In this paper, in order to optimize the dynamic performance of the permanent magnet synchronous motor (PMSM) speed regulation system, a nonlinear speed-control algorithm for the PMSM control systems using sliding-mode control (SMC) is developed. First, a sliding-mode control method based on a new sliding-mode reaching law (NSMRL) is proposed. This NSMRL includes the system state variable and the power term of sliding surface function. In particular, the power term is bounded by the absolute value of the switching function, so that the reaching law can be expressed in two different forms during the reaching process. This method can not only effectively suppresses the inherent chattering, but also increases the velocity of the system state reaching to the sliding-mode surface. Based on this new reaching law, a sliding-mode speed controller (SMSC) of PMSM is designed. Then, considering the large chattering phenomenon caused by high switching gain, an improved anti-disturbance sliding-mode speed controller(ADSMSC) method, called SMSC+ESO method, is developed. This method introduces an extended state observer (ESO) to observe the lumped disturbance and adds a feedforward compensation item based on the observed disturbances to the SMSC. Finally, simulation and experimental results both show the validity of the proposed control method.

Three configurations of voltage source converter multi-terminal HVDC transmission for large offshore wind farms are studied. The voltage-current characteristics of the converters are shown and the operation of the converters with different power output from the wind farms is discussed. The control system is designed to achieve automatic coordination between the converters without the need of fast communication between them. The operation of the multi-terminal HVDC transmission with the control system is tested through simulations using PSCAD/EMTDC and through experiments on a three-terminal test rig. The simulation and experimental results show good agreement.

In this study, a combined simulation and experimental approach is utilized to investigate the influence of hatch spacing on the microstructure and as-built quality of 316L stainless steel (SS) samples fabricated by selective laser melting (SLM). A three-dimensional finite element model (FEM) is employed to investigate heat transfer and melt pool during the SLM of 316L SS. The phase transformation and variation of the thermo-physical properties of the materials are considered in this model. The effects of hatch spacing (H) on the temperature field, microstructure and melt pool size, overlap rate, surface quality, and relative density during the SLM of 316L SS are investigated. The simulated results indicate that, as the hatch spacing increases, the depth increases and the width of the melt pool decreases. Meanwhile, with the increase of hatch spacing, the simulated temperature of the subsequent tracks falls below the melting temperature of the first track. Moreover, the microstructures were found to coarsen with the increasing hatch spacing due to the reduced cooling rate. The optimized hatch spacing and overlap rate between adjacent tracks were obtained from numerical simulations. Simulation results illustrate that, when the optimized hatch spacing of 100 μm is adopted, fully dense parts with a smooth surface can be fabricated by SLM, thus experimentally validating the simulation results.

The macroscopic mechanical performance of additive manufactured structures is essential for the design and application of multiscale microlattice structure. Performance is affected by microstructure and geometrical imperfection, which are strongly influenced by the size of the struts in selective laser melting (SLM) lattice structures. In this paper, the effect of size on microstructure, geometrical imperfection, and mechanical properties was systemically studied by conducting experimental tests. A series of AlSi10Mg rod-shaped samples with various diameters were fabricated using SLM. The uniaxial tensile test results show that with the decrease in build diameter, strength and Young’s modulus of strut decreased by 30% more than the stable state. The main reasons for this degradation were investigated through microscopic observation and micro X-ray computed tomography (μ-CT). In contrast with large-sized strut, the inherent porosity (1.87%) and section geometrical deviation (3%) of ponysize strut is greater because of the effect of thermal transform and hydrogen evolution, and the grain size is 0.5 μm. The discrepancy in microstructure, geometrical imperfection, and mechanical properties induced by size effect should be considered for the design and evaluation of SLM-fabricated complex structures.

This paper analyzes the frequency support characteristics of multiterminal voltage source converter HVdc (VSC-HVdc) (MTDC) schemes using the energy transferred from wind turbine rotating mass and other ac systems. An alternative coordinated control (ACC) scheme that gives priority to a frequency versus active power droop fitted to onshore VSCs is proposed to: transfer wind turbine recovery power to undisturbed ac grids, and allow correct control operation of MTDC systems during multiple power imbalances on different ac grids. The fast frequency response capability of MTDC systems equipped with the proposed ACC scheme is compared against a coordinated control scheme, which uses a frequency versus dc voltage droop. The frequency control schemes are demonstrated on an experimental test rig, which represents a three-terminal HVdc system. Also, the MTDC frequency support capability when wind farms do not provide extra power is tested using a four-terminal HVdc system.