Things you should know before buying a wind inverter

When looking for a grid tied wind inverter, you need to know more than just the amount of power required. In particular, you need to consider the working voltage and current of your turbine, and have an idea of the amount of power that it can harvest at different speeds or voltages.

You will also need to match the wind inverter to a wind turbine controller (sometimes called a rectifier or a wind interface) which takes the wild AC from the turbine and turns it into a smooth DC using a rectifier and capacitor. This wind interface or controller usually also includes some other functions such as switching on a dump load resistor if the wind speed gets too high, or the grid fails.

Most wind inverters have been developed from grid tied solar inverters. Companies such as SMA and Aurora / Power One have an extensive range of solar inverters, and have modified these to work with small wind turbines. So for example the SMA Sunny-boy inverter has equivalent products called SMA Windy Boy. The Aurora UNO 2kw inverter is also available as an UNO 2kw wind inverter.

The original version of the inverter will have MPPT tracking which optimises the energy harvested, depending on the voltage coming from the solar panels. This MPPT tracking is automatic and constantly seeks to optimise power. This type of power point tracking does not work for wind turbines or hydro. Here you need to be able to program the inverter with a table listing a series of voltages, and the power harvested at that voltage. This table is generally known as the Power Curve, but we prefer to refer to it as a “wind inverter power point table”. It is called a curve because generally the amount of power available is roughly proportional to the cube of the turbine voltage or RPM.

Aurora / Power One have developed a wind interface to match their inverters which can tell the inverter the RPM of the turbine. The Aurora wind interface measures the frequency of the incoming power from the wind turbine and gives a frequency reading to the wind inverter. You can then programme a table of frequency (or RPM) and power into the inverter. This is a much more accurate way of managing the power harvested, as you know the exact RPM at the time. Voltage is a less accurate determinant of turbine speed and power available.

Some wind turbine inverters have been developed from motor drives (which also use a different type of inverter). Generally these are slightly less efficient – the maximum efficiency is usually to be found in inverters that were designed specifically to convert DC power into grid voltage. So check the efficiency of your inverter from its data sheet. More importantly, the efficiency will not be constant. Depending on the DC voltage being fed into a wind inverter, its efficiency might range from 50% to 98%. If the data sheet just says “maximum efficiency 98%”, you need to find out more about what the efficiency level is at the sort of voltage and power your turbine spends most of its time working at.

The Aurora / Power One range of wind inverters between 3kw and 6kw can work in a voltage range of 50V DC up to 530V DC, and is able to withstand up to 600V DC without damage. Their 20kw and 27Kw Trio range of inverters work on voltages up to 800V DC, but start at higher voltages (you would be mad to have a 20kw turbine working at lower voltages anyhow).

Other small inverters may have a starting voltage of over 200V DC, and as such they may miss out on a lot of energy that is available at low wind speed. So you may find the turbine spinning, but no power being exported to the grid by the inverter.

You also need to consider where the inverter is to be used. The Aurora / Power One and SMA Windy Boy range of wind turbine inverters are IP66 rated, meaning that they can be used outdoors. Many inverters based on motor drives need to have open ventilation and have a lower IP rating. This may not be great in a shed with a lot of dirt, dust and damp.

Lastly, you want to be sure that your system is configured correctly. This requires expertise in designing the parameters for the wind interface or wind turbine controller. The resistance of the dump load, the voltage at which it may cut in or out must all be carefully designed to ensure that the system can be reliable, that the wind turbine is kept under control at all times, and that the grid tied wind inverter never receives DC voltage outside its limits, which would invalidate the warranty. There are also other settings in the inverter that need to be considered carefully such as start-up voltage, ramp rate, and of course the power point table needs to be optimised for the turbine being used.

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