Adeola Balogun

Decoupled Direct Control of Natural and Power Variables of Doubly Fed Induction Generator for Extended Wind Speed Range Using Feedback Linearization

The electromagnetic torque, stator power, and the grid side converter power outputs of a doubly fed induction generator (DFIG) are differentiated to obtain a feedback linear relationship with the rotor voltage vectors serving as the control inputs. The derived model is such that can allow for a transition of DFIG to operate at low wind speed by shorting its stator terminal to one another. The torque and power quantities are the state variables of the system, which are insensitive to coordinate system orientation. Steady-state models are used to obtain optimal operating regimes with respect to minimal electrical losses. Decoupled torque control is developed by field orientation, while decoupled power control is derived from voltage orientation of the DFIG. Transients originating from transitioning into low wind speed operation are investigated. Robustness of each of the decoupled controllers against parameter variations is investigated by locating the poles of its closed-loop transfer function. The results for a 5-Hp machine are presented.

Determination of Steady-State and Dynamic Control Laws of Doubly Fed Induction Generator Using Natural and Power Variables

A doubly fed induction generator model is presented whereby the natural and power variables are the state variables. The natural variables are the electromagnetic torque (Te), the reactive torque (Tr), the magnitude of the rotor flux linkage (λr), the magnitude of the stator flux linkage (λs), and the rotor speed (ωr). The power variables are the real power (Pf) and reactive power (Qf) generated/absorbed by the grid-side converter into/from the grid. Simulation of the dynamic natural variable model of the induction machine is compared with a vector variable simulation. Steady-state operating regions are established for various power factor operations. The optimal stator power factor operation is estimated. A direct control of torque and power variables is developed. The robustness of the developed control against rotor parameter variation is investigated using small signal analysis and is compared with vector control. Results are shown for a 5-hp machine.

Efficiency optimization of doubly-fed induction generator transitioning into shorted-stator mode for extended low wind speed application

The rotor voltage of a doubly-fed induction generator (DFIG) increases with decrease in wind speed. Hence, much decrease in wind speed can result in the rotor voltage exceeding its rated value. In order to extend the operation of the DFIG for low wind speed application, the stator terminals of DFIG are shorted to one another whereby the stator loses its synchronous angular frequency characteristics. Transitioning into a shorted-stator mode ensures an operation whereby the rotor voltage decreases with decrease in wind speed. Since the stator of the shorted-stator mode is disconnected from the grid, the voltage of the grid no longer contributes to the development of the stator flux linkage. Therefore, for an effective transitioning, a control scheme is developed to ensure that the resulting transients and stator flux linkage are regulated within permissible limits with the objective of minimizing copper and magnetization losses. Some results are presented for a 5Hp machine.

A five-phase interior permanent magnet generator-diode rectifier with a non-integer number of stator slots per phase as the front end of a wind generation system

This paper presents the models of a five-phase interior permanent magnet machine (IPM) feeding a rectifier resistive load. The model predicts the performance of a machine with a non-integer number of stator slots per pole. Using the stationary and rotor reference frames, the approximate diode switching functions are derived. Then harmonic balance technique is used to obtain the steady state average and ripple models for the prediction of the systems performance. The steady state model helps in determining the equivalent ac resistive load across the five-phase generator output capacitor bank. The obtained results reveal that despite the imbalance of the stator windings, the machine is still able to produce near balanced voltages at normal operation, and also the fraction of the rated output power with very little harmonic components in the output load voltage when a rectifier leg is open.

Shorted stator induction generator for low wind speed power application

Shorted stator induction generator (IG) is presented for low wind speed power application. An analytical model is derived for predicting electrical performance of the generator while operating in regions 1 and 2 of turbine power versus wind speed curve. This paper determines an explicit optimal stator electrical speed of operation since the stator is disconnected from synchronism with the grid. A vector control scheme is developed for the IG. Simulation and experimental results are presented for a 5hp machine.

Determination of steady state control laws of doubly - fed induction generator using natural and power variables

A doubly - fed induction generator (DFIG) system is presented in terms of its natural and power variables as the state variables. The natural variables are the electromagnetic torque (T<inf>ℯ</inf>), the reactive torque (T<inf>r</inf>), the magnitude of rotor flux linkage (λ<inf>r</inf>), the magnitude of stator flux linkage (λ<inf>s</inf>), and the rotor speed (ω). These are used in modelling the induction machine and the machine side converter (MSC) of the system. The real power (P<inf>ƒ</inf>) and reactive power (Q<inf>ƒ</inf>) that the grid side converter (GSC) exchanges with the grid make up the power variables of the system and are used to model the GSC. Simulation of the dynamic natural variable model of the induction machine is carried out and the results are compared with a conventional induction machine simulation technique. Steady state analysis is presented for various power factor operations. A method of predicting an optimal stator power factor operation is proposed.

Decoupled control of natural and power variables of doubly-fed induction generator using feedback linearization

The electromagnetic torque, stator power, and the grid side converter (GSC) power outputs of a double-fed induction generator (DFIG) are differentiated to obtain a feedback linear relationship with the rotor voltage vectors serving as the control inputs. The derived model is such that the torque and power quantities are the state variables of the system, which are insensitive to coordinate system orientation. The steady-state equations from the derived model are used to obtain the optimal operating region with respect to minimal electrical losses. Decoupled torque control is developed by field orientation, while decoupled power control is derived from voltage orientation of the DFIG. Robustness of the decoupled controllers against parameter variation is investigated by locating the poles of its closed-loop transfer function. Results for a 5Hp machine are presented.

Design and analysis of a five-phase interior permanent magnet generator with a non-integer number of stator slots per phase

Traditional electric machine design requiring the balancing of the phases both in phase voltage magnitudes and the angles between adjacent phases informs that the number of stator slots per phase must be an integer. There are situations however, where it is cheaper to use existing laminations for designs with non-integer number of slots per phase to build generators for specialized applications especially for DC power. In such cases a multi-phase design ensures a measure of fault tolerance and the unbalances in the phase angles between the winding phases have little effect on the quality of the output DC voltage. The methodology for the design of a five phase, 4-pole interior permanent magnet (IPM) generator using 36 slots as a source of DC power is outlined. Experimental results for the operation under healthy and faulty conditions (one and two phase windings open) are presented for verification of fault tolerance of multi-phase machines.

Influence of non-linear loads on the operation and power flow of induction generators

This paper considers the analysis and operation of an autonomous three-phase squirrel cage induction generator when it is feeding linear and non-linear loads. The nonlinear load comprises of a static voltage and frequency dependent component and a three-phase diode rectifier supplying a composite load containing constant power, constant current and constant resistance components. The dynamic model of the generator accounts for the effect of the main air-gap flux linkage saturation which with the excitation capacitors determines the steady-state operating points. Extensive simulation results are provided showing the starting transients and loading dynamics of the generator. The inducement of oscillations in the frequency and waveforms of generator variables due to the presence of the three-phase diode rectifier when the generator runs at a constant rotor speed are exemplified. The flows of the real and reactive powers with the rectifier load show that although the average active powers in capacitors and inductances are zero over the switching period of the three-phase diode rectifier, they are non-zero instantaneously and time-varying. Some experimental steady-state characteristics results obtained from an induction generator feeding a rectifier and an ac load are provided.

Natural variable simulation of induction machines

A method for obtaining natural variable model of induction machines (IM) is presented. The variables of the model are electromagnetic torque (Te), reactive torque (Tr), magnitude of rotor flux linkage (λr), magnitude of stator flux linkage (λs), and rotor speed (ωr). These variables do not change in any reference frame of transformation. The model obtained is expressed in a state-space form with the natural variables as state variables. Simulation is carried out and the results are compared with a conventional induction machine simulation technique. Simulation results show the efficacy of this model in predicting electrical behavior of induction machines. Flux linkage orientation is done to facilitate the simulation, which is similar to method of alignment in vector control.