Martina Kutija

Comparison of maximum torque per ampere and loss minimization control for the interior permanent magnet synchronous generator

In this paper maximum torque per ampere and loss minimization control methods for an interior permanent magnet synchronous generator are compared. Permanent magnet synchronous generators have wide application in wind energy conversion systems due to high efficiency, high power density, high power factor and wide speed operating range. Maximum torque per ampere control method finds optimal d-and q-axis stator currents that produce desired electromagnetic torque while minimizing stator current magnitude. Loss minimization control method takes into account both copper and iron losses and finds optimal d- and q-axis stator currents that produce desired electromagnetic torque while minimizing total power loss. Characteristic curves for both control methods considering magnetic saturation are presented. Feed-forward torque control model developed in MATLAB/Simulink was used to verify described control methods.

Control of generator- and grid-side converter for the interior permanent magnet synchronous generator

Permanent magnet synchronous generators have wide application in wind energy conversion systems. If interior permanent magnet synchronous generator (IPMSG) is connected to the grid by a full scale AC-DC-AC converter, the wind turbine can be operated to extract maximum wind power at different wind speeds by optimally adjusting shaft speed. In this paper the model of the AC-DC-AC converter and related control structures for the control of IPMSG were developed in MATLAB/Simulink. Control of generator-side converter was achieved using field oriented control (FOC) which separately controls the electromagnetic torque and the magnetic flux. Optimal stator current references were calculated using maximum torque per ampere algorithm (MTPA) which ensures minimum stator current magnitude for electromagnetic torque reference. Control of grid-side converter was achieved using voltage oriented control (VOC) which separately controls active and reactive power injected to the grid. Outer control loop, with slower dynamics, ensures that DC circuit voltage is kept at reference value which ensures transfer of the active power from the DC circuit to the grid. Inner control loops, with faster dynamics, control the d- and the q-axis grid currents. Using developed control structures, the stator and the grid current PI controllers were designed using technical optimum while DC voltage PI controller was designed using symmetrical optimum. Proposed control methods were verified by simulation using laboratory model parameters.

Rotor flux estimation method for cage induction generators used in wind power applications

This paper describes the rotor flux estimation method for squirrel-cage induction generators (SCIGs) used in wind power applications. The proposed method estimates the rotor flux angle and magnitude in rotor reference frame by using measured stator currents, rotor angle and rotor time constant. The performance of the proposed method is compared to the performance of the voltage based estimator which is commonly used flux estimation method in sensorless field oriented vector control (FOC). The pure integrator in voltage based estimator is replaced by a low-pass filter with adaptive compensation and a phase locked loop (PLL) based position controller. Both methods are implemented in a digital control system and verified on the laboratory model. Experimental setup consists of a squirrel-cage induction machine (560 kW) connected to the grid via back-to-back converter and a permanent magnet machine (375 kW) driven by the industrial frequency converter and used as a wind turbine emulator. The advantages and disadvantages of the proposed method are demonstrated by experiments.

PLL-based Rotor Flux Estimation Method for Sensorless Vector Controlled Squirrel-Cage Induction Generators

This paper presents a new rotor flux estimation method for sensorless vector controlled squirrel-cage induction generators used in wind power applications. The proposed method is based on a phase-locked loop (PLL) and the orthogonality between the rotor flux space vector and its back electromotive force (EMF) space vector. Rotor flux is estimated using stator voltage equations without integrating the back EMF components in the stationary reference frame and the well-known difficulties with the implementation of pure integrators are thus avoided. Moreover, the proposed method ensures successful magnetization of a speed-sensorless squirrel-cage induction generator at nonzero speeds which makes it suitable for wind power applications. Experimental results on a 560 kW squirrel-cage induction generator are presented to confirm the effectiveness and the feasibility of the proposed method.

A current limiting technique for V/f controlled induction machines

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 squirrel-cage induction machine (560 kW) connected to the grid via back-to-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.

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.