Existing wind turbine (WT) control strategies realize damping support mainly by active power modulation, which may affect wind energy harvest. To improve energy efficiency and robustness, this paper proposes a two-stage coordinated control of type-4 WT for damping support using the cost-effective WT resources, where the grid-forming control is applied. In the first stage, two reactive power-based dampers are designed using DC voltage synchronization control. The impacts of the dampers on stability are analytically evaluated. In this stage, the reactive power modulation, along with the partial DC capacitor energy, are jointly utilized for damping provision, while the WT is operated at maximum power point tracking (MPPT) mode. The second stage that seeks active power modulation is only activated when the DC voltage exceeds the threshold. An MPPT recovery process is further designed to eliminate the droop errors induced by the damper. The proposed scheme is validated under various contingencies compared with existing controls. Numerical results show it does not require a frequency estimator or the active power modulation for WT synchronization and grid-forming purposes; meanwhile is relatively energy-efficient and robust to provide strong damping support.

建立局部三维丝网模型研究斯特林回热器在振荡流下的换热特性,分析不同热端温度、入口流速及工作频率下回热器中振荡流周期内的瞬态换热规律.结果 表明工质在振荡流的加速与减速期间内的加热及冷却过程的换热特性存在较大差异;冷却过程的最大Re数比加热过程高133％～474％;相同Re数下,加速与减速期间内的Nu数最大差别为14.6％;冷却过程的Nu数比加热过程高1％～22％;整体而言,一个周期内振荡流的换热量比对应工况下的单向流高20％左右.通过实验对比数值模拟结果,提出加热及冷却过程的Nu-Re关联式:Nu=3.56+ 0.36Re0.66和Nu=17.67+ 0.37Re0.64,误差在7％以内.

基于孔隙尺度,建立局部三维丝网模型,对丝网回热器换热特性进行数值研究.分析不同工质、压力、温度、流速、丝网目数条件下,回热器换热特性的变化规律,并对模拟结果进行总结,拟合出特定雷诺数范围内,单向流动下Nu数与Re数的经验关联式(Nu=9.74+ 3.78Re0.36),与前人实验所得关联式进行对比,在低雷诺数范围吻合较好,最大误差在10％内.此外,定性分析孔隙尺度下的温度场分布图,发现工质流经丝网时产生的温降及扰动会对换热过程产生双重影响,而通过非稳态过程的分析,总结了对流换热强度随时间的变化规律,为完善斯特林循环分析方法及设计和优化斯特林发动机提供参考.

The flow characteristics of an oscillating flow including porous media are very important for designing oscillating-flow-based devices. A combined experimental and simulation study is carried out to provide a comprehensive quantitative understanding of the oscillating flows. The cycle rate through porous media, a particularly basic parameter, is proposed based on the dimensionless pressure drop as a correction factor for the similarity parameters of the oscillating flows. A modified dimensionless fluid displacement is introduced based on the cycle rate, and a correlation equation is proposed to calculate the cycle-averaged friction factor of porous media. Significant phase difference (7-71 degrees) is observed between the movements of gas inside porous media and pistons, which increases with the increasing pressure drop of porous media. Another correlation equation is proposed to predict the phase difference. It should be noted that the pressure drop of porous media would also affect the flows inside other heat exchangers between cylinder and porous media. The mass flow rate next to the cylinder is very different from that adjacent to the porous media in terms of amplitude and phase difference. Mass flow rates at different cross sections vary linearly with the volume between the piston upper surface and the monitored cross section. Finally, a quantitative method is proposed to describe the flow characteristics of an oscillating flow including porous media, which is expected to improve the design methods of oscillating-flow based devices. (C) 2017 Elsevier Ltd. All rights reserved.

In this paper, a new voltage balancing strategy based on improved carrier-phase-shifted pulsewidth modulation (CPS-PWM) scheme and fundamental frequency sorting algorithm (FFSA) is proposed. FFSA is a salient method which sorts the capacitor voltages at fundamental frequency and utilizes the different charging ability of driving signals to converge the capacitor voltages. Unfortunately, under the CPS-PWM scheme with high carrier frequency, 250 Hz or higher, FFSA cannot bring the capacitor voltage to the nominal value once the unbalance occurs, because there is a little difference remaining in the driving signal's charging ability. To avoid this disadvantage, an improved CPS-PWM scheme is proposed. In this scheme, driving signals with proper phase angle differences are selected to be dealt with logic AND/OR processing according to the sorting result of FFSA. With the logic processing, new driving signals with obviously different charging abilities are synthesized, which are assigned to the corresponding unbalanced submodules to achieve the balance of capacitor voltages. And several regions for logic processing are set up, where the proper phase angle difference is selected for each region in order to both guarantee the convergence speed and avoid the voltage oscillations. With the proposed scheme, FFSA can be adopted under CPS-PWM with high carrier frequency. Moreover, the merits of FFSA are inherited, such as the reduced computational burden and the elimination of arm current detection. Performance and effectiveness of the proposed strategy are validated by both simulations and experimental results.

Second-order and third-order models are the two main methods for Stirling cycle analysis. A second-order one has relatively reliable accuracy without detailed cyclic information, while a third-order one provides relatively comprehensive operational information with uncertainty. In this work, a third-order model, Sage, is tried to be improved by a second-order model, Improved Simple Analytical Model, and 100 W beta-type Stirling engine is used as a modelling prototype engine. As suggested by Improved Simple Analytical Model, the gap of the piston seal and the empirical multiplier for heat transfer of cooler in Sage should be adjusted. This is accomplished by combining these two models so that the effects of seal leakage loss, gas spring hysteresis loss, and piston friction loss could be considered in an improved Sage. The improved Sage indicates that the overall relative errors for the indicated power output and the thermal efficiency are reduced by over 30 percentage points and 20 percentage points, respectively. Pressure-volume diagrams provided by the improved Sage are much closer to the experimental ones, and the pattern similarities of pressure-volume diagrams increase from 78.6-81.4% to 80.8-85.0%. Performance simulation of the Stirling engine is carried out by the improved Sage and the results show that regenerator takes a major role in improving the performance of the 100 W beta-type Stirling engine. Lengths of regenerator, heater and cooler are optimized for the maximum indicated power output and the maximum thermal efficiency respectively, and the optimized values are increased by 15.9% and 25.2%, respectively.

This paper proposes a fundamental frequency sorting algorithm for balancing the floating capacitors in modular multilevel converters with low-frequency carrier phase shift modulation. The relationship between the driving pulses and submodules (SMs) is rebuilt in every fundamental period by sorting the voltage increments and present voltages. Besides, an improved and simplified strategy is proposed to predict the charging abilities of the driving pulses, so as to avoid sorting the voltage increments. Based on the proposed method, the switching frequency of each power device and the carrier frequency are identical, which reduces the operational losses for applications regarding high power. At the same time, the sorting frequency is as low as the fundamental frequency, which alleviates a large amount of computational cost, and the necessity to measure arm currents is omitted. In this case, the arm current sensors are eliminated, and simplified communication among the central controller and the local ones is set up. Finally, a three-phase 1-MVA simulation platform with 120 SMs and a down-scaled 6-kVA experimental prototype with 48 SMs are constructed to validate the proposed approach.

Regenerator is one of heat exchangers (heater, regenerator and cooler) in a Stirling engine, whose flow characteristics are very important for developing Stirling engine design methodology. A combined experimental and simulation study is carried out to investigate characteristics of regenerator in an oscillating flow, using steady flow as reference. It is found that the oscillating flow can share the same correlation equations of steady flow for friction factor within the measured kinetic Reynolds number range (2.59×10−2–2.04×10−1 for 100mesh, 6.60×10−3–5.22×10−2 for 200mesh, 2.74×10−3–2.16×10−2 for 300mesh and 1.43×10−3–1.13×10−2 for 400mesh) and dimensionless fluid displacement, and the maximum deviation between the experimental data and simulation results is found less than 9.6%. It is also found that the effect of gas compression can’t be ignored for the steady and oscillating flows through a regenerator with the increase of mass flow, and the pressure drop per unit length decreases as the length of regenerator increases. The velocities at both ends of regenerator are also affected by the compression of gas, and it is found that theoretical velocity based on piston velocity can be used if the dimensionless pressure drop of the whole system is less than 4.22×10−2 in this experiment. After all, it should be noted that oscillating flow has its own characteristics, and obvious flow perturbations are found at downstream of regenerator, which are exactly detected by a hot wire anemometer. The effect of mesh size, frequency and mean pressure on the perturbation is investigated, and a correlation equation for the duration of perturbation is proposed based on dimensionless pressure drop of regenerator.

Modular multilevel converters (MMCs) become more and more attractive in high-voltage and high-power applications. The balance of the floating capacitor voltages is vital for MMCs. In order to balance the capacitor voltages, a fundamental frequency sorting strategy with Phase Disposition PWM is proposed in this paper. A dual sorting method, which sorts the capacitor voltage increments and the present capacitor voltages simultaneously, is implemented to change the relationship between the carriers and the sub-modules once in every fundamental period. Due to the low sorting frequency, the intensive computation of the controllers is avoided. Moreover, the switching commutations are evenly distributed among the power devices. Furthermore, the measurement of the arm current is eliminated, which further simplifies the communication among different controllers. Finally, the proposed voltage balance strategy is validated by simulation and experiment.

Stirling engine has become preferable for high attention towards the use of alternate renewable energy resources like biomass and solar energy. Stirling engine is the main component of dish Stirling system in thermal power generation sector. Stirling engine is an externally heating engine, which theoretical efficiency is as high as Carnot cycle's, but actual ones are always far below compared with the Carnot efficiency. A number of studies have been done on multi-objective optimization to improve the design of Stirling engine. In the current study, a multi-objective optimization method, which is a combination of multiple optimization algorithms including differential evolution, genetic algorithm and adaptive simulated annealing, was proposed. This method is an attempt to generalize and improve the robustness and diversity with above three kinds of population based meta-heuristic optimization techniques. The analogous interpreter was linked and interchanged to find the best global optimal solution for Stirling engine performance optimization. It decreases the chance of convergence at a local minimum by powering from the fact that these three algorithms run parallel and members from each population and technique are swapped. The optimization considers five decision variables, including engine frequency, mean effective pressure, temperature of heating source, number of wires in regenerator matrix, and the wire diameter of regenerator, as multiple objectives. The Pareto optimal frontier was obtained and a final optimal solution was also selected by using various multi-criteria decision making methods including techniques for Order of Preference by Similarity to Ideal Solution and Simple Additive Weighting. The multi-objective optimization indicated a way for GPU-3 Stirling engine to obtain an output power of more than 3 kW and an increase by 5% in thermal efficiency with significant decrease in power loss due to flow resistance. (C) 2016 Elsevier Ltd. All rights reserved.

A key issue in designing and optimizing Stirling engines is to build a precise thermodynamic model to predict the output power, thermal efficiency, and detailed performance properties and provide useful information for further improvement. In this study, a thermodynamic model called Improved Simple Analytical Model (ISAM) was proposed, carefully considering heat and power losses in Stirling engines. A 100 W beta-type Stirling engine was built and tested with helium and nitrogen when pressure and rotary speed ranging from 1.6 MPa to 3 MPa and 260 r/min to 1380 rimin, respectively. Experimental information on performance, such as PV diagrams and temperatures of the heater and cooler, was much detailed. Increasing rotary speed brings a "thin" PV diagram because it made compression and expansion processes become more imperfect, indicating heat transfer enhancement was necessary for compression and expansion chamber in a high speed Stirling engine. Shaft power reached the maximum value of 30.1 W for helium and 21.0 W for nitrogen at rotary speeds of 1000 r/min and 650 r/min, respectively. Improving the mean pressure of gas increased the indicated power, cycle efficiency, shaft power, and electrical power. The maximum indicated power and cycle efficiency were 165 W and 16.5% for helium and 139 W and 12.2% for nitrogen in the same working conditions of 2.96 MPa and 1120 r/min. The ISAM agrees well with the experimental data with a deviation of 4.3-13.4% for helium and 1-7.1% for nitrogen. Analysis of energy losses with ISAM indicated that helium had larger shuttle and seal leakage losses and smaller flow resistance and regenerator heat transfer losses than nitrogen under the same working conditions. Flow resistance and regenerator heat transfer losses, which increased much more rapidly than seal leakage or shuttle heat losses with the increase in rotary speed and pressure, played an important role and resulted in different performances with the two working gases. This study provides comprehensive understanding of the influence mechanism of rotary speed, pressure and working gas in the view of heat/power losses for Stirling engine performance, and recommends that more work (e.g., mechanisms of heat and power losses and PV diagrams) should be performed to improve the precision of second-order models. (C) 2016 Elsevier Ltd. All rights reserved.

A fundamental frequency-sorting algorithm with staircase modulation is proposed to balance the floating capacitors for modular multilevel converters. The driving pulses are assigned to the submodules at every fundamental period according to their charging capabilities for the capacitors. The charging capabilities of the driving pulses can be evaluated by sorting the voltage increments of the capacitors or derived from the symmetrical characteristic of the curve between the voltage increments and the pulse numbers. With this method, all the power devices switch only once per fundamental period, which is suitable for high-power applications. Meanwhile, the sorting frequency decreases to the fundamental frequency. Hence, a large number of calculation resources can be saved. Moreover, it does not need to measure the arm currents so that several current sensors can be saved and the communication protocol between the central and local controllers can be simplified. At last, a three-phase simulation platform with 20 submodules per arm and a down-scaled experimental prototype with eight submodules in each arm are built to validate the proposed voltage-balancing method.

The real performance of Stirling engine always deviates from prescribed theoretical potential. This is mainly because of complex interaction of different engine parameters, heat transfer process of oscillating flows and fluid dynamics. In order to develop an effective methodology to optimize geometric design of Stirling engines a beta-type rhombic drive Stirling engine was investigated whose initial experimental efficiency and output power was relatively low. This paper presents a sensitivity analysis of p-type Stirling model and proposed a combined method to carry out multi-objective optimization of a Stirling engine using detailed information of pressure and volume provided by CFD analysis. The geometric parameters of heat exchangers including heater and cooler tubes diameter with length, regenerator length, matrix mesh and wire diameter were considered for maximizing thermal efficiency, output power and minimizing flow resistance power loss in Stirling engine. CFD analysis covers a detailed study of real and optimized model with best experimental agreement. The CFD results include the detailed description of Stirling cycle With temperature contour, velocity vectors and pressure-volume variation in compression and expansion spaces. The proposed modification results an increase of 2 percentage points in thermal efficiency and more than 80 W in power output when the dead volume of heater, cooler and regenerator is reduced up to 54%, 42% and 24% respectively. The additional dead volume leads to a phase shift of the pressure which is also the main reason of lowering output results. (C) 2016 Elsevier Ltd. All rights reserved.

提出一种采用光纤复用技术的模块化多电平变换器(MMC)三级系统控制架构:第一级主控制器实现系统功率和输出电压控制,并通过一个半双工串行光纤通信网络将相应的调制信息传给第二级控制器;第二级相控制器实现脉宽调制(PWM)和电容电压平衡算法,并通过一根上行的单工串行光纤通信线得到子模块的电容电压及状态信息,不同种类的命令信息调制成不同频率的方波信号后,通过一根下行的采用复用技术的光纤传递给第三级子模块控制器(SMC).该架构可节省大量光纤,且无需子模块同步,大大简化了系统结构,也降低了控制器间协同控制的难度.最后通过实验验证了其可行性.

With the background of power supplies for high-precision apparatuses, an isolated high-frequency dc-ac converter based on a differential output structure is proposed. It is composed of two phase-shifted full bridges (PSFBs) that share a common half-bridge as the lagging leg on the primary side. Two current-doubler synchronous rectifiers (CDRs) constitute the differential output structure on the secondary side. The ac output is synthesized by the two PSFB-CDRs generating the same dc bias and phase-reversed ac components. Compared with traditional solutions, the new circuit has the potential capability to obtain an ultralow distortion output voltage without using special control or compensations. Moreover, zero voltage switching is realized in the primary circuit within a wide load range, whereas zero current switching is achieved in the secondary circuit. Operation principles are analyzed, and a 1-kW prototype is built to validate the proposed scheme.

An isolated inverter based on differential output is proposed. The inverter is composed by two phase-shifted full bridges (PSFB) converters. The two PSFBs share a common leg which works as the lagging one on the primary side. Two current-doubler synchronous rectifiers (CDR) are on the secondary side and constitute differential structure. With the same DC bias and phase-reversed AC components in the two PSFBs, an AC output voltage is acquired. Compared to traditional H-bridge inverters which claim high-quality outputs, the new configuration has the potential capability to obtain ultra-low distortion in output voltage without using any special control or compensations, because there is no duty cycle loss in each PSFB. Moreover, switches on the primary side can realize zero voltage switching (ZVS) within a wide load range, while the ones on the secondary side can achieve zero current switching (ZCS). Finally, simulation model and a 1kW prototype are built to verify the effectiveness of the proposed scheme.

We propose omnidirectional reflective color filters based on metal-dielectric-metal subwavelength grating structure. By particle swarm optimization, the structural parameters of three color filters (yellow, magenta, cyan) are obtained. The optimized filters can present the same perceived specular color at unpolarized illumination for a broad range of incident angles. The reflectance curves at different incident angles keep almost invariable and the color difference is less than 6 in CIEDE2000 formula up to 45°. Angle-insensitive properties including the incident angular tolerance, azimuthal angular tolerance and the polarization effect are investigated thoroughly to construct a real omnidirectional color filter. Through the analysis of the magnetic field, the physical origin is verified that the total absorption band at specific wavelength results from the localized surface plasmon resonance responsible for the angle insensitive spectral filtering.

采用动网格技术,结合试验测量研究不同Reω数下斯特林机管内可压缩性流体的振荡流动特性.利用热线风速仪实验测量不同Reω数下管内瞬时速度,验证了截面速度的“环状效应”.通过对比不同Reω数和径向位置处的热线实验结果与数值解,发现Reω数较低时二者偏差小于5％.相同Reω数不同截面位置和相同几何位置不同Reω数等工况,实验值与数值解之间的最大偏差小于20％,平均偏差接近11％.管中心区实验结果与数值解曲线吻合度较高,而近壁面区二者偏差偏大.实验结果与数值求解曲线的吻合度表明,动网格技术在斯特林机管内振荡流动特性研究中具有较高的适用性和准确性.

An optimized modulation is proposed for the bidirectional DC-DC converter based on the full-bridge/push-pull circuit. With the proposed strategy, the zero-voltage-switching (ZVS) range of the lagging-leg switches in the conventional full-bridge topology can be extended. Moreover, severe voltage overshoots across main switches caused by the leakage inductance of the transformer in the current push-pull circuit are suppressed. Furthermore, the modulation is unified for both the buck mode and the boost mode, which is easy to realize compared with the conventional methods. In addition, no auxiliary circuits are employed, resulting in low cost and small volume. Detailed analysis and design of the proposed modulation are described. The simulation results validate the operation principle and a \kW prototype is built to show the effectiveness of the proposed strategy.

Based on the 2D-PC wave multilayer film structure, a method to broaden the bandwidth of polarization beam splitter is proposed, which is composed of two different thickness periodic film stacks. Combined with the evaluation function of polarization splitting characteristic, the particle swarm optimization method is employed to design the optimal structural parameters. A broadband and compact polarization beam splitter is acquired, in which the center wavelength is 565 nm and its working range has achieved 220 nm with the average extinction ratio over 30 dB. In addition, by using the finite difference time domain method, the band structure and transmission spectrum of the wave-structure multilayer film are calculated, the angle sensitivity of the structure is investigated in detail. And we also study the electromagnetic field in the wavy-structure. Simulation results prove that the structure composed of the two different thickness periodic film stacks can avoid the discontinuity of bandgap, and PSO method can accelerate the convergence of the optimization algorithm and extend the bandwidth effectively.