Author Topic: Grid tie inverters, a million and one questions  (Read 27557 times)

SCOTT

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Re: Grid tie inverters, a million and one questions
« Reply #15 on: September 22, 2008, 11:13:50 PM »
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.

Scott
net metering with a 6/1 in Connecticut
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MacGyver

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Re: Grid tie inverters, a million and one questions
« Reply #16 on: September 23, 2008, 03:29:34 AM »
So you just rectify and filter the 120VAC and feed the resulting DC (which should be nearly 170VDC) into the inverter?
Steve

JKson (PS) 6/1 'roid & ST 7.5

adhall

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Re: Grid tie inverters, a million and one questions
« Reply #17 on: September 23, 2008, 05:54:29 AM »
Jens:

Actually, you get the same peak voltage no matter how you rectify the AC. (Approximately 1.414 times the nominal AC voltage.) Full wave rectification just gives you a pulsating DC voltage that is easier to filter. (More "bumps" per second.)

Best Regards,
Andy Hall
JKSon 6/1, 5 kW ST Head, 1992 Dodge RAM Cummins 5.9L Turbodiesel, 2001 VW TDI 1.9L Turbodiesel, 2006 Jeep CRD Turbodiesel, Yanmar FX22D Diesel Tractor

SCOTT

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Re: Grid tie inverters, a million and one questions
« Reply #18 on: September 23, 2008, 02:04:23 PM »
jens to answer your questions:



Cool - what are you using for inverters ?   Sma sb 3000
Could you be a it more specific on the setup ?
 Are you using any batteries ? no
 What DC voltage are you running ? about 250-300v under load
How do you feed your DC to the inverter - is there anything electronic involved no, just a rectifier and capacitors
or are you just hooked up to the battery terminal on the inverter ? no  battery
What size Lister ? 6/1
 How much power are you selling ? 2500w at the inverter is about max
 What happens when the gid fails the inverter disconnects automatically it is part of the code that they have this failsafe.-
do you have local power from the inverter ? it feeds the grid at the same time I draw from the grid.
 How are you controlling engine speed - do you just set it at whatever and let it run in the knowledge that any excess power is sold ? Yes the inverter draws whatever wattage I program it to down to the watt.
What happens when the grid is down and you can't sell the excess power - does your DC level change at all ? Does anything change ? If the grid goes down the dc will rise because the demand on the engine is reduced and it speeds up, making more ac and thus DC.  As long as the dc is below 400v there is no problem

On the co-gen side, do you have a mall coolant loop through the engine and then through a heat exchanger to feed the house ? If so, how many sq feet of heat exchanger area do you have to carry the heat away from the engine ? I am assuming that you use circulation pumps. I have not set this up yet.  I have a buried 4” corrugated pipe from the gen shed to my basement that PEX will be housed in.
net metering with a 6/1 in Connecticut
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MacGyver

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Re: Grid tie inverters, a million and one questions
« Reply #19 on: September 23, 2008, 03:21:48 PM »
Jens:

Actually, you get the same peak voltage no matter how you rectify the AC. (Approximately 1.414 times the nominal AC voltage.) Full wave rectification just gives you a pulsating DC voltage that is easier to filter. (More "bumps" per second.)

Best Regards,
Andy Hall

Nope .....
Jens


Nope?  Since when?  The filtered output voltage is the same whether it's full wave or half wave.
Don't believe? Grab a bridge rectifier, cap, and your favorite voltmeter and give it a try...
Steve

JKson (PS) 6/1 'roid & ST 7.5

carlb23

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Re: Grid tie inverters, a million and one questions
« Reply #20 on: September 23, 2008, 05:58:19 PM »
Scott,

Could you please provide us with the size and number of capacitors you are using?

Thanks

Carl

MacGyver

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Re: Grid tie inverters, a million and one questions
« Reply #21 on: September 23, 2008, 06:38:59 PM »

MacGyver - since the day Christ was borne :)
Snicker...  were rectifiers invented on that day too?  Wow, what a coincidence?
But seriously.....

Quote
The peak to peak voltage of 120V AC is about 340V and that is what you get out when you run it through a bridge rectifier. Yes, You can run the AC through a single diode and get 170V at 60 Hz but as soon as you rectify both sides of the AC you get essentially the peak to peak AC voltage as DC on the output (assuming a cap on the output). This applies to using two diodes or a full wave bridge rectifier with four diodes. Two diodes give you 60 Hz output, a full bridge rectifier gives you 120 Hz output.

Nahhh, stop and think about this a minute. (or take a refresher course in electronics 101)

Sure, 120VAC gives about 340V P-P.
But think about what a rectified, unfiltered waveform looks like. The "bottom half" of each cycle gets flipped over so that all parts are above the 0 volt reference. (or below, depending on how you measure) So now you still have 120VRMS, but only 170V P-P because half the peaks are flipped over and when you filter it you've STILL got 170VDC.

Half wave or full wave rectified and filtered AC give the SAME output voltage, but full wave does it with twice the ripple frequency, and requires a lot less capacitance to give the same percentage of ripple at the output.
Steve

JKson (PS) 6/1 'roid & ST 7.5

MacGyver

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Re: Grid tie inverters, a million and one questions
« Reply #22 on: September 23, 2008, 06:46:02 PM »
Cool - what are you using for inverters ?   Sma sb 3000

 What DC voltage are you running ? about 250-300v under load

Hmm. Interesting. What's the maximum DC input voltage those inverters are rated for?
Steve

JKson (PS) 6/1 'roid & ST 7.5

mkdutchman

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Re: Grid tie inverters, a million and one questions
« Reply #23 on: September 23, 2008, 07:18:31 PM »
where's GuyFawkes when you need him?  ;D ;D

lifted this off wiki
http://en.wikipedia.org/wiki/Rectifier

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply, or with three diodes in a three-phase supply.


A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. Four rectifiers arranged this way are called a diode bridge or bridge rectifier:
« Last Edit: September 23, 2008, 07:21:55 PM by mkdutchman »

MacGyver

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Re: Grid tie inverters, a million and one questions
« Reply #24 on: September 23, 2008, 08:28:43 PM »
Yes, both the circuits (half wave and full wave) that mkdutchman posted will give 170VDC if fed with 120VAC.
The second schematic is the standard  full wave bridge circuit.

You can also rearrange things s bit to make a voltage doubler, but it takes TWO caps, and it really only needs 2 diodes.

Steve

JKson (PS) 6/1 'roid & ST 7.5

mkdutchman

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Re: Grid tie inverters, a million and one questions
« Reply #25 on: September 24, 2008, 03:37:41 PM »
Yes, both the circuits (half wave and full wave) that mkdutchman posted will give 170VDC if fed with 120VAC.

Um...yes, but that's not the whole story

All other things being equal the half wave rectifier will give only half the power and filtered voltage of the full wave rectifier, if I understand correctly

Let me resort to reason and logic  ;D

First, some term definitions

RMS=root mean square, the power, rated in DC, that the AC delivers. For example 120V RMS AC will deliver the same amount of power that 120 DC will

Vpp=voltage peak to peak, the distance in volts between the negative peak and positive peak on an AC waveform if viewed on an 'oscope

Power=the power delivered to the circuit

Voltage and current is constantly changing in AC, so power to the load is constantly changing, that's why we need RMS

Now look at this wiki image again

The half wave rectifier eliminates half the waveform, (either the positive or negative component) so you only get half the power that you had in the original waveform AND if you want pure DC you will have to filter the heck out of it as compared to full wave

Now if we look at the full wave rectifier

it doesn't eliminate the waveform, it makes the components ALL (in this case) positive. So you have the full power available in the rectified waveform (discounting rectified losses) that you had in the original waveform

Now some calculations
this formula is used for a pure AC waveform.

let's look at common 60Hz AC

Common AC is rated at 110/120V, but keep in mind that is it's RMS value. Looking at the formula above we do this

RMS=110
the square root of 2 = aprox 1.414

so 110 x 1.414 = approx 155
so Vpeak = approx 155

now Vpeak is only one side of the waveform. In other words, a load will see the voltage climb from zero up to 155+ then go back to zero and continue on down to 155- So the total voltage swing that your load will see is 155+155=310volts

Now back to our rectifier
The half wave will eliminate one of those 155 sides as will the full wave. BUT the difference is that the full wave will put the 155 side that it eliminates onto the  side that it does not eliminate. Vpeak will stay the same. What will not stay the same is the power and the filtered voltage.

If you filter the output from the half wave rectifier down to pure DC you will only get half the power and voltage that you get with the full wave rectifier because only half the waveform is being utilized

Now, if I need to eat crow on any of this please enlighten me. It would be too bad to have to go through life with an errornous opinion just because no one bothered to point out my mistakes ;D

<dubiously eyes nearest cawing crow and thinks about asking jens for his crow recipe>

Jim Mc

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Re: Grid tie inverters, a million and one questions
« Reply #26 on: September 24, 2008, 11:39:54 PM »
The half wave will eliminate one of those 155 sides as will the full wave. BUT the difference is that the full wave will put the 155 side that it eliminates onto the  side that it does not eliminate. Vpeak will stay the same.

Right


Quote
What will not stay the same is the power and the filtered voltage.

Sorta right, sorta wrong.  Depends on the filter...


Quote
If you filter the output from the half wave rectifier down to pure DC you will only get half the power and voltage that you get with the full wave rectifier because only half the waveform is being utilized

Again, depends on what we agree is meant by 'filter'.  The wikipedia schematic you show does not include any filter.

In this thread so far, 'filter' has referred to a capacitor input circuit.  In the case of a proper capacitor-input filter, the output of the filter is not the average of the rectifier output.  Rather it is the peak of the rectifier output.  Therefore, it will be the same regardless of whether the rectifier is full or half wave.  Of course in the real world, there is ripple voltage to contend with.  But given a sufficiently large capacitor, you won't see much difference between full and half wave.

Half wave rectifiers do have draw backs, mainly they provide high peak currents relative to the average current, so they promote excessive heating in the AC transformer they're connected to.

mkdutchman

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Re: Grid tie inverters, a million and one questions
« Reply #27 on: September 25, 2008, 12:08:29 PM »
Quote
Again, depends on what we agree is meant by 'filter'.  The wikipedia schematic you show does not include any filter.

You're right, it doesn't. Shoulda clarified that. What I meant by filter was "any filter that will take all the ripple out and produce pure DC" fed by the rectifier output, like this

transformer......rectifier......filter....pure DC

Jim Mc

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Re: Grid tie inverters, a million and one questions
« Reply #28 on: September 26, 2008, 01:52:54 AM »
... What I meant by filter was "any filter that will take all the ripple out and produce pure DC" fed by the rectifier output, like this

transformer......rectifier......filter....pure DC

Uhhh, OK.  Like a capacitor, right?

With a capacitor filter, the DC output is about the same regardless of whether you use full or half wave rectifier...

mkdutchman

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Re: Grid tie inverters, a million and one questions
« Reply #29 on: September 26, 2008, 12:14:39 PM »
... What I meant by filter was "any filter that will take all the ripple out and produce pure DC" fed by the rectifier output, like this

transformer......rectifier......filter....pure DC

Uhhh, OK.  Like a capacitor, right?

With a capacitor filter, the DC output is about the same regardless of whether you use full or half wave rectifier...

Umm......no. 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. (To compensate) The power source is working on half of one cycle and loafing on the other. PWM is based on this principle....although PWM uses a square wave instead of a sine wave the same principle applies

http://en.wikipedia.org/wiki/Pulse-width_modulation
http://www.netrino.com/Embedded-Systems/How-To/PWM-Pulse-Width-Modulation
« Last Edit: September 26, 2008, 12:34:34 PM by mkdutchman »