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
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
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
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
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 (𝐸𝑘)
• Maximum exciting current (𝐼𝑒) at rated (𝐸𝑘)
• Maximum Resistance (𝑅𝑐𝑡) at 75°C
June , 2020 | Slide 23
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
33. Current
Transformer
Testing Turn Ratio Error%
Current Error% =
𝐾𝑛𝐼𝑠−𝐼𝑝 ×100
𝐼𝑝
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 1st 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
