Transformer Secondary

[Sparky1981]

My company is designing the electrical system for an industrial installation. The utility company is providing the transformers. It is standard for them to have a delta to wye configuration however my motors do not require a neutral. I bounced the idea off them to provide delta to delta transformers, which they will but if a transformer goes down they wont be able to instantly replace it. With the delta to wye configuration the plant wont stay down but for a few hours, obviously preferable from an owners standpoint.

Is there anything wrong with providing a delta to wye transformer and just not use the neutral?

 

[iwire]

If the power company leaves XO floating you do not have to bring it into the building.

On the other hand if they bond XO to ground you must bring it in as far as the service gear and tie it in

 

[Smart $]

What iwire stated is required by Code, but only if it is a system operating at less than 1000V. See 250.24(C).

Whether the supply is delta or wye makes no difference for 3? 3-wire supplied motors. Where you will have differences is if the delta and wye phase voltage is different.

 

[templdl]

My company is designing the electrical system for an industrial installation. The utility company is providing the transformers. It is standard for them to have a delta to wye configuration however my motors do not require a neutral. I bounced the idea off them to provide delta to delta transformers, which they will but if a transformer goes down they wont be able to instantly replace it. With the delta to wye configuration the plant wont stay down but for a few hours, obviously preferable from an owners standpoint.

Is there anything wrong with providing a delta to wye transformer and just not use the neutral?

You are correct. It is not all that uncommon for MCCs to be feed with just 480 3ph with out a neutral. They usually have only 3 horizontal and vertical bus. However the are feed with a 480y/277 transformer where the X0 is grounded and the X0 is borough out to the MCC as an EGC. What is nice about this is that the L-G voltage in 277v and should there be a L-G fault the EGC provided a path for the fault current to flow back to the X0 in order to trip the OCPD.
Personally I would stay away from a D-D transformer unless there is a reason for using an ungrounded delta which does have some advantages. It is of my opinion that there really is no advantage of a corner grounded delta.

 

[Julius Right]

I agree with Smart $.
If Y neutral is grounded in the case of insulation fail the current to ground will be elevated as phase-to-neutral[ground] short-circuit. The protection will trip the breaker .In Delta supply system if a phase insulation fail the other two insulation with respect to ground will be stressed since the ground potential will raise to the fail phase voltage and the potential difference between healthy phase to ground will be square 3 times more. The motor could work in this way for a while. You have to control the insulation fail and to limit the time .

 

[Sparky1981]

So lets say I have the utility supply the delta-wye. It seems like it would be advantageous to bring the neutral not only to my service disconnect but I should bring it all the way to the motor control center as well.

I know the contractor wants to save money by not supplying a neutral but the argument about supplying one to use as a pathway to trip the breaker in case of a L-G fault is pretty convincing.

Anybody want to play devils advocate and convince me I should save the money and not supply the neutral and eother have the power comapny float the neutral on a wye secondary or provide a delta secondary?

 

[don_resqcapt19]

The ungrounded system will require a ground fault alarm of some type, 250.21(B), and a new 2011 section, 250.21(C), requires labeling at the first disconnect to indicate it is an ungrounded system.

 

[jumper]

Anybody want to play devils advocate and convince me I should save the money and not supply the neutral and eother have the power comapny float the neutral on a wye secondary or provide a delta secondary?

Confused here. How can you float the secondary of a wye? I thought all wyes were grounded systems.

 

[templdl]

Confused here. How can you float the secondary of a wye? I thought all wyes were grounded systems.

You could float the X0. Good design? In my opinion no. I would think that if you wanted on ungrounded system it would be a delta.

 

[GoldDigger]

Confused here. How can you float the secondary of a wye? I thought all wyes were grounded systems.

Technically, the center of the wye could even be used as a neutral conductor without being a grounded conductor.
I do not see any logical reason to prohibit use of a four-wire (plus EGC) ungrounded system, but the code is not always logical and I have not searched very hard.

 

[jumper]

You could float the X0. Good design? In my opinion no. I would think that if you wanted on ungrounded system it would be a delta.

An ungrounded wye? New to me. Never heard of it, though transmission lines are beyond my scope, but a building?

 

[Smart $]

Confused here. How can you float the secondary of a wye? I thought all wyes were grounded systems.

An ungrounded wye? New to me. Never heard of it, though transmission lines are beyond my scope, but a building?

For systems 50 to 1000V, X0 of a wye is not required to be grounded if there is no circuit conductor connected to it. See 250.20(B)(2).

 

[Sparky1981]

Seems like I should provide the delta-wye as its abundant and easily replaced. It also seems advantageous to bring the neutral to the service disconnect and into the motor control center.

Appreciate the opinions on this.

 

[GoldDigger]

For systems 50 to 1000V, X0 of a wye is not required to be grounded if there is no circuit conductor connected to it. See 250.20(B)(2).

But see also 250.20(B)(1), which can be argued to require the grounding (using X0) of a 208/120 wye, whether the neutral is used as a circuit conductor or not.
Since it is possible to use that as the ground and meet the 150 volt criterion, it becomes mandatory.

 

[iceworm]

My company is designing the electrical system for an industrial installation. The utility company is providing the transformers. It is standard for them to have a delta to wye configuration however my motors do not require a neutral. I bounced the idea off them to provide delta to delta transformers, which they will but if a transformer goes down they wont be able to instantly replace it. With the delta to wye configuration the plant wont stay down but for a few hours, obviously preferable from an owners standpoint.

Is there anything wrong with providing a delta to wye transformer and just not use the neutral?

Assumptions:
This is industrial ("motors" comment),
And it is 480V, 3ph (you never verified)

Yes, the non-standard xfm is a real problem. Two of my customers opted to spend ~$2M each to have a spare on hand - choke... that's definitely some real money.

You already have good responses on using a wye grounded or ungrounded. And un-grounded Delta is hard to handle. The voltage to ground drifts around, suspectable to lightning surges, and highly suspectable to arcing, restriking ground faults. Not one of my favorites. Only use it if equipment requires, and keep it really short.

There are a couple of other things to consider.

1. What level of power are you looking at? Different design criteria if the system is 500KW than if it is 1MW, or if 5MW.
2. Are you looking at ungrounded systems to save a few dollars on copper? Or is this for continuty of power issues?

If the issue is the copper neutral cost, I won't have any help beyond what has already been said. Bolt the main to the xfm, run the neuetral to the main - and no farther. If not:

At 500Kw, grounded Wye is pretty easy to handle. SCC is ~ 12KA. Equipment costs are low, grounded Wye is simple, well understood, and easy to T-shoot. 277V is available for lighting, but you will still need 480/208/120 xfm for hotel loads. again, you can put the main as close as you want to the xfm and the neutral has to go no farther.

At 1MW and up, I would seriously consider HRG. Utilities don't normally supply HRG, but they also don't usually want to supply un-grounded systems. If they will supply an un-grounded wye, ask if they will use the same transformer installed as an HRG. If it will work for one, it will work for the other. Offer to buy the resistor and if it bothers you, stock a spare (although I have never heard of one failing - except from a ground fault caused by a backhoe driving one into the ground):roll:

Set the resistor at (or on) the xfm. The neutral does not have to go past the resistor. Just take the three phases and the grounding/bonding conductor to the the switchgear.

HRG has a lot of advantages. Phase to ground faults are really damaging on grounded systems - and these go away. Phase to phase faults clear quickly. Continuity of service is good - the plant will stay running with one ground fault. But one has to go find it before too long.

Can't use 277V lighting, so one can either scatter a few small 480D/480Y xfm around for lighting or run the lighting from the 208/120 hotel load xfms - minor design decisions.

Up in the 5MW range you may want to consider buying the xfm and taking the service at 4160V or 13.8KV, or 69kV. This is a whole new set of design issues.

Give us a bit more on what size you ar looking at

ice

 

[Smart $]

But see also 250.20(B)(1), which can be argued to require the grounding (using X0) of a 208/120 wye, whether the neutral is used as a circuit conductor or not.
Since it is possible to use that as the ground and meet the 150 volt criterion, it becomes mandatory.

That is correct. And thank you for pointing it out.

In defense of my post's shortcoming, I assumed being industrial it was going to be 480V or greater supply... which would be not less than 277V to ground (if grounded).

 

[templdl]

An ungrounded wye? New to me. Never heard of it, though transmission lines are beyond my scope, but a building?

I haven't heard of it either but it is of my experience on this forum the it is not unusual for things to end up on rabbit trails that end up far from the subject of the OP involving theory that is of no practical use.
Coult you leave the X0 float, that leaving ungrounded and/or not used as a neutral? Why not or why would you?

 

[Smart $]

I haven't heard of it either but it is of my experience on this forum the it is not unusual for things to end up on rabbit trails that end up far from the subject of the OP involving theory that is of no practical use.
Coult you leave the X0 float, that leaving ungrounded and/or not used as a neutral? Why not or why would you?

Same reasoning you'd want any ungrounded system.

 

[iceworm]

You might want to consider invesing in ieee 242 (2001), Buff, Protection and Coordination, and ieee 141 (1999), Red, Electrical Power Distribution, and maybe even ieee 399 (1997), Brown, Power systems Analysis. They are old - may have to find second hand. Still, math models are true - good references.

ice

 

[robbietan]

If the power company leaves XO floating you do not have to bring it into the building.

On the other hand if they bond XO to ground you must bring it in as far as the service gear and tie it in

What iwire stated is required by Code, but only if it is a system operating at less than 1000V. See 250.24(C).

Whether the supply is delta or wye makes no difference for 3? 3-wire supplied motors. Where you will have differences is if the delta and wye phase voltage is different.

the utilities here in my neck of the woods usually provide transformers to their customers with a delta-wye transformer with 3 cans and a ground. four wires are usually provided to the customer. and this is in 460V or 230V. the customers often use the four wires without ever using a neutral.