Environmental Engineering Reference
In-Depth Information
diesel-generators and photovoltaic units, using also energy storage (buffer)
devices. Dependent on the grid configuration, the task of providing a set-value
frequency and a controlled nominal voltage has to be fulfilled by one or more
components.
Stand-alone wind systems for low rating in the kW-range are known as “battery
loaders”, where an accumulator is supplied from a directly driven synchronous gen-
erator with PM excitation via a diode rectifier and loading controller. Systems with
a.c. output may be organized as shown in Fig. 5.2b, where the battery is inserted
in the converter's d.c. intermediate circuit. Induction generators may also be used
for stand-alone systems, as long as a self-controled (active front-end) machine-side
inverter is used to provide the magnetization reactive power.
5.2.4 Wind Pumping Systems with Electrical Power Transmission
A special case of a wind system with electrical output is the one with one load
device directly connected, such as a water pumping system, driven by an induction
motor. The electric power transmission (“electrical shaft”) allows erecting the wind
system in a suitable elevated place for supplying a deeper situated pump in a well.
The characteristics of a centrifugal pump match favourably with those of the wind
rotor, since for both the torque varies in proportion to the speed squared, and simple
dedicated control concepts may be used [Mul00].
5.3 Systems for Feeding into the Grid
5.3.1 General
It was already mentioned that, parallel to the increase in installed power in wind
parks, the rating per unit has seen considerable growth. Figure 1.3 shows the values
for Germany as an example. Statistical values allow a closer look on the distribution
of turbine systems within classes of diameter values; Fig. 5.3 applies to the same
country for systems installed in 2004 and 2007, respectively [End08]. It is obvious
that turbines with pitch control and variable speed systems prevail, especially for
larger diameters.
The concepts of constant and variable speed may be discussed in view of the
principal characteristics of power and torque over rotational speed. The turbine char-
acteristics in Fig. 5.4 are similar to Fig. 2.8, scaled in normalized values; however
wind speed parameter values are in m/s with an assumed rated value of 12 m/s.
Variation of the pitch angle is not considered in the Figure. Note that the maximum
power values follow a third power curve. The relevant points on the torque curves
are found on the falling slopes.
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