The Issues of Traditional DC Pressure Testing Methods
We get asked this question a lot: What is the correct testing for my new HV cable or installation?
Traditionally industry has followed the principle of overvoltage testing conductors with DC voltages being applied well above the normal operating voltage. That's the way we have always done things so it must be ok..........
Overvoltage DC Pressure Testing of XLPE cables damages the cables. It is not recommended over 5KV.
The reasoning for the recommendation for not exceeding 5KV DC for insulation resistance testing on XLPE cables is due to the 'stress' that high voltage DC currents apply onto cables designed to operate on AC networks. The electromagnetic field generated by AC and DC currents are very different; with DC not generating a rotating EMF like AC, instead producing a uni-directional EMF. The DC EMF will at higher test voltages, decay the integrity of the XLPE cable insulation is it 'pushes' the insulation away from the conductor.
In worst cases this can create tiny cracks or 'treeing' within the insulation, which over time will enable water to ingress within the cable and eventually partial discharge to occur. XLPE cables suffer worse than older paper insulated cables due to their ability to act as excellent capacitors, with extremely low leakage rates.
It is worth noting that DNO's will provide the rationale for cable testing free to the public and are available via the internet. You will find that they either do not permit DC Insulation Resistance testing on XLPE cables operating under 33KV, or limit the test voltage to 5KV DC.
Cable manufacturers also advise against using DC voltages over 5KV, this can be found on the data sheets provided by them.
An alternative to high voltage DC testing is AC Very Low Frequency (VLF) testing. This enables overvoltages to be applied to the conductors without stressing them as the frequency used can be as low as 0.1Hz. VLF testing can also enable you to monitor the integrity of the cable insulation resistance over time through Tan-Delta testing. The end result test figure grades the cable condition, with the measurement increasing over time which can enable accurate predictions over the conductors operational life to be made. The one drawback of VLF testing is that is it struggles to provide accurate results on short cable runs as there is not enough capacitance within the conductor for meaningful results. This is why on short runs, 5KV DC Megger testing is commonplace.
Unfortunately within HV contracting there does not seem to be the same joined-up approach that is found at DNO level. The issues overvoltage DC testing can cause are not always immediately apparent and lead to partial discharge some time after the tests were completed. Usually well outside of any defect liability periods, leaving the customer to fund the repair or replacement of the cable.