Grid-Forming Control for VSC-HVDC Links Connected to Weak Grids – a Comparison Study to Vector Current Control

Power-synchronization control (PSC) is a previously proposed grid-forming control for grid-connected voltage source converters (VSCs). A prominent feature of the PSC is that the VSC synchronizes with the grid through a transient transfer of power instead of the traditional phase- locked loop (PLL). By such a control strategy, the VSC mimics the dynamic behavior of a synchronous machine. The PSC is particularly useful for VSC-HVDC links connected to a weak grid, or an island system. This paper intends to demonstrate the advantage of the PSC comparing to the traditional vector current controller (VCC) for VSC-HVDC links connected to a weak grid. By a mathematical equivalent modeling of a VSC using either PSC or VCC, the paper shows the inherent difficulty of the VCC when the short circuit ratio (SCR) of the ac grid becomes low, while the PSC can maintain relatively stable operation for a weak grid with a SCR as low as 1.0.