Introduction to Power System Faults

Demonstrative Video

Contents

  • 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

Introduction to Power System Faults

  • : 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.

\[\begin{aligned} \text{3-phase faults}& = 5\%\\ \text{LLG faults}& = 10\%\\ \text{LL faults}& = 15\%\\ \text{L-G faults}& = 70\% \end{aligned}\]

Methods to Analyse PS Faults

  • 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

Fault Calculations

  • 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.