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Ideas - Detect when feeding back to grid


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Hi all

I have an Imeon now, which can feed back to the grid. Since I had to get extra panels to generate the VDC, I now generate nearly 1.5kW more than ness.

How can I detect when there is power feeding back to the Grid to be able to turn a relay/switch on/off? Eg: to turn on a pump to move water from lower tank to high tank...

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 I don't know the imeon and how clever it is

I asked almost the same question a while back as I have the axpert /infini v series which can also grid tie .

but I run my set up in self consumption mode  to keep things simple, this way means my bats are not used or cycled

a cheap idea is to place the item on a timer  and when you know that there is excess then set the timer to switch (self consumption or normal solar  ) not fool proof though

another Heath  Robinson is to keep the grid tie on , but switch the inverter off with a timer  and run the system off batteries during the day and get the timer  to switch back on  at a preset time knowing you now have enough excess


someone mentioned maybe trying   a  single phase reverse power monitor http://www.rhomberg.com.au/downloads/SP510.PDF relay  which I liked the idea

the second part of the problem is that   you may need to place some sort of timer in the system to stop it reacting too quick ie   the relay has to be on for say 30 seconds before it will switch the dump load on

third part of problem is how much are you exporting . pointless to switch a 1000w dump load if you only exporting 100w


 can you not look at watchpower or similar program and use that can do what you need .  I wish I could


also look at my posts and read the one about bi directional current  sensing,   there were some other ideas there that you might like

please keep me updated




Edited by maxomill
because i can
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8 hours ago, maxomill said:

someone mentioned maybe trying   a  single phase reverse power monitor http://www.rhomberg.com.au/downloads/SP510.PDF relay  which I liked the idea

If I recall, the issue with that relay was it's either too slow or not sensitive enough for meters like the BEC23/44.

Edit: But of course it might be perfect for switching a pump on to do some work!

Edited by plonkster
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This is @plonkster theory

from my old post

I've thought of this before. The context was a little different, it was specifically for people like @Wetkit with the tripping prepaid meter, the idea being that you turn on an array of loads (a bunch of 100W incandescent lamps seems like a good idea) to cancel out backfed power. In this application it has to be extremely fast. The effect, when used with a slowly adjusting inverter, is that occasionally some lights go on for a few seconds to absorb the energy.

First though, have to ask this one question. Can your V-series avoid feedback (even if by a slowly adjusting feedback loop), and if it can, will your prepaid meter tolerate it even if it does charge you a small amount for it? If the meter will tolerate it, the simply truth is that whatever you pay for those few seconds in the day when it feeds back a little will cost way less than anything you can buy or build to try and avoid it. Just live with it... or add a time switch to turn on some loads.

With that said, the idea I had in the back of my mind is this, to adapt the Open Energy Monitor project for this. What that project does is literally sample the voltage and current waveforms several hundred times per second, and then calculate the power and the power factor.

Now my thinking is that if it samples fast enough, it should be able to detect whether the current is in phase (mostly) or out of phase (mostly) with each other, and summing the vector product of these readings will tell you which way the power is flowing.

The default setup of the arduino sets the ADC clock to 125khz, which allows it to read around 9000 samples per second. You can also mess with the prescaling registers and things yourself and increase this to significantly more.

The example code in that project takes about 2000 samples, or 10 full AC cycles, taking into account zero-crossings, before it starts calculating. So in theory you can have a reading every 0.2 seconds... though I suspect in practice it is going to take a bit longer.

I had another idea, which is more analog. With this idea, you use a transformer to derive an AC voltage that is within some range that you can measure. Then connect a current transformer (that's placed around the conductor you want to sense) in "series" with this AC voltage source, together with a burden resistor change current into a voltage. The idea is that when your current is in phase with the voltage, the CT adds to the voltage, and if it is out of phase, it subtracts from that voltage. Now, you simply need to measure the difference between these points, average it over a few cycles, and that will give you an idea. If power is flowing in one direction, the CT will add to the voltage and you'll get more positive readings than negative ones, if the power goes the other way the CT will subtract from the voltage and you'll get a more negative values. You have to sum this over a number of cycles because an inductive load factor (for example) will always push back a little on each cycle, but you're interested in the overall value for the whole cycle. The prepaid meter (by the way) does exactly the same thing.

You could also just get one of those Rhomberg reverse detection relays from ACDC, though I'm told they aren't very sensitive.


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Bear with me for a moment  , so while surfing I came across this

Optical Utility Meter LED Pulse Sensor


£18.97 (inc VAT) £15.81 (exc VAT)
50.00 Grams
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Optical pulse sensor for detecting LCD pulses from Utility Meters. 

The sensor can be easily stuck on to the front surface of any utility meter with an LED pulse output. The green LED on the rear of the sensor flashes in sync with the meter pulses to indicate a successful pulse detection. 

The Optical Pulse Sensor works by sensing a utility meter's pulsed LED output. Each pulse corresponds to a certain amount of energy passing through the meter. The amount of energy each pulse corresponds to depends on the meter. By counting these pulses the meters KWh value can be calculated.

Unlike clip-on CT based monitoring, pulse counting is measuring exactly what the utility meter is measuring i.e. what you get billed for. The pulse counting cannot provide an instantaneous power reading like CT based monitoring can. Where possible, we recommend using pulse counting in conjunction with CT monitoring. The emonPi and emonTx can simultaneously perform pulse counting and CT based monitoring.

In the case of an electricity meter, a pulse output corresponds to a certain amount of energy passing through the meter (kWh/Wh). For single-phase domestic electricity meters (eg. Elster A100c) each pulse usually corresponds to 1 Wh (1000 pulses per kWh). For water and gas meters they will usually be marked to show the quantity of water (litres/gallons) or of gas (cubic meters/cubic feet) that each pulse represents.


Compatible with emonPi (all versions) and emonTx V3.4 (firmware V1.8 and above, July 2015 onward)

With the removal of the RJ45 connector or use of RJ45 breakout and manual terminal block wiring the sensor will also work with the emonTH (firmware V1.6 and above) see emonTH wiki and emonTx V3.2 (firmware V2.0 or above) see emonTx V3.2 Wiki.



  • Pulse Output: TTL synchronised with the LED light pulses of energy meter.
  • Sensing Spectrum: IR to Light
  • Indicator: LED light at the top which synchronises with the pulses
  • Suitable for: all types of utility meters with pulse output: single-phase, multi-phase meters, water gas and electricity
  • Sticky attachment 
  • IP56 Waterproof (sensor unit, not connector) 
  • Power supply: 3.3V - 5V 
  • Diameter: 22mm 
  • RJ45 Connector
  • 1m Cable


  • Current Consumption - no Pulse (sensor in darkness): 0mA
  • Current Consumption - Pulse: 5.3mA for duration of pulse (standard meter 100ms)
  • Output: Positive pulses output in TTL, is synchronized with the LED light pulses.

RJ45 Connector Pin-out:

  • RJ45 Pin 2 - VCC
  • RJ45 Pin 5 - GND
  • RJ45 Pin 6 - TTL Data
The cable can be extended using RJ45 Extender and standard RJ45 cable (not cross-over).
 idea for GTI   export limiting
So now that that has sank in  the bottom line is that the more power a house uses the faster that the led blinks  so what if we place a counter to count the pulses (  the greater the power from gti the less power drawn from grid ) and at a set count then a contactor  is somehow switched  to switch on a lets call it a dump load
 this could probably be done using a arduino thingy  or now the rhomburg could be triggered
thanks @Chris Hobson for the interesting site



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Ok so after a bit of thinking

my last post may work but quite high tech ,    I think I may have a KISS  keep it super simple  aka Heath Robinson

what happens if we monitor the power to the house 

   SCENARIO  we are using  grid power and as the gti starts feeding power the grid power goes down  (gets less)

now check this out  a LOAD SHED RELAY http://mce.co.za/product/electronic-load-shed-relay/     These guys also do them but min 5a http://www.electrodev.co.za/product/load-shed-relay-dls/

so now we place a load shed relay    on the incomer to  your house

when  grid power  reaches down to  say 1or 2 amp just before we start exporting  the load shed relay  allows a dump load  to be switched



since the LSR is only capable of taking 32A and  the screw connectors will not accept a general 16mm incomer,  you will have to split the incomer into say     2 or 3    10mm wires of equal  length   in parallel which is a slep   BUT  offers you the opportunity to place a LSR  on each wire . what this means is that now you can have a number of Dump loads that switch on proportionally  to the amount of power you need to dump and your systems outputs  .  so for example at 3a dump load 1(small) comes on and at 1 amp dump load 2(big) comes on , the skies the limit.


you may also have to install a timer on each dump load  so at night when drawing less power nothing comes on (possibly a bypass switch as well for convenience )


The LSR should also be installed after your main switch ( usually stops those ugly sparks , burnt fingers and arc eyes while installing)


the thing that does worry me is timing       when the dump load come on then  all of a sardine the grid power goes up  and the dump load may be switched  off but the LSR does have 0 to 120 delay setting which should give the gti enough time to catch up or it could just flip flop until the gti has enough sun power to keep the dump load on


there are probably other kinks that need to be ironed out  so lets chat

please shoot me down if  and where you can so that we can improve on  it and  patent this the "Power forum limiter"

 I used to have one connected in my old system which only allowed the pool pump to be on when the inverter (house)was drawing less than 1400 w  so it was impossible to overload the inverter if say the hair drier/dishwasher was on . I believe it was made for low power houses(RDP) and such so that  stove got priority over geyser  and could never both be on at the same time  or if you were running you house off a genie (inverter)

I took it out as my new inverter runs in self consumption mode  although it can grid tie


 Yes I know what you are thinking  like this guy is so clever that he should have been twins but  I do accept credit cards ,blank cheques ,beer and pizza 

cheers Max

Edited by maxomill
after thought
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7 hours ago, maxomill said:

when  grid power  reaches down to  say 1or 2 amp just before we start exporting  the load shed relay  allows a dump load  to be switched

That assumes that the power goes down gradually towards zero. If the power abruptly switches direction, for example if you had a 2kw load (a kettle) and 1kw local production, so 1kw was imported, and then the kettle switches off so a 1kw import instantly turns into a 1kw export. If you cannot sense direction, and you aren't super fast in your sampling, you're going to miss it.

This is more common than you might think. Washing machines that turn this way... switches off... then turn that way.

7 hours ago, maxomill said:

when the dump load come on then  all of a sardine the grid power goes up

Put the dump load before the measuring point for the load shed relay, so it doesn't "see" it.

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Hi @plonkster

I see your logic

Luckily for me my  washing machine is on my solar side but having said that I do have a "real power " monitor  on the incomer to the inverter and the bastard washing machine does do exactly as you say. I see it jump up and down by about 200w when on/off

luckily for me that my prepaid meter "Hexing type" does not trip but does charge I think when exporting . it only  shows a diode  symbol when exporting

I had a kaco gti  with a microcare grid tie limiter  and  it worked but there was a couple of seconds reaction time  while things switched on/off

I believe that worked by monitoring the phase angle and adjusting the gti output to suit   ( it has "one direction" ct s to monitor the coming and goings, anyone know their correct name please tell me ) it also always allowed 100w to be draw from the grid as a  buffer I guess

I still have the grid tie limiter in my cupboard and not sure what to do with it ( the kaco has been if for repairs forever at a  name given to me but someone on this forum, so if you need repairs done please call me and I will tell you where not to go )

I believe the end justifies the means as long as your municipal meter is not too sensitive


now back to the LSR,it does have an off delay but  I have no cooking clue how fast it will react(sample). I believe it will react quick but how fast is fast  I would say that the kettle scenario is okay , washing machine not so much ,again  back to the sensitivity of municipal meter

I had a quick look and the BEC23/44.  seem to have only a 40w export  tolerance  please check me up

having said that, in my case of the Vseries and @KLEVA imeon which are  inverters with grid tie functionality  it would be wise for us to install the LSR (s) at the incomer to the  inverter

I think this means the Dump load would be switched on first before  the exported power goes back to the house assuming the DB is split into 2  meaning the house( something must be drawing some power) would be  a buffer before being exported

4 hours ago, plonkster said:

Put the dump load before the measuring point for the load shed relay, so it doesn't "see" it.

  it goes somewhere along the lines of your point

 just another point is that the Dump loads must be equal  to or greater than the amount you can export

 I  think  I will do this at my house as a test . I have  a 650w geyser element and about 600 ish w of excess  power  and let you know how it performs



ps @KLEVA you could just change the wiring around so that the dump load   is an extra load on the inverter if the inverter is powerful enough to handle it ( back to timers ?)

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27 minutes ago, maxomill said:

I had a quick look and the BEC23/44.  seem to have only a 40w export  tolerance  please check me up

BEC23 is 40W over a 15 second window. BEC44 is less, though I cannot remember if it's the wattage or the window size.

28 minutes ago, maxomill said:

monitoring the phase angle

Correct. If your cos(phi) goes negative you're pushing into the grid.

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Hi, maybe I need to clarify more...

1. I have the old type disk meter - so have no issues about not getting back what I put in, have checked and it spins backwards just as much as it spins in (only did this for the test, still not sure if it's legal to supply back to my municipality yet). It does have a block though - so it can't run backwards and read accurately all the time... So can have a point where I feed in to Metro and the Meter doesn't register. Thats actually the point of this question, I don't want to give Metro my power free, but at the same time I don't want to screw them over.

2. my Load (the pump), I only want to kick in when the Imeon is feeding back to the grid. Have no issues with a minute or so delay. The pump is on the INPUT side of the Imeon (IE: it will not affect the readings on the Imeon, the Imeon will always still see a load pushed back to the grid). The load can be anything that trips when the PV input Power exceeds the AC output power... I just want to trip a relay. (eg: Imeon feeds back 500W to grid, I want it to flick a switch to turn on a "load").

3. All my tools and software that operate outside of the Imeon, can only tell me Current(A) values. But they all don't care which way the current is following, so I cannot detect with my  tools whether I am feeding back to the grid or using from the grid. Does that make better sense of what I am trying to do?

I have no idea how to use/impliment the above scenarios (PS: Please bear in mind that my budget is NILL, so a 5k+ option is unrealistic for me)

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1 hour ago, KLEVA said:

... still not sure if it's legal to supply back to my municipality yet)

Best is to be 'kleva' and find out. :D

Connecting, Durban has regulations already it seems. 

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3 hours ago, KLEVA said:

I have no idea how to use/implement the above scenarios (PS: Please bear in mind that my budget is NILL, so a 5k+ option is unrealistic for me)


I anticipated that this may become an issue and ordered 4 current transformers, 4 Arduino etc. My own client with a Goodwe has been battling with a Conlog BEC23 and I am halfway with  (my own install and now tax returns have interupted play) with a Arduino/Triac based energy diverter. Our municipality has after an age promised to install a solar friendly pre-paid meter so the project may not be needed. I am in Zambia this coming week and when I return I will try and complete the project. The transformer, Arduino, current transformer ,Triac and enclosure  should come in at about R800 so not hugely expensive.

The fact that you are on a disk meter would be an advantage and if this sheep farmer's programming skills are not right on the money then there is a bit of leeway.

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  • 2 weeks later...

@Chris HobsonSorry to hear mate.

On 2018/08/07 at 10:13 PM, maxomill said:

someone mentioned maybe trying   a  single phase reverse power monitor http://www.rhomberg.com.au/downloads/SP510.PDF relay  which I liked the idea

Had a look at this, and this seems to be almost exactly what I need although would like a bit more flexibility with the options (current, seconds, etc) and is available from AC/DC for around R550 (although there are none in stock at the moment) - https://www.acdc.co.za/collections/rhomberg-electronic-timers-relays-panel-instruments/acd300?page=89. If I live with those values and limitations would this do the job if I place it on the Invertor AC input side and put the Load before this?

AC Input -> Excluded Loads -> SP510 -> Invertor -> Included Loads
SP510 relay would turn on a load on the Excluded Loads side, so from what I can ascertain the SP510 will still see a reverse load?

Edited by KLEVA
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Whoops - Stop the bus!

The Rhomberg units all seem to be limited to 5A forward current, this could be an issue when connected inline to an Invertor that can do 3-6kW (unless a shunt is used)... So will wait for Chris's idea.

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