What is the impulse test of a transformer?

Transformer Impulse Test is a high-voltage dielectric test performed to assess the insulation strength of a transformer against transient overvoltages, such as those caused by lightning strikes or switching surges in a power system.

Purpose

To ensure that the transformer's insulation can withstand impulse voltages that may occur during its service life without failure.

Test Procedure

Standard Impulse Waveform
A standard lightning impulse voltage (e.g., 1.2/50 µs wave: rise time 1.2 µs, decay to 50% in 50 µs) is generated by an impulse generator and applied to each winding terminal, one at a time.

Test Levels
The test voltage is determined by the transformer's rated voltage and insulation class (according to standards like IEC 60076-3 or IEEE C57.12.00).

Application

Full wave impulse: Applied to verify basic insulation strength.

Chopped wave impulse: Simulates a lightning strike with a sudden voltage collapse, testing insulation under steep voltage changes.

Front-of-wave impulse: Tests insulation response to very fast rising voltages.

Monitoring
The test is considered passed if:

No sudden drop in test voltage (indicating insulation breakdown).

Comparison of applied impulse wave and transferred wave (measured across other windings or parts) shows no significant deviation.

Post-test power frequency and turns ratio tests show no change.

Why It Is Important

Lightning Protection: Confirms the transformer can survive lightning-induced surges.

Insulation Coordination: Ensures insulation matches system protection levels.

Design Validation: Detects weaknesses in major insulation, turns, or bushings before field service.

Industry Standards

IEC 60076-3: Power transformers – Insulation levels and dielectric tests.

IEEE C57.12.00: Standard general requirements for liquid-immersed distribution, power, and regulating transformers.

GB/T 1094.4 (China): Similar insulation and testing requirements.

In summary, the impulse test is a key type test (usually done in a laboratory on prototype or sample units) to validate the dielectric robustness of a transformer's insulation system against high-voltage transients.