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Auto-sensing disconnect of PV from grid??.....  RSS feed

 
John Weiland
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Going over some projected planning for a future PV installation, probably starting with grid intertie and backing away from the grid down the road if need to. One thing I recall when talking to the rural electrical cooperative representative was the fact that some sort of shut-off switch that is accessible to a coop lineman would need to be integrated. This, so that they could shut off the PV power if local lines need to be worked on. I assume that this would mostly be the case when power from the utility was also down for whatever reason. And was then wondering if there was some sort of automatic sensing switch that would kill the feed from the PV panels in the event that power coming from the utility was no longer sensed. I could not find such a beast with some internet perusal ..... has anyone come across such a device? Is manual shut-down the only current way to take care of this issue? Thanks.
 
Steve Oh
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Every grid tie inverter, that I've seen, does this automatically. I believe it is a code requirement for grid tie systems, at least in the USA...not sure where you are located.

Edit: Yes, It's an NEC code requirement. Perhaps search on "grid tie inverter"?
 
John Weiland
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Thanks, Steven. I think it was more than 15+ years ago when I put this question to the Coop representative and the idea of "automatic" did not arise...he seemed to think it had to be done manually, so maybe this has become industry-standard at this point. Two other questions that came up as I was researching this further.

1) If one went with "micro inverters", one per panel, and these were delivering 220VAC to a grid-intertied system, would they auto-shutdown as well? Is some additional box needed for this interface/intertie? I'm sure the coop will have an idea of how it is to meet up with their components at the power pole.

2) I've read that having to service a panel/micro-inverter combo can be a bit of a pain. Since I would be installing this array on a metal roof of an uninsulated building, I was thinking of positioning the micro-inverters inside the roof, just behind each panel. This way, if a panel were to go down, I could test the inverter from inside the building before having to test the panel from the outside. Does this make sense? Can one separate the micro inverter from the panel in this way? Sounds nice that these inverters can be separately monitored for their function using software.

Thanks for helpful comments....
 
Steve Oh
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John, sorry, I don't have any experience with micro-inverters, though I agree that they are a very attractive concept.
Grid tie inverters are pretty standard fare these days, and I've seen "grid tie micro-inverters" advertised, but I don't know if they would meet your local codes or any codes, for that matter. The auto-shutoff feature, to me, would seem absolutely vital. Protecting human life should be a top priority.

Many solar companies have help lines to assist you in designing a system, you might look into that. Just be aware that their knowledge base is likely limited to their offering and may not be applicable to the offerings of other vendors.

You probably already know this, but I'll state it for the sake of others reading the thread. One of the main benefits of micro-inverters is limiting the length of the low voltage DC cables, thus limiting cable losses. Higher voltage and AC have far lower losses than lower voltage and DC, on the same cable size, so micro-inverters, installed very close to the solar panels, convert 12,24,48 volts DC to 110/220 volts AC and that AC is what traverses the long cable run to the breaker panel and building distribution. This is a basic design concept behind efficient solar installations. You always want to minimize the distance the low voltage DC needs to travel, due to losses in the cabling (or the high cost of big gauge cabling) .

Your panel outside, micro-inverter inside is intriguing, if you could do it with very short cable runs on the low voltage side (between the panel and the inverter). Not knowing your design intent, I don't know if it will work well. In my mind I envision a lot of holes in the roof, to keep the runs short, and roof + holes usually = bad. But there are far brighter men than I in this world, and you may have a better plan.

 
R Scott
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Of the micro inverters I have seen (only a few), there was the standardized plugs and short lead. Enough to get the inverter mounted on a separate rack under the panels on an outdoor rack. It would be enough to mount inside the roof. But that may be a lot of holes in the roof to weather seal, depends on the size of the system.
 
Troy Rhodes
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Most grid tie inverters cannot run independent of the grid. There are exceptions that can use batteries and can run independent of the grid at need, but they are rare and complex. You need to have detailed and frank discussion with manufacturers before you go forward. The vast majority automatically detect a grid down scenario and shut down automatically. If a grid tie inverter does not have this feature, you can be sure that the utility will not approve it, and using it will assuredly be breaking several laws and codes.

Also many of the grid tie inverters have no provision for charging/monitoring batteries. Again, a few do, but you need to be 100% sure your supplier knows what you want, and can do all of the needed functions.

Here is one of many discussions of this issue:

http://forum.solar-electric.com/discussion/16885/can-i-use-a-grid-tie-inverter-on-an-off-grid-system
 
John Weiland
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Thanks for recent responses. Yes, Steven, I'm hoping that when the time comes, between the coop having had to ....er..."cooperate"....with more homeowners in recent years on intalling grid-interties, that they will be more schooled at it. Minnesota is a "net metering" state and the public utility commission is pretty strong on getting renewable energies online at the industrial, community, and residential levels. Even the governor of our state was ready to go to lawsuit battle with the state of North Dakota over Minnesota's mandate to have a certain percentage of electrical energy be renewable: Since North Dakota is a big coal producing and coal-sourced electrical generator for the region, they were balking at Minnesota's sudden decreased market for their power. And thanks for the DC/AC differences on line loss. Somewhat humorously, I first came across that concept in a history course where I think it was Edison wanting to go with DC, but someone else noting AC would be better for long-distance power transmission.....Ah, here it is: https://en.wikipedia.org/wiki/War_of_Currents

With regard to your's and R Scott's notion of " roof + holes usually = bad".....LOL.....Yes, I quite agree as would most. What I would envision is one hole centrally located for 4 panels. By using the right combination of cabling, conduit, couplers, and caulk, I think I could get the wiring to the inside of the building with minimal damage and leakiness. If my math is right, which it tends not to be, assuming the panels to be 12V, 4 X ~250W panels would deliver an inverted 220VAC @ ~4 amps total. Initially I would stick with about 8 panels to deliver ~2 kW (8 amps?) during peak output. Could expand in a modular fashion from there if things were working out and finances allowed. All much hand-waving at this point of course...

Thanks, Troy, for that additional information.....in fact I was noting that I was not seeing much battery charging being indicated with these systems. Thanks for the link as well. The one pretty well-known company that does seem to offer integrated storage is Enphase: https://enphase.com/en-us/products-and-services/storage/our-system But I have no experience with them, just noted that they offer this. On the one hand, it would be unfortunate if components of these systems became too "proprietarily integrated".....like Apple computer's *own* OS and *own* software running only or best on it's *own* hardware. (Since I use both Apple and non-Apple systems, I've appreciated how well this integration can work, but it does come at a cost.) And hopefully such integration would not exclude using other storage systems like the Tesla Powerwall or other options coming down the road.
 
frank li
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John Weiland wrote:Thanks for recent responses. Yes, Steven, I'm hoping that when the time comes, between the coop having had to ....er..."cooperate"....with more homeowners in recent years on intalling grid-interties, that they will be more schooled at it. Minnesota is a "net metering" state and the public utility commission is pretty strong on getting renewable energies online at the industrial, community, and residential levels. Even the governor of our state was ready to go to lawsuit battle with the state of North Dakota over Minnesota's mandate to have a certain percentage of electrical energy be renewable: Since North Dakota is a big coal producing and coal-sourced electrical generator for the region, they were balking at Minnesota's sudden decreased market for their power. And thanks for the DC/AC differences on line loss. Somewhat humorously, I first came across that concept in a history course where I think it was Edison wanting to go with DC, but someone else noting AC would be better for long-distance power transmission.....Ah, here it is: https://en.wikipedia.org/wiki/War_of_Currents

With regard to your's and R Scott's notion of " roof + holes usually = bad".....LOL.....Yes, I quite agree as would most. What I would envision is one hole centrally located for 4 panels.


You can usually sneak in under the ridge vent or hang out over the side or eave and leave a strain relief/drip loop telecom/catv/dish installers do. You will mount in a way that allows the wiring to loop down to the wall or soffit without touching the roof. Pay attention to abrasion points and use pv wire or use watertight flex conduit for the around the roof edge bit. The proper solution is an electrical pass-through enclosure or ugly worst case a no calk flashing with an elbow and gobs of calk like i stop the electricians trying to get away with. (Not on my job you dont!)

These are the best of the best and they make one for solar thermal plumbing. The enclosure can be installed under the modules for neatness or be exposed for ease. I have installed about a hundred of these and it they are fast clean and sure. Also they are configurable as combiners with din rail fuses and breakers plus it uses din rail! I like boxes listed as machine panel enclosures because we can do more and access the industrial automation catalogs for enhancing functionality and streamlining appearance.

 
frank li
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"And was then wondering if there was some sort of automatic sensing switch that would kill the feed from the PV panels in the event that power coming from the utility was no longer sensed. I could not find such a beast with some internet perusal ..... has anyone come across such a device? Is manual shut-down the only current way to take care of this issue? Thanks."

All ul, etl, ce, tuv etc., listed grid tie inverters automatically shut down during grid failure and cannot possibly have power on the output side during a power out or line service where linemen disconnect a line or source connected to that inverter. The inverter is echoing the signal from the power line as a beat frequency to sync with and without a beat freq to amplify there is no oscillation and no forward power can result.

The code is ul1471 or 'anti islanding'

http://www.metlabs.com/blog/emc/ul-1741-safety-testing-of-inverters-includes-anti-islanding-requirement/
 
frank li
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John Weiland wrote:One thing I recall when talking to the rural electrical cooperative representative was the fact that some sort of shut-off switch that is accessible to a coop lineman would need to be integrated. This, so that they could shut off the PV power if local lines need to be worked on.


Switching to off and locking out the main disconnect for the system will do the same for anyone doing electrical work on or near your house. Only the inverter ac side conductors need be switched open. The pv can stay connected and in the case of microinverters, switching would be impractical for most situations. But the code advances so here are auto disconnects for pv,ac power,batteries, inverters, whatever and by the good guys at midnite solar.


 
John Weiland
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Thanks for these excellent extra comments, Frank. Yes, I can see now where if the inverter(s) is looking for the echo from the main service that this would serve to kill power feed in the event of a utility power outage. I'm looking forward to putting this all together on the roof of a newer outbuilding with a south-tilting roof. It has a 4/12 pitch to it and I'll have to look into whether the panels will need to be positioned different from this pitch for a northern Minnesota latitude. But it's really good to know that there are solutions for cabling the microinverter feeds under the roof and consolidating some of those feeds inside before the main combined feed (disconnectable) goes to the power pole (if I'm envisioning this right). Like so many things, this technology really seems to be moving rapidly and in a more plug-n-play direction. Again, thanks for your expert input here along with that of the others.
 
frank li
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There is nothing wrong with microinverters inside and modules outside except possibly fat wire and multitudes of wire. If you want to be able to switch the pv module output to a storebought ups for extended outages or other use it can be less expensive and less dangerous than a relay setup and sometimes not.

An enphase system will have a proprietary bus cable to plug inverters into in paralell. I would use it as it is uv and abrasion resistant and has the needed conductors bundled inside one jacket about 1/2" in diameter plus sockets. There are weather-tight splices that are of automotive quality or better and the cables can be combined from all different directions into a common combiner or sub panel with common electrical techniques an equipment. Use jumpers from electrical supply multiconductor tray cable and the engage couplings instead of the $20 every 3 foot with sockets dangling in sight if you are going around/over the edge and down to a wall mounted service disconnect that goes inside and feeds a breaker at the bottom of your house mains panel. Or the soladeck ($65) and through a conduit or on heavy house wire to your mains panel through the house structure, a chase or closet...

Love to help. I stayed outa here because i thought it was a site for dirt castles made of old tires and colorful glass bottles! There is nothing wrong with dirt castles but the site is all about appropriate technology and i love the community here.
 
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