Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...

NP Control with Carrier based PWM 111
Essentially, there are 4 new types of states indicated with type 3 in TABLE 43. All others
correspond with the states of the type 1, type 2 converters or are available in all ML ANPC
converters. Two of these extra states generate a zero output (neutral point output) without
generating a NP current. These states alone cannot balance the FC voltages, so that they need to be
combined with the other remaining new states as indicated in Figure 70.
1
2
3
4
1
2
Figure 70, ANPC type 3 with type 3 zero voltage output states, coloring corresponds with TABLE 43
The resulting NP current for NP voltage output is ½ as can be seen from the table in Figure
71 (b). This corresponds with the NP current generated for –U DC/4 and +U DC/4. It can be
expected that a proper modulation schemes making a linear combination of the required states will
yield a constant NP current of ½ for V out from –U DC/4 to +U DC/4. Outside of that range, there are
no type 3 states available and the standard NP current function applies (see Figure 72 b).
5.3.1.1 Design of a suitable modulation scheme
The following modulation schemes apply single commutations between adjacent states only
and make use of the available redundant states. The carrier based PWM applies phase shifted
carriers for the 4 cells. There are 6 different sequences possible; always two of them are equivalent
as the sequence is simply reversed. The 3 fundamentally different sequences are shown in Figure
71.
A current is flowing in the NP if the first cell of either stage 1 or stage 2 and the first cell of
stage 3 are in different states. This corresponds with regions enclosed by carrier 1 and carrier 3 in
Figure 71. Obviously, this region is larger in the case of a large phase shift and smaller for the case
of a small phase shift, which is reflected in the average NP currents. Not all possible carrier
sequences lead to a balanced operation. Sequence {C 1C 4C 2C 3} applies type 3 zero outputs and
results in all balanced flying capacitors. Sequence {C 1C 2C 3C 4} and sequence {C 1C 2C 4C 3} use type 3
and standard zero outputs and generate unbalance in the flying capacitors. All capacitors will be
balanced if the two types of sequences are combined, resulting in a long sequence
{C 1C 2C 3C 4C 1C 2C 4C 3}.