Bus layout with delta wye transformers
Bus layout with delta wye transformers
(OP)
After a recent phasing snafu when interfacing with another utility, I learned that there are at least two philosophies. Our friends build their bus ABC north to south and east to west, and configure their reconfigurable transformers to fit the system. Our method is to build the bus to fit our ANSI standard transformers, and then make sure we hit the station correctly. Just thought I'd throw it out there for discussion. What do you do, why, and have you had this sort of communication problem?






RE: Bus layout with delta wye transformers
RE: Bus layout with delta wye transformers
‘ACB’ was the serving utility’s standard; while the customer had extensively documented ‘ABC’ for its transmission feeds, medium- and low-voltage lines and buses. This somehow had been overlooked during construction of the ‘ACB-phased’ station.
During the design stage of the tie, phasing was discussed and dismissed as unimportant, for it was inconceivable that they could be different, and besides, any difference could be easily fixed by flopping cables at the MV level. {We realized before tie-line completion that flopping cables would be a lot of work; the ‘ACB-phased’ station had bus duct on the transformer secondary and multiple 750MCMs per phase in the ‘ABC’ yard.} After a number of line patrols and lots of phasor sketches, we found that for the two ANSI-standard delta-wye power transformers, no flopping or rolling of low-side connections could fix a high-side phase reversal. In the end, it became necessary to build a model using six 100VA machine-tool transformers to demonstrate the misconception of configuring the two 20MVA transformers.
Before tie-line heat up, the serving utility had to come on site, take an outage and swap two 4/0 ACSR spans into the ‘ACB’ station. Of course, it then became necessary to flop phases at cable terminations on that station's eight MV feeder breakers. It was a costly lesson. Phasing across the open tie breaker was triple-checked before closing the first time.
RE: Bus layout with delta wye transformers
In general, the connection of the primary supply line to the substation will be such that Red phase is to the H1 transformer bushing, Yellow phase to the H2 bushing, and Blue phase to the H3 bushing.
We recently rebuilt a sub with three existing transformers and discovered one transformer had secondary cables swapped to correct primary phase rotation with respect to the other two transformers. The existing bus was such that phase rotation on the primary was not maintained within the substation(!!!???) We designed new rigid main and cross busses to provide uniform phase rotation within the sub, (no swapping of secondary cables required) and a new incoming structure. It was relatively straightforward to obtain correct phase connections as the utility line (72 kV) had phase conductors in the vertical plane on the poles, rather than horizontal. Therefore, transitioning to our incoming structure, with conductors in the horizontal plane, was very easy. We did verify the phase rotation within our sub and with the utility line very early in the design.
All I can say is, take nothing for granted and verify with the utility before getting too far into the design.
RE: Bus layout with delta wye transformers
RE: Bus layout with delta wye transformers
Don(resqcapt19)
RE: Bus layout with delta wye transformers
It is what we try to do, anyway. To an observer facing the control side (low side) of the transformer, phases should connect ABC left to right (H1 to H3). Unfortunately, this same observer is looking to the west, and the real phasing is ABC north to south. A short flat span connects the two stations with no place to role it. The solution we're implementing is to add a third level of bus to make the turn. Energization was delayed by about two weeks.
RE: Bus layout with delta wye transformers
Decades ago my company bought several small utilities, each with its own unique method of phasing. Most of these stations were 46/12 kv and they were usually isolated so that tying with another station was not a concern. Most are now gone, but some are still around. Some of the NON-STANDARD connections can be a real challenge to the field engineer trying to connect a mobile substation and make it tie.