delta wye to wye wye change

[wilbur1]

our plant is changing from a 3 wire to 4 wire system . Our utility is telling us this. How will this affect or loads . Most are three phase loads with no need for a neutral. However questions were asked about our protection ? New utility service and switchgear are 4 wire compatable. From there on there are alot of fused disconnects and some three wire distibution panels . What problems might occur when we change over?

 

[DTR2011]

your ratings of surge protection may be undersized at the bus connected to the utility

 

[waross]

Such a change implies new transformers. You may wish to check the available fault current, particularly to ground and re-evaluate your protection settings.
You should check your ground grid to make sure that it is still up to code. A ground fault on a wye system with a poor or faulty grounding system may result in lethal voltages on exposed metal surfaces through out the plant.
You haven't mentioned grounding method but if your system is solidly grounded you can expect a lot more hazard and damage from ground faults.
It may be well to hire a suitable consultant to familiarize your electricians and any one else who may be affected with the arc energy available to ground with a grounded wye system.
About 40 years ago, many of the 440V industrial plant in my area were delta fed. Although 120/208 volt systems were common, many industrial electricians were unaware of 277/480 volt industrial systems. Some of the old timers would use a jump wire to ground from one phase of a delta system to check for power. They would scratch the wire and watch for the capacitive discharge spark.
We were building a new mine mill, 277/480 volt system.
And experienced electrician tried to check for power by grounding one phase of a 1000 KVA 277/480 volt transformer with a #14 AWG jumper. He survived but he was wearing bandages like boxing gloves for a few weeks until the burns on his hands healed somewhat.
You don't want someone to find out the difference between Wye and Delta the hard way.


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

 

[alehman]

You may need to revise your ground fault protection system. If your new system is to be solidly grounded you may be required to add ground fault protection. That is a question for a consultant who is familiar with your codes.

Some old delta systems were corner grounded and used fuses in only two of the three phases. If you happen to have one of those, you would have to replace any 2-pole fuses with 3-pole.

As waross suggested, you need have an experienced engineer take a look at this conversion.

 

[sobeys81]

Smallgreek,

Just out of curosity, when you mentioned that Surge protection may be undersized. Why do you say that.

If it is a Delta system or Floating Wye, surge arrested must be sized with MCOV equal to the Line-Line Vol. of the system and that is the max. protection.

On the other hand if was wye with resistance grounded than Arresters might be sized for Nominal vol/ Sqrt(3) > MCOV of the arrester.

So what prompted your comments.
Thanks

 

[wilbur1]

i appreciate the replies. I think the utility thinks the inside wiring is older than it is. We have no grounded phases or unprotected phases. They also mentioned damage to motors which after checking with suppliers I Don't feel is an issue either.

 

[waross]

What was your old voltage and what will your new voltage be? Often there is a small voltage change when changing from delta to wye. Mostly of a result of changing standards. eg: very old delta systems were 440 V, newer deltas were 460 V. Wye systems are generally 277/480 V.
A small change in voltage may affect the odd motor that is now working at the extreme edge of the performance envelope.

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