Power system strength assessment with high penetration of inverter-based resources-A conceptual approach

The power system strength has been conventionally evaluated by finding the short circuit ratio (SCR) at nodes of a power network. In systems with inverterbased resources (IBRs), the short circuit index is still usually used although several other indices are defined to improve its accuracy for systems with IBRs. However, even these indices do not represent-to a good accuracy-the actual value of the system strength for a system with many IBRs, which have the capability of injecting reactive power to the network in addition to absorbing reactive power from the network. There is obviously a pressing need for finding a new metric for correctly evaluating the system strength in modern power systems dominated by IBRs such as wind farms, solar power plants with storage, other renewable energy sources, microgrids, and active distribution networks. Although this can be done by detailed system modeling through EMT-based simulations, but there is a need for a metric that can evaluate system strength with a good approximation without having to do detailed modeling, similar to the way that SCR can be easily calculated. This chapter discusses the concept of power system strength in modern power grids in contrast with the system strength in conventional systems and provides the big picture of this concept. As such, it serves as an introduction to other chapters of this book as each of them will discuss some specific aspects of the power system strength notion for the power systems with IBRs.