50 Hz slope finds cable faults fast

Hein Putter - product manager, testing and diagnostics

It’s no surprise that the new 50 Hz slope cable test technology is attracting a lot of attention from power network operators worldwide, as it offers a faster, more convenient and more reliable way not only to find faults on underground power cables, but also to evaluate the condition of those cables.

The new technology is based on VLF testing with a cosine-rectangular waveform that has rise and fall times almost identical to those of a 50 Hz sine wave. Extensive evaluation has demonstrated conclusively that the results produced by this test waveform are directly comparable with the performance of cables operating at power frequency.

When used for quality control of newly installed cables, the compact lightweight test units based on the new technology deliver immediate results in almost every case, enabling repair work to be started without delay, thereby minimising disruption to customer supplies. To make them even more versatile tools for cable condition evaluation, the units incorporate DAC (damped AC) test facilities that complement their VLF test capabilities.

These options for determining the condition of cables are particularly important at the present time as many of the cables in use in the power networks of developed countries have reached or exceeded their design lives. In addition, the networks are having to be reconfigured to cope with the changes resulting from the increasingly widespread introduction of renewable energy sources.

The ideal solution would be complete renewal of older network cables, but practical constraints – not the least of which is the very high cost involved – mean that this is rarely possible. In fact, many parts of the cable network may still be in good condition, but a key challenge is deciding which cables are fit for further service and which have truly reached the end of their reliable service lives.

The new test sets offer a fast, reliable and convenient way of obtaining the data needed to make fully informed decisions in such cases, thereby enabling a high level of security to be attained in the supply system, at a reasonable cost.

With the new and innovative 50 Hz slope technology, cable faults are identified by the occurrence of partial discharge at the fault location. Because of the unique waveform used and its special relationship to power frequency sine waves, the test voltage reliably triggers these partial discharges and PD inception is directly comparable to that exhibited by cable at power frequencies.

This contrasts sharply with VLF testing carried out using sinusoidal test voltages, where experience and extensive evaluation have shown that critical partial discharge effects cannot always be directly related to the effects exhibited by the cable when it is operating at power frequency.

Partial discharge measurement has, in recent years, established itself all over the world as one of the most successful diagnostic methods for evaluating cables because it allows faults and defects of almost all kinds to be accurately identified and located. Testing with DAC voltages is well proven and is valued as it poses minimal risk to the cable under test. On the other hand, many standards now recommend that voltage withstand tests are carried out at VLF.

Since they provide facilities for both of these types of test, the new test sets with 50 Hz slope technology are a particularly attractive option. In fact, as well as their unrivalled test and diagnostic capabilities, they embody other benefits such as a system that, when the test voltage is reversed, recovers most of the energy stored in the cable. This allows the test sets to be made smaller and lighter. Sophisticated PD evaluation algorithms are also used, which automatically map the precise location of discharges, eliminating the time, inconvenience and cost of post-processing the data generated during testing.