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Author Topic: Accuracy of CT Clamps/Monitors  (Read 1448 times)
Mike123
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« on: April 15, 2018, 05:48:15 PM »

Hi

I have an Owl energy monitor (I have 2 actually, and they both read the same), plus an Apollo Gem immersion diverter, and current clamp meter/multimeter and 2 x Growatt SP2000's

All these items, including the Gem, seem to agree with an import/export of at least 3kw - they all read about the same

However when the Apollo Gem says, for example, I'm at 0 import/export, the Owls, current clamp and Growatts all say I'm still importing 1.5kw. This is also the case say, if I'm exporting 1.5kw (according to the Owl/Clamp/Growatts), the Gem will say I'm at 0 import/export

Of course if I am still importing 1.5kw, and the Gem says I'm at 0, then I am using paid-for electric to heat the Towel rails

So is the Gem reading wrong, I can't see that all the other clamps would all read wrong? Anyone have any experience with these/this?

Thanks
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Tinbum
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« Reply #1 on: April 15, 2018, 06:39:02 PM »

What does the DNO meter show?
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Mike123
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« Reply #2 on: April 15, 2018, 06:51:57 PM »

Unfortunately it isn't a spinning meter - I wish it were

But thank you for reminding me - I'd forgotten this bit...

About a week or so ago, we had good sun for a few hours, and I took a reading off the meter. I re-read the meter 3 hours later and the reading hadn't changed at all (it only shows integer reading, no 1/10, or 1/100 of kwh). I also watched the LED for a couple of minutes and there was no flashing at all. I was estimating that if I were importing about 1.5kw I should see it flash approx once every 30 seconds??

So that made me trust the Gem, but that would mean all the other items are all wrong, yet all agree with each other, which seems unbelieveable

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sam_cat
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« Reply #3 on: April 16, 2018, 08:30:31 AM »

Clamp meters have a working range, and are (generally) more accurate away from 0..
Having looking at the documentation for a few over the years it feels like they are most accurate between 15% and 100% of their rating.. With accuracy falling off as you get closer to zero.
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TheFairway
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« Reply #4 on: April 16, 2018, 08:55:57 AM »

I cannot remember specifics as it was a long time ago, but before I had solar I had and Owl type device - EcoEye iirc. When I got Solar, it behaved oddly so I contacted the manufacturer who explained that it would be getting confused with I think the phase shift or something that an inverter would introduce - it all made perfect sense at the time and my system was behaving exactly as the manufacturer had described it would. So I changed to a Solar friendly energy monitor and the problem went away. However, any readings under a couple of 100kW I take as a purely indicative value, especially since CT clamps are current based and my displays are in kWh which requires the voltage which fluctuates depending on generation et. al.

Worth also double checking the direction of the clamps. iirc one of my clamps also had to go on neutral rather than live.

[edited for sake of confusion]100kW to 100W
« Last Edit: April 16, 2018, 02:06:12 PM by TheFairway » Logged

-Ian

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All posts are my own personal thoughts and opinions and do not represent those of my former employer, clients or partners.

marshman
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« Reply #5 on: April 16, 2018, 09:03:25 AM »

Hi

I have an Owl energy monitor (I have 2 actually, and they both read the same), plus an Apollo Gem immersion diverter, and current clamp meter/multimeter and 2 x Growatt SP2000's

All these items, including the Gem, seem to agree with an import/export of at least 3kw - they all read about the same

However when the Apollo Gem says, for example, I'm at 0 import/export, the Owls, current clamp and Growatts all say I'm still importing 1.5kw. This is also the case say, if I'm exporting 1.5kw (according to the Owl/Clamp/Growatts), the Gem will say I'm at 0 import/export

Of course if I am still importing 1.5kw, and the Gem says I'm at 0, then I am using paid-for electric to heat the Towel rails

So is the Gem reading wrong, I can't see that all the other clamps would all read wrong? Anyone have any experience with these/this?

Thanks

I have an Apollo Gem and a "freebie" British Gas energy monitor. The Gem CT is on the incoming supply as it should be and I use the British Gas one to monitor the output from my PV. As a quick check to answer your question I have just moved the CT for the British Gas unit to be on the incoming supply. next to the Apollo Gem CT. Nice steady bright sunshine here (for a change) and PV generating around 2kW as we speak (confirmed by the reading on the inverter and the British Gas monitor). After moving the CT the Gem showing 0 export and around 1500W to the immersion. British gas monitor is all over the place, changing between about 600W up to a max of 2.1kW.  

I suspect that the "dirty" switching of the Apollo Gem as it "modulates" it's output to the load upsets the British gas monitor. I still have an incandescent lamp in the cupboard where the consumer unit is and this flickers when the Gem is diverting. I also suspect that yours all works OK at 3kW and above as the Gem is then switched hard on and not modulating the power to the immersion or towel rail) sending nasty switching spikes down the incoming supply.

In short I think that when the Gem is diverting it's reading is the one to follow. I wonder if you can apply some filtering to the outputs of the the other CT's to make them read correctly?

Roger

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Philip R
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« Reply #6 on: April 16, 2018, 09:03:48 AM »

Mike,
You have described the 1.5kW offset shown on the Gem.
As a short term rearrangement, could you reverse the Gem current sensing clamp on the cable. This should change the sign of the power flow from forward to reverse or vice versa. Note the readings. Does the 1.5kw change to a 0 or 3kW error?.
Trying to ascertain if the fault is in the current clamp or the instrument.
Is the clamp a hall effect device or a proper wound current transformer. I would wager that if HE type device, has an incorrect bias offset, and should be changed.
Ps do not want to run your immersion on mains too long! Could you load up your system with say some conventional incandescent lamps, or electric cooker ring and note the respective readings of your instruments.
Philip R
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Countrypaul
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« Reply #7 on: April 16, 2018, 09:08:57 AM »

... However, any readings under a couple of 100kW I take as a purely indicative value, especially since CT clamps are current based and my displays are in kWh which requires the voltage which fluctuates depending on generation et. al.

"under a couple of 100kW" what size supply do you have? Grin  I presume you mean "under a couple of 100W"

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marshman
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« Reply #8 on: April 16, 2018, 09:11:36 AM »



Worth also double checking the direction of the clamps. iirc one of my clamps also had to go on neutral rather than live.

Just checked mine again - The British Gas monitor gives the same "incorrect" readings no matter which way round the CT is or whether it is on Live or Neutral.  As I said I am sure it is the fact that when diverting the GEM is introducing switching harmonics etc onto the line and this upsets/confuses the other monitors.


Roger
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TheFairway
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« Reply #9 on: April 16, 2018, 09:23:42 AM »

... However, any readings under a couple of 100kW I take as a purely indicative value, especially since CT clamps are current based and my displays are in kWh which requires the voltage which fluctuates depending on generation et. al.

"under a couple of 100kW" what size supply do you have? Grin  I presume you mean "under a couple of 100W"


facepalm
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-Ian

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JohnS
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« Reply #10 on: April 16, 2018, 10:37:58 AM »

... However, any readings under a couple of 100kW I take as a purely indicative value, especially since CT clamps are current based and my displays are in kWh which requires the voltage which fluctuates depending on generation et. al.

"under a couple of 100kW" what size supply do you have? Grin  I presume you mean "under a couple of 100W"


facepalm

Could you please re-edit your previous post to include 'edited to from 100kW to 100W'.
Otherwise the correcting posts don't make sense to other readers.
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Mike123
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« Reply #11 on: April 16, 2018, 11:39:52 AM »

Thanks to all!

Okay that all makes a lot of sense

I did change the Gem to the Neutral over the weekend - it doesn't make any difference to the reading of the Gem. Obviously I reversed the direction of the clamp. I did actually initially forget to reverse the clamp (doh) and it correctly misread the import as export

So this leaves with me the pressing issue - the Growatt SP2000's are all reading incorrectly, just like the Owl. That isn't good. I know there are others here who have SP2000's and have immersion diverters, so how are theirs working? My Gem is merely heating a few towel rails, not an immersion

I suppose the next step is to switch off the Gem when we get some good sun, and then see how all the other clamp then read, that would prove whether the Gem is introducing interference

To enlarge upon the setup, I have a DNO meter outside the house, in a black box. In there I have the Owl, and the SP2000 clamps. The supply then goes into a wall, up to the loft, and then down into a large CU in an upstairs cupboard. The Gem clamp is there and also a Clamp (and electronics I built) connected to a computer. Then there's a Henley block and the supply then splits to 2 x CUs

I'm hoping for some sun today so will take more readings with the Clamp Meter, with and without the Gem turned on
« Last Edit: April 16, 2018, 11:41:43 AM by Mike123 » Logged
chris_n
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« Reply #12 on: April 16, 2018, 11:43:03 AM »

Power is shunted back and forth to the meter to stop it indexing. It doesn't index until a certain amount has been used, I have seen it described as a bucket, you use the bucket until it is nearly empty then use your generated power to fill it back up. None of the simple CT type meters work with diverters.
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« Reply #13 on: April 16, 2018, 12:53:10 PM »

Mike123,

You’ll be aware of my Growatt charging issues both from forum postings and our direct PM’s. I have wondered if the Immersun was ‘interfering’ in some way.

I just need to be at home for one day of complete sun. Yesterday was a good example of another opportunity missed.

Regards

Richard
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Mike123
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« Reply #14 on: April 16, 2018, 03:56:10 PM »

I have turned the Gem off, and have been monitoring the Owl via the Owl USB -> PC thingy

The sun has been in and out so a lot of export, then import, then export, etc

At no point does the Owl ever read below 1kw on the graph. This seems to prove that it is desperately inaccurate at and below 1kw. I use a lot of electric so to me 1kw is not a lot, but I know most people would see that as high useage, so if these electric monitors are so inaccurate why isn't everyone complaining about them?

I have today ordered a monitor that uses the LED pulses from DNO meter, so I can use that to 100% prove what's what
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