Introduction

Modern vector control techniques enable the torque of alternating current (AC) generators to be controlled with a rapid dynamic response. Unfortunately, investigation has shown that the bandwidth of the frequency converter controller implemented in the Lagerwey LW-50/750 wind turbine is too small to utilize the potential benefits of model based control. Consequently, we have decided to develop a new frequency converter controller for the direct-drive synchronous generator of the Lagerwey LW-50/750 wind turbine.

The frequency converter of the Lagerwey LW-50/750 wind turbine consists of a rectifier, DC bus with a smoothing capacitor, and an inverter as illustrated in Fig. 6.1. The frequency converter is located at the tower base. The DC bus capacitor is used to make the DC input appear as a DC voltage source with a very small internal impedance at the switching frequency. The rectifier and inverter are connected back-to-back via the DC bus. The rectifier converts the three-phase alternating currents to 1100 V DC by connecting each of the three stator phases to the positive or negative side of the DC link voltage. The inverter in turn converts the DC to 690 V AC 50 Hz of the utility grid. Because all power is rectified to DC and then inverted to AC the generator is decoupled from the utility grid allowing variable speed operation.

Both the inverter and rectifier are composed of six power semiconductor devices or power switches. Each switch consists of an insulated gate bipolar transistor (IGBT) with a reverse diode in antiparallel. The antiparallel diodes provide paths for the current independent of the switching state of the IGBT's.

Rectifier DC link Inverter

Rectifier DC link Inverter

Figure 6.1: Schematic of back-to-back converter, with SG: synchronous generator, and Si: switch i.

The frequency converter controller in Fig. 6.1 actually consists of two controllers, viz. a rectifier controller and an inverter controller. The rectifier controller forces the electromechanical torque to equal the torque set-point specified by the wind turbine controller. The power quality requirements (e.g. power factor) are controlled by the inverter (the power factor is often set to unity to obtain zero reactive power). Both goals can be achieved independently of each other by the capacitor decoupling between the rectifier and inverter.

Renewable Energy 101

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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