Bar-Type CT Failures

[Kigif]

We have experienced several CT failures that have us scratching our heads. The CTs are bar-type, 5kV, 150/5A. Each set is in a motor starter operating on a 2.3kV system.

The first failure occurred about three weeks ago. Motor was running, tripped, continued to trip on re-start. Technician determined the B-phase CT was open. Replaced it, problem solved. The starter is 20 years old, so we did not really dig into the failure.

One week later, different starter, different feeder, same scenario. The B-Phase CT failed open.

Yesterday, ditto. Anyone seen this before? Thanks, K

 

[waross]

The center phase in a starter tends to run hotter than the outside phases.
It may be age related.
Nevertheless, I would put the scope on the system, phase to phase and phase to ground. You may have a discontinuous fault or other anomaly putting high voltage RF on the system.
If so, it may be present only when a particular faulty piece of equipment is operating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Is the 2.4 kV system ungrounded? (Fed by a delta transformer winding.) Is it possible that A or C phase has a ground fault and is elevating B phase to full phase-phase voltage above ground? Could a voltage surge have come through the system on B phase recently?

 

[Kigif]

Guys,

Thanks for the input.

RC,

The secondary is wye, resistance ground. The transformer is a 10 MVA utility owned unit, I am not sure of the relaying setup.

Thanks, K

 

[GordS]

What is the neutral resistance let thru amps with respect to the LV system charging current ( i.e capacitive amps due to cables, surge caps, etc). If the former is less than the latter, than transient overvoltages are possible during an arcing groundfault which would impact all equipment on a particular phase equally.

 

[Kigif]

GordS,

I am not sure exactly what the ratio is, but this is a fairly large system with many cables, surge caps, and PF caps. I beleive the former is greater than the latter and understand your comments on overvoltage with an intermittent fault.

This is what is so curious to me. The primary of the bar-type CTs checks "good" with a 5000-Volt megger. The secondary is open. There does not appear to be any evidence of overheating or physical damage.

Thanks for your input, K

 

[stevenal]

Are your CT secondaries all properly grounded? Although there may be disagreement over where to ground, most of us will agree that they must be grounded; and at one place only.

 

[scottf]

Since the failure mode is open circuited secondaries, system voltage is most likely not the cause.

The possible causes for a secondary open-circuit are as follows:

1) Thermal damage to the winding based on excessive current flow. This could be prolonged exposure to continuous currents above the max current rating of the CT (150A times rating factor). This could also come from fault currents in excess of nameplated thermal or mechanical current ratings.

2) Internal crimps that have aged to the point of breaking.

Internal open circuits on the secondary windings of CTs are pretty rare. Shorted-turns as a result of turn-to-turn insulation failure is much more common.

Failure of all 3 CTs at roughly the same time after 20 yrs is certainly curious and would lead me to investigate if the CTs have seen excessive currents.

Just curious, how are you determining that he secondary windings are open? DC resistance/continuity?

 

[Kigif]

Scottf,

Yes, we are checking for DC resistance/continuity. I am at somewhat of a disadvantage in trouble-shooting this problem as I work in a corporate group at HQ and am relying on a technician in the field to provide the information.

The technician disconnected the CTs completely to make the check. All the motor trips were due to phase imbalance. Replacing the CTs solved the problem. This leads me to beleive that the technician is accurate in determing the problem is an open secondary.

I will ask the tech to verify grounding.

Thanks for your input, K

 

[scottf]

Kigif....is a head-scratcher for sure. I can't imagine grounding would have anything to do with it myself.

 

[stevenal]

A possible scenario for scottf to shoot down:
Excessive voltage on ungrounded secondary causes insulation failure causing shorted turns. Shorted turns cause localized excessive current which open circuits the secondary. Why now after 20 years? Because tech lifted the ground on B phase and failed to return it.

 

[scottf]

Stevenal-

Don't think so. The capacitively coupled voltage that could be induced on the secondary winding (resulting from losing ground reference) is not "across the winding", but rather the entire winding floats up. The secondary turn-to-turn voltage drop doesn't change. The core also floats up, so the winding to core insulation sees the same stress too.

About the only feasible thing that could change the turn-to-turn voltage drop/stress is increase in current or increase in burden and even then, the core likely saturates before the insulation is stressed to the point of failure.

Disclaimer...the above is certainly theoretical