use of delta-wye distribution transformer in step up mode
use of delta-wye distribution transformer in step up mode
(OP)
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
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






RE: use of delta-wye distribution transformer in step up mode
Some earlier discussion can be found at Thread238-11717 and Thread238-9287
99NEC 450-3(b) dictates secondary overcurrent protection based on transformer size.
RE: use of delta-wye distribution transformer in step up mode
I repeat my specific doubt>> Do I hv top float the neutral in the supply 208 volt side wye. If yes, for waht specific purpose and Do I invite any additional problem by doing so.
Another corollary to above, If I have a control trafo on the load circuit, which will ultimately make the drawn load current a little unbalanve, does it create any problem.
Shall aprreciate a path forward.
Thanks
RE: use of delta-wye distribution transformer in step up mode
Julian
RE: use of delta-wye distribution transformer in step up mode
There is no effect on the low voltage side, and the unbalanced load shouldn't be a problem as long as it isn't large enough to cause unbalanced voltage. (It will obviously have to be a phase to phase load since your 480V system has no neutral.
Gord
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
How far apart are the calculated/actual starting characteristics? Are the UV trips with one unit starting, one running/one starting, or both units starting? Over what time interval is the ‘400 volts’ measured? Has the 27 device dropout/time been verified accurate? Are there warrantee/reduced-life issues if the 27 settings are changed? Are the 208V-bus sags unacceptable?
RE: use of delta-wye distribution transformer in step up mode
2) The measured undervoltage is immediate upon strating the unit.
3) On supply side, the voltage dip is down to 195volt.
Not to mix up various issues, I faced a situation as follows, which could not give me any answer.
Based on transformer supplier advise and some published article, I kept the step up transformer(208wye/480delta),225 kva Transformer in f;loating neutral mode and measured the open circuit voltage on HV DELTA.two of the phase to phase voltages are 470~480 volt, while third one is 27volt.(twtenty seven only)I am not able to get any clue out of it. My questiuon is does that answers/confirms that we have a fault in the supply 208 side. If yes, we have other building load in the supply circuit connected and remains unaffected, or there is a hidden message here. Request for sincere advise and help.
Thanks
RE: use of delta-wye distribution transformer in step up mode
I'm puzzled with 27 volts phase to phase, with others still 480V. I would suggest closely checking transformer connections on both sides, including winding Wye and Delta connections.
Did the transformer run 5 months open neutral, or is that a change you recently made?
As to supply problems you'll quickly know with suggested measurements.
Gord
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
Gord, I will request you to share your views on this, while others are most welcome to provide a valuable solution.
RE: use of delta-wye distribution transformer in step up mode
This may have little bearing on the problem at hand, but it’s hard to know if, in the past, the overcurrent was marginally close to the trip value and has recently increased only a small degree, or if it was always ‘normal’ and something changed in the system to cause a much-larger increase in the starting current. Does it seem like one of these conditions may be more prevalent than the other? The severely imbalanced voltages reported would tend to point to the second condition. [With motors, something like a 3½% voltage imbalance can equate to a 25% current imbalance.]
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
X0-system neutral measurement, possibly high shift could not be measured today out of panic results, will do that tomorrow morning under floating neutral condition and let you know.
I have ruled out measurement at starter load terminal and motor terminal box in view of non-satisfactory open circuit voltage at transformer 480 terminal switch fuse box (2 feet away from the 480v end).
I have no reason, but now compelled to doubt the supply end cable connection raceway (this is a 6 feet cable raceway, spliced from our existing 208 volt building supply and connected to the 208 side of the transformer through a switch fuse box.
I performed all the measurements at both side of transformer opening the Switch fuse unit box.
I am convinced about your doubt(2nd one), something changed in the system, remained unnoticed till this frequent tripping.But would like to exactly smoke out the fault with all your advise, and go for a corrective action plan to run the unit. The detection will probably become easy, once I measure the neutral shift voltage magnitude and inform you.
RE: use of delta-wye distribution transformer in step up mode
- phase-to-phase and phase to ground measurements on hi and low-side
- X0-to-ground measurements.
Beyond that I have nothing practical to add. At some point down the road you might consider a transformer test (winding resistance, ratio). Also at some point down the road might want to check for resistance to ground on hi-side and low-side buses while deenergized. But more likely there is some problem on low-side... corner grounded? not a true three-phase system? . I'd do those voltage measurements before chasing the transformer etc
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
I’m sure readers will be interested in your findings and resolution.
RE: use of delta-wye distribution transformer in step up mode
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.\\\
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
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.
RE: use of delta-wye distribution transformer in step up mode
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 .
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode
RE: use of delta-wye distribution transformer in step up mode