CT polarity

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bejoyengineer

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As i was surfing down the forum ,i had noticed an attachment that really confused me with the CT polarity connection. for the first page of the attachment - the CT that is placed between the Incoming Line and CB has the X1(dot) at the bottom indicating the flow of current from the CT would be from X1 to the terminal 1B - FLB - SCTB :1(4AX1) but when it comes after the Circuit Breaker,the X1(dot) comes at the Top indicating the flow of current from the CT would be from X1 to the terminal 1B - FLA - SCTB :1 (1AX1). so the question would be that - why is the CT placed differently before and after the breaker. It could have been placed the same way?

**************Relay*****************************
 
090316-1613 EST

bejoyengineer:

I have no idea. Now some questions.

Are the current transformers being used for any other purpose than to measure current magnitude? In other words is phase relative to any reference of importance?

Do the voltage transformers get used in any interactive way with the current measurements? In other words is power being measured?

What are the circles with A, B, and C? Are these meters, coils, or something else? What is 1A AS, MFM, and 2A VS? AS is apparently a current monitor of some kind. Does AS treat each of the three inputs independently, sum them, or do some other algorithm? Same question for VS?

The designers are consistent on the current transformers in making X2 connect to ground.

.
 
CT Polarity

CT Polarity

bejoyengineer said:
As i was surfing down the forum ,i had noticed an attachment that really confused me with the CT polarity connection. for the first page of the attachment - the CT that is placed between the Incoming Line and CB has the X1(dot) at the bottom indicating the flow of current from the CT would be from X1 to the terminal 1B - FLB - SCTB :1(4AX1) but when it comes after the Circuit Breaker,the X1(dot) comes at the Top indicating the flow of current from the CT would be from X1 to the terminal 1B - FLA - SCTB :1 (1AX1). so the question would be that - why is the CT placed differently before and after the breaker. It could have been placed the same way?

**************Relay*****************************
Click to expand...

My answer to you is you are correct, as long as the devices that each set of Cts supply are separate systems, ie: ammeter, power meter, etc.
If the CT set is part of a larger differential scheme (87) then the polarity
of one set has to be the opposite of the second set since on set is "looking at " the other set to cancel out the currents to zero or no trip condition.
Make sense? :wink:

TT
 
As Gar and I are trying to explain. We have to know what kind of devices the CTs and PTs are suppling before we can give you an intellegent answer.

TT
 
gar said:
090316-1613 EST

bejoyengineer:

I have no idea. Now some questions.

Are the current transformers being used for any other purpose than to measure current magnitude? In other words is phase relative to any reference of importance?

Do the voltage transformers get used in any interactive way with the current measurements? In other words is power being measured?

What are the circles with A, B, and C? Are these meters, coils, or something else? What is 1A AS, MFM, and 2A VS? AS is apparently a current monitor of some kind. Does AS treat each of the three inputs independently, sum them, or do some other algorithm? Same question for VS?

The designers are consistent on the current transformers in making X2 connect to ground.

.
Click to expand...
In answer to your questions AS (ammeter switch), VS (voltmeter Switch), MFM (multifunction meter), PQM (Power Quality Meter), 1A and 2A are probably circuit identification numbers. The ammeter switch and the voltmeter switch select the phase that is to be read on the ammeter or voltmeter, this uses one meter for all three phases. The circles with A, B, and C are the current coils for the SR750 protective relay. On CTs the X1 is normally the polarity of the CT. The upper set of CTs are used for metering and relaying. The lower set is use to supply a Schwetizer Engineering Labs Line Differential Relay (SEL387L).
 
090317-1255 EST

richxtlc:

Thanks.

The AS and VS and their associated meters are of no importance to the question of CT's polarity unless they are phase sensitive meters. Whether MFM requires a particular polarity would depend upon what the MFM does. Assuming that MFM has nothing to do with both the upper and lower A, B, and C, then it is of no concern to the question of the differing polarity of the upper and lower CTs.

Remaining is the relationship of ABC of the top CTs to ABC of the bottom CTs. Whether K1 (key interlock) is open or closed it does not change the the current relationship between the top and bottom CTs. If the switch is open, then none of the CTs have any current flow. If closed, then the CTs on each line see the same current.

Therefore, it would look like the only purpose of the paired CTs in this phasing arrangement would be as a means to detect a CT circuit failure. So what does the Schwetizer Engineering differential relay do and does it make use of the information from the upper and lower CTs. If not, then why two sets of CTs?

Both the MFM and SR750 rectangles show the 3 phase input voltages, but no description of any other inputs and how the data is processed. If MFM is calculating power, and related items that need phase information, then polarity at various points is important.

It seems the real question is what the MFM, SR750, and SEL387L do and what are all the inputs and outputs? There seems to be an RS485 connection to SR750 and one to MFM for output.

If the lower ABC does not relate to the upper ABC, then why would the lower CTs have their polarity reversed?

Without a priori information this diagram is difficult to follow. Rich with your information and after I printed out the diagram, then some items were traceable.

.
 
CT Polarity

CT Polarity

I guess the answer to my question was got from tonytonon answer,the other answers too gave me a better idea of the system. so now i have understood that the CT can be placed in any way if the Cts supply's separate systems, ie: ammeter, power meter, etc. and in case of differential scheme (87) ,the polarityof one set has to be the opposite of the second set since on set is "looking at " the other set to cancel out the currents to zero or no trip condition. Just want to know if i understod it right.
thanks a lot for all the answers
 
090317-1358 EST

Not quite. If you are dealing with an ordinary AC voltmeter or ammeter measuring AC, then the phase of the value being measured is of no importance. In fact for phase to enter in to the subject you need a phase reference. For example the phase angle of a voltage relative to itself is 0. But the phase angle of a current relative to its associated voltage can be anything. The phase angle of a sound wave 10 ft from a speaker might be 20,000 degrees from the excitation of the air at the speaker. This large of a phase angle implies a substantial time delay.

If you have a phase sensitive ammeter with the reference phase being the voltage, then you have a way to measure real power. In fact this is how a wattmeter works. It has a voltage coil and a current coil, and it is somewhat more than a phase sensitive ammeter. It does not require a separate voltage measurement and measures power directly.

So for power or energy measurements the CT phasing is important or you will read negative power when it should be positive or vice versa.

When phase relationships are required, then the phasing of the current transformer or voltage transformer is important.

Note: If some differential measurement was required and the processing was done with electronic circuits, then it would be entirely possible to do the phase inversion in the electronics.

.
 
gar said:
090317-1255 EST

richxtlc:

Thanks.

The AS and VS and their associated meters are of no importance to the question of CT's polarity unless they are phase sensitive meters. Whether MFM requires a particular polarity would depend upon what the MFM does. Assuming that MFM has nothing to do with both the upper and lower A, B, and C, then it is of no concern to the question of the differing polarity of the upper and lower CTs.

Remaining is the relationship of ABC of the top CTs to ABC of the bottom CTs. Whether K1 (key interlock) is open or closed it does not change the the current relationship between the top and bottom CTs. If the switch is open, then none of the CTs have any current flow. If closed, then the CTs on each line see the same current.

Therefore, it would look like the only purpose of the paired CTs in this phasing arrangement would be as a means to detect a CT circuit failure. So what does the Schwetizer Engineering differential relay do and does it make use of the information from the upper and lower CTs. If not, then why two sets of CTs?

Both the MFM and SR750 rectangles show the 3 phase input voltages, but no description of any other inputs and how the data is processed. If MFM is calculating power, and related items that need phase information, then polarity at various points is important.

It seems the real question is what the MFM, SR750, and SEL387L do and what are all the inputs and outputs? There seems to be an RS485 connection to SR750 and one to MFM for output.

If the lower ABC does not relate to the upper ABC, then why would the lower CTs have their polarity reversed?

Without a priori information this diagram is difficult to follow. Rich with your information and after I printed out the diagram, then some items were traceable.

.
Click to expand...

I put the information for the VS,AS etc in answer to a question as to what they were. It is true that the ct polarity has nothing to do with the switches, but the polarity has a definite relation to the function of the MFM (multifunction meter) which includes, watt, vars, pf, amps, volts, watt-hours, etc, the RS485 is for a scada input for both the MFM and the and the SR750.
The SR750 is a multifunction Feeder Management Relay containing the following functions Phase Timed Overcurrent (51P)
 Voltage Restraint
 (12 selectable curves plus 2 Flexcurves)
 Ground Timed Overcurrent (51N)
 Phase & Ground Loset Overcurrent (50PL, 50NL)
 Phase & Ground Hiset Overcurrent (50PH, 50NH)
 Phase & Ground Directional Control
 Zero Sequence (67P)
 Negative Sequence (67N)
 Negative Sequence Instantaneous (46/50) and Time (46/51)
Overcurrent
 Voltage Phase Reversal (47)
 Negative Sequence Overvoltage (59Q)
 Undervoltage & Overvoltage (27-1, 27-2, 59-1, 59-2)
 Underfrequency & Overfrequency (81U-1, 81U-2, 81O-1, 81O-2)
So it serves many functions, which may or may not be used by the user.
The K1 key interlock is designed to allow or prevent the circuit breaker 5-GM1 350 to be opened or closed depending on the arrangement of the circuit breakers on the bus (eg. 2 out of 3 breaker scheme).
The SEL387L is a line current differential relay used for the following:
Protect two-terminal transmission lines and subtransmission lines with the same ratio CTs on each end. Apply a 64 Kb digital communications channel for complete phase and ground fault protection with no settings.
The cts for this relay and their polarities are set for the relationship between the current flow on this end of the feeder and the remote end. It looks at the current flow and converts it to a signal that is proportional to the level and direction at this end and compares it with the signal for the level and direction at the remote end and if it determines that a fault exists on the feeder it opens the circuit breakers at both ends.
The relationship between the polarities between the upper and lower cts has nothing to do with each other, they each serve a different purpose and are used as installed.
 
090317-1830 EST

richxtlc:

If I understand what you are saying the bottom CTs are associated with some CTs somewhere else and that determines what their phasing should be.

Are the RS485 circuits electrically isolated or maybe converted to fiber?

.
 
gar said:
090317-1830 EST

richxtlc:

If I understand what you are saying the bottom CTs are associated with some CTs somewhere else and that determines what their phasing should be.

Are the RS485 circuits electrically isolated or maybe converted to fiber?

.
Click to expand...

Yes, the SEL387L encodes the feeder information and this is sent to the other location and compared with the signal that was produced there. They inturn do the same at their end. This information is usually sent via Power Line Carrier (PLC) between the two stations.

The RS485 can be connected via telephone line, ethernet, or fiber optics, the user specifies what they want the output to be channeled over.
 
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