Ungrounded wye connected CTs

[HamburgerHelper]

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?

 

[stevenal]

Why should the voltage be close to open circuit? You have one CT feeding the excitation branches of two others in parallel. The voltage won't rise much above the knee point of the first CT to saturate. Ever do an excitation test? Voltage cannot get very high, since the impedance drops once you get above the knee.

 

[scottf]

Depends on what you're calling the "excitation" branch. Can you be more specific?

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

 

[stevenal]

The branch that is in parallel with the ideal transformer, and the one that carries the excitation current. The branch that is high impedance in the linear region, but becomes very low impedance past the knee point voltage.

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?

 

[scottf]

Stevenal-

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.

 

[stevenal]

Consider the un-faulted phase CTs to have open circuit primaries. It is common to ignore load for fault conditions since load current is relatively small and will have little effect.
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.