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Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

(OP)
This is a theoretical question.

I'm considering a three-phase transformer bank, described as follows:

HV Primary - Ungrounded Wye Configuration

LV Secondary - Center-tapped, grounded Delta, ab, bc & ac=240V, aN & bN=120V, cN=208V.

A diagram is included as an attachment, or can be found here: http://files.engineering.com/getfile.aspx?folder=1d732e6b-0c88-48df-8416-e58905135bea&file=Y-D_Transformer.jpg]

What is the effect on the secondary voltages if a phase is lost on the primary?

The clearest information I've been able to find regarding this ungrounded primary set-up states that a lost phase on the primary results in reduced voltages on two phase-to-phase readings and zero volts on the third p-p.  I've come across a lot of specific information on losing a phase on a grounded wye primary and the open-delta effect it creates, but no specific calculations for the ungrounded case.  If anyone has insight, experience and/or calculations for this scenario, I'd be grateful for your help.

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

If you lose one of the primary phases, the remaining phases are energized by a single phase-to-phase voltage, giving 1/2 of the ph-ph voltage on the 2 corresponding secondary windings.  The output of the other secondary winding corresponding to the open primary phase will be 0 V.

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

Normal voltage on the primaries will be equal to phase to neutral voltage. If one phase is lost the other two transformers go in series, line to line. With balanced loads, the voltage on each of the two secondaries will be 1.73/2 of normal or 86.6% of normal. For unbalanced loads, the secondary voltages will be in inverse proportion to the loads. The sum of the voltages will be 173% of the normal voltage. This is similar to a single phase circuit with a floating neutral except that the sum of the voltages is 173% of the normal rather than 200% as in a floating neutral.
For your 240 volt secondaries, a missing phase will result in the two energized phases sharing 416 Volts not 480 Volts.

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

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

(OP)
Thank you both for the helpful information.  I appreciate it.

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

A further point that may be usefull if you are faced with trouble shooting;
The simple formula says that the voltages across the two transformers will be in inverse proportion to the impedances of the loads and that as one load becomes less in relation to the other the voltage will approach 173% of normal. .
In real life, as the voltage rises on the lightly loaded the transformer may saturate magnetically. As the transformer saturates, the impedance drops and limits further voltage rise.

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

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

(OP)
A follow-up question: does anyone know what might be a good method or program to simulate this scenario?

My power system design software (Windmil) won't do it properly.  I've been playing around with some SPICE software trying to simulate it that way, but the analysis doesn't want to run; it might be that the program doesn't agree with my paths to ground, or it might be that it doesn't like looped inductors, or a host of other reasons.  Any thoughts on this would be appreciated.  Should I be modeling the transformers using inductors rather than the built-in transformer models?

Are any other available power system programs (ETAP, DAPPER, etc.) sophisticated enough in their transformer functions to model this scenario?

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

SAKufel, I don't believe ETAP or DAPPER will give you the output that you are looking for. IMOH the best program for this type of simulation is ATP/EMTP. It does have a learning curve but it is free (though you do have to sign an agreement with them) See http://www.emtp.org/.

RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank

(OP)
Thank you.  I'm sending in the licensing agreement now.  I appreciate the tip.

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