Current Transformers.pdf
- 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 (𝐸𝑘) • Maximum exciting current (𝐼𝑒) at rated (𝐸𝑘) • Maximum Resistance (𝑅𝑐𝑡) 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% = 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
- 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