Power Electronic Concepts in Wind Turbines.

In recently years, we have lived big discoveries on the Power Electronic field that unveil new opportunities to improve the performance in the electric circuits and therefore, in Wind Power industry. Power Electronic can be defined as the brand of the electrical engineering which studies the control and conversion of electric power. Therefore, the power conversion systems can be classified according to the type of the imput and output power:

Here we are the basic components in the Wind Power industry.

Soft Starter

It is a simple and cheap device. Its functions are to reduce the in-rush current and to limit the disturbance to the grid, close to the rated current. Without the soft-starter, the in-rush current can be up to 7-8 times the rated current, which can cause severe voltage disturbance on the grid. In the above figure, we can see the soft-starter contains two thyristors as commutation devices in each phase.

Capacitor Bank

In electric power distribution, capacitors are used for power factor correction. They usually come as three capacitors connected as a three phase load (see above figure).  The value of these capacitors is given in VAr (Volt-Amperes) and not in Farads. It supplies reactive power to the induction generator. Thus, the reactive power absorbed by the generator from the grid is minimized.

Rectifiers and Inverters

As we said, a rectifier is an electrical device that converts AC to DC (process known as Rectification). Diodes can be used for this purpose (the rectification), whereas electronic switches can be used in the Rectification as well as the Inversion processes.

In the rectification, we usually use the Diode Rectifier because is simple, cheap and with low losses. However, it generates harmonic currents, allows only a unidirectional power flow, and cannot control the generator voltage or current. Therefore, it can be used in a system with a generator that controls the voltage and with an inverter that controls the current.

The new inverters allow higher performances and low prices. Firstly, the wind turbines manufacturers paid their attention on thyristors, but looking for better performances thyristors were substituted by GTO (Gate Turn-Off thyristor) and IGBT (Insulated-Gate Bipolar Transistor). Firsts can handle more power but their valve is more complicated. Furthermore, new improvements on IGBTs make them more attractive on the power electronic circuits.

Frequency Converters

There are different ways to combine a rectifier and an inverter into a frequency converter. The purpose of a Frequency Converter is to change the frequency of an AC to another frequency (it could change the voltage, but it is not its principal goal). In recent years different convertes topologies have been investigated to apply in the wind turbines.

  • Back-To-Back Converters
  • Multilevel Converters
  • Tandem Converters
  • Matrix Converters
  • Resonant Converters

Structure of the Back-To-Back Frequency Converter.

The most important in the Wind power field is the Back-To-Back Converters, but the Multilevel and Matrix Converters have interesting properties. We can see the structure of the back-to-back frequency converter on the next figure.

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