Sensing Current for Solar

by Ben N on November 13, 2017

About a week or so ago, I started playing around with a DIY current sensor. The initial experiments seemed to work, so I took it out to the garage, and hooked it up to my grid-tie solar array.

I first removed the cover from my garage circuit breaker panel so that I could get to the actual wiring. Next, I clamped the CT (current transducer) onto one of the two HOT wires from my solar combiner box to the circuit breaker that feeds the solar power in to my garage. I’m using micro-inverters, so the power is already 240V AC. There’s no neutral used, and both legs have the same current flowing through them, so I can get an accurate measurement with just one CT.

I used some double-sided tape to stick my Arduino, breadboard, and servo onto a piece of cardboard. That way, I could just hang the whole unit on the wall near the breaker panel. For testing, the whole thing is just powered off a 5V USB battery pack.

IMG_7270

 

So far, this project is based on concepts from the Open Energy Monitoring project. Take a look at their web page. There’s all sorts of great information there! This link will take you straight to the section on sensing current. LINK

In a nut-shell, the CT creates a small current proportional to however much current is traveling through the wire the CT is clamped around.

That current then effects the voltage of the circuit it’s connected to, based on the voltage supplied and resistance of that circuit. In this way, a large current gets converted to a low current and then into a low voltage which can be read by the Arduino. (For details of how this happens, please visit this page.)

Inside a micro-controller, it’s very easy to do a little math and then use that to output to the real world. In the tutorial I was following, the output is just using a USB cable to display data on a computer screen through the Arduino software. Once I had done that, I modified the code to instead move a servo to a position, based on the current measured. (I did that by essentially just cutting and pasting some code from a servo tutorial.)

The whole idea is to create data (the current sensed) and then be able to interact with the real world (position a pointer on a servo.) In this case, I’d like to make a simple “Speedometer” style physical display which would just show how much solar power I’m making.

The next step after that would be to use the data to interact with an electric car charging station (EVSE) or other power use that I would want to control, depending on how much solar power I have available. For example, I could have an electric car charging station which would allow the car to charge at full power, but have it charge more slowly when a cloud comes in front of the sun. That way, I would maximize my solar use yet NOT draw power from the grid for my car charging.

Anyways, I’m still just in the early stages of this monitoring system, but it’s pretty cool to learn how things work, and then figure out neat new ways to use them!

Until next time, stay charged up!

-Ben

PS: To be technically accurate, at this point, I am simply measuring current. I’m really trying to figure out the POWER of my solar, which would be measured in Watts. Right now, I’m just doing some multiplication times 240V to get power. The finished version of this project will include a voltage sensor so that truly accurate power can be measured, as well as tracking which direction power is flowing. That aspect is important back at the meter to see if I’m importing energy from the grid or exporting to it.

PPS: Here’s the video I shot when I was first starting to play with this:

 

 

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