Emonlib Measure Solar Feed on mains power with negative values from CT Clamp

BrutalBirdie · 18 June 2020 13:02

Hallo there and thanks for reading this.

First I want to excuse myself if I took the wrong category or tags.

The Start

Let’s start with what I have done so far.

I have created a little esp32 board with staying close to the docs:

I wanna do a revision since I can only flash it via. GPIO for now which is painful and other reasons coming up later

Only ampere measurement integrated into my openHAB setup:
puush-linux+(2020-06-18+at+02.38.07)
Phase One, Phase Two and Wattage (Yes I only got 2 phases)

Everything working so far, I get accurate enough measurements.

The Problem / The Question

I got a solar power installation and want to measure if I am feeding to much into the grid.
Too much is kinda wrong.
Let me try to explain.
I can measure the load of my local power 290 Watt (above picture).
Now I can tell my solar power system to feed 290 Watt into my system.
So far so good, the ampere / watt measurement drops to approximately zero.

Now comes the “bad” part.
If now a device powers off and removes 50 Watt, the new measurement of the load would be 50 Watt, since the solar power would feed 50 watt too much.
The CT Sensor would read 0.21 Ampere / 50 Watt and the solar feed would try to feed 50 Watt.

With the next measurement it would read 190 Watt (290-50=240 the lost 50 watt power off | 240-50=190 the new feed value) and would then try to feed 190 which would be correct.

I would like to measure -50 Watt / -0.21 Ampere and then adjust the feed like that.
As far as I understand it the CT Sensor can not read negative currents.
To do so I would have to add the voltage sensor, right?
Docs
currentvoltage_bb

TL;DR

I need/want to measure negative current with CT Clamps since I got a solar power installation.
As far as I understood it I need to add a voltage sensor to achieve this.
Did I miss anything?

Thanks for reading

Cheers
BrutalBirdie

Robert.Wall · 18 June 2020 16:40

Your understanding is slightly wrong, but the conclusion is correct. The ‘Learn’ section explains (Learn→Electricity Monitoring→AC Power Theory→Introduction→An Introduction to AC Power), but in very few words, it’s the relationship between voltage and current that is important. That lets you know the direction of power flow. If you measure current only, you can never know that (like a multimeter on a.c. - that can’t tell you the direction of power flow).

BrutalBirdie · 19 June 2020 09:02

Ahh I remember after reading that doc part again.
I still a bit unsure tho.

After looking at the example, will the reading automatically change to negative?

github.com

openenergymonitor/EmonLib/blob/master/examples/voltage_and_current/voltage_and_current.ino

// EmonLibrary examples openenergymonitor.org, Licence GNU GPL V3

#include "EmonLib.h"             // Include Emon Library
EnergyMonitor emon1;             // Create an instance

void setup()
{  
  Serial.begin(9600);
  
  emon1.voltage(2, 234.26, 1.7);  // Voltage: input pin, calibration, phase_shift
  emon1.current(1, 111.1);       // Current: input pin, calibration.
}

void loop()
{
  emon1.calcVI(20,2000);         // Calculate all. No.of half wavelengths (crossings), time-out
  emon1.serialprint();           // Print out all variables (realpower, apparent power, Vrms, Irms, power factor)
  
  float realPower       = emon1.realPower;        //extract Real Power into variable
  float apparentPower   = emon1.apparentPower;    //extract Apparent Power into variable

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Robert.Wall · 19 June 2020 11:48

Yes, the real power will be positive or negative depending on the direction of power flow. Apparent power, Vrms & Irms are always positive.