In this article we will discuss about:- 1. Introduction to Power System 2. Description of Power Supply System 3. Single Line Representation.
Introduction to Power System:
Power is the basic need for the economic development of any country. Availability of electricity has been the most powerful vehicle of introducing economic development and social change throughout the world. The process of modernisation, increase in productivity in industry and agriculture and the improvement in the standard of living of the people basically depend upon the adequate supply of electrical energy. Appropriately the programmes relating to the generation, transmission and distribution of electrical energy have been the highest priority in the national planning process.
Generating stations, transmission lines and distribution systems are the main components of an electrical power system. Generating stations and distribution systems are connected through transmission lines, which also connect one power system to another. A distribution system connects all the loads in a particular area to the transmission line.
The transmission system of an area (or state) is called the grid. The different grids are interconnected through tie lines to form a regional grid and the different regional grids are further interconnected to form a national grid. Each grid operates independently. However, power can be transmitted from one grid to another, via the tie lines under conditions of sudden loss of generation or increase in load.
The distribution system in India is quite extensive. The voltages used in distribution systems are 11, 6.6 and 3.3 kV at ht level (or for primary distribution) and 415 volts at lt level (or for secondary distribution).
The country has been divided into five regions (Northern, Western, Eastern, Southern and Northern-Eastern), each with a regional electricity board, so as to promote integrated operation of the constituent power systems. Each State has a State Electricity Board, responsible for generation, transmission and distribution of electrical power in their respective states.
Central Government has also control over many generating plants, transmission lines and substations through central organisations like NTPC (National Thermal Power Corporation), NHPC (National Hydroelectric Power Corporation), PGCIL (Power Grid Corporation of India Ltd.) etc.
Regional load despatch centres have been established for coordinating the activities of state load despatch centres so as to ensure optimum utilization of generation and transmission facilities in the regions. Setting-up of a National Load Despatch centre is under active consideration.
Electrical energy is normally generated at the power stations far away from the urban areas where consumers are located. The problem, therefore, is to transport (or more usually called transmit) the large blocks of power over long distances economically. In this volume of the book we will deal with the type of equipment, techniques and problems involved in transmitting large blocks of power over long distances from the generating stations to the load centres.
Description of Power Supply System:
Electrical energy is generated at the power stations (hydroelectric, thermal 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. The generation, transmission and distribution system of electrical power is called the electrical power supply system.
The transmission system is to deliver bulk power from power stations to the load centres and large industrial consumers beyond the economical service range of the regular primary distribution lines whereas distribution system is to deliver power from power stations or substations to the various consumers.
Although electrical power can be transmitted and distributed by either ac or dc but in practice 3-phase 3-wire ac system is universally adopted for transmission of large blocks of power and 3-phase 4-wire ac system is usually adopted for distribution of electrical power.
The transmission system may be further divided into primary and secondary (or sub) transmission. Similarly the distribution system may be divided into primary and secondary distribution.
Generation voltages are 3.3, 6.6, 11 or 33 kV, most usual value adopted in practice is 11 kV. The primary transmission voltages are 110, 132, 220, 400 or 765 kV depending upon the distance, the amount of power to be transmitted and the system stability. Secondary transmission voltage is normally of the order of 33 or 66 kV. The voltages for primary distribution are 11, 6.6 or 3.3 kV depending upon the requirements of the bulk consumers and for secondary distribution usable voltage is 415/240V.
The distribution system may further be divided into:
(ii) Distributors, and
(iii) Service mains.
Feeders are the conductors which connect the stations (in some cases generating stations) to the areas to be fed by those stations. Generally from feeders no tapping is taken to the consumers, therefore, current loading of a feeder remains the same along its length. It is designed mainly from the point of view of its current carrying capacity.
Distributors are the conductors from which numerous tappings for the supply to the consumers are taken. The current loading of a distributor varies along its length. Distributors are designed from the point of view of the voltage drop in them.
(iii) Service Mains:
Service mains are the conductors, which connect the consumer’s terminals to the distributor. Elements of a distribution system are shown in Fig. 1.2.
A distributor is subject to the legal requirement that the voltage at the consumer’s terminals should be maintained within ± 6% of the declared (or rated) voltage. However, there is no such legal restriction on a transmission line and the voltage can vary as much as 10% or even 15% due to variations in loads.
Modern ac electrical power supply system usually consists of the following elements:
(i) Generating stations
(ii) Step-up transformer substations
(iii) Primary transmission lines
(iv) Primary transmission substations
(v) Secondary (or sub) transmission lines
(vi) Secondary transmission substations
(vii) Primary (high voltage) distribution lines (or networks)
(viii) Service transformer banks, and
(ix) Secondary (or low voltage) distribution lines (or networks).
Essentially elements (ii), (iii), (iv), (v) and (vi) fall in the transmission system while the elements (vii), (viii) and (ix) fall in the distribution system.
All systems of power transmission and distribution may or may not include all elements enumerated above; for example, some systems 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.
Single Line Representation of Power System:
A complete diagram of power system representing all the three phases becomes too complicated and cumbersome for a system of practical size, so much so that it may no longer convey the information it is intended to convey. It is much, more practical to represent a power system by means of simple symbols for each component resulting in what is called a single line diagram.
Single Line Diagram:
The single line diagram of a power system network shows the main connections and arrangements of the system components along with their data (such as output rating, voltage, resistance and reactance etc.). In case of transmission lines sometimes the conductor size and spacing’s are given.
It is not necessary to show all the components of the system on a single line diagram, e.g., circuit breakers need not be shown in a load flow study but are must for a protection study. In a single line diagram, the system components are usually drawn in the form of their symbols.
Generators and transformer connections-star, delta and neutral earthing are indicated by symbols drawn by the side of the representation of these elements. Circuit breakers are represented by rectangular blocks. Figure 1.4 represents the single line diagram of a typical power system. The ratings of generator, motor and transformers are given below the diagram.