Basic information of Electrical Technology

A.C generator

A.C power can be generated as a single phase or as a balanced poly-phase system. However, it
was found that 3-phase power generation at 50 Hz will be economical and most suitable. Present
day three phase generators, used to generate 3-phase power are called alternators (synchronous
generators). An alternator has a balanced three phase winding on the stator and called the
armature. The three coils are so placed in space that there axes are mutually 120° apart. From the terminals of the armature, 3-phase power is obtained. Rotor houses a field
coil and excited by D.C. The field coil produces flux and electromagnetic poles on the rotor
surface. If the rotor is driven by an external agency, the flux linkages with three stator coils
becomes sinusoidal function of time and sinusoidal voltage is induced in them. However, the
induced voltages in the three coils (or phases) will differ in phase by 120° because the present
value of flux linkage with R-phase coil will take place after 120° with Y-phase coil and further
120° after, with B-phase coil. A salient pole alternator has projected poles. It has non uniform air gap and is generally used where speed is low. On the other hand a
non salient pole alternator has uniform air gap and used when speed is high.

Frequency, voltage & interconnected system

The frequency of the generated emf for a p polar generator is given by
f = pn/2 where n is speed
of the generator in rps or f= pn/120 when n is in rpm. Frequency of the generated voltage is
standardized to 50 HZ in our country and several European countries. In USA and Canada it is
60 Hz. The following table gives the rpm at which the generators with different number of poles
are to be driven in order to generate 50 Hz voltage.
Number of poles of Generator 2 4 6 8 10
rpm at which generator to be driven 3000 1500 1000 750 600
A modern power station has more than one generator and these generators are connected in
parallel. Also there exist a large number of power stations spread over a region or a country. A
regional power grid is created by interconnecting these stations through transmission lines. In
other words, all the generators of different power stations, in a grid are in effect connected in
parallel. One of the advantages of interconnection is obvious; suppose due to technical problem
the generation of a plant becomes nil or less then, a portion of the demand of power in that area
still can be made from the other power stations connected to the grid. One can thus avoid
complete shut down of power in an area in case of technical problem in a particular station. It
can be shown that in an interconnected system, with more number of generators connected in
parallel, the system voltage and frequency tend to fixed values irrespective of degree of loading
present in the system. This is another welcome advantage of inter connected system. Inter
connected system however, is to be controlled and monitored carefully as they may give rise to
instability leading to collapse of the system.
All electrical appliances (fans, refrigerator, TV etc.) to be connected to A.C supply are therefore
designed for a supply frequency of 50 Hz. Frequency is one of the parameters which decides the
quality of the supply. It is the responsibility of electric supply company to see that frequency is
maintained close to 50 Hz at the consumer premises.
It was pointed out earlier that a maximum of few hundreds of volts (say about 600 to 700 V)
could be developed in a D.C generator, the limitation is imposed primarily due to presence of
commutator segments. In absence of commutators, present day generated voltage in alternator is
much higher, typically around 10 kV to 15 kV. It can be shown that rms voltage induced in a coil
is proportional to f and n i.e., Ecoil . f n where f is the flux per pole and n is speed of the
alternator. This can be justified by intuition as well: we know that mere rotating a coil in absence
of magnetic flux (f) is not going to induce any voltage. Also presence of flux without any
rotation will fail to induce any voltage as you require rate of change of flux linkage in a coil. To
control the induced voltage one has to control the d.c field current as speed of the alternator gets
fixed by frequency constrain.
Thermal plant

We have seen in the previous section that to generate voltage at 50 Hz we have to run the
generator at some fixed rpm by some external agency. A turbine is used to rotate the generator.
Turbine may be of two types, namely steam turbine and water turbine. In a thermal power station
coal is burnt to produce steam which in turn, drives the steam turbine hence the generator (turbo
set).
In a thermal power plant coil is burnt to produce high temperature and high pressure steam in
a boiler. The steam is passed through a steam turbine to produce rotational motion. The
generator, mechanically coupled to the turbine, thus rotates producing electricity. Chemical
energy stored in coal after a couple of transformations produces electrical energy at the generator
terminals . Thus proximity of a generating station nearer to a coal reserve
and water sources will be most economical as the cost of transporting coal gets reduced. In our
country coal is available in abundance and naturally thermal power plants are most popular.
However, these plants pollute the atmosphere because of burning of coals.

Hydel plants

In a hydel power station, water head is used to drive water turbine coupled to the generator.
Water head may be available in hilly region naturally in the form of water reservoir (lakes etc.) at
the hill tops. The potential energy of water can be used to drive the turbo generator set installed
at the base of the hills through piping called pen stock. Water head may also be created
artificially by constructing dams on a suitable river. In contrast to a thermal plant, hydel power
plants are eco-friendly, neat and clean as no fuel is to be burnt to produce electricity. While
running cost of such plants are low, the initial installation cost is rather high compared to a
thermal plants due to massive civil construction necessary. Also sites to be selected for such
plants depend upon natural availability of water reservoirs at hill tops or availability of suitable
rivers for constructing dams. Water turbines generally operate at low rpm, so number of poles of
the alternator are high. For example a 20-pole alternator the rpm of the turbine is only 300 rpm.

Nuclear plants


As coal reserve is not unlimited, there is natural threat to thermal power plants based on coal. It
is estimated that within next 30 to 40 years, coal reserve will exhaust if it is consumed at the
present rate. Nuclear power plants are thought to be the solution for bulk power generation. At
present the installed capacity of unclear power plant is about 4300 MW and expected to expand
further in our country. The present day atomic power plants work on the principle of nuclear
fission of 235U. In the natural uranium, 235U constitutes only 0.72% and remaining parts is
constituted by 99.27% of 238U and only about 0.05% of 234U. The concentration of 235U may be
increased to 90% by gas diffusion process to obtain enriched 235U. When 235U is bombarded by
neutrons a lot of heat energy along with additional neutrons are produced. These new neutrons
further bombard 235U producing more heat and more neutrons. Thus a chain reaction sets up.
However this reaction is allowed to take place in a controlled manner inside a closed chamber
called nuclear reactor. To ensure sustainable chain reaction, moderator and control rods are used.
Moderators such as heavy water (deuterium) or very pure carbon 12C are used to reduce the
speed of neutrons. To control the number neutrons, control rods made of cadmium or boron steel
are inserted inside the reactor. The control rods can absorb neutrons. If we want to decrease the
number neutrons, the control rods are lowered down further and vice versa. The heat generated
inside the reactor is taken out of the chamber with the help of a coolant such as liquid sodium or
some gaseous fluids. The coolant gives up the heat to water in heat exchanger to convert it to
steam. The steam then drives the turbo set and the exhaust steam from the
turbine is cooled and fed back to the heat exchanger with the help of water feed pump.
Calculation shows that to produce 1000 MW of electrical power in coal based thermal plant,
about 6 × 106 Kg of coal is to be burnt daily while for the same amount of power, only about 2.5
Kg of 235U is to be used per day in a nuclear power stations.

Non conventional sources of energy

The bulk generation of power by thermal, hydel and nuclear plants are called conventional
methods for producing electricity. Search for newer avenues for harnessing eco friendly
electrical power has already begun to meet the future challenges of meeting growing power
demand. Compared to conventional methods, the capacity in terms of MW of each nonconventional
plant is rather low, but most of them are eco friendly and self sustainable. Wind
power, solar power, MHD generation, fuel cell and power from tidal waves are some of the
promising alternative sources of energy for the future.

Transmission of power

The huge amount of power generated in a power station (hundreds of MW) is to be transported
over a long distance (hundreds of kilometers) to load centers to cater power to consumers with
the help of transmission line and transmission towers.

Substations


Substations are the places where the level of voltage undergoes change with the help of
transformers. Apart from transformers a substation will house switches (called circuit breakers),
meters, relays for protection and other control equipment. Broadly speaking, a big substation will
receive power through incoming lines at some voltage (say 400 kV) changes level of voltage
(say to 132 kV) using a transformer and then directs it out wards through outgoing lines.
Pictorially such a typical power system . At the
lowest voltage level of 400 V, generally 3-phase, 4-wire system is adopted for domestic
connections. The fourth wire is called the neutral wire (N) which is taken out from the common
point of the star connected secondary of the 6 kV/400 V distribution transformer.

Some important components/equipments in substation

As told earlier, the function of a substation is to receive power at some voltage through incoming
lines and transmit it at some other voltage through outgoing lines. So the most important
equipment in a substation is transformer(s). However, for flexibility of operation and protection
transformer and lines additional equipments are necessary.
Suppose the transformer goes out of order and maintenance work is to be carried out.
Naturally the transformer must be isolated from the incoming as well as from the outgoing lines
by using special type of heavy duty (high voltage, high current) switches called circuit breakers.
Thus a circuit breaker may be closed or opened manually (functionally somewhat similar to
switching on or off a fan or a light whenever desired with the help of a ordinary switch in your
house) in substation whenever desired. However unlike a ordinary switch, a circuit breaker must
also operate (i.e., become opened) automatically whenever a fault occurs or overloading takes
place in a feeder or line. To achieve this, we must have a current sensing device called CT
(current transformer) in each line. A CT simply steps down the large current to a proportional
small secondary current. Primary of the CT is connected in series with the line. A 1000 A/5 A
CT will step down the current by a factor of 200. So if primary current happens to be 800 A,
secondary current of the CT will be 4 A.
Distribution system

Till now we have learnt how power at somewhat high voltage (say 33 kV) is received in a
substation situated near load center (a big city). The loads of a big city are primarily residential
complexes, offices, schools, hotels, street lighting etc. These types of consumers are called LT
(low tension) consumers. Apart from this there may be medium and small scale industries
located in the outskirts of the city. LT consumers are to be supplied with single phase, 220 V, 40
Hz. We shall discuss here how this is achieved in the substation receiving power at 33 kV.
Power receive at a 33 kV substation is first stepped down to 6 kV and with the help of under
ground cables (called feeder lines), power flow is directed to different directions of the city. At
the last level, step down transformers are used to step down the voltage form 6 kV to 400 V.
These transformers are called distribution transformers with 400 V, star connected secondary.
You must have noticed such transformers mounted on poles in cities beside the roads. These are
called pole mounted substations. From the secondary of these transformers 4 terminals (R, Y, B
and N) come out. N is called the neutral and taken out from the common point of star connected
secondary. Voltage between any two phases (i.e., R-Y, Y-B and B-R) is 400 V and between any
phase and neutral is 230 V(= 400/squareroot 3). Residential buildings are supplied with single phase
230V, 50Hz. So individual are to be supplied with any one of the phases and neutral. Supply
authority tries to see that the loads remain evenly balanced among the phases as far as possible.
Which means roughly one third of the consumers will be supplied from R-N, next one third from
Y-N and the remaining one third from B-N. The distribution of power from the pole mounted
substation can be done either by (1) overhead lines (bare conductors) or by (2) underground
cables. Use of overhead lines although cheap, is often accident prone and also theft of power by
hooking from the lines take place. Although costly, in big cities and thickly populated areas
underground cables for distribution of power, are used.