alternative testing techniques for current transformers...• an instrument transformer that is...

SEPTEMBER 5 - 7, 2018

Alternative Testing Techniques for Current Transformers

Dinesh Chhajer, PETechnical Support Group

MEGGER


Agenda Current Transformer – Definition and Fundamentals

– Current Transformer Applicationso Metering / Revenueo Protection

– Accuracy Classification IEEE

Field Testing Practices

Alternative Testing Techniques– Concurrent Testing Method– DC Excitation Technique

Summary


Applicable Standards

IEEE C57.13 IEEE Standard Requirements for Instrument Transformers

IEEE C57.13.1Field Testing of Relaying Current Transformers

IEC 60044-1 Instrument Transformers Part 1: Current Transformers

IEC 60044-6 Requirements for protective current transformers for transient performance

IEC 61869-2 Instrument transformers - Part 2: Additional requirements for current transformers


Application & Classification (IEEE)

CURRENT TRANSFORMER DEFINITION & FUNDAMENTALS


Current Transformer - Definition

• 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.

• 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.

IEEE C37.110 - 2007 IEC 61869-2:2012 (IEC 60050-321:1986)


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

Current Transformers


Current Transformer – Ideal vs. Real

• Ideal CT

21 NIsNI p ×=×

• Real CT

IeIsIpNN

+=×2

1


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.

Current Transformer – Ideal vs. Real


Current Transformer - Applications

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Metering CT - IEEE

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

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Protective Relaying

– Protection class CTs provide input information for the protection of a power system.

– IEEE C57.13.1-2006 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.

Current Transformer - Protection

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Protection CTs – Classification IEEE

• 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.

• 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.

Class T Class C

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Protection CTs - Standard Burden IEEE

Burden R (Ω) L (mH) Z (Ω)VA

(at 5A)PF

B – 1 0.50 2.30 1.0 25 0.5

B – 2 1.00 4.60 2.0 50 0.5

B – 4 2.00 9.20 4.0 100 0.5

B – 8 4.00 18.40 8.0 200 0.5

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FIELD TESTING PRACTICES

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Standard Field Electrical Tests for CTs

Saturation/ Excitation

Ratio and Phase Deviation

Polarity

Winding Resistance

Insulation Resistance

Burden

CT Demagnetization

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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

CT Insulation Resistance Test


Winding and Lead Resistance

– 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.

WR test Factors Affecting WR

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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

CT Demagnetization

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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.

CT Demagnetization


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.

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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.

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Polarity Test

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

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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.

Burden Test

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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

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Protection CT Assessment


Efficiency and AccuracyALTERNATIVE TESTING TECHNIQUES

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Multi-tap CT Field Testing

Traditional Non-Concurrent

CT Test Set

CT Test Set

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Concurrent Method of Testing

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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.

Alternative: Concurrent Measurement

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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 (Is)


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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


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Concurrent Vs Non-Current Saturation Testing

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

Inter Winding Taps

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Multi-tap CT Test: Concurrent Method

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INSULATION RESISTANCE TEST

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WINDING RESISTANCE TEST

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Winding Resistance Test Results

Comparison Between Different Methods

X2-X3 X1-X4 X3-X5 X2-X5

Four Concurrent Measurements

50 405 454 504

50 405 454 504

50 406 455 505

51 405 454 505

Four Non-Concurrent Measurements

53 405 455 503

52 406 456 504

51 406 456 504

53 406 458 504

Readings in mohms

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Saturation Test Results

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Saturation Test Results


X1-X3 (V/I) X2-X3 (V/I) X2-X4 (V/I) X2-X5 (V/I)


68.363/0.1733 23.026/0.5252 136.61/0.0866 226.90/0.0519

69.049/0.1753 22.90/0.5233 138.40/0.0878 232.60/0.0532

68.472/0.1739 22.691/0.5187 137.57/0.0873 226.76/0.0519

69.604/0.1767 23.455/0.5359 139.15/0.0883 230.47/0.0528


66.099/0.1680 23.852/0.5413 135.23/0.0849 231.46/0.0526

67.551/0.1716 23.804/0.5423 135.84/0.0851 237.84/0.0541

67.907/0.1724 23.644/0.5388 136.47/0.0856 232.95/0.0529

69.325/0.1761 23.766/0.5415 136.16/0.0854 225.68/0.0513

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Ratio & Polarity Test

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Ratio Test Results


X1-X2 (200:5) Error (%) X3-X4 (500:5) Error (%) X1-X4 (800:5) Error (%) X2-X5 (1000:5) Error (%)


200.095 : 5 0.048 500.156 : 5 0.031 800.386 : 5 0.048 1000.246 : 5 0.025

200.095 : 5 0.048 500.188 : 5 0.038 800.366 : 5 0.046 1000.257 : 5 0.026

200.088 : 5 0.044 500.144 : 5 0.029 800.247 : 5 0.031 1000.194 : 5 0.019

200.094 : 5 0.047 500.171 : 5 0.034 800.331 : 5 0.041 1000.263 : 5 0.026


200.021 : 5 0.01 499.904 : 5 0.012 799.892 : 5 0.013 999.999 : 5 0

200.020 : 5 0.01 499.852 : 5 0.03 800.002 : 5 0 999.931 : 5 0.007

200.031 : 5 0.015 499.845 : 5 0.031 799.868 : 5 0.017 1000.073 : 5 0.007

200.025 : 5 0.012 499.878 : 5 0.024 799.904 : 5 0.012 999.858 : 5 0.014

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Concurrent Vs. Non-concurrent Ratio Method

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DC Excitation Technique

For Saturation Test

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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

Issues during Saturation of Protection Class CTs

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The AC method - Core Saturation

0

2

4

6

8

10

12

0 50 100 150 200

Exci

tatio

n Vo

ltage

, V [k

V]

Excitation Current, Iexc [mA]

45 Hz 50 Hz 60 Hz 120 Hz

Effect of 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 𝑣𝑣𝑣𝑣𝑓𝑓𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑓𝑓

B ∝ 𝑉𝑉𝑓𝑓


Alternative method for Saturation

IEC 61869-2: 2012 – The most suitable

procedure for the determination of the saturation flux ψsat 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.

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DC Excitation Method

The core saturation can be achieved by applying DC voltage. IEC 60044-6“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 ∗ 𝑑𝑑𝑣𝑣

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.

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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 voltagecan be kept constant as the time increases.

The conventional method used over the years has been to keep the timeconstant (or fixed frequency at 50/60Hz) as the voltage is increased until itreaches saturation.

Alternatively, the voltage can be kept the same; thus DC voltage, and the timecan be prolonged until the core becomes saturated. By integrating theconstant DC voltage over time the core saturation can be determined.

This saturation can then be mathematically converted back to an equivalent 50Hz / 60 Hz saturation. This will achieve the same result as conventional ACexcitation test technique.

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The advantage of DC Excitation method is:

Eliminates the need of high voltage AC and achieve thesame results by utilizing a DC voltage at or below theavailable line voltage. Safer operation !

The technique allows to test CTs with higher knee pointvoltages utilizing the same concept although with aslightly longer test duration.

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AC Vs DC Excitation Methods

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Concurrent DC Excitation Curves

Comparison of concurrent AC and DC excitation testing methods

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AC and DC Saturation Alternative DC Technique for Saturation/Excitation Testing

– AC Saturation up to 2kV – DC Saturation up to 30 kV

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DC Saturation Testing

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Recommended Testing Sequence

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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.

Summary and Conclusions

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Questions?

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