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Author Topic: Large 3 phase CT meter. Is it right?  (Read 2587 times)
Scruff
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« Reply #15 on: June 12, 2018, 09:33:07 AM »

 Huh CTs don't have a polarity.

Hall effect sensors do.
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Iain
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« Reply #16 on: June 12, 2018, 09:43:09 AM »

Huh CTs don't have a polarity.

Hall effect sensors do.

If they are used in conjunction with voltage sensing to measure phase angle and current I am sure they are.

They will give the correct current reading either way around but if used with a voltage reference I am sure they are directional sensitive.

https://www.schneider-electric.co.uk/en/faqs/FA203850/

Quote
.CT's installed backwards provide inverted signals to the meters to which they are connected. Most meters note the polarity of the signal received from the current transformers, and use that polarity to help determine the direction of the flow of electricity, assigning a positive or negative value to calculated power and energy. It is imperative that current transformers are installed correctly with respect to their orientation on the load conductors that pass through them.

Iain
« Last Edit: June 12, 2018, 10:14:11 AM by Iain » Logged

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Scruff
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« Reply #17 on: June 12, 2018, 09:48:13 AM »

Most cheapy OWL type meters with only one CT are calculating export using pf, so I'm sure there's something to it.

Strictly speaking AC doesn't have a polarity either just an earth reference.

Seems easier for sparkies to think power enters the live and exits the neutral instead of oscillating at 50hz. ....I blame facebook... tumble
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Stig
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« Reply #18 on: June 12, 2018, 10:47:54 AM »

Seems easier for sparkies to think power enters the live and exits the neutral instead of oscillating at 50hz. ....I blame facebook... tumble

Nah, with Facebook the power flows in the same direction as the data, from the user to the big company...
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Scruff
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« Reply #19 on: June 12, 2018, 11:09:24 AM »

Data.  hysteria
...I'd consider Monty Python a more reliable source..
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roys
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« Reply #20 on: June 12, 2018, 06:11:53 PM »

In days gone by I remember (only just 😀) flick testing ct's in 3 phase systems to ensure they were all connected with the same polarity.  Probably the last time I used an anolgue meter.
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Westie
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« Reply #21 on: June 12, 2018, 06:13:41 PM »

Huh CTs don't have a polarity.

Hall effect sensors do.

CT Polarity is applicable and is  important in a 3 phase power measurement system. In a single phase system it wouldn't make any difference to measured power but in a 3 phase measurement system you need to get each CT mounted the same way around physically and connected the same way around electrically, if you get one wrong the output current from that CT will be 180 degrees out of phase which will completely screw up the power measurement circuit.  If you look at Guys picture you can see the CT's connections are 'polarity' marked (sometimes marked with colours , other times X1,X2). Also CT's are marked to show their correct physical orientation,  physical sides are marked line and load, sometimes marked H1 on the line side and H2 on the the load side.

Also check the three phase connection to the meter they must also be in sequence with the CT inputs ie Red CT - Red phase , Blue CT - Blue phase...etc

As I said before if your meter is showing a Pf of .78 leading the meter is almost certainly connected incorrectly.

« Last Edit: June 12, 2018, 06:50:13 PM by Westie » Logged

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Westie
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« Reply #22 on: June 12, 2018, 06:21:20 PM »

In days gone by I remember (only just 😀) flick testing ct's in 3 phase systems to ensure they were all connected with the same polarity.  Probably the last time I used an anolgue meter.

Yep,  used the same method to detect polarities (ie winding direction) on 3 phase motors that had 6 ends out and no cable markers.
« Last Edit: June 12, 2018, 06:24:25 PM by Westie » Logged

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Scruff
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« Reply #23 on: June 12, 2018, 09:13:46 PM »

Hrmmm tis a most vexing subject because the terminology has equal and opposing meanings I guess....and I'm a bit thick when it comes to triangles

Alternating current does not have a polarity or it can be argued it has but it inverts at 50hz.
To me polarity is the direction of a magnetic field or electron flow. I guess in this context it means front and back?

Like earth and ground. Earth = planet reference, ground = chassis reference. They can be the same (if you connect them) but they are not. 
I've only ever used CTs on single phase and they never read negative in either orientation.

It would help to be specific we are talking about apparent power measurement not real power. P = IVpf not P=IV....
Guy are you being billed for reactive power?

I'm not entirely getting how being 180 outtov phase will affect measurement but probably straight forwards if it was drawn.
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guydewdney
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« Reply #24 on: June 12, 2018, 09:25:15 PM »

I'm billed in kwh not kvah. There's no reactive power element to the bill. It's essentially a domestic bill but with an extra zero.....
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roys
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« Reply #25 on: June 12, 2018, 10:25:16 PM »

Why have you got a Japanese fighter plane on your bill😀
Anyways so if you are not getting billed for any reactive (kVAr) then surely your PF doesn't matter, however doesn't answer the question of why your PF is leading, which could be pointing to a connection error somewhere, what is your main loads? If you say you are running a big sychronous motor with lead PF, of happen to have connected up a large bank of capacitors you had lying about, then that would explain it. But if it is normal domestic or motor loads then it should be lag.
By the way your leccy bill is huge.
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roys
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« Reply #26 on: June 12, 2018, 10:31:38 PM »

Think I would also be borrowing 3 tong testers and putting them on the tails and monitoring for say the odd hour over different periods during the day and week as a spot check to see what:
the current draw was,
Identify what the loads were,
Work out roughly what your daily then quarterly consumption would be, multiply by tariff and see if it i roughly tallies with bill.
I you were near me you could borrow my testers but I think you are at the other side of the country.
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roys
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« Reply #27 on: June 12, 2018, 10:34:50 PM »

And another thought.
The meter reading hasn't been read wrong has it? Maybe a big digit entered wrongly.
Have you checked meter reading tally with bill.
Sorry if this is obvious I am just thinking out loud.
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knighty
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« Reply #28 on: June 12, 2018, 10:36:20 PM »

you could push for a smart meter?  (or 3 of them if they have no 3 phase ones? - my 3 phase at work is 3x single phase meters)

there's no point/need for the CT clamps unless you have massive loads?


does summer/winter make much difference to your bill?

with your water wheel and the pile of solare you have I thought you're bill would be much less than that
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Westie
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« Reply #29 on: June 13, 2018, 08:55:05 AM »

I'm billed in kwh not kvah. There's no reactive power element to the bill. It's essentially a domestic bill but with an extra zero.....

Okay, so your bill is in kwhr.   Looking at the picture you posted the CTs all seem the right way around and the cabling to the JB bleow the meter all looks okay, obviously we can't vouch for the meter connections as we can't see behind the cover.  So, assuming the connections are correct we should think of other options.  #

Have you changed supplier recently?

Are you on single rate or E7?

The Mk10A meter is smart, is it read remotely or do always provide the reading?

If you provide the reading do you always give the them the T0 register figure for Total kWHr?

The MK10 meter is able to read MD, Export, kVAh and kVArh.  This is stored on Set B registers. They can be
read by holding down the Display Select button for 3 seconds this switches the meter
between Set A registers and Set B registers. If they are reading remotely the metering agent (not always the same as the supplier!)  could be transposing the T0 ans S6 registers so your billed for reactive power instead of real power.  I ask if you changer supplier recently because this sort of error seems to crop up when a new supplier takes on your smart meter.  

What is the kVArh reading on your meter, ie the S6 register, also what does S5 read - does the S5 or S6  value tally with the billed value?
 
« Last Edit: June 13, 2018, 08:59:27 AM by Westie » Logged

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