Single Breaker, Triple Bus Scheme 2

[Mbrooke]

What would be the advantage of this system over say breaker-and-a-half? This is supposed to be for a new major substation build outside of North America, but I can't see why someone would choose this over a more conventional approach.

 

[davidbeach]

Somebody owns stock in a motor operated disconnect manufacturer? Based simply on a two bay snippet, I can't see any benefit.

 

[Mbrooke]

LOL, I would say so! Here is whats being used as the disconnects, vertical reach pentograpghs:

http://coelme-egic.com/en/prod/23/Pantograph_(VR2D)/

They come single phase, so as a bonus you get 9 motor operators per bay. Station will have 14 bays.

 

[davidbeach]

Full employment for some group of people to keep all of those working. 126 of them plus any on the lines. Seems more complex than necessary.

 

[Mbrooke]

Complex... you should see the low impedance bus differential protection. Believe it or not the 487B manual makes mention of it and can be set to my knowledge. The older B90 manuals address it specifically (see pic) The only plus is that each line and transformer can be treated as straight bus since its only 1 breaker per element.

 

[Mbrooke]

Pic, edit didn't catch it:

 

[davidbeach]

Just because you can, doesn't mean you should...

I see no point, but somebody obviously does.

 

[Mbrooke]

Yup. I'd like to think I'm the one missing something, at least that would be my instinct.

Already a potential down side being noted that if the bus relay malfunctions it has the potential to clear the entire station. This has happened before:

http://www.nerc.com/pa/rrm/ea/Lesso...202_Bus_Differential_Power_Supply_Failure.pdf

 

[bacon4life]

Since you ask only about advantages:
[ul]
[li]An option to run as 3 separate buses to reduce fault levels.[/li]
[li]For a bus 1 fault during bus 2 maintenance, several lines would stay connected together on bus 3. With a breaker and a half scheme, only pairs of terminals stay connected.[/li]
[li]Ability to use line relays with only 1 CT input without paralleling CTs. Not sure if the SEL411L is ahead or behind other relay products. [/li]
[li]Lower tower heights as the incoming terminals do not have to cross over 2 levels of bus.[/li]
[/ul]

 

[Mbrooke]

Ok, that makes much more sense now that I see it.


However, I am willing to challenge this one:

For a bus 1 fault during bus 2 maintenance, several lines would stay connected together on bus 3. With a breaker and a half scheme, only pairs of terminals stay connected.

1. While theoretically true, in most bulk systems (345kv and over) line pairs tend to be far more stable then loosing 2/3 of all circuits. Should both buss bars be lost in breaker and a half, the center breakers will keep power flowing east to west, north to south or visa-versa. At least this is the design topology implemented around here- lines from generation one on side, line to load centers on the other with auto transformers and cap banks on each bus bar.

2. At first glance bus bar maintenance appears more difficult. With the center bus for example you have to stay clear of both end busses close by on each side, and even when dealing with the end buss you have live bays running close below them. Failure or maintenance of a photograph requires taking a bay out of service. Where as compared to BAAH, bus maintenance is a breeze in terms of clearance since one side has no parts of any kind, the other isolated via horizontal disconnected, and live parts kept at a good distance (the feeder cross overs are very high up on the stations I've seen).

3. With centralized bus bar protection common to all 3 busses clearing the entire station is possible if one of the differential relays malfunction. Clearing 2 or more busses is also a possibility since zone discrepancies can occur if any of the pentograph switches malfunction, and generally when bus isolator position switches give contradictory readings the relay is set to simply combine any of the two bus zones in question so a fault on one bus will take out the other over loose connection or broken micro switch.


4. During switching of bus bars a short circuit on a pentograph can also clear two bus bars since zones are summed (combined) during the transfer switching process for obvious reasons.

Just my 2 cents in observation. Some might prove me wrong however.

 

[Marmite]

Space is another benefit of the triple bus. The bay length is about half of the equivalent breaker and a half bay length.

Regards
Marmite

 

[davidbeach]

But you can't have two positions share a single bay.