An Introduction to Power System Vol-I

A typical electric power supply system comprises : (a) Generating units that produce electricity ; (b) High voltage transmission lines that transport electricity over long distances ; (c) Low voltage distribution lines that deliver electricity to consumers ; (d) Sub-stations which are the part of the electricity transmission and distribution system where the voltages are transformed to lower levels for distributing power to end-users and (e) Energy control centres to coordinate the operation of the components.

The electrical power is drawn by various consumers from the distribution network as per their prevailing demand. The consumers demand certain MW and this demand acts as the load on supply system. Hence, the terms load and demand are used in similar context. Most of the complexities of modern power plant operation arise from the inherent variability of the load demanded by the users. Unfortunately, electrical power cannot be stored and therefore the power station must produce power as and when demanded to meet the requirements of the consumers. On one hand, the power engineer would like that the alternators in the power station should run at their rated capacity for maximum efficiency and on the other hand, the demands of the consumers have wide variations. This makes the design of a power station highly complex. In this chapter, we will study the variable loads on power system with reshaping of load curves by various management techniques.

An electric transmission line has four parameters namely resistance, inductance, capacitance and conductance. In this chapter, we will discuss first two parameters. These four parameters are uniformally distributed along the whole line. Each line element has its own value and it is not possible to concentrate or lump them at discrete points on the line. Thus, these parameters are known as distributed parameters. Their values are given per unit length of line and are denoted as R, L, C and G. The line resistance and inductance form the series impedance of the line. The capacitance and conductance form the shunt admittance of the line.

Capacitance of a transmission line is the result of the potential difference between the conductors, it causes them to be charged in the same manner as the plates of a capacitor when there is a potential difference between them.

The performance of a power system under normal balanced steady state conditions is of primary importance in power system engineering. We are aware that in 3-phase circuit problems it is sufficient to compute results in one phase and subsequently predict results in the other two phases by exploiting the 3-phase symmetry. Although the lines are not spaced equilaterally and not transposed, the resulting asymmetry is slight and the phases are considered to be balanced. As such the transmission line calculations are carried out on per phase basis.

Neutral grounding doesnot have any influence on the operation of a 3-phase system under balanced steady state conditions. However, the voltages and currents during ground fault conditions are greatly affected by the method used to connect neutral to earth. Protective relaying and stability analysis are also influenced by neutral grounding. In most modern HV systems, the system neutral is solidly (effectively) grounded i.e., the neutral is connected directly to the ground without any intentional impedance between the neutral and the ground. Generally, the generators are grounded through a resistance to limit the stator fault currents and also for stability considerations.

Corona phenomenon is the ionization of air surrounding the power conductor. Air is not a perfect insulator and contains a minute number of electrons and ions as a result of various effects such as ultra-violet radiations from sun, cosmic rays etc. When an electric gradient is set up in the air between two large parallel conducting planes, the electrons and ions are set in motion by the electric field and they maintain a small current by convection between the conducting planes. When the electric field intensity is less than 30 KV/cm, this current is very small and hence negligible.

The overhead line conductors should be supported on the poles or towers in such a way that currents from conductors do not flow to earth through supports i.e., line conductors must be properly insulated from supports. This is achieved by securing line conductors to supports with the help of insulators. The insulators are used to separate line conductors from each other and from the supporting structures electrically.

A transmission line is a set of conductors that run from one place to another with support on transmission towers. These lines have four distributed parameters — series resistance and inductance, shunt capacitance and conductance. The electrical power is transmitted over the overhead lines at approximately the speed of light.

The design of a transmission line has to be satisfactory from electrical as well as mechanical considerations. As far as electrical aspects are concerned, the line should have sufficient current carrying capacity and the line losses should be small. As far as mechanical aspects are concerned, the line conductors, supports and cross-arms should have sufficient mechanical strength to cope with the worst probable weather conditions. The line conductors, supports and cross-arms must be strong enough to give satisfactory service over a long period of time without the necessity of too much maintenance.

In big cities and densely populated areas, overhead lines become impractical owing to safety regulations. In such places insulated conductors are usually laid underground and are called cables. All electric cables consist of 3-essential elements. (a) the conductor for transmitting electrical power, (b) the insulation needed to insulate the conductor from direct contact with earth or other objects and (c) external protection against mechanical damage, chemical attack, fire etc.

The initial application of electricity started with the use of direct current. The first central electric station was installed by Edison in New York in 1882 which operated at 110 V DC The invention of transformer and induction motor initiated the use of AC. The polyphase induction motors which serve the majority of industrial and residential purposes are simpler and rugged in construction and cheaper as compared to DC motors of the same ratings. The advantages of 3-phase AC almost eliminated the use of DC systems except for some specific applications in electrolytic processes and adjustable speed motor drives. The main technical problems of long distance power transmission using AC are :- voltage regulation associated with reactive power balance, steady state, transient state and dynamic stability of the system under different load conditions and also under outage conditions. In view of these problems with AC, the DC transmission has staged a come back in the form of high voltage DC transmission. The first commercially used HVDC link (20 MW, 100 KV), in the world was built in 1954 between the mainland of Sweden and the island of Gotland. In 1961, an under water DC link was set up between England and France. Since then the technique of power transmission by HVDC has been continuously developed. In 1970, thyristor valves replaced the valves based on the mercury-arc technique. To date the biggest HVDC transmission is ITAIPU in Brazil (two bipoles, 6300 MW and  300 KV). In India, the first HVDC line is Rihand-Delhi ( 500 KV, 800 MW).

Electricity is generated at 11 KV by electrical generators which utilize the energy from thermal, hydro nuclear and renewable energy resources. To transmit electricity over long distances, the supply voltage is stepped up to 132/220/400/800 KV as required. Electricity is carried through a transmission network of high voltage lines usually, these lines run into hundreds of kilometers and deliver the power into a common power pool called the grid. The grid is connected to load centres through a sub-transmission network of usually 33 KV (or sometimes 66 KV) lines. These lines terminate into a 33 KV (or 66 KV) substation where the voltage is stepped-down to 11 KV for power distribution to load points through a distribution network of lines at 11 KV and lower.

In a modern power system, electrical energy from the generating station is delivered to the ultimate consumers through a network of transmission and distribution. Practically all equipment used in power systems is rated for a certain voltage with a permissible band of voltage variations. For satisfactory operation of motors, lamps and other loads, it is desirable that consumers are supplied with substantially constant voltage. Too wide variations of voltage will affect the performance of equipments and the life of most of the equipments is also sacrificed. The picture on a television set starts rolling if the voltage is below a certain level. The fluorescent tube refuses to glow if the voltage is below a certain level. Thus the necessity of controlling the voltage on the system is very much strong. To safeguard the interest of the consumers, the government has enacted a law in this regard. The statutory limit of voltage variation is ± 6% of declared voltage at consumer’s terminals.

Distribution System is that part of power system which distributes power to the consumers for utilization. AC 3-phase 4-wire is the standard distribution all over the world.