1. Field of Invention
The present invention relates to the field of wind power generation technology. More particularly, the present invention relates to an excitation control circuit, a control method and an electrically excited wind power system including the same.
2. Description of Related Art
With the ever-increasing seriousness of energy-related problems in recent times, much attention has been given to power generation realized through renewable energy, such as wind power generation and solar electrical energy generation. Taking wind power generation as an example, from a stall-regulated wind power system to a variable-speed-and-constant-frequency (VSCF) wind power system, and from a wind power system with a gear case to a direct-drive wind power system without a gear case, the installed capacity of wind power generation in Taiwan and other countries is developing rapidly. Specifically, with the continuous increase in the unit capacity of a current wind power generator, VSCF technology is gradually playing a dominate role. In a current megawatt wind power generator, the gearbox is a component with a relatively high fault rate, so that much attention has been given to the development of the direct-drive wind power system without a gearbox. This is also the case due to such a system's advantages of low noise, long turbine operation time and low operation maintenance costs.
There are two main types of direct-drive wind power systems, namely, the permanent magnet-type system and the electrical excitation-type system. However, with the high cost of rare earth materials, the manufacturing costs associated with a permanent magnet-type wind power system are high. As a result, research and development is increasingly being focused on the electrically excited wind power system.
In the prior art electrically excited wind power system, the input voltage of a DC-DC converter is provided by a DC (direct current) bus of a single set of back-to-back converters (formed by a generator-side converter and a grid-side converter), and an appropriate DC voltage is outputted after being bucked by the DC-DC converter for use as supply the excitation device. However, when the DC-DC converter malfunctions, the wind power generator is suddenly de-excited, causing the disappearance of torque, and therefore the converter cannot provide a brake torque to the wind power generator, resulting in damage to the components in the wind power generation system. In another structure of an electrically excited wind power system, by using the AC (alternating current) voltage of the electric grid, a main excitation switch, a power frequency transformer and a controlled rectifier bridge are sequentially arranged to output a rectified DC voltage. Thus an excitation winding is directly provided, or the rectified DC voltage is provided to a DC-DC converter and then an appropriate DC voltage is outputted after being bucked by the DC-DC converter for use as supply to the excitation device. However, the size of the power frequency transformer is large and the cost thereof is high. More seriously, when the voltage of the electric grid drops or when a power failure of the electric grid occurs, the DC input voltage of the DC-DC converter disappears, and the wind power generator is suddenly de-excited, causing a sudden change in torque.
When the wind power generator is suddenly de-excited, causing a disappearance of the torque in the electrically excited wind power system of the prior art, this significantly reduces the reliability of electrical excitation and the operation stability of the system. In view of this, many in the industry are endeavoring to find ways in which to design a reliable excitation control circuit, so as to improve the operation stability of the excitation device and ensure that the magnetic field does not disappear suddenly.