Margin Balancing Control Design of Three-Phase Grid-Tied PV Inverters for Stability Improvement

Frequency responses are nonidentical between the positive and the negative frequency range for symmetric three-phase systems modeled with complex transfer functions, which shows unbalanced stability margins. The unbalanced stability margins limit the system performance, as the system dynamics are restricted by the worst one. This article extends the discussion of unbalanced stability margins to asymmetric three-phase systems, i.e., three-phase grid-tied inverters, which are described by transfer matrices. The reason for unbalanced stability margins is analytically investigated, and different impacts are analyzed. It is found that both the control delay and asymmetric control loops can lead to the unbalanced stability margins. To improve the performance limited by the worst stability margin, a margin balancing control is proposed. By implementing a phase correction term, the worst stability margin can be increased by reducing the best one. As a result, the overall stability margin of the system can be improved. The design procedure is elaborated based on a 2-MW inverter system. Verifications are presented based on a 1-kW prototype including simulations, experimental results, and Nyquist analyses. This method applies to all types of grid-tied inverters with multiple control loops.