DQ-axis current-based droop controller

During an islanding operation of a microgrid comprised of multiple distributed generation (DG) units serving a load, DG units interfaced with voltage-sourced converters (VSC) are regarded as the only energy sources. Hence, DG units should be controlled to form the frequency and voltage, pick up the full load, and respond to any load changes. Conventional P — f/Q — V droop controller enables a communication-less load sharing among DGs while maintaining close deviations of frequency and voltage. Utilizing the dq-frame power flow analysis, this paper presents an equivalent primary droop controller id — f/iq — V based on the direct and quadrature components of the feeder currents, which are already existent in each voltage controller for disturbance rejection. More emphasis is placed on the phenomenon of inaccurate reactive power sharing caused by mismatched feeder impedances. Analysis is presented in terms of the droop equation, and compensation methods are examined with focus on computation and communication requirements, and PCC voltage profile. For validation of the controller's response, a testbed model exhibiting the dynamics of a two-DG microgrid is developed and simulated in Matlab/Simulink.