Harmonic Analysis of Droop-Based Islanded Microgrids

Islanded microgrids have specific features that should be considered in their harmonic analysis. First, microgrids' distributed generators (DGs) are typically droop-based, and accordingly, the steady-state frequency is different from the nominal one. This affects the harmonics produced by DGs and nonlinear loads. Second, there is no slack bus, which affects the distribution of the fundamental currents, and thus, the harmonics. Third, DGs usually have voltage source converters (VSCs) as an interface for energy transfer to microgrids. The control structures and output filters of DGs affect the voltage and current distortions, and hence, they should be considered in the harmonic analysis. This paper introduces a sequential algorithm for the harmonic analysis of droop-based islanded microgrids that considers these features. For being accurate and yet computationally efficient, DGs' cascaded control loops are modeled in the proposed algorithm through their transfer functions. Due to its quadratic convergence, a Newton-based approach is proposed for modeling nonlinear loads that relies on an analytical Jacobian matrix to further speed up its convergence. The meritorious advantage of the proposed approach is the consideration of the mutual coupling between different harmonics. The effectiveness of the proposed algorithm is confirmed through comparative evaluations with time-domain simulations.