Study On Control Technology Of Three-level Wind Power Converter At Low Switching Frequency

Offshore wind power is considered to be a new trend in the development of wind power due to its advantages of small land,environmental impact and wind speed stability,etc.Due to the high cost of offshore wind power,which leads to offshore wind power has the tendency of high power of single to save installation cost.In addition,offshore wind power converter mostly adopts direct drive and medium voltage sheme.With the increase of the grid connected voltage and current,the switching losses of power devices are also increased.In order to ensure the safe operation of the medium voltage converter,the switching frequency of MW class medium voltage converter is generally less than 1kHz.With the converter switching frequency decreased,it will lead to the output current containing a large number of switching frequency band harmonic,and increasing of filter design parameters.In addition,PWM modulation pulse delay will be more serious,which will result in poor control performance.In order to improve the control performance of medium-voltage wind power converter with low switching frequency,this paper takes full-power three-level medium voltage wind power converter as the research object.Following are the major research aspects of this dissertation:(1)In order to improve the equivalent switching frequency of the system,the characteristics of different kinds of discontinuous pulse-width modulation strategy are compared and analyzed.The general calculation method of loss of the device under discontinuous pulse-width modulation is given.The effects of different kinds of discontinuous pulse-width modulation strategy on switching losses and conduction losses of power devices are analyzed,which provides a theoretical basis for improving the equivalent switching frequency of the system.(2)The detailed process of different kinds of discontinuous pulse-width modulation strategy based on space vector systhesis method is given.A neutral point balance control method of discontinuous pulse-width modulation strategy is proposed.The equivalent switching frequency of the system can be maintained constant in the process of adjusting the neutral point potential.(3)The feedforward decoupled PI strategy of the grid-side converter is analyzed.The influence of the low switching frequency on the decoupling performance of current regulator is analyzed by the zero-pole distribution method.The influence of continuous and discontinuous strategy on the control performance of gird-side converter is analyzed and compared,and the modulation strategy with the best switching loss under different power factors is given.(4)The rotor flux linkage detection method of permanent magnet synchronous motor based on low-pass filter intergral compensation is introduced.On this basis,the zero-d-axis current control strategy of the permanent magnet synchronous motor at low speed is analyzed.In order to prevent the terminal voltage from being too high when the motor is running at high speed,a method of field weakening control by double current regulator is given.(5)Based on the above work,a 25kW three-level full-power wind power converter experimental platform is built,and the control strategy of the machine side and grid side converter is verified experimentally.It is proved that the control strategy is effective at low switching frequency.