What is short-circuit ratio (SCR)?
The short-circuit ratio(SCR) of a synchronous machine is defined as the ratio of field current to produce a rated voltage on the open circuit to field current required to circulate rated current on a short circuit while the machine is mechanically driven at synchronous speed.
From OCC (open circuit characteristics) and SCC (short-circuit characteristics). According to the definition we have,
since the triangle OAC and OBD are similar triangles,
The SCR is the reciprocal of per unit synchronous reactance Xs. The value of synchronous reactance depends upon the saturated conditions of the machine but SCR is specific and defined at rated voltage.
Significance of short-circuit ratio
A small value of SCR indicates a smaller value of current under short-circuit conditions owing to the large value of synchronous reactance.
SCR also measures the transient stability of the unit. If SCR increases Xs(synchronous reactance) decreases and hence the power capability increases,
i.e Higher the ratio provides greater stability.
A generator with increased SCR above the nominal value requires large field winding which in this case requires an increase in the size of the machine, increasing the cost.
Lowering the value of SCR results in an increase of Xs, resulting in larger voltage drops and poor voltage regulation.
With a high value of SCR, stability limit increased, voltage regulation improved and machines with a low value of SCR have difficulty during parallel operation owing to the small value of synchronizing power.
The SCR of a high-speed turbo-alternator usually lies between 0.5 and 0.75 while that of low-speed salient pole generators lies between 1.0 and 1.5.
Why an alternator with a low value of SCR has lower limit stability?
Ans: An alternator with a low value of SCR has a high value of Xd
and therefore has a slower stability limit because the maximum power output of a machine is inversely proportional to Xd.