On Thu, 6 Feb 2003, N. Richard Solis wrote:

I don't know of too many electrical distribution networks that use DC interconnection to limit AC failures from propogating.

There are 3 power grids in the continental US: Western, Eastern, and Texas. (http://www.eia.doe.gov/cneaf/electricity/page/fact_sheets/transmission.html) These are interconnected using high voltage DC ties, for reasons including (but not limited to) what Sean said. - d. -- Dominic J. Eidson "Baruk Khazad! Khazad ai-menu!" - Gimli ------------------------------------------------------------------------------- http://www.the-infinite.org/ http://www.the-infinite.org/~dominic/

On Thu, 6 Feb 2003, N. Richard Solis wrote:

The main cause of AC disruption is a power plant getting out of phase with the rest of the power plants on the grid.

This is typically a result of sudden load change (loss of transmission line, short, etc) changing the electromagnetic drag in generators, and, therefore, the speed of rotation of turbines.

When that happens, the plant "trips" of goes off-line to protect the entire grid.

Some difference in phase is tolerable, the resulting cross-currents generate heat in the trasmission lines and transformers. It is not sufficient to disconnect a generator from the grid. Since water gates or steam supply can not be closed off fast, the unloaded turbine would accelerate to the point of very violent self-destruction. So the generators are connected to the resistive load to dump the energy there. Those resistors are huge, and go red-hot in seconds. If a gate or valve gets stuck, they melt down, with the resulting explosion of the turbine.

You lose some generating capacity but you dont fry everything on the network either.

Well... not that simple. A plant going off-line causes sudden load redistribution in the network, potentially causing overload and phase shifting in other plants, etc. A cascading failure, in other words. --vadim

N. Richard Solis wrote:

Yeah yeah yeah. I know that everything isn't simple. I actually worked at a power plant so none of this is new to me. Can cascading failures occur? Yes. Witness the Great Blackout in NYC. My point was that there are places where the electrical network is designed to "blow the bolts" to TRY and protect everything. Does it work? Most of the time, yes. All of the time? NO.

Bringing this back to topic. What you are refering to is similar to a failure within an AS. When you start having problems within one section of your network that could jeapordize the rest of your network, you cut it off until the problem can be fixed. Does it work? Most of the time, yes. All the time? No. Sometimes the failure is too rapid to avoid the cascade failure within the AS. This practice is seprate from grid and AS interconnects.

It is a complicated problem but you'd be suprised at how fast things can happen when you HAVE to keep the system running. There is a tremendous amount of skill concentrated in that field and they do a good job of keeping everything running well. How many turbine overspeed events do <snip>

I agree. The same can be said for many networks. The difference is that dealing with some networking problems is new to many engineers. Without proper training and expecting a cascade failure, how do you know the fastest method to deal with it? I've had lots of practice on my network. I have an average stabalization rate of about 5 minutes now, but then, I redesigned my network a long time ago to effectively deal with such problems in a shorter time span.

The loss of a single transmission line isn't going to cause a whole station to trip. If you're losing a bunch though, you've probably got lots of other problems to worry about.

Also true with many networks today. However, this topic falls within a single grid. The original analogy was dealing with grid interconnects which have different requirements and must be protected at all costs. IF, and I don't think it's happened in a very long time, an entire grid lost integrity, it would be unacceptable for the grid to cascade into the other two grids. Extra percautions are put into place. In the same reguards, many Autonomous Systems do have different policies reguarding their interconnects compared to their internal network. -Jack