Richard Clark wrote:


Hi Nick,

Phase is necessarily found in Impedance. As you allow that multiple
generators share a line, they are also across the load as a load if
they do not present the right phase.

As for the "bad idea" of matching, you are appealing to Edison's old
and deliberate misreading of Thevenin. Matching does NOT require a
resistance, this is a mis-read of conjugate matching that follows the
fact (in antennas). It does not drive the need (in power delivery).
Power stations only need perform a Z Match, not a Conjugate Match.
Any form of X is sufficient to accomplish the task and they do it far
simpler through field excitation control.

Hello Richard,

Even with your redefined version of matching, individual generating
stations don't explicitly match to the load reactance or (certainly
not) load resistance. A little generation 101. (This goes a little
beyond the scope of discussion, but I throw it out because I think it's
interesting.):

Generators operate in two modes: Isochronous (single generator
supplying local bus) and parallel (multiple generators in parallel;
connected to a grid).
The operator has two main controls: Steam (or whatever) flow to the
turbine or other driver, and current to the field.

In the isochronous mode, varying steam flow causes the speed of the
generator to change. It also varies the amout of power delivered. In
the parallel mode, the generator can't measurably push the speed of the
grid, so increasing steam flow only increases the electrical power
output.

In the isochronous mode, varying field current changes the terminal
voltage of the generator. In parallel mode, varying field current
can't significantly change grid voltage. But it does change the
reactive power output (MVAR or kVAR) of the generator, as you said.

The normal mode of operation for a large generator is in parallel with
the grid, so the operator is using steam (diesel, water, hamsters,
etc.) to regulate real power output and field current to regulate
reactive power output.

Now some anecdotal stuff about how generators are operated. The system
dispatcher requests individual generators to adjust their power and
VARs to match load. This isn't impedance matching, it's simply
supplying the demand. In the case of VARs, the goal is both to supply
the demand and to equalize voltage across the system, not to cause any
kind of mathematical match between the generator's internal X and the
system's X. Oh yeah, I said earlier that individual generators don't
appreciably affect grid voltage. That's true, but locally they do have
an effect, like tent poles in a big canvas. So the local stations are
both supplying their share of the total reactive load and propping up
voltage in their area. (The operator increases VAR output by taking
his excitation switch to the "raise voltage" position.)

Anyway, I digressed from my anecdotal stuff. At my plant, the
generator puts out 1050 MW 24/7, but MVAR may vary between 0 (or
slightly negative) and 200 MVAR. So we're not matching to any specific
impedance, but supplying load and maintaining voltage.

A story transmission guys like to tell is how they may use open ended
transmission lines as a kind of capacitor bank. Say there's a line 100
miles long from my plant to somewhere that's not needed to carry load.
The system controller might connect it at my plant's end but leave the
breakers open at the far end. A line has both capacitive and inductive
reactance of course, but when unloaded, the capacitive dominates. So
the trick of the trade is to use it to supply reactive MVARs. The
point of the story in this context is that the controller isn't
concerned about SWR on this extremely mismatched line.

Another possibly relevant story. We connect our emergency diesel
generator to the grid for testing and load it to about 3000 kW and
typically from 0 to 100 kVAR. But to fully test the excitation system,
the kVAR is at some point raised to 1400. The point being that the
generator can be operated anywhere within its rating, with no need to
match to any mysterious impedances out there in the world. Makes sense
when you think about it. Who would want a generator that was
constrained to operate at some fixed ratio of real to reactive power?
73--Nick, WA5BDU


73's
Richard Clark, KB7QHC