Question

3) Refer to the one line diagram on the next page. Assuming copper conductor and using the Bussman method, calculate the symmetrical three-phase short-circuit current occurring at fault a F1 b. F2.

Answer 1

Sol

(a) To calculate the fault at F1:

Network Data

SYSTEM DESCRIPTION				VALUE	UNIT
Service Voltage Un				13.2	kV
Short Circuit Current at 11kV System Ik				4.37	kA
Short Circuit Apparent Power of System Sk = √3*Un*Ik				100	MVA

Transformer Data

Apparent Power Transformer Pn				2000	kVA
Utilization Current Rating In				2406	A
Impedance Uz				5.75	%
Voltage VL-L (cU'n)				480	V
Voltage VP-N				277	V

DESCRIPTION		SYMBOL		EQUATION				VALUE			REMARKS
Network
Impedance of Source Network		Zs		( cU'n/1000)^2/Sk				0.002304		Ohm	Refer IS 13234 page no 17 eq. 5a
**Voltage Factor,c of 1.1 has been considered for medium voltage (> 1KV to 35KV) as per IS 13432,Pg-41,Table-1
0 = tan-1X/R				X/R=12				1.487655095
Reactance of Source		Xs		X = Z sin e				0.002296041		Ohm
Resistance of Source		Rs		R= Z cos e				0.00019134		Ohm

Transformer
0 = tan-1X/R				X/R=5				1.373400767
Impedence of Transformer		Ztr		(VL-L)^2*Uz/((Pn*100)*1000)				0.006624		Ohm	Refer IS 13234 page no 18 eq. 6
Resistance of Transformer		Rtr		R= Z cos e				0.001299073		Ohm
Reactance of Transformer		Xtr		X = Z sin e				0.006495366		Ohm
Total Resistance		Rt		Rs + Rtr				0.00149041		Ohm
Total Reactance		Xt		Xs + Xtr				0.008791408		Ohm
Cable Impedance		Zb						0.00013		Ohm
Total Impedence		Zt		Sqrt(Rt^2+Xt^2)				0.009046848		Ohm
3 Phase Fault Just After Transformer (in kA)		Isc 3PH		VL-L/(3^0.5*Zt*1000)				30.63		kA	Refer IS 13234 page no 22 eq. 14

2) To calculate the fault at F2

225KVA transformer Data

Apparent Power Transformer Pn				225	kVA
Utilization Current Rating In				625	A
Impedance Uz				5.75	%
Voltage VL-L (cU'n)				208	V
Voltage VP-N				120	V
X/R, Ratio				1

Transformer -225 KVA
0 = tan-1X/R				X/R=5				0.785398163
Impedence of Transformer		Ztr		(VL-L)^2*Uz/((Pn*100)*1000)				0.011056356		Ohm	Refer IS 13234 page no 18 eq. 6
Resistance of Transformer		Rtr		R= Z cos e				0.007818024		Ohm
Reactance of Transformer		Xtr		X = Z sin e				0.007818024		Ohm
Total Resistance		Rt		Resitance of Network + Resistance of transformer 2000KVA & Resistance of 225KVA transformer				0.00930843		Ohm
Total Reactance		Xt		Network Reactance + 2000KVA Reacatce + 225KVA Reacatnce				0.016609432		Ohm
Cable Impedance		Zb						0.00026		Ohm
Total Impedence		Zt		Sqrt(Rt^2+Xt^2)				0.019299962		Ohm
3 Phase Fault Just After Transformer (in kA)		Isc 3PH		VL-L/(3^0.5*Zt*1000)				6.22		kA	Refer IS 13234 page no 22 eq. 14