The Effect of A.C. Bias Fields on the Impulse Strength of Point-Plane and Sphere-Plane Gaps

Present laboratory impulse tests do not simulate the large power-frequency voltages which may be present in practice. Experiments up to 850 kV (impulse) show that such bias fields have no significant effect on impulse breakdown. The 50% breakdown strength for both fast "lightning" impulses (1.7 sμ rise time) and for slower "switching" impulses (80μs rise time) was measured on 1.75 m point-plane and sphere-plane gaps with bias voltages from zero up to 200 kV r.m.s. and at various points-on-wave. A laser triggered gap was used to protect the impulse generator from the power frequency voltage.