bi directional sesor relay

[maxomill]

I can have a fair amount of excess solar in summer and looking to use it some how . I have a v series  which runs in self consumption mode

it has gti functionality but I always run the risk of sending back to the grid and that would cost me .

I'm playing with ideas and wondering if anyone has a solution of some sorts

what about a bi directional current sensor controlled relay to switch on a load when it reaches a set point

so I keep the machine in gti mode running my normal stuff ,when it senses excess current in reverse , it switches.

I have a dumb current sensing switch but it cant tell if current is coming or going

anyone got better ideas please

cheers

 

[___]

25 minutes ago, maxomill said:

what about a bi directional current sensor controlled relay to switch on a load when it reaches a set point

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.

[maxomill]

thanks @Plonkster

I'm an electrician so any more than 4 wires and I'm stuffed plus the fact that I was born BC which is before computers so the arduino idea is out until my daughter finishes studying elect engineering at varsity.

 the rhomburg relay on the other hand sounds promising since I'm on single phase and my prepaid doesn't even flinch about backfeeding  but does charge me for it

my solar geyser is only using about 2 kw a day so if I can boost it up to a higher temp during the day using my excess then it will use even less in the morning. I generally have about 6 to 800 w excess  and I have a small 680w geyser element  so in  a couple of hours   I can make a couple of kws extra if placed on a timer

thanks again

cheers

 

 

[flamegrilled]

 

5 hours ago, plonkster said:

You could also just get one of those Rhomberg reverse detection relays

Don't know if this is still available.

https://libra.energy/wp-content/uploads/sites/14/Export-power-manager_V1.0-7.pdf

[maxomill]

Nice but probably comes in at a nice price too but I will look at it

cheers

[___]

Looking at the manual, it seems that program 9 controls whether it feeds back or not. If all your loads are on the output, that should be all you need to worry about. You could use a contactor (powered by the mains) that drops out if the mains goes out, ensuring that heavy loads are disconnected if the grid fails.

Something else that I must also note: I don't think the V-series complies with NRS097-2-1, so you'd be be going cowboy :-)

[Chris Hobson]

On 11/15/2017 at 8:55 PM, maxomill said:

....... I'm on single phase and my prepaid doesn't even flinch about backfeeding  but does charge me for it

Hi @maxomill

What prepaid meter do you have?

 

 

[maxomill]

a hexing 101 

doesn't mind feeding back at all but does register it

cheers