A simple Four Quadrant power meter

Why?

I have built a Solar Diverter which diverts excess solar power to an electric hot-water tank. The Solar Diverter needs to know whether "the house" is exporting or importing power so it can adjust the amount of power it sends to the tank.

In my case the Diverter gets the imported/exported power value from an http call to a rather complicated four-quadrant meter employing a NodeMCU/ESP8266 to sample and multiply the voltage and current waveforms.

I felt that if I'm going to publish the design for the Solar Diverter, I should at least present a simpler power meter to provide the imported/exported power values.

How?

The system averages "mains" current during the positive half of the mains voltage cycle: If the current averages positive, "the house" is consuming power. If negative, "the house" is exporting excess power.

Details:

T2 clamps over the incoming power line, and generates the "current" wave-form over R2. This is fed into the RC network R3/C1 via the photo-relay U1. U1 is "on" during most of the voltage's positive part of the cycle, giving us a measure of the average current, and therefore power during the positive half of a cycle at FAST.

In case of power import, the current will be (predominantly) positive while U1 is closed, resulting in a proportional positive voltage at FAST. In case of export, the average current, and therefore FAST will be negative.

Modeled for passive load, reactive load and export below.

Disclaimer:

The design has not been used in anger, it is a breadboard tested proof-of-concept only.