VLF Testing for MV Cables

Very Low Frequency testing for Medium Voltage and High Voltage cables is a method for verifying the integrity of the cable insulation. VLF testing is a withstand test usually carried out at a frequency of between 0.1 Hz and 0.01 Hz rather than 50/60 Hz. The test is conducted on entire drum of cable rather than a test sample length.

Although the frequency is very low, it is an alternating current with polarity reversals every half cycle. At 0.1 Hz output, it takes 600 times less current and power to apply an AC voltage to a capacitive load.

VLF testing of solid dielectric cable is supported in IEC 60502 (up to 35kV) and in IEEE 400.2 (up to 69 kV). Acceptance test voltages are generally 2.5-3 times the line to ground system voltage. VLF tests should last between 15 and 60 minutes with a recommended minimum duration of 30 minutes.

The tested MV or HV cable must withstand an AC voltage for the specified testing time without flashover. VLF cable testing  on MV and HV cables uses different wave shapes, typically sine and square, so care must be taken when describing the voltage to be used. RMS and peak voltages have different relationships to each other depending on the wave shape - IEEE 400.2 uses the peak voltage level to equate the wave shapes.

IEEE 400 embraces VLF testing as a viable alternative to DC testing. It is testing methodology widely adopted by Distribution Network Operators (DNOs) as it exposes serious cable defects, thus preventing in-service failures, while not harming healthy insulation or aggravating minor defects.

MV Cable Test Facility for VLF Testing

VLF Testing on MV and HV cables is undertaken in our specialist MV test facility, under strictly controlled test conditions. As with all testing undertaken within The Cable Lab, our technical experts precisely match proscribed test methodology with results recorded impartially and automatically within our systems.

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