| Along with the increasingly mature of AC transmission technology, the high voltage direct current transmission(HVDC) technology is also has a great development with the development of high-power electronic devices and high voltage inverter technology, solving the technical difficulty in early HVDC technology development. Flexible HVDC technology(VSC-HVDC)play an important role in large-scale renewable energy grid and the construction of smart grid.Recent years, a series breakthrough were made, of which the most striking is the successful application of modular multilevel converter(Modular Multi-level Converter, MMC) in HVDC project. MMC-HVDC not only improve the efficiency of flexible HVDC project, but also greatly promote the development of flexible HVDC technology and its engineering application. Making breakthrough in key technologies of high voltage large capacity MMC-HVDC need deeper theory and the control scheme. So the stable modular multilevel converter internal energy is vital for the normal operation of the MMC- HVDC. The purpose of this paper is to study the internal energy of MMC dynamic behavior and additional control, and in the PSCAD/EMTDC, 11 level double side-MMC HVDC system model is set up and implement the control strategy. This paper is organized as follows:First of all, the basic topological structure and working principle of MMC were introduced,and the recently level approximation with the carrier phase shift modulation strategy expounded;On this basis, adjusting the MMC HVDC system in ac voltage, active power, reactive power and other important system parameters for effective formula derivation and complete analysis of the characteristics of MMC, and then analyze the MMC bridge arm current, expounded current bridge arm unit contains different component of MMC and the effect of energy in the bridge arm,and a mathematic model of the MMC and analyze its dynamic characteristic of the capacitor voltage module were derived. Secondly, understand the physical nature of transient energy and related physical quantities and the mathematical relationship, analyze the wave characteristics of transient energy, and energy fluctuation of multilayer deduction, providing effective control strategy for MMC-HVDC stable operation. In view of energy fluctuation in the deduced analytical formula, analysis the mechanism of the MMC internal energy balance, and put forward the corresponding energy control strategy; With a focus on the energy balance bridge arm controller, analyze its design process, finally MMC internal capacitor voltage control,making the internal energy fluctuations decrease and achieve stability, as well as in the application of MMC power operation interval optimization. Finally, set up double-end MMC HVDC system model in PSCAD/EMTDC, and validate the proposed in this paper the energyvariation rule of analytical and bridge arm energy balance accuracy and effectiveness of the controller. |
