1 Bushing vs 2 Bushing PT Required

[rockman7892]

I'm specifying wye-wye PT's on a 34.5kV system that is a Delta system but grounded through a zig-zag transformer with NGR. Can these PT's be standard 20125/35000 GRD Y single bushing PT's or do they have to be a 2-bushing PT for potential elevated L-G voltage during a ground fault?

I know typically on ungrounded or impedance grounded systems the PT's require 2-Bushing PT's rated for full L-L voltage that would be present during a ground fault. In this case it would be a 20125/35000 Y rated PT.

Also I was looking at a PT that had a rating nomenclature of 34500/34500 GRD Y. What does this rating indicate? Why is the L-L voltage repeated twice on here?

 

[RRaghunath]

"standard 20125/35000 GRD Y single bushing PT's" - yes. The system is ineffectively grounded and the overvoltage on healthy phases during an SLG fault will be addressed by selecting the voltage factor to be 1.9.
"nomenclature of 34500/34500 GRD Y" - the rating says that phase-to-ground insulation is same as Phase-to-phase insulation in this case.

R Raghunath

 

[Kiribanda]

Even though your the 34.5kV system is DELTA ungrounded during normal operation, it transforms to an effectively grounded
system (provided that the ZZ transformer is properly sized) during a single line to ground fault. Therefore, in my opinion,
you can use single bushing VTs even with voltage factor 1.1.

 

[rockman7892]

Thanks for the response.

What does the Rated Voltage Factor indicate? For a particular PT I'm looking at I see them as 1.1 or 1.25 with some of them listed as "@64% burden"

So based on your description above of the 34500/34500 GRD Y PT this would be a PT that would could be applied on an ineffectively grounded system? It appears this would be a 1-bushing PT but would be rated for L-L during ground faults as mentioned?

 

[scottf]

34500/34500 GRD Y is an odd-ball rating normally seen in switchgear applications where the VT is spec'd to be able to take the line-to-line voltage indefinitely, but is a single-bushing VT connected line-to-ground....so it's normally energized at 58% rated voltage. To my understanding it's rated like that for ratio/relay input reasons and not because of fault-condition/OVF reasons, but I could be wrong on that. Not sure what the "@64% burden means".

To answer your original question, from a practical perspective, I would recommend purchasing a 2-bushing VT with line-to-neutral rating, e.g. 34500Y/20125V with a 175/300:1 ratio. The price difference between a 1-bushing and 2-bushing VT with line-to-neutral ratio is pretty small (<10%).

Or, if it's a delta system, consider going with a true L-L rated unit like 34500Y/34500V, 300:1 ratio and connect them L-L. We're seeing a lot of ferro-resonance coming up now at 34.5kV applications on renewables and data centers because of a lot of cable capacitance (we think)...connecting the VTs line-to-line reduces the chance of ferro-resonance to some degree.

 

[RRaghunath]

Rated Voltage factor of 1.2 continuous and 1.9 for 30s is recommended by IEC 61869-3 for VTs meant to be used in non-effectively earthed systems that include automatic tripping in case of SLG fault.

R Raghunath