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.e ngineering .com/getfi le.aspx?fo lder=1d732 e6b-0c88-4 8df-8416-e 58905135be a&file =Y-D_Trans former.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.
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
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
RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank
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
RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank
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
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
RE: Effect of Lost Phase on Ungrounded Wye/Center-Tapped Delta Bank