What is the significance of short circuit ratio?

Atta,

It is one measure of the relative transient stability of a synchronous generator. It is defined as the ratio of field current required to produce rated armature voltage at no load to the field current required to circulate rated armature current with the armature short circuited.

Most electrical engineers never encounter the term unless they are taking an advanced AC machines course, even fewer ever use it during their career.

Alan

It is the ratio of field current required to produce open circuit voltage at the armature terminals to the field current required to produce current in the armature with the armature terminals short-circuited. Ioc;Isc or Ioc/Isc

This is similar to a normal short-circuit, It can be easily understood by looking at a normal 120V light circuit @ .5Amp suddenly short-circuited and voltages after the short becomes 1V and current reaching a high of 600Amps +

RGDS

Thanx , but what is the effect of SCR if increases or decreases? According to grid code it value must be greater than 0.5

Increasing or decreasing means smaller or larger machine/alternator , which is directly proportional to stability of the system during a fault. the maximum is 0.6. so 0.4, 0.5 or 0.6.

Now you will realize that this is not something you change by the touch of a button.

RGDS

Thanx Mr. Norman,
I mean during design if the ratio kept on higher or lower side.In other words, if one alternator having 0.5 and the other 0.6 SCR, which one is better and how? Waiting for your response , Atta

0.6 provides betters stability, however, system stability is not just based on this factor, it is also based on a number of other factors such as speed.

It would be best for you do a more in-dept research on the subject other than this forum.


RGDS

The definition of short circuit ratio given here is correct, but remember that higher short circuit ratio implies that you have lower reactant , which means that higher fault currents. Hence you improve stability by higher short circuit ratio , while sacrificing safety during faults. Generators with higher short circuit ratios (often  large hydro generators) shows immense ability to work in the leading power factors with larger load angles, absorbing larger reactive powers. Such generators have the ability of line charging  of  long lines, with greater stability .  

short circuit ratio relates to excitation requirements for stable operation.

A higher SCR makes the machine inherently more stable with less excitation, but results in a more expensive machine design.  A lower SCR requires a better excitation system to preserve stability, but allows a smaller, less expensive generator design.

P.Kundur's book, Power System Stability and Control provides a good, but brief, treatment of this in Chapter 3.