FAULT LEVEL CALCULATION

Dinesh Kumar Sarda

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.

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.

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.

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)

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.

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.

Resistance of short transmission line are neglected.

Fault calculations are always done assuming Three Phase bolted short circuit fault.

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-

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.

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)

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.

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.

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.

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.

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.

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.

GENERATOR REACTOR:-

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.

BUSBAR REACTOR-

FEEDER REACTORS-

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?