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The causes of a number of explosions of EHV current transformers in service on the Hydro-Québec 765 kV network have been investigated. Thermal runaway related to unexpectedly high dielectric losses in one of the oils used was diagnosed. Electrical instability of the oil was related to dissolved-metal content rather than to oxidation. A new way of stabilizing the oil is proposed, consisting in the...
Some discharging effectiveness was found in the oil tank system equipped with 100 or 200 mesh stainless screen fixed on top of the diffuser at the oil inlet. To study the discharging mechanism in detail, the potential distribution along the axis of the cylindrical oil tank, the earth current in each part of the oil-flow system, and surface potential of the oil tank were measured.
Using several specially designed discharge chambers embodying point-point, point-plane and point-sphere electrode systems, and filled with dielectric liquid, a zone of low luminescent intensity was detected between the electrode corona and the point in the liquid at which the discharge channel originated. This dark zone is evidently the location of the mechanisms which are the critical precursors...
Pre-breakdown light emission, current and photo current are measured, simultaneously. For a negative needle point, figures of light emission are divided into two patterns. They change from a bush-type to a tree-type as the applied voltage and the oil surface pressure increases. In the case of the tree-type, they are caused by streamer only at the initial stage. In the case of the bush-type, the lights...
Results are presented for charging and reverse currents in spectral grade carbon tetrachloride, toluene and transformer oil. The decay of charging current with, time can be changed at first by exponential lair but after that as I∼ t−n where n = 0.33 for carbon tetrachloride and n = 0.5 for toluene. The voltage-current characteristics have been studied over a range of time interval from 0.3 sec to...
The results presented here refer to a tungsten point anode and stainless steel plane cathode. Figure 1 relates maximum positive streamer growth to pulse duration and pulse amplitude in nhexane. The figure shows a scatter in experimental points on the graph typical of such a set of curves. A smooth curve is drawn through each set of points, by eye, to approximate a best fit curve with a degree of uncertainty...
A comparison of current vs time relationships obtained experimentally and calculated on the basis of Thomson-Thomson and weak electrolyte models. It was found that the weak electrolyte model properly describes the state of nematic liquid crystals and the Thomson model can be utilized for the description of smectic liquid crystals. This agreement is observed only in “pure” materials for times comparable...
The temperature curves of dc current on liquid unsaturated polyester (UP) resins diluted by various amounts of styrene exibit several straight-line segments. From this the activation energies, glass and liquid-like transition temperatures have been evaluated. The transitions have been conformed by thermally stimulated discharge (TSD) measurements. The current intensity and transitions have been varied...
Fluids used for the impregnation of medium voltage power capacitors made with polypropylene film must exhibit suitable gassing tendency, appropriate compatibility with the film in terms of wettability and limited solvent capacity, but in addition should withstand an endurance test. This test consists of two phases: during the first, repeated voltage impulses are applied which may generate partial...
The gassing properties of some dielectric liquids in the presence of different gases have been investigated by means of the experimental apparatus described in IEC n. 628 Specification. The tests have been carried out at different temperatures and for different durations; at the end of each test the volume variation of the gas phase has been measured and its composition has been analyzed. Further...
Past studies of the electrically modified forced flow through a cell with a wire two plate electrode geometry modelling the injection zone of an E.H.D. generator showed the existence of two flow regimes. We present here some more refined results based on velocity measurements by L.D.A‥ For both regimes, the velocity profiles allow us to explain how the injected space charge modifies the flow and how,...
Studies were carried out for needle-plane gaps with a comparatively long separation in transformer oil. Relations between needle voltages and space charge quantities due to coronas by lightning impulse and AC voltages were obtained. It was found that the charge quantities for AC voltages were larger by 1.2 times and 6 times than that for the positive and the negative impulse voltages respectively...
Using Kerr effect charge injection from point cathode and anode in nitrobenzene in prebreakdown electric field under pulse voltage of microsecond range has been recorded. Cathode emission started at Ei=0.5MV/cm and anode - at Ei=1.0MV/cm. Negative space charge was approximately uniformly distributed around the point cathode top, and anode positive charges injection was of local nature. Prebreakdown...
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