Performance evaluation of distance protection of transmission lines connected with VSC-HVDC system using closed-loop test in RTDS

- Impact of reactive current support to the ac network from VSC-HVDC connection on the commercial distance relay is evaluated.
- Active current blocked and reactive current priority current limitation strategies for the VSC during the fault is implemented.
- Performance of the commercial distance relay is analyzed using a closed-loop test in RTDS and compared with the simulation.
- Reactive current support from the VSC causes under reach effect to the distance relay.
- The distance relay mis-calculates the location of fault and issues a wrong pickup and trip signals.

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) can control voltage and power flow on ac transmission lines with the priority of reactive current support during the fault as suggested in the grid codes for HVDC connections. Furthermore, the fault current contribution from the converter is limited in magnitude and modified in the waveform in order to protect the power electronic devices during the fault, which on the other hand affects the apparent impedance seen by the distance relay. This paper addresses the challenges faced by the distance protection of transmission lines connected with VSC-HVDC system by performing a closed-loop test of a commercial distance relay using Real Time Digital Simulator (RTDS). The power system with VSC is modeled in detail under a more realistic scenario in RTDS so that the performance of the distance protection can be tested under near to actual field conditions. The responses of the commercial distance relay for balanced and unbalanced faults are analyzed. The oscillograph data of the voltage, current and protection bits recorded by the relay are extracted and compared with the simulation results.