1. Department of Electrical and Electronic Engineering Ariel University Israel, Eilat, 2014 A. LIPSKY, N. MITEVA, E. LOKSHIN Frequency Response of Voltage Transformers and Power Quality in Medium Voltage Power Networks (Measurement Accuracy )

2. … … Today, the development of electric power networks and systems is determined by the concept of “smart grids”.

3. Report contents 2. In which modes VTs have to operate? 3. When one needs to know amplitude frequency responce of VT from practical point of view? 4. Frequency response of inductive voltage transformer. 5.Practical aspects of the problem. 1. How to measure voltage and its parameters in the high-voltage power networks. 6. Conclusion.

4. An example of metering in power network ~ ● ● ●●● S ● 161 kV T2 T1 3.15 - 36 kV ● ● Cab ● 0.4 kV U, I, P, Q, EPQI … measurements ● T3 I” U, I, P, Q, EPQI … measurements Load ●●

5. We need voltage sensors in HV PN. The most widespread of them are induction voltage transformers (VT). In what modes of power supply network VTs have to operate? (Standard IEC 60044-2 ) Voltage mode of power supply network is determined by: To answer the question of the accuracy of measurement by inductive voltage transformer, it is necessary to answer the question,

6. B.B. U(t) - The modes of load operations connected to the network where VT is installed, - The regimes and processes themselves in the networks. (dips, swells, different types of transients, voltage harmonics, … ). VT

7. 0.4 0.6 1.0 0.8 Time, h Power, p.u. Load operation mode may be static or dynamic. The static mode. The maximum frequency in the spectrum of envelope voltage graph of static load does not exceed 0.001 Hz, Voltage variations in the network may be static or dynamic. Effect of load

8. 1 3 5 7 Time, sec Voltage, V The dinamic mode To obtain accurate results of measurements, frequency responses (!) of voltage transformer must comply with the spectrum of voltage distortions in its network.

9. Spectra of voltage distortion in medium voltage power networks Effect of energy consumers on voltage modes in power supply network.

10. 5101520 0 40 60 80 100 Current spectral density, % Frequency, Hz Active current spectrum of current main harmonic of electric arc furnace. Dynamic mode. The maximum frequency in the spectrum of envelope voltage graph of dynamic load (arc furnace) does not exceed 25 Hz. Based on the the theory of amplitude modulation voltage spectrum in this case is in the range 25-75 Hz.

11. At the same time equipment connected to the power network can to have linear or nonlinear volt-ampere characteristics. In the latter case this equipment is the sources of voltage harmonics ultimately. Loads U

12. n Current harmonic spectrum of the electric arc furnace Electric arc furnaces

13. The harmonic frequency spectrum of bridge rectifiers Harmonic order: is indicator of rectifier rippling. for 6- pulse rectifier, for the 12-pulse rectifier, etc.; k = 1, 2, 3... is a series of natural numbers. Amplitude of the current harmonic is defined as For example, for a 24-pulse rectifier harmonic 49 (2450 Hz) of the current has a value not exceeding 2%. It can be ignored. Thus, frequency spectrum of the harmonics for rectifiers does not exceed 2500 Hz.

14. There are some problems with the spectrum of the inverters.

15. Graphs of the instantaneous values ​​of voltages in the 6 kV power supply network of 12-pulse rectifiers of 4 MW frequency converters (the results are based on measurements using inductive VT). Time, sec Voltage, V http://www.news.elteh.ru/arh/2005/32/22.php

16. Voltage harmonics, % Harmonics number, n n < 40 The voltage harmonic spectrum of 6 kV power network of the above frequency converter http://www.news.elteh.ru/arh/2005/32/22.php (10 kHz)

17. According to EN 50160 the total harmonic distortion factor THD, calculated using the following expression: Australian Standard AS 4777.1 – 2005 recommends measure harmonics up to n = 50 (2500 Hz). Thus, the power network voltage of different types of industrial installations are usually contain harmonics with frequencies which not exceeding 2500 Hz. Although exceptions may be there.

18. Regime changes and transients in the power networks themselves are characterized by changes in the effective and the instantaneous values ​​of voltage. These changes are characterized by different types of electric power quality indies in the power supply networks. These distortions represent dips, swells, and different types of transients. Short interruptions in the power supply are considered separately. Effect of power supply network

19. Residual voltage U, % Duration t ms 10 ≤ t ≤ 200 200 < t ≤ 500 500 < t ≤ 1000 1 000 < t ≤ 5000 5 000 < t ≤ 60000 90 > u ≥ 80CELL A1 CELL A2CELL A3CELL A4CELL A5 80 > u ≥ 70CELL B1 CELL B2CELL B3CELL B4CELL B5 70 > u ≥ 40CELL C1 CELL C2CELL C3CELL C4CELL C5 40 > u ≥ 5 CELL D1 CELL D2CELL D3CELL D4CELL D5 5 > uCELL X1 CELL X2CELL X3CELL X4CELL X5 Classification of dips according to residual voltage and duration

20. As it can be seen from the table, the minimum width of a voltage dip is 10 ms. The ideal dip (a) and its spectral density (b): The equation of the spectral density:

21. 90% of the pulse energy is in the frequency range of. Therefore, we can assume that the maximum frequency, which must be considered, is or. When the pulse duration is 10 ms, the maximum frequency of voltage envelope is 100 Hz. Because it is carried out the amplitude modulation of 50 Hz voltage, the spectrum of the signal contains frequencies of 150 Hz. Taking some reserve, we can say that the spectrum of the voltage distortion, characterized by dip and swell, does not exceed 300 - 400 Hz.

22. 5.3.3overvoltages Transient overvoltages in MV supply systems are caused by switching or, directly or by induction, by lightning. Switching overvoltages generally are lower in amplitude than lightning overvoltages, but they can have a shorter rise time and/or longer duration. EN 50160 – 2010 Transients

23. Transients. They fall into two subcategories: - oscillatory, - impulsive.

24. Overvoltages in power networks Voltage, P.U. Time, ms 0 20 40 Lightning impulse (the duration of the order of 100 microseconds) Commutation overvoltage (f = 0.1 – 1 MHz)

25. 25 Oscillatory transients associated with switching of capacitor banks of 10 kV The oscillation frequency of the transient is 920 Hz ≈ 0.001 MHz Current and voltage during 50 ms Current and voltage during 10 ms

26. There are another problems, such as fault location in the overhead and cable lines. There are methods in which one have to analyze spectra with the components in units of GHz. Thus, the voltage distortions with frequencies from tens Hz up to few GHz, exist in the MV power networks.

27. Note that disturbances at frequencies of 9 kHz - 30 MHz are traditionally studied by specialists in communication and CISPR (International Committee on Radio Interference). It is assumed that the range up to 2÷9 kHz must be supervised by personnel of electrical systems. Therefore, it is not clear whether we should deal with HF problem.

28. When one needs to know amplitude frequency responce of VT from practical point of view? 1.Verification of compliance (network operators, customers, regulators) – Quality of produced electrucal energy (EN 50160) – Comparison of compatibility levels and planning levels IEC 61000-2-x, IEC 61000-3-6 – Troubleshooting in case of malfunctions (network operators, manufacturers).

29. 2. Researches: - Cancellation effects of harmonics. - Transfer of harmonics between voltage levels. - Propagation of high frequency harmonics. - Assessing the impact of voltage distortion on the electrical equipment. - Network resonances. - Harmonic filter effectiveness.

30. The intermediate conclusion: 1. From a practical point of view, frequency spectrum of voltage distortion and most transients in medium- voltage power network does not exceed 2500 Hz. 2. Thus, for a voltage transformer used in medium voltage power networks, from a practical point of view, the frequency range up to 2500 Hz is important.

31. Voltage transformers, classification

32. 32 A X a x 50 Hz The equivalent circuit of the voltage transformer for static mode for 50 Hz The presence of parasitic capacities: - The capacities between coils, - The capacities between windings, - The capacities between windings and body of transformer. There are no serious problems in static mode for 50 Hz The problems of voltage transformer are under presence of harmonics.

33. 33 A X a x A X a x Scheme the parasitic capacitance location in the induction voltage transformer for bandwidth up to 2-2.5 kHz Equivalent circuit taking into consideration capacities between coils and between windings Equivalent circuit taking into consideration ground connection

34. The results of experimental investigations of frequency responces The calculation of the frequency responses of inductive voltage transformers is rather complicated and problematic. It requires knowledge of the values ​​of all the parasitic capacitances, which can only be found through experimentation. Therefore, various experimental investigations are carried out by different firms and universities.

35. The errors of the VT transformation ratio for different voltage harmonics.

36. Experimental investigations of the frequency responses of voltage transformers for 22 kV and 33 kV power networks Functional diagram of the setup

37. There is the problem of accurate measurement of high voltages without using voltage transformers. There are different types of voltage dividers (R, RC. C), which should be correct for different frequencies. For medium voltages it is possible to use the standard voltage dividers, for example, probe.

38. U, kVM OUNTING Type 22IndoorUCJ-24, 24 kV OutdoorVJL-24, 24 kV 33indoorVCN - 36, 36 kV outdoorVJN - 36, 36 kV Results of investigations: - Only for frequencies above 50 Hz, - 50 Hz, plus a higher frequency. -Secondary burden: from nominal load up to idling -(Fluke 1750 – 2MΩ, 5 mW → 300 W), cosφ = 0.8-1.0

39. 1- unloaded mode; 2- loaded mode (basic + high harmonics) VJL-22 VJN-33 UCJ-22 UCN-33 indoor outdoor Experimental results

40. The carried out experimental studies of the amplitude frequency response of VTs with cast resin insulation for 22 and 33 kV power network showed that the error of VT transformation ratios depend mainly on the frequency range of the higher harmonics or disturbances in power networks and the burden of the VT secondary winding. In general, the errors of the transformation ratios for each harmonic do not exceed ±10% in the range up to 2500 Hz. Intermediate conclusions

41. Practical aspects of the problem: How can we evaluate the real measurement error of the voltage harmonics in the existing medium voltage power networks using serial VT with cast resin?

42. Standard IEC 61000-4-7 gives permissions ONLY for measuring instruments. For both classes of harmonics measurements permission level is +/- 5%. Corresponding Russian standard GOST 13109 gives +/- 5​​% as the measurement error of voltage harmonics. When exists uncertainty of VT characteristics allowed error is 1.5 times greater.

43. What happens in practice? Taking into account the VT uncertainty, reaching up to ±10%, in relation to the European standard we can talk about the maximum error in the measurement of voltage harmonics in ±(10% + 5%) = ±15%. In relation to the Russian standard ±[10% + (instrument error)] = 15% > (5% 1.5 = 7.5)%

44. Really, measuring instruments ​​of the EPQ values measurement and voltage harmonics in particular, have a smaller error, for example ±(0.2-0.3)% (like for Fluke). Therefore, we can say that in the medium voltage power networks using VT with cast resin insulation, measurement error of voltage harmonics practically does not exceed ±10.5% ≈ ±11%.

45. Conclusion 1. The performed experimental studies of VT AFR with cast resin insulation for 22 kV and 33 kV power networks revealed that in the bandwidth up to 2.5 kHz the relative error of the transformation ratio for voltage harmonic in comparison with the VT transformation ratio on the industrial frequency does not exceed ±10%. 2. The measurement errors of the voltage harmonics in 22, 33 kV power networks practically does not exceed ±(10-11) % when using VT cast resin insulation.

46. 3. An increase the accuracy of measurement results up to standard demand values ​​can be reached by introducing a correction factor in the calculation program, for example, levels of higher harmonics, of electric power quality monitoring devices. 4. There is the problem of creation of the voltage sensor with suitable frequency characteristics in the range up to 1-2 GHz

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