Standby Generator Sizing vs Service Transformer Rating

[Tank7900]

Greeting all, this is my first post to the forum, and perhaps someone here can help me with a generator sizing question. I am trying to size a stanby generator for a facility that contains some devices the require startup current (RLA). After doing the calcs based on motor code specs, I find that the generator size that is required is larger than the utility service transformer. Is it true that if given an equally sized generator and transformer, that the electrical stress to the windings would be different? Or is the generator more stressed because its output is driven by an engine(which can droop when loaded) vs commercial power that is not hampered by the loads of one facility. Thank you for any insight that can be provided.

Tank

 

[fmtjfw]

Sure, a utility transformer is sized for average heating (and often undersized from the size of a service based on utility's assumption of load diversity). If you overload a transformer for a short period of time with high startup current it just heats a little more but cools down when temporary overload is over.

A generator may stall out if overloaded because the prime mover for the generator doesn't have the torque to meet the load.

 

[chris kennedy]

Welcome to the Forum.

Could we see your calcs and could you provide the size of the POCO tranny?

 

[rcwilson]

Tank - What are the voltage and frequency droop limits you are using in the calculations? If they are too stringent, the generator may be oversized.

A 750 kVA transformer can usually start a larger motor than a 750 kVA generator. The source impedance of the generator is much higher than the transformer/utility combined impedance. Motor inrush current flowing through the generator source impedance causes the output voltage to drop. If it drops too low, the motors can't start.

One soluton may be to start motors or add loads sequentially, instead of in one big block.

Most generator suppliers have free programs to analyze the various scenarios to get teh best match of generator to the load and budget.