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What I have, is a common 12-pulse rectifier with its bridges in series and I included a snubber between DC-link and rectifier output. Is important to note that all inductances before the rectifier are small in order to minimize its impact on this snubber performance. Also, I cleaned the circuit from scopes and control of IGBTs in order to visualize in a better way this system.

enter image description here

The main Idea of this snubber is to give a semi-triangular shape to the current by following a reference (each snubber has it reference which is the same shape but shifted in 30°). The background theory and reason for obtaining this currents, is that doing so, I should have sinusoidal currents at the transformer's primary winding or the AC busbar which is connected this rectifier.

Now, the snubber is supposed to work like this: If output current from snubber is greater than reference, then IGBTs must be turned OFF in order to charge this capacitor and lower output current. Otherwise, if output current is less than reference IGBTs must be turned on, discharging this capacitor and increasing this current. Until now, I can only get this working using DC voltage sources instead of rectifiers (250kV each bridge) as you can see in this picture:

And when using a proper 12-pulse rectifier I get this.

enter image description here

Here the tracking of the reference (green) is really great but for some reason the voltage across the capacitor increases until my simulation crashes. The main idea of this snubber is to turn off both IGBTs when voltage across capacitor is aprox 1V in order to reduce losses while switching, however, if I add that condition in both cases (with DC Voltage Sources and 12-pulse rectifier) the output current from snubber (red) takes a long time to discharge when the snubber have to track the lower sections of this triangular reference.

Any suggestion or recommendation would be truly appreciated as I don't really know what to do instead of changing values of my system without knowing why.

EDIT: I'm using PLECS 3.5.2, if you think something in the logic control should be wrong then I can upload pictures of it too.

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  • \$\begingroup\$ Kindly explain where you got this design and why you call this a 'snubber', to me it sounds more like a power factor corrector. \$\endgroup\$ – HarryH Jun 12 '18 at 3:14
  • \$\begingroup\$ You write that "...all inductances before the rectifier are small...", yet I see "L: 1e-3", doesn't that mean 1 mH, and how is that 'small'--or is this common in HVDC transmission? \$\endgroup\$ – HarryH Jun 12 '18 at 3:18
  • \$\begingroup\$ @HarryH 'Method and apparatus to reduce current distortion in line-commutated rectifiers' M.E Villablanca, published in IET Power Electronics (2007). There he called this circuit (IGBTs + Diodes + Cap configuration) a 'snubber' which basically at the time was just an IGBT with huge losses. That is why he designed this 'snubber' configuration to achieve soft-switching. As he is my teacher I just get used to the term, however I would call it a current converter. \$\endgroup\$ – Zrakk Jun 12 '18 at 3:22
  • \$\begingroup\$ Have you got a link to that article please? What I understood is that a snubber is a contraption that reduces switching losses, but I could have missed a more general definition. \$\endgroup\$ – HarryH Jun 12 '18 at 3:25
  • \$\begingroup\$ Your input voltage is 66kV, your transformer looks to me as 1:1, how does each bridge make 250 kV out of this? \$\endgroup\$ – HarryH Jun 12 '18 at 3:26
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The main idea of this snubber is to turn off both IGBTs when voltage across capacitor is aprox 1V in order to reduce losses while switching, however, if I add that condition in both cases (with DC Voltage Sources and 12-pulse rectifier) the output current from snubber (red) takes a long time to discharge when the snubber have to track the lower sections of this triangular reference.

So this is your question?
When the transistor is off, current through the contraption should continue to flow, only against an inverse voltage. I.e.: the diodes should conduct and the capacitors discharge; about as fast as they have charged.
So I think there's a fault in your model and/or steering logic.
I don't have the time to analyse the logic and simulation model, so maybe someone else can help you with that.

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