Impact of system strength and HVDC control strategies on distance protection performance

This paper presents comprehensive studies and tests for evaluating the impact of reduced system strength and different control strategies used by HVDC systems on the performance of distance protection. A Hardware-In-the-Loop (HIL) test setup is established to enable realistic testing of physical relays being used in the system, where simulated voltage and current waveforms are injected into the distance protection relay via an analogue amplifier, and the relay tripping signal is fed back to simulation and recorded for protection performance analysis. In the simulation, a reduced but representative transmission network model, which includes a Modular Multilevel Converter (MMC) based HVDC system, a synchronous condenser (SC), and a two-level converter representing non-synchronous generation (NSG), is developed in RSCAD for the RTDS simulator. The model can be flexibly configured to reflect different levels of system strength and synchronous compensation applied at the HVDC site. The HVDC system is implemented with a flexible controller, which can replicate typically used control strategies during faults (e.g. balanced current mode to eliminate negative sequence current, and constant active and reactive power modes to suppress the oscillations on the active and reactive power respectively), allowing the user to inject different levels of negative sequence current. From the studies, it was found that with decreased system strength, the impact of the HVDC system on the distance protection becomes apparent, i.e. protection performance could be compromised with delayed operation, and such impact, to some extent, is subject to the control strategies applied in the HVDC system. It was also observed that the installation of SC could facilitate the protection response, and such support is dependent on the SC capacity.