A Two-step Reactive Power optimization Method of OffShore Wind Farms with Discrete Devices Considered

Off shore wind farms have experienced rapid growth in China, providing adequate power for coastal load centers. While, the investment of off shore wind farms is much higher than that of land-based wind farms. Meanwhile, the transmission loss inside an off shore wind farm (typical value >3%) is much higher than that in a land-based one (typical value <1%) for larger installed capacity, wider space distribution and significant capacitive reactance of long submarine cables. Therefore, minimization of transmission loss becomes important to reduce the cost of off shore wind farms. Due to nonlinear relation and uncertainty of wind, minimizing active power loss caused by reactive power transmission remains a challenging task. This paper presents a two-step optimization strategy to reduce transmission loss inside off shore wind farm. Discrete devices such as on load tap changing transformer (OLTC) and static var compensator (SVC) are built in, and the uncertainty of wind speed is considered. Step I controls discrete devices with a scenarios based method, while step II fine tunes the reactive power generation of wind turbines. In the optimization, second-order cone relaxation (SOCR) is applied for better computational performance. In contrast with automatic voltage control (AVC), transmission loss decreases by30% without significant increase of voltage fluctuation.