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Short Circuit Current Calculations – Infinite
Bus Method
The short circuit is essentially an abnormal condition within a power system in which a heavy
amount of current flows through the circuit.
A short circuit usually occurs as a result of a fault in a power system. The fault may be a
conductor breaking and falling to the ground, or two or more electrical conductors coming in
contact with each other.
Such faults result in the formation of a low resistance path for the current. This is a short circuit
condition.
That is why short circuit calculations are mandatory as well as recommended practice of
electrical preventive maintenance according to NFPA. (NFPA 70B chapter 9)
Impact of Short Circuit Current
A short circuit is followed by a flow of extremely high current known as short circuit
current. The high magnitude of the short circuit current makes the working
environment critically dangerous.
The excessive heat generated from the high current causes the conductors to burn or catch fire.
Not only these currents damage equipment like generators, motors, and other electrical
appliances but can also burn the motor windings.
Another hazardous effect of short circuits is arc flashes that destroy the equipment and can
prove lethal to surrounding people and equipment.
It is, therefore, necessary to perform short circuit calculations in order to be prepared for an
unfortunate short circuit event.
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Having adequate knowledge of short circuit current helps in determining the protection of a
system before an incident occurs.
Infinite Bus Method
A simple method for the approximation of short circuit current is the infinite bus short circuit
calculation method.
This method calculates the worst possible or maximum current that propagates from the
transformer in case of a short circuit. We get maximum value because the source and any
other impedances are ignored or considered equal to zero except the transformer impedance.
The transformer impedance plays a vital role in the calculation of SCC as it limits the maximum
permissible SCC which can be transferred to the LV side.
Data required:
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The infinite bus calculations are performed across a 3-phase transformer in a power system.
Therefore, we should have the data on the transformer KVA rating, primary and secondary
voltage, and percentage impedance. This data can be easily obtained from the transformer
nameplate.
Calculation
This calculation is performed in two simple steps which are as follows:
Step 1:
Calculate the full load ampere (Current) rating at the transformer secondary.
FLA secondary =
=
(KVA3 phase)
-----------------KVL-L x √3
Where :
FLAsecondary = Secondary Full Load Amps
KVL−L = Secondary Voltage in kV
KVA3 phase = Transformer Three Phase kVA
Step 2:
Calculate the short circuit current on the secondary of the transformer.
SCA secondary =
(FLAsecondary x 100)
=
--------------------%Z
Where :
SCAsecondary = Short Circuit Amperes on Secondary of transformer
% Z = Percentage Impedance of transformer
Sample calculations:
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Service entrance single line diagram
With 3 phase bolted fault
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Utility available short ckt (Isc u) current gets larger utility %z aproaches to zero
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Since
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Now if we consider as infinite bus per unit impedance is disappears because it is negligible
(or) equals to 0 then we can able to remove from thevinins equalent ckt.
Most possible fault current (infinity) we could get from secondary of the transformer hence it is
infinite bus.
Limitations of Infinite Bus Method:
Infinite bus method, being a simple method of calculation, overlooks some important factors
and has its limitations, which are:
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The effect of current contributed by the motor.
The decrease in current due to source and line impedance.
Short Circuit Current and Arc Flash Studies:
Short circuit calculations using infinite bus methods are not suitable for arc flash studies.
Infinite bus method gives the worst possible current in the event of a short circuit, therefore a
relay or protection system configured using infinite bus method current will trip the circuit in
minimum time.
However, for a smaller value of fault current, the inverse time characteristics of that relay will
delay the operation of protection systems (delayed tripping time).
The prolonged time will release greater incident energy in the event of an arc flash at such a
current value. So infinite bus methods are not suitable for arc flash studies.
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