Notes Topic 8

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Unformatted text preview: uch as isolated gate bipolar transistors (IGBTs). This may be done in sequence so that with adequate filtering the output waveform is of the desired frequency. The advantages of using this style of variable speed control is that a relatively cheap, squirrel cage induction motor may be used and that the speed variation is infinitely controllable. Another advantage is that by controlling the inverter switching, the torque on the rotor may be also controlled, allowing aerodynamic control of the rotor. The main disadvantage of this design is the expense of using two full-power inverters in series. 8.2.2.2 Wound Rotor AS Generators Partial Variable Speed As mentioned above, variable speed turbines using a short-circuited rotor design require power electronics that must be capable of controlling the full turbine current continuously, which proves to be a costly solution. By using a wound rotor generator, reduced capacity power electronics may be used to control the speed of the generator to a certain extent; this reduces the cost of the power electronics. Section 7.2.5.2 described ways the generator may be constructed to enable partial speed variation through rotor power dissipation as heat. This method of speed control is generally used for controlling transients in rotor speed due to wind gusts. A more efficient way of controlling the speed of a wound rotor generator is by using slip recovery where the surplus energy is converted to electricity rather than be dissipated as heat. 8.2.2.2.1 Slip Recovery A common method of asynchronous generator slip control is often referred to as the Static Kraemer Topology. In this system, an induction machine with a wound rotor (and slip rings) is used but without a rotor resistive load. Slip variation is achieved through the control of power transferred between the rotor and the grid instead of being dissipated in the resistor. This control method improves the overall system efficiency. Interestingly this system was used in early elevators to recover energy as the elevator car was descending. The operating principal is relatively simple – a diode bridge is used to rectify the variable AC from the rotor to DC. An inverter then converts the DC to AC that is utility-compatible for the grid. The power produced through the stator will be synchronous hence here no conversion is needed. The capacity of the power electronics is proportional to the amount of slip required, hence high degrees of slip are cost prohibitive and is generally limited to approximately 25%. 8.2.2.2.2 Cycloconverter The cycloconverter is similar in principle to the Kraemer system except rotor energy can be transferred in both directions: from grid to rotor and rotor to grid. This system does not use an intermediate AC-DC-AC converter. Instead variable rotor excitation is provided by a network of thyristors referenced to the grid frequency via a control system. The power electronic converter capacity is proportional to the slip range, and is generally limited, by cost, to approximately 50%. This type of system allows the asynchronous generator to generate below synchronous speed because the...
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This note was uploaded on 06/09/2011 for the course PV 5053 taught by Professor Aasd during the Three '11 term at University of New South Wales.

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