Negative power reported

[Dawid Ferenczy]

Hi, I just bought a Shelly Pro EM-50 device to measure the energy consumption of my whole apartment. I have physically connected the device and it's successfully reporting measurements. I see the measurements on the device's web interface, in my Home Assistant and also in the Shelly Cloud/Smart Control. The problem is that it reports the current power as a negative number and therefore it's counted as an "active returned energy" instead of an "active energy" (like I'm sending energy to the grid instead of consuming it). I can see it negative on all places (web interface, RPC, Home Assistant, Shelly Cloud) so I'm sure that it's what the device reports and it's not just bad interpretation by the service. 

I have noticed that I can probably change this in the settings of each measurement transformer on the device's web interface - it's called "Reverse CT measurement direction". But I would like to understand why it's measuring the power as negative in the first place. I don't think it's a default behavior, because majority of people just consume energy, they don't produce it. So I suspect I may have the decice connected incorrectly or something, but I could find anything wrong.

Could anyone help me, please?

[Olsche]

The CT transformer is probably installed at a 180° angle. Note the direction marked K-L

[Dawid Ferenczy]

Oh, damn, I didn't realize there may be any difference in the physical orientation of the CT. I thought it just measures a magnetic field and that should be same not matter in what direction energy actually flows. Do you know how exactly (physically/electrically) it actually works, please? And what's the meaning of "K" and "L", please?

Anyway, thank you for the information, I really appreciate it!

[thgoebel]

A current clamp („Stromwandler“ in German) is a simple transformer with ONE primary winding and (in the case of the Shelly clamps) 3000 windings on the secondary side. Thus, a 3000-fold smaller current flows in the circuit at the secondary side. This side must be shunted by a small resistor. Otherwise, the clamp will be immediately destroyed by overvoltage at the secondary side. In the case of the Shelly clamps, a TVS diode is incorporated in the clamp, which limits the secondary voltage to some 10 volts.

Changing the direction of the clamp gives negative power value due to the fact, the secondary current is now phase shifted by 180°. Power in alternate current systems is defined as P = U * I * phase angle (cos φ).

The meaning of „K“ and „L“ is a mystery. In fact, the arrow must show to the source with Shelly 3EM; with Shelly pro 3EM and EM50, the arrow has to be in line with the power flow (it must show to the load).

[Dawid Ferenczy]

So the device compares the phase of the current in the phase (L) wire and the CT and from the difference, it's able to establish the direction of the current. Do I understand it right?

Anyway, thank you for the very detailed answer! 👍

[thgoebel]

The energy measurement device (the Shelly) compares the phase angle of the voltage path with the phase angle of the load current and determines the power factor (e.g. cos φ). This is the reason why the current clamps must be mounted at the appropriate wires:

 

Otherwise, the value of power and energy are not consistent.

 

Edited July 26 by thgoebel

[Dawid Ferenczy]

Yep, that's exactly what I mean. Awesome, thank you for a very detailed answer!