Synchronous island control with significant contribution from wind power generation

Power system islanding is gaining interest as a way to improve power supply security and power quality. This has been facilitated by continuing advancements in communications and digital control. For maximum flexibility, power system islands must be capable of operating with different generation and power network topologies. Furthermore, the implication of high capacities of renewable resources on the ability of power system islands to keep within frequency and voltage limits must be considered. This is especially the case for synchronous islanded operation, whereby the island´s reconnection to the grid is aided, and out-of-synchronism re-closure avoided, by holding the island in phase with the main power system while not electrically connected. As this requires tight frequency control to remain within even relaxed synchronization limits, shown by the authors to be in the order of plusmn 60deg, the issues surrounding continuously variable renewable power sources is evident. Building on previous work, including a demonstration of synchronous island control on a single-set system, this paper investigates phase control in a multiple-set island with significant generation from a fixed-speed wind farm. A distribution system has been modelled using PSCAD/EMTDC. Fully controllable synchronous machine interfaced generators are used to balance any variation of wind energy. The multiple machine system is capable of island control functions, such as load sharing, and this requires a suitable communication system. Results show that phase control can indeed be achieved. The limits on the penetration of fixed-speed wind energy in the synchronous island, and on the maximum load disturbance that can occur while remaining within an acceptable phase difference, are explored. Two areas are then considered to reduce frequency and phase difference variations. The first is a more sophisticated control strategy for synchronous machine connected distributed generation, such as the use of - a real power input to provide faster response. The second is to reduce the short-term variability of renewable energy by replacing fixed-speed induction generators (FSIG) with doubly-fed induction generators (DFIG).