Comparative Analysis of System Strength Indices in Weak Electrical Grids

Short-circuit strength of the present power grid is subjected to large variations due to the high penetration of renewable energy generation. Due to the complex behavior of power electronic-based inverters, they are unable to supply a large short-circuit current compared to conventional synchronous generators. Therefore, innovative methods are required to characterize the grid strength under fast evolving conditions. The generic short circuit ratio is unable to interpret complex grid conditions accurately. Hence, this paper presents a comparative analysis of advanced short-circuit ratio methods proposed in published literature and analyzes them using the New England (NE) 39-bus system. The synchronous generator dominated NE 39-bus system is updated with high renewable penetration to understand the effects of inverter-based resources on existing system strength indices. Two wind farms (496 MW & 248 MW) and a single solar farm (40 MW) are integrated into the system to represent renewable energy generation. Short-circuit power levels at each connection point are obtained following the IEC-60909-0 calculation standard. The study analyzed the composite short-circuit ratio (CSCR), weighted short-circuit ratio (WSCR), short-circuit ratio with interaction factor (SCRIF) and site-dependent short-circuit ratio (SDSCR). The results are validated using time-domain dynamic simulations. The advantages and drawbacks of these advanced short-circuit ratios, and the context under which they can be best applied are discovered in this study.