paralleling co-gen units and PV systems

[b2000s81]

How do I tie how to tie a PV system and a generator into a building supply if I am paralleling co-gen units and PV systems into a single building service?:?

 

[jaggedben]

Broadly speaking, if the generator is designed to island (i.e. operate without the utility) then the PV must be tied in on the utility side of the transfer switch.

If the generator is designed to automatically shut down in case of utility power loss, then it may not matter.

More details about your installation would help get you answers specific to your situation.

 

[b2000s81]

thank you for the reply.

 

[harborrick]

Back Feed Breaker

Back Feed Breaker

We installed a 800 amp backfeed breaker with a shunt trip.
When utility shut down power then breakers trips.
That way solar does not see generation power
Only problem is breaker will have to be reset.
Just trying to protect generator with any kind of backfeed from solar.
Hope this helps

 

[pv_n00b]

If you are paralleling co-gen and PV you connect the PV as you would on any PV install taking the total back feed current into account from all generation sources. If the system is designed to island with a backup generator the PV system needs to shut down when the system is islanded unless you are intentionally building a microgrid with multiple sources and the required control systems. How the co-gen is interconnected is typically a complex issue, much more so than interconnecting PV systems.

 

[ggunn]

Interconnect the PV outside the transfer switch or switches that connect the critical loads to the generator so that the PV and generator cannot be connected to each other.

 

[GoldDigger]

Interconnect the PV outside the transfer switch or switches that connect the critical loads to the generator so that the PV and generator cannot be connected to each other.

Perfectly reasonable advice for a backup generator, but the OP is talking about co-gen, which is by nature grid interactive, just as grid tie PV is. With the notable difference that co-gen may normally not try to produce at its maximum output capacity at all times.
Co-gen which is designed to allow islanded operation switches to a voltage regulating operation mode when the grid is not present rather than synchronizing to the grid for both voltage and frequency. Part of the provision for islanded operation may involve load shedding to keep the connected load within the generator's capacity. And, of course, islanded operation will require either a simple transfer switch or a network protection relay to keep from backfeeding a "dead" grid. Actually, a simple ATS will not work in this situation, and instead a network protection relay or other backfeed detector would be required. That would not allow backfeed from PV if the PV were connected downstream of it. Does co-gen commonly involve net metering or does it always have backfeed prevention? If a particular co-gen installation can operate islanded with backfeed protection, one way to combine PV with that co-gen is to put the PV on the grid side of that protection, whatever form it may take.
But a more comprehensive approach would allow the PV to also contribute to islanded operation with a provision to throttle or disconnect the PV when the local load is less than the PV output, to avoid backfeeding the co-gen.

 

[ggunn]

Perfectly reasonable advice for a backup generator, but the OP is talking about co-gen, which is by nature grid interactive, just as grid tie PV is. With the notable difference that co-gen may normally not try to produce at its maximum output capacity at all times.
Co-gen which is designed to allow islanded operation switches to a voltage regulating operation mode when the grid is not present rather than synchronizing to the grid for both voltage and frequency. Part of the provision for islanded operation may involve load shedding to keep the connected load within the generator's capacity. And, of course, islanded operation will require either a simple transfer switch or a network protection relay to keep from backfeeding a "dead" grid. Actually, a simple ATS will not work in this situation, and instead a network protection relay or other backfeed detector would be required. That would not allow backfeed from PV if the PV were connected downstream of it. Does co-gen commonly involve net metering or does it always have backfeed prevention? If a particular co-gen installation can operate islanded with backfeed protection, one way to combine PV with that co-gen is to put the PV on the grid side of that protection, whatever form it may take.
But a more comprehensive approach would allow the PV to also contribute to islanded operation with a provision to throttle or disconnect the PV when the local load is less than the PV output, to avoid backfeeding the co-gen.

A solar company I was with for a while did the solar part of a project which had DC generators connected in parallel with PV on the inputs of 4 hefty inverters. I have no idea how well it worked or how they controlled the output of the generators, and they wouldn't tell us.

 

[texie]

co-gen

co-gen

I think a little clarification is in order here. Co-generation has nothing to do with paralleling with another source. Rather it is producing 2 forms of energy with the same prime mover, typically electricity and heat. The OP is referring to grid interactive/paralleling of a POCO, genset and PV source. The genset may, or may not be a co-gen source. A co-gen system may not even connect to another source in many applications. I realize you probably get that, but co-gen is not the correct term.

 

[ggunn]

I think a little clarification is in order here. Co-generation has nothing to do with paralleling with another source. Rather it is producing 2 forms of energy with the same prime mover, typically electricity and heat. The OP is referring to grid interactive/paralleling of a POCO, genset and PV source. The genset may, or may not be a co-gen source. A co-gen system may not even connect to another source in many applications. I realize you probably get that, but co-gen is not the correct term.

I see this (or something like it) a lot; some people are adamant that they want their grid tied PV system to keep producing power when the grid is down. Whatever you call it, in most cases it's really not worth it, especially if they also have a generator. Run the numbers. How much of the time do they reasonably expect the grid to be down? How much energy, cumulatively, over the life of the system, would the PV system produce during those times, and how much money does that represent? How much would it cost to enable parallel production by the generator and the PV system, i.e., sustainable off grid operation? Subtract the latter from the former. I'll hazard a guess that most of the time it will be a negative number. If the grid is reliable it could be a rather large negative number.