How to increase switchgear's short circuit capacity? 1

[jlazucena]

I have a Siemens switchgear with the nominal values of 15kV, 3000A and 750 MVA short circuit capacity at old STD C37.06-1987.
There is cell available with only 500 MVA short circuit capacity (K factor 1.3)with the same nominal voltages and currents.
Problem: The expected 3phase sc current is 25660 Amps at 13.8kV (600MVAsc)
Question: What do I have to do to increase the short circuit capacity of this cell?
By adding supports to the buses?
By increasing the insulation of the buses?

Cloud you please help my?

Thank you

 

[dpc]

If you are talking about increasing the short circuit interrupting rating of a circuit breaker, then there is not much you can do. The breaker is an engineered item, built and tested by manufacturer to a certain rating. There is nothing an end-user can do to increase this.

You could install current-limiting reactor ahead of the switchgear to reduce the fault current.

 

[jlazucena]

Thank you dcp, my problem is not the breaker.
I need to increase the sc capacity of the buses of the new cell. This cell is to add another breaker, (the existing cells are 750MVA and all the breakers are 750MVA).
I cannot use reactors because this new breaker is to energize a static var compensator for motor starting.

 

[RonShap]

Current limiting fuses will not reduce short circuit current availability. They might help with a series rating for AIC or withstand.

 

[swgrmfg]

JL:

I would not attempt this at any cost if I were you! The liability is tremendous. But, if you insist, you must first check with Siemens to confirm, in writing, that the cell you have can handle the 750MVA AT THE STABS (the primary disconnects of the drawout mechanism). From there on, supporting the bus is easy. Just make sure than the bus is supported every 18".

Good luck,

GH

 

[DanDel]

jlazucena, the cell(switchboard section) is a factory manufactured item, also. Its ratings can only be approved by the manufacturer. Any modifications you make will only serve to cancel the factory ratings for your liability.
Do not add any components to an existing swithgear that are not equally rated.

 

[jbartos]

Suggestion: The adequate MVAsc of the needed switchgear cubicle should be greater than the calculated 600MVAsc. Normally, the switchgear bus would have all switchgear rated at 750MVAsc. If there is the upstream power distribution that will accept current limiting reactors, than the short circuit current level and MVAsc could be reduced below 500MVAsc from 750MVAsc. Also, an upstream transformer impedance increase can reduce the MVAsc and current fault level, if this is in the engineering or design phase.

 

[peebee]

I agree with the general trend of the above posts.

The only way to increase the cell from 500 to 750 would be to replace it with a 750, or to have the original manufacturer make & certify modifications, which would likely be either expensive or impossible. If you do this yourself, you are taking on the liability. You'd better check with your insurance company and see what they think -- although I already know what their answer will be.

Current limiting fuses will limit the current as indicated by slowstart. That's their whole reason for being, that's why they're called current limiting.

You might be able to get CL fuses installed in the 500MVAA breaker, it seems to me that the tap rule would allow this. I could be mistaken. The manufacturer could easily answer this for you over the phone.

Any upstream impedance would also help reduce the available fault current to 500MVA, as previously indicated. Typical sources of impedance include transformers, cables, and reactors.

 

[jlazucena]

I have great respect for the time that you have volunteered to answer my questions, and if I asked for advice (And your time) is because I want to make sure that I am considering all the variables to increase the rating of this cell (You may read my original thread). In my opinion, this Forum is to share concerns and experiences so we can provide good advice to our clients or employers. In some Bylaws you may find under “professional scope of practice” that the professional can undertake only the tasks that his/her knowledge, skills and abilities will allow him/her to successfully complete them. That does not mean that the professional must know everything before undertaking the project, but the final product must be completed with acceptable engineering quality (The final user must be advise about the limitations of the product if any).

My client has a dead line to meet and the nice 750 MVA cell may not be on time from the factory, and I wanted to have a back-up plan. This cell is a temporary solution only. I said that I want to increase the SCC from 500MVA to 600MVA that was to give an idea of the problem.

By adding supports or increasing the size of the cell-to-breaker connectors I am increasing the strength of the cell.

About liability, the value of 600MVA leaves some margin as follow:
Maximum 3phase short circuit calculated (Under abnormal conditions) is 520MVA.
Maximum 3phase short circuit calculated (Under normal conditions) is 480MVA.
Maximum 2phase-to-ground short circuit calculated (Under abnormal conditions) is 450MVA.
Maximum 2phase-to-ground short circuit calculated (Under normal conditions) is 410MVA.
The contribution from Utility is less than 55% of the total short circuit capacity of the system.
3phase have the lowest probability (Most of them are from leaving grounds on after maintenance), and it is acceptable to put more attention to 2phase-to-ground faults.

 

[hbendillo]

There are many things you can do to limit the available fault current at a switchboard. As mentioned in other posts the best thing to do would be to install some kind of current limiting device ahead of the gear. Why go to the trouble of trying to modify the bussing of the gear to brace for a higher short-circuit? Chances are you would have to change out all of the circuit breakers if they were not properly rated.

 

[RonShap]

Not to mix discussions, but .....
My statement that current limiting fuses do not limit fault current was not typed in the way that my brain planned it.
When applying withstand interrupting capacity ratings, it is important not to apply the let through curves of a current limiting device upstream, and assume that the remaining current is all that is left to deal with regarding the rating of a downstream piece of equipment.
The consideration of dynamic impedance has been documented to the extent, that you must utilize a combination of OCPD devices or OCPD device and equipment that has been tested, called series rating. Without the series rating on a pair of items, you don't know the extent of limitation by the current limiting device. So in essence, the current limiting device is not reliably limiting current, unless it is previously tested with that downstream equipment in question.