Prediction model of voltage–time characteristics for SF 6 long gap under VFTO and lightning impulse voltage

Voltage–time ( V – t ) characteristics of the SF 6 long gap under very fast transient overvoltage (VFTO) draw wide attention at the high‐voltage transmission systems. In this research, a generating system of VFTO and lightning impulse (LI) up to 2.5 MV was set up. The V – t characteristics for SF 6 gaps with hundreds of millimetres length under negative LI and VFTO were researched. Experimental results indicated the rate of rising of V – t curve for the sphere‐plane gap under VFTO was moderate, and V – t curve under LI was rapid, resulting in the intersection of the two types of V – t curves. However, for the rod‐plane gap, the V – t curve under VFTO was below that under LI at 0.5 MPa. The shape of V – t curve had many relationships with discharge propagation velocity in the SF 6 gap, which was determined by the gap distance, field non‐uniformity, gas pressure and applied voltages. Based on analysis of discharge process and physical background of V – t curve, the semi‐empirical formulae were derived and the prediction model was established. It was demonstrated the predicted V – t curves could accurately predict breakdown characteristics of long SF 6 gas gaps