Research On Cascaded H-Bridge SVG Control Strategy Under Abnormal Grid Conditions

Wind,light,etc.renewable energy is fluctuant and random.With a large number of renewable energy power plants access power systems,these properties will not only cause fluctuation of active power in power systems,but also cause fluctuation of reactive power.It will lead to fluctuations in the grid voltage and seriously affect the stability of the power system.Among the reactive power compensation devices,the cascaded H-bridge static var generator has become the focus of current research because of its rapid response,strong reactive power compensation ability,easy expansion of modular structure and convenient access to high-voltage bus.It has been widely used in the renewable energy power plant area.Therefore,the thesis conducts research on the control strategy of cascaded H-bridge SVG in the renewable energy power plant area.Firstly,the topology and working principle of SVG are analyzed.In order to make the findings more actual value,the thesis refers to the specific parameters of a renewable energy power plant,line,transformer and other devices in Xilingol League.Considering the cost and device performance,this thesis uses the directly suspended star connected cascaded H-bridge SVG as the main circuit structure.The parameters of SVG device are designed,the SVG modulation strategy is analyzed,and the mathematical model of SVG is built.For small disturbance conditions,the overall control strategy is studied,including outer loop control,inner loop decoupling control and direct side three-level balance voltage control.The control strategy is verified by RTLAB real-time simulation platform under the conditions of increasing load and changing bus voltage.Transient large disturbance conditions include three-phase unbalanced voltage drop and three-phase balanced voltage drop.The control strategy of SVG under three-phase unbalanced condition is studied,including the separation strategy of positive and negative sequence components and the phase-to-phase voltage balance strategy with injecting zero sequence voltage,which can realize the stability of capacitor voltage and the normal operation of SVG under three-phase unbalanced condition.According to the three-phase balance condition,the strategy of changing the outer loop control of constant voltage and constant reactive current according to the voltage drop amplitude is designed,which can meet the requirements of control objectives such as response speed and safe operation of the device in case of small and large drop of bus voltage.Considering the extremely poor working environment,the traditional fixed amplitude limiting strategy of SVG is relatively conservative,and generally does not reach the limit output capacity,resulting in a great waste of resources.In order to fully develop the over compensation capability of SVG device,a SVG dynamic amplitude limiting strategy is proposed from the perspective of transient stability and switching device characteristics.The device model,loss model,thermal model and dynamic limit amplitude controller of IGBT module are designed.Considering all aspects of device safe operation,the allowable transient current value is calculated according to the steady-state working condition data,so that SVG can improve the output capacity and support the bus voltage within the transient time and safety range.