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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)

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