A Unified Control Scheme Based on a Disturbance Observer for Seamless Transition Operation of Inverter-Interfaced Distributed Generation

A generalized hierarchical structure is proposed for inverter-interfaced distributed generation in a microgrid that enables both grid-connected and islanding modes with no need of forced switching between district controllers. The proposed control scheme consists of a fixed voltage reference generation algorithm and a modified voltage-current controller, where the former is based on droop technique to accommodate all operation modes, and the latter aims for an offset-free voltage control. To improve the control dynamics of the well-accepted cascaded voltage-current method, a disturbance observer based feedforward scheme is proposed by introducing the estimation of fluctuations into the reference of the inner current loop, which does not need remote measurements and communication in existing schemes. Based on the proposed strategy, disturbance effects both in structure and external power are remarkably attenuated. Details on the design of the observer and controllers as well as the robustness and sensitivity to parameter perturbation are provided. The effectiveness of the control scheme is verified by the simulations of an island demonstration project and experimental results.