- Dielectric Test Definition: The dielectric test of a transformer checks the insulation’s ability to handle voltage without breaking down.
- Separate Source Voltage Withstand Test: This test evaluates the main insulation’s ability to withstand voltage between winding and earth.
- Induced Voltage Withstand Test: This test assesses the insulation between turns and line ends, and between windings and earth.
- Test Procedure: The test involves applying voltage and checking for insulation breakdown, using specific steps and equipment.
- Test Voltage Values: Specific voltages are used for different equipment ratings, ensuring the transformer can handle operational stresses.
The dielectric test of transformer checks winding insulation in two steps: a separate-source voltage withstand test and an induced-voltage withstand test of the transformer. IEC 60076-3 covers those dielectric tests. The steps below follow the teaching sequence on this page.
Separate Source Voltage Withstand Test of Transformer
This dielectric test checks whether the main insulation can hold voltage between the winding under test and earth without breakdown.
Procedure
- All three line terminals of the winding to be tested are connected together.
- Other winding terminals not under test and the transformer tank should be connected to the earth.
- Then a single-phase power frequency voltage of shape approximately sinusoidal is applied for 60 seconds to the terminals of the winding under test.
- The test shall be performed on all the windings one by one.
- The test is successful if the insulation does not break down during the test.
In this transformer testing the peak voltage is measured, so a capacitor voltage divider and a digital peak voltmeter are used as in the diagram. Peak times 0.707 (1/√2) is the rms test voltage for a sine wave.
Typical short-duration power-frequency test voltage values for fully insulated windings are in the table. Treat them as a teaching set. The applicable IEC or national table for the transformer rating governs a factory test.
| Nominal system voltage rating for equipment | Highest system voltage rating for equipment | Rated short duration power frequency withstand voltage |
| 415V | 1.1 KV | 3 KV |
| 11 KV | 12 KV | 28 KV |
| 33 KV | 36 KV | 70 KV |
| 132 KV | 145 KV | 230 / 275 KV |
| 220 KV | 245 KV | 360 / 395 KV |
| 400 KV | 420 KV | 570 / 630 KV |
| Winding with graded insulation, which has neutral intended for direct earthing, is tested at 38KV | ||
Induced Voltage Test of Transformer
The induced voltage test of transformer checks inter-turn and line-end insulation, and main insulation to earth and between windings.
- Keep the primary winding of transformer open circuited.
- Apply three phase voltage to the secondary winding. The applied voltage should be twice of rated voltage of secondary winding in magnitude and frequency.
- The test should last for 60 seconds.
- The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to desired value.
The test passes if no breakdown occurs at full test voltage. The frozen steps use twice rated voltage and twice frequency for 60 s. IEC 60076-3 raises frequency to avoid overfluxing and sets duration from that frequency, so 60 s at twice frequency is a teaching version of the test.
