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This article proposes a multi-resonant three-port DC-DC converter (MRTPC) that can achieve power decoupling in the presence of deviations in the parameters of the resonant elements. The MRTPC consists of a four-element resonant tank (FERT), a three-element resonant tank (TERT), and an LC resonant tank (LCRT). Due to the fact that the FERT can be configured with first and third resonant frequencies (RF), it can transmit the energy of the fundamental and third harmonic components. By reasonably setting the RFs of the FERT, TERT, and LCRT, decoupling control of primary and tertiary energy transmission can be achieved. Due to the characteristics of the FERT, TERT and LCRT at different frequencies, decoupling the transmission of the power of the primary and tertiary frequency components can be achieved even when there is a certain deviation in the parameters of the resonant elements of the FERT. This feature can effectively reduce the cost of selecting resonant elements. To improve the efficiency of the MRTPC, an optimized triple-phase-shift modulation strategy (OTPSMS) is proposed to minimize the root mean square value of the resonant current (RMSRC). For the nonlinear problem of minimizing the RMSRC, the KKT condition is used for the optimization solution. Finally, an experimental prototype with a power level of 1 kW is built to verify the theoretical correctness of the MRTPC and its OTPSMS. [ABSTRACT FROM AUTHOR]

Low-frequency pulsating ripples exist on the input side of a single-phase inverter, which bring some adverse effects and harm to the inverter and photovoltaic power generation system. In order to suppress the low-frequency pulsating ripple and reduce the filter circuit parameters, a novel single-stage boost single-phase inverter is proposed, which can suppress low-frequency ripple. And a three-closed-loop compound control strategy that can suppress input low-frequency ripples under the limitation of an energy storage inductor current and buffer capacitor voltage is proposed. The circuit topology, control strategy, key circuit parameters design, system modeling, and simulation of the inverters are deeply analyzed and studied. Simulation and experimental results show that the inverter has a good ability to suppress input low-frequency ripples. [ABSTRACT FROM AUTHOR]

Copyright of Electric Machines & Control / Dianji Yu Kongzhi Xuebao is the property of Electric Machines & Control and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

When a grid-forming (GFM) inverter is connected to a low- or medium-voltage weak power grid, the line impedance with resistive and inductive characteristics will cause power coupling. Typical GFM decoupling control strategies are designed under nominal line impedance parameters. However, there are deviations between the nominal line impedance and actual parameters, resulting in poor decoupling effects. Aiming at this problem, this paper proposes a power decoupling strategy based on a reduced-order extended state observer (RESO). Firstly, the power dynamic model of the GFM is established based on the dynamic phasor method. Then, the model deviation and power coupling due to line impedance parameter perturbation are estimated as internal disturbances of the system, and the disturbances are compensated on the basis of typical power control strategy and virtual impedance decoupling. Good decoupling performance is obtained under different impedance parameters, improving the control strategy's robustness. Finally, the effectiveness of the proposed method is verified by the results of RT Box hardware-in-the-loop experiments. [ABSTRACT FROM AUTHOR]

Copyright of Electric Drive is the property of Electric Drive Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Multiport DC–DC converters based on a dual-active-bridge (DAB) topology have attracted attention due to their high power density and bidirectional power transfer capability in DC microgrid systems. In addition, connectivity is high for various distributed resources (DRs). However, power coupling among ports magnetically connected by single or multiple transformers induces poor power control and voltage regulation performance. To mitigate these technical issues, an enhanced power decoupling method for a four-port DAB converter is proposed, which does not have supplementary power decoupling algorithms under the grid-connected mode and islanding mode operations of a DC microgrid system. In addition, a modular structure can be applied to individually connect a system with various loads. Finally, a 3-kW prototype four-port DAB converter is implemented to verify the validity and performance of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a single-stage buck-boost differential inverter is optimally designed for applications with varying input DC voltage (e.g., photovoltaics and fuel cell systems). The designed inverter has multiple functionalities, including power decoupling and AC output filtering, and it can operate with a wide DC voltage range without adding extra power conversion stages or filters. Hence, it is naturally compact and highly efficient. To fully exploit its benefits, the proposed inverter operating principle and mathematical model were first developed to form the foundation of an optimal design. The criteria for selecting the inverter's key components have been presented. This ensures that the developed inverter meets the aforementioned functional requirements without being overly sized. A digital design procedure based on artificial neural networks is followed for further multiple objective optimization, targeting high efficiency, high power density and low cost. A 1.8kW prototype of the inverter was fabricated through the digital design. The inverter's operating functionality with varying DC voltage, power decoupling, and filtering was demonstrated by both simulation studies and experimental tests on the prototype. The accuracy of the optimal design was also validated.

Capacitance reduction in single-phase rectifiers has been the focus of several works, allowing, in many cases, the replacement of short-lifetime electrolytic with film capacitors, prolonging the lifespan of the converter. Among the different techniques used to promote this capacitance reduction, the use of modulation and control strategy has the advantage of not adding extra elements to the circuit. Thus, this work proposes the application of a hybrid modulation in a converter derived from integrating an interleaved boost rectifier and a full-bridge Zero Voltage Switching (ZVS) DC-DC converter to reduce the output low-frequency ripple actively. Besides the active low-frequency voltage ripple reduction allowing employ only film capacitors, the proposed solution presents low DC bus voltage, reducing the voltage stress, low input current ripple, simple control strategy, and fast dynamic response. Experimental results of a prototype with a rated power of 300 W are presented to validate of the proposed solution, operating with an efficiency of 92.4%, a power factor of 0.9936, and a total harmonic distortion (THD) of 10%. The adopted hybrid modulation and control strategies allow for reducing the voltage output ripple with a capacitance/watts ratio of $0.18~\mu $ F/W, a 33 times capacitance reduction, avoiding electrolytic capacitors, and maintaining a fast dynamic response of output voltage.

Low-frequency pulsating ripples exist on the input side of a single-phase inverter, which bring some adverse effects and harm to the inverter and photovoltaic power generation system. In order to suppress the low-frequency pulsating ripple and reduce the filter circuit parameters, a novel single-stage boost single-phase inverter is proposed, which can suppress low-frequency ripple. And a three-closed-loop compound control strategy that can suppress input low-frequency ripples under the limitation of an energy storage inductor current and buffer capacitor voltage is proposed. The circuit topology, control strategy, key circuit parameters design, system modeling, and simulation of the inverters are deeply analyzed and studied. Simulation and experimental results show that the inverter has a good ability to suppress input low-frequency ripples.

Grid-forming inverters, which are represented by droop control and virtual synchronous generator control, have been widely studied and applied because of their excellent grid-supporting ability and smooth off-grid switching. When a grid-forming inverter is connected to a microgrid or utility grid, the control loops of active power and reactive power will be coupled because of the voltage phase difference, which will affect the power control performance. This paper first derives the small-signal linearized model of the system, based on which a frequency feedforward control and an amplitude feedforward control are proposed to decouple the active power and reactive power control loops, respectively. The proposed decoupling strategy directly modifies the reference values through feedforward with an easily implementable principle that is applicable to various control coordinate systems, control coordinate systems, and control structures. By comparing system models with and without the proposed decoupling strategy, its effectiveness can be theoretically proven. Time-domain simulations and hardware experiments are presented to further validate its effectiveness. [ABSTRACT FROM AUTHOR]

Under low-voltage microgrid conditions, when virtual synchronous generators (VSG) are connected in parallel with loads, there is power coupling between the output active and reactive power due to the resistive characteristic of the equivalent transmission impedance, and the output power suppression effect cannot be accurately distributed with poor current circulation, which seriously affects the steady-state performance of the system; Secondly, the line impedance of each inverter control system often does not match the capacity ratio, making the traditional virtual impedance decoupling strategy somewhat limited. The paper proposes a power decoupling strategy based on improved virtual impedance to address this problem by using a fuzzy algorithm to monitor the VSG power angle to adjust the virtual impedance in real time and still meet the requirements of VSG parallel operation power equalization and circulating current suppression during sudden load changes. Finally, a VSG simulation model is constructed in MATLAB/Simulink to verify the feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

This paper discusses the power coupling problem of the traditional virtual synchronous generator (VSG) control strategy in medium and low-voltage microgrids, analyzes the cause of power coupling, and discusses the necessity for decoupling. In addition, a power decoupling control strategy based on an adaptive diagonal matrix compensation matrix is proposed. By compensating for the coupling component, the output power of the VSG is completely decoupled. To cope with the operating characteristics of frequent load switching on the grid side of a microgrid, the proposed strategy tracks variations in the static operating point of the VSG and automatically adjusts the compensation component. Simultaneously, the stability of the VSG is analyzed, using the transient instability criterion of the power system, to ensure decoupling. Finally, the effectiveness of the proposed control strategy is verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes an effective VAR source established as a cascaded H-bridge (CHB) static compensator (STATCOM), which is based on the flux cancellation method. The conventional CHB-STATCOM uses a low-frequency large capacitor as a source. The required capacitance value of the capacitor increases as a function of the amount of VAR produced to compensate the power system. The proposed topology is based on flux cancellation. Therefore, the source of VAR in this case is not limited by the capacitors or the power decoupling transformer size. The sub-module (SM) of the proposed topology comprises of a CHB module with power decoupling circuits. The double-frequency ripple powers on the sub-module (SM) capacitors, which have a phase shift of 120° with respect to each other. The ripple powers are derived from each of the three phases toward a common magnetic core to cancel each other out. An isolated bidirectional triple port dual half-bridge converter is utilized for the flux cancellation process. In this converter, the main challenges are leakage inductances and the high voltage insulation among the three windings of the high-frequency transformer. An αβ-frame-based model is proposed, using the generalized state-space averaging method, for the flux cancellation circuit. Furthermore, the size is significantly reduced by the proposed method, since a small sub-module capacitance of a few microfarads is sufficient to operate the CHB-STATCOM. The analysis and the controller design process are presented, followed by simulation and hardware validations. [ABSTRACT FROM AUTHOR]

This study compares ripple port, stacked switched capacitor, and capacitive energy storage architectures for active power decoupling, comparing the number of components, performance, energy density, DC-link capacitor reduction, efficiency, and frequency operation to highlight their main benefits and drawbacks for single-phase grid-connected applications. The overview reveals equivalent effective energy density without electrolytic capacitors, as well as enhanced simplicity, performance, and durability, thereby providing stacked switched capacitors as an attractive power-decoupling alternative for multi-stage connected applications, based on the principle that its individual buffer capacitors absorb and deliver energy without tightly constraining their individual terminal voltages, while maintaining a narrow range voltage at the buffer DC port. [ABSTRACT FROM AUTHOR]

With the rapid growth of the power grid load and the continuous access of impact load, the range of power system frequency fluctuation has increased sharply, rendering it difficult to meet the demand for power system frequency recovery through primary frequency modulation alone. Given this headache, an optimal control strategy for battery energy storage participating in secondary frequency regulation of the power grid is proposed in this paper based on a double-layer structure. Besides, a coordinated control framework is constructed for energy storage battery joint units engaged in automatic generation control (AGC). At the dispatching level, the power allocation principle is set to coordinate the fast and slow resources of energy storage and conventional thermal power units, and the power decoupling of the two types of frequency modulation (FM) resources is completed. At the system level, a power allocation model representing the real-time frequency modulation capability of energy storage is established to realize the division of frequency modulation responsibilities of each unit and state of charge (SOC) consistency management, and the proposed control strategy is simulated and verified to provide a reference for the energy storage battery to participate in the secondary frequency modulation design of the power grid. [ABSTRACT FROM AUTHOR]

Grid-forming inverters, which are represented by droop control and virtual synchronous generator control, have been widely studied and applied because of their excellent grid-supporting ability and smooth off-grid switching. When a grid-forming inverter is connected to a microgrid or utility grid, the control loops of active power and reactive power will be coupled because of the voltage phase difference, which will affect the power control performance. This paper first derives the small-signal linearized model of the system, based on which a frequency feedforward control and an amplitude feedforward control are proposed to decouple the active power and reactive power control loops, respectively. The proposed decoupling strategy directly modifies the reference values through feedforward with an easily implementable principle that is applicable to various control coordinate systems, control coordinate systems, and control structures. By comparing system models with and without the proposed decoupling strategy, its effectiveness can be theoretically proven. Time-domain simulations and hardware experiments are presented to further validate its effectiveness.

Second-order ripples occur in the voltage and current during any DC–AC power conversion. These conversions occur in the voltage source inverters (VSIs), current source inverters (CSIs), and various single-stage inverters (SSIs) topologies. The second-order ripples lead to oscillating source node currents and DC bus voltages when there is an interconnection between the AC and DC microgrids or when an AC load is connected to the DC bus of the microgrid. Second-order ripples have various detrimental effects on the sources and the battery storage. In the storage battery, they lead to the depletion of electrodes. They also lead to stress in the converter or inverter components. This may lead to the failure of a component and hence affect the reliability of the system. Furthermore, the second-order ripple currents (SRCs) lead to ripple torque in wind turbines and lead to mechanical stress. SRCs cause a rise in the temperature of photovoltaic panels. An increase in the temperature of PV panels leads to a reduction in the power generated. Furthermore, the second-order voltage and current oscillations lead to a varying maximum power point in PV panels. Hence, the maximum power may not be extracted from it. To mitigate SRCs, oversizing of the components is needed. To improve the lifespan of the sources, storage, and converter components, the SRCs must be mitigated or kept within the desired limits. In the literature, different methodologies have been proposed to mitigate and regulate these second-order ripple components. This manuscript presents a comprehensive review of different effects of second-order ripples on different sources and the methodologies adopted to mitigate the ripples. Different active power decoupling methodologies, virtual impedance-based methodologies, pulse width modulation-based signal injection methodologies, and control methods adopted in distributed power generation methods for DC microgrids have been presented. The application of ripple control methods spans from single converters such as SSIs and VSIs to a network of interconnected converters. Furthermore, different challenges in the field of virtual impedance control and ripple mitigation in distributed power generation environments are discussed. This paper brings a review regarding control methodologies to mitigate and regulate second-order ripples in DC–AC conversions and microgrids. [ABSTRACT FROM AUTHOR]

Due to fewer conduction devices in operating condition, the bridgeless power factor correction (PFC) converter is more efficient than the traditional PFC circuit. However, to achieve a low output voltage ripple on the DC side, a large electrolytic capacitor must be connected in parallel to the output end. To reduce the value of capacitance, this paper proposes a dual-boost bridgeless PFC converter with a bidirectional buck/boost power decoupling converter in the latter stage. The bidirectional converter absorbs double-line-frequency ripple, lowering the power pulsation at the output end while realizing power decoupling. The one-cycle control is adopted in bridgeless PFC converter, so that the input current can follow the input voltage to achieve power factor correction and decrease harmonic pollution. The power decoupling circuit is designed with a voltage outer loop using PI control and a current inner loop using model predictive current control, which alleviates the output voltage fluctuation caused by the reduction of the capacitance value of the filter capacitor, for the purpose of realizing non-electrolytic capacitor. Finally, the topology and control strategy involved in this paper are simulated and experimented to verify the validity and superiority of the theory. [ABSTRACT FROM AUTHOR]

In a microgrid system, the dc bus contains a significant amount of undesired low-order harmonic currents (LHC) when multiple converters of various types and with nonlinear behavior are connected. To address the issue, this article proposes an iterative control strategy based on a negative-order capacitor (NOC) branch on dc bus. In the proposed method, the reference voltage of decoupling capacitors in the NOC circuit is obtained based on the dc bus current. The voltage difference between the two decoupling capacitors is taken as the control objective, leaving little dc difference between the two capacitors and resulting in maximum LHC suppression ability. To refactor the reference voltage, a singularity points hunting strategy is proposed, helping to reduce the current spike. More importantly, the iterative control strategy is employed in the virtual current loop, decreasing steady-state error from device deviation or control deviation. Finally, an 800 W prototype is built to validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This work introduces a compensation scheme that uses a genetic algorithm to mitigate the 2nd voltage harmonic component of hybrid microgrids. The foremost advantage of the proposed scheme, is its capability to effectively compensate the 2nd harmonic component regardless of the feeder impedance, the location of the equipment and without any need for real-time communications. A graphical representation of the genetic algorithm parameters is also presented, being a handily tool for designing it for any microgrid application and the mathematical analysis of hybrid microgrids under the prospect of harmonic compensation is given. Simulations and experimental tests verify the effectiveness of the proposed method, indicating that its efficiency is maintained at about 90%, while the efficiency of the other methods falls below 35% when the impedance of the connection line increases at $1+\,\,\text{j}0.16\,\,\Omega _{\mathrm {(\text{@}100 Hz)}}$ . Finally, a comparative analysis is also performed between the proposed method and existing ones.

The design of single-phase differential buck inverters has two important considerations, including reducing second-order ripple power using decoupling capacitors and increasing inverter performances. Using larger decoupling capacitors will improve the performance of ripple power reduction and efficiency while reducing power density. Such trade-off has not been fully modeled and investigated, leading to the sub-optimal design of inverters. To address that, in this paper, the trade-off among decoupling capacitance, inverter efficiency and power density are investigated through detailed mathematical modeling and sensitivity study. The trade-off of the volume and power loss of essential inverter components, including power switches, inductors and heatsinks, are also studied to facilitate the inverter design. A fast multi-objective design optimization method based on geometric programming is presented to optimize the inverter efficiency and power density. A 1 kW prototype of a Gallium Nitride (GaN) based inverter has been designed based on the proposed method. A hardware prototype of the inverter has been built and tested, which has an efficiency of 98.02% and power density $ {4.54\;\mathrm{{kW/dm^{3}}}}$ and matches 99% to the presented multi-objective design method. This validates the accuracy and effectiveness of the presented design approach considering detailed trade-off analysis.

This paper proposes an adaptive grid impedance shaping approach, implemented through a variable structure virtual impedance control strategy (VS-VIS), to improve the power flow control of grid-forming power converters (GF-PC). In this strategy, an equivalent grid impedance estimator (EGIE) evaluates the feeder X/R ratio to determine the required virtual impedance to reach the grid shaping objective. Besides, the virtual impedance definition also considers the converter power rating to ensure the system’s feasibility. A virtual negative resistance series interconnected with a positive virtual reactance implements the virtual impedance. Moreover, the required virtual impedance is only updated when the X/R ratio extrapolates deviation limits determined by allowed grid voltages boundaries specified by the standards. Besides, the virtual reactance is split into two parts that use the conventional and sliding mode (SMC) approaches. The first implements the main part of required virtual reactance, while the second provides a fine-tuning action matching the X/R control objectives and compensating grid impedance uncertainties. This grid-shaping method provides a smooth compensation increasing the GF-PC resilience under grid impedance variation. These features constitute a novelty compared to the shaping methods presented in the literature. Experimental tests obtained from a GF-PC laboratory setup demonstrate the effectiveness and robustness of the proposed grid impedance shaping method.

In many applications of wireless power transfer (WPT) systems, there are situations where the transmitter needs to supply power to receivers of different frequencies or powers. Therefore, it is very important to improve its compatibility and adaptability. In this paper, a dual-frequency compatible WPT system with a single transmitter and multiple receivers is proposed. The transmitter current contains two main frequency components obtained by current modulation and resonance compensation. The receivers of different frequencies pick up the corresponding components by frequency selective resonance. Therefore, decoupled transfer and independent control can be achieved. In this paper, a dual-frequency compensation topology is constructed and an adjustment method for receiver power decoupling control is proposed. Through theoretical and experimental analysis, the efficacy characteristics of the system have been evaluated, and its dual-frequency compatibility and power decoupling control capability have been verified. Compared with the single-frequency multiple-receiver system, this system can avoid the problems of mutual coupling: induced output power and system efficiency reduction, transmitter detuning, reactive power increase and inverter output capacity reduction.

Medium-voltage (MV) motor drives have become an appealing application for modular multilevel converters (MMCs). Starting and operation at low speeds result in wide fluctuations of the low-frequency ripple components in the sub-module (SM) capacitors DC link voltages, which can adversely affect system performance and system lifetime. A solution for this problem is to replace the low-frequency (LF) SM capacitor with a power decoupling circuit (PDC) that is independent from the converter line frequency. In this paper, a power decoupling approach based on the flux cancelation method is proposed. A three-winding high-frequency transformer (HFT) is employed to magnetically couple and cancel the three-phase symmetrical ripple power. However, this approach has two main challenges. (1) The ripple powers through the HFT are a function of the value of the leakage inductances. (2) Different leakage inductances and ripple power unbalance between phases cause unequal ripple voltages. As a result, phase-shift ripple rejection control is needed. Conventional liner controllers have several problems, such as bandwidth limitations, stability margins, and slow dynamics near-zero-speed operation. In addition, linear controllers are designed for a specific ripple frequency. In this paper, a frequency-independent ripple rejection sliding mode controller (SMC) is proposed to overcome the limitations of linear controllers. The SMC is applied to pass the SM capacitor voltage ripple into the HFT. Thus, the ripple is canceled out in the HFT magnetic core regardless of the converter line frequency. The proposed control is suitable for adjustable-speed applications. The performance of the proposed scheme is verified via simulation and experimental tests. [ABSTRACT FROM AUTHOR]

Energy storage technology has become critical for supporting China's large-scale access to renewable energy. As the interface between the battery energy storage system (BESS) and power grid, the stability of the PCS (power conversion system) plays an essential role. Here, we present a topology of a 10 kV high-voltage energy storage PCS without a power frequency transformer for the establishment of a large-scale energy storage system. We analyzed the energy storage converter's mechanism and characteristics and also introduced the power-control strategy of the HVAC (high-voltage AC) and LVDC (low-voltage DC) converter module. On this basis, a 10 kV/1 MW high-capacity PCS prototype was designed. Additionally, by simulation and experiment, we proved the correctness of the PCS scheme. The topology and control strategy proposed in this paper can provide cases and technical support for the subsequent promotion and application of new energy and power station energy storage. [ABSTRACT FROM AUTHOR]

Photovoltaic energy is one of the most widespread renewable sources of electrical energy. At the heart of the power conversion system is the grid-connected interface converter based on power electronics. Single-stage systems offer higher efficiency and lower cost and size. However, they suffer from a reduced operating range due to the effects of irradiation and temperature. In addition, especially for low-power single-phase systems, pulsating power is an additional handicap. A dual-function decoupling circuit is proposed to mitigate these drawbacks. It compensates for the pulsating power and increases the lifetime of the system. In addition, the proposed low-power interface compensates for the voltage drop due to low irradiation and increases the operating range. A 1 kW test bench is designed, and experimental results show that when injecting power into the grid, the compensator attenuates 85% of the power oscillations and perfectly compensates up to 20% of the voltage drop due to low. [ABSTRACT FROM AUTHOR]

Transformer and electrolytic capacitors (Ecaps) in light-emitting diode (LED) drivers lead to increased cost and shortened lifespan. Therefore, eliminating the transformer and Ecaps of the LED driver can not only achieve a lightweight design but also extend the service life. In this article, a novel single-phase transformerless capacitorless power factor correction rectifier is proposed. The proposed topology shares a common ground with the grid and the load. Thus, the reduced common-mode current is obtained, and the demand for an isolation transformer is eliminated. In addition, this article presents a unidirectional active buffer circuit to isolate the pulsating power, and the capacitor of the buffer circuit is multiplexed as an output filter. Compared with similar works, a lower voltage on the decoupling capacitor, fewer power components, and a simple control unit are obtained. Then, the operation principle and switching modes as well as the parameter design and implementation of the control strategy are elaborated systematically. Finally, an 80-W experimental prototype is built, and the experimental results confirm the feasibility of the proposed rectifier by reaching the efficiency up to 97.1% and the power factor to 0.984 at a wide range of operating voltage. [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]

With the rapid growth of the power grid load and the continuous access of impact load, the range of power system frequency fluctuation has increased sharply, rendering it difficult to meet the demand for power system frequency recovery through primary frequency modulation alone. Given this headache, an optimal control strategy for battery energy storage participating in secondary frequency regulation of the power grid is proposed in this paper based on a double-layer structure. Besides, a coordinated control framework is constructed for energy storage battery joint units engaged in automatic generation control (AGC). At the dispatching level, the power allocation principle is set to coordinate the fast and slow resources of energy storage and conventional thermal power units, and the power decoupling of the two types of frequency modulation (FM) resources is completed. At the system level, a power allocation model representing the real-time frequency modulation capability of energy storage is established to realize the division of frequency modulation responsibilities of each unit and state of charge (SOC) consistency management, and the proposed control strategy is simulated and verified to provide a reference for the energy storage battery to participate in the secondary frequency modulation design of the power grid.

This letter presents a transformerless single-phase PV inverter, which combines with common-ground and active power decoupling techniques. The leakage current is eliminated and bulky electrolytic capacitors are removed, thus improving the system safety and reliability. In addition, it needs only four switches and can be implemented conveniently by a commercial H-bridge module. The validity of the inverter is verified with a 2-kW prototype. [ABSTRACT FROM AUTHOR]

The traditional power decoupling control of modular multilevel converter (MMC) is based on the phase synchronization achieved by the phase-locked loop (PLL), and MMC intricate inner dynamics are ignored, which jeopardizes the system stability and power decoupling. To improve the performance, a nonlinear phase-unsynchronized power decoupling control for MMC is proposed without PLL. At first, the MMC model as a nonlinear multi-input multi-output system is developed. The developed model illustrates that the MMC power coupling is caused by the ac current as well as the MMC capacitor voltages. Then, through the input–output linearization method, the proposed method decouples the MMC output powers without the phase synchronization. Involving integral control, the dynamics of MMC output powers are designed as second-order systems so that the control parameters can be easily determined based on the required transient performance. To verify the accuracy and correctness of the developed model, a comparison of MMC dynamics with the proposed control is conducted between the numerical solution in MATLAB and the EMT simulation in PSCAD/EMTDC. Moreover, seven cases in the EMT simulations and four cases in the experimental results demonstrate the desired phase asynchronization and power decoupling performance of the proposed method compared with the conventional $dq$ control. At last, the system zero dynamic stability is investigated and the influences of MMC control parameters, ac-side voltage, dc-side voltage and equilibrium points are analyzed to evaluate the stability performance of the proposed method. [ABSTRACT FROM AUTHOR]

This study proposes a matrix converter (MC) control method capable of power decoupling (PD) for a single‐phase input isolated AC‐DC converter. The proposed converter can also be fed by a three‐phase source without any additional components. The PD circuit shares its capacitors with the input filters and its switches with the MC, and absorbs the source power ripple to supply a constant DC voltage for the load. Moreover, the MC control method achieves a sinusoidal source current, a high output frequency, and soft switching. Additionally, the feedback control to compensate for the DC output voltage error is applied. The proposed method is evaluated experimentally. [ABSTRACT FROM AUTHOR]

Three-port intelligent conversion systems, interfacing photovoltaic (PV) arrays and electric vehicles (EVs), with an electrical grid, are a promising flexible infrastructure for future smart grids. In this system, multi-energy, direct current (DC), and alternating current (AC) power are decoupled by a larger DC-link capacitor. However, bulk capacitors prevent the achievement of high power density and high reliability of this system. To address this issue, active power decoupling control for a single-phase three-port conversion system is proposed in this paper. Owing to its low internal impedance, the inherent double-line-frequency ripple power in a single-phase system can be actively absorbed by a Li-ion battery pack, with a sinusoidal charging/discharging technique, in the proposed control strategy. Therefore, the capacitance of the DC-link capacitors can be significantly reduced. Further, a DC-link voltage ripple resonant compensation control loop was developed to ensure single-phase ac power quality on the grid side. Finally, the experimental results obtained from a 2.5 kVA single-phase three port power conversion prototype verify the feasibility and performance of the proposed control strategies.

This paper focuses on a comparative analysis between the conventional single-phase H-bridge inverter and an inverter implementing a minimalist active power decoupling method, which is used to eliminate the power ripple of twice the fundamental frequency at the DC link. This method allows minimising the need of large electrolytic DC link capacitors, only two smaller film capacitors being required on the hardware side. The implemented 1 kW inverter makes use of silicon carbide (SiC) MOSFETs, offering the main advantage of high switching frequencies, reduced size and improved efficiency. The performance analysis, including switching and conduction losses and the inverter efficiency, is done through the results obtained from simulations and experiments. [ABSTRACT FROM AUTHOR]

This article proposed a grid-connected photovoltaic (PV) converter based on modular multilevel converter (MMC), which can realize long-distance dc transmission while realizing the local consumption of PV. In addition to the advantage that there is no PV power mismatch between the upper and lower arms, the isolation transformer couples the two submodules of the upper and lower arms on a three-port transformer, and the differential-mode components of the capacitor voltage ripples in the upper and lower arms can be canceled between each other. Besides, through the double fundamental frequency circulating current control, the power exchange among MMC phase units can be realized, thereby eliminating the common-mode component of the capacitor voltage ripple. Through the combination of topology and control strategy, the low-frequency components of the voltage ripple can be completely canceled. Since the decoupling capacitance is reduced dozens of times, the volume and cost of the system are significantly reduced. Therefore, the life and reliability of the system are increased. Simulation and experimental results verify the effectiveness of the proposed topology and its control strategy. [ABSTRACT FROM AUTHOR]

For the single-phase high-power railway traction system, the inevitable second-order resonating power has become a critical issue to the traction system. In this article, a power-decoupling solution is proposed to replace the conventional passive LC resonance filter by utilizing the buck-type bidirectional dc/dc converter (BBDC) of hybrid electric multiple units, which is designed to power the train in nonelectrified routes using the on-board battery. By considering both the current ripples in the battery mode and the power decoupling in the catenary mode, the corresponding parameter design process of the BBDC is presented. Based on the power coupling phenomenon and the spectrum characteristic of the BBDC, a direct resonance control method is proposed to decouple the low-order resonating power. Simulations and experiments are carried out to validate the effectiveness of the new proposal. The results demonstrate that the proposal is almost the same as using the conventional passive LC resonance filter for power decoupling in both steady and dynamic operation scenarios. [ABSTRACT FROM AUTHOR]

Power decoupling is always a strong desire for single-phase voltage source inverter (VSI) due to the inherent double-line (2ω) 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 2ω 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 2ω 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. [ABSTRACT FROM AUTHOR]

This paper presents a modular grid-connected single-phase system based on series-connected current-source module integrated converters (MICs). The modular configuration improves the reliability, redundancy and scalability of photovoltaic (PV) distributed generators. In this system, each PV panel is connected to a dc/ac inverter to permit individual Maximum Power Point Tracking (MPPT) operation for each panel. Thus, the harvested power from the PV system will increase significantly. There are four different inverter topologies suitable to be used as MICs with different performances in terms of filtering elements size, power losses, efficiency, output voltage range, and high frequency transformers' size. For the MPPT control, the oscillating even order harmonic components should be eliminated from the inverter's input side otherwise the maximum power cannot be extracted. The proposed modulation scheme in this paper will ease the control of inverter's input and output sides. Therefore, the 2nd order harmonic in the input current can be eliminated without adding new active semiconductor switches. A repetitive controller coupled with proportional-resonant controllers are employed to achieve accurate tracking for grid side as well as input side currents. Comparisons and performance evaluations for the proposed MICs are presented and validated with 1 kVA prototype controlled by TMS320F29335 DSP.

Power coupling caused by high impedance ratio and nonzero power angle is a crucial issue for virtual synchronous generator (VSG), which may result in output power error, slower power response and even invalid operation. To solve this issue, a novel enhanced power decoupling method by using virtual steady-state synchronous impedance (VSSI) and current dynamic decoupling compensation (CDDC) is proposed. VSSI with the unique characteristic that the value of virtual reactance maintains constant in all frequencies, can solve the power coupling caused by impedance ratio. CDDC can solve the power coupling caused by nonzero power angle in the fastest way by the designed compensation in inner current loop. The reshaped output impedance characteristics are also analyzed in detail. Then, by state-space modelling and analysis, it is proved that, compared to the traditional methods, the proposed method can improve the dynamic performance and enhance the system stability further. Finally, the results of comparative experiments demonstrate that the proposed method can solve the power coupling problem more effectively and improve the dynamic and steady-state responses of active power and reactive power.

Power electronic transformer (PET) with modular cascaded structure has become a flexible interface between AC and DC grids. Nonetheless, the notable 2nd-order ripple power introduced from the AC source requires considerable energy storage in the module capacitors, which is likely to degrade the overall power density and increase system cost. In this paper, a power channel based four-winding PET (PC-PET) is proposed to address such an issue. Owing to the phase-crossing power channels, the low-frequency ripples can cancel each other through magnetic flux counteracting. To improve the accuracy of power decoupling, the effect compromised by inconsistent windings is considered. Accordingly, a proportional-resonant (PR) control based power decoupling strategy for PC-PET incorporated with power balance is proposed to decouple the three-phase low-frequency ripple powers, and minimize the voltage ripple in module capacitors. Additionally, the improvement of power density is discussed based on analyses on power losses, capacitor volume and transformer volume. Simulation and experimental results verified the validity of proposed control strategies.

Second-order ripples occur in the voltage and current during any DC–AC power conversion. These conversions occur in the voltage source inverters (VSIs), current source inverters (CSIs), and various single-stage inverters (SSIs) topologies. The second-order ripples lead to oscillating source node currents and DC bus voltages when there is an interconnection between the AC and DC microgrids or when an AC load is connected to the DC bus of the microgrid. Second-order ripples have various detrimental effects on the sources and the battery storage. In the storage battery, they lead to the depletion of electrodes. They also lead to stress in the converter or inverter components. This may lead to the failure of a component and hence affect the reliability of the system. Furthermore, the second-order ripple currents (SRCs) lead to ripple torque in wind turbines and lead to mechanical stress. SRCs cause a rise in the temperature of photovoltaic panels. An increase in the temperature of PV panels leads to a reduction in the power generated. Furthermore, the second-order voltage and current oscillations lead to a varying maximum power point in PV panels. Hence, the maximum power may not be extracted from it. To mitigate SRCs, oversizing of the components is needed. To improve the lifespan of the sources, storage, and converter components, the SRCs must be mitigated or kept within the desired limits. In the literature, different methodologies have been proposed to mitigate and regulate these second-order ripple components. This manuscript presents a comprehensive review of different effects of second-order ripples on different sources and the methodologies adopted to mitigate the ripples. Different active power decoupling methodologies, virtual impedance-based methodologies, pulse width modulation-based signal injection methodologies, and control methods adopted in distributed power generation methods for DC microgrids have been presented. The application of ripple control methods spans from single converters such as SSIs and VSIs to a network of interconnected converters. Furthermore, different challenges in the field of virtual impedance control and ripple mitigation in distributed power generation environments are discussed. This paper brings a review regarding control methodologies to mitigate and regulate second-order ripples in DC–AC conversions and microgrids.

Active pulsating power buffering (APPB) is an emerging technology that can effectively minimize the energy storage requirement of single-phase power conversion systems, potentially leading to high density and high reliability design. Nonetheless, the implementation of APPB generally requires the addition of excessive number of sensors in the circuit. Employing many sensors not only increases the system's volume and cost, but also undermines the system's robustness. Existing methods of reducing the sensor count in single-phase converters with an APPB suffer from issues such as high design complexity, noise sensitivity, and/or computational complexity. In this article, a simplified algebraic estimation technique is proposed to tackle the high sensor count problem. The proposed technique is intuitive to design and applicable to different topologies. It can effectively reduce the number of sensors while yielding similar or even better system's performance than that with a full set of sensors. Moreover, the technique features very low computational complexity, and can thus be easily implemented by low-cost microcontrollers. Experiments are conducted to verify the feasibilities of the proposed estimation and sensor reduction method. With this method, the sensor count can be reduced by 50%, while achieving a nearly 20-times computational time reduction as compared to that of the conventional method. [ABSTRACT FROM AUTHOR]

The existence of the double grid frequency (100 Hz) ripple current in dc/ac dual active bridge (DAB) power converters can degrade their performance and as such these ripples need to be compensated. An active power decoupling method is proposed in this paper to attenuate the 100-Hz current ripple in single phase dc/ac DAB converters. Compared to other current ripple reduction methods for which additional switches are used, the proposed method does not require any additional switching components thus decreasing the system cost. In the proposed method, the existing switching devices of the primary side full bridge converter and an LLC circuit integrated to form a 100-Hz ripple current reduction circuit, which is controlled by regulating the duty cycle of the primary full bridge converter switching legs. The mathematical analysis and the circuit parameter design methods are proposed. In addition, a current sharing method is proposed to keep the currents of the inductors in the LLC circuit are balanced against any possible circuit parameter deviations. The effectiveness of the developed method is evaluated through the simulation and experimental tests.

Energy storage technology has become critical for supporting China’s large-scale access to renewable energy. As the interface between the battery energy storage system (BESS) and power grid, the stability of the PCS (power conversion system) plays an essential role. Here, we present a topology of a 10 kV high-voltage energy storage PCS without a power frequency transformer for the establishment of a large-scale energy storage system. We analyzed the energy storage converter’s mechanism and characteristics and also introduced the power-control strategy of the HVAC (high-voltage AC) and LVDC (low-voltage DC) converter module. On this basis, a 10 kV/1 MW high-capacity PCS prototype was designed. Additionally, by simulation and experiment, we proved the correctness of the PCS scheme. The topology and control strategy proposed in this paper can provide cases and technical support for the subsequent promotion and application of new energy and power station energy storage.