Why the negative voltage is not displayed?

I try for the example of Implement synchronized pulse generator to fire thyristors of six-pulse converter ( click this link ). If I try to change alpha degree >90, output voltage (Udc) should be negative voltage. But, the negative voltage is not displayed. The result is shown in figure below (alpha degree=120).

So, How I can see the negative voltage? Thanks

Answer 1

[Patrik Ek]

Are you 100% sure of this? It seems to quite counter purpose to have a thyristor convertor outputing negative voltage, since the point with a dc converter is to rectify the current. I can say that it is also what a thyristor converter is designed for. The thyristor converter works similar to a diode rectifier, but you have a better control of the output voltage. By looking at the graph in the link you provided, you can also see that the output voltage is positive. Of course it is possible to back current a thyristor as well, if the voltage is high enough, but then you would indeed have real problems.