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
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
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.
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.
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
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
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
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.
RE: PT Primary Connections
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
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:The PTs will now reflect ground fault conditions.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: PT Primary Connections
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
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
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.
Nonsense. Vector control is entirely unrelated to the grounding of the PT.
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
I also agree with davidbeach's comments above.
RE: PT Primary Connections
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
RE: PT Primary Connections
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
RE: PT Primary Connections
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
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
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
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
Can you provide some more information on why a VT connected in parallel with the NGR will cause a problem?
RE: PT Primary Connections
RE: PT Primary Connections
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
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
RE: PT Primary Connections
RE: PT Primary Connections
RE: PT Primary Connections
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
RE: PT Primary Connections