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PT Primary Connections

PT Primary Connections

PT Primary Connections

(OP)
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

RE: PT Primary Connections

I don't see why this would cause trouble.

RE: PT Primary Connections

(OP)
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?

RE: PT Primary Connections

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

RE: PT Primary Connections

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.

RE: PT Primary Connections

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.

Quote (collies99 )

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.

Quote (timm33333 )

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.

RE: PT Primary Connections

(OP)
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.

RE: PT Primary Connections

Quote (krisys)

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.

RE: PT Primary Connections

(OP)
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.

RE: PT Primary Connections

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.

RE: PT Primary Connections

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

Quote (timm33333)

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.smile

RE: PT Primary Connections

(OP)
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/files/littelfuse...

RE: PT Primary Connections

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:

Quote:

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

RE: PT Primary Connections

Quote (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?

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.

RE: PT Primary Connections

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.

RE: PT Primary Connections

Addressing your other questions now:

Quote (timm33333)

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.

Quote (timm33333)

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.

RE: PT Primary Connections

(OP)
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?

RE: PT Primary Connections

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.

RE: PT Primary Connections

Quote (LiteYear)

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.

RE: PT Primary Connections

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.

RE: PT Primary Connections

I should have said:
Unless you connect a neutral as well as a ground to the VT primary, it will not be shorting out the NER.

RE: PT Primary Connections

(OP)
Yes I had figured it out, thanks!

RE: PT Primary Connections

Quote (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.

Quite right, thanks for the criticism - my scenario only concerns the magnetising current of the VT. Yes, provided the VT is perfect and the magnetising current is zero, there can be no ground current on the primary without ground current on the secondary. But consider these three scenarios:

  • each VT contributes some magnetising current in parallel with the NER current. How many VTs does it take to become significant?
  • once the VT is saturated, the magnetising current becomes very large. How much headroom do your VTs have under fault conditions?
  • magnetising current at DC is very large. Do you have any DC sources that can contribute to fault current?
Once you've considered and eliminated those possibilities, and any others I haven't thought of, you can go back to your ideal VT model. Or you can just stop introducing additional ground paths on resistance grounded networks and sleep easy.

RE: PT Primary Connections

The VT magnetizing current is so negligible (in the order of few mA) that it need not be taken into account while dealing with the subject issue. We are comparing mA with the earth fault current thru NER which is the order of 200 to 400 Amps.

RE: PT Primary Connections

Wow.

Even on a high resistance grounded generator with only 5-10 amps of ground current the VT magnetizing current is several orders of magnitude lower.

On resistance grounded systems the VT primary should be rated for phase-phase voltage, so saturation is not an issue.

DC is going to cause enough problems all over the power system that any sources will be quickly rooted out and corrected. Half-wave saturation of a power transformer gets lots of attention and the DC (more likely even harmonics rather than actual DC) source gets removed to stop the power transformer from growling.

RE: PT Primary Connections

Good-o, glad you've got it all sorted. Where I'm from people use things like VSDs and very sensitive ground fault relays, buy their VTs from the cheapest supplier and use lots of them. Ergo, somewhere in the world it's a problem and has led to failures in the field. Fortunately not in your part of the world.

RE: PT Primary Connections

(OP)
DavidBeach, the manufacturer says that the test current that they will use for testing of NGR will be DC current, so it will pass through transformer primary as the transformer primary will act as short circuit to DC. It still does not make sense to me because transformer primary is inductor which will delay the passage of current and in the mean time the NGR will trip. What do you think about it?

RE: PT Primary Connections

Test the NGR when? Under what conditions?

RE: PT Primary Connections

(OP)
Instead of waiting for the ground fault to occur, they will intentionally inject external DC current into the NGR circuit. This external DC current will act as ground fault current, and will trip the NGR. The test will pass if the NGR trips on this external DC current.

RE: PT Primary Connections

So they test it and put it in service. If you want to test it again, open the disconnect to the VT. It's not like that's something that you're going to need to worry about very often.

RE: PT Primary Connections

Sounds like you're mixing a few concepts here.

A pure inductor does act like a short to DC. However, in reality that's speaking to the inductive reactance. The VT's primary also has a very high DC resistance...probably in the 1-5 k-ohm range depending on the voltage class.

With all of that said, it's always a good idea to remove VTs from the circuit when doing DC tests. It's more important if doing DC voltage stress tests, but also a good idea when doing DC current tests, especially if the voltage will be anywhere over a few volts. Dc can send the VT into deep saturation and could damage the unit.



RE: PT Primary Connections

How does an NGR trip? Breakers trip, resistors melt.

RE: PT Primary Connections

timm33333, for what it is worth, I am familiar with neutral-grounding resistor monitors and high-resistance grounded networks. I understand there is a continuous online test. I've measured the waveform produced by the SE-330 in particular. I understand, as does Startco, that putting the primary of your VTs in parallel with the neutral-grounding resistor by grounding the VT primary neutral is problematic for the reason they mention and many others. You're free to try to convince yourself otherwise but the facts remain. scottf is completely correct, but is not familiar with the SE-330 operation. It is not just as you describe.

RE: PT Primary Connections

Liteyear-

Can you provide some more information on why a VT connected in parallel with the NGR will cause a problem?

RE: PT Primary Connections

(OP)
If we wind the primary of PT in delta, then zero sequence voltage will not be passed on to the relay. The only practical way to pass the zero sequence voltage to the relay is if the PT primary is Y-grounded.

RE: PT Primary Connections

Quote (scottf)

Can you provide some more information on why a VT connected in parallel with the NGR will cause a problem?

If you specifically mean with regard to a NGR monitor, then I will have a go. For an incomplete list of other potential problems, see my posts at 28 May 14 1:15 and 28 May 14 18:37.

NGR monitors attempt to confirm the resistance of the NGR is within tolerance (typically -50%/+100%) by continuously injecting a square wave voltage across the NGR. Naturally the rest of the system is in parallel with the NGR, and therefore must be discounted. A substantial DC component is used so that the system capacitance can be neglected, but it is assumed that the DC resistance to ground of the rest of the circuit is very high. In fact, NGR monitors are typically coupled with ground fault lockout devices that ensure phase to ground resistance is >1MΩ. At medium voltage levels the primary resistance of Y-Y VTs might be a few kohms, but there is no guarantee. Every VT combines in parallel, and it doesn't take much for them to make a significant impact on measuring the few hundred ohms of the NGR.

The fact that NGR monitors and ground fault lockout devices exist should suggest how important it is to keep stray paths to ground out of high-resistance grounded networks.

RE: PT Primary Connections

Or, it should suggest that NGR monitoring systems should recognize the realities of real life. In this day and age of numeric relays there is no excuse or justification for VTs with any primary connection other than a grounded wye. Period. Full stop.

On high resistance grounded systems the secondary of VT can go with a phase grounded rather than the wye point; anything else the secondary should also have the wye point grounded.

To hobble the rest of your protection simply to make it easier for an NGR monitor manufacture seems to be getting priorities screwed up. We have many NGRs and nary a monitoring system that precludes proper protection on the system.

RE: PT Primary Connections

The diagram shows a CT (EFCT-X) monitoring the current. Not sure how the manufacturer expects DC to properly test this setup.

RE: PT Primary Connections

(OP)
It looks that DC current is injected out from terminal 6.

RE: PT Primary Connections

(OP)
DC current out at terminal 6, pass through sensing resistor, transformer star point, NGR, ground soil; and then back into terminal 7.

RE: PT Primary Connections

Quote:

The diagram shows a CT (EFCT-X) monitoring the current. Not sure how the manufacturer expects DC to properly test this setup.

The CT is actually deliberately designed to be ignorant of the test current. Notice how the current path travels in both directions through the CT in order to cancel the effect. The current is monitored by the device internally. The CT is there for traditional earth fault detection (hence the EFCT designation). This is a combined earth fault detection and NGR monitor device.

There seems to be some scepticism about these devices. FWIW, we've been using them in underground coal mines in Australia for years. The latest update to the applicable standards actually requires their use in some situations. Startco are not the only manufacturers. They are bread and butter devices in our industry.

RE: PT Primary Connections

I can see that DC could be used to monitor NGR continuity. I don't think this injection, filtered by the CT, will result in tripping the breaker.

RE: PT Primary Connections

The device is designed to trip the breaker via its trip coil or undervoltage coil. If the NGR goes out of spec, the device's internal relays change state, closing or interrupting the circuit to the trip or u/v coil respectively, and the breaker opens its main contacts. Often the devices are paired with a contactor instead, and can then just interrupt the contactor's main coil.

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