Bus layout with delta wye transformers

[stevenal]

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?

 

[gordonl]

I encountered this problem in the recent past. We were installing a tie line between two busses fed independantly from the utility. They were both delta-wye but were facing in opposite directions in the substation. (One east and other west) To get the two busses in phase we had to change the feeder tap locations on one of the transformers to get the sources in phase. (And change the differential connections)

 

[busbar]

Here, too. About a half-dozen years ago phasing of a radial transmission feed into two distribution substations became a problem when it was decided to build a secondary tie line between the two stations. One was first energized in 1981; the other in 1985.

‘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.

 

[redtrumpet]

Our local utility (SaskPower) designates phases with colors Red, Yellow, and Blue. Phase rotation is positive R-Y-B (ABC).

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.

 

[gordonl]

busbar's comments brought back to memmory that we had to swap secondary connections as well. In our case we had secondary bus duct which we replaced with cable in tray. We've had many bus duct failures and replace them with cable quite often, so we didn't view this cost as a total loss.

 

[resqcapt19]

What about the requirements in the NEC in sections 384-3(f) and 430-97(b) for switchboards and motor control centers that require the phase rotation to be A,B,C front to back, left to right or top to bottom?
Don(resqcapt19)

 

[stevenal]

NEC excludes "Installations, including associated lighting, under the exclusive control of electric utilities for the purpose of communications, metering, generation, control, transformation, transmission, or distribution of electric energy." Nothing about it in the NESC.
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.

 

[NormGA]

When facing the transformer low side, connecting high side phases in the sequence ABC, BCA, or CAB will all produce the same low side STANDARD phasor with counterclockwise rotation. The phases just come out on different bushings. However, by connecting the transformer in any other sequence such as CBA, BAC, or ACB will produce the same voltage and the same phase rotation, but the Y will be inverted from the STANDARD ABC sequence. We call this a NON-STANDARD connection. No amount of rolling of the low side will correct this so that it can be tied with a transformer with a STANDARD connection. If you were trying to tie a NON-STANDARD with a STANDARD connected transformer you can only drop load from one and pick it up on the other. All three phase loads will then rotate in the proper direction. The voltage inversion can only be corrected by rolling the high side.

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.