Impulse Voltage Withstand Testing for Power Transformers Using HV Lightning Impulse Generator Systems

Power transformers are among the most critical and expensive assets in electrical transmission networks. Their insulation must withstand transient overvoltages caused by lightning strikes and switching operations. Impulse voltage withstand testing using HV lightning impulse voltage generator systems is therefore a mandatory type test and, in many cases, a routine test for new and repaired transformers. This article explains the purpose, standards, procedures, and result interpretation of impulse testing for power transformers.

Purpose of Impulse Testing on Transformers

The primary objective of impulse testing is to verify the dielectric strength of the transformer insulation system under transient overvoltage conditions. Specific goals include:

• Confirming that the insulation design can withstand standard lightning impulses without breakdown.
• Detecting manufacturing defects such as weak insulation, poor impregnation, or incorrect clearances.
• Verifying the integrity of the insulation after transportation, assembly, or repair.
• Providing a baseline for future diagnostic comparisons.

Applicable Standards

Impulse testing of power transformers is governed by several international standards:

• IEC 60076-3: Defines insulation levels, dielectric tests, and external clearances for power transformers.
• IEC 60060-1: Specifies standard impulse waveforms and tolerances for high voltage testing.
• IEEE C57.12.90: Provides test procedures for liquid-immersed distribution and power transformers.
• GB 1094.3: Specifies insulation levels and dielectric tests for power transformers in certain regions.

These standards define the waveform parameters, test voltage levels, and acceptance criteria used during impulse testing.

Test Configuration for Transformers

Transformer impulse testing requires careful circuit configuration because the test object presents a complex network of capacitance, inductance, and resistance. Key considerations include:

• Terminal Selection: Impulses are applied to each high voltage terminal in turn, with other terminals grounded or connected according to the test plan.
• Waveform Shaping: Front and tail resistors are adjusted to compensate for the transformer winding capacitance and produce the standard 1.2/50 microsecond waveform.
• Reduced Full Wave: A reduced-level impulse is applied first to verify the circuit and measurement system.
• Chopped Wave Testing: Where specified, a chopping gap is used to simulate insulation flashover and produce a chopped impulse.
• Neutral Connection: The neutral terminal is grounded or connected through a current measuring shunt.

Test Procedure

A typical transformer impulse test follows a defined sequence:

• Preparation: Inspect the transformer, clean bushings, verify oil levels, and record ambient conditions.
• Calibration: Verify the divider ratio and recording system using a low voltage impulse.
• Reduced-Level Impulse: Apply approximately 50 to 75 percent of the test voltage and record the waveform.
• Full-Level Impulses: Apply the specified test voltage, typically three impulses per terminal.
• Chopped Impulses: Where required, apply chopped impulses at a higher peak value.
• Comparison: Compare waveforms at reduced and full levels for consistency.
• Documentation: Record all waveforms, voltages, and observations.

Waveform Interpretation and Fault Detection

The key to successful impulse testing is the comparison of waveforms. A healthy transformer produces consistent waveforms at reduced and full levels. Indicators of insulation problems include:

• Sudden Changes in Waveform Shape: Alterations in front or tail characteristics may indicate partial discharge or breakdown.
• Increased Oscillation: Changes in winding resonance behavior can signal insulation damage.
• Neutral Current Anomalies: Abnormal current waveforms suggest internal faults.
• Audible Noise or Visible Discharge: Any unusual sound or flash during testing requires immediate investigation.
• Deviation in Transfer Function: Comparison of input and output waveforms reveals changes in winding behavior.

Modern digital analysis tools assist in detecting subtle differences that may not be visible to the naked eye.

Common Challenges

Transformer impulse testing presents several practical challenges:

• High Winding Capacitance: Large transformers require high generator energy to achieve the required waveform.
• Waveform Distortion: Winding inductance and capacitance can distort the impulse, requiring careful resistor selection.
• Overshoot Control: Overshoot must be limited to acceptable levels without compromising front time.
• Measurement Accuracy: Accurate measurement of fast transients requires high bandwidth dividers and recorders.
• Safety: High voltage and stored energy demand strict safety procedures.

Post-Test Evaluation

After impulse testing, the transformer should be inspected for visible damage, and oil samples may be analyzed for dissolved gases. Any indication of insulation failure requires further investigation, including repeat testing at reduced levels or internal inspection. A successful impulse test provides confidence that the transformer can withstand transient overvoltages in service.

Conclusion

Impulse voltage withstand testing using HV lightning impulse voltage generator systems is essential for verifying the insulation integrity of power transformers. By following international standards, preparing the test object carefully, and interpreting waveforms rigorously, engineers can ensure that transformers will perform reliably under lightning and switching overvoltages throughout their service life.