IMO, I don't see a risk in opening an air disconnect when de-energized, in some case even energized. Its done all the time automatically in all voltage ring buss applications to restore the original ring structure, circuit switches are often used to remove a failed, but none bolted fault transformer from service. There is an arc interrupting full load but it breaks.
At smaller substations like a 34 to 13.8kv its done all the time. When a supply line fails to reclose (lock out), motorized air breaks open to isolate the normal feed line from the buss, and normally open air breaks from the alternate line close to pick the substation back up on line. In this scenario there is risk for older none communicating (SCADA) situations since the line may have tripped on a buss fault within the station itself. However one solution has been either SCADA with fault logic or operating a split buss design where all energized reclosing is done via a center breaker that can automatically open in case restoring a buss fault.
There is one disadvantage to air breaks which also creates a risk. During an ice storm air breaks can be iced over, so opening them can do harm as well as trying to close one.
But other than that I am not aware of any risk. Unless your company knows something?
FWIW, There is this operating procedure that might make some of you go
In feed through station applications, where no breaker is present at a transmission to distribution substation, if the lines fault on either side of the transmission to distribution substation breakers will open at both supply substations. At the T-to-D substation both circuit switchers open automatically. A reclosing sequence begins on both lines. When the first of either two lines successfully auto reclose, the circuit switcher on that line will be re-closed to pick the T-to-D station load back up. If the second line successfully recloses the second circuit switcher closes to restore the loop, if no successful reclose takes place the second circuit switcher remains open.
Im not sure how common or safe this method is, but it is routinely done.
Yep, we call it a sectionalizing station, have a bunch of them on our system. Plus a lot more of the transfer stations that will auto transfer from a tripped source to the available source. Once upon a time that was all done with MODs. We've been replacing the MODs with circuit switchers but still have quite a few MODs remaining.
Ok good to know that. The comment "At some point there has to be a device used as an isolation point. That's where the danger is." raised the question.
One danger of remote operation of disconnect switches is false indication of switch position due to mechanism failure. If the auxiliary switch indicates the wrong position to SCADA and status is not verified visually, damage can occur.
One generating station destroyed a turbine generator when the ring bus breakers were opened to take the generator off line and the GSU transformer’s disconnect was opened to isolate the generator prior to reclosing the ring. The switch was stuck closed due to a broken mechanism but the control board indicated an open switch. The operator did not verify switch status due to nasty weather. When the first ring breaker closed, the turbine generator was torn apart by the out of synch inadvertent energization.
In most T&D situations, having a stuck switch with incorrect indication will not cause that much damage. But it is something to consider, especially around generation.
I have been thinking about this extensively and so far one uncertainty has sparked debate. In breaker and a half, where should supply transformers be terminated? My understanding has been on the busses themselves, however I came across this online design manual that recommends terminating supply transformers in the individual bays:
"The reason 345-kv autotransfomers may not be terminated on the main busses is because of the design problem
imposed by breaker failure contingencies for breaker-and-a-half stations containing 3 or more autotransformers.
With 2 autotransformers at a station (1 on each main bus), installation of the 3rd is problematic because wherever
the 3rd autotransformer is terminated, a breaker failure contingency could result in a tripping of 2 autotransformers.
Terminating the autotransformer in individual bays eliminates this potential operating problem."
We never connect the transformers to the bus in breaker-and-a-half, nor in double bus-double breaker. One of the major drivers for this type of configuration is to allow breaker maintenance without removing lines or transformers from service. Single breaker to the bus forces the transformer out for breaker maintenance. Not sure what possible benefits (other than one less breaker) there could be to a single breaker connection; though I'm sure someone will proceed to enlighten me.
I wish my utility did not regularly connect the low side of our 500/230 and 230/115 autotranformers directly to the busses [with only an intervening disconnect switch] instead of into breaker-and-a-half bays; unloading a tranformer means opening one end breaker in each bay plus any bus tie breakers where so equipped, opening the transformer low-side [motorized] disconnect, then returning the bus to service.
This configuration becomes especially problematic during contingency situations where there are pre-existing outages already in effect in some of the bays involved...
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
I would say both of you are correct in seeing that way.
For reasons I cant always explain there are many substations where transformers are connected directly to the busses. Its tempting when the buss ends up directly in front of the transformer, especially when the buss spacing is near equal to transformer spacing. The only advantage is cost (that I know of); using only one secondary breaker or none at all.
If loosing 1 transformer does not have a significant impact I could understand the cost being worth it, but if not, I will admit that can defeat the whole breaker redundancy plan to some degree or another.
Here is another foot note from the guide:
"The elements in the 345-kV grid tend to be extremely critical to the transmission grid reliability, thus the added
reliability of a breaker-and-a-half scheme will be considered good utility practice if 4 or more transmission
connections are forecasted."
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