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This paper presents an instantaneous torque control method for cascaded doubly fed induction machines (CDFIMs) with a rated torque-sharing ratio feature under steady-state operation. A CDFIM includes two coupled main and auxiliary doubly fed induction machines, and the rated torque-sharing ratio herein means that the machine torques are proportional to the machine ratings. The proposed method is established based on transferring the torque equations of the two machines into the rotor-flux reference frame of the auxiliary machine. Then, the instantaneous torque and the rated torque-sharing ratio are controlled via the stator current components of the auxiliary machine, while machine fluxes are within allowable ranges. To verify the method and evaluate its capabilities, the suggested method is used for the speed control of a CDFIM experimental setup including two 4-kW doubly fed induction machines and a 4-kW dc machine to apply a sudden mechanical load change to the CDFIM. Further, a case study including 12-kW and 4-kW DFIMs is also simulated to evaluate the performance of the torque control method for a CDFIM with different machine ratings. The test results confirm that the proposed control schemes achieve the desired performances without exceeding the ratings of the machines under various test scenarios. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of various reference frames and their relative speeds in cascaded doubly-fed induction machines (CDFIMs). Then, a compact space-phasor model of CDFIMs in the natural and arbitrary reference frames is presented that is useful for performance analysis of CDFIMs based on rotating phasors. The merits of the proposed model compared with existing models are using a common (single) reference frame for all machines and parts of CDFIMs, and the model formulation is independent of the rotor angle position. The reference frames in the suggested model can be fixed on the common rotor of the CDFIM machines for dynamic analysis or it can be fixed on the stator/rotor flux of each cascaded machines useful for CDFIMs controller design. A two-axes equivalent circuit for a CDFIM is also presented, which enables analysis of CDFIMs in commercially available power circuit simulation software tools. The validity of the model is verified via the investigation of an experimental setup and consists of two mechanically coupled 4-kW DFIMs and a 4-kW dc machine as a load. The setup is investigated under various test scenarios, including free acceleration, load (torque) change, and frequency step change conditions. [ABSTRACT FROM AUTHOR]

This article proposes and demonstrates a fault-detection method to locate interturn short circuits (ITSCs) in the stator windings of a brushless doubly fed induction machine (BDFIM). The detection of ITSC is important in machine health prognostics as ITSC is an early stage fault that may lead to other faults such as coil-to-coil and coil-to-ground. BDFIM consists of power and control windings in the stator that are magnetically coupled through a nested-loop rotor winding. Existing ITSC detection algorithms use rotor slot harmonics in stator current spectra as fault indicators for only conventional doubly fed induction machines. However, these algorithms cannot be applied to BDFIM due to its different stator/rotor winding structure. This article primarily proposes a new analytical formulation for the nested-loop rotor slot harmonics (NRSHs) as ITSC fault indicators in BDFIMs. Using NRSHs, a detection algorithm is also proposed to detect and locate ITSC in power/control windings of BDFIMs. The proposed algorithm is verified based on numerical analysis of a BDFIM using the finite-element method. The accuracy of the proposed fault-detection algorithm is also experimentally investigated and demonstrated using a BDFIM test rig. Both numerical and experimental test results confirm effectiveness of the proposed ITSC fault-detection algorithm. [ABSTRACT FROM AUTHOR]

A new inverter control approach, called enhanced virtual synchronous machine (eVSM), is proposed based on the VSM concept. Unlike existing VSM approaches, the eVSM does not emulate the rotating inertia based on an assumption of the unlimited energy storage, but it deploys the physically existing inertia of the dc-link element. The eVSM adopts an innovative way of enlarging the inertia utilization range, which obviates the need for having a large dc-link element or a dedicated battery storage system, while still providing the same inertia response of an equivalent synchronous machine. Theoretical developments and numerical results presented in this paper confirm that the proposed eVSM can present a stabilizing support to the grid similar to a synchronous machine despite the small size of its dc-link element. Moreover, its transient responses can be improved beyond those of conventional synchronous machines thanks to the possibility of more flexibly adjusting damping and governor functions. [ABSTRACT FROM AUTHOR]

This paper presents a power transfer matrix model and multivariable control method for a doubly-fed induction generator (DFIG) wind energy system. The power transfer matrix model uses instantaneous real/reactive power components as the system state variables. It is shown that using the power transfer matrix model improves the robustness of controllers as the power waveforms are independent of a dq frame of reference. The sequential loop closing technique is used to design the controllers based on the linearized model of the wind energy system. The designed controller includes six compensators for capturing the maximum wind power and supplying the required reactive power to the DFIG. A power/current limiting scheme is also presented to protect power converters during a fault. The validity and performance of the proposed modeling and control approaches are investigated using a study system consisting of a grid-connected DFIG wind energy conversion system. This investigation uses the time-domain simulation of the study system to: 1) validate the presented model and its assumptions, 2) show the tracking and disturbance rejection capabilities of the designed control system, and 3) test the robustness of the designed controller to the uncertainties of the model parameters. [ABSTRACT FROM AUTHOR]