A VLF hipot tester is a crucial piece of equipment for high-voltage electrical testing.Its primary purpose is to substitute power frequency (50Hz/60Hz) high voltage for withstand tests, especially suited for electrical equipment with very large capacitance. For equipment with large capacitance, using traditional power-frequency test transformers for withstand voltage testing requires very large and heavy power supplies and transformers, which are highly inconvenient for field use.
In simple terms, a VLF hipot tester is mainly used to test the insulation strength of power equipment with significant ground capacitance.
The following are the most common types of equipment tested with VLF hipot testers:
1. Power Cables (This is the most common and primary application)
Types: Including cross-linked polyethylene (XLPE) cables, polyethylene (PE) cables, polyvinyl chloride (PVC) cables, rubber-plastic insulated cables, etc., covering medium-voltage cables from 6kV to 35kV and even higher voltage levels.
Reason: The longer the cable, the greater its ground capacitance. Using a 0.1Hz VLF for withstand testing requires a current only 1/500th of that needed for 50Hz power frequency testing. This significantly reduces the required capacity, size, and weight of the test equipment, making it ideal for field use after cable installation, such as for acceptance tests or preventive maintenance tests.
2. Rotating Machinery (Large Generators and Motors)
Types: Hydro generators, turbine generators, large high-voltage motors, etc.
Reason: Generator stators have many turns of windings, resulting in large ground capacitance and inter-turn capacitance, creating a huge equivalent capacitance. Power frequency withstand testing requires equipment with immense capacity. VLF testing effectively checks the main insulation strength and is less damaging to the insulation compared to power frequency testing.
3. Power Capacitors
Reason: Capacitors are inherently capacitive devices with very large capacitance values, making VLF withstand testing necessary.
4. Surge Arresters
Although arresters themselves do not have large capacitance, they are often tested as part of a system connected to cables or as insulators for routine insulation tests.
5. Other Capacitive Equipment
For example, long gas-insulated lines (GIL), certain types of bushings, etc.
