reverse power meter bodge?

[guydewdney]

Reverse power meters / relays are expensive and complex.

BUT - theres one in every household meter - the one that lights the 'export' LED. For those that havent seen this - any meter with a LED that flashes when its importing (one flash per xkwh) also will light up continuously when power is flowing the 'wrong' way.

What I wish to do is turn on a (say) 1kw load (eg water heater) when I am exporting 1.1kw

generator--total gen meter--T----bodge meter-----T----normal meter
                                        I                              I
                                    Normal house                 'dump' load



So - how to I determine that I am exporting 1.1kw? I assume a current clamp meter (CT clamp) across a resistor giving a known voltage. Feed that voltage to a ?comparator? to get an on / off signal - combines with a signal from the bodge meter through an AND gate - firing a relay to fire up the 'dump' load.


If this is good - can someone make it for me?

[Alan]

Uses a current transformer on generated and house load.

calculate_excess:
available_power = generated - used
If used >= generated Then available_power = 0
If available_power > 1023 Then available_power = 0
If available_power < 1 Then available_power = 0

PWMduty 1,  available_power

Above exports generated power in excess of house load to pulse width modulator. ( Heater )

'If available_power >= 10 And available_power <= 255 Then RD4 = 1
'If available_power < 10 Then RD4 = 0

10 in the above line would be sized for what ever heater you have.

Above switches on a fixed load when available power is greater than used and equals the heater load.

You could digitally step available load in various stages.



Yellow bar graph is used.

Red bar graph is generated.

http://steamboilers.homecall.co.uk/1239.jpg

Regards

Alan

[Alan]

Been doing far tooo many 96 hour days with no time off.

will try and get one built for you over Xmas.

You will need two current transformers.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0351106

and a solid state relay.

This one is good for 4 amps at 240 V.A.C.

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2508644563

This one is good for 50 amps at 240 V.A.C.

One transformer will go on the mains live cable from the consumer unit to the inverter.

The other one will have all the other live cables from each M.C.B. in your consumer unit. ( Total house load )
passing through the centre.

I will program the device for eight different out put loads.

No out put Leds on will mean exported is less than used.

Regards

Alan

Forgot to paste the second solid state relay

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4914868

[guydewdney]

You are a God of Electronics and Niceness in one.


What do you drink again? I shall have the factory shipped to your front garden, and the output to a third tap in your kitchen....

[noah]

Having binary combinations of resistance elements is a pain. What do you use that is easily available in smallish steps and robust enough to withstand constant switching?
Instead of binary combinations could you not use a large dimmer module  (eg http://www.quasarelectronics.com/ci0013.htm ) which would be wired up to one simple load.

[guydewdney]

several heaters around the house?

but 3 x 200w heaters is useless, whereas 1 x 500w heater is (just) enough to heat one decent size bedroom (Im using a cold watch typre greenhouse heater (500w) to warm the spare bedroom for the oldies when they come to stay tomorrow - its just about warm enough in there after a week (going from stone cold - literally).

But I see your thinking. I have a huge rheostat that I am not using - I could wire the heaters to that   

[Paulh_Boats]

Alan,

How will all the non-inverter circuits go through the second current transformer? They are too big and spread out??

-Paul

[guydewdney]

I think the heaters are just connected to the main Cons Unit - and when it calculates excess power, it switches them on until balance is achieved. therefore only two CTs are required - a) generated power and b) consumed power and if a>b then switch on extra loads until a=b.

Not sure how I am going to get the power wires actually there... might look at somthing wireless... (or powered through the mains like those home network plug things you get)

[knighty]

do you have central heating ?

if you do, I'd think it should be pretty easy to add an extra pump, and a few heaters, or do something a bit more fancy, so you can use say a 4KW heater, wired into your rheostat ?

advantage is, you could stick it anywhere you find a pipe for the central heating, might have one close to where you're going to install the electronics etc.. ?

[Alan]

The current transformer has a 10.7 mm center hole. Big enough for
doubled insulated meter tails.



http://steamboilers.homecall.co.uk/DRAWING1.bmp


If you only have one consumer unit then one transformer will go on
the generated M.C.B. ( Fuse )
 
The other transformer ( House load ) will have all the other load
cables passing though it.

With relay heating you do not switch on the heater unless generated
power is greater than house load + the heater load.
 
If you had say a one kilowatt heater and only generated 990 watts
above your house load the heater will not switch on.
You could end up with periods of time
where you give / sell power to the country.

Here I use pulse width modulation so that any power generated in
excess of power used in the house is utilized as heating for water
or under floor heating.

I can include a proportional 0 to 5 volt D.C. voltage which would
only be generated when generated is greater than house load.

You could use 0 to 5 volt controller.

http://www.united-automation.com/_ProductDetail.phuse?PhuseAction=PyrUUhn1O8sELPSFzFKQvF2fptvIHFGbmMb4An06h9CTdmMLrle8uqrtGZI6eaYm

http://www.united-automation.com/res/media/pdf/X10532EVR25A_Bpdf.pdf

Regards

Alan

[ecogen]

I can include a proportional 0 to 5 volt D.C. voltage which would
only be generated when generated is greater than house load.

You could use 0 to 5 volt controller.

http://www.united-automation.com/_ProductDetail.phuse?PhuseAction=PyrUUhn1O8sELPSFzFKQvF2fptvIHFGbmMb4An06h9CTdmMLrle8uqrtGZI6eaYm

http://www.united-automation.com/res/media/pdf/X10532EVR25A_Bpdf.pdf

I considered this approach too. Then all of a sudden from nowhere, the ghost of Electrical Power Engineering appears. The air fills with static, hair stands on end, a faint whiff of ozone, my pants fall down! Shocked by my exposed vulnerabilities, ouch, a feeling of smallness, a cold tingling chill down the spine. An eerie voice calls out... BLONDEL.
I bend down, grab my pants and mutter....Bu9 9er.

[guydewdney]

Correct me if I'm wrong-

PWM gives an average power level as required - ie the 990 watts scenario the heater is not quite fully hot.

BUT - isnt the instantaneous power still 1.0kw then 0.0kw then back to 1.0kw? I know this is right for DC applications - but how does it apply for AC? If the pwm frequency is considerably higher than the mains, then the system could effectivly trim off the peaks of the voltage graph - sort of like clipping on a music amp - so my heater gets 210v instead. Or is this a load of cobblers?

I would be happy at 500 watt loads - I would only ever be exporting 499 watts, which I can live with. With 8 switchable loads, thats 4kw - more than enough.



Guy
Feeling bloaty after xmas 

[Alan]

Software on test. I was using it for pulse width modulation.
So did not use so many I.O. pins



With binary out put a lot more I.O. pins have been used up.

was trying offset and span analogue adjustment for binary loads.
Bit more testing required. But works well with fixed loads.

Regards

Alan

[guydewdney]

<rubs hands gleefully>

How difficult would it be to add extra CTs for house load and generated load? The reason I ask, is that the house actually has two consumer units - one downstairs, one up - and the cable isnt that easy to move. Also - I will (should) be generating on two phases soon - the import meter just gives a running total (not a per-phase total).

I feel this has gone beyond a crate of beer, and into the realms of a cheque.

[Alan]

Two generated inputs and two used inputs is Doooable.

Regards

Alan