Shunt reactors causing CT failures?

[KevinWEvans]

We have a 69kV delta / 24.9kV gnd wye substation with a feeder that goes to an island. In the fall of 2008, we installed a 3 MVAR shunt reactor bank to counteract the ~3 MVAR of capacitance from the ~24 miles of submarine cable that feeds the island. This seemed to fix all issues we had.

This last fall, (Oct '09) we had an issue where a 50:5, RF 3.0 CT in the substation failed and fried our entire metering circuit. We replaced everything, but had no idea of the cause. A few months later a VT failed, and we chalked that up to damage sustained from the CT failure. A few days ago, we again had a total CT failure on B phase that again fried the entire metering circuit. We can find no data to suggest what might be the cause of these problems. There was no fault on the feeder, and from everything we can find, the sub was operating in normal conditions, no excessive loading. We have just replaced all of the metering transformers as well as the metering circuit.

It has been brought up that perhaps the reactors might be causing an abnormality. I can't think of any reason that they would since they are in continuous operation and the only time they are de-energized is then there is a recloser operation, in which case the capacitive cable is also dropped. But again, there was no reclosing event when the CT failed.

Any help, insight, or general conjecture on what might be happening would be greatly appreciated!

Thanks,
Kevin

 

[dpc]

What was the failure mode of the CTs? I'm having hard time seeing a connection between the shunt reactors and CT failures. Not sure I see much connection between CT failure and VT failure for that matter. A little more info on the CTs might be helpful.

David Castor

http://www.cvoes.com

 

[magoo2]

Let me make sure I understand your system. I'm assuming the reactor is at the island-end of the cable, correct?

Also, the recloser is in your substation, correct?

Do you have any switched capacitors in the substation or at the island-end?

Do you have any voltage monitoring in the substation, like Dranetz?

Some details on the cable would also be of interest like size, insulation type and mil thickness (260 mils is standard for 25 kV).

 

[LSpark]

I can't see how you would connect the failure of the CTs and VTs unless there had been a primary side fault on the system taht would affect them both.

As the metering circuits were fried it suggests that the output of the CT went high. If it was a wound CT then I would say that there may have been an HV interturn fault but I'm not familiar with the construction of RF CTs. Have you spoken to the CT manufacturer or sent a failed CT back to them? It may a type fault.

 

[KevinWEvans]

dpc-
CT failure mode is unknown. We cannot pinpoint any one mode of failure because everything was ruined. I would agree with your initial thoughts, as I cannot see any relation to the reactors and the failures. We are leaning toward poor grounding as the culprit, but the reactors were mentioned so I am trying to get any info I can.

magoo2-
The reactor is on the mainland side of the cable.

The recloser is in the substation (it is a 3 phase).

We have no switched capacitors to my knowledge on the circuit. We maintain the sub, but not the line, so we have limited information on anything outside the fence.

We just installed a PQ meter yesterday. We did this after the first CT failed but it never showed anything useful.

The only info I have on the cable is 4/0 copper. Again, not ours.

LSpark-
No primary fault that I was informed of. We are sending this CT in for an analysis. The RF is just rating factor, not type. The type was a JKW-6. Sorry for the poor clarification.


It is shaping up to appear that my initial thoughts that the reactors had nothing to do with the problem are true. Thank you all for your input.

 

[scottf]

What is the nominal and max current through these CTs? A 50:5A, 25 kV CT is likely to have a thermal current rating of 5 kA - 1 second...any chance this has been exceeded during their service?

What kind of VT do you have and how is it connected? Any time I hear "cable" and "reactor" ferroresonance is the first thing that comes to mind. Have you looked into this? Normally VTs and xfmr bushings are the failure source due to ferroresonance, but if the CT has very poor partial discharge performance (and the one you have likely does...biased opinion disclaimer) then a voltage rise due to ferroresance could have cause PD inception without extinction.

 

[marks1080]

Why are you leaning towards poor grounding? Don't just pull ideas out of a hat or this will be a very long process for you. Reactors don't fail CT's under normal operating conditions. I have a hard time believe poor grounding would fail a phase CT too - That being said I don't know your system, nor have I seen a schematic.

Are they poorly made IT's? Were both CTs and the VT made by the same manufacturer? is a 50:5 ratio appropriate to use for your application? I'm assuming you know your steady state load, but have you analyzed any transients on the system or cold load pick ups? Ferroresonance is an interesting theory, though I would tend to doubt it in the situation that you have outlined. I would hope that your ITs aren't blowing up because they are overloaded. You do have them connected to protective relays correct? Are the relay settings accurate? More information please.

First thing you should do is install a proper recording device for logging key information.

 

[LSpark]

Another aspect that should be easy to check is how heavily burdened the CTs are, the burden on an instrument CT is mainly from the resistance of the wiring (Isq*R). It's very easy to overburden a 5A CT.

I normally run two cores in parallel to reduce the burden on 5A CTs and most of our new builds have 1A secondaries for this reason.

 

[davidbeach]

What? It is not at all easy to over burden a properly selected 5A CT. Two cores in parallel doesn't reduce the burden, in fact (ratio unchanged) two CTs in parallel will develop twice the voltage across the burden and be more likely to saturate.

If you want to reduce the effects of the burden, two CTs in series will work far better. With the CTs in series each sees only half the total voltage and is therefore further from saturation.

 

[LSpark]

Sorry I wasn't clear, if you have 2 signal cores (not 2 CTs) in parallel you will halve the resistance of the signal cores so halve the burden. If the CT burden rating has been correctly selected then it isn't easy to over burden but if the system wasn't designed properly it is easy to do and sadly we don't live in an ideal world.

 

[davidbeach]

No. If you have two secondaries in parallel you will have twice the current through the burden and twice the voltage and therefore more saturateration. If they are in series, the current through the burden remains the same, the voltage remains the same, but each secondary sees only half the voltage therefore there will be less saturation.

 

[scottf]

Davidbeach is correct on all points.

I seriously doubt the CT failure has anything to do with being "over-burdened". About the only burden you can put on a CT that would cause it to fail (fail meaning insulation failure) would be an open-circuit.

 

[LSpark]

I think the terminolgy I'm used to using (UK) is different to the terminology used in the US, I'm new to this forum and have only talked tech speak with European engineers before. I'm attempting to refer to the cable cores between the CT secondary and the meter/instruments not the CT windings.

One of my sites (fairly old one) has got a CT with a probable HV interturn fault but it hasn't failed (yet) but the output has gone high. It's only because it is a metering CT rather than protection CT that it's been noticed. It seems to be stable so not a catastrophic failure like the ones Kevin has had problems with.

If a CT open circuit is suspected then I would carry out a tightness check from the CT through the instrument/metering panel. I've removed burden resistors that had been added to a CT circuit recently as erratic power readings made me suspect a problem in this area. I'm not sure why the installers had felt it necessary to add the burden resistors.

 

[electricpete]

Just throwing out a thought on possible link between instrument transformer failures and installation of a shunt reactor:

maybe you have created conditions favorable for ferroresonance?....that could create voltage excursions that threaten PT installation and perhaps CT ?

Again just a thought for your consideration - I haven't studied this post carefully.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[davidbeach]

OK, define "core". The only place I encounter the term in normal usage is the innards of a power transformer. I've run into usage of "core" in reference to individual secondaries of a multi-secondary CT - metering core or protection core. It also shows up occasionally in reference to multi-conductor cables.

"Two cores in parallel" could only mean CT secondaries, since running two conductors in parallel in a CT circuit is just asking for trouble. The solution to impedance in a CT circuit is bigger wire, not multiple wires in parallel. Never heard of such a thing and I'd never permit it on any installation I have anything to do with. Any reasonably arranged switchyard can do with 10 AWG or 8 AWG conductors for the CT circuits, but there would be no reason that a long run couldn't use 2 AWG or even larger conductors.

 

[Marmite]

Davidbeach, what LSpark described above, running secondary conductors in parallel to reduce resistance is common practice in the UK. Physically the cable could be a 12 core (conductor), or 19 core cable with 19x 2.5 sqmm copper conductors. In order to reduce burden 2 or even 3 of the cores in the cable may be connected in parallel. The approach is to use a standard cross section regardless of rating.
Regards
Marmite

 

[scottf]

davidbeach-

Paralleling leads between CTs and instruments (meters/relays) is something I've seen a fair bit of in the US and Canada.