Zero-Sequence CT and regular CT
Zero-Sequence CT and regular CT
(OP)
Does anyone know how to tell whether a currnet transformer is for zero sequence sensing and just a normal CT? For example, I am looking at the product page for one mfr. The product page did not mention anything related to zero sequence, so how do I know if it is for the particular application i am looking for or not, even though it is lised under ground sensing product category. I observed that the few product data sheets where it specifically says for ground fault, the ratio rnage is like 50:0.025A, which is very sensitive. My app is looking for 50:5 ZCT, do they make that?






RE: Zero-Sequence CT and regular CT
The 50-0.025 CT's must be interfaced with protective devices designed to accomodate that ratio.
old field guy
RE: Zero-Sequence CT and regular CT
What OFG saied.
No problem with this ratio of CT ring type.
If you are located in Europe, ABB Poland have a serious range of such CT's. Very important point is only size of window.
Good Luck.
Slava
RE: Zero-Sequence CT and regular CT
RE: Zero-Sequence CT and regular CT
RE: Zero-Sequence CT and regular CT
Is it residual phase input or separate CT input. The "ZCT" or residual will always see the imbalance as well as the phase to ground fault current, but this is also imbalance. Generally when I have a separate neutral CT I use a dedicated relay for the input. Some relays have what is called a "sensitive earth" input. Dedpends on the application. In either case, you must decide what you have and how to set the relay.
Sounds like you might need some help?
Hope this post helps!
Alan
RE: Zero-Sequence CT and regular CT
RE: Zero-Sequence CT and regular CT
Alan
RE: Zero-Sequence CT and regular CT
The 750 also allows you to setups 50/51N elements using a derived value from the three phase inputs. These are set up as "Neutral Current" in the setpoints.
The ZCT is treated like any other CT: ratio, polarity, accuracy class (saturation). The only consideration that is often different is the physical size of the window since you're going to run all three phase cables through that window.
Also, you might want to note that the shield grounding is correctly routed. There is a picture in the SR-750 manual that shows this. Failure to properly route the shield grounding will disable ground detection.
old field guy
RE: Zero-Sequence CT and regular CT
I am attaching the 750 connection diagram, it shows the connection method which matches our installation with the shield grounding routing inside the CT window. Thanks for your tip, oldfieldguy!
Because what we have is a 3-ph delta system, why would you want to setup the relay to detect a calculaed 50/51N vs a straightforward 50/51G?
The original reason I made this post was that during a recent ground fault in the system, we discovered that some of the relays were tripping incorrectly (or mysteriously), that is, some of the upstream breakers were tripped thinking it was a forward fault current, when in fact, and we thought, it should be reverse (i.e. opposite to from source to fault location). And yes, we were investugating the polarity of the setup of the whole syste, and so far, everything checks out, and then we come to the ZCT and thought perhaps somehow the ZCT element was selected incorrectly which was my original post(but, polarity of installaion checked out).
RE: Zero-Sequence CT and regular CT
No diagram attached?
Are you supplied from a utility?
Is the supply truly a 3 wire or 4 wire system?
How are you detecting the ground faults?
Radial or networked system?
Please describe your system in more detail.
Alan
RE: Zero-Sequence CT and regular CT
I noticed in this post that a metering class cannot be used for protection. What problems could possibly either way if the CT's are replaced in applications.
Thanks
RE: Zero-Sequence CT and regular CT
"Protection" CT's are constructed in order to maintain accuracy at current levels far in excess of their nominal values, typically 20 times nominal, in order to insure proper representation of currents under fault conditions.
Metering CT's don't have to work under these conditions.
old field guy
RE: Zero-Sequence CT and regular CT
a10jp,please provide addtional info:
1. What is a setting of 51N function, current and time.
2. What is a lenght of cable, that protected by 750 relay and with is a setting range of 51N function in this relay.
It saied once, but I ask again, are you check that grounding concentrator of cable back to grounding via ZCT?
Best Regards.
Slava
RE: Zero-Sequence CT and regular CT
RE: Zero-Sequence CT and regular CT
RE: Zero-Sequence CT and regular CT
RE: Zero-Sequence CT and regular CT
I meant, grounding concentrator or shield of 15kV cable.
Firstly check, if it back via ZCT or not touched grounding between ZCT and bushing.
Second, check if capacitive current of cable is less than your setting. 2A seems is low setting, for cable size, from 120mm^2 is less than 1km lenght.
Best Regards.
Slava
RE: Zero-Sequence CT and regular CT