Why the converter output is delayed with respect to the PWM modulating voltage wave?

Good morning all, please find attached the file power_PWMGenerator3Level from Matlab-Simulink library. I have modified it just by adding a Three-phase VI Measurement block plus one scope in order to compare the reference modulating voltage wave for generating PWM pulses to the resulting converter output voltage – said changes are highlghted in blue.

I note that during the first moments of the simulation there is a slight delay with the converter output voltage (in green) lagging with respect to the modulating voltage (in yellow), total delay time is less than 0,4 miliseconds. If I extend the simulation time the delay is further reduced it is still noticeable in the graphs.

My impression is this time difference should not be significative still I would like to have your opinion to discard any possible error that could affect future simulations.

My questions:

- Why does this time difference appear? Is it a delay introduced by the converter switching times, a phase difference due to the converter output choke impedance or both?

- In your experience, if the converter were connected to an external grid, could this time difference be significant enough to cause recirculating currents? If so how could they be avoided?

Thanks in advance!

Juan Cabeza

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Research Gate: Juan Cabeza

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[Graham Dudgeon]

Hi Juan - The phase difference on the load voltage is due to the choke impedance. If you overlay the modulation wave on the converter terminal voltage, you will see that the converter output voltage is aligned correctly. As for recirculating currents - I would need to see a specific example where circulating currents are being observed, in order to provide a suitable response to that question.

[Juan Antonio Cabeza López de Tamayo]

Thank you very much, Graham - I will try as you kindly suggest.

[Juan Antonio Cabeza López de Tamayo]

Good morning, apologies for having added my response to "Answers" and not to "Comments" - could you please kindly review and reply? Thanks in advance!

Juan

Answer 1

[Juan Antonio Cabeza López de Tamayo]

Dear Graham, I have add a Three-Phase VI Measurement block to the converter output and superimposed the measured voltage over on the reference voltage waveform. They actually match as you have rightly advised.

Still it leads me to a new query. As you can see, the original power_PWMGenerator3Level file includes measurements of converter output voltage Va by means of a voltage measurement block,

If a compare the lecture of Va measurement block to that of said Three-Phase VI Measurement block, I note that the earlier is a square wave while the latter rather resembles a sinusoidal wave.

Both measurement blocks are set to register the phase-to-ground so they actually should show the same waveform. The only difference I note is that while Va measurement block is comparing phase Va voltage to node 10 voltage, the 3-Phase VI measurement block is comparing phase Va voltage to ground – could you please revise and provide with an explanation?

Thanks in advance and BR!

Juan