Direct torque control of a doubly fed induction generator working with unbalanced power grid

Abstract

Summary The paper describes direct torque control of a doubly fed induction generator (DFIG) executed in the stator stationary reference frame. A full model of DFIG is presented, and relations for the rotor voltages are derived. The case with symmetrical and asymmetrical grid voltage operation is presented along with the corresponding simulation and experimental results. As a control target, the minimization of the torque oscillations at sinusoidal stator current is chosen. No decomposition into positive and negative voltage sequence for presented algorithm is necessary. The proportional‐resonance (PR) regulator tuned for fundamental harmonic of grid voltage frequency is applied. As the torque value is independent of the reference frame, the variables are not synchronized neither with the grid voltage nor with the stator flux vector. The control method is presented, and its performance for balanced and unbalanced voltage is given. To obtain the required target, the auxiliary scalar variable calculated as a dot product of stator flux and stator current is used.

Rotor Current Feedback Based Direct Power Control of a Doubly Fed Induction Generator Operating with Unbalanced Grid

“…A possible DPC modification is given in (40), (41) for control in the rotating reference frame and (42), (43) for the stationary reference frame. The relations are derived from (3), ( 4) and (11). Similar principles were published in [8] for stator voltage oriented rotor current vector control.…”

“…Taking into consideration flux Equations ( 3) and ( 4) and Kirchhoff's current law for DFIG circuit (11) stator and rotor fluxes can be expressed as the relation where only the other side measured signals are used, as given in Equations ( 12) and ( 13).…”

“…Electromagnetic torque generated by DFIG due to quite easy access to stator and rotor variables measurements can be calculated in many different ways, one of which is presented in Equation (15). Taking into consideration flux Equations ( 3) and ( 4) and Kirchhoff's current law for DFIG circuit (11) stator and rotor fluxes can be expressed as the relation where only the other side measured signals are used, as given in Equations ( 12) and (13).…”

“…Currently, there are a few control approaches to DFIG operation with an asymmetrical power grid. One is connected with current control (stator or rotor) [7][8][9] the second with torque control [10,11] and the last with instantaneous power control [5,12]. Each approach has some advantages and disadvantages connected directly with the controlled variables and control targets.…”

Improvement of direct torque control applied to doubly fed induction motor under variable speed

“…The switching table selects the appropriate voltage vector of the inverter in order to orient the stator flux and electromagnetic torque in their desired values, also based on the position of the source voltage vector and the errors of the stator flux and electromagnetic torque [16,17]. In this work, we adopted a modified switching table compare to ensure optimal minimization in error.…”