Research On Simulation Technology Of Modular Multilevel Converter With Different Step

Power system is a large time-varying complex system, occupying a very important position in the national economy. With the increasing of system size, the traditional physical simulation analysis has been greatly limited. The digital simulation of power system becomes an irreplaceable tool to study the dynamic characteristics of the whole power system, and becomes an important means of power system research, planning and design. The modular multi-level converter with its excellent characteristics, in recent years the rapid development of the main support for the flexible HVDC power, then the simulation technology of MMC research is particularly important.The simulation adaptability of MMC-HVDC under different step size is studied.Based on the analysis principle of electromagnetic transient analysis, two kinds of electromagnetic transient simulation methods based on nodal analysis and state equation method are analyzed. Combined with the structural characteristics of MMC, especially for the synchronous electromagnetic simulation flaws. The necessity and rationality of hybrid simulation step on MMC-HVDC is put forward, which provides a theoretical basis for the follow-up research.To get higher simulation precision, this paper proposes an algorithm which considering hybrid simulation step in MMC-HVDC system electromagnetic transient simulation. First, it is built on the MMC detailed equivalent model. Second, according to hybrid simulation interface method, the interface model is attained between the primary system and MMC. Owing to these two steps, it can realize that the primary system is simulated in big simulation step as well as the MMC is simulated in small step. Finally,101-level MMC-HVDC system and two-terminal back to back system are built in PSCAD/EMTDC to testify the transient and dynamic response characteristics.It is found that when the network structure changes owing to fault or switch of breakers calculation error may be introduced when using trapezoidal integration is used because of the mutation of the capacitor current, which is proved by the results of PSCAD simulation. To solve the problem, a method to eliminate the calculation error of MMC arm equivalent trapezoidal integration is proposed in this paper.The capacitor voltages before and after the elimination of the calculation error at the time of switch change are compared in PSCAD/EMTDC. A single-terminal 101-level MMC-HVDC is established and simulation results show that the proposed method is effective and correct.