Showing papers in "IEEE Transactions on Energy Conversion in 2016"

Internal Model Control for a Bearingless Permanent Magnet Synchronous Motor Based on Inverse System Method

Abstract: To effectively enhance the control accuracy and dynamic performance of a bearingless permanent magnet synchronous motor (BPMSM), this paper presents a novel control scheme combining the inverse system method and the internal model control. By cascading the inverse model of the BPMSM with the original BPMSM system, a decoupling pseudo-linear system is constituted. Moreover, in order to improve the robustness of the whole system and reject the influence of the unmodeled dynamics and system noise to the decoupling control accuracy, the internal model control scheme is employed for the pseudo-linear system to design extra closed-loop controllers. Consequently, the proposed decoupling control scheme incorporates the advantages of both the inverse system method and the internal model control. The effectiveness of the proposed control scheme is verified by experimental results at various operations.

Coordinated Microgrid Frequency Regulation Based on DFIG Variable Coefficient Using Virtual Inertia and Primary Frequency Control

Abstract: This paper proposes a variable coefficient combined virtual inertia and primary frequency control strategy for doubly fed induction generators (DFIG) in coordination with diesel generator to participate in wind/photovoltaic/diesel microgrid frequency regulation. The frequency response characteristic is analyzed under different wind speeds with corresponding inertia control parameters different. A 10% wind power margin is preserved through overspeed control and pitch angle control to offset the decrease of wind output power after temporary extra power surge, and provide a permanent frequency support for microgrids. The influence of droop control gain setting is also illustrated under different wind velocities. By continuously adjusting the control parameters according to wind speed variation, a variable coefficient method is realized. The method can guarantee an efficient implementation of this strategy in time-varying conditions. Finally, this coordinated control strategy is tested in a storage-independent microgrid with solar, wind, and diesel generators.

An Enhanced Power Sharing Scheme for Voltage Unbalance and Harmonics Compensation in an Islanded AC Microgrid

Abstract: In this paper, an enhanced hierarchical control structure with multiple current loop damping schemes for voltage unbalance and harmonics compensation (UHC) in ac islanded microgrid is proposed to address unequal power sharing problems. The distributed generation (DG) is properly controlled to autonomously compensate voltage unbalance and harmonics while sharing the compensation effort for the real power, reactive power, and unbalance and harmonic powers. The proposed control system of the microgrid mainly consists of the positive sequence real and reactive power droop controllers, voltage and current controllers, the selective virtual impedance loop, the unbalance and harmonics compensators, the secondary control for voltage amplitude and frequency restoration, and the auxiliary control to achieve a high-voltage quality at the point of common coupling. By using the proposed unbalance and harmonics compensation, the auxiliary control, and the virtual positive/negative-sequence impedance loops at fundamental frequency, and the virtual variable harmonic impedance loop at harmonic frequencies, an accurate power sharing is achieved. Moreover, the low bandwidth communication (LBC) technique is adopted to send the compensation command of the secondary control and auxiliary control from the microgrid control center to the local controllers of DG unit. Finally, the hardware-in-the-loop results using dSPACE 1006 platform are presented to demonstrate the effectiveness of the proposed approach.

Distributed Smart Decision-Making for a Multimicrogrid System Based on a Hierarchical Interactive Architecture

Abstract: In this paper, a comprehensive real-time interactive energy management system (EMS) framework for the utility and multiple electrically coupled MGs is proposed. A hierarchical bi-level control scheme (BLCS) with primary and secondary level controllers is applied in this regard. The proposed hierarchical architecture consists of sub-components of load demand prediction, renewable generation resource integration, electrical power-load balancing, and responsive load demand. In the primary level, EMSs are operating separately for each microgrid (MG) by considering the problem constraints, power set-points of generation resources, and possible shortage or surplus of power generation in the MGs. In the proposed framework, minimum information exchange is required among MGs and the distribution system operator. It is a highly desirable feature in future distributed EMS. Various parameters such as load demand and renewable power generation are treated as uncertainties in the proposed structure. In order to handle the uncertainties, Taguchi $^\prime$ s orthogonal array testing approach is utilized. Then, the shortage or surplus of the MGs power should be submitted to a central EMS in the secondary level. In order to validate the proposed control structure, a test system is simulated and optimized based on multiperiod imperialist competition algorithm. The obtained results clearly show that the proposed BLCS is effective in achieving optimal dispatch of generation resources in systems with multiple MGs.

Synthesis of Flux Switching Permanent Magnet Machines

Abstract: This paper investigates the nature of a flux switching permanent magnet (FSPM) machine based on the flux harmonic theory. First, analytical expressions for air-gap flux density distribution, back-electromagnetic force (EMF), and torque are developed to analyze the electromagnetic features of FSPM machines. It is found that the conventional winding theory, such as star of slots, can be directly employed to analyze FSPM machines without any modification. Combinations of stator and rotor teeth, winding connection, and conditions for symmetrical-phase back-EMF waveform are analyzed and obtained using star of slots. The armature winding pole pairs of FSPM machines are redefined. Based on this new concept and the proposed analytical expressions, slots per phase per pole and winding factor in FSPM machines are derived, a new type winding configuration with overlapping windings for FSPM machines is predicted and verified to be with ∼90% larger back EMF than those of conventional FSPM machines with nonoverlapping windings. All these analysis results are validated by finite element analyzes.

Power Oscillations Damping in DC Microgrids

Abstract: This paper proposes a new control strategy for damping of power oscillations in a multisource dc microgrid. A parallel combination of a fuel cell (FC), a photovoltaic system, and a supercapacitor (SC) is used as a hybrid power conversion system (HPCS). The SC compensates for the slow transient response of the FC stack. The HPCS controller comprises a multiloop voltage controller and a virtual impedance loop for power management. The virtual impedance loop uses a dynamic droop gain to actively damp the low-frequency oscillations of the power sharing control unit. The gain of the virtual impedance loop is determined using a small-signal analysis and the pole placement method. The Mesh analysis is employed to further study the stability of low-frequency modes of the overall dc microgrid. Moreover, based on the guardian map theorem, a robust stability analysis is carried out to determine a robustness margin for the closed-loop system. The main advantage of the proposed method is its robustness against uncertainties imposed by microgrid parameters. This feature provides DG units with plug-and-play capability without needing the exact values of microgrid parameters. The performance of the proposed control scheme is verified using hardware-in-the-loop simulations carried out in OPAL-RT technologies.

Improved Vector Control of Brushless Doubly Fed Induction Generator under Unbalanced Grid Conditions for Offshore Wind Power Generation

Abstract: Brushless doubly fed induction generators (BDFIGs) are promising alternatives to doubly fed induction generators due to high reliability and low maintenance cost for the absence of brushes and slip rings. As a typical application in offshore wind energy generation, a feedforward method is applied to the grid-voltage oriented vector control to make the active and reactive power decoupled. Another challenge in practice is how to meet the demanding requirements of grid codes. The dynamic behavior of BDFIG is analyzed in detail under unbalanced grid conditions with the proposal of four different targets. To deal with these issues, an improved vector control strategy based on the proportional-integral resonant controller (PI + R) in a single synchronous reference frame is developed. The stability and robustness of the proposed control scheme under parameters uncertainties and variations are discussed as well. The advantage of this control strategy is that it is simple and fast in transient response. The effectiveness of the proposed control strategy is validated by the experiment results of a 3-kW prototype machine.

On the Accuracy of Fault Detection and Separation in Permanent Magnet Synchronous Machines Using MCSA/MVSA and LDA

Abstract: In this paper, the motor current/voltage signature analysis and linear discriminant analysis (LDA) are evaluated with respect to the accuracy to detect the status of permanent magnet synchronous machines (PMSMs) whether it is healthy or faulted, determine the type of that fault, and estimate the severity in the case of static eccentricity or turn-to-turn short-circuit fault. Three types of faults are discussed: static eccentricity, turn-to-turn short circuit, and partial demagnetization fault. Two-dimensional finite element analysis (FEA) is used to model and simulate the machine under healthy and faulted conditions. Fast Fourier transform is applied to the phase voltage or current signals to obtain the frequency spectrum. A combination of the amplitude of the harmonics of the stator voltage or current signals are used as detailed features for the classifier for fault detection. LDA is chosen as a classification method for both detecting the fault and estimating its severity. Two different winding types of PMSMs are tested: a concentrated and a distributed winding machine. To validate the simulation results, experiments at different operational points are carried out and the results are compared with the sFEA.

Small Signal Instability of PLL-Synchronized Type-4 Wind Turbines Connected to High-Impedance AC Grid During LVRT

Abstract: Instability phenomenon of type-4 wind turbine during grid faults is increasingly concerned by researchers and may result in failure of low-voltage ride through (LVRT), especially when connected to a high-impedance ac grid In order to carry out the stability analysis for type-4 wind turbines during deep voltage sag, dynamic model, which reflects the interaction between phase locked loop (PLL) and alternating current control (ACC), is first developed Eigenvalues and modal analysis show that there is a risk that a pair of poorly damped poles may become unstable, which is mainly dominated by PLL as well as the interaction between PLL and ACC The mechanism of small-signal instability during LVRT is identified as the insufficiency of damping Complex torque coefficient approach is further proposed to study the characteristic of the damping component, which is composed of the inherent one determined by PLL and the additional influenced by ACC Finally, simulated results are presented to verify the analytical results

Global Identification of a Low-Order Lumped-Parameter Thermal Network for Permanent Magnet Synchronous Motors

Abstract: Monitoring critical temperatures in permanent magnet synchronous motors (PMSM) is essential to prevent device failures or excessive motor life-time reduction due to thermal stress. A lumped-parameter thermal network (LPTN) consisting of four nodes is designed to model the most important motor parts, i.e., the stator yoke, stator winding, stator teeth, and the permanent magnets. An empirical approach based on the comprehensive experimental training data and a particle swarm optimization are used to identify the LPTN parameters of a $\mbox{60}$ -kW automotive traction PMSM. Varying parameters and physically motivated constraints are taken into account to extend the model scope beyond the training data domain. Here, a so-called global identification technique for linear parameter-varying systems is innovatively applied to a thermal motor model for the first time. The model accuracy is cross-validated with independent load profiles, and a maximum estimation error (worst-case) of $\mbox{8}\,^\circ$ C regarding all considered motor temperatures is achieved. Also, a comprehensive residual statistical analysis proves suitable estimation results in terms of model robustness and accuracy.

Stable Short-Term Frequency Support Using Adaptive Gains for a DFIG-Based Wind Power Plant

Abstract: For the fixed-gain inertial control of wind power plants (WPPs), a large gain setting provides a large contribution to supporting system frequency control, but it may cause over-deceleration for a wind turbine generator that has a small amount of kinetic energy (KE). Further, if the wind speed decreases during inertial control, even a small gain may cause over-deceleration. This paper proposes a stable inertial control scheme using adaptive gains for a doubly fed induction generator (DFIG)-based WPP. The scheme aims to improve the frequency nadir (FN) while ensuring stable operation of all DFIGs, particularly when the wind speed decreases during inertial control. In this scheme, adaptive gains are set to be proportional to the KE stored in DFIGs, which is spatially and temporally dependent. To improve the FN, upon detecting an event, large gains are set to be proportional to the KE of DFIGs; to ensure stable operation, the gains decrease with the declining KE. The simulation results demonstrate that the scheme improves the FN while ensuring stable operation of all DFIGs in various wind and system conditions. Further, it prevents over-deceleration even when the wind speed decreases during inertial control.

Improved Rotor Flux Estimation at Low Speeds for Torque MRAS-Based Sensorless Induction Motor Drives

Abstract: In this paper, an improved rotor flux estimation method for the Torque model reference adaptive schemes (TMRAS) sensorless induction machine drive is proposed to enhance its performance in low and zero speed conditions. The conventional TMRAS scheme uses an open loop flux estimator and a feedforward term, with basic low pass filters replacing the pure integrators. However, the performance of this estimation technique has drawbacks at very low speeds with incorrect flux estimation significantly affecting this inherently sensorless scheme. The performance of the proposed scheme is verified by both simulated and experimental testing for an indirect vector controlled 7.5-kW induction machine. Results show the effectiveness of the proposed estimator in the low- and zero-speed regions with improved robustness against motor parameter variation compared to the conventional method.

Discernment of Broken Magnet and Static Eccentricity Faults in Permanent Magnet Synchronous Motors

Abstract: This paper deals with the discernment of broken magnet and static eccentricity faults in permanent magnet synchronous motors through the stator phase current. Broken magnet and static eccentricity faults exhibit very similar fault patterns in back-electromotive force (emf) and flux spectrums. Therefore, it is essential to separate these faults from each other for a true diagnosis. In this study, stator emf and phase current waveforms are analyzed in detail to identify the discerning components and characterize their dynamic behaviors. Two-dimensional time-stepping finite-element simulations and experimental results show that the fault classification process can be implemented by using fault-dependent in-phase current fault signatures.

Assessment and Enhancement of a Full-Scale PMSG-Based Wind Power Generator Performance Under Faults

Abstract: A full-scale permanent-magnet synchronous generator (PMSG)-based wind turbine with dc-link voltage control via the machine-side converter has the potential to provide inherent low-voltage ride-through (LVRT) performance without additional hardware components. However, several important performance aspects related to this topology are not addressed in this literature. This paper investigates the impacts of the LVRT control on the stability and risk of resonance, successful operation, and fatigue in a full-scale PMSG-based wind power generation system. An analytical model, considering the double-mass nature of the turbine/generator and typical LVRT requirements, is developed, validated, and used to characterize the dynamic performance of the wind generation system under LVRT control and practical generator characteristics. To enhance the operation and reduce the fatigue under LVRT control, two solutions, based on active damping control and dc-link voltage bandwidth retuning, are proposed, analyzed, and compared. The detailed nonlinear time-domain simulation results validate the accuracy of the developed model and analytical results.

Nonlinear Performance Degradation Prediction of Proton Exchange Membrane Fuel Cells Using Relevance Vector Machine

Abstract: Environmental issues, especially global warming due to the greenhouse effect, have become more and more critical in recent decades. As one potential candidate among different alternative “green energy” solutions for sustainable development, the proton exchange membrane fuel cell (PEMFC) has received extensive research attention for many years for energy and transportation applications. However, the relatively short lifespan of PEMFCs operating under non-steady-state conditions (for vehicles, for example) impedes its massive use. The accurate prediction of their aging mechanisms can thus help to design proper maintenance patterns of PEMFCs by providing foreseeable performance degradation information. In addition, the prediction could also help to avoid or mitigate the unwanted degradation of PEMFC systems during operation. In this paper, an advanced self-adaptive relevance vector machine (RVM) has been developed and demonstrated to predict the performance degradation of PEMFCs. In order to prove the versatility of proposed RVM method, the predictive results are experimentally validated using two different PEMFC stacks aging data under different operating patterns. Furthermore, the obtained results are compared with results from both classic support vector machine and original RVM methods in order to highlight the effectiveness of the proposed self-adaptive RVM method with a modified design matrix. A comparison between single-step-ahead and multiple-step-ahead predictions of the proposed method is also given and discussed. The results show that the proposed novel RVM method is powerful and effective for PEMFC degradation prediction.

A Suboptimal Power-Point-Tracking-Based Primary Frequency Response Strategy for DFIGs in Hybrid Remote Area Power Supply Systems

Abstract: Due to the presence of a power electronic converter, the doubly fed induction generator (DFIG)-based wind generators are isolated from grid frequency variations, which will impose significant burden on conventional generators to regulate frequency in a hybrid remote area power supply (RAPS) system. Thus, participation of wind generators in frequency control is increasingly demanded. In this paper, a primary frequency response strategy is proposed for the DFIG to regulate the RAPS system frequency. A droop control loop without conventionally used high-pass filter is integrated to generate the torque reference for the DFIG to provide primary frequency response, and a supplementary control loop is proposed to enhance the primary frequency response with torque feedback control. Furthermore, the suggested suboptimal power point tracking strategy is capable of reserving wind power to improve the frequency response. The proposed strategy enables DFIG to participate in RAPS system frequency regulation while alleviating high rate of change of power and thus stress on the diesel generators under highly variable load demand. The effectiveness of the proposed strategy is validated through simulations.

Combined Vector and Direct Thrust Control of Linear Induction Motors With End Effect Compensation

Abstract: Linear induction machines dynamics may not be as fast as that of their rotary counterparts due to a larger air gap, a bigger leakage flux, and end effect. Direct thrust control can speed up their dynamics substantially with rather high pulsations, while vector control facilitates a smoother performance but slower dynamics. In this paper, an analogy between the two control methods, when they are applied to the linear machines, is proved first. The analogy is then used to justify combining the selected parts of the two control methods to come up with a control method that provides faster dynamics than vector control and smother performance than direct thrust control. The proposed control method is realized by getting rid of demanding parts of the two conventional control methods. End effect is also considered in the modeling and control system design. Extensive simulation results confirmed by experimental results prove improved motor performances under the proposed control method. The proposed control is particularly suitable for linear applications that need fast dynamics and smooth operation.

Adaptive Maximum Power Point Tracking Control Algorithm for Wind Energy Conversion Systems

Abstract: This paper presents an adaptive maximum power point tracking (MPPT) algorithm for small-scale wind energy conversion systems (WECSs) to harvest more energy from turbulent wind. The proposed algorithm combines the computational behavior of hill climb search, tip speed ratio, and power signal feedback control algorithms for its adaptability over wide range of WECSs and fast tracking of maximum power point. In this paper, the proposed MPPT algorithm is implemented by using buck–boost featured single-ended primary inductor converter to extract maximum power from full range of wind velocity profile. Evaluation of the proposed algorithm is done on a laboratory-scaled dc motor drive-based WECS emulator. TMS320F28335, 32-bit floating point digital signal controller, is used to execute the control schemes of the in-lab experimental setup. Experimental results show that tracking capability of the proposed algorithm under sudden and gradual fluctuating wind conditions is efficient and effective.

Multi-Level Energy Management System for Real-Time Scheduling of DC Microgrids With Multiple Slack Terminals

Abstract: Multi-level energy management system (EMS) is proposed for dc microgrids operations to ensure system reliability, power quality, speed of response, and control accuracy in this paper. System distributed control is scheduled as the primary control. Battery energy storages (BESs) operate in voltage regulation mode with droop control for power sharing. However, bus voltage deviation and power tracking error due to line impedance are the main drawbacks of distributed control. To enhance the system power quality and control accuracy, bus voltage restoration and power sharing compensation are implemented in secondary control. Economic dispatch based on comparison of system units' marginal operation costs is carried out in tertiary control to minimize the system operation cost. Detailed elaboration has been derived to quantify BES utilization cost due to the cumulated lifetime degradation. In case of communication failure, system operation can still be retained with primary control without operating mode change so that to enhance system reliability. A lab scale dc microgrid is developed for the verification of the proposed multi-level EMS and relevant control techniques.

Magnetic Field Analysis in Eccentric Surface-Mounted Permanent-Magnet Motors Using an Improved Conformal Mapping Method

Abstract: This paper presents an improved conformal mapping (ICM) method for magnetic field analysis in one typical surface-mounted permanent-magnet (SMPM) motor considering the static, dynamic, and mixed rotor eccentricities. The ICM method accurately accounts for the slotting effect, the winding distribution, the armature reaction, the magnetic saturation effect, and the working point variation of PMs throughout their volume. The ICM method is also very rigorous to separate the onload air gap field components. First, the slotless model of an eccentric SMPM motor is mapped to one concentric model using a new form of bilinear conformal mapping, and the field solution is then obtained through applying Hague's solution in the main canonical domain. Second, the slotted air gap magnetic field is obtained through the modulation of air gap magnetic field in the slotless physical domain using the complex relative air gap permeance. The ICM method is also used to investigate the influence of rotor eccentricity on the flux linkage of the stator coils. This developed model is validated through comparing with the corresponding results obtained through finite element method and the frozen permeability method. This method can also be used as a tool for design and optimization of electrical machines.

Analysis and Improvement of a Hybrid Permanent-Magnet Memory Motor

Abstract: This paper proposes a hybrid permanent-magnet memory motor. Using finite element method, the utility of AlNiCo and ferrite as low-coercive-force (LCF) magnets in the motor is comparatively analyzed. The results demonstrate that the positive magnetization of the ferrite magnets is difficult, while the field-variation range with the AlNiCo magnets is not sufficient, and the motor-power density and the positive magnetization characteristic of the LCF magnets cannot be improved simultaneously. On the other hand, the q -axis current may cause irreversible demagnetization in the LCF magnets, especially in the AlNiCo magnets. To solve the problems, an improved rotor topology with magnetic barriers is designed and considering that the demagnetization curve of ferrite magnets is mostly linear, ferrite magnets are used as the LCF magnets in the improved motor. Performance of the improved motor is analyzed and compared with that of the original configuration. Simulation results show that the positive magnetization characteristic of the ferrite magnets is significantly improved and the irreversible demagnetization in the ferrite magnets is avoided under load conditions. A prototype is fabricated and tested, verifying the analysis results.

Principles of Stator DC Winding Excited Vernier Reluctance Machines

Abstract: Stator dc winding excited Vernier reluctance machines (DC-VRMs) are one novel kind of Vernier reluctance machines, and have doubly salient structure and additional dc field windings in their stators to generate the exciting field. These machines advantages include a wide speed range, due to the flexible exciting field by the dc winding and a robust rotor structure without permanent magnets or windings. In this paper, the nature and principles of DC-VRMs are first illustrated theoretically with winding function and harmonic theories. First, by considering the permeance modulation function, the equations and harmonics of the exciting field are obtained. Next, based on these results, the stator/rotor pole combinations and armature winding configuration methods are proposed. Additionally, the expressions for the self-inductance, mutual inductance, the back electromotive force (back-EMF) of armature windings are summarized with the winding function theories. Also, the effects of permeance, field, and armature winding harmonics on inductance harmonics are analyzed. The equation for electromagnetic torque is also given, and the design parameters that may influence the machine's torque are provided. Finally, the inductances and torque in synchronous reference frame are analyzed. All the analytical results are validated by finite element analyses and some experimental results are also given to validate the theoretical analysis.

Recursive-Least-Squares-Based Real-Time Estimation of Supercapacitor Parameters

Abstract: The letter suggests utilizing a recursive-least-squares identification algorithm for real-time estimation of supercapacitor equivalent capacitance and resistance. Estimation is required since both parameters are subject to age, temperature, and terminal-voltage-based variations in addition to typical 20% tolerance of manufacturer provided values. The proposed approach allows calculating the device instantaneous state of energy used as a fuel gauge instead of the commonly adopted state of charge. Experimental results are given to verify the feasibility of the proposed method.

Acoustic Noise Analysis of a High-Speed High-Power Switched Reluctance Machine: Frame Effects

Abstract: This paper examines the effect of frame on the acoustic noise and vibration of a high-speed and high-power switched reluctance machine (SRM). Five types of frame/ribs are investigated, where different frame thicknesses, types of cooling ribs, and frame shapes have been analyzed. For this purpose, a 12/8 SRM has been designed at 22 000 r/min and 150 kW and two stages have been applied. In the first stage, a new equivalent stiffness of the frame is utilized in order to estimate the resonant frequencies for different frame shapes. The paper provides a mathematical calculation method and evaluates its effectiveness with finite-element simulations. The results indicate an improvement between 4% and 25% in the estimation natural frequencies. The second stage includes the vibration and acoustic noise analysis using 3-D finite-element method. Considerations for acoustic noise reduction in high-speed SRM using different frame types and cooling ribs are discussed. Particularly, radial and screw-type configurations represent a better solution to decrease the sound pressure level up to 12 dB.

Design of Mutually Coupled Switched Reluctance Motors (MCSRMs) for Extended Speed Applications Using 3-Phase Standard Inverters

Abstract: Design of a mutually coupled switched reluctance motor (MCSRM) competitive to the third generation interior permanent magnet synchronous motor (IPMSM) of Toyota Prius 2010 is presented. Compared with conventional SRM (CSRM), the proposed machine utilizes the standard six-switch voltage source inverter, which can facilitate the adoption of reluctance machines for traction applications. The structure of the MCSRM has been optimized for 60 kW output power over wide speed range of operation (2768–13500 r/min). Design details to improve machine torque density and to achieve the wide constant power speed range are presented. Performance evaluation under the targeted benchmark shows that the designed MCSRM can attain competitive performance metrics as that of the third generation IPMSM.

A Novel Method for No-Load Magnetic Field Analysis of Saturated Surface Permanent-Magnet Machines Using Conformal Mapping and Magnetic Equivalent Circuits

Abstract: The paper presents a method for calculating the back electromotive force and cogging torque waveforms of surface permanent-magnet motors using a model based on conformal mapping and magnetic equivalent circuits. The model takes into account the saturation of the iron core and its variation due to rotor movement. Because of the shorter execution time and achieved accuracy, it represents a good alternative to time-consuming finite-element-based models, especially in an initial stage of motor design. The proposed method has been implemented and evaluated on selected examples of 36-slot / 6-pole motors with different levels of saturation in teeth and stator yoke, and shows excellent agreement with the results obtained using finite-element method.

The Impacts of Distributed Energy Sources on Distribution Network Reconfiguration

Abstract: Thanks to the recent improvements in renewable energy technologies throughout the world, distributed energy sources are now playing an undeniable role in supplying the electricity in distribution networks. This paper studies the impacts of utilizing distributed generation units on the task of network reconfiguration in distribution systems. Considering the importance of reducing voltage drops and voltage sags in distribution systems, network reconfiguration is formulated as a multiobjective optimization problem in this study to minimize these two objective functions. A Pareto-based metaheuristic optimization algorithm is proposed to identify a Pareto frontier representing the alternative high-quality suboptimal configurations. The proposed optimization method is tested on a 69-bus distribution system to demonstrate the performance of the algorithm.

A Novel Design of Rotor Contour for Variable Reluctance Resolver by Injecting Auxiliary Air-Gap Permeance Harmonics

Abstract: This paper proposes a novel rotor contour design for variable reluctance (VR) resolvers by injecting auxiliary air-gap permeance harmonics. Based on the resolver model with nonoverlapping tooth-coil windings, the influence of air-gap length function is first investigated by finite element (FE) method, and the detection accuracy of designs with higher values of fundamental wave factor may deteriorate due to the increasing third order of output voltage harmonics. Further, the origins of the third harmonics are investigated by analytical derivation and FE analyses of output voltages. Furthermore, it is proved that the voltage harmonics and the detection accuracy are significantly improved by injecting auxiliary air-gap permeance harmonics in the design of rotor contour. In addition, the proposed design can also be employed to eliminate voltage tooth harmonics in a conventional VR resolver topology. Finally, VR resolver prototypes with the conventional and the proposed rotors are fabricated and tested respectively to verify the analyses.

Design and Implementation of a Highly Robust Sensorless Sliding Mode Observer for the Flux Magnitude of the Induction Motor

Abstract: This paper presents the design and analysis of a sliding mode observer for the flux magnitude of the induction machine. The design is done using a modified model of the motor in the rotating reference frame. The inputs of the observer are the voltages and currents in the dq frame, and an estimate of the speed (which is not necessarily accurate). Using a speed estimate allows us to eliminate the speed measurement and yields a sensorless observer. The novelty is that, despite using an inaccurate input speed, the design of the feedback gains allows us to obtain an accurate flux estimate. The sliding mode observer is compared with a similar open-loop flux observer—it is shown that the proposed design is much more robust to parameter variations. The theoretical derivations are supported with simulations and experimental waveforms.

A Distributed Control Method With Minimum Generation Cost for DC Microgrids

Abstract: This paper proposes a fully distributed control method for dc microgrids to realize power balance and bus voltage recovery without central coordination. Based on peer-to-peer communication, distributed generators and energy storage systems need only exchange information between neighbors over a sparse communication network, and hence there is a minimal communication burden. By combining the equal increment rate criteria and a subgradient algorithm, this distributed control method can efficiently regulate the bus voltage to the nominal value with minimum generation cost. In addition, the proposed method can improve the utilization of renewable energy generation via reasonable power sharing among distributed generators. Both islanded mode and grid-connected mode were simulated as case studies. The results indicate that the method is reliable, scalable, and flexible over centralized schemes. Influences of communication delays and failures of communication links are also discussed in numerical tests.