Connecting two zero sequence CT in parallel

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Mousa

Member
Location
Amman, Jordan
Hello friends
I have a strange case that I would like to share with you and take your advice
I have a MV switchgear divided in two sections (A&B) and a Tie breaker between them
each section has a feeding breaker
The strange side that both feeders of the switchgear will be connected to one breaker (one breaker supplying power to two breakers) ... actually this is the provided design and it is out of my scope.
a zero sequence CT shall be connected on the outgoing cables (6 cables) of the source breaker for earth fault protection , but as i mentioned before the 6 cables will be divided, each three cables will go to breaker
the cables size is 630 mm2 and the provided CT is not enough to put 6 cable inside (the CT inner diameter is 200mm), so can two CTs be used ( one CT for each circuit) and connect the CTs secondary together then to the protection relay ??!

DRW2.jpg

Please advice and do not hesitate to ask if the subject is unclear
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
170921 EDT

Theoretically yes.

You can parallel ideal current sources to a finite impedance, but not series them. With practical devices and instrumentation will the function work well, and will you get the answer you want?

You can series ideal voltage sources, but you can not parallel them.

Another question to ask is: if you parallel measurement current transformers do you get the instantaneous or average current value for your intended purpose?

.
 
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GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
The other minor point is that the paralleled CTs will detect only a ground fault current in the total circuit. Not likely to happen, but equal but opposite fault currents in the two branches would not show up on the combined CT output.
This is not a practical problem, nor is it even a technical quibble if the specification goal is only to detect zero sequence current on the three wire source circuit.

FWIW (which is not a lot unless you are touching the terminals), your CT placement would not detect a ground fault in the splitter wiring connecting the single input run to the two output runs.
 
I

Ingenieur

Senior Member
Location
Earth
http://m.energy.siemens.com/us/pool...standard-products/techtopics/techtopics50.pdf

When there are too many cables for onezero-sequence ground CT, multiple zero-sequence CTs can be used. It is essentialthat each set of three-phase cables and thecorresponding ground cables pass througha single, zero-sequence CT. For example,with three cables per phase, two three-phase sets of cable and the correspondingground cables could pass through one
CT, and the remaining three-phase set
of cables and the corresponding groundcables would pass through a second CT.This assures that all of the currents arebalanced, and that the continuous current-carrying capability of the zero-sequenceground CT is not exceeded.
 
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Ingenieur

Senior Member
Location
Earth
After thinking about it he likely has 2 0 seq ct's already
that would give him better resolution than 3 phase ct since the ratio could be much smaller
may be moot if unlimited gf (vs ngr system)
both will saturate on thecway to tripping

a bunch of ways to skin this cat
 
I

Ingenieur

Senior Member
Location
Earth
GoldDigger said:
But depending on the location of the fault, the two CT system will not even necessarily detect a fault where you propose.
Click to expand...

not talking about detecting but clearing

not sure why the relay anyways
ngr?
inst only cb?
lower trip than afforded by cb?
leakage?

a gf will trip the cb if the system is grounded
should be in the inst range and I doubt the relay would be faster
 
M

Mousa

Member
Location
Amman, Jordan
gar said:
170921 EDT

Theoretically yes.

You can parallel ideal current sources to a finite impedance, but not series them. With practical devices and instrumentation will the function work well, and will you get the answer you want?

You can series ideal voltage sources, but you can not parallel them.

Another question to ask is: if you parallel measurement current transformers do you get the instantaneous or average current value for your intended purpose?

.
Click to expand...

The most thing i care about that the gf will work properly or not, the CT shall trip CB1 only,if any unbalance current in one of the two circuits happened, will it trip CB1?
 
M

Mousa

Member
Location
Amman, Jordan
Ingenieur said:
http://m.energy.siemens.com/us/pool...standard-products/techtopics/techtopics50.pdf

When there are too many cables for onezero-sequence ground CT, multiple zero-sequence CTs can be used. It is essentialthat each set of three-phase cables and thecorresponding ground cables pass througha single, zero-sequence CT. For example,with three cables per phase, two three-phase sets of cable and the correspondingground cables could pass through one
CT, and the remaining three-phase set
of cables and the corresponding groundcables would pass through a second CT.This assures that all of the currents arebalanced, and that the continuous current-carrying capability of the zero-sequenceground CT is not exceeded.
Click to expand...

thanks you dear , i check the TechTopics and i found it useful ^^
 
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