Improved Control and Stability Analysis of a Microgrid Connector Controller Under Unbalanced Network Conditions

Abstract

The microgrid connector controller has been shown to be a more cost-effective alternative for a microgrid-grid interface than a back-to-back connector. This paper proposes an improved control system for this controller under unbalanced operation, as distribution feeder and microgrid loads are normally unbalanced. The controller regulates the utility interface while minimizing the impact of load unbalancing on both the utility feeder and the microgrid. An improved reference current generation strategy is designed to suppress the fluctuations of the dc link voltage, thereby preventing them from being introduced into the control system and affecting the controller operation. Furthermore, a four-stage start-up strategy is proposed to avoid an external dc source for charging the controller’s dc link capacitor, making the interface more practical and cost-effective. The paper also presents a comprehensive investigation of the effects of the controller parameters and microgrid load unbalance on the small-perturbation stability of grid-connected microgrids. The performance of the presented controller is compared with its original controls, a back-to-back with existing unbalance control techniques, and a switch interface through detailed simulations in a benchmark test system. The results illustrate that the proposed controls can minimize the impact of feeder and microgrid unbalancing by eliminating the negative-sequence components and reducing the fluctuations in the transferred powers and dc link voltage, showing similar overall performance to a back-to-back interface.