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The authors' previous paper presents the complete model of parasitic capacitances in a high-voltage (HV) multiplier in the HV generator in a medical X-ray machine. The equivalent parasitic capacitance of the multiplier Cem based on the model is exhibited to interpret the role of the parasitic capacitances in circuit operation of the generator. Without derivation of the analytical expression of Cem, its dependence on the component parameters is directly shown, which leads to guidelines for minimization of Cem. However, the complete capacitance model has a complicated structure and contains voltage-dependent capacitances and voltage sources, which gives complexity to derivation of Cem as well as difficulties to clear understanding of the voltage dependence of Cem. Thus, this paper presents the detailed derivation and analytical expression of Cem. Exactly speaking, the key part of Cem, namely, the total chain capacitance CDcht that determines the voltage dependence of Cem, is conducted. A simple way is presented for the derivation of CDcht. The derivation enables a clear understanding of the voltage dependence of CDcht. Moreover, a design procedure for minimization of Cem is created, accompanied by a case study. It gives designers step-by-step rules to make an optimal HV multiplier with a minimum Cem. [ABSTRACT FROM AUTHOR]

This paper develops a robust tool for handling ac side stability issues occurring in $LCL$-filter-based grid-connected converters. $LCL$ filters give advantages in cost and dynamic as smaller inductors employed compared with $L$ filters for the same required damping of the switching harmonics. Nevertheless, the additional poles introduced by the $LC$ part induce resonance in the system leading to stability problems. To deal with this stability issue, a new damping method based on the generalized circle criterion is proposed in this paper. The large signal nature of the applied theory allows obtaining high damping efficiency and robustness under large variations of the grid impedance. A theoretical analysis and the experimental results are presented to show the effectiveness of the proposed compensating method. [ABSTRACT FROM PUBLISHER]

This paper presents an asymmetrical cascaded multilevel inverter based on a five-level transistor-clamped H-bridge power cell. A two-cell configuration with a dc link voltage ratio of 4 : 1 is presented. The modulation method is based on the fundamental frequency switching. An analysis of the inverter output voltage and harmonics is presented. Injection of the third harmonic voltage as to maximize the output voltage and the issue of the inverse power flow or regeneration in the low-voltage cell are presented. The prototype of the proposed inverter was built, and its functionality was verified through experimental results. [ABSTRACT FROM AUTHOR]

Finite control set model predictive control (FCS-MPC) is emerging as a powerful control scheme in the control of power converters, because it takes the discrete nature of power converters into account and offers a flexible way to consider various constraints. However, conventional FCS-MPC requires to evaluate a cost function for each discrete switching states, which poses high computational burden. This paper proposes a low-complexity MPC (LC-MPC), which only requires one prediction to find the best voltage vector. The principle of LC-MPC is inherited from prior direct current control (DCC), but has been generalized by identifying its advantages, limitations, and potential application areas. Furthermore, the relationship between LC-MPC and FCS-MPC is studied and it is found that in some cases, the LC-MPC is completely equivalent to FCS-MPC. This paper presents the application example of LC-MPC in power control of three-phase ac/dc converter. To make it a success, the negative conjugate of complex power in synchronous frame is selected as the control variable. Detailed principle of vector selection is introduced and the reason for requiring only one prediction in the proposed LC-MPC is strictly proven using mathematical tools. The proposed LC-MPC is compared with conventional FCS-MPC and its effectiveness is verified by both simulation and experimental results from a two-level ac/dc converter. [ABSTRACT FROM AUTHOR]

Recently, the low-frequency oscillation (LFO) frequently happens in the vehicle-grid system of high-speed railways when electric multiple units (EMUs) are put into operation. As a possible solution, a single-phase cascaded H-bridge static synchronous compensator (CHB-STATCOM) directly connected to the traction network is applied to dynamically compensate the reactive power of vehicle-grid system. This paper focuses on the impedance modeling and stability analysis in the vehicle-grid system with CHB-STATCOM. Firstly, the vehicle-grid system is simplified to a single-phase rectifier with large grid inductance. The accurate dq-frame impedance of the rectifier system with CHB-STATCOM is derived in detail. Secondly, through the generalized Nyquist stability criterion (GNSC), the influence of CHB-STATCOM and its different control parameters on the stability of rectifier system is revealed. Thirdly, the excellent performance of CHB-STATCOM in suppressing the LFO and the accuracy of theoretical analysis are verified via time-domain simulations. Finally, to further verify the effectiveness of CHB-STATCOM in real applications, a dSPACE based controller hardware-in-the-loop (HIL) simulation platform is established, which includes the physical control circuits of eight EMUs. The experimental results show that the CHB-STATCOM has a good dynamic compensation effect and can efficiently suppress the LFO. [ABSTRACT FROM AUTHOR]

Using HVDC grids for interconnection of ac grids across countries posses questions regarding interactions and stability. Important aspects that may be considered are the different short circuit ratio (SCR) values of the corresponding ac grids and outages. The analysis of such mixed SCR will be time-consuming since; detailed models and multiple simulations of the entire system will be required in order to develop a complete understanding of the modes of interactions and existing stability margins. Use of small signal model will allow for easy parametric variation after the initial barrier of model development is overcome. The article presents an analysis technique that uses the mode identification and participation factor augmented with eigen-sensitivity to analyze modes of interactions and stability margins of the HVDC grids when system parameters and controller gains are varied. The modes are classified as local and inter-area modes based on the participation of controller states for HVDC grid. The method does not require detailed switched models to be developed. The paper also proposes an adaptive control technique which results in a reduction of interactions and prevents instability when system operating conditions are changed. The results obtained using the proposed analysis technique are verified with a detailed time-domain simulation of the HVDC grid. The interaction modes and participation of states are dependent on the converter operation modes, loading characteristics, SCR values, PLL gains and loading conditions. [ABSTRACT FROM AUTHOR]

To determine the operating region of modular multilevel converter (MMC) station under unbalance grid conditions, this paper proposes the analytical and point-scanning methods. First, the imbalance of PCC voltages is expressed in terms of two proposed indexes for single phase fault (SPF), which can be directly calculated from the grid parameter and grounding impedance. Next, the algebraic equation to solve the PCC voltage for the given PCC power is presented. The existence of PCC voltage is considered as one constraint. Moreover, the other constraints for transformer rating, converter rating, ac current and output voltage are also analyzed to adapt the unbalance conditions. Based on the proposed method, the effect of the grid imbalance on the operating region of MMC is analyzed and then the common constraints to confine the operating region are determined. Following that, the boundary control is proposed to limit the operation of MMC on the boundary of the operating region under unbalanced conditions. The open and modular control structure allows more constraints to be considered for the boundary control. The simulation results validate the correctness of the proposed methods to define the operating region and the effectiveness of the boundary control. [ABSTRACT FROM AUTHOR]

Sudden start of an IM load and frequent change of nonlinear load in a standalone distributed generation system (DGS) cause the dip in ac voltage and frequency. Moreover, these loads distort DGS currents. Therefore, in this paper, an improved-reweighted zero-attracting quaternion-valued least mean square (I-RZA-LMS)-algorithm-based voltage source converter (VSG) control is proposed for voltage and frequency regulation, and power quality improvement in DGS. Moreover, the control algorithm of dc–dc bidirectional converter (BDC) is used for dc link voltage regulation and MPPT of the solar photovoltaic array at IM starting and nonlinear loading. In this DGS, the I-RZA-LMS-based control algorithm estimates the active and reactive component currents of distorted load currents for effective harmonics mitigation, reactive power compensation, and point of common coupling voltages regulation. The proposed control algorithm rejects the dc-offset component from load currents and gives the fundamental load component. The dc link voltage of VSC is regulated using BDC with a solar feedforward term, which is used to enhance the capability of dc link voltage control under solar power variation, IM starting, and unbalanced loading. The effectiveness of the proposed DGS is validated through the simulation as well as experimental results obtained on a developed prototype of DGS. [ABSTRACT FROM AUTHOR]

In this paper, eight ultrahigh-voltage gain dc–dc converters are proposed. These sets of converters are derived by the association of boost cells, coupled inductor, voltage multiplier, switched capacitors, and LC filter cells. The nonpulsating input and output converters are evaluated in terms of: principle of operation, voltage gain, voltage and current stress, estimated power losses, and efficiency. Following full analysis prototypes built in the laboratory show experimental results verifying the theoretical analysis. [ABSTRACT FROM AUTHOR]

Differential power processing (DPP) is a promising architecture to solve the issue caused by mismatches among photovoltaic (PV) submodules. To eliminate the mismatch power losses, this paper presents an optimized PV-to-bus DPP system with submodule-level individual maximum power point tracking (MPPT) and module-level minimum-power tracking (MPT) simultaneous implementation according to the built mathematical model of the power processed by DPP converters with respect to the string current. Furthermore, a flexible time-sharing algorithm is adopted to reduce the number of MPPT units. Compared with other solutions, such as PV optimizer, the proposed scheme can reduce the effects of mismatches to the minimum regardless of mismatch conditions. The system cost is reduced since only one adaptive MPPT controller is required. Besides, the system efficiency is improved due to the true MPPT implementation and MPT control. Both simulation and experimental tests are provided to validate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper proposes a very simple analog control for quasi-square-wave zero voltage switching (QSW-ZVS) and triangular current mode bidirectional converters. The proposed circuit controls the inductance current, thanks to the use of a variable-width hysteretic current mode control. The upper and lower bounds of the hysteretic band are clamped to ensure QSW-ZVS operation with a single current command and independently from power flow direction. This enables the control to achieve a seamless transition between source and sink modes. The advantages and disadvantages of the proposed analog control are analyzed and two different power converters are built to demonstrate the validity of this solution: a 50 W, 48 to 24 V bidirectional buck converter and a 100 W, 24 to 48 V bidirectional boost converter. [ABSTRACT FROM AUTHOR]

The power efficiency and weight of present point-of-load (POL) dc–dc converters for drone applications are often compromised because they suffer from large switching losses at continuous conduction mode for heavy loads and excessive hardware overheads at discontinuous conduction mode for light loads. This paper presents a boundary conduction mode (BCM) control scheme for POL converters embodying a single operation mode. This is achieved by means of a hysteresis voltage controller to turn on/off the output power stage when necessary. The proposed BCM control scheme achieves high power efficiency (≥91.2%) over a wide load range (5 mA–1 A) by means of fully soft switching. Specifically, a hysteretic current controller is proposed to realize ZCS, and an adaptive dead time controller is proposed to realize ZVS. Further, the proposed BCM control scheme requires a small output inductor (0.82 μH) by means of designing the customizable peak inductor current. To verify the proposed BCM control scheme, we realize a BCM-based POL converter that features an input voltage range of 5–16 V, output voltage range of 2.5–8 V, switching frequency of 1.5 MHz, peak power efficiency of 96.8%, and ≤35 mV output voltage undershoot/overshoot for 1-A load step. When being benchmarked against state-of-the-art counterparts, the proposed design features the lowest voltage undershoot/overshoot, the highest switching frequency, ∼5.7× smaller inductor, and ∼11% higher power efficiency at light loads. [ABSTRACT FROM AUTHOR]

The nonresonant single-phase dual-active-bridge (NSDAB) dc–dc converter has been increasingly adopted for isolated dc–dc power conversion systems. Over the past few years, significant research has been carried out to address the technical challenges associated with modulations and controls of the NSDAB dc–dc converter. The aim of this paper is to review and compare these recent state-of-the-art modulation and control strategies. First, the modulation strategies for the NSDAB dc–dc converter are analyzed. All possible phase-shift patterns are demonstrated, and the correlation analysis of the typical phases-shift modulation methods for the NSDAB dc–dc converter is presented. Then, an overview of steady-state efficiency-optimization strategies is discussed for the NSDAB dc–dc converter. Moreover, a review of optimized techniques for dynamic responses is also provided. For both the efficiency and dynamic optimizations, thorough comparisons and recommendations are provided in this paper. Finally, to improve both steady-state and transient performances, a combination approach to optimize both the efficiency and dynamics for an NSDAB dc–dc converter based on the reviewed methods is presented in this paper. [ABSTRACT FROM AUTHOR]

In this paper, a multisource switched capacitor based boost multilevel inverter is proposed. The proposed topology features less number of components, high gain, self-charging capability of capacitors, reduced size of filter components. The modes of operation of the proposed converter and design guidelines of converter components are discussed. A comprehensive comparative study of the proposed converter topology with the topologies in the literature is given. Analytical expression for capacitor energy utilization (CEU) factor for the proposed topology is derived and the feasibility of the proposed converter with respect to CEU for the designed power rating is verified. The analytical performance of the proposed converter topology is verified through simulation studies. The common mode voltage and leakage current in the proposed topology are analyzed. Experimental prototype of the proposed converter topology with two sources and two capacitors for power rating of 500 W is developed and tested with resistive, inductive, and nonlinear loads. The experimental and simulated results are in close agreement with the analytical steady-state performance of the proposed converter. [ABSTRACT FROM AUTHOR]

This paper proposes a new control strategy of microgrids for the improved voltage quality. In the existing control techniques, the droop control is commonly adopted as a decentralized power sharing method at the cost of voltage deviations. Besides, the conventional cascaded control featuring relatively slow dynamic response shows difficulties in handling the fluctuation of renewable energy outputs, leading to further voltage quality deterioration. In this paper, an advanced model predictive power control strategy by considering the battery constraints is proposed for the bidirectional dc–dc converters to smooth the solar photovoltaic (PV) outputs and stabilize the dc-bus voltages. A model predictive voltage control scheme taking into account the voltage changing trend is then developed to control the distributed inverters to improve the output ac voltages. Furthermore, a washout-filter-based power sharing approach with the plug-and-play capability is adopted to achieve a proper load sharing among parallel inverters and mitigate the voltage deviation. The proposed control strategy is numerically simulated in MATLAB/Simulink and experimentally verified by hardware-in-the-loop tests under the condition of fluctuating PV outputs and variable power demands. (This paper is accompanied by a video showing the hardware-in-the-loop test.) [ABSTRACT FROM AUTHOR]

This paper proposes the design of gray wolf optimizer (GWO) algorithm-based fractional order proportional integral (FOPI) controller for power factor correction (PFC) using interleaved dc–dc single ended primary inductance converter converter with improved performance characteristics such as speed, accuracy, and stability. The detailed analysis and modeling of the proposed converter are presented. The optimal parameters of FOPI controller are obtained using GWO algorithm to improve the speed of response of the system. FOPI controller provides better performance in terms of robustness and stability since it has one more adjustable parameter than the conventional proportional integral (PI) controller. The cascade control strategy is implemented for the proposed converter with GWO-based FOPI controller as inner current and outer voltage controllers for PFC and load voltage regulation. The performance characteristics of the proposed system are analyzed and compared with conventional and other optimization techniques using MATLAB/Simulink software. A prototype of the proposed converter controlled by TMS320C2000 microcontroller for an output power of 200 W is tested and validated with the simulation results. The proposed system is used for powering devices like robots in critical applications such as military, space craft, etc., where stability, speed, and accuracy are major concerns. [ABSTRACT FROM AUTHOR]

The algorithm complexity of model predictive control (MPC) for cascaded H-bridge (CHB) static synchronous compensators (STATCOMs) was optimized to a polynomial level in previous studies. However, implementing MPC with conventional approaches still suffers from long execution time, where a low cost digital signal processor (DSP) is used to execute optimization algorithms and a small-sized field-programmable gate array (FPGA) is used to extend gate signals and sampling functions. This paper presents a parallel implementation approach to reduce the execution time of MPC, where the FPGA is configured to be used for accelerating the high-complexity sorting tasks. By running the DSP and FPGA in parallel, the algorithm execution time is only determined by the DSP, and the resource consumption is only determined by the FPGA. To minimize the resource consumption and requirement in FPGA, a parallel comparison-based sorter with multiple comparison and accumulators is proposed. The proposed implementation method can reduce the time and space complexity of the MPC from the polynomial level to a linear level without additional hardware cost, which means the MPC for medium-voltage CHB STATCOMs (e.g., 20 stages) can be implemented online with existing control platforms. Experimental results for various CHB STATCOMs are presented to validate the proposed configuration and algorithm. [ABSTRACT FROM AUTHOR]

This paper proposes an improved LLC resonant converter with reconfigurable hybrid voltage multiplier for wide output voltage range applications. By adopting three legs and two active switches, the rectifier can be reconfigured to three configurations, i.e., full-bridge rectifier, hybrid voltage-multiplier rectifier and voltage-multiplier rectifier, covering the range of more than three times of the minimum output voltage. Compared with conventional LLC converter, the operating range of each configuration is narrowed so that the magnetizing inductance is optimized and switching frequency range is narrowed to achieve higher efficiency. Pulsewidth plus pulse-frequency modulation strategy is employed to achieve smooth transition between these operation modes. Zero-voltage switching of all power mosfets and zero-current switching of all secondary-side diodes are achieved as well. Detailed operational principles, characteristics, design consideration, and control stagey of the proposed converter are analyzed. A 3.3 kW 150–450 V output prototype is built and tested to verify the effectiveness and advantages of the proposed converter. [ABSTRACT FROM AUTHOR]

In the no-load condition, LLC converter usually fails to regulate its output voltage although it operates at a high switching frequency. Till now, it is hard to obtain the exact relationship between design parameters and the maximum switching frequency for no-load regulation capability. In this paper, a specific criterion for no-load regulation of LLC converter is provided, without using active components or other modulation schemes. By analyzing the macroscopic switching period and microscopic switching transition in the no-load condition, it is shown that not only the peaking resonant current during the switching transition, but also the resonant tank design affect the no-load regulation of the LLC converter, which affects the no-load regulation capability. Furthermore, the relationship among design parameters is analyzed and the design guideline is also provided to achieve no-load regulation at the specified maximum switching frequency. To verify the effectiveness of the proposed design, 400 V input and 50 V/200 W output prototype is built and tested. [ABSTRACT FROM AUTHOR]

Cascaded-connected string converter is an effective way to directly interconnect distributed low-voltage dc sources into higher voltage ac grids. When this system is switched to islanding operation, an important task is to obtain decentralized power sharing among power cells according to the corresponding dc source power capacity, and at the same time, maintaining a good regulation of supply voltage quality to local critical loads. In this paper, a hierarchical control approach is proposed to simultaneously realize the abovementioned two tasks. First, the central controller at point of common coupling (PCC) is responsible for supply voltage magnitude and frequency deviation suppression and it also distributes the key control signals to all power cell local controllers with only low bandwidth communications. At the same time, a modified inverse power factor droop regulator with adjustable offset is adopted by each power cell. As the offset is relevant to the PCC load power factor and the state of charges of power cell backstage batteries, it can easily obtain decentralized proportional power sharing. The proposed system and the control strategy are compared with the conventional parallel converters-based system and the potential applications of the system are also given in this paper. [ABSTRACT FROM AUTHOR]

An isolated bidirectional dc-dc converter, which combines multiple two-level converters (TLCs) in parallel on the medium-voltage side and a modular multilevel converter (MMC) on the high-voltage side, namely the TLC-MMC converter, is a promising candidate for the interconnection of MVdc and HVdc grids. In utility applications, the availability of power converters is of great importance, which makes the fault-tolerant operation (FTO) capability a required feature. In this paper, an FTO scheme of the TLC-MMC converter in case of a TLC failure is addressed. The FTO capability is enabled by employing only mechanical disconnectors. The faulty TLC can be bypassed and isolated for maintenance, and the TLC-MMC converter can continuously operate with a reduced power capacity. A dedicated mode-transition strategy is developed, whereby the TLC-MMC converter can transit to the fault-tolerant mode seamlessly without resulting in overvoltage or overcurrent despite the involvement of low-speed mechanical disconnectors. Thus, the proposed approach can effectively enhance the availability of the TLC-MMC converter. The validity of the proposed approach is verified by both simulation and experimental test with a down-scale prototype. [ABSTRACT FROM AUTHOR]

This paper presents a new highly integrable hybrid step-down converter that merges switched-inductor and switched-capacitor operations and significantly reduces onboard loss by using the input cable's parasitic inductance as its main inductor. This converter has the inductor placed at the input with a smaller voltage swing, leading to possible use of a smaller inductor and low-voltage rating switches that generally translate to reduced conduction losses. Analyses of converter operation and losses to reveal its original characteristics and design guidelines are presented to facilitate the components optimization. The converter architecture is verified by a proof-of-concept 15-W inductor-less lithium-ion battery charger prototype that uses a 1-m USB 3.0 cable as inductor. The converter, switched at 2 MHz from a 5-V input, experimentally achieves 89.7% peak efficiency and 6% higher efficiency at full load than a Buck converter counterpart. This high efficiency and zero onboard inductor yield a relative 45.7% onboard loss reduction at full load, promising excellent integration feasibility and superior system thermal management. [ABSTRACT FROM AUTHOR]

The articles in this special issue focus on the latest achievements of modular multilevel converters regarding the development of new circuit configurations, converter models, modulation strategies, and control schemes. [ABSTRACT FROM AUTHOR]

Conventionally, a proportional-integral (PI)-based dual-loop control strategy, in which both the dc-bus voltage and the inductor current are regulated, is utilized to control ac/dc converters with power factor correction (PFC). Due to the characteristics of the PI controller, converters experience poor control performance such as current distortion and slow dynamic response. This paper proposes a novel cascaded proportional control in conjunction with algebraic estimation method for PFC ac/dc converters. Different from the conventional control system, the average dc-bus energy and the instantaneous input power of the converter are regulated. Moreover, system nonlinearity, load power and uncertainties are estimated by incorporating algebraic estimators. A holistic design of controller parameters and stability analysis are presented based on the z-domain transfer functions. A 1 kW PFC ac/dc prototype is designed and experimental results demonstrate the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In this paper, an improved floating interleaved Boost converter is proposed for fuel cell applications. In order to achieve zero-ripple input current and keeping others merits such as high voltage gain and low voltage stress of the conventional floating interleaved Boost converter (FIBC) unchanged, a pair of input current ripple cancellation networks composed of a small inductor and a small capacitor are added into the conventional FIBC. Then, the operating principle and the steady-state and dynamic performances of the proposed converter are performed and compared with the conventional FIBC. Finally, a 240 W experimental prototype has been created for further evaluating the expected performances of the proposed converter. The simulation and experimental results demonstrate that the input current ripple of the proposed converter can be totally removed in all power range without significantly increasing the current stress and power loss. [ABSTRACT FROM AUTHOR]

This paper deals with the design of a converter-machine system considering a set of operating points (speed, torque) composing a routine cycle. From the 1-D analytical modelling of the Permanent Magnet Synchronous Machine (PMSM) and the power electronic converter, a design methodology minimizing the power losses-to-volume ratio is established where the flux weakening control structure is considered within the system sizing. Based on a deterministic approach developed for PMSM sizing only, we propose a methodology to include the converter losses in the machine design process. Hence, we demonstrate how the computing time of calculation can be significantly reduced limiting the calculation of the flux weakening mode to a few critical power points. This method is applied to a liquid piston mechanism as part of a Compressed Air Energy Storage (CAES) system. We then validate the converter-machine design method from a 2-D analysis applied to the most significant points. Finally, the PMSM design from both methodologies with and without considering the converter losses are compared. Results highlight how the converter affects the machine sizing and the distribution of power losses to reduce the total losses of the system. [ABSTRACT FROM AUTHOR]

Hybrid cascaded-parallel microgrid is becoming a new emerging structure to integrate multiple low-voltage power sources. This paper presents a unified distributed control strategy to implement power sharing control in hybrid cascaded-parallel microgrid under both resistive-inductive (RL) and resistive-capacitive (RC) load, where a sign function is introduced to automatically match load characteristic. Active power and reactive power regulators without frequency drop are developed, and low bandwidth communication network is employed to support power management and improve system redundancy. Furthermore, small signal model of hybrid cascaded-parallel microgrid with RL load and RC load is established. Also, small signal stability and dynamic performance of the proposed distributed control strategy is investigated. Simulation results show that the unified distributed control strategy is able to implement desirable power sharing under different load types with superior control performance. Also, the proposed control strategy is able to improve system redundancy and support plug-and-play operation of microgrid. [ABSTRACT FROM AUTHOR]

Phase-shift full-bridge (PSFB) converter is a widely used topology for high-efficiency and high-power applications, thanks to low voltage/current stress of all components and zero-voltage-switching operation of primary mosfets. The critical drawback of the PSFB converter is hard-switching characteristics of primary mosfets at a very light load, which causes severe thermal explosion and extremely high input power consumption, especially with the latest super-junction mosfet. To minimize the switching loss of the PSFB converter at a very light load, a new optimal dead-time control scheme with burst-mode operation is proposed in this paper and its validity is verified experimentally with the implemented 2.2-kW server power supply unit. [ABSTRACT FROM AUTHOR]

In single-phase grid-connected converters, unexpected perturbations and variations of system parameters can deteriorate the performance of grid current and dc-link voltage control and even jeopardize the stability and operation of loads. In this paper, a robust control strategy for the single-phase grid-connected converter operating under multiple system perturbations is proposed. An adaptive quasi-proportional-resonant (AQ-PR) controller for the grid current is adopted in combination with an equivalent inductance identification algorithm, which assures that the parameters of the AQ-PR controller can be online calibrated to attain accurate current regulation under ac-side inductance uncertainties. A super-twisting sliding mode (ST-SM) controller for the dc-link voltage is proposed to enhance system behavior under both internal and external disturbances. The convergence of the controller is ensured by utilizing Lyapunov theory. Experimental prototype and rigorous tests are presented to validate the feasibility and robustness of the proposed approach under various perturbations. [ABSTRACT FROM AUTHOR]

Two-stage isolated battery-driven grid interface (IBDGI) with front-end three-phase dual active bridge (DAB) dc–dc converter is promising to be applied in the residential battery energy storage system for supporting photovoltaic (PV) integration, load shifting, and backup power purposes. However, the second harmonic current (SHC) caused by the pulsating power of the downstream single-phase inverter will increase the battery's degradation and the component stress of the front-end converter. In this paper, a reduction method is proposed for the IBDGI with three-phase DAB converter using a load current feedforward control. The proposed idea is to incorporate virtual impedance to the output impedance of the front-end converter. The easy-to-be-implemented method does not need to modify the original voltage closed-loop controller but suppress SHC while keeping good dynamic performance. As the first step, the small-signal model of the three-phase DAB converter is provided. The parameter sensitivity analysis is proposed to demonstrate that the SHC can be well-suppressed within $\pm \text{20}\%$ parameter error of the feedforward control. Finally, a 1 kW experiment setup is built to verify the performance of the proposed SHC reduction method. [ABSTRACT FROM AUTHOR]

High penetration of renewable energies through fast-response power converters results in a considerable displacement of conventional synchronous generators and losing of system inertia for frequency control. Modular multilevel converters (MMCs) can be employed serving as an interface between the large-scale renewable generation and power grids. In a microgrid with high shares of renewables integrating through MMCs, submodule (SM) capacitors can be used as energy storage to provide a degree of synthetic inertia for system frequency support. This paper quantitatively exploits the MMC synthetic inertia in a microgrid with flexible renewable penetration levels. An MMC inertia coefficient concept is proposed. It is mainly affected by the penetration ratio, the SM capacitance, and the modulation index of an MMC, with the system operation constraints. According to the analysis, a substantial portion of system inertia can be provided by properly designed MMCs. Detailed MMC frequency control loops are presented. The capacitor average voltage is proportionally linked to the frequency deviation, in order to flexibly adjust the capacitor energy during frequency events. The MMC output active power is deliberately and simultaneously regulated according to the capacitor energy change rate. By controlling the MMC capacitor voltage, dc side power, and output active power, an amount of energy can be released or absorbed by SM capacitors to improve the system frequency response during the frequency event transients, while the renewable generation is scarcely influenced. The proposed concept is experimentally verified and the results show that, with proper system parameters and control loops, the line frequency deviation and the rate of change of frequency can be significantly reduced. Good agreements of the system frequency characteristics have been achieved between the theoretical calculation and experimental results. [ABSTRACT FROM AUTHOR]

In this paper, a two-stage transformerless multiple-output converter is proposed for applications requiring a high voltage step-down ratio. The proposed circuit transfers power via a current interface and the use of current-source-mode (CSM) converters, resulting in low voltage stress and drastically simplified control. The first stage of this configuration is regulated to provide a constant current to the second stage. The second stage consists of series-connected CSM boost converters. The high step-down ratio of the proposed configuration reduces the voltage stress in the switches of the second stage. The series-connected CSM converters are inherently independent of each other, leading to a very simple control scheme without the need for decoupling of the input voltage of the interconnected converters. Input voltage variation to the system will not affect the input voltage of the second stage. Load variation in one output will not affect the other outputs. If one or more CSM converters are shorted, there is no impact on other converters. These advantages enable a high scalability. Besides, the first stage of the proposed configuration can operate both in continuous conduction mode (CCM) and discontinuous conduction mode (DCM) with the same control strategy. This feature leads to a low control complexity and elimination of inductors. The performance of the proposed converter is illustrated with a laboratory prototype driving light emitting diodes (LEDs). [ABSTRACT FROM AUTHOR]

This paper discussed an active-bridge-active-clamp (ABAC) topology that is suitable for highpower more-electric-aircraft applications. Though conventional modulation techniques can be applied to ABAC converters, they present drawbacks, such as increased low-voltage (LV) current ripple and limited power transfer capability. To address these problems, a phase-shift-modulation (PSM) scheme is proposed to provide clean dc terminal current at the LV side while maintaining high power transfer capability and efficiency in a wide operating range. The proposed PSM has a complete switching harmonics cancellation on the LV terminals independent of the operating conditions. This results in high quality power without any ac components, thus minimizing passive filtering. In addition, when terminal voltages vary from their nominal values, the proposed PSM can improve the maximum power transfer capability of the ABAC converter compared to the conventional approach. The theoretical claims are validated by both simulation and experimental results on a 10-kW 270-V/28-V ABAC converter. [ABSTRACT FROM AUTHOR]

In this paper, a systematic and analytical study of non-isolated topologies belonging to the family of reduced redundant power processing (R2P2) converters is presented. Based on this study, all the non-isolated R2P2 topologies that can be implemented in reality are derived. Detailed examples clarify all possible cases of this analysis. In addition to this, the voltage ratio and the efficiency for each one of the R2P2 configurations are calculated, being further illustrated with examples. A comparison among configurations is also discussed with criteria the high step-up voltage gain and efficiency, aiming for high-power applications. One of these R2P2 converters is selected for further examination, as the most suitable for such applications. The theoretical analysis, as well as the calculations of the step-up voltage ratio and efficiency of the selected converter, is compared to experimental results conducted on a 2-kW laboratory prototype designed to operate in various step-up voltage ratios and in a high-power range, proving the effectiveness of the proposed investigation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An important issue in the photovoltaic (PV) power generation systems is the massive amount of heat wasted by PV modules/panels. For the first time, this paper provides a new solution to recover a portion of the mentioned waste heat by utilizing a proposed electrochemical heat engine. A thermally regenerative electrochemical cycle (TREC) system has been designed, constructed, and embedded in a PV power generation system including a 200-W PV module to harvest the waste heat of its PV module, so a PV-TREC hybrid system has been organized and built. The hot cell of the TREC system has been positioned in direct contact with the backside of the PV module, and the cold cell is in direct contact with a heat sink located in the shade of the PV module. Experimental verifications demonstrate that the proposed TREC system efficiently converts the waste heat into electric power ranging from 0.1 to 54.5 W. In this manner, up to 6% of the waste heat of the PV module is recovered. It also substantiated that the daily energy efficiency of the PV-TREC hybrid system is improved by 27.5% compared to a usual PV system, and this is the main contribution and novelty of this paper. [ABSTRACT FROM AUTHOR]

The mineral insulating oil and pressboard are important insulation dielectrics in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the oil and oil-impregnated pressboard. In this paper, the influences of the temperature, field strength and aging on the resistivity of oil and oil-impregnated pressboard have been investigated. Samples used in the experiments are new oil, aged oil, pressboard immersed in new oil and aged oil respectively. The measuring device of resistivity was full of oil and isolated from outside. All samples were measured at temperature range of 25-80 °C under the electric field strength range of 2-10 kV/mm for oil and 3-30 kV/mm for oil-impregnated pressboard. The results showed that the resistivity of oil-impregnated pressboard decreased along with the increase of field strength and temperature. For all oil specimens and pressboard immersed in aged oil, the resistivity decreasing rate ( σ lgρ / σE ) along with electric field strength showed a downtrend with the increase of temperature; while, for pressboard immersed in new oil, the decreasing rate showed an uptrend with the increase of temperature. For oil, the resistivity reduction rate with aging (r) decreased along with the increase of field strength and temperature. For pressboard immersed in oil, the reduction rate with aging (r) showed an uptrend with the increase of temperature. [ABSTRACT FROM AUTHOR]

This paper proposes a technique to reduce the low-frequency dc-link energy oscillation due to the pulsating single-phase energy flow in a single-phase-to-single-phase voltage source converter (VSC) and current source converter (CSC), which operate connected to the grid. This pulsating energy is the major factor for increasing the size of the passive components and power losses in the converter. Pulsation reduction is achieved by incorporating two auxiliary active switches and one passive energy storage element to the rectifier and inverter sides. Additionally, this paper presents the comparison of these converters (VSC/CSC) regarding efficiency and power density. Details of the control strategy are presented. Simulation and experimental results are provided to validate the theoretical approach. [ABSTRACT FROM PUBLISHER]

Fault-tolerant permanent magnet synchronous generator (PMSG) drives for wind turbine applications play a major role in improving reliability and availability levels, since power converters are very prone to fail. In this paper, a fault-tolerant converter with the ability to handle power switch open-circuit faults is addressed. The main concern of the proposed converter topology is the minimization of the hardware requirements, leading to a low increase of the system cost. First, the employed fault diagnostic technique does not require additional measurements, nor high computational effort. Secondly, the circuit topology reconfiguration implies a minimum number of extra components as well as minimal oversizing of the standard ones. Accordingly, a four-switch three-phase converter with the dc bus midpoint connected to the transformer neutral point and a three-switch three-phase rectifier are adopted for post-fault operation of the grid- and PMSG-side converters, respectively. Vector control strategies are proposed for both converters under analysis, focusing the issues of capacitor voltages balancing and torque ripple minimization. The performance of the proposed fault-tolerant PMSG drive is analyzed by means of experimental results. [ABSTRACT FROM PUBLISHER]

This paper discusses a new circuit configuration and a new control method for a single-phase ac–dc converter with power factor (P.F.) correction and a power pulsation decoupling function. The proposed converter can achieve low total harmonic distortion (THD) on the input current and the power pulsation decoupling function between the input and output sides, which allows low output voltage ripple even on a small output energy buffer at the same time using an active buffer. Therefore, the proposed converter does not require large smoothing capacitors or large smoothing inductors. The buffering energy is stored by a small capacitor, which controls the variation of the capacitor voltage through the active buffer. In this paper, the fundamental operations of the proposed converter are investigated experimentally. The experimental results reveal that the input current THD is 1.44%, the rate of the output voltage ripple is 6.33%, and the input P.F. is over 99%. In addition, a maximum efficiency of over 96% is obtained for a 750-W prototype converter. [ABSTRACT FROM PUBLISHER]

A transformerless wind power generator concept with the potential to achieve 100 kV dc output is proposed in this paper. In this paper, the modular ac/dc converter system suitable for such a high-voltage generator is analyzed for normal operation and fault-tolerant modes. The control synthesis is addressed, based on an assumption that the control can be designed modularly. Additionally, the investigated control system can facilitate faulttolerant operation without changing the main structure. The robustness of the module control system is analyzed, and the limits for fault-tolerant operation are investigated. The theoretical studies are supported by transient analysis through simulations of the turbine in EMTDC/PSCAD. Finally, the generator/converter solution is verified experimentally. The laboratory results were obtained using a 45 kW prototype with three generator segments and three converter units in series. [ABSTRACT FROM PUBLISHER]

In this paper, a new compensation strategy for receiving clean power of a conventional 18-pulse ac/dc converter formed by three 6-diode bridges is proposed. According to the proposed strategy, a three-phase current-controlled inverter injects the compensation currents into the three positive terminals of the three six-diode bridges. The goal of injecting currents at dc side is to improve the quality of the ac line currents. Compared to the conventional active filter deployed at the ac side, the three-phase inverter used in this paper is with lower kVA rating, and the 18-pulse converter draws nearly sinusoidal currents from the ac main by the proposed compensation strategy. The theoretical compensation command is derived in this paper, and then an approximate approach is recommended to simplify the calculation. A digital-signal processor is employed as a digital controller to calculate the compensation command, and a 3-kW prototype including the 18-pulse converter and the current-controlled inverter is built for evaluation and measurement. Moreover, the performance affected by unbalanced ac source is investigated. The experimental results demonstrate that the proposed method not only improves the line current quality but also mitigates the effects caused by unbalanced source voltages. [ABSTRACT FROM AUTHOR]

This paper discusses the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems. The paper introduces a control model with a cascaded DC voltage control at every converter that allows a two-terminal VSC HVDC system to cope with converter outages. When extended to an MTDC system, the model naturally evolves into a master-slave set-up with converters taking over the DC voltage control in case the DC voltage controlling converter fails. It is shown that the model can be used to include a voltage droop control to share the power imbalance after a contingency in the DC system amongst the converters in the system. Finally, the paper discusses two possible model reductions, in line with the assumptions made in transient stability modeling. The control algorithms and VSC HVDC systems have been implemented using both MatDyn, an open source MATLAB transient stability program, as well as the commercial power system simulation package EUROSTAG. [ABSTRACT FROM PUBLISHER]

This paper introduces a novel five-level nested neutral point piloted (NNPP) converter and analyzes the operating principle of five-level NNPP converter. This paper presents a novel space vector pulse width modulation (SVPWM) algorithm based on gh coordinate for five-level NNPP converter. First, the common-mode voltage is reduced by choosing the appropriate redundant switching states. After that, the floating-capacitor voltage balance control strategy is presented. The appropriate switch combinations of each phase are determined by the control requirements of floating-capacitor voltages respectively and the hardware mapping method of the switching states is presented. Furthermore, the neutral-point voltage balance control strategy is presented. In order to balance the dc-link capacitor voltages, the seven-segment switching sequence is selected according to the control requirements of dc-link capacitor voltages and the neutral-point voltage regulatory factor is introduced to regulate the durations of the redundant switching states in a switching cycle. Finally, the validity of the novel SVPWM algorithm with decoupling control strategies of floating-capacitor voltages and dc-link capacitor voltages is verified by the experimental results of five-level NNPP converter under steady-state and dynamic conditions. [ABSTRACT FROM AUTHOR]

A new static synchronous compensator (STATCOM) based on the diode-clamped modular multilevel converter (DCM2 C) is proposed in this paper. In this converter topology, the capacitor voltage is clamped by using a low power rating diode in each submodule. The quantity of voltage sensors is significantly reduced and is free from the number of voltage levels. Furthermore, the voltage balancing control method becomes very simple and the capacitor voltage balance speed is fast. Based on the structure of modular multilevel converter, the DCM2C-STATCOM has the capability of Var compensation and negative-sequence current compensation. The topology characteristics and compensation control method of DCM2C-STATCOM are investigated in this paper. Experimental results obtained from a laboratory prototype validate that the capacitor voltage of the proposed DCM2C-STATCOM can be well balanced and the Var and negative-sequence current compensations are effective. [ABSTRACT FROM AUTHOR]

To achieve high-performance control of modern dc–dc converters, using direct digital design techniques, an accurate discrete model of the converter is necessary. In this paper, a new parametric system identification method, based on a Kalman filter (KF) approach is introduced to estimate the discrete model of a synchronous dc–dc buck converter. To improve the tracking performance of the proposed KF, an adaptive tuning technique is proposed. Unlike many other published schemes, this approach offers the unique advantage of updating the parameter vector coefficients at different rates. The proposed KF estimation technique is experimentally verified using a Texas Instruments TMS320F28335 microcontroller platform and synchronous step-down dc–dc converter. Results demonstrate a robust and reliable real-time estimator. The proposed method can accurately identify the discrete coefficients of the dc–dc converter. This paper also validates the performance of the identification algorithm with time-varying parameters, such as an abrupt load change. The proposed method demonstrates robust estimation with and without an excitation signal, which makes it very well suited for real-time power electronic control applications. Furthermore, the estimator convergence time is significantly shorter compared to many other schemes, such as the classical exponentially weighted recursive least-squares method. [ABSTRACT FROM AUTHOR]

An outer-permanent magnet tubular linear generator has been designed to increase the power density and distance of relative movement. The design of the two buoys has been completed. This paper describes the detailed design process of the proposed generator. The magnetic network model of the proposed generator is implemented to analyze the performance. Considering the cogging force and voltage, the linear generator has been optimized and the final parameters have been determined. Based on the design results, the integrated wave converter system has been manufactured and put into China’s Yellow Sea. The measured voltage is highly consistent with the simulation data. All of results show that the proposed generator is well suitable for wave energy conversion. [ABSTRACT FROM AUTHOR]

Magnetic bearing has been developed for high-speed rotational machinery and its power electronics drive is its key technology. In this paper, a novel converter with reduced switches for magnetic bearing drive is described. The principle of the magnetic bearing and its test rig were introduced and graphically illustrated at first. Then, based on the current unidirectional and phase-leg sharing principles, the novel converter topology for magnetic bearing drive together with the current control functions is introduced, including the steady state and dynamic performance. Less power electronics devices are required for this converter in comparison with conventional magnetic bearing drives. Simulation and experimental results have successfully validated the current control capability of magnetic bearing with the proposed converter. With the novel converter and the controller, magnetic bearing can deliver superior levitation and dynamic control performance. In the rotational test, the magnetic bearing has been controlled with position error less than 20% of the air gap up to 24 000 r/min on the test rig. [ABSTRACT FROM PUBLISHER]

Current-transformer (CT) saturation causes severe distortion in the measured current waveform which may lead to maloperation of the protective devices. This paper proposes a low-cost, power-electronic device to prevent the CT from saturation. The proposed compensator is inserted in series with the relay in the CT secondary circuit and acts as a controlled voltage source (CVS). The proposed CVS generates a time-varying voltage to cancel the voltage developed across the CT burden; therefore, the CT magnetic flux remains almost constant and undistorted during the power system transients. It will be shown that this device can precisely compensate fault current, inrush current, and other probable transient currents despite its simplicity. The proposed device can be employed to compensate the already in-service CTs connected to nondigital and digital relays. Comprehensive computer simulations are carried out to validate the effectiveness of the proposed compensator. The performance of a sample compensator of this type is validated through special high-current laboratory experiments (carried out over several hundred amperes up to 1.6 kA), and the obtained results illustrate the capability of the proposed compensator to prevent CT saturation. [ABSTRACT FROM PUBLISHER]