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Online transformer condition monitoring techniques based on transfer function methods, such as transmission-line diagnostics and swept frequency-response analysis, require the injection of a known test signal into the transformer. On larger power transformers, a practical method is to use the available bushing tap connection. In this paper, we will discuss a custom high-power signal generator that injects high-frequency signals on the bushing tap of the transformer under investigation, as well as a circuit to replace the bushing tap short and allow online operation of the system. Finally, the system is demonstrated on a 650 kV transformer. Index Terms--Bushing tap injection, frequency response, frequency-response analysis (FRA), online diagnostics, power transformers, swept frequency-response analysis (SFRA), transmission-line diagnostics (TLD), winding signature.

Sulphur Hexaflouride ([SF.sub.6]) is generally found to be very sensitive to field perturbations such as those caused by conductor surface imperfections and by conducting particle contaminants. A study of CIGRE group suggests that 20% of failures in Gas Insulated Substations (GIS) is due to the existence of various metallic contaminations in the form of loose particles. The presence of contamination can therefore be a problem with gas-insulated substations operating at high fields. If the effects of these particles could be eliminated, then this would improve the reliability of compressed gas insulated substation. It would also offer the possibility of operating at higher fields to affect a potential reduction in the GIS size with subsequent savings in the cost of manufacture and installation. The purpose of this paper is to develop techniques, which will formulate the basic equations that will govern the movement of metallic particles like aluminum and copper in a bus duct. The simulation considers the electric field effect on particle movement and the results have been presented and analyzed. Index Terms--Metallic particles, Electric field effects, Gas Insulated Substations.

FRA tests in the frequency range up to 10 MHz were investigated using simulations and measurements. It was found that the FRA test was more sensitive to changes in the transformer at frequencies above 1 to 3 MHz than at lower frequencies. However, the effects of the conventional test circuits mask the changes at the higher frequencies. A new test method is proposed that is effective up to at least 10 MHz. Index Terms--Frequency response, modeling, transfer functions, transformers, windings.

This paper presents an exploratory experimental study of discharge initiation on surface of ice. The phenomena underlying these processes are, as yet, poorly understood. To elucidate the initiation process in particular, ultra high-speed streak photography was used to observe and analyze the first visible discharges and a number of parameters were investigated. It could be shown that ambient temperature, conductivity of ice surface and the presence of imperfections on the ice surface influence not only the critical voltage for discharge initiation, but also the velocity of discharge development. Some parameters could be identified, which should be investigated further. The present paper is the first stage of a long-term research effort in this new field. This study should enable further experimental investigations and mathematical simulations to gain better understanding of ice surface discharges, specifically in the context of electrical insulation integrity of support insulators in power systems operating under severe atmospheric conditions. Index Terms--Ice surface, streak camera, visible discharge initiation, surface charge, breakdown voltage, outdoor insulators.

Coating the inside surface of gas insulated switchgear (GIS) enclosures with a dielectric film reduces the deleterious effect of electrode surface roughness, impedes the development of metallic particle initiated microdischarges, increases the field required to lift particles, and reduces the charge acquired by particles, all of which help alleviate the adverse effect of contaminating metallic particles on insulation withstand. The performance of particle-contaminated compressed gas systems with dielectric coated electrodes is analyzed. Two mechanisms for the transfer of charge from electrodes to contaminating particles are considered, namely, conduction through the coating layer and microdischarges in the surrounding gas. The paper presents an electrostatic study of the particle lifting fields with dielectric covered electrodes. The overall breakdown strength of the system is evaluated and the results are discussed in the light of experimental findings. Index Terms--Breakdown, charging mechanisms, electrode coating, gas-insulated switchgear, particle contamination.

Results of an investigation to clarify the process of surface flashover in gases are reported. Two hemispherical capped rods of 12 mm diameter, separated by a half-cylindrical spacer of 50 mm diameter, formed the samples. The spacer materials were polytetrafluoroethylene, acrylic and nylon. High-speed streak photographs of impulse flashover indicate that the presence of a spacer not only lowers the impulse breakdown voltage but also reduces the breakdown time. The discharge on spacers may be initiated anywhere between the electrodes and develops rapidly into a flashover in [approximately] 5 ns for an 18 mm gap.

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and B.C. Hydro. [V.sub.50] (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 [[micro]second]), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The [V.sub.50] results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The Volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns.

The insulation performance of gas insulated transmission line (GITL) systems can be substantially affected by the presence and movement of contaminating metallic particles. Dynamics of wire and spherical particles in a coaxial-electrode system under ac voltage have been studied under different conditions. At voltages just above the lifting voltage the particle moves near the outer enclosure. Under appropriate conditions the particle can move into the high field region and eventually cross the inter-electrode gap, thus causing a potential insulation hazard. Contaminating particles in a GITL system move randomly in the electric field. Control of contaminating particles is crucial for reliable operation of GITL systems. In this study a numerical model of the contaminating particle's motion in the inter-electrode gap in a GITL system is used to study particle control methods in gas insulated power apparatus.