Accurate CMU Readings on EDLT

Hi guys,

I've had a scroll through previous posts and couldn't find a recent answer.

I have a CMU and 2 x CT's connected. 1 is direct on the solar production and the other is on the load side of my main switch.

On my EDLT I have displayed the live solar reading and I'd like to also display the amount being drawn from the grid. I know with solar the CT's are working 2-way, so the reading is not accurate. Can anyone give me some ideas on how to display an accurate reading for power being drawn from the grid?

Thanks

 

I'm not familiar with solar and what you refer to as 2-way but if you want to do any kind of calculation based on the readings then something with a Logic Engine will do it easily.

Use the Measurement Application and read the values using GetIntSystemIO. Do whatever calculations are necessary and then broadcast it on another measurement channel using SetIntSystemIO which the eDLTs are configured to display. Make sure controllable is set otherwise it won't actually send it back out on the C-Bus network.

 

Im not exactly sure, but I think your asking how to measure/display the difference between total power used and solar power generated so you know what your drawing from (or feeding to) the grid?

I'm pretty sure this can be achieved by passing the 2 active wires through the same current transformer in opposite directions. The effect is to cancel each other out with the output equal to the difference.
It's something like that anyway and someone here will correct me if I haven't got it quite right.

Cheers

Rohan

 

Get a 3rd CT on the main incomer..

Then use logic and basic maths to get the tallies you are looking for..

Cheers..

 

Put the first ct directly on the output of the inverter. This measures the output of the solar array (power can only go from the inverter out). Put the second ct on the input to the house downline of where the inverter connects. This measures power going into the house which can also go only one way (i.e. into the house). In logic, subtract the house power from the solar power. A -ve value tells you the power you are drawing from the grid. A +ve value tells you the power you are exporting to the grid.

 

Not sure about that image. All inverters I have seen have a single AC output that just connects up to the meter on the house side. Connecting up the CT's to the DC side wouldn't get you very far anyway as they are AC only devices. (Even if you could measure the DC input current it wouldn't do you much good as the DC input voltage varies all over the place due to the mppt tracker so the power calculation would be meaningless).

 

I currently do this, but a different way (digital meters with a Wh pulse).

Two CT's needed.

One anywhere from the outgoing side of the main switch to the first distribution point (i.e. where the power is split up into the different circuits in the house).

The second on the inverters LV side - anywhere between the output breaker (which all have installed adjacent to the inverter) and the solar power meter in your board.

You are only measuring current so you will get a resultant current flow when you add them both up. This may not equate to cost.

e.g. I get 60c/kwh for my solar, but pay 26c/kwh for my power. So through the day even though the power coming in is more than the power being generated, I am still making money - so depending on your situation, you may see a reading of current that indicates you are drawing more from the grid than the solar, your solar feed in tariff may actually mean you are making money.

 

Hi Ashley,

Thanks for your help. Just a quick follow up, The CT's are installed in those exact areas u suggested. The issue I have is looking at the readings, it seems the CT installed downline (Load side) of where the solar connects to the to the main switch is relative to the solar array readings. For instance, if solar CT is reading 2500W, the house usage CT would read 1500W. If the solar would suddenly drop 600W, so would the house CT. Same would happen if the array went up. Have you got any suggestions as to what would this issue be? Also, I have done plenty of logic, just none involving meters or monitoring. Could you set me on the right path of possible.

Many Thanks

 

I think Master Guru Darren covered your situation in his post here.

Brad

 

That's the way to do it if you can't get to point C in his diagram. The whole post is worth reading as it covers both options:

http://www.cbusforums.com/forums/showthread.php?t=6803

The solar output cannot affect the house consumption so either the CT's aren't in the right place (most likely), they are interfering with each other (unlikely), or a load in the house is being changed.


To use them in logic you need to set up the measurement channels first under PROJECT/APPLICATION/MEASUREMENT. Then in logic use

Code:

GetRealIBSystemIO("Measurement App Real Value", network, device, channel);

with the appropriate values for network, device and channel

 

Agree, well spotted.


Whilst probably not P.C. as not a C-Bus product .........
Attached is the connection diagram from the Wattson Solar Plus, a solar current measurement device from a company that (unfortunately as it was a good product) went into liquidation.

....... but it does show how the CTs can be arranged and linked to give 1. Power use from grid/Generation sent to grid of the whole house system and 2. Solar Output Generation of the Solar alone, but in a physical way (like Darren's post) rather than using logic for the summation, so it may be useful to you.

As per Darren's post, the 'Y' cable CT linkage in the drawing offsets the current of the solar output against the total so the generation power is not 'measured twice', ie by both 'Solar' and 'House Use/Grid Conn' CTs. The third CT is the offset current, so this method uses 3 CTs, not 2 if you can't do it with logic.

Clear as mud right!!