PT Primary Connections 2

[timm33333]

If we feed a transformer-relay (digital) with Y-Y connected PT’s, with primary Y of PT’s ungrounded and secondary Y grounded; will the fact that the primary of PT’s is not grounded cause malfunction of the relay? Thanks

 

[unclebob]

I don't see why this would cause trouble.

 

[timm33333]

That is what I was thinking. But some people think that if the primary of PT's is ungrounded, the relay might mistakenly start reading Phase to Phase 208V voltage from PT-secondary (instead of phase to ground 120V). And as the transformer feeds variable speed drives and the transformer neutral is high resistance grounded, so vector control of variable speed drives will not work properly in case ground fault occurs when PT primary is ungrounded. Is it correct?

 

[collies99]

Capacitive coupling between the primary to secondary winding can make it morr unsafe and a higher shock potential hazard.

 

[jghrist]

You will not have a source of zero-sequence voltage on the VT secondary, so any protection function depending on zero-sequence voltage will not work.

 

[krisys]

jghrist,
I have a small clarification on this.
The zero-sequence voltage on the VT secondary can be derived only if you have a broken delta secondary side connection. That has nothing to do with the primary side VT connection.

Now coming to timm33333’s query of VT primary connection which is wye (star) and not earthed. The VT primary is conventionally earthed in order to reduce the size. The primary connection P1 will be connected to the phase and the secondary will be directly earthed. Thus the P2 connection will be at earth potential and need not be terminated at a terminal having insulation level of line voltage.

Capacitive coupling between the primary to secondary winding can make it more unsafe and a higher shock potential hazard.

timm33333 has not mentioned the rated voltage of primary winding. If the VT is at low voltage (LV) level, it should be fine. As it is resistively earthed system, I presume it is a medium voltage system. Hence from the safety point of view it is recommended to solidly earth the neutral of VT primary winding.
Please do not forget that solidly earthing the neutral of VT primary winding would also eliminate the risk of ferroresonance.

so vector control of variable speed drives will not work properly in case ground fault occurs when PT primary is ungrounded

As far as the question of downstream VSD malfunctioning, I do not see any issue there. Please note that the occurrence of fault is not an everyday phenomena. Additionally, the earth fault is generally expected to be cleared within 150 ms with the operation of primary protection. Even with the back up protection the fault clearing time should not be exceeding say 500 ms.

I cannot imagine a possibility of any malfunction of variable speed drives (VSD). At least the risk is not perceivable. VFD controls are not having such a fast response time to act with in 150 ms.

 

[timm33333]

The PT's are 4160V-120/208V, as primary voltage is less than 15 kV so ferroresonance should not be an issue.

The secondary of VT is grounded, so how will the grounding (or not grounding) of VT-primary effect the insulation level on the already grounded secondary?

Solidly grounding the neutral of VT-primary makes sense, but the problem is that the manufacturer of neutral grounding resistor requires the primary of VT to be ungrounded.

 

[davidbeach]

The zero-sequence voltage on the VT secondary can be derived only if you have a broken delta secondary side connection.

Completely untrue. Any decent relay can calculate the sequence components from the phase voltages and use (3)V0 values in the protection calculations. It just requires that the system side of the VT have the wye-point grounded.

 

[timm33333]

Yes, but that is what I am trying to understand that how the grounding/un-grounding of the system side (primary) of the VT affects the load side (secondary) of the VT.

 

[davidbeach]

If the primary is grounded and you lose on high side voltage you will have two good voltages on the low side and one zero voltage. If you float the high side and then lose one voltage, the low side will have two half voltages that are 180 degrees out from each other.

 

[krisys]

I agree with davidbeach. The new generation relays don't need the broken delta secondary VT winding. Thanks for the clarification.

Solidly grounding the neutral of VT-primary makes sense, but the problem is that the manufacturer of neutral grounding resistor (NGR) requires the primary of VT to be ungrounded

I do not understand what could be the effect of VT primary earthing on the normal resistor (i.e. NER). You should ask the NER supplier to substantiate his claim. At least we will learn something new in the process of working.

 

[timm33333]

Actually the manufacturer is saying that if you ground the primary of VT, it will act as an alternate path and will bypass the current, and the current will not pass through the neutral grounding resistor at all.

What do you guys think about it?

Please see section 9.4–c (page 32) of the SE-330 manual at the following link:

http://www.littelfuse.com/~/media/f...otectionrelays_se_330_manual_new_revision.pdf

 

[waross]

What are you using the PTs for?
If you connect the PT wye point to the transformer wye point, the PTs will accurately reflect the phase to phase and phase to neutral voltages but will not reflect ground fault conditions.

You may ground the PT wye point subjet to the following instruction:

Verify proper installation of the ground-fault current
sensor. Ensure the cables pass through the ground-
fault-current-sensor window
.

The PTs will now reflect ground fault conditions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Actually the manufacturer is saying that if you ground the primary of VT, it will act as an alternate path and will bypass the current, and the current will not pass through the neutral grounding resistor at all.

What do you guys think about it?

I'm surprised this is not widely recognised. You absolutely cannot introduce additional low impedance grounding locations on a resistance grounded network. You're effectively shorting out the NER. Y-Y PTs are a perfect example. Don't ground the primary. It's up to the relay manufacturer to decide whether they can handle it. I can't see why they couldn't.

 

[davidbeach]

How is that shorting out the NER? The primary of a VT has a quite high impedance. A wye-wye VT, grounded on both sides, can't contribute any ground current to a fault unless there is a ground source on the secondary side, something I've never seen. That's all a bunch of hog wash.

When you have a system, such as a generator, that is high resistance grounded, there is some risk of a ground fault on the secondary of the VT being seen, through 3V0 sensing, as a primary ground fault, typically worried about a 100% stator ground fault scheme responding to a VT secondary fault. This is typically handled by grounding one of the phases (typically B) on the secondary and thus reducing the number of possible secondary faults that might trip the unit. But even then, it is 3V0 from a secondary fault that is of concern, not 3I0 from the VT back onto the high side, and the high-side wye-point is grounded. A properly connected (includes grounded wye-point) VT can never contribute current to a system fault.

Find a different resistor manufacturer, one who actually understands power systems.

 

[LiteYear]

Addressing your other questions now:

But some people think that if the primary of PT's is ungrounded, the relay might mistakenly start reading Phase to Phase 208V voltage from PT-secondary (instead of phase to ground 120V).

There's no "mistake" and it's not a matter of opinion. The secondary voltages only reflect the primary voltages in phase to phase measurements, because the secondary is not referenced to the primary (the transformer is doing its job and isolating the two circuits). This does mean that if you have a ground fault on the primary, it will barely be perceptible on the secondary because the primary phase to phase voltages won't change much. Relying on phase to ground voltage measurements on the secondary would be a very bad idea. The neutral of the secondary can be very different to the neutral on the primary.

And as the transformer feeds variable speed drives and the transformer neutral is high resistance grounded, so vector control of variable speed drives will not work properly in case ground fault occurs when PT primary is ungrounded. Is it correct?

Nonsense. Vector control is entirely unrelated to the grounding of the PT.

 

[timm33333]

Thanks Davidbeach! So to put it in simple words, if we have a transformer with NGR feeding a load, and VT (with primary Y grounded) are put in between transformer and load. If a ground fault occurs at phase A, the current would pass through ground soil and would like to go back to transformer neutral. It can come back to neutral either from VT primary or through NGR. But the only available path is through NGR because VT primaries will not let the current pass through it because VT primary is a coil and would resist the passage of current and in the meantime the NGR will trip. Is it how it will work?

 

[davidbeach]

In even simpler, and more accurate, words - current doesn't flow in the primary because current has no path to flow in the secondary. The amp-turn balance between primary and secondary will be maintained and with zero amp-turns in the secondary there are only two options for the primary; zero amps or zero turns. Zero turns seems highly unlikely leaving zero amps as the only reasonable outcome.

 

[jghrist]

I'm surprised this is not widely recognised. You absolutely cannot introduce additional low impedance grounding locations on a resistance grounded network. You're effectively shorting out the NER. Y-Y PTs are a perfect example.

A grdY-grdY connected VT is not a ground source, so there will no contribution to a ground fault from the VT. Unless you connect a neutral as well as a ground to the VT secondary, it will not be shorting out the NER. You wouldn't be running a neutral on a resistance grounded circuit anyway, so that would not be a possibility.

 

[marks1080]

Make sure you tell your relay that the primary Y connection is ungrounded. I've seen many transformers trip out because this simple settings wasn't changed.

I also agree with davidbeach's comments above.