Introduction
The function of a power station is to deliver power to a large number of consumers. However, the power demands of different consumers vary in accordance with their activities. The result of this variation in demand is that load on a power station is never constant; rather it varies from time to time.
Load curve
A load curve (or load graph) is a graphic record showing the power demands for every instant during a certain time interval. Such a record may cover 1 hour, in which case it would be an hourly load graph; 24 hours, in which case it would be a daily load graph; a month in which case it would be a monthly load graph; or a year (7860 hours), in which case it would be a yearly load graph. The following points are worth noting:
1. The daily load curve shows the variations of load on the power station during different hours of the day.
2. 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.
3. The highest point on the daily load curve represents the maximum demand on the station on that day.
4. The area under the daily load curve divided by the total number of hours gives the average load on the station in the day.
5. The ratio of the area under the load curve to the total area of rectangle in which it is contained gives the load factor.
6. The load curve helps in selecting the size and number of generating units.
7. The load curve helps in preparing the operation schedule of the station.
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.
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.
Figure: Load curve and Load duration curve
The following points may be noted about load duration curve:
a. The load duration curve readily shows the number of hours during which the given load has prevailed, i.e., it gives the data in a more presentable form.
b. 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.
c. 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.