Research On Modeling And Double Closed Loop Control System Design Of Three Phase Voltage Sourced Converter

Power electronic devices play an increasingly important role in people’s production and daily life, nevertheless, these nonlinear devices has caused some serious problems like the current harmonic pollution and reactive power consumption in the power lines, etc. While for many precision electrical equipments, they are very sensitive to the change of the quality of electric energy. Under this background, the three-phase voltage-sourced converter(VSC) based on PWM technology has become a hot spot of research because of unique performance like realization of single power factor converter and four quadrant operation. In this dissertation, two cases including the ideal three-phase symmetry and the three-phase asymmetry often appearing in practice are considered. The modeling of VSC and design of double closed loop control system are studied and the validated through simulation.(1) Under the condition of three-phase symmetryThe foundation to study and analyze the characteristics of three-phase VSC is the mathematical model. Merits of the common model of three-phase VSC described by switch function are clear physical meaning and the accurate description of the system, so it is suitable for simulation. But this model is time-varying, discontinuous and strong coupling. For the sake of the thorough analysis and in the view of these problems, the general mathematical model based on switch function of VSC is transformed through Fourier transform, and two continuous models are obtained: high frequency model and low frequency model. Through the further derivation of high frequency model, the dynamic characteristics description of the system can be obtained, for example, the quantitative relationship between voltage ripple and current harmonics of AC side and the system parameters such as the harmonic number and switching frequency. For the low frequency model, using the principle of coordinate transformation makes the analysis and control of the model are simplified. In the double closed loop control system, the inner current ring uses hysteresis control which will make the current’s change quickly track the command value and obtain better robustness. PI control is introduced to outer voltage loop which can achieve no static error control in dq coordinate system. It is indicated by simulation experiment that the VSC has a good performance on bidirectional convert under the control of double closed loop control. The relationship between the input and output harmonic amplitude and the system parameters reflected by the high frequency model is correct. It has guiding significance to the selection of system parameters.(2) Under the condition of three phase asymmetricalThe asymmetrical case of amplitude and phase of three-phase voltage often occurs in the practical application. In this case, the existing literatures mostly just focus on the description on the control of static working point of VSC, but there is little research on input and output instantaneous power flow and the intrinsic relationship between harmonic of AC side and harmonic of DC side. Moreover, in many cases the instantaneous power exchange of AC side on the inductance is neglected, but this neglect is not reasonable when power consumption is high and switching frequency is the limited. In this dissertation, when there is negative sequence component, the AC side and DC side instantaneous power flow and harmonic generation mechanism is deeply analyzed. The analysis is reasonably simplified and control basis is put forward. Compare the different positive and negative sequence isolation methods and choose the delay method to realize precise and fast separation of positive and negative sequence, the independent double PI control strategy of positive and negative sequence component is designed to suppress low frequency fluctuation of DC side. Through the analysis of output instantaneous power, an improved voltage loop control strategy is been proposed. Through the SVPWM(Space Vector Pulse Width Modulation) the current loop is modulated. Simulation results show that the improved model has superiority on the response speed and overshoot suppression under the condition of high degree imbalance.Simulation results show that the improved double closed loop control strategy is effective.