Computer based investigation of the load influence on DFSM drive stability

This paper presents a general analysis of the steady-state stability of doubly fed synchronous machine (DFSM) drives with emphasis on the load torque influence. The study concerns the voltage-controlled DFSM with a voltage source in the rotor circuits, and the current-controlled DFSM with a current source in the rotor circuits; where in both machines the stator circuits are directly connected to an infinite bus. The stability analysis is carried out using the rigorous eigenvalue method. It has been found that the stability of the current-controlled DFSM increases considerably when the load torque is a function of the speed, which is not the case with the voltage-controlled DFSM where the drive stabilization brought by the load remains relatively low. A simple assessment of the DFSM drive stability using the synchronizing and damping torque approach is also presented. We have found that the influence of the load concerns only the damping criterion, and the synchronizing criterion remains the same as under no load operation.