Hi Mohammad,

There are two aspects to power generated by wind turbines (and indeed, many other renewable energy based generators) that require active power control - both have to do with compatibility of the generation unit to the load or the grid at point of connection:

Hope that helped !

-Sanjay

Basically, we have two types of wind turbine: constant an variable speed.

A variable speed wind turbine is one in which the speed can be changed depending on the operational conditions. It means that the wind is capable of changing its shaft speed, whereas the other one is designed to keep the same shaft speed no matter which speed its blades develop. Take a look in the firs chapter of the book:

Doubly Fed Induction Machine: Modeling and Control for Wind Energy Generation Applications

https://ieeexplore.ieee.org/xpl/bkabstractplus.jsp?bkn=6047757

Cutting the long story short: the speed of the blades and the speed of the shaft are linked in a variable speed turbine.

Remember that the electric conversion is made by the electric machine that is connected to the turbine's shaft. So, if you make this machine produce torque, it will interfere in the shafts dynamics. Basically, we can mathematically represent this as follows:

J dw/dt = Tm - Te

where J is the inertial moment, w is the angular speed, Tm is the torque produced by the blades (with the help of the wind) and Te is the torque produced by the electric machine (which will operate as generator). It tell us that the electric machine is able to control the shaft speed (and the blade, as well).

So we get to the point: why using the generator for controlling the shaft (and blade) speed. The answer is quite simple: to make the turbine produce the maximum power possible.

To understand this you should take a look on the first chapter of the aforementioned book. Summarizing, the amount of energy generated depends on how much energy we take away from the wind and this amount depends on the speed of the blades. So, we use the electric generator for controlling the shaft (and blade) speed in order to get to the speed in which the system produces its maximum power. This is the active power control that you referred because changing the shaft seed we also change the amount of energy generated by the turbine.

I think the short answer is that the active power control adjusts the IG real power export (by adjusting the phase of the rotor-side mmf and the magnitude of the stator voltage) so as to balance the power extracted by the aero turbine blades (less losses). Otherwise the rotor speed would vary form that at which the blade pitch control optimises the wind powe extraction.

The active power of DFIG is derived from the power flow analysis performed on the equivalent circuit. The stator winding joule losses, as well as the iron losses, are neglected because these are few percents of the rated power, and ignoring them simplifies the power flow analysis. From the equivalent circuit, you will derive

1. The mechanical active power, Pm

2. Stator Power, Ps

3. Total power to the grid, Pt

4. Slip

5. Rotor Power, Pr

So, from the equivalent circuit, you derive

Ps=Pm/1-s

Pm=Pt=Ps-Pr

Pr=sPs=(s/1-s)Pm

Edward,

And what do you do when the 'equivalent circuit' value of slip (which implies you know Ps) gives a value of turbine tip speed that is incompatible with the optimum operating point of the wind-speed/ blade pitch/power extraction coefficient? Re-run your loadflow?

Anyway, I think you're missing the point of Mohammed's question: what's the conceptual meaning of DFIG active power control?

Hi James,

I presume you asked a question concerning high-speed control in DFIGs because I do not argue.

The machine is normally shut down during extreme weather conditions. Typical DFIGs use VSI back-to-back converters to control the torque, and consequently, the speed.

Thanks @Cleiton Freitas

I can summarize that the aim of active power control in a DFIG-based wind turbine is two-fold:

1) It means implicit control of rotor speed, that is our final goal for active power control may be to keep the operating point on the MPP to have maximum aerodynamic efficiency.

2) From the grid point of view, active power should be controlled so that balance between production and consumption is held.

Would you please comment on my deductions?

Mohammad,

I'll try to answer your points and your original question.

It's not really practical or economic for a type 3 (DFIG) or type 4 (Directly-coupled WTG) to adjust its electrical output. Its blade pitch control aims to operate the turbine at the optimum pitch angle and tip speed so as to extract maximum wind power at the given wind conditions (not necessarily max efficiency). With this level of power input, the IG electrical power generated (either via rotor or stator) is controlled by varying rotor excitation amplitude, phase and frequency so as to attain steady-state power balance at the IG airgap (otherwise the rotor speed would vary up/down and the gearbox would risk mechanical failure). Essentially it dumps generated electrical power into the ac system roughly equal to the extracted windpower, irrespective of system load or frequency.

Dear James Toal ,

Thanks for your kind response.

If this is so (electrical output is not adjusted), then why many research works concentrate on different control approaches for active/reactive power regulation of DFIG's/PMSG's, etc?

Mohammad, no way would I try to interpret the mindset of research workers ;-)

But just consider the economics (i) of producing less power than you can extract from the available wind, and (ii) of modifying and quality-assuring peak-tracking software for the blade pitch control.

Hi Mohammad,

Thank you for having sought my inputs once again in order to complete the discussion.

The points I offered in my earlier reply to your question still stand; that is, continuous conversion of mechanical power to electrical is an important requirement, while compatibility to rest of the utility system is another one.

Having said that, your more focused query (as it has emerged from the discussions with different respondents) has to do with the practical importance to control the active power output of the electric generator in wind energy applications, that is (going by your discussion with James), why go for maximum efficiency control (or MPP, such as vector control) at all if everything is decided from the turbine side ?

Frankly, from one fundamental viewpoint your doubt is a natural one, since in vRES (such as WECS) one doesn't spend money on the energy source. That is the so called fuel cost (hence the running cost) is negligibly small. So since the energy source is essentially for free, how does it matter if we employ inefficient control - maybe losing some of the power as mechanical or electrical conversion losses ?

I would offer two different reasons to address this doubt - hope you find them to be of some help. To initiate the thought, consider a turbine that is inputs some variable quantum of aerodynamic energy from nature. Now for the utility that aims to use the power output,

• it makes sense to extract the maximum possible power, simply because it is green power ! That is, if the wind turbine can be made to output the most, then connected fossil fuel generators will accordingly need to generate less, with associated environment friendly benefits. If you check out point #2 in my original reply, you will realise that this is a detail of the compatibility aspect I tried to highlight.
• maximum power extraction implies minimal losses (both mechanical and electrical) within the overall WECS unit. Thus lower dissipation within the WECS, easier cooling, and on the whole less thermally stressful performance by the unit. Hence follows improved reliability, and possibly longer lifespan of the WECS. This relates to #1 in my original reply.

Did that help ? (Liked your persistent curiosity, by the way !!)

Cheers !

-Sanjay

Dear Professor Sanjoy Roy , Thanks a lot for your informative response.

There is one point in your last statement: " maximum power extraction implies minimal losses (both mechanical and electrical) within the overall WECS unit... "

As you know, the MPPT control has to do with active power control (or the corresponding variables, i.e. torque and speed) but the loss minimization control (LMC) has to do with reactive power control (or reactive current component control). It is evidenced in many papers, where the aim is simultaneous MPPT and LMC. For instance, you can refer to:

Article Optimal Efficiency Control in a Wind System With Doubly Fed ...

So, how could you justify your statement (that is, the MPPT necessitates LMC)?

Thanks a lot for guiding me.