Rotor flux estimation for speed sensorless induction generator used in wind power application

This paper compares two methods for rotor flux estimation of sensorless vector controlled squirrel-cage induction machine used in wind power applications. The compared methods are based on the integration of stator back electromotive force (back-emf) which is obtained from the machine parameters and measured voltages and currents. Rotor flux estimation based on the integration of back-emf leads to an estimation error due to dc drift and initial value problems associated with pure integrators. In order to overcome these problems, two methods for rotor flux estimation are compared in this paper. In first method the pure integrator is replaced by a low-pass filter (LPF), whereas the second method has a quadrature detector to detect the orthogonality between the estimated flux and back-emf and a PI controller to generate an appropriate compensation feedback level. Both methods are implemented into digital control system and verified on the laboratory model. The experimental setup consists of a squirrel cage induction generator connected to the grid via back-to-back-converter and the permanent magnet motor driven by industrial frequency converter and used as a wind turbine emulator. Experimental tests carried out on the induction generator connected to the grid demonstrate advantages and disadvantages of both methods.