Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattern recognition in terms of either PD pulse-height/discharge epoch (phase) distributions or discharge pulse shape attributes.

Partial discharges. Their mechanism, detection and measurement

Condition monitoring of rotating electrical machinery has received intense research interest for more than 30 years. However, electrical machinery has been considered reliable and the application of fast-acting digital electrical protection has rather reduced the attention operators pay to the equipment. The area based upon current literature and the author's experience is reviewed. There are three restrictions: only on-line techniques for rotating machines are dealt with; specific problems of variable speed drives are not dealt with, except in passing; conventional rather than emerging brushless, reluctance and permanent magnet machines of unusual topology are concentrated upon. The art of condition monitoring is minimalist, to take minimum measurements from a machine necessary to extract a diagnosis, so that a condition can be rapidly inferred, giving a clear indication of incipient failure modes. The current state of the art is reviewed in the following ways: survey developments in condition monitoring of machines, mechanically and electrically, over the last 30 years; put that work in context alongside the known failure mechanisms; review those developments which have proved successful and identify areas of research which require attention in the future to advance the subject.

Review of condition monitoring of rotating electrical machines

Partial discharge (PD) measurement has long been used as a test to evaluate different insulation system designs, and as a quality control test for new equipment. However, in the past 20 years, PD measurement has been widely applied to diagnose the condition of the electrical insulation in operating apparatus such as switchgear, transformers, cables, as well as motor and generator stator windings. Improvements in the capabilities as well the lower cost of sensors, electronics and memory is partly the reason for the increased popularity of PD diagnostics. Another reason has been the development of methods-including the use of ultrawide band detection-to improve the reliability of the PD measurement in the presence of noise. In addition, rapid progress is being made in automated pattern recognition techniques that also helps to suppress noise. This paper reviews the various PD measurement technologies that have been specifically developed to improve PD diagnostic methods, and outlines how they have been implemented for stators, cables, transformers and switchgear. Areas for further research are also presented.

Partial discharge diagnostics and electrical equipment insulation condition assessment

The objective of the paper is to discuss a tool which is proving extremely efficient in partial discharge measurement studies. Though the technique itself is not new, its application to partial discharge studies is. It will be demonstrated in this paper that it has tremendous power and this accounts for its rapid growth as an application in this field. The paper begins with the description of the fundamentals of wavelet analysis, wavelet categories and the properties of the associated wavelet transforms. PD pulses as acquired from detectors composed of different detection circuits are investigated and numerically simulated, and a method on how to select optimally the wavelet corresponding to the representative forms of PD pulse is then presented. Finally, applications of wavelet analysis to partial discharge studies are explored. The paper demonstrates that the wavelet based denoising method proposed in the paper can be employed in separating PD pulses from electrical noise successfully and can be used in pulse propagation studies of partial discharge in distributed impedance plant to provide enhanced information and further infer the original site of the PD pulse.

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Interpretation of wavelet analysis and its application in partial discharge detection

This paper describes briefly the main ageing and failure mechanisms and will indicate the advantages and limitations of the diagnostic tests available for the different insulation systems used in distribution and transmission cable systems. SC21 of CIGRE has addressed the subject of ageing factors and diagnostics of cable systems and has published three reports covering both fluid-filled and extruded cables. The reports describe ageing factors and several diagnostic methods, their purpose, and guidelines for use.

Ageing mechanisms and diagnostics for power cables - an overview

It is shown that significant improvements in partial discharge (PD) measurement can be achieved through the use of ultrawideband (UWB) coupling techniques. These enhancements are principally in the areas of increased sensitivity, external noise rejection, and better understanding of the physics of the PD process. Examples of the improvements achievable by the use of UWB techniques are discussed for the cases of rotating machines and GIS. A new PD coupler which has a frequency response extending into the GHz range is demonstrated to enable reliable discrimination of PD signals from noise on operating turbine generators. Preliminary results from a fundamental study on PD in epoxy resins, show that there are two types of PD phenomena, 'fast' and 'slow'. >

Practical implementation of ultrawideband partial discharge detectors

SF6 gas-insulated switchgear is a sealed system, but low- level contamination can accumulate and affect spacer performance. The degradation of a number of spacer materials exposed to either direct arcing conditions or arc by-products was investigated to correlate their surface molecular features with changes in electrical performance. Electron Spectroscopy for Chemical Analysis, Scanning Electron Microscopy, Infrared and Thermal Analysis were used to probe the surface of the dielectrics. In the immediate vicinity of the spark, spacer surfaces undergo irreversible chemical, morphological, and physical changes with extensive pyrolysis and surface erosion. There is clear evidence of chemical reactions with high concentrations of fluorine bonded to the organic resin. Exposure of spacer materials to SF6 arcing by-products results in significant reduction in electrical properties with profound changes in the chemical structure of both epoxy moieties and/ or filler particles. This paper will describe the surface characterization techniques as applied to degraded spacer surfaces and discuss the possible degradation mechanisms.

Characterization of GIS Spacers Exposed to SF6 Decomposition Products

A system was assembled which can perform high-sensitivity partial discharge (PD) measurements under combined electric stress and X-ray irradiation on 138-kV class gas-insulated switchgear (GIS) post insulators. Under X-ray irradiation, the PD pulse repetition rate is increased and the discharge inception voltage (DIV) is sharply reduced. The DIV of these insulators under X-ray irradiation was recorded at values at low as 25 kV, although some samples could not be brought into discharge at 400 kV without X-rays. The modulation effect of X-rays on PD activity permits the location of the discharge side. In general, pulse-height and pulse-phase patterns are more tightly clustered with X-ray irradiation, but the patterns display a board range of features depending on the type of defect, voltage, and time on voltage. >

Modulation of partial discharge activity in GIS insulators by X-ray irradiation

More demanding partial discharge (PD) acceptance criteria for gas-insulated switchgear (GIS) insulators are required at the higher voltage classes to maintain high levels of reliability. In addition to increased operating stresses and greater manufacturing difficulties, fundamental limitations exist which lessen PD detection sensitivity of large physical objects. PD scaling relationships based on a recently developed theory that enable prediction of PD magnitude in equivalent defects in different insulators are presented. For example, PD magnitudes of 2 to 5 pC (the current acceptance criterion) in a 138 kV insulator correspond to magnitudes of 0.5 to 1.2 pC for 550 kV insulators, based on certain assumptions. In effect, defects with greater severity would thus be passed in present factory tests for 550 kV insulators. To maintain the high levels of reliability that manufacturers and users have come to expect, PD acceptance criteria of 0.5 to 1.0 pC are proposed for 550 kV insulators to obtain a standard of severity equivalent to that in existence at 138 kV. >

Reliability of GIS EHV epoxy insulators: the need and prospects for more stringent acceptance criteria

When polymeric insulation systems are subjected to high electrical stresses, they are susceptible to localized degradation at stress enhancements caused by contaminants within the insulation or protrusions at electrode/insulation interfaces. The degradation is referred to as electrical treeing, tree-like structures of gas-filled channels growing from the stress concentration. There are three phases of electrical treeing, tree initiation which is degradation leading to formation of the first channel, tree growth which is the extension of the channels and the final breakdown when one or more of the channels bridge the electrodes to produce failure. The growth phase is caused by partial discharges (PD) within the channels. Although the PD characteristics have received some attention in the past these studies have used conventional narrow band PD detection equipment which gives information on the apparent discharge magnitudes and repetition rates of the pulses. However, valuable information about the discharge processes, which is available from the pulse shape, is lost. This paper describes the initial results of a study of the amplitudes, repetition rate and shapes of the discharge pulses observed during the growth of electrical trees in XLPE cable insulation using a wide band PD detector.

J.M. Braun

Papers

Practical implementation of ultrawideband partial discharge detectors

Characterization of GIS Spacers Exposed to SF6 Decomposition Products

Modulation of partial discharge activity in GIS insulators by X-ray irradiation

Reliability of GIS EHV epoxy insulators: the need and prospects for more stringent acceptance criteria

Partial discharge characteristics of electrical trees in polymeric cable insulation