PV array and back up generator

[bluesmoke...oops]

What effects can a PV array have on a back up generator? I'm thinking in terms of performance and load balance.

 

[Real_Bostonian]

If it is a grid tied array it is designed to operate in-sync with utility power. If you lose utility power the array's inverter should kick out. My guess is you would then have to isulate the grid via a transfer switch and then sync the grid with your generator.

 

[K8MHZ]

What effects can a PV array have on a back up generator? I'm thinking in terms of performance and load balance.

I'm not sure I get what you are asking. Circuitry in the PV's create 60 Hz AC so it would be just like any other 60 Hz AC source. When the PV's aren't supplying energy they are electrically disconnected and wouldn't create a load or affect power factor from a generator.

Now, if what you are asking is about using PV and a generator at the same time, the biggest issue is synchronization and it would be far better to use DC to AC inversion, that is using the DC from the PV along with a DC generator (alternator) to charge a battery bank and then invert to the premises AC. That is how we set up emergency field radio stations, except there is no 'premises', the 120 VAC from the inverters is used for stuff in the field. The gennys are noisy so we fire them up when there is no radio traffic and then rely on the batteries and the PV cells during radio operation. This arrangement also allows for seamless transfers.

Of course, we don't know the scale of your endeavor, either, and that can affect the design substantially.

 

[bluesmoke...oops]

I'm wondering the effect of the PV array working at the same time as the generator. Yes once the utility goes the inverter kicks out, but once the gen is up and running, the inverter would see a signal again and turn back on correct?

The system is an 800A, 240V, single phase. The generator backs up the entire facility. The client wishes to add a PV array to backfeed into the system. I'm wondering if there could be sufficient oscillation in the system from the PV array changing it's current output (and if it would happen fast enough) to prematurely wear out the generator. Current thinking on the design end for this is that an isolation switch is warranted to protect the longevity of the generator. I'm wondering what the forum thinks about this.

 

[broadgage]

The inverters used for grid tied PV detect the presence of the grid supply before they will work.
They wont work in conjunction with a small or portable generator since the voltage and frequency are unlikely to be stable enough.

In the case of a large generator, then the grid tied inverter MIGHT work, if the generator has a stable enough output then the inverter wont "know" the difference between grid and generator.

A potential problem exists if the load is less than the PV output, since the generator unlike the grid cant readily absorb power, it can only supply power.

If a generator is rated at say 600KVA, and loaded to say 400KW, then the addition of perhaps 20KW of PV is of no consequence, the generator "thinks" that the load has been reduced from 400 to 380KW.
If however the load on the generator is only 5KW, and 10 KW of PV is introduced, then what happens to the other 5KW? It might trip any reverse power protection on the generator, or it might cause damage.

 

[iceworm]

What effects can a PV array have on a back up generator? I'm thinking in terms of performance and load balance.

If it is a grid tied array .

Yes once the utility goes the inverter kicks out, but once the gen is up and running, the inverter would see a signal again and turn back on correct?

So, it is a grid tied PV tied to a utility, with a transfer switch to a gen? The PV is connected to the load side of the transfer switch?

The system is an 800A, 240V, single phase. The generator backs up the entire facility.

That is an interesting gen. I don't think I have ever seen a 200kw single phase gen. Hummm. Probably is a 300kw, 12 lead. So the driver is likely 500hp.

The client wishes to add a PV array to backfeed into the system. I'm wondering if there could be sufficient oscillation in the system from the PV array changing it's current output (and if it would happen fast enough) to prematurely wear out the generator.

We don't know the size of the proposed PV array, nor anything about the proposed controls - so there is no way to even guess.

Current thinking on the design end for this is that an isolation switch is warranted to protect the longevity of the generator. .

You may well need an isolation switch, but with the limited information we have, I would not tend to base the decision on gen longevity. I would guess that a 500hp driver is going to be hard to jerk around to the point of damage. As for backfeed/reverse power damage, that is what protective relays are for.

A potential problem exists if the load is less than the PV output, since the generator unlike the grid cant readily absorb power, it can only supply power.

We don't know who (or what) is (are) your "design end" - don't know her capabilities, knowledge, or resources. And , of course, we don't have any design specs. So from here, the best I could suggest is controllable micro inverters. That way the controls could switch the PV out in stages to keep a minimum load on the gen.

I'd say you have a really interesting DIY engineering project. And I suspect it will mostly be a controls design

iceworm

 

[bluesmoke...oops]

Yeah it's turning into controls/hook up pain. The client keeps changing their mind about how much of the array they want to feed one building, and how much they want to feed another.

What they did on the generator is have a high leg alternator where the high leg isn't used.

And after talking to another engineer at work, an isolation switch is being put in to prevent harmonic buildup from happening.

 

[iceworm]

And after talking to another engineer at work, an isolation switch is being put in to prevent harmonic buildup from happening.

Holy weasel words, Batman! I can't believe we escaped the harmonic buildup vortex generator.:grin:

One may choose to use an isolation switch because the control issues of getting the PV inverters to play nice with the gen are not easily solveable, but I can't think of any context for using "harmonic buildup" to describe the problems.

ice

 

[bluesmoke...oops]

his view was that he considered both sources to be non linear and was worried about harmonics.

Me, I'd more worry abut the control issues.