Lecture-38
Introduction to Power System Faults
Methods to Analyse PS faults
Lecture-39
Transients due to short circuit in TL and Alternators
Lecture-40
Analysis of Symmetrical faults in PS
Selection of Circuit Breakers
: A fault is any failure which interferes with the normal flow of current.
Faults occur in PS due to
insulation failure of equipments
flashover of lines initiated by lightning stroke
permanent damage to conductors and towers
accidental faulty operation
Most faults on transmission lines of 115 kV and higher are caused by lightning, which results in the flashover of insulators causing a low impedance path to ground.
Faults may be broadly classified into:
Shunt faults (short circuits)
between conductor and ground or between two or more conductors
Series faults (open conductors)
one or two broken conductor
circuit controlled by fuses or breakers which do not open all the 3-phases
Shunt faults can be classified as:
Line-to ground fault
Line-to Line fault
Double line-to-ground fault
Three phase faults
Shunt faults are characterised by increase in current and fall in voltage and frequency
Series faults are characterised by increase in voltage and frequency and fall in current in the faulty phases
Only 3-phase fault is symmetrical fault and all other faults are unsymmetrical faults.
Line-to‐line faults not involving ground are less common.
Experience shows that 70 to 80% of transmission line failures are single line-to-ground faults.
Permanent faults are caused by lines being on the ground, insulator strings breaking, ice loads, and equipment failure.
Roughly 5% of all faults involve all three phases and these are called symmetrical three-phase faults or just symmetrical faults.
\[\begin{aligned} \text{3-phase faults}& = 5\%\\ \text{LLG faults}& = 10\%\\ \text{LL faults}& = 15\%\\ \text{L-G faults}& = 70\% \end{aligned}\]
Symmetrical faults are analysed on per phase basis using Thevenin’s theorem or using Bus-impedance matrix (Z-bus)
Unsymmetrical faults are analysed using symmetrical components
When fault occur in a part of power system, heavy current flows in that part of circuit which may cause permanent damage to the equipments
Faulty parts should be isolated from the healthy part immediately on the occurrence of a fault
This can be achieved by providing protective relays and circuit breakers
Protective relays sense the faulty condition and send signal to circuit breakers to open the circuit
The currents which flow in a power system immediately after the occurrence of a fault differ from those flowing a few cycles later just before the circuit breakers are called upon to open the line on both sides of the fault.
Two factors which determine the proper selection of circuit breakers are the current flowing immediately after the fault occurs and the current which the breaker must interrupt.
In fault analysis values of these currents are calculated for different types of faults at different locations in the system.
The information obtained form these calculations are used to determine the relay setting that control the circuit breakers.
The depends on:
normal operating current
the current it has to interrupt
the maximum current it may have to momentarily carry
Short circuits occur in power system due to various reasons
equipment failure,
lightning strikes,
falling of branches or trees on the transmission lines,
switching surges,
insulation failures
other electrical or mechanical causes.
All these are collectively called faults in power systems.
First we shall discuss the effects of symmetrical faults on the system.
Symmetrical fault refers to those conditions in which all three phases of a power system are grounded at the same point.
For this reason the symmetrical faults sometimes are also called three-line-to-ground (3LG) faults.