Research On Start-up Inrush Current Suppression And Cascaded Stability Of PWM Rectifier

Compared with traditional uncontrolled rectifing and phase-controlled rectifing, pulse width modulation(PWM) rectifing has the advantages of low harmonic components in grid-side input current, high power factor, etc., it is widely used at present, becoming a hotspot in power electronic field. In practical application, PWM rectifier usually adopts the double closed loop proportional-integral(PI) control method, causing inrush current during the start-up process from uncontrolled rectifying to PWM rectifying, thus, influencing the security and reliability of system. On the other hand, PWM rectifier cannot obtain an output voltage adjusting to wide range by itself, so it is necessary to cascade with a DC/DC converter. But because of the interaction effect between the impedances of the two converters, the cascaded system they constitute maybe unstable.To solve the problem about start-up inrush current, this paper establishes a mathematical model of three-phase voltage source PWM rectifier, analyzes the generation mechanism of inrush current in the double closed loop PI control mode with feedforward decoupling, and determines the factors that generate the inrush current. A slow given method making the given value of direct current(DC) voltage gently change according to continuous three-section curve is proposed to suppress the inrush current, and its reasonableness and effectiveness are validated by the simulation and experiment results.Aiming at the stability problem of cascaded system, this paper takes full bridge DC/DC converter as an example, letting it cascade with PWM rectifier as a load converter. The input impedance of full bridge DC/DC converter using peak current control method is deduced in detail, as well as the output impedance of three-phase voltage source PWM rectifier using double closed loop PI control method, and the parameters that impact the impedances of converters are provided.Based on the impedances of converters, it is analyzed and confirmed that the cascaded system is stable under the original parameters, and the parameters of PWM rectifier cannot improve the relative stability of the cascaded system. For this purpose, this paper proposes an approach, which connects a virtual impedance in parallel with the output port of the source converter, reducing the output impedance of PWM rectifier, increasing the distance between dot(-1,j0) and nyquist curve of impedance rate of converters, thus, attaining the aim to improve stability margin and relative stability of system. Its reasonableness and effectiveness are validated by the simulation and experiment results.