Inverter - Technique Digital

For m=0.577 or less, the operation of conventional, radial and virtual SVPWM are similiar, they use the same space vectors to synthesize the ref. Vector, so we obtained that the THD also very close.

When m=1.15, actually Radial and virtual SVPWM use the nearly the same vectors for synthesis the voltage output, the duty of medium vector become smaller, that’s why their THD value get closer at m=1.15

The first dominant harmonic is appeared at twice the switching frequency (2*3600Hz). This occurs because adjacent state selection effectively doubles the switching rate of the Line-Line voltage. When the state adjacency is lost in radial state and Virtual SVM, the frequency of the line-line voltage is limited to the switching frequency.

Besides the 3D scheme, when m=0.577 or lower, the switching no longer use large vectors and that the line-line voltage waveform will not higher than 400V. As 3D scheme still use large vectors in this modulation index value, the harmonics of the scheme must worse than others.

After reviewing the three different loading condition, we found that even in unbalanced loading condition, the Radial State SVM and Virtual SVM can keep the neutral point potential steady, and for the 3D scheme, although the scheme is designed for unbalanced condition, it only have a better performance in some condition compare with conventional scheme.

From the result of unbalanced voltage condition, 3D SVM actually perform better result on neutral point balancing then other conditions. However, apply an unbalanced voltage into a balanced loading seems not very useful. As the objective of 3D scheme should be balancing the unbalance voltage loading. If the loading is already balanced then this kind of work become meaningless.

The aims of the simulation only on the prove of the 3D SVM can modulate an arbitrary 3-phase sine wave.

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