Purpose of Pre-Commissioning Impulse Testing for Power Transformers

No-Load Full-Voltage Switching Impulse Testing for Newly Commissioned Transformers

For newly commissioned transformers, besides conducting necessary tests according to handover test standards and protection/secondary system tests, no-load full-voltage switching impulse tests are typically performed before official energization.

Why Perform Impulse Testing?

1. Check for Insulation Weaknesses or Defects in the Transformer and Its Circuit

When disconnecting a no-load transformer, switching overvoltages may occur. In power systems with ungrounded neutral points or neutral points grounded through arc suppression coils, overvoltage magnitudes can reach 4-4.5 times the phase voltage; in systems with directly grounded neutral points, overvoltage magnitudes can reach 3 times the phase voltage. To verify whether the transformer's insulation strength can withstand full voltage or switching overvoltages, no-load full-voltage impulse tests must be conducted before transformer commissioning. If there are insulation weaknesses in the transformer or its circuit, they will be exposed when breakdown occurs due to switching overvoltages.

2. Check for Maloperation of Transformer Differential Protection

When energizing a no-load transformer, magnetizing inrush current occurs, which can reach 6-8 times the rated current. The inrush current decays rapidly initially, typically reducing to 0.25-0.5 times the rated current within 0.5-1 second, but complete decay takes longer—several seconds for small and medium transformers, and 10-20 seconds for large transformers. During the initial decay period of magnetizing inrush current, differential protection may maloperate, preventing transformer energization. Therefore, during no-load impulse switching, the wiring, characteristics, and settings of differential protection can be practically checked under the influence of magnetizing inrush current, allowing evaluation and conclusion on whether this protection can be put into service.

3. Evaluate the Transformer's Mechanical Strength

Due to the significant electrodynamic forces generated by magnetizing inrush current, no-load impulse testing is necessary to evaluate the transformer's mechanical strength.

Why Typically Five Impulses?

For new products before commissioning, five consecutive no-load full-voltage impulse tests are generally required. Since the closing angle differs at each switching moment, the corresponding magnetizing inrush currents also vary—sometimes large, sometimes small. Generally, five no-load switchings are required to comprehensively test the transformer's insulation, mechanical strength, and the operation of differential protection.

What Are the Characteristics of Magnetizing Inrush Current?

Characteristics of magnetizing inrush current:

• Contains significant non-periodic components, often causing the inrush current to be biased to one side of the time axis, with typically one phase opposite to the other two phases

• Contains substantial high-order harmonics, with the second harmonic component being the largest

• There are intermission angles between the inrush current waveforms

• The inrush current value is very large in the initial stage, reaching 6-8 times the rated current, and then gradually decays