CT grounding
CT grounding
(OP)
After reading this thread thread240-191665: current transformer grounding? and this article: http://www.netaworld.org/files/ItemFileA877.pdf, I need to ask one question to get clear something to myself.
Article mentioned above, recomends grounding of CT's in protective relay house. I also found same recomendation in some other sources.
Why is better to ground CT in relay house then on one secondary terminal of CT itself?
Article mentioned above, recomends grounding of CT's in protective relay house. I also found same recomendation in some other sources.
Why is better to ground CT in relay house then on one secondary terminal of CT itself?






RE: CT grounding
We always use only one rule of thumbs.
1. grounding CT's only in one place , near to CT as possible. At the first place, where you can easily connect or disconnect this grounding.
Regrds.
Slava.
RE: CT grounding
RE: CT grounding
The instrument transformer secondary circuits are generally grounded at one point. The circuits shall
preferably be connected to ground at the first point of application of the circuit (switchboard or relay
panel). This practice provides maximum protection for personnel and connected equipment at the
switchboard, which is the location where overvoltages are likely to be experienced.
RE: CT grounding
I fully agree with that.
RE: CT grounding
RE: CT grounding
That's means in the switchyard. IEC practice.
I have talked right now with some colleagues about your post, jghrist, and they commented that your post is "typical ANSI". Am I right?
"We" do not earth at cubicle location. Of course "we" do earth the cubicles, but this is another issues. From the CT, we get 4 cables: L1,L2, L3 and N. L1, L2, L3 enter the relay input CTs. The output of the input CTs are connected together and this is where we connect "N", which goes back to the main CT, which is earthed close to the CT.
I hope I have managed to explain this..
but of course, forgetting that "everybody believes he's doing right", it should be interesting to know the motivations from both proposals, if you can remember them. because for me it has just been accepted as "normal practice".
RE: CT grounding
RE: CT grounding
For example, if its generator, grounding usually connected at the junction box near to generator neutral point and on the ( if I remeber right) on the glad box for the stator Ct's.
If its GIS, usually is LCC
If its outdor swg, first CT junction box.
Regards.
Slava
RE: CT grounding
RE: CT grounding
RE: CT grounding
Looks like you assumed that GPR at both locations is the same. I was looking at the transfered potential that is there because of unequal ground potential rise.
RE: CT grounding
The reason that CT secondary circuits need to be grounded is to give the secondary winding some form of ground reference to prevent transferred voltages via capacitance coupling between the secondary winding and the primary winding. From this perspective, where the ground is applied, really doesn't make a difference.
From a current measurement accuracy perspective, it's very important to only ground at one point. From a strictly theoretical perspective, to best guard against EMI, it's best to run both leads all the way to the control house and then ground one there. That way there is opposing current throughout the entire wiring run. However, it's not commonly done that way except in EHV and UHV applications.
RE: CT grounding
We also follow this convention from the IEEE. It is important to have only 1 ground in the system. and the ground should be at the first point in the relay bldg. We do not ground at the breaker.
I would add, we follow this same guide for CCVTs/PTs. I find a lot of situations where many people are not familiar with the IEEE spec. on this.
The thing I always keep in mind is that you are grounding first and FOREMOST for personell protection.
RE: CT grounding
RE: CT grounding
The transfered potential is my way of explaining how the first point of application provides for "maximum protection for personnel." as the standard states. Anyone have other ideas on this? Or is IEEE full of excrement?
This is not a new exercise for me. Older versions of the ANSI metering standards had the CT secondary grounded as close to the CT as possible. The newer versions have aligned with IEEE, but habits die hard and I had to go further than "IEEE says so" in defending the change.
RE: CT grounding
Many times people understand it not so right and connect groundung on out of CT's and on each phase.