Unbalance Current when no Neutral Provided

[faresos]

Hello Everyone,

In a normal situation where we have a utility transformer (MV-Pri & 480Y/277V-Sec) and few step down transformers from (480 Delta- 208Y/120V). The unbalance load at 208Y/120V level will cause the unbalanced current to flow through the neutral back to the source (Star at transformer sec). My questions

1. Does the delta winding (primary side) of the 480-208Y/120V transformer sees unbalanced load similar to the secondary side? if so, how is this unbalanced load is handled at the primary side since there is no neutral?
2. If most the loads from the main service are transformers, 3-phase motors (balanced loads) and few lighting panels (277V circuits), does that mean the main service neutral conductor can be reduced to lets say half the size since not much 277V circuits used? My concern is just the unbalanced load coming from the step down transformers but if the unbalanced current is handled at the transformer secondary (208Y/120v level) then I will think there is not unbalanced current seen at the upstream utility transformer.

Any thoughts?

Thanks,

 

[texie]

1) The unbalance on the load side of the 208Y is seen on the primary as line to line unbalance. The neutral is not involved on the primary side of the 208 transformers.

2) The neutral size for the service must be based on the calculated neutral load but not less than the value in 250.102. The minimum size allowed is to make sure it is large enough to carry line to neutral faults or in the case of service conductors it is large enough to carry line to ground faults.

 

[Carultch]

The delta side handles the imbalance with phase shifts, and the imbalance generally decreases when a delta primary supplies the imbalanced load on the wye secondary. The delta side ends up needing to handle this with phase shifts, in order to get three non-equal magnitude current vectors to add up to zero. I can't upload a spreadsheet, but here is the output of it, and several key formulas.

Wye side currents, assuming unity power factor
Ia	100​	A
Ib	70​	A
Ic	80​	A
Max percent imbalance =max(abs(Ia-Ib), abs(Ia - Ic), abs(Ib-Ic))/max(Ia, Ib, Ic)	30%​
	Phase-to-Neutral	Phase-to-Phase
Wye side voltage (V)	Vsn = 120​	Vsp = 207.846​
Delta side voltage (V)	Vpn = 277.1281292​	Vpp = 480​
Assign the following transformer couplings:
A on WYE : AB on Delta
B on WYE : BC on Delta
C on WYE : CA on Delta
Delta side phase-to-phase currents inside the windings
Iab = Ia*Vsn/Vpp	25​	A
Ibc = Ib*Vsn/Vpp	17.5​	A
Ica = Ic*Vsn/Vpp	20​	A
Line currents on delta side
Iap = sqrt(Iab^2 + Ica^2 + Iab*Ica)	39.05124838​	A
Ibp = sqrt(Iab^2 + Ibc^2 + Iab*Ibc)	36.99662147​	A
Icp = sqrt(Ica^2 + Ibc^2+ Ibc*Ica)	32.5​	A
Max percent imbalance =max(abs(Iap-Ibp), abs(Iap - Icp), abs(Ibp-Icp))/max(Iap, Ibp, Icp)	17%​