Notes Topic 8

An inverter is used here rather than a controlled

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Unformatted text preview: y of the inverter on the generator side of the DC link. An inverter is used here, rather than a controlled bridge rectifier, because an inverter has the flexibility to be able to alter the power factor, which is especially beneficial when an asynchronous generator is used. This inverter converts the generated AC to DC, which is then stabilised on the DC link by a capacitor bank. The inverter on the grid side then converts the stabilised DC to grid compatible AC (at fixed voltage and frequency). For each of these transformations both from AC to DC and then back to AC again, a large number of circuits and design topologies may be used some of which have been briefly described above. Describing the operation of all of the combinations is outside the scope of this course, however a general overview of the most common methods used is presented. The type and capacity of the power converter used is also largely dependent on whether a short-circuited rotor (squirrel cage) or wound rotor generator is used. If a squirrel cage generator is chosen, the power converter must be capable of handling the full power of the turbine resulting in a relatively costly system. This is due to the expense of the power electronic components that dramatically increase with current capacity. In contrast, if a wound rotor generator is chosen, a lower capacity converter may be used to control the current in the rotor only. The capacity of the power electronic converter is also related to the range of variable speed required. 8.2.2.1 Squirrel cage AS with full variable speed The number of poles and the frequency of the connected AC supply govern the rotor speed of an asynchronous generator. Since pole changing machines only allow very coarse speed variations, the only practical option to allow variable speed is to alter the frequency of the stator voltage. One approach to achieve this is to use an AC-DC-AC converter as described above. 8.2.2 Indirect Connection of Asynchronous Generators In order to operate an asynchronous (or synchronous) generator at variable speeds, the generator must be electrically de-coupled from the influence of the electricity grid. A number of manufacturers offer the conventional asynchronous generator/gearbox topology with a power converter to achieve variable speed The wind turbine supervisory controller determines the optimal rotor speed for the given wind conditions. The controller then computes the ideal frequency of the stator voltage for the rotor to operate at the predetermined rotor speed. To achieve this operation two inverters are used on either side of a DC link. The inverter on the generator side of the DC link has a variable frequency output to control the generators speed of rotation. The inverter on the grid side exports the power at a stabilised voltage and frequency, compatible to that of the grid. Large capacitor banks are commonly used to stabilise the voltage of the DC link. 8-5 8-6 Pulse Width Modulation (PWM) is generally used to synthesise the AC waveform, whereby the microprocessor-based controller can vary the on-off time of high frequency semi-conductor switches s...
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