Power generating stations are located at feasible places - according to the availability of the fuel, the dam site or an efficient location for renewable sources. Hence, they are often located quite away from the populated areas. This is very practical since the transmission of electrical power over longer distances is a lot much economical than the relative transmission of any other fuel. Also, a hydroelectric plant must be located according to an appropriate dam site or a wind power plant may be located off-shore to harvest additional energy from the wind. Thus, a long distance transmission system is needed to transmit the generated electricity to the populated areas. And a distribution system is needed to distribute the power to every consumer at appropriate voltages.
How does a power grid work?The following diagram shows a typical layout of an electrical power grid.
A power grid can be divided into three stages: Power generation, transmission and distribution. Each of these stages is explained in details below.
Power Generation:Electricity is generated in power plants which are often located far away from the populated areas. There are various types of power generating stations such as thermal, nuclear, hydro, solar, wind etc. A power plant may consist two or more 3-phase alternators which are operated in parallel. Electricity is generated in power plants at voltages ranging from 11kV up to 25 kV. Generation voltage can not be much higher due to technical limitations.
Transmission:For the transmission of power over longer distances, the generated voltages are stepped up to a much higher level. A step up transformer is used for this purpose, which increases the voltage level with the corresponding decrease in the current. Stepping up the voltage is necessary to increase the transmission efficiency by reducing I2R losses in the transmission lines. Higher the transmission voltage means lesser the current and, hence, lesser the I2R loss. Transmission voltages are generally 220kV or greater up to 765kV. Transmissions lines are often seen running over tall towers at the outskirts of a city.
Most commonly 3-phase AC power at very high voltage is used for the power transmission. But due to the advancements in power electronics, HVDC (High Voltage DC) has proved many advantages for longer distance transmission. So, HVDC transmission systems are being employed for very longer distance power transmission. AC power is converted into HVDC at a converter station for the transmission, and then it is converted back into AC at the other end. Also, HVDC link is the only option today for interconnecting grids with different frequencies.
Distribution:Power from the transmission system is then stepped down to a considerably lower voltage (say 33 to 66kV) using a step-down transformer in a primary step-down substation. The power is then carried to distribution substations or directly to very large industrial consumers. At distribution substations, the power is further stepped down (say at 11kV). Power distribution is carried out using overhead or underground distribution lines which are usually interconnected in a ring or mesh network types. Distribution transformers are used to lower the voltage further to the utilization voltage (120 volts or 230 volts) and supply several consumers using the secondary distribution lines.
This is, however, a general overview of an electric power grid. But, practically, a power grid is much more complex. The voltage levels for transmission and distribution vary widely.