Coordinated control strategy of Grid-connected converters based on maximum power optimization during grid voltage sags

Renewable energy grid-connected converters are sensitive to unbalanced voltage, which can cause overcurrents. As a traditional control strategy, grid current is maintained within its rated limit by adjusting the current reference value. While, this method limits the OAP (output active power, OAP) of the converters and affects their reliability, which can cause OAP shortage in grid-connected system of renewable energy. To address this problem, this paper proposes a feasible and effective control strategy, which can optimize and enhance the output power of grid-connected converters. First, the toughness is defined and elaborated. Second, considering various constraints such as the grid current harmonics, power fluctuations and DC side voltage fluctuations, the operation toughness range of the converters is calculated. Based on the toughness range, an optimal operating point is selected and an optimal strategy is developed. Then, the performance of adopted PCI controller is deeply analyzed and its parameters are designed in detail. Compared with the traditional strategy, the proposed strategy allows improving the maximum active power output capacity and toughness 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, the power conversion capacity has been optimized, experimental results show the effectiveness and feasibility of the proposed strategy.