Generator + PV greenfield install

[six73]

I have an inspection client (electrician) who's embroiled in a dispute with a property owner and I'm trying to give him some guidance. The root of the problem revolves around a PV install that the PO undertook separately from the work my client has done, without consultation with my client, and neither my client nor I have much experience with those systems. It's not even clear to me that this setup is entirely sensible, so opinions are welcome. In short, my client installed a new service with hardwired generator and ATS, and then at some point the PO went behind and had a PV system installed and now everybody is pointing fingers at each other about who is responsible for what. Nothing is energized currently (maybe the PV system, but utility power hasn't been connected yet for the service).

Question 1: Does the ATS count as the disconnect for the PV install, or should there be a separate disconnect? My gut feeling is that the ATS would not suffice for that, but any competent PV installer (I don't know for a fact that that's what we're dealing with) should handle that as part of their install, right?

Question 2: Which side of the ATS should the PV feed be on? This also seems like something the PV installer should be able to answer, but the situation devolved very quickly and now they're refusing to even speak to my client. Is this something that the utility provider should specify? I am aware of article 705 for interconnected power production systems, but does that apply in this case?

 

[ggunn]

Question 2: Which side of the ATS should the PV feed be on?

The PV system must interconnect on the line side of the ATS. If it were to be connected on the load side of the ATS the PV system could backfeed the generator, which would be bad news for the generator.

 

[jaggedben]

What he said. ^
Another option could be to have a contactor drop the PV system when the grid goes out, but that's hardly preferable.

Also the ATS cannot be the PV disconnect.

 

[six73]

The PV system must interconnect on the line side of the ATS. If it were to be connected on the load side of the ATS the PV system could backfeed the generator, which would be bad news for the generator.

Thanks. I kind of thought that was the case, but wanted to confirm.

 

[Joethemechanic]

The PV system must interconnect on the line side of the ATS. If it were to be connected on the load side of the ATS the PV system could backfeed the generator, which would be bad news for the generator.

Not that I'd do it, but would the voltage and frequency from the inverter stay in step with the generator output?

 

[Birken Vogt]

The solar will drive the generator faster like a motor. At some point it will disconnect because V and Hz are out of spec. Then the gen will slow down. The solar will reconnect. Cycle repeats. Some solar systems will lock out on a permanent fault after many of these cycles. Also bad things could happen to the generator. Uncharted territory. Should not be tried to see what happens. Solar must be tied to utility at all times.

 

[BackCountry]

What he said. ^
Another option could be to have a contactor drop the PV system when the grid goes out, but that's hardly preferable.

Also the ATS cannot be the PV disconnect.

If the ATS is service entrance rated, then you’d want a line side tap ahead of it, and your utility will likely have some special requirements on top of the AHJ requirements.

Not a big deal though, pretty common, hopefully you/they are able to tap right by the existing ATS and keep it simple tap rule wise.

In my area, the PV AC disconnect will have to be line of sight and 10ft or less from the meter, with appropriate placarding. If greater than 10ft it requires a variance, again this is the utility. Yours may be different.

 

[ggunn]

Not that I'd do it, but would the voltage and frequency from the inverter stay in step with the generator output?

There is no reason why it wouldn't; within limits a PV inverter matches the voltage and frequency that it sees at its terminals. The problem arises when the generator is running and the demand from the loads falls below the output of the PV system. In that case the PV system will try to backfeed the generator, and most generators will not tolerate that; something will fail.

There are ways to coordinate PV systems with generators but all the ones I know of include batteries, battery inverters, and special devices in place of a traditional ATS. It's not cheap or simple. If you do a cost - benefit analysis of setting up a PV system that will play nice with a generator, unless grid outages are frequent and of long duration, you will likely find that it isn't worth the trouble and expense.

 

[ggunn]

The solar will drive the generator faster like a motor. At some point it will disconnect because V and Hz are out of spec. Then the gen will slow down. The solar will reconnect. Cycle repeats. Some solar systems will lock out on a permanent fault after many of these cycles. Also bad things could happen to the generator. Uncharted territory. Should not be tried to see what happens. Solar must be tied to utility at all times.

I don't think that's correct.

 

[Joethemechanic]

The solar will drive the generator faster like a motor. At some point it will disconnect because V and Hz are out of spec. Then the gen will slow down. The solar will reconnect. Cycle repeats. Some solar systems will lock out on a permanent fault after many of these cycles. Also bad things could happen to the generator. Uncharted territory. Should not be tried to see what happens. Solar must be tied to utility at all times.

I was kind of thinking something along those lines. I was thinking voltage would go up, field excitation would drop, with the possibility of slip.

Then the inverter not seeing power would do something (probably bad). Being I don't really know solar inverters, I hit a dead end

Typically if you are parrelling (lets say a generator to the grid) you match voltage and frequency, get them in sync, close the switch, and then adjust the governor to get to your desired output.

I have no idea how solar inverters sync to the grid. and then start delivering power

 

[hillbilly1]

It would be like paralleling generators, except one would be a solar generator. The inverter should sync just like it does with the utility, but as others have said, what happens when the load drops below the output of the solar generator?

 

[electrofelon]

I was kind of thinking something along those lines. I was thinking voltage would go up, field excitation would drop, with the possibility of slip.

Then the inverter not seeing power would do something (probably bad). Being I don't really know solar inverters, I hit a dead end

Typically if you are parrelling (lets say a generator to the grid) you match voltage and frequency, get them in sync, close the switch, and then adjust the governor to get to your desired output.

I have no idea how solar inverters sync to the grid. and then start delivering power

The voltage, and even moreso frequency limits, (59.3 to 60.5 hz) on a grid tie inverter are quite tight. I would guess the inverter will shut down long before it damages the generator. I would also conjecture a majority of the time the inverter won't even connect to the generator. ( But I'm still not trying it).

 

[electrofelon]

Regarding the OP, the situation seems cut and dry to me. If the electrician did the service upgrade and ATS first and was not told to do anything specific in regards to providing for the PV system, then the PV installer is is responsible for properly connecting the PV system. If he connected it to the load side of the ATS than that is on him. I guess there is the possibility that the PV installer looked at it and designed it before the ATS went in and didn't notice that that work had been done upon install (seems unlikely).

 

[ggunn]

I was kind of thinking something along those lines. I was thinking voltage would go up, field excitation would drop, with the possibility of slip.

Then the inverter not seeing power would do something (probably bad). Being I don't really know solar inverters, I hit a dead end

Typically if you are parrelling (lets say a generator to the grid) you match voltage and frequency, get them in sync, close the switch, and then adjust the governor to get to your desired output.

I have no idea how solar inverters sync to the grid. and then start delivering power

Why would one ever try to parallel a generator to the grid? It seems to me that unless the generator has a phase locked loop to keep it in synch with the grid it will not stay in perfect synch no matter how well you match it initially. PV inverters match voltage, frequency, and phase with whatever AC voltage they see at their terminals within their operational limits. If it strays out of their limits, they shut down.

 

[ggunn]

Regarding the OP, the situation seems cut and dry to me. If the electrician did the service upgrade and ATS first and was not told to do anything specific in regards to providing for the PV system, then the PV installer is is responsible for properly connecting the PV system. If he connected it to the load side of the ATS than that is on him. I guess there is the possibility that the PV installer looked at it and designed it before the ATS went in and didn't notice that that work had been done upon install (seems unlikely).

If a customer has an existing simple grid tied PV system and decides to add a generator later, the PV system must be rewired to accommodate it. If the PV system is line side connected the rewiring is simple; you just install the ATS such that the PV interconnection is on the line side of it. If the PV system is interconnected through a backfed breaker, then it's a bigger job.

 

[electrofelon]

If a customer has an existing simple grid tied PV system and decides to add a generator later, the PV system must be rewired to accommodate it. If the PV system is line side connected the rewiring is simple; you just install the ATS such that the PV interconnection is on the line side of it. If the PV system is interconnected through a backfed breaker, then it's a bigger job.

In the OP's case, it sounds like the ATS went first.

 

[Joethemechanic]

Why would one ever try to parallel a generator to the grid?

I was just using the grid as an example of a large generator and load. Typically it would be two trailer mounted portables hooked in parallel with no grid connection. And with droop type governors one will typically lead a bit until a load is connected

 

[ggunn]

I have an inspection client (electrician) who's embroiled in a dispute with a property owner and I'm trying to give him some guidance. The root of the problem revolves around a PV install that the PO undertook separately from the work my client has done, without consultation with my client, and neither my client nor I have much experience with those systems. It's not even clear to me that this setup is entirely sensible, so opinions are welcome. In short, my client installed a new service with hardwired generator and ATS, and then at some point the PO went behind and had a PV system installed and now everybody is pointing fingers at each other about who is responsible for what. Nothing is energized currently (maybe the PV system, but utility power hasn't been connected yet for the service).

Question 1: Does the ATS count as the disconnect for the PV install, or should there be a separate disconnect? My gut feeling is that the ATS would not suffice for that, but any competent PV installer (I don't know for a fact that that's what we're dealing with) should handle that as part of their install, right?

Question 2: Which side of the ATS should the PV feed be on? This also seems like something the PV installer should be able to answer, but the situation devolved very quickly and now they're refusing to even speak to my client. Is this something that the utility provider should specify? I am aware of article 705 for interconnected power production systems, but does that apply in this case?

I am not sure what the problem is unless the PV system is already load side connected through a backfed breaker in the MDP. If it is it must be changed to a line side interconnection on the line side of the ATS, but either way it is a simple decision and there is no reason for unprofessional animosity between the installers. The property owner is ultimately responsible for any incurred expense, since he apparently approved the PV system installation without considering what effect a generator would have on it.

 

[ggunn]

In the OP's case, it sounds like the ATS went first.

If that is the case, then it is on the PV installer to design the solar interconnection correctly so that it cannot backfeed the generator. The property owner might be insistent that the PV system continue to run during an outage, but if that is the case it is incumbent on the PV installer to explain to the PO why this would be a bad idea. If the PV installer does not understand this, then I cannot help.

 

[Birken Vogt]

I don't think that's correct.

I've seen it happen. Lots of trunk slammers interconnect generators and solar out here like that and then we get called on to fix it.

An engine driven generator can also be a motor and it will drive the engine faster than spec. The engine controls will see the overspeed and go closed throttle but speed just keeps rising. It doesn't rise very high before the sensitive routine in the inverters go off spec and drop.

I can't point to any damage caused but I would like to hear any other scenarios you think may happen as an EE. I would love to have more ammo to point at people who say, it's OK don't worry about it.