The Fault Analysis Method of Islanded Microgrid Based on the U/f and PQ Control Strategy

The fault calculation of microgrid is an important method for fault characteristic analysis. The traditional fault analysis methods ignore the connection of inverter interfaced distributed generation (IIDG) and are not suitable for microgrids. A new fault analysis method suitable for the master-slave controlled microgrid is proposed in this article, which considers the control strategy of IIDGs, low voltage ride through (LVRT) and the fault current limiting. The fault models of PQ-controlled IIDG (PQ-IIDG) and U/f-controlled IIDG (U/f-IIDG) are analyzed, then an equivalent Norton model of microgrid is established, and a new fault calculation method is presented. By analyzing the fault models, the working state of U/f-IIDG is divided into a constant voltage state and a constant current state. In the constant voltage state, a grid-connected microgrid fault calculation method is adopted. In the constant current state, the improved nodal voltage equation based on the current limiting is adopted. Without complicated calculation, high precision results can be obtained after simple iterative calculation. The results of the microgrid simulation model established in PSCAD/EMTDC and the fault calculation method programmed on MATLAB are compared to verify the effectiveness and accuracy of the proposed fault analysis method.