Over-current relay coordination for the protection of offshore wind power grid with HVDC connection

An offshore wind power plant is spread over a large area as there are tens or hundreds of turbine units connected to a common collector system. Large and distant offshore wind power plants can be connected to the onshore grid using voltage source converter (VSC) based high voltage direct current (HVDC) transmission. The whole wind power plant will be affected if there is an electrical fault in the offshore collector grid. Therefore, an efficient relay protection scheme is necessary. This paper describes the use of overcurrent relays for the detection and discrimination of faults in the offshore wind power plant grid. When the turbines are equipped with full scale converters, and there is a VSC-HVDC connection, the offshore wind power plant grid has power electronic converters at all the terminals. Relay coordination is a problem in such a scenario, as the converters operate in current limiting mode during faults. This paper presents a relay coordination scheme using overcurrent relays for the offshore wind power grid protection. There are some overreach cases which can be discriminated using the IEC-61850 based communication capabilities of modern relays. The proposed scheme has been demonstrated through real time simulation studies. A test wind power plant grid has been modelled in Real Time Digital Simulator (RTDS) and a physical over-current relay (with IEC 61850 communication) is used to detect the fault. Relay setting parameters are based upon maximum nominal currents.