ZCT Cracking Due to Cold Temperature-QUERY

[ThePunisher]

HI all,

We experienced two 600V ZCTs cracks which we strongly believe caused by freezing temperature (Canada). There were no evidence of human causes to them and we have coordinated with the transformer vendor to replace them with suitable ones for the application and conduct root-cause-failure analysis report.

The ZCTs will be replaced but the energization will take place prior to the shipment date.

The transformer vendor advised us that we can proceed with the energization even for a cracked ZCT. This concerns me and we have sent out our query to them to justify that no issues would occur energizing the system with these ZCTs in place.

I am hoping I could get some comments here based on anyone's experience and technical capability which I respect so much.

Regards,

 

[ThePunisher]

Second Photo

 

[unclebob]

Can you remove them and live without ground fault protection for awhile?
But as the zct is not directly on energized parts, I would leave them there.
Make sure the secondary circuit is not open.

 

[cuky2000]

Before energize the transformer, seen to be a good idea to test the CT to verify if is within the manufacturer parameters.
Usually CT ratio tests could identify that the voltage ratio is within expected parameters and CT insulation resistance testing for any open or short winding.
Check with the manufacturer to see if they recommend a seal compound or silicon to prevent further damage until the CT be replaced.

 

[Kiribanda]

ThePunisher,
1)The photo shows it is installed inside a covered box.Am I correct? In that case you may have
a thermostatically controlled small heater inside the enclosure.
2)If the CT is used in Canada in outdoor, then one has to specify -50C as the
ambient temperature as a special operating condition per CSA 60044 in the data sheet.

 

[ThePunisher]

Thank you all.

The ZCT is inside the transformer secondary termination compartment. Is it feeding the secondary protection relay at the switchgear which the relay asserts itself to trip the secondary circuit breaker and block bus transfer (Main-tie-Main).

The idea of providing thermostatically-controlled heater inside the compartment but this would be a maintenance disaster as we have to de-energize the transformer every time the heater is required to be accessed. At the same time, varying temperature fluctuations here in Canada during Winter times may cause condensation concerns.

Yes the ZCT, if replaced must be -55 Deg C. I believe the construction of the ZCT specified was not the right one.

WE decided to use conventional ZCT and will add it to the X0 connection to the NGR and will be inside the NGR enclosure instead. The ZCT would be like the "plastic" construction like the one already in the NGR which would have -50 to -55 Deg C rating and is readily available.

 

[waross]

Plastic in the construction of a CT has two functions.
1. Insulation. You have good clearances and the cable insulation by itself provides all needed insulation.
2. Corrosion protection. You can probably safely energize the installation and sometime in the next few years replace the CT during a scheduled turn around.
Go ahead and energize and look for a suitable replacement at your leisure.
Or, seal the crack with suitable tape to prevent the ingress of moisture. Scotch Fill splicing tape comes to mind.
The root cause is probably differential contraction between the core and the plastic in the last cold snap.
Investigate the expansion coefficients for the plastic and for the core.
When you said Canada and cold, I thought Alberta. Right?

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

 

[LionelHutz]

I'd be curious if the CT is still functional or if the crack broke the secondary winding.

 

[waross]

Good point Lionel.
If so a temporary fix may sometimes be made by winding a new secondary winding.
20 turns of a conductor suitable for 5 Amps will give you a 100-5 Amp CT.
Leave the original winding disconnected and open circuited to avoid arcing at the break.
This is not the safety issue that it once was as most safety programs would now require the transformer to be de-energized before accessing that compartment.

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