AC/DC system strength as indicated by short circuit ratios

Although from the very early HVDC schemes the designer had to contend with effects of ´weak´ AC systems, a good understanding of the interactions between AC and DC systems has been established only over the last few years. The ´strength´ of an AC system is represented by its impedance and its mechanical (rotational) inertia. System strength expressed as an absolute numerical value, such as short circuit MVA, is useful only if compared to the power and reactive power values of its load. The short circuit ratio (SCR), defined as the ratio of system short circuit level MVA to the DC power MW, has been used to indicate system strength. A brief review of the history of short circuit ratios is given. The reasons for introducing effective, operational and critical short circuit ratios and the influence of the convertor reactive power on these ratios are considered. The effects of AC systems inertia on operation of HVDC are also discussed.