In this study, a control scheme for the stand-alone generation system based on brushless doubly-fed machine (BDFM) is presented. The output voltage amplitude and frequency are kept constant under variable rotor speed and load by regulating the amplitude and frequency of the control winding (CW) current of BDFM appropriately. The control scheme utilises a CW current vector controller as the inner loop, and a power winding voltage amplitude controller and a frequency controller as the outer loop. The proposed control scheme has been implemented on a prototype BDFM designed for the variable-speed stand-alone ship shaft generation application. Moreover, the satisfactory dynamic performance and low total harmonic distortion of the output voltage in the proposed stand-alone generation system is verified by experiments with two kinds of typical loads: three-phase induction motor load and three-phase RL inductive load.

The standalone brushless doubly-fed induction generator (BDFIG) has demonstrated excellent energy saving performance in ship shaft power generation applications. As a standalone system, it exhibits unbalanced terminal voltages and poor performance under unbalanced loads. However, the existing control scheme of grid-connected BDFIGs cannot be directly applied to stabilize the amplitude and frequency of terminal voltage when the rotor speed and electrical load vary. This paper presents a new sensorless control scheme for the standalone BDFIG under unbalanced load conditions in the ship shaft power generation system. A second-order generalized integrator based quadrature signal generator is introduced to realize the rotor speed observer for the standalone BDFIG feeding unbalanced loads. The compensation method of negative-sequence power winding voltage is proposed to eliminate the negative-sequence component of the unbalanced power winding voltage. Comprehensive experiments are carried out on a prototype BDFIG with and without the compensation of negative-sequence power winding voltage. The good performance of the proposed sensorless control scheme is verified by the experimental test results.

Brushless doubly-fed induction generator (BDFIG) is a new type of ac induction machine and can be used for both grid-connected and stand-alone power generation. The conventional control methods for the BDFIG almost all rely on the encoder, which brings many limitations in terms of cost, complexity, reliability, and so on. This paper presents a new rotor speed observer (RSO) for the encoderless operation of the stand-alone BDFIG, which is based on a rotor position phaselocked loop (PLL) and a second-order generalized integrator (SOGI) and independent of any other machine parameters except for the pole pairs. Comprehensive experiments are also presented to validate the effectiveness of the proposed RSO.

SUMMARYThe effective stress principle has been efficiently applied to saturated soils in the soil mechanics and geotechnical engineering practice; however, its applicability to unsaturated soils is still under debate. The appropriate selection of stress state variables is essential for the construction of constitutive models for unsaturated soils. Owing to the complexity of unsaturated soils, it is difficult to determine the deformation and strength behaviors of unsaturated soils uniquely with the previous single-effectivestress variable theory and two-effective-stress-variable theory in all the situations. In this paper, based on the porous media theory, the specific expression of work is proposed, and the effective stress of unsaturated soils conjugated with the displacement of the soil skeleton is further derived. In the derived work and energy balance equations, the energy dissipation in unsaturated soils is taken into account. According to the derived work and energy balance equations, all of the three generalized stresses and the conjugated strains have effects on the deformation of unsaturated soils. For considering these effects, a principle of generalized effective stress to describe the behaviors of unsaturated soils is proposed. The proposed principle of generalized effective stress may reduce to the previous effective stress theory of single-stress variable or the two-stress variables under certain conditions. This principle provides a helpful reference for the development of constitutive models for unsaturated soils.

The conventional control methods for brushless doubly-fed induction generator (BDFIG) normally employ mechanical sensors to acquire the information of rotor speed, which brings many disadvantages in the cost, complexity, reliability, and so on. This paper presents an improved rotor speed observer (RSO) for the sensorless operation of standalone BDFIGs, which is based on the power winding (PW) voltage and control winding (CW) current. In order to eliminate the impact of unbalanced and nonlinear loads on the RSO, second-order generalized integrators (SOGIs) and low-pass filters (LPFs) are introduced to pre-filter the PW voltage and CW current, respectively. Through comprehensive parameter design, the response speed of the improved RSO will be not lower than that of the basic RSO with ensuring the filtering effect of these additional filters. In addition, the proposed RSO is independent to machine parameters except the pole pairs. Comprehensive experiments are conducted and results verify the proposed improved RSO applied to the standalone BDFIG. Also, the applicability of the proposed RSO on another dual-electrical-port machine, DFIG, is confirmed by simulation results. 1

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