Abstract
The insulation of the power transmission systems is exposed to stresses not only during normal operating but also by the transient overvoltages. Transient overvoltages are originated by different sources such as lightning, switching and faults. The switching surges have become dominant factor in the insulation design of Extra High Voltages and Ultra High Voltages systems. Insulation failure probability under transient surges is the main impressive concern in the studies of insulation coordination and failure risk. In this paper the failure risk of insulation under switching overvoltages generated during energization of unloaded transmission line are computed using the statistical approach. In the calculation, both the statistical characteristics of the overvoltages and statistical distribution of insulation flashover probabilities are tackled. The statistical distributions of energization overvoltages on a typical 500 kV transmission-line are derived from the results of 100 energization cases with statistical switching using Alternate Transient Program (ATP). The most important factors affecting the insulation flashover probabilities and the failure risk are demonstrated. The effects of number of towers in parallel, the switching overvoltages profile, the front time of the overvoltages waveform and the non-standard atmospheric conditions on the failure probability are investigated. Failure risk analysis is analyzed using different probability distribution schemes.