Continuous Power Flow of Islanded Microgrids with Droop-Controlled Distributed Generations

Due to the small scale and the randomness of the power of intermittent distributed generations (DG) and loads, islanded microgrids (IMGs) are vulnerable to voltage instability problems. IMGs with droop-controlled DGs are even more weak because no master DGs are responsible for the regulation of frequency and voltage. Continuous power flow (CPF) is an effective tool for static voltage stability analysis. However, a droop-regulated has its own unique features. The steady-state model of IMGs with droop-controlled DGs are different from those of traditional power systems. Moreover, the convergence of Newton-Raphson method for the load flow equations of IMGs with droop-controlled DGs is not satisfactory. Thus, a CPF calculation method based on Newton-Raphson with optimal multipliers for droop-regulated IMGs is proposed in this paper. The Newton-Raphson method with optimal multipliers is adopted to solve the initial point and the steady state solution of the correction step. The tangent prediction method and the local parameterization method is employed in the prediction step and the correction step respectively. Vertical correction is adopted in the initial steps, and then horizontal correction is adopted when approaching the critical point. Newton-Raphson method with optimal multipliers allow to track the PV curves with a larger step in the whole process. Finally, the proposed method is applied to an IMG system modified from 33-bus test system, verifying the correctness and effectiveness of the proposed algorithm.