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This paper describes a dc-link voltage control method of a single-phase back-to-back cascaded H-bridge inverter (SBCI) for a medium-voltage motor drive system. The main advantage of the SBCI topology over the conventional regenerative cascaded H-bridge topology with a three-phase active front-end (AFE) is a simple system structure, which is composed of an input transformer, a power cell, a current sensor, etc. However, the challenging points of the SBCI are larger voltage ripple in the dc-link capacitor and imbalance of dc-link voltages of each phase. The asymmetric dc-link voltage of each power cells could cause unstable operation such as over-modulation due to the lack of the dc-link voltage of a particular phase and result in over-voltage or under-voltage faults. In this paper, the control strategy of the dc-link voltage for the SBCI that uses the negative-sequence voltage of the converter is described. The proposed control method is verified with a computer simulation whose target is a 6.6-kV–1.25-MW medium-voltage drive system. Also, through the experimental setup with the prototype SBCI whose power rating is 16.2 kVA, the dc-link voltage of each AFE has been controlled within a 0.5% error of its reference value at the full load. [ABSTRACT FROM AUTHOR]

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency ($2f_{{\text{o}}}$), generating second harmonic current (SHC) in the front-end dc–dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc–dc converter and PV panel, the voltage loop gain of the front-end dc–dc converter should be high enough at $2f_{{\text{o}}}$. Since there is a −180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at $2f_{{\text{o}}}$ , proportional–integral–resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes. [ABSTRACT FROM AUTHOR]

A modular multilevel converter (MMC) is regarded as a promising topology in high-voltage direct current systems. However, the MMC consists of numerous submodules (SMs) and switching devices, which lead to a considerable switching loss, and increased cost and size of the heat sink. To mitigate these issues, this paper presents a novel modulation method composed of fundamental frequency modulation (FFM) and multicarrier-based sinusoidal pulsewidth modulation schemes. The main purpose of this modulation method is the reduction of switching loss while maintaining good harmonic performance. However, the FFM scheme leads to the unbalanced capacitor voltage of each SM in the MMC. Accordingly, this paper additionally proposes the rotation method and selective voltage balancing control for SMs to ensure effective performance of the method. Simulation and experimental results verify the effectiveness and performance of the proposed modulation scheme through switching loss and spectral analyses. [ABSTRACT FROM AUTHOR]

Constant on-time V^2 control for point-of-load buck converters has instability issues in the cases that the output capacitors' RC time constants are small. This paper intensively studies the proposed solution using capacitor current ramp compensation, which is a superior solution featuring fast response and universality. A frequency-domain small-signal model based on describing function method is proposed in this paper. The time-domain large-signal response to the load step change is analyzed. The analysis illustrates the unique transient response behaviors of the capacitor current ramp compensated V^2 control. The design optimization methodology based on frequency-domain and time-domain analysis is presented. The proposed model and the design guidelines are verified by the experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a generalized power-decoupling control scheme using a multiport isolated bidirectional converter for a multilevel inverter, which has multiple dc links inside. In the proposed method, a single power-decoupling capacitor is needed for all the dc links in the multilevel inverter cell. First, a prototype of the power-decoupling concept of individual H-bridge cells in the multilevel inverter is proposed, using a separate power-decoupling circuit. Then, a more advanced one-step power-decoupling method is proposed. The lifetime and reliability of the multilevel inverter is improved as film capacitors replace the large capacitance electrolyte capacitors. A multi-input ports/single output voltage-fed dual half-bridge converter (MDHB) is used for the power-decoupling circuit. A steady-state analysis for the peak and root mean square of the MDHB current is carried out for the loss breakdown. The currents are functions of the switching frequency, phase shift, leakage inductance, turn ratio, and output voltage, which make the multiport transformer design complex. A design methodology is proposed, which takes into account the design of the copper and core losses as functions of the switching frequency and number of turns. Furthermore, a special winding method for the input port is illustrated to obtain identical leakage inductances for the uniform current distribution in the multiport transformer. The proposed MDHB employs a current-sensorless power-decoupling control that contributes to the spontaneous ripple rejection of all the dc links without individual link current information, as well as to the cost and size reduction. Hence, the ripple-rejection controller is independent of the control configuration of the multilevel inverter, and also available for universal applications of various inverter topologies. Since the primary-input ports of MDHB share a single magnetic core for interfacing the ripple power to the unified secondary ripple capacitor, the controller design becomes difficult in considering the dynamic interaction among the ports, along with the average-voltage control loop design. In this paper, the dynamic analysis and controller design procedure of the circuit is also presented. The power decoupling is achieved even when the ripple frequency is other than the double frequency of the inverter output, since the single-pole transfer function of the small-signal model of the MDHB allows sufficient phase margin, along with high bandwidth. The proposed power-decoupling method for the multilevel inverter is validated with the help of simulation and 1.2-kW hardware prototype experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid seven-level converter based on T-type converter and H-bridge cascaded suitable for low-voltage and high-power-density applications. The operation principles and conduction paths are analyzed comprehensively. Voltage balance control of the floating capacitors is the key point of this paper. Mathematical expressions of capacitor voltage balance conditions are deduced based on sinusoidal pulse width modulation (SPWM). The results show that floating capacitor voltages are only balanced when the modulation index is below 0.82 under SPWM. In order to enlarge the modulation index ensuring the capacitor voltage balance, space vector modulation (SVM) is also applied to regulate floating capacitor voltages taking advantage of redundant switching vectors. Meanwhile, in SVM, there are no additional control requirements for the capacitor voltage. Taken both SPWM and SVM into consideration, the limit to the range of operation for this seven-level converter is presented, which is a function of the modulation index and the power factor. The merit of dc voltage utilization improvement in this topology is also analyzed based on simulation results. Simulations and experimental results under different modulation index and RL load are presented to verify the modulation strategies illustrated in this paper. [ABSTRACT FROM PUBLISHER]

An extended-duty-ratio (EDR) boost converter is studied extensively in this paper for high voltage gain applications with a wide input (and/or output) voltage range. The EDR is a unique combination of an interleaved, multiphase boost converter and switched capacitor configuration that achieves high voltage gain with significantly lower switch voltage stress and switching losses compared to conventional high-gain solutions. Most of the switches in the multiphase EDR experience only a fixed fraction of the output voltage ($1/M, 2/M$ , etc., where $M$ is the number of phases). Through extensive analysis over a wide operating range, it is shown here that the EDR boost converter has inherent current sharing among the phases only in a limited range of duty ratio— $(M-1)/M \leq D \leq 1$. As the duty ratio reduces beyond this range as required in wide input voltage applications, inherent current sharing property is lost. In this paper, techniques to ensure current sharing under all operating zones without requiring current sensors are presented. Instead of having equal duty ratio for each phase, it is adjusted for each phase according to the operating region of the converter. Extensive analysis is presented to derive the required duty ratio changes for the different phases. The proposed concept is validated with experimental results from a 250 W, 3-phase EDR boost, and GaN-based hardware prototype. [ABSTRACT FROM AUTHOR]

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]

An isolated bidirectional dc–dc converter, which combines parallel-connected two-level converters (TLCs) on the medium-voltage side and a modular multilevel converter (MMC) on the high-voltage side, namely a TLC-MMC converter, is a promising candidate for the interconnection of medium-voltage direct current (MVDC) and HVdc grids. Different from typical phase-shift controlled isolated bidirectional dc–dc converters, transformer currents of a TLC-MMC converter are regulated by the MMC in a closed-loop manner in order to achieve extremely low switching losses and low total device rating of power-semiconductor devices in TLCs. This paper addresses the dedicated operation technique and the voltage-balancing control of the TLC-MMC converter. The MMC in the TLC-MMC converter operates with six-step phase voltages and sinusoidal phase currents. Such an operation makes the MMC present remarkable differences to ac–dc applications. The direct-modulation technique, which is the most-used technique performed to MMCs in line-frequency ac–dc power conversion, has been concluded not applicable to the MMC in a TLC-MMC converter due to the significant sixth-order oscillation resulted in the dc-link voltage. Thus, the indirect-modulation technique is employed, and consequently a closed-loop voltage-balancing control is mandatory for the MMC. Possible measures for voltage balancing have been explored, and effective voltage-balancing control with a simplified implementation is proposed. Validity and effectiveness of the proposed control strategy have been verified by both simulations and experiment using a downscale prototype. [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]

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]

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]

A multiphase-interleaved non-isolated high step-up dc/dc resonant converter for high-current applications with wide load range is proposed in this paper. The converter consists of an inverter stage with multiple resonant inverters in parallel interleaving operation that is cascaded to a common diode–capacitor rectifier stage. The topology of the inverter stage is a variation of class E inverter, which is designed for voltage boosting and of which the voltage gain is insensitive to load change. The rectifier stage comprises multiple pairs (correlated to the number of inverters in parallel) of diodes in parallel and a small output capacitor, which makes it simple to implement and of low cost. Together, the voltage gain variance of the converter can be confined within a small value and zero voltage switching operation can be maintained over a wide load range. A detailed mathematical analysis of the proposed converter is conducted for systematizing its design. A prototype of the proposed converter with an input voltage of 23.6 V, a rated output voltage of 190 V, and a rated output power of 80 W at 486 kHz is constructed for verification. A frequency controller is implemented to achieve fine regulation of the output voltage. [ABSTRACT FROM AUTHOR]

A family of bidirectional switched-capacitor (SC) converters with high-gain ratio of any positive integer is proposed in this paper for distributed energy resources applications. As compared with other existing SC converters achieving a same conversion gain, the main advantages of the proposed converters are that they require a relatively lower number of switches and capacitors, have a relatively lower switch's and capacitor's stress, and that their associated driver circuits are simpler to realize. Importantly, with the achievable conversion ratio being flexible and that the input and output of the proposed converters are of common ground, the proposed converters are widely suitable for many applications. Moreover, as the proposed converters do not possess magnetic component or any component that can severely degrade the converters’ performance at high temperature, they are especially useful for high-temperature applications. Besides, the proposed converters are capable of delivering bidirectional power, which is a key requirement for emerging applications with battery storages. Different aspects of the proposed converters, including a simple auxiliary power supply circuit for the MOSFETs’ drivers, will be discussed in this paper. A nine-time SC converter prototype that operates with 20-V input voltage, 100-W output, and at 75 kHz, is constructed and tested. Experiment results show that the maximum efficiency achievable with this prototype is over 98% (without driver's loss) and the efficiency over the entire load range between 25 and 100 W is over 95.5% including the driver's loss. The output voltage ripple of the SC converter is less than 1%. When the SC converter is open-loop controlled, the load voltage regulation is relatively well kept at less than 5% between full load and no load conditions. [ABSTRACT FROM PUBLISHER]

In this paper, the Z-source converter is introduced to power factor correction (PFC) applications. The concept is demonstrated through a wireless power transfer (WPT) system for electric vehicle battery charging, namely Z-source resonant converter (ZSRC). Due to the Z-source network (ZSN), the ZSRC inherently performs PFC and regulate the system output voltage simultaneously, without adding extra semiconductor devices and control circuitry to the conventional WPT system such as conventional PFC converters do. In other words, the ZSN can be categorized as a family of the single-stage PFC converters. In addition, the ZSN is suitable for high-power applications since it is immune to shoot-through states, which increases reliability and adds a boost feature to the system. The ZSRC-based WPT system operating principle is described and analyzed in this paper. Simulations and experimental results based on a 1-kW prototype with 20-cm air gap between the system primary and secondary sides are presented to validate the analysis and demonstrate the effectiveness of the ZSN in the PFC of the WPT system. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel control strategy for three-level neutral-point-clamped (NPC) power converter. The proposed control scheme consists of three control loops, i.e., instantaneous power tracking control loop, voltage regulation loop, and voltage balancing loop. First, in the power tracking control loop, a set of adaptive sliding mode controllers are established to drive the active and reactive power tracking to their desired values via radial basis function neural network technology. In the voltage regulation loop, an efficient but simple adaptive controller is designed to regulate dc-link output voltage where the load is considered as an external disturbance. Moreover, a composite controller is developed in the voltage balancing loop to ensure imbalance voltages between two dc-link capacitors close to zero, in which a reduced-order observer is used to estimate sinusoidal disturbance improving the converter performance. The effectiveness and superiority of the proposed control strategy for the NPC power converter are compared with other control schemes through experimental results. [ABSTRACT FROM AUTHOR]

This paper explores the large-signal and small-signal dynamics of a series-capacitor (SC) buck-type converter and introduces an optimal closed-loop control scheme to accommodate both the steady-state and transient modes. As opposed to a conventional buck converter, where time-optimal control is realized by a single on–off cycle, in the SC-buck topology, there is a need to distribute the switching phases to satisfy the charge balance of the SC. The new control method merges a voltage-mode controller for the steady-state operation and a nonlinear, state-plane based transient-mode controller for load transients. A detailed principle of the operation of the SC-buck converter is provided and explained through an average-behavioral model and state-plane analysis. The operation of the controller is experimentally verified on a 20 W 12 V-to-1 V converter, demonstrating voltage-mode control operation as well as minimum-deviation and time-optimal responses for load transients. [ABSTRACT FROM AUTHOR]

In this paper, analysis and design of a three-phase-isolated Cuk-based power factor correction (PFC) converter have been proposed. The proposed converter is operated in discontinuous output inductor current mode to achieve PFC at ac input. This avoids the inner current control loop which further eliminates the sensing of current. This makes the system more reliable and robust. The converter requires only one simple voltage control loop for output voltage regulation, and all the power switches are driven by the same gate signal which simplifies the gate driver circuit. The detailed operation of the converter and design calculations are presented. And also a small-signal model of the converter by using current injected equivalent circuit approach is presented to aid the controller design. The experimental results from a 2-kW laboratory prototype with 208-V line-to-line input voltage, 400-V output voltage are presented to confirm the operation of the proposed converter. An input power factor of 0.999, an input current total harmonic distortion of as low as 4.06%, and a high conversion efficiency of 95.1% are achieved from laboratory prototype. [ABSTRACT FROM AUTHOR]

The stability and small-signal analyses of the dual series-resonant dc–dc converter are investigated in this paper. The small-signal model of the converter is derived from the extended describing function analysis. Frequency-domain analysis is then employed to derive the small-signal transfer functions of the converter. This procedure establishes an insightful understanding about the effects of the converter parameters and operating conditions on the stability and transient behavior of the converter. The results are used to illustrate the development of the single-loop, voltage-feedback control scheme for the dual series-resonant dc–dc converter. It is demonstrated that the proposed control method enables the converter to regulate its output voltage in the case of abrupt changes in input voltage and output load, and it provides improvement in the transient response. An experimental prototype of the dual series-resonant dc–dc converter is built and the performance of the control scheme is experimentally evaluated by applying abrupt changes in the operating condition of the converter. Based on the experimental results, it is shown that the single-loop, voltage-feedback control scheme is effective in regulating the output voltage of the converter. [ABSTRACT FROM AUTHOR]

Model predictive control (MPC) for modular multilevel converter (MMC) systems has drawn attention among researchers in recent years due to its straightforward implementation, ability to control multiple objectives in a single cost function, and excellent dynamic response. Even though MPC seems promising for the MMC, it suffers from an excessive increase in computational complexity when the number of submodules (SMs) per arm (N) is increased. The computational load can be reduced by decoupling the SM capacitor voltage control from the cost function and balancing them in an external voltage sorting algorithm. In this paper, a further reduction of computations in the MPC is achieved by the preselection of control options that can satisfy the control objectives in the next sampling time. The preselection algorithm generates a greatly reduced number of control options to be computed by a straightforward MPC loop at each sampling time. In addition, the MPC together with the preselection algorithm can generate $2N+ 1$ output voltage levels in the MMC while maintaining a low $dv/ dt$ in the output voltage and suppressing the circulating currents in MMCs with considerable number of SMs. The steady-state and dynamic performance of the MMC operating with the proposed method is verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a circuit structure that can improve the power-conversion efficiency \eta e of an AC–DC boost converter. The circuit uses a 1:1 transformer and a voltage boost circuit composed of an inductor, a capacitor, and a diode. The transformer forces the converter to operate in continuous conduction mode for input current, and thereby reduces input ripple. The voltage boost circuit enables the switch to operate in critical conduction mode, so the power loss due to turn-on of switch and reverse recovery of diode is minimized. The proposed converter has high \eta e over a wide range of AC input voltage $V_{{\rm{ac}}}:\,\eta _{e}= 92.9\% $ at $V_{{\rm{ac}}}= {\text{85}}\,{\text{V}} and \eta e= 97.4\% at V{\rm{ac}}= {\text{265}}\,{\text{V}}, when the converter was operated at DC output voltage of 400 V and output power of 500 W. The temperature of the switch stabilized at ∼84 °C for $V_{{\rm{ac}}}= {\text{85}}\,{\text{V}}$ , at ∼70 °C for $V_{{\rm{ac}}}= {\text{110}}\,{\text{V}}$, and at ∼42 °C for $V_{{\rm{ac}}}= {\text{220}}\,{\text{V}}$, whereas that in other AC–DC converters either failed or stabilized at much higher levels. The proposed converter is well suited to systems that require an input voltage range of 85–265 V in root-mean-square value. [ABSTRACT FROM AUTHOR]

Modularity and high reliability from redundancy are the two attractive advantages of modular multilevel converters (MMCs). This paper elaborates a switch open-circuit fault diagnosis and a fault-tolerant operation scheme for MMCs with distributed control. The proposed fault diagnosis and fault-tolerant control method can significantly improve the reliability of the MMC while maintaining the modularity of its software implementation. By distributing fault diagnosis into submodules, its local controller is capable of identifying the switching devices in open-circuit fault without extra hardware circuitry. Based on the real-time measurements of submodule terminal voltage and arm current, single, or multiple faulty switches can be identified within 3.5 ms without triggering faulty alarms. Furthermore, a new fault-tolerant operation is proposed to maintain the output current, internal dynamics, and switching harmonics unchanged after the faulty submodule is bypassed. This is achieved by resetting the period and phase registers in the local controller according to the information of bypassed submodules. The control loops of the MMC are not influenced by the proposed fault diagnosis and fault-tolerant operation, making the operation transition seamless and reliable. Experimental results show that fault identification and system reconfiguration can be completed within 5 ms, and the MMC can seamlessly and smooth ride through the switch open-circuit faults without severe malfunction and catastrophic damages. [ABSTRACT FROM AUTHOR]

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. Recent researches have proved its significant advantages for adjustable-speed motor drives compared with the back-to-back modular multilevel converter. However, the branch energy balancing in the M3C presents great challenge especially at critical-frequency points where the output frequency is close to zero or grid-side frequency. Generally, this balancing control depends on the appropriate injection of inner circulating currents and the common-mode voltage (CMV), whereas their values are hard to determine and optimize. In this paper, an optimization-based predictive control method is proposed to calculate the required circulating currents and the CMV. The proposed method features a broad frequency range balancing of capacitor voltages and no reactive power in the grid side. For operation at critical-frequency points, there is no increase on branch voltage stresses and limited increase on branch current stresses. A downscaled M3C system with 27 cells is designed and experiment results with the R–L load and induction motor load are presented to verify the proposed control method. [ABSTRACT FROM AUTHOR]

This paper introduces a hybrid modular multilevel converter (MMC) with the full-bridge submodules (FBSMs) inserted in the ac side. It is intended for redistributing arm powers of the MMC system to reduce the submodule (SM) capacitor voltage fluctuation by injecting high-frequency powers in upper and lower arms without disturbing the output voltage. New power distribution of the hybrid MMC system can be realized by controlling the SMs and FBSMs working under the high-frequency powers. The proposed FBSMs in the hybrid MMC system also perform as the harmonic power generator to eliminate the common-mode voltages in the ac side. Since the FBSMs locate in the ac side of the hybrid MMC system, the traditional MMC control method can be directly applied in arm control of the hybrid MMC system. Meanwhile, relationship between the arm powers and SM capacitor voltage has been analyzed in detail to reduce the SM capacitor voltage ripples by redistributing the frequency spectrum of arm powers. Simulation and experimental results have also verified the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents the active capacitor voltage-balancing (ACVB) methods based on the dynamic model for a five-level nested neutral-point piloted (NNPP) converter using hybrid carrier modulation. The traditional hybrid carrier modulation provides natural voltage balancing of floating capacitors and dc-link capacitors. First, the natural voltage-balancing ability under steady-state and ideal conditions is theoretically proved. However, there are accumulated errors and deviations of capacitor voltages in practical applications, especially under dynamic conditions. In order to address the accumulated errors and deviations of capacitor voltages, the dynamic models of floating-capacitor voltages and neutral-point voltage are established. Moreover, the ACVB method based on the dynamic model of the neutral-point voltage is proposed by injecting the zero-sequence voltage. And the ACVB method based on the dynamic model of floating-capacitor voltages is proposed by introducing the adjustment variables. The proposed ACVB methods have the advantages of simple formula, definite physical meaning, and high accuracy. Furthermore, the output voltage quality and current quality are evaluated in term of total harmonic distortion and the effect of model parameter mismatch on the proposed ACVB methods is presented. Finally, the comparative simulation and experimental results verify the correctness of the proposed ACVB methods based on the dynamic model under different conditions. [ABSTRACT FROM AUTHOR]

An H-bridge hybrid modular converter (HBHMC) is proposed for high-voltage direct current (HVDC) applications. It uses a wave-shaping circuit consisting of series-connected full-bridge submodules (FBSMs) at the output of the main H-bridge converter. For a three-phase system, three HBHMCs are connected either in series (series-HBHMC) or in parallel (parallel-HBHMC) across the dc-link. The operating modes of HBHMC, novel modulation strategies for voltage balancing of FBSMs, and control of HBHMC-based HVDC system are presented in this paper. A detailed comparison between HBHMC and other hybrid topologies is performed on the basis of required number of switches and capacitors. The HBHMC has the features of dc fault blocking capability, lower footprint structure, and extra degree of freedom for submodules capacitor voltage balancing. The efficacy of the HBHMC-based HVDC system for three-phase balanced and unbalanced grid conditions and its fault-tolerant capability are validated using PSCAD simulation studies. Furthermore, the feasibility of the proposed converter under normal and dc fault conditions and of the proposed capacitor voltage control scheme is validated experimentally by using a three-phase grid-connected HBHMC laboratory prototype. The results demonstrate the effectiveness of the proposed HBHMC topology, control techniques, and satisfactory responses of the HBHMC-based HVDC system. [ABSTRACT FROM AUTHOR]

Compared with conventional power transformer, the power electronic transformer (PET) or solid-state transformer has many attractive additional features. This paper focuses on an H-bridge-based three-phase three-stage modular PET, which consists of an input stage with series-connected H-bridge converters, an isolation stage with several independent dual-active-bridge converters, and an output stage with parallel-connected H-bridge converters. This PET suffers dc-link capacitor voltage unbalancing issue, and the parallel-connected module current unbalancing sharing issue. In this paper, a system control structure is proposed for the PET to deal with these issues. Different input-stage individual module dc-link voltage balancing control methods are analyzed and compared. It is found that the one implemented by directly trimming module output voltage amplitude is most suitable for PET. Moreover, a downscaled laboratory prototype is designed, built, and tested to verify the control strategy. [ABSTRACT FROM PUBLISHER]

In this paper, we introduced an improved transformer-based parallel-resonant converter, suitable for high voltage, high power dc loads. The resonant capacitors are connected in parallel with diodes of the high voltage rectifier, thus restraining dv/dt stress in high voltage circuits. The LC tank resonant frequency is well beyond the switching frequency, reducing in this way the VA rating of LC components. With rated load, the output current of the IGBT bridge is equal to zero at switching instants, and the switching losses are very low. Proposed subresonant circuit requires lower primary current for the given output power and it has reduced IGBT losses. Experimental setup delivers 79 kW with an efficiency of 96.78%. The paper explains the switching states of the devised topology, and provides the guidelines for designing the power transformer, the LC tank and controls. Analytical prediction of the performance curves is verified both in laboratory and by the field tests with an electrostatic precipitator plant. [ABSTRACT FROM AUTHOR]

This paper presents an intensive discussion on a transformerless back-to-back (BTB) system using modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC). It is expected to be applied to a power distribution system with many distributed power generators. This paper designs, constructs, and tests a three-phase 200-V, 10-kW DSCC-based BTB system installed on a 200-V, 40-kW power distribution simulator consisting of two radial feeders. Experimental results show that the BTB system can mitigate power-flow imbalance between the two feeders. Moreover, the zero-sequence current circulating between the two feeders can be suppressed to be as small as 10 mA (0.02%) in rms by connecting a common-mode choke to the common dc link of the BTB system. [ABSTRACT FROM AUTHOR]

V2Ic control provides very fast dynamic performance to the Buck converter both under load steps and under voltage reference steps. However, the design of this control is complex since it is prone to subharmonic oscillations and several parameters affect the stability of the system. This paper derives and validates a very accurate modeling and stability analysis of a closed-loop V2Ic control using the Floquet theory. This allows the derivation of sensitivity analysis to design a robust converter. The proposed methodology is validated on a 5-MHz Buck converter. The work is also extended to V2 control using the same methodology, showing high accuracy and robustness. The paper also demonstrates, on the V2 control, that even a low bandwidth-linear controller can affect the stability of a ripple-based control. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for high-voltage and high-power applications. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs), which assumes that the multiple MMCs are directly connected at both ac and dc sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions: voltage balancing of submodules and mitigation of circulating currents, where the novel transformed third-order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second-order and fourth-order circulating currents and a sixth-order harmonic resonant controller is used to attenuate the zero-sequence sixth-order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a submodule, respectively. Carefully controlling the healthy submodules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for the short-circuit switch fault operation which was seldom discussed in the published literature works with respect to the MMC fault-tolerant operation schemes. MATLAB simulations and the constructed experimental prototype have verified the performance of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Considering the synthetical effects of the ac system and the dc system, this paper proposes two novel indexes, the dynamic redundancy and the utilization ratio of the submodules. On this basis, the nearest level control is applied as the modulation method and an optimized control strategy based on the dynamic redundancy for the modular multilevel converter (MMC) is proposed. One of the main innovations is that the reference value of the capacitor voltage is derived according to the maximum output voltage of each converter arm and the safety margin which can be adjusted artificially. Unlike previous strategies, the redundancy can be adjusted dynamically, and the utilization ratio of the submodules can be effectively improved. In addition, the capacitor voltage and the inner stress are reduced, and the fault ride-through capability of the system can be enhanced. In particular, the strategy under abnormal operating conditions is also detailed in this paper. A model of two-terminal MMC-high-voltage direct current system is built in PSCAD/EMTDC, and the simulation result proves the validity and the feasibility of the proposed strategy. [ABSTRACT FROM AUTHOR]

Increasing energy demand and growth of wind generation have significantly increased the stress on the electric grid. The low investment in transmission infrastructure necessitates adoption of methods for efficient use of existing resources. Power converter-based FACTS devices, capable of dynamic power-flow control, have been the preferred solution to maximize utilization of existing infrastructure, but implementation of traditional FACTS solutions at transmission level voltages is complex and cost prohibitive. This paper proposes a power router (PR) for dynamic power-flow control in a meshed network. The proposed PR is realized by augmenting a transformer with a fractionally rated back-to-back converter. The main advantages of the proposed converter are the fractional converter rating and reduced low-frequency transformer requirement compared to the traditional FACTS solutions. This paper outlines the proposed PR topology, control range, controller design and demonstrates the functionality through simulation and experimental results. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is receiving wide acceptance in both high- and medium-voltage (MV) applications. However, due to the low (fundamental and second-order) frequency ripple powers in the submodule (SM) capacitors, large capacitance is required to smooth the SM voltage. The SM capacitors account for a large portion of volume and weight in the MMC system. Present methods (e.g., circulating current control, power channels linking upper and lower arms) cannot eliminate the fundamental and second-order ripple powers simultaneously. This paper investigates the feasibility of an active power decoupling technique for solving this issue. By adding a buck-type active power filter (APF) circuit (which contains another energy-storage capacitor), the low-frequency ripple powers can be transferred to the APF capacitor. This significantly reduces the SM voltage ripple and therefore the total capacitance of the SM. To enhance voltage ripple suppression, APF capacitor voltage reference is modified in a closed-loop manner, and a proportional-integral plus repetitive controller is proposed. Simulations and experimental results prove the validity of the method. A comparison with the traditional MMC shows that it can significantly reduce system volume and improve power density. The method is best suited for MV applications where power density is given a high priority. [ABSTRACT FROM PUBLISHER]

This paper presents a multiobjective real-time controller for a modular multilevel converter capable of parallel module connectivity, the so-called modular multilevel series parallel converter (MMSPC). The MMSPC topology allows the batteries to be dynamically rewired in various series–parallel configurations, generating a wide range of output voltage levels. The novel control method parallelizes the modules to balance their voltages without the need for individual module voltage monitoring. Additionally, the controller optimizes across the large number of feasible system configurations to minimize switching and conduction losses. Finally, the controller efficiently encodes the system configuration with module interconnection states rather than the module switch states, which substantially simplifies control. Furthermore, this work experimentally validates the MMSPC topology and concept. In the prototype, the parallel mode reduced the system losses at 5 kW output power by 18% and 24% for load power factors of 1.0 and 0.8, respectively. Sensorless balancing via parallelization maintained well-matched module voltages (standard deviation = 0.045 V) over a 5-h battery discharge with highly variable load current. The reduced conduction losses and simple balancing capability of the MMSPC can enable new applications at medium and low voltages that benefit from its high-quality output, elimination of filtering magnetics, fast response, and modularity. [ABSTRACT FROM AUTHOR]

Dynamic voltage restorer (DVR) technology has become a mature power quality product. In high-power applications, DVR using a multilevel converter is commonly used. However, DVR using a multilevel direct pulse width modulation (PWM) ac–ac converter has not been well studied. This paper presents a new DVR topology using a cascaded multilevel direct PWM ac–ac converter. In the proposed scheme, the unit cell of the multilevel converter consists of a single-phase PWM ac-ac converter using switching cell structure with coupled inductors. Therefore, the multilevel converter can be short- and open-circuited without damaging the switching devices. Neither lossy RC snubber nor a dedicated soft commutation strategy is required in the proposed DVR. This improves the reliability of the DVR system. The output voltage levels of the multilevel converter increase with the number of cascaded unit cells, and a high ac output voltage is obtained by using low-voltage-rating switching devices. Furthermore, a phase-shifted PWM technique is applied to significantly reduce the size of the output filter inductor. A 1-kW prototype of single-phase DVR is developed, and its performance is experimentally verified. Finally, the simulation results are shown for a three-phase DVR system. [ABSTRACT FROM AUTHOR]

This paper presents an isolated LLC series resonant DC/DC converter with novel frequency adaptive phase shift modulation control, which suitable for wide input voltage (200–400 V) applications. The proposed topology integrates two half-bridge in series on the primary side to reduce the switching stress to half of the input voltage. Unlike the conventional converter, this control strategy increases the voltage gain range with zero-voltage-switching (ZVS) to all switches under all operating voltage and load variations. Adaptive frequency control is used to secure ZVS in the primary bridge with regards to load change. To do so, the voltage gain becomes independent of the loaded quality factor. In addition, the phase shift control is used to regulate the output voltage as constant under all possible inputs. The control of these two variables also significantly minimizes the circulating current, especially from the low-voltage side, which increases the efficiency as compared to a conventional converter. Experimental results of a 1-Kw prototype converter with 200–400-V input and 48-V output are presented to verify all theoretical analysis and characteristics. [ABSTRACT FROM AUTHOR]

The five-level active neutral-point-clamped (5L-ANPC) converter has been widely studied for its excellent performance in high-power medium-voltage applications. This paper analyzes the space vector pulse width modulation (SVPWM) strategy of the 5L-ANPC converter in the virtual coordinate, and presents an optimized control strategy which can balance the neutral point (NP) voltage and avoid the dead-time effects for the first time. In this strategy, on one hand, the 125 space vectors are combined by 96 triangles in the seven-segment vector synthesis method, then the triangles are divided into seven categories which have different characteristics of balancing the NP voltage, and every category has its own principles to choose the vector sequence and compute the vector durations. On the other hand, the dead-time effects of the 5L-ANPC converter are studied in detail and the transitions between different switching states are constrained in the optimized control strategy to avoid the dead-time effects that cannot be compensated by traditional pulse-based dead-time compensation methods. Finally, the optimized SVPWM strategy is experimentally verified in this paper. [ABSTRACT FROM AUTHOR]

The boost converter with a voltage multiplier cell allows the static gain extension by means of the switching capacitor technique, reducing the duty cycle needed to achieve the same voltage gain when compared to the conventional boost converter. However, the modeling of this converter is complex and requires the use of advanced techniques due to the resonant inductor. Thus, this paper aims to present a reduced-order model of this converter without the resonant energy exchange between the capacitors, so that the state-space averaging technique can be applied assuming small ripple in the state variables. In addition, this paper presents the design of a control system for the boost converter with a voltage multiplier cell. The adopted strategy employs an inner loop to control the input current and an outer loop for the output voltage regulation. Extensive analysis based on simulations and an experimental prototype demonstrate that the proposed modeling, although simplified, is sufficient for an adequate control system design, ensuring good voltage regulation, and fast transient responses. [ABSTRACT FROM PUBLISHER]

This paper presents dc-bus voltage control with a three-phase bidirectional inverter for dc distribution systems. The bidirectional inverter can fulfill both grid connection and rectification modes with power factor correction. The proposed control includes two approaches, one line-cycle regulation approach (OLCRA) and one-sixth line-cycle regulation approach (OSLCRA), which take into account dc-bus capacitance and control dc-bus voltage to track a linear relationship between the dc-bus voltage and inverter inductor current. Since both of the approaches require the parameter of dc-bus capacitance, this paper first presents determination of dc-bus capacitor size and an online capacitance estimation method. With the OLCRA, the inverter tunes the dc-bus voltage every line cycle, which can reduce the frequency of operation-mode change and current distortion. The OSLCRA adjusts current command every one-sixth line cycle to adapt to abrupt dc-bus voltage variation. The two approaches together can prevent dc-bus voltage from wide variation and improve the availability of the dc distribution systems without increasing dc-bus capacitance. Experimental results measured from a three-phase bidirectional inverter have verified the feasibility of the discussed control approaches. [ABSTRACT FROM AUTHOR]

Multilevel inverters are receiving more attentions nowadays as one of preferred solutions for medium- and high-power applications. As one of the most popular hybrid multilevel inverter topologies, the five-level active-neutral-point-clamped inverter (5L-ANPC) combines the features of the conventional flying-capacitor type and neutral-point-clamped (NPC) type inverter and was commercially used for industrial applications. In order to further decrease the number of active switches, this paper proposes a seven-switch 5L-ANPC (7S-5L-ANPC) topology, which employs only seven active switches and two discrete diodes. The analysis has shown a lower current rating can be selected for the seventh switch under high power factor condition, which is verified by simulation results. The modulation strategy for 7S-5L-ANPC inverter is discussed. A 1 kVA single-phase experimental prototype is built to verify the validity and flexibility of the proposed topology and modulation method. [ABSTRACT FROM AUTHOR]

Input-series output-series (ISOS) modular dc–dc converters are suitable for applications involving high input and output voltages. Input voltage sharing (IVS) and output voltage sharing (OVS) of the constituent modules must be guaranteed for an ISOS system. A duty cycle-based model predictive control (DC-MPC) scheme is proposed in this paper to achieve IVS and OVS for an ISOS system. The employed switching frequency in the DC-MPC scheme is fixed; duty cycles are optimized and then determined by minimizing a cost function, which is derived from an ISOS system aimed at achieving IVS and OVS. IVS and OVS can, therefore, be obtained by employing the optimized duty cycle within each sampling period. In addition, challenges and disadvantages of directly using traditional MPC to achieve IVS and OVS is briefly discussed, while the proposed DC-MPC can effectively avoid them. Compared with the existing proportional integral-based schemes in the literature, the proposed DC-MPC is better for IVS and OVS because it is a very simple and intuitive approach with lower cost and higher overall dynamic performance. Simulation and experiments are conducted to verify that the proposed DC-MPC scheme works well during both steady and transient states. [ABSTRACT FROM AUTHOR]

This paper presents a new voltage-balancing controller for cascaded multilevel converters, especially for single-phase cascaded multilevel converters. It proposes a control algorithm that devotes itself not only to balancing the floating dc capacitors but also to eliminating the coupling effect between the voltage-balancing controller and the original system controller (controller without additional voltage-balancing controllers). Specifically, the average model in the d-q coordinate frame is derived and the control law is established. Then, the coupling effect between the voltage-balancing controller and the original system controller is identified and a new expression for duty cycle modification is proposed thus to eliminate the effect. Furthermore, this paper gives the design considerations of the pro- posed method, including the derivation of key transfer functions and effective voltage-balancing area, for the completeness of the discussion. Moreover, the reference generation techniques of the voltage-balancing controller are also discussed. This paper investigates the voltage imbalance in the soft-start process caused by an unsuitable reference, and presents a simple modified reference generation solution. Finally, both simulation and experimental results verify the performance of the proposed control system. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) for high-power applications may contain over a thousand submodules. In work such as submodule design or innovation to build a full system with as many levels to test the submodule is time consuming and costly. To solve the problem, testing schemes are being developed to allow an individual submodule to be tested without a complete MMC. This paper presents a new submodule testing scheme that builds on a recent development to improve the testing capability. The improved structure is introduced along with design and control techniques. A design example of a test platform to test SMs with 2000-V rated voltage is introduced, which only requires a 545-V dc supply. The voltage testing capability is increased by more than five times when compared with the original scheme. In addition, simulation results show that the proposed method can offer better current tracking accuracy than the original with the same parameter settings. A prototype test platform, designed to test submodules with 400-V rated voltage and 14-A current, has been built and tested experimentally in order to verify the design. [ABSTRACT FROM PUBLISHER]

Among the main control targets in a modular multilevel converter (MMC) is the control of the circulating currents within the phase legs of the topology. This paper presents a controller for the circulating current of the MMC that utilizes the available redundancies of the multilevel waveform in $2N+1$ modulated MMCs in order to regulate the circulating current to its reference. The main advantages of the approach are the elimination of control loops that generate the reference voltages for the control of the circulating current, simple implementation and very fast dynamic performance. The controller is implemented at the modulation stage and its operation is independent of the circulating current reference. An extension of the controller to track large deviations in the circulating current is also demonstrated. The simplicity and effectiveness of the proposed controller is illustrated through detailed simulations and experimental results from a single-phase laboratory prototype. [ABSTRACT FROM PUBLISHER]

In this paper, a novel step-up converter is presented, where the charge pump concept, combined with the traditional boost converter structure, is utilized. Although two inductors are used in such a converter, the difference in value between the two inductors affects the regulation performance of this converter slightly. Furthermore, the behavior of this converter is similar to the traditional boost converter, and hence, the control of this converter can be realized easily. Above all, the energy stored in the two inductors, connected in series with the energy stored in the charge pump capacitor and the input voltage, is released to the load during the demagnetization period. In this paper, the basic operating principles of the proposed converter are presented along with some experimental results to demonstrate the effectiveness of this converter. [ABSTRACT FROM AUTHOR]

This paper proposes a new circuit topology for a multilevel pulse width modulation (PWM) rectifier. The proposed method forms a new circuit by combining a diode clamp-type topology with a flying capacitor-type topology. The proposed circuit uses only 12 switches, despite the use of a five-level three-phase PWM rectifier. Further, the proposed circuit can obtain good performance, same as a conventional multilevel circuit. This paper describes about the features of the proposed topology; the control strategy and the loss analysis that is estimated by a circuit simulator. In addition, the basic operation of the proposed method is confirmed by simulation and experimental results. The proposed converter achieved total harmonic distortion of 3.4% for the input current and efficiency of 97.4% for a 1 kW class experimental setup. [ABSTRACT FROM AUTHOR]

This paper presents the modular multilevel cascade converter based on double-star chopper-cells, which is intended for grid connection to medium-voltage power systems without using line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional pulsewidth modulation chopper-cells with floating dc capacitors. This arm structure requires voltage-balancing control of all the dc capacitors. However, the voltage control combining an averaging control with an individual-balancing control imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation and experiment. [ABSTRACT FROM AUTHOR]

Switched-capacitor (SC) converters were initially introduced for low-power applications and monolithic integration. In recent years, SC converter has found high-power applications. However, voltage regulation remains an issue. This paper addresses regulation challenges for high-power SC converters, based on charge-balance transient-calculation (CT) modeling method and peak current stress estimation. With the help of CT model, due to its accuracy and comprehensive relationship, circuit and control parameters’ impacts on regulation become straightforward and concerns on components stresses can be addressed quantitatively. The suitability of CT method for regulation analysis is confirmed by comparison with traditional modeling methods. The CT method is used in a 1-kW 3X two-switch boosting switched-capacitor converter (TBSC) circuit for steady-state analysis and current stress estimation. The soft rising input current and nature interleaving properties of 3X TBSC make it well suited for high-power application. Finally, the small-signal model of the 3X TBSC is developed and a closed-loop operation is achieved under 1 kW power rating. [ABSTRACT FROM AUTHOR]

This paper proposes a power-factor-corrected canonical switching cell (CSC) converter-based switched-mode power supply for arc-welding applications. In the proposed arc-welding power supply (AWPS), CSC converter operating in discontinuous inductor current mode (DICM) is used to attain inherent power factor correction. The DICM operation substantially reduces the complexity of the control and effectively regulates the dc-link voltage. At the back end, a pulsewidth-modulated (PWM) isolated full bridge dc–dc converter is used to provide a high-frequency isolation, which is mandatory for the arc-welding process. A dual-loop control scheme is utilized to incorporate overcurrent protection and to regulate dc voltage at the output making it suitable for arc-welding applications. Test results are presented to confirm the viability of the proposed AWPS. The performance of the proposed AWPS is evaluated on the basis of total harmonic distortion (THD) of the supply current, power factor, dynamic response, and voltage regulation to prove its effectiveness. [ABSTRACT FROM PUBLISHER]