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    Power System

    Load Duration Curve in Power Station

    • Categories Power System, Substation
    Load Curve and Load Duration Curve

    Load duration curves are used in power stations to illustrate the relationship between generating capacity requirements and capacity utilization. A load duration curve (LDC) is similar to a load curve but the demand data is ordered in descending order of magnitude, rather than chronologically.

    Table of Contents

    • Definition
    • Load Curve vs Load Duration Curve
      • Load Duration Curve from Load Curve
    • Important Points about LDC
    • How to Plot a Load Duration Curve
    • Video
    • Frequently Asked Questions (FAQ)
      • Related Articles

    Definition

    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 Curve vs Load Duration Curve

    A LDC is derived using the same data as a load curve, but the magnitude ordinates are sorted in descending order. As a result, the maximum load of a system is represented on the left, whereas decreasing loads of a system are represented on the right in descending order. In this sense, there is no difference between the area under the load duration curve and the area under the load curve. 

    Load Duration Curve from Load Curve

    The daily load curve of a power station is shown in the figure below. You can easily obtain the daily LDC from it. 

    Load Curve
    Load Curve

    According to the daily load curve, the load elements are: 20 MW for 8 hours; 15 MW for 4 hours; and 5 MW for 12 hours.

    The daily load duration curve can be obtained by plotting these loads in descending order of magnitude. It is as shown below.

    Load Duration Curve
    Load Duration Curve

    The load duration curve is the plot between load and time. The LDC is constructed using the daily load curve. The area under the LDC represents the total energy generated by the utility‘s generating system in the considered period.

    An LDC is usually divided into the three parts.

    1. Baseload: The base load is the load below which the demand never falls and is supplied 100% of the time.
    2. Peaking load: It occurs for about 15% of the time
    3. Intermediate load: It represents the remaining load between the base load and the peaking load.

    Important Points about LDC

    The following points may be noted about load duration curves:

    • The LDC presents the data in a more understandable way. This indicator readily displays the number of hours during which a given load has prevailed.
    • The area under the LDC is equal to the area under the corresponding load curve. It is obvious that the area under a daily load duration curve (in kWh) will give the number of units generated that day.
    • It is possible to extend the LDC to cover any period of time. One curve can portray the variation and distribution of demand for an entire year by dividing the abscissa from 0 to 8760 hours. The curve thus obtained is called the annual load duration curve.  

    How to Plot a Load Duration Curve

    Now lets understand the calculation of LDC using an example.

    A power station has the following daily load cycle :

    Time in HoursLoad in MW
    6 – 820
    8 – 1240
    12 – 1660
    16 – 2020
    20 – 2450
    24 – 620
    Daily Load Cycle of Power Station

    Daily curve is drawn by taking the load along Y -axis and time along X-axis. For the
    given load cycle, the load curve can be drawn as below.

    Load curve example
    Load curve

    From the load curve, you can draw the load duration curve as the figure shown below.

    Load duration curve calculation
    LDC

    As we mentioned earlier, we can calculate the unit generated from the LDC.

    Units generated/day = Area (in kWh) under daily load curve
    = 103 [20 × 8 + 40 × 4 + 60 × 4 + 20 × 4 + 50 × 4]
    = 840 × 103 kWh

    Also,

    Units generated/day = Area (in kWh) under daily LDC
    = 103 [60 × 4 + 50 × 4 + 40 × 4 + 20 × 12]
    = 840 × 103 kWh

    Video

    Frequently Asked Questions (FAQ)

    1. Load duration curve is the plot between:
      1. load and time
      2. load and frequency
      3. load and temperature
      4. load and voltage

      Answer: Option 1 : load and time

      Detailed Explanation:
      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 the plot between load and time. The load duration curve is constructed using the daily load curve. The area under the load duration curve represents the total energy supplied by the utility’s generating system during the year.

    2. What does a load duration curve represent?
      a. The variation of load during different hours of the day.
      b. Average load.
      c. The number of hours for which a particular lasts during the day.
      d. None of the above.

      Answer: c. The number of hours for which a particular lasts during the day.

    3. What does the chronological load curve indicate?
      a. Variation in demand factor during 24 hours.
      b. Variation of demand from instant to instant during 24 hour.
      c. The total energy consumed upto different times of the day.
      d. The total number of hours for which a particular load lasts during the day.

      Answer: b. Variation of demand from instant to instant during 24 hour.

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    • Share:
    Tesla Edison
    Tesla Edison

    Tesla is an Electrical Engineer, Physicist and an Inventor in making. He is a good writer and author of many courses and articles published in this site.

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      2 Comments

    1. Testo India
      October 16, 2022
      Reply

      “The information shared in the above article is very useful
      It will be very helpful for all the people who are studying load duration curves in power stations.
      Keep sharing such a piece of quality information!”

    2. Mohamed
      January 4, 2023
      Reply

      Very Good

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