Software Sensor-Based STATCOM Control Under Unbalanced Conditions

A new control strategy for static compensators (STATCOMs) operating under unbalanced voltages and currents is presented in this paper. The proposed strategy adopts a state observer (software sensor) to estimate ac voltages at the STATCOM connection point. This way, physical voltage sensors are not needed and the hardware gets simplified, among other advantages. Using the superposition principle, a controller is designed to independently control both positive and negative sequence currents, eliminating the risk of overcurrents during unbalanced conditions and improving the power quality at the STATCOM connection bus. Two operating modes are proposed for the computation of the reference currents, depending on whether the objective is to compensate unbalanced load currents or regulate bus voltages. The proposed observer allows positive and negative sequences to be estimated in a fraction of the fundamental cycle, avoiding the delay often introduced by filter-based methods. Overall, the STATCOM performance is improved under unbalanced conditions, according to simulation results presented in the paper.