ASK THE EXPERTS: Capturing Varying Wind Energy

Intermediate

A wind turbine generates a different voltage/amperage at different wind speeds, so how does it produce usable power over these varying speeds? Do the power inverters or converters regulate a variable input voltage to a constant output voltage for usage, such as to charge a bank of batteries or feed the grid?

Paul Johnson • via email

There are actually a few different ways to convert the variable-frequency, variable-voltage three-phase “wild AC” from a typical wind turbine into a usable form, and the best choice depends on your application.

In most off-grid and grid-tied with battery backup systems, the turbine’s wild AC output is rectified to DC, which directly charges the battery bank. The only components between the wind turbine and battery bank are the turbine brake switch, rectifier assembly, and a DC circuit breaker. The batteries provide “control” by regulating turbine voltage down to their own level. This simple strategy works well until the battery bank reaches a full state of charge and can’t store any more energy. Unlike a PV array, a wind turbine can’t be disconnected from the battery bank—doing so could cause the turbine to overspeed and possibly be damaged. Most wind turbines must have an electrical load on them at all times.

The most common solution is to install a diversion-load controller (also known as a “dump-load” controller) connected directly to the battery bank to send any surplus energy into air- or water-heating elements. This keeps a load on the turbine while preventing overcharging and still provides a good three-stage battery-charging regime. A typical PV controller would shut down the flow into the battery bank to prevent overcharging, while a diversion-load controller simply dumps it directly from the battery bank.

For grid-tied wind systems with no battery bank, new wind turbine control technologies were developed using grid-tied inverters and maximum power point tracking (MPPT) electronics that send energy directly to your loads and sell excess generation to the utility. These inverters are more complicated than MPPT inverters for PV since the turbine still must always have a load on it. With grid-tied wind inverters, you must program the power curve of the wind turbine into the inverter so it can adjust how hard it loads the turbine in rapidly changing wind conditions. A dump load is still required in case the grid goes down.

MPPT control can also give substantial power boosts, and allows you to “tweak” power curve settings for maximum performance—gaining 15% to 20% is common. One controller manufacturer, MidNite Solar, has developed an MPPT wind controller for battery-based systems so you can dial in that same extra performance

Dan Fink • Buckville Energy Consulting

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