Testing a medium-voltage cable at 50 Hz calls for a source able to supply all the reactive power its capacitance demands, which on long runs means equipment the size of a truck. Dropping the frequency to 0.1 Hz cuts that demand by a factor of 500, and the same test is done with a case one person can carry.
VLF subjects the cable to a very low frequency AC voltage for a standardised time, typically 15 to 60 minutes, to bring out defects that do not show at service voltage: water trees, poorly made joints, contaminated terminations.
It is applied mainly to dry-insulated XLPE and EPR cables, after installation and as periodic diagnostics, and it is usually combined with partial discharge or tan delta measurement during the same test to tell a cable that barely holds from one that holds with margin.