Short Circuit Calculations — Infinite bus method
Short Circuit:
Short circuit is essentially an abnormal condition within a power system in which 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, satisfying certain NFPA standards.
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 short circuit current makes the working environment critically dangerous.
The various effects of a high magnitude Short Circuit current can include excessive heat generation and Arc Flashes. The after effects and damages caused by such incidents can great vary as well, amongst which equipment damage and appliance burns are the most frequently occurring ones.
It is, therefore, necessary to perform short circuit calculations in order to be prepared for an unfortunate short circuit event.
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 always get maximum value because of a number of different factors involved in the calculation. The transformer impedance also plays a vital role in the calculation of SCC by influence of the same factors.
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Data required:
The infinite bus calculations are performed across a 3-phase transformer in a power system. Therefore, we should have a list of data related with the transformer which 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.
FLAsᴇᴄᴏɴᴅᴀʀʏ = ( KVA₃ ₚₕₐₛₑ )/KVₗ₋ₗ×√3
Where
FLAsᴇᴄᴏɴᴅᴀʀʏ = 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.
SCAsᴇᴄᴏɴᴅᴀʀʏ = ( FLAsᴇᴄᴏɴᴅᴀʀʏ × 100 )/ % Z
Where
SCAsᴇᴄᴏɴᴅᴀʀʏ = Short Circuit Amperes on Secondary of transformer
% Z= Percentage Impedance of transformer
The Infinite bus method, being a simple method of calculation, possesses a set of limitations as well.
Short Circuit Current and Arc Flash Studies:
Short circuit calculations using infinite bus method 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 response of that relay will be different as compared to the previous scenario.
We, at AllumiaX, have a standard procedure to make use of 2 cases:
- First, we study using the normal short circuit current given by infinite bus method.
- Then a second study is carried out by using depressed currents at 40% to 50% of the worst possible current to cater the prolonged trip time.
AllumiaX, LLC. specializes in conducting arc flash studies and complies with the global standards set by OSHA and NFPA. Following the acts suggested above, we provide “Arc flash Reports” along with “Arc Flash Labels” indicating an equipment’s risk category along with associated PPE clothing required for employees to interact. Moreover, we also provide trainings on arc flash, educating the employees on how to work with an equipment.
Let us know if you have any queries regarding this topic and do provide us with your feedback in the comments.
About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.
Abdur Rehman is the CEO and co-founder of allumiax.com and creator of GeneralPAC by AllumiaX. He has been actively involved in various roles in the IEEE Seattle Section, IEEE PES Seattle, IEEE Region 6, and IEEE MGA.
