Will this work? (CT Metering Question)

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iwire

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Massachusetts
Your drawing is how the POCOs meter multiple drops to a customer for a single bill. I'm sure you see this all the time at risers and at the transformer.

No I have not seen that, few power companies around here will meter at the transformers. :)


You are correct. My curiosity is killing me, but I'll resist the urge to ask questions.

You get a free pass, your an electrician and you already gave your opinion on my original question so ask away.:)

The job has two small PV arrays supplying two 13 KW 208 V 3 phase inverters that I have to back feed into the buildings MDP. But somewhere along the way I have to provide revenue grade metering of the PV systems total AC output. This is to satisfy some rules of an incentive program.
 

mivey

Senior Member
The job has two small PV arrays supplying two 13 KW 208 V 3 phase inverters that I have to back feed into the buildings MDP. But somewhere along the way I have to provide revenue grade metering of the PV systems total AC output. This is to satisfy some rules of an incentive program.
As long as the wires going through the CT are in the same direction (you would be surprised how often this gets screwed up by linemen not paying attention), and the sources are synced, you will be able to meter the net power flow. You will not have the ability to tell which source is supplying (or taking) how much power unless this is logged elsewhere.

PS: Make sure this complies with the connection agreement and that you are not required to separately meter the sources.
 

iwire

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Massachusetts
You will not have the ability to tell which source is supplying (or taking) how much power unless this is logged elsewhere.

Each of the inverters has a data output that supplies all kinds of specific information that will be used to drive an interactive display on a large touch screen. This is in a new public library so it's more show than go.

However none of that metering meets the rules of the organization providing the rebates. They require revenue grade metering of the total PV AC output. They would allow me to use multiple meters but do not require it. The power company is on their own for their metering.
 

rbalex

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Not sure, too many other factors involved, but a 2% variance is possible.

Maybe that Engineering DIY forum would know :)
Assuming the CT secondary circuit had a sufficiently low burden, I believe it would take quite a few "other factors" to generate a 2% deviation error. At worst, making sure the conductors were paired AA, BB, CC and closely bound as they passed through the CT would reduce a substantial part of any error.
 

zog

Senior Member
Location
Charlotte, NC
Assuming the CT secondary circuit had a sufficiently low burden, I believe it would take quite a few "other factors" to generate a 2% deviation error. At worst, making sure the conductors were paired AA, BB, CC and closely bound as they passed through the CT would reduce a substantial part of any error.

And be sure they are centered. I agree, most likely it will be within 2%, but it is "possible" it will not be. Maybe some testing is in order, Bob?
 

SG-1

Senior Member
Bob, in your diagram the loads are really sources & the source is really the load. If this is true the diagram looks good to me. Keep the conductors centered in the CTs for maximum accuracy. Possibly lash them together there.

Steve
 

gar

Senior Member
090930-1939 EST

iwire:

Your method will work assuming everything is phased correctly. If not phased correctly you have enough experience to know of the problem and how to correct it.

On accuracy. If the CT is a split core type, then centering is probably a greater problem. Following are two experiments.

1. Fluke 8100Y Hall type split core probe with a 3/4" diameter hole. Current wire was #12. Load my 1500 W heater. Max-min readings with the wire in different positions was 11.02 to 11.15. This probe is out of calibration, but that does not affect this experiment. The error here from position differences is about 1.2%. The maximum error is not from center to one side but from one side to another. In other words this split core has substantial leakage flux.

2. A home made current transformer. This is using a continuous tape wound core with no split air gap. Very high permeability material, probably Supermalloy, bought probably 40 or more years ago from Arnold. Since my purchase of the core Arnold sold their tape wound core business to Magnetic Materials.

This transformer is very crudely made. I wound 10 turns loosely on only about 1/8 to 1/4 of the toroid circumference. The center hole of the core is about 1.2". The total variation in current measured for any position of the one #12 wire thru the toroid hole was 10.56 to 10.55 or about 0.1%. And this might be more a result of line voltage variation than wire position in the core.

http://www.magmet.com/tapewound/
http://www.magmet.com/tapewound/intro2.php

I suggest you experiment with one of your CT units and evaluate the variation in current output for position variations.

A question. When you normally apply a fixed CT are there components supplied with the CT to hold the wire in the center, or is there normally no problem with wire position? I suspect that good commercial CTs for fixed applications use continuous tape wound cores and thus wire position is not a major factor except for very high precision.

.
 

rbalex

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Location
Mission Viejo, CA
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Professional Electrical Engineer
And be sure they are centered. I agree, most likely it will be within 2%, but it is "possible" it will not be. Maybe some testing is in order, Bob?
Testing of course, but I'm not qualified to do that. I was just thinking about how I would have designed it.:D I didn't give too much thought to centering either, but probably should have. I had also assumed a fairly tight window for metering grade CTs.
 

mivey

Senior Member
On accuracy. If the CT is a split core type, then centering is probably a greater problem.
I agree.

Don't use a split core. Just make sure there is a connection near so you can open the circuit if you have to change the CT (or used a CT with bolted terminals).

I don't see centering as a big issue with a properly sized CT.
 

iwire

Moderator
Staff member
Location
Massachusetts
The CTs I have are not split and they are nice and small. I actually have to double check and see if they are rated enough for both loads.
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Bob, may I ask what the actual application is here? Metering two 3? 50A circuits doesn't sound like a service.
 
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iwire

Moderator
Staff member
Location
Massachusetts
Bob, may I ask what the actual application is here? Metering two 3? 50A circuits doesn't sound like a service.

Sure, here is a recap of the thread. :)


It is actually metering for a grid tied PV system and in truth I have two sources and one 'load' but if I drew it that way I think the focus would have been on the two sources to one load and not the metering. :)

The job has two small PV arrays supplying two 13 KW 208 V 3 phase inverters that I have to back feed into the buildings MDP. But somewhere along the way I have to provide revenue grade metering of the PV systems total AC output. This is to satisfy some rules of an incentive program.

Each of the inverters has a data output that supplies all kinds of specific information that will be used to drive an interactive display on a large touch screen. This is in a new public library so it's more show than go.

However none of that metering meets the rules of the organization providing the rebates. They require revenue grade metering of the total PV AC output. They would allow me to use multiple meters but do not require it. The power company is on their own for their metering.


:)
 

dbuckley

Senior Member
Kirchoff's Law applies here; the total current through the wire(s) will be the same whether you split before or after the transformers, and its current that generates the magnetic flux that the CTs measure.

So yes you can do what you want todo.
 
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