Sizing a zero sequence CT
Sizing a zero sequence CT
(OP)
I have a 480V switchgear with Eaton Magnum DS breakers equipped with Digitrip 1150 units.
The system ground fault is limited to 5A via a high resistance ground on the upstream transformer. As such, the Digitrip 1150 will never see the ground fault since its lowest ground fault pick-up setting is 0.25 x In.
One of the proposed solutions to detect ground fault in the 480V switchgear is to add zero-sequence CTs on each feeder, as this would help identify which feeder the ground fault exists on, and using this zero-sequence CT with the Digitrip 1150 or an external device like an I-Gard, Bender, etc.
How do I go about sizing the zero-sequence CT such that it is sensitive enough to pick up a ground fault current of <5A? Do they come in 5A:1A configurations?
The system ground fault is limited to 5A via a high resistance ground on the upstream transformer. As such, the Digitrip 1150 will never see the ground fault since its lowest ground fault pick-up setting is 0.25 x In.
One of the proposed solutions to detect ground fault in the 480V switchgear is to add zero-sequence CTs on each feeder, as this would help identify which feeder the ground fault exists on, and using this zero-sequence CT with the Digitrip 1150 or an external device like an I-Gard, Bender, etc.
How do I go about sizing the zero-sequence CT such that it is sensitive enough to pick up a ground fault current of <5A? Do they come in 5A:1A configurations?






RE: Sizing a zero sequence CT
RE: Sizing a zero sequence CT
(OP)
1. " One of the proposed solutions to detect ground fault in the 480V switchgear is to add zero-sequence CTs on each feeder, as this would help identify which feeder the ground fault exists on, and using this zero-sequence CT with the Digitrip 1150 or an external device like an I-Gard, Bender, etc."
I propose for (Type A.three-phase or Type B.three-phase and neutral systems) each feeder, you may install three CTs for Type A [one on each line]; or four CTs for Type B. [one on each line and one on the neutral].
These CTs primary rating shall be 1.25 x line current. The secondary may be 1A or 5A
These CTs are wired in [parallel] (wire all S1 together and all S2 together) then wire S1 to the interposing CT* P1 where S2 is wired to interposing CT* P2.
Wire interposing CT* S1 and S2 to the protection relay.
The [very small size] interposing CT* can be 5:1 or 1:1 to suit the protection relay input current rating.
2. With this method you are measuring the [spill current] similar to the zero-sequence CT; but avoid requiring a [CT with big window and very low current rating].
Hope this proposal is helpful to you
RE: Sizing a zero sequence CT
i started looking into pulsing systems. do most clamp type ammeters do zero sequence measurements as well? i suppose what you implied is that instead of installing zero sequence CTs in the switchgear for every feeder, you can use the clamp type ammeter to determine if there is zero sequence current in that feeder?
RE: Sizing a zero sequence CT
I wouldn't go with KuanYau's suggestion. It's what works well for a solidly grounded, or even low impedance ground system but won't provide the sensitivity necessary for a high impedance grounded system. There also cannot be a neutral in a high impedance grounded system.
Personally, I'd go with 50:5 CBCTs and feed them into a relay that can work with 0.1A secondary. That 5A ground current would be 0.5A secondary, well above the minimum setting.
Alarming would be the typical response to the first ground fault; otherwise most people would avoid the costs associated with high impedance systems, but nothing says you can't trip on the ground fault.
RE: Sizing a zero sequence CT
"No, a clamp on meter can't measure zero-sequence (unless it is around all 3 phases), but with the pulse system it would see the step changes in current if clamped on between the pulser and the ground fault."
Everything I've read so far indicates that a pulse system would cause zero sequence current changes in the feeder that feeds the fault. And that's what you need to detect.
RE: Sizing a zero sequence CT
You can find the fault without any tools. Just start flipping off loads one at a time until the ground clears. That implies that turning off the loads is not a problem. Have someone checking to see if it's cleared. That also doesn't require the pulsing system. Just have an alarm when there is a fault. The concern is that if there is a a grounded phase the other two phases are now at phase to phase voltage equals the phase to ground voltage. If a second ground fault happens it will be a phase to phase fault.
RE: Sizing a zero sequence CT
RE: Sizing a zero sequence CT
i will be checking with a technical applications rep at littelfuse startco to confirm implementation of their SE-330 product.
they have an excellent technical note (http://www.littelfuse.com/~/media/files/littelfuse...) which says in once place "The location of the ground fault can be determined by observing the zero-sequence currents" which lead me to believe we need zero-sequence CTs to measure the pulse change.
But the same document later states "The CT on the faulted feeder will detect the sum of the charging currents of the unfaulted feeders and the resistor current. When the pulsing contactor is turned on, this CT will detect a modulating current that indicates the faulted feeder." This leads me to believe the regular phase CTs will detect the pulse change.
RE: Sizing a zero sequence CT
To detect the ground fault just put a voltage relay across the neutral grounding resistor. If your going with the pulse system a lot of companies make them that you can buy off the shelf with the ground fault alarms built in. Type in "neutral ground resistor alarm" into a search engine and 5 different companies come up that want to sell a pulsar system.
RE: Sizing a zero sequence CT
But not so under IEC rules. It's common to see UNgrounded 400Y230 and 690Y400 in Europe and Asia. With LN loads on the system.
I suspect that's where KuanYau is coming from.