Study On Control Strategies Of Battery Energy Storage System In Microgrid

As a effective form to take use of renewable energy and distributed power generation, microgrid technology is becoming the focus of power system research in recent years. Microgrid which consists of micro sources, energy storage devices, power electronics converters, load and control system, can either be connected to distribution grid as a special load, or disconnected from the utility to operate in islanding mode. Due to the smaller capacity and inertia, microgrid is vulnerable to the fluctuations of the distributed power and load, so it is faced with many problems such as the large intermittency and fluctuations of the power output, complex network flow calculation, difficult relay protection and stability control. With the development of energy storage technology, effective ways are provided to solve these problems and make energy storage technology become the important part of microgrid.In the existing energy storage form, Battery Energy Storage System is more and more applied in microgrid because of its matured technology, low cost and high reliability. Power balance is a decisive factor of the stability of the system in microgrid. Taken Battery Energy Storage System as the research object, its control strategies of constant power charge and discharge in microgrid and the key supporting role for microgrid were studied in this paper.According to the requirement of energy management system in microgrid, the whole design plan of constant power charge and discharge for the battery energy storage system was proposed. On the basis of comparative analysis, charging and discharging circuit of three-phase voltage source PWM converter was determined, operation mode was analysed, and overall control strategy of the constant power charging and discharging was established.Getting accurate information of voltage phase was the precondition for energy storage system to control constant power charging and discharging. In view of imbalance of three phase voltages that may be emerged in microgrid, and. based on the comparison with a variety of phase locked loops, the application of the three-phase software phase locked loop technology used in three-phase voltage source PWM converter was researched. The effect of phase locked loop in the three-phase voltages balance and imbalance were compared through the simulation, the results show that, the three-phase software phase locked loop designed in this paper can follow the change of accurate phase, when the given frequency changes and the feasibility was proved through the experiments.The control of battery energy storage system corresponds to the control of three phase voltage source PWM converter. To accomplish the constant power control of battery energy storage system, first, the mathematical models of the voltage source PWM converter in three-phase static coordinates and in rotary two-phase coordinates were offered in this paper, the current calculation formulas in control of constant power charging and discharging were derived; Second, the feed-forward decoupling control mode was used, the current regulator was designed, the parameters of regulator was determined by the engineering design method. SVPWM space vector modulation was introduced in the switch control to get a better control performance. Based on the design above, the simulation models established in PSEM proved that constant power charge and discharge control of battery energy storage system could be realized through controlling three-phase voltage source PWM converter,In order to validate the supporting action of battery energy storage system in microgrid, first, microgrid simulation model consisted of de voltage source as equivalent simple micro power was established, and the actual operation mode of the power grid was simulated. The results of simulation show that, the battery energy storage system can effectively adjust the balance of power in microgrid, and keep voltage and frequency constant when the load was changed. Second, microgrid simulation model mainly comprised of the wind generator, diesel generating system and battery energy storage system was perfected for further consideration of the actual working condition, The situations of energy storge system connected to the microgrid and disconnected from the microgrid were simulated respectively by this model, the results of the study show that, system stability can be maintained effectively by energy storage system.