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This paper proposes a parallel-connected system where two three-phase three-level T-Type photovoltaic inverters with common ac and dc sides to improve the power rating. However, zero-sequence circulating currents (ZSCCs) will occur, which will distort currents, decrease efficiency, and increase loss. In this paper, the fundamental mechanism of ZSCCs based on the derived model of ZSCCs is analyzed in details including the dead time effects. It is can be seen that the ZSCCs generated in three-level case are more complex than parallel two-level inverters due to existing more space vectors. Then, a synchronizing starting small vector control method with PI controller and feed-forward control method is proposed to suppress the ZSCCs. As a result, a good circulating current suppression performance can be obtained. The effectiveness of the analysis and proposed method is validated by experimental results.

This paper presents the digital implementation of a boundary controller with unipolar switching characteristic for single phase voltage source full bridge inverters, which will be used as an interface between a low voltage DC microgrid and AC loads. This paper expands the second order switching surface control method for single phase inverters by unipolar switching with the use of a finite-state machine. Compared to bipolar switching, unipolar switching has the advantage of lower switching losses and filter size. Besides, the switching losses can be equally distributed among semiconductor switches compared to conventional three-level inverter switching scheme. The operating principles of proposed control scheme and mathematical derivation of the switching functions will be given. A 550 VA, 120 V, 60 Hz voltage source inverter (VSI) with a digital signal processor has been implemented to verify the theoretical predictions. Experimental results for steady state operation at different loading conditions show a tightly regulated output voltage, for large-signal disturbances the control is fast and precise.

This paper introduces an open-loop control scheme for Cuk-based universal converters that have recently been proposed by the author. Cuk-based universal converters extend the principles of the operation a dc-dc Cuk converter to systems with single-phase or multi-phase ac sources and/or loads. These power converters can provide galvanic isolation by adding a single-phase high frequency transformer to the link. This eliminates the need for low frequency transformers that have large volume and weight. Being single-stage, the Cuk-based universal converters do not require decoupling components, and a small capacitor is used for transferring the power from input towards output. This eliminates the need for large electrolytic capacitors that have high failure rates. In prior works, a closed-loop control system was used for the Cuk-based universal converters. These converters were controlled such that they always operate at the boundary of continuous and discontinuous conduction modes, which leads to a variable switching frequency and a large voltage ripple across the link capacitor. The closed-loop operation along with variable switching frequency result in a complex control algorithm, which can limit the maximum switching frequency of the converter. This paper proposes an open-loop control scheme that allows the converter operate in discontinuous conduction mode and with a fixed switching frequency. Implementation of the proposed control scheme is simpler than the closed-loop control method. This paper presents details of the proposed method and evaluates its effectiveness.

Power production by conversion of salt gradient energy (osmotic power production) has potential for global commercialization. Research on the net-power output and plant configuration provide viable methods for efficient plant operation. In this paper, a novel equivalent electric-circuit model of the pressure retarded osmosis process (PRO) is described and, is used to develop a power hardware-in-the-loop (PHIL) emulator to represent the osmotic power plant which includes impulse turbine and synchronous generator for supplying power to off-grid or isolated loads. The response of the hydraulic, mechanical and electrical components of the system is observed given changes in the source flow-rate and electrical loading. The proposed PHIL emulator provides insight into the operational dynamics and behavior of the PRO system. The proposed real-time emulator is a powerful tool which can advance research and development of pressure retarded osmotic power generation system. Simulation and experimental results are presented in the paper to validate the operation of the proposed PHIL PRO emulator.

The paper presents a low-loss auxiliary circuit in a power factor correction (PFC) circuit to achieve zero voltage transition and hence improving the efficiency and operating frequency. The high dynamic energy generated in the switching node during turn-on is diverted by providing a parallel path through an auxiliary inductor and a transistor placed across the main inductor. The auxiliary devices operate at zero current switching and this addition has no effect on the control of the main PFC circuit. The paper discusses the operating principles, design and merits of the proposed scheme with hardware validation on a 3 kW/ 500 kHz PFC prototype.

In this paper, based on a double-sided LCC compensation network, the leakage inductance equivalent model of the loosely coupled transformer (LCT) is built and the constant current (CC) output with load independent is analyzed in an inductive power transfer (IPT) system. This paper proposes four resonance conditions for achieving constant voltage (CV) output in various load conditions for the compensation network. The Zero Voltage Switching (ZVS) operation for the primary side H-bridge converter can be achieved in CC mode and CV mode to minimize the switching loss. A seamless transfer control strategy is also investigated to realize transmission from CC mode to CV mode for IPT EV (electric vehicle) charging system. An IPT EV system with 3.3 kW transmission power is built. The theoretical analyses are confirmed by simulation and experiment results.

This paper proposes a novel dc-dc buck converter that is composed by an active switched capacitor cell and a conventional buck converter. The proposed converter is named as hybrid buck converter and it presents static gain from 0 to 0.5, maintaining the typical linear characteristics of the buck converter and it reduces the voltage stress on the power components. A theoretical steady state analysis to obtain the steady values of all capacitor voltages and the inductor current are presented. A generalization of the switched capacitor cell is also proposed to show the extensive applicability of this structure to dc-dc high step-down voltage ranges. Simulation and experimental results are reported herein to corroborate the proposed structure and the theoretical analysis. An efficiency result is also reported in this paper for the rated power. The prototype specification designed was: 1 kW, 600 V in input voltage and 150 V in output voltage.

With increasing demand for higher efficiency, engineers are looking for an alternative solution to replace induction motors (IMs) in many applications. One of the very promising topologies is the permanent magnet (PM) motor with a squirrel cage on the rotor, which is referred to as the cage rotor PM motor in this paper. Based on the literature survey, the cage rotor PM motors have been studied and compared to IMs for direct on-line applications. Some studies also investigated the performance of line start PM motors (LSPMs) using a conventional VSD by comparing it to an IM using the same VSD. The comparative evaluation shows that, in most operating conditions, the LSPM drive provides better performance (usually higher efficiency) than the IM drive. In this paper, a d-q dynamic analytical model of the cage rotor PM motor is derived. The model is intended to be used in place of finite element analysis (FEA) and for purposes of control development. The model is implemented in Matlab/Simulink and used to study the behavior of different types of the cage rotor PM motor; line-start PM and light cage PM motors, when fed by a standard, scalar controlled drive. The analytical results are verified against the FEA, which shows excellent agreement.

This paper proposes a method to suppress the resonant current circulating between integrated half-bridge modules with phase-leg capacitors in two legs. This circuit configuration has the advantage of low stray inductance between power semiconductor devices and dc-link capacitors (phase-leg capacitors). This paper presents an analysis of the equivalent circuit for the circuit configuration that suggests that the dc-link capacitor current will increase depending on the relationship between resonant and switching frequencies. The capacitor current was experimentally evaluated under pulse-width modulation operation at switching frequencies up to 100 kHz using SiC- mosfet s. As a result, the resonant current may occur in the dc side when the resonant frequency between phase legs was close to either the fundamental or the third harmonic of the switching frequency. In order to suppress the circulating resonant current, a graphite bus bar is applied to the phase-connecting bus bar. Using the graphite bus bar, the capacitor current can be reduced by 35% without any influence from the ac-side output current.

This paper presents the results of experimental tests designed to verify analytical predictions of the rotor demagnetization characteristics of a 0.6 kW (cont.) 9-slot/6-pole fractional-slot concentrated winding (FSCW) interior permanent magnet (IPM) synchronous machine. The demagnetization characteristics of the rotor magnets in this commercially produced FSCW-IPM machine are measured using a test configuration that is designed to conduct multiple demagnetization tests on the same test machine under controlled temperature conditions. In this paper, finite-element (FE) predictions of the rotor demagnetization characteristics of the experimental machine during three-phase symmetrical short-circuit and single-phase asymmetrical short-circuit faults are presented. These results are compared with experimental test measurements of the postfault currents and the magnet flux density distribution following demagnetization, demonstrating very good agreement of many key features. These comparisons also confirm that 3-D effects and magnet material properties such as the magnet thermal coefficients have a significant impact on some details of the FE predictions of the machine's fault-mode response characteristics.

It is expected that wide-bandgap devices like silicon carbide MOSFETs and gallium nitride HEMTs could replace Si devices in power electronics converters to reach higher system efficiency, e.g., a 3-phase 380VAC bidirectional battery charger for electric vehicles. This paper uses the conventional half-bridge LLC topology to build a 10kW all-SiC bidirectional charger. As a well-known topology for the unidirectional charger, it has not been comprehensively explored for the usage of the bidirectional energy flow, which falls into the scope of this paper. A double-pulse-test platform is utilized to provide the accurate power losses, which, combined with the state-space model deriving the accurate switching current waveforms eventually accurately, estimates the system efficiency. Based on this model, to further enhance the system efficiency the DC-bus voltage is varied while keeping the LLC DC/DC converter running at the resonant frequency through the whole power range. Experimental results validated our proposed approach that such topology could realize the bidirectional power flow with zero-voltage-switching turn on. With varying the DC-bus voltage, the V2G and G2V modes reach ∼96% wall-to-battery efficiency.

To support the integration of more PV sources with utility grids, there is a need for low cost and efficient power electronics converters. In this paper, the structure of cross-switch multilevel inverter (CRM) is proposed to interface more than one PV modules with a resonant converter. CRMs treat each pair of PV modules as one unit during the positive and negative half cycles and this could go against the need to track the maximum power point of each PV module individually. In this paper, a control method is proposed to achieve MPPT of individual modules. Basically, the paring of PV modules in the positive half cycle is made different from that in the negative half cycle which allows the MPPT of each module to be achieved effectively. Moreover, the MPPT of the modules is achieved through distributed controller to enhance the system scalability. Through simulation studies, the effectiveness of the proposed system is verified and its advantages are clearly demonstrated.

This paper adopts a circuit model with temperature-dependent parameter values to investigate the current unbalance risk in parallel-connected lithium-ion (Li-ion) battery cells and uses this circuit model to calculate heat dissipation. The paper first investigates use of the battery model to explain the impact of temperature on current unbalance in parallel-connected Li-ion battery cells. Experimental results confirm the model's accuracy, as well as demonstrating the serious effect that the negative temperature coefficient of Li-ion cells can have on current unbalance, particularly at low temperatures below 0°C. The second half of the paper focuses on the use of a calorimeter to collect battery heat dissipation data that is then used to confirm the battery model parameters derived from Electrochemical Impedance Spectroscopy (EIS). The effectiveness of the battery circuit model for estimating the pack heat dissipation is experimentally verified.

This paper presents an active compensation device for common-mode (CM) voltage elimination in 3-phase space-vector pulse-width-modulated (SVPWM) inverters. The proposed device consists of a single-phase 2-level inverter (H-bridge) which supplies a compensating voltage to the inverter via a step-up common-mode transformer tied to all three phases at the output. The H-bridge active filter is supplied by a low voltage bus and switched several orders of magnitude faster than the inverter switching frequency. This device takes advantage of the direct knowledge of the switching pulses sent to the inverter to predict and generate the compensating voltage. A technique is employed to subtract the low frequency harmonics from the modulation of the H-bridge which allows for the size of the common-mode transformer to be reduced significantly. Small passive components are added to attenuate the active filter's PWM frequency content and thus produce an effective compensating voltage. This paper will review existing common-mode voltage compensation techniques and demonstrate that the proposed method is a logical choice for certain drive applications. Design considerations are included to provide understanding and guidance for implementation of the device, as well as MATLAB/Simulink simulation results to demonstrate the operation of the active compensation device. Final validation is presented through experimental results from a hardware prototype.

Increased load demand and penetration levels of renewable energies are imposing great challenges in maintaining the voltage profile of the distribution feeders at high quality. Various solutions such as volt/VAR control and tap changers are employed in the system to regulate the secondary voltage within a typical band of ± 5%. This paper presents a novel device for distribution grid control. The proposed device, called flexible transformer, consists of a fractionally-rated converter and a slightly modified medium voltage (MV) distribution transformer with additional taps. The converter can be wired to either the primary or the secondary side of the transformer, resulting two different configurations. The proposed device successfully meets the requirements of great performance, long lifetime, high efficiency, and low cost for typical utility assets. This paper presents the concept of the flexible transformer as well as the schematic and operating principle of both configurations. Simulation and experimental results of the proposed device rated at 7.2 kV / 50 kVA is also provided.

Doubly fed induction generators are presently used in the large majority of high power wind turbine applications. The current doubly-fed induction generator technology specifies that the turbine operates sub-synchronously to follow the desired power vs. wind speed profile and to operate super-synchronously only during brief periods where wind gusts exceed the nominal maximum value of thrust. This paper suggests that improved operation is possible if the generator is confined to always operate in the super-synchronous state resulting in improved efficiency and greater output power, and the cost effective power converter arrangement and control scheme for the super-synchronous operation are proposed and investigated in this paper.

Stator DC winding Excited Vernier Reluctance Machines (DC-VRMs) are a kind of novel machines, which are investigated more and more owing to absence of expensive permanent magnet material. However, DC-VRMs are found to have large torque ripple if they are not carefully designed. There are a lot of existing methods to reduce the cogging torque in permanent magnet (PM) machines, but the design techniques for DC-VRMs have only been mentioned recently in few papers. In this paper, some new methods are proposed including shifted rotor tooth, stepped rotor skewing with multiple modules, T-shape rotor tooth, various rotor tooth arc, rotor tooth-chamfering, different length of air gap, and these methods are based on the techniques for PM machines to reduce torque ripple. The influence of the proposed methods on torque ripple is examined by using Finite Element Analysis (FEA) and it is found that the torque ripple can be reduced significantly.

This paper presents a retrospective of electric motor developments in General Motors (GM) for electric vehicle (EV), hybrid electric vehicle (HEV), plug-in hybrid electric vehicle (PHEV), and fuel cell electric vehicle (FCEV) production programs. This paper includes i) the progression of electric motor stator and rotor design methodologies that gradually improved motor torque, power, and efficiency performance while mitigating for noise, ii) Heavy rare earth (HRE) mitigation in subsequent design to lower cost and supply uncertainty, iii) Design techniques to lower torque ripple and radial force to mitigate noise and vibration issues. These techniques are elaborated in details with design examples, simulation and test data.

This paper develops the N-paralleled converters with LCL filter system model and expands the virtual resistor control method for harmonic current control. Power electronic converter can be controlled as a current source. For some application the current source can be used to test the inductor characteristic, such as temperature rise, noise or vibration. While in some cases, several converters are connected in parallel to increase the output current amplitude. The N-paralleled converters with LCL output filter system presents complex resonances feature. And the output current of the system becomes out of control. This paper develops the closed-loop model of N-paralleled converters with the LCL filter system and reveals the output current amplification by analyzing the resonance feature of the system at first. Then the virtual resistor based current control method is expanded in the harmonic current control. For the application of N-paralleled converters with an LCL filter system to test the inductor characteristic, the amplitude of output harmonic current need be considered carefully. With the optimal virtual resistor value the output harmonic current can also be controlled well. Simulation and experimental results prove the validation of the system model and optimization for damping resistor value.

This paper proposes a double-sided LC compensated capacitive power transfer (CPT) system, which is the dual of the conventional series-series (SS) compensated inductive power transfer (IPT) system. Four metal plates are arranged as the capacitive coupler, and there are coupling capacitances between each pair of plates. At each side, a compensation capacitor is connected in parallel with the plates, and its value is usually much larger than the coupling capacitance between the plates, which results in a loosely coupled CPT system. In this paper, the fundamental harmonics approximation (FHA) method is used to analyze the circuit working principle. It shows that the system can work at constant current mode. An expression for the system output power is expressed, which is similar to that of the SS compensated IPT system. A 150W input power CPT system is therefore designed as an example to validate the compensation topology. The experimental results show that the dc to dc efficiency of the system is 66.67% when the air-gap distance is 180 mm and the switching frequency is 1.5 MHz.

With the increasing penetration of nonlinear loads into distribution system, stable operation of power distribution system suffers challenge by harmonic voltage propagation and resonance amplification which is also known as whack-a-mole phenomenon. However, until now this phenomenon has not been well investigated and discussed. This paper starts from theoretical analysis of harmonic propagation and how it is triggered when DG unit interfaced to the grid. Moreover, harmonic damping performance of various types of impedance seen from DG at selected frequencies are analyzed and compared by introducing microwave transmission line theory. In addition, this paper proposes a control algorithm with DG unit where virtual impedances at selected frequencies are individually designed to mitigate the harmonic amplification. The validity of the control strategy has been verified by the case study results.

This paper focuses on the analysis and design of a triple-rotor axial-flux spoke-array vernier permanent magnet (TR-AFSAVPM) machine. By cooperating with spoke-array rotor and coil-wounded winding, power factor and torque density of the proposed machine are much improved and its copper utilization is reduced comparing with conventional vernier machines. First, the machine structure and its operation principles are reviewed. After that, analytical equations of its back-electromotive force and torque are derived to reveal the machine characteristics. Based on both quasi-3-D finite element analysis (FEA) and 3-D FEA, its high torque density performance is verified and a set of optimized machine sizing specifications is ultimately settled. A fractional slot axial-flux permanent magnet (VPM) machine and an axial-flux surface VPM machine are designed to compare with the proposed configuration. Analysis results show that the TR-AFSAVPM machine has high power factor viz., 0.96 and high torque density viz., 24.2 kNm/m3. A prototype has been designed and tested to validate the results. No-load test result is illustrated in this paper and the rated load experiment will be added in the future.

This paper introduces and generalizes a class of multilevel dynamic voltage restorer (DVR) for voltage sags/swells compensation of high-power sensitive loads. Such a device can improve the power quality of sensitive loads located in stiff systems. The proposed DVR is based on three-phase bridge converters series-connected by means of cascaded transformers using the concept of open-end winding (OEW). Hence, two dc-links can provide either symmetrical (i.e., equal dc-link voltages) or asymmetrical (i.e., different dc-link voltages) operation of the DVR converters. A generalization for $K$ -stages is presented as well. The proposed configuration is named DVR-cascaded open-end winding (COEW). Such a topology permits a generation of a maximized number of voltage levels per converter leg. An alternative solution with multiple dc-links (i.e., multiple dc sources), named DVR-MSCOEW, is briefly introduced. The multilevel waveforms at the output voltages of the converter are generated by using a suitable pulse-width modulation (PWM) strategy associated with both dc-link voltage and transformers turns ratios. In the scenario, where common mode currents (CMC) for DVR-COEW are introduced, and if such currents are significant, a modified PWM strategy is described to compensate for CMC issues. Since it uses two-level cells, the modularity feature makes the proposed DVR-COEW an attractive solution compared with some conventional configurations. Modeling and PWM control are addressed in this paper. Simulation and experimental results are presented.

This paper presents a neural network (NN) based wind estimator and related Maximum Power Point Tracking (MPPT) technique for high performance wind generator with induction machine. The target is to develop an MPPT system, embedding an adaptive virtual anemometer which is able to correctly estimate the wind speed even in presence of variations of the wind turbine characteristic, caused by aging or even damages. This paper proposes the use of the adaptive properties of feed-forward neural networks to address the on-line estimation of the wind speed even in case of slowly time-varying wind-turbine parameters. The method is inspired to the inverse adaptive control but it is used for parameter estimation and not for control purposes. Once the wind speed is estimated, the machine reference speed is then computed by the optimal tip speed ratio. For the experimental application, a suitably developed test setup has been used, with a back-to-back configuration with two voltage source converters, one on the machine side and the other on the grid side.

A number of appliance manufacturers have recently replaced the motor stator winding material from copper (Cu) to aluminum (At) to reduce the product cost in the competitive global appliance market. Although reduction in the motor cost could be achieved, a noticeable increase in the failure rate due to stator insulation breakdown was observed. In this paper, increased failures in At winding stator insulation is investigated for 230 V, 50 W single phase induction fan motors used in the outdoor unit of air conditioners (A/C). Since the motors with defects or failures could not be screened out with the quality assurance tests (QA) that were applied to Cu winding motors, new QA test methods are evaluated for screening out the units with high risk of failure. The investigation shows that partial discharge (PD) measurement under surge excitation can serve as an effective means of screening out the units that are likely to fail. The claims made in the paper are verified through comparative experimental surge PD testing on At and Cu winding motors before and after accelerated degradation.

This paper analyzes a cost-effective modification of the rectifier plus boost converter power topology which is commonly found in small wind turbine systems. The paper proposes removing the input capacitor after the rectifier and the boost converter inductance. This will be done without adding any LC filter between the permanent magnet generator and the passive rectifier. This paper discusses both the power topology and the control strategy as well as a performance comparison with the usual topology. Simulation and experimental results are presented to demonstrate the technical viability of this proposal.

This paper proposes a novel coupler structure for wireless power transfer, which takes advantage of both magnetic and electric fields. The coupler contains four metal structures, two each at the primary and secondary sides, which are capacitively coupled. Each structure consists of long strips of metal sheet to increase its self-inductance, which is then inductively coupled with the other three structures. The structures are vertically arranged, and the outer structures are larger than the inner ones to maintain the capacitive couplings. An external LCL compensation network is proposed to resonate with the coupler. The resonance provides conduction current flowing through each plate to establish magnetic fields, and displacement current flowing between different plates corresponding to electric fields. A 100W output power prototype is designed and implemented to operate at 1 MHz, and it achieves 73.6% efficiency from dc source to dc load at an air-gap distance of 18 mm. The contribution of this paper is to propose a concept to transfer power using magnetic and electric fields simultaneously. The potential application of this system is the charging of low power portable devices, such as laptops. After the coupler structure is optimized, it can be extended to higher power applications, such as vehicle charging.

This paper proposes an LC-compensated electric field repeater to extend the transfer distance of a capacitive power transfer (CPT) system. The repeater contains two metal plates, connected with an external capacitor and an external inductor. The plates are used to generate electric fields to transfer power. The external inductor and capacitor are used to resonate with the plates to increase the voltage levels. The repeater is placed between a transmitter and a receiver, which also contains metal plates compensated by an LC network. The repeater can increase the transfer distance of the CPT system without significantly influencing the system power and efficiency. In this paper, the capacitive coupler structure and dimensions are designed and simulated using Maxwell software. Considering all the capacitive coupling between plates, an equivalent circuit model is derived. The fundamental harmonics approximation method is used to analyze the working principle of the circuit. A 150W input power CPT system is designed as an example to validate the proposed repeater structure and compensation circuit topology. The system can achieve an efficiency of 66.9% from dc source to dc load, when the transfer distance is 360 mm and the repeater is placed between the transmitter and receiver.

Electric drive system for the Cadillac CT6 plug-in hybrid electric vehicle (PHEV) comprises a power-split hybrid transmission utilizing two motors, a traction power inverter module (TPIM) having two inverters and a liquid-cooled lithiumion battery pack [1, 2]. The resulting propulsion system provides a combination of exceptional fuel economy, crisp acceleration, and strong electric-only driving range. To meet all these performance, reliability and robustness targets and package the TPIM into a compact under hood environment is a challenge. This paper describes indispensable and collaborative design efforts by Hitachi Automotive and General Motors to meet this challenge. Enhancing heat dissipation performance and using high efficient power semiconductor enables reduced inverter size and increased power output. Elimination of heat impeding thermal grease and developing direct water cooled power module made generational improvements in this area. Hitachi went further by developing a direct double-sided cooling power module to achieve ∼35% improvement in heat dissipation utilizing a proprietary cooling structure that immerses the module in coolant [3]. This paper elaborates on the integration, structure and the selection of power transistor switch for this TPIM to meet Cadillac CT6 PHEV tractive and special maneuver needs. Among other coherent steps are to design low inductance busbar, optimize the gate drive circuit elements, provide protection functions for detecting over-temperature, over-current, over-voltage ranges and thus maximizing the performance from the power devices. Design efforts were also directed to surround the Bulk DC capacitor by the vertical water channels of the power module and maintain life expectancy by not exceeding rated hot spot temperature. This paper describes predictions, component and vehicle level test results to justify design competence.

The distribution system voltage volatility caused by changing load conditions and penetration of renewable resources presents an increasing challenge in the grid integration of renewables. This paper presents a novel voltage regulator, or field upgradeable transformer (FUT), for the distribution system, targeting at direct voltage regulation at the customer side with a fast dynamic response. Proposed system consists of a conventional medium voltage (MV) distribution transformer interfaced with a fractionally-rated power converter. Compared to other power electronics solutions for grid applications, the proposed device has advantages of low cost, high reliability, and high efficiency. The paper presents the FUT topology, principle of operation, control and protection scheme verified through simulation studies. Experimental results at 7.2 kV, 50 kVA, demonstrating FUT technology are also presented.

This paper focuses on the distributed control method of the power module (PM) capacitor voltage balancing in modular multilevel converters (MMCs). The large number of PMs imposes great computational challenge on the capacitor voltage balancing in MMC based high voltage direct current (HVDC) transmission system. In this paper, a distributed voltage balancing method is proposed to reduce the computational load of control strategy of MMCs with hundreds of PMs per arm. With this method, all PMs in one arm of MMC are grouped first. The PMs in each group is balanced independently by voltage sorting and arm current polarity. The sorting all the PM voltages in this arm is avoided. Meanwhile, a close-loop control strategy is also introduced to balance the average PM voltage of each group dynamically. Simulation results on a ±350 kV/ 1000 MW MMC based HVDC system show the effectiveness of the proposed distributed control method.

This paper investigates energy storage configuration strategy for virtual synchronous machine (VSM). The proposed VSM provides virtual inertia and damping to maintain stability of grid. Virtual inertia and damping need to be established by energy storage system (ESS). So that a strategy of energy storage configuration has been investigated through theoretical analysis and simulations in this paper. In addition, this paper also proposed influences of virtual inertia, damping factor and reference of reactive power on energy storage configuration. Simulation results verified the proposed energy storage configuration strategy.

This paper investigated the post-fault operation of a novel six-phase double-stator axial flux permanent magnet machine with detached winding configuration, which was found to be superior to existing winding configuration in previous study. However, the unbalanced magnetic force problem still remains unsolved. In this paper, the axial force balancing control principle is proposed and a group of specific current waveforms are deduced. When applying these currents under post-fault condition, magnetic torque, axial magnetic force and rotor losses of the machine are calculated in finite element analysis. The results are compared with normal condition and commonly-used post-fault current waveforms. It is verified that this method reduced the unbalanced axial magnetic force immensely and the torque ripple was also kept at a low level. In order to achieve the proposed current waveform, finite control set model predictive control (FCS-MPC) is adopted. This paper proposed the post-fault model of dual three-phase permanent magnet machines and designed a cost function to track the desired current waveforms. The model of the machine is used to predict the future behavior of the controlled variables and the cost function decides the next step of the inverter by evaluating all the predictions. At last, it is verified by simulation results that the control strategy performs well in both dynamic and steady-state situations.

This paper describes a theoretical with experiment study on a control strategy for the parallel operation of threephase voltage source inverters (VSI), to be applied touninterruptible power systems (UPS). A circulating current suppression strategy for parallel VSIs is proposed in this paper based on circulating current control loops used to modify the reference currents by compensating the error currents among parallel inverters. Both of the cross and zero-sequence circulating currents are considered. The proposed method is coordinated together with droop and virtual impedance control. In this paper, droop control is used to generate the reference voltage of each inverter, and the virtual impedance is used to fix the output impedance of the inverters. In addition, a secondary control is used in order to recover the voltage deviation caused by the virtual impedance. And the auxiliary current control loop is added to acquire a better average current sharing performance among parallel VSIs, which can effectively suppress both of thecross and zero-sequence circulating currents. Experimental results are presented in order to verify the effectiveness of the proposed control strategy.

A new brushless doubly-fed double stator double rotor (DSDR) generator based on permanent magnet field modulation for the wind power application is proposed in this paper. This generator adopted double stator to achieve the decoupling of the control winding and power winding. To improve the power density, a freely rotating rotor is introduced. To achieve the variable-speed constant-frequency operation and cancel the brush and slip ring, rotating field modulation ring is adopted to link with the wind turbine directly. In this paper, the operation principle of DSDR generator is detailed described. To verify the correctness of the principle, the performance of the DSDR generator is analyzed by the means of finite element analysis (FEA). The proposed DSDR generator owns the advantages such as low cost, low maintenance, high power density, high reliability, low speed direct driven, and good performance on grid fault ride-through. It has the potential on application of future large-scale wind power generation systems.

Due to the realization of zero voltage switching (ZVS) under the full load range, LLC resonant converter is widely adopted in the telecom, battery charger and several applications, characterized with high efficiency, high frequency and high power density, to realize DC conversion. Recently, by using Gallium Nitride (GaN) HFETS, switching frequency of LLC converters is further increased. However, ZVS failure cannot be predicted accurately in the high switching frequency condition by only considering traditional constraints generally applied in the low frequency design. The traditional constraints result in a too optimistic estimation of the dead time to obtain ZVS without considering the reverse resonance under the dead time and the design of resonant parameters at high resonant frequency and high load condition. The experiment shows the LLC converter loses ZVS even through the converter satisfies the ZVS constraints proposed by previous paper. In this paper, the failure mode will be investigated in detail and an accurate ZVS boundary is proposed for high frequency LLC converter design. The proposed theory was verified on a 1 MHz, 1500 W LLC prototype.

In this paper, it is described how the circulating current flows between inverters which have isolated DC-link such as Cascaded H Bridge (CHB) topology, when they operating in parallel. In this case, Zero Sequence Circulating Current (ZSCC) flowing through shared DC-link cannot flow between inverters in parallel. However, circulating current is provoked by asynchronous switching instant of devices and difference of DC-link voltages of parallel inverters. The circulating current should be suppressed by sharing inductor which is inserted between inverters. Generally, all of the inverters should synthesize the same output voltage reference for load current control, to minimize the size of this sharing reactor. However, this conventional method cannot guarantee to diminish circulating current in the transient state and even in the steady state. In this paper, to reduce circulating current, after deriving circulating current model, circulating current control method is devised based on the model. This proposed algorithm is applied to Active Front End 5Level-CHB inverter system for medium voltage drive. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.

This paper discusses and compares phase-shifted pulsewidth modulation (PS-PWM) and level-shifted PWM (LS-PWM) in a modular multilevel single-delta bridge-cells (SDBC) inverter for utility-scale grid-tied photovoltaic (PV) applications. A unique characteristic of this inverter application is the capability to operate even under imbalanced power distribution among its bridge cells. An imbalanced energy distribution may degrade harmonic performance and limit the inverter operating range. The focus of this research lies in analyzing the following two fundamental properties; waveform quality and energy balancing capability when either PS-PWM or LS-PWM is applied. This paper highlights the benefits of LS-PWM over PS-PWM in this particular application, and demonstrates that the SDBC inverter is suitable for applications with imbalanced power distribution. Experimental results obtained from a three-phase 10-kW downscaled SDBC inverter with six bridge cells per cluster show good agreement with those from theoretical analysis.

This paper proposes a high step-up interleaved converter with coupled inductor and voltage-lift technique. The proposed converter achieves high voltage gain by employing coupled inductor and voltage-lift technique. Moreover, low input current ripple is realized by using interleaved technique. The energy in the leakage inductance of the coupled inductor is recycled to the output load. The voltage stresses on the main switches are reduced and the voltage spikes across the main switches are alleviated. This paper shows a theoretical analysis in steady-state and experimental results with a small-scale prototype circuit.

Back-EMF self-sensing is commonly used in induction machine (IM) drive systems for its maintenance cost and safety considerations. In recent years, deadbeat-direct torque and flux control (DB-DTFC) has been shown to be a highly effective method for induction machine control. It has advantages such as the fastest possible torque dynamics, dynamically loss manipulation capability independent of torque dynamics, and parameter insensitivity. This paper evaluates the synergy between back-EMF self-sensing technology and DB-DTFC for IM drive systems. In this paper, an observer-based closed-loop back-EMF tracking self-sensing control in an IM DB-DTFC drive system is presented. It includes a back-EMF state filter, back-EMF tracking observer, and cascaded motion observer. Back-EMF harmonic decoupling is explored to improve the low speed performance. The bandpass filter method (BPF) for back-EMF self-sensing is also presented. Finally, the closed loop system dynamic stiffness at very low speeds, with and without the BPF method, is experimentally evaluated.

To date, a family of series-connected boost converter (SCBC) with enhanced efficiency has been presented as the front-end dc/dc stage of distributed photovoltaic (PV) inverter, which suffers the severe PV voltage/power oscillation when operated under unbalanced grid voltage condition. This paper proposes an operational scheme to enhance the performance of one type SCBC, namely the dual-input central capacitor (DICC) converter, under unbalanced grid voltage condition. The PV voltage/power oscillation is eliminated using the propose scheme, resulting in the maximum PV energy harvest and enhanced grid current quality even under severe unbalanced condition. Also the overall dc-link voltage oscillation amplitude is minimized. The analytical representation of double line-frequency current along with the DICC circuitry model are studied, so that the DICC circuitry parameters could be designed accordingly. This paper further proposes the control strategy of DICC-PV inverter under unbalanced grid voltage condition. All the theoretical findings and the control method proposed can be derived to other type SCBCs. Finally, the theoretical findings were verified through both Matlab simulation and an experimental prototype.

As renewable energy resources such as wind become more prolific, the challenges in utilizing these resources to their full efficiency come to the forefront of power system engineers. Although the sustainability of such alternative energy resources is attractive, the ability to obtain satisfactory efficiency has been difficult until recent advances in nonlinear control technologies. The subject of this paper pertains to sliding mode control and its application in nonlinear electrical power systems as seen in wind energy conversion systems. Due to the robustness in dealing with unmodeled system dynamics, sliding mode control has been widely used in electrical power system applications. This paper presents first and high order sliding mode control schemes for permanent magnet synchronous generator-based wind energy conversion systems. Application of these methods for control using dynamic models of the d-axis and q-axis currents, as well as those of the high speed shaft rotational speed show a high level of efficiency in power extraction from a varying wind resource. Computer simulation results have shown the efficacy of the proposed sliding mode control approaches.

This paper focuses on a Modular Multilevel Converter grid interface for a klystron modulator system that behaves as a pulsed DC load. With such a load, and without mitigating control, the MMC suffers from cell capacitor voltage imbalance between the converter arms which leads to distortion (fluctuation) of the absorbed AC power. This paper proposes an augmented modulation strategy, with a tailored distribution of the modulation signals between the arms within a phase, to ensure low AC power fluctuation. The effectiveness of the method has been verified through simulation and has been experimentally proven on a 7 kW MMC prototype operating with a 3 kA pulsed DC load.

The purpose of this paper is to analyze and investigate the influence of temperature variation on the characteristics and performance of interior permanent magnet (IPM) machines. The impact of temperature variation on the materials of IPM machines is discussed to show the sources of performance variation. The flux linkages and torque output capability variation as functions of the temperature of are analyzed and discussed. The paper also shows the influence of temperature variation on key IPM machines performance including constant torque curves, voltage limit ellipses, maximum torque per ampere and maximum torque per volt trajectories and torque-speed curves. The results and trends shown in this paper set a foundation for developing control algorithm which takes the temperature effects into consideration, especially in the applications where operating temperature varies significantly.

This paper presents a physics based comprehensive dynamic model of a future power distribution system, termed as the FREEDM system, for plug-and-play interface of distributed renewable energy resources and distributed energy storage devices. The system allows for high penetration of renewable generation with energy storage at the distribution level. FREEDM system consists of an energy router, which is the power electronics based solid-state transformer (SST) that interfaces distributed generation, storage and local loads on the low voltage side with the medium voltage node of the distribution grid. In this paper, state-space modeling and dynamic performance of the SST is analyzed along with the renewable generation sources and storage components with the goal of studying the feasible operating points of the FREEDM system. The actual model of the single-SST system amounts to highly complex dynamics with more than hundred state variables. Singular perturbation based model reduction techniques are applied, thereby leading to a 70th order state-space average model suitable for AC and DC energy cell system sizing, stability analysis, and controller design. The analysis with the system model revealed the SST input stage system parameters have the dominant effect on the feasible operation region.

In this paper, an alternative layout for small scale back-to-back variable speed wind turbine systems is proposed. This novel layout is created by splitting the converters at their common DC bus and linking them through the wind turbine tower with a DC rated cable. The grid-tie voltage source converter (VSC) is placed at ground level and the generator VSC is placed in the nacelle. Not only does the DC transmission cable allow electrical power to be transferred with minimal copper losses, it allows the nacelle to be more compact and maintenance friendly. The control of a reluctance synchronous generator (RSG), variable speed, fixed pitch wind turbine and grid-connected system is detailed in this paper with specific attention to the control of the DC bus voltage with a DC-link cable. DC bus voltage dynamics are investigated with practical measurements on a 3 kW wind turbine emulator, RSG and LCL-filter grid-tie system. Power input steps at rated power are used to perturb the DC bus system for two different cable lengths. Two minutes of emulated wind speed using a 150 m cable length is also tested. The results show that the DC-link dynamics are controllable.

This paper presents electromagnetic analysis of two permanent magnet transverse flux outer rotor machines with and without magnetic shunts. The research started with designing and analysing a permanent magnet transverse flux machine with inner rotor, previously patented by J. Giearas in 2010. However, the results obtained from the prototype test differed significantly from the estimated results. Applying 3D Finite Element Method tool main problem of the machine was detected. The problem was in high flux leakage which weakened magnetic flux density in the stator poles. Such observation led to designing two machines with outer rotor; with and without magnetic shunts. The role of the magnetic shunts is to minimize flux leakage in the stator in order to improve performance of the machine. Electromechanical parameters of both outer rotor machines, with and without magnetic shunts, are compared in the paper. 3D Finite Element Method results are supported by laboratory analysis of the machines' prototypes. Experimental measurements proved that the machine with magnetic shunts demonstrated better performance comparing to the machine without magnetic shunts. Presence of the magnetic shunts in the machine resulted in reducing of flux leakage and cogging torque.

This paper deals with the analysis of average torque and torque ripple in synchronous reluctance machines assisted by Ferrite permanent magnets, for applications requiring high torque at low speed. The machines distinguish themselves by an anisotropic rotor and a high number of poles. As a peculiar feature, the analysed machines are characterized by fractional-slot non-overlapped coil windings. When anisotropic rotors are employed in stators with fractional-slot winding, there is a worsening of the torque quality: the average torque decreases and the torque ripple increases with respect to the distributed winding machines. This paper aim is to analyse the nature of the torque generation in such machines, that is, to investigate the magnetic phenomenon causing the average torque decrease and the torque ripple increase.

This paper proposed a modified pulse energy modulation (PEM) technique for a single-phase single-stage three-switch buck-boost inverter, solving the issue existed in practical applications that the requirement for sampling circuits is too high to achieve. The three-switch buck-boost inverter was investigated for both standalone and grid-connected operations. Simulation and experimental results verified that the newly proposed PEM technique enables the buck-boost inverter to transfer the demanded energy from a DC source to AC side and achieve a sinusoidal output current with low total harmonic distortion (THD). Therefore, it can be concluded that the proposed modified PEM technique is suited for buck-boost inverters in distributed generation systems, with lower requirement for the sampling circuits. The new contributions presented in this paper include advancing the PEM concept as an alternative to PWM for inverter modulations; and applying the proposed PEM technique to a low component-count inverter in standalone and grid-connected applications.

Induction motor rotor designs are investigated in this paper to reduce secondary winding harmonic losses in hybrid electric vehicle (HEV) applications. The induction motor is more and more popular to use for the traction drive, but its' secondary winding harmonic losses are significantly higher than the usual when having high frequency and high power density in the HEV application. Thus, the major cause of the high secondary winding losses is analyzed and evaluated in the paper. The losses of rotor bars due to harmonic fields are calculated and analyzed for a typical eight pole induction motor by using the finite element method (FEM). Several rotor structures are proposed to decrease the harmonic losses and improve efficiency. Finally, the rotor design methods of the induction motor are compared to achieve a high efficiency and high power density based on the reduced harmonic losses, increased efficiency and output torque. The results show the secondary winding losses of the induction motor can be reduced by up to 43% when using the new rotor structure design.