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Aiming at the influence on the suspension control accuracy caused by the suspension windings rotor induced current of a squirrel-cage bearingless induction motor (SBIM), a novel three-speed wound bearingless induction motor (WBIM) is designed based on establishing the stator-rotor equivalent circuit, deriving the exciting current-suspension slip ratio equation and analyzing the magnetic field distribution with odd pole pairs and dipole pairs. The stator is embedded with a fixed 4 pole winding and a 2/6 pole changing winding, while the rotor is wound with series-parallel windings connecting at a slip ratio ring. By changing the connection method of the stator coils and adjusting the three-phase voltage source, and controlling the short circuit of the rotor winding central point, it can realize that the rotor can only induce the torque winding magnetic field under three speeds. Through the finite element analysis (FEA), the induced current, air gap magnetic density, suspension force and electromagnetic torque of the traditional SBIM and the new WBIM are quantitatively calculated and compared. Finally, an experimental platform is built for further verification.

T-type three-level inverters have attracted extensive attention for their utility in many applications, especially photovoltaic power generation. For these inverters, parallel operation offers superior performance in efficiency, reliability, and scalability. However, the introduced circulating current (CC) may cause overcurrent in line, which seriously threatens the safety and stability of the system. The mechanism of CC lacks detailed analysis in single-phase modular inverters. Thus, an accurate mathematical model is established and a CC suppression method is proposed in this paper for parallel-operated single-phase T-type three-level inverters. Firstly, the characteristics of port degradation are analyzed, and an improved equivalent circuit is introduced based on the findings. Then, according to the improved equivalent circuit, a detailed mathematical model is established. Using this model, excitation sources of CC are analyzed and divided into three parts. After that, a CC suppression method involving carrier-based pulse width modulation with offset voltage injection is proposed, and a deadbeat controller is designed to calculate the offset voltage. Moreover, the proposed method can balance the neutral point voltage across DC-link capacitors as well. Simulation and experimental results validate the theoretical analysis and effectiveness of the proposed scheme.

The state-of-charge (SOC) estimation and remaining-dischargeable-time (RDT) prediction are critical and challenging questions to safe operation of lithium-ion batteries. The main challenges are the limited accuracy of traditional equivalent circuit model and computation-inefficiency of electrochemical battery models. To address this problem, this paper proposes a Lebesgue-sampling-based extended Kalman filter (LS-EKF) approach that integrates the high-fidelity of a simplified first principle (SFP) model with the low computation of Lebesgue sampling (LS) in a Bayesian estimation framework. In this framework, the SFP battery model is first introduced along with its design and validation. The LS-EKF is employed with the SFP model to estimate the SOC and predict the RDT. The proposed method is verified with a series of experiments under different operating conditions. The results and comparison with the traditional Riemann-sampling-based extended Kalman filter demonstrate the effectiveness of the proposed method in terms of the accuracy of estimation and prediction, as well as the computational efficiency.

SiC MOSFET is a promising power device for electric drive system due to its various advantages such as high switching frequency and low loss. However, fast switching speed results in high voltage stress and can potentially cause insulation damage of the winding. Few studies have analyzed and evaluated the voltage stress, especially for the hairpin winding, which is the mainstream winding type for electric vehicle traction machines. For this purpose, this paper proposes a high-frequency equivalent circuit model to predict the inter-turn voltage stress of hairpin windings driven by SiC MOSFETs. Firstly, the working principle of the overvoltage at the winding ends is analyzed and validated by experiments. Then, the parasitic parameters are calculated before building the high-frequency equivalent circuit. A field-circuit coupled finite element method is used to consider the nonlinear parameters and to evaluate the voltage stress. Finally, a prototype is built, and the voltage stress is tested. The resultant error between the calculated and experimental maximum inter-turn voltage stress is only 1.3%, which supports the inference on the validity of the presented equivalent circuit and the method. Based on the above, a novel winding connection scheme is proposed to reduce effectively the voltage stress of hairpin winding.

Online parameter identification is vital for boosting the accuracy of the battery Equivalent Circuit Model (ECM) under dynamic profiles. However, traditional Recursive Least Squares (RLS) method easily decays with the noise corruption from sensors or insufficient exciting signal in reality, which further limits the performance of ECM in battery modeling and states estimation. This paper thus proposes a reliable online parameter identification method for battery ECM, which utilizes a well-designed Information Appraisal Procedure (IAP) based on the Fisher-information-based Cramer-Rao Lower Bound (CRLB). Without increasing much computing complexity, a comprehensive appraisal indicator, derived recursively from CRLB, enables a new mechanism for online parameter updating. Simulation and experimental results prove the validity of the proposed method under different driving cycles, temperatures, and aging conditions. The results show that the identification accuracy of the proposed method has been significantly improved comparing with a typical RLS and a Multiple Adaptive Forgetting Factors RLS (MAFF-RLS) method.

This work describes a frequency-dependent bearing voltage model for squirrel-cage induction motors. Previous researches related to bearing voltages commonly considered the bearing voltage ratio (BVR) as a constant. However, it is important to analyze the frequency-dependent behavior of BVR when dealing with systems excited by high switching frequency PWM voltages. First, we measured the BVR characteristic for a 1.5-kW test motor. The measurement data shows that, in the mid-frequency range (100 kHz to 6 MHz), the BVR values are smaller than the conventional low-frequency constant value, and at higher frequencies, the BVR value increases above the low-frequency value. Considering this BVR characteristic, an equivalent circuit has been built and analyzed from the perspective of BVR. The proposed equivalent circuit has been verified by comparing simulation data with measurement data. Based on the analysis results of the equivalent circuit, a parametric study was done to examine the effects of motor system parameters on bearing voltage. For the study, variations of the bearing voltage on the changes of the input voltage waveforms and the winding-to-stator capacitance are analyzed by using measurement and simulation data.

This article introduces an efficient modeling approach based on the Wiener structure to reinforce the capacity of classical equivalent circuit models (ECMs) in capturing the nonlinearities of lithium-ion (Li-ion) batteries. The proposed block-oriented modeling architecture is composed of a simple linear ECM followed by a static output nonlinearity block, which helps achieving a superior nonlinear mapping property while maintaining the real-time efficiency. The observability of the established battery model is analytically proven. This article also introduces an efficient parameter estimator based on extended-kernel iterative recursive least squares algorithm for real-time estimation of the parameters of the proposed Wiener model. The proposed approach is applied for state-of-charge (SoC) estimation of 3.4-Ah 3.6-V nickel-manganese-cobalt-based Li-ion cells using the extended Kalman filter (EKF). The results show about 1.5% improvement in SoC estimation accuracy compared with the EKF algorithm based on the second-order ECM. A series of real-time tests are also carried out to demonstrate the computational efficiency of the proposed method.

This paper presents a joint state-of-charge (SOC) and state-of-available-power (SOAP) estimation method based on online battery model parameter identification. First, the SOAP of lithium-ion batteries is analyzed thoroughly, and a safe operating area border-based (SOAB-based) SOAP estimation is proposed. Second, based on the adaptive battery-state estimator (ABSE) and improved ABSE, a joint SOC and SOAB-based SOAP estimation method is proposed. The joint estimation results show that the improved ABSE achieves higher accuracy than the ABSE at different battery aging states. The open-loop accuracy evaluation results show that the improved ABSE identifies the battery model parameters more accurately, and the ABSE algorithm error source lies in its identified Rp being much higher than the actual value when the battery is charged/discharged at a high current. The ABSE does not consider the influence of load current on the equivalent circuit model parameters, so it is not suitable for SOAP estimation in theory. The improved ABSE proposed by our team can eliminate this modeling error, identify the battery model parameters, and estimate the SOC and SOAPSOAB more accurately. This improved ABSE is an effective algorithm for estimating the battery state when the battery is charged/discharged with a high current.

Free positioning wireless charging for consumer electronics allows the devices to be charged at arbitrary positions and angles to improve user experience. However, a user-initiated sudden movement of the device during charging can cause hazards due to the abrupt variation of the coupling coefficient. To solve this issue, the coupling coefficient variation at different positions should be mitigated, which is also good for the design and high-efficiency operation of power electronics converters. This article proposes a design methodology to employ antiparallel windings to smooth the coupling coefficient variation over different positions by reducing the coupling coefficient at central positions and enhancing it at boundary positions. Two optimization methods are proposed: turn-by-turn optimization and winding-by-winding optimization. A design flow is offered. The hexagonal coil is compared with the square coil and proved to achieve better performance than the latter. An experimental prototype is implemented to validate the effectiveness of the proposed design methodology.

Expanded use of power electronics opens the possibility that input characteristics presented to the conventional utility as well as ac and dc microgrids could be controlled in ways that help support utility operation. Power factor correcting interfaces are one wide-spread example, used around the world to meet regulatory requirements for both power factor and also harmonic content injected to the utility. More support for ensuring healthy utility operation might be demanded from these power electronic interfaces. For example, the potentially destabilizing effects of constant power loads (CPLs) could be remediated on time scales useful for supporting utility operation. This article presents an equivalent circuit model for limited-bandwidth CPLs that quantifies the intrinsic damping properties of regulated converters. A control architecture based on this equivalent circuit model is then analyzed. The control scheme limits the impact of CPL operation by emulating the internal damping of CPLs through programmable input behavior. An intermediate, internal energy buffer is used to support CPL operation while enabling the programmable or selectable input impedance on designed time scales. The effectiveness of this strategy on system stability is shown using a stability analysis and experimental results.

Existing nonunit protection schemes inevitably require setting, which is a serious problem in practical engineering. Faults occurred at different fault zones will result in different equivalent models, therefore, the fault zone can be determined by recognizing which equivalent model the fault fits well with. In this article, this “model recognition” idea is introduced in fault identification and a “setting-less” protection method is proposed. First, the Peterson equivalent circuits when faults occur at backward external zone, internal zone, and forward external zone are presented, respectively. Accordingly, the corresponding three fault voltage expressions are derived, which are defined as three fault modes. Then, the three fault modes are used to approximate the measured fault voltage using Levenberg-Marquardt optimal approximation method. The fault mode that best fits the measured fault voltage is recognized as the final determined fault mode, which is used for fault identification without setting threshold value. Numerous test studies carried out in Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) and real-time digital simulator have demonstrated that the proposed method can be utilized for different multiterminal dc systems and is effective under different fault locations, different fault types, and high fault impedances. The proposed method does not require high sampling frequency and has good robustness against measuring noise.

This article proposes a universal zero-voltage switching (ZVS) circuit design method for a family of interleaved dual boost diode-capacitor (IDBD) step-up converters with different voltage multiplier cell (VMC) structure and number. By utilizing one small inductor and two resonant capacitors, all the power devices in the hard-switching (HS) IDBD converters can achieve soft-switching. Besides, the increment of voltage and current stresses imposed on the semiconductor devices and passive components is minimized in comparison to the HS counterparts with the conventional SS design. Due to the voltage-clamped ability of the diode-capacitor network, the voltage stress of one switch in the constructed topologies is less than the output voltage and decreases further when the VMC number increases. Hence, the low on -state resistance switches can be used to further reduce the conduction loss. Consequently, the constructed ZVS topologies demonstrate improved efficiency with low cost and small volume. Furthermore, the high voltage gain, low-input current ripple, and low-voltage stress characteristics of the HS IDBD converters are also conserved. Finally, a 320 W prototype is built and tested to verify the theoretical analysis and efficiency improvement.

Lithium-ion battery packs are critical to ensure the operational reliability and service life of spacecraft. The battery pack health diagnosis is meaningful for reasonable mission plan and proper control actions of spacecraft. However, the characters of space application scenarios include periodic charge and discharge, nearly constant discharge current, low electric current rate, and fixed depth of discharge, which bring challenges to battery pack state-of-health estimation. Considering these characters, this article proposes a health diagnosis approach for lithium-ion battery pack based on an improved equivalent circuit model. First, an empirical model is introduced into the standard first-order equivalent circuit model to improve the accuracy of discharge process modeling under constant current. Furthermore, a parameter lookup table is developed to simulate the complete discharge based on the in-orbit partial discharge. Finally, the battery pack complete discharge process is virtually conducted to realize battery pack capacity estimation. The proposed method just satisfies the special operating conditions in space applications. The proposed method is validated based on a real tested lithium-ion battery pack. And the results indicate that the proposed method can realize the high-accurate health diagnosis for both battery pack and its inner cells under space operating conditions, which illustrate the adaptability and applicability of the proposed method in space applications.

A front-end bidirectional symmetric bipolar outputs (SBOs) LLC dc-transformer (DCX) for half bridge class-D audio amplifier (HB-CDAA) system is proposed and studied in this article. The bidirectional SBO LLC DCX provides SBOs for the HB-CDAA without producing voltage pumping on the outputs of the SBO LLC DCX. The operation of the proposed bidirectional SBO LLC DCX is analyzed, and the effects of the HB-CDAA on the operation of its front-end bidirectional SBO LLC DCX are studied. Soft switching of the proposed bidirectional SBO LLC DCX can be realized both in the positive and negative audio frequency cycles, which improves efficiency of the proposed HB-CDAA system. A 100-W prototype is built up to verify the analysis results. The experimental results show that SBOs are obtained and soft switching of all switches are achieved.

Many methods have been studied to balance the capacitor voltage of the cascaded H-bridge rectifier (CHBR). However, these balancing control methods will fail while the difference of the dc-load connected in each cell exceeds the operating limitation. Therefore, it is necessary to analyze and optimize the limitation of the balancing control quantitatively to avoid the failure of the control system. In this article, the voltage balancing limitation (VBL) is quantitatively derived and the stable operation region (SOR) is analyzed. Moreover, essential factors affecting the balancing ability are screened out. Then, the VBL and balancing dynamics of two typical methods are elaborated. And three novel methods (Methods 1–3) are proposed to improve the balancing dynamics and extend the VBL. The effects of the five methods are compared. Finally, the correctness and effectiveness of the theoretical calculation and optimization methods of VBL are verified by the simulation and experimental tests. The work of this article can provide a theoretical reference for the system parameter design of CHBR and the alternative control scheme under extreme load conditions.

Switching losses including reverse recovery losses, have a significant impact on total losses of DC–DC converters especially in high voltage applications. This article introduces a new family of DC–DC converters with two degrees of freedom, which can handle the mentioned issue. The proposed family of DC–DC converters contains a bidirectional and a unidirectional cell, and provides an additional control parameter. This parameter can be adjusted so that there are soft switching conditions for all the switches of the converter within the entire range of loading. In addition, properly tuning the new parameter can decrease the circulating current and consequently decrease the conduction losses considerably. Besides, the unidirectional cell contributes to power transfer, especially under full load condition. Moreover, the filter capacitor is one fourth in comparison with the conventional dc-dc converters. The proposed converter is modeled and its operation modes are studied. Also, in order to verify the findings, a 500 W prototype with a frequency of 100 kHz is implemented, and the results are compared.

In this article, an autotransformer coil for domestic IH adopting single-ended resonant inverter (SE-IH) is proposed. The proposed SE-IH can simultaneously meet the impedance requirements of ZVS and high induction efficiency at a higher frequency, resulting in a higher efficiency, increased output power and improved compatibility with nonferromagnetic vessels. Based on the proposed accurate wide-frequency impedance model of the coil-vessel system, the induction efficiency of the proposed SE-IH is analyzed, as well as the impedance condition for ZVS. The selection of the operating frequency and the design of the proposed coil for induction efficiency optimization are then given. Finally, a prototype of the proposed domestic SE-IH is fabricated, achieving 2269-W output power with 99.26% induction efficiency for GB45 steel vessel and 932-W output power with 90.46% induction efficiency for aluminum vessel.

The concept of open-current vector for sparse matrix converter is put forward along with its application constraints in this article. Based on the open-current vector, an improved space vector modulation (SVM) strategy is proposed to reduce the peak magnitude of common-mode voltage (CMV) by 42.3% without sacrificing the performance of input and output waveforms and the voltage transfer ratio (VTR). First, the application constraints and common-mode equivalent circuit of the open-current vector in the rectifier stage of the sparse matrix converter are analyzed. After that, in accordance with the principle of minimum switching action and two adjacent active vector synthesis, the improved SVM strategy used a suitable open-current vector instead of the active-current vector in the rectifier stage when the zero-voltage vector is used in the inverter stage, which contributes to CMV reduction. The improved SVM strategy not only reduces the peak magnitude of the CMV, but also ensures that the performance of input and output waveforms and the VTR are the same with those of traditional SVM. Finally, the effectiveness of the improved SVM strategy is verified by simulation and experiments.

Parasitic capacitance has a considerable impact on conductive electromagnetic interference (EMI), especially on the common mode (CM) noise. In vehicular applications, such as more electric aircraft and electrical vehicles, the value of CM stray capacitance can vary during the drive cycle, due to some change in environmental, and operating conditions, resulting in fluctuations in EMI level. Designing EMI filters according to potentially overestimated CM capacitance results in additional weight, volume, and cost which should be avoided. In this paper, for the first time, the parasitic capacitance relevant to CM noise is estimated in real-time using unscented Kalman filter (UKF). The UKF is employed due to its ability in dealing with nonlinearity and stochastic nature of the EMI as well as the measurement noises. The proposed method is tested on a typical dc–dc buck converter in different operating conditions. The effectiveness of the proposed method is tested using some hardware-in-the-loop experiments using dSPACE1104 development platform.

Accurate measurement of electric fast transient (EFT) voltage is very important for insulation design and fault analysis of electric apparatus. In this article, a new simple method for EFT voltage measurement is proposed. Based on the linearity between EFT voltage and electric field intensity on the surface of electrode, the principle of the method is introduced and the corresponding equivalent circuit is built. By using finite-element method (FEM), the capacitances are computed and the relationship between output and input voltages is simulated when we apply Laplace transformation. Experiment model with coaxial structure for verification of the proposed method is built. Based on this model, experiment is conducted. According to the equivalent circuit, by using measured voltage, the parameters of capacitances is also calculated, and they are good coincident with those computed by FEM. The calibration for these capacitances is also done, and their validation is further verified. The proposed method can be used to measure very fast transient overvoltage in gas insulated switchgear and transformer or other apparatus. It can also be used in insulation design and fault location for electric apparatus in power system.

Modular multilevel converters (MMCs) are very promising and attractive in medium-/high-voltage applications. One of the main technical challenges associated with MMCs is to precharge all submodule (SM) capacitors to their nominal voltage values during a startup process. This article proposes a simple and effective startup strategy for MMCs based on a deadbeat predictive current control. This strategy can realize a fast precharge for all the SM capacitors from either ac-side or dc-side main voltages without any inrush current and, thus, guarantees a rapid and safe startup of MMCs. With the developed strategy, precharge currents from both ac-side and dc-side can be easily regulated in a closed-loop manner with an excellent dynamic performance. Moreover, conventional controller design and parameter tuning are not necessary, which greatly reduces design burdens and structural complexities of the control algorithm, and consequently facilitates its implementations. Detailed design processes and considerations of the proposed startup strategy are introduced and analyzed. The effectiveness and performance of the developed strategy are verified by sufficient experimental results.

In this article, a modular multilevel dc–dc converter philosophy is proposed which can form an effective interlink to exchange power between line commutated converter (LCC) based high voltage direct current (HVdc) link and voltage source converter-based HVdc grid of same or different voltage levels. The converter exhibits symmetric four quadrant operation irrespective of the voltage polarity in the LCC-based HVdc link. In the event of reversal of power flow by reversing the voltage polarity in the LCC-based HVdc link, the power flow direction/magnitude through the interlink can be made to remain unchanged without the need of any extra switching arrangement. The working principle is explained analytically and its operation is validated by simulation as well as experimental results.

Overmodulation techniques of asymmetrical six-phase machine achieve higher dc-bus utilization by applying voltage in the nonenergy transfer plane. This results into unwanted current and associated copper loss. Existing overmodulation technique minimizes this voltage from space-vector perspective with predefined set of four active vectors. To find the best technique, one needs to perform the above minimization problem with all possible sets of active vectors with which higher voltage gain can be attained. Therefore, this requires evaluation of large number of cases. This article formulates the above minimization problem in terms of average voltage vectors of two three-phase inverters, where active vectors need not to be specified beforehand and, thus, the analysis is more general. Sixteen possible techniques with different set of active vectors are derived following the above analysis, which attain minimum voltage injection in the nonenergy transfer plane. This article also identifies one of these 16 techniques, which can be implemented without involving complex six-dimensional transformation. The above technique is validated through experiments on six-phase induction motor and simulations in MATLAB at 3.5-kW power level.

In this article, a fast circuit-field coupling method is proposed for the analysis of the skewed induction motor. Based on the multislice model, the equivalent circuit of the skewed induction motor is established. The distributed magnetic field method is used to calculate air-gap flux density and deal with the core saturation at each axial slice. Based on the coupling analysis of the equivalent circuit and magnetic field, a two-layer iterative process is used to calculate the magnetic field distribution and performance indexes of the skewed induction motor. By comparing with finite element analysis and experiment results, the proposed method is proved to be correct and effective. The proposed method is characterized by fast computation speed and good accuracy, so it is an ideal approach to analyze the skewed induction motor.

The bidirectional buck-boost current-fed isolated dc–dc converter (B3CFIdc) has the advantages of low current stress, high efficiency, and wide power/voltage conversion range. The linear models of buck mode and boost mode of B3CFIdc are originally derived using state-space averaging method. The dual-closed-loop control configuration with the inductor current and output voltage is designed. In order to ensure the stability of both operation modes at the same time, the parameters of the controllers are designed using the method of Hurwitz criterion plus frequency response. The asymptotic stability of the operating conditions with periodic switching between the two modes caused by the periodic fluctuation of input voltage is verified using the linear switching theory. The detailed experimental results verify the correctness of the built small-signal models and the feasibility of the designed closed-loop control configuration and the design rule of the controller parameters.

As is the case with several other mechanical power takeoffs (PTOs), the mechanical-motion-rectifier-based PTO consists of components, such as one-way clutches, gears, a ball screw, mechanical couplings, and a generator. Equivalent circuit models have been created in this article to describe the dry frictions, viscous damping, and mechanical compliances in these components, so the nonideal efficiency and nonlinear force of the PTO can be predicted in electrical simulations by integrating these subcircuit models. The circuit model is simplified, and its parameters are categorized as dc and ac unknowns. The dc and ac tests on the PTO are performed sequentially to extract two sets of parameters through linear regression or nonlinear curve fitting. Then, the model is validated through its prediction capability over 25 test conditions on input forces, output voltages, and efficiencies, with correlation coefficients of 0.9, 0.98, and 0.981, respectively.

The concept of vehicle-to-grid has received considerable attentions over the past decade. Electric vehicle (EV), a common load of dc microgrids, can serve as distributed energy storage system to improve the reliability and stability of the dc microgrid, support integration of renewable energy source, and improve the overall system efficiency. In order to achieve bidirectional power transmission between the dc microgrid and EV, this article proposes a novel three-level CLLC resonant converter for off-board EV charger. By adding resonant CLLC components and by combining the working modes of the two three-level full bridges, the proposed converter adapts to the wide voltage range of EV, from 200 to 700 V. An equivalent circuit model with first harmonic approximation approach is established to analyze the frequency characteristics of the resonant converter. Moreover, an algorithm of working mode selection is proposed on the principle of minimum transformer RMS current. Finally, a 3.5 kW hardware prototype was built to verify the feasibility and the advantage of the three-level CLLC resonant converter. The efficiency of the proposed converter changes little over a wide output voltage range, and the flying capacitor voltages of the proposed converter are balanced well.

In this article, a nonisolated bidirectional dc–dc converter is proposed based on coupled inductor (CL) and switched capacitor (SC). The converter can achieve a high voltage conversion ratio (VCR) by using CL and SC. The CL is operated not only as a filter inductor to regulate voltage but also as a transformer to improve the VCR. Furthermore, the voltage stresses of capacitors and switches are reduced by introducing SC structure. The pulsewidth modulation plus phase-shift control is employed, where the duty cycle is used to ensure that the voltages of two sides are matched and the phase-shift angle is used to control the direction and amount of power flow. In addition, all the switches can achieve zero voltage switching in both forward and backward modes under a wide operating range, which can improve the efficiency of power conversion. The operating principle and steady-state analysis are examined, and a 400-W prototype is designed to verify the validity of the proposed converter.

Short-circuit current calculation and its characteristic analysis are the foundation of selecting circuit breakers and designing protection systems. First, a companion circuit-based method is presented to fast calculate the short-circuit current of asymmetric bipolar modular multilevel converter-based multiterminal direct current (MMC-MTDC) grids. With the reduction of the whole MTDC grid into an equivalent RLC circuit, the short-circuit current of the grid is efficiently and accurately solved by companion circuit method instead of the numerical integration. In addition to the efficiency, the proposed method can readily adapt to various types of MTDC grids and all kinds of dc faults. Second, unique short-circuit current characteristics of a bipolar MTDC grid following different types of dc faults are theoretically analyzed. A fault type discrimination method is then proposed for bipolar MMC-MTDC grids with dedicated metallic return. Finally, intensive numerical results of two MTDC grids validate the accuracy, adaptability, and efficiency of the presented dc grid short-circuit current calculation method as well as the correctness of the short-circuit current characteristic analyses.

In this article, basic cell (BC) of a novel swi-tched capacitor converter (SCC) has been proposed first. After that, the generalized structure of proposed SCC is developed. The developed SCC requires reduced number of switches, drivers, diodes, capacitors, and lower number of conducting switches in current flow paths and capacitor charging paths as compared to the other recently developed SCCs. A switched capacitor multilevel inverter (SCMLI) utilizing two numbers of generalized SCCs is developed next. Further, cascaded extension of proposed SCMLI is realized and analyzed for symmetric and asymmetric dc source configurations. A detail analysis of optimum selection of capacitance for switched capacitors of 13-level SCMLI is presented. An extensive comparison study shows that the proposed SCMLI requires lower number of components as compared to other SCMLIs. Further, the proposed structure has minimum cost function per level per boosting factor as compared to the other SCMLIs. Extensive experimental results considering fundamental switching frequency scheme are presented to validate the merits and effectiveness of the proposed structure.

Oil-immersed transformer is one of the most important electrical equipment in power distribution and transmission systems. Dielectric response method is a well-recognized method to diagnose the insulation defect of oil-immersed transformers. However, the applicability of this method is restricted due to the long testing time. Under some special field conditions, the method is not even applicable. In this article, a novel testing method is proposed based on the following ideas: first, the low-frequency dielectric parameters are extracted by using mixing frequency excitation; then, parameters of the extended Debye equivalent circuit are determined based on cuckoo search optimization algorithm; finally, the specific parameters are used to the established simulation model and obtain the recovery voltage curve. Compared with the traditional method, the testing time of the proposed method has been greatly reduced. Besides, dielectric parameters in both frequency domain and time domain can be obtained simultaneously. The applicability of the proposed method is verified by the dielectric response measurements on a laboratory transformer and a real power transformer in a substation.

Extraction of the current commutation loop (CCL) inductance is necessary for the overvoltage analysis of power devices. In general, each transient process of a converter corresponds to a specific CCL. However, there is no such single CCL in the full-bridge module in hybrid dc circuit breakers during the turn- off transient. First, this article proposes a coupled inductance model of full-bridge modules. The model has less number of inductance compared to the traditional partial inductance model. Second, this article proposes the extraction method of inductances that can be easily obtained by both simulation and measurement. Moreover, the measured overvoltage waveforms of insulated gate bipolar translators (IGBTs) in the full-bridge module agree with the simulation results. Third, we propose an equivalent circuit of the series full-bridge modules in a 500 kV hybrid circuit breaker based on the proposed coupled inductance model. The mutual inductances among different modules are considered due to the compact structure of the series modules. The measured results show that the overvoltage of the IGBT in the series module could be well-predicted by the proposed equivalent circuit.

Ultrasonic wireless power transfer (UWPT) has been a promising technology to recharge the sensor batteries that locate in enclosed metal containers. An accurate and comprehensive analytical model is the key enabling tool for designing the UWPT system. In this article, a novel equivalent circuit model has been proposed for a typical UWPT system with two symmetric piezoelectric transducers bounding onto an aluminum plate coaxially. The proposed model is a two-port network equivalent circuit that can be described by T parameters. By considering the losses of the UWPT system, the accuracy of this model has been enhanced in comparison with the widely used equivalent circuit model, which ignores its losses. The experimental results show that the average error of output power decreases about 6.4 times in the proposed model compared to the existing model. Moreover, the proposed model is tested under various working conditions to validate the enhanced accuracy.

The zinc–nickel single-flow battery is a new and special type of flow battery with a number of promising features, such as membrane free and high scalability, and thus has attracted substantial interests in recent years. However, the cyclability of alkaline zinc cells is rather poor, with sharpened capacity degradation resulted from undesirable zinc deposition formation. Yet, little has been done so far to investigate how to effectively and reliably manage this new type of battery. In this article, an open-circuit-voltage estimator based online joint estimation of both the state of charge (SoC) and the state of health (SoH) is proposed. A second-order equivalent circuit model is applied to improve the accuracy. Meanwhile, an extended Kalman filter with reduced state dimension is formulated to estimate the SoC assisted with the proposed estimator, which solves the increased complexity issue in a higher order model. An SoH indicator is then derived from capacity estimate and employed to determine the time for reconditioning maintenance, which is a key stage to update capacity and prolong service life. The method is finally applied to a bench-scale cell demonstrator, and the experimental results confirm the efficacy of the proposed method.

Energy efficiency estimation for in-service induction motors has become a crucial area of research due to the increasing need for energy savings and demand-side management. As a result, several in-service efficiency estimation methods for induction motors have been proposed. This article presents a nonintrusive method for estimating the efficiency of induction motors operating on-site using limited terminal measurements and motor nameplate data. The method uses a modified inverse Г-model equivalent circuit with series core loss arrangement to mitigate the inherent problems of higher computational burden and parameter redundancy in the conventional T-model. Furthermore, a new method is presented for estimating the friction and windage loss using the airgap torque and motor nameplate data. The proposed nonintrusive field efficiency estimation technique was validated experimentally on standard and premium efficiency induction motors for both balanced and unbalanced voltage supply. The results demonstrate the accuracy of the proposed method and confirm its advantage over the conventional method for in-service efficiency determination.

Induction-based reaction spheres have been presented in many references and their performances are normally investigated through experiments or numerical simulations which are time-consuming. Here, an analytical way is presented and it enables researchers to evaluate a new design quickly. In this article, the presented performance analysis is conducted through the classical equivalent circuit approach. Involved circuit parameters are determined through the magnetic flux density distribution which is a function of design variables. Based on this, the steady-state torque–speed curve and the achievable maximum driving torque $T^\ast$ are identified. $T^\ast$ deviates from the maximum torque obtained from numerical simulations by only 3%. For validation, the presented performance analysis method is applied to an experimental case. Mean absolute percentage errors of predicted torque–speed curves are within 23% and mainly caused by end effects. The presented performance analysis method is generally applicable to induction-based spherical actuators, not only limited to reaction spheres. Additionally, since influences of design variables of the actuator have been formulated analytically through the determined circuit parameters, performance optimizations could be greatly facilitated.

This article addresses a single-stage bidirectional step-up inverter designed from the integration of a differential boost inverter and switched-capacitor (SC) cells. The conventional boost inverter, even being a step-up topology, presents a gain limitation due to losses, and therefore, does not fully attend the step-up specifications. The insertion of SC multiplier cells into it allows increasing its static gain without increasing the voltage stresses on its components. However, the resulting topology is nonlinear and contains a high amount of energy storage elements, which implies high-order models. In this article, static and dynamic analysis of the SC differential boost inverter is performed under different types of modulation. A generalized and reduced order equivalent circuit and a small-signal average model are proposed, as well as a static gain linearization technique that reduces the harmonic distortion of the output voltage, regulated in the closed loop by a resonant controller. This article also presents the main waveforms, equation, and a comparison between the differential boost inverter and its version with SC multiplier cells. To verify this study, a 250-W prototype of the rated power, input voltage of 60 V, switching frequency of 50 kHz, and the output voltage of 220 V is experimentally evaluated.

The widespread application of nonlinear power electronic devices in residential loads has resulted in significant harmonic distortions. Harmonic source modeling is of great importance since it is a prerequisite to estimate the harmonic pollution. In this paper, a dynamic harmonic coupled matrix model (HCMM) is proposed for the nonlinear home appliances. This model is adaptive to the load supply voltage conditions; therefore, the dynamic harmonic generating characteristics of nonlinear home appliances can be demonstrated by the proposed HCMM. Moreover, the HCMM derivation process is based on measurement, independent of the load equivalent circuit, which makes the proposed method most suitable for nonlinear home appliances with unknown circuit topologies. The widely linear complex partial least squares method, which has a fast speed and high robustness, is used to estimate the HCMM. A procedure to establish the HCMM for multistate home appliances is also proposed in this paper. Lab experiments were performed to verify the accuracy and validity of the proposed methodology. The aggregate harmonics caused by multiple nonlinear residential loads have been analyzed based on the proposed HCMM.

This paper investigates the real-time estimation on the state-of-charge (SoC) and state-of-health (SoH) of lithium-ion (Li-ion) batteries for the purpose of achieving reliable, safe, and efficient use of batteries. Three terminal sliding-mode observers (TSMOs) are designed; each observer is used to estimate one variable of a Li-ion cell for developing a real-time SoC estimation algorithm. To estimate the SoH, two additional TSMOs are subsequently presented. Finally, a set of complete estimation algorithms for SoC and SoH are formulated. The output injection signals of the proposed TSMOs are designed to be continuous. This can attenuate chattering that exists in the traditional sliding-mode observers and simplify the estimation algorithms. The main advantage of the proposed algorithms is eliminating the low-pass filter in the estimation algorithms. Therefore, higher estimation accuracy and faster response speed are obtained. The proposed methods are tested and evaluated using the acquired dynamic stress test and federal urban driving schedule test data, which demonstrate the effectiveness and feasibility.

An improved division–summation (D–Σ) digital control strategy without high-speed communication is proposed in this paper. Unlike the traditional zero-sequence circulating current control method, the interphase circulating current suppression method is discussed and implemented on the basis of the single-phase equivalent circuit in this strategy. The generation mechanism of the interphase circulating current is emphatically analyzed. Output voltage compensation and virtual impedance compensation are studied and integrated with the D–Σ digital control method to suppress the circulating current. Experimental platform with three paralleled three-level T-type grid-connected inverters has been set up. The experiments are carried out under the conditions of the impedance match or mismatch among the inverters, the unbalanced grid voltages, and the dynamic process such as start-up and step current change. The experimental results verify that the improved D–Σ digital control strategy may effectively restrain the circulating current under various conditions.

Battery safety is one of the most crucial issues in the utilization of lithium-ion batteries (LiBs) for all-climate electric vehicles. Short circuit, overcharge, and overheat are three common field failures of LiBs. In this paper, online fault diagnosis for external short circuit (ESC) of LiB packs is investigated. The experiments are carried out to obtain and compare ESC characteristics of 18650-type NMC battery pack and single cell. Based on the analysis of experimental results, a two-step equivalent circuit model is established to describe the ESC process and an online model-based scheme is proposed to diagnose ESC faults of battery packs. The proposed scheme is evaluated by experimental data. The results show that it can effectively diagnose ESC faults in 3.5 s after their occurrences with the terminal voltage error less than 25 mV. The proposed scheme has shown great generalization ability. ESC faults of battery packs under different number of cells connected in series and unavailable current information can also be diagnosed at the terminal voltage error less than 48 and 60 mV, respectively.