Unbalanced Voltages of Open-Wye Open-Delta Banks

[boyaxski]

sirs,

i've encountered an open-wye open-delta connected transformer bank which gave unbalanced voltages. thinking that its the primary that's causing the problem, i energized the bank without load and got line-to-line readings of ab=232, bc=232 and ca=220volts (ca being the output across the two xfs), equivalent to a 3.5% unbalance which of course is already detrimental to 3-phase motors. i concluded that the primary voltages magnitude are ok but are somehow not exactly displaced by 120 degrees. this theory, however, was disproved when i checked a nearby wye grded-wye grded bank that only showed a very small unbalance. they say that unbalanced secondary voltages is inherent in an open-wye open-delta bank. what could have caused this problem? i really would appreciate if you can refer me to a site that has a detailed technical explanation for this. thanks so much....

boyax
meralco, philippines

 

[rempman]

The bank you describe is usually provided for small commercial facilities that typically have single phase load in addition to a limited amount of three phase load.

If you look at the bank I would guess that one transformer is quiet a bit larger than the other. Something like a 50 kVA (likely connected 120/240)and a 15 kVA (likely connected as 120V - paralleled secondary windings). Your unbalance is likely due to the mismatching of transformer winding impedance. In order to understand this you need to work out a per unit circuit diagram and I would bet you can see the problem.

 

[prc]

Can you give the rating and voltage ratio of the transformers involved?What is meant by open star-open delta means?Does it means neutral unearthed star and ungrounded delta 3phase system? If so this is due to a phenomenon called neutral inversion which is caused by the oscillation of line capacitance with line inductance at certain critical values of C & L-some sort of a ferroresonance.Is the unbalance voltage same or different with time ?Most probably it will be varying.
You will find technicall explanation of this some AIEE papers published in 1940's.I have noticed it in 11 KV lines ,but cannot vouch for 230 V lines.

 

[davidbeach]

By open-wye, open-delta he means a three phase bank built of only two transformers. Take a wye delta bank of three transformers and remove one.

 

[rbulsara]

Is the wye side grounded?

 

[boyaxski]

thanks for the replies sirs..

here are my answers to your queries..

1. yes, it is a bank of two transformers with sizes of 100KVA and 25KVA. both are rated 19.92KV-240/120V (our system is multi-grounded wye 34.5KVY/19.92KV). the 100KVA serves the other 1-phase residential loads. by the way, the 100KVA and 25KVA xfs were connected to phases B and C respectively of the primary source. the primary connection is open-wye with the common point connected to ground/neutral (basically a wye grded-delta connection with one xf taken out).

2. i dont think impedance is the main culprit since when the bank was energized unloaded, both the 100KVA and 25KVA xfs gave identical 232V output. the phase CA (=VAB+VBC), however, only read 220V.

3. i have recently corrected though the problem by transferring the pimary phase feed from B and C to A and B, but still in the dark on what caused the unbalance?

 

[jghrist]

Have you checked to see that both transformers are on the same tap?