Is there a strategy for setting up a MAIN relay on high voltage switchgear ?

[bdn2004]

See the attached sketch. We’ve a 12.47kV feeder coming into a switchgear from the Utility. The Utility’s feeder has an ampacity of about 2500 Amps. It feeds into a single 12.47kV double ended switchgear through a main breaker. There are three more switchgears exactly alike this one that can tie into this switchgear through tie breakers.

The 12.47 kV switchgear is rated for 2000 Amps. We’ve discovered that all of the LT pickup settings of the main breakers are set at 1200 Amps, for all 4 switchgears – well below the bus rating, and less than one half the rating of the incoming feeders.

This was done by a very experienced company many years ago and it’s not nuisance tripped ever. Is there an underlying strategy for setting this main breaker up like that ? What would be any detrimental effects of bumping it up to 2000A to match the bus rating?

 

[marks1080]

Do you have fault values and load profiles?

 

[bdn2004]

The symmetrical amps at the main is about 15,290 Amps, X/R is 2.95.
And no I don't have the actual load data at this time.

This question is prompted because they are adding two more line-ups and a new utility feed in order to shed load from these 4 substations.
And looking at the existing the question arises - why is it done like that? and should we do it the same way?

 

[davidbeach]

That may have been a point that provided sufficient loadability while adequately coordinating with the upstream devices. You may be able to change it with little consequence or you may not, but without a broader picture it would be impossible to know.

 

[Sn00ze]

Unable to tell solely based on equipment rating. Need to know system loads.

 

[bdn2004]

the maximum load is 1300 amps

 

[RRaghunath]

The 2500A rating of CB and 2000A rating of bus are all IEC (or IEEE) ratings, which mean ratings at rated IEC ambient of 40degC or IEEE ambient. A derating factor is applicable over these ratings based on client's specified design ambient temperature.
Further, after arriving at the derated value, the closest higher rating is chosen based on IEC (or IEEE) standard ratings.
Some times the higher ratings are deliberately selected to meet the anticipated future needs. But, the protection settings are based on actual load and the need for ensuring coordination with upstream utility settings.

Rompicherla Raghunath

 

[marks1080]

Don't constrain yourself with existing settings. That happens way too much in our industry.

If you have the fault data and load profiles don't be afraid to adjust your settings when your system changes. I hate it when someone says "well that's the way it's always been" as opposed to doing proper engineering. Your system is pretty simple in terms of applying protections. You have fault data, load data and you have equipment ratings.

The simple answer to your fundamental question is: No, the folks who did this work before you came along didn't have special knowledge. They chose their set points with a certain amount of 'artistic interpretation.' If you ask ten different protection engineers to give you settings for your system you very well might get ten different settings and all are still correct.

 

[bdn2004]

From stuff I've seen on the internet...one of the main breaker's functions is to provide backup protection for the feeder breakers.
To insure the main won't trip there should be at least a 6 cycle delay between the main and any feeder breaker.
This is the kind of answer I was looking for - but referenced.

We've also been told, but haven't confirmed, that the Utility sets their trip based on the ampacity of the conductors it supplies to the Plant. They aren't particularly worried about coordination with your system.