Lock-Up Problem in the Current-Limiting Mode of the Grid-Forming Converter With Priority-Based Limiter and Its Solution

As a voltage-source-type device, the output current of a grid-forming converter without a current limiter is determined by the external circuit status. When a short-circuit fault occurs in the grid, the converter may burn out because of the large current. Therefore, it's necessary for grid-forming converters to be equipped with current limiters. For the grid-forming converter employing the priority-based current limiter, this study found that the lock-up problem in the current-limiting mode may occur; that is, the converter cannot switch back from the current-limiting mode to normal mode if the fault is cleared in a relatively short period of time. This is caused by the impact of the line current dynamics. It causes the converter to enter the current-limiting mode early, making the maximum power angle smaller than the angle in which the converter can exit the current-limiting mode. A strategy to overcome this lock-up problem by reasonably setting the proportional gain of the proportional-integral (PI) controller in the voltage loop is provided, which essentially reduces the lower boundary of the angle at which the current-limiting mode can exit to be equal to or less than the fault occurrence power angle. The effectiveness of the strategy is demonstrated through numerical simulations.