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Energy meter with fast cycling load

Energy meter with fast cycling load

Energy meter with fast cycling load

(OP)
I have on my bench a kWh meter that is supposed to have a systematic (all meters of that type seem to have the problem) error.

Calibration works out fine, but when you cycle the load with a few seconds on and a few seconds off, the meter registers more energy than is actually used by the load.

The reason this deviation was detected is that a guy with a (I think) badly designed thermostat in an around 2 kW application thought he paid too much for electricity. Utility did a check and found that the meter actually registered too much energy. Not when testing with steady load, but when checking with parallel meter from other manufacturer.

I have some theories about this. But feel genuinly unsure. My first thought is that some accumulating or filtering software is asymmetrical so that 'attack' is faster than 'decay' - if I am allowed to use musical language.

Anyone has any experience with this? Heard about a similar thing? I am open for all possibilities - the weirder the better (well, within limits).

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Possibly a slow sample rate? Some interaction between the sample rate and the load cycle time?
Did you hear about the retired railway man with the sleep disorder? He would wake at random times for a few hours and go back to sleep to wake again at a truly random time. When he awoke he would go down to the tracks and wait for the next train to pass. He would note the direction of the train and return home to sleep again. He saw three times as many east bound trains as west bound trains. How could this be? Actually there were the same number of trains each way, but the eastbound train passed on the hour and the west bound train passed 15 minutes later.
He had a 15 minute window of opportunity to see a west bound train and a 45 minute window of opportunity to see an east bound train.
Random sampling produced a 300% error.
Is it possible to have a similar interaction between the sampling rate and the load cycling rate?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Energy meter with fast cycling load

(OP)
Have been thinking about that. The meter manufacturer says he samples 1000 times/second. And I really hope that this is for all six (three voltage and three current) channels. That would be 20 samples/period (50 Hz) and it does not seem possible to have an error in the 15 percent range with such a tight sampling. Well, not exactly tight, but good enough for a meter, I think.

If the sampling is spread over all six channels, then anything could happen, 3.33 Sa/period and switching load on/of in seconds could produce some interesting interference patterns. But ought to equal out in the long run. Also, this is a well-known manufacturer and the specs (checked by independent lab) are good. So, I think we have 20 Sa/period on each channel.

I am setting up a test with triacs for fast cycling and a bank of 9 times 300 W lamps. It is getting bright (and hot) when we turn them on.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Don't forget to check your own kWHr meter and bill the customer for the power consumed during testing. LOL

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Energy meter with fast cycling load

(OP)
Good tip! I usually lose more than I gain. Billing this energy, which is heating my house, I will perhaps come out even.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Could be they are getting spikes coupling in.  Essentially blunders in data.  Is the customer using random SSRs?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Energy meter with fast cycling load

(OP)
I shall have a closer look at the current recorded. Stand by.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

(OP)
Looks like turn-on is completely random. Turn-off is, of course dictated by zero crossing. The currents (channels) are 1, 3 and 5 (resistor colour code). Upper recording shows turn-on and it looks like thyristors. Lower recording confirms that. The load is obviously connected L2 - L3 with a small load connected L1-N. A small switch-mode power supply seems to be connected from L2 to N (can be seen in lower picture after cursor CIII.

The length of this packet is 8 seconds (Delta C3-C2) and the off time is 7 seconds (Delta C2-C1).

Not much distortion.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

I'm a little confused what we're looking at as well.  We are evaluating a test signal used to check a meter?

But interesting to see the waveforms do not go to zero during the off-time.  Is there maybe measured energy accumulating during the off-time?

=====================================
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RE: Energy meter with fast cycling load

(OP)
Thanks for your comments!

First, yes, this is what the customer's load looks like. It is an ordinary one-family house at around ten o'clock in the morning. There are very few other loads except the heater that is connected between L2 and L3. When the heater is on, current flows from L2 to L3, so channel 3 and 5 shall be 180 degrees apart.

Then, when the heavy load is off, there is something with o little crest. Probably a switchmode power supply (TV set or computer) in channel 3.

Channel 1 was off in the upper picture, but is activated in lower picture. Looks like a modest linear load. It was mentioned that a small heater with a fan was on in addition to the heater.

There seems to be a DC offset in channel 5 lower picture. I hadn't noticed that before. Me to wonder from where that comes. Shall look for it in my following recordings.


I am now trying to load the meter with a set of incandescent lamps controlled by a three-phase thyristor bridge that is, in its turn. controlled by a function generator so I can have rapid cycling and really nasty waveforms. Trying to provoke bad behaviour.

My first test will be to check if there really is a fast 'attack' and a slow 'release' when power is cycled. If there is a linear filter involved, I think that cycling should have no effect on the accumulated energy. If the filter behaves differently when switching power on and switching it off, I think that there could be a net error.


Thanks again. Six eyes are a lot better than two.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

What is the actual measurement device in the meter?  Is it a uC or an application specific device that does-it-all?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Energy meter with fast cycling load

(OP)
We are not supposed to know. There's a seal on the meter and I do not want to break it.
These meters are made in millions, so I guess that there's a lot of application specific stuff inside.

Good thing: Got my thyristors working, really nasty waveforms and cycling at several Hz. My lab has turned to a disco.

Now I have a 'slight' Excel problem; how to measure distance between the metering pulses. Each pulse is around 60 rows in the sheet. I have done edge detection and have a resulting '1' for each positive edge. Now looking for a way to measure distance (seconds) between the edges. Never done that before. Guess I can use row number when I find a cell with '1' in it and subtract row number of previous cell with a '1'. Not what I usually do.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Sounds like the excel part would be easy - let us know here or in excel forum if you want ideas on that.

I don't work with meters, but the whole thing is mysterious to me. I would have thought a kw-hr meter would simply accumulate continuously using simple stepwise integration of instantenous power (V*I) to get energy W:

Wk+1 = Wk +  (Ik+1 * Vk+1)*DeltaT ....  where DeltaT is time between samples.

Apparently it is different than that because there is something called a "metering pulse".  What is the purpose of a metering pulse? ... When  pulse occur the power is computed (based on a half cycle or cycle interval), and used as representative of the entire interval between pulses?

In either case, I'm not sure what would be the purpose of a filter.  I guess I can see the 2nd algorithm (metering pulse) where we look at a short shapshot 1-cycle or so would be susceptible to small deviations in power frequency... if the samples do not occur at exactly submultiples of power frequency period than a snapshot 10 or 20 samples would have some error based on looking at an interval slightly different than a full or half cycle.... and it power frequency was consistently different in same direction that error would not be random.  Might need something more complicated to deal with that which I suppose could include a filter. But nothing here to do with cycling loads.

Do you know yet the approximate time between pulses?

What is the magnitude of the error that is seen?

=====================================
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RE: Energy meter with fast cycling load

(OP)
The metering pulse is used to transmit energy data to a remote meter reading system like Turtle or some other PLC (Power Line Communication) system. Or to a radio communicatiob channel.

Metering pulses are short (20 - 40 ms) pulses that corresond to a certain amount of energy. In this case 1000 pulses correspond to 1 kWh. Distance between pulses can be seen from attached recording. It is from the same measurement as the ones shown before. But this is taken over longer time. The metering pulses are on channel 6 (blue). Currents in the three phases are shown in channels 1, 3 and 5 as before.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Thanks for explaining that.... makes sense now, although I still don't see the need for any filtering.  Another observation I'm sure you noticed, it appears there is a one or two second delay in generating the pulses.  Not a big deal for the application but just complicates your analysis task slightly.

=====================================
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RE: Energy meter with fast cycling load

(OP)
That's the problem. I don't see any need for it either. And still, it looks like there is something that influences the pulse output. And, if the load is cycled, it looks like it affects the accumulated energy. That is why I am doing a test with faster cycling.

The viewer program is available at http://www.gke.org/test/ and I will put the original file on http://www.gke.org/pub for download. Data files with faster cycling will also be uploaded soon.

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

The following user-defined function may help.  Can find the row number of a 1 anywhere within specified range.  

= FindOne(3,A1:A5,False)  would find the 3rd occurence of the number 1 from the bottom in range A1:A5

CODE

Function FindOne(Occurence As Integer, myrange As Range, SearchDown As Boolean) As Integer
' Finds the Occurence_th occurence of the number 1 within range,
' (For example if Occurence = 1, it finds the first occurence, if Occurence=2, if finds 2nd)
' it searches downwards from top if SearchDown is true
' it searches upwards from bototm if SearchDown is false


' Note you can use the built-in match function if you simply want to search down for first occurence
' Assumes the input range is a single-column range... no error checking is done

Dim rowcounter As Integer ' count rows of range
Dim foundcounter As Integer ' count number of 1's found
Dim thisrow As Integer ' identifies row number for the loop
Dim thiscell As Range ' identifies cell for the loop

    FindOne = 0   ' Error flag - if function returns 0 there are not enough 1's in the range
    foundcounter = 0 ' Initialize

    For rowcounter = 0 To myrange.Rows.Count - 1
        ' Set thisrow according to User Preference in SearchDown
        If SearchDown = True Then
            thisrow = rowcounter
        Else
            thisrow = myrange.Rows.Count - rowcounter
        End If

        ' Identify the cell of interest
        Set thiscell = myrange.Offset(thisrow, 0).Resize(1, 1)

        ' Increment foundcounter if cell contains 1
        If thiscell.Value = 1 Then
            foundcounter = foundcounter + 1
        End If

        ' If foundcounter matches target, output results and terminate
        If foundcounter = Occurence Then
            FindOne = thiscell.Row
            Exit For
        End If
    Next rowcounter
End Function
 

=====================================
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RE: Energy meter with fast cycling load

(OP)
Thanks a lot, Pete!

I had a feeling you would jump in and help. Much appreciated. Only, I have never done user defined functions. Something new to learn!  smile

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Glad if I can help to return the favor for ideas about checking motor rotation among other things.

Here's how to use the function (at least in my excel 2003)

In the workbook of interest:

Select Tools / Macro / VisualBasicEditor

Select Insert / Module

That should open a subwindow which says "General" near the top.
Paste the entire code listed in my post above into that subwindow

Perhaps double check on left side that the selected module (module 1 probably) is in a folder named modules which appears underneath something called 'VBA Project ("MyFile.xls")'.... where MyFile.xls is the name of your spreadsheet.  This just verifies you are working in the right place, rather than inserting into your personal.xls or some other vba window.

Select File/CloseAndReturnToMicrosoftExcel
 

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Energy meter with fast cycling load

There is one small error - as written, the code does not see a "1" if it is located in the very first row of the data, and the search is upwards. To correct this:
"thisrow = myrange.Rows.Count - rowcounter"
needs to be changed to
"thisrow = myrange.Rows.Count - rowcounter - 1"

Attached is a spreadsheet I used to test the corrected function.  Sorry for the error.
 

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Energy meter with fast cycling load

I was thinking about that 1-2 second delay that is apparent.  It seems unusual... surely there is no intentional delay.   Makes me thing maybe there is a microprocessor busy taking care of some housekeeping details required to shift from monitoring mode to pulse transmission mode, handling communications, switching back to monitoring mode and somewhere along the line reset the integrator=accumulator back to zero watts.   One wouldn't think that would take a long time, but there has got to be some reason for that delay, most likely during the communication part.

So let's say during that busy period after reaching to limit trigger, the microprocessor is in fact busy... so busy that it cannot continue to monitor the incoming power to update the totals.

One presumes that the unit tries to compensate by adding an amount of energy based on elapsed time (I presume there is a clock) as well as last known power level.    That would be an accurate compensation provided there is not a transition (off-to-on or on-to-off) occuring during the critical period while last-know power is used as a substitue for actual power.  

At first glance, that would seem like a random error, since the probability of hitting off-to-on is same as hitting on-to-off, isn't it?  No, actually not.    An off-to-on transition can only occur when the heater was initially off, and an on-to-off transition can only occur when the heater was initially on.    Considering the duty cycle and difference in power levels, the majority of pulses are sent (or at least the accumulator limits reached) during the on-state.    So if we monitor for 1 day, we might have 1000 pulses sent from on-state and 100 pulses sent from off-state (just numbers for talking purposes).    Since duration of off state and on-state are roughly the same, the probability of hitting a transition during that time interval is the same for both types.... let's say 10%.     So 10% of the 1000 pulses while on experience an on-to-off transition during the critical time period and 10% of the 100 pulses while off experience an off-to-on transition during the critical time period.  That is a lot more on-to-off type errors during a critical transition period than off-to-on type errors during a critical transition period.... ends up overcharging.

Or, maybe not winky smile
 

=====================================
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RE: Energy meter with fast cycling load

(OP)
Very good. That is something I am beginning to think also. I have done tests with very fast cycling and the apparent energy/pulse is dropping from nominal 3600 Ws/pulse to around 2400 Ws/pulse when cycling with 500 ms period and 20 % duty cycle. Resulting in a 50 % extra charge for the energy.

I admit that this is an abuse of the meters. But the intent of this exercise is to provoke bad behaviour - and I think we have just that. The customer's load also cycles, but with a lower rate and over months and years. Customer says that he thinks he has payed around 15 % too much, but data from parallel meter says that it could also be more.

I attach Excel sheets (sorry, I am in a hurry so I will use your code when not so pressed for time). I have reduced row heights so you can only see the first three - four rows, and after that only the rows where pulses are output.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

(OP)
Should be 'paid', I think?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

At 300 watts continuous,  you would have 1 pulse/12 seconds.
At 2900 watts continuous, you would have 1 pulse/1.24 seconds.

It says 500 sec cycle on top, but below it says 2hz.  I assume it is 500 millisecond cycle (?).
Than for 50% duty cycle, you have only .25 sec in each state.....so you have many heater on-off cycles between each pulse.

If I stick with the "theory" that I had suggested above, it would require that every single one of the pulse "triggers" (when integrator reaches the limit) occurred during on states (otherwise if pulse trigger occurred during offstate, we would have had a high watt-second reading).

For 50% duty cycle that is plausible.  There is 2900/300 ~ 10x as much probability that each pulse trigger is reached during on state compared to off state.

For 20% duty cycle, it is not as much.  4x as much time in off state as on state.  Only 2.5x as much probability that each pulse trigger is reached during on state compared to off state.  I would have thought at least one of those 7 pulses during the 20% duty cycle would have shown a total which was at or above the 3600.  So it does not seem very consistent with my theory from that standpoint.    Given that there is some unknown delay in the neighborhood 1-2 seconds, there is no hope of going back to the data to figure out the condition when the trigger occurred.

Another thing to note: the average power is 1600 watts in 50% duty cycle and 820 watts in 20% duty cycle.   If the trigger occurred during on state, the difference between "assumed" (under my theory) and average power would be 2900-1600 = 1300 watts at 50% duty cycle or 2900-820= 2080 watts at 20% duty cycle.   Assuming the time for which the error occurs is the same, the ratio of errors for the 20 to 50 duty cycle should be 2080 to 1300... the actual ratio would be higher.

It would be interesting to see results if the cycle time were much slower such that you get let's say average of two or three pulse triggers in an on period and one pulse trigger in an off period.   Maybe 3 seconds on, 12 seconds off.   Then if timing of pulse relative to transition (or to state at time of pulse) is important, you should get some normal pulses (3600 watt seconds) and some pulses with lower watt seconds, and perhaps pulses with higher higher watts seconds.  Focusing on the timing characteristics which are unique to the abnormal pulses and what preceded them might help define the mechanism better.  Just a thought.

Of course that experiment probably means data collection over longer period of time which could become difficult to manage.   And whether it's worth the effort depends in part on judgement of the likelihood of uncovering useful data.  And I'm not sure if your goal is to recreate the behavior and that is all that is needed, or to how much additional effort would be warranted to understand the mechanism.   
 

=====================================
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RE: Energy meter with fast cycling load

(OP)
Working on it. I am rechecking all calibration now.

I can handle 60 second's worth of data. And there is no problem recreating a longer cycle - like the one we saw at the customer's place. Stand by! I am still confused. Want to make 100 % sure that this isn't a red herring.  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

(OP)
Yes, of course, 500 milliseconds.

I have checked the data acquisition and accumulation in my measuring device. It delivers very consistent data when I run without switching. All pulses are then worth between 3498 and 3505 Ws. Instantaneous power is in line with expectations and also with an electrodynamic wattmeter with class 0.2, so I feel confident about the measurements. What I think could be a problem is if the sampling interval isn't correctly specified. Then, the accumulated energy could be off with a few percent - which it perhaps is. Or, since the utility guys say that the meters can have errors up to 5 percent (which is hard to believe), it could also be that I am spot on - or close.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

Congratulations on the Tip Master award, E-Pete.
Yours
Bill  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Energy meter with fast cycling load

Thanks. I am in good company.

I think stretching out the duration a little more may make the pattern clearer.

=====================================
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RE: Energy meter with fast cycling load

(OP)
I made a 'macro' recording with all voltages and currents removed and over 60 seconds. Duty cycle is still 50 % but period is 10 seconds. The delayed reaction is clearly visible and I think that the meter is favoring short pulse duration, also when level is low.

Actually, that is what we also saw on customer's site and what we have noticed in other recordings.

Do you know anything useful about sampling theory? Does this have anything with the retired railway man and the trains, that Bill mentioned in his 26 Feb 10 2:22 post, to do?

BTW, I am not so sure about the exact power levels in previous recordings. But, in this one, the levels are 180 W and 2700 W.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

(OP)
Finally! I see what the problem is. When those meters (I now have three of them) are feeding a load that is dimmed (using thyristors), I get very high watt-hours. The excerpt from the report may be of interest.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

So, the most recent data is similar to the earlier data, except that now the cycling time is very fast (dimming), correct?

What is the new insight that it gives?

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RE: Energy meter with fast cycling load

(OP)
I have no real insight. But I think (I may have to eat this) that the algorithm somehow makes a mistake. It seems that the manufacturer looks for the negative zero crossing and then assumes that conduction angle always is 180 degrees.

That strategy works in most circumstances but not when conduction angle is small. We will probably never know. The whole thing is now subject to discussions on a national level. Big money involved. And probably lawsuits.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Energy meter with fast cycling load

OK, I understand now... the finding that it overcharges for dimmers helps define the scope of the problem (how many customers might be affected), but not necessarily the mechanism.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Energy meter with fast cycling load

... although it's another clue in the puzzle.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Energy meter with fast cycling load

Clearly the meters aren't doing what I'd expect for a revenue grade watt-meter.  That's measuring the instantaneous current and voltage and multiplying them either continually or a thousand times a second.  Instead they're using some Mickey Mouse zero crossing detection multiplied by assumptions.

No-doubt they got the bid to supply the bazillion meters because their POS was much cheaper than meters that actually work.

Nice work Gunnar.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Energy meter with fast cycling load

I don't know how the Euro standards deal with this, but ANSI tests all the way to + or - 2% rated frequency. I'm sure most manufacturers do better, but until there are some appropriate standards we cannot expect very much.

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