Hello Roy,

Good question and one I had considered addressing in my already over long
post. In general "the grid" is viewed as an idealized source or sink of
both real and reactive power. So we can theoretically supply it as much
power as we wish, and supply or take in as much reactive power as we wish.
No reactive load banks needed.

So when I said generation (of both watts and VARs) is matched to demand,
that's not necessarily *exactly* the case when it comes to VARs, as you
guessed. Generators can both supply and absorb them to meet the need, and
the net VAR output doesn't necessarily have to equal whatever the customers
are offering as the load at any given time. BTW, in the power biz, we have
the convention of "supplying", "outgoing", or positive VARs to describe
reactive power out from the generator to a lagging (inductive) load and
incoming, or negative VARs to leading (capacitive) loads. Incidentally,
real power must flow *out* only. We have reverse power (anti-motoring)
relays to trip the unit off line if this rule is broken.

The tendency of generators to exchange VARs when in parallel leads to a
stability problem in excitation control. A slight mismatch in excitation
systems can lead to a huge exchange of VARs and resulting overcurrent. So
excitations system incorporate what is known as a "droop" feature which
essentially provides a negative feedback based on reactive current.
Increased VARs out tends to reduce excitation, stabilizing the system.
Droop is typically switched "off" in isochronous (one generator isolated)
mode. There's an analogous "droop" feature on the governor for speed
control when in parallel.

Not sure if your question included this, but it's interesting to consider
just how a generator produces out of phase current when connected to what
we're essentially considering to be equivalent to an ideal voltage source,
since by definition the generator's terminal voltage must equal that of the
source (grid). As I see it, the key is that the generated voltage, Eg, is n
ot the same as the generator's terminal voltage, Et. There's a drop across
the armature reactance, so Et equals Eq minus that drop. Interesting that
out of phase currents produce drops in phase with Eg ... Well, I thought so
anyway. Current is Et minus Eg divided by Za (armature impedance).
Changing excitation changes the magnitude of Eg (Et is fixed by the grid and
so is an anchor point). By fooling with the phasors, I think you can see
how changing excitation changes the phase angle and therefore controls VARs.

How *power* is controlled is beyond the scope of this discussion (and maybe
of my understanding). But it actually is related to the angle of the
rotor's physical position relative to the rotating field of the armature.
That angle is dependent upon the torque supplied by the driver.

73--Nick, WA5BDU
in Arkansas


"Roy Lewallen" wrote in message
...
Thanks very much for the interesting and informative tutorial from
someone in the industry. I have one question:

Nick wrote:
. . .
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. . .

If your customers' loads were, for the sake of argument, purely
resistive as seen at your power plant output, then the voltage and
current would be in phase at that point. But in order to make your
generator produce "reactive power", the voltage and current have to be
forced out of phase at the generator. How is this resolved? Is that
reactive power "delivered" to (actually swapped back and forth between)
other generators in the system -- that is, do the other generators in
the system shift their own phase angles so that the V and I can be at
some angle other than zero at your generator output (and, necessarily,
also at the outputs at other generators in the system) yet in phase at
your customers' loads? Or do you have some local bank of reactance that
you can switch in to feed the "reactive power" back and forth to when
you run this test?

Roy Lewallen, W7EL