Environmental Engineering Reference
In-Depth Information
lated at 50 or 60 Hz. This modulation is achieved by use of a more sophisticated switching
pattern on the input (left-hand) inverter and is designed to make the current
i
x
take the shape
of a rectifi ed 50 or 60 Hz sine wave. The output (right-hand) bridge then 'un-rectifi es ' or
'steers' this to produce a full sine wave current into the grid. The advantage of this, over the
arrangement shown previously in Figure 4.40, is that the steering bridge switches at just 100
or 120 Hz, and only when the instantaneous current is zero, making it more effi cient than the
PWM inverter on the right of Figure 4.40.
4.6.2 Applications to Wind Power
Fixed Versus Variable Speed - Energy Capture
[4]
This section deals with average changes of wind speed over tens of seconds or minutes and
the wind turbine response to maximize energy capture over this timescale. In Section 2.4.3
mention was made of the fact that for maximum energy extraction the wind turbine speed
should be adjusted to vary in sympathy with the wind speed. Figure 4.42 shows typical wind
turbine power versus rotational speed characteristics with wind speed
U
as a parameter where
U
1
= 0) and at
runaway speed when the turbine is rotating without any restraining torque. In between these
two extremes the power rises to a maximum. It stands to reason that for each wind speed it
is desirable to operate the wind turbine at this maximum power point. For this to be possible
as the average wind changes, the turbine torque must be adjusted so that the locus
aa
that
passes through the maximum power points is tracked. This is an alternative way of expressing
<
U
2
<
U
rated
. For each curve the converted power is zero at standstill (
ω
b
a
U
rated
c
U
2
b
a
c
U
1
0
w
2
w
1
Ω
(rad/s)
Figure 4.42
Wind turbine power -rotational speed relationship with wind speed as a parameter
