Coordinated Frequency Regulation using Wind Turbine and Battery Energy Storage

Wind power can participate in frequency regulation of the power system using stepwise inertial control after a disturbance by releasing rotor kinetic energy. During the stepwise inertial control, the system frequency drop caused by the disturbance can be decreased, then the buffering ability of the system to deal with disturbance is improved. However, the rotor kinetic energy cannot be released indefinitely. Once the allowable kinetic energy is released, the wind turbine (WT) will instantly reduce the output power and terminate the stepwise inertial control. In this case, the instantaneous drop of WT's output is equivalent to a new disturbance to the system, which will cause a secondary frequency drop (SFD) and threaten the safe operation of the system. Energy storage technology can quickly and flexibly adjust system power as an important controllable resource in the system. This paper proposes a coordinated control method using battery energy storage (BES) to minimize the SFD and improve the system's ability to buffer against disturbances. The variable droop control method is applied to the BES and the selection principle of droop coefficients is analyzed. Furthermore, the effectiveness of the proposed strategy is verified by case studies.