Faulty Electricity meter

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cyril7

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#430451 24-Jan-2011 09:37

I wonder if it has anything to do with the power factor of computers etc and other appliances with AC motors etc that have a certain amount of non-sinewave feedback, as opposed to pure resistive loads. I thought power meters were actually supposed to measure the average power.

Having designed a few power meters in the past of which there are many thousands in current use throughout the world I might comment here.

Modern electronic power meters have the ability to record both active and reactive power, however in single phase domestic situations only your active power is registered for consumption, so if your load presents a poor power factor then that reactive component does not contribute to your bill. Naturally its in the retailer (and lines companies) interest to ensure that the power factor is within a manageable limit.

Older ferraris meters in theory do the same but with nowhere near the accuracy over the same dynamic range as modern electronic solutions.

Cyril

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#430460 24-Jan-2011 10:04

@sattv - just making sure we get this right - your Elster gREX D AMi meter was replaced with another Elster gREX D AMi meter. The replacement reads high. Is this correct?

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Damager

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#430730 24-Jan-2011 21:12

cyril7:
I wonder if it has anything to do with the power factor of computers etc and other appliances with AC motors etc that have a certain amount of non-sinewave feedback, as opposed to pure resistive loads. I thought power meters were actually supposed to measure the average power.

Having designed a few power meters in the past of which there are many thousands in current use throughout the world I might comment here.

Modern electronic power meters have the ability to record both active and reactive power, however in single phase domestic situations only your active power is registered for consumption, so if your load presents a poor power factor then that reactive component does not contribute to your bill. Naturally its in the retailer (and lines companies) interest to ensure that the power factor is within a manageable limit.

Older ferraris meters in theory do the same but with nowhere near the accuracy over the same dynamic range as modern electronic solutions.

Cyril

Been quite interesting about the comparison's of the new smart meters and their accuracy.

Cyril can you please give an example of an appliance presenting only active power for consumption against a non smart meter? Just trying to understand the situation in layman's terms. As i understand it, a pure resistive load presents a constant load that is easily measured. An unbalanced load presents varying levels load of which only some or the average load is recorded as consumption on the meter.

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richms

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#430738 24-Jan-2011 21:29

With AC you cant just average the current and the voltage and get a true watt reading. While none were dumb enough to do that (unlike the first versions of some plug in meters seemed to) there is still an issue of how often you sample. They have to take great care not to overcharge, which would happen if they just used the peak current in the sample period so there is some really agressive lowpass filtering going on in them.

You need to get the current and voltage at that moment and calculate the instantaneous wattage and then average that. The higher the sample rate that is done at then the more accurate you get. Its not uncommon to find the peak wattage of some loads to be kilowatts. I am suprised that the questionable power supply companies dont put that on the side of them instead of the the current BS of several 100 watts.

I get the feeling that some of the plug in type meters are not even sampling continuously either which is another gotcha if you have something like my friends DIY reflow oven that is only on for some cycles and not others.

If you want to see how bad you can do it and still get somewhat usable results, look at the tweet-a-watt project, on AC cycle every 2 seconds is sampled, 17 samples (I think from memory) and then calculated on. And its not that bad for accuracy compared to the first revision of the elto plugin meter.

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SATTV

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#430760 24-Jan-2011 22:01

@gzt We had an ADR remote read meter for about 3 years, then a new Elster gREX D AMi meter was replaced with another Elster gREX D AMi meter. It was the first Elster that was reading High.

John

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richms

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#430762 24-Jan-2011 22:03

I am wondering if they are calibrating the new ones lower and will slowly edge it up to accurate over time to appease those that dont like to suddenly pay what they are actually using. I am assuming they can remotely read and manipulate the calibration registers inorder to create a fudge factor in them.

Richard rich.ms

MikeSkyrme

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#430771 24-Jan-2011 22:21

Shouldn't fiscal / tariff meters be built to a specification? It would interesting to see the spec that they are manufactured to. Anyone have any ideas where this might be found?

Michael Skyrme - Instrumentation & Controls

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richms

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#430774 24-Jan-2011 22:26

When I suggested the same thing for internet traffic I was laughed at.

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MikeSkyrme

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#430782 24-Jan-2011 22:34

I do not understand why you would be laughed at.....
A similar example would be petrol / diesel pumps. The meters used on these are built to a specification and have to be calibrated at regular intervals.

Michael Skyrme - Instrumentation & Controls

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#430881 25-Jan-2011 09:33

MikeSkyrme: Shouldn't fiscal / tariff meters be built to a specification? It would interesting to see the spec that they are manufactured to. Anyone have any ideas where this might be found?

Designed, built, and tested to these standards:

ANSI C12.20-2010

ANSI C12.1-2008

IEC62053-11

IEC62053-21

IEC62053-22

Note: The links go to table-of-contents style previews.

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gzt

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#430887 25-Jan-2011 09:59

And for communication:

[quoting wikipedia:]ANSI C12.18 is an ANSI standard that describes a protocol used for two-way communications with an electricity meter, mostly used in North American markets.The C12.18 standard is written specifically for meter communications via an ANSI Type 2 Optical Port, and specifies lower-level protocol details. ANSI C12.19 specifies the actual data tables.ANSI C12.21 is an extension of C12.18 written for modem instead of optical communications, so is better suited to automatic meter reading.

ANSI C12.18-2006

ANSI C12.19-2008

ANSI C12.21-2006

ANSI C12.22-2008

Again, previews only.

gzt

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#430888 25-Jan-2011 10:05

The Elster gRex D also claims compliance with an Australian standard:

NMI M 6* (Metrological and Technical Requirements)

*complete document

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gzt

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#430926 25-Jan-2011 11:23

webwat: I wonder if it has anything to do with the power factor of computers etc and other appliances with AC motors etc that have a certain amount of non-sinewave feedback, as opposed to pure resistive loads. I thought power meters were actually supposed to measure the average power.

With respect to comments already made by [richms] and [Cyril7] about power factor and sampling, your comments could also refer to metering challenges created by harmonics and harmonic distortion generated by switchmode power supplies etc.

Anyone with an IEEE membership or library access like to summarise the metering conclusions from these documents? : )

Revenue metering in the presence of distortion and unbalance: myths and reality
Efficiency of active electrical power consumption in the presence of harmonic pollution: a sensitive analysis
Should a utility meter harmonics?

savag3

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#430991 25-Jan-2011 12:49

MikeSkyrme: I do not understand why you would be laughed at.....
A similar example would be petrol / diesel pumps. The meters used on these are built to a specification and have to be calibrated at regular intervals.

Yes its actually a legal requirement for anyone selling goods by weight or measure (Weights and Measures Act)
I can't see how you can justify requiring the scales at the supermarket checkout to be certified but yet there are no requirements for selling data by the gigabyte.

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