A flexible real time network model for evaluating HVDC systems' impact on AC protection performance

This paper presents a reduced but reprehensive real time network model constructed in RSCAD for RTDS to evaluate the impact of HVDC systems and Non-Synchronous Generation (NSG) on the protection performance in the AC grid. The proposed network model could be flexibly configured to evaluate key factors that could affect the protection performance, including the level of system strength, different control strategies adopted in the HVDC system, different levels of synchronous compensation installed at the HVDC site, etc. The developed network model contains a Modular Multilevel Converter (MMC)-based HVDC system, a NSG unit representing the converter-interfaced generation and a Synchronous Condenser (SC) representing the level of synchronous compensation. A flexible controller is designed for the HVDC system to realise various typically used control strategies, including balanced current control, constant active power control and constant reactive power control, and inject a desired level of the negative sequence current as required. The NSG employs the widely-adopted PQ control strategy. Three typical controllers, comprising the Automatic Voltage Regulator (AVR), constant reactive power and droop controller, are implemented for the SC to realistically emulate the SC behaviour under different control modes. Case studies on the application of the model for testing distance protection performance are presented. The developed model is suitable for both pure simulation-based studies and also hardware-in-the-loop test when connected to an external physical relay, thus providing an ideal testing platform for identifying the potential critical protection issues and the potential solutions in future power networks with high penetration of renewables.