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A new tool for averting failures in HV and MV systems 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.
ailures 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 10 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, 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 produces 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 equip-
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12 Electrical Review | March / April 2021
ment 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.
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 10 months before the cable could be returned to service 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 plants. 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.