CT placement and phase angle

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mull982

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I have been doing some work with some protective relaying and metering and have come to learn that the placement and orientation of a CT on a circuit determines the current phase angle for relaying purposes. If I understand it correctly, if the CT is oriented in one direction it will give a phase angle 180deg out of phase in which it was mouned the opposite direction. If the CT is mounted incorrectly then this phase angle can lead to errornaous readings. What causes this phase angle difference to be seen by the relay with the orientation of the CT?

I understand it has something to do with the polarity of the primary and secondary of the CT. The primary winding of the CT is the circuit or cable being measured, so is the polarity of this winding positive for the direction in which current flows. What dertermines weather the secondary winding is positive or negative polarity. How does this polarity matter for relaying and metering?
 

jim dungar

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mull982 said:
I understand it has something to do with the polarity of the primary and secondary of the CT. The primary winding of the CT is the circuit or cable being measured, so is the polarity of this winding positive for the direction in which current flows. What dertermines weather the secondary winding is positive or negative polarity. How does this polarity matter for relaying and metering?

Every transformer has polarity markings (even power transformers), they are commonly called H1 and X1. A CT is wound so that when primary current enters H1 then secondary current leaves X1.

Keeping it very simple (KISS). If the CT is installed with H1 pointing toward the source (electrically) then then current coming out of X1 = I (or I @ 0?), but if the CT is installed pointing away from the source (electrically) then then current coming out of X1 = -I (or I @ 180?).

The orientation of CT's in a circuit is dependent on what they will be doing. If you are feeding single ammeters then orientation is not important, but if the currents must be algebraically combined, like in a KWH meter or a ground fault relay, then need to know what direction any one current is flowing versus the other ones.

For example (remember KISS), we know that in a GF relay balanced currents must sum to zero: Ia+Ib+Ic=0
But if the C phase CT is installed "backward" our relay is actually measuring Ia+Ib-Ic.
 

mull982

Senior Member
jim dungar said:
Every transformer has polarity markings (even power transformers), they are commonly called H1 and X1. A CT is wound so that when primary current enters H1 then secondary current leaves X1.

Keeping it very simple (KISS). If the CT is installed with H1 pointing toward the source (electrically) then then current coming out of X1 = I (or I @ 0?), but if the CT is installed pointing away from the source (electrically) then then current coming out of X1 = -I (or I @ 180?).

The orientation of CT's in a circuit is dependent on what they will be doing. If you are feeding single ammeters then orientation is not important, but if the currents must be algebraically combined, like in a KWH meter or a ground fault relay, then need to know what direction any one current is flowing versus the other ones.

For example (remember KISS), we know that in a GF relay balanced currents must sum to zero: Ia+Ib+Ic=0
But if the C phase CT is installed "backward" our relay is actually measuring Ia+Ib-Ic.

This was the exact explanation that I was looking for Thanks! I have a metering module that is giving me a negative kW reading. I am suspecting that it is because the CT's are installed backwards. (I will try to post a drawing) If the CT's are installed backwards (reverse polarity) can I simply just swap the CT wiring going to the metering module or do I need to physically flip the CT around the other direction giving it the correct polarity?

This might me digging into more than I bargined for, but is there a good scientiic explainatin why the phase anges change from the orientation of the CT.
 

jim dungar

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mull982 said:
This was the exact explanation that I was looking for Thanks! I have a metering module that is giving me a negative kW reading. I am suspecting that it is because the CT's are installed backwards. (I will try to post a drawing) If the CT's are installed backwards (reverse polarity) can I simply just swap the CT wiring going to the metering module or do I need to physically flip the CT around the other direction giving it the correct polarity?

This might me digging into more than I bargined for, but is there a good scientiic explainatin why the phase anges change from the orientation of the CT.

When measuring KW you must make sure that all of your CT's are pointed in the same direction and that they correspond to the orientation of the VT's as well as that they are on the correct conductors. Ideally VT's would be connected with their H1 lead on the first line; the VT for Vab should have its H1 lead on A, Vbc has H1 on B, and Vca on C. The CT's should have their H1 side pointing toward the source of the current.

In many cases you can swap the CT secondary leads instead of remounting the CT.

The scientific explanation (using KISS) is: this is very similar to the polarities of the leads in a DC circuit. We want to measure all of the variables at the same relative points of the sine waves (measure I going up at the same time V is going up), if our leads are reversed we get the correct magnitude but the wrong sense of direction.
 

mull982

Senior Member
Jim

Thanks for the explanation! Its starting to become clear.

I've attached a drawing of the particular circuit for which I am experience negative power readings. If you look at the metering CT's in relation to the direction of current flow it appears that the H1's are pointing away from the source of current (or negative polarity) thus giving a negative phase angle.

(You may have to zoom in on the dwg to see the H1 polarity dots)

The X1's appear to be heading directly into the relay input which you mentioned is correct in a previous post. I'm thinking I can just swap the CT leads and have the X2 leads go into the relay rather than change the CT orientation in order to get the correct phase angle. Can this be done in this case?

The PT's appear to be correct, with H1 facing the direction of current flow as you mentioned.

Am I getting a negative kw and kVar reading on the meter because it is multiplying a positive voltage times a negative current? The kVA value that I am getting is positive however. Is this because kVA is an absolute value quanity?
 

jim dungar

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mull982 said:
Jim

Thanks for the explanation! Its starting to become clear.

I've attached a drawing of the particular circuit for which I am experience negative power readings. If you look at the metering CT's in relation to the direction of current flow it appears that the H1's are pointing away from the source of current (or negative polarity) thus giving a negative phase angle.

(You may have to zoom in on the dwg to see the H1 polarity dots)

The X1's appear to be heading directly into the relay input which you mentioned is correct in a previous post. I'm thinking I can just swap the CT leads and have the X2 leads go into the relay rather than change the CT orientation in order to get the correct phase angle. Can this be done in this case?

The PT's appear to be correct, with H1 facing the direction of current flow as you mentioned.

Am I getting a negative kw and kVar reading on the meter because it is multiplying a positive voltage times a negative current? The kVA value that I am getting is positive however. Is this because kVA is an absolute value quanity?

From your diagram it appears that the CT's are wired correctly, however you PT's are backwards. The H1 leads should be connected to Lines A and C with the H2 leads both connected to line B.
 

jim dungar

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jim dungar said:
From your diagram it appears that the CT's are wired correctly, however you PT's are backwards. The H1 leads should be connected to Lines A and C with the H2 leads both connected to line B.

Oops, exceeded the editing time.

What I meant to say was:
From your diagram it appears that the Metering CT's are wired backward (pointing toward the load), however your PT's are most likely wired incorrectly. The PT H1 leads should be wired to Lines A and C with the H2 leads both connected to Line B.

Is this an electronic power meter? Do the rest of your values (i.e. V12=V23=v31, Ia=Ib=Ic, PF, and KVAR) look correct?

Yes, KVA is an absolute value.
 

mull982

Senior Member
Jim

I agree the CT's appear to be backwards. The thing that helps solidfy this, is that in a feeder breaker section of this switchgear I have the exact same meter and am getting a positive kW and kVar power readings. The CT in this section oriented the same direction as the one in the main section except because this section is a feeder breaker the current is flowing the opposite way and therefore the H1's of the CT are facing the direction of the current source. (See attached). Can I just switch the CT leads on my main breaker to correct the reversal issue?

You are right there is most likely a problem with the VT's. I am getting a weird phase angle for Vca=300deg rather than the 240deg one would expect. I see what you are saying about the way the VT's should be wired, but the confusing part is, I have this exact same meter elsewhere in the plant with the same VT arrangement and my phase angles look correct. I'm not sure if it may be the wiring then. Just for reference how are only two PT's used in this arrangment to give all three voltages references:

Also for reference here are the phasors I read from the meter (PQMII) on my main breaker where I am getting the negative readings:

Voltage: Vab=4137@ 0deg Lag
Vbc=4181@ 120 deg Lag
Vca= 4172@ 300deg Lag

Current: Ia=27A@ 255deg Lag
Ib=27A@ 18deg Lag
Ic=27A@ 130deg Lag.

On the meter that mentioned was reading correctly the voltage phasors where the same as above and the current phasors were:

Ia= 27A@ 70deg Lag
Ib= 27A@ 189deg Lag
Ic= 27A@ 311deg Lag

As you can see these current phase angles are a 180deg out of phase from the ones listed above b/c of he CT orientation as you explained.
 

jim dungar

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What matters is the relationship between any 1 CT and its associated VT. You may swap the secondary leads of any transfromer rather than swapping the primary (or turning it around).

If you have a 3-Phase 3-Wire source (a transformer with no X0 connection) then you only need 2 VT's connected in an open-delta arrangement. most meters have a menu that lets you setup the number of VT's and CT's connected to it.
 

djohns6

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Louisiana
I see that the relay is an SEL 387L . This is a line current differential relay , used to protect two terminal transmission lines . Why is it being used in this application ? I only see one set of current inputs being used . Am I missing something here ?
 

djohns6

Senior Member
Location
Louisiana
Yeah , I missed something . The 387L only uses one set of current inputs .
It communicates with another 387 via fiber optic cable . I was thinking of a 387 used in Transformer diff applications , with multiple current winding inputs . Never mind . :cool:
 

mull982

Senior Member
jim dungar said:
What matters is the relationship between any 1 CT and its associated VT. You may swap the secondary leads of any transfromer rather than swapping the primary (or turning it around).

If you have a 3-Phase 3-Wire source (a transformer with no X0 connection) then you only need 2 VT's connected in an open-delta arrangement. most meters have a menu that lets you setup the number of VT's and CT's connected to it.

Jim

When you say the relationship btwn 1 CT and it associated VT I'm assuming this means that:

Ia CT must match up with Vab VT
Ib CT must match up with Vbc VT
Ic CT must match up with Vca VT

Is this correct? The power factor that I am seeing is about .76 so I figure that I should see about a 40deg phase shift btwn the voltage and current.

For example if Vab is at 0deg then I should see Ia at about 320deg. Is this correct?

djohns6 said:
Yeah , I missed something . The 387L only uses one set of current inputs .
It communicates with another 387 via fiber optic cable . I was thinking of a 387 used in Transformer diff applications , with multiple current winding inputs . Never mind . :cool:

you are looking at the wrong relay. The CT's we are refering to are the top set which are connected to a Multilin 750 and PQMII meter.
 

jim dungar

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mull982 said:
Jim

When you say the relationship btwn 1 CT and it associated VT I'm assuming this means that:

Ia CT must match up with Vab VT
Ib CT must match up with Vbc VT
Ic CT must match up with Vca VT

Is this correct? The power factor that I am seeing is about .76 so I figure that I should see about a 40deg phase shift btwn the voltage and current.

For example if Vab is at 0deg then I should see Ia at about 320deg. Is this correct?

That is the way I do my troubleshooting.
 
Just a observation.
The CT's are wired Wye.
The PT's are wired Delta.
Wouldn't this result in a 30% phase shift between the two?

I've never used any delta configured PT's except for some of the older directional schemes that use an open delta for polarizing voltage or something like that.

Peter
 

quogueelectric

Senior Member
Location
new york
Xformer polarity can sometimes be critical especiallyin higher voltages. If the xformer is wound on a grounded iron core base you dont want the hv output side too close to the grounded frame. When the winding originates close to the iron core and winds itself outwards from the center you want the voltage gradient to increase gradually. Not like putting the 2000 volt output of the winding right next to the grounded core. You want to start with the grounded side of the winding close to the grounded core and gradually increase voltage levels as you wind outward.
 
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