Grid tie inverters, a million and one questions

[Jim Mc]

Just because you started the thread, you think you can bring it back on topic???

...If all other conditions stay the same the DC output goes down with the half wave.  But in the real world what usually happens is that the voltage and/or current in the half wave that goes through is increased.

I really don't understand that statement.  It is unclear.

Here's my position:

Go out in the real world.  Take a full wave bridge rectifier and filter capacitor.  Apply 120 V rms.  Measure the filter capacitor voltage.  You'll see about 165 Vdc.

Now, repeat the experiment, but take the full wave bridge out, and put in a single rectifier (a half wave circuit).  Again you'll see about 165 Vdc. 


 

The power source is working on half of one cycle and loafing on the other.

yes, true.

 

PWM is based on this principle....although PWM uses a square wave instead of a sine wave the same principle applies

There's one big difference between  60 Hz rectifier circuits we're discussing and typical PWM power supplies.  PWM is typically set up with an inductor input filter.  The 60 Hz sine wave circuits we are discussing are typically set up with a capacitor input filter.  Big difference.  A capacitor will quickly charge to the peak voltage, and hold that voltage, even while supplying current through the 'off cycle' if the cap is large enough.  An inductor will more closely provide an output voltage that's the average of the input voltage.

Figure 8-5 on P152 might help: 
http://books.google.com/books?id=gmCSDP3Fp5EC&pg=PT165&lpg=PT165&dq=full+wave+half+capacitor+inductor+filter+peak&source=web&ots=-w38JaWJN4&sig=ECckIKpuUIR0slk2vJt0gIRBlmo&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPT165,M1provide an average output voltage, not peak.

[LowGear]

Gosh I love this intellectual talk.  I can even kinda follow some of it from my superhyperdine something-ah-rather days in the Army.  Yup, we had diodes and capacitors in those days as well.

But I’m far more interested in the fact that SCOTT has a grid tied Lister with a ST-head AC generator.  I’m in Hawaii and am considering being the first in my state to do a solar – biomass hybrid system.  I’m looking at a Witte with AC generator.  (No stones please – it’s not quite as pretty as a Lister but it’s a big brusier with giant flywheels, factory electric start and Made In USA stamped on it.)  So when I read

Jens I may be able to help with your quest for a grid tied lister plant.

I currently have a lister that is attached to the grid using a grid tied inverter.

Ac from St head> rectifier with capacitor bank> clean dc to inverter> clean ac from inverter to grid.

It works fine the key is cleaning up the dc out of the rectifier.  I believe my capacitor bank is 12-15k mf  the measured ripple is around 2%

In the last several months I have not done anything with the project, but it is there waiting to be run when it gets cold and there is a use for the heat.

The above install was inspected by both my city and my local power company.

 
I thought SCOTT and I should be friends or at least better acquainted.  SCOTT would you consider writing about your experience perhaps in a new thread?  Do you have any information about the efficiency costs to do the rectify to inverter process?  Will you share the expenses of such a project?  And really important;  Has it actually farted and slowed the meter or even better ran it backwards? 

Casey

[Tom]

So let me see if I have this correct? You are going to run your big "browser" on bio-mass. Sounds like you are farming. 

[LowGear]

Hi Tom,

Well, actually I am practicing share crop farming but Waste Vegetable Oil qualifies as biomass according to the Hawaiian Electric Company.  I've checked with the National Resource Conservation Service for any grants that might be available for solar-biomass hybrid programs.  If any of you know of such a program please let me know.  My Bruiser (spelling corrected) should do well doing a tank of mostly non-petro oil most evenings.

Casey

[jzeeff]

Yes, you can use a AC generator to supply a grid-tie inverter.  UL approval for the inverter is needed to have any hope that the utility will let you connect it.

It's best to use a 3 phase generator + full wave rectifiers.  Second best is a single phase generator with a full wave rectifier.  Don't bother trying a single rectifier.   Both need capacitors, the 3 phase generator needs fewer of them.

I believe that  grid-tie inverters can be set to put a constant load on the generator but this needs confirmation.  You would have to put your in-house AC loads on the output of the inverter, not directly connected to the generator.  All remaining power would go to the utility.

If

1) you already have a generator
2) you don't mine putting hours on it and rebuilding it some day
3) you burn the same cost (or cheaper) fuel as you use to heat your building
4) you recover most of the waste heat
5) you don't mind the noise
6) you operate only when you need building heat

then generating electricity even when you have grid power available can make economic sense

[LowGear]

Gosh, I hate to think of myself as fat, dumb and lazy but my war is with the Oil Cartels - you know - the 1st cousins to the Opium Cartels and not so much with the local privately held electricity corporation.  Pick your wars is one of the mantras at Camp Aloha. 

I hate to break it to you but they can inspect you just about any time they want with threat of disconnection on the excuse that your use looks "irregular".  We all know it will never happen; besides you’d have time to pull your equipment for storage at your brothers.  And should you get caught, say your dog bites the neighbor child and they know you've got a bandit inverter on the line, you would most likely need a second mortgage to pay the attorney's fees.  Let’s not forget the dead worker on the pole just outside your property line.  Perhaps your liability insurance agent can explain it better.

Keep It Simply Smartass is also an important concept here in 44 cents a kilowatt land.  I'm not too worried about a 6 to 10 hp electrical plant running my meter too far backwards so dynamic control isn't really in the cards.  In Hawaii, at 1000 foot elevation, there isn't much demand for heat except heated water and the house with hot water showers is over yonder from the energy lab.  The Menehune Energy Lab, MEL, roof will have solar as well.  I believe Outback Grid-Tie inverters accept two sources of DC - one at a time.  Anyone wanting to contribute to my knowledge base is very much invited to do so at this time.

Casey

[LowGear]

We're getting off subject.  Wait a minute.  What was this thread about three days ago? Anyway, I guess some of us prefer to be in compliance with regulations when monopolistic powers are involved.

The dog.  When you play games that involve illegal or contrary to regulation practices you’re at the mercy of everyone that knows.  I once got a very nice side sewer job because someone had turned my client in for adding a bedroom without a permit.  The permit process required that he connect to the public sewer – hence his reluctance to get the permit.  He couldn’t figure out who complained to code enforcement.  He was also pissed at the neighbor for expecting my client to pay the emergency room fees his son incurred when he had to get some stitches from this client’s dog biting the boy.  Daahh!  I thought he was lucky the neighbor didn’t ask for the title to his home including all bedrooms permitted or not.

The dead guy on the pole.  Did I mention anything about it actually being your fault or responsibility?  (The neighbor kids had found an early radar oven, hooked it up to a Dish Network antenna, plugged it into 240, aimed it at the cat in the window which also lined up with the lineman and his pace maker.  The rest is conjecture.)  If I did I was wrong.  I meant to imply that the attorney’s fees and looking for a new insurance company could be costly.  And being off the grid until the utility and governing agency could schedule a very thorough inspect at your expense might be quite inconvenient as well.  All worst case of course.

But hey!  This is all outhouse something a rather and I’ve become obedient to the law.  It's nice to welcome all comers onto my property and into my home.

Does anyone have a good guess as to what this Witte engine and single phase generator on factory stand are worth?

Casey

[LowGear]

So, in review of my questions:

• Inverters that take two DC inputs?
• Estimates of worth on the Witte 6 HP diesel with factory generator and support frame?

Casey

[jzeeff]

A system that would allow a generator to be connected to a buildings utility power, but would never actually feed any power back to the utility makes sense.   Accounting for the reverse flow (sell back to the utility) may be too much trouble or not make economic sense.

It would have to have a sensor on the connection to the utility to know at what point reverse flow starts to happen. 

I would guess that grid-tie inverters may have everything they need (sensor, computer, separate connections to utility and internal power) to implement such a thing.    Worth asking them if they could or do offer this feature.

On a slightly related subject, I know of a data center that has some oversized diesel generators that are seldom used and the local utility is contracting with them to generate power during peak load periods.