Derived three-phase with SCT006/SCT013


#1

Hello IoTaWatt community,

I just got my IoTaWatt and SCT006/SCT013 installed and have some questions to validate my setup and resulting data:



(all inputs had to be reversed because the input0 seems to be reversed)

As you can see from the config I’m measuring each phase on the panel input (L1/L2/L3) using 3xSCT013 and a subset of the panel breakers using SCT006.

I’m wondering about the difference in measurements between L3 (SCT013) and ILP (SCT006), they are on the same phase and ILP is connected behind L3, so L3 should be greater-or-equal to ILP. At first I suspected derived-three-phase for the skew (pf) of L3, but that would impact both measurements and the pf of ILP in the same way?

Any hints in addition to checking the CTs?

Thanks,
Aarno


#2

Graph of the L3 measurements:

They do seem to correlate :slight_smile:


#3

Hi @arska,

I’ve taken a look at your setup.

This isn’t the problem, but for the sake of anyone else looking at this for tips: You seem to have an EU type 9V transformer and I believe you can just reverse it in the socket. If not, or that is not convenient, you could check the reverse box in the input_0 configuration rather than in all of the individual circuits. :slightly_smiling_face:

Looking at the L3 specifically, and the status display that you posted, I see that L3 is 740Watts and that ILP is 1,023 Watts. That would appear to be somewhere in the graph around 22:00-23:00. It’s interesting that although the ILP appears to be the entire load (an understatement), the power factor for the main L3 is quite a bit different. That’s not right.

Looking at the graph, I can see that when a well defined load like kattoOH comes on, the main does not increase by the same amount, and it varies. Something changed at about 20:45 because there was a load that was a 200W load varying rapidly that stopped.

At about 1:15, the main L3 is less than half of the ILP power, which is very steady at that point.

My first guess is that the CTs are not correctly oriented. There are 42 possible combinations of CT orientation with derived three-phase (more with direct) and only one of them is correct. So it requires attention to detail and a methodical approach to get it right.

Another possibility is that the SCT013 is either not what you think it is, defective, or not seated fully as installed.

  • The SCT013 comes in one several flavors. The one you configured is SCT013-000 which is a current type. I have had problems with folks mistaking the SCT013-050, 030, etc and installing them as 000’s. The impact is that they read low…
  • There is also the all too common instance of these CTs having cracks in the core, or debris or case misalignment causing the two core halves to not contact fully. The impact is that they read low and the power factor is off…
  • The other possibility is that the L3 main CT is simply reversed. You know how to change that in the configuration app.
    Regardless of the detail circuits, there are a couple of things you can do to validate your mains:
  • Using the local graph app, you can plot energy for your “total” output. The “max” value shown when you create the graph is the IoTaWatt’s current kWh total for that output. If you record that and the reading on your meter, you will have a basis for comparison over the course of the next few days as you work through this.
  • Another quicker and more immediate approach requires a quality handheld clamp on RMS ammeter. If you have one of these, you can check any of the CTs directly, including the SCT006’s. You can create an Amps output for the desired CT, and compare the reading on the clamp on meter to the IoTaWatt output. This reading will be independent of phase or CT orientation and will serve to validate that the CT is good. After that, you can try to reconcile the power and load factor with the CT reversed and not.

Please let me know how you make out.


#4

Thanks!

Exactly. If the derived three phase was off then both L3 and ILP (finnish for air heat pump btw) would be off-phase by the same amount but ilp being at pf=0.99 seems to be quite on-phase.

what do you mean with “orientation” ? it does not seem to be backwards (negative current), I checked the socket for input0 being wired on L1 and also tried swapping L2/L3 transformers to eliminate getting the phase rotation wrong but that resulted in pf=0.3 readings which were worse than this.

Yes. will need to power down the whole house to open up the pdu :slight_smile:

I got them from https://openenergymonitor.com/iotawatt/ where they are labeled SCT006-000/SCT013-000 but I’ll double-check when opening the pdu again.

Will check.

Wouldn’t the power reading then just be negative but with the same absolute value and pf?

Thanks, will do! as I have a smart meter I should be able to get hourly metering data from my power company somehow…

Haven’t got one of those - yet :slight_smile:

Thanks for you help and I’ll report my findings as soon as I get the PDU opened again.

-A


#5

In a single phase installation. With derived three-phase that’s not the case. It’s complicated, but this could easily be the result of a reversed CT.


#6

I opened the pdu today and moved the L3 CT closer to the other L1/L2 CTs and checked the power/pf before closing up, it reads much better now and the calculated “rest” is now almost always positive!

Thanks for your support!


#7

You might want to check the total against the meter in a few days. Careful not to overdrive those SCT006’s more than about 15Amps. If you need more capacity, I have good 50Amp CTs.