I need help with a current transformer diagram

[iwire]

I hope someone can help me out here. :smile:

What do the black dots (red circle) represent on this diagram?

Would a CT manufacturer call that black dot 'X1'?

Would it be a problem if all three CTs where wired the opposite way as shown?

Thanks in advance, Bob

 

[iwire]

Here are the CTs being used.

H681x-V Install

 

[zog]

The black dot is showing polarity, CT's have to be installed the right way physically or the ratios will be off. Not the same as X1, the dot is not a connection point, think of it as an arrow showing diretion of current flow.

 

[qcroanoke]

I wonder if it is indicating source side of the CT?
Just a guess..

 

[iwire]

The black dot is showing polarity, CT's have to be installed the right way physically or the ratios will be off.

OK, they are physically installed in the correct direction.

That much I know, at least the labels are facing the right direction.

Not the same as X1, the dot is not a connection point,

So what is the significance of the CT company providing a white and black with a label indicating "White is X1" and where would X1 have to land on that meter diagram?

 

[jim dungar]

If the CT is being used to feed a single current relay or ammeter, the polarity dots are not needed. However, if the polarity dots CT's feeding a 3-phase device are not consistent then two of the CT's will be connected in an additive configuration and one in a subtractive one. This mismatch of will cause the subtractive mode CT to be 180? out of phase which means any device, like Power Factor, that depends on phase angles will be very confused. The next problem is if all three polarity dots face the same way but in a different direction than the VT's. Again this would cause an unexpected phase shift, a typical result is the KW readings being negative.

 

[iwire]

Yes KW readings are messed up.

This job has about 20 meters and not all are working properly, also some meters are running two sets of CTs into one set of inputs to watch the combined total of two different feeders.

How is power factor determined in that case? Average?

 

[whillis]

So what is the significance of the CT company providing a white and black with a label indicating "White is X1" and where would X1 have to land on that meter diagram?

The label should indicate the line and load side of the CT or there will be a painted dot on the line side of the CT opening. When current flows from the line to load side then the secondary current will flow from the white to the black wires. In your diagram the white wire is the one with the dot beside it.

 

[drbond24]

Just for educational purposes, this is out of my "Electric Machinery Fundamentals" textbook:

1. If the primary voltage is positive at the dotted end of the winding with respect to the undotted end, then the secondary voltage will be positive at the dotted end also. Voltage polariites are the same with respect to the dots on each side of the core.

2. If the primary current of the transformer flows into the dotted end of the primary winding, the secondary current will flow out of the dotted end of the secondary winding.

From another section of the same textbook:

...the dots help determine the polarity of the voltages and currents in the core without having physically to examine its windings. The physical significance of the dot convention is that a current flowing into the dotted end of a winding produces a positive magnetomotive force, while a current flowing into the undotted end of a winding produces a negative magnetomotive force. Therefore, two currents flowing into the dotted ends of their respective windings produce magnetomotive forces that add. If one current flows into a dotted end of a winding and one flows out of a dotted end, then the magnetomotive forces will subtract from each other.

Now, a lot of that doesn't apply here since there is only one black dot on each transformer. Looks to me like they just want to make sure you get all three CTs wired with the same polarity so they are additive. Terminals I11, I21, and I31 should be connected to the line side of each phase, respectively.

How is power factor determined in that case? Average?

As you know, the power factor is a ratio of real power to apparent power. If you were combining two circuits into one set of readings, the power factor will be the the real/apparent ratio for the combination of the two feeders. It would be a completely different value than the power factor for either of the two feeders themselves.

As long as it took me to type all of that, someone else has probably already responded with a much more informative and helpful post in the meantime...(edit: I was wrong!)

 

[weressl]

I hope someone can help me out here. :smile:

What do the black dots (red circle) represent on this diagram?

Would a CT manufacturer call that black dot 'X1'?

Would it be a problem if all three CTs where wired the opposite way as shown?

Thanks in advance, Bob

Depends on the device. A watthourmeter would indicate reverse flow.(If it is capable to do so, many are not.)

I presume that you mean that the marked terminals would face the opposite direction, but all the wiring would remain the same and not that the unmarked leg of the A phase CT would be wired to I11.

 

[mikeames]

Yes KW readings are messed up.

This job has about 20 meters and not all are working properly, also some meters are running two sets of CTs into one set of inputs to watch the combined total of two different feeders.

How is power factor determined in that case? Average?

Should those CT's then be wired in series?

 

[gar]

081202-1434 EST

iwire:

Simplify the circuit to single phase and look at A and N.

Connect the voltage A-N to one scope channel, N being connected to common, and synchronize at a positive going zero crossing of the voltage. Connect the current transformer output to the other channel with I12 to common. Place a resistive load on the circuit. Both displayed waveforms will be in phase. Essentially superimposed but of different amplitudes.

Next reverse the current transformer leads or invert the orientation of the current transformer. Now the current waveform will be 180 deg out of phase with voltage.

Next take two in phase sine waves and multiply these together point by point. The result is a double frequency sinewave displaced from the zero axis by 1/2 of the peak to peak value. All instantaneous values are positive and the average value is 1/2 the peak to peak value.

Do the same thing where one waveform is 180 deg out of phase. The result is essentially the same except now the average is negative.

+1*+1 = +1, and +1*-1 = -1.

In one case we indicate positive power flow, and in the other negative power flow, load to source instead of source to load.

A three phase wattmeter will simply added the three separate single phase power measurements. This might be done all in one watt-hour meter mechanism.

.

 

[quogueelectric]

As I understand it it is to correctly orient the cts in the same direction. They should all be oriented the same way around the load as per mfgs install instructions. I have seen these installed wrong a couple of times.

 

[weressl]

.....also some meters are running two sets of CTs into one set of inputs to watch the combined total of two different feeders.

That is an ABSOLUTE no-no.

 

[Cold Fusion]

That is an ABSOLUTE no-no.

Why is that? The currents from paralleled CT add just fine (vector-wise).

cf
(Hummm ... Is "vector-wise" a word)

 

[djohns6]

Why is that? The currents from paralleled CT add just fine (vector-wise).

cf
(Hummm ... Is "vector-wise" a word)

This is done all the time in Bus differential relaying .

 

[weressl]

This is done all the time in Bus differential relaying .

That is incorrect. Each CT is to be bought into a separate input. See attached diagram from BE1-87T.

 

[weressl]

Should those CT's then be wired in series?

Current is additive in parallel connection, voltage is additive in series connection.

But it is not applicable to CT's and PT's in metering or protective relaying functions.

 

[djohns6]

That is incorrect. Each CT is to be bought into a separate input. See attached diagram from BE1-87T.

I think we're talking about two different things . What I'm thinking about is multiple breakers on a bus , with a " sum point " . Each phase current then goes to a bus relay , Example , an ABB type KAB . Maybe I got away from the original question . :smile:

 

[iwire]

Thanks to everyone that has responded so far. :smile:

That is an ABSOLUTE no-no.

Well I am going to need a bit more than 'Laszlo says it is an ABSOLUTE no-no.'

The equipment and design was by a Siemens Engineer using Siemens meters. An EC walking in and saying it's all wrong without something to back that up will be laughed all the way out the door.

This metering is not for protection, it is to monitor energy consumption of various feeders to assign electrical costs to the right departments in a large company.