Notes Topic 8

The effect of harmonics may be noticeable as noise or

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Unformatted text preview: ed as sinusoidal voltage and frequency oscillations. The effect of harmonics may be noticeable as noise or electromagnetic interference on electronic equipment and have the potential to cause damage to both utility distribution and consumer load equipment. Most modern grid interactive inverters have harmonic distortion devices fitted as standard and produce outputs with very little harmonic distortion. (Source: Courtesy of Bernie Brix, Westwind Turbines) 8-9 8-10 8.4.4 Islanding Islanding refers to the situation that occurs if a section of the electricity grid becomes disconnected from the main supply grid. The disconnection may be a result of lightning or line faults blowing fuses or for safety if maintenance is needed on the line. If a wind turbine continues to provide power to a section of the grid, a small ‘island’ of consumers will continue to be energised by the wind turbine. This causes significant safety hazard to linesmen and other maintenance personnel who believe that the entire line has been disconnected. In addition, the frequency of the island network may ‘wander’ from that of the main grid. If the grid were reestablished while the generator was not correctly synchronised, large current surges would result risking serious damage to the generator or the electrical interface with the grid. To prevent this situation occurring, the controller constantly monitors the voltage and frequency of the utility grid and will shut down the turbine if any significant difference is measured. 8.5 Battery Charging Control Systems flow to the battery as the voltage reaches float level. This type of controlled charging greatly increases battery life and reduces the maintenance requirements (less cell gassing) than simple on/off switching where the current cannot be limited. When battery float voltage is reached, the circuit opens preventing current flow from the generator. When this occurs the generator’s opposing torque is also lost. This results in a rapid acceleration of the rotor- to what is called free running condition. In moderate wind speeds (>7m/s) the rotor may still accelerate to quite high rotational speeds, as the only opposing torque acting on the rotor is aerodynamic drag and rotor bearing friction. An increase in rotor speed normally results in a dramatic increase in aerodynamic noise and obviously increased mechanical wear and loading on the turbine. This type of control system is often used on small turbines (<1kW) because of the systems inherent simplicity and low cost. Bergey Windpower use this type of control system in their range of turbines up to 10kW. Figure 8-10 Block diagram of series regulation controller Many small wind turbines of capacity less than 30kW are often designed primarily for battery-charging RAPS applications. For reduced maintenance direct driven (no-gearbox) permanent magnet, synchronous generators are commonly used. See section for generator description. The output voltage and frequency of a permanent magnet synchronous generator are both directly proportional to rotational speed hence a...
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