Probabilistic voltage quality evaluation of islanded droop-regulated microgrid based on non-intrusive low rank approximation method

Islanded microgrids (IMGs) are more vulnerable to voltage quality issues caused by the integration stochastic characteristics of intermittent distributed generations (DGs) owing to the small scale. Meanwhile, traditional load flow model is not suitable for decentralized droop-regulated IMGs considering their specific features, i.e. the absence of slack bus and variation of frequency. Therefore, a probabilistic voltage profile evaluation method for decentralized droop-regulated IMGs based on the canonical LRA method is proposed in this paper. Firstly, the deterministic load flow models of both balanced and unbalanced droop-regulated IMG are built and Newton-Raphson method is applied for solutions taking angular frequency as an unknown. Then, step size optimization is employed to improve the convergence performance of Newton-Raphson method, especially for load flow of the unbalanced IMGs. Finally, to evaluate the impact of uncertainties of the intermittent DGs and loads on voltage profiles in droop-regulated IMGs, probabilistic load flow calculations based on a non-intrusive low-rank approximation (LRA) method are performed. The response of voltage profile is represented statistically-equivalently by a sum of rank-one functions based on a small number of deterministic load flow calculations, The probabilistic characteristics of voltage quality metrics including mean value, standard deviation, limit violation probability, PDFs and CDFs of node voltage as well as three-phase voltage unbalance factor (VUF) can be evaluated analytically and accurately without evaluating a large number of samples. Test results on balanced and unbalanced IMGs demonstrate the effectiveness and accuracy of the proposed method.