TOPIC:- UNSYMMETRICAL FAULT

introduction

• Generator, transformer , and transmission line

• The fault which gives rise to unsymmetrical fault current are known as unsymmetrical fault.

• It makes system unbalanced.

How can fault occur

• Falling of tree on line

• Wind and ice loading on transmission line

• Vehicle colliding with supporting structures

• Birds shorting on lines

• Insulation failure

Unsymmetrical fault

• Single line to ground fault

• Line to line fault

• Double line to ground fault

Analysis of unsymmetrical faults using the bus impedance matrix

• we use the positive sequence bus impedance matrix for calculation of current and voltage during balanced three phase faults.

• This method is extended here to unsymmetrical fault by representing each sequence network as a bus impedance equivalent circuit.

• A bus impedance matrix can be computed for each sequence network by inverting the corresponding bus admittance network.

• ForSimplicity,resistance,shuntadmittance,non-rotating impedance load and pre fault load current are neglected.

• Show in the fig. the connection of sequence of equivalents for both symmetrical and unsymmetrical fault at bus n of N- bus three phase power system each bus impedance element are show subscrip = 0,1 and 2 that identified the sequence equivalent in which it is located.

• The prefault voltage Vf is included in the positive-sequence equivalent.

• show in the fig..the sequence components of the fault current for each type of fault at bus n are aa follows:

Balanced three phase faults :

Ia1 = Vf / Znn – 1

Ia0 = Ia2 = 0

Single line to ground fault : Ia0 = Ia1 = Ia2 = Vf / (Znn-0 + Znn-1 + Znn-2 +3Zf)

Line to line ground fault:

Ia1 = -Ia2 = Vf / Znn-1 + Znn-2 + Zf

Ia = 0

Double line to ground fault :

Ia1=Vf / Znn-1 [Znn-2 ( Znn-0 + 3Zf ) / Znn-2+Znn-0 + 3Zf ]

Ia2 = -Ia1 [Znn-0 + 3Zf / Znn-0 + 3Zf +Znn-2]

Ia0 = -Ia1 [Znn-2 / Znn-0 + 3Zf +Znn-2]