Dear Varun,

balancing those capacitors is a matter of PWM-generation.

Typically, getting to one point of operation can be done by using

a) all capacitors --> no balancing needed

b) dedicated capacitors --> balancing desired.

Balancing can be achieved by draining energy from the corresponding capacitors equally. To do so, a point of operation generated by m vectors, draining energy from n capacitors has to be shared n times.

Simple example 3-Level-Inverter:

A point of operation can be reached by two vectors (combination of switches on or off) draining power from two capacitors.

If this point is driven by vector "1" and "2" for 50% of the instantaneous pulse duration, the energy taken from the capacitors will be equal - balancing done.

This assumes, that in no-load condition, voltage and capacity of the capacitors was well balanced to start from.

Hello, 

Maybe this paper could help:

@ARTICLE{6863645,

author={T. Soong and P. W. Lehn},

journal={IEEE Journal of Emerging and Selected Topics in Power Electronics},

title={Internal Power Flow of a Modular Multilevel Converter With Distributed Energy Resources},

year={2014},

volume={2},

number={4},

pages={1127-1138},

abstract={This paper examines the internal power flow mechanisms that exist within a generalized modular multilevel converter (MMC) and examines alternatives for integration of distributed energy resources (DERs) within the MMC structure. Each phase leg of the MMC consists of two series connected strings of submodules, where each string of submodules is referred to as a phase arm. Based on analytically developed inter-arm power flow relations, a control methodology is proposed, which provides fully independent control of each arm's real power flow, facilitating extreme levels of inter-arm power transfer. This eliminates the need for uniform integration of DER units across all submodules of the MMC. Among others, viable operating configurations are shown to include DERs only integrated in the upper (or lower) phase arms and DERs integrated only in the upper and lower phase arms of a single phase leg. Power balance is maintained internal to the MMC via dc and ac currents circulating within the converter without distorting input dc or output ac currents. To maximize conversion efficiency, a mechanism for minimizing the necessary circulating ac currents under any inter-arm power flow condition is also identified. Comprehensive simulation results validate both the developed model and controls.},

keywords={AC-DC power convertors;energy resources;load flow control;MMC structure;conversion efficiency;distributed energy resources;inter-arm power flow relations;inter-arm power transfer;internal power flow mechanisms;lower phase arms;modular multilevel converter;power balance;single phase leg;upper phase arms;Capacitors;Density estimation robust algorithm;Legged locomotion;Load flow;Power electronics;Reactive power;Vectors;AC-DC power converters;AC???DC power converters;Energy resources;Energy storage;Multilevel systems;Power conversion;energy resources;energy storage;multilevel systems;power conversion},

doi={10.1109/JESTPE.2014.2342656},

ISSN={2168-6777},

month={Dec},}

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6863645&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel7%2F6245517%2F6507303%2F06863645.pdf%3Farnumber%3D6863645