Phase-to-phase insulation part II: Required clearances and coordination with phase-to-ground insulation

Optimization of the air insulation of high voltage systems is obtained by studying the overall behavior of the combined gap formed by phase-to-phase (Ph-Ph) and phase-to-ground (Ph-G) clearances. The coordination between Ph-Ph and Ph-G gaps is investigated both experimentally and statistically for a given electrode arrangement. The transition from Ph-Ph to Ph-G breakdown, following a gradual increase of the Ph-Ph clearance (with a fixed Ph-G clearance), is expressed by charts of relative probabilities and is related to the parameter α = Vn/(Vn+ Vp). Recommendations are offered to help select the Ph-Ph clearance which, combined with the Ph-G clearance of the system, gives a predetermined total breakdown probability for the combined gap as well as predetermined relative Ph-Ph and Ph-G breakdown probabilities.