Frequency Converters

From Wind wiki

A traditional frequency converter, also called an adjustable speed drive, consists of a rectifier, an energy storage device, and an inverter:

• A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification.

• Energy storage in the form of capacitors.

• An inverter is an electrical or electro-mechanical device that converts direct current (DC) to alternating current (AC)

The most common rectifier is the diode rectifier – because of its simplicity, its low cost and low losses. It is nonlinear in nature and, consequently, it generates harmonic currents. Also the diodes only allow power to flow in one-direction; it cannot control the generator voltage or current. Thus, it can be used only with a generator that can control the voltage and with an inverter that can control the current.

The generator and the rectifier must be selected as a combination, i.e.) the generator and rectifier must match. A diode rectifier can be used together only with a synchronous generator, and it does not require a reactive magnetizing current.

In order to convert the DC power from the rectifier to AC power with exactly the same frequency as the public electrical grid, an inverter is used. This conversion is accomplished with either thyristors or transistors. The thyristor (grid commutated) based inverter solution is a cheap inverter, with low losses and, as its name indicates, it needs to be connected to the grid to be able to operate. The disadvantage of the thyristors is that it consumes reactive power and produces large harmonics.