----- Original Message -----
From: "Magnus Danielson" [EMAIL PROTECTED]
snip
BTW, measuring the 53rd overtone frequency may not give a clear
picture of the
frequency deviations at the base frequency. Overtone spectras
experience quite
a different phase shift from the way it is produced by a number of
devices
which vector-add to become the seen frequency and phase. Turn on or
off a
strong producer of that frequency may shift that phase quite a bit
while the
fundamental is barly shifted.
Quite true, the majority of loads that produce high order harmonics
back into the grid are almost always varying considerably in the short
term-thus they produce short term variations in the resultant harmonic
spectra that renders the lines somewhat broader than you might think.
Ski lift motor drives are a good example of this. Such loads, without
adequate filterimg at the point of supply for the load, produce high
harmonic levels in the associated supply network, to the detriment of
any telecom cable network that happens to run parallel for significant
distances-as they always do in rural areas!
The old manual method that used to be used for power grid frequency
checking involved a comparison of two clocks, one driven from a
reference and the other from the grid itself-more usually the output
of a local generator in the days prior to strong grid linkages. The
comparison was typically done once or twice a day and appropriate
adjustments to the generation plant made to correct the grid driven
clock and keep its reading 'syncronised' to the reference clock.
I have here the remains of an attempt in the early seventies to bring
this type of system up to date - it comprised an HP 105 series quartz
reference, a K20- HP 5280A up down counter and an HP 5321B clock.
The up down counter was driven from 100 Hz signals derived from both
the 105B and the power grid. An HP 6933A D/A converter on the BCD
output from the counter had its plus/minus 10 volt output interfaced
to the generation plant. The 5321B clock reading was initialised from
the local time service(radio time pips) and then used to fine tune the
control system (D/A conveter to generator coupling) so the up down
counter stayed at or near zero reading and grid time ran in sync with
national standard time.
I dont know how successful this system was but I think it was in
service for several years.
I have the up down counter and the D/A converter almost fully
operational again- but only have parts ratted circuit boards from the
5321B clock and I never got the 105B.
DaveB, NZ
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