Research And Implementation On The SVG Based On PWM Rectifier

Environment protecting and energy saving has become the hot topics concerned by all countries; however, reactive power consumed by various electrical equipments is obtained from grid to maintain their own normal operating. A large number of reactive power loss to public grid will be produced because of the existence of reactive power, therefore it is imperative to compensate reactive power. Compared with the traditional reactive power compensation device, Static Var Generator (SVG) has become an important component of modern flexible AC transmission system.SVG is a device which makes the bridge converter circuit parallel to grid by connecting reactor, the current of AC side of bridge circuit can be controlled directly or the phase and amplitude of voltage of AC side of bridge circuit can be controlled indirectly to make the SVG absorbing or generating appropriate reactive current, which can adjust the reactive power of grid to meet the requirements and achieve the goal of dynamic reactive power compensation. The SVG based on three-phase voltage source PWM rectifier is focused on in this paper, main research contents are as follows:1、The research of three-phase voltage source PWM rectifierThe working principle of three-phase voltage source PWM rectifier is analyzed, its mathematical model is deduced in static three-phase (a,b,c) coordinates and in rotary two-phase (d,q) coordinates. On the basis of the comparison between various current control strategies of PWM rectifier, SVPWM current control with fixed switching frequency is adopted to analyze the double loop control system with inner current loop and outer voltage loop.The startup process of three-phase voltage source PWM rectifier means that the uncontrollable rectifier bridge of diode stops its charging and PWM rectifier starts to be controlled when the voltage of DC side reaches the specified value. During the startup period, the larger inrush current is emerged, the larger peak current can lead to the action of over-current protection of system, or even might damage the switch device, so it is particularly important to research on the startup inrush current. The impact of the initial phase of supply voltage on startup current of three-phase voltage source PWM rectifier is researched using the analytic method, the analyzing results show that various initial phase angle of the grid electromotive force has various influence to startup current. Reasonable choice of initial phase angle of the grid electromotive force can makes the three phases of AC current not varying much between each other, which makes the current threshold of switch device decreases and the cost reduces. Experimental results are consistent with theoretical analysis.The inherent causes of inrush current during startup period of three-phase voltage source PWM rectifier are analyzed and the voltage feedforward compensation is adopted to reduce the startup inrush current. Experimental results show that startup inrush current decreases obviously after adopting the voltage feedforward compensation.The influence caused by dead time to current and voltage is analyzed and dead time effect is compensated. Experimental results show that the steady-state current waveform is improved evidently when dead time effect is compensated.2、The research of SVGThe working principle of SVG is deeply studied, the mathematical model in rotary two-phase (d,q) coordinates is deduced, as well as PWM rectifier, SVPWM current control with fixed switching frequency is adopted, the three closed loop control system with power loop, current loop and voltage loop is designed. On the basis of the studies above, two kinds of situations, which are the given reactive power is constant and the given reactive power step changes, are simulated and analyzed respectively in steady and dynamic state in MATLAB/Simulink simulation platform, the effectiveness of reactive power compensation by SVG is verified by simulation results. The three-phase asynchronous motor is adopted as load in laboratory condition, experimental results shows the effectiveness of reactive power compensation by SVG, in the dynamic process in which the given reactive power step changes, the SVG system regulates fast, operates steady and the power factor of grid is improved.The software programming and related experiments of the control of above parts is conducted on the hardware platform which use TMS320F2812digital signal processor (DSP) of TI Company as the controller, the correctness of theoretical analysis is verified by the experimental results.