CT Clamp Emulation

Projects General Guidance
21

Thanks for the RST link. I was confused by the naming. In Germany it’s typically called L1, L2, L3. But it seems to be the same, I’ll try to get these values from the inverter tomorrow to have a starting point :slight_smile:

I agree 8bit will have probably tor poor resolution.
I do not understand why you expect higher current and higher voltage? Are my calculations wrong?

I would expect only 0.014A in the secondary circuit for 35A load in the primary.

When using higher voltages I would need an additional power supply. I was hoping to keep it small and simple and using the 3V3 from the ESP32. But it seems that it’s not a good idea?!

I have to take a closer look to your low pass filter.
I am confused, why you’ve mentioned pwm. I thought, that we need to generate current ac, don’t we?

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Oh, it was only a suggestion ... if you end up needing higher resolution you could do something like this (except use an ESP32's output). You would essentially be creating your own PWM controlled DAC with higher bit resolution (13-bit for example). The PWM duty value would set the voltage. To get a sine wave, you would vary the duty cycle like this, but using an ESP32.

Probably best to keep it simple and start with using the ESP's DAC as you've suggested.

23

Probably your ideas are the better ones :wink:
Thanks for the links. I understand your approach.

I need to build something for the DAC anyways. ESP32 has only 2 DAC and as you mentioned only 8bit each. As we have 3 phases it's not sufficient ...
My first idea was to use an external DAC via i2c. Something like this: Adafruit MCP4728 Quad DAC with EEPROM [STEMMA QT / Qwiic] : ID 4470 : $7.50 : Adafruit Industries, Unique & fun DIY electronics and kits
The breakout board has 4x 12bit DAC. What do you think?

24

I've managed to read the voltage and current from the inverter.
I get these values for example:

  • R Phase Grid Voltage: 234,399993 V
    R Phase PCC Current: 4,869999886 A

  • S Phase Grid Voltage: 234,6999969 V
    S Phase PCC Current: 3,390000105 A

  • T Phase Grid Voltage: 233,8999939 V
    T Phase PCC Current: 3,089999914 A

To me it looks quite reasonable.
What do you think?

I ordered the DAC from Adafruit.
Not sure, if this is the best solution.

Do you agree with my calculations?
I am expecting 0,014 A current to be induced, when 35 A are drawn in the primary circuit.
This is the max per phase, the fuse is capable of.

So, the 35A/0,014A are my max current, and in that case I want to apply 3V3.
U = RI
3.3 * 0.014 = 0.0462 ohm

This is probably also the biggest problem of this solution.
I took a fast look, and it's hard to find a suitable resistor.

I've only found this one, and it's not available.

Following your suggestion with the low pass filter, might be easier then ...

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