Y:Y transformer primary wiring - 4 or 5 wire?

[mayanees]

If the Uitlity is providing 5-wire service, 13.8 kV, 3 hots, 1 neutral and and EGC as the power source for a Y:Y transformer.
They have insisted that our distribtion transformers be yg:yg.

The Utility service transformer will be grounded and bonded at the primary and secondary.

I think I need to run 5-wire to the primaries, and bond the neutral to the egc at each primary. It sounds wrong, since the egc and neutral are the same point.
I know if I'm running the secondary to a switchborad with no single phase loads I can get away with 4-wire, carrying the neutral to the geqar.

Is there any way to legally use 4-wire on the prinary?

Thanks for any responses.

John M

 

[templdl]

If the Uitlity is providing 5-wire service, 13.8 kV, 3 hots, 1 neutral and and EGC as the power source for a Y:Y transformer.
They have insisted that our distribtion transformers be yg:yg.

The Utility service transformer will be grounded and bonded at the primary and secondary.

I think I need to run 5-wire to the primaries, and bond the neutral to the egc at each primary. It sounds wrong, since the egc and neutral are the same point.
I know if I'm running the secondary to a switchborad with no single phase loads I can get away with 4-wire, carrying the neutral to the geqar.

Is there any way to legally use 4-wire on the prinary?

Thanks for any responses.

John M

In the 21 years that I have been in the industry providing transformers to industrial/commercial customers I have never once was required to provide a Y-Y transformer. To the best of my knowledge it is common for utilities to use Y-Y transformers for transmission but for a service entrance D-Y are commonly used.
Is this a transformer that you are required to purchase and will be owned by you? How about the utility metering? I am assuming that there will be CTs amd PTs involved. Are they on the 13.8kv side which they probably are?

I agree with you to question the Y-Y transformer requirement.

 

[Hv&Lv]

In the 21 years that I have been in the industry providing transformers to industrial/commercial customers I have never once was required to provide a Y-Y transformer. To the best of my knowledge it is common for utilities to use Y-Y transformers for transmission but for a service entrance D-Y are commonly used.
Is this a transformer that you are required to purchase and will be owned by you? How about the utility metering? I am assuming that there will be CTs amd PTs involved. Are they on the 13.8kv side which they probably are?

I agree with you to question the Y-Y transformer requirement.

Most of the padmount three phase transformers used by a utility are Wye-Wye five leg core.

 

[templdl]

Most of the padmount three phase transformers used by a utility are Wye-Wye five leg core.

Utilities yes but since my customers have been typically the own the transformer and not the utility.
If the utility allows the customer to supply the transformer that would be interesting.

 

[mayanees]

These transformers are owned by the campus, but the Utility has dictated that for them to connect to them, they need to be Y to Y.
We're running medium voltage concentric neutral condcutors in pvc conduit in concrete-encased ductbanks, so I think the logical/best thing to do is add a 600V-insulated wire for the ground/egc.
John M

 

[kwired]

Does the center point of the primary actually connect to and use the neutral? I am not all that familiar with medium voltage stuff, but with low voltage a wye primary winding typically you would not connect the neutral - just three phases and ground the enclosure with the equipment grounding conductor.

 

[mayanees]

Yes kwired, the 33% concentric neutrals tie together on the neutral bushing, which is then bonded to the GEC, primary and secondary.
John M

 

[kwired]

Yes kwired, the 33% concentric neutrals tie together on the neutral bushing, which is then bonded to the GEC, primary and secondary.
John M

So what is the point of having a neutral conductor and a separate grounding conductor if they get tied together at the load end? This effectively makes them a single conductor with parallel elements. Or am I still not understanding what we have here?

 

[mayanees]

No kwired, you're understanding it correctly, and it's why I'm asking the question. With the neutral being the grounded conductor, can I get away without running an EGC along with the phase and neutral conductors? It's easy enough to just run a 600V class #6 awg egc, and that just looks right. or does it...
.. with the somewhat unique aspect being that these are connected YG:YG

 

[kwired]

No kwired, you're understanding it correctly, and it's why I'm asking the question. With the neutral being the grounded conductor, can I get away without running an EGC along with the phase and neutral conductors? It's easy enough to just run a 600V class #6 awg egc, and that just looks right. or does it...
.. with the somewhat unique aspect being that these are connected YG:YG

I am no medium voltage expert, but the concept of equipment grounding should not change. The general idea is to keep grounded metal objects from becoming neutral current carrying paths. Utilities use the neutral for equipment grounding, but if this is load side of service the grounded/grounding conductors need to be separate conductors beyond the service disconnecting means - I believe that voltage does not matter.

If this transformer primary neutral terminal is not able to be "unbonded" then I don't think this transformer is able to comply with NEC, but would work for NESC applications - which is generally utilities owned installations.

 

[mayanees]

thanks for the update/insight kwired.
NYSEG is requiring the primaries to be bonded for GWye:GWye with the MV neutral being the same point as the equipment case.
I suppose as long as we get them to put that requirement in writing, then that's what we'll design.
I think their belief is that it makes for a more stable system, less subject to transient overvoltages.

John M

 

[kwired]

thanks for the update/insight kwired.
NYSEG is requiring the primaries to be bonded for GWye:GWye with the MV neutral being the same point as the equipment case.
I suppose as long as we get them to put that requirement in writing, then that's what we'll design.
I think their belief is that it makes for a more stable system, less subject to transient overvoltages.

John M

I see it as pointless no matter what the voltage is. You do not have a separate grounded and grounding conductor, you have one parallel conductor. There is no increase in safety or reliability over using a single conductor for same purpose. If a connection of one of the conductors fails - you never realize it as the other one takes over. It is only after both have failed that a problem will be noticed.

 

[mbrooke]

I would try and fight for a delta-wye unit. If this unit is owned by the university they should not be dictating what the connection is. My guess the Utility is stuck to tradition and wants a wye-wye out of fear of ferroresonance. However the risk is only there if the unit is small, left unloaded and single phased, which the unloaded is not something that will happen at a university building. Even this aside, there are more issues with Ygr-Ygr than Delta-wye. I have specd and seen hundreds of delta wye padmounts up to 34.5kv operate for years with out incident.

As for the Ygr-Ygr bonding causing objectionable current that is true, the others are totally right, unfortunetly unlike other industries utilitys treat ground, neutral, earth and bonding as the same thing. And its not without problems either.

I would ask to speak with the utility engineer and have his reasoning behind it, explaining yours as well.

 

[meternerd]

Here in California we supply 12,470/7200 padmounts to our customers. They are Wye/Wye. The primary concentric neutrals tie to the ground stud, which is bonded to the core and the enclosure. The secondary X0 bushing is also bonded to the same point. All are tied to a driven ground rod. We do not run any EGC along with the service wires. 4 Wire only. Any bonding and grounding of the load takes place at the service panel where the service disconnect is located. I have worked at utilities from Arkansas to Alaska and they all do it the same way.

 

[kwired]

Here in California we supply 12,470/7200 padmounts to our customers. They are Wye/Wye. The primary concentric neutrals tie to the ground stud, which is bonded to the core and the enclosure. The secondary X0 bushing is also bonded to the same point. All are tied to a driven ground rod. We do not run any EGC along with the service wires. 4 Wire only. Any bonding and grounding of the load takes place at the service panel where the service disconnect is located. I have worked at utilities from Arkansas to Alaska and they all do it the same way.

But does the transformer primary coils actually tie to the neutral conductor or are they just internally connected and otherwise isolated from the supply neutral conductor? If the neutral point of the primary coils do not connect to anything external, then your grounded supply conductor is not carrying any current, and functionally is just an equipment grounding conductor.

 

[RB1]

HO and XO are often the same terminal for wye-wye transformers. This is not unusual at all. A single-point grounded neutral system would have five wires. I have never seen one, but that appears to be the method described by 250.180(B).

 

[kwired]

HO and XO are often the same terminal for wye-wye transformers. This is not unusual at all. A single-point grounded neutral system would have five wires. I have never seen one, but that appears to be the method described by 250.180(B).

I think:

A single point grounded neutral would have the neutral grounded at one point. There could be other grounding elsewhere on an equipment grounding conductor. Just like for under 600 volts where we do not bond neutral to equipment ground beyond service equipment, or at the source on separately derived systems.

Multigrounded neutral systems would be like what the POCO usually does, the neutral is grounded at every structure, and there is no separate equipment grounding conductor.

One of my main questions throughout this thread has been if the neutral is grounded in multiple locations, then what is the purpose of the fifth wire? Yet people keep bringing it up as if it is a general practice to install a fifth wire and have multipoint grounding.

 

[mbrooke]

Where did the 5 wire requirment come from? (just read it now) I cant see the utility wanting 5 a wire secondary.


From my experience this boils down to who owns what. If this transformer is being owned by the Utility than a 4 wire secondary is ok and the padmount is not of worry.

However, if this is owned by the campus then their rules apply. The 5 wire secondary would make sence in this case along with the delta primary.

 

[kwired]

Where did the 5 wire requirment come from? (just read it now) I cant see the utility wanting 5 a wire secondary.


From my experience this boils down to who owns what. If this transformer is being owned by the Utility than a 4 wire secondary is ok and the padmount is not of worry.

However, if this is owned by the campus then their rules apply. The 5 wire secondary would make sence in this case along with the delta primary.

It makes no sense at all if the neutral and ground are tied together at the load end, which is what is happening if the HO is bonded to the transformer case and can not be "unbonded". All you are doing in that instance is connecting the grounded neutral and equipment grounding conductor in parallel making them effectively one conductor.

 

[mbrooke]

It makes no sense at all if the neutral and ground are tied together at the load end, which is what is happening if the HO is bonded to the transformer case and can not be "unbonded". All you are doing in that instance is connecting the grounded neutral and equipment grounding conductor in parallel making them effectively one conductor.

The exact goal is fuzzy by the op, but the way I saw it as having both neautral and ground bonded at the pad mount and having the neutral isolated and unbonded at the first disconect while the EGC is grounded the way the neutral normally is in a regular service. Im seeing this the way you would ground a seperatly derived 480 to 120/208 in a comercial building.