Alternative Testing Techniques for Current Transformers. Dinesh Chhajer, PE Technical Support Group MEGGER

Transcription

1 Alternative Testing Techniques for Current Transformers Dinesh Chhajer, PE Technical Support Group MEGGER

2 Agenda Current Transformer Definition and Fundamentals Current Transformer Applications o Metering / Revenue o Protection Accuracy Classification IEEE Field Testing Practices Alternative Testing Techniques Concurrent Testing Method DC Excitation Technique Summary 2

3 Applicable Standards IEEE C57.13 IEEE Standard Requirements for Instrument Transformers IEEE C Field Testing of Relaying Current Transformers IEC Instrument Transformers Part 1: Current Transformers IEC Requirements for protective current transformers for transient performance IEC Instrument transformers - Part 2: Additional requirements for current transformers 3

4 CURRENT TRANSFORMER DEFINITION & FUNDAMENTALS Application & Classification (IEEE) 4

5 Current Transformer - Definition IEEE C IEC :2012 (IEC :1986) An instrument transformer that is intended to have its primary winding connected in series with the conductor carrying the current to be measured or controlled. An instrument transformer in which the secondary current, under normal conditions of use, is substantially proportional to the primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections. 5

6 Current Transformers Definition Ideally, CT is a transformer with the secondary short-circuited. The secondary terminal voltage is zero and the magnetizing current is negligible. 6

7 Current Transformer Ideal vs. Real Ideal CT Real CT I p N = Is 1 N 2 N N 1 2 Ip = Is + Ie 7

8 Current Transformer Ideal vs. Real Current transformers (CTs) exhibit two primary errors: Accuracy errors (related to gain or linearity) Phase angle errors The error in the current reproduction from primary to secondary will be reflected in both amplitude and phase. 8

9 Current Transformer - Applications 9

10 Metering CT - IEEE High degree of accuracy at the specified standard burden at 10% and 100% of the rated primary current. 10

11 Current Transformer - Protection Protective Relaying Protection class CTs provide input information for the protection of a power system. IEEE C specifies that protection CTs must maintain a ratio error of no more than ± 10% in a range from 1 to 20 times rated secondary current at the specified load. 14

12 Protection CTs Classification IEEE Class T Covers CTs in which the leakage flux in the core creates a 1% difference between the actual ratio correction and the calculated ratio correction within the current and burden limits. In this case the ratio must be Tested. 15 Class C Covers CTs in which the leakage flux in the core has a negligible effect on the ratio within the limits of current and burden, so that ratio can be Calculated.

13 Protection CTs - Standard Burden IEEE Burden R (Ω) L (mh) Z (Ω) VA (at 5A) PF B B B B

14 FIELD TESTING PRACTICES 17

15 Standard Field Electrical Tests for CTs Saturation/ Excitation Ratio and Phase Deviation Polarity Winding Resistance Insulation Resistance Burden CT Demagnetization 18

16 CT Insulation Resistance Test DC voltage applied across the insulation and IR is measured CT Primary (H1-H2) CT Secondary (X1-X2) CT Primary (H1-H2) Ground CT Secondary (X1-X2) Ground 19

17 Winding and Lead Resistance WR test Factors Affecting WR Static stress: weight of the connected conductor Dynamic stress: wind / seismic activity vibrations in CB operation Electrodynamic forces: under short-circuit conditions Forces in the system This test confirms that: the DC resistance of the CT is within specification there is no high resistance connection in the CT or the wiring connected to it Temperature correction shall be made to meet 75 C.

18 CT Demagnetization The demagnetization of transformer cores can be performed in several ways: Variable Voltage Constant Frequency (VVCF) source Constant Voltage Variable Frequency (CVVF) source Decreasing the amplitude of an alternating DC current 21

19 CT Demagnetization Demagnetization needs to be done to eliminate the effects of residual magnetism due to DC current injection. Achieved by slowly ramping the secondary RMS voltage and taking the CT to saturation region, then slowly decreasing the voltage back to zero. 22

20 Excitation / Saturation Curve This test confirms that: The CT is of the correct accuracy rating The CT has no shorted turns No wiring or physical short circuits have developed in the primary or secondary windings of the CT after installation. An AC voltage is applied to the secondary winding of the CT while the primary winding is left open circuited.

21 Excitation / Saturation Curve 24

22 Excitation / Saturation Curve 25

23 Ratio Tests Current Method Voltage Method This test is not intended to prove the accuracy of the ratio, but simply to prove that the ratio, as installed, is as specified, and if taps are available, that they also have the correct ratio and have been wired to the correct terminals. 26

24 Polarity Test Polarity tests prove that the predicted direction of secondary current flow is correct for a given direction of primary current flow. 27

25 Burden Test External burden is measured by injecting AC current into the circuit connected to the CT and measuring the voltage drop across it. Both resistive and reactive components are determined from the phase angle difference between V and I. 31

26 Verifies that the CT will maintain its accuracy under a specified set of loading conditions. Ensures that the CT is able to operate the devices linked within their operating characteristics. Burden 32

27 Protection CT Assessment 33

28 ALTERNATIVE TESTING TECHNIQUES Efficiency and Accuracy

29 Multi-tap CT Field Testing Traditional Non-Concurrent CT Test Set CT Test Set 35

30 Concurrent Method of Testing 36

31 Alternative: Concurrent Measurement Inject voltage on full secondary winding of a multi tap CT. Example X1 X5 Simultaneously measure voltage on all CT secondary taps Simultaneously measure voltage on the CT primary Measure current passing through CT secondary and phase angle between primary and secondary. 37

32 Alternative: Concurrent Measurement Various data points are measured as voltage applied to CT secondary is increased to saturate the CT Ratio is still calculated as X1-X5/H1-H2 This same calculation is used for all other ratios (i.e. X1-X3/H1-H2) Saturation curve is plotted as X1-X5 voltage vs current through secondary winding (I s ) 38

33 Alternative: Concurrent Measurement Simultaneously measuring all values allows the calculation of saturation curves, knee points, ratio, polarity and winding resistance on all taps The concurrent measurement on all other taps significantly reduces testing time 39

34 Concurrent Vs Non-Current Saturation Testing 40

35 Inter Winding Taps Inter-winding taps saturation and ratio s (i.e. X2-X3) can be calculated from other measurements obtained. 41

36 Multi-tap CT Test: Concurrent Method 42

37 INSULATION RESISTANCE TEST 43

38 WINDING RESISTANCE TEST 44

39 Winding Resistance Test Results Comparison Between Different Methods Four Concurrent Measurements X2-X3 X1-X4 X3-X5 X2-X Four Non-Concurrent Measurements Readings in mohms 45

40 Saturation Test Results 46

41 Saturation Test Results Comparison Between Different Methods X1-X3 (V/I) X2-X3 (V/I) X2-X4 (V/I) X2-X5 (V/I) / / / / Four Concurrent Measurements / / / / / / / / / / / / / / / / Four Non-Concurrent Measurements / / / / / / / / / / / /

42 Ratio & Polarity Test 48

43 Ratio Test Results Comparison Between Different Methods Four Concurrent Measurements X1-X2 (200:5) Error (%) X3-X4 (500:5) Error (%) X1-X4 (800:5) Error (%) X2-X5 (1000:5) Error (%) : : : : : : : : : : : : : : : : : : : : 5 0 Four Non-Concurrent Measurements : : : : : : : : : : : :

44 Concurrent Vs. Non-concurrent Ratio Method 50

45 DC Excitation Technique For Saturation Test 51

46 Issues during Saturation of Protection Class CTs An excitation test on protection class CTs requires to reach saturation of the core. A C800 CT will require a minimum of 400 V AC test voltage and under some instances can take around 1300V to achieve 1A of excitation current. For transient type CTs, the test voltage can easily reach 4,000V or more to achieve 1A saturation 52

47 The AC method - Core Saturation Effect of ffffffffffffffffff vvvvvvvvvvvvvvvvvv 12 Excitation Voltage, V [kv] B VV ff Excitation Current, Iexc [ma] 45 Hz 50 Hz 60 Hz 120 Hz 53

48 Alternative method for Saturation IEC : 2012 The most suitable procedure for the determination of the saturation flux ψ sat with the d.c. saturation method is given DC method: An alternative method to obtain saturation / excitation curves of a CT is to use a low DC voltage. This method eliminates the need for dangerous high AC voltages during testing.

49 DC Excitation Method The core saturation can be achieved by applying DC voltage. IEC Instrument Transformers Part 6: Requirements for Protective Current Transformers for Transient Performance in Annex B-3 explains this alternate way to perform CT excitation. φφ = dddd dddd The integral of voltage over a period of time would be a measure of the flux produced. It can be generated by using AC or DC excitation voltage. The area under the curve reflects the flux produced. 55

50 DC Excitation Method The flux can be increased by utilizing either of the two methods. Either the time can be kept constant as the voltage is increased, or the voltage can be kept constant as the time increases. The conventional method used over the years has been to keep the time constant (or fixed frequency at 50/60Hz) as the voltage is increased until it reaches saturation. Alternatively, the voltage can be kept the same; thus DC voltage, and the time can be prolonged until the core becomes saturated. By integrating the constant DC voltage over time the core saturation can be determined. This saturation can then be mathematically converted back to an equivalent 50 Hz / 60 Hz saturation. This will achieve the same result as conventional AC excitation test technique. 56

51 DC Excitation Method The advantage of DC Excitation method is: Eliminates the need of high voltage AC and achieve the same results by utilizing a DC voltage at or below the available line voltage. Safer operation! The technique allows to test CTs with higher knee point voltages utilizing the same concept although with a slightly longer test duration. 57

52 AC Vs DC Excitation Methods 58

53 Concurrent DC Excitation Curves Comparison of concurrent AC and DC excitation testing methods 59

54 Alternative DC Technique for Saturation/Excitation Testing AC Saturation up to 2kV DC Saturation up to 30 kv AC and DC Saturation 60

55 DC Saturation Testing 61

56 Recommended Testing Sequence 62

57 Summary and Conclusions New measurement techniques proposed utilize the concepts well described in electrical textbooks. DC excitation and concurrent method of testing offer an alternative approach for testing CTs that provide same measurements and results as conventional techniques recommended in various international standards. The comparative analysis between different methods indicate that DC excitation and concurrent method of testing can be utilized in place of AC excitation and individual tap by tap testing techniques without compromising the accuracy and reliability of the results. Demagnetization is highly recommended after DC excitation to minimize any residual magnetism in the core of the CT. 63

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