Types Of Loads

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:

(i) 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 12%).

(ii) Commercial load. Commercial load consists of lighting for shops, fans and electric appliances used in restaurants 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.

(iii) 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.

(iv) Municipal load. Municipal load consists of street lighting, power required for water supply 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.

(v) 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.

(vi) 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.

Load Duration Curves

Load  Duration  Curve:

When the load elements of a load curve are arranged in the order of descending magnitudes, the curve thus obtained is called a  load duration curve.

Load Duration Curves

The load duration curve is obtained from the same data as the load curve but the ordinates are arranged in the order of descending magnitudes. In other words, the maximum load is represented to the left and decreasing loads are represented to the right in the descending order. Hence the area under the load duration curve and the area under the load curve are equal.Fig.( i) shows the daily load curve.The daily load duration curve can be readily obtained from it.  It is clear from daily load curve [See Fig. (i)], that load elements in order of descending magnitude are: 20 MW for 8 hours; 15 MW for 4 hours and 5 MW for 12 hours. Plotting these loads in order of descending magnitude, we get the daily load duration curve as shown in Fig.(ii ).

The following points may be noted about load duration curve :

(i) The load duration curve gives the data in a more presentable form.In other words, it readily shows the number of hours during which the given load has prevailed.

(ii) The area under the load duration curve is equal to that of the corresponding load curve.

Obviously, area under daily load duration curve (in kWh) will give the units generated on that day.

(iii) The load duration curve can be extended to include any period of time.By laying out the abscissa from 0 hour to 8760 hours, the variation and distribution of demand for an entire year can be summarized in one curve.The curve thus obtained is called the annual load duration curve.

Load Curves

Load  Curves:

The curve showing the variation of load on the power station with respect to (w.r.t)  time is known as a Load Curve .

The load on a power station is never constant; it varies from time to time.These load variations during the whole day (i.e.. 24 hours) are recorded half-hourly or hourly and are plotted against time on the graph. The curve thus obtained is known as daily load curve as it shows the variations of load w.r.t.  time during the day. Figure shows a typical daily load curve of a power station.It is clear that load on the power station is varying, being maximum at 6 P.M. in this case. It may be seen that load curve indicates at a glance the general character of the load that is being imposed on the plant. Such a clear representation cannot be obtained from tabulated figures.

Daily Load Curve

The monthly load curve  can be obtained from the daily load curves of that month. For this purpose, average values of power over a month at different times of the day are calculated and then plotted on the graph.The monthly load curve is generally used to fix the rates of energy.The yearly load curve is obtained by considering the monthly load curves of that particular year.

The yearly load curve is generally used to determine the annual load factor.

Importance Of Load Curves:

 The daily load curves have attained a great importance in generation as they supply the following information readily :

(i) The daily load curve shows the variations of load on the power station during different hours of the day.

(ii) The area under the daily load curve gives the number of units generated in the day.

                             Units generated/day = Area (in kWh) under daily load curve.

(iii) The highest point on the daily load curve represents the maximum demand on the station on that day.

(iv) The area under the daily load curve divided by the total number of hours gives the average load on the station in the day.

                           Average load = Area (in kWh) under daily load curve/24 hours

(v) The ratio of the area under the load curve to the total area of rectangle in which it is contained gives the load factor.

     Load factor = Average load/Max. demand= Average load ×24/Max. demand ×24

                        =Area (in kWh) under daily load curve/Total area of rectangle in which the load curve is contained
(vi) The load curve helps in selecting the size and number of generating units.
(vii) The load curve helps in preparing the operation schedule of the station.