The ac excitation control current to provide three-phase frequency conversion device to the rotor slip frequency excitation this way the frequency conversion device commonly used CYCLOCONVERTER matrix converter or AC-DC converter.
CYCLOCONVERTER thyristor commutation natural, stable, reliable work, suitable as a doubly-fed machine rotor windingpower, the highest output Cycloconverter frequency is 1/3-1/2 of the grid frequency, high-power low-frequency range There are a lot of advantages. Cycloconverter DC link, high conversion efficiency, simple main circuit without DC circuit and part of the filter, and power between reactive power handling as well as active power feedback easily. Although the doubly fed Cycloconverter universal application, but because of its low power factor, high more harmonic output frequency range is narrow, so use the number of components and more applications subject to certain restrictions.
The matrix converter is an AC-AC directly inverter, consists of nine connected directly to the switch between the three-phase input and output array. Matrix converter does not have intermediate DC link, the output consists of a three level harmonic content is relatively small; power circuit is simple, compact, and can output the frequency, amplitude and phase controllable sinusoidal load voltage; matrix converter input controllable power factor, four-quadrant. Although the matrix converter has many advantages, but is not allowed in the commutation process in the presence of two switches are simultaneously turned on or off the phenomenon, the more difficult to achieve. Matrix converter maximum output voltage capability of the device to withstand the high voltage is also a big drawback of such a converter. Used in wind power generation, since the input and output of the matrix converter is not decoupled, i.e. either the load or the power supply side asymmetry will affect the other side. Further, the input terminal of the matrix converter must be connected to the filter capacitor, the capacitor than the LCI the middle of the storage capacitance is small, but since they are AC capacitors, to withstand the switching frequency of the AC current, its volume is not so small.
AC-DC converter can be divided into two kinds of voltage and current control methods and hardware design of a variety of factors, the voltage source inverter applications. Current AC-DC-AC inverter using the natural commutation thyristors as the power switch, the DC side of the inductor is relatively expensive and used in double-fed speed, synchronous speed commutation circuit, low slip frequency under the conditions of relatively poor performance is not much in the doubly-fed induction wind power applications. Voltage type AC-DC-AC inverter This the rectifier inverter device has a simple structure, less harmonic content, power factor of the stator and rotor adjustable excellent characteristics, can significantly improve the running state of the doubly-fedand output power quality and the structure through the side of the DC bus capacitors to fully achieve the separation of the network side and the rotor side. Voltage type AC-DC-AC inverter doubly-fed generator stator magnetic field oriented vector control system based on the the fan maximum power point tracking generator decoupling control of active and reactive power, VSCF wind power is a representative of the direction.