The message <
[email protected]>
from Neon John said:
On Tue, 14 Oct 2008 21:48:29 +0100, Johnny B Good
You wouldn't want to put a little wager on that would you? I'm REALLY
hoping
that one of you alligators (all mouth, no brain) will back one of your
claims
a little loot, as I could use some extra spending money.
Really? So you're saying that since I can over-excite the field of a
grid-connected generator to supply leading KVARS when needed, then I can do
the same thing with a little isolated generator? What do you think
happens to
the output of an isolated generator when its field excitation is increased?
After you explain that, please explain how that is the same only in
microcosm.
You can't make a generator produce 'reactive' VA, that's purely an
effect of the load characteristics. All "over-excitation" does is raise
the generator voltage which makes it drive more current into whatever
load it is seeing regardless of whether that load is purely resistive or
happens to include a portion of reactive loading. This, in turn, means
it will place a higher mechanical loading on its PM [1] due to it now
supplying more _power_.
PSUs [2] don't use individual gensets (or even whole power stations) in
this fashion to 'compensate' for lagging currents in the 'Grid', they
simply spread the load over whatever capacity they have on-line
regardless of the PF of the total load. In the end, it is the current
that sets the upper limit of operation.
Ignoring the secondary effect of increased I2R losses, the fuel
consumption of the PMs will actually drop when driving a low PF load.
Since the system is essentially limited by the current carrying capacity
of the wires and the associated circuit breakers, the PSUs penalise
commercial users with low PF loads since they can't sell as much power
without bringing more gensets on line which means a less profitable
operation.
Legislation of standards applied to domestic appliances is the
mechanism used to keep the PF above an acceptable minimum for the PSU's
domestic customers which is why they only ever meter for real power
consumption.
Medium to large commercial users, otoh, if allowed to, can present the
PSU with very low PF loads since the motors associated with any large
machinery are often custom builds or configurations where the PF
correction kit might be considered an 'optional extra'. The PSU will
find it worthwhile to also meter VA and peak VA demands as well as the
KWH consumption with appropriate penalty charges included in the
'contract to supply'.
However, in the context of this discussion, when you're generating your
own power, your main consideration is whether the VA loading is likely
to exceed the genset's VA rating for any protracted periods by more than
a few percent. Please, also keep in mind that a purely reactive current
with purely resistive current doesn't add up in a simple sum; for
example, a 4 amp 'wattless' current plus a 4 amp resistive current gives
a total current of 5.656 amps (a PF of .707). It's also worth
considering that correcting the PF to a 'perfect' figure of 1 is not an
economic exercise and correcting to a figure of 0.9 is deemed entirely
satisfactory in most practical cases (correcting that last 10% usually
involves a doubling or tripling up of PF correction capacitors).
[1] PM = Prime Mover
[2] PSU = Public Supply Utility (in this case, the Electric Power
Company or companies)