DC-side faults in PV plants

[tricard]

Hello,

I am performing an arc flash study of a PV plant with EasyPower and noticed something curious during analysis. If a DC bus is faulted, there is no contribution from the AC-side of the inverters that I have modeled. This wouldn't be true though as far as I can reason, since if the IGBTs are turned on in the inverter, then they can conduct in either direction. Am I interpreting this correctly? In everyone else's experience, should there be some contribution to the DC fault from the AC side?

Tim

 

[cranky108]

If the electronics are working correctly there should be no contribution from the AC side. Basicly if there is no DC voltage the inverter should shut off.

On the other hand, because most solar panels are current type devices, it won't hurt them to be short circuited, so a fast was to remove a DC fault voltage is with a crow-bar circuit.

 

[Skogsgurra]

You may see a very short contribution from grid via the IGBTs, but all modern IGBT based VFDs have a very fast (microseconds) turn-off function if there is an uncontrolled current rise and that makes the thermal effect of the contribution from the grid very miniscule.

Do you model the VFD in detail? Or are you using a ready-to-go model? The latter may not show detail down to the single electron.Temporal step size may also be important.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[tricard]

Thanks for the help Skogsgurra and Cranky. I am required to use EasyPower in the analysis, which has a limited number of fields to enter data. It includes Full load rating and KVA of the inverter plus short circuit rating (based on a multiple of FLA) and the duration. Just as you were stating, these values are only used when the inverter feeds current into an AC side fault. There are no fields representative of the DC side of the inverter. Since this is inverting DC-generated power, it is just a DC-AC converter, not an AC-DC link-AC converter like AC VFDs. There is surprisingly little to no literature on this topic (at least not free to find on the internet ).

Thanks again!