Studies on Coroaa in Transformer Oil by Measurement of Space Charge

Studies were carried out in needle-plane gaps with a comparatively long separation (50 to 150 mm) in transformer oil. Relations between needle voltages and space charge quantities due to coronas by lightning impulse and ac voltages were obtained. The charge quantities for ac voltage are 2 times larger than those for positive impulse voltage, and 7 times larger than those for negative impulse voltage. The polarity effect on the charge is not observed in case of ac voltages. Back-discharge phenomena take place, accompanied by sudden space charge reductions, at the wave tail of the impulse voltage. The space charge due to initial coronas at the wave front are reduced to 57 to 85% by the phenomena.