Short circuit calculation

Making Short-Circuit Calculations Easy

A general rule of thumb would be:

Transformer short circuit current =

Transformer Full Load Current / Percentage Impedance of the Transformer

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Dear Sir,

I present the simple short-circuit calculation for transformer.

Base MVA = 1 MVA, and Base Voltage = 480V

I base at 480 V =

I_B = 1MVA/(

V_B = 1 p.u

Assumed there are motors connected on LV system.

I_M-RMS = V_B/Xm= 1/(0.20) = 5 p.u.

I_M-RMS actual = I_M-RMS × I_B = 5 × 1202 = 6010 Amperes = 6.01 kA.

I_M-RMS is zero if there is no any motor operated on this system.

X_SC source = (MVA base)/(MVA available) = 1 MVA/500MVA = 0.0020 pu.

X_SC network = X_T + X Source = .065 + 0.002 = 0.0652 p.u.

I network = 1/(0.0652) = 15.32 p.u

I actual = 15.32 × 1202 A = 18435 A = 18.44 kA.

Fault current at point of fault : I network + I_{M RMS} = 18.44 kA + 6.01 kA = 24.45 kA.

So, 30 kA bus rating selected is meet the requirements.

Dear Sir,

I am sorry the equation cannot be accepted on this picture, so;

I base at 480 V= [1000000/(√3 x 480)] = 1,202 A.

I have a software for short circuit calculation. the software is created myself. you can download from my website

www.shakhawathossain.yolasite.com