WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–14
easily be shown to be zero in the balanced case.
I n = I a + I b + I c = 0 A (25)
The total power supplied to the load is three times the power supplied to each phase.
|S tot | = 3|E a ||I a | VA
P tot = 3|E a ||I a | cos θ W (26)
Q tot = 3|E a ||I a | sin θ var
The total power can also be expressed in terms of the line-to-line voltage E ab .
|S tot | = √ 3|E ab ||I a | VA
P tot = √ 3|E ab ||I a | cos θ W (27)
Q tot = √ 3|E ab ||I a | sin θ var
We shall illustrate the use of these equations in the discussion on synchronous generators
in the next section.
3 THE SYNCHRONOUS GENERATOR
Almost all electrical power is generated by three-phase ac generators which are synchronized
with the utility grid. Engine driven single-phase generators are used sometimes, primarily for
emergency purposes in sizes up to about 50 kW. Single-phase generators would be used for
wind turbines only when power requirements are small (less than perhaps 20 kW) and when
utility service is only single-phase. A three-phase machine would normally be used whenever
the wind turbine is adjacent to a three-phase transmission or distribution line. Three-phase
machines tend to be smaller, less expensive, and more efficient than single-phase machines of
the same power rating, which explains their use whenever possible.
It is beyond the scope of this text to present a complete treatment of three-phase synchronous
generators. This is done by many texts on electrical machines. A brief overview is
necessary, however, before some of the important features of ac generators connected to wind
turbines can be properly discussed.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001