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.

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.

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.

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)

The Load Flow Analysis is done to determine the flow of real and reactive between different buses in a power system.  It also helps in determining the voltage and current at different locations.

To conduct a Load Flow Analysis, the components in a power system need to be modelled.  The modelling is done by developing equivalent circuits of the components, such as the generator, transmission lines and line capacitances.

The Generator equivalent circuit is shown below.

The Thevenin equivalent circuit is as shown below.

This consists of a voltage source and a resistance and an inductance in series with the load.

E = V + IZ


Z is the steady stage impedance
V is the voltage and
I is the current

The Norton equivalent circuit

The Norton equivalent circuit consists of a power source and an admittance in parallel.

INorton = V/Z

INorton = YV


The load is modelled as a resistance and inductance in a series circuit that is earth

Transmission lines

Transmission lines are modelled as

Short transmission lines (less than 80 km)

Short lines that are less than 80 kms long are modelled as a resistance and reactance in series with the load.  The line capacitances are neglected.

Medium (80 to 250 km)

Medium lines are modelled as a resistance and reactance in series.  The admittance is in parallel in two sections.

Long lines ( 250 km and above)

Long lines are also modelled as a resistance and reactance in series.  The admittance is in parallel in two sections.

Circuit breakers are used in a wide range of applications.  They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria.  Some of the classifications are below

Based on the interrupting medium
  1. Air circuit breaker
  2. Oil circuit breaker
  3. SF6 circuit breaker
Based on type of action
  1. Automatic circuit breakers
  2. Non-automatic circuit breakers
Based on the method of control
  1. Locally controlled circuit breakers
  2. Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)
Based on the type of mounting
  1. Panel mounted circuit breakers
  2. Remote from panel mounted circuit breakers
  3. Rear of panel mounted circuit breakers
Based on location
  1. Outdoor circuit breakers
  2. Indooor circuit breakers
Based on voltage
  1. Low voltage circuit breakers
  2. Medium voltage circuit breakers
  3. High voltage circuit breakers

The relay in a protection system should be sensitive enough to operate when a fault occurs.  A sensitive relay improves the reliability of the system.

When the parameter exceeds the set value, the relay should start operating.

The sensitivity of a relay is mentioned as a ratio of the minimum value of short circuit current to the minimum value of the quantity for the operation.

The sensitivity is indicated by a sensitivity factor Ks

Sensitivity of a Relay

Is is the minimum short circuit current in the zone and
Io is the minimum operating current for the relay.

The sensitivity of a relay is also related to the VA of the input to the relay.  Lesser the VA of the input, greater will be the sensitivity and vice versa. For instance, a relay which has 1 VA as its measuring input will be more sensitive than a relay, which has 5 VA as its measuring input.