Load curves in electrical power systems

Load curve

The curve showing the variation of load on the power system with respect to time is known as a load curve.

The load on a power generating station varies widely from time to time and these load variations are recorded under
regular intervals and are plotted with respect to time forms a load curve. The load curve may be daily load curve, monthly load curve or a yearly load curve.

The daily load curve

The variation of load during the whole day for 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.

Figure showing typical daily load curve

Monthly and yearly load curves

The monthly load curve can be obtained from the daily load curves of the given month. To obtain the monthly load curve the average value of loads over a month at different times of the day are calculated and then plotted. The monthly load curve can be used to fix the energy rates. The yearly load curve is obtained by considering the averages of monthly load curves of that particular year. The yearly load curve is generally used to determine the annual load factor.

Importance of Load Curves

1. The daily load curve shows the variations of load during different hours of a day.
2. The area under the daily load curve gives the number of units generated a day.
3. The highest point on the daily load curve represents the maximum demand on the station on that day.
4. The average load on the power station can be found by dividing the area under the daily load curve by the total number of hours.
5. They help in selecting the size and number of generating units, and in preparing the operation schedule of the station.