Curves Uses in System Operations of Electrical Power Plant

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Curves Uses in System Operations of Electrical Power Plant

Load Curve

  • The electrical load on a generating station varies from time to time; depending upon the requirement of different types of consumers. The load curve is a graph, which represents on the generating station recorded at the interval of half-hour or hour against the time chronological order.
Typically Daily Load Curve
Figure A

Figure A represents the typical dally load curve. The load curves supply much used information, which are as follows :

  1. The area under the dally load curve represents the total number of units generated by the plant.
  2. Area under the daily load curve, divided by the total area of the rectangle in which is contained, gives the load factor for that day of the generating plant.
  3. The maximum (peak) value on the curve represents the simultaneous maximum demand on the station on that day.
  4. The area under the curve divided by number of hours of the day, gives the average demand on the generating station on that day.
  5. It also represents the minimum load on the station.
  6. From such curves, annual maximum and minimum values of load are determined. Also the average annual load factor can be determined.
  • The daily load for a station is not same for all days, it differs from day to day and season to season.

Load Duration Curve

  • The load curve drawn in Figure A represents the load in kW / MW at any hour of the day. The same curve can also be changed into load duration curve by arranging the ordinates as shown in Figure B.
  • The ordinate representing the maximum load is represented to the left and the decreasing loads are represented to the right in the descending order.
    • Load Duration Curve
    Figure B
  • The total load supplied during a day is divided into number of sections, may be equal to number of generators used in the plant.
  • Suppose that 5 generators are used in the plant. The generator which supplies load OA works almost for 24 hours and supplying its full load and has a load factor of nearly 90 to 95% . Hence, it is a generator supplying the base load on the plant.
  • When the load on the station increases and becomes OB, second alternator (generator) will work in addition to first and it works on nearly 50-55% of load factor.
  • When the load increases to that represented by OC, third generator works in addition to the previous two, and its load factor may be 30 to 35%.
  • Similarly, fourth and fifth generators will start depending upon load requirement and may have load factor of about 10 to 15% and 5 to 10% respectively.
  • The area under the load duration curve represents the total number of units consumed in that time. Also the load factor of the station can be determined from it. From this curve it is possible to distribute the load between different units installed in a station or decide the size of different units to be installed in a station.

Integrated Duration Curve

    Integrated Duration Curve
    Figure C
  • The curve representing total number of units generated for the given demand in kW is called as Integrated duration curve. The ordinate represents the the demand in kW and the abscissa represents  units generated in kWH. Such a curve can be drawn from the load duration curve.
  • Consider Figure C, let the load demand be represented by a point A and it corresponds to line AG on the load duration curve.
  • The number of units generated (P1) corresponding to this load demand are represented by the area OAGF, it corresponds to point P1, on the integrated duration curve. Similarly, point P2 on this curve corresponds to load demand B and kWH generated (P2) area given by the area OBHF.

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