The wind turbine is another core component of the wind power generation system. Its basic function is to operate under the driving of a rotating wind turbine, and then convert the mechanical energy converted by the wind turbine into electrical energy.
Traditional wind turbine
Traditional wind generators are directly connected to the grid or supply power to users through stator windings, and the applications of various generators are different. In the past, low-power wind generators generally used DC generators (actually, AC power is generated inside the DC generator, and then the AC power is converted into DC power through brushes and collector rings or diode rectifier bridges), but they are gradually being replaced by synchronous or asynchronous AC generators. Most of the medium and large wind turbines use AC generators.
In general, the efficiency of a synchronous generator is higher than that of a DC generator, and it can generate more power than a DC generator at low speeds, and can also generate electricity at high speeds. When using a synchronous generator to generate electricity, not only can it provide the current of its own magnetic field, but the speed range of the wind wheel is also larger. However, complex grid-connected control systems are usually required, which increases costs and affects economic benefits.
In modern wind power generation, asynchronous generators are sought after by the majority of wind power plants because of their simple structure, low cost, no need to rotate connecting parts, easy start-up, convenient grid connection, and no oscillation. Generally, when the rotational speed of the asynchronous generator is different from the synchronous rotational speed by a few percent, it will not have a serious impact on the grid connection, and there is no need for too long overload time. Of course, the asynchronous generator also has its significant shortcomings, that is, it cannot provide the excitation current by itself and also absorb reactive power. The grid-side parallel capacitor can be used to compensate the reactive power absorbed by the asynchronous generator, thereby improving the power factor. The asynchronous generator can be used as a motor to start the wind wheel. If the wind speed is lower than the initial wind speed, the automatic device will disconnect it from the grid. This is very helpful for starting high-speed wind wheels with fixed blades with low torque.
The electricity required to be fed into the grid is alternating current with a fixed frequency. The power grids of various countries have strict regulations on the frequency of the voltage. For example, the US power frequency is a fixed 60Hz, while in China, the power frequency is a fixed 50Hz. It should be noted here that for any power generating equipment connected to the power grid, the output AC voltage frequency should always be consistent with the frequency of the grid voltage.
Due to the volatility and randomness of wind energy itself, it is difficult for the output voltage frequency of traditional wind turbines to consistently meet the frequency requirements of the power grid. At present, most wind turbines are connected to the grid through inverters based on power electronic technology. The difference in grid connection between synchronous generators and asynchronous generators has become insignificant, and some new types of wind turbines have been designed and used.
Electromagnetic induction mechanism in generator
All types of generators include two parts: one part is a stationary stator part, and the other part is a rotor that is fixed on the shaft of the generator and can rotate with it. The stator and rotor of the generator are composed of an iron core and windings (coils made of metal conductors or other structures with the same function). The function of the winding is to conduct current and form a magnetic field as a metal conductor, or to induce voltage and current when moving in a magnetic field. The role of the iron core constrains the stator or rotor magnetic field within a certain space to help the stator and rotor windings complete the electromagnetic induction process.
In the process of wind power generation, the rotating wind turbine drives the rotor of the generator to rotate through the transmission mechanism. Since the generator rotor is placed in the magnetic field of the stator, the conductor of the upper winding starts to cut the magnetic induction movement, so an induced current is generated. At the same time, when current flows through the winding coils on the rotor, a corresponding magnetic field will be formed. Compared with the rotor, the winding coils of the stator are also cutting the magnetic induction movement, so induced current will also be generated. In actual practice, the stator winding is usually directly or indirectly connected to the power grid, and the electric energy induced in the stator winding is sent to the power grid. Therefore, the stator winding is sometimes called an armature.