Research On Control Strategy And Junction Temperature Of Permanent Magnet Wind Power Converter Based On Working Condition Of Grid Voltage Sag

Due to its advantages of clean,efficient and abundant resources,wind power construction scale has been continuously expanded in recent years.However,wind farms are mostly built in remote areas or on the sea where the weak grid is easy to have voltage drop fault.Therefore,wind turbines are required to have low-voltage ride through capability.Moreover,frequent voltage fluctuation will cause the junction temperature fluctuation of insulated gate bipolar transistor(IGBT)module in the converter.The reliability of IGBT as the core device of converter will be greatly tested.Therefore,in order to improve the low voltage ride through capability of the permanent magnet wind power system,accurately analyze the junction temperature variation characteristics of the permanent magnet wind power converter under grid fault condition and improve the operation stability of the permanent magnet direct drive wind power generation system,this paper takes the permanent magnet wind power converter as the object and establishes the permanent magnet direct drive wind power system model and the permanent magnet wind power converter junction temperature model considering the electrothermal coupling characteristics on the Simulink simulation platform.The control strategy and junction temperature variation law of the wind power converter under fault conditions are mainly studied.The topic selection is in line with the development direction of vigorously promoting wind power construction in China.And the research results have academic significance and application value to improve the operation reliability of permanent magnet wind power system and reduce its maintenance cost.The main contents of this paper include:(1)In order to provide the basic control system model for the following paper,the operation characteristics of the wind turbine under different wind speeds and the mathematical model of the wind turbine are analyzed firstly.Then,the mathematical model of grid-side converter in dq coordinate system and the vector control strategy based on grid voltage orientation are analyzed,Then the mathematical model of generator-side permanent magnet generator in dq coordinate system,maximum power tracking control based on hill climbing method and vector control strategy based on rotor field orientation are analyzed.Finally,the overall model of permanent magnet direct-drive wind power generation system is built in MATLAB,and the excellent control effect of the vector control strategy at generator side and grid side is verified by simulation.(2)In order to study the low voltage ride through control strategy of permanent magnet wind power system under voltage sag,the dynamic response characteristics of permanent magnet direct-drive wind power generation system under voltage sag are analyzed through theory and simulation.And the influence of grid voltage sag fault on grid-side current,DC voltage and grid-side power is determined.The working principle and control method of common DC-side crowbar circuit are described.Considering comprehensively,the protection scheme of DC-side crowbar circuit with energy dissipation resistance under the control of DC voltage deviation through PI regulation is selected,and the control scheme is verified by simulation under two different voltage drop depths.In view of the deficiency of using DC-side protection circuit alone,an improved control scheme is proposed,which combines the strategy of reactive power provided by grid-side converter with crowbar circuit.The effectiveness of the combined protection scheme in low voltage ride through of permanent magnet wind power system is verified in Simulink.(3)In order to deeply and comprehensively analyze the junction temperature variation law of the permanent magnet wind power converter IGBT module under normal and fault conditions,the linear interpolation method is used to model the loss calculation at any temperature on the basis of mastering the electric thermal coupling characteristic and heat transfer process of the IGBT module.Then based on the lumped parameter method,the foster thermal network model for predicting the junction temperature of IGBT is established.The thermal loss simulation model of IGBT module of converter in permanent magnet wind power system considering the electric thermal coupling characteristic of switching devices is built in Simulink.The characteristics of loss and junction temperature of IGBT modules of converter at generator side and grid side under normal condition are analyzed.Finally,based on the combined low voltage ride through control scheme in Chapter 3,the junction temperature variation law of IGBT module of grid-side converter is simulated.The above research content can not only provide a relatively perfect LVRT control strategy for permanent magnet wind power system,but also provide support for real-time and accurate analysis of IGBT module junction temperature in case of fault,which has reference significance for operation and maintenance optimization of wind power converter.