Delta vs Wye.

[Lancie1]

Bottom line: I believe floating 480V networks should be unlawful to install due to the safety issues.

I strongly agree, specially where we are talking about buildings where normal (untrained) people work.

The DELTA power system design has been the workhorse of the industry for years, but it also has its draw backs:

What industry are you talking about where Delta connections are considered the "workhorse"? I don't think this statement has been true for at least 50 years now. For most factories, industrial, and commercial uses, a grounded Wye system is closer to being the workhorse in the US.

The only place where I know that using an ungrounded Delta connection makes any sense at all is for utility transformers feeding pole lines. These are only serviced by highly trained electricians, and it is important that the first ground (tree limb brushing the line) does not necessarily take out the power line.

 

[DickDV]

Tim, with the three phase 480V secondary on the distribution transformer having no ground connection or reference of any kind, any stray voltage from any source will raise the whole network to that voltage.

Especially where you have dusty or high humidity conditions, you can get leakage from welders, cathode ray tubes, DC welders, etc. etc. etc and whatever leaks into the network will lift its voltage. Even if the leakage is only, say, 50 milliamps, it can raise the network voltage to the source voltage of the leaking current.

I once had a voltage monitor attached to the power network in an automotive plant with floating delta power (they were blowing up 250hp AC drives at the rate of one every 10 days). It was connected to record voltage to ground. In 24hrs, depending on what other machines were running in this plant, the voltage went from balanced (277VAC) to 800VDC, at one point inverting the polarity to -800VDC, and then going from 400VAC to 1000VAC. None of this was intentional, it all was the result of various kinds of leakages and induced voltages throughout the system. The installation of a 480/480V delta/wye drive isolation transformer with the center of the wye grounded ended the drive problems but, as far as I know, the plant power network is still out there floating in the breeze (or worse!).

 

[sthompson]

What industry are you talking about where Delta connections are considered the "workhorse"? I don't think this statement has been true for at least 50 years now. For most factories, industrial, and commercial uses, a grounded Wye system is closer to being the workhorse in the US.

It was for quite some time very popular with a number of Industries, the one I am most familiar with was Wood Processing.

Sawmills liked it, because if one Phase could go to Ground, and they could keep running until the weekend. Then the Maintenance Electricians would track it down and correct it.

The rub came when a second Phase went to Ground. Boom! Now with the complicated Electronics coming into the Plants, they are going away with the Floating Deltas to a Grounded Wye. Things like VFDs etc, though I still see lots of the older existing Mills fed with Delta.

In Canada 600/347VAC Grounded Wye is the common Industrial Voltage, but 480VAC Delta and 480/277VAC Grounded Wye is also common for Equipment purchased in the US.

Stu....

 

[leitmotif]

Dick

When I learned trade it was on board a submarine. All of our distribution was ungrounded 450 120 60 Hz and 400 Hz and DC.

It has been a few decades and I do not remember a lot of details. I do remember we did not check voltage to ground but frequently checked resistance to ground when fixing a ground. We allowed 50 kOhm for DC and uhh 1meg ohm for any AC resistance to ground. We isolated and fixed grounds as soon as we found them.

What I also do not remember is whether our alternators were wye or delta wound. I vaguely recall that checking line to line voltage gave us 450 on all 3 phases with little variation.

We were a nuc boat so most of our motors were 3 phase and we had 5 SCR variable 3 phase drives circa 1966 or so.

I grant very quickly that shipboard is a different situation than industry.
Dan Bentler

 

[Lancie1]

Sawmills liked it, because if one Phase could go to Ground, and they could keep running until the weekend.

That figures. I have worked in sawmills, and human life and safety meant very little in those places. You see guys in mills with missing fingers, even missing arms and legs. It makes sense that they would be the last to worry about having a grounded electrical system.

 

[DickDV]

lietmotif, you said the key words in your post---"if there was a ground, we immediately worked to remove it". In most land-based facilities that I have been exposed to, there was always talk of finding the ground fault but never any action. It would stay until the second ground fault took the power down.

The motive for floating delta is exactly as stated for the sawmills above---being able to continue running with a single ground fault. We'll fix the ground at the next maintenance outage---uh-huh! And the moon is made of blue cheese!!!

 

[mendonsy]

The Oldsmobile assembly plant in Lansing Mi. used to have everything floating as part of their specs. No ground references anywhere!
Their stuff was a disaster to work on!

 

[leitmotif]

lietmotif, you said the key words in your post---"if there was a ground, we immediately worked to remove it". In most land-based facilities that I have been exposed to, there was always talk of finding the ground fault but never any action. It would stay until the second ground fault took the power down.

The motive for floating delta is exactly as stated for the sawmills above---being able to continue running with a single ground fault. We'll fix the ground at the next maintenance outage---uh-huh! And the moon is made of blue cheese!!!

Aah yes the Production God stuff.

It was bit different on submarines - we needed the power to pump water out of the people tank. Pretty hard to sink a sawmill or auto plant.

Dan Bentler

 

[milldrone]

First let me say "I'm not defending the floating delta".

Pretty hard to sink a sawmill

Obviously you have never worked in a aging sawmill!

When first constructed they are generally built on high ground. After several years of hauling in gravel to fill in "low spots" the sawmill is now the low spot!

 

[I is a noob]

Did anyone mention 277V lighting? Isn't that a thing? If for some reason you just have transformers that are over sized just laying about you can use an open delta to get three phase power. I wouldn't recomment it but could get you out of a jam...

 

[leitmotif]

I was in a few Oregon sawmills as a safety inspector. In winter they seemed to be either buried in sawdust or water and mud especially so in the log yard. I seemed to only do sawmills in winter - colder than you know what. Did not take long to find out I did not want to work there - give me a warm engine room anytime.

Dan Bentler

 

[Lancie1]

When first constructed they are generally built on high ground. After several years of hauling in gravel to fill in "low spots" the sawmill is now the low spot!

Are you sure that it was gravel fill? What I saw was that the sawmills were built on a hill, sloping off to a valley where the sawdust was dumped. After 40 or 50 years, the valley fills up, then grows higher than the original hill. As the mill grows, buildings are placed on a gravel pad on top of the old rotted sawdust. Getting a low-resistance earth ground point in those buildings sitting on old sawdust is difficult. Ten-feet long rods hit a couple of taps fall out of site, never to be seen again. I once saw a case where a 250 KCM ground cable had to be run 1000 feet to a water-well casing.

In more modern times, the sawdust problem has been solved. It is now valuable for making composite wood, or for use in fuel pellets.

 

[OkiePC]

The motive for floating delta is exactly as stated for the sawmills above---being able to continue running with a single ground fault. We'll fix the ground at the next maintenance outage---uh-huh! And the moon is made of blue cheese!!!

What, and just tell everbuddy, "Don't touch no metal with yer bare skin this week..."

 

[OkiePC]

The motive for floating delta is exactly as stated for the sawmills above---being able to continue running with a single ground fault. We'll fix the ground at the next maintenance outage---uh-huh! And the moon is made of blue cheese!!!

What?, and just tell everbuddy, "Don't touch no metal with yer bare skin this week..." ?

I am glad I have not run across this type of power.