Determining ratings for 11kV switchboard

[gekko1]

We have a site requiring 2 x 1MVA diesel gensets to feed an 11kV switchboard, each through a separate 11kV CB, with 2 x 11kV CBs feeding a ring main (load around 1MVA) and a feed from the network (for when the 2 gensets are not operational) through the 5th 11kV CB.
1. How do we calculate the ratings required for the main buswork ... 630A, 1250A, 2500A ... ? Is this purely based on normal operating load? (In which case we would expect it to be ~100A, being 2 x 1MVA@11kV)
2. How do we determine the appropriate current rating for each CB?
3. How is the fault rating and short-circuit rating for such a board calculated?
4. What are the corresponding fault/short-circuit ratings for cables connected to each CB? And how is this used to determine their size?

 

[sitl]

In order to clarify this, what are ''with 2 x 11kV CBs feeding a ring main (load around 1MVA)'' Are these feeders from 11kV switchboard or you have another source?

 

[gekko1]

These 11kV CB's are both in the switchboard and feed each end of the ring from the board.
To summarise :
Supplies to the board are 1. 11kV from the local network, and 2. two gensets (each with their own 11kV CB).
Load is two 11kV CB's feeding the ring.

 

[sitl]

I suppose that the ring is one-way, meaning that energy is flowing from the source (network) to the ring. Having this in mind:
1. main busbars should be calculated according to the maximum installed power of all transformers (and/or motors) in the ring. I would carefully analyze all the circumstances before sizing gensets, having in mind before said.
2. see 1.
3. you should check three phase short circuit power Ssc (usually given in MVA) at the point of coupling of your network. This should be given to you by your utility. When you calculate Isc from it, you should choose breaking capacity (Icu) of breakers as Icu>Isc. Making capacity of breakers should be bigger than (max2)*square root(2)*Isc. If you have medium voltage motors in the ring, then they add fault current to the Isc.
4. You should calculate heating effect in cables while in short-circuit regime and determine minimal cross section from it using predefined formulas.

 

[KJvR]

You might also need to have a quick look at the X/R of the fault since you have generators feeding onto the bus. Standard breakers are rated and type tested for a time constant of 45ms (X/R of fault < 14) - dc component of the fault.