Notes Topic 8

Since both are fixed a relatively constant speed of

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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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