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Summary: In a wind energy conversion system (WECS) based on a doubly fed induction generator (DFIG), a fast and smooth automatic synchronization of the DFIG with the utility grid is always desirable. Additionally, elimination of the need for a speed encoder makes the synchronization process much more reliable. This paper proposes a modified voltage controller‐based automatic grid‐synchronization strategy for the DFIG‐WECS using a rotor‐current‐dependent model‐reference‐adaptive‐system (MRAS) for rotor‐speed/position estimation. A V/f control‐based starting strategy is used to start the DFIG. An improved decoupled DFIG stator voltage controller is proposed to enhance the transient response during the DFIG grid synchronization. The modified controller is implemented in a grid‐voltage‐oriented reference frame to synchronize the DFIG‐stator with the grid. A robust modified MRAS with higher noise immunity and reduced machine parameter dependency is used to estimate the rotor‐speed/position during synchronization control. To validate the performance of the proposed strategy, a DFIG‐based WECS is modeled and simulated using an OPAL‐RT real‐time simulator, where a motor‐generator setup is modeled to have an equivalent WECS configuration. As compared with the latest reported work, this approach reduces the synchronization time by 16.99% while ensuring a smooth synchronization with no stator inrush‐current, even without any speed/position encoder. [ABSTRACT FROM AUTHOR]

Improved reliability and fault tolerant operation of inverter-fed variable frequency ac drives are very important for critical industrial applications. The paper describes variable frequency, variable voltage operation of a three-phase induction motor drive in single-phase mode for inverter fault conditions, such as open base drive and device short circuit. The detailed mathematical analysis of the machine in single-phase mode indicates that odd harmonic voltages of appropriate magnitude and phase can be injected at the machine terminal to neutralize the large low-frequency pulsating torques so as to permit smooth drive operation. The magnitude and phase angle of the desired harmonic voltages have been derived theoretically. However, to eliminate the parameter variation effect and operating point dependencies, a general purpose search algorithm has been proposed in the paper which fabricates the desired harmonic voltages on real time basis. The search algorithm was initially exercised manually on a simulated drive system to prove its validity, and then tested extensively on a volts/hertz controlled 3-hp drive system in the laboratory. The on-line search algorithm along with estimation and control related to the project was implemented with C language on a TMS320C30 digital signal processor board. The extensive laboratory investigation shows that the proposed search algorithm performs well and successfully eliminates low-frequency pulsating torques not only in a steady-state condition but also during slow acceleration and deceleration of the drive. The search algorithm is perfectly general and can be easily extended to other induction motor drives.

Improved reliability and fault tolerant operation of converter systems are extremely important for industrial ac drives. The paper considers variable frequency variable voltage operation of a three-phase induction motor in single-phase mode for two common faults of a three-phase inverter, i.e., open base drive and device short-circuit. The motor performance has been extensively analyzed in single-phase mode and remedial strategies have been developed to neutralize large second and other lower order harmonic pulsating torques. In a single-phase open loop volts/Hz control mode of a faulty three-phase inverter, it has been demonstrated that odd harmonic voltages at appropriate phase angles can be injected to neutralize the low frequency pulsating torques so as to permit smooth drive operation. It has been shown that the pulsating torque can be further reduced by load dependent flux programming rather than operating with constant rated flux.

The reliability of power electronics systems is of paramount importance in industrial, commercial, aerospace, and military applications. The knowledge about the fault mode behavior of a converter system is extremely important from the standpoint of improved system design, protection, and fault tolerant control. This paper describes a systematic investigation into the various fault modes of a voltage-fed PWM inverter system for induction motor drives. After identifying all the fault modes, a preliminary mathematical analysis has been made for the key fault types, namely, input supply single line to ground fault, rectifier diode short circuit, inverter transistor base drive open, and inverter transistor short-circuit conditions. The predicted fault performances are then substantiated by simulation study. The study has been used to determine stresses in power circuit components and to evaluate satisfactory post-fault steady-state operating regions. The results are equally useful for better protection system design and easy fault diagnosis. They will be used to improve system reliability by using fault tolerant control in the next phase of our work.