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For obvious safety reasons my residential PV system disconnects from the grid if it notes the grid is down. The thing is it also shuts itself off so that during a grid blackout rather than providing me power but detaching from the grid the inverter disconnects itself from both the grid and the panels leaving me without power. I want to safely defeat this safety feature that is to say I want to be able to use solar power when the grid is down without frying the guy in the cherry-picker working on the lines to fix them.

Edit to add: I was hoping I could do something more clever along the lines of : 1) manually throw knife switch to disconnect grid from my home. 2) turn on little liar circuit that puts out a drip of 240vac (powered by say a motor cycle batter) to make my inverter think the grid is live and then uses the inverter power feed to keep the 240vac liar circuit powered (as well as the battery charger for the motorcycle battery). Seems safe for the line workers and simple enough to build assuming the inverter will be fooled.

Note that SolarCity now sells a battery pack along with the switch setup I was looking for.

Ram
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    I would get a licensed electrician to do this. – Brian Carlton Nov 14 '12 at 23:02
  • Reasonable advice though I want to understand the solution before I allow someone to come in and implement it. – Ram Nov 14 '12 at 23:10
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    really a -1 on the questionand no reason in the comment - bad form no? – Ram Nov 14 '12 at 23:22
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    Anti-islanding is mandated by regulations and switches which allow islanding need regulatory approval. Islanding detection can be complex and hard to fool with a "non stiff" reference source. – Russell McMahon Jul 03 '14 at 16:24

7 Answers7

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If your inverter shuts down completely when there is no line power, it is probably not capable of putting out AC without AC present. Some inverters assume the power line is essentially a 0 Ω source. They look at the voltage and decide what current to dump onto it. These types of inverters don't actually synthesize the 60 Hz themselves. They count on that being a input.

If you want to be self-powered during a blackout, you have to get the right inverter for that, which probably costs more. Then the inverter also has to deal with not being able to hold up the line to the proper voltage, and what to do about it. It gets complicated, which is why many inveters require the voltage waveform to already be there.

Olin Lathrop
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  • Thanks for that insight Olin. What am I looking for in the inverter specifications to see if that is the case? [It's an Aurora PVI-6000 with all the docs online http://www.power-one.com/renewable-energy/products/solar/string-inverters/aurora-uno/pvi-5000-pvi-6000/series]. – Ram Nov 15 '12 at 00:57
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If your inverter is currently grid-tied, I don't think the anti-islanding feature is something you'll be able to defeat, since it's integral to the safety approval of the inverter (and is a gross violation of most electrical codes)

What it sounds like you need is a separate, off-grid inverter with an automatic transfer switch, that will keep the inverter isolated from the mains when not in use but power your loads after the mains drop out (with some small delay, of course). These schemes generally also involve battery banks to stabilize / provide at least some power when clouds or other solar obstructions occur.

I cannot say for sure if the particular model you've targeted can operate in off-grid mode. (People I know who have this model use it grid-tied.)

Considering the costs involved with a second inverter, you may be better off just going with a gasoline-fired generator for your backup power.

Adam Lawrence
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    The problem with gas fired is I still have to dump substantial $. I was hoping I could do something more clever along the lines of : 1) manually throw knife switch to disconnect grid from my home. 2) turn on little liar circuit that puts out a drip of 240vac (powered by say a motor cycle batter) to make my inverter think the grid is live and then uses the inverter power feed to keep the 240vac liar circuit powered (as well as the battery charger for the motorcycle battery). Seems safe for the line workers and simple enough to build assuming the inverter will be fooled. – Ram Nov 15 '12 at 18:36
  • Hard to say if the inverter would be fooled by that. The anti-islanding feature often minutely adjusts the inverter frequency (to move the phase minutely) to detect how the grid 'pulls' and 'pushes' on the inverter ("infinte sink" or "infinite source" depending on the phase relationship). If the pushing and pulling isn't what the normal grid would do, I doubt the inverter will enable. I don't think there's *anything* safe about a knife switch grid disconnect either, but I digress... – Adam Lawrence Nov 15 '12 at 19:04
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    The knife switch grid disconnect is mandatory for all PV and generator systems where I live (I would have thought it be be NEC) this is in addition to the main service breaker in my main panel. I guess I have some reading to do to figure out how this inverter behaves and how easy it is to fool – Ram Nov 15 '12 at 19:48
  • I don't think a knife switch would fly in Ontario. – Adam Lawrence Nov 16 '12 at 02:22
  • I probably used that term in a less jargony way. I was thinking about my friend's model train set but I suspect the guts of this thing are the same... – Ram Nov 16 '12 at 21:36
  • Knife switch is fine, you dont use it to break the circuit its just for visible isolation like a HV disconnector – Raggles Jul 01 '13 at 23:20
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All grid tie inverters MUST disconnect the A/C supply when the grid is lost. This is a safety measure to prevent injuring hydro employees when they isolate systems to work on them. It should be noted that standard procedures require the linesmen to ground the secondary of transformers they have isolated so your inverter will most likely be shorted out if the hydro company is working on your system, maybe damaging your inverter. When hydro finds out you have defeated the safety feature without informing themyou will be immediately disconnected from the grid and it will cost a bundle to get back on, not withstanding any fines you will incur from ESA. However there are grid tie inverters that provide a separate A/C supply that you can power up a few appliances. This is a separate system and is always powered through the inverter. For significant loads you will need batteries to act as buffers to absorb extra power and provide sufficient power for startup, motors typically require 5 times the current to start as they need while operating. So starting your fridge on solar alone will probably not work, it will need a battery to get it going, after its running the current drops back down and the extra power goes into the battery. You can purchase a grid tie/islanding system. It will include the system you have plus a battery bank and an automatic transfer switch(ATS). The setup will insure that when the grid is lost your house is disconnected from the grid and transfered to the inverter. The inverter will then startup in 5 minutes and will run until the ATS detects the grid. At which point the inverter will shutdown and transfer your house back to the grid and startup in 5 minutes. This system insures that the inverter is NEVER connected to the grid when the grid is down. You will need the system properly engineered, inspected and tested before hydro will allow you to connect it. Keep in mind that a backfed pole transformer will provide 13,000 Volts to the primary if you make a mistake, and easily kill your local linesmen. Steve

Steven
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    Informative. Who is "hydro"? – Ram Jul 28 '14 at 18:01
  • @Ram Your local power company. –  Sep 12 '15 at 01:53
  • If you're going to use non-standard names unique to your situation like "hydro" and "ESA", you should at least say your are giving examples instead of the general "power company" or whatever you actually mean. – Olin Lathrop Sep 12 '15 at 21:30
  • @OlinLathrop I expect that duskwuff is Canadian, and is just now learning that "hydro" is a Canadian regionalism. (EDIT: Oops, four years later. Oh well, I'll leave it! XD) – Glenn Willen Nov 13 '19 at 21:06
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Short answer: AC-coupled hybrid grid-tie configuration.

Long answer: You must add an AC sub-panel for critical load, a "hybrid inverter/charger" and batteries to your system. The hybrid inverter sits between your main AC panel and sub-panel. Your grid tie inverter(s) and critical loads are connected to the sub-panel.

Under normal operation, the hybrid inverter is in standby mode and it's internal automatic transfer switch feeds grid power from your main panel to the sub-panel. It also charges batteries via grid. Your grid-tie inverter feeds the sub-panel, main panel, and grid through the transfer switch.

During a grid outage, the hybrid inverter's transfer switch toggles which disconnects the sub-panel from your main panel. The hybrid inverter now powers the sub-panel via battery and the grid-tie inverter syncronizes with the hybrid inverter's signal. Excess grid-tie inverter power is utilized by the hybrid inverter's charger to replenish batteries.

Pros: Adds battery backup off-grid capability to an existing grid-tie topology.

Cons: Not self restarting - If the batteries are depleted, all inverters shut off and will not automatically restart until batteries are recharged from another source i.e. generator. DC-coupled topology eliminates this limitation but is more complex and expensive.

Erik Friend
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You would need a separate UPS that only provides standby power to a load it can handle and deal with diverting the DC power from the PV to charge the UPS instead of the grid supply. So all the Statement of work is done at the DC side.

Tony Stewart EE75
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I have an Aurora 6000 and just spoke with their techs. What you want can be done according to them. You must have a manual transfer for input and use code compliant disconnect for isolation from grid.

Quality of signal in voltage and frequency is critical as the Aurora will shut itself down even if input signal is out of parameters. Definitely get a qualified individual to help you design and inspect prior to commission.

JRE
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Larry
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  • This answer would be much better with proper spelling ("you" not "u") and more information besides a product recommendation. As it stands, it doesn't say much about electrical engineering. – Phil Frost Jul 01 '13 at 18:27
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The main problem is that the grid-tie inverter expects to deliver all available power from your solar panels to a load - either your home and/or the grid. Aside from being picky about a clean, 60 Hz sine wave, I would expect the inverter to shut down if its output voltage dropped too low (short circuit - grid power outage) or went too high (main breaker open). So without the grid connected, the load would have to be managed carefully to keep the inverter on.

One way to manage load would be to charge batteries, as explained by Eric Friend above, but the batteries would be costly, defeating the point of trying to get value out of your existing expensive system. Trying to balance household load with available solar power would be complicated. But depending on how picky the inverter is, perhaps an inexpensive load, such as resistive heating, could be set up to automatically dissipate unused power.

So for example, if your solar panels are delivering 4kW and you have a clean sine-wave inverter providing 200W from a motorcycle battery (cheap because it is only 200W), and a household load of 2kW, then you could switch on, say, eleven 200W heaters using automated relays to dissipate the remaining 2.2kW of power.

To start such a system, you would turn off the main breaker and turn off all loads to your house except for one 200W load. Then you would turn on the 200W inverter to make it appear that the grid power has been restored to the grid-tie inverter. When the grid-tie inverter turns on, enough (i.e., 4kW) resistive load would have to be immediately connected quickly enough to keep the voltage stable. Then household loads could be connected, provided that equal amounts of resistive load automatically disconnect quickly enough to compensate.

Please correct me if I'm wrong, but I don't think this system would be likely to shock any linemen because, if the main breaker were to be closed, then the entire neighborhood would be loading the circuit, causing the voltage to fall below nominal and shutting the inverted off by its own anti-islanding protection. The 200W inverter would also hopefully shut off when shorted rather than self-destructing. If this is still a concern, it should be much more relevant to the backup generators that people are connecting to their houses in case of a power outage.

Chris Jenks
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  • Regardless of whether you think the risk is low or not it would be illegal not to isolate. Note that the question was asked in 2012. Welcome to EE.SE! – Transistor Nov 13 '19 at 20:29
  • @Transistor: Agreed, and thanks for the welcome. I know the question is old but it is also timely due to the planned power outages affecting millions of people in California. I don't mean to recommend people to do unsafe things, but rather hope they will be informed about the complexities of the current crisis so that technology and the codes governing them can be adjusted to compensate for the direction the weather is taking. I'm looking into solar myself and trying to (legally and safely) avoid the problem the OP is (or was) having. – Chris Jenks Nov 14 '19 at 15:53
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    For the record I am still keenly interested in answers too :) I had not considered the need to dissipate the excess load (I'm not an EE, I'm more of a 5v-24v hobbyist at best). The market has caught up a bit here and folks like Solar City will now sell you a batter backup system that will tie into a Solar PV system so that you have power during outages. Even more interesting would be to tie an electric car to the home system so that rather than dealing with dissipation I could charge my car with the excess power and then draw in over night when the sun isn't doing the trick. – Ram Nov 14 '19 at 17:50