AngelStoyanov
Posts: 1
Joined: Tue Feb 04, 2020 7:02 pm

Current Transformer Current/Voltage monitoring

Tue Feb 04, 2020 7:11 pm

Hello,
I want to monitor Voltage/Current on certain points and upload it to an Ubuntu server. I have 3 current transformers and a Raspberry Pi 3 Model B+. How can i get the voltage/current from the CTs into the Raspberry from where it can be uploaded to the monitoring software on my server.

P.S
I am talking about a high voltage/current. Like 70 kW per phase (~400 amps)

Thanks in advance.

JohnsUPS
Posts: 188
Joined: Fri Jul 06, 2018 2:13 am
Location: USA

Re: Current Transformer Current/Voltage monitoring

Thu Feb 06, 2020 2:39 am

Usually the mechanism to monitor power (I am saying power because you're wanting to monitor both current and voltage) is to begin as you have done, with current transformers. You will also need potential transformers. A set of each will be needed for each phase.

There are certainly other solutions, but this is how I would tackle the problem.......

In your case, I would use watt transducers. You would need a three phase (or three element) transducer. The voltage transformers within the transducer can usually be connected directly to the line (I got ~175V from your description), so the VTs would need to be rated accordingly. As for the current transformers (CTs), typical/customary outputs are 5A and 20A out at full scale (not sure what you have in this regard). Your CT output would connect to the CT within the transducer. Because of the amount of power you're talking about, you may also want to consider measuring VARS.

Transducer outputs can be either analog or digital, with the digital outputs being a variable speed pulse rate proportional to the power being seen. This would be easy to connect to a GPIO pin and count the pulses.
The transducer is calibrated, so fewer concerns there depending on the accuracy that you need.

The one below is just an example of what I am referring to. This particular one has a 4-20ma output that would need an A/D converter, but others are available with contact closure outputs (either Form A or Form C).
https://www.ebay.com/itm/AMETEK-SCIENTI ... SwV45bv2Dg

In case you're wanting per-phase quantities, the same mechanism could be used, only with single element transducers - one per phase, but system phasing would have to be taken into consideration somehow for the three phase cumulative consumption.

What I have done in my house is to install current transformers on each line coming in (residential - split single phase) that have 5 amp outputs at full scale. These CTs are connected to a pair of Scientific Columbus single phase analog watt transducers (I could have used one two element transducer). The watt transducers have built in precision VT transformers that are rated for 120v (USA), and are connected directly to the line (fused). The transducers are calibrated to provide a 0-1 ma current source output for a 0-500 watt input. A precision 500 ohm resistor on the transducer output at my A/D converter for 0 to 0.5 volts and I'm done. For the A/D converter, I am using the ADS1115 16 bit A/D, so there is plenty of resolution. The A/D has an I2C interface which is easily read by the Pi.

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