Standby Generator and Solar

[mkgrady]

I know very little about solar power connecting to a home’s service so please go easy on me. I wire a lot of standby generators but I’m not sure how generators play with solar inverters. My question is about the generator output being connected to the solar output. If I understand correctly the solar inverter can synchronize with the grid. Can it also synchronize with the generator? Or does the generator output need to be isolated from the solar? If it does what is the reason?

 

[texie]

Grid tie inverters will not play well with a generator and must be isolated. Here is a recent discussion: https://forums.mikeholt.com/threads/isolating-generator-from-solar-system.2543264/

 

[mkgrady]

Thanks for that link. So it is not a code issue. It’s a design issue. For now I will keep the generator output isolated from the solar output.

 

[ggunn]

I know very little about solar power connecting to a home’s service so please go easy on me. I wire a lot of standby generators but I’m not sure how generators play with solar inverters. My question is about the generator output being connected to the solar output. If I understand correctly the solar inverter can synchronize with the grid. Can it also synchronize with the generator? Or does the generator output need to be isolated from the solar? If it does what is the reason?

In addition to what texie said, one must realize that the purpose of grid tied solar is to offset the customer's electric bill. Although it is possible to build a system whereby the solar will run along with the generator during a grid outage, it is expensive, and one needs to run the numbers to determine if the contribution to the customer's bottom line by having the solar run during an outage is worth what it would cost. Virtually 100% of the time it is not even close.

 

[wwstrick]

I just saw on the Solaredge website that they now have interconnection with generators.

 

[jaggedben]

I just saw on the Solaredge website that they now have interconnection with generators.

Interesting, but that application seems to be only for cases where the constant load is large enough to consume the Max PV. It does not seem to address situations where the PV attempting to backfeed the generator would be an issue.

 

[ggunn]

Interesting, but that application seems to be only for cases where the constant load is large enough to consume the Max PV. It does not seem to address situations where the PV attempting to backfeed the generator would be an issue.

Yeah, apparently the inverter goes into a mode where it accepts a wider range of voltage and frequency when the grid is down.

 

[electrofelon]

But would it ever be an issue if the inverters went into a mode where they rolled back their output as necessary to keep V and frequency within a certain window? Seems easy enough and that that would work fine.

 

[synchro]

But would it ever be an issue if the inverters went into a mode where they rolled back their output as necessary to keep V and frequency within a certain window? Seems easy enough and that that would work fine.

It appears that's what they do in the "Alternative Power Source" control mode on page 6 of the document that wwstrick posted. They have a programmable straight line reduction in output vs. frequency offset, and a 5-segment piecewise linear reduction vs. voltage.

https://forums.mikeholt.com/attachments/se-inverter-with-generator-pdf.2551473/

 

[shortcircuit2]

New 2020 Code section 702.5(D) ...

(D) Inadvertent Interconnection. Transfer equipment shall be suitable for the intended use and shall be listed, designed, and installed so as to prevent the inadvertent interconnection of all sources of supply in any operation of the transfer equipment.

Similar language already exists in 700.5(A) and 701.5(A)...but also includes language that permits operation in parallel as does new 2020 section 702.5(E)

 

[jaggedben]

But would it ever be an issue if the inverters went into a mode where they rolled back their output as necessary to keep V and frequency within a certain window? Seems easy enough and that that would work fine.

I stand corrected. The doc says power will be derated first, and then I guess the inverter will still trip off if load is too low (as judged by voltage). There's quite a dearth of explanation though. Seems like you need to be an EE to figure out proper settings. And I still don't imagine generator manufacturers being too forthcoming with warranty coverage.

 

[JoeNorm]

We recently installed a relay that can distinguish between grid power and generator power. When it senses Gen power it opens to prevent the solar from back-feeding.