 V

L3

36 A

Matching transformer

519.6 V 77.9 V Test transformer

Branch 2: generation of 3rd voltage harmonic

Fig. 12. Test circuit used for testing of 420 kV equipment.

Testing equipment data are shown in Table 1.

Measuring cable

8A

24 A

Amplifier

Matching transformer

Branch 1: generation of fundamental voltage harmonic (50 Hz)

4.7 A

87.1 A

256.3 A

LV network 220 V

Pass filter for 3rd harmonic

Capacitor voltage divider HV measuring system

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Table 1. Testing equipment data. Equipment

Rated parameters

Amplifier rated power

750 W

Regulating transformer

0-250 V, 100 A

Matching transformer

400/1200 V, 130 kVA

Test transformer

750 V / 350 kV, 1 A

C1; C1p; L3b; C3b

1410 µF; 2.2 µF; 3.75 H; 0.27 µF

L3b rated current

40 A

L3; L3p; L1b; C1b

2.3 mH; 2.3 mH; 19 mH; 528 µF

Capacitor voltage divider ratio

1:2259

Measuring equipment

Power

analyzer

DEWE-571-

PNA, accuracy ±0.1 %; ±0.5 min

Rated power of the amplifier is significantly lower than the rated power of source for fundamental harmonic. An open-core test transformer is used, so compensation of fundamental harmonic is mandatory because transformer consumes relatively high magnetizing current on the low voltage side. The current of the fundamental harmonic 230.7 A is flowing through branch 1, i.e. through capacitor C1p which in series with L3b-C3b parallel provides a low impedance path for the fundamental harmonic. Slightly higher current 256.3 A is flowing through inductor L3b (blocking filter for third harmonic). Capacitors are generally more available for this range of currents compared to inductors. So, the inductor L3b is the most critical element of the filter branch regarding the current loading. In order to reduce the current loading of filters in branch 1, filters were not connected directly into test circuit as shown in Fig. 12, but via power transformer 20 kV / 400 V, Dyn5, 630 kVA which was available in HV laboratory, as shown in Fig. 13. The rated ratio of power transformer is pr=86.6. 2U

2V U

2W

1/3U

1/3U

1U

2/3U

2/3U

1V

1/3U

1/3U

1W

L3b

C3b

Impedance of filters Zf

C1p

I

Fig. 13. Connection of filters in branch 1 through power transformer.

Besides the current loading of the filters, another important parameter that should be considered is their impedance. Impedance of pass filter for fundamental harmonic should be as low as possible, while the impedance of blocking filter for third harmonic should be as high as possible. The impedance of filter as seen from the test circuit side (LV side of power transformer) Zf’ is inversely proportional to transformer ratio p:

Z 'f = Z f ⋅

1 p2

(7)

A drawback of this connection is that the impedance of blocking filter for third harmonic reduces. Therefore, an optimum ratio of power transformer should be selected satisfying both current loading and acceptable filter impedance. A special connection of filter to test circuit through power transformer is shown in Fig. 13 and the acceptable ratio of 28.9 is achieved. In this way, a current through inductor L3b reduced from 256.3 A to 8.9 A, while the impedance of blocking filter is still high enough to effectively block the third harmonic. Current of the third harmonic through L1b is 4.7 A, while current of the fundamental harmonic is 87.1 A. Inductance of 19 mH satisfying the required current loading is achieved by using a single-phase power transformer 27.5/110 kV, 15 MVA which was available in HV laboratory. HV side of power transformer is short-circuited while LV side is connected to test circuit. Experimental verification successfully confirmed all calculated voltages and currents in the test circuit. 5. Conclusions The voltage quality is gaining more importance due to the widespread use of power electronic devices needed for example to connect renewable energy sources and HVDC transmission lines to power networks. Hence network operators, customers and



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regulators carry out power quality measurements more frequently including harmonics at all voltage levels. In this paper, a method for frequency response and harmonic distortion testing of IVTs used for power quality measurements is presented. RCFs and phase angle errors of the IVT are determined at fundamental frequency and at each harmonic frequency from 2nd to 50th harmonic. These correction factors can be applied to power quality monitors connected to the secondary terminals of the IVT. Different test circuits are proposed for generation of HV consisting of fundamental voltage and harmonic voltages with amplitudes in range 5-15 % of the applied fundamental voltage. This is of great importance since high signal to noise ratios reduce measurement uncertainty. The selection of appropriate test circuit depends on Um of the test object and its required active and reactive power. In order to perform tests on equipment with Um≥123 kV, a compensation for both fundamental and harmonic voltages is proposed with special connection through blocking and pass filters. Testing of equipment for Um=420 kV with fundamental voltage and superimposed higher harmonic is successfully performed. Some practical solutions are proposed such as reducing the current loading of blocking filters for higher harmonics. References [1] J. K. Phipps, J. P. Nelson, P. K. Sen, “Power quality and harmonic distortion on distribution systems”, IEEE Transactions on Industry Applications, vol. 30, no. 2, pp. 476-484, April 1994. [2] IEC 61000-2-2: “Electromagnetic compatibility (EMC) - Part 2-2: Environment - Compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply systems”, Second edition, International Electrotechnical Commission, Geneva, Switzerland, March 2002. 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