An Analytical Method to Obtain Maximum Allowable Grid Support by Using Grid-Connected Converters

Recently, supporting the grid voltage and proper operation of the grid-connected converters (GCCs) under a wide range of grid voltage conditions have become major requirements. An analytical study is very useful for evaluating the supporting capability of the available control strategies in GCCs. This paper analytically studies, then modifies the supporting capability of three existing strategies. The contribution of this paper is two-fold: first, analytical expressions of instantaneous active/reactive powers oscillation and maximum phase currents are formulated and used to conduct several comparisons among different strategies. Second, based on the obtained formulas for the maximum phase currents, maximum allowable support (MAS) control schemes are proposed under unbalanced voltage conditions. The MAS control schemes have two important objectives: obtaining maximum active or reactive power delivery and simultaneously respecting the maximum phase currents under the unbalanced condition. The proposed equations can further estimate the maximum depth of the faulted voltage where each strategy is still able to satisfy the voltage support requirements imposed by the grid codes. The proposed expressions can also help all techniques to provide their maximum voltage or frequency support under the pre-set maximum phase current limitations. Different selected simulation and experimental tests are carried out for comparing the strategies, and validating the effectiveness of the proposed MAS equations.