ALBERT

Description: Several strategies have been proposed for operating doubly-fed induction generator (DFIG)-based wind turbine under unbalanced grid voltage conditions. This study focuses on those strategies in which the rotor-side power converter aims at eliminating the oscillations affecting the electromagnetic torque and the stator reactive power. Given the limited size of the DFIG’s power converters, and hence their tolerable current and voltage boundaries, this study analyzes which DFIG power generation capability is under unbalanced grid voltage, and therefrom derives rotor current and stator power controllable ranges. Besides, the feasibility regions of the DFIG are also deduced for different types of imbalance. Furthermore, and based on the outcome of previous analysis, a modified rotor current limiter, as also its equivalent stator power limiter, is proposed. In contrast to the conventional ones, the limiters proposed here take into account that imbalances may arise in the grid voltage. As a consequence, system’s overall performance is considerably enhanced under unbalanced grid voltage conditions. Finally, simulation results establish the validity of the treated issues.

Description: Wind turbine technology has evolved into a unique technical identity with potential to contribute significantly to the global energy mix powered by renewables. Wind energy, being a fluctuating resource, requires tight control management that addresses stability issues for it to be integrable with the grid system. Difficulty in controller construction for wind energy conversion systems (WECSs) is reinforced by sensitivity to numerical complexity, fast parameter variations due to wind stochasticity, tight performance requirements, as well as the presence of flexible modes that limit the control bandwidth. Proposed herein is a pitch control scheme and a model-based ${cal H}_{infty }$ synthesis controller that yields a multivariable control law governing operation of the power electronic converter for a megawatt-class WECS over the entire nominal operating trajectory. The ${cal H}_{infty }$ control objectives are cast as optimization programs with a unique cost function subject to linear matrix inequality constraints. Simulational analysis confirms the efficacy of the adopted technique: issues regarding uncertainties with respect to system modeling and possible adverse control due to interactions with highly turbulent winds are handled with precision, while significantly improving the quality of voltage and output power.

Description: A general dynamic hysteresis core loss model has been developed. Experimental data for different lamination thicknesses have been measured and analyzed at various frequencies. Based on the analysis, a dynamic hysteresis finite element core loss model is established and validated by experiments. The developed dynamic hysteresis model is simple and efficient, and has been shown to be very accurate compared with the experiments. Moreover, the model can calculate core losses based on the input parameters obtained from experimental measurements at only one single frequency in a thin lamination. The model, to our best knowledge, is the first one that is capable of calculating core losses for different thicknesses of materials and different operating frequencies, without a massive experimental database. In addition, only the eddy current and hysteresis models are necessary for this calculation but with the important addition of the variation of the flux density within the lamination.

Description: Conventional active islanding detection methods (IDMs) are designed for the inverter-based distributed generation systems (DGSs) with current control interface. Such strategies can hardly be extended to the DGS with power control interface because the power control loop can affect the IDMs by enlarging the nondetection zones (NDZs). This paper presents an active IDM based on negative-sequence power injections for the DGS with power control interface. Combining with the IDM, the power control of the DGS is achieved with two control loops. One is the positive-sequence power loop that satisfies the conventional power control requirements. The other is the negative-sequence power/current variation loop for the islanding detection. The positive and negative sequences are separated by a simple strategy based on an all digital phase-locked loop. Due to the differences between the grid impedance and the local load impedance, the percentage of the voltage imbalance (VI) at the point of common coupling is utilized to indicate the islanding operation. For the grid-connected DGS, the VI is dominated by the grid voltage, which is a constant. If the DGS is disconnected from the grid, the VI is determined by the injected negative-sequence power/current. The NDZs of the presented scheme with different system configurations, such as grid impedances, load quality factors, etc., are also analyzed in this paper. By injecting the negative-sequence power/current periodically, the NDZs resulting from the imbalance of the grid voltages or the local loads are further mitigated. For multi-DGSs, the IDM is still effective if combined with the conventional under/overfrequency-protection strategy. The simulation and experimental results verify the effectiveness of the IDM.

Description: This paper presents an analysis of the cost of utilizing battery electric vehicle (BEV) batteries as energy storage in power grids [also known as vehicle-to-grid (V2G)] associated with lessening battery cycle life due to more frequent charging and discharging activities and utilization in elevated ambient temperature. Comparison is made between V2G in the U.K., where annual electricity peak demand is reached in winter, and in China, where peak demand is reached in summer due to the air conditioning load. This paper presents mathematical correlations between charging–discharging, ambient temperature, depth of discharge (DoD), and the degradation of electric vehicle batteries based on manufacturer's data. Simulation studies were carried out for V2G in both the U.K. and China. Numerical results show that ambient temperature and DoD of a BEV battery play a crucial role in the cost of battery wear. Lead-acid and NiMH battery powered BEVs are not cost effective in V2G use due to the present electricity tariff. Under the present electricity tariff structure, no vehicles would be cost effective for the peak power sources in China. However, lithium-ion battery powered BEVs are cost effective in the U.K. due to a much longer cycle life.

Description: In this paper, an effective new approach for estimating the operating temperature of a photovoltaic (PV) module by using the simple diode model is presented. The developed method is simple and does not need any additional hardware. The proposed approach uses an analytical formula to derive the temperature from the maximum power point voltage and current. Since the temperature estimation (TE) is based on the model of the PV module, at first the model is explained. In the model, effects of the temperature coefficients are considered. Moreover, a new approach to find all parameters of a PV module model is described. Unlike other methods, the parameters’ extraction method that is proposed here only needs manufacturer data from the datasheet and does not need any additional information. Effectiveness of the new TE procedure is investigated through some conducted simulations in MATLAB/Simulink environment and its validity is verified by experiment on a REC-AE220 solar module.

Description: The most important reference quantities for monitoring and controlling transient stability in real time are the rotor angle and speed of the synchronous generators. If these quantities can be estimated with sufficient accuracy, they can be used in global and local control methods. In the classic state estimation methods, such as the extended Kalman filter (EKF) technique, the linear approximations of the system at a given moment in time may introduce errors in the states. In order to overcome the drawbacks of the EKF, the authors of this paper have applied the unscented Kalman filter (UKF) to estimating and predicting the states of a synchronous machine, including rotor angle and rotor speed, using phasor measurement unit (PMU) quantities. The UKF algorithm propagates the pdf of a random variable in a simple and effective way and is accurate up to the second order in estimating the mean and covariance. The overall impression is that the performance of the UKF is better than the EKF in terms of robustness, speed of convergence, and also different levels of noise. Simulation results including saturation effects and grid faults show the accuracy and efficiency of the UKF method in state estimation of the system, especially at higher noise ratios.

Description: Using a finite element (FE) model of the electrical machine in a grid solver enables us to take the machine nonlinearities into account with great precision in the simulation of a complete system. Three methods are usually used for linking the grid and FE solver: direct coupling, lumped components, and current or voltage output approach. In this article, the coupling is treated as the resolution of a nonlinear equation system that is solved with the Newton method. The proposed linking scheme has been used to simulate a two phase short circuit on a hydrogenerator. Its results have been validated by running the same simulation with commercially available software.

Description: The choice of the most suitable control strategy for wave energy converters (WECs) is often evaluated with reference to the sinusoidal assumption for incident waves. Under this hypothesis, linear techniques for the control of the extracted power, as passive loading and optimum control, are well known and widely analyzed. It can be shown, however, how their performances are fundamentally different when irregular waves are considered and the theoretical superiority of optimum control is questionable under real wave conditions. Moreover, the global optimization of WECs implies a rational design of the power electronics equipment. This requires the analysis of the instantaneous extracted power in addition to the average one. In this paper, the impact of irregular waves on the power extraction when using different control techniques is analyzed in the case of a point absorber in heave. It is also shown how a convenient tradeoff between high average power extraction and limited power electronics overrating can be obtained by applying simple power saturation techniques. Moreover, the impact of power conversion efficiency on the control strategy is analyzed.

Description: Coreless dual disks hysteresis motors (CDDHM) have been recently introduced. The CDDHM presents some improvements in power factor and efficiency compared to general hysteresis machines. However, there is not much design or manufacturing experience available and an initial design methodology has not yet been proposed for the new machine. According to this and existing inherent complexity in the model of hysteresis motors, it is difficult to achieve an optimal design solution by applying a conventional optimization method. In this paper, an initial design approach is proposed. Then, a genetic algorithm-based method is developed to find the high-efficiency solution for an optimal CDDHM. The optimized motor has been manufactured and fitted in a test facility with a torque meter. Some experiments are carried out to prove the optimal design. Furthermore, start-up characteristics of the motor are predicted from the model and validated by a routine test.

Description: This paper presents an investigation of the behavior of a 200-MVA-synchronous hydrogenerator during interturn stator faults, focusing on the affection to the electromagnetic magnitudes, such as the currents and the electromagnetic torque. Two kinds of interturn fault are examined 1) a short circuit between two conductors belonging to the same phase; and 2) a short circuit between two conductors belonging to different phases. These faults have been investigated via simulation using the finite element method. The benefit of this method is that it gives the possibility to calculate, except from the classical electrical magnitudes (e.g., the stator and field currents, the load angle, and the electromagnetic torque), other magnitudes which in general are difficult to compute during transient operation (e.g., the damper currents and the electromagnetic field inside the machine air gap). The main aim of this paper is to highlight quantitative and qualitative the condition in the interior of the machine in the cases of typical faults, which usually can occur during the machine operation. So, it is possible to find out that faults exist in the machine through the measurement of the field and the stator currents.

Description: A mathematical model of a valve-regulated lead-acid battery under discharge is presented as simplified from a standard electrodynamics model. This nonlinear reaction–diffusion model of a battery cell is solved using an operator splitting method to quickly and accurately simulate sulfuric acid concentration. This splitting method incorporates one-sided approximation schemes to preserve continuity over material interfaces encompassing discontinuous parameters. Numerical results are compared with measured data by calculating battery voltage from modeled acid concentration as derived from the Nernst equation.

Description: A significant problem of turbogenerators on complex end structures is overheating of local parts caused by end losses and complex fluid flow in the end region. Therefore, it is important to investigate the 3-D flow and heat transfer process in the end. Using a 200-MW air-cooled turbogenerator as an example, the influences of end cores and the actual shapes and material of the end coils, press finger, press plate, and copper shield are considered for the end field calculation; then, the physical and mathematical models of the coil end with involute portions are created. The 3-D electromagnetic field was calculated and the losses of different parts in the end and its distribution were obtained. Based on this, the losses from magnetic field calculations will be applied to the end as heat sources in the temperature field. For symmetry of the ventilated structure, the fluid and thermal physical models of the generator within the half-axial section were determined. A set of equations of fluid flow and heat transfer were derived from the fluid–solid conjugated heat transfer and the fluid and temperature distributions were obtained after solving the equations. All of the aforementioned will provide a theoretical basis for the generator safe operation.

Description: The calorimetric method has for a long time been applied for energy conversion efficiency measurement in electrical machines. The number of temperature sensors necessary for its successful application in the field constitutes in one of the major drawbacks of the method. This paper shows how the usage of IR thermal imaging techniques can reduce costs and the necessary time for the calorimetric method application. In addition, contribution to the heat transfer coefficient determination and a new approach to consider conduction losses in the generator shaft are also presented.

Description: This paper presents a dc-bus voltage control technique for a new power conversion topology feasible for parallel-integrated permanent magnet (PM) wind generation systems. This technique is based on a master–slave hysteresis control scheme in order to solve discrepancy problems that could happen between the controllers. A three-phase semicontrolled rectifier topology is proposed here as an effective interface circuit between each wind generator and the dc-bus. The proposed system provides interconnection extension ability of multiwind converter units sharing the same dc-bus and more economic utilization of the wind generator; by ensuring unity power factor operation. More advantages include voltage disturbance compensation capability, robustness: since a short circuit through a leg is not possible and high efficiency: due to the reduced number of switching elements. The rectifier topology concept, the principle of operation, the control scheme, and test results are presented. The developed technique is also implemented on a 12 kW parallel-connected PM laboratory setup in order to confirm the effectiveness of the proposed system.

Description: This paper presents the design and analysis of an electromechanical flywheel energy storage system to enhance rural electrification in sub-Saharan Africa. The system consists of a flywheel rotor, an electrical machine, control system, bearings, and a containment structure. With the exception of the power electronics and magnets, local materials were used for the manufacture of the flywheel system. The flywheel rotor is made from glass fiber-epoxy composite, designed using novel shape profiles and utilizes a stress based solution by introducing a central hole for shaft inclusion. The system was accelerated to 6000 r/min storing up to 227 kJ. Numerical stress analyses were performed during the design stage to ensure that the maximum tensile strength is not exceeded. A lumped parameter thermal model is used to estimate the temperature distribution to ensure safe operating conditions of the flywheel system and environment. A life cycle cost analysis performed found that by integrating a flywheel system into a Solar Home System implies a cost savings of 35% per kilowatthour when compared with lead-acid batteries.

Description: The experimental research, whose most important segments are described in this paper, is one of the first attempts to determine the influence of wind turbine blade rotation on likelihood of getting struck by lightning. The tests were conducted in the high-voltage laboratory, applying the up-and-down method for determination of the 50% flashover standard switching voltage. The impulse voltage waves were applied between the specially designed arching electrode and the blades of a reduced-size wind turbine model, driven by a frequency controlled motor. Two types of lightning protection systems and the case of absent protection system were examined. According to the theory of similarity this paper discusses the characteristics of direct atmospheric discharges for the following typical rotational speeds of the wind turbine: w  =  0 r/min (when the wind turbine is idle), w  =  250 r/min (for the winds of moderate strength), and w   =  400 r/min (rated rotor speed).

Description: The aim of this paper is to analyze the dynamic behavior of the doubly fed induction generator (DFIG) subject to symmetrical voltage sags caused by three-phase faults. A simple control algorithm is considered and assumed ideal: the rotor current $i_{rf}$ in the synchronous reference frame is kept constant. This hypothesis allows the electrical transient to be solved analytically, providing a comprehensive description of DFIG behavior under symmetrical sags. The fault-clearing physics of symmetrical sags is also analyzed. That is, the fault is cleared in the successive natural fault-current zeros, leading to a voltage recovery in one, two, or three steps. This clearing process, called discrete fault clearing in this paper, results in a more accurate sag modeling than the abrupt or instantaneous fault clearing (the usual modeling in the literature). The fault-clearing process has a strong influence on the rotor voltage required to control the rotor current after fault clearing. To compare the effects of both abrupt and discrete sags, different wind turbine (WT) operating points, which determine different generated powers, are considered. This study helps in the understanding of WT fault ride-through capability.

Description: A high-fidelity battery model capable of accurately predicting battery performance is required for proper design and operation of battery-powered systems. However, the existing battery models have at least one of the following drawbacks: 1) requiring intensive computation due to high complexity; 2) not applicable for electrical circuit design and simulation; and 3) not capable of accurately capturing the state of charge (SOC) and predicting runtime of the battery due to neglecting the nonlinear capacity effects. This paper proposes a novel hybrid battery model, which takes the advantages of an electrical circuit battery model to accurately predicting the dynamic circuit characteristics of the battery and an analytical battery model to capturing the nonlinear capacity effects for the accurate SOC tracking and runtime prediction of the battery. The proposed battery model is validated by the simulation and experimental studies for the single-cell and multicell polymer lithium-ion batteries, as well as for a lead-acid battery. The proposed model is applicable to other types and sizes of electrochemical battery cells. The proposed battery model is computational effective for simulation, design, and real-time management of battery-powered systems.

Description: Modern grid codes determine that wind generation plants must not be disconnected from the grid during some levels of voltage sags and contribute to network stabilization. Wind energy conversion systems equipped with the doubly fed induction generator (DFIG) are one of the most frequently used topologies, but they are sensitive to grid disturbances due to the stator direct connection to the grid. Therefore, many efforts have been done in the last few years in order to improve their low-voltage ride-through capability. This paper analyzes the behavior of the DFIG during symmetrical voltage sags using models in the frequency domain. A new strategy, the machine magnetizing current control, is proposed in order to enhance the system response during balanced dips. The method is derived on a theoretical basis and numerically investigated by means of simulation. Experimental results are presented and validate the proposed strategy. Finally, the practical aspects of the use of this strategy are discussed.

Description: Directly coupled electromagnetic field-electric circuit modeling approach for coupling an electrical machine to its associated external circuit and power electronics has been widely reported in the literature. However, the inclusion of a closed-loop control system, such as conventional vector (field-oriented) control, within a directly coupled electromagnetic field-electric circuit model has not been widely studied. This is partly because it is a rather complex task. This paper introduces a directly coupled electromagnetic field-electric circuit computational model in which a conventional vector control system has been incorporated for detail simulation of vector-controlled electric machinery drive systems. The model has been successfully applied in the design and analysis of a vector-controlled wound field brushless starter-generator system prototype. Comparison between the simulation and test results for both motor and generator modes show excellent correlation that validates the efficacy and computational soundness of the modeling approach.

Description: In this paper, an oscillating water column-based wave energy conversion plant is modeled and controlled by means of two complementary control strategies in order to improve the conversion of wave energy into electric power. This wave power generation system consists of a capture chamber, a Wells turbine, and an induction generator. The improvement relays on the implementation of a control scheme that combines two different control methods: a rotational speed control and an airflow control implemented by means of a throttle valve in series with the turbine. The use of rotational speed control provides a fast way to react to the abrupt and short changes in the turbine speed, ensuring that the average power of the generator is adequately adjusted according to the incident wave power level. Besides, a throttle-valve is used to control the flow through the turbine so as to increase the amount of energy produced, particularly at the higher incident wave power levels. These two control strategies complement each other, maximizing and improving the quality of supply by controlling and smoothing the generated power for different scenarios of wave oscillations, variations of wave grouping, and changes in the sea state.

Description: Interests of subsynchronous resonance (SSR) in series compensated electric networks with wind power penetration have arisen recently. To better understand the nature of such systems, modal analysis of a doubly-fed induction generator (DFIG)-based wind energy system interconnected with a series compensated electric network is conducted in this paper. The system model is built in Matlab/Simulink. The major contributions of the paper include: 1) identification of the four system modes (SSR, supersynchronous, electromechanical, and shaft modes) of a DFIG-based wind farm interfaced with a series compensated network, 2) investigation of the impacts of various parameters and operating conditions on those modes, and 3) prediction of the dynamic performance of the system using modal analysis and confirmation of such prediction via time-domain simulation results.

Description: This paper proposes a design for a renewable-energy hybrid power plant that is fed by a photovoltaic (PV) source with a supercapacitor (SC) storage device and is suitable for distributed generation applications. The PV array is used as the main generator, and the SC functions as an auxiliary source for supplying the (transient and steady-state) power deficiency of the PV array. For high-power applications, four-phase parallel boost converters and four-phase parallel bidirectional converters are implemented as a PV converter and a storage device, respectively. A reduced-order mathematical model of the PV and SC converters is described for the control of the power plant. Using a nonlinear approach based on the flatness property, we propose a simple solution to the dynamic, stabilization, and robustness problems in the hybrid power system. This is the key innovative contribution of this research paper. We analyze a prototype small-scale power plant composed of a 0.8-kW PV array and a 100-F SC module. The experimental results authenticate the excellent control algorithm during load cycles.

Description: This paper presents the analyzed results of both dynamic simulations and steady-state performance of a tidal power generation system (TPGS) containing a permanent-magnet synchronous generator (PMSG) driven by a tidal turbine through a gearbox. The stator windings of the tidal PMSG are connected to an onshore distribution system through an ac-to-dc converter, a dc-to-ac inverter, a step-up transformer, and a transmission cable. A d–q axis equivalent-circuit model is employed to establish complete dynamic equations of the studied system under three-phase balanced loading conditions. A frequency-domain approach based on eigenvalue analysis and a time-domain scheme based on nonlinear-model simulations are both carried out to systematically determine the dynamic stability of the studied system under different operating conditions. It can be concluded from the simulation results that the studied TPGS subject to different disturbance conditions can maintain stable operation.

Description: Presents the table of contents for this issue of the publication.

Description: Solid-oxide fuel cell (SOFC) power plant plays a vital role in a hybrid alternative energy based microgrid due to its reliability and flexibility in power supply. However, the control of SOFC is challenging in terms of providing a fast load tracking while maintaining the fuel utilization rate within a safe range. To this end, this paper builds two basic coordinated control strategies (CCS) for power management of SOFC-based microgrid. The first is “Fuel Cell follows Inverter” scheme, where fast tracking is preferred while SOFC may operate on the edge of the safety range. The second is “Inverter follows Fuel Cell” scheme, where high security is guaranteed by sacrificing performance in load tracking. To obtain a robust and simple scheme, energy balance principle is used in CCS such that PI controller is sufficient to fulfill the basic duties. Moreover, a simple supervisory control strategy is proposed for microgrid to provide a reasonable power reference for SOFC. The control system is designed and tuned based on an SOFC plant with inverter's average model. The efficiency of the proposed strategies is validated via a grid-connected SOFC/photovoltaic microgrid.

Description: In this paper, a wave energy converter (WEC) is designed for a wave climate condition and its interior permanent magnet (IPM) generator is dimensioned for minimum life cycle cost (LCC). The single point absorber movement is simulated and the harnessed annual energy is determined to be 325 MWh. Possible IPM generator sizes are investigated using finite element calculations. Tradeoffs between the generator cost and the annual energy efficiency is performed in order to find the best economical solution. For the economical evaluation, the net present value method is utilized in order to obtain the LCC of the electric generation system of the WEC. Increasing the axial length of the machine 1.6 times results in the lowest LCC, with a $2.5%$ efficiency increase, which means an increment of approximately 1000 €   in the investment cost. Despite the higher investment cost, the LCC of the WEC generator decreases from 8070 €   to 6450 €, due to the lower losses.

Description: This paper presents an elaborate scheme, which is different from the traditional parameter identification method, to estimate the parameters precisely for high-speed magnetic suspension motor. It is well known that the inductance and resistance of each winding play an important role in ensuring the stability of the controller for motor. There have been sufficient research works on equivalent circuit model, where the input and output data are assumed to be true value, and unknown circuit parameters are obtained by solving the equivalent circuit equations of motor. However, the output data are often interfered with noises, such as measurement error, the high-frequency components from pulse width modulation signals and sampling errors. The system response output data are reconstructed through applying in stochastic theory, which is different from the traditional filter techniques, to eliminate effectively the above-mentioned noises. Thus, the parameters can be estimated precisely. As an experimental verification, parameters estimated by the proposed method are applied in calculation of switching point between pulse width modulation mode and pulse amplitude modulation mode for magnetic suspension motor. The experimental results validate the effectiveness of the proposed method.

Description: Winding configurations that are applicable to a novel bearingless flux-switching permanent magnet motor with stacked structure are investigated, and one method to evaluate these winding configurations is also proposed in this paper. First, the motor structure and the influence of coil connections on torque and radial force are illustrated, and the fundamental radial force expressions of two diametrically opposite coils are derived. Afterward, two kinds of torque windings and four kinds of radial force windings for the motor are studied and the radial force models are derived accordingly. Following this, the radial force models are verified and the transient radial force and torque characteristics are investigated by finite element analysis. More importantly, an evaluation factor related to the winding copper losses is defined to evaluate the winding configurations. Finally, an optimal winding configuration is determined theoretically and a prototype motor is built to validate the effectiveness of the motor structure and the optimal winding configuration.

Description: This paper presents a functional observer based technique for estimating gaseous partial pressures in triple phase boundary of a high-order solid oxide fuel cell. Triple phase boundary is a nanoscale region in solid oxide fuel cells where direct measurement of partial pressure of individual gases is not possible. For a reliable and a safe operation those quantities must be monitored. This paper reports a novel functional observer based dynamic state estimation approach that utilizes a system decomposition algorithm to provide a functional observer with minimum order. Therefore, the proposed technique has a simpler structure than conventional state observer based schemes. Case studies of the proposed technique, implemented on a complex nonlinear power system, show accurate and smooth estimations in comparison to full-order state observer based techniques in terms of tracking of nonlinear partial pressures.

Description: Because excitation winding short-circuit faults that occur in steam-turbine generator will lead to heavy financial losses, accurately detecting excitation winding short-circuits (EWS) can help power generation operations. It can allow them to take timely actions and shorten downtime as well as cut the costs of troubleshooting. This paper introduces a method for detecting EWS faults online using a new detection coil. Here, a large-dimension detection coil is installed inside the generator to span over an iron-core segment in the stator. The even or fractional harmonics induced by the detection coil are used to determine whether an EWS fault has happened, and the short circuit position can also be determined. This method can reveal short-circuit faults that occur in one turn of the excitation winding, and with obviously higher sensitivity.

Description: The static exciter is dominating among large grid-connected generators due to the weak dynamic performance of conventional brushless exciters. In this contribution, a six-phase double-star outer pole permanent magnet rotating brushless exciter is evaluated with different active rectification topologies. Both thyristor-based and chopper-based topologies are considered. A high speed response brushless excitation system is obtained by replacing the conventional rotating diode bridge rectifier with the proposed active rectification topologies on the shaft. The given two-stage system generates its own excitation power directly from the shaft, contrary to static exciters. The selection of an appropriate rectification topology could minimize the rotor armature phase currents for a given generator field current. The objective is a high-power factor and a high utilization of the exciter machine. An optimal rectification topology makes higher ceiling currents was possible, improving the transient behavior of the synchronous generator. In this paper, we show that six-phase topologies add complexity, but improve exciter redundancy, increase the available ceiling voltage, and reduce the steady-state torque ripple. Experimental results are given for validating the models implemented for the analysis.

Description: In this paper, a permanent magnet alternator which presently relies on magnetic slot wedges is redesigned to give lower iron loss. The new segmented stator design is used as the basis for a validated loss study—comparing bonded lamination stacks with those held together with a punched notch. Segmentation is shown to improve the production process, reduce losses, and remove the need for wedges, while not significantly altering the acoustic performance. In the first series of prototypes, however, where notching is used to secure lamination packs, the process is shown to have adversely affected material properties of the nickel iron laminations and overall measured losses increased. In the second prototype, with thinner laminations and glued stacks, iron loss predictions and measurement agree and are less than the present slot wedge design.

Description: This paper proposes an advanced dynamic modelling approach of the mutually coupled switched reluctance motor (MCSRM) in the $dq$ reference system that can consider saturation, cross-coupling, and spatial harmonics. Different topologies and their operating principles are investigated and an idealized $dq$ -model considering the inductance harmonics is derived. A dynamic model is built based on flux-current lookup tables (LUTs) obtained from finite element analysis (FEA). A simplified method to inverse the two-dimensional LUTs is proposed. A fast computation approach is used to reduce the number of FEA simulations and calculation time to obtain the LUTs. Motor dynamic performances at different speeds are simulated by using the proposed dynamic model and the results are investigated and verified by FEA. The motor dynamic behavior can be accurately obtained in a short simulation time by using the proposed approach. Experiments are carried out on a 12/8 MCSRM, showing good accuracy of the proposed model.

Description: Fast synchronization of generators and microgrids will be a critical technology in future power systems with high penetration of nondispatchable power resources. Existing synchronization methods rely on generator controls and their performance is limited by the generator characteristics. Speed of microgrid synchronization is further limited by the communication link among generation units. These factors lead to a slow and sometimes faulty synchronization, predominantly because of the phase mismatch during interconnection. This paper frames the generator synchronization problem as a phase-locked loop (PLL) design problem and introduces a voltage-source converter (VSC) based synchronizer for implementing the PLL-based active synchronization method. The VSC-based synchronizer is connected at the point of common coupling and it relies only on local measurements for control. It ensures zero phase error during interconnection by taking the advantage of the fact that the phase is not regulated in the generator controls. Relation between the synchronizer rating, control design, and synchronization speed is developed using describing function analysis of frequency and phase control loop gains. The operation and performance of the VSC-based synchronizer is demonstrated using simulations of a 555 MVA, 24 kV synchronous generator.

Description: In this paper, an extended Kalman filter (EKF) is employed to estimate stator currents, rotor speed, rotor position, and mechanical torque of a direct-driven surface-mounted permanent-magnet synchronous generator (PMSG) based variable-speed wind turbine systems controlled by a robust predictive deadbeat (DB) algorithm. Therefore, the flux-oriented control of the PMSG is realized without mechanical sensors (i.e., no position encoders or speed transducers). The estimated stator currents are fed back to the prediction model in order to compensate the one-step delay caused by the digital controller and to reduce the total harmonic distortion of the stator currents. Thus, the torque ripples are also reduced. Furthermore, a simple disturbance observer is presented, which estimates the perturbations caused by parameter variations of the PMSG or by any unmodeled dynamics in order to enhance the robustness of the proposed DB algorithm. The proposed scheme is experimentally validated using a 14.5-kW PMSG and a dSPACE DS1007 real-time platform. Its performance is compared with that of the conventional DB control strategy for all operation conditions and under parameter variations of the PMSG.

Description: As interturn short fault (ISF) in a permanent-magnet synchronous machine (PMSM) degrades its energy efficiency and may cause other problems, accurate fault diagnosis for the PMSMs is necessary. In this paper, we present a method that uses voltage and current residual components to diagnose the phase and the severity of the ISF in PMSMs. A faulty-winding model (FWM) is introduced to analyze the fault; the model describes a relationship between the voltage decrease of the faulty winding to the characteristics of the fault such as a fault resistance and a fault turn ratio. Based on the model, we suggest a fault index (FI) that shows the severity of the ISF. As the ISF induces residual components to both voltage and current, the proposed method utilized those components with the FWM. A least squares (LS) method is used to estimate the FI and to diagnose the ISF of PMSMs. Experimental results demonstrate that the proposed method diagnose the ISF effectively.

Description: An interior permanent magnet (IPM) machine with ∇ + U shape rotor topology is proposed and its electromagnetic performance with typical V shape and ∇ shape for electric vehicle (EV) applications are compared. The pole-arc to pole-pitch ratios are optimized to maximize the fundamental air-gap flux density and minimize its total harmonic distortion (THD) of all the three IPM machines by the finite element (FE) method. It can be obtained that ∇ + U shape rotor topology has the lowest THD of the air-gap flux density. The back electromotive force and its THD orders, cogging torque, torque density, torque ripple, d - and q -axis inductances, saliency ratio, and iron loss of three machines are analyzed and compared. It is demonstrated that the ∇ shape IPM machines have the highest average torque, and the torque ripples of the V shape and ∇ shape machines are almost the same. The ∇ + U shape IPM machine has the lowest torque ripple and iron loss with only a little reduction of average torque. Finally, the ∇ + U shape IPM machine is prototyped to verify the results of the FE analysis.

Description: The rising use of renewable power generation plants is highlighting the need for an integrated research combining multiple disciplines in order to achieve a commercially competitive technology stage. The demonstrative NEREIDA wave power plant installed in the northern coast of Spain constitutes a good example of this effort. This paper deals with the design, modeling, and control of the oscillating-water-column-based wave energy converter in order to maximize the power output of the NEREIDA power plant. The power optimization relies on two control strategies proposed to avoid the stalling behavior, a characteristic drawback of the Wells turbine, which limits the system's power. The first control strategy is an airflow control using a PID controller tuned by the particle swarm optimization algorithm and its recent memetic variant called fractional particle swarm optimization memetic algorithm. This controller will control a throttle valve to regulate the airflow in the turbine duct. The second one consist of adequately controlling the rotational speed of the generator by means of the rotor-side converter of the back-to-back converter connected to the doubly fed induction generator to provide a swift way to respond to the rapid variations in the turbine speed. The results show that both controls provide a higher power generation compared to the uncontrolled case.

Description: For the purpose of avoiding the drawbacks of large volume, heavy weight, complex wiring, and low reliability caused by the mechanical position sensors in aeronautic power drives, the self-sensing control of permanent magnet synchronous machines (PMSMs) has been getting increasing attention. High-frequency (HF) pulsating current injection is an efficient method to obtain the rotor position at low speeds. However, in this method, the rotor position is extracted from the command voltage instead of the actual measurement due to the lack of voltage sensors, and the control performance of the HF current is nonideal because of the proportional-integral controller (PI) current regulators. Both these shortcomings give rise to position estimation error. To avoid the aforementioned problems, a novel current regulator consisting of the PI and second-order generalized integrator (SOGI) is presented. The different combinations of the PI and SOGI in the d–q axes current regulators are studied. The system stabilities of the proposed and existing HF pulsating current injection-based self-sensing control strategies are comparatively analyzed considering the current control error and some other aspects. The experimental results demonstrate the feasibility of the proposed strategy by a surface-mounted PMSM vector controlled system.

Description: This paper proposes an optimum design for the dc-based DFIG generation system in a dc grid to make its cost optimal. As the stator is connected to a dc bus by the stator side converter, its voltage and frequency are not enslaved to an ac grid, which makes it possible to decrease the synchronous speed, rotor speed, and gearbox ratio. Three aspects are improved. First, control targets are optimized to minimize the increased generator current with decreasing gearbox ratio. Second, optimum design of a generation system is made by regulating gearbox ratio at the cost of increase in the generator current. Significant decreases in cost and losses are analytically obtained from the elaborated cost and losses models, whose parameters are taken directly from the practical and commercial wind generation system. Third, the coordinated model predictive control is adopted in the stator flux-oriented frame with a stator flux and rotor current predictive method. Finally, as the synchronous speed and rotor speed decrease to low levels, simulation and experiment studies are carried out to verify the operational feasibility of DFIG and analyze its steady and transient performance.

Description: Magnetic-geared machines are based on the flux-modulation principle and typically constructed with a flux-modulated double rotor (FMDR). The modulator experiences high torsional stresses when designed for high torque transmission. As a result, the FMDR has high probability of failure if not designed appropriately. In this paper, the FMDR design options are investigated with mechanical and electromagnetic finite element (FE) modeling. The addition of filler material in between the poles of the modulator pole-pieces is found to reduce the shear stresses by $61.7%$ to $87.2%$ at the cost of $2.6%$ to $3.2%$ lower pull-out torque. Possible shear stress reduction with the design of bridged modulators is investigated. Increase in pole number is shown to increase localized stress levels in bridged modulators. Electromagnetic simulations show that for a given FMDR specification, the highest pull-out torque is at an optimal pole number. The tradeoff between FMDR weight, losses, and shear stress are discussed. A scaled-size prototype FMDR with a bridged modulator is constructed. The FMDR is tested and is shown to achieve comparable results with the FE simulations. At field intensity levels above 600 kA/m in the periphery, the FMDR achieves reduced pull-out torque $28.7%$ lower in comparison with three-dimensional simulations. The losses at different load levels of the FMDR are also compared with experimental and FE simulation result and are shown to confirm the FE simulation results.

Description: This paper investigates the effect of the stator winding connection on the performance of a six-phase switched reluctance machine (SRM). Five winding connection types are proposed for the machine. Finite element analyses (FEAs) of flux distribution, output torque, and core losses are presented under single-phase and multiphase excitation for each connection and the results are used to compare the average torque and torque ripple ratio characteristics and to develop understanding of the respective contributions of mutual inductance in the torque development. Experimental tests on a six-phase conventional SRM verify the torque performance and mutual inductance effects of the different winding connections. An optimum winding configuration for a six-phase SRM is proposed.

Description: A comprehensive modular state-space model of a microgrid is developed that handles basic components and loads, as well as inverter-based generation, such as PQ inverter and voltage-source inverter (VSI). Specifically, the developed model is suitable for control design utilizing the nominal set-point values of frequency and voltage of the VSI inverters, and active and reactive power set points of the PQ inverters. The model is also useful for small-signal stability assessment and enhancement. It is shown that when a PQ inverter is added to the system, the low-frequency modes of the power-sharing dynamics drift to new locations and the relative stability is improved. Meanwhile, to preserve the power-sharing stability, a decentralized droop controller of paralleled VSI inverters is designed. Finally, the dynamic model is used to design and implement distributed secondary frequency controllers taking into consideration the characteristics of storage devices. The proposed modeling and control methodologies are demonstrated using an 11 bus microgrid with and without PQ inverters.

Description: For permanent magnet synchronous generators (PMSGs) based dc microgrids working autonomously, load sharing among multiple PMSGs is necessary when the available wind power is more than the load demand. During the load-sharing process, PMSG may prone to instability due to possible over deceleration. To address this issue, this paper presents a new distributed control scheme, which can achieve stable and optimal load sharing among multiple PMSGs in a dc microgrid. This scheme has a two-layer structure. Based on the distributed model predictive control approach, the upper layer controllers coordinate the operation of parallel-connected grid-side converters, thus providing power references for each PMSG. The computed power references are sent to the lower layer PMSG controllers for execution. The salient feature of this scheme is that by incorporating the rotor kinetic energy and the generation margin into the controller design, it not only achieves the optimal load sharing, but also ensures stable operation of all PMSGs. Simulation results verify the effectiveness of the proposed scheme.

Description: This paper proposes an improved direct power control (DPC) strategy for the rotor-side converter (RSC) and grid-side converter (GSC) of the doubly fed induction generator under unbalanced and distorted voltage conditions. The proposed scheme is implemented in a synchronous reference frame with the fixed angular speed, so that the phase-locked loop can be removed and relevant potential instability issues can be avoided. Besides, the sequential separations and complex calculations of the power compensating items can be eliminated with the direct-resonant method based on a dual-frequency frequency adaptive vector PI. Thus, the negative and harmonic components can be directly eliminated, achieving balanced and sinusoidal current injected into the power grid. Meanwhile, the control strategy can achieve self-adaptive to the grid frequency variation in the practical situation with the proposed frequency estimation scheme. Finally, the experimental results validate the availability of the proposed DPC strategy.

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Description: Line-commutated rectifiers are often utilized in machine-converter systems and many energy conversion applications. Simulation of such power systems using detailed switching models of rectifiers is computationally expensive, and as an alternative for system-level studies, the so-called average-value modeling (AVM) techniques have become indispensable. The parametric AVM (PAVM) uses a computerized approach for establishing the key relationships between the averaged ac and dc variables. In this paper, a generalized PAVM (GPAVM) is proposed, which extends several previously proposed models. The new GPAVM includes the ac harmonics in thyristor-controlled rectifier models considering their nonlinear dependency on the line frequency. The new model is verified using detailed simulations and experimental results and is demonstrated to have better accuracy in a wider range of operating conditions and speeds/frequencies.

Description: Fractional-slot concentrated windings (FSCW) are becoming more and more popular in the design of permanent magnet electric machines. A well-known drawback of their adoption is the occurrence of large magneto-motive force (MMF) harmonics, which produce eddy-current losses in rotor permanent magnets. The use of a multi-layer design, with coils of different phases wound around the same tooth, is a possible countermeasure to mitigate the problem. In this paper, a new general systematic methodology is proposed to optimize the multilayer FSCW design in the form of a multi-objective quadratic programming problem. The maximization of the MMF fundamental and the minimization of total rotor losses are taken as properly weighed objective functions. Constraints are imposed to guarantee the physical feasibility and the electric symmetry of the winding. An application example to a 9-slot 8-pole machine is presented, together with extensive validations by comparison with finite element analysis (FEA) simulations, to prove the effectiveness of the proposed technique.

Description: This paper presents a novel rotor structure that enables a single-phase flux switching motor to eliminate the dead zone of torque. The basic structure and operating principle of the rotor are introduced. A three-dimensional finite-element model was built to analyze the characteristics of static torque-rotor positions. In consideration of the characteristics, a supporting winding control strategy was proposed. The validity of the novel and control strategy was proven through co-simulation analysis. A prototype was designed and manufactured on the basis of the results of finite-element analysis. A series of experiments was performed. Experimental results agree with the results of co-simulation, which verifies the effectiveness and practicability of the novel rotor.

Description: A five-phase dual-rotor permanent-magnet synchronous motor (PMSM) used for electric vehicles is investigated in this paper. The dual-rotor PMSM consists of an inner motor and an outer motor, which share a common stator yoke. The magnetic coupling exists not only within the interphases, but also between the inner and outer motors, which leads to control complexity and affects performance and fault-tolerant capability of the motor. Thus, the electromagnetic characteristics, particularly the magnetic-coupling characteristics of the dual-rotor PMSM, are investigated based on magnetic circuit and finite-element methods. The influences of magnetic coupling caused by the structural arrangement (pole/slot combination, magnetization type of PMs, armature reaction, common stator yoke thickness, and winding structure) are analyzed in this paper. Then, a magnetic decoupling design is introduced for the five-phase dual-rotor PMSM. A prototype was manufactured and tested to verify the validity and accuracy of the process presented in this paper.

Description: A broad, long-term research project is described, which will lead to the computer becoming an equal member of the system-development team, continuously making proactive contributions, akin to those expected from an experienced and knowledgeable customer or user, a conscientious QA engineer, a strict regulatory auditor, an engineering -team leader, or the organization’s CTO. The web extra at https://youtu.be/mmrv8ZACbpU describes the authors’ novel “wise computing” approach and demonstrates one possible application. The second web extra at https://youtu.be/DtpvMxMwYPM extends the discussion of the authors’ novel “wise computing” approach and presents a case study.

Description: A clear and comprehensive policy framework is needed to make data sharing more useful, robust, and efficient. Based on decades of experience as a CIO in the healthcare sector, the author recommends ways to address various existing privacy, reliability, interoperability, security, and trust challenges.

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Description: As systems increase in complexity, so too must the design principles used by software engineers. Also in this issue: The New Killer App for Security: Software Inventory (p. 60) and A Future with Quantum Machine Learning (p. 68).

Description: FreeBSD (an open source Unix-like OS) and Apple’s Mac OS use similar BSD functionality but take different approaches. FreeBSD implements a traditional compact monolithic Unix kernel, whereas Mac OS builds the BSD Unix functionality on top of the Mach microkernel. The authors provide an in-depth technical investigation of both approaches.

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Description: Assisted by centralized data collection and analytics, IoT-based mechanisms can substantially reduce food waste, improve transportation and distribution efficiency, and support quick removal of contaminated or spoiled products from the fresh food supply chain.

Description: Could combining quantum computing and machine learning with Moore’s law produce a true “rebooted computer”? This article posits that a three-technology hybrid-computing approach might yield sufficiently improved answers to a broad class of problems such that energy efficiency will no longer be the dominant concern.

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Description: Information of interest to Computer Society members. This article includes two errata. One erratum for “The New Science Wars” (H. Berghel, vol. 50, no. 11, 2017, pp. 72-76; doi: 10.1109/MC.2017.4041367). The second erratum is for the Table of Contents for the January 2018 issue (vol. 51, no. 1; doi: 10.1109/MC.2018.1151011).

Description: The accurate and fast calculation of circulating currents between strands for different transposition types is the important premise of the engineering design for stator bars in large turbo-generators. The air gap magnetic field partly enters into the stator slot, also generating circulating currents. However, there is a lack of fast simulation method for calculating the air gap magnetic field entering the stator slot in the existing calculation method for circulating currents. In this paper, the analytical algorithm of calculating circulating currents considering the air gap magnetic field entering stator slots is proposed, in which the equivalent circuit model of transposed strands in stator bars is established based on the superposition principle, the analytical algorithm of calculating the induced electromotive force is proposed based on the discrete integral method, and the analytical algorithm of calculating the air gap magnetic field entering stator slots is proposed based on the conformal mapping method. Then, a 1400-MW turbo-generator is taken as an example, circulating currents between transposed strands in stator bars under no-load and rated condition are calculated by the proposed analytical algorithm. Finally, the proposed analytical algorithm is validated by the two-dimensional finite element method.

Description: The definition of standard dynamic wind turbine models for power system stability analysis has been under development in recent years. These standard models, also known as generic or simplified models, have been developed under the framework of a new international standard, IEC 61400-27-1, the first edition of which was published in February 2015. These models are simplified representations of complex systems, such as wind turbines. Hence, the wind power industry urgently needs validation in comparison with real measurements to verify the accuracy and corresponding usability of the models. This paper presents the validation of a generic doubly-fed induction machine wind turbine model, commonly known as Type 3, based on a measurement campaign carried out in a Spanish wind farm. The wind turbine has been subjected to six voltage dips with different characteristics, magnitude, and duration, as well as several wind turbine operating conditions. The real behavior of the wind turbine is compared with the simulated response provided by the corresponding generic dynamic wind turbine model produced by the manufacturer Gamesa . Furthermore, the validation guidelines recently issued by IEC 61400-27-1 have been implemented, and the most important validation errors have been analyzed.

Description: This paper presents a fault-tolerant sensorless control strategy based on a wide-speed strong-robustness sliding mode observer (SMO) for a new five-phase fault-tolerant fractional-slot concentrated-winding interior-permanent-magnet (FTFSCW-IPM) motor, in which the motor drive system has a high reliability. The designed motor possesses a high fault-tolerant capacity. Also, the fault-tolerant space vector pulse width modulation (SVPWM) strategy and the sensorless control are adopted to further improve the fault tolerance of the control system. Moreover, unlike the existing SMO, the proposed SMO algorithm can achieve enhanced low system chattering, estimated accuracy at low speed, as well as strong robustness to motor parameters, fault and load disturbance. Numerical simulations and experiments with a 2-kW five-phase FTFSCW-IPM motor are carried out. The results verify the feasibility and effectiveness of the proposed fault-tolerant sensorless method adopted by the FTFSCW-IPM motor.

Description: This paper proposes a new approach for separation of saliency signals of the induction machine at very low speed and heavy loads. The change of current responses due to the transient voltage pulse excitation is employed to obtain the inherent impedance saliency information. The proposed separation approach uses the discrete wavelet transform (DWT) in combination with Taylor's expansion and least square error (LSE) methods. The DWT removes the higher and lower harmonic orders to extract the operating frequency band based on the reference speed. Then, Taylor's expansion modeling processed by LSE algorithm is applied to extract the slotting saliency signal from the nearby frequency of the inter-modulation signal. The extracted slotting saliency signal is employed to estimate the rotor/flux position using the phase locked loop observer and current model. Experimental results are provided to prove the effectiveness of the proposed new separation approach at very low speed including zero electrical speed operations. It is found that the proposed new approach is accurate at very low speed and heavy load operations with less noise compared to previous methods.

Description: Electric drive trains have a torsional rigid-body vibration mode at a small, nonzero frequency. If an excitation occurs close to this frequency, the vibration amplitude may grow large and the electrical machine may suffer from significant additional losses. Standards set constraints on the oscillating torque in the shaft coupling and on the harmonics of line current. They indirectly limit the vibration amplitude and losses of the machines. Time-discretized finite-element analysis was used to study the losses of six induction and six synchronous machines under torsional vibration restricted by the constraints above. All the machines were supplied from sinusoidal voltage sources. In the worst cases of the induction motors, the vibration increased the electromagnetic total loss by about 20%. The constraints on synchronous machines are milder than those for induction machines. In this case, the maximum increase of the loss was 75%. The limit on the harmonic currents is essential from the loss point of view. Without this limit, the additional loss at the rigid-body resonance would lead to a temperature rise high enough to destroy the insulation system of the machine. The method of loss analysis was validated by measured results.

Description: This paper presents an analytical approach for the prediction of no-load stator iron losses in spoke-type permanent-magnet synchronous machines. The influence of bridge saturation is considered. According to the permeability distribution, the bridge is equivalent to fan-shaped saturation region with constant permeability. The magnetic equivalent circuit and iterative method are used to calculate the permeability. The motor is divided into six regions, i.e., shaft, inner saturation region, magnet, outer saturation region, air-gap, and slot. The air-gap magnetic field distribution is first calculated on the basis of Poisson's and Laplace's equations. Then, according to the flux paths, the stator core is divided into tooth and yoke. The magnetic densities of the tooth and yoke are obtained on the basis of the continuity of magnetic flux. Finally, the Bertotti model is employed to calculate the stator iron losses. The validity of the proposed method is verified by the finite element method.

Description: This paper presents oxygen excess ratio (OER) control methods for proton exchange membrane fuel cell systems based on high-order sliding mode (HOSM) observer. The nonlinear and strong coupling characteristics make it difficult for the nonlinear uncertain systems to realize OER closed-loop feedback control. An improved HOSM observer is developed to observe the unmeasurable system states. Even under the interference of model parametric uncertainties, external disturbances, and measurement noises, the HOSM observer can still show fast convergence and good robustness. With the output error injection feature of improved HOSM algorithm, OER can be reconstructed in finite time. The proposed two stage sliding mode based on super twisting algorithm plus feedforward (TS_SM_STW+FF) controller and TS_SM_STW plus fuzzy feedforward (TS_SM_STW+Fuzzy_FF) controller are applied to regulate the estimated OER to track optimal value, respectively. The dynamic performance of OER with proposed methods is validated through the RT-LAB hardware-in-loop platform. The experimental results have shown the feasibility and effectiveness of proposed controllers as compared with conventional strategies.

Description: An increasing interest in both efficient electric machines and more extensive control strategies demands evermore faster simulation tools. In that context, Fourier-based models, which combine low computational times with a high accuracy, have already proven their value. However, even Fourier-based models may encounter problems related to CPU usage. To cope with these problems the authors present a number of simple techniques to reduce the computational effort of Fourier-based models for synchronous machines. The techniques are based on simplifying the studied geometry and a qualitative analysis of the machine's time- and spatial-harmonic content. The proposed techniques are validated and a benchmark test has shown that a great reduction in computational burden can be achieved without significant loss of accuracy. The preliminary harmonic analysis gives, by far, the largest reduction of the computational burden.

Description: The vibroacoustic characterization of an electric powertrain which consists of a permanent magnet synchronous motor and a gearbox for electric vehicles is analyzed in this paper. First, the vibroacoustic origins of the electric powertrain are investigated. The influence of current harmonics on electromagnetic force harmonics is presented. The spatial distribution of electromagnetic force exerting on stator inner surfaces is investigated by a two-dimensional magnetic transient solver. The spectrum of gear meshing force is then presented. After that, the vibration response of the electric powertrain system is predicted by a mode superposition method, based on which the acoustic noise of the electric powertrain is obtained using a direct boundary element method. Furthermore, the influence of electromagnetic force harmonics and gearbox on the vibroacoustic behavior of the electric powertrain is analyzed. To verify the calculating results, a vibroacoustic test is conducted in a semi-anechoic room and the results show that both electromagnetic forces and gear meshing forces contribute to a considerable proportion of the vibroacoustic behavior due to the integrated structure.

Description: This paper focuses on core loss reduction of a consequent-pole bearingless disk motor using a C-shaped stator core made of soft magnetic composites (SMC). The core loss of the previously built test machine was significantly high, because the C-core has a three-dimensional flux path; therefore, conventional laminated steel cannot be used. In this paper, the solid iron core is replaced by the SMC, and its effectiveness is experimentally demonstrated. The core loss with solid iron, when the bearingless motor is driven at 3000 r/min under no load condition, is 3.8 W, but that with the SMC core is reduced to only 0.6 W. The maximum efficiency with solid iron is 27%, but that with the SMC core is improved to 61%. It is experimentally verified that the SMC core is effective to reduce the core loss and improve the efficiency of the bearingless disk motor with C-shaped stator cores.

Description: The 12-phase synchronous generator–rectifier system is widely used in mobile platforms that require high system security. In the system, the stator internal short circuit is a kind of common fault that may compromise the security of the generator. This paper gives detailed analysis on the internal phase-to-phase short-circuit faults, including the faults within the same group of Y-connected phases and those between different groups of Y-connected phases in the 12-phase system. A multiloop mathematical model is built to calculate all the voltages and currents when the faults occur in the system. In the model, the additional short-circuit loop between different phases, as well as the fault effects on both the air-gap magnetic field and the circuit topology of the system, are taken into account. Then a series of fault experiments are carried out on a 12-phase model machine. In view of the good agreement of experiment and simulation results, the effectiveness of the mathematical model and the simulation method is confirmed. Finally, the electric characteristics of the internal phase-to-phase short-circuit faults are summarized, and the fault mechanism is revealed.

Description: Multiphase permanent magnet-assisted synchronous reluctance motor (PMa-SynRM) is proposed as one of the optimal machine designs for vehicular applications such as electric vehicles due to their fault tolerant operation capability. However, optimization of the multiphase PMa-SynRMs for in-wheel applications in EVs and aircrafts require more research efforts to increase their power density with the size constraint. In addition, optimization of the machine design with multiphysics structural and thermal analysis has not been intensively discussed in the literature. In this paper, an optimal design procedure which includes multiphysics analysis to design the multiphase external rotor PMa-SynRMs is presented. In specific, a five-phase external rotor PMa-SynRM with neodymium-based magnets has been proposed as a solution to produce higher power density compared to the conventional internal rotor PMa-SynRMs. Multiphysics analysis is included in the optimization procedure to determine the effects due to rotational forces and heat flow on the proposed optimal five-phase external rotor PMa-SynRM design. Detailed electromagnetic finite element simulations are carried out to depict the performance characteristics. A 3.8-kW prototype is fabricated and the experimental results are compared with same volume 3-kW five-phase internal rotor PMa-SynRM.

Description: Brushless machines with interior permanent magnets (PMs) are mostly made with PM geometries inconsistent with the polar coordinates of the rotor cross section. Therefore, without any subdomain transformation, the analytical approaches cannot precisely model the interior PM machines in a single coordinate. This paper presents an accurate two-dimensional (2-D) analytical model for electromagnetic analysis of slotted brushless machines with rectangular spokePMs assisted by hub magnets. The slotting effect is modeled via subdomain technique. However, the spoke PM is not consistent enough with polar coordinates to be approximated by one subdomain. In order to map the spoke PMs in polar coordinates, herein, the rectangular geometry is approximated by a combination of arc shaped subdomains. On this basis, the 2-D analytic model is developed for M-fraction spoke PM. To verify the accuracy of the proposed 2-D approach, analytic results are compared with numeric results of finite elements method. It is revealed that by dividing the spoke PM into arc subdomains, the 2-D model achieves higher accuracy in calculation of electromagnetic quantities such as PM and armature reaction fields, electromagnetic torque, and back electromotive force.

Description: The combined star-delta winding has been attracting the attention of industry because of its potential to improve the efficiency of electrical machines. In order to show the efficiency benefits of this type of winding, many authors have used methods to determine the improvements in winding factors, published in different papers, which have not been validated yet. Some of these methods contradict each other. Furthermore, as a demerit of this winding, a unidirectional rotation has been reported. In this paper, a set of new formulas to calculate the winding factors for combined star-delta windings is proposed. In order to check the validity of the proposed formulas, a 2-D FEM and experimental tests on a salient pole synchronous machine are presented, which allows different combined star-delta winding connections. In contrast to other publications, the measurements show that combined windings can be applied for both directions of rotation without any problems.

Description: This paper proposes a new numerical formula and a design method to reduce the torque ripple while improving efficiency and the control performance of an interior permanent magnet synchronous motor. In previous studies, the inverse cosine function (ICF) has been used for torque ripple reduction by making the air gap flux density distribution sinusoidal under a no-load condition. However, in this paper, the advanced inverse cosine function (AICF) based on the ICF is proposed. It determines an asymmetric rotor shape for rendering the air gap flux density distribution sinusoidal, considering a certain load condition. In addition to the torque ripple reduction, lower peak values, the total harmonic distortion (THD) of the induced voltage, and a lower iron loss can be achieved by applying the AICF, compared to the other conventional methods. The lower peak value and THD of the induced voltage are important because they affect the control performance of the motor. The lower iron loss can also lead to a higher efficiency, particularly, in the high-speed region. To verify the validity of the proposed design method, the characteristics of 8-pole, 12-slot motors that have different rotor shapes are analyzed using finite element analysis and experiments.

Description: The low cost of diesel generators have led to their increased presence in hybrid microgrids. The presence of low-inertia devices in such microgrids makes the stability of hybrid microgrids a pressing concern. This paper analyzes the impact of equal and unequal active power sharing conditions on the dynamic stability of a hybrid microgrid. The sensitivity of low-frequency eigenvalues to changes in active power droop gains is studied and a “domain of stability” region is proposed to assess the stability of hybrid microgrids under different power sharing conditions. The different operating regions within the domain of stability are defined. Based on the sensitivity analysis and the domain of stability, the optimal droop gain for the diesel generator to improve the stability of the microgrid is determined. Time-domain simulations are carried out in the MATLAB/Simulink software environment and are used to validate the small-signal stability analysis results.

Description: Switched reluctance motor (SRM) drives conventionally use current control techniques at low speed and voltage control techniques at high speed. However, these conventional methods usually fail to restrain the torque ripple, which is normally associated with this type of machine. Compared with conventional three-phase SRMs, higher phase SRMs have the advantage of lower torque ripple: To further reduce their torque ripple, this paper presents a control method for torque ripple reduction in six-phase SRM drives. A constant instantaneous torque is obtained by regulating the rotational speed of the stator flux linkage. This torque control method is subsequently developed for a conventional converter and a proposed novel converter with fewer switching devices. Moreover, modeling and simulation of this six-phase SRM drive system has been conducted in detail and validated experimentally using a 4.0-kW six-phase SRM drive system. Test results demonstrate that the proposed torque control method has outstanding performance of restraining the torque ripple with both converters for the six-phase SRM, showing superior performance to the conventional control techniques.

Description: Doubly fed induction machine (DFIM) is increasingly being preferred in sites where there is large variation in water head (e.g., Tehri hydropower complex in India having water head variation from 130 to 230 m), as it offers high dynamic stability and better energy efficiency at part loads. Smooth starting, unity power factor, and better rotor current profile are the reasons; back-to-back voltage source converters are used in rotor side of the machine. Speed and power factor of DFIM are controlled through power converters in accordance with surplus power available in grid and water head of dam. This paper presents dynamic behavior of a 250 MW DFIM hydro-generating unit, to be commissioned in 1000 MW Tehri pumped storage plant, operating in pumping mode. Further, the performance of pumping operation of DFIM is assessed under power converter and control circuit failures. Persevere ability of the machine is analyzed based on performance measures: Speed, current, reactive power consumption. From the analysis, energy storage loss of about 66.61 MU/year is estimated in a 1000 MW plant due to the inadequate power and control redundancy. A 2.2 kW DFIM is experimented at laboratory level to support the simulation.

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Description: This paper presents as a new contribution a proposal to optimize the performance of a switched reluctance generator (SRG) in a variable wind energy conversion system. An approach based on the design of computational experiment is applied to determinate the optimal firing angles and the optimal dc link voltage that guarantee the best system behavior for each rotor speed. A third-order response surface model based on space-filling designs is applied to build a multiobjective function considering efficiency improvement and torque ripple reduction. An interior point method is applied to find the minimum of the surface, optimizing the process. Direct power control is used to obtain the maximum power as a function of the rotor speed, employing the optimal parameters. Hysteresis and single-pulse current control are applied for low- and high-speed operation, respectively. Simulation and experimental results have shown that the proposed approach returned a good compromise between SRG high efficiency and low torque ripple. Moreover, the presented technique reduces computational effort and provides a clear massive data simulation framework.

Description: Prospective authors are requested to submit new, unpublished manuscripts for inclusion in the upcoming event described in this call for papers.

Description: Software and systems engineering combine artifacts from diverse domains and tools. This article explores how incremental consistency checking is able to automatically and continuously detect inconsistencies among these artifacts.

Description: Presents a listing of the editorial board, board of governors, current staff, committee members, and/or society editors for this issue of the publication.

Description: This theme issue includes three articles that explore various aspects of engineering complex systems that involve software. As these systems increase in complexity, so too must the design principles employed.

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