Dropped Leg From Utility/Transformer Failure

jmkeng

Member
Location
USA
Occupation
Engineer
HI all - long time lurker, first time poster (lots of collective knowledge around here - such a helpful community!), and hoping to get some feedback on a recent PV issue we've seen at one of our sites.

A PV array is installed on a premises, and feeds into the utility via a net meter.
The utility recently dropped a leg of power, but the inverter failed to shut down and a transformer between the inverter and distribution gear got burnt up.

The utility transformer is a Delta-Wye (208/120, 3P, 4W on the secondary) which feeds the buildings main distribution gear. The PV array is 480V off the inverter, which feeds into a Wye-Delta (Primary/High-side: 480V Wye, no neutral, Secondary/Low-side: 208V Delta, feeding the distribution gear).

The relationship between the Wye/Delta configurations is getting me a little confused; when the utility dropped a phase, could that cause an amperage spike in the two "live" legs and cause the PV transformer failure? I also understand that it can be a little tricky for an inverter to sense a dropped phase from the utility - is that accurate?

Thanks in advance for any feedback.
 

jaggedben

Senior Member
I'm no transformer expert, but since no else has chimed in...

I'm a little skeptical that the inverter wouldn't trip off due to an overvoltage or undervoltage, even if there were other difficulty in detecting the lost leg.

Is there really a single 3-phase inverter? Or is the PV system made up of multiple single phase inverters?

Also this part...

which feeds into a Wye-Delta (Primary/High-side: 480V Wye, no neutral,
No neutral on the PV side, is that correct?
How old is this inverter?

Do you have any idea how long it operated after the utility leg was lost?
 

jmkeng

Member
Location
USA
Occupation
Engineer
Hey jaggedben - thanks for the reply!
After doing some reading, it sounds like if a phase is lost on a delta-side of a transformer that other two legs will still energize the lost leg and, assuming the windings are beefy enough to handle it, the wye-side may not necessarily see any change. Does that sound right?

Yes, it is a 3-phase inverter installed around 2012 or 2013. And correct that there's no neutral connected on the wye side (but it is grounded). Unfortunately I'm not sure how long it operated after the utility phase leg was lost.

Really I'm just trying to wrap my head around how this type of setup would react when trying to backfeed the grid when a leg is down. It almost seems like, if anything, the utility transformer would have been the one to have trouble.
 

BillK-AZ

Senior Member
Location
Mesa Arizona
Does this PV system have remote monitoring? If so, a study of the data may show the time periods, voltages, and currents for the inverter. If the inverter dropped out, the transformer problems may not have been caused by the inverter.
 

pv_n00b

Senior Member
Location
CA, USA
The transformer might have failed due to ferroresonance caused by the loss of the phase and it had nothing to do with the inverter. There are many situations where an inverter will not see a phase loss on the primary side of an interposing transformer. It's an extremely complex area of engineering.
 
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