CT Location

[IndianTechEngg1001]

I wanted to know the location of CT whether it is on the incoming side or outgoing side of breaker.

Kindly tell me what difference it will make it it is kept at other side

 

[marks1080]

How about both sides?

Typically we want to overlap protective zones. In some cases, 500kV and above, we will see CT's only on one side due to cost. Slightly different protective schemes are required when this is done to make up for a 'blind spot' by not having complete overlapping zones.

 

[IndianTechEngg1001]

Thanks marks, what would be the case when we have to select the position of a CT for O/L Protection. What location of CT preferred Load side or Supply side.

 

[waross]

Servicing, checking and replacing are all a lot easier and safer when the CT is on the load side of the breaker.
What is the application?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[marks1080]

The best location would depend on the configuration of the station. Is this a diameter breaker? How many lines on the diameter? How many breakers? Just one CT for each breaker? There are more questions you need to ask yourself as well. What other CT's are available in adjacent zones? There are other questions you might need to ask yourself as well.

Waross has a good point about maintenance as well. I would choose to keep a more secure system keeping in mind that you do not want an outage on any single element to require an outage on another.

Pay attention to areas that do not have overlapping zones when using just one CT for a breaker. This will happen and should be an important consideration when chooisng where to put those CTs.

 

[racobb]

If it is an outdoor station and sounds as such, I prefer mine on the line side terminals for OL protection. That way if I also use bus differentials I can overlap with the second set on the load side - as marks1080 said.

Alan

 

[IndianTechEngg1001]

marks1080 you specified that 500kV and above, we will see CT's only on one side due to cost. Is such a limitation also exists when we go to low voltage sides like 400V or or 6.6kV or 11kV in indoor S/S.

 

[marks1080]

Yes, you can use the one CT per breaker scheme anywhere, it's just been my experience to see it in high voltage stations. It's always preferred to use CTs on each side of a breaker, but it is not always feasable.

 

[DTR2011]

For clarity, I believe we are discussing Live Tank breakers. In the dead tank world, it is quite common to find 2-3 CT's per pole, thus 4-6 CT's per phase, utilizing internal bushing CT's.

 

[jghrist]

In general, it is preferable to have the breaker in the protected zone. Therefore, the CT for line protection would be on the bus side of the breaker. The CT for bus protection would be on the line side.

 

[davidbeach]

What jghirst said. Breaker in the zone of protection.

 

[m3ntosan]

jghrist, davidbeach,

CB will always be in a protected zone

With the CT only on the bus side of the CB, how does the BBP work for a fault between the CT and the CB? Is it a good practice to rely only on the remote end CB trip?
Also, with the CT only on the bus side of the CB, how does it work if you have a unit protection and you want to keep the OHL/cable energized from one end only, to gain reactive support?

I’ve only seen this kind of arrangement in a indoor s/s due to space restriction.

CT on both side of the CB I’ve seen only on bus couplers, bus sections and middle CB in 1 and ½ running arrangements.


May you grow up to be righteous, may you grow up to be true...

 

[racobb]

"CB will always be in a protected zone"

What makes you think so if only 1 set of ct's are used on the bus side of the breaker? I am speaking of an outdoor substation with the breaker in question feeding radial distribution lines.

Alan

 

[racobb]

OOPS....meant to say on the outgoing line side of the breaker.

Alan

 

[m3ntosan]

You're right if you're talking about an outdoor s/s ... without any protection.

If you don't have a BBP, you may have an overreach protection on the remote end of the incoming feeders, to "see" your outdoor substation busbars (for a less important distribution s/s with radial feeders), it makes no sense not to protect a s/s busbar.

May you grow up to be righteous, may you grow up to be true...

 

[davidbeach]

I'm not aware of utility installations without CTs on both sides of all breakers. Quite often two sets of CTs on each side.

 

[m3ntosan]

David,
I am assuming you’re talking about US, in UK and continental Europe, for feeders, you’ll find CTs only on the line side of the breaker.
What’s the idea with CTs on both sides of the CB? You’ll have an overlap in the CB area and for a fault between the two CT sets (CB area),I guess both protections will operate (BBP and feeder) and open the same CB.
I can’t see right now what the big advantage for this kind of arrangement.
Could you please let me know?
What about a fault between the CB and the CTs on the line side of the CB, it will be in the feeder protection zone and also in the BBP zone, do you use time discrimination for BBP?
I am missing something ...
It makes sense for a bus section to have CTs on both sides of the CB in order to trip just one section of the busbar in case of fault, and trip both only in the unfortunately event of a fault in the overlap area between the CTs.


May you grow up to be righteous, may you grow up to be true...

 

[racobb]

David

I have seen many older installations with only 1 set, but most if not all of the newer ones have at least 2 sets of ct's.

Alan

 

[DTR2011]

On dead tank breakers / switchgear, each CT is 600V class, thus inexpensive and built into the CB or Xfmr (bushing type).

The initial cost of having 2-6 CT per phase is minimized verses having to change later on. Some CT's used for protection, some used for metering, SCADA, etc.

 

[davidbeach]

Bushing CTs, from low voltage to 500kV.

For a radial system, the feeder relay can be non-directional. For networked lines, the line protection is directional.

Fault happens on the bus side of the line CT (on the bus side of the CB) then only the bus zone responds. If there are non-directional feeder relays they see no current. If there are directional relays they see a reverse fault and don't trip. So, only a single zone trips and it includes the breaker.

If the fault is on the line side of the bus CTs (on the line side of the CB) the the bus zone will not respond and the line protection will trip the breaker.

If the fault is between the CTs the both zones will respond as is proper for a breaker fault.