Fault current calculation for 69KV Instrument Transformer

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friendlyfire

Member
Location
Comox BC
Hi, I am trying to do a basic fault calculation of a 69KV to 120V transformer and am finding GE specs on the unit confusing. This is not something I have a great deal of experience with so some advice would be great.

The unit is the JVT350. If you are interested a spec sheet is easily located by searching on that model.

Specs:
Primary: 69KV
Secondary 120V
Ratio: 600:1
Thermal rating: 4.5KV
BIL: 350KV
And something I do not understand:
"ANSI Accuracy, Burden Per ANSI, W,X,M,Y = 0.3"

What do I use as the impedance for the fault calculation?

Also, just to confirm, thermal VA is the rated secondary output maximum while maintaining accuracy?

thanks!
 

topgone

Senior Member
Hi, I am trying to do a basic fault calculation of a 69KV to 120V transformer and am finding GE specs on the unit confusing. This is not something I have a great deal of experience with so some advice would be great.

The unit is the JVT350. If you are interested a spec sheet is easily located by searching on that model.

Specs:
Primary: 69KV
Secondary 120V
Ratio: 600:1
Thermal rating: 4.5KV
BIL: 350KV
And something I do not understand:
"ANSI Accuracy, Burden Per ANSI, W,X,M,Y = 0.3"
What do I use as the impedance for the fault calculation?
Also, just to confirm, thermal VA is the rated secondary output maximum while maintaining accuracy?
thanks!
0.3% accuracy will be attained at rated voltage and frequency.
Type JVT350 has a rating of 400VA (ZZ designation). So, your impedance is = 120^2/400 VA = 36 ohms.
Thermal rating is the rating that should not be exceeded to prevent damage to VT. The VT burden is the max load on the VT that the unit remains accurate (to 0.3% in your case).
 

jghrist

Senior Member
0.3% accuracy will be attained at rated voltage and frequency.
Type JVT350 has a rating of 400VA (ZZ designation). So, your impedance is = 120^2/400 VA = 36 ohms.
Thermal rating is the rating that should not be exceeded to prevent damage to VT. The VT burden is the max load on the VT that the unit remains accurate (to 0.3% in your case).
This will give the load impedance that loads the VT to its thermal rating. You need the tested impedance of the VT to calculate the fault current.
 

friendlyfire

Member
Location
Comox BC
hey, thanks for the responses.
topgone: what does it mean ZZ designation, and where did the 400VA come from? I do not see that figure anywhere in the spec sheet.

jghrist: I think you are correct, but the tested impedance seems to be lacking in the spec sheet. It seems to me that my next step is to bug GE for that info.
 

topgone

Senior Member
VT Burden Based on Designation Letter/s

VT Burden Based on Designation Letter/s


If you doubt the attached data, you can Google it yourself using key phrases "Standard Burdens for Voltage Transformers"

Please take note:
VA = IE = E^2/Z or Z = E^2/VA = 120^2/400 = 36 ohms.
and:
R = E^2 X PF/ VA = 120^2 X 0.85/400 = 30.6 ohms
 
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rcwilson

Senior Member
Location
Redmond, WA
topgone: what does it mean ZZ designation, and where did the 400VA come from?
As shown in the chart, the ZZ designation means the VT will be within accuracy specifications when it is supplying loads up to 400VA at 0.85 power factor.

I have never seen an impedance value given for a VT, but I never looked explicitly for one either.

I am curious as to why you are trying to calculate the short circuit current on the 120V side of the VT?
 

friendlyfire

Member
Location
Comox BC
Hi,
The reason for this investigation is to determine appropriate ratings and arc flash labeling required of a service rated panel to be connected to the output of the transformer. Application is an unmetered highway sign in the middle of nowhere. The only source of power is a 69kV transmission line.
 
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