Research Of Direct Power Control Strategy On STATCOM/BESS System

The grid capacity is increasing with the development of wind power generation. However, wind power generation is undulantly and unpredictable so that the impact on the power grid system has become increasingly evident. The STATCOM with battery energy storage system which called STATCOM/BESS combines the advantages of STATCOM and battery energy storage system. So it can control the active power and reactive power in the wind turbine with power fluctuation. When used in the distribution network, it can also reduce the impact on the power grid of load fluctuations. Thus, the realization of the effective control of STATCOM/BESS is extremely important.The principle and control strategy of STATCOM/BESS in the four quadrant operation are described in this paper. And, this paper establishes a main circuit model in the α-β and d-q coordinate system, analyzes the direct power control deeply. The simulation results show that STATCOM/BESS can achieve reactive power compensation under the charge or discharge mode.When traditional DPC control based on the switching table used in STATCOM/BESS, the reactive power has a periodic fluctuation. To solve this problem, this paper considers the active power and reactive power control accuracy, redesigns the switch table. In addition, a new method is adopted to judge the sector in order to reduce the computational quantity. The simulation results show that the active power and reactive power tracking accuracy is higher in the control of the improved switching table in DPC.Traditional DPC control based on predictive current model is mainly used in PWM rectifier. When used in STATCOM/BESS, the reactive power control effect is not ideal, and without considering the different situations of active and reactive power with different importance. This paper brings the reactive power error into account value function, and draw into the power tracking weight coefficient which can adjust the active power or reactive power tracking precision independently. The simulation shows that the improved strategy can control the reactive power effectively, and can adjust the tracking accuracy easily.Traditional DPC based on predictive power can realize the goal of fixed IGBT switching frequency in STATCOM/BESS system, but it can’t control reactive power well, and predicted referance power applying linear interpolation has big error. This paper takes reactive power into control strategy, deduces STATCOM AC side voltage expression. Then, this paper uses second order lagrange interpolation to predicte referance power. The simulation results show that the STATCOM/BESS output power fluctuation is small and accurately in low switching frequency.In conclusion,the advantages of DPC based on the switching table are that the structure is simple and easy to implement. At the same time, its disadvantage is that the switching frequency is not fixed. The DPC control based on predictive current model has fast response and high power tracking. On the other hand, its disadvantage is that the algorithm is too complexity. The DPC based on predictive power can realize fixed frequency control, but its disadvantage is that the dynamic response is not as fast as the first two strategies.