Transmission of Electric Power

Transmission of electric power is a process by which the electric power produced at power plants is transported in bulk quantities over long distances for eventual use by consumers. Electric power is sent from generating power plants to the end consumer by transmission lines. Transmission lines, when interconnected with each other, become transmission networks.

This transmission network along with power stations and substations is known as ‘transmission grid’ or simply ‘grid’.

Fig 1 Typical transmission grid

The transmission networks which are interconnected at the national level are known as ‘National grid’. Energy is usually transmitted within a grid with three phase alternating current (AC). Due to the involvement of large quantity of electric power and due to the properties of electricity, transmission involving long distances normally takes place at high voltage (33 kV or above). Electric power is usually transported to a substation near the consuming point which is either a populated area or an industrial complex. At the substation, the high voltage electric power is converted to lower voltages suitable for consumer use, and then transported to the end users through low voltage electric distribution line.

Transmission efficiency and transmission losses

Transmitting electricity at high voltage reduces the fraction of energy lost to resistance, which varies depending on the specific conductors, the current flowing, and the length of the transmission line. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor.

Transmission efficiency is improved by increasing the transmission voltage using a step-up transformer which has the effect of reducing the current in the conductors, whilst keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Ohms law, the losses are proportional to the square of the current, halving the current results in a four-fold decrease in transmission losses.

Reduced current means lesser I2R (square of the current I multiplied by the conductor resistance R) loss in the system, less cross sectional area of the electrical conductor cable means less capital involvement and decreased current causes improvement in voltage regulation of power transmission system and improved voltage regulation indicates quality power. Because of these three reasons electrical power is mainly transmitted at high voltage level.

Hence electric power to be efficiently transported to long distances need high voltages. This voltage can be 33 kV, 66 kV, 110 kV, 132 kV, 220 kV, 400 kV or even higher. The generator voltage of a power plant usually ranges from 11 kV to 25 kV. The generated electric power is first transported from the generator to a transformer at the power plant. The transformer increases the voltage to the voltage of the grid. The generator is then synchronized with the grid and the generated power is transmitted to the consumer end. At the consuming point end the transmission lines are connected to a substation. Here the transformers of substation change the voltage of the electric power from high voltage to a lower level. From substation electrical power of lower voltage is distributed to the consumers of the electrical power through distribution lines.

The main components of an electric power transmission grid are as follows.