3 Stages of Power Supply

The electricity supply system is divided into three major parts: generation, transmission and distribution.

Generation means ‘production’ of electricity at various types of power plants.

Transmission means carrying the electricity produced at power plants over long distances at high voltages to locations near the point of actual use.

Distribution involves actual delivery of power to the end users.

(Distribution stage image source: https://bengaluru.citizenmatters.in/bengaluru-bescom-power-cut-transformers-poorly-maintained-metering-cstep-study-46438)

Traditionally, electricity generation has been based on the effects of magnetism. When a conductor is exposed to a varying magnetic field, a voltage develops across it which depends on the rate of variation of the magnetic field. This is stated by the Lenz’ Law and Faraday’s Law. Several different types of power plants use this principle.

Coal-fired thermal power plants burn coal to generate high-temperature steam to run a turbine which drives a generator. Nuclear power plants use heat generated from nuclear reactions to generate steam to run a turbine which drives a generator. Gas-powered plants use combustion of natural gas to run a turbine which drives a generator. Hydroelectric power plants use the power of moving water to rotate a turbine which drives a generator. The blades of a wind turbine rotate due to the wind, and in turn, drive a generator installed inside the turbine. In all these types, the turbine-generator component is common. The generator is essentially a machine containing a rotating magnetic field with electric conductors located close to it. The rotation is responsible for creating the variation in the magnetic field which is required for developing a voltage. The current flow depends on what equipment is connected to the network. Each piece of equipment has its own ‘impedance’ which determines how much current it will draw.

Solar photovoltaic power generation is different as it does not involve any rotating magnetic field, but instead uses the effect of the energy in sunlight on certain materials to generate electricity.

Traditionally, large high-capacity power plants are set up at some location and transmission lines are installed to carry the power generated to several different areas. A ‘grid’ is set up in which multiple power plants are connected to a single system and the combined power is transmitted to the vicinity of the users.

Why is transmission done at a high voltage? To understand this, let us first understand what power essentially is. It is the voltage multiplied by the current. The power depends on the appliance which is in use. Let us assume that a certain set of appliances is operating at a certain condition, and that this is the only load. These will require some specific level of current for their operating condition. Accordingly, this level of current will be drawn from the generator and fed over the transmission and distribution lines. As the current goes through the transmission and distribution lines, some losses take place due to the resistance of the wires.

The power generated by the generator is also given by the product of the voltage created across the generator and the current flowing out from it. As we are assuming the load to be constant, the amount of power required is constant, and hence increasing the voltage will decrease the current. Now, the losses occurring in the transmission lines are proportional to the square of the current. Hence, reducing the current will reduce the transmission losses by a greater magnitude. And since the transmission losses can be significant over long distances, this strategy is used to cut down the loss. Even though the load varies all the time, the concept behind the losses remains the same.

Distribution is the last stage, which involves receiving a large volume of power at one point, reducing its voltage and distributing it among the end users. A distribution network is set up in the geographical area of the end users.

Now losses in the distribution network are also proportional to the square of the current. So why is the voltage reduced and not maintained at the same high level as transmission lines? This is because high voltages have strong electric fields associated with them. These strong electric fields are a safety risk and may even prove fatal. Equipment which operates at high voltage needs a proper level of insulation to handle these strong electric fields, and providing such high capacity insulation for small, low-power equipment is not practical, hence such equipment is designed to operate at low voltages. Similarly, the distribution lines will also require high levels of insulation, with proper spacing and proper materials required for providing the required insulation. Air is an insulator and reduces the impact of the electric and magnetic fields when a certain distance is present between the conductor and the target object. Higher voltage requires larger air gap for reducing the impact. This is a reason why the high-voltage transmission lines are located at greater heights compared to the distribution networks in cities. In case of space constraints, insulating materials of proper thickness are to be used. It is to be noted that even in distribution networks, voltage of 11,000 volts is maintained until the last-stage transformer (the ones mounted on poles/pedestals along the streets or in an apartment complex, like the one shown in the figure, from where power is then fed at low voltage to the end users located nearby).