But the isolation is cancelled by the red line in post #3?Yes, the DC sources used to supply IO, control board and drivers are isolated from the grid.
Unlikely. What's the connected grids earthing type? https://en.wikipedia.org/wiki/Earthing_systemmidpoint of the DC link which should be at the same potential of grid GND
So you have no problem when connecting the two gnds... except that you do have problems....Thank you for your response.
I have no problem with IO board and control board even when their supply's GND is connected to the negative rail of DC link. The problem is that the SPI fails when the negative rails connection occurs, and I lose programmers (connected to grid's GND via USB and PC) through time ( in a month or so).
What is "the grid's ground"? You mean earth potential?I think I cannot convey my meaning.
control board's supply is isolated and connecting its negative rail to the grid's ground and USB does not cause a problem, theoretically and practically.
To clarify, you are getting "failures" in this configuration (no USB programmer connected)? What do you mean by fail? Do components get damaged, or is the SPI communication just not happening correctly?The situation that the negative rail of the DC link shorted to the control board supply's negative rail is a situation that may happen in working period (while inverter is working, no programmer is connected), this situation also is not dangerous and may not cause a problem because of isolated supply of the control board and IGBT drivers.
Again, what do you mean by "fail" with regards to the programmer. And under what conditions is the programmer connected to the system? Your diagram doesn't show this.However, I want to issue a warning alarm. my problem is kinda strange because SPI fails and programmer fails after a month or even more (more than 200-300 times of programming). there is a problem that I don't know about, but I understand that shorting negative rail of DC link to USB definitely will kill PC and other circuits (it is not the problem). I would be grateful if you share any other comment.
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