Is there any advantage to ungrounded wye wye power systems? Ungrounded Wye delta?

[bdn2004]

We are working at a plant right now, it's probably 50+ years old that has two main substations. One is fed by a wye wye 12470V - 480V, the other wye - delta, same voltages. They aren't tied together via a tie breaker. I've never seen this before just wondered what's the advantage of this? And why would they be different? They've had some issues, and are considering changing things out in order to increase reliability. Any suggestions?

 

[Fuse4]

For ground faults, the delta system has the same behaviour that wye system with isolated neutral (the low voltage side).

 

[waross]

100 years ago, delta:delta systems were the norm.
There was a period when these systems were converted to wye:delta to increase capacity.
This was followed by a period of issues with wye:delta systems.
The wye:wye system avoids the problems associated with wye:delta systems.
That is; Grounded or four wire wye:delta, circulating currents and backfeeds into primary system faults.
Floating wye:delta, switching transient overvoltages.

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

 

[dpc]

What waross said. If you're talking about ungrounded 480 V delta system, these were the norm until the 1960s. Advantage is that one phase can go to ground without the need to trip anything. Disadvantages are numerous. Grounded wye - grounded wye is common in this part of the world for utility-owned service transformers. Recommendation for replacement would depend on the nature of the plant and your commitment to maintenance. A solidly grounded wye 480 V secondary would be by far the most common approach today. However, this will require a good grounding system with functioning equipment grounding conductors.

 

[jraef]

When I worked at a steel mill decades ago, we had an ungrounded 480V delta system. In the electric shop, they had 3 light bulbs connected to the main switchgear. If there was a ground fault anywhere in the mill, one lamp glowed brighter to let us know, but other than that, nothing shut down because that would have been a MAJOR catastrophe. That was the point of having an ungrounded delta system; the first ground fault was tolerated because it just made your system a corner grounded delta system and everything (other than where the fault was) continued on normally. I used to call it the "Damn the torpedoes, full steam ahead!" approach to plant operation.

So when that lamp glowed bright, we would get on the radio and ask who in the plant was having problems or seeing smoke, or if nobody responded we would jump in golf carts and drive around like keystone cops looking for it. Good times...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[cuky2000]

Below is a summary of transformer connections. I hope this contribute with the discussion in this subject.

 

[waross]

Hi Jeff. Your post brought back memories of an old sawmill with 440 Volt delta. Our ground detector lights were configured so that one went out when there was a ground.
A lamp on an MCC went out one day. The foreman sent a good young electrician to find the problem.
Two and a half days later he stomped into the shop and threw his tool belt on the bench.
The foremen asked;
"Found the trouble yet?"
"Yes, d____ it I found it!"
"What was the problem?"
"The d___ light bulb was burnt out!"
My first thought;
"Thank God it was him and not me!"
I have also worked in plants that used a resistance grounded wye so as to be able to continue running with a ground fault on one phase.

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

 

[HamburgerHelper]

Waross,

I have always thought it scary that someone could get hurt or killed in the field without anyone doing anything incorrectly.

 

[bdn2004]

I can see the production issue being a driving force for this ungrounded system... - as in if it fails could be catastrophic to production. they wouldn't want it to immediately trip. I used for work DuPont. They had a deliberately corner grounded delta 480v systems. All the B phases were grounded with solid links in all fuse holders for the B phase. At that plant when they replaced portions of the system they went with a high resistance grounded system. Good suggestion here? And what happens on a wye wye ungrounded system when one leg goes to ground, same as delta..first one nothing?

 

[waross]

There will be charging currents to ground in an ungrounded system, delta or wye.
A wye system may be impedance grounded with a high enough impedance that the system may operate with a ground on one phase.
The impedance may limit the ground fault current to a value that does no damage.
The problem is when you have an arcing ground fault.
A persistent arc of even a few Amps may eventually do serious damage.

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