Research On Advanced Control Strategy Of Current Loop Of Cascaded H-bridge Static Var Generator

Dynamic reactive power compensator and active power filter are two primary approaches of improving and enhancing the quality of grid power,among which,static var generator(SVG)is the most widely applied reactive compensator and has aroused extensive concern and discussion due to its main features of faster dynamic response,less harmonic components,smaller energy-storing elements and lower costs.Due to these characteristics,SVG has recently become the most attractive topology for industrial applicaiton and one of the hottest research topic in the fields of power electronics.It has revolutionized the flexible AC transmission technology and significantly pushed the development of theoretical research of static synchronous compensator.But the SVG has still beset by kinds of problems,such as technongy of current-loop tracking and controlling,digital delay consideration,interference rejection,robustness improving and dynamic performance optimizing,and all these problems have severely constrained the development and application of cascaded H-bridge SVG.Under this circumstance,the mathematical model has been deduced and analysed in this thesis,based on the fundamental principle of the cascaded H-bridge SVG.In addition,the necessary and sufficient condition for the precise linearization in the state space of the whole SVG system has been derivated based on the differential geometry theory.Furthermore,according to the affine non-linear features of the SVG system,by chosing a proper output function,the whole system can be linearized to 2 linear systems,the controlling algorithm can be simplified,and the dynamic performance can be improved as well.Moreover,the controlling strategy proposed by this thesis has been analysed and confirmed theoretically,and its stability has been tested and verified by MATLAB simulations and prototype experiments.The digital sampling delay,output pulse delay etc.are the main factors of deteriorating the dynamic response of the SVG system,a systematic dead-beat controlling algorithm has been put forward to cope with the problem.First of all,the principle of the delay elements that deteriorating the dynamic performance has been analysed,and three solutions have been explored and discussed.On this foundation,a new type of state-observer has been constructed via the discrete mathematical model of SVG.Finally,through repeatingly estimation of the output voltage,the lag of tracking caused by delay factors can be eliminated,and the tracking accuracy of the current-loop can be enhanced.The dead-beat controlling can minimize the current error caused by the delay factors effectively,but it has only limited effect on the dynamic performance and anti-interference result affected by internal and external disturbance and non-linear cross coupling.For this reason,sets up a controlling structure based on acticve disturbance rejection control(ADRC),adapts the time optimal function synthesizer as the tracking differentiator,and preinstalls the tracking variable and its differential trajectory,and all these actions can avoid the high-frequency viberation and poor filtering effect.And furthermore,restrains and eliminates the internal and external disturbance by adapting the extended state observer which can extract all the status of the whole system in real time.Finally,designs linear and non-linear ADRC controller respectively,and all the constrast experiments have been conducted on prototype machine.Based on the controlling strategy of ADRC listed above,the lag problem caused by time delay has been taken into consideration.Sets up the mathematical model of SVG included the delay elements primarily,designs the time optimal controlling synthetical function and arranges the transition process as the erro feedback controlling strategy of nonlinear state to prevent the output integral saturation of SVG current-loop.In addition,designs a three-order nonlinear extended state observer to estimate the variables of current-loop and the internal and external disturbance of the whole system.Kinds of static and dynamic experiments have been carried out on the prototype machine,and have also been compared to the traditional proportional resonant(PR)control and dead-beat control.Finally,in order to evaluate and verificate the validity and effectiveness of the controlling strategy and the overall design,a 10 k V cascaded H-bridge SVG has been built.In the fifth chapter of this thesis,designs the overall scheme and main power circuit of SVG,along with the specific design of power elements,main control board,carrier phase shift vector modulator and etc.Static and dynamic experiments have been taken on the SVG prototype machine under kinds of working conditions,with the control strategy and design put forward by this thesis.