Load Characteristics of Power System

Load Characteristics of Power System


A device which uses electrical energy is said to impose a load on the system.

Connected Load

The sum of continuous ratings of all loads connected to the system is known as a connected load.


It is the load that is drawn from the source of supply at the receiving terminals averaged over a suitable and specified interval of time.

Installed Capacity

The arithmetic sum of ratings of alternators connected to a bus is called as installed capacity.

Demand Interval

It is the period over which the load is averaged.

There are two types of demands:

1. Instantaneous demand
2. Sustained demand

Maximum demand

It is also called as peak demand. It is the greatest of all demands which have occurred during the specified interval of time.

  • It helps in determining the installed capacity of a generating station.
  • The Cost of plant and equipment increases with the increase in maximum demand.

Average load = (Energy consumed ÷ total number of hours)

Note: average load < maximum demand < installed capacity

Reserve Capacity = Installed capacity − maximum demand

Demand Factor

It is the ratio of actual maximum demand of the system to the total connected load of the system.

  • The demand factor depends upon the nature of the load.
  • Lighting loads have higher demand factors than power loads.
  • The demand factor is usually less than 1.

Average Demand

It is also called as an Average load. It is the ratio of energy consumed in a given period of time to the number of hours in that time period.

Load Factor

It is the ratio of the average load over a given period of time to the peak load (maximum demand) occurring during that period.

It can also be defined as the ratio of the actual energy consumed during a given period to the energy which would have been used if the maximum demand had been continuously maintained throughout that period.

  • The higher the Load factor, the lesser will be the cost of generation per unit (kWh) for the same maximum demand.

Diversity Factor

It is the ratio of the sum of the individual maximum demands of the various subdivisions of a system to the maximum demand of the whole system.

  • Diversity factor can be defined for loads, substations, feeders, and generating stations.
  • The diversity factor can be equal to or greater than 1.
  • DF greater than 1 represents good diversity.
  • DF equal to 1 represents poor diversity.
  • Large diversity factor has the effect of reducing the maximum demand. Hence lesser plant capacity is required. As a result, capital investment on the plant is reduced and the cost of generation is also reduced.
  • A high diversity factor may be obtained by using electrical energy at night or light load periods.

Load diversity

It is the difference between the sum of the peaks of two or more individual loads and the peak of the combined load.

Utilization Factor

It is the ratio of maximum demand of a system to the rated capacity of the system.

Plant Capacity Factor

It is also known as the plant factor. It is the ratio of the total actual energy produced or supplied over a specified period of time to the energy that would have been produced or supplied if the plant (or unit) had operated continuously at maximum rating.

The maximum plant rating in the total installed plant capacity including the reverse capacity.

Loss Factor

It is the ratio of the average power loss to the peak load power loss during the specified period of time.

Note: This relationship is applicable for the copper losses of the system but not for the iron losses.

Base load

The base load on a grid is the minimum level of demand on an electrical grid over a span of time.

Peak load

A sudden increase in the load above base load for a very short duration is called as peak load.

Fig. 1 Base and Peak loads (reference: nrdc.org)

Firm Power

The power available in emergency condition is called as firm power.

Spinning Power

The power available at the busbar and delivered to the load is called as spinning power.

Cold Reserve

The power available for service but not in operation is called as a cold reserve.

Hot Reserve

The power available for operation but not in service is called as a hot reserve.

Load Curve

The variation of the load with respect to time for specified duration graphically.

Fig.2 A daily load curve

Load Duration Curve

The variation of the load with respect to the time (or percentage of time) and loads are arranged in descending order.

Fig. 3 Load duration curve


Q1. A 200 MW generator having a plant capacity factor and plant load factor is 80% and 60% respectively. The reserve capacity in MW as __________________?

Ans. Plant capacity factor (PCF) = 80% = 0.8

Plant load factor (PLF) = 60% = 0.6

PLF = (average load ÷ maximum load)

PCF = (average load ÷ installed capacity)

PLF ÷ PCF = (installed capacity ÷ maximum load)

0.6  ÷ 0.8  = (200 ÷ maximum load)

Therefore, maximum load = 166.67 MW

Reserve capacity = Installed capacity − maximum load = 200 − 166.67

= 33.33 MW


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