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.