Voltage Phase Angle Differences between two incoming sources 1

[rockman7892]

In industrial plants which are fed with two incoming utility sources (double ended main switchgear)is it a likely occurrence that the two incoming sources are usually out of phase with one another for one reason or another? I don't have much experience with utility distribution systems so I'm not sure if these two sources are usually derived from two different utility distribution transformers or are tapped at different points in the system thus causing a phase angle difference at the incoming service to the plant. I know different loading can cause this difference as well.

I know this is certainly possible due to some of the reasons I mentioned but I am curious how often this occurs? This would obviously prevent paralleling of the mains at the incoming switchgear so I was also curious how that is typically addressed?

 

[davidbeach]

Our system is designed and operated such that, if properly connected in the first place, two sources intended as alternates for each other will be in phase. Distribution Phase A at any point on the system is within a few degrees of Phase A at some other point near enough to be considered for alternate service. Two feeders from extreme ends of the system might not phase well because of transmission phase angle differences, but they are also too far apart to be of concern.

 

[wroggent]

In our system we cannot tie together busses from different transformers in the sub due to excessive fault current. For primary networks we bring the multiple feeders from the same bus. In some cases another set of feeders from another bus with a transfer switch at the customer's site. For secondary networks we use feeders from different busses in the sub however we have to take great care to ensure the feeders and the busses have similar loading otherwise differences in angles can get out of hand and we can easily overload a transformer on circulating current alone.

 

[jghrist]

Normally, if two sources can be connected by a tie breaker, they will be in phase and the same voltage. It still may not be permissible to tie them together if they are served by separate transformers of different size, taps or impedance. Unbalanced loading of the transformers or circulating currents could occur. Connection from two parts of the utility transmission system may also result in undesirable flow of utility system load through the tie breaker. You need to check with the utility to determine if the tie can be operated closed. It may be permissible to tie them momentarily to achieve a closed transition from one source to the other.

 

[Mbrooke]

The answer is: It varies and often requires careful analysis. Any two redundant feeders may be from the same transformer, same substation different transformer or different substations all together. In the latter you may or may not have different transformer vector groups (ie, sub 1 delta wye, sub 2, wye wye). In the first you can still be effected by voltage regulators out on the line, uneven loading or line lengths (though a certain amount is tolerable) contingencies which can split the supply substation, recloser loops or manual transfer, increased fault current, and more complicated feeder protection for the utility.


If you do tie the feeder together, I highly recommend relays that to sink check/phase angle before closing the tie and setting which split the tie for excessive power flows.


But lies the question: why normally closed operation desired?

 

[bacon4life]

There are cases where physically adjacent distribution feeders can be fed from different transmission lines. When transmission elements are out of service, there could be large enough phase angles to drive significant current through a medium voltage connection. The utility should be able to calculate the maximum phase difference for likely transmission configurations.

 

[waross]

In extreme cases where one feeder is heavily loaded and the other feeder is lightly loaded, you may find your tie breaker passing current from the lightly loaded feeder to the heavily loaded feeder to help support the load.


Bill
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"Why not the best?"
Jimmy Carter