Health check for high voltage gear

There are a number of characteristics that give information about a piece of high voltage equipment and each can be used to provide some form of diagnosis on its health. Andrew Stevens, business development manager of EA Technology, explains

Owners and operators of electrical networks have become increasingly interested in condition monitoring of high voltage electrical equipment. Major benefits have been obtained through preventing failures, reducing maintenance, operating costs and extending plant life. The use of testing, diagnostics and condition monitoring systems has enabled assessment of the condition of existing distribution equipment by using a number of applied techniques during maintenance or indeed initial commissioning phases. It has been recognised that simply increasing the frequency of maintenance can have a negative effect and increase failure rates.

There is no single technique that will give all the answers. The skill of the asset manager is to choose which characteristics to study and which techniques to use to provide high quality information that allows informed diagnosis and decision making.

Switchgear monitoring Monitoring the condition of HV insulation in switchgear can be achieved by the detection of partial discharge activity. Partial discharge activity leads to deterioration and subsequently failures in high voltage insulation. Partial discharge breakdown of insulation produces light, heat, smell, sound and electromagnetic waves. The sound produced by partial discharge breakdown of insulation can be detected with ultrasonic detectors. In order for the detectors to pick up ultrasonic noise, they must be in media of similar densities, as propagation of ultrasonics between low and high density mediums is poor due to most of the energy being reflected.

Airborne ultrasonic detectors are therefore successful in detecting surface discharge activity, for example on an insulator in air, and activity where there is a good acoustic path, such as on a dry termination in a vented cable box.

A capacitive probe or probes can be placed on the outside earthed metalwork of the switchgear to detect any transient earthed voltage (TEV). The TEV is generated on the metal surface when electromagnetic waves propagate out into free space at any opening in the metal cladding. Instruments using a single probe can measure the amplitude of the discharge activity while more sophisticated instrumentation determines the location of the source using a second probe.

Transformer testing Transformers are generally reliable pieces of plant; it is rare for them to catastrophically fail shortly after the first warning signals appear. What is more likely is a gradual deterioration in condition which can be adequately monitored through regular maintenance and condition monitoring. A more common practice involves routine sampling and analysis of the transformer insulating oil. This information provides an indication of the overall condition of a transformer and can help to predict their rates of ageing relative to the loading conditions. There are a number of methods for determining the condition of the oil including dielectric strength and resistivity measurement together with more specialised tests for the assessment of water, acidity, dissolved gases and Furfuraldehyde.

Failures While the failure rates of electrical distribution equipment are fortunately low, the causes and the mechanisms involved are numerous.

A failure examination strives to identify the cause and understand the mechanism involved. Often the fault damage is extensive but through attention to detail and the use of forensic techniques, it is usually possible to obtain sufficient evidence during an examination to identify the incident resulting in the failure.

Prediction of service failures is difficult due to the numbers of variables involved.

With some of the techniques, it is possible to give an indication that deterioration has taken place and failures are to be expected.

To address this problem, results of diagnostic testing are often recorded and combined with information about service performance in order that the relationship between the two might be examined. In addition to this, there is growing interest in the continuous monitoring of equipment which it is hoped will provide more detailed information about any deterioration taking place and again relate this to service failures.

If testing, diagnostics and monitoring techniques are carefully chosen, applied and interpreted, they can provide an efficient tool kit for increased reliability, reduction of costs and prioritisation of replacement programmes.