H. Moulai, A. Nacer (Algeria), and A. Beroual (France)
Mineral oil, transformer, partial discharges, current, energy, chromatography.
This paper deals with the characterisation of the partial discharge currents generated in mineral oil and their consequences. The shape of these currents, the associated electrical charges and energies are analysed in function of the number of discharges, the electrode geometry and the applied voltage. Chromatographic analyses in gaseous phase of oil samples submitted to partial discharges are achieved. The energy levels of the partial discharges are investigated as well as the types and concentrations of gases that they can generate. Two experimental devices were used. The first one consists of a test cell and an automatic setup enabling to measure the discharge voltage, the dielectric permittivity, the loss factor (tanδ) and the resistivity of oil samples. The available energy supplied by this device is not sufficient to provoke a significant deterioration of the characteristics of oil samples. It is applied through an automatic disconnecting switch for discharge currents higher than 4 mA in a lap of time lower than 1 ms. The second device enables to measure and to visualise the currents, and to generate discharges of different levels of energy (Figure 1). It consists of a test cell of 330 ml and a high voltage transformer (220V/100kV - 5kVA - 50 Hz). The voltage is applied to the oil gap through a switch which cut out when the discharge current exceeds a certain value; it’s uniformly increased (2.0 ± 0.2kV/sec). A non-inductive measurement resistor is inserted in the primary circuit. It’s characterised by a voltage drop of 120mV corresponding to a current flow of 12A. A digital storage oscilloscope (Tektronix - 100MHz Bandwidth) is used to record the current signals.
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