Variable Loads on Power System

Today’s interconnected power systems supply a variety of loads depending upon the consumer’s demands. These demands, of course, vary constantly which leads to the variable loading of the system and all its consequences.

Effects of Variable Loading on Power System

Variation in loading has certain undesirable effects, the most appreciable of which are given below:
  1. Generation of power becomes costly:
    For obvious reasons of optimum operation, alternators are designed in such a way that maximum efficiency occurs at (or very close to) their rated capacity. Hence, when the load varies and becomes low, the alternator will not be loaded up to its rated capacity and its working efficiency is reduced. This consequently increases the cost of production.
  2. Difficulty in controlling the system:
    When the load changes, the frequency of the system also varies. For proper operation, the frequency must be within the permissible limits. (Generally ± 3% deviation is permissible, i.e. 48.5 Hz to 51.5 Hz in case of system frequency of 50Hz in India.)
    In order to keep the frequency within limits, additional control equipments are required. Such equipments increase the cost and complexity of the system.
  3. Requirement of additional equipment:
    As explained above, variable loading necessitates the use of speed governors, voltage and frequency sensors, microcontrollers and other closed loop control equipments to exert control over the system and maintain all parameters within permissible ranges.
  4. Increased losses:
    Due to variation in loading conditions, various machines like transformers, electronic devices and other machines show increased losses due to magnetization characteristics, saturation and variation in parameters. This decreases the overall efficiency of the system.

Load Curves

As stated above, the load on the system varies with time. This variation can be represented graphically and is termed as “Load Curve”.

daily load curve
The above figure shows a typical daily load curve. As we can see, the maximum load demand occurs around 8 pm. Such a load curve shows the variation of load with time. We can determine the maximum demand on the system too. This maximum demand relates to the maximum load that occurs on the system. This maximum load will affect the size and capacity of the plant. We can also determine the energy (in units or kWh) by calculating the area under the curve.
Such curves are also helpful to determine important terms and factors like Average Load, Maximum Demand, Load Factor, Demand Factor, Plant use factor etc. (They’ve been discussed further in the article.)
These curves are also needed while selecting the number and size (capacity) of the generating units (alternators). Load curves are also needed in the control and management section for preparing the schedule of the station.

Another variation of the load curves is a ‘Load duration curve’. This is shown below:

Load Duration Curve

When the various loads occurring on a system are arranged in a decreasing order of their magnitudes with respect to the time period of the occurrence of these loads the graph obtained is known as a Load Duration Curve.
load duration curve
A load duration curve gives us the data in a more presentable form. We can easily determine the max demand and its duration. Also, we can determine the exact amount of time a specific load has prevailed.
This curve is basically generated from the values of the load curve; hence, the area under the curve will also give us the total energy generated. Also, similar to the load curve, the load duration curve can also be plotted for ant period of time.

Terms Related to Loading Conditions

The variation in load introduces some terms that need to be specified. These terms are:

Connected Load

It is defined as “the total sum of all the loads (ON and OFF) connected to the power system.
All the loads may not be switched ON together, but such loads have to be calculated to determine the required power and hence the capacity of the units.
For example, if one of the consumers has three lamps of 200 W each, four lamps of 100 W each and a machine consuming 5 kW, then the connected load of the consumer = 3(200) + 4(100) + 5000 = 6000 W

Average Load

As the name implies, it indicates the average value of all the loads occurring on the station for a given time period (such as day/s or month/s or year/s)
It can be expressed as
Average load =
No. of units (kWh) generated in given time period
The time period

Maximum Demand

It is defined as “the maximum value of load that occurs on the system during a specific time period.”
In the figure no. 1, the maximum demand is 40 MW and it occurs around 8 pm. Maximum demand is measured by a max demand meter.
Knowledge of max demand is necessary because the installed capacity of the plant is decided on the basis of max demand since the power station must be capable of supplying the max demand.

Factors Related to Variable Loading

Demand Factor

It is defined as “the ratio of maximum demand to the connected load of the system.”
Demand factor =
Maximum demand
Connected load
Since, all the connected loads are not ON all the time, Maximum demand < Connected Load.
Hence, Demand factor < 1
It is necessary for determination of the required plant equipment capacity.

Load Factor

It is defined as “the ratio of average load to the maximum demand in a given time period.”
Load factor =
Average load
Maximum demand
It can be daily/monthly/yearly load factor according to the time period considered. It is less than unity because Average load < Maximum demand.
Value of load factor affects the production cost too. It should be as high as possible. If the load factor is high, max demand is low and required station capacity (which depends on max demand) is reduced. This reduces cost of production. Load Factor should be as close to 1 as possible.
Also, a higher value of load factor reduces the variable loading problems. This is because, a higher value of load factor implies less variation in demands at various times. Due to this, the effects of variable loading are minimised. Hence Load factor should be as high as possible.

Diversity factor

A power station supplies a variety of consumers. Each consumer will have an individual maximum demand and such max demands may not occur all at the same time.
Diversity Factor is defined as “the ratio of the sum of the individual maximum demands to the total maximum demand on the system. It can be expressed as,
Diversity factor =
Sum of individual maximum demands
Maximum demand of the power station
Obviously, Diversity Factor is greater than 1. This factor gives us the diversification of the load and is necessary to decide the installation, transmission and distribution capacities of the plants.
It should be as high as possible. Higher diversity factor means that maximum demands of different consumers occur at different times, and hence, interchange and scheduling is easier and operation is optimum.
Another implication of higher diversity factor is that total max demand is lower. This reduces the size (capacity) of the required units and also the production cost.
In order to increase the diversity factor, following methods have been employed:
  1. Scheduling office times with certain time differences (known as staggering of timing.)
  2. Making use of different time zones.
  3. Giving incentives to particular consumers to utilize electricity at off-peak hours (such as night time.)
  4. Use of daylight savings.
  5. Using two part tariff schemes.

Plant Capacity Factor

It is defined as “the ratio of actual energy produced in a given time period to the total energy that could’ve been produced in the same time period.”
plant capacity factor

If we consider time period to be 1 day then,
Plant capacity factor =
Total kWh output of 1 day
Plant capacity X 24

Plant Use Factor

It is defined as “the ratio of actual energy produced (in kWh) in a given time period to the product of plant capacity and the number of hours the plant was in operation.”
Plant use factor =
Actual energy generated (kWh)
[Plant capacity X Time (in hours) the plant has been in operation]
Suppose 100 MW plant produces 50 × 106 kWh energy after being in operation for 2500 hrs in a year. Then,
Plant use factor =
50 x 106
[100 x 103 x 2500]
Therefor, Plant use factor = 0.2 = 20%

Solved Numerical

A plant has a connected load of 40 MW and a maximum demand of 20 MW. 73.8 × 106 kWh energy is generated in a year. Calculate: [i] demand factor, [ii] average load, [iii] load factor

Given data:
  • Connected load = 40 MW
  • Maximum demand = 20 MW
  • Generation = 73.8 × 106 units per annum.
demand factor, average load, load factor