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Short Circuit Manual Calculation

The document summarizes the manual calculation of short circuit current for a 110kV grid connected to a 45MVA transformer with a 33kV output. It provides the input parameters and calculates the per unit impedances and angles for the grid and transformer individually. It then combines the impedances to calculate the total short circuit current on the 33kV side as 6.179281kA.

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Sridharan Priyadharsen
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373 views3 pages

Short Circuit Manual Calculation

The document summarizes the manual calculation of short circuit current for a 110kV grid connected to a 45MVA transformer with a 33kV output. It provides the input parameters and calculates the per unit impedances and angles for the grid and transformer individually. It then combines the impedances to calculate the total short circuit current on the 33kV side as 6.179281kA.

Uploaded by

Sridharan Priyadharsen
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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GOWTHAM MAHENDRAN

Short Circuit Calculation


Input Parameters:

Grid Transformer:

Voltage 110 kV MVA 45


X/R 14 HV Side Voltage 110 kV
Isc 40 kA LV Side Voltage 33 kV
X/R 45
Z 12.5

ETAP Calculation:

Manual Calculation:

Grid:

MVAsc = sqrt(3)*V*I = 1.732*110*40 = 7621.024 MVA

Zpu = MVAbase/MVAactual = 100/7621.024 = 0.013122

Z^2 = R^2+X^2

Z = SQRT(R^2+X^2)

Z = R*SQRT(1+(X/R)^2)

Rpu = Z/SQRT(1+(X/R)^2) = 0.013122/SQRT(1+(14)^2) = 0.000935

R% = 0.0935

Xpu = 14*Rpu = 14*0.000935 = 0.01309

X% = 1.309
GOWTHAM MAHENDRAN

Angle = tan-1(X/R) = tan-1(14) = 85.914°

Transformer:

MVA 45
HV Side Voltage 110 kV
LV Side Voltage 33 kV
X/R 45
%Z 12.5 Zpu = 12.5/100 = 0.125

Z = R*SQRT(1+(X/R)^2)

Rpu = Z/SQRT(1+(X/R)^2) = 0.125/SQRT(1+(45)^2) = 0.002777

%R = 0.002777*100 = 0.2777

Xpu = 45*Rpu = 45*0.002777 = 0.124965

%X = 0.124965*100 = 12.4965

Angle = tan-1(X/R) = tan-1(45) = 88.72697°

Kt factor = 0.95*Cmax/(1+0.6Xt) = 0.95*1.1/(1+0.6*0.124965) = 0.972112

Rpu @ 45 MVAbase= Rpu * Kt factor = 0.002777*0.972112 = 0.0027

Xpu @ 45 MVAbase= Xpu * Kt factor = 0.124965*0.972112 = 0.12148

Zpu(new) = Zpu(old)*(MVAnew/MVAold)*(kVold/kVnew)^2

Rpu @ 100 MVAbase = Rpu(old)*(MVAnew/MVAold) = 0.0027*(100/45) = 0.006

Xpu @ 100 MVAbase = Xpu(old)*(MVAnew/MVAold) = 0.12148*(100/45) = 0.269956

As per IEC 60909, for calculating LV side Isc and Angle, we need to add the source and transformer R
and X parameters separately.

Rtotal = Rpu(grid) + Rpu(transformer) = 0.000935 + 0.006 = 0.006935

Xtotal = Xpu(grid) + Xpu(transformer) = 0.01309 + 0.269956 = 0.283046


GOWTHAM MAHENDRAN

Z = SQRT(R^2+X^2) = SQRT(0.006935^2+0.283046^2) = 0.283131

If = Ifsc actual/Ifsc base

If = V/Z = 1/0.283131 = 3.531934

Ifbase = MVAbase/SQRT(3)*V = 100/(SQRT(3)*33) = 1.749546 kA

Ifsc actual = If * Ifsc base = 3.531934*1.749546 = 6.179281 kA

Ifsc = Ifsc actual * C-factor = 6.179281*1 = 6.179281 kA

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