A power plant control system for a first renewable energy power plant comprising a plurality of wind turbine generators, the power plant control system comprising; receiving means configured to receive a power delivery demand from a transmission grid operator; control means configured to control the plurality of wind turbine generators of the first renewable energy power plant so as to supply power that targets the received power delivery demand; and communication means configured to transmit a request to one or more further renewable energy power plant control systems (80,95) to request operation of one or more power compensation units associated with respective ones of the further renewable energy power plants (20,90), characterised in that the power plant control system further includes phasor measurement means comprising at least a first phasor measurement unit configured to measure electrical phasor parameters at the first renewable energy power plant and one or more further phasor measurement units (114,116) configured to measure electrical phasor parameters at respective ones of the further renewable energy power plants (20,90) and wherein the power plant control system is configured to monitor the phasor measurement means and to determine respective electrical phases of the first renewable energy power plant and the further renewable energy powerplants, thereby to determine the phase difference and/or rate of change of phase difference between respective ones of the renewable energy power plants (20,90), and wherein the power plant control system is configured to determine the presence of a grid fault on the basis of the determined phase difference and/or the rate of change of phase difference, and in response to send a request message via the communication means to one or more of the further renewable energy power plants (20,90) to initiate a low power production mode of the power generation units (12,90) associated therewith. receiving means configured to receive a power delivery demand from a transmission grid operator; control means configured to control the plurality of wind turbine generators of the first renewable energy power plant so as to supply power that targets the received power delivery demand; and communication means configured to transmit a request to one or more further renewable energy power plant control systems (80,95) to request operation of one or more power compensation units associated with respective ones of the further renewable energy power plants (20,90), characterised in that the power plant control system further includes phasor measurement means comprising at least a first phasor measurement unit configured to measure electrical phasor parameters at the first renewable energy power plant and one or more further phasor measurement units (114,116) configured to measure electrical phasor parameters at respective ones of the further renewable energy power plants (20,90) and wherein the power plant control system is configured to monitor the phasor measurement means and to determine respective electrical phases of the first renewable energy power plant and the further renewable energy powerplants, thereby to determine the phase difference and/or rate of change of phase difference between respective ones of the renewable energy power plants (20,90), and wherein the power plant control system is configured to determine the presence of a grid fault on the basis of the determined phase difference and/or the rate of change of phase difference, and in response to send a request message via the communication means to one or more of the further renewable energy power plants (20,90) to initiate a low power production mode of the power generation units (12,90) associated therewith.