29-06-2013, 03:20 PM
POWER SYSTEM
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INTRODUCTION
The energy is neither be created nor be destroyed but it can be converted from one form to another. The generation of an electrical energy is nothing but the conversion of various other forms of energy into electrical energy. The various energy sources which are used to generate an electrical energy on the large scale are steam obtained by burning of coal, oil, natural gas, water stored in dams, diesel oil, nuclear power and other nonconventional energy sources. The electrical power is generated in bulk at the generating stations are also called power stations. Depending upon the source of energy used, these stations are called thermal power station, hydroelectric power station, diesel power station, nuclear power station etc.
This generated electrical energy is demanded by the consumers. Hence the generated electrical power is to be supplied to the consumers. Generally the power stations are located too far away from the town and cities where electrical energy is demanded. Hence there exists a large network of conductors between the power stations and the consumers. This network is broadly classified into two parts.
STRUCTURE OF ELECTRICAL POWER SYSTEM
The flow of electrical power from the generating station to the consumer is called an electrical power system or electrical supply system. It consists of the following important components:
1. Generating station
2. Transmission network
3. Distribution network
All these important networks are connected with the help of conductors and various step up and step down transformers.
A scheme shows a generating station which is located too far away from cities and towns. It is generating an electrical power at 11 kV. It is required to increase this level for the transmission purpose. Hence a step up transformer is used which steps up the voltage level to 220 kV. This level may be 132 kV, 220 kV or more as per the requirement.
COMPONENTS OF DISTRIBUTION
The distribution scheme consists of following important components:
1. Substation: Transmission lines bring the power up to the substation at a voltage level of 22 kV or 33 kV. At the substation the level is reduced to 3.3 kV or 6.6 kV. Then using feeders, the power is given to local distribution centres.
2. Local distribution station: It consists of distribution transformer which steps down the voltage level from 3.3 kV, 6.6 kV to 400 V or 230 V. Then it is distributed further using distributors. This is also called distribution substation.
3. Feeders: These are the conductors which are of large current carrying capacitor. The feeders connect the substation to the area where power is to be finally distributed to the consumers. No tappings are taken from the feeders. The feeder current always remains constant. The voltage drop along the feeder is compensated by compounding the generators.
4. Distributors: These are the conductors used to transfer power from distribution centre to the consumers. From the distributors, the tappings are taken for the supply to the consumers. The voltage drop along the distributors is the main criterion to design the distributors.
DISTRIBUTED GENERATION
When small generators of capacities around 2-50 MW output are installed at typical points in the area such that each of these generators supply power to a small number of consumers nearby then it is called distributed generation. The generators may be operated through renewable energy sources such as solar, wind or gas turbines, small hydro or micro turbines as these are most economical choices. Dispersed generation is use of still smaller generating units of less than 500 kW and used for individual houses or small business. The distributed or dispersed generators may be standalone or grid connected depending on the requirement.
Distributed generation proves to be economical as it requires no transmission network and reduced need of distribution equipments. Another advantage is this generation is portable or compact as compared to big power stations located far away from consumers. Also they are modular and relocatable. The fuel cells and micro gas turbines are two new types of techniques evolved in distributed generation.
DIVERSITY FACTOR
To improve the working of the generating station the loads must be diversed or staggered. The maximum demand of various types of consumers which is supplied by a power station does not occur at the same time. Hence the maximum demand on the generating station is always less than the sum of individual maximum demands.
The diversity factor is thus defined as the ratio of sum of individual maximum demands to the maximum demand on power station.
Diversity Factor = Sum of individual maximum demands/ Maximum demand on power station
The diversity factor is always greater than 1 if defined in above way. Greater the diversity factor, lesser is the cost of generation since more diversity factor means less maximum demand which corresponds to lesser plant capacity which reduces cost of plant and hence that of generation.
LOAD DURATION CURVE
The curve obtained by arranging the load elements of a load curve in order of decreasing magnitudes is called load duration curve. No extra data or information is required for plotting this curve as it can be plotted with the same data as that for the load curve.
The ordinates from the load curve arranged in order of decreasing magnitudes. The peak load is shown to the left while the decreasing loads are arranged to the right in the descending order. It is quite obvious that the area under load curve and load duration curve is same.
The area under load duration curve indicates the total number of units that are consumed in that time. The load factor of the station can be determined from this. It also suggests the division of load between the various generating units of the plant to operate them near their maximum efficiency points.
The load elements of 3 kW, 6 kW, 9 kW, 12 kW, 15 kW and 18 kW are arranged in decreasing order for the respective time duration on the load duration curve. The total load supplied can be divided into various sections. Let us say that there are 6 alternators corresponding to these sections.
The alternator which supplies load of 3 kW will be in operation almost continuously with high load factor. When the load is increased to 6 kW, then the second alternator will be put in service which will have comparatively low load factor. Likewise the alternators will be put in operation with decreasing load factors.