CT Problems & Testing Techniques Peter Fong OMICRON electronics Corp. USA
Content 1. Differences Between Protection and Metering CT‘s 2. What to Test? . 4. New Methods Available
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9 November 2009
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Protection CT • Need to operate at high fault current with acceptable accuracy • 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 = 10A 2. Calculate Vterm = Vexc – 100A*Rct 3. Round Vterm down to nearest 100, , , correspon ng o C100, C200, C400, C800 E.g. For Rct = 0.9Ω: exc = a exc = Vterm = 400V – 100V (0.9) = 310V
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C200 Rating
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Will the CT Saturate? Apply Anticipated Fault Current and determine if the voltage impressed 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 = 100A 310V > 2x 100A * 1 = 200V CT is o.k. √ © OMICRON
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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 lower ta . The combination of a lower kneepoint and higher secondary current could significantly bring down the saturation level of the CT or e same app ca on. © OMICRON
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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 or space constraints Small loads are put on powerful buses, and CT ratios are selected based on load current rather than fault duty
•
Low ratio zero sequence CTs are placed on systems having high ground fault duty
•
Unusual burdens (e.g. stablizing resistors) are placed in the CT circuit
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Metering CT Classes 0.3 B 0.1 Metering Class CT Accuracy in % (e.g. 0.3%) when 100% primary current is applied.
Maximum Burden (e.g. 0.1 Ω )
primary current is applied (e.g. 0.6%)
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Rating Factor (RF) Multiples of Rated Current which the CT can maintain accuracy yp ca : 1, 1.5, 2, 3, 4 E. . 200/5A CT with RF 2 means the CT will maintain it’s accuracy certification up to 400A
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Secondary Burden • CT accuracy is rated up to it’s maximum burden rating • ver- ur en ng w ten to pro uce error n avor o the customer • – loose connections – corrosion – ro en stran e w res
• Need to have excess burden capacity during installation © OMICRON
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TABLE. CT System Failure Modes Failue Mode
Te st Technique fo r F ai lu re Mo de
Possible Cause o f Fa il ure Mod e
W ire Verification W ire Verification W ire Verification W ire Verification W ire Verification Burden Test Burden Test Burden Test Burden Test Burden Test Burden Test
Installation Error Installation Error Installation Error Installation Error Installation Error Vibration/W ind/Installation Error Moisture Installation Error Installation Error Installation Error Installation Error
Wiring Problems 1 2 3 4 5 6 7 8 9 10 11
Cross Phasing of a CT Cross Phasing of a CT/PT X1X2/H1H2 Reversal of a CT X1X2/H1H2 Reversal of a PT Meter Socket W ire Error Loose Connections Corroded Connections/Terminals Insulation Stripping Error Excessive W ire Length/Small W ire Broken W ire Strands Meter Overburdened
r mary econ ary
urren
un
ro
ems
Shorted Turns (Wiring & Leakage Current) Ratio/Burden Test Shorted Turns (Turn to Turn) Ratio/Burden Test S horted Turns (Turn to Ground) Ratio/B urden Tes t Theft (Secondary Shunt) Ratio/Burden Test Theft (Primary Shunt) Ratio/Burden Test e rap a o ur en es Ratio/Burden Test 18 Theft (Primary Tap) Ratio/Burden Test 19 Theft (Secondary Tap) 12 13 14 15 16
Lightning/Ground Fault Lightning/Ground Fault Lightning/Ground Fault Tampering Tampering amper ng Tampering Tampering
Current Transformer Problems 20 Open Secondary or e econ ary 22 Magnetized CT 23 Dual Ratio Error 24 Inaccurate Ratio on CT 25 W ire W rap Error on CT 26 Inaccurate Rated Burden on CT 28 29 30 31 32
CT Mounted Backwards Polarity Dot on W rong Side of CT Overburdened CT Overdriven CT CT Phase Shift
Ratio Test a o es Ratio/Burden Test Ratio Test Ratio Test Ratio Test Burden Test
Installation Error/Tampering ns a a on rror amper ng Lightning/Ground Fault Installation Error Manufacturer Defect Manufacturer Defect Manufacturer Defect
W ire Verification W ire V erific ation Ratio/Burden Test Ratio/Burden Test W ire Verification
Installation Error Manufacturer Defec t Secondary Burden Excessive Current 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 CT back to test lab if in result is suspect
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What to Watch Out For?
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Ratio Test (Bushing CT) • • • •
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Short the bushings and windings you are not testing emagnet ze t e e ore test Test Voltage should be below e 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 to demagnetize CT Beware of insulation level of all parts of CT circuit and use safe test voltage levels
•
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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 accurately for 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)
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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
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Thank you for your attention!
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9 November 2009
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