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

 

[waross]

Further to your post, Steven, I will be spending the next few days pondering the division of current and possible circulating currents when a balanced three phase load is fed by unequal transformers.
Comments and suggestions welcome.
I am not contradicting you, but rather comparing different experiences.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

Good luck, I have been working on this for our distribution folks, it’s been pretty interesting. The fact that the current in the transformers is significantly out of phase with the applied voltage in the winding is a bit mind bending for me (transformer winding is Vab, but current in the winding is Ia for example)

 

[stevenal]

waross,

I look forward to your calculations.

It's been a challenge attempting to get rid of wye-delta banks here. 480 V isn't so bad, but existing 240 V customers don't wish to change to 208. Under "the customer is right" theory, we've even installed new delta services. Would you believe a single pole in an underground area just so we could do an ungrounded wye-delta? Primary riser on one side, secondary riser on the other. Primary neutrals are floating here, so blowing a fuse won't result in an open delta. Primary neutrals are temporarily grounded with a ground cable for single phase switching, then the ground is lifted.

 

[waross]

Steven;
Initial thoughts, open to comments:
Assumptions:
Unity PF, equal %imp of transformers, equal and balanced supply to the transformer bank in question and negligible voltage drop in the primary neutral supply conductor.
Note on open delta; For two identical transformers in open delta, the virtual transformer across the open side of the delta will have the same %imp and regulation as either of the real transformers.

Consider a 100 KVA transformer paired with two 50 KVA transformers.
The single phase load is 75 KVA.
The 100 KVA transformer supplies 50 KVA, 50%, and the pair of 50 KVA transformers supply 25 KVA, 50%.
So each transformer is loaded to 50% and so the voltage drop across each transformer will equal.

Conclusion, the rule that parallel transformers will share the in proportion to their capacities will hold true for a single phase load on a three phase bank within the previous assumptions.

I am still thinking about the case with three phase loads and more particularly, mixed single and three phase loads.

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

 

[bacon4life]

@Stevenal Pole mounted equipment can be so much more flexible that padmounted equipment, so I definitely believe your story. Somewhat similarly, we have local improvements districts that pay for undergrounding the utility owned power lines. However, the service wire to each customer is owned by the individual customer. For any customer that doesn't want to pay $$$$$ for undergrounding the service, the pole gets topped at the secondary level where the OH service is attached, and a new conduit runs up the pole from a padmounted transformer.

I am cautious about using LEDs as voltage magnitude indicators for two reasons:

1) LEDs can glow at very low voltages. When I attempted to use an LED as hot line indicator for a 115 kV substation bus, and I found the was noticeably illuminated just from capacitively coupled voltage on the 115 kV bus. I also have a LED lightbulb at home that glows noticeablely due to the tiny amount of power drawn by the smartplug controlling the lamp.
2)Identically rated LED products can have very different voltage vs lumens curves. Perhaps this can be mitigated by only using LEDS from the same production lot.​

 

[crshears]

Steve n Al [ my stupid smart phone won't let me spell your handle correctly ], I can understand not wanting to go from 240VAC to 208VAC; almost every station I worked at used 120/208 inside the buildings but then supplied standard 120/240 ranges in the control-room-adjacent kitchens; if you had to work the holidays roasting and cooking were often significantly extended affairs . . .

CR

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