Parallel generators - how does that work?

[Dave Typinski]

Random question of the day: how are generators paralleled?

DC generators, like those on older* vehicles (or alternators with built-in
rectifiers on newer* vehicles), don't have phase angle problems -- but how do
you keep one generator from driving the other? How do you tie the gens to the
same bus appropriately, given that the generator outputs may not be precisely
the same?

AC generators, same issue, but with a phase angle bonus problem. Electric
generating stations handle this somehow, and so do large airplanes with multiple
generators that all tie to the same AC bus. But how they do it is mysterious to me.

How does that work?

* older meaning pre-1965 or so, newer meaning post-1965 or so
--
Dave

[Conrad Sawicz]

When I toured the DC to AC conversion plant in Southern California in around 1978 the only instrument that the operators watched is the frequency meter to match the AC output to the power grid.

Thinking about it now if the generator did not have a pure phase match to the grid, power would be running the wrong way and being absorbed by the generator. I am sure there is a more accurate explanation.

I was also exposed to generators and ground power being added and removed from the 400Hz, 3 Phase 120Volt power bus on a 747 airframe. The only criteria discussed by the power engineers was the frequency control. For the most part I was worried about power disruptions over all phases of a flight, and they mostly did not happen. From start up on cart power to inflight engine switching to save fuel. We did not have special designs for AC power switching in our power supply designs.

I now design medical devices for use in eye surgery through out the world. The US spec for power drop out is about 200mSec. The practical number for reliable world wide operation is 2 second. This is likely due to the design of the power grid, and how back up power in a hospital is covered. Good US systems have both battery and diesel generators. They are virtually seamless.

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