Auto transformer for Motor rating abovr 4MW

[ravindranathan]

In my organization...Upto 4MW Motor ratings is fed from 6.6kV,3150A (Vacuum Circuit Breaker feeder).
Motor ratings above 4MW.... is fed from 33kV Bus through a 33/11kV auto transformer.
The circuit breaker operations does not demand frequent operations and hence Vacuumm contactor is not a must.
With 6.6kV vacuum cirucit breaker feeder, whats the highest motor rating we can opt to feed ?

 

[waross]

An old rule of thumb is when the H exceeds the voltage, consider a higher voltage.
At 6.6 kV, that would be 4.95 MW.
Your cutoff at 4 MW seems reasonable.
However, I know of no rule, code or otherwise, that states this.
If there is such a rule somewhere, please share it. I am still learning.
That said, motor HP versus motor voltage is a compromise and the selection depends on site conditions.
In a smaller plant with a 480 Volt service and only one motor above 480 HP, and after consideration of costs, a motor considerably larger than 480 HP may be installed on a 480 Volt supply.
In such cases, the motor may have an effective reduced voltage starting method.

It appears, ravindranathan, that higher voltages are available in your plant.
For the design of a new facility, where higher voltages are available, I suggest adhering to existing guidelines.
For the addition of one motor rated above 4 MW, I may consider installing a motor of above 4 MW on the 6.6 kV bus.
Without specific details, IT DEPENDS.
Be aware that there may be issues with your relations with others at the plant and possible career implications if you choose to ignore standing plant standards.
And;
Beware of unintended consequences.
Too large a motor on a given bus may cause unacceptable voltage dips on the bus with varying loading on the motor.
It depends.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

In the ANSI world the closet available is 8.25kV (rated maximum voltage) breakers with standard ratings 1200, 2000 & 3000A.
6.6kV is an IEC voltage.
Therefore, for 6.6kV the rated maximum voltage is 7.2kV with bkrs having standard ratings of 1000, 1250, 1600 & 2000A per IEC.
You may have to contact ABB, SIEMENS etc and get a catalogue.

 

[9wearSuave]

>5000HP motor on MV bus is pretty substantial. Would definitely not directly connect without a detailed engineering study. Your organizations' rule of thumb sounds pretty reasonable to avoid an in depth study. Would need to consider other factors besides voltage sags of directly connecting at 6.6kv. A motor like this would add a considerable amount of short circuit contribution, which could give reason to review equipment short circuit ratings, redo arc flash analysis, etc. If it's connected through and AT could be less of an issue depending on the impedance.