Effect of Humidity on the Breakdown of Air Gaps under Oscillatory Switching Impulse Voltages - Electrical Insulation, 1996., Conference Record of the 1996 IEEE International Symposium on

Experimental investigations were carried out to study the effect of humidity on the breakdown of rod-plane gaps subjected to both uni-directional and bi-directional oscillatory switching impulse voltages. The humidity ranged from 14.1 36 gm/m3 at a constant temperature of 40' C. The gap subjected to uni-directional 6.3, 10, lQ kHz and bi-directional 4.2, 10,20 kHz oscillatory switching impulse voltages. The gap distance was varied from 8 to 30 cm. The experiments were carried out in a controlled atmospheric room. The results indicate an increase in breakdown strength with increase in humidity. The results are of great relevance at atmospheric conditions existing in Tropical countries. INTRODUCTION Switching surges are produced when a circuit breaker closes or re-strikes. These surges are generally uni-directional in nature with double exponential wave-shapes. Recent investigations have however shown generation of oscillatory switching surges [4] because of the inductance associated with a transformer or a machine or a line and capacitance of line or cable. The magnitude and frequency of these surges depend on the system parameter and their waveform depends on the system parameter and switching conditions. Depending on the switching condition, the oscillatory switching impulse voltages (OSIV) are categorized as uni-directional f (1-cosine) and bi-directional (fsine) [3,4]. The IEC has recommended testing of all equip ment designed for operating voltages above 300 KV with standard switching impulse voltage (SSIV) of 250/2500 ps [l]. However, recent studies, have found that at least for one frequency the breakdown voltage under OSIV is lower than that of SSIV [2]. Since the breakdown of air gaps is considerably affected by atmospheric conditions, a knowledge of effect of humidity on OSIV becomes essential. Information is available in literature on the effect of humidity on rod-plane gaps subject to switching impulses of varying time to front [6,7] but data under OSIV has not been reported. The present paper proposes to study the effect of humidity on rod-plane gaps subjected to OSIV. The uni-directional5.3, 10, 20 kHz and bi-directional4.2, 10 ,19 kHz oscillatory impulse voltage was applied to the test gap. These frequencies are in the reported frequency range of OSIV [4]. EXPERIMENTAL SET-UP Experiments were conducted in a controlled atmospheric room of 6m x 6m x 6m dimensions.The humidity was varied from 14 36 gm/m3 and the temperature was kept constant during the experiment (at 40O). This was done to prevent breakdown voltage from being influenced by change in temperature. Oscillatory switching voltages were generated by modification of a 10 stage, 500 KV, 6.25 kJ impulse generator with adaption of inductance for waveshaping [5] .The uni-directional OSIV was generated by introducing an inductance in series with the front resistor. To generate the bi-directional OSIV the wave-shaping inductance was introduced in parallel to the tail resistor. The test gap used was a RodPlane with the rod of 12.5 x 12.5 mm cross-section as high voltage electrode and plane of 90 c m diameter as ground.The rod was suspended vertically from a suspension insulator inside the controlled a t m e spheric room. The waveshapes of OSIV used for the study : 0 4.2 kHz bi-directional OSIV, time to front