Re: Interfacing wind turbines to the grid
Reply #21 –
Success!
My first prototype* did its thing without any magic smoke escaping.
I haven't posted about this for several months because I spent February building the kit and deciding to drastically simplify it compared to the system I described and sketched for you all before. In late Feb I had parts of it working while I refined the code. In March the WT started making funny noises and then my charge controller quit. So it's been out of service until this week. That paused the development of this idea for 3 months. Also, instead of using discrete devices like FETs or SSRs to regulate the battery current, I've chosen to use a buck controller instead. It got me to the testing phase quickly.
The buck controller converts the 50-60V from the battery into a flat regulated 48.0V. Feeding a microinverter with that has some limitations (I'll describe later) but for now I'm satisfied that the microinverter won't run away to infinity or go berzerk. When plugged into 240V AC the microinverter's output went happily to the AC feed in my garage. I happened to have my car plugged in at the time, meaning that most of the 300W being exported actually went into the car. 
First time my car was charged from WIND.
The buck converter didn't have any trouble either. It was windy and as long as wind power was coming in, the battery was at float about 54V feeding the bucker enough that it maintained 48.00V without a twitch. This sounds great at first but there's a detail that makes this a small problem. With 6 Amps going through the buck controller it's little fan was running on the heatsink, but its little MOSFETs were below 35C (95F).
Later, while still running the buck converter + microinverter combination, I shut down the wind turbine for a while. The charge controller stopped, too, and while the microinverter was still running the battery voltage dropped to about 50V. At that point, the buck converter's output started to sag below 48v. The microinverter actually responded to this, interestingly.
Microinverters have a start-up procedure that they do every time they start up. The one I'm using has a 5-minute start where it watches the incoming voltage before it connect any current flow. It's probably making sure it's not responding to a spurious voltage pulse. Once current does start to flow, the microinverter varies the current randomly. It's strange to watch because the current swings from 2 to 6 to 4 to 5 to 3 to 4 to 6 and basically wanders all over the place. Eventually I figured out that it's building some kind of "performance table" in its memory. Once it's built the performance table then it knows how the "solar panel" responds to change in current. This makes sense because solar panels have a varying power curve. Full sun, partial sun, clouds, changing angle during the day, all affect them. So it seems the microinverter will keep trying to calculate what this performance table looks like until it has one that makes sense and can predict where the MPPT point is. While it doesn't have a solution to this problem, it keeps shifting current up and down randomly.
For all the time the buck converter was feeding the microinverter a tightly-regulated 48.00V, the microinverter couldn't do anything with the current that would affect the voltage. This gave it nothing for the MPPT algorithm to use and it doesn't stop randomly testing the current/voltage changes. During the time the WT was continuing to charge the battery and it was held at float, the buck converter could maintain its 48.0V, preventing the microinverter to settle into its full power MPPT operation. Then, when I did shut off the WT, and the battery voltage sagged, this is what allowed the battery voltage, and thus the output of the buck controller to finally start varying with current. Then the microinverter got the answer it was looking for. Once that happened, it's output went from fluctuating between 100W to 300W to suddenly rise to a nearly steady 600-630W.
* AKA "box of random bits"
Topic: Interfacing wind turbines to the grid (Read 2426 times)
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