Why a wind Mill used as a heat source needs MPPT.
Let's say that you have a 5 KW, Chinese wind mill that is used for heating purposes.
Since the generator is rated to a minimum of 5 KW, it may generate 2 or 3 more additional KWs.
The Wind mill starts to rotate and at that moment it has about 200 watts of energy, if one connects the 5 KW load the wind mill will brake immediately to a stand still.
So one needs to read the voltage and start inserting a low watts load or its equivalent, let's say that we have 1000 KW load, which by definition it is to heavy load for the 200 watts energy source.
The solution is to start to insert the load for a fraction of a second and read the voltage to determine the direction of the voltage changes, if voltage continues going up, the controller starts to increase the fraction of a second to present a greater load and the reverse if the voltage goes down with the inserted load for such fraction of time.
This reading and loading is done continuously and if the power increases, the load is inserted for a longer period of time until the generated power starts to be higher than the load so the variable time load is 100 % of the time ON.
The controller, at this moment, needs to insert another load , to maintain stability of the wind mill, and in this case the most practical way is to insert another 1000 watts load and resetting the first load to 0 time insertion.
So the first load is a PWM load that varies from 0 % time insertion to 100 % time insertion, and this variation goes up and down until an end ( 0 or 100 % ) is reached for the controller to add or remove a fixed load from the generator.
The second load is an equal fixed value load to the PWM load, and the subsequent loads can be equal or go in a binary step if the controller can do such variations.
So for this 5 KW ( 7 or 8 KW top power) the controller will have a number of loads that are inserted or removed as needed to maintain maximum power harvesting, and at the same time maintaining the stability of the Wind Mill
What happens if the wind keeps going up?.
If the power continues going up and the Furling has not started to rotate, then the controller in principle needs to have an ELC ( Electronic Level Controller) that has a set maximum generator output voltage allowed that if it is reached the controller starts to insert the loads at a pace fast enough that the generator is clamped to that maximum allowed voltage, let's say 240 volt AC.
The power, at the maximum voltage setting, may increase but the MPPT controller is not active because the ELC has taken over, it will become active once the generator voltage drops below the 240 volts limits and of course the RPM starts to slow down as well.
Regards
Nando