PT Rating

[vennivivi]

Hi,

I visited a site where they used a potential transformer to sense ground fault on their 22 kV / 400 V transformer. The PT is rated at 13 k V/ 110 V . This sound strange as the PT should have been sized at the rated line/line voltage of the transformer.
Any comment !

Grundig

 

[rockman7892]

It depends on how the PT's are arranged. If the three PT's are arranged in a "wye" configuration then the voltage across each on of the PT's would be 22/1.73 = 12.7kV. This would get you close to the 13kV necessary to produce the 110V on the secondary.

 

[davidbeach]

If the 22kV is solidly grounded then the wye connected 13kV windings should be fine. If the 22kV is not solidly grounded, or the VT is connected in delta, then the primary rating should be 22kV.

 

[rockman7892]

davidbeach

I'm curious, why does the 22kV need to be solidly grounded in order for the VT's to be connected in "wye"

 

[dpc]

Because the VT will only be rated for the line-to-neutral voltage based on the neutral side at ground potential. If the wye is ungrounded, the neutral side of the VT could be at a much higher voltage during certain situations. The insulation may be graded to reduce costs, with less insulation towards the neutral end.

 

[rcw retired EE]

David - If the 22 kv winding is not grounded, ferroresonance of the wye connected VT's is possible during a ground fault. Full phase-phase 22kV is impressed on two VT's driving them into saturation, creating a ferroresonance condition. I've burned up some switchgear doing that.

Alwasy spec the grounding VT's for full phase-phase votlage or make sure there are suitable damping resistors in the circuit.

ANSI C37.102 had a good explanation (5.2.2).

 

[vennivivi]

Hi,

Thanks for the replies. The 22 k V transformer is grounded through this PT that is the PT is connected to the star point of the 22 k V transformer. I understand that the fault current should be high enough to induce a voltage in the secondary. But my understanding is that this PT should be sized for line/line voltage.

Regards.

Grundig

 

[davidbeach]

used a potential transformer to sense ground fault

transformer is grounded through this PT

This are rather different and distinct applications. Either can be done, but generally not both at the same time. Clear, concise, and precise terminology can certainly help narrow down the range of answers and keep them more focused on the real issue.

 

[vennivivi]

Ok. Sorry for the misunderstanding. Let me clarify the issue. The transformer 22 k V start point is grounded through a potential transformer, which rating is 13k V /110 V. I believe this transformer should have been sized to the V_LL.

Grundig

 

[davidbeach]

"Potential Transformer" is an older term for what IEEE recommended practices now calls a "Voltage Transformer", but in either case, whether PT or VT, it is an instrument transformer. What you are doing does not involve an instrument transformer and using the term "potential transformer" just confuses the issue. Grounding transformer or distribution transformer used to ground the power transformer would have eliminated much confusing and misdirection in previous answers.

Distribution transformer from power transformer neutral to ground with resistor on the low side of the distribution transformer should be sized for the line-neutral voltage of the power transformer. Ground one of the phases of the power transformer and now you have L-N neutral voltage across the distribution transformer; that's as bad as it will get, no need for a higher voltage rating.