WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–1
ASYNCHRONOUS LOADS
Wake up, North Wind. South Wind, blow on my garden. Song of Songs 4.16
We saw in the previous chapter that there are at least six distinctly different electrical
generators that will allow a wind turbine to operate in a variable speed mode. The electrical
output of these generators varies from rather poor quality, in the sense of widely varying
frequency and voltage, to utility quality electricity. We saw that it is possible to have a
variable speed turbine and still operate in parallel with the utility network. This design
option needs to be considered in the design of each new wind system to determine if more
energy can be captured from the wind or if overall equipment costs can be reduced.
If the wind energy system actually operates independently of the utility grid, the character
of the load becomes very important to proper system operation. The load needs to be able to
accept the highly variable power delivered by the turbine if the system is to work satisfactorily.
We saw several instances in the previous chapter where battery or resistive loads could accept
such variable power readily. There are many other possible loads which may be proposed for
wind turbines and some knowledge of their characteristics will be helpful in any system design.
Many of these loads can be operated either with or without electricity as an intermediate step.
That is, the mechanical output of a wind turbine can be connected directly to a piston pump
for pumping water, or the mechanical output can be converted to electrical form, and then
back to mechanical by use of an electrical motor. In either case, we need the characteristics
of a piston pump to determine the loading effect on the wind turbine. In this chapter we
shall consider a number of loads which might be proposed for use on a wind turbine operating
independent of the utility network. These loads therefore can be called asynchronous loads,
whether they actually require electrical power or if they only use power in a mechanical form.
The vast majority of wind turbines built in the past have been used for non-electrical applications.
Water pumping and grain grinding are classical applications of wind power. Wind
turbines have been used for many centuries by a number of cultures for watering livestock,
land drainage, irrigation, salt production, and supplying household needs.
We might divide these turbines into two basic types: the indigenous and the American
multiblade. The indigenous windmills typically use locally available materials such as wood,
sail cloth, and bamboo mats. The American multiblade was developed in the late 1800’s and
has been used widely in North America and Australia. It has a highly evolved design, uses
mass produced steel components, and is available on the international export market. The
indigenous turbine will only be regionally available. The indigenous turbine is characterized
by locally made components, relatively low capital costs, short life, and high maintenance,
which may be a good solution in a country which is short on foreign exchange and long on
cheap labor.
These machines compete rather well with all the alternatives except an electric utility
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001