Load Curve

A load curve is a plot of the load in a network against time.  Load is plotted along the Y axis and time on the X-axis.

The curve shows the variation of load with time.  There can be daily load curves, monthly load curves and yearly load curve.

The area under the load curve shows the total units generated.  The tallest point in the curve shows the maximum demand.  The average load can be deduced by dividing the area under the curve by the total number of hours.

The load factor can be calculated by dividing the area under the curve by the area of the rectangle enclosing the curve.

Load Duration Curve

A Load Duration curve (LDC) is a curve formed by placing the loads in descending order of their magnitude.

The curve is essentially a bar graph.  Each bar represents a specific load.  The taller bars indicating the higher loads are placed to the left.  The area under the load curve indicates the total units generated. 

The Load Duration Curve can be used in economic dispatching, system planning and reliability.

Variable load and its impact

If the load on a power system is not constant but has steep variations, the cost of power generation will increase. 

More generators will have to be connected to handle the high peak loads of the system.  These generators will be idle when the load drops or they may have to operate at low loads.  This will reduce the efficiency of the prime mover and increase the fuel costs.

Increase in capital cost

More generators will have to be added to deliver the peak load.  This will result in higher capital cost.

Electric Loads - Classification and behaviour

Load - Definition

An Electrical Load is any device or component that taps energy from an electrical network.  Examples of domestic loads are domestic appliances, such as bulbs, fans and air conditioners.  Industrial loads include motors, heaters and lighting loads.

Electric loads have a range of power ratings from very low bulbs to motors which draw megawatts of power.

The vast majority of electric loads are motor loads.  The following is a classification of the types of electric loads and their quantity.

Motor devices                     - 70%
Heating and lighting loads  - 25%
Electronic devices               - 5%

Sensitivity to voltage and frequency variations

Loads are sensitive to both voltage and frequency

Motor loads are sensitive to both frequency and the voltage variations. A change in the frequency can cause the motor speed to increase or decrease.  Motor speed is directly proportional to the frequency.

When the voltage drops, a motor draws more current.  The power drawn varies as the square of the voltage.  Power is proportional to V²

Loads can be further classified based on the size, number of phases (single or three phase), and the duty cycle (constant use or intermittent use)