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Maximum Power Point Tracking (MPPT) has been used for photovoltaic (solar) power for a long time. It is more recent for wind turbines. What MPPT does is to always load the wind turbine just right, so it delivers the most energy at any wind speed.

When the wind blows through the turbine it will produce a certain amount of torque on the blades. For each wind speed there is a specific rotor RPM where this torque, and power output, will be at its maximum; speed up the blades at that wind speed (higher RPM) and drag will increase so less torque is available, slow down the blades (lower RPM) and blade lift will decrease, resulting in less torque. It helps to think of this point of maximum torque as the best lift-to-drag ratio for the blades, even if this is not completely correct it is close enough. In real life the wind speed is not constant, but changes continuously, and with it the best rotor RPM for maximum torque will change as well. What is important to remember is that there is an optimum RPM for each wind speed, and that is where we want to run the wind turbine to achieve MPPT.

Another way to think about the best power point for a wind turbine rotor is with the use of the Tip Speed Ratio (TSR) concept. The TSR is the number you get when dividing the speed at which the rotor blade tips travel, by the speed of the wind. For most wind turbines the TSR is in the range of 5 … 10, and it is frequently used with wind turbines because it carries lots of information in a single number: A high TSR means a noisy wind turbine, with tips traveling at great speeds, resulting in high wear as well. Usually the efficiency of high TSR machines is not great, because the blades are working in the higher-drag end of their range. Its usefulness for MPPT comes from the way airfoils work: The point where the airfoil works best, the best lift-to-drag ratio if you will, is at a certain angle of the blade to the wind, the angle-of-attack (AOA) of the airfoil. This AOA actually stays almost constant over a wide range of wind speeds (this is not entirely true; the Reynolds number is a factor in this equation and it changes as the wind speed changes, but for our purposes we can assume the best AOA is the same at all wind speeds). To keep the AOA constant as the wind speed changes, means that the tip speed of the blade has to change proportionally with the wind speed. Double the wind speed, double the RPM, double the tip speed, and the resulting AOA stays the same. Remember that the TSR is tip speed divided by wind speed, and if those two numbers change in proportion to each other that means the TSR needs to be constant over a range of wind speeds for the best power point!

The way MPPT is done by the wind inverter is by loading up the rotor in such a way that at each wind speed the rotor torque is at its optimum. Since power is torque times RPM, what the inverter really needs to know to track the maximum power point is how much power to extract from the rotor for each rotor RPM. The inverter does not have direct access to the rotor RPM, so it uses a different parameter that is proportional to it. Rotor RPM corresponds directly with the frequency of the voltage coming off the AC alternator of the turbine, and the Aurora inverters can use this. It also corresponds closely, not quite directly but the difference is small, to the alternator’s voltage. That is why most wind inverters and MPPT tables use the voltage of the alternator after rectifying it to DC. Programmed into the inverter is a table that has a number of points linking voltage versus inverter output power (sometimes frequency vs. output power, though Voltage is the easier one to work with). The inverter will use these points (up to 16 points the Aurora inverters) to create a curve by interpolating between points as needed. That is the MPPT table, and MPPT curve, which is different for each brand and type of wind turbine.

When the wind is blowing the turbine’s rotor will spin, and the inverter will load the rotor according to the MPPT table it has. That load on the alternator results in a torque on the blades, and if that torque matches the torque the wind exerts on the blades the rotor will not speed up or slow down. The other side of this is that if the inverter load is not enough to match the blade torque, the blades will speed up and rotor RPM will increase, the voltage will increase, and the inverter will increase the load according to the MPPT table until the blades stop speeding up (and vice-versa). The net-result is that at each wind speed the rotor will continuously reach equilibrium, where the alternator torque (created by the inverter load) matches rotor torque, and the turbine will automagically gravitate to this RPM. If the wind picks up the rotor torque will exceed the inverter torque, and the RPM will increase. The inverter sees this and increases the load accordingly, until a new equilibrium RPM is reached. If the MPPT table was made in such a way that this equilibrium RPM is the point of maximum torque for a wide range of wind speeds we will truly be tracking the maximum power point and get the most energy out of the wind turbine. Another way of saying the same is that the MPPT table should be such that the TSR of the wind turbine is kept at its optimum regardless of wind speed. That is what MPPT does: Load the turbine such that it is always running at its most efficient, extracting the maximum amount of power from it, at any wind speed.

Deriving a working MPPT curve for a new wind turbine is not rocket science (luckily!). In fact, just by using the rotor diameter, voltage characteristics of the alternator, and an educated guess of its efficiency, it would be possible to get a curve that works, at least well enough to get started. To make something a bit more precise it is good to measure at least one point high up on the power curve, and one at the lower end. While power in the wind follows a cube relationship with wind speed (and RPM of the alternator, since RPM has to vary linearly with wind speed to keep the TSR constant), most wind turbines are not efficient enough to follow a cube relationship for output power vs. voltage or frequency. In practice, most turbines end up with an MPPT curve that is between a square and a cube relationship (the more efficient ones are closer to a cube). MS-Excel can greatly help, due to its graphing ability and build-in functions that can map a smooth curve through a few points. Luckily, the efficiency of most wind turbine airfoils does not change all that much even if the TSR is off from its best value by quite a bit. That means the MPPT table can be off considerably, without noticing much difference in energy production.

To work properly the wind inverter needs an MPPT table that is specific for the wind turbine that you hook up to the inverter. The MPPT table is simply a list of numbers, it can be as little as two points (though we suggest a minimum of 3 points) to a full 16 points. Power-One makes the AuroraInstaller program available to load the MPPT table into the inverter. AuroraInstaller can also read the table that is in the inverter, or modify it. The process is very easy and the AuroraInstaller manual describes it in detail. Of course, we are here to help you with this if needed.