Research On The Optimization Method Of Submodule Capacitors For MMC Permanent Magnet Direct Drive Wind Power Converters

With the proposal of the global carbon neutrality goal,all countries in the world are striving to accelerate the energy transformation.As one of the most promising traditional energy alternatives,wind power is developing rapidly in the field of new energy.The continuous growth of wind energy demand has promoted the development of large wind turbines,as compared to small turbines,large turbines can obtain more wind power at lower installation and maintenance costs.Over the past 30 years,the scale and capacity of commercial wind turbines have grown exponentially.In multi megawatt wind energy conversion systems,Permanent Magnet Synchronous Generators with full-scale power converters are widely adopted due to their significant characteristics such as high power density,no need for excitation,and high efficiency.At the same time,as the rated power of the wind turbine increases,the system will generate greater current,so parallel converters are needed to handle the increasing current.However,parallel converters have issues such as circulating currents,and the transmission of large currents will increase losses and costs.The adoption of 3 kV-33 kV medium voltage power electric power conversion provides a feasible way to solve the problem of large capacity and large-scale wind power conversion.Modular Multilevel Converter is one of the feasible topology solutions for medium voltage power conversion systems due to its high waveform quality,strong fault handling ability,and reduced losses.Conducting research on the optimization of the operation of MMC type permanent magnet direct drive wind power converters has important theoretical and practical significance.This article will focus on the MMC-based permanent magnet direct drive wind power converter as the research object.On the basis of mastering its operation mode and control principle,a mathematical model of the MMC-based permanent magnet direct drive wind power converter will be established.Based on this model,further optimization research on the operation of the MMC type permanent magnet direct drive wind power converter will be carried out.The main research content includes:(1)A steady-state analysis method for the capacitor voltage of the MMC-based permanent magnet direct drive wind power converter submodule is proposed.By analyzing the operating principle and control method of the MMC-based permanent magnet direct drive wind power converter,and combining the coupling relationship between the electrical quantities of MMC and Permanent Magnet Synchronous Generator,a solution method for each electrical quantity in MMC is provided.In the solution model,the electrical quantities in MMC can be gradually obtained by inputting the system operating conditions,laying the foundation for subsequent optimization operation research.(2)A capacitance reduction method for the submodule of MMC-based permanent magnet direct drive wind power converter has been proposed.By analyzing the operation of a wind farm,the Weibull fitting method was used to obtain the wind speed probability fitting curve of the wind farm,and the concept of the highest probability wind speed was proposed.It was found that the inverter mostly operates at the highest probability wind speed.Then,the influence of cyclic current injection on capacitor voltage ripple was analyzed,and the relationship between cyclic current injection amplitude and phase angle and capacitor voltage ripple was established.A constant capacitor voltage ripple control method was proposed.This method can fully suppress the voltage ripple of the submodule capacitor,thereby reducing the submodule capacitor demand of MMC,(3)By studying the influence of the dc component of the MMC modulation signal on the average operating voltage of the submodule capacitor,a method for improving the capacitor life of the MMC-based permanent magnet direct drive wind power converter submodule has been proposed.First of all,the influence factors of submodule capacitor lifetime are analyzed.It is found that when the average operating voltage of the submodule capacitor decreases,the submodule capacitor lifetime will be greatly improved.Then by analyzing the principle of the submodule capacitor lifetime improvement method,it is found that when the dc signal component in the MMC modulation signal increases,the average operating voltage of the sub-module capacitor will decrease.In order to maximize the life of the submodule capacitor without over-modulation,the conditions that should be satisfied for injecting dc signal components are analyzed.Finally,the secant method is used to calculate the required dc signal component injection value Ainj under different wind speeds.Compared with the traditional method,the proposed method can reduce the average operating voltage of the submodule capacitor,thus improving the service lifetime of the submodule capacitor.