Reactive Power and Voltage Control of Offshore Wind Farms

Abstract

There are several challenges related to reactive power and voltage control of HVAC transmission from offshore wind farms to the main grid, which need to be addressed when designing wind farms. One challenge is the variation of wind speeds and thereby also power production, which can make it difficult to operate the system within the grid code requirements. This Master s thesis focuses on finding beneficial operating strategies to solve these challenges. Various locations of a static var compensator (SVC) are tested in addition to local regulation on the turbines through voltage source converters (VSCs). To investigate different operational strategies, a simulation model of a system connecting an offshore wind farm to the main grid has been developed in DIgSILENT© PowerFactory. A comprehensive examination of the behaviour of the dynamic voltage control devices has been performed to ensure the desired functionality of the model. Two different approaches for controlling the output of the wind farm have been examined; P-Q operation controls the active and reactive power output, whilst P-V control mode determines the active power and the voltage at the connected node. For these two scenarios, a static var compensator has been implemented on both sides of the transmission cables. Coordination between load tap changers (LTCs) and the static var compensator has been examined, and the active power losses for operating the system with an SVC offshore have been estimated.Simulation results show that the most beneficial operational strategy for the wind farm in both P-Q and P-V control mode is to use an SVC located offshore. This operational strategy provides better voltage control and lower cable currents than the other analysed system structures. Thus, a longer distance to shore is possible without exceeding maximum cable currents. Cable losses are also lower with the SVC implemented offshore. The SVC was barely affected by the taps from LTCs, and coordination issues between these two dynamic components can be considered negligible.