Coordinated control for distributed energy resources in Islanded microgrids with improved frequency regulation capability

With the increasing prevalence of renewable energies in islanded microgrids, wind turbine generators are becoming one of the main resources that provide frequency ancillary services by rapid and significant active power increments. Nevertheless, the substantial power increment will inevitably lead to a significant secondary frequency dip for the rotor speed restoration period and frequency overshoot for minor disturbances. This paper proposes an improved deloaded scheme to avoid the secondary frequency dip by increasing the frequency regulation reserve, which can also reduce the pitch angle adjustments. In the deloaded mode, based on the quantified frequency regulation reserve and operational limitations of wind turbine generators, an active power enhancement strategy is proposed to improve the frequency nadir by enhancing the active power contribution. The stable operation of the wind turbine generator can be ensured by varying the active power with the rotor speed. Moreover, to eliminate the frequency overshoot and improve the frequency response, a switching-based coordinated control strategy is proposed to flexibly regulate the active power outputs for wind turbine generators and distributed energy resources by switching different control strategies according to the frequency deviation. Finally, simulation results on an islanded microgrid validate the effectiveness of the proposed strategies. For instance, compared with active power reserve control, the proposed switching-based coordinated control can reduce the maximum frequency deviation by 32.2 % and 14.5 % in large load increase and decrease scenarios, respectively.