Sunday, October 06, 2002

Electric dogsleds

There's a dogsled theory of management that says "if you ain't the lead dog, the scenery never changes". But for the dogsleds that's completely irrelevant to what follows, but this is my site and you ought to be used to this by now.

Anyway, dogsleds aren't a bad analogy for what you're dealing with when you hook up multiple generators to provide power to the same electric grid. You'll have a lead dog. You want to make sure that all of the dogs are pulling their share of the load rather than just keeping up with the traces, and you sure don't want a dog that is soaking up power from the other dogs.

You can overextend the analogy though. There's never been a band called Three Generator Night, and whipping a turbine or a diesel isn't likely to affect its behavior. Instead, you adjust the governor.

What's a governor? A device to control the speed of a prime mover like a diesel generator or turbine. With the proper feedback, it can also impose a maximum load the prime mover will carry. It does this by monitoring the speed of the device, and if properly equipped, it also monitors the current power level. When it detects a discrepancy between the machine's behavior and its current settings, it manipulates the machine's controls to minimize the discrepancy.

With sophisticated modern controls this can be done in a number of ways. But for backup purposes a lot of them still depend on traditional flyball governors, which are neat little mechanical gizmos.

These mechanical governors are driven by a shaft that rotates at a speed proportional to that of the prime mover itself. This causes some weights to rotate at this speed, and centrifugal forces cause them to want to fly out. As they do, they displace a shaft which ultimately controls the prime mover. A spring resists this displacement, and the setting the speed the governor maintains actually amounts to tightening or loosening this spring's tension.

That shaft that displaces - what does it do? That depends on the prime mover. For a steam turbine, it would manipulate the main steam control valves, controlling the rate at which steam enters the turbine system. This of course affects the boiler, which will respond to the resulting increase or decrease in back pressure from the turbine (and more, in the case of a nuke) to adjust power as needed.

For a diesel generator, it controls the positions of the fuel racks. These control how much fuel is injected into each cylinder at each stroke. In essence it's like the accelerator pedal in a car, but the linkage is a lot more complicated.

OK, you're controlling the speed. But this only works if all of the generators are set for exactly the same speed. The ones set lower will take it easy while the ones set higher might overload.

So how do you whip the dogs, ie how do you make sure all the generators are carrying their share of the load? That's another post. Til then...

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