ABSTRACT The theoretical sensitivity of conventional partial discharge detectors is compared with that obtained from ultra wideband (UWB) (up to 1 GHz) detection systems. The comparison indicates that for relatively lossfree distributed systems, such -a-s SFe insulated bus, the UWB system is up to two orders of magnitude more sensitive. UWB detection also embodies additional advantages such as facilitating the location of discharge sites and the rejection of external electrical noise. For discharge detection in plastic-insulated cables, true UWB detection is not practical because of frequency-dependent attenuation effects, although certain gains in sensitivity can be achieved with a detector bandwidth of up to 10 MHz. INTRODUCTION In many high-voltage (HV) systems, partial discharges (PD) are an indication of insulation weakness which will eventually lead to catastrophic failure. For this reason, the measurement of partial discharges has become a routine procedure for acceptance testing of shielded power cables, switchgear, transformers, etc. In addition, partial discharge measurements are sometimes performed on operating equipment such as switchgear and generators to assure the integrity of such insulation systems. Since partial discharge tests are often spepiied in contracts between a manufacturer and purchaser, all aspects of the measurement must be fully understood. This paper reviews the various methods employed for partial discharge testing, especially with respect to simple distributed HV systems such as SFg bus duct and plastic- insulated concentric-neutral cable. The relative merits of a number of measurement techniques are discussed, with particular reference to the obtainable signal-to-noise ratio as compared with that obtainable in theory. PARTIAL DISCHARGE DETECTION METHODS A PD is a low of electrons and ions which occurs in a gas over a small volume of the total insulation system. This short duration event emits acoustic, optical, and radio frequency energy. PDs can be detected by measuring any of these radiations [1], In this paper only the direct-coupled measurement of the radio frequency current and voltage pulses will be considered, since this method is by far the most widely employed in industrial applications. Conventional PD Detectors The measurement system shown in Fig. 1 is the test arrangeiaent normally employed in practical situations [2,3,4]. This coniguration permits the equipment under test to be grounded in the normal fashion. The high-frequency electrical energy associated with a PD pulse lows through the coupling capacitor C 1 and detection impedance Z. Fig. 1: Conventional PD me: Buying arrangement The detection impedance is usually an RLC circuit having a large impedance to a certain frequency band in the PD spectruB, which causes a signal that can be ampliied and displayed on an oscilloscope screen. Z is usually designed to provide a low impedance path for power frequency current. Two forms of detection impedance have beconse popular in commercial PD detectors. One form of detector is referred to as “narrow band” since Z has a bandwidth of about 10 kHz, centered between 20 and 30 kHz. The other common detector, termed “wide band”, has a bandwidth of about 100 kHz with a center frequency between 200 and 300 kHz. In both cases, the output of the pulse ampliier (Fig. 1) is relatively easy to observe, even on older models of cathode ray tubes. The pulse output is usually displayed with respect to the power frequency voltage to aid discrimination between PD and electrical noise. Since the time constant of both detectors is long compared to the duration of a PD pulse, the conventional detectors integrate the current pulse. Thus the magnitude of the pulses must be measured in terms of charge (pC) [4]. On the other hand, one might reasonably expect that damage is roughly proportional to the number of ions and electrons involved in a PD so that this limitation is not serious. For the measurement of PD in distributed systems, practical dificulties arise which can lead to errors in the FUNDAMENTAL LIMITATIONS IN THE MEASUREMENT OF CORONA AND PARTIAL DISCHARGE S. A. Boggs and G. C. Stone Ontario Hydro Research, Toronto, Canada IEEE Transactions on Electrical Insulation Vol. EI-17 No.2, April 1982 143