This example shows the operation of a 1.5-MVA motor drive using a multicell power converter.
A Full-Bridge MMC with External DC Link block is used to feed a 1800-HP induction motor. this block is programmed to model five H-bridges in series. Phase-shifting carrier-based PWM technique is used to control the H-bridges.
On the rectifier side, a phase-shifting transformer and a three-phase diode bridge is used to feed each individual DC link. Using phase-shifting transformer allows significantly reducing current harmonics injected into the distribution grid.
In order to cancel harmonics produced by the five six-pulse rectifier units in each phase, voltages applied to the AC side of individual rectifiers must be phase shifted by 60/5 = 12 degrees with each others. These phase shifts are specified in the PhaseShift parameter defined in the model pre-load function (PhaseShift = [12,24,36,48,60])
Run the simulation and observe waveforms on the various scopes. The harmonics cancellation process using phase-shifting transformers works fairly well.
Now, set PhaseShift=ones(1,5) in the MATLAB® workspace to program the same phase-shifting to all transformers. Rerun the simulation. You will see that the currents injected into the grid now have very high harmonics content.