How is the impulse voltage test conducted for transformers?

The impulse voltage test comprises lightning impulse tests (including full-wave and chopped-wave tests) and operating wave impulse tests. In the latest IEC76-3 standard, for transformers with a rated voltage (U) ≤ 40.5kV, both full-wave and chopped-wave impulse tests are considered type tests. For transformers with U ≥ 72.5kV, the full-wave impulse test is a routine test, while the chopped-wave impulse test is a type test. Transformers with U ≥ 252kV undergo routine tests for full-wave, chopped-wave, and operating wave impulse tests.

The full-wave and chopped-wave impulse tests are alternately conducted, typically with negative polarity. It starts with one full-wave impulse, followed by two chopped-wave impulses, and finally two more full-wave impulses. Therefore, a breaking device is required.

In cases where the transformer capacity is large and the waveform cannot meet the requirements due to high capacitance, multiple stages of impulse voltage generators are connected in series.

When conducting impulse tests at the midpoint of the transformer, as it involves a three-phase waveform with high capacitance but generally not high test voltage, multiple stages of impulse voltage generators are connected in series and parallel to apply the voltage.

The distribution of full-wave and chopped-wave voltages along the winding is not uniform.

During testing, all terminals, except those to which the voltage is applied, are grounded. This includes grounding the midpoint of the high-voltage winding and the starting and ending points of the low-voltage winding. In the impulse test, the middle of the low-voltage winding induces impulse voltage on the core column. If the field strength is too high, conducting the impulse test on the high-voltage winding may lead to breakdown in the low-voltage winding on the core column. When using an H-L—L-H or L-H—L configuration, there may also be impulse-induced voltage at the connection point of the two low-voltage windings.

During impulse testing, attention should be paid to preventing simultaneous breakdown of instruments once a breakdown occurs. This is especially critical in chopped-wave tests where the impulse current passing through the grounding resistance may cause significant voltage drop. It is crucial to ensure that the grounding resistance value is not too high, generally specified to be below 0.5Ω. If necessary, the grounding of the product can be separated from the grounding of the instrument.

For transformers with rated voltage U ≥ 252kV, an operating wave impulse test is also required.

In the operating wave test, the test voltage is distributed uniformly along the winding turns and has an inductive effect. Therefore, during the operating wave test, each winding must not be short-circuited, and the high-voltage and low-voltage windings can only be grounded at one point.

In the operating wave test, the phase-to-phase test voltage is higher than the ground test voltage, while in the full-wave and chopped-wave impulse tests, the ground test voltage is equal to the phase-to-phase test voltage. Consequently, during the operating wave impulse test, the circuit must be reconnected.

Since the operating wave impulse voltage has an inductive effect, theoretically, it can be applied to either the high-voltage winding or the low-voltage winding. However, in practical tests, it is usually applied to one end of the high-voltage winding.

During the operating wave impulse test, attention should also be given to the fact that the test voltage between the line terminal applying the voltage and the adjacent low-voltage winding may exceed the applied voltage.

Both lightning impulse tests and operating wave impulse tests are conducted phase by phase.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

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