IIT Kharagpur notes for EEE

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Changeover from D.C to A.C:
In later half of eighties, in nineteenth century, it was proposed to have a power system with 3- phase, 50 Hz A.C generation, transmission and distribution networks. Once a.c system was adopted, transmission of large power (MW) at higher transmission voltage become a reality by using transformers. Level of voltage could be changed virtually to any other desired level with transformers – which was hitherto impossible with D.C system. Nicola Tesla suggested that constructionally simpler electrical motors (induction motors, without the complexity of commutator segments of D.C motors) operating from 3-phase a.c supply could be manufactured. In fact, his arguments in favor of A.C supply system own the debate on switching over from D.C
to A.C system.

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 as shown in figure 2.1. 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 as shown in figure2.1(a). 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 (figure 2.1(b)) and used when speed is high.

Thermal, hyddel & nuclear power stations:
In this section we briefly outline the basics of the three most widely found generating stations – thermal, hydel and nuclear plants in our country and elsewhere.

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 figure 2.2 the elementary features of a thermal power plant is shown.

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 as depicted in the figure. 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.

Stringent conditions (such as use of more chimney heights along with the compulsory use of electrostatic precipitator) are put by regulatory authorities to see that the effects of pollution is minimized. A large amount of ash is produced every day in a thermal plant and effective handling of the ash adds to the running cost of the plant. Nonetheless 57% of the generation in out country is from thermal plants. The speed of alternator used in thermal plants is 3000 rpm which means 2-pole alternators are used in such plants.

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 as shown in figure 2.4. 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.

The initial investment required to install a nuclear power station is quite high but running cost is low. Although, nuclear plants produce electricity without causing air pollution, it remains a dormant source of radiation hazards due to leakage in the reactor. Also the used fuel rods are to be carefully handled and disposed off as they still remain radioactive.

The reserve of 235U is also limited and can not last longer if its consumption continues at the present rate. Naturally search for alternative fissionable material continues. For example, plutonium (239Pu) and (233U) are fissionable. Although they are not directly available. Absorbing neutrons, 238U gets converted to fissionable plutonium 239Pu in the atomic reactor described above. The used fuel rods can be further processed to extract 239Pu from it indirectly increasing the availability of fissionable fuel. Effort is also on to convert thorium into fissionable 233U. Incidentally, India has very large reserve of thorium in the world

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 as shown in figure 2.5.

To give an idea, let us consider a generating station producing 120 MW power and we want to transmit it over a large distance. Let the voltage generated (line to line) at the alternator be 10 kV. Then to transmit 120 MW of power at 10 kV, current in the transmission line can be easily calculated by using power formula circuit (which you will learn in the lesson on A.C circuit analysis) for 3-phases

Instead of choosing 10 kV transmission voltage, if transmission voltage were chosen to be 400 kV, current value in the line would have been only 261.5 A. So sectional area of the transmission line (copper conductor) will now be much smaller compared to 10 kV transmission voltage. In other words the cost of conductor will be greatly reduced if power is transmitted at higher and higher transmission voltage. The use of higher voltage (hence lower current in the line) reduces voltage drop in the line resistance and reactance. Also transmission losses is reduced. Standard transmission voltages used are 132 kV or 220 kV or 400 kV or 765 kV depending upon how long the transmission lines are.

Therefore, after the generator we must have a step up transformer to change the generated voltage (say 10 kV) to desired transmission voltage (say 400 kV) before transmitting it over a long distance with the help of transmission lines supported at regular intervals by transmission towers. It should be noted that while magnitude of current decides the cost of copper, level of voltage decides the cost of insulators. The idea is, in a spree to reduce the cost of copper one can not indefinitely increase the level of transmission voltage as cost of insulators will offset the reduction copper cost. At the load centers voltage level should be brought down at suitable values for supplying different types of consumers. Consumers may be (1) big industries, such as steel plants, (2) medium and small industries and (3) offices and domestic consumers. Electricity is purchased by different consumers at different voltage level. For example big industries may purchase power at 132 kV, medium and big industries purchase power at 33 kV or 11 kV and domestic consumers at rather low voltage of 230V, single phase. Thus we see that 400 kV transmission voltage is to be brought down to different voltage levels before finally delivering power to different consumers. To do this we require obviously step down transformers.

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 is shown in figure 2.6 in a short of block diagram. 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 mustalso 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.

Suppose the rated current of the line is 1000 A, and due to any reason if current in the line exceeds this limit we want to operate the circuit breaker automatically for disconnection.

In figure 2.7 the basic scheme is presented to achieve this. The secondary current of the CT is fed to the relay coil of an overcurrent relay. Here we are not going into constructional and operational details of a over current relay but try to tell how it functions. Depending upon the strength of the current in the coil, an ultimately an electromagnetic torque acts on an aluminum disc restrained by a spring. Spring tension is so adjusted that for normal current, the disc does not move. However, if current exceeds the normal value, torque produced will overcome the spring tension to rotate the disc about a vertical spindle to which a long arm is attached. To the arm a copper strip is attached as shown figure 2.8. Thus the arm too will move whenever the disk moves.

The relay has a pair of normally opened (NO) contacts 1 & 2. Thus, there will exist open circuit between 1 & 2 with normal current in the power line. However, during fault condition in the line or overloading, the arm moves in the anticlockwise direction till it closes the terminals 1 & 2 with the help of the copper strip attached to the arm as explained pictorially in the figure 2.8. This short circuit between 1 & 2 completes a circuit comprising of a battery and the trip coil of the circuit breaker. The opening and closing of the main contacts of the circuit breaker depends on whether its trip coil is energized or not. It is interesting to note that trip circuit supply is to be made independent of the A.C supply derived from the power system we want to protect. For this reason, we expect batteries along with battery charger to be present in a substation.

Apart from above there will be other types of protective relays and various meters indicating current, voltage, power etc. To measure and indicate the high voltage (say 6 kV) of the line, the voltage is stepped down to a safe value (say 110V) by transformer called potential transformer (PT). Across the secondary of the PT, MI type indicating voltmeter is connected. For example a voltage rating of a PT could be 6000 V/110 V. Similarly, Across the secondary we can connect a low range ammeter to indicate the line current.

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Contact Details:
Indian Institute of Technology Kharagpur
Kharagpur,
West Bengal 721302
India

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