CT reversed in bus differential zone

[mbrooke]

How does bus protection behave when a CT is reversed polarity wise? Say normally all feeders are kept on the lower bus B. Then all feeders are placed on the upper buss A except one remaining on B with current flowing from A to B via bus coupler and then out the feeder.

 

[Phil Corso]

MBrooke,

When 3 CTs, Ia, Ib, and Ic, are configured to derive a residual-current, 51N, then 51N-Amps will be exactly twice that of the line-current measured with a reversed CT!

Following are 3 cases given Ia, Ib, and Ic:

a) Covers the case when the CT in Line A is reversed; then, 51N is twice Ia!

b) Covers the case when the CT in Line B is reversed; then, 51N is twice Ib!

c) Covers the case when the CT in Line C is reversed; then, 51N is twice Ic!

Hopefully, this will aid you!

Regards, Phil Corso

 

[mbrooke]

MBrooke,

When 3 CTs, Ia, Ib, and Ic, are configured to derive a residual-current, 51N, then 51N-Amps will be exactly twice that of the line-current measured with a reversed CT!

Following are 3 cases given Ia, Ib, and Ic:

a) Covers the case when the CT in Line A is reversed; then, 51N is twice Ia!

b) Covers the case when the CT in Line B is reversed; then, 51N is twice Ib!

c) Covers the case when the CT in Line C is reversed; then, 51N is twice Ic!

Hopefully, this will aid you!

Regards, Phil Corso

Thanks!

So if the feeder current is 1000amps, it will appear as a 2000amp bus fault?

 

[rian0201]

sorry but im having a hard time understanding the scheme. basically, when designing the 87b if should cover all possible normal connections.

when you transfer feeders, so basically, it should be stable.

when you combine bus, the bus coupler is close and there is only one bus. the CT in the bus coupler is not included.


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[rian0201]

if you are using partial 87b, the same thing will happen, only that the relay takes the polarity of CTs used.


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[mbrooke]

sorry but im having a hard time understanding the scheme. basically, when designing the 87b if should cover all possible normal connections.

when you transfer feeders, so basically, it should be stable.

when you combine bus, the bus coupler is close and there is only one bus. the CT in the bus coupler is not included.


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Normally this system would have two dynamic differential zones with half the circuits on bus one and half on bus two. A fault on either bus clears only half the circuits.

In this case all the circuits are on one buss and buss two is simply a spare, used only when bus one needs to be taken out of service. So under normal operation there is only one zone.


Technically I do not need to include the CT coupler, but one of the busses (the reserve) Id like to be protected with over current by the bus coupler, so technically under normal operation I need to exclude the reserve bar out of the differential zone. Either that my bus differential zone would need an 8 cycle delay, something I'd like to avoid.

 

[gray.one]

CT reversed in bus differential zone

To answer your question: they add making a through current appear 2x actual through current. Differential relaying works by applying Kirchoff's current law. Things should add to zero, but with reversed CTs you've multiplied by -1.


Your diagram is lousy. Check out the SEL document. Relevant stuff starts on page 7 but all is good. SEL has so many good white papers.

https://cdn.selinc.com/assets/Liter...tection_SC_20130912_Web.pdf?v=20150812-080535


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[mbrooke]

To answer your question: they add making a through current appear 2x actual through current. Differential relaying works by applying Kirchoff's current law. Things should add to zero, but with reversed CTs you've multiplied by -1.


Your diagram is lousy. Check out the SEL document. Relevant stuff starts on page 7 but all is good. SEL has so many good white papers.

https://cdn.selinc.com/assets/Liter...tection_SC_20130912_Web.pdf?v=20150812-080535


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Thanks


What if I simply opened the bus coupler test switch CTs?


I think the challenge here is that while this substation is a single breaker double bus (which are usually dynamic and run with an SEL487B) we have the case where under normal operation only a single zone is needed.

 

[rian0201]

Normally this system would have two dynamic differential zones with half the circuits on bus one and half on bus two. A fault on either bus clears only half the circuits.

In this case all the circuits are on one buss and buss two is simply a spare, used only when bus one needs to be taken out of service. So under normal operation there is only one zone. [/QOUTE]

this is what i am saying..

[QOUTE]
Technically I do not need to include the CT coupler, but one of the busses (the reserve) Id like to be protected with over current by the bus coupler, so technically under normal operation I need to exclude the reserve bar out of the differential zone. Either that my bus differential zone would need an 8 cycle delay, something I'd like to avoid.

im curious, you mean that the reserve bar is protected by overcurrent? how? is the bus coupler always closed? what coordination with the down stream OCs?



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[mbrooke]

im curious, you mean that the reserve bar is protected by overcurrent? how? is the bus coupler always closed? what coordination with the down stream OCs?



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Yup, the bus coupler is always closed. The reserve bus is protected by over current via the bus coupler. The bus coupler is set to have an SEL-351 relay which gives the definite and inverse time over current.


Normally no bays are connected to the reserve bus. The reserve bus is only used when the main bus must be taken out of service for repair or maintenance.


The reasoning behind keeping the reserve bus energized and protected is so faults on it can be detected as they happen. Otherwise the reserve bus may become faulty and nor be discovered until its needed.

 

[Ingenieur]

im curious, you mean that the reserve bar is protected by overcurrent? how? is the bus coupler always closed? what coordination with the down stream OCs?



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I think he his saying
all loads less one on A
one load on B
tie closed
use the ct's on the tie and the one load in a differential scheme
if they don't match fault on bus

but why?
need to see how buses are fed

 

[mbrooke]

I think he his saying
all loads less one on A
one load on B
tie closed
use the ct's on the tie and the one load in a differential scheme
if they don't match fault on bus

but why?
need to see how buses are fed

Normally all loads are on buss A, and the only live connection to Buss B is the coupler.

The issue I am facing is having the buss bar differential system remaining stable as loads are shifted from buss A to bus B with the coupler closed.

 

[rian0201]

Yup, the bus coupler is always closed. The reserve bus is protected by over current via the bus coupler. The bus coupler is set to have an SEL-351 relay which gives the definite and inverse time over current.


Normally no bays are connected to the reserve bus. The reserve bus is only used when the main bus must be taken out of service for repair or maintenance.


The reasoning behind keeping the reserve bus energized and protected is so faults on it can be detected as they happen. Otherwise the reserve bus may become faulty and nor be discovered until its needed.

i see..

but this will result to only 1 bus scheme. why not use partial differential? that way, both buses are protected by 87 separately.. and it is dynamic..


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[Phil Corso]

MBrooke…

I apologize for misinterpreting your query, i.e., reverse CT (1 of 3 in a set), when you really meant reverse-current flow thru the coupler’s CT!

Presuming it is a conventional system (not CT’s with air-core toroidal cores), then reverse fault-current thru the coupler’s CT will produce a current 180⁰ out of phase with that of any feeder CT monitoring the same fault current. Stability is then dependent on resultant current thru the 87 relay!

Remember, the circuit is simply a resistive one, including secondary resistance of both CT’s and their connected burdens! If the components are matched, then, the most likely outcome is that the ‘87’ won’t work! Thus, to accomplish the desired result make sure the coupler-CT polarity matches the feeder CT’s in the bus-section you want protected!

Personally, I would not do it!

Regards, Phil

 

[mbrooke]

MBrooke…

I apologize for misinterpreting your query, i.e., reverse CT (1 of 3 in a set), when you really meant reverse-current flow thru the coupler’s CT!

Presuming it is a conventional system (not CT’s with air-core toroidal cores), then reverse fault-current thru the coupler’s CT will produce a current 180⁰ out of phase with that of any feeder CT monitoring the same fault current. Stability is then dependent on resultant current thru the 87 relay!

Remember, the circuit is simply a resistive one, including secondary resistance of both CT’s and their connected burdens! If the components are matched, then, the most likely outcome is that the ‘87’ won’t work! Thus, to accomplish the desired result make sure the coupler-CT polarity matches the feeder CT’s in the bus-section you want protected!

Personally, I would not do it!

Regards, Phil

Make sense.

What if I opened the coupler differential CTs when all the loads are being transferred from bus one to bus two?

 

[Phil Corso]

Mike...

"What if I opened the coupler-breaker differential CTs .... ?"

Much too dangerous to open CT's! Some systems use limit-switches fitted on the feeder-breaker transfer switch to essentially prevent response of the '87' during transfer!

Phil

 

[mbrooke]

Mike...

"What if I opened the coupler-breaker differential CTs .... ?"

Much too dangerous to open CT's! Some systems use limit-switches fitted on the feeder-breaker transfer switch to essentially prevent response of the '87' during transfer!

Phil

Done everyday, see this:

http://www.statesproducts.com/common/documents/RackMount_DS_en_V07.pdf

 

[Phil Corso]

Mike...

I didn't say you couldn't do it! I said I wouldn't do it!

Phil

 

[mbrooke]

Mike...

I didn't say you couldn't do it! I said I wouldn't do it!

Phil

But you made it sound like it was dangerous. It is only dangerous if you do not have shunts. Test switches in substations have shunts that short the CT together when opened. No harm would be done opening the buss coupler CT under load.

 

[rian0201]

hihi.. is the OC backup in your scheme? it is always to do the backup.. for me, i would use partial 87 with check zone and finish..


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