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

[William Korthof]

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I believe a 5-jaw meter and socket would be necessary
(the 5th prong of the meter is a neutral connection for the
voltage-sense coils). L1 and L2 lines would be in phase
and 0 V. apart, rather than 240 volts and 180 deg. out of
phase. The 5th jaw-to-neutral connection would put
120 V into both voltage coils, allowing correct operation.

In "single phase" 120/208V installations, a issue problem
occurs. A 4-jaw meter would see 104 V in the voltage coils
rather than 120 V. These applications where 5-jaw meters
are required are fairly common (condo and apartment
buildings for example).

/wk

At 09:33 PM 1/19/04 -0800, you wrote:
>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
>
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