in the MPPT curve, the region to the left of the maximum is an instable region, while the right one is stable. also, if you want to limite the extracted power because of machine limitations, braking down the generator requires more torque, so the limite must be exceeded. in this case, you must accelerate the generator, so the operating point will move toward the stable region and the extracted power will be limited.

Elyazid Amirouche, But while breaking, the only possibility is to draw current from the generator. This will probably not end up in speeding up the generator, moving towards the stable region. So how to do it?

Am I correct to conclude that with a MPPT system purely dedicated in finding the MPPT, in combination with a own design stand alone braking system, it would not be possible to achieve the goal of limiting torque while operating in the stable region?

Exactly, you can not use a braking system in this case, you must reduce the electromagnetic torque instead in order to speed up the generator.

Thank you for your answer! In order to reduce the torque to prevent exceeding the maximum, the MPPT circuit should draw less current, reducing the torque, it will in this case speed up the wind turbine (move further to the bottom right along the graph.

This leaves the question, where is breaking resistor for? I could imagine if the speed is too high, the voltage is therefore too high, which could lead to exceeding inverter input maximum values. Activating the breaking resistor will increase the torque again since it draws extra current and therefore could exceed rated torque on the generator? this puzzles me

I haven't heard about braking resistor in any MPPT strategy for wind turbines, maybe it is for another purpose than what is discussed here. do you have any references?

maybe it is used when the available power is lower than the generator rating but exceeds the load demand of the grid. in this case, we prefer to store the extra power in batteries or in a water pumping system than to lose it in resistors

When the generator accelerates and exceeds its rated speed, another technique is applied, the flux weakening, where the air gap flux is reduced using the d axis current.

Thank you very much for your helpful input.

I am new to this field and I could have been misinformed about the braking resistor. What I read now is that they are used as dump load when in an off grid system to dissipate energy to when a battery is full. But if you would already have a mechanical brake you could just stop the wind turbine from spinning I guess. So I think for this purpose I do not need the braking resistor. So, the MPPT system should make sure to operate the turbine at max power and within its mechanical capability limits. When this is no longer possible because the wind speed is too high, the turbine should be brought to a standstill.

I did read that I maybe I would need a braking resistor when the wind is too high and the turbine should be brought to a stand still by a mechanical brake. This is done by initially slowing down the turbine to prevent the mechanical brake from wearing:

"A mechanical drum brake or disk brake is used to hold the turbine at rest for maintenance. Such brakes are usually applied only after blade furling and electromagnetic braking have reduced the turbine speed, as the mechanical brakes would wear quickly if used to stop the turbine from full speed."

https://www.daviddarling.info/encyclopedia/W/AE_wind_turbine_braking.html

Other examples of diversion load usage is for example in the ABB 4000/7200-WIND-INTERFACE. This load is automatically activated at 530 V according to its datasheet. In this case it is probably used to protect its corresponding inverter rated for an absolute maximum of 500 V. This would also probably effect the MPPT due to increase in torque when the diversion load is activated.

You are welcome, don't hesitate to ask for more assistance if you need it