Electrical Power- II

Switch Gear

• INTRODUCTION

A switch gear can be define as an assembly of equipment for switching on/off , controlling and protecting an electrical installation.

In simple words an ordinary switch in conjunction with a fuse makes a switch gear. The switch will on/off the circuit, were as the fuse will protect the circuit.

• ISOLATOR

By isolator we disconnecte a circuit under no load for repair and maintenance work. They are placed generally on both side of a circuit breaker. For repair, the circuit breaker on that line is first opened and then the isolator . After repair the circuit breaker is first closed then the isolator is closed .

• CIRCUIT BREAKER

The circuit breaker operates automatically through a relay under fault condition. The CB can be operate manually also for carrying repair work . The circuit is used to protect a line against a fault. The CB is connected in the line . A current transformer is also connected in the line.

• PHENOMENON OF ARC FORMATION IN CIRCUIT BREAKER

When a fault accures a heavy current flows through the contact of the CB. When these contact separate, the resistance increases and large fault current produces excessive heat. This heat ionised the air or the oil and as a result an arc is produced between the contact of the CB. There are two method which can be used extinguish this arc .

1. By increasing resistance of the arc:

This method is used in DC circuit breakers. In this method, the current is reduced to such a value that it cannot maintain the arc. This is done by increasing resistance of the arc either by separating the contents sufficiently or by splitting the arc through use of conducting pieces between the contacts.

2. By zero current method:

This method is used to extinguish arc in a.c. Circuit breakers. The current become zero after every half cycle and at that instant the arc is extinguished of its own. Efforts are made at this instant to prevent the arc from restricting for the next cycle.

• CLASSIFICATION OF CBs

The CBs are of six types:

1. Oil circuit breaker

2. Air circuit breaker

3. SF6 circuit breaker

4. Vacuum circuit breaker

5. Water circuit breaker

6. Miniature circuit breaker

OIL CIRCUIT BREAKER (OCB)

A high-voltage circuit breaker in which the arc is drawn in oil to dissipate the heat and extinguish the arc; the intense heat of the arc decomposes the oil, generating a gas whose high pressure produces a flow of fresh fluid through the arc that furnishes the necessary insulation to prevent a restrike of the arc.

The OCBs are of three types:

1. PLANE (bulk) OCB: Bulk oil circuit breaker is such types of circuit breakers where oil is used as arc quenching media as well as insulating media between current carrying contacts and earthed parts of the breaker.

2. SELF BLAST OCB: In this type of circuit breaker, the gases produced during arcing are confined to a small volume by the use of an insulating rigid pressure chamber or pot surrounding the contacts. Since the space available for the arc gases is restricted by the chamber, a very high pressure is developed to force the oil and gas through or around the arc to extinguish it.

• Self Blast OCBs are of the following types:

1. PLAIN EXPLOSION POT: It is a rigid cylinder of insulating material and encloses the fixed and moving contacts. The moving contact is a cylindrical rod passing through a restricted opening (called throat) at the bottom. When a fault occurs, the contacts get separated and an arc is struck between them. The heat of the arc decomposes oil into a gas at very high pressure in the pot.

2. CROSS JET EXPLOSION POT: This type of pot is just a modification of plain explosion pot.Itis made of insulating material and has channels on one side which act as arc splitters. The arc splitters help in increasing the arc length, thus facilitating arc extinction. When a fault occurs, the moving contact of the circuit breaker begins to separate. As the moving contact is withdrawn, the arc is initially struck in the top of the pot. The gas generated by the arc exerts pressure on the oil in the back passage.

3. SELF COMPENSATE EXPLOSION POT: This type of pot is essentially a combination of plain explosion pot and cross jet explosion pot.Therefore, it can interrupt low as well as heavy short circuit currents with reasonable accuracy.The figure below shows the schematic diagram of self-compensated explosion pot.It consists of two chambers, the upper chamber is the cross-jet explosion pot with two arc splitter ducts while the lower one is the plain explosion pot.

• MINIMUM OIL CIRCUIT BREAKER: In the bulk oil circuit breakers discussed so far, the oil has to perform two functions. Firstly, it acts as an arc quenching medium and secondly, it insulates the live parts from earth.It has been found that only a small percentage of oil is actually used for arc extinction while the major part is utilised for insulation purposes.For this reason, the quantity of oil in bulk oil circuit breakers reaches a very high figure as the system voltage increases.This not only increases the expenses, tank size and weight of the breaker but it also increases the fire risk and maintenance problems.

• CONSTRUSTION MINIMUM OIL CIRCUIT BREAKER

The figure below shows the cross section of a single phase minimum oil circuit breaker.There are two compartments separated from each other but both filled with oil.Theupper chamber is the circuit breaking chamber while the lower one is the supporting chamber.The two chambers are separated by a partition and oil from one chamber is prevented from mixing with the other chamber.This arrangement permits two advantages.Firstly, the circuit breaking chamber requires a small volume of oil which is just enough for arc extinction. Secondly, the amount of oil to be replaced is reduced as the oil in the supporting chamber does not get contaminated by the arc.

• OPERATION AND ARC QUENCHING: Under normal operating conditions, the moving contact remains engaged with the upper fixed contact.When a fault occurs, the moving contact is pulled down by the tripping springs and an arc is struck.The arc energy vapourises the oil and produces gases under high pressure.This action constrains the oil to pass through a central hole in the moving contact and results in forcing series of oil through the respective passages of the turbulator.The process of turbulation is orderly one, in which the sections of the arc are successively quenched by the effect of separate streams of oil moving across each section in turn and bearing away its gases.

AIR CIRCUIT BREAKER

This type of circuit breakers, is those kind of circuit breaker which operates in air at atmospheric pressure. After development of oil circuit breaker, the medium voltage air circuit breaker (ACB) is replaced completely by oil circuit breaker in different countries. But in countries like France and Italy, ACBs are still preferable choice up to voltage 15 KV. It is also good choice to avoid the risk of oil fire, in case of oil circuit breaker.

TYPES OF ACBs

There are mainly two types of ACB are available.

• Cross blast air circuit breaker

• Axial blast air circuit breaker

• CROSS BLAST AIR CIRCUIT BREAKER:

In these ACBs the air blast is at right angle to the arc.

The moving contact of this ACB has a number of spring loaded fingers and the moving contact is of plat blades. The contacts are made of silver tungsten alloys.

When a fault occurs an air blast thrown away at right angle to the arc. The arc splitters quench the arc quickly.

• AXIAL BLAST FOR AIR CIRCUIT BREAKER:

In axial blast ACB the moving contact is in contact with fixed contact with the help of a spring pressure as shown in the figure. There is a nozzle orifice in the fixed contact which is blocked by tip of the moving contact at normal closed condition of the breaker. When fault occurs, the high pressure air is introduced into the arcing chamber. The air pressure will counter the spring pressure and deforms the spring hence the moving contact is withdrawn from the fixed contact and nozzle hole becomes open. At the same time the high pressure air starts flowing along the arc through the fixed contact nozzle orifice. This axial flow of air along the arc through the nozzle orifice will make the arc lengthen and colder hence arc voltage become much higher than system voltage that means system voltage is insufficient to sustain the arc consequently the arc is quenched.

SF6 CIRCUIT BREAKER

Sulfur hexafluoride circuit breakers protect electrical power stations and distribution systems by interrupting electric currents, when tripped by a protective relay. Instead of oil, air, or a vacuum, a sulfur hexafluoride circuit breaker uses sulfur hexafluoride (SF6) gas to cool and quench the arc on opening a circuit. Advantages over other media include lower operating noise and no emission of hot gases, and relatively low maintenance. Developed in the 1950s and onward, SF6 circuit breakers are widely used in electrical grids at transmission voltages up to 800 kV, as generator circuit breakers, and in distribution systems at voltages up to 35 kV.

• CONSTRUCTION: A sulphur hexafluoride (SF6) circuit breaker consists of fixed and moving contacts enclosed in a chamber. The chamber is called arc interruption chamber which contains the sulphurhexafluoride (SF6) gas. This chamber is connected to sulphur hexafluoride (SF6) gas reservoir. A valve mechanism is there to permit the gas to the arc interruption chamber. When the contacts of breaker are opened, the valve mechanism permits a high-pressure sulphur hexafluoride (SF6) gas from the reservoir to flow towards the arc interruption chamber.The fixed contact is a hollow cylindrical current carrying contact fitted with an arcing horn. The moving contact is also a hollow cylinder with rectangular holes in the sides. The holes permit the sulphur hexafluoride gas (SF6) gas to let out through them after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with a copper-tungsten arc-resistant material. Since sulphur hexafluoride gas (SF6) gas is costly, it is reconditioned and reclaimed using the suitable auxiliary system after each operation of the breaker.

• WORKING OF SF6 CB

In the closed position of the breaker, the contacts remain surrounded by sulphur hexafluoride gas (SF6) gas at a pressure of about 2.8 kg/cm2.

When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronised with the opening of a valve which permits sulphur hexafluoride gas (SF6) gas at 14 kg/cm2 pressure from the reservoir to the arc interruption chamber.

The high-pressure flow of sulphur hexafluoride gas (SF6) rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers.

The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation (i.e. after arc extinction), the valve is closed by the action of a set of springs.

MINIATURE CIRCUIT BREAKER

An MCB or miniature circuit breaker is an electromagnetic device that embodies complete enclosure in a molded insulating material. The main function of an MCB is to switch the circuit, i.e., to open the circuit (which has been connected to it) automatically when the current passing through it (MCB) exceeds the value for which it is set. It can be manually switched ON and OFF as similar to normal switch if necessary.

MCBs are of time delay tripping devices, to which the magnitude of overcurrent controls the operating time. This means, these get operated whenever overload exist long enough to create a danger to the circuit being protected. Therefore, MCBs doesn’t respond to transient loads such as switches surges and motor starting currents. Generally, these are designed to operate at less than 2.5 milliseconds during short circuit faults and 2 seconds to 2 minutes in case of overloads (depending on the level of current).

• CONSTRUCTION OF MCB

An MCB embodies complete enclosure in a moulded insulating material. This provides mechanically strong and insulated housing. The switching system consists of a fixed and a moving contact to which incoming and outgoing wires are connected. The metal or current carrying parts are made up of electrolytic copper or silver alloy depending on the rating of the circuit breaker. As the contacts are separated in the event of an overload or short circuit situation, an electric arc is formed. All modern MCBs are designed to handle arc interruption process where arc energy extraction and its cooling are provided by metallic arc splitter plates. These plates are held in a proper position by an insulating material. Also, arc runner is provided to force the arc that is produced between the main contacts.

The operating mechanism consists of both magnetic tripping and thermal tripping arrangements.

• WORKING AND OPERATION OF MCB

Under normal working conditions, MCB operates as a switch (manual one) to make the circuit ON or OFF. Under overload or short circuit condition, it automatically operates or trips so that current interruption takes place in the load circuit. The visual indication of this trip can be observed by automatic movement of the operating knob to OFF position. This automatic operation MCB can be obtained in two ways as we have seen in MCB construction; those are magnetic tripping and thermal tripping. Under overload condition, the current through the bimetal causes to raise the temperature of it. The heat generated within the bimetal itself enough to cause deflection due to thermal expansion of metals. This deflection further releases the trip latch and hence contacts get separated. In some MCBs, magnetic field generated by the coil causes develop pull on bimetal such that it deflection activates the tripping mechanism.

EARTH LEAKAGE CIRCUIT BREAKER (ELCB)

An ELCB is one kind of safety device used for installing an electrical device with high earth impedance to avoid shock. These devices identify small stray voltages of the electrical device on the metal enclosures and intrude the circuit if a dangerous voltage is identified. The main purpose of Earth leakage circuit breaker (ELCB) is to stop damage to humans & animals due to electric shock.

An ELCB is a specific type of latching relay that has a structure’s incoming mains power associated through its switching contacts so that the circuit breaker detaches the power in an unsafe condition.The ELCB notices fault currents of human or animal to the earth wire in the connection it guards. If ample voltage seems across the ELCB’s sense coil, it will turn off the power, and remain off until manually rearrange. A voltage sensing ELCB doesn’t detect fault currents from human or animal to the earth.

• CONSTRUCTION:

• OPERATION:

CHAPTER: PROTECTION DEVICE

• FUSE: A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, and it must be replaced or rewired, depending on type.

Fuses have been used as essential safety devices from the early days of electrical engineering. Today there are thousands of different fuse designs which have specific current and voltage ratings, breaking capacity and response times, depending on the application. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption. Wiring regulations usually define a maximum fuse current rating for particular circuits. Short circuits, overloading, mismatched loads, or device failure are the prime reasons for fuse operation.

A fuse is an automatic means of removing power from a faulty system; often abbreviated to ADS (Automatic Disconnection of Supply). Circuit breakers can be used as an alternative design solution to fuses, but have significantly different characteristics.

Power-system protection

Power-system protection is a branch of electrical power engineering that deals

with the protection of electrical power systems from faults through the isolation

of faulted parts from the rest of the electrical network. The objective of a

protection scheme is to keep the power system stable by isolating only the

components that are under fault, whilst leaving as much of the network as

possible still in operation. Thus, protection schemes must apply a very pragmatic

and pessimistic approach to clearing system faults. For this reason, the technology

and philosophies utilized in protection schemes can often be old and well-

established because they must be very reliable.

Components

Protection systems usually comprise five components:

• Current and voltage transformers to step down the high voltages and currents

of the electrical power system to convenient levels for the relays to deal with

• Protective relays to sense the fault and initiate a trip, or disconnection, order;

• Circuit breakers to open/close the system based on relay and autorecloser

commands;

• Batteries to provide power in case of power disconnection in the system.

• Communication channels to allow analysis of current and voltage at remote

terminals of a line and to allow remote tripping of equipment.

For parts of a distribution system, fuses are capable of both sensing and

disconnecting faults.

Failures may occur in each part, such as insulation failure, fallen or broken

transmission lines, incorrect operation of circuit breakers, short circuits and open

circuits. Protection devices are installed with the aims of protection of assets, and

ensure continued supply of energy.

Switchgear is a combination of electrical disconnect switches, fuses or circuit

breakers used to control, protect and isolate electrical equipment. Switches are

safe to open under normal load current, while protective devices are safe to open

under fault current.

Protective devices

Protective relays control the tripping of the circuit breakers surrounding the

faulted part of the network

Automatic operation, such as auto-re-closing or system restart

Monitoring equipment which collects data on the system for post event analysis

While the operating quality of these devices, and especially of protective relays, is

always critical, different strategies are considered for protecting the different

parts of the system. Very important equipment may have completely redundant

and independent protective systems, while a minor branch distribution line may

have very simple low-cost protection.

There are three parts of protective devices:

• Instrument transformer: current or potential (CT or VT)

• Relay

• Circuit breaker

Advantages of protected devices with these three basic components include

safety, economy, and accuracy.

Safety:

Instrument transformers create electrical isolation from the power system, and

thus establishing a safer environment for personnel working with the relays.

Economy:

Relays are able to be simpler, smaller, and cheaper given lower-level relay inputs.

Accuracy:

Power system voltages and currents are accurately reproduced by instrument

transformers over large operating ranges.

Performance measures

Protection engineers define dependability as the tendency of the protection

system to operate correctly for in-zone faults. They define security as the

tendency not to operate for out-of-zone faults. Both dependability and security

are reliability issues. Fault tree analysis is one tool with which a protection

engineer can compare the relative reliability of proposed protection schemes.

Quantifying protection reliability is important for making the best decisions on

improving a protection system, managing dependability versus security tradeoffs,

and getting the best results for the least money. A quantitative understanding is

essential in the competitive utility industry.

Performance and design criteria for system-protection devices include reliability,

selectivity, speed, cost, and simplicity.

Reliability: Devices must function consistently when fault conditions occur,

regardless of possibly being idle for months or years. Without this reliability,

systems may result in high costly damages.

Selectivity: Devices must avoid unwarranted, false trips.

Speed: Devices must function quickly to reduce equipment damage and fault

duration, with only very precise intentional time delays.

Economy: Devices must provide maximum protection at minimum cost.

Simplicity: Devices must minimize protection circuitry and equipment.

Protective relays

These are compact analog or digital networks, connected to various points of an

electrical system, to detect abnormal conditions occurring within their assigned

areas. They initiate disconnection of the trouble area by circuit breakers. These

relays range from the simple overload unit on house circuit breakers to complex

systems used to protect extra high voltage power transmission lines. They operate

on voltage, current, current direction, power factor, power, impedance,

temperature. In all cases there must be a measurable difference between the

normal or tolerable operation and the intolerable or unwanted condition. System

faults for which the relays respond are generally short circuits between the phase

conductors, or between the phases and grounds. Some relays operate on

unbalances between the phases, such as an open or reversed phase. A fault in one

part of the system affects all other parts. Therefore relay sand fuses throughout

the power system must be coordinated to ensure the best quality of service to the

loads and to avoid operation in the non-faulted areas unless the trouble is not

adequately cleared in a specified time.

Residual Current Protective Devices (RCD)

Residual current protective devices protect against dangerous electric

shocks caused by indirect contact. They are used for personal, material

and fire protection, as well as for additional protection against direct

contact. Special applications require special solutions: for example,

personal and fire protection, as well as in networks loaded with direct

current or installed in aggressive environments. The RCD has built-

incondensation protection to ensure maximum safety and durability even

under harsh conditions such as gas or moisture in the ambient air.

RCDs can be wired to protect a single or a number of circuits - the

advantage of protecting individual circuits is that if one circuit trips, it

will not shut down the whole house, just the protected circuit.

RCDs are available in at least 4 basic configurations:

As hard wired in units, where both the inputs and outputs are wired into the unit -

ideal for a workshop etc where all the sockets within can be protected. Each

individual circuit taken from the RCD is protected by a MCB of an appropriate

value.

As protected outlets - normally a protected socket can be fitted as a direct

replacement for a standard, no protected outlet socket.

As a plug-in unit which can convert any socket into to a protected circuit - this

gives good flexibility as, for example, a lawn mower or a hedge trimmer can be

plugged in at different times. However, as the individual appliance could still be

plugged into an unprotected socket, you need to remember to fit the

As a plug for wiring on to the lead of an individual appliance, this does make it less

flexible than the plug-in unit above but it does ensure that the piece of equipment

is always protected. One very usefully use to to fit it to the end of an extension

cable, then whatever you plug into the extension lead is protected.

The basic principle of operation of the RCD

When the load is connected to the supply through the RCD, the line and neutral conductors

are connected through primary windings on a toroidal transformer. In this arrangement the

secondary winding is used as a sensing coil and is electrically connected to a sensitive relay

or solid state switching device, the operation of which triggers the tripping mechanism.

When the line and neutral currents are balanced, as in a healthy circuit, they produce equal

and opposite magnetic fluxes in the transformer core with the result that there is no current

generated in the sensing coil. (For this reason the transformer is also known as a 'core

balance transformer').

When the line and neutral currents are not balanced they create an out-of-balance flux. This

will induce a current in the secondary winding which is used to operate the tripping

mechanism.

It is important to note that both the line and neutral conductors pass through the toroid. A

common cause of unwanted tripping is failure to connect the neutral through the RCD.

RCDs work equally well on single phase, three phase or three phase and neutral circuits, but

when the neutral is distributed it is essential that it passes through the toroid.

RCD Test Circuit

A test circuit is always incorporated in the RCD. Typically the operation

of the test button connects a resistive load between the line conductor

on the load-side of the RCD and the supply neutral.

The test circuit is designed to pass a current in excess of the tripping

current of the RCD to simulate an out-of-balance condition. Operation of

the test button verifies that the RCD is operational. It is important to

note, therefore, that the test circuit does not check the circuit protective

conductor or the condition of the earth electrode.

On all RCDs a label instructs the user to check the function of the RCD at

regular intervals and to observe that the RCD trips instantly.

Residual Current Breaker with Overload protection (RCBO)

A RCBOs combines the functions of a MCB and a RCD in one unit. They are used to

protect a particular circuit, instead of having a single RCD for the whole building.

Generally these are used more often in commercial building than domestic ones.

Residual Current Circuit Breakers tests

Tripping Test and Performance of Test Device Test as per IS 12640-2000 and IEC

61008 Clause 9.1.3 (Annex D) listed below:

Tripping Test:

• Sensitivity Test: The RCCB should trip at a value of pre decided residual current

(IAn); and should not trip at a value of 0.5 IAn.

• Speed of Operation Test: During the sensitivity trip test, time of tripping is

recorded and compared with the limit.

Performance of Device Test: To check the function of the push button, whether the

RCCB trips when push button is pressed.

Salient Features and Specifications:

• Leakage Current Ranges: 10 mA, 30 mA, 50 mA, 100 mA, 300 mA and 500 mA

• Current Adjustment: Between 10 mA to 550 mA

• Time Measurement: 5 digit with ranges: a. 0-99.999s & b. 1 ms

• Control: Sequence of operation and time measurement through specially

designed micro controller based unit with PC

• Current Measurement: Digital Mill ammeter, 3 V2 digit, 3 ranges

• Fixture: Electro pneumatic fixture

• Protection: Through MCB

• Size: 850 mm X 650 mm X 500 mm

Impulse Voltage With-stand Test Eguipment (with automatic seguencing &

polarity)

The Impulse Voltage Test Equipment is designed to generate impulse voltage (both

polarity) of 1.0 KV to 20 KV depending on the model chosen. The waveform

generated has a rise time of 1.2 micro second and 50 micro second duration. This

instrument (suitable model based on peak voltage) incorporates testing as per the

procedure described in IEC 61180, IEC 61010, IEC 60950, IEC 60065, IEC 60335-1,

IEC

60230.

The equipment also incorporates testing as per the below standards:

• Low Voltage Switchgear and Control gear: IS/IEC 60947 -2004 Cl. No: 8.3.3.4.1

• Circuit Breakers: IS/IEC 60898:2002 Cl.no.: 9.7.6.1 & 97.6.2

• Residual Current Operated breakers: IEC 61008 & 61009 : Cl.no.: 9.20

The Surge Protection Device (SPD)

The Surge Protection Device (SPD) is a component of the electrical installation

protection system.

This device is connected in parallel on the power supply circuit of the loads that it

has to protect. It can also be used at all levels of the power supply network.

This is the most commonly used and most efficient type of overvoltage protection.

PROCTECTION DEVICE

Components (Equipment) Do We Protect?

• •Generators

• •Transformers, Reactors

• •Lines

• •Buses

• •Capacitors

Components (Equipment) Do We Protect

• Protective relays monitor the current and/or voltage of the power system to detect problems with the power system. Currents and voltages to relays are supplied via CT’s and PT’s.

• Current Transformer (CT)

• A device which transforms the current on the power system from large primary values to safe secondary values. The secondary current will be proportional (as per the ratio) to the primary current.

• Potential Transformer (PT) A device which transforms the voltage on the power system from primary values to safe secondary values, in a ratio proportional to the primary value

• I. Generator Protection A. Construction & Theory of Operation

Generator Protection

How Do We Protect the Stator?

A. Differential Protection (what goes in must come out)

1. Detects phase-phase faults

B. Stator Ground Protection

1. 59N (95% of Stator)

2. Third Harmonic Voltage Method (100% of Stator)

3. Signal Injection (100% of Stator)

Generator Protection How Do We Protect the Rotor?

1.Loss of Field a. Impedance

Transformer Protection

Bus Protection

Bus Differential:

Current into bus must equal current out of bus

Bus Protection

Bus Fault

• Bus Protection

Double bus, double breaker

• Feeders’s protection

• As the length of electrical power transmission line is generally long enough and it runs through open atmosphere, the probability of occurring fault in electrical power transmission line is much higher than that of electrical power transformers and alternators. That is why a transmission line requires much more protective schemes than a transformer and an alternator.Protection of line should have some special features, such as-

• During fault, the only circuit breaker closest to the fault point should be tripped.

• If the circuit breaker closest to the faulty point, fails to trip, the circuit breaker just next to this breaker will trip as back up.

• The operating time of relay associated with protection of line should be as minimum as possible in order to prevent unnecessary tripping of circuit breakers associated with other healthy parts of power system.

• Line Protection by Definite Time Relay

• This protection scheme is very simple. Here total line is divided into different sections and each section is provided with definite time relay. The relay nearest to the end of the line has minimum time setting while time setting of other relays successively increased, towards the source. For example, suppose there is a source at point A, in the figure below

• Over Current Protection of Parallel Feeders

For maintaining stability of the system it is required to feed a load from source by two or more than two feeders in parallel. If fault occurs in any of the feeders, only that faulty feeder should be isolated from the system in order to maintain continuity of supply from source to load. This requirement makes the protection of parallel feeders little bit more complex than simple non direction over current protection of line as in the case of radial feeders. The protection of parallel feeder requires to use directional relays and to grade the time setting of relay for selective tripping.

•End Of PPT