By hvhipot 
Compared to DC withstand voltage testing and power-frequency AC withstand voltage testing, VLF testing offers unique advantages:
1.Compared to DC Withstand Testing:
Better simulates operating conditions: Power equipment operates under AC voltage, and the electric field distribution under DC voltage differs from that under AC voltage.
For solid-insulation equipment like XLPE cables, DC testing can create space charges within the insulation, potentially damaging it or masking underlying insulation defects.
The electric field distribution under VLF AC voltage is consistent with the power frequency and does not cause space charge accumulation, making it a more effective and safer test method.
Better at detecting concentrated defects: Defects like partial discharges are more easily detected under AC voltage.
2.Compared to Power-Frequency AC Withstand Testing:
Portable equipment: As mentioned, the test current is drastically reduced, making the test equipment dozens of times lighter and smaller than power-frequency test sets, making it highly suitable for field use.
Low power supply requirements: Requires very little capacity from the field power series resonance.
VLF high voltage generators are primarily used for insulation withstand voltage testing on electrical equipment with large ground capacitance. Their most typical and important applications are for power cables (especially medium and high-voltage XLPE cables) and large generators.