Current Transformers.pdf

Current Transformers

1. Technical Level Training
Instrument Transformers
Current Transformers
Theory & Testing
Prepared by
Hashem Talukder
High Voltage (HV) Service Team
Reverie Power & Automation Engineering Limited
Copyright ©2020 by Reverie Power & Automation Engineering Limited. All rights reserved

2. Current Transformer
Why Instrument Transformers
▪Supervision during normal conditions
▪Energy Metering for buying and selling
▪Registration of abnormal conditions- Faults
June , 2020| Slide 02

3. Current Transformer
Main Tasks of Current Transformers
▪Transform currents from high levels to measureable levels
▪Insulate the secondary circuits from the high voltage network
▪Standardize meters and relays
June , 2020| Slide 03

4. Current Transformer
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
▪Load Control
June , 2020| Slide 04

5. Current Transformer
Basic Transformer
June , 2020| Slide 05

6. Current Transformer
In a Real CT
June , 2020| Slide 06

7. Current Transformer
CT Construction Types
▪Window or Bus Type
▪Split Core
▪Outdoor ( Live & Dead Tank)
June , 2020| Slide 07

8. Current Transformer
June , 2020| Slide 08

9. Current Transformer
June , 2020| Slide 09

10. Current Transformer
Live Tank vs Dead Tank CT
June , 2020| Slide 10

11. Current Transformer
Live Tank vs Dead Tank CT
▪The core with secondary winding is housed in the
top tank which is live ( At high voltage)
▪The core with secondary winding is housed in the
bottom tank which is earthed (Dead)
▪Core and Secondary winding - isolated ▪Only primary winding is isolated
▪Insulation is robust and reliable ▪Insulation is not robust
▪Primary winding is of shortest length possible
which offers high strength against the short time
dynamic force
▪Primary winding has to pass through the porcelain
insulator and the larger length of primary
conductor produces maxm mechanical force
during short time dynamic force
▪Minimum heat generation, maximum dissipation ▪Maximum heat generation, minimum dissipation
▪Compact and economical ▪Bulky and costly
June , 2020| Slide 11

12. Current Transformer
CT Core
June , 2020| Slide 12

13. Current Transformer
IEC Standards
June , 2020| Slide 13

14. Current Transformer
Definition as per IEC 60044-1
▪Accuracy Class : a designation assigned to a current transformer the error of which remain within
specified limits under prescribed conditions of use
▪Burden : the impedance of secondary circuit in ohms and power-factor. Usually expressed in VA
▪Instrument Security factor (FS): the ratio of rated instrument limit primary current to the rated
primary current
▪Accuracy Limit Factor (ALF) : the ratio of the rated accuracy limit primary current to the rated
primary current
▪Rated Knee-Point e.m.f : the minimum sinusoidal e.m.f at rated power frequency when applied to
the secondary terminals of the transformer, all others terminals being open circuited , which when
increased by 10% causes the r.m.s exciting current to increase by no more than 50%. The actual
knee point emf will be equal or greater rated knee point emf.
June , 2020| Slide 14

15. Current Transformer
Major Technical Specification of a Current Transformer
June , 2020| Slide 15

16. Current Transformer
CT Terminal Markings
▪Expressed in 5A or 1A Ratio
▪1200/5 or 1200/1
June , 2020| Slide 16

17. Current Transformer
CTs for Protection and Metering Applications
▪A distinction has to be made between a Metering and Protection Class CT
▪The designs of the magnetic cores are different
▪This ensures that they perform according to the needs of the particular device
connected
June , 2020| Slide 17

18. Current Transformer
Properties of Metering Core
▪High Accuracy in a smaller range ( 5-120% of rated current / 1-120% for class S)
▪Low Burden
▪Low Over Current Factor (FS) (At FS, Core saturated and Accuracy isn’t guaranteed)
▪Leads to lower Saturation Voltage ( Protects Metering Devices from overloading)
June , 2020| Slide 18

19. Current Transformer
Standard Accuracy Classes for Metering
▪0.1 – 0.2 – 0.5 – 1
▪0.2S – 0.5S
▪3 – 5
June , 2020| Slide 19

20. Current Transformer
IEC 60044 – 1 Metering Accuracy Requirements
June , 2020| Slide 20

21. Current Transformer
IEC 60044 – 1 Metering Accuracy Requirements
June , 2020| Slide 21

22. Current Transformer
Properties of Protection Core
▪Designed to transform a distortion-free signal even well into the overcurrent
range
▪Current range 100% of rated current to ALF x rated current
▪Relays are required to perform in fault current type situations
▪Low accuracy requirements
▪High saturation voltage
▪More core material is needed
June , 2020| Slide 22

23. Current Transformer
Standard Accuracy Classes for Protection
▪5P and 10P
▪PX (User defined CT performance)
•Turn Ratio error should not exceed ± 0.25%
•Rated Knee point emf (??????
??????)
•Maximum exciting current (??????
??????) at rated (??????
??????)
•Maximum Resistance (??????
????????????) at 75°C
June , 2020| Slide 23

24. Current Transformer
IEC 60044 – 1 Protection Accuracy Requirements
June , 2020| Slide 24

25. Current Transformer
June , 2020| Slide 25

26. Current Transformer
Testing CTs
When ?
▪Initial Commissioning
▪Investigation
▪Scheduled
Why ?
▪Verify Factory Tests
▪Ordered/Delivered Correctly
▪Ensure no Damages
June , 2020| Slide 26

27. Current Transformer
Test Methods
▪Primary Injection
▪Secondary Injection – Fixed Frequency
▪Secondary Injection – Variable Frequency
Test Requirements
▪Determination of Knee/Saturation Point
▪Insulation
▪Polarity
▪Winding Resistance
▪Primary/Secondary Ratio ( %Ratio Error )
▪Burden Check
▪Tan-Delta
▪Documentation / Visual Check
June , 2020| Slide 27

28. Current Transformer
Why so many Tests
▪Ensure proper Relay Operations
▪Certify Billing Accuracy
▪Reduce Possibility of Failure when Energized
▪Manufacturing Defects Do Happen
▪Installation Error Do Happens
June , 2020| Slide 28

29. Current Transformer
Before Testing Starts
▪Ask Concerned Personnel to isolate CTs ( Electrically & Mechanically )
▪DON’T Operate Yourself
▪Ensure Isolation Yourself - Physically
▪Use Caution Tape to avoid enthusiastic people from entering premises
▪Ground One-End (P1 / P2 ) to avoid Induced Voltage
▪Take notes if removal of secondary connection requires
▪Ensure Proper Grounding of Testing Kits
▪Collect FAT / Previous Test Reports
▪Revert to Original Connection after Tests end ( Ensure Yourself )
June , 2020| Slide 29

30. Current Transformer
Testing Excitation / Knee Point
▪Secondary kept Opened
▪Voltage increased until knee-
point reached
June , 2020| Slide 30

31. Current Transformer
Testing Excitation / Knee Point
June , 2020| Slide 31

32. Current Transformer
Testing Turn Ratio Error%
▪Current Injected at Primary
▪Transformed Current is
measured from Secondary
June , 2020| Slide 32

33. Current Transformer
Testing Turn Ratio Error%
Current Error% =
????????????????????????−????????????×100
??????
??????
Kn = Rated Transformation Ratio
Is = Actual / Measured Secondary Current
Ip = Actual / Measured Primary Current
June , 2020| Slide 33

34. Current Transformer
Measuring Secondary Winding Resistance
▪Usually done with AVO Meter but recommended way is to do with Test Kit / CT Analyzer
▪Convert Resistance Value at 75°C and compare with Reference / Rated Value
▪It’s the simplest test for diagnosing CT Secondary . It should be the 1
st
test to perform on CT if Core
Demagnetization option is available, otherwise, perform it at last.
▪As DC current leads to Residual Magnetism, it may jeopardize other tests Accuracy
June , 2020| Slide 34

35. Current Transformer
How CTs Secondary are Connected
▪Always Connected in SERIES
▪Don’t Connect Burden Beyond its Rating
▪Connect Cores as per Designation of
Devices
▪Don’t Interchange CT Cores
June , 2020| Slide 35

36. Current Transformer
CTs Performance at Through-Fault
▪CTs are designed to sustain a through-fault condition without being saturated
▪To avoid saturation CT should develop adequate voltage at secondary
▪Extra burden should not be imposed beyond its rating
▪For Differential Protection, PX class CTs are recommended to be used as they are
calculated to provide sufficient voltage during a through-fault
For Protection Purposes, CTs are NEVER Expected to be Saturated
June , 2020| Slide 36

37. Current Transformer
Dos and Don’ts at Commissioning
▪Do Primary Injection and check Continuity throughout the secondary circuit
▪Measure Secondary Current as per transformation ratio at each junction point with Clamp Meter
▪Ensure proper Core Allocation & Polarity
▪Avoid Double-Grounding as it leads to Wrong Measurement
▪Tighten CT secondary with Utmost care. Double Check !
▪NEVER Leave CTs Secondary OPENED
▪Once Primary Injection is done successfully, DON’T Touch CT secondary or Change Connections
▪If Changes are made, Perform Primary Injection AGAIN
June , 2020| Slide 37

38. Current Transformer
June , 2020| Slide 38

39. References
Most of the information is gathered from multiple online sources that are accessible to the public