12kV PT Acting Strange

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stephanw

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Location
Portland, OR
I'm managing a 2MW battery energy storage system tied to a 12kV grid and recently had the main breaker open on a voltage discrepancy. The AB voltage is 3500, BC is 12kV (grid voltage) and AC is 15kV. Seems to me like the A phase PT may be having issues? Seems like a phase shift where the A phase is being reported as slightly delayed, which could be due to damaged coils in the PT?

A few weeks ago we had a similar issue and one of the high side fuses was blown in one of the PTs.

Does anyone have experience with these types of issues and what may be causing them?

I have screenshots of voltages, etc. if that would help.
 
Sahib

Sahib

Senior Member
Location
India
stephanw said:
I'm managing a 2MW battery energy storage system tied to a 12kV grid and recently had the main breaker open on a voltage discrepancy. The AB voltage is 3500, BC is 12kV (grid voltage) and AC is 15kV. Seems to me like the A phase PT may be having issues? Seems like a phase shift where the A phase is being reported as slightly delayed, which could be due to damaged coils in the PT?

A few weeks ago we had a similar issue and one of the high side fuses was blown in one of the PTs.

Does anyone have experience with these types of issues and what may be causing them?

I have screenshots of voltages, etc. if that would help.
Click to expand...

Check insulation resistance of PT with a meggar and also its historic insulation resistance tests to ascertain any trending.
 
Tony S

Tony S

Senior Member
Location
Resting under the Major Oak UK
Do you have access to hot sticks? If you do check the input to the PT.

That said the fact a fuse has blown would have me looking for a replacement PT. I will say that in 40 years of working on MV gear I’ve never had a PT fail.
 
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stephanw

Member
Location
Portland, OR
Fuse was blown, PT tested fine. Insulation was good and the A Phase PT matched very well with the C Phase PT. We're replacing the fuse and will look for a replacement A Phase PT when/if this happens again.
 
Sahib

Sahib

Senior Member
Location
India
Maybe ferroresonance occurring. Are PT's lightly loaded? Any MV VCB switching taking place also?
 
junkhound

junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
Have had PT fuses blow on 3MW 11kV gen sets on shutdown due to field control staying on while rotor slows, saturated cores.

What is your tie in to the battery bank consist of?
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Sahib said:
Maybe ferroresonance occurring. Are PT's lightly loaded? Any MV VCB switching taking place also?
Click to expand...

Ferroresonance can occur in PTs that use a capacitive voltage divider followed by a wound transformer, aka Capacitive Voltage Transformers (CVTs). They are mostly used in HV but also in MV.
The resonance can be at a subharmonic (typically 1/3), and the light loading Sahib mentioned makes such a resonance more pronounced.

https://electrical-engineering-portal.com/voltage-transformers-essentials
 
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stephanw

Member
Location
Portland, OR
junkhound said:
Have had PT fuses blow on 3MW 11kV gen sets on shutdown due to field control staying on while rotor slows, saturated cores.

What is your tie in to the battery bank consist of?
Click to expand...

There's an inverter and then a 9MWh battery bank downstream of the switchgear where the PTs are located. No rotating machines on the line.
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
You mentioned that some waveforms were captured. Was this a transient event or did the voltage discrepancy persist over a sustained period?
The measured 15kV vs. a nominal 12kV seems too high to be caused by only a phase discrepancy, which would produce at most around13.9kV.
Is any datalogging done on the inverter that might show if a transient or other problem might have originated there?
 
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