Distributed wind power and photovoltaic power are the main components of distributed power, usually connected to medium-voltage or low-voltage power distribution systems, which can effectively make up for the lack of large-scale centralized power generation and transmission. The grid-connected distributed generation system has the advantages of effectively utilizing electric energy, but it must meet the technical requirements of grid connection to ensure the reliable operation of the grid. Therefore, for the large-scale application of distributed generation systems, it has become a focus of attention to clarify the technical requirements of grid connection and solve a series of related problems.
At present, the grid-connected wind power generation system widely adopts the variable speed system, and the output power of the variable speed wind power generation system is integrated into the power grid through the power electronic conversion device. The integration of wind power into the grid needs to consider the following three aspects:
(1) Power quality problems, including reactive power, voltage fluctuations, etc.
(2) Low voltage ride through problem, low voltage transition requires that the wind turbine not be disconnected from the grid when the voltage drops to a certain limit.
(3) When the power grid fails, the fault current sent by the wind turbine.
When the wind power system is connected to the grid, the amplitude and phase of the voltage must be the same as the required amplitude and direction of the power flow. The voltage can be controlled by adjusting the transformer ratio or the firing angle of the rectifier/inverter. The frequency must be very precisely equal to the grid frequency, otherwise the system will not work properly. In order to ensure the accurate frequency, only the frequency of the grid can be used as the reference value of the switching frequency of the inverter. The connected grid must have sufficient strength to provide excitation current for the induction generator.
The most widely used in wind power generation are doubly-fed asynchronous wind turbines and direct-drive synchronous wind turbines. The control of the doubly-fed wind turbine is accomplished by the cooperative control of the rotor-side converter, that is, the rectifier and the grid-side converter (that is, the inverter); the grid-connected direct-drive permanent magnet synchronous generator is mainly composed of a generator-side converter and a grid-side converter. The generator-side converter mainly realizes the decoupling control and speed regulation of the active and reactive power of the permanent magnet synchronous generator, and the grid-side converter mainly realizes output grid connection, decoupling control of output active and reactive power and DC side voltage control.