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A minimum mass design study applicable to thin circular tube is performed for various modes of eccentric compressive loading. Axial crushing failure mode, frequently noticeable in uniform axial compressive loading of thin circular tube, does not appear in eccentric compression. Hence, other compressive failure modes, e.g., global buckling, yield and local buckling are studied with respect to non-dimensional load and geometric shape factors for a fixed-free condition. These modes are predominant in ductile engineering alloys. A failure mode map in terms of non-dimensional load and shape-factor for a given load-eccentricity are obtained and the prescription for minimum mass is given.

This paper presents a probabilistic approach in the modeling of stand-alone microgrids to predict their operational adequacy performance considering uncertainty of energy storage system (ESS), photovoltaic system (PVS) and conventional generator (CG). Instead of using the daily or hourly time step, operating period a minutely time step is considered to incorporate the effect of fast ramp up/down of system components on microgrid operating adequacy through expected energy not supplied (EENS) and expected energy not used (EENU), due to load and resource variations. A time varying state of charge (SOC) model is proposed to determine power output of an ESS in reliability modeling. The reliability of a PVS is modeled in detail based on the total cross-tied configuration (TCTC) of photovoltaic (PV) cells and arrays. The proposed technique and indices will be useful for system planners to select the type and size of microgrid systems that contain alternative energy sources and storage. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Distribution systems are evolving into active networks due to the high penetration of distributed generation in recent years. Micro Phasor Measurement Unit (μPMU) is capable of providing super-fast and accurate synchronised phasor mea-surements, and eligible for distribution networks applications. This paper introduces a project aiming to develop a μPMU based monitoring system for distribution networks. In this project, parallel computation method is proposed for big data screening, gathering and processing. This paper takes linear state estimation as an example to show the improvement of parallel computation on big data processing.

The state of energy (SOE) of Li-ion batteries is a key indicator for the energy optimization and management of energy storage devices (ESDs) in electric vehicles and smart grids. To improve the SOE estimation accuracy, a hybrid Li-ion battery model is presented against the dynamic loads and the rate energy effects of the battery. In order to acquire advantages of both models, an analytical battery model is merged with an electrical model for accurately tracking the battery voltage characteristics and capturing the nonlinear rate energy effects to realize high-fidelity SOE prediction. Commercial Li-ion batteries are tested at the single-cell level at dynamic loads to validate the effectiveness of the proposed model. The simulation results show high accuracy and reliability of the proposed model for predicting of the terminal voltage and the SOE of batteries under dynamic loads.

The number of Stationary Battery Systems (SBS) connected to various power distribution networks across the world has increased drastically. The increase in the integration of renewable energy sources is one of the major contributors to the increase in the number of SBS. SBS are also used in other applications such as peak load management, load-shifting, voltage regulation and power quality improvement. Accurately modeling the charging/discharging characteristics of such SBS at various instances (charging/discharging profile) is vital for many applications. Capacity loss due to the aging of the batteries is an important factor to be considered for estimating the charging/discharging profile of SBS more accurately. Empirical modeling is a common approach used in the literature for estimating capacity loss, which is further used for estimating the charging/discharging profiles of SBS. However, in the case of SBS used for renewable integration and other grid related applications, machine-learning (ML) based models provide extreme flexibility and require minimal resources for implementation. The models can even leverage existing smart meter data to estimate the charging/discharging profile of SBS. In this paper, an analysis on the performance of different ML approaches that can be applied for lithium iron phosphate battery systems and vanadium redox flow battery systems used as SBS is presented for the scenarios where the aging of individual cells is non-uniform. Published version

The high power density converter is required due to the strict demands of volume and weight in more electric aircraft, which makes SiC extremely attractive for this application. In this work, a prototype of 50 kW SiC high power density converter with the topology of two-level three-phase voltage source inverter is demonstrated. A gate assisted circuit is further introduced to reduce the switching loss and mitigate EMI. A mathematical model with consideration of various parasitic parameters is developed. This converter is operated at a switching frequency up to 100 kHz and a narrow dead band of 250 ns. And the efficiency is measured to be 97.91% at 100 kHz and 98.68% at 60 kHz.

The reliability of power converters has become a major design concern especially for those mission-critical industrial and aerospace applications required to operate in a harsh environment. There is a need for real-time monitoring techniques that are capable of tracking health status to give an alert signal before a disastrous system failure and furthermore make the early failure prediction due to long-term wear-out possible. This paper presents a real-time isolated voltage monitoring unit (IVMU) that monitors on-state voltage of a semiconductor power switch accurately. It can be integrated with off-the-shelf power converter to perform real-time status monitoring. The design has been validated with double-pulse testing and on a H-bridge inverter that utilize SiC power switches.

The state of energy (SOE) of Li-ion batteries is a key indicator for the energy optimization and management of energy storage devices (ESDs) in electric vehicles (EVs) and smart grids. To improve the SOE estimation accuracy, a hybrid Li-ion battery model is presented in this study against the dynamic loads and the rate energy effects of the battery. Firstly, in order to take the advantages of both models, a 2nd order RC battery model is merged with an analytical kinetic battery model for accurately predicting the battery voltage characteristics and capturing the nonlinear rate energy effects to realize the high-fidelity SOE and runtime prediction. Secondly, a new method to separate the fast and slow dynamics of the 2nd order RC model is developed and presented with high-performance accuracy. Thirdly, commercial Li-ion batteries are tested at dynamic loads under various temperature to validate the effectiveness of the proposed model. The experimental results show high accuracy and reliability of the proposed battery model on the estimation of the battery SOE and the battery terminal voltage responses under dynamic loads.

The state of energy (SOE) is a key indicator for the energy optimization and management of Li-ion battery-based energy storage systems in the smart grid applications. To improve the SOE estimation accuracy, a Li-ion battery model is presented in this study against dynamic loads and battery ageing effects. Firstly, an electrical battery model is combined with an analytical model in order to take advantages of both models for accurate prediction of battery terminal voltage characteristics, SOE and remnant runtime. Secondly, a novel method to separate the fast and slow dynamics of the electrical battery model is developed, and its superior performance is presented. Thirdly, the effects of the battery initial SOC, load current rate and direction, operating temperature and ageing level are systematically scrutinized and involved into the proposed model for robust SOE and terminal voltage prediction. Commercial Li-ion batteries are then tested under dynamic loads and at an arbitrary battery ageing level to validate the effectiveness and robustness of the proposed model. The laboratory-scale experimental test results show superb accuracy and reliability of the proposed battery model for estimating battery SOE and terminal voltage under dynamic loads and battery ageing conditions. Accepted version

Reliable online estimation of state of charge (SOC) and capacity is critically important for the battery management system. This paper presents a multi-timescale estimator to dually estimate the SOC and capacity for lithium-ion battery. The first-order RC model is used to simulate the dynamics of lithium-ion battery. Based on the battery model, the open circuit voltage (OCV) is timely updated with a simple OCV, the result of which is further corrected with the Kalman filter (KF). Then the SOC is inferred from the SOC-OCV look-up table. Meanwhile, a RLS-based capacity estimator is formulated to work simultaneously with the SOC estimation in the form of dual estimation. Different timescales are adopted for the dual estimator to improve accuracy and stability. Experimental results suggest that the proposed method estimates SOC and capacity in real time with fast convergence and high precision. NRF (Natl Research Foundation, S’pore) Published version

As with all redox flow batteries, the Vanadium Redox flow Battery (VRB) can suffer from capacity loss as the vanadium ions diffuse at different rates leading to a build-up on one half-cell and dilution on the other. In this paper an extended dynamic model of the vanadium ion transfer is developed including the effect of temperature and bulk electrolyte transfer. The model is used to simulate capacity decay for a range of different ion exchange membranes that are being used in the VRB. The simulations show that Selemion CMV and Nafion 115 membranes have similar behavior where the impact of temperature on capacity loss is highest within the first 100 cycles. The results for Selemion AMV membrane however are seen to be very different where the capacity loss at different temperatures observed to increase linearly with increasing charging/discharging cycles. The model is made more comprehensive by including the effect of bulk electrolyte transfer. A volume change of 19% is observed in each half-cell for Nafion 115 membrane based on the simulation parameters. The effect of this change in volume directly affects concentration, and the characteristics are analyzed for each vanadium species as well as the overall concentration in the half-cells.

Thermal management needs to be carefully considered in the lithium-ion battery module design to guarantee the temperature of batteries in operation within a narrow optimal range. This article firstly explores the thermal performance of battery module under different cell arrangement structures, which includes: 1 × 24, 3 × 8 and 5 × 5 arrays rectangular arrangement, 19 cells hexagonal arrangement and 28 cells circular arrangement. In addition, air-cooling strategies are also investigated by installing the fans in the different locations of the battery module to improve the temperature uniformity. Factors that influence the cooling capability of forced air cooling are discussed based on the simulations. The three-dimensional computational fluid dynamics (CFD) method and lumped model of single cell have been applied in the simulation. The temperature distributions of batteries are quantitatively described based on different module patterns, fan locations as well as inter-cell distance, and the conclusions are arrived as follows: when the fan locates on top of the module, the best cooling performance is achieved; the most desired structure with forced air cooling is cubic arrangement concerning the cooling effect and cost, while hexagonal structure is optimal when focus on the space utilization of battery module. Besides, the optimized inter-cell distance in battery module structure has been recommended.

This paper presents the modeling and motion-sensorless direct torque and flux control of a novel dual-airgap axial-flux permanent-magnet machine optimized for use in flywheel energy storage system (FESS) applications. Independent closed-loop torque and stator flux regulation are performed in the stator flux ( x-y) reference frame via two PI controllers. This facilitates fast torque dynamics, which is critical as far as energy charging/discharging in the FESS is concerned. As FESS applications demand high-speed operation, a new field-weakening algorithm is proposed in this paper. Flux weakening is achieved autonomously once the y-axis voltage exceeds the available inverter voltage. An inherently speed sensorless stator flux observer immune to stator resistance variations and dc-offset effects is also proposed for accurate flux and speed estimation. The proposed observer eliminates the rotary encoder, which in turn reduces the overall weight and cost of the system while improving its reliability. The effectiveness of the proposed control scheme has been verified by simulations and experiments on a machine prototype.

Microchannel is one of several high-heat-flux removal techniques being used in electronic cooling. Double-layered microchannel heat sink (DL-MCHS) with counter current flow arrangement is found not only to be able to lower the thermal resistance of the heat sink, but also decrease the maximum temperature and streamwise temperature rise on the base surface compared with single-layered microchannel heat sink (SL-MCHS). The present paper numerically investigated the thermal resistance, pumping power and temperature distribution on the base surface of substrate of DL-MCHS in different microchannel parameters and flow conditions, so as to find the complicated relationship between the overall performance of DL-MCHS and its geometric parameters and flow conditions. The numerical results show that the optimal width ratio of DL-MCHS should be increased when the microchannel aspect ratio is increased. The effectiveness of increasing aspect ratio of microchannels on improving the overall performance of DL-MCHS is dependent on the pumping power provided. DL-MCHS with higher aspect ratio and smaller width ratio is suited to the situation when higher pumping power is provided. Compared with the situation with identical inlet velocity being assigned to the bottom and upper microchannels, adjusting the inlet velocity of upper channels to be smaller than that of bottom channels may result in the improvement of the overall performance of DL-MCHS at a given pumping power, especially when the given pumping power is lower. These strategies could be tried in the real application of DL-MCHS.

The utilization of daylight can significantly affect building performance, energy efficiency, productivity, as well as occupants' comfort and satisfaction in buildings. This paper aims to assess daylight performance for tropical office buildings in a parametric approach. Thus, four passive design categories are investigated, namely interior surface reflectance, glazing visual transmittance, light shelves and shading control. The approach is exemplified using the case study of two selected offices in Singapore. This study contributed to the assessment of the daylight performance and prediction of the consequences of retrofitting alternatives toward fostering the utilization of daylight in existing buildings in the tropics. Published version

Oceanic currents and tidal streams are a predictable and vast source of energy. The astronomical nature of the underlying forces causing such streams, results in speed variations that change at certain time intervals. There are different methods for harnessing part of this vast and renewable source of energy; however, regardless of the method used, integrating a large-scale tidal in-stream energy converter into a conventional power system poses many challenges, in particular, in weaker networks. Injection of the captured stream power to a nonstiff network changes the magnitude of the voltage at different times of the day. The impact on the voltage magnitude is commonly most critical if tidal generators and consumers share the same medium voltage feeder, which is a case that seems inevitable especially when considering the electrically remote but energy-rich areas of estuaries, oceanfronts, and natural water channels. This investigation provides designing details of a hybrid power port solution that establishes an independent active and reactive power control, enabling it to regulate the point of common coupling voltage, while transferring variable amounts of captured stream power to a nonstiff grid. To verify the feasibility of the proposed scheme, an example system is designed and tested using a common tidal stream power profile. This investigation is a timely contribution to the emerging field of marine current power generation.

While tidal stream patterns of natural channels are essentially consistent and smooth, coastal tidal flows experience much more radical speed variations. This paper proposes a new converter control strategy for maximum power extraction of the fast-changing currents that are typically seen in open waters and near the shore. The proposed method makes use of variable-speed generation and has the ability to adapt to the nature of different tidal flows, thus enabling efficient exploitation of resources that require faster dynamic responses. The main motivation behind this study has been the exploration of the possibility of combined offshore wind-tidal cogeneration. The aspects discussed in this paper include interdependences between the injection of the fluctuating generated power, grid-tie connection, and maximum power capture with a view to addressing energy conversion and power quality requirements. Detailed insights into this novel power generation system are provided with a case study that involves a method for maximum power point tracking and power injection controls in a nonstiff network.

Electrification of vehicular systems has gained increased momentum in recent years with particular attention to constant power loads (CPLs). Since a CPL potentially threatens system stability, stability analysis of hybrid electric vehicle with CPLs becomes necessary. A new power buffer configuration with battery is introduced to mitigate the effect of instability caused by CPLs. Model predictive control (MPC) is applied to regulate the power buffer to decouple source and load dynamics. Moreover, MPC provides an optimal tradeoff between modification of load impedance, variation of dc-link voltage and battery current ripples. This is particularly important during transients or starting of system faults, since battery response is not very fast. Optimal tradeoff becomes even more significant when considering low-cost power buffer without battery. This paper analyzes system models for both voltage swell and voltage dip faults. Furthermore, a dual mode MPC algorithm is implemented in real time offering improved stability. A comprehensive set of experimental results is included to verify the efficacy of the proposed power buffer.

The space vector pulse-width-modulation technique is extensively applied in the three-phase power electronics circuits because of its easy digital implementation and wide linear modulation range features. However, the attempt of this technique for the single-phase Z-source inverter has seldom been reported because of its unique topology and operational characteristics. In this paper, based on an in-depth mathematical derivation and theoretical explanation, the space vector pulse-width-modulation principles have been discussed in detail. Various implementation schemes are demonstrated, and a comparison study for selected switching patterns is conducted. In addition, the theoretical analysis is validated by both the simulation and experimental results. This work will be helpful for understanding the space vector pulse-width-modulation concept and modulation techniques of the single-phase full-bridge Z-source inverters. Copyright © 2013 John Wiley & Sons, Ltd.

Induction motors (IMs) have been extensively used in industrial applications because of their inexpensiveness, ruggedness and reliability. Generally, to improve the performance of IM drives, one position sensor, one DC-link voltage sensor and at least two AC current sensors are necessary. However, failure of any of these sensors can cause degraded system performance or even instability. Consequently, it is very important to develop a sensor fault resilient control system for IMs drives so that continuous and normal operation is maintained even in cases of sensor faults. This study proposes a compact and robust sensor fault detection, isolation and system reconfiguration algorithm based on extended Kalman filter and reduced number of adaptive observers. A comprehensive set of experimental results are provided to verify the effectiveness of the proposed algorithm.

In this study, we investigated non-ideal characteristics of a diamond Schottky barrier diode with Molybdenum (Mo) Schottky metal fabricated by Microwave Plasma Chemical Vapour Deposition (MPCVD) technique. Extraction from forward bias I–V and reverse bias C − 2 –V measurements yields ideality factor of 1.3, Schottky barrier height of 1.872 eV, and on-resistance of 32.63 mΩ·cm 2 . The deviation of extracted Schottky barrier height from an ideal value of 2.24 eV (considering Mo workfunction of 4.53 eV) indicates Fermi level pinning at the interface. We attributed such non-ideal behavior to the existence of thin interfacial layer and interface states between metal and diamond which forms Metal-Interfacial layer-Semiconductor (MIS) structure. Oxygen surface treatment during fabrication process might have induced them. From forward bias C–V characteristics, the minimum thickness of the interfacial layer is approximately 0.248 nm. Energy distribution profile of the interface state density is then evaluated from the forward bias I–V characteristics based on the MIS model. The interface state density is found to be uniformly distributed with values around 10 13 eV − 1 ·cm − 2 .

The world today is continuously striving toward a carbon neutral clean energy technology. Hence, renewable wind power systems are increasingly receiving the attention of mankind. Energy production with structurally more promising and economically more competitive design is no more the sole criterion while installing new megawatt (MW) range of turbines. Rather important life cycle analysis (LCA) issues like climate change, ozone layer depletion, effect on surrounding environments e.g. eco-system quality, natural resources and human health emerge as dominant factors from green energy point of view. Hence, the study covers life cycle impact analysis (LCIA) of three wind farms: one onshore horizontal, one offshore horizontal, another vertical axis. It appears that vertical axis wind farm generates per unit electricity with lowest impact followed by horizontal offshore and horizontal onshore farms. The study, henceforward, discovers most adverse impact contributing materials in today's multi megawatt wind turbines and subsequently substitutes copper, the topmost impact contributor, with more eco-friendly aluminum alloys and its corresponding process routes. In this process, it reduces overall life cycle impacts up to 30% for future greener wind farms. In later stages, it compares all major electricity production technologies, viz., oil, diesel, coal, natural gas, wind, solar, biomass, nuclear, hydro plant in a common platform which demonstrates the wind farms performing the best except the hydro-kinetic ones. However, as the study suggests, offshore VAWT farm may even perform better than hydro-kinetic farms because of higher capacity factors in the high sea. Findings from the study can be deployed to harness massive scale green electricity from environmentally more clean and green turbines.

It is the intention of this paper to propose a compact flywheel energy storage system assisted by hybrid mechanical-magnetic bearings. Concepts of active magnetic bearings and axial flux PM synchronous machine are adopted in the design to facilitate the rotor–flywheel to spin and remain in magnetic levitation in the vertical orientation while the translations and rotations along and about x and y axes are constrained by mechanical bearings for simple operation. In this paper, the mathematical model of the flywheel’s levitation force and rotational torque is developed. The control systems of the position and velocity of the flywheel are designed based on PID method. Experiments are carried out and its test results prove the feasibility of the proposed flywheel system concept, the correctness of the mathematical analysis and the validity of the designed control system.

The state of energy (SOE) of Li-ion batteries is a key indicator for the energy management and optimization of energy storage systems (ESSs) in smart grids and electric vehicles (EVs). In order to improve the SOE estimation accuracy, an electrical Li-ion battery model is presented in this study against the dynamic loads and the rate energy effects of the battery. Firstly, in order to take into thorough consideration of the influencing factors in the SOE estimation process, a look-up table is merged with a 2nd order RC battery model for accurately predicting the battery voltage characteristics and capturing the nonlinear rate energy effects to realize the high-fidelity battery SOE and runtime prediction; Secondly, a new method to separate the fast and slow dynamics of the 2nd order RC battery model is developed and presented with a high-performance accuracy; Thirdly, commercial Li-ion batteries are tested at dynamic loads with various temperature to validate the effectiveness of the proposed model. The testing results show high accuracy and reliability of the proposed electrical battery model on the estimation of the battery SOE and the terminal voltage responses under dynamic loads.

Solid State Power Controller (SSPC) is gaining popularity, replacing mechanical switches and circuit breakers, in niche markets like electrical power distribution systems of the more electric aircraft. Significant amount of research is being conducted to develop high power SSPCs to cater the dynamic, distributed load management. The 28V SSPCs are widely used for protection and control of the secondary distribution system. Although there are high current SSPCs for low bus voltages, the 270V SSPCs are not mature and high current 270V SSPCs are not available in the market or having limited capability, which would be used in the primary power distribution system of the aircraft. This paper presents the market availability, design challenges the design engineer should be aware of and various approaches adopted to design and develop a high current 270V DC SSPC for more electric aircraft system, its key features which may not have been available in the traditional switches and future trends. National Research Foundation (NRF) Accepted version This work is jointly funded by National Research Foundation (NRF) Singapore, Rolls-Royce Singapore Pte. Ltd, and Nanyang Technological University, Singapore.

Cooling system becomes a serious concern due to weight and volume contribution of power electronics system in aerospace sector. This research investigates the contribution of cooling system to the total weight of power electronics converter when liquid metal cooling is applied. In addition, it is compared with the traditional working fluid (water). It is concluded that cooling system could be much lighter if the liquid metal cooling is introduced. In this study the introduction of liquid metal cooling results in 12 times lighter pump.

Phasor Measurement Units have been applied on power system to improve the performance of power system state estimator for decades. As it is not practical to install one PMU at every bus in a power system due to its high cost, plenty of research focus on solving the optimal PMU placement (OPP) problem which is to determine a placement with minimum number of PMUs while ensuring network's observability. Since PMUs are mostly employed within transmission networks, most of the OPP algorithms are also designed for transmission networks. In recent years, distribution networks are evolving from passive to active because of increasing application of distribution energy resources, which raises the desire of more continuous monitoring of distribution networks. Traditional PMUs cannot be employed in distribution networks due to economic and technical constraints. The advent of μPMU makes it possible to employ phasor measurement devices in distribution networks. It is anticipated that distribution networks will be employed with μPMU in near future. This paper proposed a hybrid approach based on global search algorithm to solve the OPP problem in distribution networks. The drawback of global search method that consuming extensive computational time is overcome in this method by some preprocessing steps. Several IEEE distribution test feeders are used to examine the proposed placement method, and the results shows this placement approach is able to give the optimal solution while keeping the computational time in acceptance range.

This paper presents the benefits of interleaving three phase voltage source converters (VSCs) to improve the power density. The output harmonics are also analyzed in frequency domain, then design and optimization of coupled inductor and output filter are given. Verifications are carried out through electrical circiut and electromagnetic's FEM simualtions. Finally, the inductor weight are compared with and without interleaving.

The high power density converter is required due to the strict demands of volume and weight in more electric aircraft, which makes SiC extremely attractive for this application. In this work, a prototype of 50 kW SiC high power density converter with the topology of two-level three-phase voltage source inverter is demonstrated. This converter is driven at high switching speed based on the optimization in switching characterization. It operates at a switching frequency up to 100 kHz and a low dead time of 250 ns. And the converter efficiency is measured to be 99% at 40 kHz and 97.8% at 100 kHz.

Integration of electric vehicles (EVs) into existing power system can be carried out smoothly and efficiently using coordinated/smart charging. Time based coordination and power based coordination are the two strategies available for coordinated charging methods. Both the charging strategies operate the EV chargers at different operating power and hence the overall all charging efficiency differ from one another. In this paper, the efficiency evaluation for different coordinated charging methods is carried out for both the charging strategies. The overall energy efficiency of two types of coordinated charging methods is evaluated for a typical EV system charged in high-rise residential buildings.

The applications of Li-ion batteries used in energy storage system (ESS) in building environment is drawing more and more public attention and it is predicted to hold the most share of battery market in the year of 2020. In this paper, an equivalent circuit battery model for the ESSs applications in building environment is proposed, and the parameters are extracted experimentally. A new methodology to separate the fast and slow dynamics of the model is presented, and the parameter extraction results show a high-performance accuracy. The State of Energy instead of the State of Charge of the battery is used as model output in order for accurate battery energy state prediction during discharging. The discharging power is taken as the direct input of the proposed model, instead of the discharging current. The model is set up in Matlab environment, and the testing results of a commercial Li-ion battery are presented.

This paper demonstrates a 60kHz 50kW/400Hz grid facing three-phase power converter using commercially off-the-shelf Silicon Carbide (SiC) module. The power converter is designed to achieve high gravimetric density for possible application in future aircraft. In order to achieve high power density, the power converter is operated with high switching frequency to minimize its LCL filter size that connects to the grid. The high switching frequency is achieved by optimizing circuit design that consider minimum parasitic inductances in the power loop and high speed gate driver. As a result, the grid facing power converter with 60kHz switching frequency and total efficiency 97.7% are demonstrated. Gravimetric power density of the converter is above 6kW/kg with 50% of its total weight coming from its output inductors.

This paper presents power loss and thermal analysis of SiC power devices in a 50 kW 3-phase high power density converter during operation. The presented loss and temperature estimations include average analysis and transient analysis by using electro-thermal simulations. For this purpose, thermal impedance network of the SiC power module and cold plate prototype are extracted. Transient analysis shows dependence of temperature ripple in the devices on AC output frequency. It is found that high frequency AC (HFAC) system of 400 Hz has 7 times smaller temperature ripple magnitude compared to that in 50 Hz AC system, and therefore higher power devices' reliability in HFAC system. This adds to the list of benefit of HFAC system for future applications.

The power converter for more electric engine in aircraft requires high power density, which makes SiC extremely attractive for this application. In this work, a 50 kW SiC high power density converter with the topology of three-phase two-level voltage source inverter is developed. Meanwhile, an accurate mathematical model for the power loss and efficiency calculation is proposed, which is based on the experimental results of switching characterization. The prototype presented in this work shows an efficiency up to 97.77% at 50 kW input power and 60 kHz switching frequency. And it matches well with the result predicted by model.

The state of energy (SOE) of Li-ion batteries is a key indicator for the energy optimization and management of energy storage devices (ESDs) in electric vehicles and smart grids. In order to realize the improvement of the estimation accuracy and reliability of battery SOE, factors that influence the discharging performance of Li-ion batteries are investigated thoroughly in this study. The discharging power, previous charging conditions, and operating temperature are the influencing factors, and they should be considered comprehensively during the SOE estimation in the battery management system (BMS). High discharging power will lead to “capacity fade” and the operating temperature change is known to have critical effects on the performance, cycle life, and safety of Li-ion batteries. With a thorough consideration of these factors, modifications are made to the definition and calculating algorithm of SOE in this paper, which is being used for indicating the energy state of the battery during discharging in the current state-of-art. The estimation results of the modified SOE show higher coincidence with the real battery performance compared with that of the conventional SOE determination algorithm. This study modifies the definition and the calculation algorithms of SOE and contributes to the development of a comprehensive equivalent circuit model for the battery SOE estimation and runtime prediction in the future work.

Phasor Measurement Unit (PMU) has been applied on power system to improve the performance of power system state estimator for decades. It is anticipated that PMUs will observe distribution networks in future considering the contribution of PMUs to transmission networks. The advent of micro phasor measurement unit (μPMU) makes it possible to employ phasor measurement devices in distribution networks. Optimal PMU Placement (OPP) problem is to determine a minimal set of PMUs such that the whole system is observable. Since PMUs are mostly employed within transmission networks, most OPP algorithms are also designed for transmission networks. However, OPP algorithms designed transmission networks have not considered the radial topology of distribution networks. This paper investigated the different topology of transmission and distribution networks and proposed a customized OPP method for radial networks. The customized OPP method made use of radial topology of distribution networks to equivalently downsize the scale of the networks. Since the solution for OPP is usually difficult for large scale systems, the solution of OPP is accelerated by the customized OPP. The proposed method was tested on various IEEE distribution test feeders and transmission systems.

The high switching frequency (> 20 kHz) of SiC MOSFET module makes it as an attractive alternative of Si IGBT module for high power density applications. Since SiC MOSFET and Si IGBT have the similar MOS-gated structure, it is normally regarded that the gate driver of SiC MOSFET can directly inherit from that of Si IGBT. However, considering the different device physics properties, some special design considerations need to be taken. In addition, the higher dv/dt and di/dt in SiC MOSFET module leads to more serious EMI issues, which will comprise the switching speed in return. In this work, the high-speed gate drivers for Si IGBT and SiC MOSFET modules with similar ratings are designed and optimized. Based on the experiment of switching characterization, the considerations of gate driver design to migrate from Si IGBT module to SiC MOSFET module are presented. And the EMI issue is the major challenge in the gate driver design for SiC MOSFET module.

The increasing demand for high power density requires power converter to operate in high switching frequency. SiC power module is regarded as one of the most promising candidates for high-frequency applications due to the superior switching speed and low switching loss. The conventional strategy to optimize switching loss is normally achieved by repetitive double pulse tests, which is time-consuming to find an optimum gate resistance to achieve the trade-off between switching loss and EMI issues. In this work, an accurate SiC module subcircuit model is proposed. It considers the physical behavior of the device and can be directly extracted from the datasheet information. Good agreements are achieved between the PSpice simulation and experimental results in both switching waveform and switching loss. It also provides a guidance for gate driver design with a reasonable accuracy.

This paper presents a dynamic model predictive voltage control (DMPVC) method for the single-input dualoutput (SIDO) flyback converter. In this converter, cross regulation is one of the most critical issues because its primary winding is shared by its secondary windings so that the two output voltages are interacted with each other. The proposed method has the ability to reduce cross regulation significantly, i.e., the two output voltages can be regulated individually. The statespace models of the converter for the DMPVC method are formulated, as well as the corresponding cost function which determines the optimal control actions. In order to verify the performance of the DMPVC method, simulation platform is built in the MATLAB/Simulink environment. The results indicate that the DMVPC method is capable of controlling the SIDO flyback converter with expected line and cross regulations along with seamless mode transition among various operation modes, including buck-buck mode, buck-boost mode and boost-boost mode.

This paper presents the extraction of common mode (CM) and differential mode (DM) inductances of a common mode choke (CMC) using 3D finite element method (FEM). For optimal performance of the CMC, the effects of CM and DM currents on the magnetic core saturation are investigated. As the parasitic capacitance of a CMC is a critical parameter that affects its high frequency performance, the parasitic capacitances of the turn-to-turn and the turn-to-core capacitances of different winding layouts are also extracted for comparison. Finally, different core structures are modeled and simulated for performance evaluation.

The differential current in Modular Multilevel Converters (MMCs) is inherently subjected to even order harmonics, which are mainly caused by the voltage mismatch between phase legs and dc bus. This issue may exist in both single-phase and multi-phase MMC systems and can be eased by repetitive control based active harmonic suppression methods, which regulate the circulating current through legitimate adjustment of the voltage applied on the arm inductance. Nevertheless, conventional repetitive controllers have relatively slow dynamic response because of the presence of one fundamental period delay, and all the sampled errors in the past one cycle have to be stored for closed-loop control. This paper presents an improved even-harmonic repetitive control scheme that requires less data memory with faster dynamic response. The steady state performance can also be enhanced if system disturbances e.g. frequency deviation are considered. In order to verify its practicality and effectiveness, PLECS simulation and experimental results are presented to show the excellent dynamic response and disturbance rejection of the proposed even-harmonic elimination scheme.

For a Li-ion battery based energy storage system, the state of energy is a key index for the energy management and operation optimization. In the actual Li-ion batteries based energy storage system in building environment, the varying load current will lead to the estimated battery's residual energy deviated from the real value. To solve this problem, an equivalent circuit model is presented for the Li-ion battery's state of energy estimation during and after discharging process. The state of energy is introduced in the proposed approach to replace the traditional index SOC, to describe the electrical behaviors and the residual energy of the Li-ion battery under dynamic load current conditions, so that the battery's internal energy loss on its internal resistance and during the electrochemical reaction processes, as well as the effects of the decrease of the OCV during discharging are taken into consideration. Experimental data of the previous study regarding energy efficiency of Li-ion batteries used as energy storage devices in building is utilized in the present study. Finally, the simulation results on 20 Ah LTO batteries indicate that the proposed equivalent circuit is competent to predict the state of energy accurately. What's more, state of energy appears an improvement over SOC in terms of accuracy in capturing battery's energy changes. Thus, this study provides a viable solution for estimating Li-ion battery's SOE and capturing the battery's energy behaviors under dynamic loads.

The trend towards SiC-based high power density converter requires to drive SiC MOSFET with high-speed switching. However, it tends to aggravate dv/dt effect due to the impact of parasitic parameters, resulting in shoot-through and high device stress in the half bridge configuration. In this work, a novel gate assisted circuit is proposed to eliminate shoot-through issue without compromise on switching speed. The mathematical model for the gate assisted circuit is developed. Then the switching test experiment is set up to validate its performance with commercial SiC half bridge module, which shows a high dv/dt up to 20 and 31 V/ns at turn-on and turn-off respectively and a 34% reduction of switching loss without any shoot-through issue.

The reactive power in power converter with inductive load (motor drive e.g.) requires a current commutation path for the freewheeling current. Due to the high voltage drop of body diode of SiC MOSFET, a SiC Schottky diode is normally recommended as the anti-parallel freewheeling diode for SiC MOSFET to suppress the conduction of body diode. However, since the MOSFET can work as synchronous rectifier, the freewheeling diode only conducts during the dead time, leading to a low utilization rate of device. In this work, the three-phase SiC inverter using synchronous rectification is investigated. The analytical model for inverter power loss with and without freewheeling diode is built. Based on the switching characterization, the inverter with synchronous rectification permits a surprising higher efficiency than that with freewheeling diode due to the reduced current overshoot at turn-on. And a 5 kW prototype of three-phase inverter is developed, which shows a 99% high efficiency at the switching frequency of 40 kHz. This work confirms the possibility to remove the freewheeling diode in SiC inverter without degrading the efficiency.

In power transistor models, it is very important that device capacitances are modelled accurately so that switching losses, EMI filter requirements and gate timing requirements of the converter can be accurately determined. In this paper, a modelling technique utilizing modified sigmoid functions to describe the device capacitances is applied on a GaN HEMT and a silicon MOSFET to develop their corresponding SPICE models. Comparison of switching energies of the transistors under slow-switching conditions suggest that the technique is suitable for modelling both types of transistors. The developed model for the GaN HEMT is also shown to simulate faster than the manufacturer's model under fast switching conditions.

This study proposes two novel modularised battery systems capable of controlling the power of each module independently, and with inductive interface for convenient battery swapping. The proposed systems aid in overcoming the limitations such as unavailability of electric vehicle (EV) due to battery pack fault and lengthy battery recharging time which largely hampers the adoption of EVs for personal transportation. The proposed systems consist of a plurality of battery modules which are wirelessly coupled to the EV through inductive power transfer technology. The proposed systems are described in detail, and models are presented to analyse their steady-state behaviours. A design guideline for a 24 kWh 80 kW battery micro-pack system is discussed. Performances of the proposed topologies are investigated using simulations. To demonstrate the applicability, prototype systems of 1.5 kW are implemented and tested under various operating conditions. Results convincingly indicate that the proposed systems improve the vehicle's availability under fault condition. Published version

The effects of two-phase-flow modeling on nuclear reactor single-channel stability analysis are investigated with four two-phase-flow models, namely, the homogeneous-equilibrium model, the homogeneous-nonequilibrium model, the nonhomogeneous-equilibrium model, and the nonhomogeneous-nonequilibrium model. The models are applied to hot-channel analyses of a proposed typical supercritical-water-cooled-reactor (SCWR) design. The neutral stability boundaries derived by using the four models are compared and plotted on the traditional subcooling number versus phase change number plane. To ensure proper development of the models, they are benchmarked to the experimental data. It is found that the homogeneous models predict more conservative stability boundaries than the nonhomogeneous models and that the differences of the stability boundaries predicted by all four two-phase-flow models are reduced under higher-pressure conditions.

To avoid checkerboard pressure in numerical simulations by using collocated grids, the momentum interpolation method was initially proposed by Rhie and Chow and subsequently revised by various researchers. All the momentum interpolation methods are classified into two categories, which are called method A and method B in this article. The performance of the two kinds of methods is evaluated for both single- and two-phase flow in the present work. It is found that method B is more robust for the numerical simulation of two-phase flow.

Due to increasing wind power penetration into utility grids, system operators have instituted stringent standards for ride-through compliance of grid-connected wind turbines. Series voltage compensation can be an effective solution to meet the new standards, especially if a low-cost system can be developed. This paper first presents the characteristics of ride-through behavior of doubly fed induction generator (DFIG) using phasor analysis. The analysis is extended to include the effects of generic series voltage compensation. A new control scheme is then described and analyzed, which allows relatively low-energy ratings in the series compensator, thereby significantly lowering the overall system cost. The new controller enables the DFIG with the compensator to provide the reactive power required for system voltage recovery during grid fault, in compliance with the new grid codes. Simulation and experimental results have demonstrated the enhanced performances resulting from the application of the control scheme.

This paper presents a fast current control scheme for a dual air-gap axial flux permanent magnet (AFPM) machine based on model predictive control (MPC) approach. This AFPM machine functions as both motor and generator in the flywheel energy storage system (FESS), hence its fast acceleration or deceleration is crucial for rapid storage and release of energy respectively. MPC method, which can pre-calculate the system behaviour and subsequently select optimal control variables, has been employed to control this machine. Performance of MPC for this machine is evaluated and compared with the proportional-integral-derivative controllers. The feasibility and potential of the MPC for this AFPM machine in the FESS have been validated by both simulations and experiments on a machine prototype.