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The capacitance of sub-module capacitor has a great impact on the cost, size, and operation of the modular multi-level converter (MMC). However, the capacitance selected by the existing methods does not always meet the requirement in practice. To find out the reasons for this improper selection, the existing selection methods are analyzed in this paper. Based on the analysis, a new capacitance-selection method is proposed. In the new method, the problems of the existing methods are solved, and the calculation of capacitor voltage is based on a proposed capacitor-voltage model whose calculation error can be smaller than 0.1%. Furthermore, the Secant Method is used in the capacitance selection to quickly calculate the required capacitance. The new capacitance-selection method can be applicable to the MMC not having a circular operating region. In addition, this paper found that the capacitor voltage and the capacitor-voltage ripple reach their maximum values when the power-factor angles of the MMC are –π/2 and π/2, respectively, and the dc component in the capacitor voltage changes according to the operating condition of the MMC. Finally, the proposed selection method and theoretical analyses are verified by both simulation and experiment. [ABSTRACT FROM AUTHOR]

In this paper, new half-bridge and full-bridge topologies for a switched-boost inverter are proposed. These topologies have a high boost factor in comparison with conventional types. In addition, the proposed topologies have continuous input current and can generate zero-voltage level in output. In this paper, the operation of the proposed inverters in different operating modes is analyzed. Then, the values of inductors and capacitors are designed. Moreover, the used switching pattern and its logical diagram are introduced. A comprehensive comparison between the proposed and conventional topologies in terms of the boost factor, the number of elements, and the efficiency shows the good features of the proposed inverters. Finally, to verify correct operation of the proposed topologies and derived equations, simulation and experimental results are presented. [ABSTRACT FROM AUTHOR]

A multilevel dodecagonal voltage space vector generation scheme for variable-speed drive applications with single-dc-link operation requires a large value of capacitance for cascaded H-bridge (CHB) filters, when operated at lower speeds. In existing schemes, the multilevel dodecagonal structure is obtained by cascading a flying capacitor inverter with a CHB. In this paper, a new scheme has been proposed to minimize the capacitance requirement for full speed operation by creating vector redundancies using modular and equal voltage CHBs. Also, an algorithm has been developed to optimize the selection of vector redundancies among the CHBs in order to minimize the voltage ripple of the floating capacitors. The proposed algorithm considers instantaneous capacitor voltages and phase currents for optimal selection of vector redundancies. A mathematical model for capacitor voltage deviation is presented, and the effectiveness of the proposed algorithm is verified in both the simulation and the experiment. [ABSTRACT FROM AUTHOR]

The operating principle of the clamp circuits like clamping diodes and flying capacitor in three-level LLC converter is different from other three-level converters, because the mean value of the resonant tank current is always zero and pulse-frequency modulation control is generally used instead of pulsewidth modulation control. And the converter with only the clamping diodes or flying capacitor is difficult to realize voltage balance. In this paper, the characteristics of the three-level LLC converter with single clamp circuit are first analyzed. Then, the operation principle of the hybrid-clamped circuit under the unbalanced voltages on input capacitors is described with the mode analysis, based on which the mechanism for voltage autobalance is elaborated and the effects of two clamp circuits are derived. Then, the design of the clamping diodes and the flying capacitor is also involved. Finally, a three-level LLC converter prototype is built and the experimental results verify the validity of the theoretical analysis and design. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has attracted extensive research in recent years. An appropriate model is necessary to analyze stability or to design MMC controllers. Several published MMC models have been derived in single-phase form to simplify the modeling mathematics. However, little attention is given to the zero-sequence voltage, which introduces coupling in the single-phase model and leads to significant error. In this paper, after revealing the mechanism behind the zero-sequence voltage, a more accurate generalized single-phase MMC model is proposed, which eliminates the zero-sequence voltage coupling effect and is linearized based on harmonic state space (HSS) theory to precisely characterize the internal harmonic features of MMC. A systematic HSS modeling process is presented for both open-loop and closed-loop conditions. And the proposed model is generalized as it can incorporate different control strategy by controller transfer function substitution. Hence it is valuable to analyze MMC stability and dynamics. Model effectiveness is verified by simulation results. [ABSTRACT FROM AUTHOR]

This paper presents the flexible dc capacitor voltages balancing control method for star-connected cascaded H-bridge converters. In this paper, the power flows are analyzed by investigating all the converter's control freedoms. Based on the analyses, users can accomplish the dc capacitor voltage balancing with both negative-sequence current and zero-sequence voltage. These two kinds of control options are further integrated together with the defined weighting factor $K$. The low-voltage ride-through operation of a proposed control method is also discussed in this paper. The control method is verified with a 220-V 1-kVA static synchronous compensators based on the star-connected cascaded H-bridge converters, and the experimental test results show that all the dc voltages are well regulated at the commanded value with different weighting factor $K$. [ABSTRACT FROM AUTHOR]

Charge control is a well-known sensorless approach to operate piezoelectric actuators, which has been proposed for more than 30 years. However, it is rarely used in industry because the implemented controllers suffer from the issues of limited low-frequency performance, long settling time, floating-load, and loss of stroke, etc. In this paper, a novel controller circuit dedicated to overcome these issues is presented. In the proposed scheme, a grounded-load charge controller with decoupled configuration is developed, which separates high-frequency and low-frequency paths, thus achieving arbitrarily low transition frequency without increasing the settling time. Based on this, a self-compensating configuration is further proposed and integrated into the controller circuit, which makes full use of controller output to improve its own control performance at low frequencies. Experimental results show that the presented charge controller can effectively reduce more than 88% of the hysteretic nonlinearity even when operating close to the transition frequency. To demonstrate its practical value, we then integrate it into a custom-designed high-speed atomic force microscope system. By comparing images obtained from using voltage drive and charge controller, it is clear that the piezoelectric hysteresis has been significantly reduced in a wide bandwidth. [ABSTRACT FROM AUTHOR]

This paper presents an accurate steady-state initialization method for electromagnetic transient (EMT) models of modular multilevel converters (MMCs). The proposed method uses steady-state arm voltages and currents to initialize internal electrical variables and control systems of the converter, including capacitor voltage and gating signals of each sub-module. In addition to typical voltage source converter upper level control, MMC specific controls are also considered, including circulating current control, dc ripple control, and capacitor voltage balancing algorithm. EMT simulations on 101-level and 401-level MMC-HVDC systems confirm the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

This paper introduces the control of the parallel operation of two voltage source converter (VSC)-HVdc links interconnecting an offshore wind farm. The aim of the study is to propose and validate a control system that allows the parallel operation of two VSC-HVdc links by controlling the currents injected by the VSC converters. The currents set points are established by a voltage controller in order to maintain constant voltage and frequency in the capacitor of the output filter and therefore within the offshore wind farm (OWF). It is demonstrated that the decoupled control of the d-q component of the voltage at the capacitor allows achieving the direct control of voltage and frequency, respectively. The voltage and frequency control is implemented by orienting the capacitor voltage toward a synchronous axis that is generated within the controller and therefore is not subjected to any grid disturbance. Both converters collaborate therefore in maintaining constant voltage and frequency, achieving in this way the parallel operation of the converters. The validation of this approach is demonstrated by simulation where the OWF and the VSC-HVdc rectifier have been modeled. Simulation results demonstrate that the proposed control system allows the parallelization of the converters while maintaining constant voltage and frequency within the OWF, even during transient faults. [ABSTRACT FROM AUTHOR]

There are technical challenges when the grid-connected current source converter operates under unbalanced grid voltage conditions. The negative sequence component of the unbalanced grid voltage would result in the low-frequency ripple at dc side, as well as current harmonics at ac side. It has an adverse impact on the system operation. In order to solve the problem, the system mathematical model is established in this paper. The limitation of the conventional control strategy is clearly clarified. And then, the improved control strategy is discussed from the viewpoint of constant instantaneous active power. In order to further enhance the system performance, a new control strategy is proposed. Instead of the complicated instantaneous power theory, the proposed solution is a direct control strategy. Both the low-frequency dc-side ripple and ac-side current harmonics can be significantly reduced with a simple control structure, due to the direct control concept. Finally, the comparison evaluation of the conventional and proposed control strategies is carried out. The experimental results verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

The five-level nested neutral-point-clamped (5L-NNPC) converter is one of the most promising topologies for medium-voltage (2.3–7.2 kV) high-power applications such as medium-voltage drives, wind energy conversion systems, and grid-connected systems. The 5L-NNPC requires a fewer number of switching devices, freewheeling and clamping diodes, and flying capacitors compared to the existing five-level multilevel converters. In the 5L-NNPC topology, each flying capacitor voltage is regulated at one-fourth of the dc-bus voltage to obtain the five-level operation. Due to the lack of redundant switching states, it is difficult to control the flying capacitor voltages by using the pulse-width-modulation-based classical control methods. This paper proposes a model-predictive current control (MPCC) approach to control the flying capacitor voltages along with the output currents of the 5L-NNPC converter. The discrete-time model of 5L-NNPC is developed to implement the MPCC scheme. The simulation and experimental studies are conducted to verify the dynamic and steady-state performance of 5L-NNPC with the MPCC scheme. The performance of the proposed MPCC approach is compared with the conventional space-vector-modulation-based voltage-balancing approach. Furthermore, the flying capacitor voltage control capability of MPCC is verified at different load power factors. [ABSTRACT FROM AUTHOR]

This paper proposes a multi-timescale volt/var optimization for the optimal dispatch of battery energy storage system in smart distribution grids. It aims to coordinate the substation on-load tap changer operation on slow-timescale (hourly basis) with the photovoltaic inverters and battery storage operations on fast-timescale (15 min basis). This coordination is achieved by using two-stage stochastic programming and implemented via model predictive control. The power loss and energy purchase cost are reduced while maintaining voltages within limits. The forecasting uncertainties are modeled by generating a large number of random scenarios and then subsequently reducing scenario numbers to establish a tradeoff between computational burden and accuracy of the solution. The mixed-integer second-order cone program (MISOCP) is formulated with reduced scenarios, which achieves global optimum. Simulation results demonstrate the effectiveness of proposed MISOCP model in keeping the voltages within limits under forecasting uncertainties. [ABSTRACT FROM AUTHOR]

This paper presents a new methodology to manage the initialization process of fuel cells (FCs) as a main source in a microgrid. The startup aims to reduce the warming-up time, increase the FC efficiency, and decrease the time response. As a solution, the use of storage systems combined with an adaptive gain scheduling to absorb the power transients improve the microgrid stability and avoid the dc-link voltage collapse is proposed when we perform short circuits at the FC terminals. In the first analysis, we evaluate the mathematical model to obtain the gain of the interleaved boost with voltage multiplier (IBVM) as a function of the hydrogen pressure, membrane resistances, and temperature, i.e., the effect of the chemical variables on the IBVM terminal voltage. In the second part, an adaptive gain scheduling to compensate the short-circuit effects on the FC terminals is designed. To analyze the feasibility of the proposed technique, the frequency and step response for different gain scheduling is performed. Finally, we present a set of experimental results to support the theoretical study and demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

Pollution problems caused by fossil fuels have led to more investigations on renewable energy systems. Photovoltaic cells and fuel cells output low-level voltage than required; therefore, high gain dc–dc converters are used to boost this low voltage. The Z-source converter can be employed as dc–dc converter to boost the photovoltaic panel voltages. It also offers other advantages, such as clamped switched voltage, high voltage gain, isolation of energy source from the load, and positive polarity for output voltage; therefore, this is a good choice for high step-up applications. This paper presents analysis of a novel high step-up Z-source based dc–dc converter that has higher voltage gain than the Z-source converter. Furthermore, high efficiency, low device voltage stress, and wide voltage gain range make it a good candidate for photovoltaic and high voltage step-up applications. The proposed dc–dc converter is evaluated experimentally for converting 24-V dc input to 300-V dc output at 100 W and to validate the simulation results. [ABSTRACT FROM AUTHOR]

This paper presents the development and performance evaluation of a new approach for detecting voltage flicker events in power systems hosting distributed generating units. The proposed approach is based on extracting the magnitudes and phases of the different frequency sub-band contents present in the voltage signal(s). Desired magnitudes and phases are extracted by processing the voltage signal(s) using nine phaselet frames that are realized by a modulated filter bank. The modulated filter bank is designed using nine digital high-pass filters (HPFs), each of which implements one phaselet frame. The coefficients of HPFs are determined by bi-orthogonal phaselet basis functions. Extracted high-frequency sub-band contents provide signature information for accurate detection and quantification of voltage flickers. The performance of a phaselet transform based approach is experimentally tested for different conditions, including voltage flicker events that are triggered by an interconnected photovoltaic system. Performance results show accurate, reliable, and fast detection and quantification of voltage flickers, along with a negligible sensitivity to the source producing voltage flicker events. [ABSTRACT FROM AUTHOR]

This paper presents a dc capacitor voltage balancing control method for the star-connected cascaded H-bridge pulsewidth modulation converter in the static synchronous compensator applications. With the zero-sequence voltage injection as a basis for dc capacitors voltage balancing, this paper investigates the detailed power flow of the converter as a whole and within individual modules under unbalanced grid voltages and then proposes a method to control the reactive power output and the dc capacitors voltage by precisely managing the power flow. This approach enhances the low-voltage ride-through capability, which is very critical as the penetration of distributed energy resources grows rapidly. The proposed control method is verified by a scaled-down prototype in the laboratory. [ABSTRACT FROM PUBLISHER]

A modular multilevel converter control system, based on converter energy storage, is proposed in this paper for two different control modes: active power and dc voltage. The proposed control system decouples the submodule (SM) capacitor voltages from the dc bus voltage. One of the practical applications is the management of active redundant SMs. A practical HVDC system with 401-level MMCs, including 10% redundancy in MMC SMs, is used for validating and demonstrating the advantages of the proposed control system. This paper also presents a novel capacitor voltage balancing control based on \max–\min functions. It is used to drastically reduce the number of switchings for each SM and enhances computational efficiency. [ABSTRACT FROM AUTHOR]

In this paper, the design and development of a novel interleaved tri-state boost converter (ITBC), which produces lower ripple and exhibits better dynamic response, is discussed. Boost converters are frequently connected in parallel and operate in an interleaving mode for the reduction of ripple content in source current and in output voltage. In this way, interleaved boost converter (IBC) is conceived, which improves the power handling capabilities and increases the overall system rating. It also has the advantage of reduction of the ripple content in source current and output voltage, but when control-to-output transfer function of IBC is derived under continuous conduction mode of operation, then a right-half-plane (RHP) zero appears in the transfer function. Due to the presence of RHP zero, IBC has nonminimum phase problem, which deteriorates the dynamic performance. The tri-state boost converter (TBC) is the best choice for RHP zero elimination, but due to the extra freewheeling mode, ripple content will also be increased. The proposed converter is a parallel combination of two TBC and operates in an interleaving mode. Therefore, the proposed converter has both of the advantages of TBC and IBC. The performance analyses of ITBC, TBC, and IBC have been studied based on simulation and experimental results. From the comparative analysis, it is observed that ITBC is performed better than other two converters. The ripple comparisons between three converters have also been done. It is found that the ripple content in ITBC is slightly greater than IBC but is less than TBC. [ABSTRACT FROM PUBLISHER]

This study adopts the method of third-order harmonic voltage injection (THVI) to move the peak value of a reference voltage toward the high capacitor voltage and increase the utilization rate of the capacitor voltage. This method can lower the cost or enlarge the capacity of a modular multilevel converter (MMC) when applied in high-voltage direct-current transmission. In this paper, the principle of THVI is introduced. Two calculation methods for modulation waves are studied, and the mathematical expressions of the capacitor voltage under all operating conditions are deduced. Thus, the optimal magnitude and phase of the third-order harmonic voltage are determined under all operating conditions. The mathematical expressions of the optimal magnitude and phase of the third-order harmonic voltage are obtained, and their online calculation burden is reduced. Simulation and experimental results show that the proposed method of THVI is feasible and effective. [ABSTRACT FROM PUBLISHER]

The analysis of circulating current in modular multilevel converter (MMC) has aroused extensive research interests recently. Most studies on the derivation of harmonic components of circulating current neglect the harmonics in the line currents, but the harmonics evidently distort the line currents in many MMC applications where a circulating path for the dominant third-order harmonic current exists. So far, little research has been done on the model of the harmonics in line currents of MMC and neglecting them inevitably causes large error in the previous analytical expressions of circulating current. To solve these problems, this paper proposes a novel accurate steady-state mathematical model of harmonic components existing both in arm and line currents, and presents the detailed relations between the inner and outer harmonic characteristics of MMC. Thus, the derived instantaneous voltages and currents both in arm and line from the proposed models are far more accurate than the present research. What's more, each existing higher order harmonic is precisely calculated for the first time, filling up the blank of the conventional studies, and providing a better reference for the understanding, designing, and controlling of the MMC engineering applications. The correctness and high accuracy of the proposed models are verified by simulation results in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thévenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission. [ABSTRACT FROM AUTHOR]

A fractional-order capacitor (FOC) plays a vital role in fractional-order circuits. However, the FOC with order $1< \alpha < 2$ based on existing manufacturing schemes can be only operated in milliwatt power level. Thus, the applications of an FOC in power conversion are limited. To solve the problem, this paper proposes a novel method for realizing a high-power FOC with order $1< \alpha < 2$, in which a power converter is applied to emulate the current–voltage characteristic of an ideal FOC, so the rated power of the FOC can be increased greatly and depended on the power converter. The corresponding topology and its control strategy are provided. The available experiments verify the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper proposes a star-channel modular multilevel converter (MMC). It overcomes the zero/low-fundamental-frequency operation issues of conventional MMC while not injecting the common-mode voltage (CMV). The proposed converter is characterized by the cross connections of upper three arms and lower three arms through two sets of star-channel branches. These star-channel branches rebalance the power among three phase legs, resulting in minimized submodule capacitor voltage ripples even at zero/low-fundamental-frequency operation without CMV injection. This feature enables the proposed converter to interface medium/high-voltage DC and variable-frequency AC systems. A 4160-V 1-MW simulation model is performed in MATLAB/Simulink, and a 208-V 5-kW experimental setup is constructed in the lab. Both of the simulation and experimental results verify the feasibility of the proposal. [ABSTRACT FROM PUBLISHER]

In this paper, a single-phase multilevel ac/ac converter with reduced number of power semiconductor devices is proposed. For the simple case of a three-level converter, only three bidirectional switches and two capacitors are required. The proposed converter provides a highly sinusoidal and regulated output voltage with a simple topology. The efficiency is high because in each state of operation only one semiconductor conducts. The converter can be easily extended to any desired levels in the output voltage to even more reduce the output total harmonics distortion. Another advantage of the proposed converter is that by increasing the voltage levels, the stress on the switches will decrease. The proper performance of the proposed converter is confirmed through extensive simulation and experimental tests. [ABSTRACT FROM PUBLISHER]

This paper presents a transformerless high-voltage dc–dc converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage dc grids. The proposed dc–dc converter consists of two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the output voltage of the cross-connected branch, a trapezoidal ac current is induced to interact with the ac voltage components in the upper and lower arm for rebalancing the power amongst converter arms. The features of modular design, single-stage power conversion, and transformerless structure make the proposed dc–dc converter gain the outstanding merits of wide voltage ratio range, high system efficiency, and light converter weight. A control scheme is also elaborated for guaranteeing the normal operation of the proposed dc–dc converter. A 100-kV 100-MW simulation model performed in MATLAB/Simulink verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1-kW laboratory setup also confirm the validation of the proposal. [ABSTRACT FROM PUBLISHER]

This paper presents a feedback/feed-forward control strategy to improve the voltage balancing of modular multilevel converters (MMCs) under unbalanced grid conditions. The inclusion of a feed-forward compensation improves the disturbance rejection capability against asymmetrical faults and sudden voltage imbalances in the ac grid. The floating capacitor voltages of the MMC are controlled in two stages. The first one equally distributes the voltages in each arm, and the second one balances the energy among the six arms. This last control stage is also divided into the called horizontal and vertical balancing. Based on a detailed analysis of the power terms disturbing the MMC arm energies, the control strategy is tailored to improve the voltage transient response by adding feed-forward terms to both horizontal and vertical balancing controls. In addition, various approaches to manage the MMC under unbalanced grid conditions are compared regarding several aspects, such as capacitor voltage ripple, balancing performance, and negative-sequence current injection. Advantages and disadvantages of both the proposed and the conventional energy-based control schemes are also studied, showing that the voltage control performance can be improved by enhancing the existing MMC control systems. [ABSTRACT FROM AUTHOR]

Diode–capacitor-based dc–dc converters provide a simple and low cost solution for high step-up voltage regulation in solar and fuel cell generation. Transient modeling analysis reveals their worse influence of nonlinear and nonminimum-phase system characteristic due to right-half-plane (RHP) zero, especially in high voltage gain application. However, the process of energy transfer for diode–capacitor-based dc–dc converter is different from basic dc–dc converter. Based on the unique feature, this paper proposes an improved main circuit structure with parallel connection of resistive–capacitive damping network across the intermediate capacitor to achieve good dynamic performance. By optimal parameter design according to Routh–Hurtwitz criterion, all the RHP zeros in the transfer function of control-to-output voltage are eliminated completely. Then, by the case of diode–capacitor-based boost converter, the adaptive PI controller is designed to deal with nonlinear characteristic of voltage gain. It gets good dynamic performance under wide range output voltage. All the theoretical findings and design approaches are verified by simulation and experiment results. The existing diode–capacitor-based high step-up dc–dc converters with slight main circuit modification are more promising in renewable energy application. [ABSTRACT FROM AUTHOR]

Plug-in electric vehicle (PEV) charging equipment incorporates protection that ensures that grid disturbances do not damage the charger or vehicle. When the grid voltage sags below 80% of nominal, undervoltage protection is likely to disconnect the charging load from the grid. Most PEV charging will occur overnight, when non-PEV load is at a minimum. This paper argues that PEV voltage-sag response, when synchronized across large numbers of PEVs, could result in the loss of a significant proportion of the total load. It is shown that this load loss can lead to unacceptably high voltages once the initiating event has been cleared. This paper explores the nature of this voltage-rise phenomenon. Analysis tools are developed to assist in determining PEV loading conditions that demarcate acceptable postdisturbance voltage response from unacceptable outcomes. Two examples, based on standard distribution test systems, are used to illustrate PEV-induced overvoltage behavior, and demonstrate applications of the analysis tools. [ABSTRACT FROM AUTHOR]

As penetration of distributed generation increases, the electrical distribution networks may encounter several challenges mainly related to voltage control. The situation may deteriorate in case of a weak grid connected to an intermittent source such as photovoltaic (PV) generation. Fast varying solar irradiance can cause unacceptable voltage variations that may not be easily compensated by slow-responding utility equipment. The PV inverter can be used to control the grid voltage by injecting/absorbing reactive power. A small-signal model is derived in order to study the stability of a PV inverter exchanging reactive power with the grid. This paper also proposes a method that utilizes the available capacity of the PV inverter to support the grid voltage without violating the rating of the inverter and the maximum voltage that the inverter's switching device can withstand. The method proposed in this paper is validated using PSCAD/EMTDC simulations. [ABSTRACT FROM PUBLISHER]

In this paper, a high-frequency ac-link photovoltaic (PV) inverter is proposed. The proposed inverter overcomes most of the problems associated with currently available PV inverters. In this inverter, a single-stage power-conversion unit fulfills all the system requirements, i.e., inverting dc voltage to proper ac, stepping up or down the input voltage, maximum power point tracking, generating low-harmonic ac at the output, and input/output isolation. This inverter is, in fact, a partial resonant ac-link converter in which the link is formed by a parallel inductor/capacitor (LC) pair having alternating current and voltage. Among the significant merits of the proposed inverter are the zero-voltage turn-on and soft turn-off of the switches which result in negligible switching losses and minimum voltage stress on the switches. Hence, the frequency of the link can be as high as permitted by the switches and the processor. The high frequency of operation makes the proposed inverter very compact. The other significant advantage of the proposed inverter is that no bulky electrolytic capacitor exists at the link. Electrolytic capacitors are cited as the most unreliable component in PV inverters, and they are responsible for most of the inverters' failures, particularly at high temperature. Therefore, substituting dc electrolytic capacitors with ac LC pairs will significantly increase the reliability of PV inverters. A 30-kW prototype was fabricated and tested. The principle of operation and detailed design procedure of the proposed inverter along with the simulation and experimental results are included in this paper. To evaluate the long-term performance of the proposed inverter, three of these inverters were installed at three different commercial facilities in Texas, USA, to support the PV systems. These inverters have been working for several months now. [ABSTRACT FROM PUBLISHER]

In a modular multilevel converter (MMC), the zero-sequence currents at twice the fundamental frequency and its multiples flow through the dc bus and cause dc-bus power fluctuations during unbalanced conditions. These fluctuations appear in dc-bus current, considering that the dc-bus voltage is stiff. In this paper, a model-predictive control is proposed to minimize the ripple in dc-bus current. The proposed approach takes the advantage of internally generated zero-sequence voltage to mitigate the zero-sequence current during unbalanced conditions. To implement the proposed approach, a generalized mathematical model of the MMC, which includes the zero-sequence voltage, is presented. The performance of the proposed approach is verified through MATLAB/Simulink simulations on a 4.16-kV/5-MW system and dSPACE/DS1103-based experiments on a 208-V/3-kW laboratory prototype. [ABSTRACT FROM AUTHOR]

A zero-current switching full-bridge isolated dc/dc converter based on a semiresonant L-C cell has been developed in this paper. With respect to the classical full-bridge configuration, no additional switches are required, thus by preserving its simplicity and robustness. The proposed architecture allows achieving zero-current switching at both primary and secondary sides. A 50-kW prototype has been developed and tested in different operating conditions. The experimental results are in agreement with the simulations by confirming the goodness of the proposed solution. Efficiency up to 97% at full load operating condition has been reached in the laboratory prototype. [ABSTRACT FROM PUBLISHER]

As more distributed renewable energy sources are installed in the utility grid, the static synchronous compensators are applied to manage the power factor and the grid voltage in the medium-voltage level of the power system. The cascaded multilevel converter with single-star topology is well applied to this application. The dc-capacitor voltage balancing control is a fundamental issue, and it can be accomplished by negative-sequence current and/or zero sequence voltage injection. Unfortunately, the zero sequence voltage injection increases the risk of over-modulation and the negative-sequence current injection results in high peak current. This paper provides the voltage balancing strategy to manage the peak current or the modulation index. Besides, all the asymmetrical grid voltages are considered in the proposed method to satisfy the grid fault operation. Laboratory test results verify that the proposed method limits the peak current or manages the modulation index during the grid fault operations. [ABSTRACT FROM PUBLISHER]

In this paper, a new cascade switch-ladder multilevel inverter topology is presented which can generate a large number of output voltage levels. First, a fundamental switch-ladder multilevel inverter structure is described. Then, the structure of recommended cascade topology based on series connection of fundamental switch-ladder topologies is presented. To generate maximum number of levels with minimum number of switching elements, dc sources, and voltage on switches, the proposed cascade topology is optimized. Comparison results prove that the presented cascade topology requires fewer numbers of components. Also, the value of voltage rating on switches is less than other structures. Experimental results for two topologies are analyzed to verify the performance of the proposed topology. [ABSTRACT FROM PUBLISHER]

Leakage current reduction of the single-phase transformerless cascaded H-bridge PV inverter is investigated in this paper. The high-frequency common-mode loop model of a typical single-phase cascaded H-bridge PV system is established. Based on the model, the main factors that affect the leakage current are discussed. The reason why the typical single-phase cascaded H-bridge inverter fails to reduce the leakage current is explained. In order to solve the problem, a cascaded topology based on the H5 inverter is presented, along with a new modulation strategy, which can ensure that the stray capacitor voltage is free of high-frequency components. In this way, the leakage current can be effectively reduced. Finally, a prototype with TMS320F28335DSP + XC3S400FPGA digital control is built. The performance tests of cascaded H-bridge and the proposed topologies are carried out. The experimental results verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

A new class of modular multilevel dc/dc converters has recently emerged that eliminates the traditional full-rated intermediate ac stage inherent to two-stage dc/dc converter structures, that is, two cascaded dc/ac modular multilevel converters (MMCs). This paper introduces the first averaged dynamic state-space model for this new class of single-stage dc/dc converter, called the DC-MMC, which captures all input and output terminal dynamics as well as internal capacitor dynamics. Based on the developed analytical model, a dynamic phasor model is derived to accommodate the multifrequency components that exist at steady state. The phasor model enables: 1) solving for the full steady-state solution of the DC-MMC under arbitrary loading and 2) evaluation of DC-MMC open-loop stability via eigenvalue analysis. These analyses are not possible with existing DC-MMC models. Simulations of a comprehensive switched model verify the accuracy of the presented analytical models, which represent valuable study tools for the analysis of emerging modular multilevel dc/dc converters containing multiple frequency components. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) has become one of the most promising topologies for high-voltage applications. Although the MMC has a modular and redundant structure, which can significantly enhance its reliability and fault tolerance, its fault handling capability can be further improved by adopting a postfault strategy. In this paper, a postfault strategy based on the neutral-shift method is proposed, which, subsequent to bypassing the faulty submodule(s), aims at improving the maximum attainable line-to-line voltages of the MMC. Based on a newly developed internal variable analysis method, the impact of the neutral-shift strategy on the operation of the MMC is analyzed. In conjunction with the proposed postfault strategy, a modified circulating current suppression (CCS) method is used to mitigate the double-frequency component of the circulating currents. Performance and effectiveness of the proposed neutral-shift strategy with and without the CCS method is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. [ABSTRACT FROM PUBLISHER]

A new cascade topology for multilevel converter is introduced in this paper which comprises series connection of several submultilevel units. To determine the magnitudes of the dc sources, two new methods are described. The proposed topology is optimized to generate any levels with minimum number of components and peak voltage on switches. The presented topology can be used in high-voltage applications due to using the switches with low voltage rating. To indicate the merits of the proposed structure, comparison studies are provided with other topologies in terms of the number of elements and peak voltage on switches. The comparison results prove that the presented cascade multilevel converter requires fewer components. Moreover, it is shown that the total peak voltage on switches in the proposed topology is less than other structures. Experimental work is presented to demonstrate the performance of the presented multilevel converter. [ABSTRACT FROM PUBLISHER]

The flying-capacitor-based multilevel converter is one of the well-known breeds of the multilevel power converters. This paper proposes a new flying-capacitor-based multilevel converter to minimize the number of flying capacitors (FCs) and power switches. The advantage of the proposed FC-based multilevel converter in comparison with the conventional flying-capacitor multicell converter is that it needs fewer FCs. Also, in comparison with the stacked multicell converter, the proposed multilevel converter requires fewer semiconductor switches. In order to balance the voltage of the FCs in proposed multilevel converter, a new active voltage balancing method which is fully implemented using logic-form equations is presented. The proposed voltage balancing method measures output current and FC voltages to generate switching states to produce the required output voltage level, as well as balance the FCs voltages at their reference values. The output voltage of the proposed multilevel converter controlled with suggested active voltage balancing method can be modulated with any pulse-width-modulation (PWM) method, such as phase-shifted-carrier PWM or level-shifted-carrier PWM. Simulation results and experimental measurements of proposed FC-based multilevel converter are presented to verify the performance of the proposed converter, and its novel switching and modulation strategy, which is based on the active voltage balancing method. [ABSTRACT FROM PUBLISHER]

This paper presents a new formulation of a finite control set model-predictive control (FCS-MPC) algorithm, allowing to determine the best commutation instant in order to optimize the converter current tracking. This algorithm, mixing properties of FCS-MPC method and time resolution related to pulse width modulation schemes, is illustrated in a parallel flying capacitor converter. The parallel association of classical commutation cells is a good solution for increasing the output current driven by the converters, while the standard semiconductor technology is used. The control strategy used with this type of connection must be able to ensure not only a good distribution of the output current among the different legs, but also the balancing of the floating capacitor voltages. [ABSTRACT FROM PUBLISHER]

The decoupled control of the ac-side, dc-link and circulating current of a modular multilevel converter (MMC) can significantly improve its operation and control performances. This paper presents a new mathematical model for MMC, in which six intermediate controllable voltages (ICVs) corresponding to the ac-side currents, the dc-link current, and the circulating currents are introduced. Using the ICVs as the intermediate control variables, the MMC can be decoupled into six single phase circuits, and the current in each circuit responds to its corresponding ICV. Based on the ICV controls, the direct control of dc-link voltage and circulating current and the decoupling of dc side and ac side can be achieved. These conditions can improve the performances and stability of the MMC-based high-voltage direct current (HVDC) systems. Simulations and experiments were implemented to verify the proposed method. [ABSTRACT FROM PUBLISHER]

This paper studies microgrids where loads are supplied by parallel-connected inverters controlled by decentralized active power/voltage frequency and reactive power-/voltage-magnitude droop control laws. A paralleled ac system, such as a multiinverter microgrid, is susceptible to circulating currents due to differences in voltage magnitude, frequency, phase angle, or dc offset. Circulating currents due to differences in voltage magnitude and dc offset have been known issues reported in literature. However, an in-depth analysis of the problem is required to ascertain the deviation of the system-operating condition from the desired condition. This paper provides a mathematical model that predicts the effect of voltage-magnitude offsets on reactive power sharing between inverters. Simulation and experimental results verify the accuracy of the analytical results obtained from the mathematical model. We examine the effect of dc-circulating currents and propose a simple capacitor emulation control law implemented in software to eliminate dc-circulating currents. This solution is a possible alternative for hardware implementation to eliminate dc-circulating currents. The effectiveness of the capacitor emulation control law has been verified through experimental results. [ABSTRACT FROM PUBLISHER]

The number of semiconductor switches in a modular multilevel converter (MMC) for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level voltage-sourced converter (VSC). The large number of devices creates a computational challenge for electromagnetic transient simulation programs, as it can significantly increase the simulation time. The paper presents a method based on partitioning the system's admittance matrix and deriving an efficient time-varying Thévenin's equivalent for the converter part. The proposed method does not make use of approximate interfaced models, and mathematically, is exactly equivalent to modelling the entire network (converter and external system) as one large network. It is shown to drastically reduce the computational time without sacrificing any accuracy. The paper also presents control algorithms and other modelling aspects. The efficacy of the proposed method is demonstrated by simulating a point-to-point VSC-MMC-based HVDC transmission system. [ABSTRACT FROM AUTHOR]

The modeling, modulation, and control of the three-phase four-switch (TPFS) PWM rectifier are investigated in this paper. Three space vector pulse width modulation methods using different equivalent zero vectors are developed, where sector identification and the trigonometric function are not required. Then, the high-frequency model for the current ripple analysis is proposed, and the effects of three SVM approaches on the ac current ripple are investigated. According to the analytical results, the method introducing the smallest current ripple is selected. With the optimized SVM approach, a control-oriented model, considering the capacitor voltage oscillation and deviation, is built in the dq synchronous frame to facilitate the controller design. Furthermore, a control strategy implementing the proportional controller is developed to eliminate the capacitor voltage deviation. Meanwhile, the dual-loop control of the TPFS is not affected by the proposed strategy as the capacitor voltage deviation is eliminated. Finally, a novel linear modulation index function is defined to reject the low-frequency harmonic current introduced by the overmodulation. Experimental results demonstrate that excellent current performance is achieved with comprehensive considerations of the modeling, modulation, and control strategy. [ABSTRACT FROM AUTHOR]

This paper introduces a multiple-input multiple-output modular multilevel dc–dc converter (MIMO-MMC) and its associated control scheme. The proposed topology has a bidirectional structure and may be utilized in both low- and high-power applications ranging from approximately 100 W–10 MW. The modular structure of the MIMO-MMC enables efficient component utilization through module voltage and current sharing capabilities. The topology can supply or extract regulated power from an arbitrary number of controllable voltage nodes without requiring source or load multiplexing, resulting in minimized filtering requirements. The MIMO-MMC's structure is described, and the steady-state operation of the converter is theoretically analyzed. The dynamic models of the converter for both step-down and step-up configuration are derived and employed to devise an effective control algorithm for closed-loop operation. A general method is provided for stability analysis of the proposed closed-loop system followed by a case study verifying stability of the system at different operating points. The steady-state operation and dynamic response of the converter under both configurations is investigated through simulation and experiment. [ABSTRACT FROM PUBLISHER]

The purpose of neutral-point voltage balancing algorithms in the three-level neutral-point clamped (3L-NPC) topology is to eliminate the voltage unbalance of top- and bottom-side dc-link voltages (neutral-point voltage). These algorithms are divided into dc unbalance and ac unbalance compensation methods. The dc unbalance causes distortion of output currents; therefore, this problem has to be solved. The ac unbalance implies that there is an ac ripple in the top- and bottom-side dc-link voltages. In applications using 3L-NPC topology, a large dc-link capacitor value is required to mitigate this ac unbalance problem. As a result, the size and the cost of applications become large and expensive. In this paper, a compensation method for mitigating the ac unbalance of the neutral-point voltage is proposed, which causes the dc-link capacitor value to be small, when a carrier-based pulse width modulation method is applied. The proposed method compensates the ac unbalance of the neutral-point voltage by adding an optimal compensation value to the reference duty signals. This compensation value is calculated simply by currents and reference duty signals. The calculation process of the optimal compensation value is discussed, and the effectiveness of the proposed method is verified with simulations and experiments. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a novel transformer-less direct current (dc) converter that features low switch voltage stress and automatic uniform current sharing. An interleaved four-phase voltage divider operating from a 400 V dc bus is used to achieve a high step-down conversion ratio with a moderate duty ratio. Based on the capacitive voltage division, the proposed converter achieves two major objectives, i.e., increased voltage conversion ratio, due to energy storage in the blocking capacitors, and reduced voltage stress of active switches and diodes. As a result, the proposed converter permits the use of lower voltage rating MOSFETs to reduce both switching and conduction losses, thereby improving the overall efficiency. In addition, due to the charge balance of the capacitors, the proposed converter enables automatic uniform current sharing of the interleaved phases without adding extra circuitry or complex control methods. The operation principles and performance analyses of the proposed converter are presented, and its effectiveness is verified by a 500 W output power prototype circuit that converts 400 V input voltage into 24 V output voltage. [ABSTRACT FROM AUTHOR]

In this paper, selective harmonic mitigation (SHM) and selective harmonic elimination (SHE) modulation techniques have been combined to produce appropriate pulses for a four-leg three-level neutral point clamped (NPC) inverter. In the proposed hybrid method, the SHM technique is applied to phase legs in order to mitigate non-triplen harmonics (fifth, seventh, $\ldots$) , and the fourth leg is controlled by the SHE principle to eliminate important low-order triplen harmonics (third, ninth, $\ldots$) completely. In this context, it is demonstrated that using SHM instead of an SHE modulator for phase legs leads to a reduced vast range of harmonic orders in the NPC output voltage, significantly. Moreover, it is proved that the presented hybrid switching technique has the ability of dc-bus capacitor voltage balancing, inherently. Furthermore, analyses are performed to calculate the dc-bus capacitances accurately to show 5% voltage ripple acceptably. Simulation and experimental results validate a good dynamic performance of the proposed hybrid modulation technique in firing the NPC inverter switches to generate low-harmonic voltage waveforms supplying balanced and unbalanced loads while balancing the dc capacitor voltages in a four-wire network with small dc-link capacitors. [ABSTRACT FROM PUBLISHER]

This paper describes the implementation of a load voltage observer into the control algorithms of a Rapid Capacitor Charger controlled by a digital signal controller (DSC) with a fast DSP core. A hybrid peak current mode control algorithm with digital control of the PWM signal and high speed analog current sensing is used to control the inverter of the charger. The DSC controller keeps the current loop stable using adaptive slope compensation. In addition to providing fast adaptive digital control of the current loop, the DSC can perform additional functions such as estimation of the voltage level of the load capacitor. This avoids the cost, bandwidth limitations and insulation challenges of a conventional HV-sensor. To evaluate the performance of the load voltage observer, we measured the output voltage with a laboratory grade HV probe to establish a reference to test the results of the digital algorithm. [ABSTRACT FROM PUBLISHER]

The behavior of the series–parallel resonant converter with a capacitor as output filter is analyzed when operating under optimum switching conditions. A polynomial is deduced to quickly compute what switching frequency and duty cycle are necessary at the control to obtain the desired output voltage and power in the converter. This polynomial is so simple that it is suitable for being implemented in low-cost digital processors. It could provide, for instance, a start-up strategy guiding the power supply to a known set point, or it could generate alarms if the currents or voltages measured are larger than expected. In this paper, the steps to obtain such a model are described in detail. As an example, the coefficients of the polynomials are deduced when the series and the parallel capacitor are the same. The grade of accuracy of the model has been demonstrated through experimentation on a prototype. [ABSTRACT FROM AUTHOR]