Research On Control Strategy And Application For STATCOM/BESS

STATCOM has a large number of engineering applications in power systems. It can output fast, continuous and two-way reactive power which is not affected by voltage of the point in connection with the system. Traditional STATCOM is made up of DC capacitor and voltage-source inverter. To guarantee the amplitude of the output voltage, Traditional STATCOM needs to absorb a certain amount of active power from the grid to compensate its own losses in order to maintain DC voltage when it is in steady state. So it can’t adjust active power and inject active power to the grid. STATCOM with battery energy storage(STATCOM/BESS) can not only compensate reactive power fast, but also compensate active power. So STATCOM/BESS provides a more flexible power adjustment function. Therefore, this paper researches the main circuit structure, mathematical model, control strategy and applications in wind farm of STATCOM/BESS in detail.This paper researches the composition of STATCOM/BESS, then establishes mathematical model of STATCOM/BESS in dual-stage structure, establishes switch function model in three-phase coordinates and dq stationary coordinates, and establishes the overall small-signal model of STATCOM/BESS. By analyzing the small-signal model, this paper puts forward overall control strategy of battery energy storage side controls the size of the active power output and STATCOM side stabilizes DC voltage. The current-voltage dual closed-loop combined with front-fed power control strategy applied in STATCOM side can stabilize DC voltage better when active power fluctuates.The LCL filter which always use in the traditional DC/AC converter is applied to the DC/DC converter of battery energy storage module. The LCL filter applied to the charging and discharging circuit of the battery energy storage module can reduce current ripple of battery charge and discharge and improve battery’s life. Through structureanalysis and control strategy research of LCL filter, this paper puts forwarda more suitable control strategy based on the filter capacitor voltage differential feedback for engineering applications, which can suppress resonance peaks and improve the stability of control system.When STATCOM/BESS DC side parallels plurality of battery energy storage modules, power balancing control strategy becomes an important research. First, this paper introduces detection of the state of charge of the battery. Then based on the state of charge of each battery energy storage, this paper analyzes the reason why equilibrium process of traditional scale factorcontrol power balancing strategy is slow and puts forward a variable scale factor control power balance control strategy, which can complete power balance faster and introduce key parameters in the control stragety.Prediction direct power control can quickly achieve active and reactive power decoupling control, but its disadvantage is that it is greatly impacted by system parameters. This paper applies recursive least squares method with forgetting factors to STATCOM/BESS parameters online identification, which can identify parameters online, incorporate results which has been identified into prediction direct power control, improve the control precision of active and reactive power and reduce the harmonic distortion of output current.Through analyzing mechanism of wind farm power fluctuations, this paper applies STATCOM/BESS to wind farm to stabilize power. Based on smoothing time constant control strategy, this paper uses nine-zone diagram comprised of the active power which should be compensated for wind farm and the level of STATCOM/BESS device storage battery’s state of charge to determine smoothing time constant. This control strategy can suppress the fluctuation of active power in wind farms as much as possible under the premise of extending the service life of the storage battery.