Emonpi with 200a cts

Hardware
1

I installed an emonpi with 200a cts. How do I configure the emonpi?

2

That cannot be answered until you say exactly which c.t. you have used.

3

Robert, we used the sct-019.

4

Which one - there are 13 variants. Two of those are good for 200 A. Are yours the SCT-019-000 or the SCT-019-200?

5

Robert, we used the sct-019-000.

6

Now we’re getting somewhere.

The ratio of that is 200 A : 33 mA, the burden resistor fitted in the emonPi is designed for 50 mA secondary current from your c.t. If you connect the SCT-019-000 without changing anything, you will be using only ⅔ of the available range at 200 A. The consequence of that is at low currents, accuracy and resolution will suffer. And it’s questionable whether you’ll actually be using 200 A anyway - my colleague here @Bill.Thomson has written about that many times.

The ideal solution is to disassemble your emonPi, and locate the burden resistors - they are the larger SMT component immediately behind the c.t. sockets - and change them for a higher value wire-ended resistor (there are holes ready to accept it). The value should be that which will give you no more than 1.1 V at the maximum current you will measure.

When you’ve done that, or if you decide not to, you need to alter the calibration. The easy place to do that is in emonhub.conf, which is accessible via a web browser.

This method uses the scale factors in emonHub to adjust the values passed on to emonCMS. It is the easiest and should be adequate for most purposes. (It is also applicable to any input received from other sensor nodes via emonHub and the radio module or serial port, but not to data received directly by emoncms via an Ethernet connection.)

Open emonCMS in a web browser.
Navigate to Setup > EmonHub > Edit Config
Scroll down to Node 5 – it looks like:

[[5]]
  nodename = emonpi
  [[[rx]]]
    names = power1,power2,power1pluspower2,vrms,t1,t2,t3,t4,t5,t6,pulsecount
    datacodes = h, h, h, h, h, h, h, h, h, h, L
    scales = 1,1,1,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1
    units = W,W,W,V,C,C,C,C,C,C,p

The numbers you will change are the ones in “scales = …”. These are scale factors for the values listed in the “names = …” line above, and correspond 1 : 1 with those. Note that the voltage and power scale factors are quite independent, equally “power1pluspower2” is independent of both “power1” and “power2”.
You should proceed generally following the instructions for the emonTx. First, set up the voltage accurately, so that, if you are using a multimeter to measure the current, you can use the displayed voltage when calculating the power. Next, set up the currents – you will need to adjust the power scale factors to read the same as the power you calculate, or measure if you are using (say) the power display on your inverter.
There is no mechanism available via emonhub.conf to correct the power factor for phase errors in the transformers.

If you are not changing the burden resistor, the values for scales should be 3.03 This will get you close, if you’re able to measure the actual current accurately, then you can adjust that number to give you the correct power readings.

If you do decide to change the burden, you’d best post your proposal and I’ll confirm it and calculate the numbers for you.

7

Robert,
Thank you very much for taking the time to explain exactly what I need to do. Sharing your expertise is very much appreciated.
I am glad to hear that changing the burden resistors is not necessary. The system is at a K-12 private school in Central America, I am back in the United States for a few months. I zoned out on the scaling as we were short on time and had IT issues to resolve. I can pass along instruction on how to change the EmonPi scaling to the IT manager or have them redirect a port so that I can do it remotely.
Absolute accuracy is not critical as the energy monitor is being used as a science teaching tool, to monitor net power usage with their PV system, and verify results from conservation experiments. They love the idea of having real-time data available for science classes.
Not being able to sense up to 200A should not be a problem. We can tell from the time-of-use demand charges on their electric bill that they peak somewhere between 60A and 80A. They don’t have any huge loads, just a pile of small AC units and some appliances on this panel. The 200A CTs were necessary due to the wire diameter. The 100A CTs would not fit. The main feeds on the panel use aluminum wire that is a bit larger than what one would encounter with copper.
I think I get the scale factor change as you explained. The 100A:50ma CT has a turns ratio of 2000. The 200A:33mA CT is 6060, hence the sensed current is actually 3.03 times larger. We will make the following change:
scales = 3.03,3,03,3.03,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1
Thanks again for the help.
Sincerely,
Kevin

1 Like