Frequency is the change in direction of the current flow in an AC(alternating current).In US and many other countries grid frequency is 60 Hz.In India and Europe and many other countries it is 50 Hz(cycles per second). Electric power generators connected to the electricity transmission and distribution grid function not individually but as part of a team of generators. The key factor that is common to the grid and the individual generator is the frequency.The frequency is directly linked to the speed of rotation of the generators. The generators on the 50 Hz systems rotate at a speed of 3000 rpm. This is because the rotor in the generator is a single magnet with two poles. 3000 rpm is 50 revolutions per second, or in every second the single magnetic field cuts the stator coils 50 times. In US and other countries where the frequency is 60 Hz the generators rotate at 3600 rpm.
The grid frequency is not a fixed value; it keeps changing within a narrow range. Allowable variation of the grid frequency is in a small range of ± 0.5 Hz or less. This is ± 30 rpm. At any point of time all the generators connected to the grid run at the same speed or in a "synchronized" mode.
Governors or speed controls on turbines or diesel engines control the speed of individual generators.
As the electrical load on the grid increases the generators tend to operate at a lower speed. This is compensated for by the turbine governors which feed more steam to the turbines, thereby increasing the speed. If one of the turbine generators cannot increase the speed due to steam capacity limitations, another generator on the grid will compensate for this. When all the generators reach their input capacity limitation, the grid will start operating at a lower frequency than the 50 Hz. This is an indication that the grid is overloaded and demand changes are required.
Coal fired thermal power plants are slow to respond to load changes because of thermal inertia, whereas hydropower plants or gas turbines respond much faster. A good grid will have a proper mix of all types of generators so that it can quickly control the load.
A frequency grid is a table of all the central frequencies (and corresponding wavelengths) of channels allowed in a communications system.
The most common frequency grid used for Fiber-optic communication is that used for channel spacing in Dense Wavelength Division Multiplexing (DWDM) at wavelengths around 1550 nm and defined by ITU-T G.694.1. The grid is defined relative to 193.1THz and extends from 191.7 THz to 196.1 THz with 100 GHz spacing. While defined in frequency the grid is often expressed in terms of wavelength in which case it covers the wavelength range of 1528.77 nm to 1563.86 nm with approximately a 0.8 nm channel spacing.
For practical purposes the grid has been extended to cover 186 THz to 201 THz and sub divided to provide 50 GHz and 25 GHz spaced grids.
Famous quotes containing the word frequency:
“One is apt to be discouraged by the frequency with which Mr. Hardy has persuaded himself that a macabre subject is a poem in itself; that, if there be enough of death and the tomb in one’s theme, it needs no translation into art, the bold statement of it being sufficient.”
—Rebecca West (1892–1983)