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Unformatted text preview: the frequency of the grid. Since both are fixed, a relatively constant speed of rotation will result. An
increase in wind/electricity conversion efficiency may be achieved by operating the generator at variable
speeds. This requires indirect connection and a power converter must be used between the grid and the
generator. operation. The most common method used to decouple the generator from the grid’s characteristics is
achieved by using an AC-DC-AC converter as in Figure 8-6.
Figure 8-6 Schematic of an inverter system used in variable speed, grid connected, turbine 8.2.1 Direct Grid Connection of Asynchronous Generators
The asynchronous generator is termed directly connected if the stator windings are directly in circuit with the
electrical grid. This is not quite accurate, as the ‘generator’ will act as a motor in winds below cut-in speed current will be drawn from the grid and the generator will turn the rotor like a large fan. To prevent this, the
generator is disconnected from the grid in periods of low wind allowing the blades (and generator) to rotate
slowly independently of the grid frequency. The turbine’s controller determines the rotor speed via a signal
from a sensor on the generator’s shaft. As the wind speed increases, the freewheeling rotor will also increase
in speed. The controller then signals to close the circuit to the grid when synchronous speed is reached. Since
the voltage and frequency is tied directly to that of the grid/generator interfacing components are little more
in principle than switches. The obvious advantage of direct grid connection is the simplicity of the control
system and its low cost.
In low capacity turbines (less than 100kW) this switching is commonly done with contactors (contacts which
may be opened/closed via a control signal). Even though the contactors close the circuit very near the
synchronous speed of the generator (negligible export power), a large surge of current from the grid occurs as
the iron core of the generator is magnetised. This current surge may cause voltage drops on the nearby
electrical network. In some early turbine designs with inferior blade aerodynamics with low starting torque
this problem was exacerbated as the generator was used as a motor to bring the rotor up to speed at cut in.
In order to produce a ‘softer’ cut-in, thyristors or silicon-controlled rectifiers (SCRs) are often used. These
devices enable a controlled transition to full conduction. This type of connection is often termed soft start.
Most manufactures employ contactors to override the SCR after a full connection has been made. This
reduces the power losses associated with the semi-conductor switching components. Using a soft start also
reduces the sudden mechanical shock loads in the drive train that are caused by bringing the generator ‘online’ suddenly. Figure 8-5 Schematic of circuitry showing silicon-controlled rectifiers (SCR’s) using for soft-starting
(one phase shown only) The generator’s rotational speed is controlled by the frequenc...
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- Three '11