Thread: Home solar output throttling

PV inverters will usually disconnect from the grid when the grid is out of spec. It is only when you have a battery that you can make use of the PV coming off the roof if you lose the connection to the grid.
Sure would like some batteries but the economics of batteries are still not there for most people. I will go that way for ideological reasons to do my bit to reduce our carbon output.

Originally Posted by slug_burner

PV inverters will usually disconnect from the grid when the grid is out of spec.

They are legally required to disconnect.

Originally Posted by Graeme

They are legally required to disconnect.

Agreed. My point was that when the grid drops out the inverter turns off rather than try to supply the home only. If the grid voltage is too high, does the inverter turn off or try to supply the home only?

Originally Posted by cjc_td5

Agreed. My point was that when the grid drops out the inverter turns off rather than try to supply the home only. If the grid voltage is too high, does the inverter turn off or try to supply the home only?

How can the inverter “try to supply the home only”?

Originally Posted by cjc_td5

Agreed. My point was that when the grid drops out the inverter turns off rather than try to supply the home only. If the grid voltage is too high, does the inverter turn off or try to supply the home only?

It cannot do so unless you have a battery style system with some significant smarts- then it can charge the battery and the battery supplies the home.

A conventional unit is essentially 2 wires attached to the house side of the meter. If it’s producing it contributes and what isn’t “taken” by the house is forced through the meter into the grid.

There’s no control system to lock out the network and regulate power to the house.

Originally Posted by BradC

How can the inverter “try to supply the home only”?

I think Gav (Homestar) made an observation earlier in the thread that the inverter supplied the home first then cut supply to the grid if the voltage was too high. I don't think they are that smart, if the grid voltage is too high the inverter simply turns off? I made the comparison to a "grid off" scenario where there would be an opportunity for the inverter to supply the house only (subject to balancing, brown out and all sorts of issues of course) isolated from the grid, but it chooses to just turn off completely.

Originally Posted by cjc_td5

I think Gav (Homestar) made an observation earlier in the thread that the inverter supplied the home first then cut supply to the grid if the voltage was too high. I don't think they are that smart, if the grid voltage is too high the inverter simply turns off? I made the comparison to a "grid off" scenario where there would be an opportunity for the inverter to supply the house only (subject to balancing, brown out and all sorts of issues of course) isolated from the grid, but it chooses to just turn off completely.

Tombie described it pretty well. These inverters are "grid-tie". They are designed to feed power back into the grid. As they are on the consumer side of the meter, any power consumed locally doesn't register on the meter. Any power produced above that consumed locally causes the meter to record it as being fed back to the grid. Conversely, any power consumed which exceeds the output of the solar system comes from the grid and is metered as power consumed. There is no methodology by which these devices can disconnect the grid from the house, so there is no way they can operate when the grid fails. If they were to operate, they would be trying to power the segment which is disconnected with potentially catastrophic results.

Being able to be on-grid/off-grid is a more complex arrangement with requirements for transfer switches and a whole host of safety interlocks to prevent islanding. No different to the use of whole house generation really.

Likewise, when the grid voltage rises above the threshhold, these inverters have to stop producing to protect the grid (and connected devices) from uncontrolled voltage rise. There's no way to say "the grid voltage is too high, so I'll just supply the house".

The inverter, if correctly configured, will throttle back to effectively produce nothing rather than shut-down, then ramping up quite quickly as the voltage drops either by loads from household appliances or the grid voltage drops. My inverter had incorrectly had its voltage limits changed by the installer attempting to account for a 5-6V drop between the inverter's sensor at the meter box and at the inverter itself some 20m away, but the incorrect parameters caused the inverter to shut-down within 1 second of the voltage hitting the disconnect limit seen at the meter box which then took a few minutes to restart.

After I discussed my suggested voltage limits with the local Essential Energy inspector, the installer reset most of the voltage limits to my suggestions and in 18 months the inverter has only dropped-out once.