On-Line Partial Discharge Testing Applications — An Overview

Introduction

Medium- and high-voltage insulation failure is generally attributed to partial discharge activity which is a partial failure of the insulation that ultimately completely bridges the insulation between electrodes. The partial discharges produce unique phenomena that can be detected by several means. By detecting this activity, corrective actions can be performed so that complete failure is avoided. Electrical equipment should be regularly tested for partial discharge activity in order to prevent failure and increase reliability. This article shall review some of the common methods and applications for on-line partial discharge testing.

Partial Discharge

Flaws, imperfections, contamination, improper design, poor workmanship, and other problems in medium- and high-voltage insulation systems create localized stresses in the electrical field that cause limited breakdown in this isolated area. The discharge is essentially a spark which destroys the local insulation and eventually spreads across or through the insulation until complete failure occurs.

These partial discharges produce light, sound, electromagnetic signals, and chemical reactions. Depending on the type of equipment that is being tested, various methods can be applied to best detect the partial discharge activity. Each of the following unique characteristics of partial discharge activity allows for optimum detection using specific sensors (see Figure 1) and techniques best suited for the type of apparatus under evaluation.

Chemical

As noted above, partial discharge activity creates chemical changes in the insulation. The generation of hydrogen in liquid-filled transformers is often associated with partial discharge activity, and dissolved gas analysis is very successful in detecting these events. Partial discharge activity in gas-insulated equipment can be similarly detected by analyzing a sample of the gas (see Figure 2). Unfortunately, nonintrusive chemical testing of solid insulation does not exist, but it is possible to occasionally detect severe surface insulation damage visually or by observing the strong smell associated with severe ionization of the air. However, much better detection methods exist for solid insulation evaluation as discussed below.

Sound

Surface partial discharge activity creates a small pressure wave in the air or other insulation medium which surrounds it. Two similar techniques are used to detect this pressure wave, depending on the type of equipment under test. When a fairly direct line of sight or a strong reflected signal is available, ultrasonic sensors can be used to detect the signals. When the signals are completely contained within an enclosure, acoustic emission sensors placed on the outside of the enclosure can be an effective means for detection. The ultrasonic method has been applied successfully for many years to detect surface problems in switchgear, while the acoustic emission method has been very popular for the detection and location of partial discharges in oil-filled transformers and has also been used more recently to evaluate SF6circuit breakers and isolated-phase bus condition.

Light

The light produced by surface partial discharge activity or corona can be detected provided that an absolute direct line of sight to the insulation under examination exists. Partial discharge activity primarily produces light in the ultraviolet (UV) spectrum. Solar UV light that occurs naturally during daylight conditions significantly impedes viewing partial discharge with the naked eye. However, daylight corona cameras have special filters that block solar UV but allow harmful electrical UV to pass so that detection during daylight hours is now possible. This technology is ideal for outdoor substations, switchyards, transmission lines and distribution lines. Pinpointing surface discharges in switchgear is another useful application.

Electromagnetic

The light produced by surface partial discharge activity or corona can be detected provided that an absolute direct line of sight to the insulation under examination exists. Partial discharge activity primarily produces light in the ultraviolet (UV) spectrum. Solar UV light that occurs naturally during daylight conditions significantly impedes viewing partial discharge with the naked eye. However, daylight corona cameras have special filters that block solar UV but allow harmful electrical UV to pass so that detection during daylight hours is now possible. This technology is ideal for outdoor substations, switchyards, transmission lines and distribution lines. Pinpointing surface discharges in switchgear is another useful application.

Two primary methods are used to decouple partial discharge signals. For equipment evaluation, a small metallic plate is placed very near the outside equipment enclosure. This forms a temporary capacitor which allows the high frequency partial discharges to be detected. For cable evaluation, a clamp-on high frequency current transformer sensor is typically placed around the cable shield to observe the partial discharge pulses much in the same manner that load current is measured using 60 hertz current transformers. Figure 3 displays the electromagnetic signatures of a good and a bad cable termination.

Conclusion

On-line detection of partial discharge activity is a very effective method to evaluate medium- and high-voltage insulation condition. Different types of equipment require different detection methods. By having an array of technologies available, the best methods can be applied for the specific equipment being evaluated in order to obtain optimum test results.