Study Of Hvdc System Based On Modular Multilevel Converter

Full-controlled switching device is used in HVDC-flexible technology. By the device, it can be realized for the independence decoupling control of active and reactive power. It is of the characteristics of strong transmission capacity and high reliability and can solve many shortcomings of conventional HVDC technology. Therefore, it has obtained considerable development and wide application. In recent years, a new converter topology of modular multilevel converter (MMC) gets more and more attention and study, and also becomes the development direction of the flexible HVDC. As a novel type of high-voltage power converter, MMC has advantages of high level of output waveform, no filtering device, and supplying power to the passive system, etc. Although MMC has become one of the hotspot technologies in recent years, but relative to the two-level voltage source converter, MMC’s research is still in its infancy.The specific working principle and operating characteristics of modular multilevel converter are analyzed. On this basis, a mathematical model of a modular multilevel converter is established in the two-phase synchronous rotating coordinate system. Several principles which are suitable for modular multilevel converter modulation strategies are introduced, and simulations of a carrier phase-shifted PWM modulation method are carried out and simulation results prove its effectiveness at a low converter level. The control principles of common pre-charge control of modular multilevel converter, capacitor voltage balancing control and suppression of circulating current also are illustrated. Voltage balancing and circulating current control based on carrier phase-shifted pulse width modulation (CPS-PWM) is adopted and the modeling and simulation are carried on. The results show that the effectiveness of voltage balancing and suppression of circulating current in the controller by simulation.The dual-loop vector controller is designed on the basis of the mathematical model above to achieve decoupling control of currents in the inner loop of the controller and the independence control of active and reactive power in the outer loop of the controller. On the basis of Back-stepping method, decoupling controller is designed to replace the current vector control in inner current loop to enhance the stability of a system disturbance. Finally, the both ends of the MMC-HVDC system are established and the performance of the controller which is designed to achieve the active and reactive power decoupling is testified by the simulation. Results show that the controller has good dynamic performance, with robust when the svstem is disturbed.