On Energy Storage Requirements in Modular Multilevel Converters

The modular multilevel converter is a promising topology for high-voltage and high-power applications. By using submodules equipped with dc-capacitors excellent output voltage waveforms can be obtained at low switching frequencies. The rated energy storage of the submodule capacitors is a driving factor of the size, cost, and weight of the submodules. Although the modular multilevel converter has been thoroughly investigated in the literature, a more detailed analysis of the energy-storage requirements will provide an important contribution for dimensioning and analysis of modular multilevel converters. Such an analysis is presented in this paper. The analysis relates the power transfer capability to the stored energy in the converter and the findings are validated by both simulations and experimental results. The required size of the submodule capacitors in a 4.5 MW grid-connected converter is first calculated and the calculated operating range is then compared with simulation results. The experimental results show that if the average capacitor voltage is allowed to increase 10% above the nominal value an energy storage to power transfer ratio of 21 J/kW can be achieved. It is concluded that the presented theory can relate the power transfer capability to the energy storage in the converter and is thus a valuable tool in the design and analysis of modular multilevel converters.