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 rated voltage on open circuit to field current required to circulate rated current on 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,

Short-circuit ratio,

since the triangle OAC and OBD are similar triangle,

The SCR is the reciprocal of per unit synchronous reactance Xs.The value of synchronous reactance depends upon 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 large value of synchronous reactance.

SCR also measures the transient stability of 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 increase SCR above nominal value requires large field winding which in case requires the increase in size of machine ,increases the cost.

Lower the value of SCR results in increase of Xs,results in larger voltage drop and poor voltage regulation.

With the 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 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.

Question:

Why an alternator with low value of SCR has lower limit stability?

Ans:An alternator with low value of SCR has high value of Xd

 and therefore ha slower stability limit because maximum power output of a machine is inversely proportional to Xd.