Unbalanced Voltages of Open-Wye Open-Delta Banks
Unbalanced Voltages of Open-Wye Open-Delta Banks
(OP)
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
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






RE: Unbalanced Voltages of Open-Wye Open-Delta Banks
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.
RE: Unbalanced Voltages of Open-Wye Open-Delta Banks
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.
RE: Unbalanced Voltages of Open-Wye Open-Delta Banks
RE: Unbalanced Voltages of Open-Wye Open-Delta Banks
RE: Unbalanced Voltages of Open-Wye Open-Delta Banks
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?
RE: Unbalanced Voltages of Open-Wye Open-Delta Banks