Symmetrical components as it relates to inverters

[mcintyrefd]

My understanding of symmetrical components is that it involves unbalanced loads or generation. Inverters are current sources that follow grid voltage and they inject equal current into all 3 phases. So there should be no unbalanced generation from a 3 phase inverter. I am helping a customer fill out an interconnection application with Duke Energy where they are asking for positive, negative, and zero sequence components of the inverter. My explanation to the customer is that the inverter only has positive sequence components.

Is my understanding correct?

 

[ggunn]

My understanding of symmetrical components is that it involves unbalanced loads or generation. Inverters are current sources that follow grid voltage and they inject equal current into all 3 phases. So there should be no unbalanced generation from a 3 phase inverter. I am helping a customer fill out an interconnection application with Duke Energy where they are asking for positive, negative, and zero sequence components of the inverter. My explanation to the customer is that the inverter only has positive sequence components.

Is my understanding correct?

To the best of my knowledge, yes, with one caveat. Some three phase inverters are internally three single phase inverters operating phase to neutral. I would assume that in such cases if one phase fails the other two shut down, but I don't know that as an absolute fact.

 

[pv_n00b]

Inverters can supply fault current, at least until they trip off due to the voltage being out of bounds. The fault current is typically 1.1 to 1.2 times the full load current. What that means is that inverters can supply negative and zero sequence currents to a fault. Personally, I have not seen a utility interconnection application that asked for these values for a static inverter, only for a rotating generator. So I'm not sure what you should put down, maybe a good question for the inverter manufacturer.

Under normal generation conditions, the 3 phase currents are fairly balanced but are not always equal. I walked down a system recently where the 3 phase inverters could display the individual phase currents and they were within 10% but not all the same. I had thought they would be the same but once again the real world failed to agree with me.