Switching overvoltage when disconnecting a combined 400 kVcable/overhead line with permanently connected shunt reactor

This paper demonstrates the results from a detailed study of the dynamic behavior of a shunt reactor compensated 400 kV combined cable/overhead line. This is accomplished by means of theoretical considerations, simulations and measurements. The work presented here is based on a real-life operating 400 kV power system and initiated by Danish TSO Energinet.dk after having measured the voltage of the cable/overhead line after disconnection. A particular decaying dynamic phase voltage containing voltages higher than the voltage before disconnection appeared. A simulation model for the entire system consisting of overhead lines, crossbonded cable sections and shunt reactor has been created in PSCAD/EMTDC and verified against measurements with good results. Main focus has been put on the generation of switching overvoltages. It is shown that the generation of switching overvoltages is caused by slightly different resonant frequencies of the three phases which are reflected between the phases by mutual couplings in such a way that a low-frequent modulation appears in the phase voltages.