# Synchronous generator with DClink converter

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Modern power electronics can avoid the disadvantages of a synchronous generator that is directly coupled to the mains. The synchronous generator is connected via a DC link or frequency converter to the mains (see Figure 5.38). In this configuration, the system can operate the generator at a frequency independent of the mains frequency. Changing the generator frequency varies the generator speed. Hence it is possible to vary the speed over a wide range and to run at the optimal speed to obtain the maximum power use depending on the wind speed.

Figure 5.39 shows clearly that it is possible to take the maximum power from the wind at low and medium wind speeds by changing the rotor speed. At high wind speeds it is necessary to limit the power. Two possibilities exist. In the first solution, the power electronics keeps the frequency and therefore the speed constant. Then a stall control system can limit the power (see also Figure 5.14). In the second possibility, an inverter limits the power at high speeds.

However, neither solution can completely avoid the risk of rotor overspeeding at very high wind speeds; therefore, an additional power limiting mechanism is used, for instance by adjusting the pitch of the rotor blades. This shifts the rotor curves to smaller speeds than shown in Figure 5.39.

Since the frequency converter can achieve other rotor frequencies than that given by the mains, a gearbox for adjusting the rotor speed to the generator speed is no longer needed. Gearless wind turbines are produced in high numbers even in the megawatt class today. Generators for these turbines are high-pole synchronous generators with 80 or more poles. Besides the reduced material requirements and resulting cost reductions, gearless wind power

 1 ""s. V = 13 m/s / V = 12 m/s \ ^--yL-^io m/s \ \ \ --m/s \ \ â/----- m/s'Vw \ \ \ \ \ y ---Â£5 \ X \ \ \ \ ^âTâ^^^ \ \ \ \ \ \ \

Hotor speed n

Hotor speed n

Figure 5.39 Operating Points for a Variable-Speed Wind Generator with Power Limited by Constant Speed (1) or by a Converter (2)

plants have other advantages such as lower noise levels. If pulse width modulated inverters are used, the power electronics can also control the reactive power demand.

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## Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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