Syncronous motors and the grid

[mobile_bob]

ok so maybe it is set a bit wider
and maybe it then disconnects,  waits a few seconds, resamples the grid and then reconnects if the
grid is good.

or maybe we just set it wider and if it trips, it will require a manual reset?

the latter is probably safer

anyway, it has been my experience around here an in western washington the grid freq is pretty stable, certainly never a half hz off.   as opposed to a single cylinder engine driven generator, which likely will shift above a half hz should the grid drop out.

just kicking some idea's around.

bob g

[BruceM]

You can get an analog chip that does accurate RMS ac voltage to dc voltage level, AD737, with about 0.1 second lag.  $7

I think it would be likely that a local grid down, unknown capacitance on the line situation, with not enough load to trip the breaker, that that both frequency and voltage would be off more than hair. 

A frequency to voltage converter, plus the RMS ac converter, a couple comparators and perhaps a big solid state switch to short the hot legs and blow the breaker, or just interrupt the power to a contactor.  Other guys might figure how to work a time delay relay in there, or I'd use a 555 timer.   Power it with a wide range input switching wall wort so it keeps working if power sags or surges. Thus it will operate even if things go bad, but not if the power drops quickly.

I just don't think there's much of a market for this sort of thing; not that many folks backfeeding via homebrew systems.  But adding an islanding safety circuit sure isn't too hard and makes good sense.