Research On Unified Control Of Brushless Doubly-fed Power Generation System Under Stand-alone And Grid-connected Dual Modes

In recent years,environmental pollution and energy crisis have caused more and more attention to new energy power generation.Among them,wind energy has become one of the most promising solutions since it is abundant,environmentally friendly and renewable.With the development of power electronics technology,the doubly-fed power generation systems composed of doubly-fed induction generator and power electronic converters have been widely used,but brushes and slip rings lead to high maintenance costs.The brushless doubly-fed induction generator(BDFIG)uses a special winding structure to remove the brushes and slip rings,which greatly enhances the reliability of the power generation system.Therefore,the brushless boubly-fed power generation systems built with BDFIG as the core have great application potential in harsh environments,such as offshore and remote areas.However,the special structure of BDFIG also leads to complicated electromagnetic coupling and increases control difficulty.In addition,in order to meet the requirements of modern power generation systems,the power generation system should have a stand-alone and grid-connected dual-mode operation capability,and the impact on the grid should be minimized during mode switching.However,there is a significant difference between the stand-alone operation mode and the grid-connected operation mode.Thus different orientation methods and control structures are usually adopted.Although the direct combination of them can achieve dual-mode operation,it will inevitably lead to complicated control structures and a series of problems such as increased computational burden and oscillation during mode switching.To solve these problems,this paper takes the stand-alone grid-connected dual-mode unified control of the brushless doubly-fed power generation system as the research goal,establishes a mathematical model suitable for dual-mode analysis,and then realizes dual-mode control under the unified control-winding current orientation framework.The in-depth research on the existing problems under the unified control framework is conducted:(1)Currently for brushless doubly-fed power generation system,the modeling analysis and controller designs for stand-alone and grid-connected operations are relatively independent,which is not conducive to the design of dual-mode control.This paper first reviews the mathematical model of BDFIG,and summarizes stand-alone operation,mode switching,and grid-connected operation as the constraints for generator operation.Based on this model,the mathematical modeling and analysis can be completed by selecting different constraint conditions in different operating modes.This paper also summarizes and compares the combination of different operation modes and orientation modes.After comprehensively considering various factors,the control-winding current orientation is selected as the unified orientation framework of dual-mode operation,which lays the foundation for subsequent design;(2)For stand-alone operation mode of brushless doubly-fed power generation system,there have been many control strategies on the voltage regulation control under normal-load conditions,but few for overload conditions.However,if the overload current cannot be effectively controlled,it will cause the overheat of load bus and even the thermal stability problem of the entire system.Considering the characteristics of control-winding current orientation,this paper analyzes the mathematical relationship between the control-winding current and the load current under different speed and grid-side converter compensation conditions,and then proposes a new control strategy,which changes the limitation of control winding current to achieve load current limiting.This strategy only needs to add a modular limitation unit,which fully takes the control performance and implementation cost into account,and neither structure modification of the existing control system nor the increase of load current sensor is required;(3)For the mode switching process of brushless doubly-fed power generation system,the instantaneous inrush current when switching from stand-alone operation to grid-connected operation is particularly serious,but currently there is few systematic modeling and analysis about it.Therefore,this paper first derives the relationship between the voltage-amplitude error,the phase error and the inrush current,and then designs the amplitude tracking link and the incremental phase lock link before grid connection according to the analysis conclusions.Furthermore,the Bode diagram is used to analyze the frequency characteristics of the inrush current,and a new strategy of inrush current suppression is proposed by only adding high-pass feedforward compensation links.The above design is implemented under the proposed unified control-winding current orientation framework.The amplitude tracking,incremental phase lock and high-pass feedforward compensation can all be realized by modular designs,so that low cost and smooth switching process can be achieved,which effectively enhances the operation reliability of dual-mode system;(4)For grid-connected operation of the brushless doubly-fed generation system,the grid voltage is usually taken as the orientation criterion.If two sets of orientation systems are directly used,it will cause problems such as complicated control structure,increased computational burden and difficulty in mode management.For this reason,this paper proposes a control strategy to realize grid-connected control also under the unified control-winding current orientation framework.Firstly,the steady-state and small-signal models between the amplitude and angle of control-winding current and between the active and reactive powers are established,and then the active and reactive power controllers are designed,which are combined of the steady-state calculation unit and the error fine-tuning unit.This control strategy also only needs to add several modular units into the existing control framework,so that the precise control of grid-connected active power and reactive power can be achieved.Finally,based on above studies,this paper designs a complete dual-mode control system for the 32 k W brushless doubly-fed power generation system experimental platform.The system includes basic control units(consisitng of current inner loop,voltage outer loop,and frequency control loop for the stand-alone operation),as well as an overload constant-current control function module for output current control,an amplitude tracking and phase lock loop(PLL)control function module for grid synchronization,a high-pass feedforward control function module for smooth grid-connected switching,a control winding current amplitude-angle control function module for grid-connected operation control,and a state machine for managing the above modules.The experimental results fully demonstrate the effectiveness of the design.