## Short Circuit Ratio Calculation

## Short Circuit Ratio Calculation

(OP)

Not familiar with short circuit ratio (X/R) of synchronous generator calculation.

Given generator reactances (Xd, X'd, X''d, ....), time constants, stator/rotor resistance, etc.

How would on compute the short circuit ratio ? (if it is possible with the above info.)

Thanks

Given generator reactances (Xd, X'd, X''d, ....), time constants, stator/rotor resistance, etc.

How would on compute the short circuit ratio ? (if it is possible with the above info.)

Thanks

## RE: Short Circuit Ratio Calculation

The short circuit ratio (SCr) is different than the X/R ratio. For generator applications, the SC ratio is the reciprocal of the synchronous reactance Xs

SCr = 1/Xs.

Please note that for SC purposes, the sub-transient reactance( X”d) should be used in the calc.

I hope this could help.

## RE: Short Circuit Ratio Calculation

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Example data sheet,

Xd = 2.6265 p.u

Xd = 1.6625 ohms

X''d = 0.1414 p.u

X''d = 0.0895 ohms

short circuit ratio = 0.4947

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From the info. on the generator data sheets (i.e. Gen. resistances (stator, field), reactances, time constants), can I determine the X/R ratio ?

Thanks again

## RE: Short Circuit Ratio Calculation

Field Amps for producing rated stator voltage on open circuit/Field Amps for producing rated stator current on three phase short circuit

For 2 pole turbogenerators, it is about 0.5 and for salient pole machines, it varies from 0.9 to 1.1

## RE: Short Circuit Ratio Calculation

MG Say (Performance and Design of AC machines) says that the unsaturated reactance is not affected by the operating conditions and hence quoted in the data sheets and is higher than 1/SCR. SCR is defined as reciprocal of synchronous reactance (saturated).

It appears the problem is you are looking at unsaturated reactance and the SCR in the datasheets of generator manufacturer and that explains the discrepancy you are finding. If you can obtain the saturated reactance value from the vendor, it should clear the air.

(the reactance values shall be in PU)

Trust the above is helpful.

"The un

## RE: Short Circuit Ratio Calculation

The AVR often has a short circuit maintenance facility which increases the excitation to maintain terminal volts under fault conditions in order to allow the protection to operate.

Just looking at the electromagnetic characteristics of the machine will not give you the whole picture.

_______________________________________

Regards -

Colin J Flatters

Consulting Engineer & Project Manager