Bil Rating on MCC why different values for different components?

[majesus]

BIL ratings for medium voltage MCC are typically ~60kV. However, I noticed that the CPTs installed in these MCCs are typically rated at ~25kV. See Rockwell Literature for example:

http://samplecode.rockwellautomation.com/idc/groups/literature/documents/sr/1500-sr020_-en-p.pdf

Isn't this counterintuitive? Common sense says that the BIL rating for all the components in the switchgear must be the same value. I found Siemens’ techsheet on the subject:

http://www.energy.siemens.com/us/po...topics-application-notes/techtopics47rev0.pdf

I trying to understand why it is done like that. Seems a waste of money/resource to build an MCC rated at 60kV BIL then install 25kV BIL rated CPTs.

 

[magoo2]

I would rephrase your sentence to say that common sense is that the minimum BIL requirement for a given voltage class should be the same. It doesn't mean that the BILs of all equipment must be the same.

There is a case with PTs where you can develop ferroresonance if you stick with a line-to-neutral rating. Many users specify line-to-line rated PTs so that you keep the PT in the linear range of operation rather than allowing the PT to saturate and aggravate or increase the likelihood of the overvoltage condition.

 

[majesus]

You can have different BIL ratings for the same voltage class . IE, one 5kV MCC doesn't require the same BIL rating as another 5kV MCC. It's application specific.
That's not the question, I am asking when you do rate MCC for a specific BIL rating... Then common sense say that all the equipment (CPT, PT, CT, contactors, whatever) within that MCC switchgear should be rated for the same BIL. But as I just pointed out above, it's not that simple. This is highlighted in the Siemens' Techtopic, where the MCC can be rated for 60kV BIL, but its CPT can be rated at 30kV (for example).
The Techtopic states that the downstream motors have poor insulation, and " CPTs, autotransformers, and reactors in MVCs are allowed to have the BIL capability allowed by their respective standards."
As I mentioned, it seems a waste of money/resource to build an MCC rated at 60kV BIL, then install 25kV BIL rated CPTs.

 

[LionelHutz]

I didn't read the links. In North America, magnetics are allowed to be disconnected during testing. Likely, because at one time they could not reach the proper levels similar to how motors still do not have high BIL ratings. Today, you can easily find CPT's and CT's that will withstand 60kV BIL.

I'm guessing the theory or arguement behind this is that the switchgear can survive a lightning strike without catastopic failure. It was likely considered an acceptable loss if the CPT fails and the CPT fuses clear.

We tested our soft-starter enclosures for 60kV BIL as built without disconnecting things.

 

[jraef]

BIL ratings on gear are tested, not calculated. So you can use individual components that may have lower ratings or even no rating, but if the entire assembly passes, it passes.

Conversely if it fails, it fails, regardless of what the labels on individual components say. I have seen it go that way more often than the other way.


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