VSC transmission limitations imposed by AC system strength and AC impedance characteristics

The paper investigates power transmission limitations in a VSC-HVDC system imposed by the ac system's impedance characteristics. An important observation from the study is that the operation of the converter is affected, not only by the magnitude of this impedance (quantified by the Short Circuit Ratio or SCR), but also significantly by its angle. An interesting result is that the larger the resistive component (i.e. impedance angle less than 90°), the larger is the minimum SCR required at the rectifier. The trend at the inverter end is the opposite, and the inverter is able to operate into systems with smaller SCR. Although operation is theoretically possible at the analytically calculated minimum SCRs, the paper shows that as the ac system approaches this value, the gains of the phase locked loop (PLL) used for angle reference generation must be reduced.