Main Tie Main Closed Transition Re-Transfer
Main Tie Main Closed Transition Re-Transfer
(OP)
Hi Everyone,
We have a main-tie-main configuration for our system. When reverting back to normal service we want to use a closed transition re-transfer so that we don't have stop all the motors in the facility and cause a process upset. Both A and B sources on the M-T-M are fed from a utility source. When carrying out the short circuit study, I noticed that the fault levels are quite substantial when paralleling up the two transformers (which makes sense). It is around 78kA and the gear is only rated for 65kA. Since this re-transfer would only take a couple of cycles, is it recommended to beef up all the switchgear and mcc ratings to Sym. 85kA? The parallel operation will never be used in a Continuous Operation. Please let me know your comments.
Thanks in advance.
We have a main-tie-main configuration for our system. When reverting back to normal service we want to use a closed transition re-transfer so that we don't have stop all the motors in the facility and cause a process upset. Both A and B sources on the M-T-M are fed from a utility source. When carrying out the short circuit study, I noticed that the fault levels are quite substantial when paralleling up the two transformers (which makes sense). It is around 78kA and the gear is only rated for 65kA. Since this re-transfer would only take a couple of cycles, is it recommended to beef up all the switchgear and mcc ratings to Sym. 85kA? The parallel operation will never be used in a Continuous Operation. Please let me know your comments.
Thanks in advance.






RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
I am aware of organisations which do allow short term paralleling in excess of the capability of the equipment, but under British legislation there is no provision for breaking the rules for a short period and the regulations require that equipment shall be rated for the duty. I'm not going to try to save money and by doing so potentially put myself and my employer in a very awkward position in a courtroom. Perhaps more importantly I don't ever want to have to explain to the family of a dead workmate why I decided to cut corners and put cost ahead of people. Bad enough if an accident ever did happen, but if it ever did happen I'd like to know that it happened in spite of me doing everything I could to make things safe. I'm not old enough to have been to the funerals of three workmates - it changes your perspective on taking risks at work.
RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
Somewhere in the difficulty range between a nameplate change through replacement of breakers to virtually rebuilding the busbar structure. Depends on the design.
RE: Main Tie Main Closed Transition Re-Transfer
It was a stack of transformer laminations about 5 inches square and about 10 inches high with a hole in the center. One secondary conductor passed through the hole and a few percent impedance was added.
Another possibility is to consider the impedance of the feeders. Perhaps the feeders have enough impedance to bring the actual fault level at the switchgear below the rating of the gear.
Maybe you have already done this.
Bill
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"Why not the best?"
Jimmy Carter
RE: Main Tie Main Closed Transition Re-Transfer
Please let me know your thoughts. Thank you once again for taking the time to respond. Much appreciated.
RE: Main Tie Main Closed Transition Re-Transfer
I remember seeing an installation where the primary feeder was used to increase the impedance. Calculations showed that 100 feet of primary feeder cable would add enough impedance to bring the fault current below the rating of the switchgear. The spec was that no feeder shall be LESS than 100 feet.
Depending on the layout, cable impedance may be used on the primary, the secondary or both.
I have also seen an air core reactor installed to limit the fault current. If you consider an air core reactor you may want to consider the stray magnetic field. An air core reactor installed close to a concrete wall may cause unacceptable heating of the rebar under fault conditions.
I would ask the transformer vendor for advice on availability,price and shielding.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Main Tie Main Closed Transition Re-Transfer
Some designs are as simple as a timer that trips the selected breaker x cycles after all three breakers are closed. More sophisticated designs use breaker failure relaying plus bus differential and fiber optic arc sensing to trip the Mains if a fault occurs during the switching. Or the controls are designed to allow all breaker operations from a remote location so no personnel need to be near the switchgear during the transition.
Another design is to use a bus transfer relay like the one made by Beckwith. It can do an open transfer (Trip Tie, Close Main or Trip Main, Close Tie) without dropping loads if there are enough large motors on the bus. It can also do the other schemes mentioned above plus breaker failure, plus delay manually initiated breaker operation long enough for the operator to leave the room.
Bottom line, is closed transition against Codes? Yes. Do many industrials do it and provide other protections to minimize risk? Yes. Your call.
Adding impedance to reduce fault levels does work, but it may affect your motor starting voltage drops.
RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
RE: Main Tie Main Closed Transition Re-Transfer
Bill
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"Why not the best?"
Jimmy Carter