DFIGRHS:

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   Doubly fed induction generator model.
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   Form:
   [xDot, tE, pD] = DFIGRHS( x, t, d )
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   ------
   Inputs
   ------
   x          (6,1)  States [iDS;iQS;iDR;iQR;omega;angle]
   t          (1,1)  Time (not used)
   d          (1,1)  Machine data structure
                     .p      (1,1) Number of poles
                     .lM     (1,1) Mutual inductance (H)
                     .lR     (1,1) Rotor inductance (H)
                     .lR     (1,1) Stator inductance (H)
                     .rR     (1,1) Rotor resistance (Ohm)
                     .rS     (1,1) Stator resistance (Ohm)
                     .u      (4,1) Phase voltages [uDR;uDQ;uDS;uQS] (V)
                     .omegaS (1,1) Synchronous speed (rad/s)
                     .tM     (1,1) Mechanical torque (Nm)
                     .j      (1,1) Inertia (kg-m^2)

   -------
   Outputs
   -------
   xDot       (5,1)  State Derivatives  [iDS;iQS;iDR;iQR;omega;angle]
   tE         (1,1)  Electrical torque (Nm)
   q          (2,1)  Reactive power [s;r] (W)
   p          (2,1)  Power [s;r] (W)

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   Reference: T. Demiray, F. Milano and G. Andersson,"Dynamic Phasor
              Modeling of the Doubly-fed Induction Generator under
              Unbalanced Conditions 
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