### Selection of Size and Number of Generating Units

For deciding the size and number of generator units, firstly the load curve and other relevant parameter and factors from the load curve are to be determined. In order to calculate the size of the units, the station auxiliary load and the line losses should be considered. It can be approximately taken as 20 % of the consumer load.
The minimum number of units will be one. But there drawbacks in selecting a single unit to meet maximum demand are as given below
1. The rating of the unit should be such as to meet the maximum demand. But as the load on station is variable and load factor is less than 100 % there considerable time during which the load will be much less than maximum demand. During this period the unit may run at half load or even practically at no load. Hence it is not running all time to give maximum efficiency. It is also not economical to run the set at light load as fuel consumption will be more.
2. With only one unit, reliability of operation is reduced even though power can be obtained at cheaper rate. If the same unit is under repair or maintenance then continuity of supply is lost unless there is second unit present which may increase capital cost. Thus in the environment of variable load where reliability of supply is important it is neither practical nor economical to use a single unit.
The number of units are to be selected in such a way as to fit in the load curve as closely as possible. Then each unit can be made to operate in such a way that it runs almost at full load or at a load which gives maximum efficiency. The reverse capacity required in this case will only be a single largest unit which would be much smaller than the maximum reverse capacity that would be required with a single unit case. Thus plant capacity and plant use factor is improved. But there are following shortcomings of this system.
1. With increase in number of units, the floor area required is more and so corresponding cost is also more.
2. The cost for maintenance also increased.
3. With increase of number of units, there will be frequent starting, stopping and parallel operation of units which needs increased persons for handling the equipments.
4. Capital cost for large number of units is more than that of same capacity with smaller number of units of large size.
Thus selection of number of units is a critical task as a single unit as well as large number of smaller units would be unreliable and uneconomical. Thus a compromise is made in selection of these units. The best compromise between plant capacity and plant capacity factor also gives choice for selecting the units.
The following points should be considered for selecting of units for various stations
i) The load factor of the units should be high.
ii) The minimum number of units selected should be two.
iii) The plant must have some reverse capacity under abnormal conditions.
iv) With two units selected, both must be able to supply maximum demand or load.
v) The future demand and expansion should also be considered as the load on the station always increases.
vi) For large number of small units, the space and capital cost required is more.
As far as possible, the units of equal capacities are selected which will have following advantages.
i) The parts can be interchanged.
ii) The maintenance will be easier.
iii) The working time of each plant regulated.
iv) The spare parts required to be stored are less.
In summary, the selected units should work at high efficiency as the capital cost and running cost will then be minimum.

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