Capacitance Reduction Mechanism And Improved Control Of Cascaded H-Bridge STATCOM

As the core component of modern power system,the reliability of power electronic converter directly affects the long-term stable operation of equipment.The DC-link capacitor is the most vulnerable component in the converter.Although replacing electrolytic capacitors with film capacitors can improve the reliability of converter,equivalent replacement will lead to increased cost and excessive volume due to the low power density of film capacitors.Therefore,it is of great significance to study reducing the capacitance of capacitors in converter to promote the application of film capacitors.Considering that the reduction of capacitance will limit output performance of the converter and lead to poor transient characteristics of DC-link voltage,this dissertation focuses on the implementation method and improved control strategy of low capacitance cascaded H-bridge static synchronous compensator(STATCOM).Although increasing the DC-link voltage fluctuation can reduce the capacitance demand of the cascaded H-bridge STATCOM,it will limit the inductive compensation capability of STATCOM.In order to solve this problem,this dissertation analyzes the key factors affecting the operating range of the cascaded H-bridge STATCOM,and takes the filter inductor as a control factor to improve the inductive operating range of STATCOM.The relationship between the DC-link capacitance and the rated power of the STATCOM is deduced with a constrain of the filter inductor by the way of system impedance standardization.Then,a low capacitance design method of STATCOM for all working conditions is proposed.In addition,the control strategy and regulator parameter design method of low capacitance cascaded H-bridge STATCOM are given.When the per unit impedance of the filter inductor is 15%,the proposed capacitance design method can reduce the capacitance by about 66%.The traditional cluster voltage balance control strategy will lead to poor capacitor voltage transient characteristics of low capacitance cascaded H-bridge STATCOM.In order to solve this problem,this dissertation analyzes the factors affecting the dynamics of cluster voltage balance control and proposes a model predictive fast cluster voltage balance control strategy with a large fluctuation on DC-link voltage.Firstly,a cost function is constructed to quickly evaluate the cluster voltage equilibrium with DC-link voltage utilizing the symmetrical characteristics of three-phase fluctuating capacitor potential energy.Then the over-modulation and transient overshoot of capacitor voltage can be effectively prevented by adding constraints to the subset of zero sequence voltage.The experimental results show that the proposed model can reduce the adjustment time by more than 50%.An auxiliary capacitor voltage control loop is needed to provide preconditions for the normal operation of Buck-type low-frequency pulsating power decoupling circuit.Although the low bandwidth regulator can avoid the influence of the capacitor voltage fluctuation component,it is easy to cause the auxiliary capacitor voltage beyond the effective working range and affect the normal operation of the power decoupling circuit.In order to solve this problem,this dissertation firstly expounds the advantages of using capacitive electric potential energy as the control variable.Secondly,according to the double frequency sinusoidal fluctuation characteristics of capacitor potential energy,an observer is proposed to rapidly extract the average potential energy where the average potential energy is taken as a disturbance state variable.The dynamics of auxiliary capacitor voltage of the decoupling circuit is improved with the proposed observer to ensure stable operation of the decoupling circuit.The experimental results show that about 67% of the transient surge voltage can be suppressed using the capacitor average potential energy observer.The peak value of the inductor current of the Buck-type low-frequency fluctuation power decoupling circuit will increase sharply with the increase of power under discontinuous conduction mode(DCM),which is not conducive to the hardware design of the decoupling circuit.This dissertation analyzes the influence of the inductance value of auxiliary inductor on the working mode of the decoupling circuit.Then,the working principle of the mixed conduction of the decoupling circuit is clarified.Next,the control difficulties of mixed conduction mode(MCM)are clarified by deriving the corresponding control models of continuous conduction mode(CCM)and DCM mode.Aiming at the problem that the mixed conduction control method based on capacitor voltage estimation easily suffers from the accuracy of capacitor voltage estimation,a pulse power based duty cycle calculation method for decoupling circuit under MCM mode is proposed.The proposed method is easy to implement without mode detection and can meet the working requirements of DCM mode under low power and MCM mode under high power.Simulations and experiments verify the correctness of the pulse power based mixed conduction control.Compared with DCM operation at full power,the decoupling circuit with MCM operation can reduce the device loss by about 13.5%.There are 115 figures,9 tables and 175 references in this dissertation.