CR4® - The Engineer's Place for News and Discussion®


Previous in Forum: Braking Unit   Next in Forum: Can IECEx Zone Rated Equipment Be Used in NEC Division Area?
Close
Close
Close
13 comments
Member

Join Date: Sep 2017
Posts: 8

What is the Importance of CT Polarity in HV Substations and How is it Decided?

09/21/2017 12:20 AM

What factors are taken in consideration while deciding the CT polarity in HV substations (400 kV and above) say for one and half breaker scheme? What are the effects if CT polarity is reversed by mistake during installation in HV substations?

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
United States - Member - Hobbies - Fishing - New Member Hobbies - Target Shooting - New Member Hobbies - RC Aircraft - New Member Engineering Fields - Aerospace Engineering - New Member

Join Date: May 2009
Location: On the road, USA
Posts: 2165
Good Answers: 68
#1

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 12:33 AM

What does lecture notes say? Or did you skip class that day?

If you're asking this type of question, your over your head. We generally don't do homework assignments here.

__________________
Confucius once said, “ Ability will never catch up with the demand for it".
Register to Reply Score 1 for Good Answer
Guru

Join Date: Oct 2008
Posts: 39877
Good Answers: 1587
#2

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 12:49 AM
Register to Reply
Member

Join Date: Sep 2017
Posts: 8
#3
In reply to #2

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 1:23 AM

Thanks a lot Lyn..!!

I am aware about these basics of CT polarity - the dot convention, P1 and P2 etc. But wanted to know its effects on metering and protection when used in HV Substations.

Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.
Posts: 26693
Good Answers: 700
#4

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 3:00 AM

<...What factors are taken in consideration while deciding the CT polarity in HV substations...?...> The need to get it right so that the equipment to which it is attached works properly.

<...What are the effects if CT polarity is reversed by mistake during installation in HV substations?...> The equipment to which it is attached won't work properly, of course.

Why would anyone choose to do this incorrectly (rhetorical question - NNTR)?

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
Member

Join Date: Sep 2017
Posts: 8
#5
In reply to #4

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 4:15 AM

Dear PW slack,

Thanks a lot for sparing your time..!!!

Yeah obviously, polarity is taken into consideration so that the connected equipment works properly and if the polarity is reversed they won't work as intended.

Normally, metering instruments and Protection instruments are connected to secondary cores of CT in HV substations. It is for sure that if the polarity of CT is incorrect due to some reason the instrument - metering and protection relays would not work as per their desired function.

But exactly what discrepancies will we face in operation of metering and protection instruments due to polarity of CT? and what will be its overall impact on substation?

Register to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.
Posts: 26693
Good Answers: 700
#6
In reply to #5

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 4:17 AM

Er: <...the instrument - metering and protection relays would not work as per their desired function...>.

"Go get 'em, Floyd".

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Register to Reply
2
Guru

Join Date: May 2010
Location: Liverpool, NY
Posts: 860
Good Answers: 116
#8
In reply to #5

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/21/2017 8:41 AM

Since that affects the phasors of the currents, it will have an impact on anything where current direction is an issue (power flow, reverse current or power tripping, etc.). Omnidirectional measurements will not necessarily be affected (overcurrent tripping, etc.). But transmission line protection, which is usually based on impedance measurement, will not operate properly. You may have a trip when you don't want one, or even worse, your breaker(s) may not trip when they really need to. Your power flow measurements will be all screwed up. Transformer current differential protection may false trip or fail to trip, possibly causing a multi-million-dollar piece of equipment to have a catastrophic failure. So the impact could be HUGE!

So a big lesson: Make sure you have your CT polarities correct!

__________________
To get the right answers, first you need to ask the right questions.
Register to Reply Good Answer (Score 2)
Member

Join Date: Sep 2017
Posts: 8
#13
In reply to #8

Re: What is the importance of CT Polarity in HV Substations and how is it decided?

09/26/2017 2:47 AM

Thanks a lot PeterT..!!

Register to Reply
Guru

Join Date: Apr 2010
Posts: 5744
Good Answers: 576
#7

Re: What is the Importance of CT Polarity in HV Substations and How is it Decided?

09/21/2017 8:02 AM

AC current is represented by a rotating vector called a phasor. To get the vector sum of these currents you connect the CTs in parallel. If you hook one up backwards, it subtracts instead of adds.

Register to Reply
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 12217
Good Answers: 774
#9
In reply to #7

Re: What is the Importance of CT Polarity in HV Substations and How is it Decided?

09/21/2017 9:00 AM

I always liked the modular design of the old phaser.

__________________
"Don't disturb my circles." translation of Archimedes last words
Register to Reply Off Topic (Score 4)
Guru

Join Date: Oct 2008
Posts: 39877
Good Answers: 1587
#10
In reply to #9

Re: What is the Importance of CT Polarity in HV Substations and How is it Decided?

09/21/2017 11:24 AM

We have clearance, Clarence.

Roger, Roger. What's our vector, Victor?

Register to Reply Off Topic (Score 4)
Guru
Safety - Hazmat - New Member United States - US - Statue of Liberty - New Member Engineering Fields - Chemical Engineering - Old Hand

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 13438
Good Answers: 152
#11
In reply to #7

Re: What is the Importance of CT Polarity in HV Substations and How is it Decided?

09/21/2017 12:31 PM

...and might let the smoke out?

__________________
If it ain't broke, don't fix it. Just build a better one.
Register to Reply
Guru
United States - Member - Hobbies - Fishing - New Member Hobbies - Target Shooting - New Member Hobbies - RC Aircraft - New Member Engineering Fields - Aerospace Engineering - New Member

Join Date: May 2009
Location: On the road, USA
Posts: 2165
Good Answers: 68
#12
In reply to #11

Re: What is the Importance of CT Polarity in HV Substations and How is it Decided?

09/21/2017 12:40 PM

I always hate when that happens

__________________
Confucius once said, “ Ability will never catch up with the demand for it".
Register to Reply
Register to Reply 13 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

BT (3); dj95401 (2); James Stewart (1); lyn (2); PeterT (1); PWSlack (2); redfred (1); Rixter (1)

Previous in Forum: Braking Unit   Next in Forum: Can IECEx Zone Rated Equipment Be Used in NEC Division Area?

Advertisement