An Improved PLL of VSC-HVDC System under Unbalanced and DC Distorted Voltage

In long-distance transmission, the advantages of the VSC-HVDC system become more and more obvious, but on the grid-connected side, the unbalanced component and DC component contained in the grid voltage will seriously affect the phase-locking accuracy of the phase-locked loop (PLL) and thus affect normal operation of grid-connected inverter. In this paper, an improved PLL is proposed to deal with the unbalanced grid voltage and DC components. First, the principle and defects of the traditional PLL are briefly introduced. Then according to the shortcomings of the PLL, the structure of the PLL is modified to enhance its ability to suppress the DC component. By introducing an adjustment factor into the existing system, it has a greater attenuation of the DC component, and at the same time, the dynamic performance and filtering effect of the system are not lost. Finally, the correctness and effectiveness of the algorithm are verified in the simulated power grid environment.