EEC 122 Complete

7/28/2019 EEC 122 Complete

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GENERATING PLANTS

Need for Electrical Energy

Electrical energy is vital for all living being on earth, its need cuts across every aspect of human

life as other nearly developed technologies ride on its back. Without electrical energy it wouldbe impossible to make telephones, computers, television, sound systems, etc. It is also used in

illumination system (for lighting purposes), manufacturing processes and in some advanced

countries for heating or transportation applications.

(1) Ex 1: Outline other areas where electrical energy is highly useful.

Sources of Electrical Energy.

The law of conservation of energy states that energy cannot be destroyed but only changed

from one form to another chemical energy is converted to electrical energy in potential energy

of water is concerted to electrical energy in hydro electric stations, or thermal energy to

electrical in steam power plants. In same view, electrical energy is obtained from sourced like

wind energy, potential energy of water, nuclear energy, solar energy, heat energy, chemical

energy (coal) etc.

This conversion of energy takes places at electric power stations / generating stations. Electric

power station / generating stations is a plant where electric energy is produced from some

other forms of energy by means of suitable apparatus.

Electrical generating plants can be grouped into two broad groups, namely:

(1) Thermal Stations

(2) Non - Thermal Stations.

Diagram

Thermal Stations: are power station in which electricity generation involves the production of

heat energy. Examples are Steam power and Nuclear power stations.


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Non Thermal Stations: Here, generation of electricity is not accomplished throgh heat energy

production e.g. hydroelectric power station.

STEAM POWER STATIONS

In steam power plants, heat is produced from combustion of either coal, gas or oil to produce

super-heated from water in a boiler at elevated temperatures and pressures (about 541C,

pressure of 13mpa). The steam is then passed through steam turbines to drive the blades of the

turbine and hence drive a coupled alternator, the output of which is electrical energy.

The turbine shaft is mechanically coupled to the rotor of a generator and as the rotor revolves,

the generated voltage is collected at the stator terminals. The spent steam is cooled down at

the condenser and the condensed water goes back to the hot well for continuous repeating of

the cycle.

In order to reduce the running cost which could have been laid linking the power station with

an oil refinery and through these pipelines, gas and oil are directly pumped into the power

station.

Where coal is chemical - thermal - mechanical - electrical is used, the coal when received at the

power station is conveyed to a mill for crushing into powder, that is, the coal is pulverized

before being made to undergo combustion. Pulverization aids in complete combustion of thecoal and increases the system efficiency. The heat of combustion 20% is used to convert water

in the boiler into super heated steam which at a very high temperature is used to drive the

steam turbine that has been mechanically coupled to drive the rotor of a generator.

Diagram

Large volumes of ash have to be handled after ensuring that ash is extracted to the maximumpossible content (up to 99%) by using electrostatic / Electrolyte precipitators.

Heat losses are experienced during electricity generation in a steam power plant. Typical

thermal power plant are shown


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Plant's Unit Heat loss (%)

Boiler 15.9

Condenser 53.7

Alternator 0.9

Output Power 29.5

Total Unit 100%

It could be observed that a large % of heat loss is experienced at the condenser. In order to

increase the efficiency of the steam plant (which is usually < 40%) called bleeding is employed.

Bleeding is a process whereby a small quantity of steam is tapped off from the turbine to pre-

heat water flowing through the pipe linking the hot well to the boiler as shown below:

Diagram

The efficiency of the station is improved because through bleeding, a lesser quantity of fuel isconsumed for the same magnitude of power generated.

The thermal and overall efficiencies of a steam power plant are expressed respectively as:

A unit of electricity is 1kwh = 1000w x 3600s = 3.6 X J

In terms of Heat energy and mechanical energy, 1 caloric = 4.18J

Since 1kwh = 1000 X 3600s = 3.6 X J


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(KCal = Kilocaloric)

Thermal and Overall efficiency could also be rewritten as:

Thermal Efficiency = Boiler efficiency X Turbine Efficiency

= boiler X turbine

Overall efficiency = Thermal efficiency X Electrical Efficiency

= thermal X electrical

ADVANTAGES OF STEAM POWER PLANTS

1) The fuel (i.e. coal) is quite cheap.

2) It requires less space as compared to the hydroelectric power station

3) The cost of generation is less than that of the diesel power station

4) It can be installed at any place irrespective of the existence of coal

DISADVANTAGES OF STEAM POWER PLANTS

1) It is costlier in running cost as compared to hydroelectric plant

2) It pollutes the atmosphere due to the production of large amount of smoke and fumes

GAS TURBINE POWER PLANT

In a gas turbine power plant, air is used as the working fluid. The air is compressed by the

compressor and is led to the combustion chamber where heat is added to air, thus raising its

temperature. This heat is added to compressed air either by burning fuel in the combustion

chamber or by the use of air heaters. The hot and high pressure air from the combustion

chamber is then passed to the turbine. The air drives the gas turbine. The gas turbine then

drives the alternator which converts mechanical energy to electrical energy.


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It is worthy to note here that compressor, gas turbine abd the alternator are connected on the

same shaft so that a part of mechanical power of the turbine can be utilized for the operation

of compressor.

Below is the schematic arrangement of a gas turbine plant.

Diagram

A simpler arrangement arrived at by removing some auxiliaries is drawn below.

Diagram 5

From the previous diagram (figure 2), the air at atmospheric pressure is drawn by the

compressor via the filter which removes the dust from air. The pressure of the air is raised and

air at high pressure is available at the output of the compressor. Next is the regenerator. The

regenerator is a device which recovers heat form the exhaust gasses of the turbine. The

exhaust is passed through the regenerator before wasting into the atmosphere and in this way

compressed air (from the compressor) is heated by the hot exhaust gasses.

The air at high temperature from the compressor is led to the combustion chamber via the

regenerator. In the combustion chamber, heat is added to the air by burning fuel. Oil is injected

through the burner into the chamber at high pressure. The combustion gasses are then

delivered to the gas turbine. The products of combustion comprising a mixture of gasses at high

temperature and the pressure are then passed to the gas turbine. These gasses in passing over

the turbine blades expand and thus do the mechanical work. The gas turbine is connected to

the alternator. The alternator then converts mechanical energy of the turbine into electrical

energy. The output from the alternator is given to the bus-bars through transformers, circuit

breakers and isolators. Before starting the turbine, compressor has to be started. For this

purpose, an electric motor is mounted on the same shaft as that of the turbine

ADVANTAGES OF GAS TURBINE POWER PLANTS

1) It is simpler in design, construction and operation and smaller in size as compared to steam

power station of the same capacity since gas turbine plant does not require boiler, feed water

pump and condenser.


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2) The initial and operating costs are much lower than that of equivalent steam power station.

3) The maintenance charges are quite small

4) It can be started quickly from cold condition

5) There are no standby losses.

DISADVANTAGES OF GAS TURBINE POWER PLANT

1) There is a problem for starting the unit before starting the turbine, the compressor has to be

operated for which power is required from some external sources. However, once the unit

starts, the turbine itself supplies the necessary power to the compressor.

2) Since a greater part of power developed by the turbine is used in driving the compressor, the

net output is low hence the overall efficiency is low (about 20%)

DIESEL POWER STATION

A generating station in which diesel engine is used as the prime mover for the generating of

electrical energy is known as diesel power station. In a diesel power station, the prime mover is

a four-stroke, internal combustion engine. The diesel burns inside the engine and the products

of this combustion act as the working fluid to produce mechanical energy. The diesel engine

drives the alternator which converts mechanical energy into electrical energy. The fuel used is

diesel and the running cost is high. This explains why it is not used to supply base load but only

switched on in time of emergencies when unexpected peak load demand arises on the power

system. It is called an internal combustion engine because fuel combustion takes place inside

the engine and not externally as it is the case with steam power plants

The four-stroke cycle consist of:

1) INTAKE: A valve opens and let's in atmospheric air into the cylinder.

2) COMPRESSION: The piston moves up and compresses the atmospheric air thereby reducing

its volume and increasing the temperature of the compressed air inside the cylinder.

3) COMBUSTION: The increased temperature of the compressed air ignites the combination of

fuel and air.

4) EXHAUST: The second valve in the cylinder opens to let out the product of combustion which

is the observable fume, the cycle again repeats itself from step 1 to step 4.


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The diesel engine is also called a compression- ignition engine as it does not require a spark

plug to ignite the fuel.

ADVANTAGES OF DIESEL POWER PLANTS

1) The design and layout of the plant are quite simple

2) It occupies less space as the number and size of the auxiliaries are small

3) It can be located at any place

4) It can be started quickly and can pick up load in a short time

5) There are no standby losses

6) It require less quantity of water for cooling 7) The overall initial cost of installation is much

less than that of steam power station of the same capacity

8) It requires less operating staff

DISADVANTAGES OF DIESEL POWER PLANTS

1) The plant has very high running charges as the fuel (diesel) used is costly.

2) The plant does not work satisfactory under load conditions for a long period.

3) The plant can only generate small power.

4) The cost of lubrication and maintenance charges are generally high.

SOLAR POWER PLANT

This is a power plant that harnesses the energy of the sun to generate electricity. Two methods

are used to convert solar energy into electrical energy.

The direct method involves the use of solar cells. These are photo-voltaic cells which generate

an emf when exposed to sunlight instantly. To collect an appreciable magnitude of generated

voltage, many of the solar cells are connected in series and arranged in a panel. The technology

required for the manufacture of solar cell is highly sophisticated and for this reason, solar cells

are expensive. Also, sunshine is intermittent changing with atmospheric conditions. This


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explains why solar cells are always connected to a bank of batteries for the electrical loads to

receive a steady supply of electricity.

Another military factor is the large expanse of land required to set up a generating station

based on the use of solar energy.

The indirect method makes use of curved reflecting surfaces, minors or lenses to concentrate

sun energy on a receiver to help raise water to superheated steam, which is used to drive a

turbine and generate electricity. The concentrators are designed with self-adjusting mechanism

to enable them track the movement of the sun as it rises in the east and moves to set in the

west. This is thermal method of electricity generation.

Power generation based on the solar principle does not enjoy a large scale application. Further

scientific developments on the solar power plant are on-going.

WIND POWER PLANT

The wind power plant harnesses wind energy to generate electricity. It consists of a wind mill

which is mechanically coupled to drive the rotor of a dc generator. The generated voltage

collected is not continuous but rather it is intermittent since the blowing of wind is irregular

depending on atmosphere changes.

However, to collect a steady supply of electricity, the dc generator is made to charge a bank of

batteries and the electrical load in turn is connected to the bank of batteries so that anuninterrupted supply of electricity can be ensured. This method of power generation is still on

an experimental scale and not on a large scale use.

Diagram

AC POWER SUPPLY SYSTEM

Electrical energy is generated at the power stations (thermal, hydro-electric or nuclear) which

are usually situated far away from the load centres. Hence, an extensive network of conductors

between the power stations and the consumers is required. This network of conductors may be

divided into two main components called the TRANSMISSION SYSTEM and the DISTRIBUTION SYSTEM.


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The generation, transmission and distribution system of electrical power is called the

ELECTRICAL POWER SUPPLY SYSTEM.

In Nigeria, power is generated at a voltage of 16KV the generated voltage is stepped up by a

station transformer to 330KV. The generated power at 330KV is transported by means of

transmission lines to a transformer sub station ( e.g. Ipaja in Lagos, Ayede at Odo - ona,

Ibadan, Akangba at Surulere). The transported power is stepped down to 132KV and

transported by the means of secondary/ sub - transmission lines to 132/33KV transformer

substation (e.g. Ojere, Jericho, Ewekoro). At the utilisation level, i.e. At consumer's point,

further stepping down occurs such that 415V is the voltage between live conductors (line

voltage) between a live conductor and a neutral is 240V (Phase voltage)

[Check the diagram of a typical layout of power supply scheme in Nigeria on the Next page]

FACTORS AFFECTING THE CHOICE OF SYSTEM VOLTAGE

Two factors determine the choice of system voltage. They are:-

(1) Geographical Reason

(2) Historical Reason

Diagram of TYPICAL AC POWER SUPPLY SCHEME

NOTE :- All system of power transmission and distribution may or may not include all elements

enumerated above, for example, some system may have no primary transmission, some may

not have secondary transmission and the others may not have transmission at all,, being very

small and so on.

GEOGRAPHICAL REASON

The level of industrialization of a country and the land mass over which power will betransported all combine to determine the voltage level for power transmission. For example, in

America as at 1975, power is transmitted at 750KV and by now 1000KV lines should have been

operational. However, in Nigeria, the relativity less developed country, the highest voltage level

for power transmission is 330KV.


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HISTORICAL REASON

In the western countries, where free economy is practised, private ownership of generating

station is encouraged. However, in socialist countries and most African countries, state

ownership of generating station is the tradition and so the government recommends the voltage at

every level of the power system just as it is the case presently in Nigeria. However in countrieswhere private ownership is encouraged, the government regulates the statutory limit of voltage

and frequency at the utilisation level to ensure standardisation of electrical equipment.

Statutory Units are: 240V 5% & 50HZ 1%

ADVANTAGES OF TRANSMITTIN AT HIGH VOLTAGE

For a single-phase system, the apparent power is given by: S = VI--------(a)

Where S = apparent power, V= voltage(v), I = Current (A)

The following points could be summarized:-

1) The higher the voltage of power transmission (from equation (a), the lower the currentflow on the line and hence less voltage drop (IR) is experienced on the line

2) Less power losss (IR) is experienced if the voltage is very high and this meansimproved efficiency.

3) The lower the current flow on the line, the higher the resistance of the transmission lineand the lesser the conductor material required from the construction of the line.

DISADVANTAGES OF HIGH VOLTAGE TRANSMISSION

1)

With the increase in voltage of transmission the insulation required between theconductors and the earthed tower increases the cost of line supports

2) With the increase in voltage of transmission, more clearance is required betweenconductors and ground. Hence higher towers are required

3) With the increase in voltage of transmission, more distance is required between theconductors. Therefore longer cross-arms are required

TRANSMISSION AND DDISTRIBUTION

The transmission system is to deliver bulk power from power stations to the load centres

and large industrial consumers beyond economical service range of the regular primary

distribution lines whereas distribution system is to deliver power from power stations or

substation to the various consumers.Electrical power can be transmitted and distributed by either ac or dc.

[Students can find out the advantages and disadvantages of using ac and dc transmission

and distribution]


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The transmission system can be further divided into primary and secondary (or sub)

transmission. Similarly, the distribution system may be divided into primary and secondary

distribution

COMPARISM BETWEEN TRANSMISSION AND DISTRIBUTION

Transmission Distribution

1) Usually 3-phase, 3-wire system Usually 3-phase, 4 wire sysytem

2) Has fewer substation with high capacity There are several with low capacity

3) Rated voltages are higher Rated voltages are lower

4) Line parameters are R, L & C Predominants line parameters are R and C

5) There are no tappings There are tappings on line at several points

6) Configuration is mostly radial Configuration can either be radial or ring

7) Line lengths cover several hundred (and

in some cases thousands)

Line lengths usually cover a fewer km to

fraction of km.

PERFORMANCE OF SHORT AND MEDIUM TRANSMISSION LINES

The transmission line is the main energy corridor in a power system. Hence, the performance ofa power system depends mainly on the performance of transmission lines in the system. The

important consideration in the design and operation of the transmission lines are voltage drop,

line/power losses and efficiency of transmission. The performance of a transmission line is

governed by its four parameters, viz:

1) Series Resistance, R: the opposition the conductor offers to the flow of current

2) Series Inductance, L: Due to the fact that the current carrying conductor is surrounded my

magnetic lines of force.

3) Shunt Capacitance, C: Due to the fact that the conductor carrying current forms a capacitor

with the earth which is always at lower potential than the conductor and the air between forms

a dielectric medium.


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4) Shunt Conductance, G: Due to the flow of leakage currents over the surface of the insulators

especially during bad weather. Shunt conductance is very small in the case of overhead lines

and may be assumed zero

CLASSIFICTION OF TRANSMISSION LINES

Depending on the manner in which capacitors is taken into account, transmission lines are

classified as:

1) Short transmission lines : having length up to about 50km and operating voltage lowerthan 20KV are usually considered short transmission lines. Due to smaller distance and

lower line voltage, the capacitance effects are extremely small, and therefore, can be

neglected. Hence the performance of short transmission lines depend upon the

resistance and inductance of the line. Though in an actual line, the resistance and

inductance are distributed over the whole length, but in case of shunt lines, the total

resistance and inductance are assumed to be lumped at one place. In case of single

phase circuit, the total loop resistance and inductance is considered, whereas in case of

3- phase circuits only resistance and inductance to neutral i.e per phase is required to be

taken into account

For 3 phase short transmission lines, given voltages and currents are line to line

values while the powers (VA, VAR & KW) are for the three phases. Here, the 3 phase

short transmission line is believed to be carrying equal load on each phase.

2) Medium Transmission Lines: Lines having lengths between 50Km and 200Km and linevoltage between 20KV and 100KV are referred to as medium transmission lines. Owingto appreciable length and voltage of the line, the charging current is appreciable and

therefore capacitance effect cannot be ignored. Though the capacitance is uniformly

distributed over the entire length of the line, yet the capacitance may be assumed to be

one or more points.

3) Long Transmission Lines: When the length of an overhead transmission line is morethan 200Km and line voltage is very high (usually more than 100KV), it is considered as a

long transmission line the treatment of such a line assumed that the line constant are

considered uniformly distributed over the whole length of the line and rigorous

methods are employed for solution.

NOTE: The exact solution of any transmission line must consider the fact that theconstants of the line are not lumped but are distributed uniformly throughout the

length of the line. However, reasonable accuracy (obtained from approximate solutions)

can be obtained by considering these constants as limped for short and medium

transmission lines.

IMPORTANTS TERMS


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1) VOLTAGE REGULATION: when the load is supplied, there is a voltage drop in the linedue to resistance and inductance of the line and therefore, receiving end voltage, VR, is

usually less than the sending end voltage, Vs. This voltage drop (i.e. Vs VR) expressed as

a percentage of receiving voltage VR is called the REGULATION. Voltage Regulation is

therefore defined as the change in voltage at the receiving (load) and when the full load

is thrown off, the sending end (or supply) voltage and supply frequency remainingunchanged. It is usually expressed as a percentage of the receiving end voltage.

Alternatively, Voltage Regulation can be defined as the difference in voltage at the

receiving end of a transmission line between conditions of no-load and full-load,

expressed as a percentage of the receiving and voltage, VR

NOTE: At no load, there is no drop in the line so that at no load, Vs = VR. However, at full

load there is a voltage drop in the line so that receiving end voltage is VR. Difference in

voltage at receiving end between no-load and full-load = Vs VR.

Mathematically,

The lower the voltage regulation, the better it is, because low voltage regulation means

little variation in receiving end voltage due to variation in load current.

2) TRANSMISSION EFFICIENCY: The power obtained at the receiving end of a transmissionline is generally less than the sending end power due to losses in the line resistance.

Efficiency of a transmission line is defined as the ratio of the power delivered at the

receiving end to the power sent from the sending end mathematically

3)

X 100%

=

Or

X 100%

Where andare the receiving end voltage, current and power factor (all phasevalues) while andare the sending end voltage, current and power factor (all phasevalues) respectively.


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SHORT TRANSMISSION LINES

(OC) = (OD) + (DC)

= (OE + ED) + (DB + BC)

= ( ) + ( )

(a) ( ) ( )

(b)

(c) Sending end pf, ( ) (d)Power delivered =

Line Losses = IR

Power sent out =

% transmission efficiency =

=

X 100%

Using the diagram of fig 1.2, an approximate expression for the sending end voltage, Vs, can be

obtained as follows:


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OC = OF = OA + AF = OA + AG + GF

= OA + AG + BH

The solution can also be obtained using complex notation as shown below:

Taking

( )

Z= R + j

+

( ) ( ) (R + j) ( ) ( )

( ) ( )

The second term under the root is quite small and can be neglected with reasonable

accuracy. Therefore, the approximate expression for becomes:

DISTRIBUTION SYSTEMS

Power is usually generated at power generating stations, then transmitted and distributed up

to find point of consumption by the consumers. That part of power system which distribution

electric power for local use (Consumers find point) is known as distribution system. In general,


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the distribution system is the electrical system between the Sub station fed by the

transmission system and the consumer meters.

A distribution system consists of feeders, distributors and service mains. In trying to define their

terms, a tabular comparison is made between a feeder and a distributor and finally,Service

mains is defined.

FEEDER DISTRUBUTION

1. A feeder is a conductor which connects the

sub station (or localized generating station) to

the area where power is to be distributed.

2. No tappings are taken from the feeder soCurrent in it remains the same throughout

3. The main consideration in the design of

feeder is the current carrying capacity

A distributor is a conductor from which tappings

are taken for supply to the Customers.

The current through a distributor is notconstant because tappings are taken at various

places along its length

While designing a distributor, voltage drop

along its length is the main consideration

(because statutory unit of voltage variations is

16% of rated value at the consumers

terminal)

SERVICE MAINS: A service mains is generally a small cable which connects the distributor to the

consumer terminals

Diagram


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CLASSIFICATION OF DISTRIBUTION SYSTEM

A distribution system may be classified according to:-

1 Nature of current DC distribution system

AC distribution system

2 Type of construction Underground system

Overhead system

3 Scheme of connection Radial system

Ring system

However, only classification category according to scheme of connection (i.e Radial System) are

of interest to us in this chapter.

RADIAL DISTRIBUTION SYSTEM

The radial system is serviced by a single transformer substation. In this system, separate

feeders radiate from a sub station and feed the distributors at one end and only. Any fault on

the transformer substation renders all consumers out of supply of electricity, hence the radial

system suffers from insecurity of the electricity supply.

Diagram

The diagram above shows a radial system whereby the distributor AB is only fed at one end

(point A) by feeder OC.

DEMERITS OF RADIAL DISTRIBUTION SYSTEM

1.

The end of the distributor nearest to the feeding point will be heavily loaded2. The consumers are dependent on a single distributor. Any fault on the feeder and single

distributor cut off supply to the consumers who are on the site of the fault away from supply

from the sub station.


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3. The consumers at the distant end of the distributor would be subjected to various villagefluctuations when the load on the distributors changes.

RING & MAIN SYSTEM

In this system, the primaries of distribution transformers form a closed loop. A sub station

supplies to the closed feeder ABCEFGHI according to the diagram that follows

The distributors are tapped from different place C, F and H of the feeder through distributor

transformers.MERITS OF RING DISTRIBUTION SYSTEM

i. There are less voltage fluctuation at consumer terminal

ii. The system is very reliable as each distribution is fed via two feeders. This means in the rent

of fault on any section of the feeder, the continuity of supply is maintained.

For example, suppose any fault occurs at any point P of section ABC, of the feeder, then section

ABC of the section can be isolated for repairs and at the same time. Continuity of supply is

maintained to all the consumer via feeder AIHGFEC.However, it is possible for a feeder ring to be energized by two or more substations (or

generating stations) as shown below.

Diagram


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Here, distributors are connected to points O< P< R & Q of the feeder ring through distribution

transformers.

VOLTAGE DROP IN DC DISTRIBUTION LINES

When DC Voltage is transported, voltage drop is only due to line resistance and the circuit

representation becomes:

Vs, = Sending end voltage

Vr = Receiving end voltage

In calculation, involving dc distribution, consideration is given to the type of distributor. The

most general method of classifying is the way they are fed by the feeder on this basis, dc

distributors are classify:

a. Distributor fed at one end.b. Distributor fed at both ends.c.

Distributor fed at the centre.

d. Ring distributor.In addition to the methods of feeding, a distributor may have

i Concentrated loading

ii Uniform loading

iii Both concentrated & Uniform loading

VOLTAGE DROP IN AC DISTRIBUTIONAC distribution calculations differ from the dc distribution in the following respects:

1. In case of dc system, the voltage drop is a resistance alone. However in ac system, the drops aredue to the combined effects of resistance inductance and capacitance as can be seen in the ac

distribution representation below:-


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Vs = Sending (supply) end voltage

Vr = Receiving end voltage

L = Line inductance per km per phase

Iron, stoneware, asphalt or treated wood. After the cable is laid in position, thetroughing is

filled with a bituminoun or asphatic compound and covered over.

DISADVANTAGES

1. It is more expensive than direct laid system.2. It requires skilled labour and favorable weather conditions.3. Due to poor heat dissipation facilities, the current carrying capacity of the cable it required.

LINE INSULATOR AND SUPPORTS

LINE INSULATORSThese are items that prevent current from flowing between conductors (overhead lines) and

their supports i.e the poles or towers. Line insulators are so place and arranged in such a way

that leakage currents from conductors do not flow to earth through line supports (poles and

towers).

Some of the desired properties of a good insulator are high mechanical strength to withstand

conductor load are high mechanical strength to withstand conductor load, high electrical

resistance to avoid leakage currents to flow to earth, high relative permittivity, etc.The most commonly used material for insulators of overhead line is porcelcum or china clay.

Porcelam is produced by firing at a high temperature a mixture of kaolin, feldspar and quarte in

a kiln. Other types of insulator material are glass, steatite, etc.

TYPES OF INSULATORS


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1. PIN TYPE INSULATOR: - The pin type insulator is secured to the cross arm on the pole. There isa groove on the upper end of the insulators for housing the conductors proper binding of the

conductor is done by an annealed wire of the same material as the conductor pin type

insulators are used for transmission and distribution of electric power at voltages from 11kv up

to 33kv. Beyond operating voltage of 33kv, the pin type insulators become too bulky and

2. SUSPENSION INSULATORS: - These are employed for high voltage i.e. beyond 33kv. (>33kv).They consist of a number of porcelain disis connected in series by metal links in the form of a

string. The conductor is suspended at the bottom end of this string is secured to the cross

armof the tower. Each unit or disc is designed for low series voltage, say 11KV. The number of

discs in series would obviously depend on the working voltage. For instance, if the workingvoltage is 66KV, then six discs in series will be provided on the string.

ADVANTAGES OF SUSPENSION INSULATORS

1. They are cheaper than pin type insulators for voltages beyond 33KV. The bottom insulator. Theguard ring introduces capacitance between metal fittings line capacitance currents L1, L2, etc

resulting in the same charging current I through each unit of string. This brings about

uniform potential distribution across the units.


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3. STRAIN 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 time of excessive tension, strain

insulators are used for low voltage lines (C11CV), shackle insulators are used as strain

insulators. However, for high voltage transmission lines strain insulators consisting of an

assembly of suspension insulators use in the vertical plane are employed.

4. SHACKLE INSULATORS: - They are used for low voltage distribution lines such insulator caneither be used in the vertical or horizontal 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. They are used

for 415V distribution lines.

POOR PERFORMANCES OF INSULATORS


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Any electrical insulator may be subjected to poor performance due to the following reasons: -

1. Cracking 2. Puncture Insulator

3. Porosity of material4. Improper glazing

5. Mechanical stress 6. Short Circuit

TESTING OF INSULATORS

In accordance with the British standard, the insulators most undergo the following tests: -

1. Flash over tests2. Performance Tests3. Routine Tests

LINE SUPPORTS.

These are supporting structures for overhead line conductors. The line support used for

transmission and distribution of electric power are of various types. These include wooden

poles, Reinforced concrete poles and steel towers. The choice of supporting structure for a

particular case depend on the line span, cross sectional area, line voltage and cost.

WOODEN POLES: - These are made of seasoned wood and are suitable for lines of relatively,

shorter spans (distance between two poles) of less than 50meters. They are mostly used in therural areas for low distribution voltages. The wooden poles generally tend to rot below the

ground level, causing foundation failure. In order to prevent this, the portion of the pole below

the ground land is impregnated with preservative compounds like composite oil.

Advantages

1 They are sharp 2 They are easily available

3 Provide Insulating

Disadvantages1. Have the tendency to rot below ground level

2. Comparatively shorter life span.

3. Cannot be used for high voltages

4. Have less mechanical strength


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2.REINFORCED CONCRETE POLES: - They are for high distribution voltages of up to 33KV Lines.

They have greater mechanical strength, longer life span and permit longer spans than wooden

poles.

They are more expensive than wooden poles.

3. STEEL TOWERS: - Where as wooden and reinforced concrete poles are used for distribution

purposes at voltages between up to 33KV, Steel tower are used for long distance transmission

voltages (above 33KV). Steel towers have greater mechanical strength, longer life, permit

longer spans (more than 300m) and can withstand most severe climatic conditions. Tower

footings are grounded by driving rods into the earth. This minimizes the lightning troubles as

each tower acts as a lightning conductor check diagrams of various types of steels towers on

the page that follows.


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PROTECTION IN POWER SYSTEM

A power malfunctions or experiences a form during conditions of short circuits. A short circuit is

always accompanied by an excessive flow of current which if left unchecked can damage circuit

component and electrical devices.

Several protective devices exist in power system. They include fuses, circuit breakers, Relays

lighting arresters switchgear, isolators (Reclosers and sectionalizers) etc.

However, the scope of this course limited to fuses and circuit breakers (moulded case type) and

as such only these two will be discussed.

FUSES

A fuse is a protecting device which has an element made of silver or tinned copper (tinned in a

circuit and melts, (or blows out) excessive current flows through it and breaks the circuit. The

fuse element is generally made of materials having low melting point, high conductivity

andleast deterioration due to oxidation. Fuses are inserted in series with the current to beprotected whenever a short circuit or overload occurs, excessive current flows through the fuse

thereby raising its temperature and fuse element blows out, disconnecting the circuit protected

by it.

The time required to blow out the fuse depends upon the magnitude of excessive current. The

greater the current, the smaller is the time taken by the fuse to blow out.

For small currents up to 10A, tin or an alloy of lead and tin is used for making the fuse element.

For larger currents, copper or silver is employed.DEFINITION OF IMPORTANT TERMS.

1 CURRENT RATING: - This is the maximum value of current, stated by the manufacturer, that

the fuse can carry without overheating or melting.


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It depends on the temperature rise of the contacts of the fuse holder, fuse material and the

surrounding of the fuse. It is also called the continuous rating of the fuse.

2 FUSING CURRENT: _ This is the maximum current which causes the fuse to blow or melt.

Value of fusing current is always more than the current rating of the fuse element. It is also

called the interrupting current of the fuse.

3 FUSING FACTOR: - This is the ratio of the minimum fusing current rating of the fuse element

i.e.

Fusing Factor = Minimum fusing current

Current Rating of fuse

Its magnitude is always greater than unity

Types of fuse Fusing Factor1. Rewirable fuse

2. Cartridge fuse

3. HRC fuse

4. Current breakers

1.8

1.25 1.75

1.25

TYPES OF FUSES

There are 3 major types of fuses. THEY ARE:1 REWIRABLE FUSES: They are commonly used in domestic installation. They are used where

low values of fault current are to be interrupted. It consists of

I a base and ii a fuse carrier. The base is of porcilin and carries the fixed contacts to which the

incoming and outgoing phase wires are connected. The fuse carrier is also of porcelain and

holds the fuse elements (tinned copper wire) between its terminals. The fuse carrier can be

inserted in or taken out of the Saxe when desired.

Whenever a fault occurs, the fuse element is blown out and the circuit is intempted. The fuse

carrier is taken out and the blown out fuse element is replaced by the new one. The carrier is

then re- inserted in the base to restore the supply.


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Advantages of Rewirable Fuse

1 Cheap and easy to repair on the cost of replacement is negligible.

2 Easily accessible.

Disadvantages of Rewirable Fuse

1 It can be replaced with a wire of incorrect size

Produces an electric are when it melts to open a circuit, hence cannot be used in environment

containing chemicals and flammable gases.

Since it is semi enclosed, it oxidizes while carrying normal current, thereby reducing in cross

sectional area and with time, fuse will trip when carrying rated current below the rated value.

2 CARTRIDGE FUSE: This fuse also finds its plication in domestic installations. It is the type

usually in 13A fused plugs used in homes and offices. Cartridge fuse overcomes the

disadvantage sociated with the rewirable fuse as the rating of a placement fuse element is

determined by the manufacturer.

ADVANTAGES OF CARTRIDGE FUSE

Fuse element is totally enclosed in a glass or porcelain tube and hence does not oxidize while inoperation therefore breaking the circuit only at rated values

It is easily replaceable.

DISADVANTAGES OF CARTRIDGE FUSE

It is more expensive than a rewirable fuse. It does not have an arc extinguishing mechanism,

hence unsuitable in areas where inflammable gases

HIGH RUPTURING CAPACITY (HRC)

This type of fuse has its characteristics trolled by the manufacturer. These fuses are used to

protect large industrial loads, mainly in other situations where very large fault can occur.


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The fuses element made of silver is in a fibre tube. When short circuit current the fuse

element melts and the resulting heat up the fibre tube. It releases gases and high pressure, the

high pressure in turn extinguishes the electric arc. The HRC Fuse, unlike the carrier and

rewirable fuses, does not heat up its surrounding circuit. It is safe for use in industries and

refineries where inflammable gases exist in the environment. However, it is the most expensive

fuse.

CIRCUIT BREAKERS

A circuit breaker is a piece of equipment which can:

i. Make or break a circuit manually or by remote control under normal conditionsii. Break a circuit automatically

A circuit breaker can make a circuit either manually or automatically under all conditions, VIE

no load, full load and short circuit conditions.

Thus, a circuit breaker incorporates manual (or remote control) as well as automatic control for

switching functions. The automatic control employs relays and operates only under fault

condition.

PRINCIPLES OF OPERATION OF CIRCUIT BREAKERS

A circuit breaker essentially consists of fixed and moving contacts called electrodes, undernormal operating conditions, these contacts remain closed and will not open automatically until

and unless the system becomes faulty. However, the contacts can be opened manually or by

remote control whenever desired. When a fault occurs on any part of the system, the trip coils

of the circuit breaker get energized and the moving contacts are pulled apart by some

mechanism than opening the circuit.

When the contacts of a circuit breaker are separated under fault conditions, an arc is struck

between them. The production of arc not only delays the current interruption process but italso generates enormous heat which may cause damage to the system or to the circuit breaker

itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the

shortest possible time so that heat generated by it may not reach a dangerous value.

Based on the medium of arc extinction, circuit breakers are classified in: -


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1. Oil circuit Breaker2. Air blast circuit Breakers3. Sulphur hexafluoride (SF6) circuit Breakers4. Vacuum circuit breakers

ISOLATORS

Isolators (sometimes referred to as disconnect switches) are simple pieces of equipment

employed only for isolating circuit when the current has already been interrupted. They ensure

that the current is not switched into the circuit until everything in order.

Isolators Circuit Breakers

1. They operate under no load or off-loadcondition

2. They are not equipped with arcquenching devices

3. They do not have specified currentbreaking capacity or current making

capacity

1. They operate when the circuit is still onload

2. They are equipped with arc quenchingdevices

3 They are specified interrupt capacity (current

breaking and making capacity).

Isolators are employed in addition to circuit breakers and are provided on each side of every

circuit breaker to provide isolation. While opening a circuit, the circuit breaker is opened first,

then isolator. If an isolator is opened carelessly, when carrying a heavy current, the resulting arc

could easily cause a flash over to ground. This may shatter the supporting insulators and may

even cause a fatal accident to the operator.

When closing a circuit, the isolator is closed first, then circuit breakers. Isolators are necessary

on the supply side of the circuit breakers in orders to ensure isolation (disconnection) of the

circuit breaker from the live parts for the purpose of maintenance.


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CONDUCTORS AND CABLES USED IN POWER SYSTEMS

In power systems (transmission and distribution this time around) transfer of power is done

from the generating stations to points of use via conductors. Conductors can either be bare

overhead conductors or underground cables.

Any material which offers free or easy passage of an electric current is known as a conductor.

Conductors offers less/ minimal resistance to the flow of electric current.

Some typical materials used as conductors are : -

1. SILVER: - This is the best known conductor but it is too expensive for general use. The

contacts of some switches are plated with silver to reduce the contact resistance

2. COPPER: - This material is widely used for the manufacture of electric wires, cables and bus

bars. Its conductivity is second only to silver.

Some of major advantages of using copper as conducting material are: -

a. Low resistance and high electrical conductivity,b. It is ductile and therefore easily formed into wires.c. Copper has high current denity i.e. current carrying capacity of copper permit of cross

sectional area is quite large.

However, due to its higher cost and non availability, it is rarely used for general purposes.

3. ALUMINIUM: - This is the most widely used conductor material when considering properties

like cost, conductivity, tensile strength, weight, etc. Aluminum is cheap and light as compared

to copper but it has much smaller conductivity and tensile strength. Advantages of using

aluminum could be summarized as follows: -

a. It is cheap

b. it has light weight

c. It is readily available.The relative comparison between copper and Aluminum is presented in the table that follows.

Copper Conductor Aluminum Conductor

1. Has less resistance, hence higher

conductivity.

Higher resistance, lower conductivity. The

conductivity of aluminum is about 60% that of


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2. Weighs more than aluminium hence

supporting structure for copper need to be

strong

3) Higher mechanical strength and so needs

no reinforcement

4) More expensive than Aluminium

copper

Weighs less, 1/3 of copper

Less mechanical strength and is reinforced

with steels

Less expensive than copper

STEEL CORED ALUMINIUM: Pure aluminium has low tensile strength and produces greater

sag. In order to increase the tensile strength, the alluminium conductor is reinforced with a

core of galvanized Steel wires to obtain Aluminum conductor steel Reinforced (ACSR). Thegalvanized steel is used in order to prevent rusting and electrolytic corrosion. The

reinforcement with steel keeps the composite conductor light.

Other types of conductor materials include gold, brass, nichrome, manganin, cadmium, copper,

etc. There are various sizes of conductors. 35mm2, 70mm2, 95mm2, 100m2, etc. The type to be

used depend on the amount of current to be carried.

CHOICE OF CONDUCTOR MATERIALS

The choice of conductor materials for use as overhead lines depends on the following factorsa. Cost b. Electrical properties c. Mechanical properties d. Local condition

COST: - it plays a prominent role in determining what material to use for an engineering design

since the costlier the material used, the more expensive the finished product will be. This

certainly will inhibit the marketability of a finished product.

2. ELECTRICAL PROPERTIES: This refers to the voltage and the power being transported by the

line. Losses of the transported power must be drastically reduced. The choice of material for

overhead construction has a bearing on it.

3. MECHANICAL PROPERTIES: - Overhead lines are subjected to swinging and vibration due to

the blowing of the wind; the overhead lines are designed to have adequate mechanical stress

to withstand the strain placed on it by high velocity winds.


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4. LOCAL CONDITION: - In the temperate regions of the world, ice or snow formation on

overhead lines are common features. The choice of materials used for the overhead lines

construction must be able to sustain the weight of the ice deposits.

Overhead lines are usually stranded to make them flexible and mechanically strong to

withstand strains from ice formation and high velocity weights. Otherwise, unchecked swinging

of the line leads to mechanical fatigue and an eventual fracture of the line stranding helps to

overcome this type of damage.

A stranded conductor comprises a central wire around which other strands of wire are twisted.

The equation for determining the numbers of wires in a stranded overhead wire is = 3n (n+ 1) =

1

Where n = no of layers.

The overall diameter of the conductor is given

as D = (2n + 1) d

where d = diameter of a strand. Stranding reduces skin effects.

CORONA

This is a phenomenon that arises as a result of lonisation of air around Overhead conductor

lines above a particular applied voltage called the critical disruptive voltage. Corona effects areusually observed at a working voltage. Corona effects are usually observed at a working voltage

of 33KV above. Effects of corona are maximum at the conductor surfaces. These effects include

violet glow, hissing noise and production of ozone gas. Corona formed from these effects are

always accompanied by energy loss which is dissipated in form of light, heat, sound and

chemical action.

Corona is dependent on the following factors: -

1. Atmosphere i.e. Physical state of atmosphere, whether stormy, humid or dry.2. Conductor size: The rough and irregular surface will give rise to more corona because

unevenness of the surface decreases the value of breakdown voltage

3. Spacing between conductors: The more the spacing, the less the effect of corona


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4. Line voltage: If it is low, no corona is formed but as the value increases (above 33KV), the

more the tendency for corona to be formed.

Critical disruptive voltage is defined as the minimum phase mental voltage at which corona

occurs. It is given by =

Where r= radian of the conductor,

d= spacing between lines.

Corona effects are reduced by: -

1. Increasing conductor size.

2. Increasing conductor spacing.

CONCEPT OF BUNDLE CONDUCTORS

Bundle or multiple conductors is an arrangement of two or more conductors per phase

supported by one insulator assembly. The idea behind the concept is to reduce the effect of

corona discharge.

CABLES

Underground cables, or simply cables, used in power systems have several advantages such as

less liability to damage through storms or lightning, low maintenance cost, les chances of faults,

smaller voltage drop and aesthetic value i.e. better general appearance when compared with

bare overhead conductors.

However, their major drawback is that they have greater installation cost and introduce

insulation problems at high voltages. For this reason, underground cables are employed where

it is impracticable to use overhead lines. E.g thickly populated areas or areas where

maintenance conditions do not permit the use of construction.

The chief use of underground cables for many years has been for distribution of electric power

in congested urban areas.

Recently underground cable are now employed for transmission of power for short or

moderate distances.

CONSTRUSTION OF CABLES

Cables essentially consist of one or more cores or conductors 1. Covered with suitable

insulation 2.And surrounded by a protecting cover. Cables are made of 3 main parts.


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1. CONDUCTORS / CORES: A cable may have more than one core (conductor) depending upon

the type of service for which are intended. The conductors are made of tinned copper or

aluminum and are usually stranded in order to provide flexibility to the cable

2. INSULATION: Each core / conductor is provided with a suitable thickness of insulation,

depending upon the voltage to be withstood by the cable. The most commonly used materials

for insulation are impregnated paper, rubber mineral compound, polyvinyl chloride (PVC),

varnished cambric, etc.

3. PROTECTING CORES: This are usually in different layers.

These layers include: -

a. METALLIC SHEATH: used in order to protect the cable from ingress of moisture, damaging

liquids and other gases in the soil and atmosphere. They are made of either lead or aluminum.

b. BEDDING: This is a fibrous material to protect the metallic sheath against corrosion.

c. ARMORING: consists of one or two layers of galvanized steel were steel tape to protect the

cable from mechanical injury while laying it. It is laid over the bedding. Armouring may not be

done in the case of some cables.

d. SERVING: This is a layer of fibrous material similar to bedding laid over the armoring to

protect the armoring from atmospheric condition

TYPES OF CABLESAccording to the voltage for which they are manufactured, cables are divided into the following

groups:

1. LOW VOLTAGE CABLES ( 3KV): The cable insulation is PVC (polyvinyl chloride) and can

withstand moderate temperature up to 70C. PVC cables are mainly used in domestic wiring.

2. HIGH VOLTAGE CABLES ( 11KV): This type of

Documents

EEC 122 Complete