Potential Transformer in 3W Switchgear

[jimgineer]

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.

 

[jghrist]

Typically on a 3W system, you would connect 2 VTs open delta, 35:1 (4200V/120V), with secondary B phase grounded.

 

[dpc]

Traditionally it would be two PTs connected open delta - 4200 V - 120 V (35:1).

 

[jimgineer]

Can you guys further explain the open delta?

Are there other options you can use?

 

[slavag]

Hi.
Please see attached thread.
In end of thread you can find pict of few types of connections.
thread238-216248
Good Luck.
Slava.
Jghrist and Dave, next time, could you please coordinate your posts, same grading

 

[davidbeach]

Read the thread referenced by slavag. I certainly wouldn't recommend open delta VTs. Sure you can save a buck, but when you need to really figure out what happened you will find that some of the necessary information was never recorded when using open delta, and that information can never be recovered.

 

[dpc]

On an ungrounded delta system, I'd be somewhat cautious about using wye-wye PT connection due to the potential for ferroresonance.

 

[davidbeach]

3W setup with ground

Probably won't have to worry about ferroresonance.

 

[dpc]

Yup, missed that little tidbit. For some reason I had a delta system in mind.

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.

 

[davidbeach]

Or use 20:1 (2400:120) and have 120V line-neutral and 208V line-line. Any other ratio that floats your boat but keeps the secondary voltage low enough for your equipment should be fine.

 

[jimgineer]

I hope you guys get paid handily for all of your knowledge. One day hopefully I'll be there... So the natural follow up involves, what is ferrorresonance and why does it apply (I assume) to a 4W but not a 3W system?

Thanks for the resources

 

[dpc]

It can occur when wye PTs are connected to an otherwise ungrounded (delta) system. The inductance of the PTs resonates with the capacitance to ground of the system and creates very high voltages, and sometimes PT failures.

If your system is grounded at the source, this shouldn't be an issue.

 

[rockman7892]

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.

 

[slavag]

Hi Rockman7892.
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

 

[rockman7892]

Slavag

I see it now. Thank You.