what are the chances of a CT failure in a High voltage system? Our CT was connected before VCB (34.5KV), while VCB is supplying 34.5Kv input to furnace trafo. Suddenly one day we found one CT was opened. No cracks on the body. No loose connection on sec side. What are the chances??? Means it's difficult to analyse why it failed. Does ambient temperature also plays a role in CT failure? CT ration was 2000/1 Amps. Share your view plz...

Regards,
Sandy

 

Heat can play a role. Anything with copper wire windings is subject to hear failure of the "lacquer" type insulation on the wire windings.

Once the insulation fails, coil windings short and poof . . .

In fact, I've been in a couple of plants where they coat wire with insulation. In one, the plant managers and in the other, the plant engineer told me that wire is sold according to grade: how well the coating is applied, poor, good, better and best.

Once I knew that, I understood why consumer electrical stuff, like kids' train transformers burns out so quickly: poor material.

Then again, any commercial unit has a ambient rating.

 

Responding to Sandy's Aug 19 query... probability is low:

Since you can't open the unit for inspection, and there is no noticeable odor of burnt insulation, try x-ray!

Some additional food for thought:

1) Is this a new or existing installation? If the latter how many inrush cycles has the CT experienced?

2) What is CT type? Bushing? Wound? Doughnut?

3) Is CT burden relatively large?

4) Is system X/R relatively high? Greater than 10-15?

5) CT's having a current ratio of this magnitude are more susceptible to saturation effects! Especiallly if inrush duration is long, and burden
is large!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Further to my earlier reply to Sandy's query about chance (probability).... following are reliability figures taken from IEEE Std 500-1984:

"IEEE Guide To The Collection of Electrical, Electronic, Sensors, and Mechanical Equipment Reliability Data For Nuclear-Power Generating
Stations"

Category: Current Transformers over 10 kV.

All Failure Modes (Catastrophic, Degraded, Incipient):

0.14 (min) to 1.32 (max) failures/10E6 hrs.

Catastrophic (No Output Due To Open-Circuit):

0.03 (min) to 0.18 (max) failures/10E6 hrs.

There is no listing for failures/10E6 cycles!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

As well as there being different grades of insulation, standard wire specifications allow for a certain number of bare spots per length of wire. They are simply counting on successive bare spots being not likely to contact each other in a coil.

In addition to this, the wire can be damaged when winding the coil or in subsequent processing steps. Finally , the wire will be fused, soldered, or crimped to the terminals, and this is a common point of failure.

 

Sandy,

All electrical equipment may fail. For the chances and probabilities,please refer to IEEE Gold Book. The best thing you can do is to prevent problems like this applying a Preventive and Predictive Maintenance. Try to inspect your substation each six months, using ultra-sound equipment and thermographic inspection.

BR
Cláudio