3-phase energy meter with CTs

energypower-meter

I need to measure 3-phases. Usually I would use CTs on the differential port on a LTC1966, then connect Vout+ to an ADC port on a uC.

I do that on a 1-phase application. The problem is now, that we have only one available ADC port in our existing application and it cannot be changed. However, we could use I2C (RX/TX) communication.

As example, I could use three LTC1966 and some uC with more ADC and send data through RX/TX or maybe even a standalone ADC. But if possible I would like a SoC.

We cannot touch existing installed cables therefore the usage of CTs. I also know of the problem not actually being able to measure the real voltage level on each phase, but as said, cannot go into existing installation.

Best Answer

Given that you can accept the error induced by not measuring the voltage, it may be that you are looking for a reasonable estimation, rather than actual usage.

In 3 phase systems, the power is supposed to be drawn in a balanced manner. Putting more load on one or another phase than the others results in system inefficiencies.

So a well designed piece of equipment, or plant, will generally be drawing the same power from each phase. As such you can simply measure one phase, and multiply it by three to obtain an estimated power usage.

If you know your device will be used in situations where the three phase usage is unbalanced, then a simple method would be to use the one ADC, but add an analog switch such as the 4052 CMOS multiplexor to switch the current transformers into the ADC:

CMOS chip 4052, 2 of 4 multiplexor/selector

You'll need to keep the resistors on the current transformers, and only switch the ADC input to each one, never leave the current transformers "open" which attached to the AC lines. It'll require 2 I/O from your existing board, but rather than a complex communication protocol and additional code for another microcontroller, it should be pretty simple to control and use.

This will allow you to take sequential readings of the current transformers, so you won't get simultaneous instantaneous data, but you can certainly find average power usage over all three phases several times a second. With careful timing you can read each one at its positive and negative peak in order, and get nearly as accurate results as if you had three ADCs reading more frequently.

Beyond that, your solution of an offboard "expansion" device that employs another microcontroller to do the job would certainly work well. I'm not sure it would be much more expensive than the switched chip above either (the cost difference between the MCU and the cmos chip would be swamped by the labor and PCB costs), but it would involve more development time than the simple multiplexed solution.