Question.. Battery Bank to Home Usage?

[Jim Mac]

Ok so I have a question... I am in the process of installing a permitted 1 KW Whisper 200 windmill. It will be on a 65 Ft guyed tower, and 8/3 UF underground wire will run through a conduit to a shed where I will have the charge controller and battery bank.

After the energy is stored to the 24V battery bank, it will be inverted and ran to the basement of my house..

My question is, how do I use this electricity from this point? Is there any devices which allow you to tie it into my electrical breaker box without feeding back into the grid? Can I use my existing house electrical circuitry to power my home and still have the grid for when the batteries are low?

Thanks for any info..

[Rob Beckers]

Hi Jim,

The options are to either use a transfer switch, much like those used for generators, that isolates part of your house grid (ie. the basement) and switches it to the inverter for power, or you can wire up a separate 'grid' for the basement that is not connected to the regular electrical house grid. Interconnecting the two grids is not only illegal, but will likely fry your inverter (as both do battle to set the voltage).

-RoB-

[Jim Mac]

Hi Rob, Thanks for the reply.. So it sounds like I need a Transfer Switch. If you can, I would appreciate some more information regarding this.. Please excuse my

When you say "Much like those used for generators", does this mean there are specific ones for Wind usage? If so, could you point me in the direction of some options.. Or would a Generator Transfer Switch like the one you linked work?

If I am understanding this right, the Generator Transfer Switch diverts the circuit to the Generator if the Grid goes down. But can this work in reverse? Can the battery bank be used as the primary source, and only use the grid if the batteries are too low?

Thanks for the help

[Rob Beckers]

Hi Jim,

A generator transfer switch would do the job, but there is a simpler way (come to think of it), by using a bypass breaker you can also power the basement from either the batteries/inverter, or the house grid (utility company). Bypass breakers use a mechanical lockout that makes it impossible to switch on both breakers at the same time, so only one breaker is feeding the circuit. Here is a picture that hopefully explains it better (http://solarconduit.com/shop/media/catalog/product/cache/1/image/5e06319eda06f020e43594a9c230972d/f/i/file_7_53.jpg). The same setup can be used with a regular sub-panel.

The more expensive inverters come with their own build-in transfer switch (Outback, Xantrex etc.). They can be set to automagically switch to grid power when the batteries go below a set level, and they can charge the batteries from the grid when needed. However, with those you are in essence grid-tying the inverter and your utility company may not be too happy about that (unless you have a net-metering agreement and get the whole thing inspected).

-RoB-

[Jim Mac]

Rob, Thanks for taking the time to help.. There's still a few things I'm unclear on..

With the Bypass breaker, this looks like a manual solution.. I would have to physically decide what source to use and manually control it. I am looking for automation..

SO back to the Generator Transfer Switch Idea.. Am I correct to assume that when the inverter is ON and Powered, the basement will run from the inverter, but when the inverter is OFF the transfer switch connects me back to the Grid?

If this is the scenario, what controls when the inverter goes ON and OFF? Do I just leave the inverter Always ON and let the inverter kick on and off as the battery bank charges and discharges? Will running the batteries that low often cause a loss of life in the batteries?

Thanks again

[Stewart Corman]

What is wrong with a simple DPDT power relay where the coil is energized by a preset voltage ie 30volts from the battery bank?
When the battery is being charged by the wind turbine, it's voltage is higher than 24volts, so it clicks on that part of the basement you have hooked up and disconnects the grid.

http://www.kpsec.freeuk.com/components/relay.htm

Stew Corman from sunny Endicott

[Steven Fahey]

A "bang-bang" switch has some limitations. A big issue is synchronization.

If you switched out the inverter while it was at one peak and the grid switched on at the opposite peak, then any motor currently running would be out-of-phase. This situation involves noise, damage, and heat, in unpredictable quantities.

When you buy an inverter that can "grid-tie", then presumably it has electronics on board that maintian a constain synchronization with the grid. Your inverter times its waveform to match constantly. Nobody's PV system is strong enough to re-sync the grid's frequency!

You can buy automatic transfer switches that flip you over, but only AFTER a 5 second delay, for this reason. If you want an instantaneous transfer, having it on-board the inverter is the way to go. I've tried it with my SW4024 and it's seamless. Probably the same with the Outback's and Mag's, but I don't have those for trying it out.

[Stewart Corman]

Steven,
If you switched out the inverter while it was at one peak and the grid switched on at the opposite peak, then any motor currently running would be out-of-phase. This situation involves noise, damage, and heat, in unpredictable quantities.

You may be right but do you have link to substantiate your claim??
heat? in one cycle of 1/60th second?
I can't believe that one cycle could cause havoc with all the fields involved with a spinning motor armature....in theory maybe, but realistically?
Ever see a motor brake that puts straight DC into an AC motor?
Anyway, I wasn't thinking along those lines since I am switching in resistance loads.

Stew

[Steven Fahey]

Well I don't think I'm an expert on this, but from what I've learned, synchronization is a critical issue when switching two AC power supplies. I don't think a resistor would care, either, nor a DC power supply receiving AC mains. But if there's a motor in the circuit, or something else that is "reactive" or has "inertia", then watch out.


A motor is turning at a specific speed at the time of switching. That speed is close to synchronous frequency, minus a little slip because most motors are induction motors. Let's not forget the capacitor that's maintaining the phase difference in the secondary winding. (I'm talking about house wiring and single-phase AC here like air conditioners and well pumps).

The moment you switch from one power source to another, the motor's inertia prevents it from instantaneously changing its phase relationship to the new incoming source of AC. If the two sources of AC are in phase with each other, then the motor doesn't notice. If they are NOT in phase with each other, then the motor is some number of electrical degrees out of phase with its power supply.

This leads to one of a couple of possible conditions: If its phase leads the AC, the motor suddenly becomes a generator. The motor-come-generator now back feeds the new AC supply. That blows up inverters. The spike might zap the run capacitor.

Alternatively: If its phase lags the AC, it is back in start-up mode. Locked-rotor amps can flow for the time it takes to accelerate. Motors with a starting capacitor will not be able to engage the start cap because the rotor is already above the clutch speed of the centrifugal switch.

The physical load that is being driven by the motor also receives a torque impulse. Maybe not important - can't think of an example where it would be disastrous off the top of my head, but the fact is there.

Different transfer switches deal with these issues in different ways. Some keep the inverter in sync (such inverters/generators that are capable of being synchronized, that is), while some require a delay to let everything settle out:

http://www.eaton.com/EatonCom/Markets/Electrical/Products/AutomaticTransferSwitches/index.htm

http://en.wikipedia.org/wiki/Transfer_switch

http://www.gen-tran.com/assets/pdfs/Ovation_SpecSheetFINAL.pdf

[Jim Mac]

Thanks for the great information guys.

Sounds to me that the best, but most expensive, way is through an advanced inverter. So until I can be certain that I am producing enough energy to justify the expenditure, I will manually switch from Grid to Battery Bank as needed.

If I see that my alternative energy system is generating an unmanageable overflow of electricity, I will invest in the upgraded equipment.

I'd love to hear some first hand reviews of the inverters you guys have used.

[Steven Fahey]

I have a classic Xantrex (aka Trace) SW 4024 pure-sine inverter. It comes from the days way back, when human beings who spoke english wrote user's manuals, ten control panel buttons controlled ten features, and the settings that could really get you into trouble required gaining access to the DIP switches. The user was cautioned that NOT opening the case could cause damage and void the warranty.

Those days are gone, of course. New equipment costs 1/2 as much per watt, 2x as many warning labels on it, and three buttons control 127 features sorted among 25 menu branches.

It's not all bad, I guess. You seem to have caught me in "one of those moods". :sick:

[Joe Blake]

What is wrong with a simple DPDT power relay where the coil is energized by a preset voltage ie 30volts from the battery bank?
When the battery is being charged by the wind turbine, it's voltage is higher than 24volts, so it clicks on that part of the basement you have hooked up and disconnects the grid.

http://www.kpsec.freeuk.com/components/relay.htm

Stew Corman from sunny Endicott


Wouldn't a simpler (if clumsier) option be to manually disconnect the grid (ie remove the fuses/circuit breakers) for complete isolation, then using the batteries, run a small inverter and put grid-level voltage power back into the domestic circuit and plug in a battery charger attached to the batteries? Then the locally generated power (whether wind, hydro, solar) could feed into batteries via the charger?

Since there wouldn't be the clash between two different sources of power trying to synchronise either frequency or voltage, there shouldn't be any danger to the inverter.

Further, given sufficient battery capacity, it seems that this would eliminate (or at least reduce) the chance of motor burn out etc when the generator output drops off.

Obviously stressing the safety aspects of totally isolating the domestic circuit from the grid, that would seem to me to be a "DIY" method of converting from on-grid to off-grid without installing a parallel system.

I think there is a catch in my idea somewhere, but I can't put my finger directly on it.


Joe

[Steven Fahey]

Probably not.
You would only do that if 100% reliability is mandatory (hospitals, perishables, fire halls) and the cost of losing power justifies the cost of using the grid to top up batteries.

If you did this with RE, you pay for the grid to do your float charge, and the solar panels' charge controller turns them off because the batts are already at float... oops.

There may be RE inverters sophisticated enough to juggle these priorities. The chargers you might find at the hardware store - forget it!

[Joe Blake]

Steven,

As I said, the method is fairly clumsy, and not envisaged for day-to-day use, but I was thinking of a situation where (say) a major catastrophe occurs and the grid has suffered accordingly. It would seem to be a way of harvesting a resource which could be put to good use.

I probably wasn't clear in my writing (the concept itself is fairly fuzzy), but I was looking at the problem this way.

An on-grid RE system which is mandated to disconnect from the grid when the grid loses power, will shut down the domestic circuit, presumably by some sort of solendoid, even if there is still power being generated by the RE. So in a way reminiscent of using some external power to generate an exciter field in an automotive alternator, the battery powered small inverter would feed back into the domestic circuit, which would then (presumably) close the solenoid which would enable the RE power to feed back into the domestic circuit, which would then do its normal function. The battery charger I mentioned would merely be to ensure that the batteries which powered the small inverter which is keeping the "exciter field" active.

In a manner I suppose similar to a spring loaded push button. As long as the button is pushed in and held the power will flow.



Hope that clarifies it.

Joe