Grid Tie Solar or Wind Turbine with Backup Generator

mnittler

I have 5.1kw grid tie Solar, 2.5kw grid tie Wind Turbine, and Solar hot water heater. No batteries, straight grid tie. I am adding an 18RES 18kw Kohler propane backup generator to the mix (with electronic governor and voltage regulator, 0.5% regulation between 0 to full load per specs).
If I connect the 200 amp auto transfer switch at the service entrance between the utility meter and the main breaker panel and set in auto what will happen when the grid goes down and the generator kicks on?
Scenario: Grid goes down, both SMA UL-1741 grid tie inverters turn off, backup generator starts, transfer switch transfers to generator. Now I assume after 5 minutes or so (assumming the generator is stable enough) the UL-1741 grid tie inverters will try and come back on thinking the grid is back up? Here is the problem. What if the Solar and Wind grid tie inverters produce more kw than the house consumption with the backup generator running and online?
My gut feeling says to run the 200 amp transfer switch in manual and turn the inverter breakers off prior to starting the generator. But I would like the UL-1741 inverters to help the generator (to use less fuel). What I do not want to do is release all the trapped smoke in the generator and/or inverter electronics. What are my options?
Does anyone have any experience with this? Another option I thought of was to throw a blanket over the solar cells until the production is under consumption and let the generator take up the slack. This just does not fell like it is 100% fool proof. I am up for suggestions.

AntronX

Re: Grid Tie Solar or Wind Turbine with Backup Generator

mnittler wrote: »

...Another option I thought of was to throw a blanket over the solar cells until the production is under consumption and let the generator take up the slack. This just does not fell like it is 100% fool proof...

That will be one large blanket...

On a serious thought, if you do not want to get Sunny Island inverter and batteries, you should be able to use large AC voltage controlled resistive dump load to absorb excess power from solar array and to regulate your micro grid. I don't see why this cannot work, but there is probably no plug-and-play solution like this out there. I like Sunny Island idea better instead of propane generator. How much does 18KW generator cost? Is it significantly cheaper than Sunny Island system with batteries?

AntronX

Re: Grid Tie Solar or Wind Turbine with Backup Generator

Oh boy, Sunny Island is a pricey beast at $5,200 for inverter only. Then there is battery cost. Your Kohler is $1000 cheaper and needs no batteries, but you don't get a whole house uninterruptible power supply effect as you would get using S.I. system.

mnittler

Re: Grid Tie Solar or Wind Turbine with Backup Generator

I agree SI would work great but in my case I would need 2 SI to get 240vac split phase. $5100 times 2 plus a boat load of batteries. Anybody know how to auto shut down SMA SB50000US and WB3000US? Is there anyway to limit output of SMA grid tie inverters?

stephendv

Re: Grid Tie Solar or Wind Turbine with Backup Generator

If the SMA inverters are configured for off-grid use (it's a setting you can change) - then their output can be controlled by the frequency of the generator. If they are NOT configured for off-grid use then they should not come on when the generator comes on. I know that is true for the European versions, but not sure about US. The EU versions will check grid frequency, voltage and impedance if configured for normal grid use. So while your generator might be ok for frequency and voltage, the impedance wouldn't be right. If you configure them for off-grid use, then they will only check voltage and frequency - and they'll regulate their output based on the frequency.

The higher the frequency, the lower the output. This is how the SI control their output. But I think without an SI, you're playing with fire trying to get the output to match your loads all controlled by the frequency of the generator.

A safer idea would be to install a transfer switch between your home and the grid (I assume you have one, so as not to electrify the grid from your generator?). Then connect the inverters on the grid side of this switch. So in the event of power failure you'll run on pure generator power, no PV. Safe. Simple.

mnittler

Re: Grid Tie Solar or Wind Turbine with Backup Generator

Well, I definitely do not want to be playing with fire in any shape or form.

Answers: Cost of equipment:
Generator setup
$4,200 18kw generator (240vac split phase, 18,000 watt)
$1,000 Auto Transfer switch 200 amp
$1,200 Propane tank 500 gallon
$ 800 Propane cost to fill tank
Plus wire, piping, and my installation time
Total cost: $7,200 to run whole house with air conditioners during extended outage like after a Hurricane.

Sunny Island Setup
$6,200 above cost less Transfer switch (? still might need transfer switch)
$10,390 Sunny Island (2 each required for 240vac split phase (U.S.) @ $5,195.44 each)
$2,240 Batteries T105’s 8 each/ string (2 strings minimum) for 48vdc.
Plus wiring, piping and my installation time (same as above)
Total cost: $18,830 all of above
Or Total cost: $12,630 with 2 ea Sunny Islands and 2 strings of batteries and use my old original 9.5 kw 240vac split phase diesel generator (this generator will not start my 4 ton air condition but it probably would with the help of the Sunny Islands and batteries.) Already tried it without SI and the Air Conditioner will not start.


I already have 5.1 kw solar using SMA SB5000US and 2.5 Wind using SMA WB3000US so these would work well with the Sunny Island. I am assumming that the Sunny Islands would charge the battery banks from the grid tied inverters during outages and/or generator or grid when available.

It just seems a shame to have that much renewable energy available and not be able to use it during an outage. There has to be another way to do this.

Plan "B": Install a relay on the outputs of the existing SMA grid inverters using normally closed contact. The coil of these relays could be tied to the outputs of the generator and would disconnect the inverters whenever the generator comes on.

Plan "C": Operate the transfer switch in manual and do everything manually (Not as fool proof)

Somebody out there has to have done this successfully before. SI is the best approach but fairly pricey total project. Any more suggestions?

Have generater, grid tied inverters (sun & wind) and willing to look at other options.

BB.

Re: Grid Tie Solar or Wind Turbine with Backup Generator

Another method is to use the Xantrex/Schneider XW system... Basically, the XW is a hybrid Grid-Tied/Off-Grid inverter with its own internal battery charger and transfer switch, and a second AC2 input for an AC generator.

For the wind and solar, you just connect it to the battery bank and the XW inverter takes care of the rest of the issues. (you may need a shunt regulator for the wind generator on the battery bank--for use during utility outages and low power use in your home).

It should do pretty much anything you ask of it, and the price should be less.

-Bill

dsp3930

Re: Grid Tie Solar or Wind Turbine with Backup Generator

Another option is to just put your critical loads on a sub-panel that you need to power in an outage. Then, put your generator transfer switch between the main panel and the sub-panel. If the power goes out, your alt energy equipment will fall out of spec and shut down. Your auto transfer switch will kick in, and power your sub-panel and your critical loads.

In a normal situation, your solar will backfeed into your main panel. The sub panel will get its power via its connection to the main panel. (and your alt energy system) Any excess would be sent to the grid like normal.

stephendv

Re: Grid Tie Solar or Wind Turbine with Backup Generator

There's been some discussion about backfeeding a Xantrex XW from grid tie inverters, but then you have to take care of disconnecting the inverters when the batteries are fully charged.
See BB's post here: http://forum.solar-electric.com/showthread.php?t=7068

The Xantrex is a more affordable solution, especially since 1 unit will give you split phase.

mnittler

Re: Grid Tie Solar or Wind Turbine with Backup Generator

Well, what I have done so far is install the Kohler 18kw propane 18res generator, 500 gallon propane tank, and 200 amp auto transfer switch, 200 amp disconnect switch just after the utility meter. (along with grid tied 5.1kw solar and 2.5kw wind turbine) The utility is the main input to the auto transfer switch and the generator is the stand by. The 18kw generator is rated .5% stability 0 to full load (uses digital governor). 60Hz * .005 = .3Hz which would be 59.7Hz which is well within the UL1741 spec. I actually started my 4 ton air conditioner unit using only the generator and the frequency only dropped to 59.8Hz then imediately back to 60.0Hz. The generator auto starts 3 seconds after a power failure and automatically switches load 5 seconds later. I went ahead and connected the grid tied wind & solar inverter equipment to the grid side of the auto transfer switch to be safe for now. This way the renewable energy equipment cannot backfeed the generator. I am in a position to experiment with the addition of new equipment by turning off the 200 amp disconnect to the grid thus making an island, but chicken at the moment. I talked to a motor rewind and generator shop and he indicated that he felt that if the renewable energy generated was greater than the load with the generator running that it might fry the generator exciter which I do not want to do. I am still looking at Xantrex but have not seen anybody actually waving their hands and say they have done/seen it actually working. I talked to Kohler and they indicated that they did not have any problem with the setup I wanted to try (grid tied inverters on the generator side) as long as I guaranteed that the load would be double the solar + wind outputs. I cannot guarantee the load to be doubled output for any length of time. Murphy would probably have his way sooner or later.

AntronX

Re: Grid Tie Solar or Wind Turbine with Backup Generator

So, you want to connect grid-tie PV to your generator, but are afraid to blow it up. You need to figure out exactly how the governor operates, what would it do if inverters pushed AC voltage and frequency up. How would governor compensate and can that damage the exciter? Would the generator become the motor and overspeed the gas engine untill the frequency rise would trip the inverters?

If you can modify the governor to allow AC frequency to wander a bit proportionally to power load, you should be able to simulate what Sunny Island inverter does to govern power output from grid-tie inverters. So when your AC loads are less than available solar power, the inverters would force your generator to overspeed slighly and will self-regulate due to resultant increase of AC frequency, by reducing their output power. If you keep at least few hundred watts of guaranteed load, like few light bulbs always on, that could provide enough AC current sink to never allow the generator to be backfed, and keep inverters from fully shutting down.

System2

Re: Grid Tie Solar or Wind Turbine with Backup Generator

It just seems a shame to have that much renewable energy available and not be able to use it during an outage. There has to be another way to do this.

I talked to Kohler and they indicated that they did not have any problem with the setup I wanted to try (grid tied inverters on the generator side) as long as I guaranteed that the load would be double the solar + wind outputs. I cannot guarantee the load to be doubled output for any length of time.

mnittler, i had the EXACT same question, and am glad i stumbled on your posts. Are currently using a Sunpower SPR-5000m inverter and a Caterpillar XQ20 generator with the following characteristics: ± 0.5% voltage regulation at steady state from no load to full load, ± 0.25% frequency regulation for constant load from no load to 100% load, and THD <4.0%, at no load. Have been told by another person that SMA feels that this might work, but no guarantees. It would be a simple thing to just use the generator (no PV), and not worry about this at all, but it seems a shame to not be using the available PV.

Have you checked with Kohler (or SMA) to see what they think would happen if the load < solar/wind output ?

Regards,
Sridhar

mnittler

Re: Grid Tie Solar or Wind Turbine with Backup Generator

Yes, I checked with SMA and Kohler. SMA said to use a Sunny Island and it would do exactly what I wanted to do (I would have to use 2 each of the SI for 240vac split phase), and Kohler said to guarantee that the solar & wind generation was 1/2 of the load.