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

The outstanding electrical and arc-quenching characteristics of sulphur-hexafluoride (SF6) have already set new trends and standards for the construction of high voltage equipment. Intensive research is being carried out on the electrical properties of SF6, other electronegative gases and their mixtures with more common gases. The aims are to obtain an economical insulation with optimal electrical properties and to acquire a better understanding of their prebreakdown and breakdown behavior. Recently [1] we reviewed the breakdwon mechanisms operating in compressed gases and discussed the theoretical results in the light of experimental measurement in SF6. This paper deals with the breakdown and prebreakdown behavior of SF6 mixtures with other electron attaching and non-attaching gases. Uniform and non-uniform field breakdown under the applications of direct, alternating and impulse voltages is discussed. Improvement in properties such as higher breakdown voltages in non-uniform field gaps, less sensitivity to the electrode surface roughness and to the presence of contaminants and improved arc interruption characteristics of SF6-gas mixtures are discussed.

A Review of Electrical Breakdown in Mixtures of SF6 and Other Gases

A critical review is presented on the various electrical methods available for partial discharge detection and measurement in terms of their chronological development and application on cable, capacitor, transformer, and machine insulation specimens. A substantial portion of the effort is devoted to corona pulse detection techniques, because these constitute the most popular forms of corona measurement. It is pointed out, however, that under some conditions discharges may assume a pulseless or pseudo-glow type character and that in such circumstances pulse detection procedures become ineffective and must be replaced by appropriate bridge methods.

A Commentary on Partial Discharge Measurement and Detection

The development of the various partial-discharge test methods as applied to cables, capacitors transformers, and rotating machines is examined in an historical context. Special emphasis is placed on various discharge pulse analysis techniques, although it is pointed out that integrated charge measurements should receive greater attention, in particular as concerns their capability to respond to glow and pseudoglow discharges. >

Detection of partial discharges (Corona) in electrical apparatus

Experimental results are presented of ionisation ( alpha ) and electron attachment (( eta ) coefficients evaluated from the steady-state Townsend current growth curves for SF6N2 and CCl2F2-N2 mixtures over the range 60<or=E/P<or=240 (where E is the electric field in V cm-1 and P is the pressure in Torr reduced to 20 degrees C). In both the mixtures the attachment coefficients ( eta mix) evaluated were found to follow the relationship; eta mix= eta (1-exp(- beta F/(100-F))) where eta is the attachment coefficient of pure electronegative gas, F is the fraction of the electronegative gas in the mixture and beta is a constant. The ionisation coefficients ( alpha mix) generally obeyed the relationship alpha mix= alpha N2(1-F/100)+ alpha A(F/100) where alpha N2 and alpha A are the ionisation coefficients of nitrogen and the attaching gas respectively. However, in case of CCl2F2-N2 mixtures, there were maxima in the alpha mix values for CCl2F2 concentrations varying between 10% and 30% at all values of E/P investigated. Effective ionisation coefficients ( alpha - eta )/P obtained in these binary mixtures show that the critical E/P (corresponding to ( alpha - eta )/P=0) increases with increase in the concentration of the electronegative gas up to 40%. Further increase in the electronegative gas content does not seem to alter the critical E/P.

Ionisation and attachment in binary mixtures of SF6-N2 and CCl2F2-N2

Impulse breakdown voltages have been measured for negative rod-plane gaps in hydrogen and in H2/SF6 mixtures for pressures in the range 0.5-3.0 bar and electrode spacings up to 0.25m. In hydrogen, the negative-impulse breakdown voltages are 60-100% higher than those previously reported for positive polarity: However the increase in breakdown voltage with the addition of small quantities (?1%) of SF6 is smaller than for positive-impulse conditions and is confined to pressures below~H2 bar, with the result that the positive and negative impulse strengths of rod-plane gaps in such low SF6-concentration mixtures are almost equal. The results are discussed with reference to observations of the effect of SF6 on the prebreakdown corona activity.

Impulse breakdown of positive rod-plane gaps in hydrogen and hydrogen-SF 6 mixtures

After discussing transformer types, ?values, and location of voltage generators in capacitive ladder networks and equivalent transmission lines, a new electrical technique for locating transformer partial discharges or corona is described. It is based on the measurement of the radio frequency spectra at the transformer terminals. Provided the transforner winding can be approximated to a uniform transmission line for the frequency range considered, the method is shown to be theoretically sound. Measurements on model networks and actual transformers suggest that, at least over certain frequency ranges, the winding can be regarded as a uniform transmission line. Apart from locating corona, the method provides a quick and accurate means of measuring actual transformer ?values.

A. M. Sletten

Papers

Impulse breakdown of positive rod-plane gaps in hydrogen and hydrogen-SF 6 mixtures

Corona Location in Transformers by Radio Frequency Spectrum Analysis, Part I: Theory

Electric Breakdown of Long Gaps in Transformer Oil