DC high-voltage generators are indispensable tools for field service engineers and power utilities. They serve two primary functions: verifying cable insulation integrity through Hipot (withstand voltage) testing and pinpointing high-resistance faults. This article explains standard procedures and critical safety considerations for both applications.

1. DC Hipot Testing of Power Cables

The DC withstand voltage test applies a voltage higher than normal operating levels to detect insulation weaknesses. For medium-voltage cables (5–35 kV), the test voltage is typically 4–5 times the rated phase-to-phase voltage, applied for 15 minutes. A well-designed DC high-voltage generator provides a smooth ramp-up function (e.g., 1 kV/s) to avoid stress spikes. During the test, leakage current is monitored – a sudden rise indicates impending breakdown.

2. Interpreting Leakage Current

Modern DC high-voltage generators include a microammeter with 0.1 µA resolution. Acceptable leakage values depend on cable length and insulation type. As a rule, stable current below 10 µA per kV of test voltage suggests good condition. Rising current without voltage increase points to moisture or contamination. Pulsating current often indicates partial discharges or voids in the insulation.

3. Locating High-Resistance Faults

Standard time domain reflectometers (TDR) struggle with high-resistance faults (over 1 kΩ). A DC high-voltage generator solves this by applying a high-voltage pulse (up to 80 kV) that breaks down the fault point, creating a transient arc. Using a surge detector or arc reflection method, operators can locate the fault within 1% of cable length. This method works for open conductors, splices, and deteriorated insulation.

4. Safety Discharge and Grounding

After testing, cables retain a dangerous charge equivalent to the applied voltage. Automatic discharge resistors built into the DC high-voltage generator must be engaged for at least twice the charging time. A visible grounding stick should then be used to verify zero voltage. Modern generators feature automatic grounding relays and interlock systems that prevent access while high voltage is present.

5. Field vs. Laboratory Considerations

Portable DC high-voltage generators designed for field use offer:
- Battery operation or wide-input AC (110–240V) for site flexibility.
- IP54 or higher enclosure against dust and moisture.
- Digital logging of test voltage, current, and duration for report generation.
- Lightweight cascaded multiplier stacks (e.g., 20 kg for 80 kV).
Laboratory units prioritize ultra-low ripple (<0.1%) and computer control over portability.

Conclusion

A DC high-voltage generator configured for cable testing combines high-voltage sourcing with precise current measurement and fault pinpointing capabilities. When selecting a unit, verify that it includes programmable ramp rates, automatic discharge, and data recording. Proper use according to IEEE 400 or similar standards extends cable life and reduces unplanned outages.