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This paper proposes a novel three-phase hybrid rectifier topology and control method for high power electrolytic hydrogen production application. It is composed of a primary rectifier, i.e., a 6-pulse silicon controlled rectifier (SCR) and an auxiliary converter, including a pulse width modulation (PWM) voltage source converter (VSC) and a phase-shift-full-bridge (PSFB) converter. SCR undertakes most of the power consumed by electrolysis cell (EC), and the auxiliary converter can compensate dc current ripples and absorb ac current harmonics of primary rectifier simultaneously. Compared with high power PWM rectifier, the proposed hybrid rectifier can keep the main benefits of PWM rectifier, such as the low dc current ripple to realize improved efficiency of electrolytic hydrogen production and high grid current quality at ac side, while significantly reducing the cost. The effectiveness of proposed hybrid rectifier is verified by the simulation as well as experimental results.

As the most widely used power semiconductor devices, insulated gate bipolar transistor (IGBT) modules are normally composed of parallel IGBT chips to achieve the desired current capability. However, the electro-thermal behavior of each chip is significantly different due to the asymmetric layout, increasing the overheating risk of single chip. Therefore, this paper proposes a novel approach to describe the thermal safe operation area (TSOA) of multichip IGBT modules in the inverter application, considering the uneven temperature distribution among parallel chips. The novelty of the approach is that it applied the proposed analytical model describing the uneven electro-thermal behavior, thus avoiding the traditional averaging process. Comprehensive investigation has been made to reveal the deep mechanism of the inconsistent temperature distribution, which are the uneven dynamic current sharing and thermal cross-coupling effects. Correspondingly, the analytical model of the uneven switching loss and the standard form of thermal resistance matrix related to cooling performance, are proposed and further verified, respectively. Based on the proposed approach, the recommended operation area of different rated devices is fully described. The contribution of this paper enables more well-founded device selection to achieve a safe and cost-efficient design for the inverter application.

In an autonomous AC microgrid system, a peer-to-peer decentralized solution would be a preferred choice for the coordinative control of distributed generation units. However, due to the dispersed sources and loads, and mismatched line impedances, it is hard to achieve satisfactory power sharing performance and power quality without communication links. Therefore, a comprehensive decentralized solution based on dual-frequency-droop is proposed in this article. With only local information, appropriate virtual impedances could be obtained to compensate the mismatched complex line impedances in the required frequency range, and hence active, reactive, unbalanced, and harmonic powers could be shared accurately among parallel inverters under various load conditions. Then, the frequency deviation caused by the droop control is effectively attenuated without affecting power sharing through a dynamic droop-slope regulator. Moreover, the causes of steady-state ripples on virtual impedance waveforms are carefully analyzed, and the design criterions to eliminate these ripples are provided. Finally, case studies are provided in simulations and experiments to validate the effectiveness of this method.

Multiphase interleaved boost diode-capacitor (MIBD) converters, as the combination of multiphase interleaved input and voltage multiplier cells (VMCs), provide a preferred solution for both high voltage gain and high-efficiency application. This article proposes a universal zero-voltage transition (ZVT) circuit design method for a family of MIBD converters with different VMC structure and cell number. Compared with other soft-switching (SS) design methods, there is no extra voltage stress for all the semiconductor devices. An auxiliary circuit consisting of two low-voltage rating switches, two diodes, two capacitors, and one small inductor is integrated into the hard-switching (HS) two-phase interleaved boost diode-capacitor (TIBD) converters to provide zero-voltage switching (ZVS) turn- on and turn- off for the main switches and zero-current switching (ZCS) turn- off for all the diodes. Besides, the auxiliary switches turn on under ZCS. All the SS is accomplished within a wide duty cycle and power rating. From the comprehensive comparison, the constructed converters’ performance exceeds that of ZVS converters published in the recent literature. Furthermore, the extendable high voltage gain, low input current ripple, low voltage stress, and simple parameter design characteristics of HS MIBD converters are also conserved. Finally, a 320-W laboratory prototype is implemented to validate the theoretical analysis.

Power decoupling is always a strong desire for single-phase voltage source inverter (VSI) due to the inherent double-line (2 ω ) frequency ripple caused by the instantaneous power unbalance between dc- and ac-side. The existing solutions either apply a large capacitor or employ a small power converter to compensate it, which inevitably increases the system cost and control complexity. To overcome these drawbacks, this article performs the transient modeling analysis and it reveals that 2 ω frequency power component in the converter achieves minimum value when the inductor current ripple is minimized. A novel capacitors’ voltage ripple complementary control algorithm based on three-level boost fed single-phase VSI is proposed. By mitigating the 2 ω power component from the inductor and redistributing the stored energy between two intermediate series-connected capacitors, the oscillating power is absorbed with notably eliminated input current ripple and significantly reduced dc-link voltage ripple. Thus, the good power quality of both dc- and ac-side is guaranteed. Furthermore, the total capacitance requirement is minimized under the given dc-link ripple tolerance without any more electric stress increment and lifetime reduction. Simulation and experiment results verify the theoretical analysis and proposed control method. It is promising in applications, where both input current ripple is strictly limited and ac output voltage quality is highly required with small dc-link capacitance.

The proportional-integrator (PI) controller in the synchronous reference frame (dq-frame) and the proportional-resonant (PR) controller in the stationary reference frame (αβ-frame) are two typical linear current control schemes for three-phase voltage source converters. It is generally believed that the plant model is mathematically uncoupled in αβ-frame. Hence, the decoupling control required to attenuate the axis cross-coupling effect in dq-frame is regarded as not necessary in αβ-frame. However, detailed analysis with complex coefficient transfer functions reveals differences in the control performance between the fully decoupled PI control scheme in dq-frame and the PR control scheme in αβ-frame. When regulating sinusoidal currents in αβ-frame, the α-axis and β-axis current references are strongly correlated with each other, which implies a cross-coupling effect equivalent to that of dq-frame. In analogy to the dq-frame decoupling control, cross-coupling terms can be employed together with the conventional PR control scheme. It can bring the benefit of improving the transient response of current tracking and reducing the sensitivity of the resonant controller to frequency variation, especially for low-switching-frequency applications with limited current control bandwidth. Furthermore, the cross-coupling terms based on reference current feed-forward are proposed to improve the performance of both positive- and negative-sequence current control. Experimental results are shown to validate the analysis and the performance of the proposed control scheme.

The trial and error method is the conventional method for LCL parameter design. Its efficiency is low and it is difficult to obtain the optimal parameter. Furthermore, due to the lack of detailed analysis of the high-frequency harmonics in the inverter output voltage, the designed LCL filter cannot effectively filter them, resulting that the quality of the grid-connected current cannot fully meet IEEE 519 standard. Based on the above-mentioned problems, the high-frequency harmonics of inverter output voltage is analyzed in detail in this article, and the multiobjective optimization function, including five optimization objectives, is constructed. Combined with the particle swarm optimization, a screening method is proposed to realize the multiobjective optimization process. Under the proposed method, the LCL parameter, which happens to meet the requirement of IEEE 519 for the high-frequency harmonic distortion rate of the grid-connected current, can be obtained, and the greatest tradeoff of LCL filtering performance and filter cost is achieved. In addition, the proposed method considers the problem of weak grid and provides a complete engineering solution for LCL parameter design in different degrees of weak grid. Finally, the effectiveness and accuracy of the proposed design method are verified by contrast simulation and experiment.

Insulated gate bipolar transistor (IGBT) modules in modular multilevel converters (MMCs) have inherent unbalanced power loss distribution and temperature swing. Some chips may become the hotspots and have much larger temperature swing than others, which seriously compromises the lifetime of the IGBT modules. Therefore, effective improvement measures that are low cost become crucial for the IGBT modules in MMC. Reverse conducting IGBT (RC-IGBT) constructs the IGBT and freewheel diode in a single chip, which has the advantages of lower thermal resistance, lower switching loss, and larger heat capacity. In this article, a new generation of RC-IGBT module and conventional IGBT (Con-IGBT) module from Fuji Electric with the same packaging are both used in the MMC. A newly defined current density for RC-IGBT and Con-IGBT module is proposed. The thermal performances and annual damages of the two modules under different power factors are comprehensively compared at the same module current density. The results show that when the power factor is greater than 0.4, using RC-IGBT can significantly improve the thermal performance of MMC submodules (SMs) and extend their lifetime. In addition, the reasons for the comparison results are also discussed in this article. Finally, the advantages of RC-IGBT in terms of thermal performances are verified by experiment. This article also provides a guideline for the design of MMC SMs.

As a key parameter in microgrids, feeder impedance is closely related to power distribution, power quality, and system stability. This letter proposes a novel noninvasive feeder impedance estimation method, which uses the microgrid central controller (MGCC) and inherent harmonic signals introduced by a nonlinear or unbalanced load. First, the sequence impedance model for a droop-controlled inverter is built to demonstrate that the inverter output impedance is negligible at low-order harmonic frequencies. Then, the bus voltage and output current at a specific dominant harmonic frequency are extracted in MGCC, based on which feeder impedance can be obtained. Finally, the estimated feeder impedance is used to generate proper virtual impedance to enhance power sharing and voltage quality as an example, based on a hierarchical control structure. Experimental results are provided to validate the proposed method.

In order to achieve equivalent modulation of the multilevel current converter, based on the topology of three-phase multilevel current-source converters, this article presents a method to realize the output characteristic of space vector pulsewidth modulation (SVPWM) with carrier-based PWM, which can be applied in the topologies from five-level to n -level space vector modulation strategy. Different from the strategies of traditional unity theory, the proposed method analyzes the relationship directly according to the switching states of SVPWM in three-phase multilevel current-source converters. By constructing the equivalent relationship between the output level of each phase and switching states, the carriers can control the working status of the switches directly. In addition, this method is generalized to the topologies and modulation strategies under n -level, which fully reflects the correctness and universality of the proposed method. Finally, the results of simulation and experiment verify the correction of the method.

Due to the mismatched line impedances in islanded microgrids, distributed generation units with conventional droop tend to have barely satisfactory reactive power sharing accuracy. To handle this problem, this article proposes a virtual impedance tuning method based on successive approximation to accurately compensate the mismatch among line impedances. First, in each distributed generation (DG) unit, an adaptive virtual impedance regulation based on the error between the actual output reactive power and its reference is established and combined with the conventional Q - V droop control. Then, these two control schemes operate alternately for several cycles, and in each cycle, the reactive power reference of the virtual impedance regulation is updated by the latest estimate from the Q-V droop control. Therefore, the virtual impedance of each DG unit will be tuned successively to an appropriate value to achieve accurate power sharing among DG units based on local information. Meanwhile, to ensure all the DG units switching between these two control schemes synchronously, a common triggering signal from the microgrid central controller is needed to start the internal time sequence of each DG unit. Furthermore, a comprehensive design procedure for key parameters is presented based on convergence analysis and small-signal modeling of the tuning process. Finally, the effectiveness of the proposed method is verified by simulation and experimental results. This article is accompanied by a video demonstrating the virtual impedance tuning process.

Multiple insulated gate bipolar transistor (IGBT) open-circuit faults severely affect the reliable operation of modular multilevel converters (MMCs). The existing literatures have not provided an effective diagnosis algorithm for MMCs when multiple IGBT open-circuit faults simultaneously appear in one arm. This article presents a diagnosis strategy to handle that condition. Fault detection and estimation of the faulty submodule (SM) number is implemented by checking the value of the arm voltage error. The faulty SMs are located by observing when the transition of their switching functions leads to an obvious change of the arm voltage error. Meanwhile, the errors between the measured and predicted capacitor voltages are utilized to accelerate the localization. All of the faulty SMs can be quickly located one by one within several control cycles by using the proposed localization method. Simulation and experimental results demonstrate the validity of the proposed diagnosis strategy.

Grid impedance information occupies a fundamental and crucial position for the operation of grid-tied converters. Consequently, accurate and fast extraction of grid impedance is vital. The impulse injection method applied under the control loop of a grid-tied converter, which is commonly used to extract linear grid impedance online, is relatively reasonable in terms of device cost, measurement speed, simplicity, and design flexibility. However, the selection of impulse and design of impulse parameters mostly depend on rules of thumb, without theoretical guidance, leading to insufficient measurement accuracy in target frequency section and greatly burdening the system. To address these issues, a systematic guidance is proposed for analysis, design, and implementation of impulse-injection-based grid impedance identification. By comparing the spectra of several basic impulses with unipolar and bipolar forms and analyzing the impact when impulse parameters are altered, a guidance for impulse selection for different measurement scenarios is presented. Based on this guidance, an asymmetric bipolar sawtooth wave with a defined asymmetry ratio of $\boldsymbol{\rho } = 0.5$ is chosen in this article for broadband measurement of grid impedance. Furthermore, a related parameter design procedure with optimal injection position and instant is also proposed to reduce system burden and eliminate overmodulation when injecting the impulse. Finally, theoretical results are validated by experiments.

This article first analyzes the effect of the level-increased nearest level modulation (NLM) to the circulating current of the modular multilevel converter (MMC). The total inserted submodule (SM) number in one phase-leg of the MMC is variable using the level-increased NLM, which can significantly increase the peak-to-peak value of the circulating harmonic currents. Then, a circulating harmonic currents suppression method is proposed by applying deadbeat control for the MMC with level-increased NLM. Compared with the existing direct circulating current control approaches for the MMC, the proposed method presents specific extension and adjustment principle of the total inserted SM number. Therefore, the proposed method can improve the dynamic control performance and handle large circulating harmonic currents at steady state. And it can well coordinate the modulation and circulating harmonic currents suppression stages to avoid disturbing the ac-side output voltage while regulating the circulating current. The effectiveness of the proposed method is evaluated by a single-phase industrial-level simulated MMC system and a laboratory experimental platform.

Digital control delay is studied as one of the leading factors causing instability in grid-tied voltage source inverter (VSI) systems. However, how this delay affects the small-signal output impedance of the VSI and how the change in impedance shape further influences the system stability are still not clear. This article, for the first time, thoroughly analyzes the impact of delay on the dq -frame output impedance of the VSI operating in both the single current loop control mode and the dual-loop control mode. It is found that when delay is considered, a distinct magnitude drop and a phase rise always exist at the high-frequency area of the impedance, which is named the “impedance trough.” By studying the dq -frame output impedance of the VSI, the origin of the impedance trough, and its analytical relationships with the current loop dynamics, voltage loop dynamics, and delay values are unveiled. Meanwhile, the influence of the impedance trough on the stability of the grid-tied VSI system under these two control modes is evaluated through d–d channel impedance matching and generalized Nyquist criterion (GNC). It is demonstrated that the joint effects of delay and control parameters result in the impedance mismatch between the grid and the VSI, which is the root cause of high-frequency oscillations in the grid-tied system. Finally, experimental results verify the analysis.

In an islanded microgrid composed of droop-controlled parallel inverters, the system frequency endures deviations as the load changes. To compensate for frequency deviation without involving communication infrastructures among distributed generators (DGs), the proportional-integral regulator based secondary frequency control (PI-SFC) method has been proposed in the literature. However, PI-SFC may incur real power-sharing errors because the integrator accumulates disturbances and noise in each DG, leading to different compensation values of nominal real power. To achieve frequency restoration while maintaining equal real power sharing among DGs, this article proposes a small-ac-signal injection-based secondary frequency control (SACS-SFC) method, which is implemented by injecting an additional ac signal into the output voltage of each DG. Furthermore, a droop relation between the frequency of the injected SACS and the compensation value of nominal real power is innovatively established to trim the output real power of each DG to be equal. Frequency deviations caused by primary droop control are thus eliminated, and even real power sharing can be maintained among DGs. Moreover, the control parameters of the proposed SACS-SFC are comprehensively designed via steady state and dynamic model of the system. Simulation and experimental results demonstrate the effectiveness of the proposed method.

A universal controller is proposed in this article to operate parallel inverters in both grid-connected (GC) state and standalone (SA) state and ensure seamless transfer between them without reconfiguring the control structure. The universal controller is mainly composed of frequency-locked-loop and three cascaded control loops: a grid current loop, capacitor voltage loop, and inductor current loop. A proportional-integral regulator is adopted in the grid current loop, and a limiter is inserted after the integrator. In the GC state, the proposed controller accurately regulates the grid current of an individual inverter. When islanding occurs, the proposed controller can automatically convert from grid current control to $v_{C}\hbox{--} i_{g}$ -based droop control; critical islanding detection is not needed. This universal controller has several advantages. First, it can realize seamless transfer of parallel inverters; microgrid reliability can be guaranteed in the SA state, and power sharing can be achieved among parallel inverters without communication lines. Second, compared with droop control, it can regulate the grid current accurately and output constant power when the grid voltage fluctuates in the GC state. Third, the grid current harmonics in the GC state and capacitor voltage harmonics in the SA state can be mitigated. Simulation and experimental results verify the effectiveness of the controller.

The modular multilevel converter (MMC) is a popular research topic in power electronics and is a promising topology for high-power applications. Many steady-state analysis models have been investigated for MMCs. The circular interaction among the electrical quantities in an MMC has been established and improved models have been proposed, with consideration of the second-order circulating current controller and line harmonic currents in the analysis. So far, no steady-state analysis models have addressed the effect of averaging voltage regulation control, which influences switching function of MMC upper and lower arms, and finally, the accuracy of steady-state analysis. In this article, we discuss the validity of existing models with and without averaging control. After extensive simulations, it is found that the accuracy is affected, showing the weakness of existing models and verifying the effect of averaging voltage regulation control on steady-state analysis of MMCs. Then, to solve this problem, an improved analysis model is proposed. In this model, averaging control is considered at the beginning of circular-interaction-based analysis. The new model presents a higher accuracy compared with the existing ones.

This article studies the modeling of high-bandwidth buck converters under constant-frequency voltage-mode control, where the traditional small-signal averaged model fails due to two major factors in the pulsewidth modulation (PWM), i.e., the large harmonics of modulation waveform and the considerable sideband components. Several modified small-signal models have been proposed to improve the predictions of the frequency domain characteristics. However, these models are less generalized or with high complexity, and may hardly be used in the closed-loop design of real applications. To handle this issue, this article proposes a generalized multifrequency small-signal model (G-MFSSM) for high-bandwidth buck converters, which makes further progress in accuracy while maintaining simplicity in deriving the analytical transfer functions for a closed-loop design. The generalized average model is adopted to precisely describe the first-order harmonic of the modulation waveform, from which a correction factor is extracted to evaluate the influence of the ripple. Furthermore, two dominant sideband components are selected optimally for extremely accurate loop gain predictions. Finally, a closed-loop design procedure is presented for wide-input, high-bandwidth buck converters based on the proposed G-MFSSM. The validity of the proposed G-MFSSM is verified by simulations and experiments.

A dc terminal impedance model of voltage-source converter (VSC) is required to analyze the stability of multiterminal high-voltage direct current system. This article proposes a comprehensive dc terminal impedance model for conventional three-phase two-level VSC under different control modes. The control strategy includes inner decoupled current control and outer control, the latter contains dc voltage control, active/reactive power control, and droop control for grid-connected VSC and ac voltage control for VSC connected to a load. When a VSC is connected to the ac grid, the phase-locked loop (PLL) is also taken into account. The influences of PLL dynamics, control time delay and dead time on the dc-side impedance are explained. Also, how the parameters of the inner current control and outer control affect the characteristics of dc terminal impedance is analyzed in detail. Finally, experimental measurements verify the proposed impedance models and analysis.

Parallel droop-controlled inverters with renewable energy sources are widely employed in islanded ac microgrids, where dynamic interactions among them may cause small-signal stability issues. Since the active power-frequency droop scheme is applied, the dynamic interactions among inverters exist not only in bus voltage and transmitted current, but also in variable system fundamental frequency. This paper introduces a novel small-signal terminal characteristic model for droop-controlled inverter. Besides conventional impedance and admittance, a new set of terminal characteristics is proposed to characterize the dynamics of fundamental frequency. Furthermore, the small-signal model of parallel inverters is constructed based on the terminal characteristics of individual inverters. Covering the fundamental frequency interactions, a stability prediction approach based on generalized Nyquist criterion is proposed for parallel droop-controlled inverters. Besides the product of impedance and admittance, an additional term is added in the system return ratio, which consists of the proposed terminal characteristics associated with the fundamental frequency. Finally, experimental results validate the effectiveness of this proposed stability prediction approach.

LCL resonance complicates the design of a current control loop and can even threaten its stability. Extensive approaches have been proposed to deal with this resonance, among which active damping (AD) schemes based on the feedback of a single filter variable have been shown to be effective and cost-efficient. This paper presents a study of such AD techniques, where a generalized approach to developing an AD controller by relating a control diagram to an equivalent circuit is proposed. Based on this approach, AD controller forms with any one of four commonly used filter variables to realize virtual resistors (VRs) in six different connections to the LCL filter are derived. Comparisons are then made between these 24 AD controller alternatives by considering the implementation complexity of the AD controller, the number of measuring sensors, and the effect of the AD controller on the power stage. Consequently, grid-side current feedback AD to realize a VR in parallel with the filter capacitor is selected as a considered alternative. Next, two issues associated with the practical implementation of the selected grid-side current feedback AD, caused by the second-order differential expression and the digital time delay, are discussed and solved. Finally, the selected AD method is analyzed in the discrete z -domain, and its effectiveness is experimentally verified.

To address the unbalanced and harmonic power sharing issue among parallel inverters caused by feeder impedance mismatch in islanded microgrids, an adaptive virtual impedance control method is proposed based on the injection of an extra small ac signal (SACS) in the output voltage of each inverter. Similar to the principle of active power–frequency droop, the frequency of the injected signal droops with the output unbalanced and harmonic power, while the active power produced by the injected SACS is detected to adjust the virtual impedance at the fundamental negative sequence and selected harmonic frequencies, which will tune the distribution of unbalanced and harmonic power in the system. When the injected SACSs of each inverter synchronize with each other and reach a common frequency in steady state, the virtual impedance of each inverter will be matched to each other for evenly sharing the unbalanced and harmonic power. This proposed method requires neither communication links among parallel inverters nor feeder impedance information. Furthermore, the control parameter design method based on modeling and stability analysis of the proposed control structure is discussed in detail. Finally, simulation and experimental results are provided to validate the effectiveness of the proposed scheme.

The solid-state transformer (SST) has the ability to operate with only two input phases, which can be called single-phase open-circuit (SPO) operation. With this ability, the SST can maintain operation under permanent single-phase-to-ground faults, where the fault phase must be isolated, providing a better power supply sustainability than that of the conventional low-frequency transformer. In the absence of research on the SPO operation of an SST, this paper presents a study of it. The influence of SPO operation on the grid and the SST itself is analyzed. Under the SPO state, the SST will inject the negative-sequence current to the grid; meanwhile, the maximum load power will decline; thus, the optimized power factor under the SPO state is unity. The maximum power of △-SST is higher than that of the Y-SST under the SPO state owing to the △ connection. A control strategy for the SST medium-voltage cascaded H-bridge stage is proposed. SPO current control can be achieved by controlling the positive-sequence current. A proper negative-sequence voltage can be injected to synchronize the cluster voltages to obtain a lower current total harmonic distortion and smaller dc voltage ripple. In the △-SST, a proper zero-sequence current can be injected to fully use the power rating of the double-cluster branch.

Small-signal stability of ac power systems is able to be assessed by measuring their terminal impedance, and a transformerless series injector is very attractive for wideband measurement of load impedances. A typical implementation of the transformerless series injector is paralleled H-bridge (PHB) converters, where dc voltage imbalance will occur when system current magnitude is larger than a critical point, and thus the operation region of the series injector is seriously restricted. To address this issue, this paper presents a deep analysis on the mechanism behind dc voltage imbalance for PHB-based series injector, and it is revealed that a positive feedback exists in the dc voltage balancing loop under high system current magnitude. Then an enhanced control scheme is proposed to balance dc voltages in full system current range, where a reactive component is injected in the output voltage while reactive circuiting current is introduced among H-bridge converters for redistributing active power. Furthermore, the system behavior with the proposed scheme is comprehensively analyzed and the reference design for the injected reactive component magnitude in the output voltage is offered. Finally, the imbalance mechanism and the proposed scheme are validated by simulation and experimental results.

The grounding of low-frequency distribution transformers is a critical concern that ensures the proper operation and protection of the distribution system. The power electronic transformer (PET), as a future alternative low-frequency distribution transformer, also needs grounding. This paper studies PET operation with arc-suppression-coil (ASC) grounding in the Y-connected medium-voltage (MV) stage, focusing on the following two aspects. 1) The MV stage current stress brought by the ASC is analyzed, and a negative-sequence current compensation method is proposed to reduce the MV stage current stress. 2) The ASC impedance adjustment range that the MV stage can provide by zero-sequence voltage injection is analyzed, and a virtual impedance method is proposed to achieve this adjustment. The effectiveness of the proposed ASC-PET as well as its control strategy is verified by both simulation and experiment results.

In this paper, a generalized droop control (GDC) is proposed for a grid-supporting inverter based on a comparison between traditional droop control and virtual synchronous generator (VSG) control. Both the traditional droop control and VSG control have their own advantages, but neither traditional droop control nor VSG control can meet the demand for different dynamic characteristics in grid-connected (GC) and stand-alone (SA) modes at the same time. Rather than using a proportional controller with a low-pass filter, as in a traditional droop control, or fully mimicking the conventional synchronous generator parameters in a VSG control, the active power control loop of the GDC can be designed flexibly to adapt to different requirements. With a well-designed controller, the GDC can achieve satisfactory control performance; unlike a traditional droop control, it can provide virtual inertia and damping properties in SA mode; unlike a VSG control, the output active power of an inverter with GDC can follow changing references quickly and accurately, without large overshoot or oscillation in the GC mode. Moreover, given specific controller parameters, the GDC can function as both a traditional droop control and a VSG control. The GDC's controller parameter design is more intuitive and flexible, and this paper provides a distinct design process. Finally, the effectiveness of the proposed control method is validated by the simulation and experimental results.

A negative-output high quadratic conversion ratio dc–dc converter with dual working modes is proposed. One working mode is a step-up mode, and the other is a step-up/step-down mode. Analysis of the circuit principle and different operation modes, steady-state calculation, and small-signal modeling are presented. Comparisons of the proposed converter with other converters demonstrate that the proposed converter has a higher conversion ratio in its two working modes and, under the same voltage gain, the proposed converter in the second working mode has the lowest switching device power rating, a higher efficiency, and small peak-to-peak value of input current. Simulation results based on PSpice and circuit experiments are given to verify the effectiveness of the proposed converter.

Z-source inverter provides a competitive single-stage dc–ac power conversion with the capability of both buck and boost voltage regulation. In order to maximize voltage gain and to increase efficiency, this paper proposes an improved pulse-width modulation (PWM) strategy. By adjusting the shoot-through duty ratio of one-phase leg, it regulates the average value of intermediate dc-link voltage, which is the same as the instantaneous maximum of three-phase line voltage in one switching time period $(T_{s})$ . And the other two-phase legs maintain the fixed switching states. Thus, the equivalent switching frequency of power devices in the inverter bridge is reduced to $1/ 3f_{s}$ ( $f_{s}$ is the frequency corresponding to $T_{s}$ ). The operating principles and closed-loop controller design are analyzed and verified by simulation and experiments. Compared with the existing PWM strategies, the improved PWM (IPWM) strategy demonstrates higher efficiency under full operation range of low voltage gain (1.27-2) application. However, with the IPWM strategy, the inductor current and capacitor voltage contain six-time-line-frequency ripples, which consequently require large size of the passive components when the output frequency is very low. Thus, it is also suitable for 400–800-Hz medium frequency aircraft and vessel power supply system due to a relatively high output line frequency. Furthermore, the idea of IPWM strategy can be extended to other kinds of three-phase impedance network-based inverters.