• Types of Loads

    A device which taps electrical energy from the electric power system is called a load on the system. The load may be resistive (e.g., electric lamp), inductive (e.g., induction motor), capacitive or some combination of them. The various types of loads on the power system are :

    1. Domestic load: Domestic load consists of lights, fans, refrigerators, heaters, television, small motors for pumping water etc. Most of the residential load occurs only for some hours during the day (i.e., 24 hours) e.g., lighting load occurs during night time and domestic appliance load occurs for only a few hours. For this reason, the load factor is low (10% to 15%).


    2. Commercial load: Commercial load consists of lighting for shops, fans and electric appliances used in restaurants, institutes, hotels, etc. This class of load occurs for more hours during the day as compared to the domestic load. The commercial load has seasonal variations due to the extensive use of air-conditioners and space heaters. The load factor for such loads varies between 25-30%.


    3. Industrial load: Industrial load consists of load demand by industries. The magnitude of industrial load depends upon the type of industry. Thus small scale industry requires load upto 25 kW, medium scale industry between 25kW and 100 kW and large-scale industry requires load above 500 kW. Industrial loads are generally not weather dependent. The load factor for such loads varies between 60-65%.


    4. Municipal load: Municipal load consists of street lighting, power required for water sup-ply and drainage purposes. Street lighting load is practically constant throughout the hours of the night. For water supply, water is pumped to overhead tanks by pumps driven by electric motors. Pumping is carried out during the off-peak period, usually occurring during the night. This helps to improve the load factor of the power system.


    5. Irrigation load: This type of load is the electric power needed for pumps driven by motors to supply water to fields. Generally this type of load is supplied for 12 hours during night. The load factor for such loads varies between 15-25%.


    6.Traction load: This type of load includes tram cars, trolley buses, railways etc. This class of load has wide variation. During the morning hour, it reaches peak value because people have to go to their work place. After morning hours, the load starts decreasing and again rises during evening since the people start coming to their homes.



    Selection of Units

    While making the selection of number and sizes of the generating units, the following points should be kept in view:

    1. The number and sizes of the units should be so selected that they approximately fit the annual load curve of the station.

    2. The units should be preferably of different capacities to meet the load requirements.  Although use of identical units (i.e., having same capacity) ensures saving in cost, they often do not meet the load requirement.

    3. The capacity of the plant should be made 15% to 20% more than the maximum demand to meet the future load requirements.

    4. There should be a spare generating unit so that repairs and overhauling of the working units can be carried out.

    5. The tendency to select a large number of units of smaller capacity in order to fit the load curve very accurately should be avoided. It is because the investment cost per kW of capacity increases as the size of the units decreases.