Q & A on testing buried and sub-sea power cables

With the growth of renewable energy sources and the general expansion of the power transmission network, more and more underground and undersea power cables are being installed. Testing these cables has, in the past, been costly and difficult, but this situation is changing, as is illustrated by the answers to these questions recently received by our technical support team.

Q: Using conventional series-resonance techniques for voltage withstand testing on HV cables is costly and, given the necessarily huge size of the test sets, inconvenient. Is there a viable alternative?

A: Standards such as IEC 60840 and IEC 62067 do in fact allow the use of soak testing at service voltage as an alternative to withstand testing, but this is almost universally recognised as a poor option that is unlikely to reveal much about the cable under test. A much better approach is to use DAC testing. The equipment for this will fit into Sprinter-sized van, in contrast to the articulated lorry trailer that is usually needed for a resonance test set. The latest DAC test sets from Megger are available in 200 kV and 300 kV versions, and can be used on cables with capacitance up to 8 µF.

Q: Why are DAC test sets so much smaller and less expensive than resonance test sets?

A: Resonance test sets energise the cable under test continuously, so they need to supply a lot of energy, which means they are necessarily very large. In contrast DAC test sets fire energy into the cable in a repetitive series of short shots, and the voltage in the cable is allowed to decay between shots. This means that much less energy is needed, so the test set can be made much smaller.

Q: Does DAC testing comply with standards?

A: At present, this useful and convenient test technology is ahead of the standards. DAC testing is, however, accepted by many cable operators and CIGRE is at the present time actively working to formalise this type of testing.

Q: Is it useful to combine withstand testing with other forms of testing?

A: In short, yes! It is widely recognised that although withstand testing is important, it is unlikely to reveal every type of fault that may exist on a cable. It is, therefore, very useful to carry out other tests, such as a sheath test and partial discharge (PD) testing, in addition to the withstand test. The DAC test method actually facilitates fast, effective and detailed PD evaluation and, with the latest software, the calculation of results and generation of reports are taken care of automatically. PD testing is a good and very sensitive way of detecting and locating potential faults of all kinds, including poor workmanship on joints and terminations, which is the most common cause of failures on power cables.

Q: Are the same test techniques applicable to MV cables?

A: Broadly, yes, but in addition VLF testing, usually at 0.1 Hz, is widely recognised and used on MV cables. VLF test sets that produce a cosine rectangular wave shape are generally accepted as being the best choice. This well-established wave shape has leading and trailing edges that closely match those of a 50 Hz sine wave, and this feature of the waveform allows it to be used for accurate and reliable PD testing. In fact, test sets for MV applications are now available that combine VLF, DAC and PD testing in a single convenient unit and these provide a very convenient, cost-effective and versatile solution for fault detection and location on buried power cables.