No abstract

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-backconverter 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.

No abstract

This paper describes a current limiting technique for V/f controlled induction machines. In V/f control method stator current is not directly controlled so the implementation of current limiter is a challenging task. The current limiter proposed in this paper is composed of a programmable ramp and two proportional-integral (PI) controllers which keep the stator current below the limit. The programmable ramp provides limiting of the stator current during the induction machine acceleration and deceleration whereas the PI controllers, if necessary, ensure a further decrease of the stator current during the transients. Moreover, the PI controllers also keep the stator current under the limit during the steady state. The proposed method is implemented in digital control system and verified on the laboratory model. Experimental setup consists of a squirrelcage induction machine (560 kW) connected to the grid via backto-back converter and permanent magnet machine (375 kW) connected to the grid via industrial frequency converter. Experimental results show the satisfactory performance of the proposed current limiting technique.

No abstract