How to perform Dielectric Test for Three-Phase Transformers?

Dielectric Test for Three-Phase Transformers

👉 Purpose of Dielectric Tests:

1-To verify insulation strength of the transformer.

2-Ensure the transformer can withstand high voltages caused by:

*Lightning strikes

*Switching surges

*System over voltage

👉 Types of Dielectric Tests: There are mainly two standard dielectric tests for transformers:

1. Power Frequency Withstand Test (Applied Voltage Test) Tests insulation between windings and earth

2. Induced Over voltage Test (Double Frequency Test) Tests winding-to-winding insulation

👉 1. Power Frequency Withstand Voltage Test (Applied Voltage Test)

👉 Procedure:

1-Apply a high AC voltage (at normal frequency: 50Hz/60Hz) between: The transformer’s windings (HV side) and earth (tank body).

2-Other windings: Either grounded or left floating (depending on standard: IEC/IEEE).

3-Increase voltage smoothly up to the specified test level.

4-Maintain voltage for 1 minute.

⚡ Test voltage is usually: Voltage applied is much higher than rated (like 2× rated + 1kV).

⚡ Example: For 11kV rated winding → apply around 28kV AC for 1 minute.

⚡ Test Conditions: Duration: 1 minute Monitor: No flashover, breakdown, excessive leakage current.

👉 2. Induced Overvoltage Test (Double Frequency Test)

👉 Procedure:

1-Apply a higher-than-normal voltage to the low voltage side (secondary).

2-Frequency is doubled (100Hz or 120Hz) to prevent magnetic core saturation.

3-The high voltage side is left open.

4-The test stresses turn-to-turn insulation inside the transformer.

⚡ Test voltage is usually: Twice the rated voltage of the secondary side.

⚡ Example: For a 400V LV side ➔ Apply 800V at 100 Hz. This checks the inter-turn insulation inside the windings.

⚡ Test Conditions: Frequency: 100 Hz or 120 Hz Duration: 30–60 seconds Monitor: No insulation failure or noise.