CT Testing: Theory and Practice
- 1. 1 10/02/2012 Slide 1 CT Testing: Theory and Practice Presented Tom Lawton, TESCO The Eastern Specialty Company For North Carolina Electric Meter School Polyphase Tuesday, June 15, 2021 at 3:45 p.m.
- 2. 2 What we will cover • Why do we test CT’s? • Shop testing • How to read and interpret a transformer face plate • Types of field tests • Magnetization effects and demagnetization
- 3. 3 Shop Testing • New Transformers • Manufacturer’s tests • Utility tests
- 4. 4 Self-Contained vs. Transformer-Rated 1S, 2S, 3S, 4S, 9S, 12S, 16S, 45S, etc., etc. What’s the Difference? Different Forms for Different Services and Applications
- 6. 6 Self-Contained Primarily Residential (1S, 2S, 12S) Relatively Low Current Example: 100A
- 7. 7 Transformer-Rated Primarily Commercial/Industrial (9S, 16S) Relatively High Current Example: 400A
- 8. 8 What is a CT? “A current transformer (CT) is used for measurement of alternating electric currents. Current transformers, together with voltage (or potential) transformers (VT or PT), are known as instrument transformers. When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.” - Wikipedia
- 9. 9 Transformer-Rated Primarily Commercial/Industrial (9S, 16S) Relatively High Current Example: 400A 400:5 5A
- 10. 10 Current Transformers Conceptual Representation Real, with core losses Ideal. No losses
- 11. 11 CT’s – Functions and Terminology Ratio For instance, a CT with a 400:5 ratio will produce 5A on the secondary, when 400A are applied to the primary.
- 12. 12 CT’s – Functions and Terminology Thermal Rating Factor A value representing the amount by which the primary current can be increased without exceeding the allowable temperature rise. For instance, a RF of 4.0 at 30° ambient on a 400:5 ratio CT would allow for a primary current up to 1600A.
- 13. 13 CT’s – Functions and Terminology Accuracy Classifications and Burden All CT’s fall within an accuracy class. IEEE Standards have defined accuracy classes.
- 14. 14 CT’s – Functions and Terminology Accuracy Classifications and Burden Example: 0.3% @ B0.1, B0.2, B0.5
- 15. 15 CT’s – Functions and Terminology Faceplate
- 16. 16 Transformer-Rated 9S Meter Installation 5A PHASE A SOURCE PHASE C PHASE B LOAD 400A 400A 400A 5A 5A
- 17. 17 Transformer-Rated 9S Meter Installation 5A PHASE A SOURCE PHASE C PHASE B LOAD 400A 400A 400A 5A 5A
- 18. 18 Meter Testing 9S Meter Installation 5A PHASE A SOURCE PHASE C PHASE B LOAD 400A 400A 400A 5A 5A Isolate the Meter from the Service P
- 19. 19 Meter Testing 9S Meter Installation 5A PHASE A SOURCE PHASE C PHASE B LOAD 400A 400A 400A 5A 5A ? ? ?
- 20. 20 Meter Testing 9S Meter Installation 5A PHASE A SOURCE PHASE C PHASE B LOAD 400A 400A 400A 5A 5A What if? 355:5 405:5 200:5 Shorted etc.
- 21. 21 CT Testing CT Testing is Important! 1) Test for correct ratio 2) Test for functionality at rated burdens
- 22. 22 Ratio Testing Ratio of Primary Current to Secondary Current 5A PHASE A SOURCE LOAD 400A 400A 400A 5A 5A Calculate Ratio
- 23. 23 Burden Testing Functionality with Burden Present on the Secondary Loop PHASE A Some burden will always be present – junctions, meter coils, test switches, cables, etc. CT’s must be able to maintain an accurate ratio with burden on the secondary.
- 24. 24 Burden Testing Functionality with Burden Present on the Secondary Loop
- 25. 25 Burden Testing Functionality with Burden Present on the Secondary Loop Example Burden Spec: 0.3% @ B0.1, B0.2, B0.5 or There should be less than the 0.3% change in secondary current from initial (“0” burden) reading, when up to 0.5Ohms of burden is applied
- 26. 26 Burden Testing Functionality with Burden Present on the Secondary Loop ANSI Burden Values 0.1 Ohms 0.2 Ohms 0.5 Ohms 1 Ohms 2 Ohms 4 Ohms 8 Ohms
- 27. 27 Burden Testing 0.0000 1.0000 2.0000 3.0000 4.0000 5.0000 6.0000 0 2 4 6 8 0.3% @ B0.1, B0.2, B0.5 Initial Reading = 5Amps 0.3% x 5A = 0.015A 5A – 0.015 = 4.985A Burden Reading 0 5.0000 0.1 4.9999 0.2 4.9950 0.5 4.9900 1 4.9800 2 4.9500 4 4.0000 8 0.8000
- 28. 28 Burden Testing 4.9500 4.9600 4.9700 4.9800 4.9900 5.0000 5.0100 5.0200 5.0300 5.0400 5.0500 0 0.2 0.4 0.6 0.8 1 0.3% @ B0.1, B0.2, B0.5 Initial Reading = 5Amps 0.3% x 5A = 0.015A 5A – 0.015 = 4.985A At 0.5Ohms of Burden the secondary current is still at 4.990A – Less than 0.3% change – Good CT! Burden Reading 0 5.0000 0.1 4.9999 0.2 4.9950 0.5 4.9900 1 4.9800 2 4.9500 4 4.0000 8 0.8000
- 29. 29 Analog Testing Application of Burden and Calculation Manual reading of initial and post- burden secondary currents
- 30. 30 Admittance Testing Admittance test results are not immediately intuitive. Some analysis and interpretation is need. What do all these mS values mean?
- 31. 31 Admittance Testing What is Admittance? Measured in units of MiliSiemens (mS) Admittance is the inverse of impedance. Impedance is the opposition to current. Therefore, admittance testing measures the overall “health” of the secondary loop of the CT.
- 32. 32 Admittance Testing Admittance testing devices inject an audio sine wave signal into the secondary loop of the CT. The resulting current is measured. The voltage of the initial signal is known. From these two parameters, the impedance, and thus the admittance can be calculated.
- 33. 33 Admittance Testing Three phase process is recommended. 1.Test each CT individually 2.Test the matched sets 3.Test over time
- 34. 34 De-magnetization CT’s can become magnetized, due to a number of reasons, including leaving the shorting clip open, near lightning strikes, and harmonic content. CT’s can be demagnetized by slowly and smoothly increasing the secondary resistance until saturation occurs, and then slowly and smoothly decreasing the secondary resistance. A resistance that will cause a secondary current reduction of 65% to 75% will typically put the CT into saturation.
- 35. 35 What we covered • Why do we test CT’s? • Shop testing • How to read and interpret a transformer face plate • Types of field tests • Magnetization effects and demagnetization
- 36. 36 Questions and Discussion Tom Lawton President tom.lawton@tescometering.com TESCO – The Eastern Specialty Company Bristol, PA 215-228-0500 This presentation can also be found under Meter Conferences and Schools on the TESCO website: www.tescometering.com ISO 9001:2015 Certified Quality Company ISO 17025:2017 Accredited Laboratory