Ungrounded wye connected CTs
Ungrounded wye connected CTs
(OP)
Can someone explain to me what happens when you have CTs for each phases connected to the relaying with an ungrounded wye connection? The neutral was left open and I suppose for a balanced system it wouldn't matter since the neutral would only carry zero sequence current. It was discovered as part of an investigation into a CT failure I believe. To me, it looks like an ungrounded wye would screen out any zero sequence current but you would have zero sequence voltages develop across both sides of the relaying. I don't think there is an issue with saturation. Can this cause that? Can the ungrounded neutral voltage bounce around so much to cause the CT to fail?






RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
Consider if you have a little unbalance current, your circuit would act like an open CT, as the current would have no where to go.
Is that the type of damage you saw?
RE: Ungrounded wye connected CTs
yeah, there is no path for zero sequence to get back to the CT neutrals. The cts to me shouldn't be able to source zero sequence currents because their neutrals are connected together but not grounded.
cranky108,
The part about that that doesn't make sense to me is why would they saturate if their is no path to even send the zero sequence current. When I look at it, it looks like to me that it should just screen out all the zero sequence current and only positive and negative currents should get to the relay. The guy that looked at it closely thought the same way that you do but I am having a hard time seeing a CT setup without a return neutral acting like an open CT. To me, it shouldn't cause saturation because the zero sequence voltage will be the same everywhere due to there not being a place to source/sink zero sequence current. The currents the relay saw looked like a phase to phase fault with a small amount on the third phase. All three phases that the relay saw look like they sum to something close to zero. There didn't appear to be any saturation but the waveforms I saw were only sampled at 4 time/cycle.
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
It is grounded. The outputs at the relay are tied together and grounded but the neutral back to the cts was broken.
RE: Ungrounded wye connected CTs
I think I agree with you. The cts would be trying as hard as they could to push the zero sequence current into the relay circuit until they saturated. The zero sequence voltage would have increased I imagine until something failed. Maybe, the part I find different about this is that they aren't open cts in the traditional sense and you would have something like this if you had any sort of component or frequency that saw a very high impedance in your secondary circuit.
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
I've linked a sketch. To the right of each ideal CT is the associated shunt excitation branch, which is carrying the return current. Note that the current through the relay on the un-faulted phases is 180 out of phase with the current through the faulted phase of the relay. I imagine saturation would be unequal so that current wouldn't divide equally through the un-faulted relay coils. The result would be as described: equal and opposite current seen at the relay.
RE: Ungrounded wye connected CTs
Your sketch of the setup is correct as to my case.
I don't agree with your assement. If you move the excitation branch to the high side of the ct, you have two paths that the fault current has to go through. It has to flow straight through the excitation branch or through the ideal transformer to the relay and back through the transformer to the highside. For negative and positive sequence current, you would have to be extremely saturated for the least impedance path to be the excitation branch. For the zero sequence current, since the neutral is broken the excitation branch is the easiest path. The relays are saturating but the component split is why i beleive you have two phases carrying current 180 degrees apart. Also, the zero sequence currents are causing the saturation and if there were slight differences in the degree of saturation i don't think they would be much due to how the saturation curves level out after the kneepoint.
RE: Ungrounded wye connected CTs
I see extreme saturation as highly probable, since on any un-faulted phase you have current on the secondary with no corresponding current on the primary (essentially an open circuit primary). As soon as one CT saturates, that will be the low impedance path that will hog the current.
RE: Ungrounded wye connected CTs
If you prefer to see the magnetizing branch on the primary side, here is another look. I see no change in drawing it this way. The magnetizing path on the faulted phase remains high impedance since matching secondary current has a path to follow. One of the un-faulted phases will be the one to saturate.
RE: Ungrounded wye connected CTs
All the cts I beleive are saturated close to equally due to all three phases being connected with to the grounded relay circuit and having a common neutral that is swinging out. So, I don't think it has anything to do with one relay saturating before or more than the others.
I suppose it depends on the ct but when I look at the C800 curves, the slope levels out but it still has to go pretty far before it is below the relays burden, which with electronic relays and cables I imagine might be just a few ohms. So, to me it makes sense why the positive and negative sequence currents go to the relay and not through the excitation branch.
I suppose I can model it in ATP-EMTP sometime to prove it this week.
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
I modeled it and your right. The split without saturation is 1.0 in one phase and 0.5 pu returned on the other two phases. To have it go out and return on two of the phases has to be due to uneven saturation like you said.
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
You mentioned that the "CT failed test", so I'm assuming it wasn't a catastrophic HV insulation failure, but some kind of insulation failure on the secondary, which makes sense. I would assume that the insulation resistance from seoondary-to-ground would show lower than normal readings and/or the excitation test would show lower than expected saturation voltage. Failure modes could be turn-to-turn short on the secondary, secondary short to the core, or secondary short to ground (either high or lower resistance).
RE: Ungrounded wye connected CTs
RE: Ungrounded wye connected CTs
In the equiv circuit of a CT, the branch in parallel with the "ideal transformer" branch is a very high impedance (which is what causes high open-circuit voltages as the primary current is pushed through that path).
And yes...I've done an excitation test before...but heck, I've only worked for instrument transformer manufacturing companies for 20+ yrs :)
RE: Ungrounded wye connected CTs
Link
Since you've done excitation testing you know that with an open circuit primary the ideal transformer path is likewise open circuited, so the only remaining path is the parallel one. Once you get above the knee, current rises very quickly. Not an open circuit in this region.
Why the quotes? Do you use different nomenclature?
RE: Ungrounded wye connected CTs
I believe you're confusing some concepts here.
What we have in the topic at hand is not an open-circuited primary winding (as you mention above concerning excitation test), but rather a secondary winding with no return path (or at best a high impedance return path)...and no reference to ground, but that's another topic.
The voltage developed across the secondary in the case of an open-circuited secondary winding is a result of the primary current being pushed across the magnetizing branch impedance, which is quite high...this results in potentially very high peak voltage...normally in the 30-50kVp range for a typical protection rated CT.
RE: Ungrounded wye connected CTs
In a subsequent posting, the OP made it clear the CTs do in fact have a ground reference, the neutral return is open. My whole point is that this open neutral is not the same as an open CT.
I posted a couple sketches above, which show the concept.
Simply stated, though, the faulted phase CT is driving excitation current through the secondaries of the two un-faulted (open primary) CTs in parallel; just like you were performing an uncontrolled excitation test on the two CTs in parallel. As you state, the voltage produced by the faulted phase CT can go extremely high if the impedance is high enough. This voltage is certainly high enough to cause at least one of the other CTs to saturate (1000V on a C800 for example), and provide a low impedance return path through the CT and relay.