Current Transformer Theory & Testing

1 Current Transformer Theory & TestingJay Anderson On Relay School 2016 Agenda Introduction Current Transformer Basics Construction & Types Industry Standards Applications TestingCurrent Transformer Basics OMICRONPage 3 Function of Current TransformersJanuary 19, 2016 Page: 4 Convert Primary Power Signals to Manageable Values for Indicating Meters Revenue Metering Protective Relay Systems Power Generation Plant Monitoring Systems Fault Recorders SCADA Overall Electric Grid Monitoring (Local Dispatch & ISO Level) Building (Energy) Management Systems (HVAC, ) Load ControlCurrent TransformersJanuary 19, 2016 Page.

2 5 Insulation from High Voltages and Currents Isolation from other systems Safety Standardization Accuracy ( Ratio & Phase) Typically Low Power Rating Thermal Considerations Burden ConsiderationsCurrent TransformersJanuary 19, 2016 Page: 6 Insulation Consistent With Voltage Use Wide Range of Current to Replicate (Unlike VTs) Metering or Protective Class Ratings Typically Unprotected Dangerous When Open CircuitedCompliance & StandardsIEEE ANSI IEC in Europe & AsiaNERC Reliability Standards in USInstrument Transformers Are Expected to Perform & ConformSupported standards OMICRONPage 8 IEEE requirements for instrument transformers IEEE for high-accuracy instrument transformers IEC 60044-1current transformers IEC 60044-6requirements for protective Current transformers for transient performance IEC 61869-2additional requirements for Current transformersBasic Transformer ~ZФФN2N1 How it

3 WorksIgnoring Magnetic and Resistive losses for the moment:122121 IInn The Current in the Seconday is Directly Proportional to thePrimary Current by the Factor of the Turns RatioIn An Ideal CTIn a Real CT - CT Analyzer ModelThe 3 parts of the CTA model: Winding Ratio (Purely the Ratio of Turns) Magnetic & Core Losses from Hysteresis, Gaps, Inductance, Eddy Currents Winding Resistance LossesZBLmP1P2S2S1 VcoreNpNsCT Construction Types Window or Bus Type Split Core Freestanding Outdoor versus IndoorConstruction Types OMICRONPage 13 Window or Bus BarConstruction TypesSplit CoreConstruction Types Wound or Inductive Could be GIS Encapsulated HV Outdoor Freestanding ShownHigh Voltage Current TransformerJanuary 19, 2016 Page.

4 16 Construction Types Wound or Inductive Bushing Mounted External GIS Breaker CT CoresJanuary 19, 2016 Page: 18CT with Multiple CoresJanuary 19, 2016 Page: 20 Insulation of a Core PileJanuary 19, 2016 Page: 21 H1 on HV side X1 on LV SidePolarity and Terminal MarkingJanuary 19, 2016 Current Transformer Secondary TypesIEEE 60044-1 Can be Expressed in 5 Amp or 1 Amp Ratio Example 2000:5 or 200/1 Page 24 OMICRONM ulti-Ratio ExampleCurrent Transformer Secondary Types19 January 2016 Page 25 CTs for Protection and Metering Applications A distinction has to be made between a metering class and a protection class Current Transformer .

5 The designs of the magnetic cores are different. This insures that they perform according to the needs of theparticular connected vs Protection ClassesJanuary 19, 2016 Page: 26 Metering core A metering core is designed to work more accurately within the ratedcurrent range designated. When Current flow exceeds that rating, themetering core will become saturated, thereby limiting the amount ofcurrent level within the device. This protects connected meteringdevices from overloading in the presence of fault level Current flows. Itbuffers the meter from experiencing excessive torques that might becreated during those faults.

6 High accuracy in a smaller range. Less core material is needed Leads to Lower Saturation VoltagesProtection core A protection core is designed to transform a distortion-free signaleven well into the overcurrent range. This enables the protectiverelays to measure the fault Current value accurately, even in very high Current conditions. Relays are required to perform in fault Current type situations Moderate accuracy over a wider range More core material is neededMetering vs Protection ClassesProtection CT Classes (IEEE )C 200C Rating: - Less than 3% ratio error at rated Current - Less than 10% ratio error at 20 times rated Current - Standard burden 200V/ (5A x 20) = 2 200.

7 - Secondary terminal voltage which the CT must maintain within the C Rating which is 200V in this In CTAnalyzer This is known as VbPage 28 OMICRONCT Classes defines the Performance of a CTActual Transformer Label (Protection Class) s name or s primary and secondary thermal Current rating factor (RF) frequency (Hz) and Basic impulse insulation level (BIL kV) Page 29 OMICRONFLEX-CORECURRENT TRANSFORMERRATIO 3000:5A. CAT 781-302 MRRF ACC CLASS C20050-400 HZ 600V INS CLASS 10kV BILO ther Protection CT Classes (IEEE ) OMICRONPage 30 C Ratio error can be determined by Calculationfrom the Excitation Curve K Same as C class, except the Knee-pointmust be greater than 70% of the VBrating T Ratio error must be determined by Testdue to significant leakage flux PX User defined CT performance ( Vk, Ik, Rct)Metering CT Classes (IEEE ) OMICRONPage At 100% rated Current , the error limit is At 10% rated Current , the error limit is (doubled)Metering Class CTMaximum Burden ( )

8 Ratio Error OMICRONPage 32 Rating Factor (RF) OMICRONPage 33 Multiples of Rated Current to which the CT can maintain its accuracyTypical RF: 1, , 2, 3, 4 Example: 200/5A CT with RF 2CT will maintain it s accuracy certification up to 400 AMetering CT Accuracy OMICRONPage 34 Actual Transformer Label (Metering) OMICRONPage 35 INSTRUMENT TRANSFOMERS, TRANSFORMERRATIO 400:5 A. CAT 115-401RF ACC CLASS C5050-400 HZ 600V INS CLASS 10kV s name or s primary and secondary thermal Current rating factor (RF) frequency (Hz) and Basic impulse insulation level (BIL kV) Actual Transformer Label (Meter Class)Error Parallelogram: Metering CTs OMICRONPage 37 Source.

9 IEEE Selection OMICRONPage 38 Residual Magnetism (Remanence flux) When excitation is removed from the CT, some of the magnetic domains retain a degree of orientation relative to the magnetic field that was applied to the core Residual magnetism in CTs can be described by amount of flux left in the corePage 39 OMICRONC auses of Residual MagnetismResidual Magnetism Can Occur Due To: High Transient Fault Currents Circuit Breaker Arc During Trip Operations DC Currents Due to Winding Resistance Measurement Other TestsPage 40 OMICRONR esidual Magnetism How to get rid of?Page 41 OMICROND emagnetization processDone automatically by the CTAnalyzer at the very end of the measurementTesting CTs When ?

10 Initial Commissioning Investigation ScheduledPage 42 OMICRON Why ? Verify Factory Tests Ordered/Delivered Correctly Insure no damageTest Methods Primary Injection Secondary Injection Fixed Frequency Secondary Injection Variable FrequencyTest Requirements Excitation to Determine Knee/Saturation Point Insulation Polarity Winding Resistance Primary Ratio Secondary Ratio Burden Check DocumentationWhy So Many Tests? OMICRONPage 45 Ensure proper Relay Operation Certify Billing Accuracy Reduce Possibility of Failure when Energized Reduce Possibility of Injury Due to Failure Manufacturing Defects Do Happen Installation Errors Do HappenWhat is This Relic ?