INDUSTRIAL TRAINING

AT

132 KV BISSAU SUBSTATION

(RVPNL)

TEJVEER CHOUDHARY

B.E. - EEE

ROLL NO.- UE 114069

4TH

YEAR

University Institute of Engineering & Technology

Panjab University, Chandigarh

ACKNOWLEDGEMENT

Training has an important role in exposing the real life situation in

an industry. It was a great experience for me to work on training at 132

kV BISSAU (RVPNL) Substation, BISSAU through which I could learn

how to work in a professional environment.

Now, I would like to thank the people who guided me and have

been a constant source of inspiration throughout the tenure of my

summer training.

I am sincerely grateful to Mr. Dilip Singh (Assistant Engineer) at

132 kV substation, BISSAU who rendered me his valuable assistance,

precious time, constant encouragement and able guidance which made

this training actually possible.

-TEJVEER CHOUDHARY

Contents

1. Introduction

1.1 Types of substation

2. Steps in designing a substation

2.1 Earthing & Bonding

2.2 Substaion Earthing Calculation Methodology

2.3 Earthing Material

2.4 Layout of Substaion

2.5 Design of Busbar

3. Single line diagram of Bissau Substaion

4. Transformer

4.1 Current Transformer

4.2 Capacitor Voltage Transformer

4.3 CVT Frequency Response

4.4 Bus Voltage Representation of CVT

5. Circuit Breaker

5.1 SF6 Circuit Breaker

5.2 Vacuum Circuit Breaker

6. Protective Relay

6.1 Differential Relay

6.2 Overcurrent Relay

6.3 Directional Relay

6.4 Buchholz Relay

7. Miscellaneous

7.1 Isolator

7.2 Wavetrap

7.3 Busbars

7.4 Substation Grounding System

7.5 Insulators

7.6 Earth Wires & OPGW

7.7 Conductors

7.8 Support Structure (POLE)

7.9 Types of Conductor for Power Transmission

7.10 DC Supply Room (Battery)

7.11 Lightening Arrester

8. Conclusion

1. Introduction

FIG- View of Substation

The present day electrical power is generated, transmitted and distributed in the form

of the alternating current. The electric power is produced at power plant stations which are

located at favorable places generally quite away from the consumers. It is delivered to the

consumers through a large network of transmission and distribution.

At many places in the power system, it may be desirable and necessary to change

some characteristics e.g. voltage, ac to dc, frequency, power factor etc. of electric supply.

This accomplished by suitable apparatus called substation. For example; generation voltage

(11 KV or 33 KV) at the power station is set up to high voltage (say 220 KV or 132 KV) for

transmission of electric power. The assembly of apparatus (e.g. transformer etc.) used for this

purpose in the substation. Similarly near the consumer’s localities, the voltage may have to be

step down to utilization level. This job is again accomplished by suitable apparatus called

substation.

The assembly of apparatus to change some characteristic of electric power supply is

called substation.

1.1 Types of Substation:

(A) According to the service requirement:

1) Transformer substation

2) Switch substation

3) Power factor correction substation

4) Frequency change substation

5) Converting substation

(B) According to the constructional features:

1) Indoor substation

2) Outdoor substation

3) Underground substation

4) Pole mounted substation

2. Steps in Designing a Substation

2.1 Earthing and Bonding

The function of an earthing and bonding system is to provide an earthing system

connection to which transformer neutrals or earthingimpedances may be connected in order

to pass the maximum fault current.

The earthing system also ensures that no thermal or mechanical damage occurs on the

equipment within the substation, thereby resulting in safety to operation and maintenance

personnel. The earthing system also guarantees equipotential bonding such that there are no

dangerous potential gradients developed in the substation.

In designing the substation, three voltage have to be considered.

1. Touch Voltage: This is the difference in potential between the surface potential and the

potential at an earthed equipment whilst a man is standing and touching the earthed structure.

2. Step Voltage: This is the potential difference developed when a man bridges a distance of

1m with his feet while not touching any other earthed equipment.

3. Mesh Voltage: This is the maximum touch voltage that is developed in the mesh of the

earthing grid.

2.2 Substation Earthing Calculation Methodology

Calculations for earth impedances and touch and step potentials are based on site

measurements of ground resistivity and system fault levels. A grid layout with particular

conductors is then analysed to determine the effective substation earthing resistance, from

which the earthing voltage is calculated.

In practice, it is normal to take the highest fault level for substation earth grid

calculation purposes. Additionally, it is necessary to ensure a sufficient margin such that

expansion of the system is catered for. To determine the earth resistivity, probe tests are

carried out on the site. These tests are best performed in dry weather such that conservative

resistivity readings are obtained.

2.3 Earthing Materials

1). Conductors: Bare copper conductor is usually used for the substation earthing grid. The

copper bars themselves usually have a cross- sectional area of 95 square millimetres, and they

are laid at a shallow depth of 0.25-0.5m, in 3-7m squares. In addition to the buried potential

earth grid, a separate above ground earthing ring is usually provided, to which all metallic

substation plant is bonded.

.

2.) Connections: Connections to the grid and other earthing joints should not be soldered

because the heat generated during fault conditions could cause a soldered joint to fail. Joints

are usually bolted, and in this case, the face of the joints should be tinned.

3). Earthing Rods: The earthing grid must be supplemented by earthingrods to assist in the

dissipation of earth fault currents and further re- duce the overall substation earthing

resistance. These rods are usually made of solid copper, or copper clad steel.

4). Switchyard Fence Earthing: The switchyard fence earthing practices are possible and

are used by different utilities. These are:

(a) Extend the substation earth grid 0.5m-1.5m beyond the fence perimeter. The fence is then

bonded to the grid at regular intervals.

(b) Place the fence beyond the perimeter of the switchyard earthinggrid and bond the fence to

its own earthing rod system. This earthing rod system is not coupled to the main substation

earthinggrid.

2.4 Layout of Substation

The layout of the substation is very important since there should be a Security of Supply. In

an ideal substation all circuits and equipment would be duplicated such that following a fault,

or during maintenance, a connection remains available. Practically this is not feasible since

the cost of implementing such a design is very high. Methods have been adopted to achieve a

compromise between complete security of supply and capital investment. There are four

categories of substation that give varying securities of supply:

• Category 1: No outage is necessary within the substation for either maintenance or fault

conditions.

• Category 2: Short outage is necessary to transfer the load to an alter- native circuit for

maintenance or fault conditions.

• Category 3: Loss of a circuit or section of the substation due to fault or maintenance.

• Category 4: Loss of the entire substation due to fault or maintenance.

2.5 Design of Bus Bars

Bus bars are Cu/Al rods of thin walled tubes and operate at constant voltage. The bus-bars

are designed to carry normal current continuously. The cross section of conductors is

designed on the basis of rated normal current and the following factors: System voltage,

position of substation. Flexibility, reliability of supply and cost. Our design must ensure easy

and uninterrupted maintenance, avoiding any danger to the operating of operating personnel.

It must be simple in design and must possess provision for future extension. Any fluctuation

of load must not hinder its mechanical characters. The sub-station bus bars are broadly

classified in the following three categories:

1. Outdoor rigid tubular bus-bars.

2. Outdoor flexible ACSR or Al alloy bus-bars.

3. Indoor bus bars.

3. Single Line Diagram of BISSAU Substation

FIG- Single line diagram

4. Transformer

Transformer is a static machine, which transforms the potential of alternating current

at same frequency. It means the transformer transforms the low voltage into high voltage and

high voltage to low voltage at same frequency. It works on the principle of static induction

principle. When the energy is transformed into a higher voltage, the transformer is called step

up transformer but in case of other is known as step down transformer.

4.1 Current Trnsformer:-

A current transformer (CT) is used for measurement of alternating electric currents.

Current transformers, together with voltage (or potential) transformers (VT or PT), are known

as instrument transformers. When current in a circuit is too high to apply directly to

measuring instruments, a current transformer produces a reduced current accurately

proportional to the current in the circuit, which can be conveniently connected to measuring

and recording instruments. A current transformer isolates the measuring instruments from

what may be very high voltage in the monitored circuit. Current transformers are commonly

used in metering and protective relays in the electrical power industry.

An essential objective of current transformer design is to ensure the primary and secondary

circuits are efficiently coupled, so the secondary current is linearly proportional to the

primary current.

The CT's primary circuit consists of a single 'turn' of conductor, with a secondary of many

tens or hundreds of turns.

Usage:-

Current transformers are used extensively for measuring current and monitoring the operation

of the power grid.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection

devices and revenue metering may use separate CTs to provide isolation between metering

and protection circuits, and allows current transformers with different characteristics

(accuracy, overload performance) to be used for the devices. The primary circuit is largely

unaffected by the insertion of the CT.

Load, or burden, of the CT should be a low resistance. If the voltage time integral area is

higher than the core's design rating, the core goes into saturation toward the end of each

cycle, distorting the waveform and affecting accuracy.

Safety precautions

Care must be taken that the secondary of a current transformer is not disconnected from its

load while current is in the primary, as the transformer secondary will attempt to continue

driving current across the effectively infinite impedance up to its core saturation voltage. This

may produce a high voltage across the open secondary into the range of several kilovolts,

causing arcing, compromising operator and equipment safety, or permanently affect the

accuracy of the transformer.

Accuracy

The accuracy of a CT is directly related to a number of factors including:

Burden

Burden class/saturation class

Rating factor

Load

External electromagnetic fields

Temperature and Physical configuration.

The selected tap, for multi ratio CTs

Phase change

4.2 CAPACITOR VOLTAGE TRANSFORMER (PT):-

A capacitor voltage transformer (CVT), or capacitance coupled voltage transformer (CCVT),

is a transformer used in power systems to step down extra high voltage signals and provide a

low voltage signal, for metering or operating a protective relay.

Components

In its most basic form, the device consists of three parts: two capacitors across which the

transmission line signal is split, an inductive element to tune the device to the line frequency,

and a voltage transformer to isolate and further step down the voltage for the metering

devices or protective relay.

The tuning of the divider to the line frequency makes the overall division ratio less sensitive

to changes in the burden of the connected metering or protection devices.

In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in

series. This provides a large voltage drop across C1 and a relatively small voltage drop across

C2. As the majority of the voltage drop is on C1, this reduces the isolation level of the

voltage transformer. This makes CVTs more economical than the wound voltage

transformers under high voltage (over 100kV), as the latter one requires more winding and

materials.

With the rated load at the voltage transformer secondary side, The output voltage of CVT

initially decrease a little bit, then reaches the resonance peak at around 800Hz. Then it

decreases drastically and remains almost level out after 2000Hz.

4.3 CVT Current Frequency Response

The C2 current is linear with frequency. The frequency response for voltage transformer

current has a resonance peak at around 800Hz. C2 current is substantially larger than voltage

transformer current.

4.4 Bus Voltage Representation of CVT

The bus voltage in frequency domain can be calculated by summing the voltages on C1 and

C2. From the calculation result it can be seen that the bus voltage only relates to C2 current,

voltage transformer current and their ratios. This result is helpful to reconstruct the bus

voltage with the C2 current, voltage transformer current. For the ratio, it can be achieved by

using a summing amplifier.

Other Applications :-

The CVT is also useful in communication systems. CVTs in combination with wave traps are

used for filtering high frequency communication signals from power frequency. This forms a

carrier communication network throughout the transmission network.

FIG- CVT

5. Circuit Breaker

There are two types of C.B.:

1) Sulphur hexafluoride circuit breaker (SF6)

2) Vacuum circuit breaker

SF6 circuit breaker are being used in 132KV and Vacuum circuit breaker are

being used in 33KV side.

5.1 SF6 Circuit Breaker

In such circuit breaker, sulphar hexafluoride (SF6) gas is used as the arc quenching medium.

The SF6 is an electronegative gas and has a strong tendency to absorb free electrons. The SF6

circuit breaker have been found to a very effective for high power and high voltage service.

SF6 circuit breakers have been developed for voltage 115 KV to 230 KV, power rating 10

MVA.

Ratings:

• Make- Crompton Greaves

• Rated voltage- 132 Kv

• Rated normal current- 3150A

• C.T. ratio- 800-400-200/1

• Gas weight- 8kg

• Gas Pressure- 7 kg/sq. cm

• Weight of oil- 150 liters

FIG- SF6 Circuit Breaker

It consists of fixed and moving contacts. It has chamber, contains SF6 gas. When the contacts

are opened, the mechanism permits a high pressure SF6 gas from reservoir to flow towards

the arc interruption chamber. The moving contact permits the SF6 gas to let through these

holes.

5.2 Vacuum Circuit Breaker

Vacuum circuit breakers are circuit breakers which are used to protect medium and high

voltage circuits from dangerous electrical situations. Like other types of circuit breakers,

vacuum circuit breakers literally break the circuit so that energy cannot continue flowing

through it, thereby preventing fires, power surges, and other problems which may emerge.

These devices have been utilized since the 1920s, and several companies have introduced

refinements to make them even safer and more effective.

FIG- Vacuum Circuit Breaker

6. Protective Relay

FIG- Relay Panel

In a power system it is inevitable that immediately or later some failure does occur

somewhere in the system. When a failure occurs on any part of the system, it must be quickly

detected and disconnected from the system. Rapid disconnection of faulted apparatus limits

the amount of damage to it and prevents the effects of fault from spreading into the system.

For high voltage circuits relays are employed to serve the desired function of automatic

protective gear. The relays detect the fault and supply the information to the circuit breaker.

The electrical quantities which may change under fault condition are voltage, frequency,

current, phase angle. When a short circuit occurs at any point on the transmission line the

current flowing in the line increases to the enormous value.This result 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. In this way, the relay ensures the safety of the circuit equipment

from the damage and normal working of the healthy.

6.1 Differential Relay

FIG- Differential Relay

A differential relay is one that operates when vector difference of the two or more electrical

quantities exceeds a predetermined value. If this differential quantity is equal or greater than

the pickup value, the relay will operate and open the circuit breaker to isolate the faulty

section.

6.2 Over Current Relay

This type of relay works when current in the circuit exceeds the predetermined value. The

actuating source is the current in the circuit supplied to the relay from a current transformer.

These relay are used on A.C. circuit only and can operate for fault flow in the either direction.

This relay operates when phase to phase fault occurs.

FIG- Over Current Relay

6.3 Directional Relay

This type of relay is in the conjunction with main relay. When main relay sense any fault in

the system, it immediately operates the trip relay to disconnect the faulty section from the

section.

FIG- Directional Relay

6.4 BUCHHOLZ RELAY

Buchholz relay is a safety device mounted on some oil filled powertransformers and

reactors, equipped with an external overhead oil reservoir. The Buchholz Relay is used as

aprotective device sensitive to the effects of dielectric failure insidethe equipment. The relay

hasmultiple methods to detect afailing transformer. On a slow accumulation of gas, due

perhaps toslight overload, gas produced by decomposition of insulating oilaccumulates in the

top of the relay and forces the oil level down. Afloat switch in the relay is used to initiate an

alarm signal.

Depending on design, a second float may also serve to detect slowoil leaks.If an arc forms,

gas accumulation is rapid, and oil flows rapidly intothe conservator.

This flow of oil operates a switch attached to a vanelocated in the path of the moving oil.

This switch normally willoperate a circuit breaker to isolate the apparatus before the

faultcauses additional damage. Buchholz relays have a test port to allowthe accumulated gas

to be withdrawn for testing. Flammable gasfound in the relay indicates some internal fault

such as overheatingor arcing, whereas air found in the relay may only indicate low oil level

or a leak.

FIG- BUCHHOLZ RELAY

7. MISCELLANEOUS

7.1 Isolator

Circuit breaker always trip the circuit but open contacts of breaker cannot be visible

physically from outside of the breaker and that is why it is recommended not to touch any

electrical circuit just by switching off the circuit breaker. So for better safety there must be

some arrangement so that one can see open condition of the section of the circuit before

touching it. Isolator is a mechanical switch which isolates a part of circuit from system as

when required. Electrical isolators separate a part of the system from rest for safe

maintenance works.

So definition of isolator can be rewritten as Isolator is a manually operated mechanical switch

which separates a part of the electrical power system normally at off load condition.

FIG- Isolator

Types of Electrical Isolators :

There are different types of isolators available depending upon system requirement such as :

1) Double Break Isolator

2) Single Break Isolator

3) Pantograph type Isolator.

Depending upon the position in power system, the isolators can be categorized

as

1) Bus side isolator – the isolator is directly connected with main bus.

2) Line side isolator – the isolator is situated at line side of any feeder.

3) Transfer bus side isolator – the isolator is directly connected with transfer bus.

7.2 Wavetrap

A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted

inline on high voltage AC transmissionpower lines to prevent the transmission of high

frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted

destinations. Line traps are cylinder like structures connected in series with HV transmission

lines. A line trap is also called a wave trap.

The line trap acts as a barrier or filter to prevent signal losses. The inductive reactance of the

line trap presents a high reactance to high-frequency signals but a low to mains frequency.

What this does is prevent carrier signals from being dissipated in the substation or in a tap

line/branch of the main transmission path and grounds in the case of anything happening

outside of the carrier transmission path. The line trap is also used to attenuate the shunting

effects of high voltage lines.

FIG- WAVETRAP

7.3 Busbars

When numbers of generators or feeders operating at the same voltage have to be directly

connected electrically, bus bar is used as the common electrical component. Bus bars are

made up of copper rods operate at constant voltage. The following are the important bus bars

arrangements used at substations: Single bus bar system Single bus bar system with section

alisation.

FIG- Busbars

Auxiliary bus bar system

In large stations it is important that break downs and maintenance should interfere as little as

possible with continuity of supply to achieve this, dupli- cate bus bar system is used. Such a

system consists of two bus bars, a main bus bar and a spare bus bar with the help of bus

coupler, which consist of the circuit breaker and isolator.

In substations, it is often desired to disconnect a part of the system for general maintenance

and repairs. An isolating switch or isolator accomplishes this. Isolator operates under no load

condition. It does not have any specified current breaking capacity or current making

capacity. In some cases isolators are used to breaking charging currents or transmission lines.

While opening a circuit, the circuit breaker is opened first then isolator while closing a circuit

the isolator is closed first, then circuit breakers. Isolators are necessary on supply side of

circuit breakers, in order to ensure isolation of the circuit breaker from live parts for the

purpose of maintenance.

A transfer isolator is used to transfer main supply from main bus to transfer bus by using bus

coupler (combination of a circuit breaker with two isolators), if repairing or maintenance of

any section is required.

7.4 Substation Grounding System

There are many parameters that have an effect on the voltages in and around the substation

area. Since voltages are site-dependent, it is impossible to design one grounding system

that is acceptable for all locations. The grid current, fault duration, soil resistivity, surface

material, and the size and shape of the grid all have a substantial effect on the voltages in

and around the substation area. If the geometry, location of ground electrodes, local soil

characteristics, and other factors contribute to an excessive potential gradient at the earth

surface, the grounding system may be inadequate from a safety aspect despite its capacity

to carry the fault current in magnitudes and durations permitted by protective relays.

During typical ground fault conditions, unless proper precautions are taken in design, the

maximum potential gradients along the earth surface may be of sufficient magnitude to

endanger a person in the area.

Moreover, hazardous voltages may develop between grounded structures or equipment

frames and the nearby earth.

FIG- GROUNDING OF SUBSTATION

Relatively high fault current to ground in relation to the area of the grounding system

and its resistance to remote earth

Soil resistivity and distribution of ground currents such that high potential gradients

may occur at points at the earth surface

Presence of a person at such a point, time, and position that the body is bridging two

points of high potential difference

Absence of sufficient contact resistance or other series resistance to limit current

through the body to a safe value under the above circumstances

Duration of the fault and body contact and, hence, of the flow of current through a

human body for a sufficient time to cause harm at the given current intensity

The relative infrequency of accidents is due largely to the low probability of coincidence of

the above unfavorable conditions.

To provide a safe condition for personnel within and around the substation area, the

grounding system design limits the potential difference a person can come in contact with

to safe levels.

7.5 INSULATORS

Insulators proposed for use on this Project

will be polymeric-type post insulators.

They provide a connection between

conductors and structures and ensure

electrical insulation between the high

voltage of the conductors and the (earthed)

pole. The length of the insulators depends

on line voltage, clearance requirements

and environmental considerations. For

example, additional length is required for

acidic or salty air, neither of these

conditions occurs within the Project Area.

Special galvanised steel of aluminium

fittings connect both the line end of the

insulator to the conductors and the pole

end to the structure. Typical post insulators

are shown on the pole structure in

Photo.

FIG- Typical post insulators

7.6 Earthwires and OPGW

Overhead earthwires provide protection to the conductors from direct strike by

lightning and also support optical fibre cables used for communication purposes. Two

earthwires, one of which will be the optical fibre pilot ground wire (OPGW) are fitted to each

structure. OPGW construction will typically be of the ‘slotted core’ type with 48 optical

fibres supported by a combination of aluminium and steel wires. The earthwire will generally

be of the AAC type, which consists of aluminium wires. For this Project, the proposed

earthwire and OPGW will each have overall diameters of approximately 14 millimetres

(mm), although this may be subject to change to suit the final design.

7.7 Conductors

Conductors (wires) are the part of the power-line which transports high voltage electricity.

Conductors proposed to be used for the power-line would consist of a strandedaluminium

alloy with an overall diameter of around 33.8 mm. Each pole structure would support six

conductors, plus two earth wires to protect the line from lighting strikes. For a typical

doublecircuit 132 kV pole, the distance between pairs of conductors on steel poles will be

approximately 1.9 m on the vertical plane and 4.6 m on the horizontal plane (Figure 3.1).

Electrical continuity on angle structures is provided by ‘bridging conductors’ which are

suspended beneath the main conductors or around the outside of the pole connecting the

terminated conductors on either side of the pole.

Where necessary, these conductors are restrained by ‘bridging insulators’ to maintain

electrical clearance. The minimum electrical clearance distance from the ground to the lowest

point of the conductors is 6.7 m; however as noted above, larger clearances are often required

by the DTMR over road and rail crossings.

7.8 Support structures( POLE) :-

Poles are self-supporting structures used to keep the high voltage conductors separate from

each other, clear from vegetation, the ground and other obstacles. Minimum clearance

requirements between energized conductors and various types of obstacles are specified by

the Electrical Safety Regulations 2002. For this Project a mixture self-supporting concrete or

steel poles are envisaged. The distance between support structures (span) and their height is

determined by topography and clearance requirements. Poles are made in a range of heights,

generally in 1 m increments, to allow optimum height of structure to be provided at each site.

The standard overall length of a typical double-circuit RVPNL concrete pole is around 25 to

40 m long.

When installed, the ‘out of ground’ height to the top of the pole will typically be in the range

of 20 to 35 m, with taller and shorter structures used as required to suit the terrain and other

constraints. Typically, shorter poles are found on elevated areas such as hills, with taller poles

in gullies or where additional clearance is required over a mid-span obstacle. The ‘duty’ of a

pole structure is related to the method by which it supports the conductors and this in turn

influences the type of structure used. The two means of conductor support are intermediate

and tension.

Intermediate structures are used where the line follows a straight line or has a very small

deviation angle, generally less than four degrees. The structures are designed to carry the

weight (vertical load) of the conductors, and transverse (horizontal) load from wind on the

conductors, earthwires and on the structure itself.

A typical intermediate pole is shown. Features of this type of pole are their relatively light

construction and include three post type insulators on each side of the upper pole

(superstructure) directly supporting the conductors.

Tension structures are used for line deviation angles greater than about four degrees, and at

line terminations. A typical tension pole is shown. Requirements for staying of poles or the

need to install double poles to address torsional moments on the structures will depend upon

the change of direction of the powerline that the pole needs to accommodate.

A suite of poles may be designed for a particular project to cover a range of angle duties and

these remain to be progressed by RVPNL. It is expected that these tension poles will look

similar in silhouette to the intermediate poles.

FIG- Typical intermediate pole FIG- Typical tension pole

FIG- Dimensions of a typical RVPNL concrete pole

Various designs of concrete and steel poles have been used in Queensland over the last

30 years. These poles have evolved into a standard form of support structure for 132 kV and

110 kV high-voltage powerline construction in non-cyclonic areas.

Each pole is manufactured off site and transported to the required location in one or two

more sections, depending on the pole’s overall length. Additional components, such as

insulators or earthwire attachment fittings, are fabricated from galvanized steel and are

attached to each pole prior to erection.

The standard foundation for each pole is formed by backfilling (with concrete) around the

pole erected in an augured (bored) hole, in a similar manner to that used for traditional

timber poles. The typical diameter of a pole at ground level is approximately 700 mm.

The double-circuit powerline will be constructed using typical RVPNL concrete or steel

poles spaced approximately 250 m apart, with two sets of three conductors, and twin

earthwires strung between the tops of the poles above the power conductors. Single-circuit

powerlines have only one set of three conductors and a single earthwire.

Ground clearance

The conductors will be strung with a minimum statutory vertical clearance of 6.7 m between

the bottom conductor and the road or ground under all operating conditions. The height and

spacing of individual supporting structures will vary depending on the terrain, vegetation and

other factors. When installed, a typical double-circuit pole will be 20–35 m high, with taller

and shorter structures being used as required to suit the terrain and other constraints.

Where required by various regulations, additional clearance, usually necessitating taller

supporting poles, will be provided to cross flood-prone areas, sensitive vegetation or facilities

such as roads, railway lines and other infrastructure.

The line will be designed to meet these clearances when the conductors are operating at their

highest temperature and hence have sagged by the greatest amount. Under normal operating

conditions, ground clearances will be significantly greater than the statutory minimum.

7.9 TYPES OF CONDUCTOR FOR POWER TRANSMISSION

In the early days conductor used on transmission lines were usually Copper, but Aluminium

Conductors have completely replaced Copper because of the much lower cost and lighter

weight of Aluminium conductor compared with a Copper conductor of the same resistance.

The fact that Aluminium conductor has a larger diameter than a Copper conductor of the

same resistance is also an advantage. With a larger diameter the lines of electric flux

originating on the conductor will be farther apart at the conductor surface for the same

voltage. This means a lower voltage gradient at the conductor surface and less tendency to

ionize the air around the conductor. Ionization produces the undesirable effect called corona.

The following sizes have now been standardized for transmission lines of different voltages:

(i) For 132 KV lines: 'Panther' ACSR having 7strands of steel of dia 3.00 mm and

30Strands of Aluminum of dia 3.00 mm

(ii) For 220 KV lines: 'Zebra' ACSR having 7strands of steel of dia 3.18 mm and 54Strands

of Aluminium of dia 3.18 mm.

(iii) For 400 KV lines: Twin 'Moose' ACSR having 7Strands of steel of dia 3.53 mm and

54Strands of Aluminium of dia 3.53 mm.

7.10 DC SUPPLY ROOM (BATTERY)

It is used to calibrate relays & other measuring meters. It is also called heart of a power

system. The room where these batteries are kept is painted black & is also fully/nearly closed

so that batteries can’t get exposed to sun light so they don’t leak or their efficiency remain as

rated.

7.11 LIGHTENING ARRESTER (132 kv)

A lightning arrester is a device used on electrical power systems and telecommunications

systems to protect the insulation and conductors of the system from the damaging effects of

lightning.

The typical lightning arrester has a high voltage terminal and a ground terminal. When a

lightning surge (or switching surge, which is very similar) travels along the power line to the

arrester, the current from the surge is diverted through the arrestor, in most cases to earth. In

telegraphy and telephony, a lightning arrestor is placed where wires enter a structure,

preventing damage to electronic instruments within and ensuring the safety of individuals

near them.

Smaller versions of lightning arresters, also called surge protectors, are devices that are

connected between each electrical conductor in power and communications systems and the

Earth. Their purpose is to limit the rise in voltage when a communications or power line is

struck by lightning or is near to a lightning strike.

If protection fails or is absent, lightening that strikes the electrical system introduces

thousands of kilovolts that may damage the transmission lines, and can also cause severe

damage to transformers and other electrical or electronic devices. Lightning produced

extreme voltage spikes in incoming power lines can damage electrical home appliances.

FIG- Lightning Arresters

Types of Lightning Arresters for outdoor application

There are several types of lightening arresterin general use. They differ only in constructional

details but operate on the same principle, providing low resistance path for the surges to the

round.

1. Rod arrester

2. Expulsion type lightning arrester

3. Valve type lightning arrester

1.Rod arrester

It is a very simple type of diverter and consists of two rods, which are bent at right angles

with a gap in between. One rod is connected to the line circuit and the other rod is connected

to earth. The distance between gap and insulator (i.e. distance P) must not be less than one

third of the gap length so that the arc may not reach the insulator and damage it.

Generally, the gap length is so adjusted that breakdown should occur at 80% of spark-voltage

in order to avoid cascading of very steep wave fronts across the insulators. Under normal

operating conditions, the gap remains non-conducting. On the occurrence of a high voltage

surge on the line, the gap sparks over and the surge current is conducted to earth. In this way

excess charge on the line due to the surge is harmlessly conducted to earth

FIG- ROD ARRESTER

2. Expulsion type arrester

This type of arrester is also called ‘protector tube’ and is commonly used on system operating

at voltages up to 33kV.

It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber

tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected

to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each

line conductor.

On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is

stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of

tube walls resulting in the production of neutral gas. In an extremely short time, the gas

builds up high pressure and is expelled through the lower electrode, which is hollow. As the

gas leaves the tube violently it carries away ionized air around the arc.

This de-ionizing effect is generally so strong that the arc goes out at a current zero and will

not be reestablished.

FIG- Expulsion type arrester

3. Valve type arrester

Valve type arresters incorporate non linear resistors and are extensively used on systems,

operating at high voltages. It consists of two assemblies (i) series spark gaps and (ii) non-

linear resistor discs in series. The non-linear elements are connected in series with the spark

gaps. Both the assemblies are accommodated in tight porcelain container.

The spark gap is a multiple assembly consisting of a number of identical spark gaps in series.

Each gap consists of two electrodes with fixed gap spacing. The voltage distribution across

the gap is line raised by means of additional resistance elements called grading resistors

across the gap. The spacing of the series gaps is such that it will withstand the normal circuit

voltage. However an over voltage will cause the gap to break down causing the surge current

to ground via the non-linear resistors.

The non-linear resistor discs are made of inorganic compound such as thyrite or metrosil.

These discs are connected in series. The non-linear resistors have the property of offering a

high resistance to current flow when normal system voltage is applied, but a low resistance to

the flow of high surge currents. In other words, the resistance of these non-linear elements

decreases with the increase in current through them and vice-versa.

Non-linear resistor discs

Under normal conditions, the normal system voltage is insufficient to cause the breakdown of

air gap assembly. On the occurrence of an over voltage, the breakdown of the series spark

gap takes place and the surge current is conducted to earth via the non-linear resistors.

Since the magnitude of surge current is very large, the non-linear elements will offer a very

low resistance to the passage of surge. The result is that the surge will rapidly go to earth

instead of being sent back over the line. When the surge is over, the non-linear resistors

assume high resistance to stop the flow of current.

FIG- Valve type arrester

POWER LINE CARRIER COMMUNICATION:-

Power line communication (PLC) carries data on a conductor that is also used simultaneously

for AC electric power transmission or electric power distribution to consumers. It is also

known as power line carrier, power line digital subscriber line (PDSL), mains

communication, power line telecommunications, or power line networking (PLN).

A wide range of power line communication technologies are needed for different

applications, ranging from home automation to Internet access which is often called

broadband over power lines (BPL). Most PLC technologies limit themselves to one type of

wires (such as premises wiring within a single building), but some can cross between two

levels (for example, both the distribution network and premises wiring).

Typically transformers prevent propagating the signal, which requires multiple technologies

to form very large networks. Various data rates and frequencies are used in different

situations.

A number of difficult technical problems are common between wireless and power line

communication, notably those of spread spectrum radio signals operating in a crowded

environment. Radio interference, for example, has long been a concern of amateur radio

groups

8. CONCLUSION

The inplant training was completed successfully.I learnt things like how actually

substation works. I also learnt maintenance and tests carried out in substation.I understood

the operation and real time working of different equipments in the substation.The training

will definitely help me in my future.