Research On Low Voltage Ride Through Technology Of Doubly-fed Wind Power Generation System

With the depletion of fossil energy and climate change becoming increasingly prominent,the challenges facing human society have become more and more severe,and the search for clean,environmentally friendly,and renewable alternative energy sources has become a top priority.The wind power industry,which is widely distributed globally,has huge reserves,and matures in technology,is currently one of the power generation methods that have the most large-scale development and application and commercial prospects.In recent years,the installed capacity of wind power has continued to increase,and the impact on the power grid has become increasingly important.At present,countries all over the world have formulated gridconnected standards for wind turbines,which require the generators to achieve Low Voltage Ride-Through（LVRT）under certain grid fault conditions.Doubly Fed Induction Generator（DFIG）has the advantages of excellent performance,low cost,variable speed and constant frequency operation,etc.It occupies a high proportion in the field of wind power.However,because the stator windings are directly connected to the power grid,and the capacity of the converter used by the unit is small,the system is more sensitive to grid faults.When the grid voltage drops,the rotor is more likely to produce overvoltage and overcurrent,which will affect the stable operation of the unit.To realize the low voltage ride-through of the doubly-fed unit,this dissertation first analyzed the working principle of DFIG,and analyzed the Rotor Side Converter（RSC）and the Grid Side Converter（GSC）in different coordinate systems,carried out mathematical modeling and determination of control strategy.And according to the transient performance analysis of DFIG at the time of the fault,the reasons for the overvoltage and overcurrent of the generator set when the grid voltage drops are obtained.For this reason,an improved flux tracking control strategy that takes into account the voltage and current on the rotor side is proposed.According to the analysis of unbalanced power on the DC side,in order to suppress the large and frequent fluctuations of the bus voltage during the fault,this dissertation adds a twoway DC/DC and supercapacitor protection circuit,and cooperates with GSC to control it.This dissertation simulates the doubly-fed wind power generation system model built on the Matlab/Simulink platform with different control strategies.The results show that the improved flux tracking control strategy proposed in this dissertation can better suppress the rotor overvoltage and overcurrent,and greatly reduce the rotation speed.Moment pulsates and provides reactive power support for the grid,and the synergy of GSC and the protection circuit can keep the bus voltage basically constant during failure.That is,compared with the low voltage ride-through technology proposed in other documents,the scheme designed in this dissertation not only meets the requirements of my country’s wind power grid-connected guidelines,but also can better improve the LVRT capability of the unit.Finally,the hardware circuit and software programming of the bidirectional DC/DC converter and the prototype construcyion work are completed.Through debugging and experiments,the working waveforms of the converter in Buck and Boost modes are obtained,and it is verified that the protection circuit can realize the bidirectional flow of energy and keep the bus voltage stable,which further improves the low voltage ride-through capability of the doubly-fed unit.