Do Canceling Meter Legs Work? (Was Re: ratcheting cyclometers) [RE-wrenches]

[Robert Nuese]

<x-flowed>
Fellow Wrenches

I posted a message Friday afternoon raising a question about whether 
Doug Pratt's method  "to distinguish between grid feed and PV feed to 
the subpanel", in metering an SW gridtie system, will work. I think 
maybe nobody noticed my question because of my bad timing - 5:45 
Friday, just after the end of the work week. Or maybe my question was 
just too dumb to answer - but not as dumb as if I'd left it unasked: I 
need to know since I'm in the middle of installing 3 SW systems that 
need performance metering. Please someone, let me know if Doug's system 
will work, and if so, where my understanding is faulty.

Doug's idea would allow a single meter to measure the total system 
output, both that used on the site through the dedicated subpanel and 
that sold to the grid, and would automatically combine the amounts and 
ignore grid power used through the dedicated subpanel, say at night. 
Doug explained his suggestion quite clearly on Dec. 11, 2003:

> -----Original Message-----
> From: Doug Pratt [mailto: dmpratt at sbcglobal.net]
> Subject: RE: ratcheting cyclometers [RE-wrenches]
>
> …Simple. Using a standard 240v utility meter, wire the 120v leg from 
> the
> SW output to the subpanel so that power passing to the subpanel spins
> the meter forward. Wire the other 120v leg so that power passing from
> the main panel to the SW input spins the meter backwards.
>
> If the SW is simply passing utility power straight thru to the 
> subpanel,
> you'll have no meter action. The two currents cancel each other out. If
> the SW is pushing solar power to the subpanel, the meter spins forward.
> If the SW is pushing solar power to the subpanel *and* the utility, the
> meter spins forward faster.
>
> This ought to still work just fine with a ratcheted meter so that night
> time battery charging isn't deducted.
>
> *Caution*, I must admit I haven't DONE this, but I can't see any 
> problem
> with it. We run meters forward and backward like this all the time. If
> I'm missing something obvious, I'm sure we'll all hear about it. :-)
>
> Cheers,
> Doug Pratt


I hope Doug is right, this looks good to me - But I'm uncertain. The 
information I've found so far suggests that a 240v watthour meter, in 
effect,  multiplies the sensed current by the sensed voltage difference 
between the two line terminals. Some 240v meters can be adjusted for 
use on 120v systems by changing the wire routing and installing a 
jumper to neutral, as has recently been discussed. In the case Doug 
describes, we seem to be doing a 240v installation where the direction 
of one leg is reversed to allow in and out currents to cancel one 
another under some circumstances. My problem is that the voltage 
difference the meter will sense across the two line terminals will 
actually be zero. We are not talking about 2 different 120v legs of a 
240v, 1-phase system, that are 180 degrees out of phase and thus would 
have 240v difference between them. We are talking about 2 different 
sections of the same leg of the grid, one of which has passed through 
part of the inverter circuitry and then back to the meter, which have 
0v difference between them. I suspect this means the proposed setup 
won't work.

I'm hoping others will show I'm wrong, because it looks like a good 
system otherwise.

So, I'm hoping someone on this list will please tell me I'm wrong, and 
explain why. Barring that, does anyone else have a better solution than 
two separate meters?

Thanks very much for your help
Robert Nuese

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9.bWljaGFl
Or send an email to: RE-wrenches-unsubscribe at topica.com

TOPICA - Start your own email discussion group. FREE!
http://www.topica.com/partner/tag02/create/index2.html
--^----------------------------------------------------------------


</x-flowed>