Based On The Mmc Hvdc System Control Strategy Research

With the development of HVDC-Flexible system in decades, especially the applications of realistic projects and accumulations of operational experience, it has been proved that the feasibility and superiority of HVDC-Flexible system is obvious. Currently, it takes mostly two-level or three-level VSC in HVDC-Flexible transmission. In order to raise the voltage level, converter valve, which is a complex technology and its parts are monopolized, consists of a high number of direct series connected IGBTs-devices. And meanwhile, a complex control algorithm is needed to solve the static and dynamic voltage-sharing problems and a huge capacity filter is needed due to the harmonic in output voltages.MMC is designed as modularized to avoid the direct series connected devices. The output voltage is closed to sinusoid, so a large filter isn’t needed. The average switching frequency of each power switch is low so that the loss is obviously down, which is the vital factor to make HVDC-Flexible available in higher power range. At the end of2011, two practical projects have been running, which indicates that MMC-HVDC will have a bright future and to break business monopolies of ABB. This paper is aiming at some further research of MCC in HVDC-Flexible. The exact contents are listed as follow.MMC’s bridge arms consist of numerous series sub modules. And the key point of making it work well is to ensure the stability of capacitance voltage. Based on the operation principle of MMC, this paper is focused on the regularity of the capacitance voltage fluctuation and the control method. Sort the modules based on the capacitance voltage which is monitoring at any time. When the current is negative, it is forced through capacitor with highest voltage to make the capacitance discharge, and vice verse. The principle is a guarantee to distribute the energy evenly, and the capacitance voltage balanced. This job provides the basis for the operation of system.A math model is based on alternating-current system is a premise. Above that, we can get the MMC equivalent through analysis. This circuit scheme is nearly the same to the conventional VSC; therefore, it simplifies the research on the control of converter station to use the direct-current strategy of double closed-loop. The grid voltage is not always balanced. To avoid this, we establish the MMC mathematical model of positive and negative sequence and design the double-sequence current controllers to make the whole system secure. And the details of the launch steps which ensure the system can start smoothly are discussed in the text.Building a MMC-HVDC model with11levels in MATLAB to testify the method of capacitance voltage control and the strategy of systematic control, and the result is positive. In the last part, a MMC-HVDC physical model is designed.