Analysis of a Midpoint Series Compensated Line

Many electrical utilities worldwide are increasingly using series compensation to meet the need to add large amounts of renewable energy resources such as wind and solar to the existing power system network. In series compensated lines, the effective line inductive reactance becomes small due to the capacitive compensation. This can lead to voltage and current inversion and subsynchronous oscillations during disturbances along the series compensated line. For distance relays, special consideration must be taken for series compensated, and adjacent lines due to the series capacitor bank (SCB) alters the line impedance. This can cause a protection relay to operate for faults beyond its normal reach. A 400 kV series compensated network is modelled using DigSilent Power Factory. Electromagnetic transients (EMT) analysis was performed on distance protection devices on the network. The results presented in this paper show that a series compensated line presents challenges for setting distance relay protection where a three-phase fault can lead to overreach, voltage, or current inversion at different network locations.