Resilience analysis and optimal control of grid-connected converters under unbalanced voltage conditions

The operational stability of grid-connected converters is affected by unbalanced grid voltage, which often leads to overcurrent. The traditional control method maintains the grid-into current within the rated limit by adjusting the current reference value. However, this method limits the output active power of the converters and affects their reliability, which can cause active power shortage or even disconnection of the power grid. To deal with this issue, this paper proposes an effective control strategy. First, the interaction between AC and DC sides of the converters under unbalanced voltage conditions is analysed. Second, the resilience is defined and elaborated. Third, considering various constraints such as the grid-into current harmonics, power fluctuations and DC side voltage fluctuations, the operation resilience range of the converters is calculated. Based on the resilience range, an optimal operating point is selected and an optimal strategy is developed. Compared with the traditional strategy, the proposed strategy allows improving the maximum active power output capacity and resilience of the grid-connected converters under unbalanced grid conditions, while simultaneously limiting the power fluctuations and grid-into current distortions within an allowable range. Last, simulation and experimental results show the effectiveness and feasibility of the proposed strategy.