Coupler in Double Bus Single Breaker 3

[Power0020]

The need for bus coupler in double bus single breaker scheme seems a bit unclear for me.

why should a coupler be closed if all circuits can be switched to any of the two buses? would that be to add more flexibility in case of section fault?

 

[Mbrooke]

1. A buss coupler is needed if you want to switch between bus bars while the feeder/bay breaker is closed and carrying load. The reason being that with the two busses electrically isolated there might be significant voltage potential between them which would cause serious arcing when the bus disconnects are opened/closed. The bus coupler effectively "short circuits" both busses together greatly reducing the voltage potential between them. In cases where you have buss CTs for step distance protection wired for redundancy (voltage balance relay that toggles each relay to any healthy CT set) it is essential the voltage be the same relative to both bars.

2. Further, if either bus faulted, the bus coupler would open interrupting the short circuit current without taking out the other healthy bus bar. Only half the circuits would be lost. Where the station is non dynamic (one bus is simply a reserve/transfer/backup/service/emergency/auxiliary buss and the other a main bus protected via single buss differential zone) the coupler protects the auxiliary buss and in turn trips when there is a fault on it, thus indicating a problem on the aux bus that would otherwise go unnoticed if the aux bus was run normally de-energized. Remember, you want your backup equipment operational when you need it.

3. Also, if every station in the system is single breaker, double bus; having a coupler "nits" what would essentially be two separate electrical systems perhaps only loosely linked via transmission lines. This would create phase angle differences between every bus bar in the system which makes steady state / load flow operation far more difficult; unequal loading on generators and transformers; and if you had paralleled transformer secondaries with each transformer primary on a different bus there could be "loop flows" where power flows into one transformer and out the other. These phase angle difference are greatly exacerbated with transmission lines or other elements going out of service.

4. A bus coupler can be used to manually remove a known stuck breaker from service while under load. All the other circuits are transferred to one bus bar (I'll pick buss A) while the effected breaker is left on buss B. The bus copuler is then opened dropping the circuit or element with the stuck breaker. The disco leading to the stuck breaker is opened. The bus coupler is then closed and circuits that should be on bus B are put back in that buss.



5. In some cases the coupler is deliberately operated normally open to control load flow or reduce short circuit currents but closed (desired) when system conditions change. An example would be a 115kv / 34.5 kv substation with two 40/50/60 MVA transformers feeding a single breaker double bus secondary. TX1 is normally connected to bar A, and 6 34.5kv outgoing circuits also normally connected to bar A. TX2 is connected to bar B, and another 6 34.5kv circuits connected to bar B. TX1 and TX2 are not run parallel for two reasons in this example: 1. Both have on bard tap changers which are not coordinated with one another and thus differing relative tap potions will result in circulating currents. 2. Both units in parallel will exceed desired short circuit currents, possibly over burdening some equipment. Thus the coupler is normally open. Now, if TX1 or TX2 unexpectedly fail (high and low side TX breakers trip respectively), voltage transformers will sense the loss of voltage on the de-enrgzed bus and after a predetermined delay (say 15 seconds) the bus coupler will close restoring the effected bus and in turn remaining circuits via the other remaining TX.

This is far quicker, easier and cheaper then the logic required without the coupler for circuit restoration. Without a coupler a TX failure would require opening all the breakers on the effected 34.5kv circuits, then opening all the motorized disconnects to the denergized bus, closing the mods into the energized bus, and then closing the 34.5kv breakers there after. Far more that can go wrong...

In addition to the above returning the station to normal would also be a challenge in that the last set of disconnects to open bridging bus A to B may have significant voltage and current between them for a number of reasons (tap position, TX R/X differences, loading variants on each bus). Remember, disconnects are not designed to interrupt high voltage high current conditions, and such the resulting arc will destroy them at best- at worst leading to a flash over arc on the disco that clears both bus bars. Thus the coupler should always be the last object that opens (splits) the two bars after the final electrical bridge (via disconnect) is broken. Ditto for closing discos. The coupler must be the first object to close before a bay discos closes into either bar while the other disco is also closed. The ONLY granted exception to this rule is when opening or closing into a bus bar with absolutely nothing else on it. Generally speaking the disco make/break the small charging current of the bus bar without issue.

 

[Mbrooke]

And oh, to add. Where a breaker bypass exists the coupler breaker is designed to substitute the protection of the bypassed breaker.