Research On Vector Control Strategy Of Doubly-fed Wind Turbine Under Voltage Drop

In recent years,wind power has developed so rapidly,with the continuous expansion of the system scale and the increase of the single unit capacity of the unit,the derivative wind power generation problem has become increasingly prominent.When the voltage drop occurs,it will have a great impact on the wind turbine running in the grid.During the voltage drop of the power grid,the rotor side current is too large and the DC bus voltage is too high due to the sudden change of the voltage amplitude of the stator side of the unit,which can shorten the service life of the power electronic devices and mechanical components of the unit.When the power grid voltage drop capacity of wind turbine itself is weak,it will lead to off-grid operation in serious cases,which will destroy the stable running state of the system.First of all,the basic mathematical model of doubly-fed wind turbine is established and its operating characteristics are analyzed,especially during the fault period and the whole transient process after fault removal,so as to prepare for the next study of the control strategy of grid-side converter and rotor-side converter.By studying the influence of voltage drop on the output characteristics of doubly-fed wind turbine,the importance of reactive power needed to compensate the system during the failure period to speed up the voltage recovery process is obtained.The doubly-fed wind turbine outputs reactive power from the stator side and the grid side converter.since the stator side is directly connected to the large grid,the voltage variation is consistent with the large grid.There is mutual inductance between the stator winding and the rotor winding of the generator,which makes the transient transition process of the stator flux be affected by its own electromagnetic characteristics and the excitation current applied by the rotor side converter at the same time.Based on the analysis of the traditional vector control strategy of doubly-fed wind turbine,considering the transient characteristics of stator flux,the rotor side converter increases the stator flux change term caused by voltage drop.When the grid voltage drops,the stator flux change term is introduced to reduce the rotor current amplitude and restrain the DC voltage too high.Then the output characteristics of the network-side converter are studied and analyzed.It is concluded that the network-side converter can operate under the non-unit power factor and can effectively adjust the reactive part of the network-side converter when the voltage drops to support a certain reactive power of the system.Finally,when the grid voltage falls,the improved vector control strategy is combined with the doubly-fed wind turbine rotor-side and grid-side converters to provide the required reactive power of the system the simulation model is built with Matlab/Simulink to verify the feasibility of the theory and the improved vector control strategy.It is concluded that the combined vector control strategy reduces the rotor current amplitude and suppresses the high DC voltage when the voltage drop.