Help! Transformers connection groups 1

[JuanBC]

Hello,

I am very confused with this topic, I would really appreciate any help

In the attached diagram,

1. Where do you think I should draw the delta and where the star of this elevation transformers?

Until now I thought that the capital letter would correspond to the high voltage side, but now I'm not sure

2. Should the wye be solid grounded?

3. Do you think the data next to the transformers is wrong?

4. What requirement does the zigzag transformer impose?

Thank you!
Juan

 

[davidbeach]

In an ideal world they'd be Ynd transformers, solidly grounded wye on the high side, delta on the low/generator side. The 400/6351V designation doesn't make sense either, although it would tend to suggest a wye on the high side. 6351V would be the wye winding voltage on an 11kV system, the diagram says 13.2kV. Probably best to ask whoever drew it up, hard to tell the actual intent. If it has already been built then the transformer nameplates would be instructive.

 

[ScottyUK]

Adding to the mix, the generator neutrals all look like they are solidly grounded which would negate some of the benefits of the LV delta. I have a feeling this is going to be a horrible system design with Dyn* distribution transformers driven in reverse. Hopefully the LV neutral isn't grounded at both the generator and the transformer.

 

[JuanBC]

Thank you both!

Yes, IMO this diagram is awful too

I would like some clarification if it is not too much trouble:

"In an ideal world they'd be Ynd transformers, solidly grounded wye on the high side"
Q: If the wye is grounded then the zigzag is useless (it can't limit earth-fault current), isn't it?

"delta on the low/generator side"
Q: What are the advantages of using the delta on the generator side? Harmonic issues?

"Adding to the mix, the generator neutrals all look like they are solidly grounded which would negate some of the benefits of the LV delta"
Q: Same as above

"I have a feeling this is going to be horrible system design with Dyn* distribution transformers driven in reverse"
Q: Do you think they are trying to use distribution transformers instead of elevation transformers? (They are different, aren't they?)

"Hopefully the LV neutral isn't grounded at both the generator and the transformer. "
Q: What are the advantages of avoiding grounding the transformer?

I would like to understand this concepts so I can try to influence the designer to change this layout

Many thanks!

 

[waross]

The four pole generator breakers suggest that the generators are wye connected.
The zig-zag transformer and the three pole high side breakers suggest that the high voltage is delta. The zig-zg will be to provide a ground reference for the delta.
Grounding at both the generator and the transformer allows any ground current and/or neutral current to divide between the neutral conductor and the grounding conductor.
This makes it a nightmare to discriminate between ground fault currents and neutral currents.
With the generators looking into a delta, any unbalanced current will cause equal currents to flow in all three phases.
This gives a better sharing of unbalanced currents between the generator phases.
Scotty, we had a mix of wye and delta generators running in parallel and feeding the delta side of the transformer bank.
We never had an issue. Not to say that there may have been the potential for an issue.
One thing that concerned me. When the load was very light and one of the delta generators was running alone, the system was ungrounded. Fortunately that never caused an issue.
That's the way it had been running for years when I came onboard and I was forced to accept it.
I see that the generator bus is labelled 420 Volts so the 400 Volts must be the low side.

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

 

[JuanBC]

Thank you Bill.

"The four pole generator breakers suggest that the generators are wye connected."
=> You are totally right, i did not think about it.

"Grounding at both the generator and the transformer allows any ground current and/or neutral current to divide between the neutral conductor and the grounding conductor. This makes it a nightmare to discriminate between ground fault currents and neutral currents"
=> Nice point too, makes totally sense

"With the generators looking into a delta, any unbalanced current will cause equal currents to flow in all three phases.
This gives a better sharing of unbalanced currents between the generator phases."
=> I did not know that, thank you

" we had a mix of wye and delta generators"
=> What are the advantages/disadvantages of a delta-connected generator? I always saw generators connected in wye

Regards,
Juan

 

[davidbeach]

Didn't notice the 4-pole breakers, in my world they've always been a theoretical possibility but not something anybody would actually spring for. Their mere existence, though, suggests that elsewhere things are different and other people find them useful.

To me, that is one totally hosed up design. It might help if the OP clarifies his relationship to the design and whether it is an actual system or a prospective system.

 

[JuanBC]

Hi David,

Why do you say 4-pole breakers are useless? If you do not have the neutral, what happens when the load is unbalanced?

I work for an engineering and construction company and the single line diagram shown corresponds to the acquisition of a "generation package" that will be rented for an Oil&Gas client.

The design was made by the company to which our client will rent the generation package


Juan
--
"The more I read, the more I acquire, the more certain I am that I know nothing" - Voltaire (or Jon Snow, not sure)

 

[davidbeach]

In my world the neutral is always solid, not switched. I'm sure that someone who works with switched neutrals can come up with what seem like good reasons to switch the neutral but never having done so I can't imagine what they'd be. One of those ANSI vs. IEC thing I imagine.

 

[JuanBC]

Oh, alright

Here in Argentina we use IEC, and neutral must always be switched.

I do not really understand why IEC requires it and ANSI does not

Juan
--
"The more I read, the more I acquire, the more certain I am that I know nothing" - Voltaire (or Jon Snow, not sure)

 

[FreddyNurk]

I wouldn't expect that IEC necessarily mandates 4 pole switching for all applications, although I've seen a few arguments based on certain standards being referenced (sometimes incorrectly). Its possible that there's not the same implementation of IEC standards here, but in all cases I can recall, it was never a preferred option to switch the neutral, and there were normally a heap of clauses about only switching the neutral when the phase connection is switched, to avoid inadvertently floating the neutral.

All the installations I'd been involved with used 3 pole breakers for LV generation, with neutrals being connected to a dedicated neutral bus in the main switchboard, and a single Neutral - Earth connection. There would have been issues if every generator had its star point locally grounded and also connected to the Neutral bus though.

The LV cable in the attached link appears undersized to me as well, and I don't know how one would get a 100/1 Neutral CT on the high side of a Dyn11 either.

EDMS Australia

 

[waross]

Hello David. I agree with you completely on solid neutrals.
That said, I have seen a number of standby systems with a four pole transfer switch.
The reason for switching the neutral in a transfer switch is to facilitate ground fault detection.
I have always been able to find a way to achieve ground fault detection without switching the neutral.

Juan; "What are the advantages/disadvantages of a delta-connected generator? I always saw generators connected in wye"
A wye connected generator has a neutral point for grounding. (Either solidly or impedance grounding.)
A delta connected generator has no neutral point for grounding.
If the system fed from a delta connected generator must be grounded, some other method of grounding must be used, such as a zig-zag transformer or a wye delta transformer. For a four wire (120:240 Volt) generator running islanded the mid point of the 120:240 Volt winding may be used.
The delta generators in my system were very old, dating back to the time when floating delta was the preferred system as a floating delta could often stay in service with a ground fault on one phase.
The origin of that dates back to the days of rubber insulated conductors and cotton insulated windings. Ground faults were much more common 80 years ago than they are now. Ideas change slowly and the popularity of floating delta systems persisted for some time after better quality insulations came into use, and the frequency of ground fault events was sharply reduced.

Wye:delta transformer banks with the delta facing the generator;
Consider a 100 kW resistive load connected line to neutral.
The delta winding will split this so the the generator A phase will supply 50 kW, B phase will supply 25 kW and C phase will supply 25 kW,
However, equal current will flow in all phase windings.
A phase 50% of the current @ unity PF.
B phase 50% of the current @ 0.5 PF.
C phase 50% of the current @ 0.5 PF.

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

 

[ScottyUK]

Hi Juan,

Some of my comments are from the HV generator world, and there are a couple of regulars here (Bill and MikeL) who work with LV sets a lot more than I do, so where their opinion contradicts mine I would listen to them.

I would like some clarification if it is not too much trouble:

"In an ideal world they'd be Ynd transformers, solidly grounded wye on the high side"
Q: If the wye is grounded then the zigzag is useless (it can't limit earth-fault current), isn't it?
Yes, that would be the outcome.

"delta on the low/generator side"
Q: What are the advantages of using the delta on the generator side? Harmonic issues?
On an HV machine we always limit the earth fault current to a handful of amps to protect the stator core in the event of an earth fault. Fault limitation is usually done by a single phase transformer and resistor connected from the machine neutral point to the earth electrode. On LV machines it is usually a plain resistor from neutral to earth. This method of earthing the neutral would generally preclude connection of any phase-neutral loads, which isn't a problem on HV machines but should be considered on LV machines depending on what your local code allows.

"Adding to the mix, the generator neutrals all look like they are solidly grounded which would negate some of the benefits of the LV delta"
Q: Same as above
A delta winding is desirable for the generator side of the transformer because it allows the generator to operate with its own high resistance earthing scheme. The earth fault level is defined by the earthing resistance, not the number of running generators and the available system fault level.

"I have a feeling this is going to be horrible system design with Dyn* distribution transformers driven in reverse"
Q: Do you think they are trying to use distribution transformers instead of elevation transformers? (They are different, aren't they?)
Distribution transformers with Dyn windings are cheap and easily available. Generator step-up transformers with YNd windings are less common and likely more expensive / longer leadtime. If an EPC contractor can make the scheme more-or-less work with a cheap and easy solution which is technically poor then they will, if they can get away with it.

"Hopefully the LV neutral isn't grounded at both the generator and the transformer. "
Q: What are the advantages of avoiding grounding the transformer?
Don't earth a neutral in two locations, otherwise neutral current enters in the earthing system. It's very easy to do this with careless design of the system, especially if the generator can operate in both stand-alone and system-parallel modes, if the location of the neutral-earth bond is not carefully selected. Neutral current in the earthing system can cause mis-operation of the protection relays, and it is a code violation.

I would like to understand this concepts so I can try to influence the designer to change this layout

Many thanks!

 

[wroggent]

I think 4 pole breakers can be used to isolate an offline generator from elevated neutral voltages when in parallel with other generators on an impedance grounded system. Speaking of which, shouldn't a high impedance ground be used here on each generator to limit third harmonic current between generators as well as to limit damage during ground faults?


Edit: Well if they're rented I suppose you may not care what could happen to them.

 

[JuanBC]

Hi everyone, thank you very much for your excellent explanations

Freddy: "only switching the neutral when the phase connection is switched, to avoid inadvertently floating the neutral"
We also do not switch the neutral alone, we only switch neutral when we have a 3 Phases + Neutral scheme (for example, a feeder to a distribution panel board)

"a single Neutral - Earth connection"
Do you mean a PEN conductor on a TN-C earthing scheme?

Bill: "The delta generators in my system were very old, dating back to the time when floating delta was the preferred system as a floating delta could often stay in service with a ground fault on one phase"
Makes sense

"Consider a 100 kW resistive load connected line to neutral..."
Nice example, thank you

Scotty: "A delta winding is desirable for the generator side of the transformer because it allows the generator to operate with its own high resistance earthing scheme. The earth fault level is defined by the earthing resistance, not the number of running generators and the available system fault level."
Can't you get the same thing using the not-grounded wye side of a transformer on the generator's side?

Also, i've seen generation schemes in LV where generator is wye not grounded and the delta side of the transformer is in the generator side.
I suppose there will be other reasons beyond the short circuit current

Thanks
Juan

Juan
--
"The more I read, the more I acquire, the more certain I am that I know nothing" - Voltaire (or Jon Snow, not sure)

 

[prc]

As per IEC standard conventions, when a transformer connection is indicated as Dyn11, HV side is delta , LV side is star with neutral brought out .(usually earthed) In this case, since HV is delta, to get a neutral, a zig-zag connected grounding transformer is provided.
It is true that with high voltage generator transformers, usual connection is Ynd1(in US) and Ynd11 in IEC world. The reason for a LV delta is, during a LG fault on HV side, zero sequence current will be trapped in delta and will not enter the generator windings. As I understand, large generators cannot stand zero sequence currents and stator windings will be subjected to vibrations so severe to tear them apart.
But with low capacity generators Dyn is common, reason I am not sure. It cannot be due to cost aspects of transformer. In these units high voltages is usually medium voltage only and a transformer with D on HV is cost wise not a problem. But a D with 220 or 400 KV is prohibitively costly and a grounding transformer also will be necessary at that voltage.

 

[FreddyNurk]

wroggent, there's a good chance that the winding pitch of the units selected minimises 3rd harmonic currents, although it doesn't do much for the overall fault current.

I've never seen a high resistance grounded system for units of this size, perhaps catserveng or waross has though. Its possible that one reason for that is that the magnitude of the current doesn't tend to stay high for very long, unlike the machines ScottyUK deals with. Its also more complicated for people used to dealing with solidly grounded LV systems.

JuanBC, I can never remember the TN-C / TN-S definition exactly, as I don't deal with it often. I suspect what I'm referring to is TN-S though. ScottyUK has indicated the issue though, in his last paragraph.

EDMS Australia

 

[JuanBC]

Thank you prc and Freddy. Happy new year everyone

Thank you all for your continued contribution to my ongoing learning





JBC
.......
"The more I read, the more I acquire, the more certain I am that I know nothing"

 

[waross]

Re solid versus resistance grounding:
Most of my experience (but not all) has been with standby generators.
For a standby generator, grounding is a plant design issue.
A design decision will be made as to the method of grounding at the time that the plant is designed.
The standby generator grounding method will match the plant grounding method.
The larger a transformer or system, the more likely it is to have resistance grounding.
A very large plant may have resistance grounding on the higher voltage transformers, 4160 Volts or 13,000 Volts and above.
480 Volt loads may be supplied from relatively small unit substations that are solidly grounded to allow the use of 277 Volt lighting.
Generator grounding will be determined by the grounding method in place at the voltage level at which the generators connect.

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

 

[Hoxton]

in the UK we have just had new Distribution Code and Generator Connection standards / specifications

In view of the wider implications, I am putting up a separate post