VSC-HVDC control without PLL for unbalanced grid

When unbalanced faults such as single-phase voltage drop occur in the power grid, VSC-HVDC will fail or become unstable due to sudden changes in amplitude, phase, frequency, etc. of the power grid voltage. Therefore, the role of PLL in a non-ideal power grid is particularly important, but often PLL needs to make a compromise between dynamic response and system stability. Therefore, this paper proposes a free-PLL control strategy under non ideal power grid. First, according to the power transmission model of VSC-HVDC under power grid imbalance, the control command required by the inverter is given, which is determined in the environment free-PLL. Therefore, PLL is not required to retrace the phase of the grid voltage. In addition, in order to overcome the impact of the sudden change of power grid frequency on the free-PLL strategy, a frequency locking strategy is proposed in the free-PLL environment, the response time is about 1 grid cycle. Finally, in order to verify the effectiveness and correctness of the proposed strategy, the VSC-HVDC system’s active power transmission stability is guaranteed when the grid voltage encounters different degrees of single-phase sags.