Constant-Coupling-Effect-Based PLL for Synchronization Stability Enhancement of Grid-Connected Converter Under Weak Grids

The phase-locked loop (PLL)-caused couplings on the grid-connected converter (GCC) are strengthened by increasing its bandwidth and then incur synchronization instability. This article proposes a constant-coupling-effect-based PLL (CCE–PLL) to resolve the aforementioned issues. Initially, the feature of the PLL-caused couplings on GCC is explored. It exhibits that the couplings are varied along with the PLL bandwidth hence jeopardizing the system's stability. Subsequently, the CCE–PLL with a constant and low coupling regardless of its bandwidth on the GCC is proposed. It illustrates that the CCE–PLL bandwidth does not influence the GCC's impedance response, bringing the GCC and CCE–PLL can be designed separately. Last, the experiments confirm that using CCE–PLL, the system has high robustness against grid impedance variation, and the GCC is capable of injecting/absorbing 1.0 per unit active power in a very weak grid. Moreover, the system attains good anti-interference ability and transient performance under system disturbances thanks to the permission usage of the high CCE–PLL bandwidth.