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

84 3-L DC Link ML Converter Properties
A similar table as above can be drawn based on the actual switching function s x(t) instead of
abs(s x(t)). However, such a table includes much more components, as all the harmonic terms in
abs(s x(t)) can be generated by multiple terms in s x(t). TABLE 28 includes the most important
components only; in reality an infinite number of higher harmonic terms apply to each field, with
decreasing importance. All positive and negative sequence NP currents will cancel out in the neutral
point, if the system is symmetrical and balanced. The remaining harmonics visible in the NP
current and consequently in the NP voltage are 3 rd , 9 th , 15 th etc. Therefore only zero sequence NP
currents are shown in TABLE 28. On the line side only positive and negative sequence currents are
considered, as zero sequence currents are not present in three wire systems.
TABLE 28, RELATIONSHIP BETWEEN HARMONIC ORDERS OF LINE SIDE CURRENT, THE
SWITCHING FUNCTION S(T) AND THE OVERALL NP CURRENT
I-NP
AC side current DC 1 2 3 4 5 6 7 8 9
DC
1 DC / 2 nd / 6 th 1 st / 3 rd 4 th / 6 th 7 th / 9 th
2 1 st / 3 rd DC / 2 nd / 6 th 3 rd / 5 th 6 th / 8 th
3
4 3 rd / 5 th DC / 2 nd / 6 th 1 st / 3 rd 4 th / 6 th
5 4 th / 6 th 1 st / 3 rd DC / 2 nd / 6 th 3 rd / 5 th
6
7 6 th / 8 th 3 rd / 5 th DC / 2 nd / 6 th 1 st / 3 rd
8 7 th / 9 th 4 th / 6 th 1 st / 3 rd DC / 2 nd / 6 th
9
s(t)
This overview helps to understand both how the NP can be disturbed and how it can be
controlled. The input ideally only contains a fundamental and high order harmonics generated by
the switching function as it is actively controlled. In reality, the input voltage (line voltage or motor
back EMF) may contain low order harmonics also generating input current harmonics if not
actively eliminated. These harmonics are typically odd harmonics. The convolution of input current
and rectified switching function then leads primarily to odd NP current harmonics, as the rectified
switching function contains primarily even harmonics (see TABLE 27). This is not very significant as
there is no DC and amplitudes are low. If however there are odd harmonics in the rectified
switching function, a DC NP current can be generated. If there are even harmonics present (2 nd , 4 th ,
etc.) in the output current, they interact with the fundamental and the 3 rd of the switching function
and result in a DC NP current.
To control the NP, it is best to focus on a scheme using the fundamental input current, as the
presence of current harmonics is uncertain. This means that mainly a fundamental in the rectified
switching function can be used to generate a DC NP current. A fundamental in the rectified
switching function can be obtained by applying a DC offset or an even harmonic term to the