Simulink asynchronous generator voltage input clarification

Hi there.

I've found a lot of models on simulink of DFIG's using the simscape 'asynchronous machine block'. The left side of the machine with "Tm, A, B, C" inputs represents the stator side.

When the machine is operating as a generator why is it that we supply a voltage to the stator, this doesn't really make sense to me as the stator is producing active power fed to the grid/load. In operation as a machine it makes sense however.

I also undertsnd that the rotor requires an input current for magnetisation and therefore to induce a voltage.

See example here: DFIG Wind turbine Model - File Exchange - MATLAB Central (mathworks.com)

Could anyone possible clarify why is it that there is a voltage fed to the stator when operating as a generator as conceptually I don't understand it. Any response would be greatly appreciated.

Thanks,

Abd

Answer 1

[Peter O]

Hi Abd,

For an induction machine to operate, you must be able to generate a rotating field on both the rotor and stator. The fields couple through the air gap, giving you power transmission through torque and speed. The machine cares less about the direction of power travel than you'd expect!

The connections from Simscape set up both voltage and current constraints for the underlying electrical net. The machine itself appears to the network essentially as a grouping of inductors and speed-dependent source terms. So by connecting the speed, torque, and electrical terminals, Simscape is constructing a circuit network where it simultaneously solves for all the voltages and currents that make a consistent operating point and satisfy the power/speed inputs, too. If it helps, you can think of Simscape networks operating a little more like SPICE than an explicit ODE solver.

Contrast this with a purely Simulink driven system, where you'd generally begin by formulating the machine equations to solve for the output of the flux linkage differential equations in dynamic operation. The ODEs would still have source inputs of voltages, but there could be a feedback network with a load on the stator output that responds to current flow with the induced voltage on the stator side. In that way you would still be supplying the stator network with the terminal voltages. For steady-state operation, you can remove the terms and solve the characteristic equations for the voltages/currents/speeds/torques/etc explicitly from either voltages or currents or a combination of them. If you take the simulink-only approach it is often easier and more numerically advantageous to solve for the system in the rotating (qd) reference frame instead of abc variables.

Hope this helps!

[Abd Al-Rahman Al-Remal]

Hi Peter,

Thank you so much for your very detailed answer! That actually makes total sense now and your way of explaining that it's more of a network is really interesting!

I think my problem was expecting the system to look more like the general layout of a DFIG and it's connectionsl; however I took another look and, like you said, the model does have all the output values as it should (but just in the bus selector instead!).

I also didn't know that the direction of travel isn't as important as I had thought and I'm going to loook more into this actually as I never knew but is really cool!

Thank you again for your answer it really helped me a bunch!

Best,

Abd

[Peter O]

Abd, Glad it was helpful. Motors run as generators all the time in EVs when they're performing regenerative braking. That's probably the most practical example of it.

Answer 2

[Dr. Pemendra Kumar Pardhi]

Hello

Dear Abd Al-Rahman Al-Remal

The DFIG in wind energy conversion system worked as generator. Therefore it is able to deliver power to the utility grid from stator as well as rotor (At starting time it takes power from grid, to develop excitation voltage) also. From the stator it delivers power to the grid at grid supply frequency. But at the rotor end it generates the power at super synchronous speed. And hence at the rotor end AC-AC (AC/DC-DC/AC) converter is required.

[Abd Al-Rahman Al-Remal]

Hi @Pemendra Kumar Pardhi,

Thank you for your resposne and information on the operation of the DFIG it really helped and I appreciate you taking the time!

Best,

Abd