Generator vs solar

Zee

Senior Member
Location
CA
Can a grid-tied solar homeowner have a backup generator installed that turns on automatically in an outage?
What happens when there is a power outage?
Is there a danger the solar continues to feed....because generator automatically supplies voltage?
 

jaggedben

Senior Member
Usually the approach is to connect the solar on the supply side of any transfer switch for the generator.

It gets more interesting if there's also battery backup.
 

GoldDigger

Moderator
Staff member
Can a grid-tied solar homeowner have a backup generator installed that turns on automatically in an outage?
What happens when there is a power outage?
Is there a danger the solar continues to feed....because generator automatically supplies voltage?
Assuming that the generator output is stable enough at nominal voltage and frequency and has a low enough impedance to satisfy the anti-islanding tests to allow that GTI to run, the problem (yes, danger) will come whenever the local load is less than the power output of the panels at current light conditions. The grid interactive inverter has no provisions for throttling its output so it will attempt to backfeed current into the generator. This can cause serious problems, depending on the details of the auto-throttle and voltage regulation systems of the generator. The best outcome is that the generator frequency will go out of range and the grid interactive inverter will shut down. The worst outcome will be damage to the generator.
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
Assuming that the generator output is stable enough at nominal voltage and frequency and has a low enough impedance to satisfy the anti-islanding tests to allow that GTI to run, the problem (yes, danger) will come whenever the local load is less than the power output of the panels at current light conditions. The grid interactive inverter has no provisions for throttling its output so it will attempt to backfeed current into the generator. This can cause serious problems, depending on the details of the auto-throttle and voltage regulation systems of the generator. The best outcome is that the generator frequency will go out of range and the grid interactive inverter will shut down. The worst outcome will be damage to the generator.
How is this possible with the inverter and utility isolated from the generator by a transfer switch?
 

jaggedben

Senior Member
How is this possible with the inverter and utility isolated from the generator by a transfer switch?
He's describing what would happen if you connected the solar on the load side of the transfer switch, and then the generator came on.

Usually a load side connection is much easier than a supply side tap so one might be tempted to do it if one didn't understand how it could go wrong.
 

GoldDigger

Moderator
Staff member
He's describing what would happen if you connected the solar on the load side of the transfer switch, and then the generator came on.

Usually a load side connection is much easier than a supply side tap so one might be tempted to do it if one didn't understand how it could go wrong.
:thumbsup:
The OP stated:
Is there a danger the solar continues to feed....because generator automatically supplies voltage?
That sounds to me as if the OP is considering what would happen with a PV connection on the load side of the transfer switch. Most likely a backfed breaker in the main panel.

BTW, if the inverter is load side connected, I would expect it to drop out when the switch transferred and then go into the normal delay to qualify the new source of "grid" power, but I can't rule out that some might ride through the transfer.

If it is not practical to make the PV interconnect supply side at the service, then an alternative is to add a critical loads panel downstream of the transfer switch with the PV connected at the next level up (typically the main panel.) Or I imagine that the generator ATS's load shedding capability could be leveraged to disconnect the inverter.
 
Last edited:

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
How is this possible with the inverter and utility isolated from the generator by a transfer switch?
It's not; the inverter is on the line side of the ATS. When the grid goes down the inverter stays connected to the grid and shuts off.
 
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