Grid Tied Inverter With a Backup Generator

[Sparks.4.All]

I want to install a Power One PVI 3.0 grid tied inverter. The house has a Gereac automatic generator installed out at a pump house. The automatic transfer switch is located out there as well. The ATS then feeds a 200A panel located inside the house. The generator is clean enough to activate the inverter. I'm concened that this would damage the generator when backfeeding it from the house. I cannot guarentee the house would use all the power produced by the inverter. Installing a relay controlled by the grid side of the transfer switch is not feasible because the switch is remote from the home. Anyone have any thoughts about this? Any thoughts would be helpful.

 

[SolarOH]

Solar Pro has an article that covers the interaction between Solar inverter and Backup Generator.

Here is a copy: (From Solar Pro Magazine Issue 2.5, Aug/Sept 2009)
View attachment Solar Pro Generator Q and A.pdf

For a home, if you are concerned at all about the inverter providing more than the home needs, then I'd find a way to keep the inverter separate from the generator. This is my standard practice. I've done this on an industrial facility, with no issues so far (only a few months old though). But in that case it was a 60kW inverter, paired with a 1MW generator on a university campus coal heating plant " Irony?" where the load would never approach something as small as 60kW.

 

[iceworm]

I want to install a Power One PVI 3.0 grid tied inverter. The house has a Gereac automatic generator installed out at a pump house.

... I cannot guarentee the house would use all the power produced by the inverter.

... Any thoughts would be helpful.

It looks like an economics problem. Here's what I think are the issues:
The gen only runs if the utility power is out.
The inverter will tie to (parallel) the gen as though the gen were the utility.
The inverter may put out more power the the house will take.
This will reverse power the gen.

If I got this right, here are a couple of thoughts:

There are bluetooth and radio links to solve the distance issues. Put in multiple small inverters and a plc. Switch the inverters in and out as needed to maintain some generator output. Surely less than $10K.

Is the issue to use the solar array to offset the generator fuel during a power outage? If so, and the utility outages are not very long, maybe the savings are not enough to pay for the equipment. Just shut off the inverter if the utility goes out.

Interesting project - wish I could be more helpful

ice

 

[jaggedben]

If a relay from the ATS to the house is not feasible, then no other reliable and safe solution is likely to be feasible at a better price, let alone as simple. You've made the question really hard by stipulating that that is not feasible. How far is the distance anyway? Is it really too far to install a low power, low voltage circuit? (Note that the inverter you have specified has a pair of terminals for Remote Control that could be connected to a relay with a very small rating.)

A radio link is certainly an idea, but personally I would not want to rely on a radio link to keep my generator from being damaged. Another idea would be to choose an off-grid capable inverter instead, but that would likely cost more (for batteries) than you would save by not digging a trench to the pump house.

So I think that leaves one other set of questions to consider: Can you not put the solar system near (or on) the pump house? Is there a site there with suitable sun exposure? And if it can't go on the pumphouse itself, would turning this into a ground-mount cost more or less than digging a trench?

 

[jaggedben]

One further thought: I'm not even sure that the relay control circuit would have to be installed in a trench. You could maybe use overhead poles or something like that.

 

[ggunn]

The safest way to do this is to have your interconnect outside the ATS so that when the grid goes down the PV shuts off. If the grid doesn't go down that often or for very long at a time, the loss of the contribution from the PV during those times would most likely be inconsequential.

 

[BillK-AZ]

There are two sides to this problem.

First you need to determine how to shut off the inverter when the grid is not present, then you need to implement this with Listed equipment and parts to the satisfaction of the local AHJ and perhaps the utility.

I had a related problem with an off-grid home with stand-alone inverters that could backfeed to the grid (Trace SW4024) and a backup generator with a transfer switch. The generator and garage were located 150' from the home (to reduce noise) and turned out to be closest to the grid when the grid eventually arrived. When the home was constructed they installed two conduits, one for the 60KW generator and another smaller conduit for lighting circuits and power to the pump that was near the generator. When the grid was connected at the generator we converted the lighting circuit to a control circuit (power was now generally available for lights) and powered a contactor on the grid output of the inverters from the grid such that the inverters were disconnected when the grid was not present. The major problem we had was with the utility and their requirement for a visible blade lockable safety switch on the PV inverter output. The compromise was a whole service safety switch (200A) the utility could lock out if needed. It did not matter to the homeowner because the standalone inverters and generator would automatically supply power to the home if the utility went down or the disconnect was locked off. It also took some discussions to have the AHJ accept the contactor assembly because it was not a Listed assembly, but assembled from recognized components by a UL508 shop.

The situation on this thread is more complicated due to the lack of a means to detect grid presence at the inverters. Suggestions are a radio or carrier current link to get a contact closure that can then activate a contactor on the inverter output.

 

[Sparks.4.All]

I do not want to use any kind of radio to control the contactor. The 400 Amp service and disconnects are located next to the well house. Does the code prevent me from using the same conduit as the feeder to the house? The control wiring would be either 240V or 120V. The feeder has an over-current device at the well house.

 

[jaggedben]

Does the code prevent me from using the same conduit as the feeder to the house?

I 'm not sure if I can follow what you mean. Install a separate feeder to the house just for the inverter? I think this would be allowed if the conduit is large enough. It might work. I didn't suggest if before because I assumed you wouldn't want to re-pull the existing feeder.

The control wiring would be either 240V or 120V.

What control wiring? This doesn't seem to jive with your question above. It would seem to be a different strategy.

 

[ggunn]

There are two sides to this problem.

First you need to determine how to shut off the inverter when the grid is not present, then you need to implement this with Listed equipment and parts to the satisfaction of the local AHJ and perhaps the utility.

Possibly I am not fully understanding the problem, but if the interconnect is on the grid side of the transfer switch there is no additional equipment necessary. When the grid goes down the inverter(s) shut down and the generator powers whatever loads are on the generator side of the switch. As discussed above there are potential problems if the inverter interconnect is on the generator side of the switch.

Some folks want to allow the PV to continue operating during a grid outage by placing the interconnect on the generator side of the switch, but IMO the benefits of doing this in most cases are not worth having to deal with protecting the generator.

Or am I missing something?

 

[jaggedben]

Or am I missing something?

That the OP wants the inverter in a remote location from the ATS, and is therefore inquiring about different options?

 

[Sparks.4.All]

The house comes directly off the transfer switch. So it is fed by the grid or by the Gen. The inverter and modules will be installed on the house. I was thinking of istalling a contactor that cuts power to the inverter when the Gen kicks on. I would need to rus control circuits off the grid side of the transfer for N/O or tie to the Gen side for N/C on the contactor. Then try to run the line voltage control circuit throught the feeder conduit.

 

[ggunn]

The house comes directly off the transfer switch. So it is fed by the grid or by the Gen. The inverter and modules will be installed on the house. I was thinking of istalling a contactor that cuts power to the inverter when the Gen kicks on. I would need to rus control circuits off the grid side of the transfer for N/O or tie to the Gen side for N/C on the contactor. Then try to run the line voltage control circuit throught the feeder conduit.

I assume that means you have the inverter backfeeding a breaker in your MDP and the transfer switch is between the service and the MDP. You cannot connect the inverter through a supply side tap between the transfer switch and the service?

 

[jaggedben]

The house comes directly off the transfer switch. So it is fed by the grid or by the Gen. The inverter and modules will be installed on the house. I was thinking of istalling a contactor that cuts power to the inverter when the Gen kicks on. I would need to rus control circuits off the grid side of the transfer for N/O or tie to the Gen side for N/C on the contactor. Then try to run the line voltage control circuit throught the feeder conduit.

Okay. Sounds acceptable to me as long as you don't have a conduit fill or derating issue.

There is an argument for connecting to the grid side and using a N/O contactor. That way if either the relay or the control circuit fails, the inverter will be positively cut off. Whereas if you do it the other way, the control could fail without anyone knowing until the next power outage and possible damage to inverter or generator. Just make sure that the control circuit is cut in on the load side of the (non-ATS) grid disconnecting means in case someone wants to denenergize all the conductors in that conduit.

I will again repeat that the inverter you've mentioned (at least some versions of it) has dedicated terminals for remote control, so you could use a relay rated for way, way less than the power output of the inverter. Look in the manual for remote control.

 

[Sparks.4.All]

I spoke with the customer today. He said he wants a siwtch that shuts off the inverter when there is a power outage. Then in order to turn the inverter back on he would press a manual switch to restart the Inverter. I looked briefly online and saw a few devices used to shut off gas lines that do exactly this. I would still need to istall a contactor in addition to the device. Does anyone know of another alternitive that may be an Ice cude relay that accomplishes the same thing? It would need to be 20A, 2 Pole.

 

[electrimate]

Get a three pole relay and a normally open Start pushbutton switch. Use the third pole of the relay as a latching contact. You will have to push the Start button to turn the inverter on. When it loses power, the relay will drop out.
Or use a standard two pole relay and use the second pole to power the contractor.
Here is a diagram of a latching relay for a motor starter. Just don't use the Stop unless you want a way to shut down the inverter.
Use the other two poles for the inverter control.

 

[Sparks.4.All]

That's what I ended up doing. I was overthinking it. I used an ice cude latching relay that controls a general duty contactor. Installing an LED to let the customer know when the contactor has lost power. Thanks guys

 

[Sparks.4.All]

The inverter will be tied into the house panel, which is fed from the output from the transfer switch, which is located out at the well house.

 

[DanS26]

I want to install a Power One PVI 3.0 grid tied inverter. The house has a Gereac automatic generator installed out at a pump house. The automatic transfer switch is located out there as well. The ATS then feeds a 200A panel located inside the house. The generator is clean enough to activate the inverter. I'm concened that this would damage the generator when backfeeding it from the house. I cannot guarentee the house would use all the power produced by the inverter. Installing a relay controlled by the grid side of the transfer switch is not feasible because the switch is remote from the home. Anyone have any thoughts about this? Any thoughts would be helpful.

"The generator is clean enough to activate the inverter." Are you sure about this statement? Almost all, if not all inverters, listed by US and Canadian authorities will detect any non-utility voltage sources.

These listed inverters will shut down automatically without utility power, thus your issue is a non-issue. I seriously doubt that any Generac home based generator will produce enough clean power to confuse a listed inverter.

 

[ggunn]

"The generator is clean enough to activate the inverter." Are you sure about this statement? Almost all, if not all inverters, listed by US and Canadian authorities will detect any non-utility voltage sources.

These listed inverters will shut down automatically without utility power, thus your issue is a non-issue. I seriously doubt that any Generac home based generator will produce enough clean power to confuse a listed inverter.

Actually, that's not necessarily true. In a laboratory setting I have gotten a Sunny Boy, a Sunny Island, and a Generac generator to play nicely with each other in a small off grid system. Initially, the Sunny Island was acting as the grid and the Sunny Boy didn't know the difference. When the batteries dropped below the SOC trigger point I programmed in, the Sunny Island started up the generator and went into bypass mode, and the Sunny Boy kept running. The Sunny Boy recognized the power from the generator as grid and kept producing power to assist in charging the batteries and running the AC loads. When the batteries were topped off, the Sunny Island shut down the generator, went into inversion mode, and munged the frequency of its output to throttle back the Sunny Boy. It's a pretty cool system.

But please note that there are batteries and a battery inverter (Sunny Island) at the heart of the system. The Sunny Island prevents the Sunny Boy from backfeeding the generator.