Megger TDS40, Operating Manual, Page: 16 / 71

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2.5 Description of the Test Procedure with VLF-Voltage
Because of the space charges formed in the dielectric of the cable, DC test methods
prove to be dangerous when applied to PE and VPE cables and should therefore never
be used on such types of cables.
Nowadays we can take it for granted that the 0.1 Hz VLF method for testing cables with
plastics insulation outclasses any other test method using DC voltage or power-
frequency AC voltage as used so far.
A 0.1 Hz VLF test system must meet the following demands:
• The repetition rate has to be so low, that the power that is set free in any
existing PD-channel (PD = partial discharge) is small enough not to cause
further erosion and so increase gas pressure.
• The inversion of polarity, on the one hand, must be slow enough to
exclude any transients caused by travelling waves. On the other hand, it
must be fast enough that any space charge at the tip of a PD-channel from
where it grows in the direction of the opposite electrode is preserved.
A system that meets these requirements has to supply a 0.1 Hz oscillation, with the quick
polarity reversal taking place within a frequency close to the power frequency.
Fig. 1: Time frame of polarity reversal (type Plus)
Load-dependend frequency during polarity inversion:
TDS40Basis 40 ≤ f < 410 Hz
TDS40Plus 30 ≤ f < 410 Hz
TDS60Basis 55 ≤ f < 475 Hz
TDS60Plus
40 ≤ f < 475 Hz
VLF CR 0.1 Hz
polarity reversal
Oscillation with
30 … 475 Hz
(see below)