[stevehull]

Long time readers know of my interest in putting up some 15 kW of roof mounted PV in a grid tied configuration here on the farm. That plan is to use EnPhase microinverters.

These inverters automatically shut off if there is loss of grid power. This protects line workers from PV initiated shocks during grid power restoration.

Other readers also know of our recent experiences in an ice storm where we were out of power for several days and I ran the farm on my tractor power take off 16 kW 240 V AC generator. I have a manual transfer switch that I first flipped to prevent back feed of the grid.

All OK . . .

But what happens if I combined the two? Assume the PV system is attached to my farm electrical infrastructure AND I have a Generac generator (or the generic equivalent) that senses grid power loss and turns on to provide generator power within a couple minutes of power loss.

First, the Generac flips an transfer switch and isolates the farm wiring from the grid. The PV system is now shut off as the inverters do not see grid power.

Then, the Generac turns on and provides the now isolated farm electrical system with power. This unit has a voltage modulator and provides only the power required for the farm needs - up to the rated capacity of that generator. If the farm uses only 2 kW it will provide that - if you need 10 kW if will provide that, up to the rated power of the generator.

Here is the problem . . . The PV system inverters now detect AC power and turn ON! If in mid day, the PV will turn on full blast, potentially pushing some 15 kW into my farm electrical system all the while the Generac is also pushing power in.

Something is clearly going to fry!

I am sure I am not the only one that has dealt with this. The simple solution is that you must manually disconnect the PV system FIRST before the generator turns on. That would entail shutting OFF the automatic Generac grid power detection circuit.

Or . . . you can put the PV feed ahead of the transfer switch so that it remains connected to the grid. When the transfer switch goes off, the PV system is attached to the grid and not to the generator.

Or, I could create a huge "dump load" using oven electrical resistance elements together with detection circuitry.

Thoughts from those of you that have PV and a generator?

Bottom line question from an EE standpoint. Is a PV system a current source, a voltage source or a power source? I surmise it is a mix of the three . . .


Steve