Switchgear with 2 Mains and no Tie

[rockman7892]

I'm looking at a project where the customer is requesting a new MV Switchgear lineup that will have two mains fed from two alternative upstream sources. These two mains will feed onto a common bus and will be interlocked so that only one can be operational at a time.

I'm used to seeing lineups that have a tie between the two mains in order to isolate the one side of the gear if needed. In absence of the tie breaker you still have backup/redundant sources through the two mains but you need to take down the entire lineup if there is maintenance or an issue that requires de-energization of the bus.

I'm curious if anyone had typically seen this arrangement and what some of the pros/cons are?

Thanks

 

[oldfieldguy]

I've seen it several times.

The interaction between the two feeders varies.

One had a keyed interlock system. One main had to be opened and racked out to free the key for the other breaker to rack in and close. Obviously this is very slow, but the client did not see a need for something faster.

Another couple had a manual operating handle to transfer. On one, the operation of the handle would trip the closed breaker and close the other. Wasn't bumpless, but was faster than the manual, keyed system.

The third the manual operating handle. Operation would close the open breaker and upon its close, the opposite breaker would trip. There's a few cycles of both incomers feeding the bus, but this was considered a small risk.

old field guy

 

[bacon4life]

We have some stations with main-tie-main and some stations with just two mains breakers and a solid bus between them. Even though tie breaker seems like it should theoretically add reliability, I have not seen a big practical benefit since the lineup may still be totally de-energized for reasons such as:
-work on the bus tie cubicle
-loss of both incoming sources
-leaky roof dripping on multiple cubicles
-relay misoperation
-while doing major upgrades at the substation, choice to de-energize entire substation for a short, safe duration. The option to keep half the substation energized which would require more complicated safety procedures making the upgrades take much more time, while also exposing workers to electrocution and arc flash risk.
-testing of complicated relaying scheme
-single battery system feeding the entire switchgear

 

[mparenteau]

We have that configuration(s) at the site I work. Two mains - no tie. Ahead of each service lateral (and main for that matter) is a network protector.

pros: service reliability
cons: arc flash considerations

Mike

 

[ppedUK]

The Main-Tie-Main configuration only really has its advantages if you follow that philosophy all the way down the system (i.e. you have 2 supplies to each downstream board, fed from each side of the main board). Where you have a single supply to a plant or piece of equipment with the Main-Tie-Main set-up, you have the problem of which side to feed it from, and then what happens when you want to take down a section for maintenance? You could get around such issues by having a double bus switchboard where you can select which bus to feed each outgoing supply from.

@ bacon4life....you can get around the problem of not being able to work on the Bus Tie by having two of them side by side.

 

[xnuke]

Some customers have a tie breaker with equal rating to the main breakers so that if they have to, they can replace a failed main breaker with it and continue operating both sides.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

I saw a system once for starting two motors together at 13 kV that may work and may be a cost saving.
The system used Magna-Blast draw-out bbreakers.
Two cubicles were cross connected on the load side so that one cubicle ran the motors forward and the other cubicle ran the motors in reverse.
One draw-out breaker was used.
A hole was cut in the barrier between the cubicles.
A short nipple was welded into the hole.
A 3/4" pipe was captive in the sleeve.
When a breaker was installed in a cubicle, the pipe was slid into the other cubicle and blocked the insertion of a breaker.
To reverse, the breaker was racked out of the cubicle.
The pipe was slid across into the forward cubicle.
The breaker was installed in the reverse cubicle.
This interlock scheme could be used with incoming feeds if the customer can accept the time required to rack and move a breaker.

Bill
--------------------
"Why not the best?"
Jimmy Carter