Maximum kVA Loading For Generator

[ThePunisher]

Hi all,

I understand that the maximum kW loading that an engine can take (assuming an existing generator) is based on its rated kW rating. For example, a 5.5MVA, o.68P.F., 4.16kV generator will have a maximum engine rating of 3.74MW. With rated 3.74MW loading, the equivalent MVAR would be 4450MVAR. This is during steady state loading.

I ran a static motor starting on 2500HP, 4kV motor connected to the MCC bus where this generator is connected and considered an islanded situation. The geneartor is pre-loaded before this motor is started. The resulting bus voltage drop at the bus and at the motor terminals was 15% and 16% respectively. This was ok.

However, the power flow indicates that the generator under this motor starting condition (generator preloaded) is 9.735 MVA (1771.6kW + j9572.5kVAR). The engine is capable of starting this load. I am bothered with the generator itself.

One of my colleagues informed me that generators (alternators) have starting capabilities and as a rule of the thumb, it can be based on the generator available short circuit current let through. The way I understand it that the generator can be overloaded momentarily up to its short circuit let-through current and this can be used against motor starting as long as the voltage drop permits.

I would like to take some opinions on this...seems to make some sense.

 

[zade]

When you have short than your voltage is down to %15 of original, so your voltage drop is around %85.
Furthermore generator can sustain probably no more than 10 seconds which after that they are fried.
It affects the life of the generator rule of thumb is 100 times short you are out, generator or (transformer) dies.

 

[ThePunisher]

If the maximum let-through kVA cannot be used as a basis for motor starting kVA then I'll make another question:

I read that typical generators is capable of taking 300% of its rated kVA for 10 seconds on a 10~50% power factor.

Where did this came from? Is there a calculation basis for this?

 

[alehman]

The generator manufacturer should be able to furnish you a thermal damage limit time vs. current plot.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[ThePunisher]

Thanks Alehman, we are presently contacting the manufacturer for the needed information. I do hope it is still in their records since the generator was only installled last 2000.