B phase ct opposite to A and C??

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machasman

Member
Location
upstate NY
I am replacing GE M_90 Watthour meters with Powerlogic's PM series meters. I have 4 to replace on 2 indoor GE substations (circa 1950's). The ct's are not accesible without racking out the main breaker. The schematics are showing A and C phase ct's on the load side of the main with the polarity towards the Line (bottom of breaker) The B phase ct is on the line side of the breaker with its polarity towards the load (top of breaker) I cant explain this however it appears that my meter is in fact working please check my math on this

474V X 96A X 1.732 / 1000 = 78.81 KVA

78.81 X (PF) .865 = 68.17 KW

These are the actual numbers from the meter
Anyone please explain the CT orientation and let me know if I can trust these numbers

Thanks in advance
 
I am replacing GE M_90 Watthour meters with Powerlogic's PM series meters. I have 4 to replace on 2 indoor GE substations (circa 1950's). The ct's are not accesible without racking out the main breaker. The schematics are showing A and C phase ct's on the load side of the main with the polarity towards the Line (bottom of breaker) The B phase ct is on the line side of the breaker with its polarity towards the load (top of breaker) I cant explain this however it appears that my meter is in fact working please check my math on this

474V X 96A X 1.732 / 1000 = 78.81 KVA

78.81 X (PF) .865 = 68.17 KW

These are the actual numbers from the meter
Anyone please explain the CT orientation and let me know if I can trust these numbers

Thanks in advance

Fret not......:smile:When you consider the current flow from the supply toward the load, in effect all your CT's ARE facing in the same direction.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
When you consider the current flow from the supply toward the load, in effect all your CT's ARE facing in the same direction.
I'm sorry, but I don't follow.

A grounded-B on a Delta wouldn't change anything, and a Y would be symmetrical, too.

Whether before or after the breaker, the CT arrows should all point the same way.
 

machasman

Member
Location
upstate NY
Still confused

Still confused

If I one line the wiring the current would flow from the 300KVA 11460/480 Y Xfmr to the 600 A main onto the Bus. It was always my understanding that all the ct's should be seeing the current flow in the same direction. having one or all reversed wont effect V and I readings but they do cause problems when metering ??? please keep the ideas coming, I'm stumped especially since the ct's are on the print this way and so is the old M-90 meter. Another oddity on this gear is that it is phased ccw as per the print.
 

netaguy

Member
Location
Providence, RI
If I one line the wiring the current would flow from the 300KVA 11460/480 Y Xfmr to the 600 A main onto the Bus. It was always my understanding that all the ct's should be seeing the current flow in the same direction. having one or all reversed wont effect V and I readings but they do cause problems when metering ??? please keep the ideas coming, I'm stumped especially since the ct's are on the print this way and so is the old M-90 meter. Another oddity on this gear is that it is phased ccw as per the print.

Check out this site when retrofitting existing metering.

http://www.susiadapters.com/index.htm

I replaced many old Westinghouse and GE meters with Square D Powerlogic and these are a real labor saver.

The tech support is great and will be able to answer your questions.
 
I'm sorry, but I don't follow.

A grounded-B on a Delta wouldn't change anything, and a Y would be symmetrical, too.

Whether before or after the breaker, the CT arrows should all point the same way.

The CT's on a diagram point the same way, with the polarized side pointing toward the power 'flow' direction. If you put the B phase to the incoming side of the breaker, it will point toward the breaker and the two others, on the load side of the breaker, will point away from the breaker. Capise?;)
 

ohmhead

Senior Member
Location
ORLANDO FLA
The schematics are showing A and C phase ct's on the load side of the main with the polarity towards the Line (bottom of breaker) The B phase ct is on the line side of the breaker with its polarity towards the load (top of breaker)



Well looking at what hes saying the B donut is not in the same direction of current flow ? A & C are facen the line bottom and B is facen the load top ! most power companys say that when you install current transformers they will all face the line side incoming only not the load side to me this will effect the B power ct .

ALL DOTS WILL BE THE SAME ? comments Question why are you replacing these ?
 
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machasman

Member
Location
upstate NY
Meter adapters

Meter adapters

Thanks for the link to the susi page, I thought there was something like that availiable, I still dont think it will record correctly with b phase ct out of polarity, Also this gear did not have shorting blocks so I added them. then set a terminal strip and moved all 14 wires off the old meter and landed them on the new strip, then out to the rest of the control circiuts.

I will scan the drawing and post a link to it so everyone can see

I want to do another shut down and look at the cts but this gear feeds some law enforcement test labs and assorted courtrooms and parts of the jail security systems so its a major headache to schedule.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The CT's on a diagram point the same way, with the polarized side pointing toward the power 'flow' direction. If you put the B phase to the incoming side of the breaker, it will point toward the breaker and the two others, on the load side of the breaker, will point away from the breaker. Capise?;)
Yes, but:
The schematics are showing A and C phase ct's on the load side of the main with the polarity towards the Line (bottom of breaker) The B phase ct is on the line side of the breaker with its polarity towards the load (top of breaker)


The OP said that, ignoring the breaker, the B CT faces one way, while A and C face the other.
 

machasman

Member
Location
upstate NY
Done with this

Done with this

Was back to work today, thought I'd throw my amprobe on the bus to see what the ct's were doing, all my amprobe readings matched what the pm 750 meter was registering. add the math and Bang-----Accurate Kwh readings!

Once upon a time there must have been a reason for inverting the polarity on B phase but over the years someone must have switdhed it, or; the print is wrong. (like that never happens LOL

Thanks
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Once upon a time there must have been a reason for inverting the polarity on B phase but over the years someone must have switdhed it, or; the print is wrong. (like that never happens LOL)
Maybe that CT's wires are connected reversed, too.
 

mull982

Senior Member
I know that reversing a CT will cause it to be 180deg out of phase from the correct orientation and thus cause errors with metering and relaying but I never really understood physically from a theoretical statndpoint how exactly this reversed orientation causes this 180deg difference?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090527-0811 EST

Mull982:

See p 8 of "Alternating-Current Machinery", by Bailey & Gault, McGraw-Hill, 1951.

According to the Ann Arbor News of a day or so ago Google has agreed to make much larger sections of books under copyright available online. I believe this will take considerable time. I tried to look at this information via Google and obviously the court decision has not taken effect yet.

So far Google has scanned about 3 million of the U of M's 8 million books.

You can use Google Books to find libraries nearby that have the selected book.

Following is a quote from p 8:
When a current flows in the secondary, the primary current increases immediately to counteract it magnetically, so that the flux in the core remains practically unchanged.

Note: this holds even when you short the secondary. Most current transformers operate with a 1 turn primary and a many turn secondary. If you have a particular transformer and know the ratio for 1 turn, then loop the primary wire back thru the core a second time and the ratio will be cut in half.

When I did some bench tests on the TED power monitor system I tricked it in two ways.

First, I ran the same current wire thru both current transformers with the transformers phased to oppose each other. This allowed me to see the difference between the two.

Second, I ran 20 turns thru one current transformer to allow me to use 10 A to simulate 200 A thru the system.

.
 
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