Controlling an off-grid solar system

[ScottyNeal]

I am in the process of installing a 15KW solar system. The system will sometimes supply more electriciy than I will use in my house. During those times, the system will send the excess current onto the grid. All this is standard and ok, from my view point.

If the grid is down, the solar system detects and opens the grid connection. Of course, with the grid down, there is no power for the solar-panel inverters, and no power is delivered to the house circuit panel. I have a backup power generator which I could use to power the solar-panel inverters. However, there is no voltage reglator with the system to regulate solar output.

Is anyone aware of a voltage regulator I could either acquire or build to keep solar output consistent with household usage (during hours when solar is generated).

 

[blueroomelectronics]

You can buy a grid tie inverter.

 

[ScottyNeal]

I don't understand. The solar system comes with a grid tie inverter. When the grid is down, (and the solar system is disconnected from the grid), I'd like to power the individual solar panel inverters with my backup generator, and use some kind (unknown) of voltage regulator to control the output of the solar system to keep output at or below the needs of my house.

 

[RCinFLA]

A regular generator will unlikely be or stay within the grid tie inverter -0.7 Hz to +0.5 Hz frequency spec so that is the first show stopper.

Another problem is the grid tied inverter does grid presence testing by attempting to wobble its phase. The grid is like the immovable object and resists any attempt by grid tie inverter to move it phase. A generator will likely not have the low impedance resistance like the grid.

Even if gen stayed within freq spec there is no way to regulate the PV grid tie inverter to back off when loads are less then PV's are producing. This can damage generator.

One way it would be possible is to have an inverter-generator that sensed backwards power flow and moved its inverter frequency outside of grid tie spec to cause PV inverter to shut down. Unfortunately no one makes one like this. All the inverter-generators I have examined (Honda, Yamaha) have their inverter module potted so you cannot even attempt to tap into frequency clock oscillator to modify it.

 

[tcmtech]

You would be best off dumping the solar panel power into a battery bank when the grid is down and then running regular inverters of those to get your power from and just leave the grid tie units off at that point.

 

[ScottyNeal]

The large number of batteries is quite expensive, and so pushes the solution to leaving the solar system turned off (when the grid is down) and using a (propane-fired) backup generator. This is ok for shorter durations, but had wanted to make use of the solar system if the grid was down for an extended period. Seems a shame to not use a power source when nothing else is available.

 

[avoidsanity]

you can buy a charge controller that will regulate the solar panels either 12 or 24 volts. First i'd make a circut that turns on the charge controller when the grid goes down. Then I'd feed that 12 or 24 volts into a ups (battery side). It won't run your entire house but you could probibly dedicate a couple of rooms to it. you can get a ups that has a 24v battery input that could run a couple of kw. The ups will automaticly know when the grid goes down so really all you'd have to worry about is isolating the output side of the charge controller when the grid is up. Also you'll have a couple of batteries in the ups for when the clouds come. Honestly I havn't tried this yet but I plan too. I'm not sure if it would work in your situation but it is a cost concious alternative.

 

[nsaspook]

What you need is a grid-tie/backup system. **broken link removed**
Something like this can sell to the grid or go off-grid with it's own battery backup. You don't need a huge battery array, just a few hundred Ah packs as an energy buffer.

The unless you already have a unit like this it's cheaper just to keep fuel for a generator in case of outages.

 

[bryan1]

Hi ScottyNeal,
As you haven't filled in your location people can only guess, but there is an Australian made Selectronic SP Grid Tie/ Standalone, start the genset when the power goes down, or the batteries need a charge and turn on the toaster and kettle in the morning. Yes if you expect to run a house when the grid goes down a genset is an expensive way to got but a decent bank of deep cycle batteries of a good known brand will serve you well over a decade of FULL time use not just backup.

A 15KW gridtie array won't be cheap to start with so for some capital investment a SP inverter along with with a 24 or 48 volt battery array will be a good investment. Look at it this way, In the morning your batteries are down so it take say 3 hours with 15KW to fully charge them depending on size. Once charged you export PV power to the grid while the inverter keeps an eye on discharge. What does this mean your house is basically off the grid and from the money you'll get out it MIGHT pay the service charge monthly. Factor in 10 years of power bills and the cost of a SP inverter and a set of batteries. What you will find the off grid part will work out cheaper.....

Cheers Bryan

 

[AlexD]

I am in the process of installing a 15KW solar system. The system will sometimes supply more electriciy than I will use in my house. During those times, the system will send the excess current onto the grid. All this is standard and ok, from my view point.

If the grid is down, the solar system detects and opens the grid connection. Of course, with the grid down, there is no power for the solar-panel inverters, and no power is delivered to the house circuit panel. I have a backup power generator which I could use to power the solar-panel inverters. However, there is no voltage reglator with the system to regulate solar output.

Is anyone aware of a voltage regulator I could either acquire or build to keep solar output consistent with household usage (during hours when solar is generated).

It sounds like you have a straight grid-tie inverter (with no battery backup ability). Like RCinFLA said a generator's output is actually not "tight" enough to satisfy the strict sine wave signal that the grid-tie inverter has to see. You would, of course, first want to disconnect your solar (and generator) from the grid itself (using a bypass switch) so as not to electrocute the poor lineman trying to fix the broken utility line.

The product you want, as far as I know hasn't yet been invented or at best has not yet passed UL listing (it'd be great if someone would invent it and bring it to market). You'd want some kind of device that could provide even a weak stable AC signal (to keep the grid tie inverter awake) and simultaneously be able to dump or offload the entire output of your because your grid-tie inverter only knows about converting the entire input of whatever the array is producing (it doesn't know how to back off like a grid-tie inverter that supports battery backup, e.g. from manufacturers like Outback Power and Schneider Electric/Xantrex).

Batteries are expensive, as is replacing your current inverter with one that's capable of working with a battery bank. But if you're feeling the pains of power outages frequently it might be worth the investment in a system and just get a minimal battery bank to power your most critical home appliances (e.g. maybe 400-600 Amp-Hrs?). Then sell yer grid tie inverter used on ebay.