Frequently Asked Questions on Lightning Impulse Testing of Power Transformers - IEC 60076-3 / IEC 60060-1
Q: What is lightning impulse testing and why is it mandatory for power transformers?
Answer: Lightning impulse testing applies a standard steep-fronted high-voltage surge to the transformer winding to prove that its insulation system can withstand the overvoltages caused by lightning strokes and switching operations on an overhead transmission line. It is a mandatory dielectric test because the insulation level of the winding cannot be verified by any other means. The test is performed to a specified Basic Impulse Level (BIL) in accordance with IEC 60076-3 and IEC 60060-1, confirming the transformer will survive real overvoltages in service.
Q: What is the standard lightning impulse waveform, and how is it applied?
Answer: The standard full lightning impulse has a virtual front time of 1.2 microseconds and a time to half-value of 50 microseconds, designated 1.2/50 microseconds. The wave is generated by a multi-stage Marx impulse generator and applied to the high-voltage terminal, with the neutral earthed through a low-inductance current shunt. The applied peak value, front time, and time to half-value must all stay within the standard tolerances. Typically one calibrated reduced wave is recorded as a reference, followed by three full waves at the specified peak voltage.
Q: How do I interpret the recorded Oscillograms after an impulse test?
Answer: The Oscillograms are the primary evidence of insulation integrity. A healthy transformer produces a smooth, single-peaked voltage wave and a matching neutral-current trace consisting of a sharp initial capacitive spike followed by low, decaying oscillations. The full-wave records are compared against the reduced-voltage reference wave. Any abrupt collapse of the voltage, a chopped waveform, sustained high current, or abnormal high-frequency oscillation on the tail indicates a possible internal insulation fault and requires investigation.
Q: What are the acceptance criteria for a successful lightning impulse test?
Answer: The test is considered successful when all three full waves at the specified BIL show no evidence of breakdown. The current and voltage Oscillograms must be identical in shape to the reference record except the expected amplitude scaling. Any change exceeding the measurement noise tolerance, in either trace, requires the test to be repeated and, if confirmed, an internal inspection of the winding to locate the defect.
Q: What are the most common causes of a distorted or failed impulse waveform?
Answer: The most common causes include winding resonance that produces overshoot or oscillation, a turn-to-turn or partial discharge fault that distorts the tail of the wave, and series or shunt impedance faults inside the winding. External factors such as poor grounding, an incorrect divider ratio, or improper front-resistor selection can also distort the record. Careful comparison with the reference wave, together with checking the test circuit and earthing, distinguishes a genuine insulation defect from a test-setup problem.