A novel approach for energy balanced operation of submodules in multilevel VSC HVDC systems

The analysis of the characteristics of the operating behavior and functionality is essential to understand the MMC (modular multilevel converter) topology. Along with the basics of the operation mode the analysis of the stationary operation is the main interest. The operating performance is characterized by the switching events in order to balance the capacitor voltages. The balancing operations are based on the energy stored in the submodule capacitors. To reproduce the reference signal (sinusoidal signal) with a loss optimized switching frequency an adjustment of the balancing operations is necessary. In this paper different submodule drive methods were analyzed. Along with the classic drive methods (PWM) a new approach - the so called dynamic charge allocation (DCA) - was developed. In contrast to the PWM drive method the DCA drive method supplies a stable converter operation for low switching frequencies. As a result it is possible to calculate every capacitor voltage within the MMC converter. The DCA method allows a stable converter operation for low converter switching frequencies due to the developed predictive switching necessity function. Based on the capacitor voltages it is possible to define the necessity of energy balancing mechanisms which will be an indicator of the converter stability for each different drive method. It is shown that for stationary operation no dynamic energy balancing mechanisms are necessary using PWM or DCA drive methods.