Medium voltage (MV) cables are critical components in electrical power systems, responsible for safe and efficient energy transmission between substations, transformers, and distribution networks. However, their performance and reliability can degrade over time due to various factors, including aging, mechanical stress, environmental conditions, and manufacturing defects. One of the most effective diagnostic techniques used to assess the insulation condition of MV cables is Partial Discharge (PD) testing. PD testing can be performed both "on-line" during a U0 Soak Test or while load energized, or "off-line" during an overvoltage AC Hipot test at various frequencies, usually VLF when field testing
Off-line Partial Discharge test method
An overvoltage test to stress the cable system to look for PD-producing defects. Measurement and interpretation of partial discharge signals below and above normal operating voltages are made. Magnitude, location, and inception & extinction voltages are all important indicators of cable condition.
Apply high voltage to de-energized and disconnected cable up to a pre-determined level while observing the results, to stop test or voltage increase if necessary, to prevent failure if PD is present. Record test data for later analysis. It is an offline test that can be used:
1. 50/60 Hz. AC, conventional hipot or resonant
2. VLF AC @ 0.10 Hz. – 0.05 Hz.
3. Damped AC @ 20 Hz. – 500 Hz.
PD off-line diagnostic evaluations are generally considered efficient and reliable in detecting defects, their locations, and assessing their severity. Offline PD testing has the benefit over online by providing a controlled overvoltage to detect defects that initiate PD at levels above nominal.