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ericw
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« on: December 31, 2011, 03:47:05 PM » |
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The circuit and picture is of a very simple indicator which shows whether you are consuming or supplying power to the grid.
A couple of circuit variants are shown, a battery powered one and a mains powered one.
Only one current transformer clamped around one of the supply meter tails is needed. This avoids any problems of adding a CT around the PV output which may have been be wired directly into the consumer unit.
In order to determine whether you are supplying or consuming power the phase of the current is compared with the mains voltage, on the battery powered version it is obtained by wrapping a few turns of wire around the live mains feed, in the mains powered version it can be obtained from the transformer.
A PIC12F683 is used to measure the current at a time determined by the positive going zero crossing of the voltage signal and converts it into a PWM signal to drive the analog meter. This is setup so no input gives a 50% mark space ratio and hence the centre zero on the meter.
Component cost is around £15
While such a crude display probably has little practical value, it is the "proof of concept" of using a single CT to measure bidirectional power.
The same principle can be used to detect when to switch a load on only when enough surplus power is being fed to the grid.
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StBarnabas
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« Reply #1 on: December 31, 2011, 04:29:57 PM » |
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Very nice Eric. Would it be worth using a PIC with higher pin count? you could then add an LCD display very easily I think?
Sean
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 Gestis Censere. 40x47mm DHW with TDC3. 3kW ASHP, 9kW GSHP, 3kW Navitron PV with Platinum 3100S GTI, 6.5kW WBS, 5 chickens. FMY 2009. |
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Richard Owen
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« Reply #2 on: December 31, 2011, 04:30:21 PM » |
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EricW
Hero member indeed.
I do miss the applaud button.
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44 Yingli 230Wp panels feeding into 2x Solar Edge SE5000 inverters .20x 58mm SE, 20x 58mm SW, Solar Thermal feeding 320l thermal store. 10kW heat pump. 300W of Hydro Power . |
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Pat_
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« Reply #3 on: December 31, 2011, 06:47:17 PM » |
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Nice job, Eric.
I might even consider making a simple one of those just to indicate whether I'm supplying or drawing from the grid. We have great plans for something more comprehensive, but that may take a little longer...
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Davo
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« Reply #4 on: December 31, 2011, 10:18:41 PM » |
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Very clever and useful concept.
I only have a very basic knowledge of electronics and zero experience of PICs - so haven't really got a clue how your diagram/circuit functions.
Am I correct in assuming you are measuring and comparing the difference in phase between the voltage and current ?
If that is the case, if the house load is greater than the PV will the current be lagging the voltage ?
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« Last Edit: December 31, 2011, 10:31:07 PM by Davo »
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20 x BP 190W Mono (3.8kWp), SMA Sunny Boy SB4000. SE facing.
Grantham, Lincs, UK
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jotec
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« Reply #6 on: January 01, 2012, 11:28:09 AM » |
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As I have very basic knowledge of electronics (and non of PIC chips) but would like to have a go is there any chance of an idiots guide and parts list to making one of these?
Dick
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Aiming to reduce dependency on 'mains energy'. Own bio for 25000 miles, solar water heating (DIY), CHP done jotec.co.uk for info
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pj
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« Reply #7 on: January 01, 2012, 12:25:38 PM » |
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Eric,
Nice simple circuit, looks good. Can I ask, how does it differentiate between the power factor of the equipment in use (fluorescent tubes versus vacuum cleaners for example), and the phase changes due to import/export? Or are these of different magnitude?
regards PJ
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StBarnabas
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« Reply #8 on: January 01, 2012, 02:01:33 PM » |
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PJ I can answer for Eric here having worked with him in the past regarding the HalfBee project. The power monitor samples the voltage and current waveforms 256 times per 50Hz cycle, multiplies them, accumulates and then calculates the true RMS power. This not only takes account of power factor, but any higher harmonics up to the 128th. It is really very neat and much better than in most commercial systems. Iain Driving bar graph modules would be very simple to implement. I would use something like the 16F1827 which has enough pins to do this directly. A separate LED driver would not be needed. http://uk.rs-online.com/web/p/microcontroller/6988959/Dick the most difficult thing would be programming the PICs, but if interested I could program one for you Sean |
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radiogareth
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« Reply #9 on: January 01, 2012, 02:12:16 PM » |
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I like properly thought out minimalist circuits and with a junk box full of old meters I could re-use one.
Is the hex code available or a pre-programmed chip?
TIA
Gareth
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pj
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« Reply #10 on: January 01, 2012, 04:21:46 PM » |
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Sean,
Thanks for the explanation, that does describe a great method for determing true power. But, I think I must be missing something here. This device measures and indicates phase difference between current and voltage. This phase difference is normally caused by the power factor of the connected load when there is no PV. Added to this is the phase difference introduced by the PV/Invertor. My question is how does this circuit discriminate between the two phase effects?
Please forgive me if I'm being dumb!
regards Jonathan
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Pat_
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« Reply #11 on: January 01, 2012, 04:34:58 PM » |
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I'm about to do what I know from experience I should never do, which is reply to a forum post without knowing anything about the subject.
Surely grid-connected PV inverters must very closely match the phase of the supply voltage they see, and therefore cause almost no phase shift themselves. What remains is a phase shift caused by loads. The only possible assumption here is that this doesn't exceed 90°. If it did then you couldn't distinguish import versus export. But power factor can only be caused by a capacitive or inductive load and therefore has a max of 90°.
OK. Ready to learn something new...
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StBarnabas
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« Reply #12 on: January 01, 2012, 07:19:05 PM » |
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Pat
Yes there is a PLL which matches the supply voltage. However the current flow will be changed if the house is exporting. It will flow in the opposite direction giving a phase shift of 180 degrees.
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Gestis Censere. 40x47mm DHW with TDC3. 3kW ASHP, 9kW GSHP, 3kW Navitron PV with Platinum 3100S GTI, 6.5kW WBS, 5 chickens. FMY 2009. |
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pj
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« Reply #13 on: January 01, 2012, 07:47:43 PM » |
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Pat
Yes there is a PLL which matches the supply voltage. However the current flow will be changed if the house is exporting. It will flow in the opposite direction giving a phase shift of 180 degrees.
Doh! Good point - that's definitely a change in phase. |
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wookey
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« Reply #14 on: January 01, 2012, 09:36:12 PM » |
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Any chance of some code eric? A schematic with a PIC in the middle is a very hollow thing (and increasingly what all modern circuits look like)
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Wookey
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