Effects of system contacts between 400 kV and 110 kV systems in transmission and distribution networks

New 110 kV and 400 kV overhead lines are often installed on a line tower. Contact between the systems leads to large overvoltages in the 110 kV system and may not be reliably detected by the protection devices. Based on real faults, the paper shows the consequences of system contact between a 400 kV and a 110 kV overhead line. Fault records and equipment failures in the networks involved are analysed. Analytical calculation methods based on symmetrical components are presented to estimate the maximum overvoltages. Theoretical considerations are complemented by simulation calculations. The effects on the insulation co-ordination of the equipment are shown.