Search

Aiming at the problem of unstable output voltage and rapid reduction of efficiency caused by coil misalignment in the wireless charging process of wireless chargers, a hybrid wireless power system with anti-misalignment and constant current (CC) output characteristics is proposed. The system uses a high-order compensation network and a four-coil bipolar (BP) coupler to implement high anti-misalignment and CC output characteristic that is decoupled from the load. By inserting a high-order compensation network, the two overlapping coils in the same side of the transmitter and receiver sides of the BP coupler can be effectively decoupled and the design methodology can be simplified. Moreover, by adjusting the inductance value of the transmitter compensation network, the BP coupler can be made available for the occasions with different equivalent mutual inductance (MI) fluctuation requirements. While the receiver coil is suddenly removed, the system has a self-protection function, which can effectively limit the current flowing through the inverter. The design ideas of the coupler and compensation network are also discussed in this paper. Ultimately, a 1-kW experimental prototype is built to validate the proposed scheme. [ABSTRACT FROM AUTHOR]

The single-stage high-frequency (HF) isolated ac–dc converter has the advantages of high-power density, long life, and high efficiency. It has a broad application prospect in distributed power generation, ac microgrid, and energy storage system. In this article, its topologies are classified according to the energy storage components. The structure and operating principle of various topologies are introduced. Furthermore, the development of extended multiport single-stage HF isolated ac–dc converter is introduced. On this basis, the advantages and disadvantages, typical characteristics, cost, and power loss of various topologies are analyzed and summarized. Finally, the key issues that need to be solved are pointed out and the development trends are explored. This article aims to provide a reference for the subsequent research on single-stage HF isolated ac–dc converters and the advancement of their practicability. [ABSTRACT FROM AUTHOR]

Detecting and locating the open-circuit (OC) fault of an insulated-gate bipolar transistor (IGBT) rapidly and accurately is very important to improve the security and reliability of the drive system. However, due to the symmetry of the dual-inverter topology, it is difficult to achieve fault detection for open-end winding permanent magnet synchronous motor (PMSM). Thus, this article proposes a fault diagnosis technology for open-end winding PMSM fed by a dual inverter with a common dc bus. In this method, the faulty phase and the faulty case are identified by the average value of the output of the closed-loop controller, which is used to eliminate the differential-mode current in the system. Then, the faulty phase is isolated by removing the gate signals. Finally, the faulty IGBT is located by restructuring the faulty phase and analyzing the current of the faulty phase. In this method, the OC fault can be detected and the faulty IGBT can be located accurately. Experimental results are shown to validate the proposed detection method. [ABSTRACT FROM AUTHOR]

The single-phase isolated quasi-single-stage AC-DC converter has many virtues, such as high power density and efficiency; however, its grid current closed-loop control has not been solved. This paper aims to solve the remaining gap based on a large-signal model. In this paper, the large-signal model of this converter under triple-phase-shift modulation is built for the first time. It is verified that the built model is a zero-order linear system. Based on this built model, the effect of grid harmonics on this converter grid current is analyzed. The theoretical analysis reveals that the grid voltage low-order harmonics will cause the same order grid current harmonics and only varying the parameters of the filter is not an effective method to solve this problem. For the purpose of eliminating the effect of grid voltage harmonics on the grid current and realizing the zero-error control of the fundamental component of the grid current, a grid current closed-loop control strategy based on the proportional-resonant compound odd-mode repetitive controller is proposed. The operation principle, parameter constraint, and design rule of the proposed compound control strategy are analyzed comprehensively. The theoretical analysis and the compound control strategy put forward in this paper are tested, with detailed experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a triple-active-bridge resonant dc-dc converter with the ability of topology-level power decoupling is proposed. The power coupling between the two dc ports is eliminated by adding a resonant capacitor to the common port. The operation principle and the steady-state power characteristics are analyzed. On this basis, a multi-phase-shift-based global optimization modulation is proposed to minimize the RMS values of the transformer currents in the entire power and voltage range, thus increasing the global efficiency. An experimental prototype is built to verify the correctness and availability of the proposed power decoupling topology and optimized modulation. [ABSTRACT FROM AUTHOR]

In order to solve the problems of existing three-port isolated dc-dc converter such as power decoupling and the nonzero instantaneous power of the standby port, a three-port isolated bidirectional current-fed series-resonant dc-dc converter is proposed in this article. The operational principle and general transfer power expressions of various ports are analyzed with the duty-cycle plus phase-shift control. Then, the power decoupling at the topology level is realized by setting the switching frequency equal to the resonant frequency of the resonant tank on the high-voltage side; thus, the transfer power of the two ports on the low-voltage sides can be controlled independently. Moreover, it is proved that the proposed converter realizes the zero instantaneous power of the standby port. On this basis, an optimized modulation strategy by taking the minimum root mean square value of the resonant current as the objective function is presented. The exact relationship between the desired transfer power and various phase shifts is accordingly obtained. The detailed experimental results verify the correctness of the proposed converter and corresponding modulation strategy. [ABSTRACT FROM AUTHOR]

In this article, a hybrid three-level current-fed series-resonant isolated bidirectional dc–dc converter is proposed, containing an interleaved parallel bidirectional buck/boost converter at the low-voltage side and a half-bridge three-level converter at the high-voltage side. The operation principles and characteristics of the proposed converter are thoroughly analyzed with the duty-cycle plus dual phase-shift control. The relationship between the general case of three independent control variables and the transfer power is derived. Then, a modulation strategy optimization algorithm by taking the minimum root mean square (rms) value of the resonant current as the objective function is presented. Under the optimal operation point, the converter operates efficiently with the minimum rms value of the resonant current and accordingly increases the efficiency. The detailed experimental results verify the correctness and feasibility of the proposed converter and modulation strategy. [ABSTRACT FROM AUTHOR]

The bidirectional buck–boost current-fed isolated dc–dc converter is an emerging topology and has the advantages of low current stress, high efficiency, and wide power/voltage conversion range. However, it suffers from the inductor current mutation and transient voltage spike when the converter is directly switched between the buck and boost modes in the actual operation. By analyzing in detail the process of directly switching the two operation modes, it reals the reason of causing these transient problems is the duty ratio, and shoot-through duty ratio is not correctly set when the operation mode is switched. In order to solve this problem, a smooth switching scheme between the buck/boost operation modes is proposed by setting a special relationship between the closed-loop controller output of the inductor current and the two control variables based on the system model. The detailed simulation and experimental results verify the theoretical analysis and the proposed solutions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The input-series converters based on integrated-transformer are suitable for the high-input voltage multiple-output applications. In these converters, the integrated-transformer is an important device, through which active input voltage sharing of each series-module is achieved. Aiming at this type two-transistor flyback converter, stray capacitances effects of the integrated-transformer are analyzed, based on which windings arrangements of the integrated-transformer are investigated in this article. First, the conventional and input-series flyback converters are presented, and design principle of the integrated-transformer is introduced. Second, the main stray capacitances of the integrated-transformer and their influences are analyzed in detail. Third, various windings arrangements of the integrated-transformer are analyzed and compared. Finally, experimental study has been done on two laboratory-made prototypes with various integrated-transformers, and the theoretical analysis is verified by the experimental comparisons. [ABSTRACT FROM AUTHOR]

The maximum phase current (the maximum transfer power) of the dual-active-bridge based three-phase single-stage (DAB-3P1S) ac–dc converter with the present dual-period-decoupled space vector phase-shifted modulation (DPD-SVPSM) is analyzed for the first time in this article. It shows the equal switching period assignment way of the present DPD-SVPSM reduces the utilization of the control period, thus limiting the maximum transfer power of DAB-3P1S ac–dc converter. To enhance the maximum transfer power, a variable switching period based dual-period-decoupled space vector phase-shifted modulation (VDPD-SVPSM) is proposed. Besides, the current stress of DPD-SVPSM and VDPD-SVPSM are analyzed and compared. And it indicates that by properly designing the transformer leakage inductor, VDPD-SVPSM improves the maximum phase current (maximum transfer power) without increasing the current stress of the device, thereby improving the utilization of the system hardware rated capacity then increasing the power density. Detailed experimental results validate the correctness of the theoretical analysis and the feasibility of the proposed VDPD-SVPSM. [ABSTRACT FROM AUTHOR]

When the conventional space vector phase-shifted modulation is applied to the dual active bridge (DAB)-based three-phase single-stage (DAB-3P1S) ac–dc converter, the ac-side current error is caused due to the coupling of transformer currents in the adjacent switching periods and the imprecise dwell times of the active current vectors. By analogizing the DAB-3P1S ac–dc converter into the DAB-based dc–dc converter, a dual-period-decoupled space vector phase-shifted modulation based on three phase shifts is proposed to decouple the transformer currents, thus eliminating the ac-side current distortion. Moreover, taking the minimization of the rms value of the transformer current per switching period as the constant, the exact relationship between the desired ac-side current and the three phase shifts is derived to realize the efficiency improvement. Detailed experimental results validate the correctness of the theoretical analysis and the feasibility of the proposed modulation strategy. [ABSTRACT FROM AUTHOR]

In this article, a bidirectional buck–boost current-fed isolated dc–dc converter is proposed to realize the bidirectional and buck/boost voltage conversion and accordingly extend the operating range. The basic modulation is proposed and the operation principle is analyzed in detail. Furthermore, the voltage conversion ratio as the functions of the duty cycle ratio of the buck unit and shoot through ratio of the H-bridge converter is derived. The control rule of the two controllable variables is determined based on minimizing the average inductor current. In addition, the optimal starting moment of active period of the buck unit in the switching cycle is determined based on minimizing the inductor current ripple. The detailed experimental results verify the correctness and feasibility of the proposed topology and modulation. [ABSTRACT FROM AUTHOR]

In this paper, a modified single-carrier and multimodulation sine pulse width modulation (MSCMM-SPWM) is proposed to simplify the implementation of the single-phase asymmetrical neutral-point clamped grid-connected inverter (ANPC-GCI) for the PV generation system application. Furthermore, the expressions of the total switching loss and grid current total harmonic distortion (THD) of ANPC-GCI controlled by the proposed MSCMM-SPWM are derived. Then taking the grid current THD meet the grid-connected standard in the entire operational range as the constraint, an efficiency enhancement scheme for ANPC-GCI based on online adjusting the carrier frequency and the obtained THD expression is proposed. The control configuration of ANPC-GCI, including the proposed MSCMM-SPWM and online efficiency enhancement scheme, is designed. The detailed experimental results verify the correctness and feasibility of the proposed MSCMM-SPWM and the efficiency enhancement scheme. [ABSTRACT FROM AUTHOR]

In this article, an improved triple-phase-shift (ITPS) modulation for the single-phase isolated quasi-single-stage ac–dc converter is proposed to realize the current decouple between the adjacent switching periods and to reduce the grid current distortion. Moreover, the dead band effect on the proposed ITPS is analyzed for the first time. It reveals that the dead band causes not only the current couple and accordingly the grid current distortion but also the current amplitude loss. A two-step phase-shift compensation scheme is proposed based on the analysis result to solve the problems caused by the dead band. All of the theoretical analysis, the proposed ITPS modulation, and the dead band compensation scheme are validated in detail in the experimental way. [ABSTRACT FROM AUTHOR]

In this article, an improved modulation strategy for the single-phase single-stage isolated bidirectional ac–dc converter is proposed. The modulation of ac side cycloconverter is based on the unipolar idea to reduce the switching quantity and the ac side current ripple and provide the bidirectional current flow path. The dc side H-bridge converter is driven by the pulsewidth modulation signals cooperatively to speed up the transformer current and thus to eliminate the voltage spike. Furthermore, the power reversion zone (PRZ) of the converter is verified. The action of the proposed modulation strategy in PRZ is analyzed for the first time. It reveals that the reverse current overshoot is caused in ac-to-dc direction and the voltage spike is caused in dc-to-ac direction. An online switching scheme of the proposed modulation strategies in two power directions is proposed to solve these problems. The detailed experimental results verify the correctness and reliability of the proposed modulation strategy. [ABSTRACT FROM AUTHOR]

In this paper, a cooperative triple-phase-shift (CTPS) control for the isolated dual active bridge dc–dc converter is proposed first to eliminate the dual-dc side flow back currents. Furthermore, the current characteristics of the proposed CTPS control with the main variable (the primary side inner phase shift) tuned in two different regions are derived in detail and compared with each other. The tuning region of the main variable with the optimal current characteristics is finally determined based on the comparison results. As a result, the current stress is reduced and the efficiency is enhanced. The detailed experimental results of CTPS control verify its validity and practicability. The advantages of CTPS control are also highlighted by comparing it with some conventional phase shift controls. [ABSTRACT FROM AUTHOR]

The doubly fed induction generator (DFIG) has insufficient capacity to respond to the frequency inertia of the power grid due to the isolation between the rotor kinetic energy and the power grid by the generator-side converter. At the same time, the 'source' of the DFIG cannot be as stable as the thermal generator, so sudden changes in the wind speed on the ultra-short time scale can affect the system inertia. In this study, the inertia of a doubly fed wind generator with a super capacitor energy storage system (SCESS-DFIG) is defined from the standpoint of a single machine. Based on fuzzy logic controller, an inertia response control strategy of multi energy coordinated control is proposed. Then the SCESS-DFIG inertia response control strategy under variable wind speed is proposed at the wind field level. Based on the establishment of DFIG, DC bidirectional buck–boost converter, and supercapacitor energy storage unit model, a wind farm with three SCESS-DFIGs is simulated. When the grid frequency is dropped, the effectiveness of the proposed control strategy is verified by simulating their operation under the same rated parameters, different wind speed curves, and different initial voltages of the supercapacitors. [ABSTRACT FROM AUTHOR]

Power electronics technology is playing an increasingly important role in modern energy utilization systems. In order to improve the power quality and power conversion efficiency, various power electronics technologies are being researched. This Special Issue aimed to collect and present breakthrough research on various kinds of the power electronics converters, including topologies and modulations, modeling and control, fault diagnosis, and power-conversion technologies for wireless power transmission in emerging power-conversion technologies. [Extracted from the article]

As one of the most promising solutions to mitigate the power quality (PQ) problem of the modern power systems, the unified PQ conditioner (UPQC) draws considerable attentions. Since the UPQC consists of two sets of power converters, it will greatly increase the manufacturing investment of the setup. In this paper, the optimal volt-ampere (VA) ratings of the converters in the UPQC are investigated due to system compensating requirements. The phase angle control (PAC) method is discussed and illustrated to have the feature of changing the online VA loading by adjusting the corresponding displacement angle. On the basis of the variable PAC method, a two-stage algorithm is utilized to optimize the ratings of the shunt and series converters in order to obtain the maximum utilization rates of the power converters in the UPQC. Moreover, the corresponding control algorithm is utilized to reduce the proposed UPQC online VA loadings for the different compensating operations. The proposed UPQC is compared with other approaches to highlight the advantage of the proposed optimization algorithm. The proposed algorithms are also validated with the simulation and the real-time control hardware-in-loop results of the designed system. [ABSTRACT FROM AUTHOR]

The unified power quality conditioner (UPQC) has become the most attractive solution to improve the power quality (PQ) in the microgrids. However, the high manufacturing cost is a huge challenge of popularizing the UPQC system. This paper investigates the optimal size of the UPQC system based on the compensation requirements. A generalized strategy is proposed to optimize the size of the UPQC system, which determines the fundamental ratings of the shunt converter, series converter, and series transformer. A data-driven control (DDC)-based controller is developed using the variable phase angle control method to realize the implementation of the designed UPQC system under the different compensating conditions. Moreover, the MATLAB and OPAL-RT simulation results validate the practicability of the generalized strategy and the DDC-based controller. [ABSTRACT FROM AUTHOR]

This paper proposes an instantaneous power control method for high speed permanent magnet synchronous generators (PMSG), to realize the decoupled control of active power and reactive power, through vector control based on a sliding mode observer (SMO), and a phase locked loop (PLL). Consequently, the high speed PMSG has a high internal power factor, to ensure efficient operation. Vector control and accurate estimation of the instantaneous power require an accurate estimate of the rotor position. The SMO is able to estimate the back electromotive force (EMF). The rotor position and speed can be obtained using a combination of the PLL technique and the phase compensation method. This method has the advantages of robust operation, and being resistant to noise when estimating the position of the rotor. Using instantaneous power theory, the relationship between the output active power, reactive power, and stator current of the PMSG is deduced, and the power constraint condition is analysed for operation at the unit internal power factor. Finally, the accuracy of the rotor position detection, the instantaneous power detection, and the control methods are verified using simulations and experiments. [ABSTRACT FROM AUTHOR]

The effect of input harmonics, unbalance, and dc offsets on the two-phase stationary frame enhanced phase-locked loop (\alpha \beta \rm - EPLL) is theoretically analyzed first. It indicated that the nonideal grid conditions cause the periodic ripples in the estimated amplitude, frequency, and phase angle. An improved \alpha \beta \rm - EPLL by introducing two pure integrators and moving average filter (MAF) into both the amplitude and frequency estimation loops of \alpha \beta \rm - EPLL is proposed to eliminate the effect of the aforementioned nonideal grid conditions. An improved implementation of MAF to eliminate the effect of the variation of the grid frequency is proposed as well. The linear model of the improved \alpha \beta \rm - EPLL is built and the design rule of the control parameters is determined. Detailed experimental results are presented to verify the validity and feasibility of the improved \alpha \beta \rm - EPLL. [ABSTRACT FROM PUBLISHER]

The multi-inverter-based configuration is usually adopted in the renewable energy source grid-connected generation system (RES-GGS) to obtain a wide operational range and independent output power control of the renewable energy sources (RESs). The complicated configuration inevitably reduces the European efficiency. This paper proposes a multi-topology-mode inverter suitable for RES-GGS, which possesses five equivalent topology modes. When the output voltage and power of the RESs increase gradually, the system transforms its actual topology mode from the complicated ones to the simpler ones to increase the European efficiency. Furthermore, a system-level control strategy that can achieve the independent generation power control of each RES and the grid current closed-loop control is proposed. By online transforming the system among the five topology modes, the characteristics of wide operational range, independent output power control of each RES and high European efficiency are all achieved. The detailed experimental results verify the feasibility and advantages of the proposed multi-topology-mode grid-connected inverter. [ABSTRACT FROM AUTHOR]

This paper proposes a modified cascaded neutral-point-clamped grid-connected inverter (MCNPC-GCI) to achieve a wide operational range and high European efficiency in the photovoltaic generation system. Two bidirectional thyristors are added into the standard single-phase cascaded neutral-point-clamped (NPC) inverter to, respectively, connect the positive and negative poles of the two NPC inverters to achieve the transformation between the cascaded NPC inverter and the equivalent single NPC inverter. The losses and grid current total harmonics distortions of the two topology modes are derived to determine their optimal operational ranges and an approach of online smoothly switching the two topology modes is proposed. Detailed experimental results of the proposed MCNPC-GCI are shown to verify its accuracy and feasibility. [ABSTRACT FROM AUTHOR]

The transient response of the single‐phase enhanced PLL (EPLL) under the step change of amplitude, phase angle, and frequency is derived. It is revealed that there are transient periodic ripples in the estimation results of the standard EPLL. Furthermore, a virtual quadrature‐coordinate EPLL combining a second‐order generalised integrator with a quadrature‐coordinate EPLL is proposed and the transient response is derived. The proposed EPLL possesses a better dynamic performance than the standard EPLL. The detailed experimental results are shown and compared with each other to validate the theoretical analysis and the advantages of the proposed EPLL. [ABSTRACT FROM AUTHOR]

In this study, the transient process when the single‐phase open‐line fault (SPOLF) occurs in the three‐phase AC–DC pulse width modulation rectifier with LCL filter (LCL‐PWMR) is theoretically analysed for the first time and the corresponding diagnosis scheme is proposed. First, the equivalent circuit of the LCL‐PWMR is built and the detected phase voltage (DPV) detected by the three‐phase grid voltage sensors under fault conditions are derived in s‐domain. The result shows that the DPV in the open‐line phase is not equal to zero, while consists of a fundamental component plus a resonant component with specific frequency. Furthermore, a diagnosis scheme for the SPOLF based on 2‐order band‐pass filter (BPF) connected in series with enhanced phase‐locked loop (EPLL) is proposed. BPF is used to eliminate the fundamental component of the sum of three‐phase DPVs and EPLL to estimate the amplitude of resonant component. Then the SPOLF can be detected when the amplitude of resonant component is beyond the allowed threshold. Furthermore, the adaptability of the proposed diagnosis scheme for the system with different rated power is verified theoretically. Detailed simulations tests based on Matlab/Simulink software and experimental tests are carried out to demonstrate the validity of the theoretical analysis and feasibility of the proposed diagnosis scheme. [ABSTRACT FROM AUTHOR]

This paper proposes a modified cascaded multilevel grid-connected inverter (MCM-GCI) suitable for photovoltaic grid-connected generation system, which considers wide operation range, low grid current total harmonics distortion (THD), and high European efficiency. In the proposed topology, a bidirectional power switch is added to the standard single-phase cascaded multilevel inverter (CMI) to implement the transformation between CMI mode and H-bridge inverter (HBI) mode. An online topology transformation approach of the MCM-GCI is proposed to guarantee the topology modes transformed safely and smoothly. The proposed MCM-GCI operates in CMI mode when the PV arrays’ output voltage and power are low, and transforms into HBI mode when the PV arrays’ output voltage and power are high. Experimental results of five-level MCM-GCI are represented to validate the feasibility of the proposed topology and it is also compared with other three classic grid-connected inverters to highlight its advantages. Furthermore, more structures based on hybrid CMI, which possess the topology transformation ability, are shown to further improve the generation range and obtain a higher efficiency of the system. [ABSTRACT FROM PUBLISHER]

In this paper, the generation reason and the improved elimination scheme of the current zero-crossing distortion (ZCD) of single-phase grid-connected inverter controlled by unipolar hysteresis current control containing band are investigated in detail. A current ZCD elimination scheme by exchanging the control signal of low-frequency leg of inverter ahead of the zero-crossing points of grid voltage is proposed. The effect of the variation of the amplitude and frequency of tied grid and the filter inductor on the optimal leading exchanging phase angle (LEPA) is analyzed. A single-phase hybrid filter-based enhanced phase-locked loop is proposed to estimate accurately the information of the grid, even under nonideal input conditions. Then, an online filter inductor estimation scheme is proposed to obtain the accurate actual inductor value. The former estimated parameters are used to calculate the exact optimal LEPA online to improve the performance of the current ZCD elimination scheme. Comprehensive experimental results are represented to verify the accuracy and feasibility of the theoretical analysis and the proposed improvement. [ABSTRACT FROM PUBLISHER]

In photovoltaic (PV) grid-connected generation system, the key focus is how to expand the generation range of the PV array and enhance the total efficiency of the system. This paper originally derived expressions of the total loss and grid current total harmonics distortions of cascaded inverter and H-bridge inverter under the conditions of variable output voltage and power of the PV array. It is proved that, compared with the H-bridge inverter, the operation range of the cascaded inverter is wider, whereas the total loss is larger. Furthermore, a novel online variable topology-type grid-connected inverter is proposed. A bidirectional power switch is introduced into the conventional cascaded inverter to connect the negative terminals of the PV arrays. When the output voltages of the PV arrays are lower, the proposed inverter works under cascaded inverter mode to obtain wider generation range. When the output voltages of the PV arrays are higher, the proposed inverter is transformed into equivalent H-bridge inverter mode to reduce the total loss. Thus, wider generation range and higher efficiency are both implemented. Specific demonstration of the proposed inverter based on five-level cascaded inverter is presented. The detailed experimental results verify the accuracy and feasibility of the theoretical analysis and the proposed inverter topology. [ABSTRACT FROM PUBLISHER]

In this paper, the dynamic expressions of the amplitude and frequency estimated by the standard enhanced phase-locked loop (EPLL) and the ones with input dc offset estimation integrator (DCEI) are derived originally and reveal that the DCEI enlarges the amplitude of the periodic ripples caused by the dynamic disturbances and prolongs the dynamic process. To achieve correct estimation when the input signal contains dc offset and harmonics but without deteriorating the dynamic performance, an improved EPLL combined with delayed signal cancellation (DSC) is proposed. A DSC operator is employed to the input of the EPLL to eliminate dc offset and even-order harmonics. A cascaded DSC (CDSC) module is applied in both frequency and amplitude loops to remove the effect of most of the residual odd-order harmonics. The structure of the CDSC module and delay coefficients are designed in detail. Experimental results of all the three PLLs are presented and compared to validate the theoretical analysis results and the proposed EPLL. [ABSTRACT FROM PUBLISHER]

In this brief, a new two-phase stationary-frame-based enhanced phase-locked loop (TPSF-EPLL) is proposed for detecting the amplitude, phase angle, and frequency of a three-phase grid. The differential equations of the proposed TPSF-EPLL are originally derived based on minimizing the cost function with a gradient descent algorithm. Moreover, the system structure, the design guidelines of each parameter in both frequency and amplitude loops, and the stability analysis of the TPSF-EPLL are presented. The main advantage of the TPSF-EPLL is that it requires less calculation since transformation into a rotating reference frame is not involved, and all the calculation is performed in a two-phase stationary frame. Experiment results of steady state and dynamic performance of TPSF-EPLL are presented and compared with some classical PLLs to verify the validity of the theoretical analysis and its advantages. [ABSTRACT FROM PUBLISHER]

In this paper, a seven-level single-carrier and multi-modulation-wave sinusoidal pulsewidth modulation (SCMM-SPWM) strategy is proposed. In the negative half cycle of the modulation waves (MWs), dc offsets related to the amplitude of the carrier are set on the three MWs, respectively, to apply the same comparison logics of the MWs and carrier during positive and negative half cycles of the MWs. Thus, it is implemented with only one digital signal processor chip without any other attached logical circuit or controller. The reason for generating the zero-crossing voltage pulse disturbance (ZCVPD) in this strategy is analyzed, and the elimination of the ZCVPD is proposed and verified by experimental results. The spectral characters of the conventional multi-MW-based SPWM and the proposed one are originally derived and compared with each other by simulation in detail. The theoretical analysis, simulation, and experimental results indicate that the output characters of the proposed strategy are identical to those of the conventional one; it means that the proposed strategy can replace the conventional one while with the benefit of significantly reducing the cost and bulk of the implemental platform. [ABSTRACT FROM PUBLISHER]

In this paper, a simple digital hysteresis current control (DHCC) with quasi-constant switching frequency (SF) for the current control of a single-phase grid-connected inverter (SPGI) is proposed. The hysteresis band is cancelled, and the SF is fixed by online adjusting the sampling period. The principle of the DHCC with constant sampling period is analyzed, and the distribution of the switching period (SP) corresponding to the phase angle of the grid is derived. The rule of online adjusting the sampling period to fix SF is then determined according to the former theoretical analysis results. Furthermore, the effect of variation of the filter inductor on the SP is analyzed. An online estimation of the filter inductor based on the linear model of the current increment within one switching cycle is developed to eliminate this kind of effect. The proposed scheme shows the features of simple control, easy implementation in a digital way, robust to the filter inductor, and excellent steady and dynamic performance. The detailed simulation and experimental results verify its accuracy and feasibility. [ABSTRACT FROM PUBLISHER]

In this paper, a kind of zero-crossing distortion (ZCD) generated in the grid-connected current of a single-phase grid-connected inverter with unipolar hysteresis current control is investigated. By establishing the mathematical model of the grid-connected inverter around the zero-crossing points (ZCPs), the reason for causing the current ZCD is presented. It is ascribed to the slope of the actual current less than that of the reference one around the ZCPs. The expressions of the critical phase angle (the starting point of the current ZCD) and the amplitude of the current distortion corresponding to the parameters of the inverter and connected utility grid are obtained, and the solution for the ZCD based on on-line leading phase angle compensation is proposed. Simulation and experimental results verify the accuracy and feasibility of the theoretical analysis and the proposed solution. [ABSTRACT FROM PUBLISHER]

Abstract: The high sensitivity, fast response and the high quality reconstructions are observed in various [Li]/[Nb] ratios In:Fe:Cu:LiNbO3 crystals at 488nm wavelength based on the two-beam coupling experiment. The strong blue photorefraction is contributed by the two-center effect and the remarkable characteristic of being in phase between the two gratings recorded in shallow and deep trap centers. The blue photorefraction is enhanced significantly with the increasing of [Li]/[Nb] ratios under the same experimental conditions. The sensitivity S" is reduced to 0.46J/cm, simultaneously the response time is as fast as 4.4s and the erase phenomenon is not obvious in In:Fe:Cu:LiNbO3 crystals which [Li]/[Nb] ratio is 0.986 in crystal. Increasing [Li]/[Nb] ratios improve the damage-resistant ability of the crystals, but lead to a more serious beam fanning. Experimental results definitely show that the near-stoichiometric In: Fe: Cu: LiNbO3 crystal becomes a promising candidate for blue photorefractive holographic recording. [Copyright &y& Elsevier]

The article presents a study on In:Fe:Cu:LiNbO3 crystals grown by the Czochralski method in air. It discusses details of the experiment to look into the effects of reduction/oxidation treatments on the blue photorefractive properties and the two-wave coupling gain coefficients of the crystals. It explores the significance of the research for its applications in blue photorefraction.

Insufficient frequency regulation capability and system inertia reduction are common problems encountered in a power grid with high wind power penetration, mainly due to the reason that the rotor energy in doubly fed induction generators (DFIGs) is isolated by the grid side converters (GSCs), and also due to the randomness and intermittence of wind power which are not as stable as traditional thermal power sources. In this paper, the frequency inertia response control of a DFIG system under variable wind speeds was investigated. First, a DFIG system topology with rotor-side supercapacitor energy storage system (SCESS-DFIG) was introduced. Then a control strategy for frequency inertia response of SCESS-DFIG power grid under fluctuating wind speed was designed, with two extended state observers (ESOs) which estimate the mechanical power captured by the DFIG and the required inertia response power at the grid frequency drops, respectively. Based on one inconstant wind speed model and the SCESS-DFIG system model adopting the control strategy established, one power grid system consisting of three SCESS-DFIGs with different wind speed trends and a synchronous generator was simulated. The simulation results verified the effectiveness of the SCESS-DFIG system structure and the control strategy proposed. [ABSTRACT FROM AUTHOR]