Q7Electrical Machine Design

The Short Circuit Ratio (SCR) of a synchronous machine is defined as the ratio of the field current required to produce rated voltage on open circuit to the field current required to circulate rated armature current on a sustained (steady-state) short circuit:

SCR is also related to the reciprocal of the per-unit synchronous reactance (SCR ≈ 1/Xd(p.u.), approximately, under the simplifying assumption of an unsaturated characteristic), making it a direct indicator of the machine's internal reactance.

Significance of SCR: a machine with a high SCR (low synchronous reactance) exhibits better inherent voltage regulation (less voltage drop for a given load current change), a higher steady-state stability limit (able to transfer more power before losing synchronism), and lower sensitivity to sudden load changes — but achieving a high SCR requires a larger air gap, which increases the machine's physical size, weight, and cost (since a larger air gap demands more field MMF and hence a larger field winding and pole structure) and also results in a lower per-unit short-circuit current contribution during faults being less of a concern in one sense, but a larger air gap machine can be substantially more expensive for the same power rating. Conversely, a machine with low SCR (high synchronous reactance, smaller air gap) is more compact and economical but has poorer voltage regulation and lower stability margin, being more prone to instability under large disturbances. Selecting an appropriate SCR is therefore a key economic-versus-performance trade-off in synchronous machine design, with turbo-alternators (having small air gaps for mechanical reasons) typically having lower SCR (~0.5-0.7) than salient-pole hydro-generators (~1.0-1.5), which affects the type and amount of external reactive compensation/control equipment needed in the associated power system.