Research On Stand-alone Generation And Virtual Synchronous Operation Of Brushless Doubly-fed System Under Control Winding Current Orientation

China is rich in renewable resources,and the installed capacity of renewable energy generation has been growing rapidly in recent years.The brushless doubly-fed generator eliminates the brushes and slip rings due to its special structure design,and thus is ideal to operate in severe environments where the maintenances are difficult.Besides,it has variablespeed constant-frequency(VSCF)operation capability,and can be used as a wind turbine.However,the specially designed dual-stator structure of the brushless doubly-fed generator makes the mathematical model of the generation system more complicated,and poses more challenges for the control system design.Nowadays,the main control method of the brushless doubly-fed generator is vector control,and the control strategies are different when under stand-alone or grid-connected operations.When under the stand-alone operation,the control targets are the amplitude and the frequency of the generator terminal voltage.When under grid-connected operation,the control targets are the active and reactive currents flow into the grid by the generator.At present,most of the control strategies are designed separately for stand-alone and grid-connected operations,which increases the difficulty of the overall system design,leading to complicated system structure.Therefore,based on the control-winding-current orientation of the brushless doubly-fed generator,this paper makes an in-depth study of the model characteristics,the orientation accuracy of stand-alone operation and the characteristics of grid-connected operation,including:(1)Taking the geographical characteristics and energy distribution of Xinjiang Uygur Autonomous Region as an example,this paper discusses the application potential of the brushless doubly-fed generation system in remote wind farms,and points out the necessity that the system can achieve both stand-alone and grid-connected operations.And focusing on the relative isolation of stand-alone and grid-connected control designs,a modified design idea is put forward,where the stand-alone control system is taken as the fundament of the overall control system,based on which the grid-connected control function is further expanded,so as to reduce the system complicacy;(2)The model of the stand-alone brushless doubly-fed generation system is constructed,according to which the advantages and disadvantages of modellings based on power winding,control winding and rotor winding are analyzed.Then the model characteristics under control winding voltage,control winding current and control winding flux linkage orientations are further discussed.With the analysis and comparison,this paper bases on the control winding current orientation to construct the brushless doubly-fed generation control system,and analyzes the dual-loop control structure under this orientation;(3)In order to improve the vector control performance,it is necessary to ensure the coordinate transformation accuracy of the power winding electric quantities.Usually,the rotor winding initial phase angle of the brushless doubly-fed generator is requires,which needs to be measured by an encoder;the displacement angle between two stators is also required,which needs to be provided by the manufacturer.In this paper,by utilizing the characteristics that the control winding q-axis current is zero after control winding current orientation,the selfcorrection link of coordinate transformation is designed,which can achieve accurate coordinate transformation without the rotor winding initial phase angle(i.e.no encoder)or displacement angle,so that the control performance of the vector control system can be improved;(4)In order to meet the requirements of grid-connected operation,the virtual synchronous control strategy of the brushless doubly-fed generator is proposed under the control winding current orientation structure.The proposed strategy converts the the brushless doubly-fed generator mathematical model under control winding-current orientation into the equivalent synchronous-generator form,so as to simulate the behaviors of synchronous generator;and then provides the references of the control winding current orientation control system,so that the grid-connected operation of the brushless doubly-fed generator with the characteristics of the synchronous generator can be realized under the control winding current orientation.