AQ: Avoid generator overload
Two buses of 11kv, 750MVA, 3000A each fed by a transformer of 40MVA and connected through a tie breaker, now connect a generator of 18MW,11kv, 0.8 PF. How to avoid overloading?
The generator is being used as a backup power source in case utility power is lost, based on such info presented, you are going to have a hard time getting this to work with only ONE 18MW gen. In order to connect the 18MW gen to both buses, the total demand should not be more than 80% of 18MW or 14.4MW at .8 power factor. For short run times (10 or 15 minutes), you can load the gen up to 90% for continuous load, but for long run times, you need to keep it at 80%.
Demand is the diversified connected load. Not all 54.22MVA of connected load will be on at the same time, so this is why you “diversify” the load to get your actual demand load. You can look at your power bill or call your utility to find out your total demand. Or you can install a power quality monitor for a couple of weeks to get it.
A general rule of thumb is to assume that 67% of the connected load will be your demand load. But this depends on your operation. Based on this, one generator will not be sufficient for BOTH buses. However, if you are supplying each bus with its own generator, you may be ok.
Another issue is motor starting flicker. Make sure your generator can start your largest motor and that your disconnect breaker or fuses can handle the inrush. I have seen this as an issue, especially when soft starters are used. Soft starters lower the inrush by exploiting the time characteristic. If the soft starter settings do not bring the motor up to speed quickly enough, the overload trip setting on your generator may trip.
The bottom line is, you are going to have to look at this installation much closer in order to make this work with one 18MW gen. You may even have to disconnect some load when you are running on generator.
