This document discusses fault level calculations in electric power systems. It explains that fault level calculations are necessary to select protective devices, circuit breakers, and equipment that can withstand short circuit currents. The document outlines the procedure for calculating fault levels, which involves representing the system with a single line diagram, choosing a base MVA, calculating per unit reactances, determining the equivalent reactance to the fault point, and using formulas to calculate fault MVA and current. It also discusses how current limiting reactors can be used to insert additional reactance and reduce short circuit currents to match circuit breaker ratings.

1. FAULT LEVEL CALCULATION
Dinesh Kumar Sarda

2. Fault level at any given point of the
Electric Power Supply Network is the
maximum current that would flow in case of
a short circuit fault at that point.
The circuit breaker should be capable of
Breaking & Making current as per their
ratings & should also have Rated short time
capacity. So, for proper selection of circuit
breaker & other switchgear components,
Knowledge of current during normal &
abnormal conditions is necessary.

3. The design of machines, bus bars, isolators,
circuit breaker etc. is based on the
consideration of Normal & Short Circuit
Currents.
The Protective Relaying Schemes can be
selected only after ascertaining the fault
levels and normal currents at various
locations. Fault calculations are also
necessary for System Design, stability
considerations, selection of Layout etc.

4. Purpose of Fault Level Calculations:-
a) For selecting short circuit protective
devices of adequate short circuit Breaking
capacity.
b) For selecting circuit breakers and
switches of Adequate short circuit Making
capacity.
c) for selecting Bus Bars, cable and
switchgear, designed to withstand thermal
and mechanical stresses because of short
circuit.

5. Types of Short Circuits:-
1) L-E (Single Line to Earth)
2) L-L ( Line to Line)
3) L-L-E ( Line to Line to Earth)
4) L-L-L ( Three Phase)

6.  In Fault Calculations, many assumptions
are made for simplifying the calculations.
Resistances are neglected when their
values are negligible as compared to the
Reactance.
Capacitance is neglected. Machine
reactance are assumed to be constants.
Saturation effects are neglected.

7. Generated voltages are assumed to be
constant. Contribution of Shunt capacitor
banks are neglected.
 Impedences of BusBar, Switchgear, C.Ts
are neglected.
Transformer tap is in the main position.
Short circuit current waveform is a pure
Sine wave.

8. Resistance of short transmission line are
neglected.
Fault calculations are always done assuming
Three Phase bolted short circuit fault.

9. Procedure for Fault Level Calculation.
 Fault calculations begin with drawing
Single line or One line diagram of the given
network with Ratings of all the
transformers & Generators in the network.
 Suitable Base MVA is chosen. For ex. 100
MVA.
 Per Unit reactance (impedence) is
calculated for every Transformer &
Generator in the given network by using-

10. Formula P.u reactance =
[(% impedence)/100] x [Base MVA/Actual
MVA]
 Then redraw the network putting the P.u
reactance values.
 Calculate the Equivalent reactance for the
for the network as seen from the source to
the fault point.

11.  Then, Calculate Fault MVA using formula-
Fault MVA= Base MVA/ Eq. Impedence
 Calculate Fault Current using formula=
Fault Current =
Fault MVA / (1.732x Phase to Phase
Voltage at the fault point)

32. Current Limiting Reactors:-
Current Limiting Reactors ( Series Reactors)
are inserted in series with the line, to limit
the current flow in the event of a short
Circuit & thereby, bring down the fault
level. The current Limiting Reactors are
also called Series Reactors.
A current Limiting Reactor is an inductive
coil having a large inductive reactance (wL)
and is used for limiting short-circuit
currents to be interrupted by circuit
breaker.

34.  If X is the reactance of a circuit , E is
the voltage then by neglecting the
resistance , the short circuit current is
given by Isc= E / X.
Therefore, by increasing the series
reactance of the system, the short circuit
current can be decreased.
The short circuit current depends upon the
generating capacity, voltage at the fault
point and the total reactance between the
generator and the fault point.

35. The circuit breaker should have enough
breaking current capacity such that the
fault current are less than the breaking
current capacity. IF the fault currents are
beyond the capacity of the circuit breaker,
the circuit breaker may not interrupt the
fault current.
Then the fault current should be limited by
some means so that the existing circuit
breaker can be used safely.

36. Also when the system is extended by
adding more generating stations, the Fault
current to be interrupted by the same
circuit breaker will be greater than before.
In these cases, the circuit breaker should
be replaced by another of higher breaking
current capacity or the fault current can
be limited by the means of REACTORS.
The current limiting reactors are useful in
limiting short circuit current so that the
circuit breaker can interrupt them.

37. 1) Reactors limit the Short Circuit Current.
2) Reactors are used in the systems when
extensions are made and the circuit
breaker breaking current capacities
become inadequate.
3) Reactors are employed in Large Systems
so as to limit the short circuit MVA to
match with the breaking current capacity
of the circuit breaker.

38. Location of Series Reactor:-
 Generator Reactors-
Reactors are inserted between the
generator and the generator bus. The high
reactance is provided to safeguard the
generators in case of dead three phase
short circuits at its terminals.
When new generators are installed ,
generator reactors can be added to the old
generators.

39. GENERATOR REACTOR:-

40. Feeder Reactors-
The reactors in this case are connected in
series with the feeders. The advantage is
that the voltage of the bus does not drop
substantially in the event of fault on the
feeder.
 The Busbar Reactor-
The busbar reactor is placed between two
sections of busbar.

41. BUSBAR REACTOR-

42. FEEDER REACTORS-

43. QUESTIONS:-
1) Why Fault Calculation is required?
2) What is meant by Breaking Current
capacity & Making Current Capacity of
Circuit Breaker?
3) What is the formula for Calculating
Fault MVA?
4) What is the Formula for Calculating
Fault Current?
5) What should be used to decrease
Fault Level at any point?