Choice of Size and Number of Generation Units, Difficulties Involved in Economics of Power Generation

The line losses and station auxiliary load may be taken as approximately 20% of the consumer load. The capacity of the generator units must be so selected that it meets the peak load demand.

The minimum number of generating units that can be chosen could be one, having capacity equal to maximum demand on the system. When we select single unit to meet maximum demand, it has the following drawbacks :

(i) As the load on the station varies from time to time its load factor is less than 100%, hence during a considerable period of the day, the load on the station is less than M.D. During this period the generating set is either working at half the load or even practically no load. Under such condition it is not working to give its maximum efficiency, which is in most of machines at about its full-load condition. It is also uneconomical to run the set under low load condition as fuel consumption would be more.

(ii) For providing reliability of supply to consumers another set of equal capacity must be installed for use, when the first set is out of order and is under repairs. This will increase the capital cost; hence it is not practical and also economical.

Hence, another method is to decide the size and number of generator units in such a way that they are capable of supplying load as per the load curve as closely as possible. Each unit can be made to operate at its full load, or at a load at which it gives maximum efficiency. The reserve maximum reserve capacity required in case of a single unit plant. As the reserve capacity required would be less, it results in better plant capacity factor.

When the number of generating units are more, we have to face the following difficulties :

(a) When number of units are more it requires more space for their erection, which increases the cost of building.

(b) As number of units is more, it will increase the cost of maintenance.

(c) Due to more number of sets, it involves more starting, stopping, parallel operation of equipment, which need more personnel for these operations.

(d) Capital cost of more number of units is greater than a single unit of same capacity.

Hence, at the time of selecting units (sets) for a generating plant following points should be considered :

1. Number of units be so selected that they work at high load factor.
2. Number of units should not be many but they should not be less than two in any case.
3. There should be a reserve unit of capacity equal to largest capacity of the units in the plant.
4. If units selected are only two , then each must be capable of delivering maximum demand.
5. The future demand and expansion should be considered.
6. Mostly sets of equal capacity are selected, which have the following advantages :

(a) Less spares are to be stored.

(b) Parts can be interchanged.

(c) Maintenance will be easier.

(d) Working time of each unit can be well regulated.