Potential Transformer in 3W Switchgear
Potential Transformer in 3W Switchgear
(OP)
All,
We have 4160V switchgear L-L, a 3W setup with ground (no neutral bus).
We have to input voltage for relay operation of overvoltage, undervoltage, as well as other relay settings we're picking up on. My question involves suggested connection of potential transformers. We usually use single phase transformers for each phase, with a fuse on both the primary and secondary sides of this transformer.
For a 480V system, I haven't physically seen how it's wired but I've always assumed that we go line to neutral on the primary side, and that's where our 2.3 turns ratio comes from... 277/120 ~ 2.3 ... But for a 3W system us younger engineers were questioning both physically where you would connect the primary of the PTs to, and what turns ratio you would use.
Thanks in advance for shedding light on this.
We have 4160V switchgear L-L, a 3W setup with ground (no neutral bus).
We have to input voltage for relay operation of overvoltage, undervoltage, as well as other relay settings we're picking up on. My question involves suggested connection of potential transformers. We usually use single phase transformers for each phase, with a fuse on both the primary and secondary sides of this transformer.
For a 480V system, I haven't physically seen how it's wired but I've always assumed that we go line to neutral on the primary side, and that's where our 2.3 turns ratio comes from... 277/120 ~ 2.3 ... But for a 3W system us younger engineers were questioning both physically where you would connect the primary of the PTs to, and what turns ratio you would use.
Thanks in advance for shedding light on this.






RE: Potential Transformer in 3W Switchgear
RE: Potential Transformer in 3W Switchgear
RE: Potential Transformer in 3W Switchgear
Are there other options you can use?
RE: Potential Transformer in 3W Switchgear
Please see attached thread.
In end of thread you can find pict of few types of connections.
thread238-216248: Open Delta PT vs Wye PT
Good Luck.
Slava.
Jghrist and Dave, next time, could you please coordinate your posts, same grading
RE: Potential Transformer in 3W Switchgear
RE: Potential Transformer in 3W Switchgear
RE: Potential Transformer in 3W Switchgear
RE: Potential Transformer in 3W Switchgear
Three wye-wye PTs would be OK. You could still use the 35:1 PTs and would have 69 volts line-neutral and 120 V phase-to-phase.
RE: Potential Transformer in 3W Switchgear
RE: Potential Transformer in 3W Switchgear
Thanks for the resources
RE: Potential Transformer in 3W Switchgear
If your system is grounded at the source, this shouldn't be an issue.
RE: Potential Transformer in 3W Switchgear
I am confused as to why the B phase on the secondary of an open delta PT is grounded. I would think that you would just connect the B phase or corner of the delta directly to the B phase input on whatever meter is being used. I cant seem to see exactly why this is grounded.
So if you are reading Vab on the secondary I'm assuiming this should be a L-L measurement but it seems like it would be a L-G measurement with B grounded.
RE: Potential Transformer in 3W Switchgear
Please draw for you voltages vectors and you will understand, why you confused.
If you maes Vab 120V w/o grounding of B, you will meas same voltage with grounding.
If you have wye connection ( star connection) you can connect grounding to star point or to B phase, but L-L and L-N voltage will be same with or w/o grounding point.
Grounding is safety and reference issue only.
Hope that helps.
Best Regards.
Slava
RE: Potential Transformer in 3W Switchgear
I see it now. Thank You.