Short circuit ratio (electrical grid)

In an electrical grid, the short circuit ratio (or SCR) is the ratio of the short circuit apparent power (SCMVA) in the case of a line-line-line-ground (3LG) fault at the location in the grid where some generator is connected to the power rating of the generator itself (GMW). Since the power that can be delivered by the grid varies by location, frequently a location is indicated, for example, at the point of interconnection (POI):

${\displaystyle SCR_{POI}={\frac {SCMVA_{POI}}{GMW}}}$

SCR is used to quantify the system strength of the grid (its ability to deal with changes in active and reactive power injection and consumption).^[1] On a simplified level, a high SCR indicates that the particular generator represents a small portion of the power available at the point of its connection to the grid, and therefore the generator problems cannot affect the grid in a significant way.^[2] SCMVA is defined as a product of the voltage before the 3LG fault and the current that would flow after the fault (this worst-case combination will not happen in practice, but provides a useful estimation of the capacity of the circuit). SCMVA is also called a short circuit level (SCL),^[3] although sometimes the term SCL is used to designate just the short-circuit current.^[4]