vacuum circuit breaker - breaking current

[panthripu]

hi
Once again I need some information from experts here about the circuit breaker. A vacuum circuit breker with current rating 2500A is feeding a big transformer. I am trying to understand the difference ,if the breaker is switched off at almost 2200A and in other case switching off breaker after unloading the transformer untill the primary current reaches 500A.
The voltage on primary side is 33kV.
There is no doubt that switching off the breaker at 500A will increase life of the breaker. Any other comments please...benefits & drawbacks.

 

[zog]

You may want to research VCB pre-strike effects on transformers if you are doing this often enough to be concerned about the effect on the breaker. That would be a bigger concern IMO. I really don't think the load will have much of an effect on the life span of the breaker as long as it is within design ratings, too many other factors to consider. Your best bet if that is your concern is to conduct Magnetron Atmosphere Condition (MAC) testing on your VCB's to predict remaining life.

 

[templdl]

I've sold the first (W) VCP which went into a steel mill in East Chicago and a number of VCP-W as well as numerous AMGARD that had vacuum contactors in them.
I have also sold numerous 480 vacuum contactors from 140a to 540a.
I have never ran into is a question that you raised. This may be a long post but I wanted to share what I know about the vacuum bottle technology. I still keep a couple of NEMA size 4 140a vacuum bottle because I find them to be most fascinating.
I really can't find a concern that you have brought up. Where in the world would you be turning a transformer on and off at such a frequency that you would be concerned about the contacts of the vacuum bottle?
One of the roughest applications that I ever had is when using a 480v reversing vacuum contactors on an overhead crane which the guys plugged it as it traveled down the plant almost as if they were constantly ringing a door bell. The phenomena that we discovered that that the contactors were so darned fast that one bottle was getting its butt kicked out of it. We concluded that by having the control voltage taken off of one phase that the contactor was dropping up at the exact time in the cycle so a single bottle was always taking the hit and wearing it out. I forget how we addressed the problem as that was back in the mid 1990s. Otherwise the vacuum technology was quite reliable.
I never have had an issue with the Ampgard as we often provided them to control portal crane transformers.
I never had an issue with the medium voltage switchgear either that used either the VCP of VCP-W vacuum breakers.

One thing that is a bit unusual with vacuum bottle technology is when you hold a component vacuum bottle in your hand that natural state of the contacts is closed because the vacuum pulls the contact closed. Then the mechanism of the breaker pulls the contacts open. Because the contacts break the arc in a vacuum they don't have to separate nearly as much as contacts that have to breaking an arc in open air. Being that the contacts can not be visibly checked you check contact wear buy measuring the change on the gap by checking a gap between the bottle and the mechanism that holds the contact open after the mechanism has allowed the contacts to close. That gap decreases as the contacts wear.
As such contact condition is measures by checking the gap. Also, another check of the health of a vacuum bottle is a high pot check.

 

[zog]

I've sold the first (W) VCP which went into a steel mill in East Chicago

Cool, I have worked on those breakers in that plant. Serial#0000001 or something else?

Also, another check of the health of a vacuum bottle is a high pot check.

That is just a go-no go test, the VI could fail the next day, we now do MAC testing on all VCB's and are able to predict remaining life of the VI's, very valuable condition assesment tool for plant engineers.

 

[templdl]

Cool, I have worked on those breakers in that plant. Serial#0000001 or something else?

That is just a go-no go test, the VI could fail the next day, we now do MAC testing on all VCB's and are able to predict remaining life of the VI's, very valuable condition assesment tool for plant engineers.

Wow!! That's almost unbelievable. Did you work with a (W) service engineer in that area? I think his name was Dante.

If that was the same job I got the (2) double ended 2500kva power centers, about 11 MCCs which I personally did all the wiring diagrams for, numerous transformers with LI switches, a PFC capcitor bank which I drew the footing requirements for because my plant in Bloonington couldn't get them drawn fast eneough so I did them my self.
That was a job worth just a tad less then $1m back in about 1992 or so.
As I recall my customer specified pilot wire relays for the VCP gear which we built and shipped it as and then the steel mill really wanted differential relays. That cost my customer abiut $10k to have it retrofitted and I also had to have all of the drawings redone.
I even personally customized the drawing for the entire job with customer specific drawing numbers.
That was heck of a fun job.

 

[zog]

Don't recall the contact, just doing the usual NETA type testing, used to do a lot of work there. I think one of the members here recently got a job there. Small world. I always like looking at the test stickers on the breakers that come into our shop to see if I know the guy that last tested them.