Research On The Control Strategy Of 1140V Cascaded H-bridge Based STATCOM

With the improvement of the automation degree of coal industry,a large number of threephase asynchronous motors and power electronic equipments in coal mine bring a lot of reactive power and harmonic current to the power grid of coal mine.And the fact that many equipments start and stop frequently when in use may cause grid voltage fluctuation.At the same time,aging of power supply line and unbalanced load will cause three-phase voltage imbalance.Static synchronous compensator(STATCOM)based on cascaded H-bridge is an advanced equipment to solve the problems mentioned above.In this thesis,the control strategy of reactive power compensation and voltage compensation for cascaded H-bridge based STATCOM is studied.Firstly,the mathematical model of STATCOM in abc coordinate system,??? coordinate system and dq0 coordinate system is established for the design of control system.And the positive and negative sequence mathematical model of STATCOM is deduced according to the mathematical model in dq0 coordinate system.After that,parameters of the main circuit are designed to meet the requirements of the system performance.Secondly,the decoupling control strategy based on dq0 coordinate system that STATCOM is in reactive power compensation mode is studied.PI controller and Vector proportional integral controller(VPI)are both analyzed and designed.The combination of PI controller and VPI controller is called PI-VPI controller.Theoretical analysis and simulation results indicate that the PI-VPI controller has better instruction current tracking performance than the PI controller,which is more suitable for the comprehensive control of harmonic current and reactive power in coal mine.The DC-link voltage control adopts three-layer control scheme including the overall control,clustered control and individual control.In this thesis,zero sequence voltage injection is adopted to achieve clustered control,and modulation wave is modified to achieve individual control.The effectiveness of the DC-link voltage control scheme is verified by simulation.Thirdly,in order to improve the dynamic response speed of STATCOM,the model voltage predictive control strategy(MVPC)based on ??? coordinate system that STATCOM is in reactive power compensation mode is studied and the model voltage predictive strategy for cascaded H-bridge based STATCOM combined with multilevel space vector modulation technology is proposed,which has the ability of instruction current tracking and DC-link voltage control.The simulation results indicate that the proposed control strategy can effectively compensate the reactive current,harmonic current and unbalanced current.Compared with the decoupling control strategy,except for the weakness of the harmonic compensation effect,this strategy has advantages of omitting the modulation link,dacilitating the control system and the speed of dynamic response.Finally,in allusion to voltage imbalance and voltage deviation of the power grid,the positive and negative sequence separation principle based on dual second order generalized integrator frequency-locked loop and the method to achieve positive and negative sequence separation of load current are given.Zero voltage injection method that is mean to achieve clustered control of DC-link voltage under unbalanced condition is given after the analysis of STATCOM DC-link enegy flow.An improved positive and negative sequence dual-loop control strategy is proposed by adding the load current detected to the positive and negative sequence instruction current.Simulation verify that compared with traditional positive and negative sequence dual-loop control strategy,the proposed stategy can decrease the PCC voltage deviation and voltage imbalance rapidly so that the PCC voltage ibalance is reduced to the allowable range in the face of the unbalanced,impact load.