Breaker and Bus bar ratings

[power2020]

Dear Folks,
Pl. advice me for breaker and Switchgear ratings.
We have 9-AC generators are connected in parallel without grid supply. All neutrals are solidly grounded without neutral breaking during parallel operation. Fault levels are calculated by ETAP software and these values are as follows.
ANSI calculations (Device duty- 3ph):
Fault currents on Main Bus:
Interrupting Duty: 156.506kA symmetrical rms
Momentary Duty:245kA Assymmetrical kA rms & 410.080kA Assymetrical kA crest.
Steady state current:56kA rms (calculated manually)
Based on the above values, pl. advice for switchgear outgoing breakers MAKING AND BREAKING CAPACITY, SHORT CIRCUIT WITH-STAND CURRENT for 1-sec and Switchgear BRACING CURRENT & SHORT-CIRCUIT WITHSTAND CURRENT for 1-sec.
Advanced thanks for your replies and discussion.

 

[power2020]

In addition to my previous post, pl. find the following details for reference.
AC generators- 6no.s
Rating: 1312kVA, 400V, 60HZ, 0.8pf, 1800 rpm
Xd:2.68 p.u
X2:0.187 pu
X0:0.0161 pu
X"d:0.101 pu
X'd:0.127 pu

Other AC generators- 3no.
Rating: 1031 kVA, 400V, 60HZ, 0.8pf, 1800 rpm
Xd:2.29 p.u
X2:0.183 pu
X0:0.054 pu
X"d:0.026 pu
X'd:0.13 pu
Total 9-generators are conencted to common switchgear without any intermediate transformers.

 

[davidbeach]

Eh gads man, what are you trying to do? I don't know the answer to the switchgear rating question, but I don't believe it to be the right question at this time. Have you really thought through all the implications of the proposed connection, over 10MVA of generation, at 400V, all connected to the same switchgear line up, all solidly grounded? Where are you going to find switchgear with a 20kA cross bus rating, as in 20kA continuous current? Your initial post makes no mention of how you are going to get the load off this bus? It would seem that a higher voltage would make things far more manageable.

I'm curious about your 400V, 60Hz as 400V is usually a 50Hz voltage rather than a 60Hz voltage.

 

[ScottyUK]

Totally agree with David. This is crying out to be an MV application at 3.3kV or 4160V or whatever your local equivalent is. Cost alone will drive this - standard MV switchgear will be cheaper than some massive custom-built LV board, assuming you can find LV breakers to handle the fault level you're considering. I've never seen such breakers for LV service, although I have a photo in an old book of a 40,000A (thermal rating) 250V DC breaker. It is the size of a van or light truck.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[power2020]

Thank you for your replies. Voltage rating is 380V not 400V. The above values of generators (Diesel engine) are stand-by rated at 480V. For continuous load, the value is still less (each like 900kVA at 380V) and some will be stand-by. Actual Max. Demand is 7500A (3 outgoing feeders)
But still I am more concern about Switchgear for fault levels. If not possible, I would like to propose the loads to divide into two switchgear units.
Project is already finalised and purchage orders released with low fault levels (75kA/1sec short-time withstand rating) and it came to us for review.
What is best option at this stage?
Devide the loads in to two switchgears and distribute the generators into these two switchgears.
or
Provide fault limiting reactors
Advanced thanks for your reply.

 

[power2020]

Continuous to my previous post, I recommended to use single generator neutral solid ground instead of all generators neutral solid ground to reduce zero sequence reactance.

 

[ScottyUK]

What is best option at this stage?

a) Fire the designer.
b) Sue the designer.

Split the load down into smaller blocks. It sounds like your switchgear is under-rated for the application unless you can get split in to three lumps. How did this mess occur?



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
Scotty, nice options ;-).
Regards.
Slava