CHAPTER-1

1. INTRODUCTION

Electric power systems are designed such that the impedances between generation

sources and loads are relatively low. This configuration assists in maintenance of a stable,

fixed system voltage in which the current fluctuates to accommodate system loads. The

primary advantage of this arrangement is that loads are practically independent of each

other, which allows the system to operate stably when loads change. However, a

significant drawback of the low interconnection impedance is that large fault currents (5

to 20 times nominal) can develop during power system disturbances. In addition, the

maximum fault current in a system tends to increase over time for a variety of reasons,

including:

• Electric power demand increases (load growth) and subsequent increase in generation.

• Parallel conducting paths are added to accommodate load growth.

• Interconnections within the grid increase.

• Sources of distributed generation are added to an already complex system.

In an effort to prevent damage to existing power-system equipment and to reduce

customer downtime, protection engineers and utility planners have developed elaborate

schemes to detect fault currents and activate isolation devices (circuit breakers) that

interrupt the over-current sufficiently rapidly to avoid damage to parts of the power grid.

While these traditional protection methods are effective, the ever-increasing levels of

fault current will soon exceed the interruption capabilities of existing devices.

Shunt reactors (inductors) are used in many cases to decrease fault current. These

devices have a fixed impedance so they introduce a continuous load, which reduces

system efficiency and in some cases can impair system stability. Fault current limiters

(FCLs) and fault current controllers (FCCs) with the capability of rapidly increasing

their impedance, and thus limiting high fault currents are being developed. These devices

have the promise of controlling fault currents to levels where conventional protection

1

equipment can operate safely. A significant advantage of proposed FCL technologies is

the ability to remain virtually invisible to the grid under nominal operation, introducing

negligible impedance in the power system until a fault event occurs. Ideally, once the

limiting action is no longer needed, an FCL quickly returns to its nominal lowimpedance

state.

Superconducting fault current limiters (SFCLs) utilize superconducting materials

to limit the current directly or to supply a DC bias current that affects the level of

magnetization of a saturable iron core. While many FCL design concepts are being

evaluated for commercial use, improvements in superconducting materials over the last

20 years have driven the technology to the forefront. Case in point, the discovery of high-

temperature superconductivity (HTS) in 1986 drastically improved the potential for

economic operation of many superconducting devices. This improvement is due to the

ability of HTS materials to operate at temperatures around 70K instead of near 4K, which

is required by conventional superconductors. The advantage is that there frigeration

overhead associated with operating at the higher temperature is about20 times less costly

in terms of both initial capital cost and O&M costs.

1.2 ELECTRICAL POWER SYSTEMAn electric power system is a network of electrical components used to supply,

transmit and use electric power. An example of an electric power system is the network

that supplies a region's homes and industry with power - for sizable regions, this power

system is known as the grid and can be broadly divided into thegenerators that supply the

power, the transmission system that carries the power from the generating centres to the

load centres and the distribution system that feeds the power to nearby homes and

industries. Smaller power systems are also found in industry, hospitals, commercial

buildings and homes. The majority of these systems rely upon three-phase AC power -

the standard for large-scale power transmission and distribution across the modern world.

Specialised power systems that do not always rely upon three-phase AC power are found

in aircraft, electric rail systems, ocean liners and automobiles.

2

1.3 FAULT IN ELECTRICAL POWERSYSTEMFAULT:

“ The flow of current to the undesired path and abnormal stoppage of current are

termed as fault”.

In an electric power system, a fault is any abnormal electric current. For example,

a short circuit is a fault in which current bypasses the normal load. An open-circuit fault

occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may

involve one or more phases and ground, or may occur only between phases. In a "ground

fault" or "earth fault", charge flows into the earth. The prospective short circuit currentof

a fault can be calculated for power systems. In power systems, protective devices detect

fault conditions and operate circuit breakers and other devices to limit the loss of service

due to a failure.

In a polyphase system, a fault may affect all phases equally which is a

"symmetrical fault". If only some phases are affected, the resulting "asymmetrical fault"

becomes more complicated to analyze due to the simplifying assumption of equal current

magnitude in all phases being no longer applicable.

Basically the faults are divided into two type:

 Symmetrical fault

 Asymmetrical fault.

Symmetrical Fault:

When the  fault current   in three phases are   symmetrical that is they are 120

degree displaced and of same magnitude then the fault reason behind this fault current  is

called Symmetrical Fault.

DETECTION OF FAULT:

3

If the fault occur then it should be removed or cured as soon as possible,

otherwise it can damage and even burn the equipment’s.

1.       Faults in the electrical equipment’s can be detected by using sensors which

sense the fault.

2.       Now a days the faults can also be detected automatically by using automatic

fault detection and diagnostic software. Researches are still going on related to the

use of this software. CSIRO researchers are still working on this software.

EFFECTS OF FAULT:

Generally the fault occur either within the devices or in the cables which is being

used for the supply purpose.

For example:

The cable used for supplying the power becomes worn because of long or prolonged

rubbing against other objects like branches of trees, with another wires etc.

In the similar way the motor also get burn out and causing short circuit.

Electrical fault may cause the risk of fire, damage the equipment, and may cause

potential electric shock to the people.

If some equipment’s are placed near the main cables and the fault occur then it may

damage that equipment, the short circuit that occur in the main windings or cables if

not removed then it also damages the walls as well as the floor.

1.4 Open Circuit Faults:

Open circuit faults occur either by overhead line parting or pole of the circuii

breaker not fully closing. This results in load imbalance on generators and motors lead to

negative phase sequence commponents in the stator current. This negative phase

sequence component current s rotate at twice the supply frequency in the opposite

direction in relation to the rotor and causes additional eddy current losses, results in

temperature raise in the rotor

4

Interturn faults:

Interturn faults occurs in machines i.e, Transformers, Motors and Generators. An

Interturn fault occurs due to the insulation breakdown between the turns of the same

phase or between the parallel windings belonging to the same phase of the machine. The

cause of the interturn fault is usually an overvoltge or mechanical damage of the

insulation.

Interturn Faults are more severe on large alternators (generators), High voltage

motors and power transformers. Interturn fault is most ofen experienced in rotating

machines where multiple windings are present in the same groove. For large generators

generally single winding rod per groove is designed in such cases interturn fault can

occur only in the winding head region.

Interturn Fault can occur at both stator and rotor for rotating machines like

generators and motors.

When an interturn fault occurs on stator of a rotating machine there is a high

probability that such fault can lead in to the ground fault.

When Interturn faults occur on the rotor winding following symptoms are observed:

When such fault occur high excitation current is required and this is compensated by

the voltage regulator.

Machine runs less smoothly, because of the asymmetry of the excitation curve

magnetization of the shaft due to asymmetrical flux

Bearing damage due to current flowing in the bearings

Interturn faults on power transformers can be occured due to the overvoltages

accompnying ground ground faults or deterioration of the insulation due to chemical

influence of the transformer oil.

5

Interturn fault current depends on the number of the turms shorted and fault currents

will be several times higher than the rated current of the windings and thus damages the

windings.

Overload

Faults due to overload will occur due to exceeding the maximum permissible load

current throught the windings, cables, or transmission lines or due to reduction in the

cooling offered to the windings.

Electrical conductor is designed in such a manner that the conductor allows

permissible amount of current without getting over heated. In this manner the current

carrying rating of the conductor is decided. When the current passed through the

conductor is above permissible level, no immediate damage occurs but over a period of

time conductor insulation will be damaged due to the excess heat generated.

In large generators and power transformers of large MW ratings, the heat

generated is enormous, so forced fooling is provided in such cases. For large generators

hydrogen cooling is provided and for large transformers forced cooling is provided. This

part is nicely presented in Transformer Cooling Methods. When these cooling methods

fail then the damage to the equipment is certainly fast compared to the other case.

Symmetrical Faults

1.Line-to-ground fault: this type of fault exists when one phase of any

transmission lines establishes a connection with the ground either by ice, wind,

falling tree or any other incident. 70% of all transmission lines faults are classified

under this category.

6

2. Line-to-line fault: as a result of high winds, one phase could touch anther phase

& line-to-line fault takes place. 15% of all transmission lines faults are considered

line-to-line faults.

3.Double line-to-ground: falling tree where two phases become in contact with the

ground could lead to this type of fault. In addition, two phases will be involved

instead of one at the line-to-ground faults scenarios. 10% of all transmission lines

faults are under this type of faults.

Unsymmetrical Faults

1. Three phase fault: in this case, falling tower, failure of equipment or even a

line breaking and touching the remaining phases can cause three phase faults.

In reality, this type of fault not often exists which can be seen from its share of

5% of all transmission lines faults.

Method of Analysis In order to analyze any unbalanced power system, C.L. Fortescue introduced a

method called symmetrical components in 1918 to solve such system using a balanced

representation. This method is considered the base of all traditional fault analysis

approaches of solving unbalanced power systems .

The theory suggests that any unbalanced system can be represented by a number

of balanced systems equal to the number of its phasors. The balanced systems

representations are called symmetrical components. In three-phase system, there are three

sets of balanced symmetrical components can be obtain; the positive, negative and zero

sequence components. The positive sequence consists of set of phasors which has the 9

same original system sequence. The second set of phasors has an opposite sequence

which is called the negative sequence. The zero sequence has three components in phase

with each other.

7

Phase Faults:Electrical Phase faults are characterised as:

• Phase to Ground Fault

• Phase to Phase Fault

• Phase - Phase to Ground Fault

• Three Phase Fault

Phase to Ground Fault:

In this type of Electrical fault all the three sequence components (positive,

negative and zero sequence components ) are present and are equal to each other. In case

of isolated neutral connection to the generator, there will be no return path for the current.

So for such fault, fault current is zero.

Phase to Phase fault:

These are unsymmetrical faults as these faults give rise to unsymmetrical currents

(Current differ in magnitude and phase in the three phases of power system).In case of

Phase to Phase fault positive and negative sequence component of current are present,

they are equal in magnitude but opposition in phase. zero sequence components are

absent.

Phase - Phase to Ground Fault:

These faults are of unsymmetrical nature. In this type of faults negative and zero

sequence faults are in opposition with positve sequence cmponents.

Three Phase Fault:

This type of faults are called symmetrical fault. This type of faults occur very

rarely but more severe compared to other faults. In this faults negative and zero sequence

component currents are absent and positive sequence currents are present.

8

To summarize:

• positive sequence currents are present in all types of faults

• Negative Sequence currents are present in all unsymmetrical faults

• Zero sequence currents are present when the neutral of the system is grounded

and the fault also involves the ground, and magnitude of the neutral currents is equal to

3Io

9