A new tool for averting failures in HV and MV systems

Electrical Tester – 15 December 2017

UHF partial discharge detection is a new technology with a lot to offer in terms of

 capability, convenience and cost. It is quick and easy to apply, it can be used to survey HV and MV plants without taking them out of service, and it can discriminate between conditions that are dangerous and those that are less serious. Hein Putter of Megger explains.

Failures in the HV and MV transmission and distribution networks frequently have costly and disruptive consequences. For instance, a recent phase-to-ground fault in the termination of a 380 kV cable at a GIS installation caused so much damage that it was ten months before the cable could be returned to service. And in another instance, a network fault resulted in loss of power to most of the city of Munich. Up to 450,000 people were affected, with the failure of traffic signals and other services creating conditions akin to chaos.

Thankfully, events like these don’t happen every day, but they are far from uncommon and, as the power infrastructure in the developed world ages, they are becoming more frequent. This is a major concern for power utilities, who go to considerable lengths to monitor plants and detect incipient faults so that they can be addressed before they cause major failures and supply outages.

This is no easy task, for two principal reasons. The first is that it is very difficult – often virtually impossible – to take equipment out of service for routine testing, and the second is that electrical noise levels around HV and MV installations are invariably high, which makes many test techniques difficult or even impossible to use.

There is now, however, a new technique that overcomes both of these problems: UHF partial discharge (PD) detection. The basis of this technique is using a specially designed UHF receiver to detect emissions produced by PD, concentrating on frequencies above 300 MHz. The use of these high frequencies has important benefits, the first being that above 300 MHz electrical noise levels in substations are much lower. The second is that emissions above 300 MHz are characteristic of internal PD, which may point to an impending failure, whereas emissions below 300 MHz are typically indicative of corona or surface PD, which is usually less of a concern.

Corona and surface PD typically generate pulses in the nanosecond range, which produce an RF spectrum extending up to no more than 300 MHz. In contrast, internal PD, which is particularly hazardous for the future “health” of the equipment under test, produces much faster pulses in the picosecond range. The fast pulses have a frequency spectrum with components up to at least 1 GHz and often much higher.

In addition to having the ability to discriminate between dangerous and less serious conditions, UHF PD testing has two other important benefits. The first is that no connection needs to be made to the equipment under test – the detector works by receiving radiated emissions. The second is that testing is carried out with the equipment under test energised, so the inconvenience and high costs associated with taking equipment out of service are eliminated.

UHF PD testing is useful in a very wide range of applications including checking HV terminations, evaluating the condition of HV components such as potential transformers, current transformers and surge arrestors, and monitoring power transformers. This form of testing is also a valuable aid when carrying out maintenance work on HV and MV switchgear, not least as a final check to show that the work has been completed correctly and safely.

The latest UHF PD detectors, such as those in the new UHF PDD range from Megger, are very

 versatile instruments. They offer a choice of antennas, and can also be used in conjunction with UHF termination, TEV and HFCT sensors, as well as with sensors that are permanently installed on HV and MV equipment as an aid to routine monitoring. The best detectors also make provision for analysing PD signals at frequencies below 300 MHz, for power frequency synchronisation, and for listening to demodulated PD signals using stereo headphones.

When used with a dipole antenna, UHF PD detectors are ideal for carrying out quick surveys on HV and MV plant. An antenna of this type can be used either as a separate handheld device connected to the instrument via a cable, or directly mounted on the instrument. It is directional, which can make it easier to locate the source of any emissions detected. An alternative is a “rubber duck” type antenna, which covers similar applications, but is sometimes more convenient because of its smaller size. It is, however, non-directional and is always mounted directly on the instrument.

The most sensitive method of detecting UHF PD is with a permanently installed sensor, although this approach requires preplanning and does not therefore lend itself to ad-hoc PD surveys. A typical sensor will be UV resistant and will have an IP67 ingress protection rating as well as a wide operating temperature range, making it suitable for use indoors or outdoors. It will be maintenance free, and will comply with IEC 60229. Sensors of this type are readily available for use on systems up to 500 kV.

In practice, UHF PD measurements are best carried out in three steps: spectrum analysis, to determine the frequencies of interest for further measurement; time domain measurement for PRPD pattern recognition to help distinguish between true PD and noise; and level measurement to localise the PD source and to determine how stable the discharge is over time. The last step is only needed if the first two steps show that PD activity is present.

The results obtained will, of course, vary with according to the application and the type of defect – if any – that’s present. In one recent case, for example, PD activity was detected but was producing emissions with frequency content only up to around 300 MHz. PRPD pattern analysis, with the detector synchronised to power frequency, showed that maximum PD activity coincided with the voltage peaks. These results are indicative of corona discharge and this diagnosis was ultimately confirmed.

In another instance, the spectral distribution of the PD emissions covered a wide frequency range and extended into the gigahertz region. PRPD pattern analysis showed that in this case, the maximum PD activity coincided not with the peak voltage, but with the maximum rate of change of voltage. These results are consistent with an internal fault, and this was confirmed when a small void was found in the insulation of the test specimen.

The latest UHF PD detectors are small, easy to carry devices that can operate either from internal batteries or from a mains supply. They offer a range of power frequency synchronisation options via direct or wireless connections. The best types have two UHF input channels to facilitate comparisons of sensors and sources, and incorporate intuitive touchscreen operation. These detectors are easy and convenient to use, they promote safety as no connections to the equipment under test are needed, and they also eliminate the need for equipment to be taken out of service for testing.

Finally, it’s worth noting UHF PD detectors represent only a very modest investment but for utility companies and other organisations that operate power networks, if such a detector averts even a single failure, it will undoubtedly deliver savings that are orders of magnitude greater than its cost.

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