Wednesday, December 05, 2012

Long lines

In the early days of electric power, major consumers were located near large generators. 

An aluminum smelter might be built next to a large hydroelectric plant.

Or a coal-burning power plant might be built next to a large city. 

The "grid" evolved in a rather ad hoc way.

Mainly to provide reliability and even out the loads on each power plant

But still most of the power was used close to where it was generated.
We would suggest that, no matter the political and engineering details, the reliance on long-distance power transmission opens up problems that we wouldn't have if most power was generated near where it's used. 

With long distance transmission you trade vulnerability to generator failure for vulnerability to grid failure. 

A generator is a relatively simple machine with a straightforward control system.

Coal or water goes in, electricity comes out, and amount of input can be throttled as needed. 

A power grid is the most complex nonlinear control system in existence. 

It must maintain both voltage and frequency within a very narrow range under an ever-changing combination of inputs and loads. 

Long transmission lines have lumped and distributed reactances that result in peaks and valleys of voltage even in what is ostensibly a single wire. 

Rather than being under the control of one controller, a power grid is a "system of systems" in which an almost fractal-like network of sub-control systems controls various small parts in ways that are impossible to accurately model and are out of the control of the highest level of the system. 

The result of all this is that unexpected nonlinearities and unanticipated feedback loops can emerge that make the whole system unstable. 

The result is usually lower-level safety supervisor circuits tripping out, and a regional cascade of blackouts.
One attempted cure for grid instability is ever-smarter, ever-more-subtle control systems. 

The other is to simply string more and heavier wire everywhere. 

Both are expensive, and both require maintenance.
We would suggest that we re-think the trend towards moving generators father and farther away from loads. 

Granted, there is an aesthetic impact from putting a coal-burning power plant near a city.

Or from damming a nearby river.

Or from covering the urban landscape with windmills.

But the environmental impacts of those developments are the same no matter where they're located. 

In fact the environmental impacts of building things on already-degraded land might be less than building them in a pristine place out of sight somewhere.

And of course nearby power plants wouldn't clutter the rural landscape with more transmission lines.
Build sufficient power plants near the main loads, or alternatively move the big loads near the plants.

Then a grid failure may only mean that that particular place needs to cut itself off from the grid for a time and power itself independently until the grid is back up. 

Perhaps there would not even be enough power to run the whole city independently, but there would be at least enough to run the mine hoist and the irrigation pumps, for example.
We're in danger of creating a power grid that's too complex for its own good. 

We'd be better off going back to the idea of it being an intertie rather than a distribution system.

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