Load torque compensator for Model Predictive Direct Current Control in high power PMSM drive systems

In drive systems the most used control structure is the cascade control with an inner torque, i.e. current and an outer speed control loop. The fairly small converter switching frequency in high power applications, e.g. wind turbines lead to modest speed control performance. An improvement bring the use of a current controller which takes into account the discrete states of the inverter, e.g. DTC or a more modern approach: Model Predictive Direct Current Control (MPDCC). Moreover overshoots and oscillations in the speed are not desired in many applications, since they lead to mechanical stress, oscillations in the power etc. For this reason the integrating action of the speed controller must be limited leading to offsets in the speed control and a poor response to load torque variations. For this reason a load torque estimator is proposed in this paper in order to further improve MPDCC dynamic behaviour. It compensates the load torque influence on the speed control setting a feed forward torque value, i.e. current reference value. The benefits are twice. The speed controller reaches immediately the speed reference value avoiding offsets which must be compensated by the weak integrator. Moreover, a better response to load torque variations which are detected and compensated leading to small speed variations is obtained.