Effect of Local Load on Stability of PLL-Synchronized Converters in Weak Grids

This paper investigates the effect of local load on the ability of the phase-locked loop (PLL) of Voltage Source Converters (VSCs) to resynchronize after a grid fault under weak grid conditions. While it has been stated that local consumption is generally beneficial for VSC stability, it has also been recognized that there is a lack of specific analyses of this subject. In this paper the positive effect of local load of the active power delivered by the VSC is illustrated through electromagnetic transients (EMT) time-domain simulations in small test systems with very low short-circuit ratio (SCR), inspired by the interconnection of Crete to the Greek mainland system. The stabilizing effect can be mainly attributed to the lower active power exchanged with the AC system, and, thus, lower sensitivity of the converter voltage angle to the error caused by the PLL frequency deviation during the transient to the injected current phase. The results reinforce the general acknowledgment that SCR-based metrics should be used as screening tools, as weak grid issues depend on the specific system and equipment.