Wye Grounded Primary and Wye Grounded Secondary Transformer

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Another thing in a wye-delta with neutral grounded is an upstream ground fault on one of the phases will be backfed by current from the wye, because the delta secondary couples in current from the other two phases of the wye. So local OCPDs protecting the primary of the transformer can be opened by faults upstream of the OCPDs.
 

GoldDigger

Moderator
Staff member
Not sure if it was me.

The issue is that the line-neutral voltages on the wye side are reflected as line-line voltages on the delta side. Unbalanced voltages on the wye side cause circulating currents on the delta side trying to force the wye side neutral into balance. If the primary neutral floats, then its voltage can shift slightly to avoid these large circulating currents.

This is desirable, for example if you use a wye-delta transformer to derive a neutral to use as a grounding transformer.
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-Jon
1. (y) If the primary neutral is grounded the voltage balance can be maintained only via the IR drops associated with circulating currents. On the wye side these currents can easily exceed the full load current of the windings.
2. The code explicitly clarifies that a system will still be SDS if the only wire connection between primary and secondary conductors comes from their common connection to the building ground system.
 
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Electricmo

Member
Location
Missouri
Occupation
Lineman
From a lineman’s knowledge of the subject. A high voltage transformer bank must have 3 coils or transformers. The primary high side must be grounded, floating would cause unbalanced voltages. Same with the secondary side. There should be a grounded neutral supplied to the panel. 3/phase loads on these banks are usually pretty evenly loaded.
 

bobby ocampo

Senior Member
Bobby, just so it gets mentioned, the primary's neutral should be left floating, not grounded or "neutral'ed", as if it was a delta.


Grounding Electrode System
Utility transformers are Multi grounded system. Both the primary and the secondary are grounded.
 

bobby ocampo

Senior Member
Maybe, I think it depends on what voltages we are talking about and why this particular transformer is being used and what for. IF the primary is an MV MGN, primary neutral would be grounded.

More details Bobby. Is this just an academic question?
No. This is not for academic discussion. Distribution Utility is multi grounded and their transformers are always grounded wye primary and secondary. I am wondering if they are considered separately derived system. What is the schematic diagram of a separately derived wye-grounded primary and wye grounded secondary?
 

bobby ocampo

Senior Member
I have usually said; do not ground the primary side of wye-wye transformers unless you are a utility.

The reasoning behind this statement is that most industrial/commercial electricians would be involved with <600V on the primary side while utilizes would be using medium voltage primaries. I kind of skipped the situations where a facility use MV as its distribution system, like the OP has.ste

To answer the OP, the primary side neutral point being connected to ground/earth/building steel is not related to the secondary neutral point being connected to the same reference points.
My question is it considered a separately derived system?
 

bobby ocampo

Senior Member
2. The code explicitly clarifies that a system will still be SDS if the only wire connection between primary and secondary conductors comes from their common connection to the building ground system.
What do you mean "If the only wire connection between primary and secondary conductor comes from their COMMON CONNECTION to the building ground"?
 

kwired

Electron manager
Location
NE Nebraska
My question is it considered a separately derived system?
Yes. Separately derived systems that are grounded still have ground as a common reference point.

outside of abnormal conditions, secondary neutral current is trying to return to the XO point and that current ordinarily would not flow on the primary neutral.

Also for other than MGN system which generally would only be on utility systems, if there is a neutral conductor on the primary windings it would usually be a code violation to ground/bond it at the transformer, you would need a separate EGC run with the primary conductors for grounding/bonding purposes. Not doing so will result on parallel primary neutral currents on non current carrying components. This would put the primary and secondary bonding point back at the service and/or GES connection point for the secondary.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The thing that makes a separately derived system is that it is a new source of electric current. Electric current always flows in a closed circuit trying to return to the source ( the thing that pumped the electrons up to high potential in the first place)

Jon
 

paulengr

Senior Member
No. This is not for academic discussion. Distribution Utility is multi grounded and their transformers are always grounded wye primary and secondary. I am wondering if they are considered separately derived system. What is the schematic diagram of a separately derived wye-grounded primary and wye grounded secondary?
They are not always wye and definitely not always grounded. The vast majority of transmission lines (115 kv and higher) are ungrounded. I haven’t even heard of one that is.

A lot of generating stations and in fact a lot of installations use either resistance grounded or ungrounded deltas or wyes precisely for the major fault current limitation and massive improvement in reliability particularly for MV subs and distribution.

Sure at the residential level they love their wye systems but that is not the norm everywhere.

And to the guy that said there is ALWAYS three two coil transformers, wrong again, even with utilities. If you have relatively small amounts of three phase loads it’s a common trick to use 2 transformers in an open delta, usually with one of them center grounded so the available voltages are 240/120 grounded single phase (Edison residential) and 240 high leg grounded delta but with a fairly high unbalanced impedance.

They are not as common but a lot of industrial customers request and often get delta-wye service due to its many advantages from their point of view.

Utilities though tend to do just one thing, over and over again. They are reluctant to do anything they are not familiar with so they tend to be very tribal in nature. With many of my customers being large industrials, Duke, Dominion, gas plants, water and waste water, and a lot of smaller independent producers especially, I see it all.
 
My question is it considered a separately derived system?
I think it is a bit tricky. If you read through that other thread I linked to we talked about this. The key phrase is " no connection except thru grounding and bonding". IMO that leaves the door open to it not being an SDS depending on how the XO and HO are are connected. If it's a factory connection, or even (arguably) a factory removable strap, then IMO it's NOT an SDS. Kinda nitpicky but to me that's what the language says.
 

paulengr

Senior Member
I think it is a bit tricky. If you read through that other thread I linked to we talked about this. The key phrase is " no connection except thru grounding and bonding". IMO that leaves the door open to it not being an SDS depending on how the XO and HO are are connected. If it's a factory connection, or even (arguably) a factory removable strap, then IMO it's NOT an SDS. Kinda nitpicky but to me that's what the language says.
To use a much simpler example say we have a single phase system. Typically you’d connect H1 and H0 to the hot and grounded (not grounding) phases respectively, and the frame to the grounding line. With X0 if it’s a separately derived system you’d connect it to its own ground rod or to the new main distribution panel and the ground connects there. You could also connect it to the grounding connection but then we get into an issue of shared grounds. With the current NEC we tie every service together. In a non-SDS system X0 is connected to the neutral, H0.

Of course in reality ground faults are shared across the transformer. But at 3 phase we have options. If we use a phase shifting transformer like a delta-wye then ground faults are isolated from each other in both systems. Again assuming separate ground grids (avoid GPR).
 

ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
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wire pulling grunt
To use a much simpler example say we have a single phase system. Typically you’d connect H1 and H0 to the hot and grounded (not grounding) phases respectively, and the frame to the grounding line. With X0 if it’s a separately derived system you’d connect it to its own ground rod or to the new main distribution panel and the ground connects there. You could also connect it to the grounding connection but then we get into an issue of shared grounds. With the current NEC we tie every service together. In a non-SDS system X0 is connected to the neutral, H0.
Huh?
Of course in reality ground faults are shared across the transformer. But at 3 phase we have options. If we use a phase shifting transformer like a delta-wye then ground faults are isolated from each other in both systems. Again assuming separate ground grids (avoid GPR).
Of course we all know about a delta wye transformer. In reality the topic of this thread is about wye wye transformers.

As best I can tell the difference between a wye wye SDS and a wye wye NonSDS is that a SDS ties HO and XO together using building steel, ground rods, or other grounding electrodes and a NonSDS ties the HO and XO together using the same lug or buss bar and then running that on out to the grounding electrodes.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
As best I can tell the difference between a wye wye SDS and a wye wye NonSDS...
If the primary windings are separate from the secondary windings thenI have always viewed the transformer as an isolation style and therefore an SDS.
The the primary windings are interconnected to the secondary windings, I consider it to be an autotransformer and therefore a Non-SDS.

According to the NEC all 'grounds' must be tied together, so they and any connection to them (i.e. a grounded conductor per 250.30, and thus 250.26) could not be the deciding factor in the SDS decision. A transformer, by itself, is not an alternate AC power source, for the most part, if it is a Non-SDS a neutral to ground bound would not be required on the transformer secondary.
 
If the primary windings are separate from the secondary windings, then I have always viewed the transformer as an isolation style and therefore an SDS.
If the primary windings are interconnected to the secondary windings, I consider it to be an autotransformer and therefore a Non-SDS.
So say I have a wye wye that has separate HO and XO, no factory connection. I say, "I dont want this to be an SDS because I dont want to run a GEC to it (or because its Tuesday or because they were out of Haagen Daz strawberry ice cream last night and Im still annoyed.....) So I connect a conductor between XO and HO. Have I succeeded at my quest?
 

bobby ocampo

Senior Member
I have usually said; do not ground the primary side of wye-wye transformers unless you are a utility.

The reasoning behind this statement is that most industrial/commercial electricians would be involved with <600V on the primary side while utilizes would be using medium voltage primaries. I kind of skipped the situations where a facility use MV as its distribution system, like the OP has.

To answer the OP, the primary side neutral point being connected to ground/earth/building steel is not related to the secondary neutral point being connected to the same reference points.
My question is if the wye grounded primary and wye grounded secondary is considered as a SEPARATELY DERIVED SYSTEM?
 
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