Short Circuit Calculations

The short-circuit current must be calculated at each level in the installation in view of determining the characteristics of the equipment required to withstand or break the fault current.

-Two values of the short-circuit current must be evaluated

1-The maximum short-circuit current, used to determine:

A-The breaking capacity of the circuit breakersB-The making capacity of the circuit breakers

2-The minimum short-circuit current, essential when selecting the time-current curve for circuit breakers and fuses, in particular when:

A-Cables are long and/or the source impedance is relatively hig

B-Protection of life depends on circuit breaker or fuse operation

Short-circuit Type Faults

3- Short-circuit Type Faults with Fault Impedance

Types of Faults

A-Symmetrical Faults

B- Unsymmetrical Faults

short-circuit (Isc) calculation procedure when designing an electrical installation.

Basic methods on short circuit calculations.

- The Direct method- The per unit method- The software method- The point-to-point method- MVA method

Procedure

Step 1X utility = (kV) 2 / MVASCStep 2X transformer = X P.U * (KV) 2 / MVAratingR transformer = R P.U* (KV) 2 / MVAratingStep 3The impedance (in ohms) given for current transformers, large switches and large circuit breakers is essentially all X, but these impedances are negligible in the substation

A-Direct method

Step 4Determine X T.L and RT.L

Step 5X motor = X P.U * (KV) 2 / MVAratingR motor = R P.U* (KV) 2 / MVArating

Step 6Determine impedance (in ohms) of the system by:

ZT = (RT2 + XT2)

Step 7Calculate short-circuit symmetrical RMS amperes at the point of fault. IS.C. sym RMS = V line-line 3(ZT)Step8Calculate (X/R) ratioStep 9Calculate - Instantaneous Peak Current = Ip =Is x Mp - Asymmetrical RMS Short Circuit Current = Ia = Is x Mm Step 10Calculate ;

MVA breaking (sym.) =3 * V line-line * IS.C. sym RMSMVA breaking (asym.) =3 * V line-line * IaMVA making = 3 * V line-line * Ip

Results of calculations

Summary of Short Circuit Calculations (Direct method) Faulted

BusItem 220 kV

Bus Bar66 kV

Bus Bar20 kV

Bus Bar

I sym (kA)9.51410.04221.29

I asym (kA)14.88915.46531.349

I peak (kA)24.9726.02553.2995

MVA breaking sym3623.5301147.89737.5

MVAbreaking(asy)5778.262040.071085.96

MVA Making 9739.6683448.321846.34

B-Software method (using Easypower program) 1) three phase faults (L-L-L):

2) single line to ground faults (SLG):

3) line to line faults (L-L):

4) line to line to ground faults (L-L-G):