The basic structure of an electrical power system mainly consists of three parts: generation, transmission, and distribution. It generates electrical power at a certain low voltage level at generating stations. Then, the generated power is stepped up to a higher voltage level and transmitted to the load center. At the load center, the power is stepped down for distribution purposes.
Needs of Power Transmissions
Consider a generating station and a residential area located in a town. The generating station is usually situated far away from the load center. This is because the location of a generating station is selected based on several factors such as availability of resources, land, and cost. Therefore, it cannot always be constructed near the load center.
Power Generation
Electrical power is generated at a relatively low voltage level, typically in the range of 11 kV to 25 kV. This is because low-voltage generation is more economical compared to high-voltage generation. It is important to note that low-voltage generation reduces the insulation requirement of the alternator or generator. As a result, the design becomes more practical and economical. Constructing a generator for very high voltage would require significantly higher insulation, making the machine bulky and costly.
Power Transmission
However, transmitting low-voltage power over long distances is not economical. It results in higher current, increased I²R losses, poor voltage regulation, and the need for larger conductor sizes. Therefore, after generation, the voltage is stepped up to a high level for transmission. High-voltage transmission reduces the current for the same power level, which in turn reduces losses, improves voltage regulation, and decreases the required conductor cross-section and weight.
Power Distribution
At the receiving end, the high voltage cannot be supplied directly to consumers. Residential and commercial users typically require 220 V (single-phase) or 415 V (three-phase) supply.
Hence, the transmitted power is stepped down through sub-transmission and distribution substations. It is first reduced to 11 kV and then further stepped down to 415 V or 220 V using distribution transformers. These transformers are commonly mounted on double-pole or four-pole structures along roadsides.
This entire process represents the basic structure of an electrical power system.
Power Grid Systems
A grid system refers to the interconnection of multiple generating stations through transmission lines over a large geographical area. Additionally, all step-down substations are also connected to this common transmission network. Together, they form what is known as a power grid.
In other words, a grid is a large transmission network in which multiple generating stations supply power, and multiple substations draw power. This is different from a simple system where a single generating station is connected to a single load center through a dedicated transmission line.
The grid system is highly flexible and reliable. If any generating station goes out of service or its generation capacity is reduced, the grid continues to supply power to consumers. This is possible because other generating stations connected to the grid continue to feed power into the system.
In a grid system, consumers receive power from the grid as a whole rather than from any specific generating station. This feature significantly enhances the flexibility and reliability of the power system.
Types of Generating Stations
Conventional power stations can be divided into two main categories: thermal power stations and hydropower stations.
Thermal power stations can be further classified based on the type of fuel used. These include coal-based thermal power stations, nuclear power stations, and diesel or natural gas-based thermal power stations.
Hydropower stations are not categorized based on fuel, but they can be classified as pumped storage plants and conventional hydropower stations. Pumped storage plants have unique characteristics, which will be discussed separately in detail.
In recent years, renewable energy generation has gained significant importance. Major sources of renewable energy include wind power and solar power.
The Government of India has set ambitious targets to reduce dependence on coal-based thermal power and aims to achieve approximately 500 GW of installed capacity from renewable energy sources such as wind and solar by 2030. Therefore, renewable energy generation is becoming increasingly important and is the need of the time.
Transmission Power Systems
Electrical power is normally transmitted in the range of 132 kV to 765 kV using AC transmission systems. For short and medium transmission distances, 132 kV to 220 kV systems are commonly used. For long-distance transmission, 400 kV systems are typically employed. Whereas, 765 kV systems are used in India for very long distances and bulk power transmissions.
HVDC (High Voltage Direct Current)
In some cases, from an economic perspective, it becomes advantageous to transmit power using DC instead of AC. DC transmission offers several advantages. Although the conversion of generated AC power into DC and reconversion back to AC after transmission involves high costs at both ends. The overall benefits can outweigh these costs in specific situations. This type of transmission system is known as HVDC (High Voltage Direct Current).
Sub – Transmission System
After transmission, the voltage is stepped down to 66 kV or 33 kV for sub-transmission purposes. Many industrial bulk consumers receive power at 33 kV or even at 132 kV levels. After supplying power to these bulk consumers, the power is routed to 33/11 kV substations of distribution utilities. At these substations, the voltage is stepped down using 33/11 kV transformers.
Distribution System
The 11 kV supply is then distributed to consumer localities through underground or overhead lines. Within the locality, distribution transformers (11 kV / 0.415 kV) further step down the voltage. Finally, the voltage is reduced to 220 V (single-phase) and 415 V (three-phase), which are suitable for consumer use.