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CT Problems & Testing Techniques Peter Fong OMICRON electronics Corp. USA

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Page 1: CT Testing Techniques OMICRON

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CT Problems & Testing Techniques

Peter Fong

OMICRON electronics Corp. USA

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Content

1. Differences Between Protection and Metering CT‘s

2. What to Test?

.

4. New Methods Available

© OMICRON 9 November 2009 Page 2

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Protection CT

• Need to operate at high fault current with acceptableaccuracy

• Must have high excitation voltage to avoid saturation

• Multiple ratios• Transient characteristic is important

• Testing mainly concerned with saturation level

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Metering CT

• Must be accurate over a range of current

• Must be accurate over a ran e of burden

• Operate in range of load current

• Desirable to saturate early to protect the meter• Testing mainly concerned with accuracy

• Burden is important

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Protection CT Classes

C x00

C Rating: Less than 10% Relay Current Error at

20x Rated Current (5x20A = 100A) into 1, 2, 4, or

x00: The terminal voltage which the CT must

ma nta n w t n t e at ng , , ,C800)

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Determining C-Rating

1. Find Vexc where Iexc = 10A2. Calculate Vterm = Vexc – 100A*Rct3. Round Vterm down to nearest 100,

, , correspon ng oC100, C200, C400, C800

E.g. For Rct = 0.9Ω:

exc = a exc =Vterm = 400V – 100V (0.9) = 310V

C200 Rating

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Will the CT Saturate?

Apply Anticipated Fault Current and determine if the voltageimpressed on the CT will be greater than the CT rating:

E.g. For maximum Fault current of 24,000A into a 1200/5 C200 CT

w secon ary ur en an sa e y marg n o :Isec = (24,000 * 5)/1200 = 100A310V > 2x 100A * 1 = 200VCT is o.k. √

© OMICRON Page 7

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What could go wrong?

,

Vterm = 200V – 100V (0.9) = 110V

Isec = (24,000 *5)/600 = 200A

110V > 2x 200A *1 = 400V->> not o. .

* Be careful about using a lowerta . The combination of a lower

kneepoint and higher secondarycurrent could significantly bringdown the saturation level of the CT

© OMICRON Page 8

or e same app ca on.

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What else could go wrong?

• Impedance is too high due to long lead lengths

• Poor quality (i.e. low burden rating) CTs are used due to cost orspace constraints

• Small loads are put on powerful buses, and CT ratios areselected based on load current rather than fault duty

• Low ratio zero sequence CTs are placed on systems havinghigh ground fault duty

• Unusual burdens (e.g. stablizing resistors) are placed in the CTcircuit

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Metering CT Classes

0.3 0.1B

Metering Class CT

Accuracy in % (e.g. 0.3%)when 100% primarycurrent is applied.

Maximum Burden (e.g. 0.1Ω )

primary current is applied

(e.g. 0.6%)

© OMICRON Page 10

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Rating Factor (RF)

Multiples of Rated Current which the CT canmaintain accuracy

yp ca : 1, 1.5, 2, 3, 4

E. . 200/5A CT with RF 2 means the CT willmaintain it’s accuracy certification up to 400A

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Secondary Burden

• CT accuracy is rated up to it’s maximum burdenrating

• ver- ur en ng w ten to pro uce error n avor othe customer

• -

– loose connections

– corrosion

– ro en stran e w res• Need to have excess burden capacity during

installation

© OMICRON Page 12

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Failue Mode Test Technique Possible Cause

for Fai lure Mode of Failure Mode

Wiring Problems

TABLE. CT System Failure Modes

1 Cross Phasing of a CT Wire Verification Installation Error

2 Cross Phasing of a CT/PT Wire Verification Installation Error

3 X1X2/H1H2 Reversal of a CT Wire Verification Installation Error

4 X1X2/H1H2 Reversal of a PT Wire Verification Installation Error

5 Meter Socket Wire Error Wire Verification Installation Error

6 Loose Connections Burden Test Vibration/Wind/Installation Error

7 Corroded Connections/Terminals Burden Test Moisture

8 Insulation Stripping Error Burden Test Installation Error

9 Excessive W ire Length/Small W ire Burden Test Installation Error10 Broken Wire Strands Burden Test Installation Error

11 Meter Overburdened Burden Test Installation Error

r mary econ ary urren un ro ems12 Shorted Turns (Wiring & Leakage Current) Ratio/Burden Test Lightning/Ground Fault

13 Shorted Turns (Turn to Turn) Ratio/Burden Test Lightning/Ground Fault

14 S horted Turns (Turn to Ground) Ratio/B urden Tes t Lightning/Ground Fault

15 Theft (Secondary Shunt) Ratio/Burden Test Tampering16 Theft (Primary Shunt) Ratio/Burden Test Tampering

e rap a o ur en es amper ng

18 Theft (Primary Tap) Ratio/Burden Test Tampering

19 Theft (Secondary Tap) Ratio/Burden Test Tampering

Current Transformer Problems20 Open Secondary Ratio Test Installation Error/Tampering

or e econ ary a o es ns a a on rror amper ng

22 Magnetized CT Ratio/Burden Test Lightning/Ground Fault23 Dual Ratio Error Ratio Test Installation Error

24 Inaccurate Ratio on CT Ratio Test Manufacturer Defect

25 Wire Wrap Error on CT Ratio Test Manufacturer Defect

26 Inaccurate Rated Burden on CT Burden Test Manufacturer Defect

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28 CT Mounted Backwards Wire Verification Installation Error

29 Polarity Dot on W rong Side of CT W ire V erific ation Manufacturer Defec t

30 Overburdened CT Ratio/Burden Test Secondary Burden31 Overdriven CT Ratio/Burden Test Excessive Current

32 CT Phase Shift Wire Verification Harmonic Loading

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What To Test?

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Tests for Protection CT

• Commissioning: – Ratio, Polarity, Rct, Saturation

– If possible for transient studies and network modeling:

Remenance Flux Factor, Secondary Time Constant• a ntenance au t nvest gat on:

– Saturation, Rct

– Secondary Burden

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Tests for Metering CT

• Commissioning: – Ratio and Phase at 0%, 25%, 50%, 100% of Rated Burden

– , , , …

– Calculation of RCF (Ratio Correction Factor)

– Secondary Burden• Routine:

– Same if possible

– -

– Can use portable tester for quick verification and send CTback to test lab if in result is suspect

© OMICRON Page 16

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What to Watch Out For?

© OMICRON Page 17

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Ratio Test (Bushing CT)

• Short the bushings andwindings you are not testing

• emagnet ze t e e oretest

• Test Voltage should be belowe sa ura on neepo n

• Beware of EM interference(use coaxial cable or test at

o - requency

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Saturation Test

• Test on full winding• If testing on tapped winding,

-effect

• Do not short any taps• a se e es vo age s ow y

• Lower the test voltage todemagnetize CT

• Beware of insulation level ofall parts of CT circuit and usesafe test voltage levels

© OMICRON Page 19

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New Methods (Saturation Test)

• Low voltage, variable frequency saturation test• Above 120Vac fre uenc is reduced to roduce

higher flux in CT to saturation level

• Result is transformed back to 60Hz• Can test effective saturation voltage of up to 30kV

with test voltage of only 120Vac

• ,

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New Methods (Ratio Test)

• Magnetizing branch of the CT is measured accuratelyfor losses

• Use measured data for ratio, saturation, Rct to

determine ratio and phase accuracy at different levels

• No need for external Burden

• On-site verification of meterin class CT ossible to

0.1 class)

© OMICRON Page 21

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Summary

• CT is not so straight-forward• Protection and Meterin CTs have different

considerations

• Choose the tests that are appropriate to the type of CT• Take care of on-site conditions when testing CT

• New methods are available to make CT testing safer

© OMICRON Page 22

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Thank you for your attention!

© OMICRON 9 November 2009 Page 23