Abstract
This paper presents a novel capacitor voltage balancing control approach for cascaded multilevel inverters with an arbitrary number of series-connected H-Bridge modules (floating capacitor modules) with asymmetric voltages, tiered by a factor of two (binary asymmetric). Using a nearest-level reference waveform, the balancing approach uses a one-step-ahead approach to find the optimal switching-state combination among all redundant switching-state combinations to balance the capacitor voltages as quickly as possible. Moreover, using a Lyapunov function candidate and considering LaSalle’s invariance principle, it is shown that an offline calculated trajectory of optimal switching-state combinations for each discrete output voltage level can be used to operate (asymptotically stable) the inverter without measuring any of the capacitor voltages, achieving a novel sensorless control as well. To verify the stability of the one-step-ahead balancing approach and its sensorless variant, a demonstrator inverter with 33 levels is operated in grid-tied mode. For the chosen 33-level converter, the NPC main-stage and the individual H-bridge modules are operated with an individual switching frequency of about 1 kHz and 2 kHz, respectively. The sensorless approach slightly reduced the dynamic system response and, furthermore, the current THD for the chosen operating point was increased from 3.28 to 4.58 in comparison with that of using the capacitor voltage feedback.
Highlights
With the help of a Lyapunov function, based on the energy stored in the grid filter and the capacitor modules, and considering LaSalle’s invariance principle, it is shown that an offline calculated trajectory of optimal switching-state combinations for each discrete output voltage level can be used to operate the inverter without measuring any of the capacitor voltages, achieving a novel self-balancing approach as well
Due to the voltage grading of adjacent converter modules, this topology is referred to as exponential modular multilevel converter (EMMC) in [27]
This paper presented a novel capacitor voltage balancing approach applicable for cascaded multilevel converters with only one DC supply per phase and asymmetric capacitor voltages, tiered by a factor of two
Summary
As stated in [14,21], the charge balance control for an asymmetric inverter with a voltage ratio of three cannot be achieved To overcome this problem, only isolated voltage sources, charged from the mains, such as described in [22], or supplied by additional DC converters, as shown in [23], could be used. With the help of a Lyapunov function, based on the energy stored in the grid filter and the capacitor modules, and considering LaSalle’s invariance principle, it is shown that an offline calculated trajectory of optimal switching-state combinations for each discrete output voltage level can be used to operate (asymptotically stable) the inverter without measuring any of the capacitor voltages, achieving a novel self-balancing approach as well. Within the scope of this paper, the converter and the grid filter design is not considered, since the major focus lies on the voltage balancing algorithm
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