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Symmetrical Three phase faults

When the Fault occurs in the power system network, the current

flowing is determined by

internal emf of the machine in the system,

by their internal impedances and

by impedances in the network between the machines and the fault.

When the fault current is equal in all the phases, the fault is symmetrical

fault.

only possibilitythree phase faults in which all the phases

are shunted to ground.

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Symmetrical Three phase faults

Symmetrical fault can be analyzed on per phase basis using

reactance diagram or by using per unit reactance diagram.

Analysis has to be performed separately for sub transient, transient

and steady conditions of the fault Reactance's and internal emf of synchronous machine will be different in

each state.

Methods:

1. using Kirchoff's law

2. using thevenins theorem

3. By forming bus impedance matrix.

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Symmetrical fault current estimation using

kirchoffslaw

Choose appropriate base values and determine the prefault condition reactance

diagram of the given power system

Calculate the internal emfs of the synchronous machines and the prefault

voltage at the fault point using prefault current.(Load current)

If the power system is unloaded ( if there is no prefault current then Vpf is 1 p.u) Also the

internal emf for subtransient and transient state are same as steady state induced emf

Draw the fault condition reactance diagram of the system include the fault

which is represented by the short circuit

Calculate the p.u value of the fault currents in the various parts of the system

and in the fault.

The actual values of the fault currents are obtained by multiplying the p.u

values by the respective base currents.

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Symmetrical fault current estimation using

thevenins theorem

1. Choose the appropriate base values and determine the prefault condition

reactance diagram of the given power system

2. Calculate the prefault voltage at the fault point using the prefault current.

If the system is unloaded, then the prefault voltage is 1 p.u. The prefault

voltage at the fault point is the thevenins voltage.

3. Determine the thevenins impedance of the system at the fault point

4. Draw the thevenins equivalent at the fault point F.

P.u Value of the fault current, If= Vth/ Zth

5. The fault current at the other parts of the network are determined from the

knowledge of the change in current due to fault and prefault current.

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Cont..

The change in current due to fault can be estimated by connecting the

thevenins source with reversed polarity at the fault. Replace all other

sources by zero value sources

Note:

In a practical power system the normal working voltage will be very close to

1p.u. Hence the prefault voltage can be assumed as 1p.u for simplified

calculations.

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Bus Impedance matrix in Fault calculations

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Bus Impedance matrix in Fault calculations

cont..

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Bus Impedance matrix in Fault calculations

cont..

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Bus Impedance matrix in Fault calculations

cont..

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Problem 1

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Problem 1

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Problem 2

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Problem 2

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Problem 2

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Problem 2