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This paper researched on the pre-synchronization grid connection method of a virtual synchronous generator (VSG), and proposed a fast pre-synchronization method based on the virtual power loop. According to the self-synchronization characteristics of the VSG power loop, the double power loop design was adopted in this paper. The two power loops had the same structure, which the power loop 1 was used to implement the basic VSG function, and the power loop 2 was used to achieve rapid pre-synchronous control. Therefore, the mathematical model of the VSG was similar to the synchronous generator in the pre-synchronization control. This method did not require a phase-locked loop and VSG has a good inertia and Damping performance. Moreover, this method was suitable for the networking of multiple VSGs.

This article describes how basing on the future behavior of microgrid system, forecasting renewable energy power generation, load and real-time electricity price, a model predictive control (MPC) strategy is proposed in this article to optimize microgrid operations, while meeting the time-varying requirements and operation constraints. Considering the problems of unit commitment, energy storage, economic dispatching, sale-purchase of electricity and load reduction schedule, the authors first model a microgrid system with a large number of constraints and variables to model the power generation technology and physical characteristics. Meanwhile the authors use a mixed logic dynamical framework to guarantee a reasonable behavior for grid interaction and storage and consider the influences of battery life and recession. Then for forecasting uncertainties in the microgrid, a feedback mechanism is introduced in MPC to solve the problem by using a receding horizon control. The objective of minimizing the operation costs is achieved by an MPC strategy for scheduling the behaviors of components in the microgrid. Finally, a comparative analysis has been carried out between the MPC and some traditional control methods. The MPC leads to a significant improvement in operating costs and on the computational burden. The economy and efficiency of the MPC are shown by the simulations.

In multi-inverter parallel system, the complex coupling between the inverters leads to the resonance of the system. In this paper, the equivalent model of parallel operation of multi-inverter is established. The resonance principle and resonance characteristics of multi-inverter parallel connection are analyzed. Aiming at the system high-frequency resonance caused by nonsynchronous control between inverters, this paper makes functional reuse for grid-connected inverters to ensure the control of fundamental frequency. Meanwhile, the virtual impedance is put into the harmonic range to improve the system damping and restrain the high frequency resonance in the specific frequency range. In addition, in the current control of the harmonic range, the Quasi-PR controller is used for zero steady-state error tracking of harmonic currents. The simulation results show that the proposed method is effective.

Conventional switching-table-based direct power control (STDPC) presents irregular power ripples and variable switching frequency, due to the use of predefined switching table and hysteresis comparators. This paper proposes an improved DPC for three-phase pulsewidth modulation (PWM) rectifier with simple calculation. The concept of duty cycle control is introduced in DPC to allocate only a fraction of control period to the voltage vector selected from conventional switching table in DPC, while a null vector is applied for the rest of time. The main advantages of the proposed duty cycle control are twofold. On one hand, it is independent of rectifier parameter such as line-side inductance, hence strong robustness against inductance variations is obtained. On the other hand, the principle to obtain the duty cycle is very simple and easy to implement. Compared to conventional DPC, significant power ripple reduction and more sinusoidal grid current can be observed in the improved DPC. The presented simulation results and laboratory tests validate the effectiveness of the proposed method.

Objective The goal of this study was to evaluate the accuracy of the integration of computerized tomography (CT)–based bone models and laser-scanned dental models by sequential point- and surface-based markerless registration to create a digital maxillofacial-dental model. Study design The integration accuracy was evaluated in normal skulls (group I) and subjects with maxillofacial deformities (group II) by measuring the distance between the integrated models (for group I and II) and between the final integrated model and the laser-scanned original skull model (for group I). Results The average error ranged between 0 and 0.2 mm without statistically significant difference in the region of maxilla or mandible and in tooth location. Conclusions We could confirm that the integration can be made with good accuracy without the aid of fiducial markers for the maxillofacial-dental composite model from the different resolution of CT and dental models.

BACKGROUND Three-dimensional printing and computer-assisted surgery demand a high-precision three-dimensional mesh model created from computed tomography (CT) imaging data using an image-based meshing algorithm. We aimed to evaluate the three-dimensional geometric accuracy of surface meshes produced from CT images with commercially available software packages. METHODS The CT images were acquired for 3 human dry skulls and 10 manufactured plastic skulls. Four commercially available software packages were used to produce the surface meshes in stereolithography (STL) file format. These CT-based STL surface meshes were registered and compared with three-dimensional optical-scanned reference mesh surface for evaluating the accuracy of the STL mesh produced with each software package. RESULTS The surface geometries produced by the CT-image-based meshing process were all relatively accurate; differences from the three-dimensional optical-scanned data were in the voxel or subvoxel range. However, when comparisons with the three-dimensional optical-scanned surface data were performed in individual anatomic regions, we found significantly different accuracies of the CT-based STL surface meshes produced by the different software packages. CONCLUSIONS We found that all 4 software packages showed reasonably good meshing accuracies for clinical use. However, the range of errors inherent in the CT-image-based meshing process demands that caution should be taken in selecting and manipulating the software to avoid potential errors in specific clinical applications.

OBJECTIVE The objective of this study was to determine the reliable midsagittal (MS) reference plane in practical ways for the three-dimensional craniofacial analysis on three-dimensional computed tomography images. METHODS Five normal human dry skulls and 20 normal subjects without any dysmorphoses or asymmetries were used. The accuracies and stability on repeated plane construction for almost every possible candidate MS plane based on the skull base structures were examined by comparing the discrepancies in distances and orientations from the reference points and planes of the skull base and facial bones on three-dimensional computed tomography images. RESULTS The following reference points of these planes were stable, and their distribution was balanced: nasion and foramen cecum at the anterior part of the skull base, sella at the middle part, and basion and opisthion at the posterior part. CONCLUSIONS The candidate reference planes constructed using the aforementioned reference points were thought to be reliable for use as an MS reference plane for the three-dimensional analysis of maxillofacial dysmorphosis.

Experimental imitation platform of the wind energy generation system with direct-driven permanent magnet synchronous generators (PMSG) was designed to simulate wind power on-site in a laboratory environment and realize controlled natural wind environment for 10kw wind turbine simulation system. The platform consists of the IPC, asynchronous drag motor, frequency converter, PMSG, power electronic conversion device and gearbox. Establishment and characterization of the wind turbine model is completed by position machine. According to torque coefficient data sets, torque coefficient characteristic curve was set by method of least squares. By given wind turbine parameters, position machine calculates torque and speed value of the wind turbine to control of asynchronous motors and deflectors. Finally, both model and non-model control mode of the test platform is achieved. Experimental results show that the experimental platform can better simulate the running state of the wind turbine to achieve maximum power point tracking. At the same time, the results verify the validity of the MPPT control strategy under laboratory conditions, and effectively improve the power generation efficiency of the generator.

Recently, in order to resolve the problem of high cost, we have proposed the stationary discontinuous armature Permanent Magnet Linear Synchronous Motor (PM-LSM) in which the armature is engaged only when accelerated and decelerated operations are necessary, when PM-LSM is used with long-distance transportation systems in factories. However, the stationary discontinuous armature PM-LSM contains the outlet edges which always exist as a result of the discontinuous arrangement of the armature. For this reason, cogging force generated between the “entrance end” and the “exit end” has become a problem. Therefore, we have examined the outlet edge cogging force by deforming the shape of the outlet edge at the armature and installing the auxiliary pole at the mover in order to minimize the cogging force generated when the armature is arranged discontinuously. This paper presents the results of 2-D numerical analysis by FEM of the cogging force exerted by the outlet edge.

Objective The aim of this study was to present our clinical experience regarding the production and accuracy of digitally printed wafers for maxillary movement during the bimaxillary orthognathic surgery. Study design Fifty-five consecutive patients requiring maxillary orthognathic surgery were included in this study. The plan for digital model surgery (DMS) was dictated by the surgical plans for each clinical case. We carried out digital model mounting, DMS, wafer printing, and confirmation of the accuracy of the procedure. Results Moving the reference points to the target position in DMS involved a mean error of 0.00-0.09 mm. The mean errors confirmed by the model remounting procedure with the printed wafer by DMS were 0.18-0.40 mm (for successful cases; n = 42) and 0.03-1.04 mm (for poor cases; n = 3). Conclusion The accuracies of the wafers by DMS were similar to those for wafers produced by manual model surgery, although they were less accurate than those produced by DMS alone. The rapid-prototyped interocclusal wafer produced with the aid of DMS can be an alternative procedure for maxillary orthognathic surgery.