Finally got around to reading our Sun Power Inverter manuals ... some 11 months after the install. Their auto disconnect feature is intriguing. Rather than strictly looking for incoming voltage, it seems they look for (and continually sync with) the approximately 60 cycle per second harmonics. If the inverters don't see what they need to see, they won't run. Of course they DO this so during a power outage (either an act of nature or scheduled by the utility company) your PV system won't electrocute someone down stream. What I'm wondering is, if someone were to put in a switch that disconnects one's property from the grid, then feeds 240 to the detached-from-the-grid property, could the PV system be 'fooled' into restarting? I'd be interested in reading up on it if someone could point me in the right direction. Just another wacky idea I'm fooling with. .
I don't see why not. Eg, any standby power source capable of near-sine-wave 60 hz output should work In theory
The inverter really wants grid quality power there before it will turn on. An electrical device designed to fool it would work, but zero power or a mechanical generator won't suffice. Early grid tie systems kicked out a lot because their safety mechanisms enforced the established power quality standards. The problem was that real grid power violated the standards so often that inverters false triggered. I think the biggest problem was the grid voltage running too high, like 125 volts or so. There was a proposal was to save energy by reducing grid voltage down to the established standard values. That didn't fly, because the house at the end of the line also needs some minimum value. The worry was that if you reduced the power next to the transformer to, say, 122 volts, the house a the end of the line could be at 108 volts. And if the voltage goes too low, you burn out things like refrigerator motors. (Actually, the house voltage is about 240 volts, split in two to 120). So, in the real world, the voltage is highest near the transformer, and drops as you get further away. If your house is at either extreme, then the inverter safety mechanism could get trigger happy. The standard was loosened up to better match reality. The safety provided is a bit overrated. The people who work on the power lines don't depend upon inverters successfully disconnecting. There could always be some bozo who plugs their gas powered generator into their house. So the workers just assume that everything is powered and ground it to make sure. But then again, just how much of your neighborhood could a 2000 watt generator power? In order to operate the electrical devices in your house, you need a power buffer, typically a battery. A batteryless grid tie inverter just isn't prepared for the variation in power demand that a house presents. The grid serves the function of an almost infinite size battery.
While you can do so, it is not a simple as it seems. As has been suggested, the inverter needs to "see" pure grid power or else it will shut down in a matter of milliseconds. There is considerable discussion here by some of the most learned EE's and Solar system designer in the world, including folks from, SMA, Outback and Midnight Solar: Grid Tie and Grid Interactive Systems - Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun Rob has the basics correct. A battery based grid tie is what you need. Having said that, a battery based grid tie adds a SIGNIFICANT level of complication to the equation, as well as expense. The battery needs to be big enough to carry the expected load during low sun conditions such as intermittent clouds, all the while maintaining critical line voltage. In the solar business, we generally suggest that it is MUCH cheaper to buy and fuel a stand by generator for those few times that the grid goes down, rather than to try to build what is in essence a large PV powered UPS. Icarus PS On the safety issue. Rob is right that it is bit overrated, but it can and does, and has happened. Sending 120/240 out your line, to the transformer to have it transformed to HV +12kv with enough current to kill someone right now. The reality is that the home generator would usually grind to a halt trying to energize the entire neighbourhood circuit, but if you were the only service on the down part of the line, AND the lineman didn't ground the conductors and open the transformer fuse,,, it could (has) happened! A simple solution is an automatic transfer switch.
Icarus, I know it's not available, but could you describe the minimum system that would allow use of a grid tie installation when the power is out? What I'd like is to be able to use as much power as the solar panels can put out, without any battery system. I accept that it's not going to work at night, or when a cloud goes over, etc. But if you have a several day outage where the sun does come out, it would be nice to make use of the solar power. Maybe you could watch TV during midday to see what's going on. Maybe recharge a few battery devices like a cell phone. Or if it's really powerful, maybe run the refrigerator for a few hours. When my system was inspected it was an overcast day and the inspector made some comment about not getting any power during such a condition. I pointed out that it was producing 300 watts, far below its max, but still a useful number in an emergency situation. The technical problems seem to be grid disconnection, convincing the inverter to operate without the grid, and getting 120 volt power from what is probably a 240 volt inverter. I really don't believe that a grid tie inverter couldn't be programmed to operate during a grid outage. I know they are currently designed to shut down, but an alternative would seem to be to operate a grid disconnect switch and keep operating. You'd probably also need to switch the output to an isolated critical circuits branch circuit. The extra equipment would be far short of a fully functional battery system. Any ideas on such a system?
I asked this question when I was looking into solar for my house and I got the standard answer: you have to have a battery in the system. The explanation that I got was not just that the inverter wants to see 120 coming in, it was more to do with the irregularities in cell output. The system would be nearly useless from an output standpoint because every time a cloud shadow crossed the panels you'd lose your power. So without something to smooth the fluctuations in power output the system would be of little use in any case -- or that's what I was told. I too thought about (e.g.) hooking up a generator to the line side of the system. I figured, hey, to the generator, the inverter would just look like a highly variable load -- generator would idle until the panels went into shadow, then produce power on demand. (Yeah, it's ugly, but the question is, would it work.) Here's what gave me pause. The solar system would not be just like any other variable load. The inverter is programmed to push any excess power from the cells out onto the line. I don't really know what that means, technically, but in practice I think it means that my solar system would be trying to drive my generator. I couldn't see that ending well.
I too, thought batteries would be a perfect fit ... but for the load variation. Wouldn't an isolation diode be the cure to proper current flow?
I'm not quite following that. You can get a factory-made grid-tie system with battery backup. That'll keep working when the grid is down, takes care of all that automatically. But I didn't want 800 lbs of lead-acid batteries in my basement. (Although, I guess, you could get a smaller battery and have lower off-grid peak output.) Even though, as I was told, they last quite a while when used strictly for standby when the grid is down, as in, something like 10 years. So, because I didn't want to go with battery-backup grid tie, I was just wondering whether I could get the system to work, in any way, without batteries, when the grid was down. Just in some extended emergency. Say it's a cloudless day -- wasn't there some way to use the PV system by feeding it some small AC signal and so fooling it into thinking the grid was still up? (That is, put aside for a second the issue of variation in output -- can I get the thing to work at all, even on a cloudless day.) In addition to the issues raised above (by people who clearly know more about it than I do), I think a problem with trying to "fool" the PV system like that is that it would try to dump any excess power it had at any given moment into whatever AC generator I had attached. Think of it this way: get a car battery and a 300 watt inverter and feed the output of that into the grid side of your system, after disconnecting it from the grid. That's a good model for "tricking" the system into working even though the grid is down. Do whatever you want to protect the battery. I'm pretty sure the PV system would end up blowing out the inverter the instant sunlight hit the solar panels. Ditto if the setup was a gas-powered genset instead of battery plus inverter. The upshot is that you can't use a low-power AC source to trick the system. I think the PV inverter will fry it. So I came to the conclusion that my only option was factory-built battery-backup grid tie. As in, yeah, the manufacturers have it right, you can't get around it. Which meant that if I want to run any kind of a big load -- when the grid is down but the sun is shining brightly with no clouds in sight -- I need some big batteries. I don't know this for a fact. The installer I talked it over with said I needed battery-backup grid tie, period. And that seems to be the only commercially available option. So I think that's probably the only way you can get it to work.
Once again, I would refer you to the people who "really" know. Grid Tie and Grid Interactive Systems - Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun (there is a very timely thread going right now on this very subject). I am certainly not expert on grid tie, as all my installations are battery based off grid. As suggested battery based grid tie inverters do exist, but another but perhaps not simpler alternative also exists. Wire in a transfer switch even an automatic one, between the PV and the inverter so that when the grid goes down, the switch switches to the second output on the transfer. Feed this into a battery based inverter and charge controller. Provide a number of batteries to act as your buffer for the load. The problem you are going to run into is the dc voltage of a typical grid tie system is going to be too high for most (if not all) charge controllers, so you are going to have to figure out how to spit the feed from your grid tie PV. Running Pv directly without a battery is a bad idea for a whole series of reasons. Pv is a constant current (amps) device, whose voltage ramps up with insolation. The battery serves to even out the net power (VxA=W) so that the hardware can deal with it. As I suggest, Read the links provided. As I suggest, these guys have forgotten more about solar than most of us will ever know. Icarus PS Like I also suggest, it is generally MUCH cheaper to buy a generator, buy fuel to excercise it once a month for a reliable back up power system than it is to build a dual purpose system. The equation gets murkier IF you have LOTS of power outages that extend for long times. (Not very common, if you REALLY look at your grid's reliability).
Not quite. Most pad mounted residential single phase transformers, like this one, Pad-Mounted will have 2,400 vac primary. A minor dip in primary voltage can translate into a large dip in secondary (120/240 split phase) voltage. A good read is this one, pay attention to page 9, figure 15.12 http://www.mhprofessional.com/downloads/products/0071467890/0071467890_ch15.pdf Hope this clears it up A manual transfer switch is far cheaper. Some panels, such as the dreaded Federal Pioneer StabLok, offer it http://www.homedepot.ca/webapp/wcs/...ToIt&utm_medium=Affiliate&utm_campaign=901371 they also offer a 200 amp 68/24 slot split between primary and emergency Schneider Electric Canada - Solutions, Products and Services in Electrical Distribution and Automation and Control
If you forget to throw the manual switch ... nothing happens. The power remains off, and the backup generator runs without load, fully isolated from the panel A transfer switch is designed to *not* accept power from both inputs at the same time - strictly "or" by design And since the neutral is bonded to the ground for most residential panels, that also means the neutral is bonded at the transfer switch.
