Current Transformers on neutral for electrical meter

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meternerd said:
Why? VA does not take into account phase angle between voltage and current, so I though for single phase, it was VA = V x A and Watts = V x A x PF. Not so? Residential metering is Line to Line, so wouldn't angle between each leg be 180 deg? Been a LONG time since I had to remember this stuff. Makes my head hurt! Not too old to learn, though.
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For pure resistance the angle would be 180 deg, but VA has a power factor angle associated with it so it is not correct to sum VA values. To sum VA directly you must use the power factor angle and make a vector addition.

We can break the VA into W and var components (i.e. the rectangular components of the apparent power) and sum them separately. We can then use the resultant W and var totals to get the resultant VA.

In your example Wh and VAh had the same value but we accumulate Wh and varh, not VAh.
 
ronaldrc said:
I think a simpler way of looking at why you don't need to monitor the neutral with a CT.
is that any current that flows through the neutral has to has to flow in either line #1 or
line #2 and has already been added.

For Three phase using Two CTs. instead of Three, the same might be true because any current that has to flow through the phase leg without the CT. has to flow through one of the other two phase legs with CTs.
But this might be debatable.
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You can prove that for an n-wire system, you can use n-1 stators as long as the voltages all tie to a common wire without the CT (Blondel's Theorem).
 
mivey said:
For pure resistance the angle would be 180 deg, but VA has a power factor angle associated with it so it is not correct to sum VA values. To sum VA directly you must use the power factor angle and make a vector addition.

We can break the VA into W and var components (i.e. the rectangular components of the apparent power) and sum them separately. We can then use the resultant W and var totals to get the resultant VA.

In your example Wh and VAh had the same value but we accumulate Wh and varh, not VAh.
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Thanks for the input. I went back to the books and refreshed my memory of the "power triangle" and vector addition. No wonder I'm glad I'm retired! Bottom line for me in all of this discussion is that I came to see why there is, in fact, a metering error on form 2S meters if the voltages are not equal. Called "Non-Blondel metering". New one on me. Thanks for the education. I think Form 2S meters have been around so long, we just assume they must register correctly. When testing them, we can only set PF on the 240 source. Unbalancing line to neutral voltages can't be simulated. I kinda doubt these meters were designed to give the POCO an error in their favor, though. More likely just a cost saving measure to eliminate one additional voltage coil and a fifth jaw. With meters being so cheap it's kind of N/A today, but not likely to change. Form 12S meters for single phase 120/208 from a 3 phase bank do use the line to neutral voltage and 5 jaws. I'm sure a residential 120/240 could use this type of meter, but it's not likely to ever happen. Old habits die hard in the power industry. Thanks to the OP who got all this started. Hope his question was answered. Have a gidday!
 
I do not have any reason to suspect that the less expensive meter was designed to give POCO an edge.
But if it gave the customer an edge POCO might have been less likely to settle for that design.
 
GoldDigger said:
I do not have any reason to suspect that the less expensive meter was designed to give POCO an edge.
But if it gave the customer an edge POCO might have been less likely to settle for that design.
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Weights and measures authorities must have decided the design was fair enough, and they generally lean toward favoring the customer and not the seller of a product or service if there is going to be any error in measurement.
 
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