Control method of a braking chopper to reduce voltage unbalance in a 3-level chopper

In this paper, the problem of unbalance between the input voltages of a multilevel converter is analysed. To reduce the problem, particularly in braking operations, a feed-forward control method is proposed in the paper. The braking chopper is displaced between the LC chopper output filter and the load. It is switched on when the DC-link voltage becomes higher than a prefixed limit value, its frequency carrier is chosen by analysing the analytical model of the system, in order to reduce the voltage unbalance of the 3-level chopper input capacitors. The proposed control method is tested by simulation. Experimental tests have been started and the results will be presented to show the performance of the feed forward control method.