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3-L DC Link ML Converter Properties 85
switching function, as pointed out in TABLE 24 and TABLE 26. This is also documented in TABLE
27, which qualitatively indicates how a suitable DC NP current can be generated. DC injection leads
to a fundamental in the rectified switching function in phase with the fundamental of the switching
function; harmonic injection leads to phase change of π/2 (note that this is only true for a
dominant fundamental term in the switching function; otherwise, higher harmonic terms are
predominantly generated). DC injection therefore works for active power, where the output current
is in phase with the voltage; even harmonic injection works for reactive power, where current and
voltage are phase shifted.
In the over modulation region, 3 rd , 5 th , 7 th , 11 th , 13 th in the switching function s x(t) are very
pronounced. Consequently, the rectified switching function has a high content of all even
harmonics.
- Odd harmonics in the line current are likely to be present due to the harmonic content of
the switching function in over-modulation. This couples back to odd multiples of three in
the NP current.
- If there are even harmonics like 2 nd or 4 th present in the line current, they generate DC and
even multiples of three in the NP current. Note that symmetrical over-modulation does not
generate even harmonics in the line current, but they can be generated by an additional even
term in the switching function.
- All mechanisms listed in the second column (I-NP / DC) of TABLE 28 can contribute to a
DC NP current. These mechanisms may add up or compensate each other depending on
the operating conditions. This is critical in over modulation, as standard NP control
schemes are weak in this operating point.