The accuracy and validity of a lightning impulse test depend as much on careful preparation and circuit setup as on the performance of the generator itself. A correctly prepared test object and a properly configured circuit ensure that the generated impulse reaches the equipment under test with the intended waveform and amplitude. This article provides a structured approach to test object preparation and circuit configuration for HV lightning impulse voltage generator testing.

Understanding the Test Object Characteristics

Before connecting any equipment, engineers should collect the essential parameters of the test object:

• Rated Voltage and Test Voltage Level: Determines the required generator output and stage configuration.
• Capacitance: The winding or bushing capacitance strongly influences front time and waveform shape.
• Inductance: Internal inductance affects oscillation and overshoot behavior.
• Insulation Configuration: Whether the object is grounded, isolated, or has a floating potential.
• Test Terminals: Identification of the correct terminals for impulse application and measurement.

Manufacturer data sheets and nameplate ratings provide most of this information, but verification by measurement is recommended for aged or modified equipment.

Pre-Test Inspection of the Test Object

The test object should be inspected before every impulse test campaign:

• Clean all external insulation surfaces to remove dust, moisture, and contamination.
• Check bushings and insulators for cracks, chips, or tracking marks.
• Verify that oil levels, gas pressure, or insulation fluid conditions are within specification.
• Confirm that all connections are tight and free from corrosion.
• Record the ambient temperature and humidity for reference.

These steps prevent premature flashover and ensure that any failure observed during testing is genuine rather than the result of contamination or poor preparation.

Circuit Configuration Principles

The impulse test circuit should be arranged to minimize stray inductance and maintain a defined current path. Key principles include:

• Short and Direct Connections: Keep leads as short as possible to reduce inductance and waveform distortion.
• Single-Point Grounding: Ground the circuit at one defined point to avoid circulating currents and measurement noise.
• Proper Positioning of Dividers: Measuring dividers should be placed close to the test object terminals.
• Adequate Clearances: Maintain sufficient spacing between live parts and grounded structures.
• Shielding: Shield measurement cables and control lines against electromagnetic interference.

Selecting Front and Tail Resistors

The front and tail resistors determine the impulse waveform parameters. Selection should follow these guidelines:

• Calculate the expected front time based on the total series resistance and the effective load capacitance.
• Adjust the tail resistance to achieve the required decay time, typically 50 microseconds for a standard lightning impulse.
• Consider the damping requirements to limit overshoot to acceptable limits.
• Verify the thermal capacity of resistors for the intended test repetition rate.

Computer simulation of the circuit before testing reduces the number of trial impulses and protects the test object from unnecessary stress.

Connecting the Measurement System

The measurement chain must be integrated into the circuit without introducing errors:

• Connect the impulse voltage divider directly to the test object terminal.
• Use matched coaxial cables to transmit signals to the recording instrument.
• Verify the divider ratio and cable termination before applying voltage.
• Check the recording instrument settings for sampling rate, resolution, and trigger level.
• Perform a low-voltage verification impulse to confirm the entire chain works correctly.

Reduced-Level and Full-Level Testing

Standard practice requires a stepwise approach:

• Reduced-Level Impulses: Apply approximately 50 to 75 percent of the specified test voltage to verify the circuit and measurement system.
• Full-Level Impulses: Apply the specified test voltage, typically three impulses per condition.
• Comparison: Compare waveforms at reduced and full levels to detect insulation abnormalities.
• Documentation: Record every impulse waveform and note any deviation from expected behavior.

Post-Test Actions

After completing the impulse test, discharge the generator and the test object using the automatic earthing switch. Inspect the test object for visible damage, and review all recorded waveforms for consistency. Any anomaly should be investigated before the equipment is returned to service or subjected to further testing.

Conclusion

Test object preparation and circuit setup are decisive factors in the success of HV lightning impulse voltage generator testing. By understanding the test object characteristics, minimizing stray inductance, selecting appropriate resistors, and integrating the measurement system correctly, engineers can achieve accurate, repeatable, and standard-compliant impulse test results.