MCC & Fuse

[satnam123]

Hi everybody,

I need a few words of advise from you experts:

Q1, I have a 1050 KVA transformer on 4160 V primary feeding a motor. I am not sure what is on secondary side in terms of fuse curves. I have to put a primary fuse and there is a switchgear feeding this transformer located at 60 m. So what I have is a 200E SM5 S&C fuse on Primary of transformer and again a 200E fuse same type located in switchgear. As the Ifl=145 A and the fuse is 200 Amp. I need the fast acting one. What my question is when we siz fuse in a situation like this where we have a cable and a transformer , Is it the right way to protect cable and transformer.


Please advise.

 

[rbulsara]

Primary side fuse is there only to protect the transformer (and in medium voltage systems its only for short circuit protection, not overload). Secondary side equipment are protected by devices on the secondary side.

Hopefully, the transformer is properly sized for its loads.

Refer to NEC Articel 450. IEEE "Buff" book for Protection and Coordinaiton. Also for free stuff refer to S&C website (sandc.com) and find the data bulletin on transformer protection.

 

[rbulsara]

Once you learn the theory behind fuse sizing, you can use short cuts such as free fuse selection program available at Ferraz Shawmut website. Choose your means wisely.

 

[RajT]

I am assuming that the transformer is only connected (feeding) the motor and also that the transformer is adequately sized (in kVA) to successfully start the motor.

If that is the case then the current arrangement (ie 200amp fuse) on the primary is fine as the fuse is only providing sc protection irrespective where the fault is.
The motor will be (I am assuming) protected by its own motor protection relay in the switchgear.
Since the motor kVA rating is less then the transf kVA, the fuse at 200 amp should grade adequately with the motor o/L.
You don't say what the motor ratg (kW) and voltage is as that would be more helpful.

 

[waross]

Is this a dual element? I doubt that a standard 200 amp fuse on a 145 amp transformer primary will take the inrush. If this is a dual element fuse there should be no problem. (I did try googleing to find out what type the fuse was, but the pages were loading so slow that I ran out of time. Sorry.)
respectfully

 

[rbulsara]

waross:

Medium fuse amp ratings are not the same as LV fuses. For exmaple, a 200E rated fuse will take 300 sec to open when the current exceeds 400A (yes, twice the E rating).

For a check, one really need to compare a fuse TCC curve and check that its inrush current point is below the curve (10-12 times FLA for 0.1 sec would be a typical inrush point). A fuse with E rating numerically equal to 100% to 125% of FLA would generally suffice for a transfromer protection.

 

[satnam123]

Thanks everybody,
So how do we size a fuse. If you go to this link
here

http://www.sandc.com/support/tccs/TCCPDFS/153_4.pdf

a 200E fuse starts at around 800 Amp. While our FL is 145 Amp. DOes this still work.
I beleive what I am missing is when a transformer starts it takes 10 to 12 times its IFL but it is for milli seconds. But what about overload, will it be suffice to say that means that fuse cannot take care of over load.
I just need a thumb for low voltage and med. voltage application. Thanks

 

[rbulsara]

satnam:

yes..read previous posts..

 

[waross]

Thanks rbulsara
Are there fuses equivalent to dual element fuses available for medium voltages?
I was in a casual conversation with a utility engineer a few months ago and had the impression that there were fuses available that could give protection similar to low voltage dual element fuses. We were talking about transformer failure from overloading on 34,500V circuits. I may have misunderstood something.
respectfully

 

[rbulsara]

waross:

As far as I know, not really, they definitely are not dual element fuses.
MV fuses are not meant for overload protection. If you really want to use it as such, like many utility co. do for smaller transformers (that does not make it right though), they would put a 100E fuse, if they really want it to open at 200A or more. Using it this way would result in miscoordination with other devices in most cases.

It really comes down to comparing your TCC, inrush and thermal damage curves and coordination with other devices in series.

In MV system overload is usually proected by design and for larger critical transformer, winding temperature monitoring is relied on for overload (thermal) protection.