Sunday, August 14, 2011

Interconnection of Power Stations

       The connection of various generating stations is parallel is called interconnection of power stations or also called interconnected grid system. There are several advantages of this grid system and the cost associated with the interconnection through extra comparable to the benefits obtained from it.
       Before studying the advantages of this grid system let us consider the concept of base load and peak load.
        We have seen that the load on the power station is never constant but keeps on changing with time. This load can be subdivided into two parts viz i) Base load  ii) Peak load
       The unvarying load or fixed load which occurs almost whole day on the plant is called base load. The various peak demands of the load over and above base load of the power plant is called peak load.
       Consider the load curve with base load and peak load shown in the Fig.1.
Fig. 1

       Now if such load curve is to be met by a single unit then is installed capacity should be equal to peak load demand or even more. As peak load occurs for short duration such solution is not economical that we have seen earlier. The other way is to divide the load into base load and peak load. Thus by interconnection of various power stations of different types, some station will supply base load while some other stations will supply peak load. Thus the co-ordination of operation of different power stations is essential which is shown in the Fig.2.
Fig. 2

       The power plant which are working as base load should be capable of working continuously for long periods. It should have low operating cost. Its repair should be economical and speedy.
       The peak load power plants should be capable of quick start, fast synchronization, quick taking of load and fast response to load variations.
       The hydro power plant serves as base load or peak load efficiency. They are normally employed as base load plants as their capital cost is high. When water is not abundantly available then the hydro power plant works as peak load.
        The cost of generation per unit steam power plant is minimum. Hence it can be employed as base load. Nuclear power plants are also employed as base load. Diesel, gas and pumped storage plants are used as peak load plants.
       The advantages of interconnected system are as given below
1) With interconnected grid system the peak load can be exchanged between the generation stations. From load curve if there is peak load demand which is more than related capacity of the plant then the excess load can be shared by other interconnected stations.
2) It is possible to use the older and inefficient plants with grid system for short duration to supply peak demands. These units may not operate independently but with grid system they can sustain peak loads. Thus older plants can be effectively used with this system.
3) With interconnected grid system the economical operation of the plant is possible. The total load is arranged in such a way that more efficient plants can be used as base load stations which can work continuously throughout the year at high load factor. The less efficient plants can be made to operate as peak load plants. Also larger generator units can be employed to reduce capital cost per kW.
4) Various interconnected plants have their load curves different due to which maximum demand on the system is reduced as compared to sum of individual maximum demands on various stations. Thus the effective capacity of the system increased as their is improvement in diversity factor.
5) It can be seen that the load curves of the two different stations are not identical. In worst conditions the peak loads may occur at a time different by few minutes. Thus the maximum demands on individual stations are not occurring simultaneously, it is possible to work with lesser installed capacity with interconnected grid system.
6) In abnormal conditions every station should have reverse standby unit to be put in operation. With grid system the reverse capacity is reduced which increases efficiency of the system.
7) The reliability and continuity of the supply is improved with interconnected grid system. With fault condition occurring in any one station, the supply can be maintained with the help of other stations.

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