480V 3 ph Service with 240V Tap

[dithomas]

When I was still working I ran across a 3 phase electrical service where the utility supplied power to a facility using three pole mounted transformers with one transformer being larger than the other two. I can not remember the details, but it seems to me that one of the transformers supplied 240v for the facility lighting. I can not remember if it was a Y or delta.
Any help to jog this old memory would be appreciated.
Dan

 

[System Protection]

High leg Delta.

https://en.wikipedia.org/wiki/High-leg_delta

 

[waross]

three pole mounted transformers with one transformer being larger than the other two.

That would be a high leg delta system.
It was common when distribution circuits were predominantly delta.
In the 40's and 50's, a lot of utilities added a neutral conductor and changed their primary distribution to wye.
When the delta/delta systems were changed to wye/delta, serious problems arose.
Wye/delta banks are extremely rare in North America.
Four fused cutouts rather than the usual three is the giveaway.
When the primary is wye, (wye/delta) there are often four primary fused cutouts.
When the bank is energized locally, a neutral cutout is closed first.
Then the phase cutouts may be closed without over voltages due to switching surges.
Once all three phases are closed, the neutral is opened and the fuse holder removed and taken away.
That prevents the transformer bank from negatively impacting single phase customers on the same primary circuit.
In Central America the wye/delta connection was common but without the neutral cutout.
It will take pages to describe the ongoing issues and damage to consumer equipment caused by that connection.
I saw a transformer burned out due to the failure of one phase of a voltage regulation bank about 10 miles away.
I made a lot of money protecting customer's equipment on those systems.
The most susceptible were refrigerators and freezers and burnouts were common.
I hope to never see another wye/delta bank on distribution circuits.
The one wye/delta bank where I was able to chat with the utility engineers was a legacy system that was originally a delta/delta bank.
The bank fed a small plant.
Rather than drop the voltage to all of the machinery from 240 Volts to 208 Volts, the plant opted to stay with a 240 Volt delta system when the grid was changed from delta primaries to wye primaries.
One large and one small transformer in open delta is not always perfect but has far fewer issues that the three transformer bank.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dithomas]

Thanks to all. My memory has been properly refreshed.

 

[jraef]

480V high leg delta is exceedingly rare though... I have only come across it once in 40+ years. 240/120V 3 phase 4 wire delta is common of course, but for 480V, they generally just give you 480/277 Wye if you want single phase lighting power.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Years ago I worked in an old mill with ungrounded delta 440 Volts and ungrounded 110 Volt control circuits.
As far as I know, the rule requiring grounding circuits if the voltage to ground would be 150 Volts or less came into effect in 1948.
This mill was grandfathered.
It was a really old mill.
Trouble shooting control circuits, everything was 55 Volts to ground.
I got into the habit of grounding one side of a circuit until I finished trouble shooting.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dithomas]

Hi Jraef,
I have seen this configuration twice. One facilty had three separate services each installed with an expansion. The oldest one was the high leg and the other two were the normal 480/277 services. That project was to combine all of the services into a new single point of connection service to the facility.

Hi Waross,
I have worked on ungrounded delta system is two facilities in this area. One was a brewery and the other was a conveyor belt plant. They were not easy to trouble shoot. The brewery was very good at finding and clearing grounds as they occurred. The conveyor belt plant not so good. Their ground detection systems were not maintained.

Dan

 

[waross]

Remember the ground detector lamps?
A ground detector lamp went out in one of our plants.
A good electrician spent over two days looking for a ground.
The third afternoon he stomped into the shop and threw his tool pouch on the bench.
The super asked if he had found the trouble.
"Ya. I found it."
"What was it?"
"The damm light bulb was burned out."
My only thought was;
That could have been me.
(Any one who has worked on an old ungrounded system with ground detector lamps will get this.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

But it could only burn out during a ground fault on a different phase…

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

I agree with you, David that in normal operation, a ground detector lamp will see only 58% of rated voltage and typical burnout is unlikely.
But voltage/current induced aging is not the only failure mechanism.
The lamp may have been in service for 20 years or more.
Eventually, in gassing may cause failure.
Failure of a solder connection between the base and the bulb leads is possible.
I have seen in gassing with a keepsake bulb, that I energized after about 15 years in storage.
It was about 300 Watts, from a 20 Amp constant current circuit.
It would light up nicely when connected to a 12 volt battery.
I came across it after an extended period of storage.
I tried it out and immediately the filament started emitting light brown fumes.
Over the years enough air had leaked past the seals to oxidize the filament.
I am sure that with your years in the industry, you have seen a few things happen that normally never happen.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[TugboatEng]

The marine industry still uses ground detector lamps. They do burn out. It really confuses the operators. I have to tell them that if you see anything other than two brights and one dark you have a burned out bulb. In some instances I have replaced the bulbs with LEDs. They're VERY sensitive.

 

[dithomas]

Tugboateng
The ground lights at the brewery had a bulb test circuit that was a good feature. They tries LED bulbs at one time an they did not work, all three lights stayed on even with a ground, so went back to incandescent. The bub filament was an important part of how the system worked. The bulbs would burn out over time but not too many or very often. There were 22 unit substations with ground lights there.

 

[crshears]

I recall seeing ground detector bulbs always paired; highly unlikely that both would blow simultaneously. Same principle was applied to the old school synchroscope "sanity check" bulbs.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wcaseyharman]

We still install the double sync lights in our generating stations, but they are doubled up in series because the maximum voltage is 230V during synchronizing but the bulbs are only rated for 120V.

We still have a couple of high leg delta systems we maintain within the power plant side of the company I work for. They are both 120/240v high leg systems with center tapped delta transformers. The company also provides that service to customers but using an open wye/delta setup. I was trying to work out the voltage drop on one last week, very odd when you try to analyze it.

 

[TugboatEng]

Our fault light systems use light with built in transformers (120v to 6v) which is likely why they still work with LED bulbs.

However! If your ground your neutral this will also cause the indicating lights to not indicate a fault.

 

[waross]

Back to the original question about a 480 delta with a 240 Volt tap.
This may have been done to provide a ground point for the system.
It's a beach to get old.
When I was young, I would have realized this almost immediately.
Now, at slightly older than a couple of presidents, the realization takes a few days.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

Simply providing a ground point doesn't explain the different size transformers. Using the larger unit to power single phase loads (in addition to the three phase loads) does.

 

[waross]

Respectfully, Steven. The designer was under the same misconception as are you.
All three transformers of a delta bank contribute to single phase loads.
With equal transformers, the same current will flow in each transformer.
For a unity PF load the in-phase transformer will contribute 50% of the kW and equal KVA.
The out of phase transformers will each contribute 25% of the kW but will each be at 50% PF, and will each contribute the same KVA as the in phase transformer.
That notwithstanding, I have seen three phase delta banks with mismatched transformers.
Consider a three phase delta bank with three equal transformers.
Now remove one transformer, leaving an open delta.
If you calculate the voltage drop under load of the virtual transformer formed by the open delta, you will find that it is equivalent to the transformer that was removed.
Consider the open delta as a virtual transformer in parallel with the in-phase transformer.
Not so for open delta banks. A larger transformer for the single phase loads is common with open delta banks.
Depending on the relative impedances, the smaller transformer in a mismatched delta bank may limit the capacity of the bank.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

Okay, but why? Sounds lossy to run two transformers at 50% pf. The alternative is to upsize the center tapped/gnded "lighter" and downsize the other two. If they should all have the same percentage impedance, the actual impedance of the lighter will be less so that it will preferentially serve the single phase load (not exclusively as you point out). In my experience, when built from three phase overhead transformers, the high leg delta is never built with similar size cans. The lighter is at least one size higher.

 

[waross]

Good point, Steven.

My experience and observations of three phase delta banks was coloured by the local culture.
Corruption was common, and it was common for distributors to pay a cash kickback to anyone ordering transformers.
Transformers tended to be grossly oversized.
I remember one that I became involved with;
Three 50 KVA transformers for a capacity of 150 KVA to support a 17 KVA three phase load and a smaller single phase load.
Power outages were frequent and switching was with fused cutouts.
Almost all three phase banks had one fuse blown and were operating open delta.
The capacity of a bank drops to 58% on open delta, but with transformers oversized by over 200%, open delta operation was not a problem.
And, most of the high leg installations were for predominantly three phase loads with lessor single phase loads.
With large single phase loads and small three phase loads, such as a large single phase distribution circuit with a small added three phase load, the open delta is the preferred connection.
There I expect to see a small transformer (5 KVA, 10 KVA 15 KVA, the smallest that the utility stocks) paired with a 50 KVA or a 100 KVA transformer.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!