Don Kelly wrote:
----------------------------
stevenal wrote:
[snip]
I agree that's what the result should be. But to be accurate, the
meter
must
either filter out the non-fundamental currents, or represent them
properly.
I am not aware of any intentional filtering used to isolate the
fundamental.
Here is what ANSI C12.1 says on the subject:
The theoretical situation, which Don mentioned, is one where the
fundamental plus the harmonic currents are multiplied (on an
instantaneous basis) by a pure fundamental voltage waveform. Each
harmonic current multiplied by the fundamental voltage will result in
a
waveform that is sinusoidal and symmetrical about the instantaneous
power (y) axis. The average over one (or more) fundamental periods
will
be zero. Only a fundamental current multiplied by a fundamental
voltage
will produce a resulting waveform not symmetrical about the y axis.
Only
this fundamental current will contribute to a net power flow
integrated
over one or more cycles.
Frequency variations in a modem power system under normal operating
conditions are insignificant. Any inaccuracies that might result
from
variations that occur are entirely negligible. However, the presence
of
voltage harmonics or current harmonics created by nonlinear loads
may
cause
measurable inaccuracies. In the vast majority of metering
installations,
the
accuracy is still within ±2%. Cases of severe harmonic distortion
must be
analyzed on an individual basis.
In reality, all power systems will exhibit some impedance to higher
order current harmonics. Given a theoretical pure fundamental
infinite
bus voltage source, the actual voltage at the metering point will
contain voltage harmonics due to the load current harmonics, plus any
other harmonic voltage drops upstream due to other loads. Now, if you
perform the same sort of instantaneous power calculation described
above, each current harmonic multiplied by the voltage of the same
harmonic will produce a net real power flow. In practice, the voltage
distortion (and subsequent voltage harmonics) is quite low and the
resulting i*e products are even smaller at higher frequencies. Keep
in
mind that these products are real power flow and are doing real work
at
the load, However, in spite of an electromechanical meter's poor
frequency response at these higher frequencies, there isn't much
power
that gets missed.
Westinghouse admits that disk driving torque is not neccessarily of
the
same
wave form as the current and voltage at harmonic frequencies
(Distribution
Handbook), but also indicates the error is usually negligible. Newer
electronic meters might be better if the cost is right. No point in
spending
dollars to save pennys.
--
Paul Hovnanian mailto
[email protected]
------------------------------------------------------------------
Misery loves company, especially this one.
----------------
Not to mention that with harmonics in both current and voltage, some of
the
harmonic torques may be retarding torques -to the benefit of the
customer!
I note that the utilities are not overly concerned about this effect on
their billing.
Only if the power is flowing back out to the utility (this may in fact
be the case). Except for the frequency response problem, the meter will
measure the power correctly.
--
Don Kelly @shawcross.ca
remove the X to answer
--
Paul Hovnanian mailto
[email protected]
--------
If the first attempt at making a drawing board had been a failure,
what would they go back to?
Paul,
Ideally, yes. But we were talking of the meter error here. If the torques
at
the higher frequencies are not in phase with their respective voltages and
currents, the resultant torque at any given frequency might be negative
for
a positive power flow at that frequency.