Generator and Windy Boy

Due to several issues (detailed further down in this post), it is somewhat difficult for us to to directly connect a generator to the SMA-based system that we're building. However, we think we have found an good option, as described below.

Generator Use

We are designing our system so that the generator will only be used as a second-last resort to avoid low-SOC damage to the battery bank. (The last resort would be to ask a friend to bring over a portable generator)

Being pragmatic, we also suspect the generator will end up being used a few times during the fall and spring when we have weeks upon end of no sun, and thus will make a conscious choice that our need for electricity outweighs our ecological desire not to consume any fossil fuels. (We'll see how we do here)

We're installing a 1000 amp-hour battery bank. Our existing generator (an SDMO ALIZE 8000 SDE) is rated 8KW, but we've found that when run with a continuous load, it is best run it at 5KW (this works out to 20 amps of current at 240 volts).

Charging the battery bank from a 50% SOC to 100% SOC would take approximately 500 amps-hours * 48 volts, or 24000 watts-hours of power (assuming 100% efficiency). Derating it down to 30,000 watts-hours of power is more realistic, which means that we would have to run the generator for six hours. (assuming our calculations are correct)

AC Coupling

In an SMA system, a generator is typically connected to the AC inputs of the Sunny Island. However, the generator MUST match the phase configuration of the Sunny Islands, and SMA systems are very touchy with respect to which generators they work with. Other folks we know have not had much success in this area. Another problem is that dirty generator power is supplied directly to the loads, which is unacceptable to us.

(As an aside, we've lived in many places where there is extremely poor power quality, and we've observed very high mortality rates of electrical and electronic equipment when it is subject to frequent spikes, surges, dips and outages. Ever since 1995, we've run all of our electronic equipment through a double-conversion UPS — neighbouring businesses typically had to replace their equipment each year, and we've never had a major equipment failure yet. Power quality saves money!)

Ideally, we'd like to be able to connect the generator much like how another solar array or wind turbine is connected, via AC-Coupling. That way, all the regulation is done by the SMA equipment.

Doing some research, it appears that this is actually a supported configuration. Quoting from the Windy Boy manual, "Furthermore, the Windy Boy can be used as inverter for power conversion based on permanent magnet generators (hydro power systems, combined heat and power plants, diesel generator, etc.). The manufacturer of the wind turbine system or generator should have his system set up for operation with the Windy Boy (also see the Windy Boy planning guidelines in the download area of www.SMA.de/en)."

Here's a slide from SMA illustrating the concept:

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What appears to happen is that the AC from the generator is fed through a high-voltage rectifier to generate DC, which is fed into the Windy Boy. It's pretty straightforward from a theoretical standpoint, though you do need current limiting, like what is done with a EV charger. This can be accomplished by adjusting the load curve in the Windy Boy, or with an external AC -> DC converter.

Based on this, the most basic configuration would be:

AC Generator -> 30 amp Breaker -> DC Rectifier -> 30 amp DC Breaker -> Windy Boy -> AC Load Centre

Using the SMA Windy Boy protection box is an option, but it is designed for a three-phase AC input.

I've done some research on the web, and it appears that while uncommon, a number of people have set up this sort of configuration.

Has anyone on this board set up or heard of a similar configuration?

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