4160/115/230 VAC Control Transformer Failure and Motor trip

[adityalagudu3291]

We have the following CPT installed for our motor circuit controls;

Primary 4160 VAC
Secondary 115/230 VAC
3KVA, 60HZ, K-Factor 1
3A fuse on primary, 20A fuse on Secondary

We installed these CPTs a few months back in 4 of our motor feeder units.

We had trip incidents on all 4 motors and didn't have any event report on the relay. We thought to test these transformers and measured winding resistance and turns ratio. Turns ratio comes as 41.54, 26.22, 27.44, etc, and winding resistance as 277 ohms, 11.1Kilo Ohms, 53.2 Kilo Ohms, etc. So these values are consistent.

We are trying to understand what makes these transformers fail and questioning their integrity? As these transformers are tested well in the factory. How we can eliminate this issue moving forward?

 

[Sahib]

Are SPD's installed?

 

[adityalagudu3291]

Are SPD's installed?

Sahib

No, there aren't any Surge Protective Devices inside the gear. How does it affect the failure of these transformers?

 

[Sahib]

Sahib

No, there aren't any Surge Protective Devices inside the gear. How does it affect the failure of these transformers?

A control transformer (dry type) may be more vulnerable to suges. A SPD affords a degree of protection.
My guess is motors tripped due to failure of control transformers by voltage surges.

 

[Jraef]

You don’t say how the transformers are failing. What do you mean by fail and how do you know that’s what the problem is?Open winding? Are the fuses clearing? If so, primary? Secondary?

 

[adityalagudu3291]

You don’t say how the transformers are failing. What do you mean by fail and how do you know that’s what the problem is?Open winding? Are the fuses clearing? If so, primary? Secondary?

Jraef,

There is this particular pattern on the motors with these CPT models installed. We didn't find any other reason so we took out these CPTs and tested them. The turns ratio, winding resistance on these CPTs are abnormal. We confirmed with the vendor for the FAT reports and found them normal. So we are currently looking for a root cause for the failure of the CPTs. These are replacements in kind for the old GE CPTs model number: 573350P053 which were no longer in production. The new replacements are made by Magnetics & Controls, Inc with Model No: 10520. Please see the attached for reference.

 

[synchro]

Is there a relatively long feeder that could have a significant capacitance providing 4160V to the control transformer? If so, then if the feeder is switched on or off an overvoltage might occur due to ferroresonance. A capacitive load on the 4160V feeder could also make ferroresonance more likely.

If this is happening then some resistive loading on the control transformer output could help. Also, an RC snubber on the primary of the transformer could be utilized.

 

[Sahib]

The OP CPT not dry type?

 

[adityalagudu3291]

The OP CPT not dry type?

It is a dry type CPT

 

[adityalagudu3291]

Is there a relatively long feeder that could have a significant capacitance providing 4160V to the control transformer? If so, then if the feeder is switched on or off an overvoltage might occur due to ferroresonance. A capacitive load on the 4160V feeder could also make ferroresonance more likely.

If this is happening then some resistive loading on the control transformer output could help. Also, an RC snubber on the primary of the transformer could be utilized.

How can we install RC snubber on the primary if this is the case? Any vendors or product recommendations?

 

[synchro]

ow can we install RC snubber on the primary if this is the case? Any vendors or product recommendations?

Do these units with the control transformers share a common 4160V feeder? If so then that could make it more likely that this feeder is experiencing an overvoltage which is causing the failures. If such an overvoltage is from switching transients then an RC snubber might help.
I've no experience with them, but this company makes RC snubbers:
http://www.hilkar.com/compactrc.html

Are you seeing mainly higher resistance measuerments on the primaries of the CPTs, but most (if not all) of the secondary windings have resistances showing that they are intact? If so then that would tend to indicate the failures are originating on the high voltage side, and therefore more likely due to overvoltages. Of course the initial failure may propagate and cause more widespread damage.
Has any failure analysis with cutting or disassembly of the transformers been done?