Selection methods of main circuit parameters for modular multilevel converters

Abstract

The modular multilevel converter (MMC) will be used in power systems more and more widely. The main circuit parameter design of the MMC impacts the initial investment and the operating performance of the system. In this study the parameter design problem of several important elements in the main circuit of the MMC, such as the link transformer, the arm reactor, the sub-module (SM) capacitor and the SM power electronic devices are analysed. Based on the fundamental equivalent circuit of the MMC, the design method for the rated valve side no-load voltage of the link transformer is proposed; and the variation range of the rated valve side no-load voltage is determined as (1.00-1.05)/2 times of the DC side voltage. By introducing the concept of the equivalent capacity discharging time constant H , the nearly constant relationship between the fluctuation ratio of the SM capacitor voltage and the H -value for different projects is discovered, which means the H -value is independent of particular projects. In this study, the corresponding parameters of several practical projects are analysed and the recommended H -value is given as 40 ms. The voltage and current stress of the SM power electronic devices is investigated for the four extreme operating conditions by the analytical formulae of the MMC; and it is discovered that the current stress of the SM power electronic devices changes greatly with the operating condition. By introducing the concepts of the phase unit series resonance frequency and the circulating current resonance frequency, the principle for designing the parameter of the arm inductor is established. Through analysing the corresponding parameters of several real projects, the recommended value of the phase unit series resonance frequency is given as the rated frequency of the connected AC system.