Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...
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72 3-L DC Link ML Converter Properties<br />
The two phase legs can be operated with zero common mo<strong>de</strong> (opposing output voltage ratios:<br />
r 1 = – r 2) to obtain symmetry; the resulting NP curr<strong>en</strong>t is zero. If the two output voltage ratios have<br />
differ<strong>en</strong>t signs, both gradi<strong>en</strong>ts have the same sign and the resulting NP curr<strong>en</strong>t is a linear function<br />
of the CM voltage. If both output voltage ratios have the same signs, the two gradi<strong>en</strong>ts have<br />
opposing signs, so that the gradi<strong>en</strong>t of the sum of the two functions is zero. The total NP curr<strong>en</strong>t is<br />
th<strong>en</strong> <strong>de</strong>fined by the DM voltage alone. The resulting function of the NP curr<strong>en</strong>t in function of the<br />
CM voltage is a piecewise linear function as shown in Figure 59 b). This function is always<br />
monotonic but can go into saturation. A good control scheme for the NP curr<strong>en</strong>t should take this<br />
into account and limit the CM voltage to the area, where there is an impact on the NP curr<strong>en</strong>t.<br />
NP curr<strong>en</strong>t<br />
NP curr<strong>en</strong>t<br />
1<br />
1<br />
0<br />
-1<br />
I-NP / I-out1<br />
0<br />
I-NP1 / I-out1<br />
1<br />
s<br />
0<br />
-1<br />
0<br />
I-NP1 / I-out1<br />
1<br />
s<br />
I-NP2 / I-out1<br />
I-NP2 / I-out1<br />
-1<br />
(a)<br />
-1<br />
(b)<br />
4.7.1.2 Three phase system<br />
Figure 59, NP curr<strong>en</strong>t range with H-bridge<br />
In a typical three phase converter system, there are three phase legs sharing one common DC<br />
link. The load is not groun<strong>de</strong>d in most application, which means there is no path for a CM curr<strong>en</strong>t,<br />
the sum of the output curr<strong>en</strong>ts is zero:<br />
i i + i 0<br />
(54)<br />
out1 +<br />
out2<br />
out3<br />
=<br />
The neutral point curr<strong>en</strong>t is the sum of the three individual NP curr<strong>en</strong>ts:<br />
i<br />
i<br />
NPtot<br />
NPtot<br />
= i (1 − abs(<br />
s )) + i *(1 − abs(<br />
s )) + i *(1 − abs(<br />
)) (55)<br />
out1 *<br />
1 out 2<br />
2 out3<br />
s3<br />
= i − i abs(<br />
s ) + i − i * abs(<br />
s ) + i − i * abs(<br />
) (56)<br />
out1 out1<br />
*<br />
1 out 2 out 2<br />
2 out3<br />
out 3<br />
s3<br />
i<br />
NPtot<br />
(<br />
out1 1 out 2<br />
2<br />
+<br />
out3<br />
s3<br />
= − i * abs(<br />
s ) + i * abs(<br />
s ) i * abs(<br />
))<br />
(57)