Since MAGLEV train has many advantages,many countries are busy in its study. The schemes of Germany,Japan are mature by far, and there are several typical schemes in America and some other countries. It is worthful to study these typical schemes.We can use these schemes for reference in development of ourMAGLEV. The article presents the three schemes of the high speedMAGLEV trains ofGermany, Japan and America, and the detailed comparison of the three schemes is given.We can see that the scheme of Germany is the mature one.

High magnetic field plays an extremely important role in developing science and technology, gestates many significant scientific discoveries and emergences of new technologies. The generation and application of extremely high magnetic field are of great significance to extreme condition scientific facilities, biomedical engineering, and special equipment for national defense, high-precision scientific instruments and agricultural applications. Currently, many research institutes and laboratories carry out high-field superconducting magnets based on high temperature superconductors. Focusing on several research institutes and laboratories in the field of high-field superconducting magnet, this paper mainly presents their researches on high-field superconducting magnet technologies, including design schemes, technical features and latest research progress, and briefly introduces the main applications of high-field superconducting magnets.

Supercapacitor is a new type of energy storage device developed in recent decades, with the characteristics of high power density, long life and good low-temperature performance, and has been widely applied in electric power, transportation, industry and other fields. This paper introduces the types and basic principles of supercapacitors, as well as the development and market status of supercapacitor industry in recent years. Then, the main application fields and current situation of supercapacitor in China are reviewed. In the end, the relative suggestions focusing on the main defect of supercapacitor, low energy density, is proposed.

This paper reviews the research status of energy storage technology for the new energy power system which contains wind power. Firstly, the great significance of energy storage technology for power system containing a high proportion of wind power is discussed. Secondly, the present research progress of various energy storage technologies are systematically compared. On one hand, the paper described the theory of various energy storage technologies and analyzed the characteristics of energy storage technologies. On the other hand, it also summarized the advantages, problem and application range of various energy storage technologies. Thirdly, it adopted a new method that coordinates a variety of energy storage technologies for new energy power system and combines energy storage technology with large capacity and quick response. Lastly, it pointed out that optimal allocation of energy storage in the new energy power system has become a subject which needs further research.

Aiming at the methaneair mixtures at room temperature and one bar pressure, the breakdown tests in 500μm～5mm range of clearance have been performed to study the effect of gas dielectric on the breakdown of short circuit discharge with the IEC safety spark test apparatus (IECSSTA). The experimental results show that the relationship between the breakdown voltage and gap distance fits to the typical gasbreakdown chart with changing the methaneair mixtures. For 85% methaneair mixtures, the lowest voltage is about 300v which is far larger than the operation voltage of the test circuits with IECSSTA. It is also shown that the discharge process does not belong to the category of gas discharge, and the effect of gas dielectric on the breakdown of the gap can be ignored. It is pointed out that the discharge process can be categorized into vacuum discharge rather than gas discharge, and the dominant breakdown mechanism of the discharge is the cathode electron emission. Short circuit discharge experiments of capacitive circuit in different media environment with the IECSSTA are performed, The experiment results are in good agreement with the analysis showing the feasibility of the proposed methods.

With the development of electric vehicles, lithium-ion batteries have been widely used. And the safety of lithium-ion batteries is the basis of the development of electric vehicles. A comprehensive review of the research on thermal runaway of lithium-ion batteries is conducted in the paper. Firstly, this paper outlines the thermal runaway of the battery under different kinds of causes. The internal short circuit and overcharge of the battery are the two major causes of thermal runaway. Meanwhile, this paper focuses on the entire thermal runaway process of the battery, including the thermal runaway mechanism of the single-cell and the propagation of thermal runaway within the battery. On this basis, this paper summarizes the way to improve the safety of lithium-ion batteries from three aspects. They are improving the temperature characteristics of battery materials, enhancing the heat dissipation capacity of the battery cooling system, and optimizing online detection, diagnosis, and prediction of battery thermal management systems. Finally, this paper summarizes the shortcomings of existing battery safety research and future research directions, aiming to provide a reference for future battery safety research.

The process of big data for power grid requires the support of big data technology. The topics such as how to store the big data for power grid and how to mining out valuable information to promote the development of the grid are very hot currently. In this paper, both data storage and data retrieval function of the graph database Neo4j are introduced in detail firstly. Secondly, a method to build panoramic database based on Neo4j is proposed, which can integrate the scattered and isolated data in power grid with large-scale orderly according to the device mapping table based on power network topology. Furthermore, with the help of graph theory algorithm packaged in Neo4j, clustering methods for analysis of big data in power grid can be put forward. Finally, two real power grids are analyzed based on Neo4j, while the performance of information retrieval and data clustering analysis with Neo4j is tested as well.

Self-resonance coupling is a new technology for wireless power transmission，which can transfer energy in middle distance. Transmitters and receivers are core parts to achieve wireless power transmission. An antenna has a part that amplifies a flux to make the antenna have a larger effective size than its actual size. This paper analyses the conversion efficiency between transmitting and receiving antenna from the point of magnetic field ，getting the relation between the parameters of antennae and efficiency. It is helpful to choose the parameters of antennae with the purpose of achieving optimal result. At last，a wireless power transmission prototype based on magnetic resonance coupling was designed in this paper for verifying the theoretical analysis in experiments. Theoretical and experimental results show that the coil turns and the size of the coil radius of the antennae have an important influence on the efficiency.

The models of wind speed distribution are the important mathematic bases applying to wind resource assessment, planning and building of wind farms, and also for the reliability evaluation of power systems. Firstly, this paper reviews some models of medium-and-long-term wind speed distribution, such as Gamma distribution, Log-normal distribution, Rayleigh distribution, Weibull distribution and Burr distribution. Secondly, on the base of the Weibull distribution, kinds of parameter estimated methods are introduced. Thirdly, the characteristics of various distribution models and estimated algorithms are compared respectively. At the end of this paper the prospects of this research area are put forward.

Abstract: As a novel multilevel topology，the modular multilevel converter (MMC) has been widely studied because of its application in HVDC. In this paper，the commonly used modulation strategies in MMC， carrier stacked PWM，nearest level modulation，and carrier phase shifted PWM are introduced. To balance the capacitor voltage of sub-module，several different capacitor voltage balancing algorithms are also analyzed. The carrier stacked PWM has the highest switch frequency which leads to the best capacitor voltage balancing performance. The nearest level modulation is easy to implement and the switching frequency can be restricted to a low rate. The carrier phase shifted PWM has a fixed switching frequency. To compare the three modulation strategies，a 31-level simulation model of MMC is built in PSCAD/EMTDC. The three modulation strategies and capacitor voltage balancing algorithm are realized in the simulation model. The performance of the capacitor voltage fluctuation，switching frequency，voltage harmonics and other aspect of performance are compared.

Energy storage is important for promoting the renewable energy consumption and improving the grid resilience. Cost of energy storage system is a key factor to determine whether the energy storage technology can be industrialized or not. In this paper, energy storage cost per kilowatthour and per mileage were calculated for capacitytype and powertype energy storage applications individually, based on the data from industrial research and life cycle cost analysis. Results show that the cost per kilowatthour of current electrochemical energy storage is about 0.6～0.9CNY, and there is still a gap from the target cost per kilowatthour of 0.3～0.4CNY for large scale applications. Hence, ‘low cost, long life, high safety and easy recycling’ should be regarded as the development target of energy storage technology. Research on innovative technologies and products should be emphasized on the basis of comprehensive consideration of system manufacturing, system life, safety and recycling. The energy storage cost analysis of this paper will help to build a marketoriented green energy storage technology innovation system.

A comprehensive review for isolated bidirectional DC-DC converter is presented in this paper. The research background of energy storage system and its requirement for high frequency and high power density isolated bidirectional DC-DC converter are introduced. Research progress of analysis method and control technology for CLLC and DAB converter are reviewed. This paper points out that both CLLC and DAB converter meet the requirement of future large scale energy storage system. At present, DAB converter is merging to its mature stage while CLLC converter needs to be further researched. However, CLLC converter has advantages over DAB converter in small and medium capacity applications. It still has a wide application prospects.

For high capacity PWM rectifier，large grid side inductor is required in order to reduce the grid current harmonics around the switching frequency． However，large inductor will increase not only the cost but also the volume of the rectifier． Furthermore，the dynamic response of the system would be depressed． In order to reduce the grid side inductance，LCL-filter can be used to replace the traditional L-filter． In this paper，the principle of LCLfilter is investigated，a design method of parameters of grid side LCL-filter for three-level PWM rectifier is proposed． For a 1kW three-level PWM rectifier，a complete parameter scheme is put forward and verified by Simulink and experiment．

Taking two-stage grid-connected photovoltaic inverter with former stage being Boost and backward stage being full bridge inverter for example, its power losses calculation method was studied．The formula for losses calculation was derived．It shows that the losses of two-stage grid-connected photovoltaic inverter have close relationship with its middle bus voltage．Different middle bus voltage impacts the losses of switch devices，diodes and inductors, and the inverter efficiency ultimately. Based on the losses formula, the specific loss values of each device are calculated in detail. Then a method of three loop control strategy with MPPT (maximum power point tracking) in former stage and MEPT (maximum efficiency point tracking) in backward stage was proposed to search the maximum power point and efficiency point. The experiment results verified the feasibility and validity of proposed loss analysis model and MEPT control strategy.

Based on the loss separation theory, the loss of ferromagnetic materials can be decomposed into hysteresis loss, eddy current loss and excess loss. To calculate the high frequency loss of the silicon steel magnetic rings, we propose a novel method to consider the influence of hysteresis characteristic and skin effect of the ferromagnetic material on its eddy current loss. The method couples the Preisach hysteresis model with finite element equations in time domain, which is based on the vector magnetic potential A, and uses the fixed-point technology which has good simplicity and robustness. The differential reluctivity is introduced to speed up the convergence of the solutions of finite element equations. At the same time, to overcome the difficulty to extract the statistical parameters of excess loss, a rational function is proposed to fit the statistical parameter extracted from the experimental measurements. Finally, the simulation results are compared with the experimental measurements, and the results show that the maximum error of the proposed algorithm is less than 10%.

Base on the idea of sliding mode observer, a new method of designing the sliding mode observer for PMSM control system is proposed. The algorithm for estimating position and speed of PMSM is presented, which is simple in structure and easy to be implemented by using a computer.Sensorlessfield orientation control systemis achieved by using thisobserver. Experimental result showed that the observer has good robustness to load disturbance and time varying parameters.

Compared with silicon devices, SiC-based power devices feature high voltage capability with low switching losses and high operating temperature. The differences of switching characteristics between SiC-based devices and Si-based devices are analyzed. Then, a buck converter test bench is built, and the switching characteristics of SiC-based devices and Si-based devices are tested and compared. Experimental results verify the correctness of the above analysis, and provide design guidelines for the proper selection and application of SiC-based power devices.

The battery management system（BMS）is an important part of the electric vehicle energy management. It provides important parameters of the vehicle control strategy. However, BMS works in an complex electromagnetic environment of the electric vehicle, so improving the anti-electromagnetic interference performance of the BMS is extremely essential to guarantee the safe and reliable operation of the vehicle. Based on hybrid electric vehicle（HEV）platform of Changan Automobile Co., Ltd, the electromagnetic environment of HEV and its coupling mechanism to BMS are researched. By the EMC optimization design the anti-electromagnetic interference performance of the BMS is enhanced. Results show that the optimized BMS can be good adaptation in electric vehicle complex electromagnetic environment.

With the development of IGBT module and its increase of power level and density, system failures caused by junction temperature problem due to power loss are more and more severe. Deep researches on device and heatsink system can contribute to power device package design, device selection, system layout and inverter stable operation. This paper models and extracts the transient thermal impedance through FEM(Finite Element Method), with simulation results in line with manufacturer’s datasheet. It also verifies the thermal impedance curve’s universality and sets up electro-thermal model based on thermal characteristic RC equivalent network, which can be used for dynamic junction temperature prediction.

Power electronic transformer (PET) is a kind of power transmission equipment combined by power electronic converters and high-frequency transformer, which has great application value in the building of Strong Smart Grid. Hot PET topologies applied to distribution system are classified and their implementations are analyzed and compared in this paper. It has been indicated that PET has great advantages in reducing its own size and weight, and power quality regulation, compared with traditional transformer. Especially for the three-stage topologies with DC link, their good controllability can make power electronic transformer achieve more functionality and also provide a channel for the integration of distributed power generation systems, which is the future development direction of PET. Finally, several key issues in practical application of PET are proposed.

As one of the key technologies and bottlenecks for development and application of DC distribution network, the protection technology is still in its infancy, lack of practical experience and standards. This paper first overviews the topology structure, grounding mode, fault type and transient characteristics of the DC distribution network, which are closely related to the protection scheme design. Then, the fault identification technology of DC distribution network is summarized, and the main principles and characteristics of each type of method are analyzed. It is considered that with the development of smart distribution network, the communication-based protection technology has certain advantages. Finally, the existing fault location methods of DC distribution network are classified, and the basic principles of various location methods are illustrated. In addition, this paper summarizes the shortcomings of various fault location methods and gives possible improvements.

In view of the difficult industrial application of repetitive controller to inverter, this paper studies the repetitive controller design and the software implementation of repetitive controller. The notch filter, the low pass filter and phase lead compensator are designed respectively to meet the requirement of the inverter stability and the steady error. In this paper, an inverter repetitive controller system is built based on TMS320F28335 chip. And the software implementation of all parts of repetitive controller is elaborated. A series of experiments about the inverter repetitive control system verify the effectiveness of the proposed strategy.

Since the 21st century，the wireless power transmission technology has entered a rapid development stage，and the related theoretical research，innovative experiments and application promotion have made great progress. With the proposal and construction of global energy interconnection，related technologies will have greater development potential and application value in the future. The paper firstly describes the birth and development of wireless power transmission. Secondly，the key research developments of inductive power transmission，capacitive power transmission，microwave power transmission，laser power transmission and ultrasonic power transmission technologies at home and abroad in the past 20 years are highlighted. Thirdly，the six major fields of electrical power system，transportation，aerospace，underwater detection，smart home and medical devices are summarized and prospected. Fourthly，specific application scenarios are discussed for different wireless power transmission technologies. Finally，the main technical challenges of different wireless power transmission technologies are discussed.

To study the efficient control strategy of Battery Energy Storage System (BESS) participating in the primary frequency regulation of the power grid, based on the analysis of the power system frequency modulation demand and the consideration of the state of charge (SOC) of the energy storage battery, this paper proposes a comprehensive primary frequency modulation control strategy that takes into account the grid frequency modulation demand and the SOC state of the energy storage battery, and the droop coefficient self-adaptive optimization and equalization control method are given under different states of charge. The battery can participate in the adaptive adjustment and state balance of the primary frequency modulation. Based on the regional power grid frequency modulation simulation model with energy storage batteries, the proposed control method is simulated and analyzed under step disturbance and continuous disturbance. The results show that the studied control strategy has advantages in both power system frequency adjustment and battery SOC maintenance.

LCL filters are widely used in grid-connected inverters. However, the injected grid current control does not solve the problem of current harmonics and low bandwidth caused by the LCL resonance peak. The paper proposes an AD control method for LCL inverters, which is only based on the feedback of the grid current. The feedback loop uses the first-order HPF plus the first-order LPF for frequency selection, thereby suppressing the resonance peak. Due to the widening of the bandwidth of the feedback loop, this method is suitable for the resonance frequency shift phenomenon caused by the impedance of the grid. The paper also uses a harmonic compensation control strategy with damping, and analyzes the influence of this strategy on the performance of the inverter. The results show that it can effectively suppress low-frequency harmonics and improve the stability of the system under the impedance change environment. The effectiveness of the proposed control and design has been verified by MATLAB/Simulink simulation and 2kW experimental platform.

When the inductor current of boundary conduction mode (BCM) Buck converter falls to zero, the output filter inductor will resonate with the capacitor across the switch called Quasi Resonant. If the switch is turned on when the voltage of resonant capacitor is zero, ZVS and ZCS turn-on can be achieved,
and the switching loss is eliminated. This thesis analyses the resonance between output inductor and the capacitor in detail, and find out the time when the voltage across the switch is zero. Then by using a proper delay circuit, zero voltage switching and zero current switching of switch are assured. When the Buck converter works in BCM mode the output filter inductor is small, and the reverse recovery loss of diode is eliminated, so dynamic performance can be improved. And there is a capacitor across the switch when it is turned off, and ZVS turn-off is achieved. At last a 3kw prototype is designed, and the highest efficiency is 987%.

The trend in intermediate bus converters is the increasing demands for output power and high operating frequencies. Due to the leakage-inductance and AC winding loss in high frequency, the traditional transformer can not guarantee the high efficiency of a converter, hence the matrix transformer is proposed. The matrix transformer can reduce the leakage-inductance and winding loss by shunting output current. This paper analyzed the characteristics of matrix transformer and its advantages at high frequency applications. Owing to core loss and winding loss of the transformer, there is a tradeoff between efficiency and the branch numbers of matrix transformer. To get the optimal branch number when designing LLC DC-DC transformer, finite element analysis (FEA) method is used to know the transformer loss. Meanwhile, layout consideration of transformer windings is taken to minimize via loss and via leakage-inductance when the secondary side of transformer is in low voltage and high output current situation. Finally, a 1.4kW the LLC-DCX prototype was designed on the mentioned approach and some experiments were made.

In the voltage feedback fluxweakening control of interior permanent magnet synchronous motor, the traditional fluxweakening control based on current phase angle has some problems, such as poor dynamic performance and large torque fluctuation in flux weakening region. The causes of the above problems are analyzed from steady state and transient state, and an adaptive fluxweakening control strategy based on current phase angle is proposed in this paper. The adaptive gain can be obtained by deriving the voltage amplitude from the current phase angle in smallsignal analysis, and the current phase angle can be adjusted online. The experimental results show that the proposed fluxweakening control strategy can effectively suppress the daxis current, torque fluctuation and improve the utilization of DC bus voltage.

As an intrinsic attribute of materials, activation energy can directly reflect the physicochemical characteristics of insulation aging and degradation, which presents a feasible technological route as to realize effective assessment of insulation aging, and has been a research focus among the academia and industrial sectors. In this paper three definitions plus their evolution regarding activation energy are briefly reviewed, and the physical connotation of activation energy is elucidated based on the collision theory and the activation complex theory. The methodologies to obtain activation energy of insulating materials are introduced, including the thermal analysis kinetics-based ones as well as the thermo-frequency dielectric spectroscopy scheme, moreover, significant impact of the reaction mechanism function on residual lifetime prediction is also discussed. The progressing scenario of activation energy-based applications in evaluating insulation aging and residual lifetime is summarized, and finally, several key issues of scientific and technological significance are proposed and highlighted for future studies to facilitate activation energy-based insulating assessment.

The reliability of IGBT power module is a hot topic today, and the failure of its bonding wire is the main cause of its failure. The occurrence of the fault will cause the module to change the characteristic parameters, such as the module’s gate voltage Miller platform and the gate threshold voltage in the onoff process. The different faults of bonding wire failure are simulated by experiments, and the change of gate voltage Miller platform and threshold voltage with the severeness of the fault is analyzed. The result provides a reliable reference for the IGBT working monitoring technology.

A new high power factor control strategy is discussed with the aim of avoiding the distortion problem of input current of bridgeless PFC when the input voltage is crossing zero line due to the boost inductor on the AC side of the grid. Under the new control strategy input current distortion is avoided and high power factor is achieved.The reasons of the distortion are discussed. The input current THD and PF expression

The integrating of EMI choke plays an important role in the miniaturization and high power density of EMI filter. In order to improve the poor characteristics of the conventional EMI choke, a new structure of integrated planar choker is proposed. Based on this, the magnetic flux coupling under difference module current and common module current is analyzed, the forming principle of differential module inductance and common module inductance is elaborated and the equivalent circuit is finally obtained. The superiorities of inductance characteristics and impedance symmetric of this structure are illustrated. The principles of insertion gain based on the S parameter method are elaborated and the effectiveness and the superiority of the new structure are verified by experiment.

Wave energy power generation is an effective means of utilizing clean and renewable marine energy. Because of the instability and dispersion of wave energy, the energy conversion efficiency of wave power generation system is low, which restricts the development of wave power generation technology. Therefore, it is necessary to adopt a certain power control method to make full use of wave resources and improve wave energy conversion efficiency in wave power generation system. In this paper, the power control theory of wave power generation, amplitude-phase control and complex conjugate control, is briefly introduced. Various power control methods proposed at home and abroad in recent years are classified and analyzed. The development trend of power control research is expounded. Wave prediction and intelligent control are the research hotspots in the future, considering the large scale in the future. It is necessary to study the multi-objective optimal control problem of combining power control objectives with other control objectives for grid-connected array wave power generation devices.

Compared with silicon (Si) MOSFET, silicon carbide (SiC) MOSFET is more conducive to meet the requirements of power electronic converters towards high efficiency, high power density and high reliability. However, the crosstalk between the upper and lower switch limits the SiC MOSFET performance in phase-leg configuration. Based on analyses of crosstalk mechanism, this paper uses an improved method of active Miller clamp based on PNP transistor for crosstalk suppression. A double-pulse test prototype is also built to verify the theoretical explanation. As is shown by the experimental results, the improved method achieves good effect in crosstalk suppression and provides guide in design of gate driver for SiC MOSFET in phase-leg configuration circuit.

Rapid growth of new energy vehicles in China appeared in 2014，and the annual sales are close to 80000. As one of new energy vehicles the core technology of power battery technology has played a very important role. This report analyses the evolution of the National standards of power battery. According to the power battery standard GB/T31384，31485，31486，10 kinds of domestic and foreign typical power battery have been tested and compared in aspects of power battery’s performance and safety performance and storage performance，etc.

Nowadays, more-electric-aircraft （MEA） technology has become a mainstream method of airplane power supply. Because of its high reliability, light in weight and energy saving, 270V high voltage DC power system has become the main development direction for airplane power supply. In the situation of high-power DC-DC converter in aeronautics, full bridge LLC resonant converter can be a relatively better topology. Traditional analog control method has some weakness such as： the control method is relatively single, the hardware of the system is complex, and hard to improve efficiency and to reduce the costs. For these reasons, this paper proposes a solution of digital control. This solution can effectively decrease the size of the power, reduce the costs and improve the efficiency. At last, this paper developed a model machine with the input of 270（1 ±10）% VDC, and the output of 28V/500W DC. The result of the experiments verified the theoretical analysis and the feasibility of the digital control.

As a new intelligent power transmission equipment, power electronic transformer (PET) shows great value in the smart grid and energy Internet applications. In this paper, typical PET topologies and related control strategies are analyzed in view of medium and high voltage application. The characteristics of various topologies in terms of system efficiency, reliability and control complexity are summarized. The three-stage PET with multi-module cascade converter in the high voltage side is easy to expand, and the control is simple, but still there is room for optimization in system volume and efficiency. In this paper, two different types of converters in the isolation stage are summarized and contrasted, and the adaptability of the two different converters in medium and high voltage applications is analyzed.

DC short-circuit fault ride through capability is one of the main issues in applications of multi-terminal voltage source converter based high voltage direct current (VSC-MTDC) transmission systems. In this paper, the model of the full-bridge modular multilevel converter (FB-MMC) is analyzed. A dc short-circuit fault protection strategy for FBMMC-MTDC is proposed, including the converter protection control strategy and the system protection scheme. Benefit from the topology property, the FB-MMC has the capability of quick fault elimination with the valves blocked. And then, the MTDC system restarts quickly after clearance of dc fault without need to cut off the ac grid. The proposed protection strategy is demonstrated on simulation model in PSCAD/EMTDC. Simulation results verify the feasibility and effectiveness of the strategy.

After years of effort, the technology of fuel cell vehicle and hydrogen fueling infrastructure in Japan is gradually matured and will deeply effect the development of fuel cell vehicle technologies and standards worldwide. In early 2015, Toyota launched the world’s first volume-productive fuel cell vehicle model—Mirai, which means the related technology and cost issues of fuel cell vehicle have been solved in Japan. With the completion of the safety standards for fuel cell vehicle and hydrogen fueling station, this cutting edge technology is ready for commercialization in Japan. In the next step, Japanese government and related industry organizations propose the targets: 1. till 2025, the inventory of fuel cell vehicles will reach 200,000; 2. build large-scale hydrogen fueling system during 2025-2030; 3. establish zero-CO2-emission hydrogen fueling system by 2040. By announcing related policies and strategic guides, Japanese government will lead and promote the development of fuel cell vehicle and hydrogen fueling infrastructure.

A PWM strategy with fixed frequency for bidirectional series resonant converter has been studied in this paper. The modulation strategy is based on the principle that voltage waves in original and secondary side have the same phase, which makes the voltage gain of the converter has nothing to do with the direction and amplitude of the transferred power. Moreover, the converter can buck or boost the voltage via regulating the duty cycle of voltage between midpoints of two bridges in both two directions. This paper analyzes the operation of the presented strategy and discusses the possible realizations of the PWM strategy. In addition, a 1kW prototype is built and tested to evaluate the feasibility of the presented solution. And the modulation strategy makes the direction of power flow can be changed automatically and smoothly since the converter has the voltage gain independent with the direction and amplitude of the transferred power.