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The modern society is so much dependent upon the use of electric energy that it has become a part parcel our life. The present day advancement in science and technology has made it possible to convert electric energy into any desired form. This has given electric energy a place of pride in the modern world. The importance of electric supply in everyday life has reached such a stage that it is desirable to protect the power system from harm during fault condition and to ensure maximum continuity of supply. For this purpose from reliable point of view the protective device used in power system takes an important role of this system. Since in our Bangladesh, Distribution Substation Distributed electric power to the total area of Bangladesh .So we want to study of electrical power protection scheme of Distribution Substation


A particular type of equipment used in electric power systems to detect abnormal conditions and to initiate appropriate corrective action known as protective device.

Equipment applied to electric power systems to detect abnormal and intolerable conditions and to initiate appropriate corrective actions. These devices include lightning arresters, surge protectors, fuses, and relays with associated circuit breakers, reclosers, and so forth.

From time to time, disturbances in the normal operation of a power system occur. These may be caused by natural phenomena, such as lightning, wind, or snow; by falling objects such as trees; by animal contacts or chewing; by accidental means traceable to reckless drivers, inadvertent acts by plant maintenance personnel, or other acts of humans; or by conditions produced in the system itself, such as switching surges, load swings, or equipment failures. Protective devices must therefore be installed on power systems to ensure continuity of electrical service, to limit injury to people, and to limit damage to equipment when problem situations develop. Protective devices are applied commensurately with the degree of protection desired or felt necessary for the particular system.


The development of any country of the world is based on electricity & without it on industry is impossible. So we should take great about reliability & stability of a power system. Protective device serve this purpose sufficiently. It is to be noted that protective device can not remove the fault, it protect the equipment from harmful damage. As the fault in the equipment in the supply system leads to disconnection of supply to a large portion of the system. If the fault part is quickly disconnected the damage caused by the fault is minimum & the faulty part can be repaired quickly & the service can be restored without further delay. Better service continuity has its own merits.

As a protection of engineer, we should have to gain a best knowledge about protective device that are used in our distribution system. For a better understanding we can denote ourselves to the study of the protective device in a distribution substation.

We can summaries the objective of the study in the following lines:

1. To Study of substation & substation equipment.

2. To Study of protective devices of a distribution substation.

3. To study of protective devices used in Vurulia 33/11kv distribution


4. To develop suggestion for better protection.


Substation is an important part of power system. The continuity of supply depends to a considerable extent upon the successful operation of sub-station. It is therefore essential to exercise utmost care while designing and building substation. The following parts are important point which must be kept in view while laying out a substation.


Substation: The assembly of apparatus used to change some characteristic (e.g. Voltage, a.c to d.c, frequency, p.f. etc) of electric supply is called a substation.

Importance of substation:

1. It should be located at a proper site as far as possible it should be at the center of load.

2. It should provide safe and reliable arrangement for safety consideration must be given to the maintenance abnormal occurrence such as possibility of explosion or fire etc. For reliability, Consideration must be given for design and constriction. The provision of suitable protection gear etc.

3. It should involve minimum capital cost.

Classification of substation:

According to service requirement:

1. Transformer substation.

2. Switching substation.

3. Power factor correction substations.

4. Frequency changer substations.

5. Converting substation.

6. Industrial substation.

According to constructional feature the substation are classified as:

1. Indoor substation.

2. Out door substation.

3. Underground substation.

4. Pole mounted substation.


Those substations which change the voltage level of electric supply are called transformer these substation receive power at some voltage and deliver it at some other voltage. Obviously, transformer will be the main component in such substation. Most of the substation in the power system is of type this.


Those substations which improve the power factor of the system are called power factor correction substation. Such substation is generally located at the receiving end of transmission the power factor improvement equipment.


These substations do not change the voltage level i.e. incoming and outgoing lines have the same the voltage. However they simple perform the switching operations of power lines.


Those substations which change the frequency are known as change substation, such a frequency change may be rewired for industrial utilization.


Those substation which change a.c power into d.c power are called converting substation. These substations which supply power with suitable apparatus (e.g. Ignitron) to supply for such purpose as traction, electroplating, electric welding.


Those substations which supply power to individual industrial concern is known as industrial substations.


For voltage upto 11kv the equipment of the substation is installed indoor because of economic consideration .However when the atmosphere is contain with impurities these substation can be erected for voltage upto 66kv .


For voltage beyond 66kv ,equipment is invariably installed out door .It is because for such voltage the clearance between conductor and the space required for switch , circuit breaker and equipment becomes so great it is not economical to install the equipment indoor


In thickly populated areas, the space available for equipment and building is limited and cost of land is high. Under such condition the substation is created underground. The reader may further discussion on underground substation.


This is an out door subs station with equipment installs over head on H-pole or 4-pole structure. It is cheapest from of substation for over voltage not exceeding 11KV (or 33 KV in the some case. Electric power is almost distributed in localities through such substation. For complete discussion on pole mounted substation.


The following equipments are essential for a distribution sub-station:

1. Transformer

2. Power Transformer

3. Instrument Transformer

4. Current Transformer (C.T)

5. Voltage Transformer (P.T).

6. Isolator

7. Insulator

8. Line Support

9. Metering and Indicating Instrument

10. Bus bars

11. Power Factor Improvement Device.

12. Voltage Regulator

13. Earthing


Transformer:A transformer is an electrical device that transfer energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled <href=”#Electromagnetic” title=”Coil”>windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. An alternating current in one winding creates a time varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.

Figure 2.1 Transformer.


A power transformer is used in a substation to step-up or step-down the voltage. Except at the power station, all the subsequent substations use step-down transformers to gradually reduce the voltage of electric supply and finally deliver it at utilization voltage. The modern practice is to use 3-phase transformer in substation, although 3-phase bank of transformer can also be used. The use of 3-phase transformer (instead of 3 single phase bank of transformers) permits two advantages. Firstly, only one 3-phase load tap changing machine can be used. Secondly, its installation much simpler than the three single phase transformers.

The power transformer is generally installed upon lengths of rails fixed on concrete slabs having foundations 1 to 1.5m deep. For rating unto 10MVA, naturally cooled, oil immersed transformers are used. For higher ratings, the transformers are generally air blast cooled.

Figure 2.2 Three phase power transformer.


The lines in substations operate at high voltages and carry current of thousands of amperes. The measuring instruments and protective devices are designed for low voltage (generally 110) and currents (about 5A). Therefore, they will not work satisfactorily if mounted directly on the power lines. The function of these instrument transformers is to transfer voltages or currents in the power lines to values which are convenient for the operation of measuring instruments and relays.

There are two types of instruments.

1. Current Transformer (C.T).

2. Potential Transformer (P.T).


A current transformer in essentially a step-up transformer which steps down to a known ratio. The primary of this transformer consist of one or more turns of thick wire connected in series with the line. The secondary line consist of a large number of turns of fine wire and provides for the measuring instruments and relays a current which is a constant fraction of the current in the line. Suppose a current transformer rated at 100/5A is connected in the line to measure current. if the in the line is 100A, then current in the secondary will be 5A. Similarly, if current in the line is 50A, then secondary of C.T. will have a current of 2.5A. Thus the C.T. under consideration will step down the line current by a factor of 20.


It is essentially a step down the voltage to a known ratio. The primary of this transformer consist of a large number of turns of fine wire connected across the line. The secondary winding consist of a few turns and provides for measuring instrument and relay a voltage which is a known fraction of the line voltage. Suppose a potential transformer rated at 66KV/110V is connected to a power line. If line voltage is 66KV, then voltage across the secondary will be 110V.


Isolator is a disconnecting switch, which operate under no load condition. It has no any specified current breaking capacity or current making capacity. Isolator is not even used for breaking load current.

Figure 2.3: Isolator.

In some case isolators are used for breaking charging currents of x-mission lines, Isolators are used in addition to circuit breakers while opening a circuit. The circuit breaker is opened first. Then isolator While closing a circuit the isolator are necessary on the supply side of circuit breakers in order to ensure isolation of the circuit breaker form live parts for the purposes of maintenance. The operating mechanism is manual plus one of the following:

i. Electrical motor mechanism.

ii. Pneumatic mechanism.


The insulator serves two purposes. They support the conductor and confined the current in the conductors. The most commonly used material for the manufacture of

Insulator porcelain:

There are several kinds of insulator (e.g. pin type, suspension type, post insulator etc.) and their use in the sub-station will depend upon the service requirement. For example, post Insulator is used for bus bars. a post insulator consists of a porcelain body, cast iron cap and flagged cast iron base. The hole in the cap is threaded so that bus bars can be directly bolted to the cap.

Types of line insulation:

a. Pin type insulators. d. Shackle insulator.
b. Suspension type insulators. e. Stay insulator:
c. Strain insulators. f. Guy insulator

Pin type insulators:

Pin type insulators are used for transmission and distribution of electric power voltage up to 33KV.


Figure 2.4 Pin type insulators.


For high voltage i.e. beyond 33KV transmission line, Suspension type insulators used. This type insulator consists of a number of porcelain discs connected in series by the metal links in the form of strength. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross?arm of the tower. Each unit or discs is designed for 11KV. The number of discs in series would obviously depend upon the working voltage.

Figure 2.5 Suspension type insulators.


When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. In order to relieve the line of excessive tension, strain insulators are used. For low voltage lines shackle insulators are used as strain insulators. For high voltage transmission lines, strain insulator consists of an assemble of suspension insulators.

The discs of strain insulators are used in vertical plane.

Figure 2.6: Strain insulators


In Distribution Substation guy insulators are used in low voltage distribution.


For low voltage lines, the stays are to be insulated from ground at a height not less than 13 meters from ground.


Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross?arm. The conductor in the groove is fixed with a soft binding wire.

Figure 2.7: shackle insulator


The supporting structure for overhead line conductors are various types of poles and tower called line supports.

Classification of line supports:

1) Wooden poles.

2) Steel tower.

3) Reinforce concrete (RCC) poles.

4) Steel tubular pole.


Wooden poles used for low voltage distribution purpose. The wooden poles generally tend to rote below the ground level, causing foundation failure.


It is used instead of wooden pole in urban area or town for increasing vision satisfactory. It is also stronger than the wooden pole. Such poles are generally used for distribution purpose in the cities. In BPDB steel tubular poles are used distribution system.


RCC poles have greater mechanical strength, longer life and permit longer spans than steel poles; they require little maintenance and have good insulating properties. In BPDB, RCC poles are used in 11KV and 33KV transmission systems.


For long distance transmission line at higher voltages, steel towers are invariably employed. Steel tower have greater mechanical strength, longer life can withstand most severe climatic conditions and permit the use of longer spans. In BPDB steel towers are used in single circuit and double circuit transmission line, which has about 132KV and 230KV.


There are several metering and indicating instrument (e.g. ammeters, volt meters energy meters etc.) install in a substation to maintain watch over the circuit quantities. The instrument transformer are invariably used with them for satisfactory operation

2.2.10 BUS-BAR:

When a number of generator or feeders operating at the same voltage have to be directly connected electrically, bus-bar are used as the common electrical component.

Bus-bars are copper rods or thin walled tubes and operated at constant voltage. Thus electrical bus bar is the collector of electrical energy from one location.

The selection of any bus bar system depends upon the following:

1. Amount of flexibility required in operation.

2. Immunity from total shut-down.

3. Initial cost of the installation.

4. Load handled by the bus bar.

Classification of bus bar:

1. Single bus bar system.

2. Sectionalized bus bar.

3. Duplicate bus bar.

4. Ring bus bar.

5. One and half breaker arrangement.

Arrangement of different types of bus bar and is advantages and disadvantages:


Figure 2.8: Single Bus bar.


1. It is cheapest arrangement as only one circuit breaker for each outgoing circuit breaker is required.

2. The relaying on this system is simple. It should be noted that in this system the relaying on each of the circuit and the bus bar is only required.

3. Due to the absence of the transfer breaker and disconnections, the operation has become simple. For de-energizing a circuit only the associated circuit breaker is to be opened.

4. The maintenance cost, which is only dependent upon the number of breakers, will be appreciably low for a single bus bar system.


1. The biggest disadvantages of this system is complete shut-down of the line in case of a bus bar fault.

2. It is not possible to have any regular maintenance work on the energized bus bar.

3. When a breaker on any circuit of a single bus bar system fails, the will be complete shut-down of the station , for however re-energizing first the effected circuit breaker is disconnected from the bus bar with the help of isolator .

4. For maintaining or repairing a circuit breaker, the circuit is required to be disconnected from the bus bar.

5. If any stage, a circuit is required to be added to the existing single bus bar arrangement,


Figure 2.9: Single Bus bar system with Sectionalisation


1. In this system, only one additional breaker will be needed, thus its cost in comparison to single bus bar system will not be much.

2. The operation of this system is as simple as that of single bus bar.

3. The maintenance cost of this system is comparable with the single bus bar.

4. For maintaining or repairs of the bus bar only one-half of the busber is required to be de-energized and possibility of complete shut-down is thereby avoided.

5. It is possible to utilize the bus bar potential for the line relays.


1. On the bus bar fault, one half of the station will be switched off.

2. For regular maintenance also, one of the bus bar is required to be de-energized.

3. For maintaining or repairing a circuit breaker, the circuit is required to be isolates from the bus bar.


Figure 2.10 Double bus bar systems with one circuit breaker per circuit


1. It ensures supply in case of bus fault, in case of any fault in one of the bus, the circuit can be transferred to the transfer bus.

2. The circuit breaker can be maintained with uninterrupted supply as the load can be transferred to the other bus through the bus coupler circuit breaker.

3. It is easy to connect the circuit from either bus.

4. The maintenance cost of substation decreased.

5. The bus potential can be used for relays.


The bus is maintained or expanded by transferring all of the circuit to the transfer or auxiliary bus depending upon the remote back up relays and breaker for eliminating faults of the circuit’s. During this connection a line fault on any pf the circuits of the bus would shut down the entire circuit.



1. It provides double feed to all the feeders at minimum cost.

2. At the time of failure of the circuit breaker of bus section only the effective circuit goes out of service while the heal by circuits are not affected .

3. The arrangement is quite economical as the number of breakers used is nearly the same as that of a single bus bar system.


1. The circuit has to be energized while the maintenance of the bus is carried out, although it may be possible to arrange tripping of supply to the concerned feeder.

2. It is necessary to supply potential to relays separately to each of the circuit.

3. The operating of any section of the breaker may cause overloading of the circuits because power can flow in one direction only.

5. It is difficult to add any new circuit to the ring


Power factor:

The cosine angle between voltage and current in an a c circuit is known as power factor.

Causes of low power factor:

Low power factor is undesirable for economic point of view. Normally, the power factor of the whole load on the supply system in lower than 0.8 .The following are the causes of low

1. Most of the c motor is of induction type which have low lagging power factor.

This motor work at a power factor which is extremely small on light load (0.2 to

0.3) and rises to 0.8 or 0.9 at full load power factor:

2. Arc lamps, electric discharge lamp and industrial heating furnaces operate at low lagging power factor.

3. The load on the power system is varying, being high during morning and evening and low at other times. During low load period, supply voltage is increased which increases the magnetization current. This results in the decreased power factor.

Methods of Power factor improvement:

Normally, the power factor of the whole load on a large generating station is in the region of 0.8 to 0.9. However, sometimes it is lower in such cases it is generally desirable to take special steps to improve the power factor. This can be achieved by the following equipments

a. Static capacitor.

b. Synchronous condenser.

c. Phase advancers.


The power factor can be improved by connecting capacitor in parallel with the equipment operating at lagging power factor. The capacitor draws a leading current and partly or completely neutralizes the lagging reactive component of load current. This raises the power factor of the load. For three phase loads, the capacitor can be connected in delta or star as shown in figure. Static capacitors are invariably used for power factor improvement in factories.

Figure 2.11 Power factor improvements by Static capacitor


A synchronous motor takes a leading current when over excited and therefore, behaves as a capacitor. And over excited synchronous motor running on no load is called synchronous condenser. When such a machine is connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the load. Thus the power factor is improved.


Phase advancers are used to improve the power factor of induction motors. The low power factor of an induction motor is due to the fact that its stator winding draws exciting current which lags behind the supply voltage by 90°. If the exciting ampere turns can be provided from some other a.c. source, then the stator winding will be relieved of exciting current and the power factor of the motor can be improved. This job is accomplished by the phase advancer which is simply an a.c. exciter. The phase advancer is mounted on the same shaped as the main motor and is connected in the rotor circuit of the motor. It provides exciting ampere turn to the rotor circuit at slip frequency. By providing more ampere turns then required, the induction motor can be make to operate on leading power factor like an over excited synchronous motor.

The electrical energy is almost exclusively generated, transmitted and distributed in the form of alternating current. Therefore the question of power factor immediately comes into picture. Most of the loads are inductive in nature and hence have low lagging power factor. The low power factor is highly undesirable as it causes an increase in current, resulting in additional losses of active power in all the elements of power system from power station generator down to the utilization devices. In order to ensure most favorable conditions for a supply system from engineering and economical standpoint, it is important to have power factor as close to unity as possible.


A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages. With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a negative feedback servo control loop. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. For some regulators if the output voltage is too high, the regulation element is commanded to produce a lower voltage; however, many just stop sourcing current and depend on the current draw of whatever it is driving to pull the voltage back down. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response.

Figure 2.12Voltage regulator

2.2.13 EARTHING:

The word ‘earth’ or ‘ground’ means many different things to many electrical engineers. In an electrical installation these words can be used to mean either the protective conductor in a mains cord; the common bonding network of the building; the earth mass electrodes of the lightning protection system, or the conductor of the mains supply that is connected to an earth mass electrode at the distribution transformer.

Method of Earthing

The useful method of earthing is to join the exposed metal to earth via continuity conductors connected to an electrode buried in the ground. Three elements required for earthing systems are Earth conductor, Earthing lead and Earth electrode.

Earth conductor:

This is the part of earthing system, which joins or bonds together all the metal parts of an installation. The earth conductor shall have a short tine capacity adequate for the fault current which can floe in the grounding conductors for the operating time of the system.

The following table gives the minimum size of copper circuit conductor:

Minimum cross-sectional area of the copper Earth conductors in relation to the area of associated phase conductors:

Table -2.1

Cross-sectional area of the phase conductors (mm2) Minimum cross-sectional area of the corresponding Earth conductors (mm2)
Less than 16 Same as cross-sectional area of the phase conductor but not less then 14 SWG.
16 or greater but Less than 35 16
35 or greater Half of the cross-sectional area of the phase conductor.

Earthing lead:

Earthing leads are the link, which provides connection between the earth conductors and the earth electrode.

Earth electrodes:

The Earth electrodes shall as far as practicable into permanently moist soil p-referable below associated ground water table. The resistance of earth electrodes shall be not more then one ohm. Some important thing for the Earth electrodes:

-Copper rod shall have a minimum diameter of 12.7,

GI pipes shall have a minimum diameter of 50mm Copper plates shall not be less then 600mm,6000 in size, with 6mm thickness.



1) Fuse

2) High rupturing capacity (H.R.C.) cartridge fuse

3) Relay

4) Circuit Breaker

5) Lightning Arrester

6) Auto Reclosur

7) Isolator

8) Earthing

9) Current Limiting Reactor

10) Insulator

3.1 FUSE

A fuse is a short piece of metal, inserted in the circuit which is melt when excessive current flow through it thus breaks the circuit. The fuse element generally made of materials having melting point & conductivity. It is inserted in series with the circuit to be protected under normal condition it carries normal current without over heating. But abnormal condition the increasing current flow through the fuse, produce high temperature and then fuse element melt & disconnecting the circuit.

Figure 3.1: Drop out fuse

Properties of a fuse element:

The function of a fuse is to carry the normal current without overheating but when the current exceeds its normal value; it rapidly heats up to melting point and disconnects the circuit protected by it.

The fuse element should have the following desirable characteristics:

1. High conductivity.

2. Low melting point.

3. Least deterioration due to oxidation.

4. Low coast e.g. lead, tin, copper.


Current rating of fuse element:

It is the current which the fuse element can normally carry with over heating or melting. It is depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundingof the fuse.

Figure 3.2: Cut off characteristics of fuse

Fusing Current:

It is the minimum current at which the fuse element melt and thus disconnects the circuits protect by it. It value will be more than the current rating of the fuse element. Mathematically it represent by the following



I = fusing current

d = Diameter of the wire. And

K = Fuse constant. Its value depends upon the metal of which the fuse element is made. Sir W.H. Preece found for the different materials the value of k given in the table

Table: 3.1

Sl. No. Material Value of K
d in cm d in mm
1 Copper 2530 80
2 Aluminum 1873 59
3 Tin 405.5 12.8
4 Lead 340.6 10.8

Fusing factor:

It is the ratio of minimum fusing current rating of fuse element.

Fusing factor =Minimum fusing current/current rating of fuse.

Its value always more than one. The fusing factor is two fore semi enclosed or rewirable fuse.

Pre –arching time:

It is time between the commencement of fault and the instant when cut-off occurs.

The pre-arching time is very low, typically 0.001 second.

Cutoff current :

It is the maximum value of fault current reached before the fuse melts.

The cut off value depends upon

a. Current rating of fuse

b. Value of prospective current

c. Asymmetry of short circuit current.

Prospective current:

It is the r.m.s. value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance.

Arcing time:

This is the time between the end of pre-arching time and instant when the arc is extinguished.

Operating time:

It is the sum of pre-arcing and arcing time. It may be operating time of a fuse is generally quit low (say 0.002sec) as compared to a circuit breaker (say 0.2sec.).

Breaking capacity:

It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with a rated service voltage.


4 Low voltage fuse

a. Semi-enclosed rewirable fuse

b. High rupturing capacity cartridge fuse

c. H.R.C. fuse with tripping device.

4 High voltage fuse

a. Cartridge type fuse.

b. Liquid type fuse.

c. Metal clod fuse.


Low voltage fuses

a) Semi enclosed rewirable fuse

Semi enclosed rewirable fuse are made up to 500A rated current, but their breaking capacity is low about 4000A on 400V service. The use of this type of fuse is domestic and lighting loads and used where of fault current are to be interrupted. It consist of

1. A base and

2. A fuse carrier.

The base is porcelain and carries the fixed contacts to which the incoming and outgoing phase wires are connected. The fuse carrier is also of porcelain hold the fuse element between its terminals.

b) High rupturing capacity (H.R.C.) cartridge fuse

Figure 3.3:High rupturing capacity (HRC) Cartridge fuse

The H.R.C. fuse consist of a heat resisting ceramic body having metal end cap to which is welded silver current-carrying element .The space within the body

Surrounding the element is completely packed with a filling powder. The filling material may be chalk, plaster of pairs, quartz or marble dust and acts as an arc quenching and cooling medium. Under normal condition, the fuse element is at a temperature below its melting point, it carries normal current without overheating. When fault occurs the current increases and the fuse element melts.

Advantage of High rupturing capacity (H.R.C.) cartridge fuse

i) They are capable of clearing high as well as low fault currents.

ii) They have high speed of operation.

iii) They provide reliable discrimination.

iv) They require no maintenance.

v) They provide reliable discrimination.

vi) They permit consistent performance.

vii) They do not deteriorate with age.

Disadvantage of High rupturing capacity (H.R.C.) cartridge fuse

i) Heat produced by the arc may affect the associated switches.

ii) They have to be replaced after each operation.

c) H.R.C. fuse with tripping device

Some times H.R.C. fuse provided with a tripping device .When the fuse blow under fault condition the tripping device cause the circuit breaker to operate. The body of the fuse is of ceramic materials with a metallic cap rigidly fixed at each end. These are connected by a number of silver fuse elements. At one end is a plunger, which under fault condition electrically connected through a fusible link, chemical charge and a tungsten wire to the other end of the cap as shown.

Figure 3.4: H.R.C Fuse

When a fault occurs the silver fuse elements are the first to be blown out and then current is transferred to the tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charges to be detonated. This forces the plunger outward to operate the circuit breaker. Low voltage H. R.C. fuses may be built with a breaking capacity of 1600A to 3000A at 440v.

4Some type of the high voltage fuses are:

a) Cartridge type

High voltage cartridge fuse are used up to 33Kv with breaking capacity of about 8700A at that voltage. Rating of the order of 200A at 6.6kv and 11kv and 50A at 33kv are available. In this device there are two fuses element in parallel; one of low resistance (silver wire) and other of high resistance (tungsten wire). Under normal condition, the low resistance element carries the normal current. And a fault condition the low resistance element blown out and the high resistance element reduce the short circuit current and finally breaks the circuit.

b) Liquid type

The liquid type of fuse are used for circuit up to about 100A rated current on system up to 132kv and may have breaking capacities of the order of 6100A. it consist of glass tube filled with carbon tetrachloride solution and sealed at both with brass caps .The fuse wire is sealed at one end of the tube and the other end of the wire is held by a strong phosphor bronze spiral spring fixed at the other end of the glass tube. When the current exceeds the limit the fuse wire is blown out.



1. Low cost.

2. They require no maintenance.

3. They have high speed.

4. They are cheaper then other circuit interrupting devices of equal breaking capacity.


1. This type of fuse has a low –breaking Capacity and hence cannot be used in circuit of fault level.

2. There is a possibility of renewal by the fuse wire of wrong size.

3. They have to be replaced after each operation.

4. Heat produced by arc may affect the associated switches.

5. The fuse operates of a lower current than originally rated.


In a power system consisting of generator, transformer, transmission and distribution circuits. It is inevitable that sooner or later some failure will be occurring some where in the system. When a failure occurs on any part of the system it must be quickly detected and disconnected from the system. There are two principle reasons for it firstly if the fault is not clear quickly. It may cause unnecessary interruption of service to the customers. Secondly rapid disconnection of fault apparatus limits the amount of damage to it and prevents the effects of fault form spreading in to the system. The detection of fault and disconnection of a faulty section or apparatus can by relays in conjunction with C.B.

Figure 3.5: Protective Relay

A protective relay is a device that detects the fault and initiatives the operation of the circuit breaker to isolate the defective element from the rest of the system. The protective relay should have the following quantities

I) Selectivity ii) Speed iii) Sensitivity

iv) Reliability v) Simplicity VI) Economy


It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system.


It is the ability of the relay system to operate with low value of actuating quantity. Sensitivity of relay is a function of the volt –amperes input to the coil of the relay necessary to cause its operation. The smaller value of the volt –amperes input is the more sensitive relay.


It is the ability of the relay system to operate under the pre-determine conditions. Without reliability the protection would be simple rendered largely ineffective and could even become a liability.


The relay system should be simple so that it can be easily maintained. The simpler the protection system, the greater will be its reliability.

Economy. The most important factor in the choice of a particular protection scheme is the economic aspect.


The high speed relay system decreases the possibility of development of one type of fault into the other more sever type.


Figure 3.6: construction of relay

This diagram shown one phase of 3-phase system for simplicity. The relay circuit connections can be divided into three parts.

1. First part is the primary wiring of a current transformer. Which is connected in series with the line to be protected.

2. Second part consists of secondary winding of C.T. and the relay operating coil.

3. The third part is tripping circuit which may be either ac or dc.

When a short circuit occurs of on the transmission line, the current flowing in the line increasing to an enormous value this results in a heavy current flow through the relay coil causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system.


Pick up current:

It is the minimum current in the relay coil at which the starts to operate.

Pick up current =Rated secondary current of CT ×Current setting

Current setting:

It is often desirable to adjust the pick – up current to any required value. This is known as current setting and is usually achieved by the use of taping on the relay operating coil.

Plug – setting multiplier (P.S.M) :

P.S.M =


Fault current in relay coil

Pick –up current

Fault current in relay coil

Rated secondary current CT ×Current setting

Time –setting multiplier:

A relay is generally provided with control to adjust the time of operation. This adjustment is known as time – setting multiplier.


According to the measurement the relay may be classified as follows

1. Over current relay 2. Over voltage relay.

3. Under current relay 4. Under voltage relay.

According to the structure the relay can be classified as follows:

1. Induction type relay

2. Attracted armature type relay

3. Balance beam relay

4. Salient and plunger type relay [Impedance].

5. Gas operated relay [Buchholz] .

6. Induction disk relay [Electromagnetic]

7. Rectifiers relay

8. Moving coil and moving iron relay [Electromagnetic].

9. Electro dynamic relay.

10. Static electronic circuit measurement relay.

11. Microprocessor Based digital static relay

12. Directional and reverse power relay.

Functional relay types:

1. Induction type reverse power relay.

2. Induction type over current relay.

3. Differential relay

4. Distance relay

5. Tran slay scheme


Attracted armature type relay:

The schematic arrangement of an attracted armature type relay. It consists of a laminated electromagnetic M, cussing a coil C and a pivoted laminated armature . The armature is balanced by a counter weight and consist a pair of spring contract fig at the end. It is basically a single actuating quantity relay.

Fig 3.7:Attracted armature type relay

Under normal operating condition the current through the relay coil C is such that counter weight holds the armature in the position shown. However when a short circuit occur, the current through the relay coil increases sufficiently and the relay armature is attracted upward. The contacted on the relay armature bridge a pair of stationary contact attached to the relay frame. This complete the trip circuit which results in the opening of the circuit breaker and therefore in the disconnection of the faulty circuit.

Distance or impedance Relay:

The operation of this relays discussed so far dependent upon the magnitude of current of power in the protective circuit. However there is another group of relays in which the operation is govern by the ratio of applied voltage to current in the protective circuit such relays are called distance relays. The relay will operate when the ratio V/I are less than a predetermined value.

Fig 3.8: Distance or Impedance Relay

Types of distance relay:

1. Definite distance relay which operates instantaneously for fault up to a predetermined distance from the relay.

2. Time distance relay in which the time of operation is proportional to the distance of fault from the relay point. A fault nearer to the distance relays are produced by modifying either than a fault farther away from the relay.

Definite distance relay:

Figure shows the schematic arrangement of a definite distance relay. The relay is designed that the torques produced by the electromagnets are in the opposite direction. Under normal operating conditions, the pull due to the voltage element is greater than of the current element. Therefore, the relay contacts remain open. However, when a fault occurs in the protected zone, the applied voltage to the relay decreases whereas the current increases. The ratio of voltage to current falls below the predetermined value. Therefore, the pull of the current element will exceed that due to

Figure 3.9: Definite distance type impedance relay

The voltage element and this causes the beam to till in a direction to close the trip contacts.

The pull of the current element is proportional to I2 and that of voltage element to V2consequently the relay will operate when,

Induction disc relay:

Electromagnetic induction relays operate on the principle of induction motor and are widely used for protecting relaying purpose involving a.c. quantities. They are not used in d.c. quantities owing to the principle of operation. An induction relay essentially consists of pivoted aluminum is placed in two alternating magnetic fields ofthe same frequency but displaced in time and space. The torque is produced inthe

Fig 3.10: Shaded pole construction

Disc by the interaction one of the magnetic fields with the currents inducted in the disc by the other.

Induction type relay:

This relay has two four or more electromagnetic in stator .This is energized by the relay coils. The rotor consists of a hollow metallic cylindrical cup. The rotor is free to rotate in the gap between the stationary iron and the electromagnets. In this type of relay, the eddycurrents are produced in the metallic cup. This current interacts with the flux produced by the other electromagnetic and torque is produced. The theory is similar to that of the disc type induction relay.

Balance beam relay:

The schematic arrangement of a balance relay. It consists of an iron armature fastened to a balance beam. Under normal operating conditions, the current through the relay coil in such that the beam is healed in the horizontal position by the spring. When the fault condition the current through the relay coil becomes greater than the pickup value and the beam is attracted to close the trip circuit. This causes the opening of the circuit breaker.

Fig 3.11: Balance beam relay

Gas operated (buchholz) relay:

Figure 3.12: Gas operated (buchholz) relay

Buchholz relay is a gas – actuated relay installed in oil immersed transformer for protection against all kinds of fault .It used to give an alarm in case incipient. When fault is disconnect the transformer from this supply in this system .It is usually installed in the pipe connecting the conservator to the main tank. It is use for excess of 750KVA.

Rectifier relay:

The moving coil relays are used with rectifier relays in such relays the quantities to be measured are rectified and then feed to the moving coil unit. The rectifier relay is not possible to against the high me