KWH meter discrepancy [RE-wrenches]

[Bill Brooks]

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Jeff and all,

My esteemed colleague, Jeff Newmiller, who is not a member of this group,
put together a very nice reply, and I think his explanation is relevant and
the most plausible at this point. His message follows.

****Jeff Newmiller's message:
I think Matt Lafferty is on the right track in this discussion. I think the
reverse-power-flow-calibration can be an issue, but it is difficult to
compare net-metered readings with inverter output readings because of house
loads so I tend to think this not where the discrepancies are showing up.
Rather, it is where there are redundant energy production meters which are
wired for forward power flow that the discrepancies will be most easily
noticed.

1547 goes to noticeable pains to describe referencing the harmonic current
content of an interconnecting device to its rating, instead of against the
measured fundamental current.  This is because they recognized that the
absolute amplitude of current harmonics is the most important information in
determining the effect on grid power quality, not the relative fraction.

This means that compliant interconnecting power systems are allowed to
produce just as much power in harmonics at low power as they would at high
power.  Measurement of energy based on
fundamental+harmonics will therefore be larger than energy measured 
fundamental+based
on I*V*PF or by meters that internally suppress harmonics (intentionally or
not). For example, a 1kW inverter reporting 100W output may be producing 60W
of 60Hz power and 40W of harmonic power at the third harmonic yet still meet
IEEE1547 (4% maximum single harmonic referenced to current level at 1kW).
While this scenario is unlikely, it shows that being off by 66% is allowable
so a 15% difference should be quite possible based only on a "include
harmonics" versus "fundamental only" difference in type of measuring device.

Since the power contained in harmonics is far more likely to be dissipated
uselessly than the power produced at 60Hz, I don't think it is fair to
include "all harmonics" power when selling energy into the utility grid.
However, depending on how an inverter manufacturer implements their energy
accumulation, they may end up doing just that.

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Jeff Newmiller, Project Engineer                       BEW Engineering
mailto:jeff.newmiller at bewengineering.com http://www.bewengineering.com
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