  Cascaded H-bridge and modular multilevel converters (MMC) are on the rise
with emerging applications in renewable energy generation, energy storage, and
electric motor drives. However, their well-known advantages come at the price
of complicated balancing, high-bandwidth isolated monitoring, and numerous
sensors that can prevent MMCs from expanding into highly cost driven markets.
Therefore, an obvious trend in research is developing control and topologies
that depend less on measurements and benefit from simpler control.
Diode-clamped topologies are considered among the more applicable solutions.
The main problem with a diode-clamped topology is that it can only balance the
module voltages of a string in one direction; therefore, it cannot provide a
completely balanced operation. This paper proposes an effective balancing
technique for the diode-clamped topology. The proposed solution exploits the dc
component of the arm current by introducing a symmetrically level-adjusted
phase-shifted modulation scheme, and ensures the balancing current flow is
always in the correct direction. The main advantages of this method are
sensorless operation, no added computation and control effort, and low overall
cost. Analysis and detailed simulations provide insight into the operation of
the system as well as the new balancing technique and the experimental results
confirm the provided discussions.
