Why are analog watt meters and demand meters wired? 2

[majesus]

I have an incoming three phase 3w system at 12.5kV. It goes into a switchgear housing that has a pf, a wattmeter, a voltmeter and an ammeter. I want to know how these things work, so I did some digging in my text book, but I am really surprised how most of my engineering books lack the detail on metering circuits.

Thus, I did go to the library and I found some diagrams from the Handbook of Electric Power Calculations however, it still lacks the details I am looking for. I did photocopied them and I shall posted them below.

Ammeter and voltmeter is straightforward. Power factor you can get by measuring the current and phase, pretty easy as well. But the wattmeter bugs me, the diagram is shown here:

http://i48.photobucket.com/albums/f223/majesus/wattmeter.jpg

It requires CTs on two legs and PTs on two phases as well.
Since W= VLL*Iph*cos(O)*3^.5
Why can't we just simply measure the Line current? Why do we need the input from two current and two PTs?


Similar, the demand meter requires three CT and two PTs.
I wonder why?

http://i48.photobucket.com/albums/f223/majesus/demand.jpg

 

[waross]

You could just measure one phase and multiply by root 3 or by 3 depending on the connection. You would be measuring one phase and assuming that the voltages and currents in the other 2 phases was equal. Not a safe assumption. The connection that you have with 2 PTs and 2 CTs accurately measures the total current even when the currents and voltages are not equal.
However;
I was in a spot trying to get a plant online and the delivery of the metering equipment was behind schedule. We used single phase metering on one leg of a 277/480 volt service. We multiplied by three for the three phases. Our total multiplier was 360. When the proper equipment arrived, we left the single phase meter connected for 2 or 3 months as a check. It was tracking within about 1 1/2%. (I knew going in that the load was well balanced.)
I was working in a very permissive jurisdiction.
respectfully

 

[majesus]

Yeah... that's what I thought, I dunno if this is a hard thing to ask, but it would it be too much technical description to ask how having two current inputs and two voltages would give you a more acurate watt meter? How does it give a power value if the system is say unbalanced.

Futhermore, why would a demand meter require three current and only two voltage?

I may have to buy a specific book on this, because I must of opened a dozen or so distribution engineering books from the CRC, IEEE, my university books and books we have at my company and the library, but I couldn't find much that would give a good description as to how they work. I did find some info on single phase, but this is three phase. Much more interesting

 

[jghrist]

Try

http://themeterguy.com/

and click on Meter Theory. This will include some connection diagrams as well as the explantion of the theory including Blondel's Theorem:

Blondel's Theorem: "In a system of N conductors, N-1 meter elements, properly connected, will measure the power or energy taken. The connection must be such that all potential coils have a common tie to the conductor in which there is no current coil."

 

[edison123]

Try "Electrical Measurements" by Golding. I do not know if this excellent book is still in print. If you can track down an old one, it would give you the theory and practice of all sorts of electrical measurements (in the analog era).

 

[majesus]

http://themeterguy.com/

great! Thank you for the resource. I will read it this evening.


Yeah I found a bunch of books online that looked great. "Electrical Measurements" by Golding, I'll do a library search and see is I can locate it.

Thank you guys for the great help.

 

[Eleceng01]

Another great resource is the "Handbook for Electricity Metering."

 

[DanDel]

Only two CTs are necessary because the third phase is the summation of the two measured phases in a three-wire system. Basically the same reason for the voltage.
The reason you need both the voltage and the current is so you can measure the angle between them to calculate Watts.

 

[stevenal]

Try searching on Blondel's theorem.

 

[majesus]

Oh thanks for reminding me... Usually when I find a great resource after you guys point me into the right direction I'll post it here. Thus it will be easier for the next Junior engineer who is following in the footsteps as well as to provide some relevant reference for other's own interest.

Anyways a great read on Blondel's theorem is here:

https://www.ohiosemitronics.com/pdf/tech_papers/Two-wattmeter-method(E).pdf

Blondel's theorm is a very neat and simple idea, Blondel was a smart guy.

 

[majesus]

Here is some more:

https://www.ohiosemitronics.com/pdf/tech_papers/Single Element on 3-phase(D).pdf

"USE OF A SINGLE ELEMENT WATTMETER OR WATT TRANSDUCER
ON A BALANCED THREE-PHASE THREE-WIRE LOAD
WILL NOT WORK. HERE'S WHY."

https://www.ohiosemitronics.com/techLibrary.asp

provides a lot of good reads.

 

[waross]

"USE OF A SINGLE ELEMENT WATTMETER OR WATT TRANSDUCER
ON A BALANCED THREE-PHASE THREE-WIRE LOAD
WILL NOT WORK. HERE'S WHY."

This statement would be more accurate if the word "load" was replaced with the word "system".
A single element wattmeter may be used to measure a phase to neutral load on a four wire system. The phase current will be in phase with the phase to neutral voltage and the 30 deg. shift is avoided.
If the voltages and currents are equal, the the single phase reading can be extrapolated to obtain the total three phase consumption.
respectfully

 

[majesus]

Will it work if the power factor is not at unity?

On page 5 of: "USE OF A SINGLE ELEMENT WATTMETER OR WATT TRANSDUCER ON A BALANCED THREE-PHASE THREE-WIRE LOAD
WILL NOT WORK. HERE'S WHY CONCLUSION."



CONCLUSION
Use of a single element watt transducer measuring the power between two phases of a three phase connected system does not work except at unity power factor (phase angle shift of 0°).



BTW, how did you do the quote insert in your post?

 

[davidbeach]

BTW, how did you do the quote insert in your post?

You mean like that? In the message entry area that looks like:

[ignore]

BTW, how did you do the quote insert in your post?

[/ignore]

To learn more click on the words "Process TGML" below.

When using any of the TGML it is always helpful to click "Preview Post" before submitting.

 

[majesus]

Thanks David