use of delta-wye distribution transformer in step up mode 4

[akm]

Would like to get some operational tips, if I need to connect a delta-wye-n(480/208) transformer in step up mode. connect to supply source of three phase line(208) from utility source and run a cyclic load of compressors at 480 volts fro the delta 480 v side.

Do I need to isolate the netral on 208 side floating, if so why? and is it a permanent feature I can have without inviting any additional problem.

Do I need to provide additional protection device on LV/HV side of the transformer.
Any additional precautions during installation.
Thanks

 

[jbartos]

Suggestions to the original posting marked ///\\\:
Would like to get some operational tips, if I need to connect a delta-wye-n(480/208) transformer in step up mode. connect to supply source of three phase line(208) from utility source and run a cyclic load of compressors at 480 volts fro the delta 480 v side.
Do I need to isolate the netral on 208 side floating,
///It depends on the system grounding upstream. If there is grounded (high-resistance, medium-resistance or solidly grounded neutral) star, it is not recommended to ground the 208V wye neutral since the zero sequence path would be created. This would cause the zero sequence current circulation with all adverse effects involved.\\ if so why?
///It is essentially the zero sequence current path that is opened (substantially) by the isolation of the neutral on 208V side.\\ and is it a permanent feature I can have without inviting any additional problem.
///Yes, it is the permanent feature per different industry standards, e.g. IEEE.\\Do I need to provide additional protection device on LV/HV side of the transformer.
///If your cyclic compressors are dedicated branch circuits, all what is needed is the three phase protection and equipment grounding/bonding of conductive surfaces. The ground fault detector is also useful. The 480V delta with a grounded corner could get some consideration.\\\
Any additional precautions during installation.
///Yes. The LRAs of the motors should not be excessive in correlation to the transformer impedance and cable impedance (causing voltage drop).\\Thanks
///The low voltage on the 480V output side (27V) could be resolved by routine troubleshooting as suggested by other postings above.\\\

 

[stevenal]

Correction to my post from yesterday. If neutral is floated, loss of a phase on the wye side will cause one of the delta side line to line voltages to collapse completely, leaving the other two slightly reduced and in phase. Sounds a little like what you are seeing. The 27 volts could be coming from inductive and capacitive coupling. Put load on it, and that 27 V may drop to zero. But tying down the neutral should restore the full line to line voltage (with reduced capacity). Maybe both a phase and neutral are open feeding the transformer, or within the transformer.

 

[peterb]

akm -
I don't see where you have given the 208/120 V system input voltages measured at the transformer LV side.
As noted above, it would be instructive to see what are the measured values for phase-phase AND phase-neutral voltage taken at the transformer (whether or not the neutral is connected or left floating)
The change in performance could very well be due to an upstream connection developing a high joint resistance, or to a faulty connection within the transformer.

 

[akm]

To my surprise and all your excellent analytical suggestion; The results are here:
At grounded neutral: upstrem phase to phase voltages, 212,212,213, delta side opencircuit voltage 476,467,481
At open/floating netral: phase to phase voltages, 192,208, 18, neutral shift(voltage between float neutral and ground) 188. I am much puzzled, but sure that the wye side has a problem(not understood). I need to contact utility for permission to unlock the raceway terminus box, so that a physical inspection of the cable in raceway and terminations can be made. Many thanks for all your support. Much thankful to all of you so far for your prompt advise, and no words can express my sincere thanks to this forum members. Got to go back .

 

[stevenal]

You have an open supply conductor,fuse, or bad connection. Tying the neutral down allows transformer to work in open wye/delta mode, which energizes the third winding from the delta side, backfeeding to the adjacent wye connected winding. Removing the neutral allows the system to single phase, and the third voltage goes away.

 

[akm]

stevenal: I admire your brief conclusion.

 

[busbar]

akm, were you able to diagnose and solve your transformer-voltage and motor-starting problems?

 

[akm]

Since Saturday last, we have taken a shut down and the 9 years old cable installation upstream is being physically examined. So far, we detected the following. Primary winding(208 side) connection brazing is pitted, insulation burnt to brittle pieces, metallic connection rigidity is almost loosen. I doubt, if this has happen due to reversal of use(i.e. use of step down trafo in step up mode). we will go slow on repair of the damage done, but apparently, the root cause was a possible loosen/burnt connection in the primary side. I will keep to all updated in this forum about any further lesson leant in our operation, but for all your co-operation.