A three-phase transformer bank is connected wye–wye. By Disconnecting one of the three transformers, can the two remaining transformers be connected open-delta?

In my opinion if the system is ungrounded then yes. An open delta is just two windings in series, so, say phase A is removed then phase B will be in series with phase C and line of phase A will be connected to the neutral point directly making it open delta transformer.


So, I made a hand sketch for a wye connection: enter image description here

After removing T1:

enter image description here

It looks an open delta for me.

To compare it this a closed delta:

enter image description here

After removing T3:

enter image description here

  • 2
    \$\begingroup\$ Draw a picture is my opinion. \$\endgroup\$
    – Andy aka
    Commented Jul 15, 2021 at 10:43
  • \$\begingroup\$ @Andy aka I think they are the same. So, open delta can be achieved if it's an ungrounded Y-Y transformer. \$\endgroup\$
    – OMAR
    Commented Jul 15, 2021 at 12:58
  • \$\begingroup\$ When you remove a component it becomes an open circuit. \$\endgroup\$
    – Andy aka
    Commented Jul 15, 2021 at 13:08
  • \$\begingroup\$ @Andy aka Agreed. But if I connect the line of the removed component to the neutral (ungrounded), then, it becomes open delta right? \$\endgroup\$
    – OMAR
    Commented Jul 15, 2021 at 13:12
  • \$\begingroup\$ There's no reason to limit it to ungrounded systems. Corner-grounded open delta systems exist all over the place. \$\endgroup\$
    – Theodore
    Commented Jul 15, 2021 at 14:01

1 Answer 1


The short answer is (as you suspect) yes.

...as long as you connect the primaries and secondaries the same way.

If it's really a bank of three single-phase transformers, taking one out of the circuit and reconnecting as you described will give you an open delta. The power rating will of course be less, as there's one less winding. There are some other considerations if it's a three-phase transformer with all windings on a common core.

There's no need to restrict it to an ungrounded system. The voltage and phase relationships are still maintained.

On the primary side of course you don't want to ground one phase if the source isn't connected that way.

On the secondary side, I have seen some industrial installations with a 3-phase, 3-wire service, supplied from a pair of transformers in open delta with the corner grounded. Also common is to have one of the transformers larger, with a center tap. It supplies 120/240V single-phase loads. The smaller transformer gives the third phase for motor loads.

See if you can discern what's going on in this Google street view image and how a center-tap-grounded open delta system is derived from two phases of a grounded wye:

Typical overhead open-delta.  Google image


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