We've just had our first taste of winter, and with it came a good eight hours without power. It was the last two hours of this outage that brings up the question:
How easily are automatic transfer switches 'fooled?'
During the last two hours, I was treated to all manner of blinking lights, dim lights, surges as power was imperfectly restored. Usually several minutes of uneven power would be followed by a period of complete power loss.
This made me speculate as to how an automatic transfer switch would react to such a restoration of power. I can imagine the controls stopping, and starting, the generator several times in rapid succession. I can imagine the controls being fooled when only one 'leg' has good power.
I am not sure what the time delay is but my brother in law's Generac runs a while after power is restored before it decides the power is really there. He still has a good part of a minute in the dark before it kicks off the first time. (I suppose to give the reclosers a chance to do their thing)
Reno: The off time delay on resi whole house gens varies from 1.5 min to 5 min of utility restoration. Most of the commercial gens are adjustable time off delay, with minimum of 2 minutes of utility power. These are all treated as non-essential
The large data center has eight 2 MW diesel, with very sophisticated computer controls, and they are set at 15 minutes time off delay, upon confirmation of 13.2KV, 3 phase, utility power. Reference to 700.12 (B), although this install is not a required system.
They also have a legally required system for the life safety systems, which conforms to <10 sec start, etc.
All the commercial ones I've installed have been designed for instant, 'seamless' power transfer. I never thought to check if there was any delay built in.
Transfer switches like everything else, come in all grades and quality.
They can be set up to transfer back to utility power when it first returns, wait for a some period of time of "good" power, or even require manual action to switch back to utility power. "Good" power could be one, both, or all three phases present and in band for voltage, frequency, and sequence.
The generator controls determine if it turns off immediately after being unloaded or if it runs for a few minutes to cool down before shutting off. Some generators will also signal if the fuel supply falls below a setpoint, so that certain loads can be dumped to extend run time for other more important loads.
