Vacuum CBs and surge arresters

[REEngineer]

While reading a specification of work for a plant, I came upon the following sentence in design criteria section for Medium Voltage(6 & 11KV):

"Surge arresters will be used when circuit breakers(or contactors) are of vacuum type".

I may add that in this plant MV loads are mostly induction motors driving compressors.

It seems SF6 breakers don't need such provision. I guess it must be because of transient overvoltages due to switching and SF6 CBs seem to be able to endure such overvoltages without need for surge arresters.

But I'd like to know more about the background theory. Are there any standards/recommended practices about this matter? Does this rule apply to ALL brands/models of vacuum type MV CBs?

 

[zog]

I think the concern for overvoltages with vacuum breakers is X-Ray production. We are limited to our test voltages for VCB's intergrity tests due to X-Ray production.

 

[Jraef]

"Chop currents" in vacuum interrupters are a well known phenomenon, but this is a somewhat old spec. The problem was solved some time ago by adding chromium to the contact material. But there may be some manufacturers out there who don't do that yet (or ever), especially now that we are seeing cheap vacuum contactors coming in from China. The specifier was probably doing a little CYA.

Here is a good article on the subject.
http://www.panickker.net/article6.htm

 

[REEngineer]

"Chop currents" in vacuum interrupters are a well known phenomenon, but this is a somewhat old spec. The problem was solved some time ago by adding chromium to the contact material. But there may be some manufacturers out there who don't do that yet (or ever), especially now that we are seeing cheap vacuum contactors coming in from China. The specifier was probably doing a little CYA.

Here is a good article on the subject.
http://www.panickker.net/article6.htm

Thanks. The article was useful, but still I prefer articles or recommendations from sources like IEC or IEEE or other widely known technical committees.

I still have problem understanding the underlying theory of this phenomenon. Articles/information/suggestions on this matter is also highly appreciated.

 

[REEngineer]

BTW is there a need for a special overvoltage analysis in design pahse of the plant or can we rely on the new technology of VCBs to save us from overvoltages?
If such analysis is required, how should it be conducted? Is any special software involved? or should we just use general formula L*dI/dt? How one can find dI/dt in such case?

P.S: Sorry for a lot of questioning. I want it to be as clear as a spotless blue sky when I discuss the matter with someone. No 'Probably's or 'Maybe's