[Lowfer] Power Outages

[Costas Krallis SV1XV]

The following description applies to european equipment
but I believe american hardware at generation and transmission
level is very similar.

> 1) How is a power generator taken off (or put back on) the power grid?
> I think some envision a large knife-switch being operated, which I
> reject, but somewhere contacts have to be broken, no?

There are two large relays in series (or they can be combined).
One of them is tripped in emergency situations by generator
protection circuits and can handle a solid short circuit. The
other can handle the nominal current of the generator and is
used to connect and disconnet the generator during normal
startup and shutdown. It is controlled by a circuit which
assures connection takes place in sync with the grid.

> 2) Do all plants generate 3-phase? What are they timed to?

Yes, all are 3-phase. 0ne-phase AC generators with a power
output of more than a few kW are uncommon and they are 
never connected to the grid. The gens are "timed" to the
grid frequency (60 Hz in America, 50 Hz in Europe). 

If the generator starts rotating slightly faster or slower,
it it pulled automatically back in sync. This is caused
by the negative slope of the frequency vs. power
characteristic of the generator. 

> 3) What controls the period, currently 60cps? Is it the
> armature speed at some integral of 60?

Yes, the armature (rotor), which carries a DC current, rotates
in synchronization with the grid. As I explained before, this
synchronization is automatic, only the power input to the
turbine is controlled, to set the power output. (Also the
excitation current is controlled in order to control generation
of reactive power).

> 4) Can a generating plant have it's load removed (say by some event
> outside the plant, unplanned) without consequence to the generator, ie,
> one moment pushing mega electrons, the next not?

In this case, we have to dump steam and cut power to the generator.
Most modern plants have bypass pipes to the turbine and they can
dump steam to the condenser. In addition , valves can open and
dump steam to the atmopshere. As the steam is at very high pressure,
there is a lot of noise.

This process causes a major electrical and mechanical shock but
the system (turbine-generator-transformer-switches) can handle it.
What it more dangerous is the thermal shock (rapid cooling)
which can reduce the life of the turbine and the boiler.

> 5) Knowing of this recent "cascading effect" of the generators going
> down, as on Thursday, is that a very robust system or an engineering
> crap shoot? If the latter, how'd they get away with dumping it on the
> public and their national security?

The basic problem is that the electrical grid cannot store energy.
Also under heavy demand conditions it can become unstable (to put it
in simple words, an increase in power demand leads to reduction of
transmitted power, insted of increase). If we are near the limit
and demand increases, the only way to ensure stability is to cut
loads. This can be done if the increase in demand is slow. If the
system is in this condition and suddenly we lose generation or
transmission capacity, there is not enough time to react and the
system collapses. This is the well-known "blackout". Then all
generators trip and are taken off-grid, steam is dumped etc. In
this situation the grid control center cuts off all loads, until
a generator can be brought back in the grid. Then, one by one, the
generators are reconnected to the grid and, at the same time,
the loads are also gradually connected. This procedure lasts
for 6-12 hours for a small grid  to up to 48 hours for a major
system of interconnected grids.

73 Costas SV1XV