Modeling and Design of Voltage Support Control Schemes for Three-Phase Inverters Operating Under Unbalanced Grid Conditions

Three-phase inverters equipped with voltage support control schemes have been successfully used to alleviate the negative impact of voltage imbalance on electric power systems. With these schemes, dc-link voltage ripple and current harmonics are significantly reduced by processing the positive and negative sequence components separately. However, the design methods for tuning the parameters of these control schemes have one or more of the following limitations: 1) the design is a very time-consuming task, 2) it is conservative, 3) it does not guarantee the specifications for all the considered situations, and 4) the system can be unstable in some abnormal conditions. As an alternative, this paper presents a design method based on the analysis of oscillations in nonlinear systems. The method proceeds by first developing simple and accurate models of the power system, second it reveals the system features through an in deep analysis of the derived models, and third it introduces a systematic design procedure for tuning the parameters of the control schemes. As an example, a voltage support control scheme for a three-phase inverter operating under an unbalanced voltage sag is designed and validated experimentally.