Electrical Equipment - Wind Energy Systems for Electric Power Generation - page 67

Regarding wind energy systems, d.c./d.c. choppers, when applied, are mostly
boost converters, e.g. in so-called battery-loaders of low rating.

4.3.6 A.c. Power Controllers

A.c. power controllers are phase-controlled electronic devices, which provide adjust-
able r.m.s voltage of a series-connected load at unchanged frequency. They come
in single-phase and three-phase devices, using semiconductor elements in form of
thyristors arranged in anti-parallel connection or triacs which conduct both a.c. half-
waves in one element. Figure 4.19a shows a typical circuit with ohmic-inductive
load, the neutrals being not connected. Dependent on control angle the follow-

ing part of the voltage-time-area is applied to the load and current begins to flow,
ending at the moment when the current through the semiconductor element reaches
zero. Since no commutation phenomena occur, the power controllers are usually not
called converters.
The r.m.s voltage applied to the load can be adjusted from full wave at

α

= 0
downwards to zero. In a three-phase controller, Fig. 4.19a, this is the case at
α

α

= 150 ◦ . The current is a function of the load, in the example it depends on its
characteristic phase angle

ω 1 L / R ). Figure 4.19b shows the current con-
trol characteristics for selected cases cos

ϕ = atan(

ϕ = 1 and cos ϕ = 0; for other load phase

angles the curves start at with maximum current I 0 , rms . Load voltage as well

as current contain harmonics, their amplitudes depending on

α = ϕ

α . Hence additional

losses occur, e.g. when the load is an induction motor.
In wind energy systems such power controllers are used for connecting induction
generators to the grid, to avoid rush-currents in so-called soft-start devices.
Phase controlled thyristor power controllers are also used as a.c. switches without
mechanical contacts.

Fig. 4.19 Three-phase power controlers ( a ) Circuit with ohmic-onductive load; ( b ) Control char-
acteristic of r.m.s. current