Modeling And Characteristic Analysis Of Doubly Fed Wind Turbine Harmonic Current

In the context of increasing renewable energy generation capacity and higher requirements on the output power quality of renewable energy power stations,the rate of harmonic current content of re-new energy power has become an important assessment indicator,and this indicator has also been written Into the grid guidelines.Accurately assessing the power quality of gridconnected currents has become a common requirement of power generation companies and power grid companies.In order to accurately evaluate the power quality before the renewable energy power station connected to the grid,this paper aims to establish a mathematical model of the harmonic current of the renewable energy power station and use the simulation method to perform the preassessment.The harmonic current outputted by the renewable energy power station is mainly caused by its own power generation unit.Because renewable energy power stations are often distributed in areas with abundant wind resources such as the Northwest and offshore,which are often the end of the power grid.Harmonic voltages of different frequencies existing in the power grid cause harmonic currents on the generating unit.Secondly,the dead time and PWM modulation of the power generation unit itself in the wind farm will also output harmonic current.Both of these harmonic current components will affect the power quality of the output power of the renewable energy power station.In order to clarify the influence of the above factors,this paper firstly takes a typical renewable energy power generation unit——Doubly-fed wind turbine as the research object to establish a Doubly-fed wind turbine output harmonic current analysis model,based on which to establish a wind farm analysis model of output harmonic current.The model is used to accurately evaluate its power quality and provide a reference and basis for subsequent power quality management.The Doubly-fed wind turbine is composed of three parts: grid-side converter,generator-side converter and double-fed motorIn the second chapter of this paper,taking the grid-side converters of doubly-fed wind turbines as research object,the analytical models of the harmonic currents caused by the background harmonics of the power grid and the dead time of the switches are established respectively.The modeling method used in this chapter is Thevenin’s equivalent modeling method.This method takes the control link of the grid-side converter and its own filter impedance into consideration.This model can be used to evaluate the harmonic current’s magnitude and phase outputted by the grid-side converter.In addition,this chapter analyzes the influence of various factors such as the background harmonic voltage amplitude phase and operating conditions on the grid-side converter’s harmonic current characteristics.Relevant simulation and experimental results verify the accuracy of the model.The third chapter of this paper takes the rotor-side converter and doubly-fed machine of the doubly-fed wind turbine as research objects and sets up the analytical model of the harmonic current caused by the background harmonics of the power grid and the dead time of the switch.In addition,compared with the overall frequency harmonics of the grid-side converter,the slip of the stator and rotor of the doubly-fed motor will change the frequency of the harmonic current caused by the dead time.In this chapter,mathematical equations are constructed to elaborate the influence of the stator and rotor slip of the doubly-fed motor on the amplitude and frequency of the harmonic current.Synthesizing the results of Chapter Two,this chapter establishes a mathematical model of the harmonic current outputted by the doubly-fed wind turbine.Also,the effects of various factors such as control delay,rotor speed,and output operating conditions on the harmonic current are analyzed.Finally,relevant experimental results verify the accuracy of the model.With the development of the MMC-DC transmission system,it provides a flexible transmission method for the wind power system to connect to the power grid.In order to accurately analyze the magnitude and law of output harmonic current,The fourth chapter of this article takes the DC power transmission system of wind farm-MMC converter station as the research object,and aims to establish a mathematical model of the output harmonic current in the wind field after connecting to the MMC converter station.First,this chapter builds a mathematical model of the wind field with a doubly-fed wind turbine as the power generation unit based on the harmonic current model of the doubly-fed wind turbine established in Chapter 3.In this chapter,a mathematical model of harmonic voltage and harmonic impedance on the AC side of the MMC converter station are established to account for the dead time to analyze the harmonic characteristics of the harmonic current outputted from the MMC converter station.In this chapter,a mathematical model of harmonic currents of the MMC converter station-wind farm interconnection system is established to accurately analyze the amplitude and phase information of the harmonic output of the wind farm.The related results can provide reference and basis for the stability analysis of the MMC converter station-wind farm interconnected system.