AC to DC: Substation Maintenance

Substation Maintenance

Index

1. SUBSTATION

1.1 Introduction

1.2 Classification

1.3 Indoor Substation

1.4 Outdoor Substation

1.5 Selection and Location of Site

1.6 Equipment of a substation

1.7 Substation Auxiliaries

1.8 Earthing

2. SWITCH GEAR AND PROTECTION

2.1CIRCUIT BREAKERS

2.1 1Introduction

2.1 2Arc phenomenon

2.1 3Principles of arc extinction

2.1 4Classification of circuit breakers

2.1 7Types of circuit breakers

2.2Protective Relays

2.2.1 Introduction

2.2.2Electromagnetic Relays

2.2.3Induction Relays

2.2.6Time/TSM curves

2.2.8Functional Relay type

2.2.9 Induction type over current relay
2.2.10Induction Type Directional over current relay

Introduction-

Substations serve as source of power for the local areas of distribution in which these are located. Their main function is to receive power transmitted at high voltage from generating stations and reduce the voltage appropriate for local utilization and provide facilities for switching. Some substation are simply switching substations where different connections between various transmission lines are made, others are converting substations which convert either ac to dc or vice versa or change the frequency of the supply. Substations provide additional features for safety devices. Voltage of the outgoing feeders can be regulated at a substation. A substation is an ideal place for installation of synchronous condensers at the end of the transmission line for the purpose of power factor improvement and to make measurements of the various operating parts of the interconnected system.

Classification of substations-

1. Classification on the basis of nature of duties-

1.1Step-Up or Primary substation

1.2Primary grid substation

1.3Step down or distribution substation

2. Classification on the basis of service rendered

2.1Transformer substation

2.2Switching substation

2.3Converting substation

3. Classification based on the operating voltage

3.1High voltage substation

3.2Extra high voltage substation

3.3Ultra high voltage substation

4. Classification on the basis of Importance

4.1Grid substation

4.2Town substation

5. Classification on the basis of design

5.1Outdoor substation

5.2Indoor substation

Indoor Substation-

In these substations the apparatus installed are within the substation building. Such substation are erected for a voltage level of 11Kv but can be erected for 33Kv and 66Kv as well, when the surrounding environment iscontaminated with impurities such as metal corroding,gasses and fumes. The switchgear on the primary side of the supply will consist mainly of Oil circuit breakers only. The high voltage supply is given to transformer and different feeders emerge out of the busbar.The panel for each feeder consists of isolator switch and circuit breakers, in addition to isolators and circuit breakers the panel also consists of different measuring instruments. Theauxiliaries of indoor substation are Dc supply and different fire extinguishing arrangements

Outdoor substation-

Outdoor substation are of two types 1) pole mounted and 2) foundation mounted substation

1) Pole-Mounted substation:-Such substation are erected for mounting distribution oftransformers of capacity of 250 kVA.Such substations are cheapest simplest and smallest of any kind of substations. All the equipment are mounted on the supporting structure of HT distribution line. Triple pole mechanically operated switch is used for ON and OFF control of HT line.ht fuse is used, to control LT side iron clad low tension switch is used for suitable capacity. The substation is switched at two or more points.

2) Foundation mounted substation:-these substation are built entirely I open and fenced from the point of view of safety. Substation for primary and secondary transmission and for secondary distribution, (above 250kVA) are foundation substation .since instruments required for such substations are heavy ,therefore sites selected for such substation must have good transport facility. Owing to exposed bus bars and other associated equipment the clearance and spacing is important and are not only governed by the operating voltage but also by the encroachment from the outside.

Selection and Location of Site

The locational requirements for substations are site specific, and are determined by practicalities of engineering constraints, connection costs, environmental issues and impacts on social attributes. Generally, the location in which a substation is to be sited is dictated by load location and transmission line arrangements for connection of the substation to the grid, and the substation to the place where the connection is required. Access to the substation sites must be capable under all but the most extreme conditions.

The following key issues are taken into consideration:-(1)General location;(2)Development;(3)Physical;(4)Surrounding land usage;(5)Accessibility;(6)Cost;(7)Customer requirements;(8)Environmental considerations; and (9)Community considerations.(10)General access to the substations for construction, operation and maintenance.

Equipment of a substation

1)Lightening Arrester-Lightening arrestors are the instrument that are used in the incoming feeders so that to prevent the high voltage entering the main station. This high voltage is very dangerous to the instruments used in the substation. Even the instruments are very costly, so to prevent any damage lightening arrestors are used.

2)C V T-A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra high voltage signals and provide low voltage signals either for measurement or to operate a protective relay. In its most basic form the device consists of three parts: two capacitors across which the voltage signal is split, an inductive element used to tune the device to the supply frequency and a transformer used to isolate and further step-down the voltage for the instrumentation or protective relay.

3) Instrument Transformer-Instrument transformers are used to step-down the current or voltage to measurable values. They provide standardized, useable levels of current or voltage in a variety of power monitoring and measurement applications. Both current and voltage instrument transformers are designed to have predictable characteristics on overloads.

4)Bus Bar-The bus is a line in which the incoming feeders come into and get into the instruments for further step up or step down. The first bus is used for putting the incoming feeders in la single line. There may be double line in the bus so that if any fault occurs in the one the other can still have the current and the supply will not stop. The two lines in the bus are separated by a little distance by a conductor having a connector between them. This is so that one can work at a time and the other works only if the first is having any fault.

5)Circuit Breaker-The circuit breakers are used to break the circuit if any fault occurs in any of the instrument. These circuit breaker breaks for a fault which can damage other instrument in the station. For any unwanted fault over the station we need to break the line current.

6)Transformer-There are three transformers in the incoming feeders so that the three lines are step down at the same time. In case of a 220KV or more KV line station auto transformers are used. While in case of lower KV line such as less than 132KV line double winding transformers are used.

7)Isolator-The use of this isolator is to protect the transformer and the other instrument in the line. The isolator isolates the extra voltage to the ground and thus any extra voltage cannot enter the line. Thus an isolator is used after the bus also for protection.

8)Control and Relay Panel-The control and relay panel is of cubical construction suitable for floor mounting. All protective, indicating and control elements are mounted on the front panel for ease of operation and control. The hinged rear door will provide access to all the internal components to facilitate easy inspection and maintenance. Provision is made for terminating incoming cables at the bottom of the panels by providing separate line-up terminal blocks.

9)Protective Relaying-Protective relays are used to detect defective lines or apparatus and to initiate the operation of circuit interrupting devices to isolate the defective equipment. Relays are also used to detect abnormal or undesirable operating conditions other than those caused by defective equipment and either operate an alarm or initiate operation of circuit interrupting devices. Protective relays protect the electrical system by causing the defective apparatus or lines to be disconnected to minimize damage and maintain service continuity to the rest of the system. There are different types of relays.

i. Over current relay

ii. Distance relay

iii. Differential relay

iv. Directional over current relay

Substation Auxiliaries

1) DC Battery and Charger-All but the smallest substations include auxiliary power supplies. AC power is required for substation building small power, lighting, heating and ventilation, some communications equipment, switchgear operating mechanisms, anti-condensation heaters and motors. DC power is used to feed essential services such as circuit breaker trip coils and associated relays, supervisory control and data acquisition (SCADA) and communications equipment.

2)Auxiliary Transformers

Auxiliary transformers shall be adequately rated to suit the requirements of the substation.The type, number, vector group and rating shall be determined by the designer to suit the Specific site requirements. The auxiliary transformer shall not be more than 500kVA.

Substation Earthing

Substation Earthing system

- Earth mat

- Earthing spikes

- Earthing risers

To provide an earth mat for connecting neutral points, equipment body, support structures to earth. For safety of personnel and for enabling earth fault protection. To provide the path for discharging the earth currents from neutrals, faults, Surge Arresters, overheads shielding wires etc. with safe step-potential and touch potential.

Circuit Breaker

Introduction-circuit breaker is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, special attention should be given during designing of circuit breaker to safe interruption of arc produced during theoperation of circuit breaker. This was the basic definition of circuit breaker.

Arc Phenomenon-During opening of current carrying contacts in a circuit breaker the medium in between opening contacts become highly ionized through which the interrupting current gets low resistive path and continues to flow through this path even the contacts are physically separated. During the flowing of electric current from one contact to other the path becomes so heated that it glows. This is called arc.

Whenever, on load current contacts of circuit breaker open there is an arc in circuit breaker, established between the separating contacts. As long as this arc is sustained in between the contacts the current through the circuit breaker will not be interrupted finally as because arc is itself a conductive path of electricity. For total interruption of current the circuit breaker it is essential to quench the arc as quick as possible. The main designing criteria of a circuit breaker is to provide appropriate technology of arc quenching in circuit breaker to fulfill quick and safe current interruption

Principles of arc extinction-

1) Heat Loss from Arc-heat loss from arc in circuit breaker is taken place through conduction, convection as well as radiation. In circuit breaker with plain break arc in oil, arc in chutes or narrow slots nearly all the heat loss due to conduction.

2)Deionization of Gas due to Increasing Pressure-If pressure of the arc path increases, the density of the ionized gas is increased which means, the particles in the gas come closer to each other and as a result the mean free path of the particles is reduced.

3)Deionization of Gas due to Decreasing Temperature-The rate of ionization of gas depends upon the intensity of impact during collision of gas particles. The intensity of impact during collision of particles again depends upon velocity of random motions of the particles.

Types of circuit breaker

.According to their arc quenching media the circuit breaker can be divided as-

1) Oil circuit breaker.

2) Air circuit breaker.

3) SF6 circuit breaker.

4)Vacuum circuit breaker.

According to their services the circuit breaker can be divided as-

1) Outdoor circuit breaker

2) Indoor breaker.

According to the operating mechanism of circuit breaker they can be divided as-

1) Spring operated circuit breaker.

2) Pneumatic circuit breaker.

3) Hydraulic circuit breaker.

According to the voltage level of installation types of circuit breaker are referred as-

1) High voltage circuit breaker.

2) Medium voltage circuit breaker.

3) Low voltage circuit breaker.

Oil Circuit Breaker-

Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and moving contact are immerged inside the insulating oil. Whenever there is a separation of electric current carrying contacts in the oil, the arc in circuit breaker is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas and ultimately creates a hydrogen bubble around the arc. This highly compressed gas bubble around the arc prevents re-striking of the arc after current reaches zero crossing of the cycle. The oil circuit breaker is the one of the oldest type of circuit breakers.

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. In America ACBs were exclusively used for the system up to 15 KV until the development of new vacuum and SF₆circuit breakers.

Vacuum Circuit Breaker-

A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter. The vacuum interrupter consists of a steel arc chamber in the center symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10^{- 6}bar. The material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. CuCr is the most ideal material to make VCB contacts. Vacuum interrupter technology was first introduced in the year of 1960. But still it is a developing technology. As time goes on, the size of the vacuum interrupter is being reducing from its early 1960’s size due to different technical developments in this field of engineering. The contact geometry is also improving with time, from butt contact of early days it gradually changes to spiral shape, cup shape and axial magnetic field contact. The vacuum circuit breaker is today recognized as most reliable current interruption technology for medium voltage switchgear. It requires minimum maintenance compared to other circuit breaker technologies.

Sulphur Hexafluoride Circuit Breaker-

A circuit breaker in which the current carrying contacts operate in sulphur hexafluoride or SF₆ gas is known as anSF₆ circuit breaker.SF₆ has excellent insulating property. SF₆has high electro-negativity. That means it has high affinity of absorbing free electron. Whenever a free electron collides with the SF₆ gas molecule, it is absorbed by that gas molecule and forms a negative ion.The attachment of electron with SF₆ gas molecules may occur in two different ways,

Hence, for heavier and less mobile charged particles in SF₆ gas, it acquires very high dielectric strength. Not only the gas has a good dielectric strength but also it has the unique property of fast recombination after the source energizing the spark is removed. The gas has also very good heat transfer property. Due to its low gaseous viscosity (because of less molecular mobility) SF₆ gas can efficiently transfer heat by convection. So due to its high dielectric strength and high cooling effect SF₆ gas is approximately 100 times more effective arc quenching media than air. Due to these unique properties of this gas SF₆ circuit breaker is used in complete range of medium voltage and high voltage electrical power system.

RELAY

INTRODUCTION

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

The relay detects the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage,current,frequency,phase angle. Through the changes in one or more of these quantities, the fault signals their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faulty circuit.

A typical relay circuit is shown below:

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

1) The primary winding of a current transformer connected in series with line to be protected.

2) Secondary winding of current transformer and the relay operating coil

3) Tripping circuit which may be either a.c or d.c it consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit occurs on the transmission line, the current flowing in the line increases 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 system. Thus ensuring safety of circuit equipment from damage.

Electromagnetic Relay

Electromagnetic relay operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by d.c. or a.c. quantities.it consists of three types.

1) Attracted armature type relay:

It consists of a laminated electromagnet carrying a coil and a pivoted laminated armature. The armature is balanced by a counterweight and carries a pair of spring contact fingers at its free end. Under normal operating condition the current through the relay coil is such that counterweight holds the armature in the position shown. However when a short circuit occurs the current through the relay coil increases sufficiently and the relay armature is attracted upwards. The contacts on the relay armature bridge a pair of stationary contacts attached to the relay frame. This completes the trip circuit which results in the opening of the circuit breaker and therefore in the disconnection of the faulty circuit

2) Solenoid type relay:

It consists of a solenoid and movable iron plunger. Under normal operating conditions, the current through the relay coil c is such that it holds the plunger by gravity or spring in the position shown. However, on the occurrence of a fault, the current through the relay coil becomes more than the pickup value, causing the plunger to be attached to the solenoid. The upward movement of the plunger closes the trip circuit, thus opening the circuit breaker and disconnecting the faulty circuit.

3) Balanced beam type relay:

It consists of an iron armature fastened to a balanced beam. Under normal operating conditions the current through the relay coil is such that the beam is held in the horizontal position by the spring. However when a fault occurs, 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 to isolate faulty circuit.

Induction Relay

Electromagnetic induction relay operate on the principle of induction motor and are widely used for protective relaying purposes involving a.c. quantities. They are not used with d.c. quantities owing to the principle of operation.an induction relay essentially consists of a pivoted aluminum disc placed in two alternating magnetic fields of the same frequency but displaced in time and space. The torque is produced in the disc by the interaction of one of the magnetic fields with the currents induced in the disc by the other.

φ1 = Φ1 sin ωt

φ2 = Φ2 sin (ωt + θ),

Where θ is the phase angle by which ø2 leads ø1. It may be assumed with negligible error

That the paths in which the rotor currents flow have negligible self-inductance, and hence

That the rotor currents are in phase with their voltages:

iφ1 α dφ1/dtα Φ1 cos ωt

iφ2 α dφ2/dt α Φ2 cos (ωt + θ)

We note that the two forces in opposition, and consequently we may write the

Equation for the net force (F) as follows:

F = (F2 – F1) α (φ2iφ1 – φ1iφ2) (1)

Substituting the values of the quantities into equation 1, we get:

F α Φ1Φ2 [sin (ωt + θ) cos ωt – sin ωt cos (ωt + θ)] (2)

Which reduces to:

F α Φ1Φ2 sin θ

1) Shaded-pole structure:

the general arrangement consists of a pivoted aluminum disc free to rotate in the air-gap of an electromagnet.one-half of each pole of the magnet is surrounded by a copper band known as shading pole. The alternating flux in the shaded portion of the pole will owing to the reaction of the current induced in the ring, lag behind the flux in the unshaded portion by an angle α.thses two a.c. fluxes differing in phase will produce the necessary torque to rotate the disc.

2) Watt-hour meter structure:

This structure gets its name for the fact that it is used in watt-hour metres.it consists of a pivoted aluminum disc arranged to rotate freely between the poles of two electromagnets. The upper electromagnet carries two windings; the primary and secondary. The primary windings carries the relay current while the secondary winding is connected to the lower magnet. The primary current induces the e.m.f in the secondary turns so circulates a current in it. The flux induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind by an angle @.the two fluxes differing in phase by @ will produce a driving torque on the disc.

3) Induction cup structure:

It resembles an induction motor, except that the rotor iron is stationary, only the rotor conductor portion being free to rotate.

The moving element is a hollow cylindrical rotor which turns on its axis. The rotating field is produced by two pairs of coils wound on four poles. The rotating field induces current in the cup to provide the necessary driving torque.

Important terms

1) Current setting:

It is often desirable to add just the pick-up current to any required value. This is known as current setting and is usually achieved by the use of tapings on the relay operating coil. The taps are brought out to a plug bridge. The plug bridge permits to alter the number of turns on the relay coil. This changes the torque on the disc and hence the time of operation of the relay. The values assigned to each tap are expressed in terms of percentage full-load rating of C.T. with which the relay is associated and represents the value above which the disc commences to rotate and finally closes the trip circuit.

Pick up current=rated current of c.t. * current setting

2) Plug-setting multiplier (P.S.M):

It is the ratio of fault current in relay coil to the pick-up current i.e.

P.S.M = (fault current in relay coil)/(pick-up current)

3) Time setting multiplier:

A relay is generally provided with control to adjust the time of operation. This adjustment is known as time-setting multiplier. The time setting dial is calibrated from 0 to 1 in steps of 0.05 sec. These figures are multipliers to be used to convert the time derived from time/P.S.M. curve into the actual operating time.

Time/P.S.M. curve

The above figure shows the curve between time of operation and plug setting multiplier of a typical relay.the horizontal scale is marked in terms of plug-setting multiplier and represents the number of times the relay current is in excess of the current setting. The vertical scale is marked in terms of the time required for relay operation.

Functional relay types

Most of the relays in service on power system today operate on the principle of electromagnetic attraction or electromagnetic induction. Regardless of the principle involved, relays are generally classified according to the function they are called upon to perform in the protection of electric power circuits. The important types of relays are mainly:

a) Induction type overcurrent relay

b) Induction type reverse power relay

c) Distance relay

d) Differential relay

Induction type overcurrent relay

This type of relay works on the induction principle and initiates corrective measures when current in the circuit exceeds the predetermined value. The actuating source is a current in the circuit supplied to the relay from a current transformer. These relays are used on a.c. circuits only and can operate for fault current flow in either direction.

Constructional details:

It consists of a metallic disc which is free to rotate in between the poles of two electromagnets. The upper electromagnet has a primary and a secondary winding. The primary is connected to the secondary of a C.T. in the line to be protected and is tapped at intervals. Thetapings are connected to a plug-setting bridge by which the number of active turns on the relay operating coil can be varied, thereby giving the desired current setting. The secondary winding is energized by induction from primary and is connected in series with the winding on the lower magnet. The controlling torque is provided by the spiral spring. The spring of the disc carries a moving contact which bridges two fixed contacts when the disc rotates through a pre-set angle. This angle can be adjusted to any value between 0 and 360 degrees.by adjusting this angle, the travel of the moving contact can be adjusted and hence the relay can be given any desired time setting.

Operation:

The driving torque on the aluminum disc is set on the induction principle. This torque is opposed by the restraining torque provided by the spring. Under normal operatingconditions, restraining torque is greater than the driving torque produced by the relay coil current. Therefore the aluminum disc remains stationary. However,if the current in the protected circuit exceeds the pre-set value, the driving torque becomes greater than the restraining torque. Consequently the disc rotates and the moving contact bridges the fixed contacts when the disc has rotated through a pre-set angle. The trip circuit operates the circuit breaker which isolates the faulty section

Induction type directional overcurrent relay

The directional power relay discussed above is unsuitable for use as a directional protective relay under short-circuit conditions. When a short circuit occurs, the system voltage falls to a low value and there may be insufficient torque developed in the relay to cause its operation. This difficulty is overcome in the directional over current relay which is designed to be almost independent of the system voltage and power factor.

Constructional details:

It consists of two relay elements mounted on a common case a) directional element b) non-directional element

1) Directional element:

It is essentially a directional power relay which operates when power flow in a specific direction. The potential coil in connected through a potential transformer to the system voltage. The current coil of the element is energized trough a c.t. by the circuit current. This winding is carried over the upper magnet of the non-directional element. The trip contacts of the directional element are connected in series with the secondary circuits of the over current element. Thus the latter element cannot start to operate until its secondary circuit is completed.

2) Non-directional element:

The spindle of the disc of this element carries a moving contact which closes the fixed contact after the operation of the directional element. Plug setting bridge is also provided in the relay for current setting. The tapings are provided on the upper magnet of overcurrent element and are connected on the bridge.

Operation:

Under normal operating condition power flows in the normal direction in the circuit protected by the relay. Therefore, directional power relay does not operate, thereby keeping the over current element un energized. However when a short-circuit occurs there is a tendency for the current or power to flow in the reverse direction. Should this happen, the disc of the upper element rotates to bridge the fixed contacts. This completes the circuit for over current element. The disc of this element rotates and the moving contact attached to it closes the trip circuit. This operates the circuit breaker which isolates the faulty section

Index

1. SUBSTATION

1.1 Introduction

1.2 Classification

1.3 Indoor Substation

1.4 Outdoor Substation

1.5 Selection and Location of Site

1.6 Equipment of a substation

1.7 Substation Auxiliaries

1.8 Earthing

2. SWITCH GEAR AND PROTECTION

2.1 CIRCUIT BREAKERS

2.1.1 Introduction

2.1.2 Arc phenomenon

2.1.3 Principles of arc extinction

2.1.4 Classification of circuit breakers

2.17 Types of circuit breakers

2.2 Protective Relays

2.2.1 Introduction

2.2.2 Electromagnetic Relays

2.2.3 Induction Relays

2.2.6 Time/TSM curves

2.2.8 Functional Relay type

2.2.9 Induction type over current relay
2.2.10 Induction Type Directional over current relay

Introduction-

Classification of substations-

1. Classification on the basis of nature of duties-

1.1 Step-Up or Primary substation

1.2 Primary grid substation

1.3 Step down or distribution substation

2. Classification on the basis of service rendered

2.1 Transformer substation

2.2 Switching substation

2.3 Converting substation

3. Classification based on the operating voltage

3.1 High voltage substation

3.2 Extra high voltage substation

3.3 Ultra high voltage substation

4. Classification on the basis of Importance

4.1 Grid substation

4.2 Town substation

5. Classification on the basis of design

5.1 Outdoor substation

5.2 Indoor substation

Indoor Substation-

In these substations the apparatus installed are within the substation building. Such substation are erected for a voltage level of 11Kv but can be erected for 33Kv and 66Kv as well, when the surrounding environment is contaminated with impurities such as metal corroding,gasses and fumes. The switchgear on the primary side of the supply will consist mainly of Oil circuit breakers only. The high voltage supply is given to transformer and different feeders emerge out of the busbar. The panel for each feeder consists of isolator switch and circuit breakers, in addition to isolators and circuit breakers the panel also consists of different measuring instruments. The auxiliaries of indoor substation are Dc supply and different fire extinguishing arrangements

Outdoor substation-

Outdoor substation are of two types 1) pole mounted and 2) foundation mounted substation

Selection and Location of Site

Equipment of a substation

3) Instrument Transformer-Instrument transformers are used to step-down the current or voltage to measurable values. They provide standardized, usable levels of current or voltage in a variety of power monitoring and measurement applications. Both current and voltage instrument transformers are designed to have predictable characteristics on overloads.

i. Over current relay

ii. Distance relay

iii. Differential relay

iv. Directional over current relay

Substation Auxiliaries

2)Auxiliary Transformers

Substation Earthing

Substation Earthing system

- Earth mat

- Earthing spikes

- Earthing risers

Circuit Breaker

Introduction-circuit breaker is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, special attention should be given during designing of circuit breaker to safe interruption of arc produced during the operation of circuit breaker. This was the basic definition of circuit breaker.

Principles of arc extinction-