How to to Isolate Propane Generator from Solar Output
unca_bill
| Posted in GBA Pro Help on
I am planning on installing a 10kw solar array to my grid-tied installation. I also want a propane backup generator for extended outages (we get the occasional multi-day ice storm with overcast skies). I’m looking for options to isolate the generator from the solar output. I already have an automatic transfer switch installed on the meter rack near the generator site.
At this point, all I can figure out is to manually disconnect the solar before manually connecting the generator. This defeats the automatic features of the backup generator in the event that I’m away.
Advice?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Effectively you want a grid tied solar inverter connected in what's called a "line" or "supply" side tap/disconnect configuration.
This means that both the solar inverter and the generator ATS "see" the status of grid power. In normal situations, the generator will be off and the solar system will be on, syncing with the grid.
If grid power fails, the inverter will power down and stop providing power as designed. The ATS will sense no grid power and fire up. But because the solar connection is on the line side of the ATS, it will not mistakenly try to feed the system.
If you have a “service rated” ATS with a main breaker in it, it may be difficult to tap off the solar system ahead of the ATS (you can sometimes add lugs between the main breaker and the ATS contractor, but that depends on the design of your particular UPS). If you have such a system, you can probably still make things work. Most ATSes have a relay contact that closes (or opens) when on “emergency” generator power (note that this is different from the “start” contact that starts the generator). Use that contact to feed an inhibit/shutdown signal to the solar system inverters to keep them locked out while you’re running on generator power.
If your ATS doesn’t have a suitable contact like I describe, you still have another option: get a relay with a 120 or 240 volt coil, and connect the coil directly to the generator power input connection to the ATS. This way the relay is activated when generator power is present, and that relays contacts can be used to signal the solar inverters to shut down. Grainger has many suitable relays. I recommend using an octal base relay and a matching socket so that you can install it inside the ATS enclosure easily. Note that it’s a good idea to put fuses in series with the coil if you connect the relay as I describe. Grainger has suitable fuse holders too. You want to use a suitable fuse for this application, something like an FLQ fuse of maybe 1/2 amp too, not one of the glass body fuses commonly used in electronic devices.
Bill
I was figuring a trough ahead of the ATS, or simply some Polaris (or similar) connectors inside the ATS housing.
In the case of a service rated ATS, you can’t tap off ahead of the main breaker in the ATS unless you’re adding another “main” breaker (the code lets you have up to 6 “mains”). I recommend against this though. It’s preferable to tap off after the main breaker by adding some extra lugs, or putting in a separate main disconnect ahead of the ATS and tapping off after that.
I think a control-side interlock would probably be both cheaper and easier to implement than anything on the power side though, especially if this is going into and existing system.
Bill
> In the case of a service rated ATS, you can’t tap off ahead of the main breaker in the ATS unless you’re adding another “main” breaker (the code lets you have up to 6 “mains”).
Not saying you're wrong, but can you provide a code reference for this? I think what I described is pretty common in residential solar + ATS installations. If it's verboten, that'd be good to know.
I meant you can’t tap off without adding another breaker or fuse (an OCPD), not that you can’t tap off at that spot at all. Maybe I wasn’t clear, and I’m thinking that’s why you’re asking? If you tap off you run into the tap conductor ampacity rules but you’re otherwise ok. The part about “not more than six mains” I could dig up if that’s what you’re asking about.
Personally, I like to have a “main” main, so there is one main that really does shut everything off. In commercial services this is sometimes difficult to do, but it’s easy with residential services. That’s my preference though, the code doesn’t prohibit multiple mains in residential services as long as you don’t exceed the magic number of six of them.
Bill
Thanks guys, I have a meeting with the generator guy and the solar guy tomorrow. The solar guy is a master electrician so I'll pass your suggestions on to him.