Analysis of the Transition Between Multiple Operational Modes for Hybrid AC/DC Microgrids

There are four basic operational modes for the hybrid AC/DC microgrid, including AC grid-connected while interconnecting, both off-grid while interconnecting, AC grid-connected without connection, and both off-grid without connection. How to achieve a seamless operational mode transition is an urgent technical need to overcome. First, this paper describes the typical structure of the hybrid microgrid, and places a detailed focus on the power balance and transition strategy. Secondly, it takes the master-slave control structure as an example, and designs the transition logic for different operational modes, and then a method for selecting the slack bus and transition time-sequence is proposed. Based on the different roles that the interlinking converter (IC) plays in the process of modes transition, a voltage-power (U-P) control method for a hybrid AC/DC microgrid is proposed, and the exchanged power is calculated based on the voltage deviation between the rating value and measured value. Finally, a control flowchart for the transition between the four operational modes in transition is designed. Using the PSCAD/EMTDC platform, this paper takes a typical seven-point microgrid structure as an example, the proposed transition strategy is carried out, and the results show that the transition method and time sequence can achieve smooth transition between different operational modes.