Fundamentals of transformer inrush
- 1. Fundamentals of Transformer Inrush Suhag Patel, P.E. GE Digital Energy Placentia, CA Texas A&M Protective Relay Conference College Station, TX April 13, 2010
- 2. Objective Understand why transformer inrush occurs Understand the characteristics of an inrush waveform Understand the impact transformer inrush can have on differential relays Discuss various methods to reliably restrain differential relay operation
- 3. Basics of Differential Relays Very Simple – Sum of All currents should be zero. Must Compensate for Phase Shift and Magnitude Difference
- 4. Problems with Transformer Differential Relays
- 5. Inrush Current Impact on Differential Relay
- 6. What is Inrush Current All transformers must establish flux in the transformer core This flux causes a current to flow known as the magnetizing current Magnetizing current appears as differential current
- 7. Steady State Magnetization Current Non-Linearity of the core results in a non-linear magnetizing current waveform Notice flux lags excitation voltage by 90 degrees Steady State Magnetizing current is in the range of 1-3% of XFMR FLA
- 8. Magnetizing Current Under Non-Steady State Conditions When an abrupt change in excitation voltage occurs, a large magnetizing current can flow. The Magnetizing Inrush Current is dependant on several factors, which will be discussed on the following slides
- 9. Impact of Switching Point Highest magnitude inrush occurs when excitation voltage is applied at zero crossing. Lowest magnitude inrush occurs when excitation voltage is applied at –90 degrees. Time e ϕ Start of event Time e ϕ Start of event
- 10. Impact of Remnant Flux Remnant Flux can be positive or negative This can lead to increased or decreased magnetizing inrush current
- 11. Impact of Power System Impedance The peak magnitude of the inrush current is dictated by the strength of the power system source The duration of an inrush event is dictated by the resistive losses in the circuit
- 12. Impact of Transformer Design Electrical steel has remained fairly constant over the years Laminated core designs lead to lower reluctance cores Lower reluctance cores are more efficient leading to lower magnetizing current
- 13. Transformer Inrush Waveform No CT Errors – Time Domain
- 14. Transformer Inrush Waveform No CT Error – Freq Domain
- 15. Transformer Inrush Waveform with CT Sat – Time Domain
- 16. Transformer Inrush Waveform with CT Sat – Freq Domain
- 17. When Does Inrush Occur? During Transformer Energization: Typically the most severe case, because excitation voltage is going from zero to maximum value During Post Fault Voltage Recovery: During a fault the system voltage is depressed, and then returns to full value Not typically as sever as Energization because Flux won’t be fully offset from excitation voltage Sympathetic Inrush:
- 18. Inrush Restraint Methods As shown earlier, high levels of inrush current can cause differential relay misoperation We need to identify this condition and stop the differential relay from operating while inrush condition is present Many methods exist, all rely on the characteristics of the inrush waveform
- 19. Percentage of Total Harmonic This method utilizes the fact the inrush waveform is rich in harmonics. EM relays applied this per phase. Problems More efficient core designs produce less harmonic content CT Saturation essentially creates a setting “floor”
- 20. CT Saturation Waveform Note that a saturated CT waveform is highly non-linear
- 21. CT Saturation Spectrum Note the high 2nd harmonic component
- 22. Typical 2nd Harmonic Ratios Typical values of 2nd harmonic to fundamental ratios in the range of 10%- 60% Can be much lower as shown Microprocessor relays have introduced methods to deal with this problem
- 23. Percentage of 2nd Harmonic This method utilizes the fact the inrush waveform has a dominant second harmonic component. EM relays applied this per phase. CT Saturation still a problem
- 24. Percentage of 4th Harmonic This method utilizes the fact the inrush waveform is not symmetric, leading to even harmonics Used in some microprocessor relays CT Saturation still a problem No significant benefit over 2nd harmonic methods
- 25. Waveshape Based Method Relies on flat spots near zero value CT saturation can compromise security and dependability Were used widely in solid-state relays
- 26. Adaptive 2nd Harmonic Method Method utilizes 2nd Harmonic Magnitude and Angle Dynamically restrains over a maximum of 6 cycles May slow operation by a few cycles if 2nd harmonic current is present
- 27. How to Apply Various Methods? EM relays typically used either % total harmonic or % 2nd harmonic methods EM relays applied them on a per-phase basis Microprocessor relays can apply many methods on a per-phase, 1 out of 3 (cross blocking), 2 out of 3, or average basis Pros and Cons to each
- 28. Considerations When Applying Harmonic Restraint Reliability – Ability for the differential relay to operate on all internal faults Security – Ability for the differential relay to restrain for all transformer inrush events Speed – How quickly internal faults are cleared No method is best, depends on user requirements
- 29. Considerations When Applying Harmonic Restraint 1 out of 3 (Cross Blocking)– Very secure, but can reduce reliability or speed: Consider fault during energization Per Phase – Less secure, very reliable: Consider low 2nd harmonic inrush 2 out of 3 – More secure then Per Phase, potentially less reliable Averaging Method – More secure then Per Phase or 2 out of 3, no compromise on reliability
- 30. Transformer Inrush Impact on Generator Differential High DC component of Inrush may saturate Gen CT’s. Using harmonic restraint is not a good solution, adds too much delay 87G
- 31. Transformer Inrush Impact on Generator Differential Flux balanced CT configuration can be used on smaller Generators
- 32. Transformer Inrush Impact on Generator Differential For problem installations, transformer CB close can be used to delay 87G 87G Transformer Close CB Command Delays 87G Relay
- 33. Importance of Good Waveform Capture Depending on specific system conditions and transformer design, varying levels of 2nd harmonic content may be present It is in the users best interest to capture inrush waveforms whenever possible If a fairly complete library of actual waveform data is available, this can be used to fine tune settings and evaluate new methods
- 34. Conclusion Transformer Inrush will occur anytime a change to the transformer excitation voltage occurs Transformer Inrush appears as differential current to the transformer differential relay 2nd harmonic based methods should not be set lower then 15% otherwise dependability is put at risk Many blocking methods exist, however, they pose various compromises to security, reliability, and speed. The right choice of blocking method depends on the individual user Generator Differential relays can also be impacted by transformer inrush