194511440 Power System Analysis

Electrical Power System

Fault Analysis

Prepared by: Jonryl P. Novicio 1



Fault Analysis

INTRODUCTION Fault Studies play an important role of the power system analysis. The problem consists of determining bus voltages and line currents during various types of faults.An electrical fault is any failure which interferes the normal flow of current. Faults on power systems are divided into three-phase balanced faults and unbalanced faults. Different types of unbalanced faults are single line to ground fault, line to line fault, and double line to ground fault.



Fault Analysis

The information gained from fault studies are used for: Proper relay setting and coordination.

Three – phase balanced fault is used to select and set phase relays Line to Ground Fault is used for ground relays

To obtained the rating of the protective switchgears (Three – phase Faults).



Now, consider the typical system below.

Fault Analysis



The magnitude of the fault current depends on the internal impedance of the generators plus the impedance of the intervening circuit.

Fault Analysis



What are the Causes of Faults? There are several causes of failures and outages which are classified according to the reasons for its occurrence: a) Deterioration of insulation which are inherent in the system. As a result

breakdown may occur at normal voltage. b) Damage due to external causes such as birds, trees and kites. Failure may also

occur at normal voltage. Example of damage on open, outdoor equipment are due to birds with widely stretching wings by spanning an insulation or phase fault by bridging the bare conductors.

c) Breakdown due to deterioration of insulation accelerated by self generated surges following switching or other disturbances.

d) Collapse of insulation resulting from either direct lightning stroke or consequent disturbances. The fault would not be at normal voltage.

Fault Analysis



Fault Analysis

Types of Faults on

3-Phase System

Faults in 3-phase system are classified as follows :( see-Fig.1) a. Balanced or symmetrical

three-phase fault b. Single-line to ground fault c. Double line-to-ground

fault d. Line-to-line fault



Fault Analysis

A fault occurs when two or more conductors that normally operate with a potential difference come in contact with each other. The contact may be physical metallic one or it may occur through an arc.

What are the Effect of Faults? The directly resulting damage from system faults can be classified as follows: a) Thermal b) Mechanical c) Dielectric



Fault Analysis

Thermal Effects. The amount of current will be much greater than

the designed thermal ability of the conductor in the power lines or machines feeding the fault. As a result, temperature rise may cause damage by annealing

of conductors and insulation charring. Please note that the thermal stress varies as a function of both

RMS current squared and the duration of current flow (I2t).



Fault Analysis

Mechanical Effects of high instantaneous current involves magnetic forces (repulsion/attraction) of hundreds of pounds per foot run of conductor or bus as normally spaced and feed by modern generators.

The mechanical stress varies as a function of the peak current

squared and the ratio of the length of the peak conductor to the spacing between conductor centers (KI2

peak L/S).



Fault Analysis

Dielectric Effects have several causes. One obvious cause of impairment or weakening of insulation strength or dielectric strength is the high temperature of the conductor carrying the fault currents, which may result to the burning or blistering of varnish insulation or cracking of ceramic ones.

Such defects may be present but not visible for months and may never be discovered due to the later breakdown destroying the evidence of its cause.

Similar danger of unsuspected latent weakness arise from surge voltage, either generated by fault switching or rapid displacement of the neutral following a ground fault or one phase.


(a) utility systems

(b) generators

(c) synchronous motors

(d) induction motors


Fault Analysis

Sources of Short-Circuit Current



Fault Analysis

The utility system usually supplies power through step-down transformers at

the costumer’s desired voltage level. Although transformers are sometimes considered a source of short-circuit current, strictly speaking this is not correct. Transformers change voltage and current magnitude, but do not generate them.

Generators in the customer’s system can be a source of short-circuit current.

They are driven by prime movers such as steam or gas turbines, diesel engines and water wheels. When a short-circuit occurs, the generator continues to be driven by its prime mover and to produce voltage, since field excitation is maintained by the generator rotating at normal speed.



Fault Analysis

Synchronous motors behave similarly to synchronous generators. When a fault occurs and the voltage of the system is reduced to a very low value, the synchronous motor stops taking power from the system to rotate its load, and starts slowing down.

But the inertia of the load tends to prevent the motor from slowing down quickly. The inertia acts as a prime mover and, with excitation maintained, the motor acts as a generator supplying short-circuit current for several cycles after the short circuit occurs.



Fault Analysis

Induction motors contribute short-circuit current because of generator action

produced by the inertia of the load and rotor driving the motor after the fault occurs.

But there is a major difference between the short-circuit current contribution

of induction motors and synchronous motors. The field flux of the induction motor is produced by induction from the stator and not from a dc field winding.

Since this flux decays rapidly after a fault, the induction motor contribution

drops off quickly and dies out completely after a few cycles. There is no steady-state fault-current contribution. Consequently, induction motors are assigned only a sub-transient value of reactance, X’’d.



Fault Analysis

The total symmetrical short-circuit current is a combination of all the sources of short-circuit current we have described.

Utility supply, generators, synchronous and induction motors all

contribute short-circuit current into a fault. The flux in the machines decays with time after the inception of

the fault, hence their fault-current contribution also decays with time.



Fault Analysis

Consequently, the resulting total short-circuit current decays with time as shown at E in Fig. 3.

The current magnitude is highest during the first half-cycle and

decreases in value after a few cycles.

After one or two cycles, the induction-motor contribution disappears.



Fault Analysis



Fault Analysis

Fault Calculation should be made to ensure that the short-circuit ratings of the equipment are adequate to handle the currents available at the location of the fault. In general, the procedure is as follows: a.) Develop a graphical representation of the system with symbolic voltage

sources and circuit impedances. b.) Determine the total equivalent impedance from the source to specified

points. c.) At each point, divide the voltage by the total impedance to that point to

determine short-circuit current.



Fault Analysis

To simplify the calculations, certain simplifying assumptions are usually made. The following assumptions are usually made: a.) The fault is bolted that is, it has zero impedance. This assumption

simplifies calculation since the resulting calculated values are maximum, and equipment selected on the basis will always have an adequate rating.

b.) A 3-phase fault is assumed because their type of fault generally

results in the maximum short-circuit current available in a system. In most systems, a 3-phase fault is frequently the only one calculated.



Fault Analysis

Most faults actually involve arcing resistance or other undefined

impedances or both not considered in the fault calculation because of the assumed bolted fault condition as mentioned earlier.

Fault which are not 3-phase will usually be less than the 3-phase fault value. These are the following: a.) Bolted line-to-line current -, 87% of the 3-phase value b.) Bolted single-line-to-ground current - About 25-125% of the 3-phase fault

depending on system parameters. This is the most common among the faults, about 75% of the faults.



Fault Analysis

Single-line-to-ground currents of more than 100% of the 3-phase value rarely occurs in industrial, institutional and commercial power systems.

In low- voltage systems, single-line-to-ground arcing fault currents are sometimes less than normal load currents and yet can be extremely destructive.



Fault Analysis

Other assumption which effected simplification of the fault calculations are also being done as follows: a.) Load currents are not considered. b.) The voltages of power company and generator sources are

assumed to be equal to their nominal values at no-load. The actual values could be ± 5% of the nominal values.

c.) Motors are running with their rated voltage at the terminal when a fault occurs.



Fault Analysis

Other assumption which effected simplification of the fault calculations are also being done as follows: d.) The transformer percent impedance values used will be actual

values or are often nominal values possibly subject to their ± 7.5% tolerance to anticipate worst case.

e.) The source X/R ratio will be assumed at relatively high value ( above X/R ratio of 15). This normally results in calculated values of short- circuit that are slightly high.



Fault Analysis

Other assumption which effected simplification of the fault calculations are also being done as follows: f.) Switchboard and panel board bus impedances can be neglected. As the Impedances of the bus are usually of small values, excluding them from the calculation, the increase of fault current values is only very slight.



Fault Analysis

Symmetrical Faults or

Balanced Faults



Fault Analysis

The balanced three phase fault and three phase to ground fault occurs very

rarely, accounting for only about 5% of the total. For the purpose of fault studies, the generator behavior can be divided into

three stages. 1) The subtransient period

2) The transient period

3) Steady state period



Fault Analysis

Subtransient exists for a period of few cycles from the instant of short circuit

followed by transient period appears for another few cycles. With less magnitude compared with subtransient and finally settling at a still higher synchronous (steady state) value called the steady state period.

In a modern large interconnected power system, heavy currents will flow through the fault which must be interrupted much before the steady state conditions are established.

The maximum current that a breaker has to carry momentarily is the peak

value of short circuited current which must also be determined.



Fault Analysis

Transient due to Short Circuit When there is a sudden change in the current, because of the inductive property of

the power system component which give rise to transients. Transient condition in power system rises due to faults on the system which is

accompanied by sudden change in current.



Fault Analysis



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Fault Analysis

Transients due to Short Circuit in a 3-Phase Alternator at No-load

Consider a three phase alternator running on no-load. Under steady state

short circuit conditions, the armature reaction of a synchronous machine produces a demagnetizing flux. The effect of armature reaction is modeled as a reactance Xa in series with induced e.m.f.

The combination of Xa with the leakage reactance X l of the synchronous

machine is called Xd synchronous reactance (direct axis synchronous reactance in case of salient pole machines).

Armature resistance being small can be neglected. The steady state short circuit model of a synchronous machine is shown in Fig. 4.3 (a).



Fault Analysis



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Fault Analysis

THREE-PHASE FAULTS



Fault Analysis



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Fault Analysis


• From the previous equations, the volume of copper is inversely proportional to the square of the transmission voltage and the power factor.

• Thus greater is the transmission voltage level, lesser is the volume of copper required.

• The weight of copper used for the conductors

• The required material is LESS for higher transmission voltage.


Fault Analysis

194511440 Power System Analysis

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