Notes Topic 8

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Unformatted text preview: power converter has a bi-directional power flow. The cycloconverter is now seldom used because the heavy filtering needed to overcome the poor quality (harmonics) of output power. As the cost of high capacity switching components has reduced, cycloconverters have been replaced by force-commutated systems using IGBTs networks enabling Pulse Width Modulated (PWM) control. By the use of IGBT PWM inverters in the rotor circuit, grid reactions caused by harmonics may be reduced to permissible levels. 8.2.2.2.3 Double fed Asynchronous Generator The double-fed asynchronous generator or (DFAG) consists of a wound rotor that is linked to the grid via an inverter while the stator is linked directly to the grid. Since there are two separate sources that feed the grid, the term double-fed or doubly-fed is used. The inverter provides a variable voltage and frequency to the rotor to enable speed control both above and below synchronous speed. Since the stator supplies the majority of active power the DFAG produced far less harmonics than designs where the converter controls the full power output. This design also enables accurate power output and also reactive power control. Figure 8-8 Schematic of the double fed asynchronous generator (DFAG) Figure 8-7 Schematic of the Static Kraemer slip recovery system (Source: Heier, S., Gird Integration of Wind Energy Conversion Systems (1998), p110) (Source: Heier, S., Gird Integration of Wind Energy Conversion Systems (1998), p110) 8-7 8-8 8.3 Grid connection of synchronous generators 8.4 Power Conditioning and Power Quality Topic 7.3 outlines the operation of synchronous generators. We should recall that when such a machine is directly grid connected, the ratio of the grid AC frequency to the rotor speed is constant at all times, independent of rotor torque. Since the synchronous generator has negligible slip, there is little damping within the system. If a synchronous generator were directly connected, wind gusts would create torque fluctuations in the drive train since the rotor is unable to accelerate due to the stiff or rigid connection with the grid. Repeated torque fluctuations would dramatically increase the wear and tear of the drive train components –similar to what would occur with a low slip asynchronous generator. It is because of this inherent characteristic that all wind turbines using synchronous generator are in-directly grid connected via a power converter. The power converter allows full or partial variable speed hence producing smoother operation. Any electrical machine connected to an electricity grid has the ability to affect the quality of power in the network. The affect may take the form of voltage or frequency fluctuations, low power factors or introduced grid harmonics. Generally an AC-DC-AC converter is used for grid connection. An inverter on the generator side of the DC link controls the frequency that determines the rotational speed of the generator. The inverter on the grid side then exports the generated power to the utili...
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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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