This paper presents an artificial intelligence (AI)-based optimization framework for improving the performance of a hybrid neutral-point-clamped dual-active-bridge (HNPCDAB) dc-dc converter under hybrid duty-ratio phase shift modulation (HDPSM). The proposed approach integrates simulation data from PLECS with the XGBoost machine learning algorithm to build predictive models for both soft-switching conditions and converter efficiency. These models are then used in conjunction with particle swarm optimization with state-based adaptive velocity limit strategy (PSO-SAVL) to identify optimal modulation parameters that achieve zero-voltage-zero-current switching (ZVZCS) while maximizing efficiency. Experimental validation on a 2-kW HNPCDAB prototype confirms the effectiveness of the proposed method across varying power levels and output voltages. Compared to conventional modulation strategies, the AI-optimized approach achieves improved soft-switching performance and higher efficiency, particularly under light-load conditions.

This paper proposes a deep reinforcement learning (DRL)-aided strategy for intelligently managing state-of-health (SoH) and state-of-charge (SoC) imbalances in a group of battery energy storage systems (BESSs) with varying SoH levels. The proposed method prioritizes SoH balancing during normal operation. However, when one or more BESSs approach full charge or discharge, it shifts priority to SoC balancing, as the aggregated system’s total power capability decreases unnecessarily if any BESS reaches its limits prematurely. A DRL agent is trained to determine when and how to switch between SoH and SoC balancing modes. Unlike state-of-the-art energy management methods for BESSs, whose performance heavily depends on predefined control parameters, the proposed strategy provides a more adaptive and robust solution.

A physics-informed deep transfer reinforcement learning (PIDTRL) strategy is proposed for enabling advanced control and modulation of a dc solid state transformer (DCSST) in a dc microgrid. The strategy involves three stages: (i) Centralized training of deep reinforcement learning agents to balance power and reduce current stress in the DCSST; (ii) effective knowledge transfer from a Source-simulation system to a Target-experimental system using minimal experimental data; and (iii) deployment of multiple agents for online control in the DCSST. The proposed method adaptively determines optimal modulation variables (duty cycles and phase shifts) in stochastic and uncertain environments without requiring accurate model information. Experimental results validate the effectiveness of the proposed PIDTRL algorithm.

This paper focuses on the reduction of the radial force harmonics of a consequent-pole permanent magnet vernier machine by current harmonic injection. The current harmonic injection has been applied based on dq-axis frame. In the proposed method, a sixth harmonic of d-axis current is injected to reduce a sixth harmonic of the radial force with zero order mode. The effectiveness of the current harmonic injection is verified by finite element analysis. The simulation results show that sixth harmonic of radial force is reduced by 45.7%, resulting in the reduction of a vibration at the respective harmonic by 42%. To demonstrate the controllability of the current regulator for the proposed method, a circuit simulation has been carried out with a proportional, integral, and resonance (PIR) controllers. The simulation results show that the PIR controller can realize superior performance of the current harmonic regulation compared with the conventional PI controller.

The hybrid neutral-point-clamped (NPC) dual-active-bridge (DAB) topology aims to mitigate the excessive voltage and current stresses experienced by semiconductor devices in high-power applications. The hybrid duty ratio phase-shift modulation (PSM) achieves improved efficiency performance. However, previous studies have not extensively explored the intricacies of the switching characteristics. In this paper, a comprehensive analysis of the switching behavior is given, particularly focusing on the turn-off loss of Si IGBTs, based on relevant formulas and switching waveforms. To enhance clarity, a comparative analysis is conducted, contrasting the obtained results with those obtained from the triple-phase-shift (TPS) modulation method. Simulations and experiments are conducted for validation of the theoretical analysis.

Advanced reserve power point tracking (RPPT) algorithms are becoming popular and are expected to replace the conventional maximum power point tracking (MPPT) and flexible power point tracking (FPPT) algorithms. RPPT enables power reserve availability while injecting flexible power into the grid. Hence, it combines the MPPT and FPPT functionalities simultaneously. However, due to inherent continuous sweeping across the PV curve, RPPT suffers from high dc-link voltage fluctuations. For this reason, this paper extends the RPPT concept from the single-string photovoltaic (PV) system to the multi-string PV system. Specifically, a two-string PV system is proposed, where two PV strings that are interleaved at the dc-link. This approach offers advantages over using a single PV string of the same capacity, notably by reducing the ripple in the dc-link voltage caused by RPPT. The effectiveness of this proposed algorithm in the interleaved PV system is demonstrated through simulation results under varying grid frequency and irradiance conditions.

Cascaded H-bridge (CHB) static compensator (Stat-Com) enhances stability of the electricity power grid, which is becoming ever increasingly vulnerable as a consequence of replacing fossil fuel-based synchronous generators by intermitten renewable sources. A CHB StatCom needs large dc capacitors in the submodules to operate, which is a major drawback that hinders a widespread use of this technology. In order to overcome this drawback, the low-capacitance StatCom (LC-StatCom) technology was developed. Other than reducing the capacitors size, improvement in power quality and reduced losses were listed as further advantages of the LC-StatCom. On the other hand, control challenges and reduced stable operating range are the main challenges of this technology. This study aims to provide a better understanding of the prospects and challenges of the LC-StatCom technology as compared to the conventional$C$HB StatComs.

Intermittent changes in irradiance caused by passing clouds inevitably lead to fluctuations in photovoltaic (PV) power generation. To ensure the stability of a power grid with integrated solar PV generation, a battery energy storage system (BESS) is an intrinsic solution to effectively process the PV power before transmitting it into the grid. Alternatively, a PV software-based ramp rate (RR) control can be applied to mitigate the PV power fluctuations without any BESS, which is known to be cost-effective. The contribution of this paper is to compare various BESS-based and cost-effective software-based RR control strategies operating with real-time measured information, and analyze their effectiveness in regulating the PV power fluctuations. Furthermore, this paper studies the BESS utilization behavior among hardware-based control strategies, and also provides insights into which strategy is more appropriate for specific applications. The investigations are carried out through simulation case studies with a real-field one-day fluctuated irradiance profile.

This paper introduces a novel physics-informed pattern recognition (PIPR) method for open-circuit fault detection in inverters. The proposed method unfolds in three stages: model analysis, offline training, and online validation. In the first stage, we construct an analytical model of power converters. This model is subsequently used to derive fault diagnosis variables. This step is followed by the collection of training samples via simulations. The gathered samples are then fed into pattern recognition neural networks, a process enabled by the prior extraction of model information. This architecture allows for efficient training of the neural network with fewer neurons and samples. The final stage involves the detection and diagnosis of faults by a well-trained online classifier. The robustness of the proposed PIPR method in dealing with unexpected conditions in classification problems shows its potential across diverse conditions.

High penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.

This paper presents a novel zero-sequence voltage injection method for discontinuous modulation (DM) in star-connected cascaded H-bridge (CHB) static compensators (Stat-Coms). Conventional DM methods in CHB StatComs suffer from control interactions during unbalanced grid conditions. This paper proposes a DM that eliminates this control interaction by inserting a clamping transition within a third of the fundamental period. The transition instant is decided by comparing a duty cycle, which is calculated to mitigate the fundamental-frequency in the zero-sequence voltage for discontinuous operation, with a triangular carrier signal with a frequency of three times the fundamental. The clamping intervals under the proposed DM are affected by the phase-shift of this carrier. Specifically, the paper analyses the effects on the harmonic content of the proposed zero-sequence voltage, switching loss, twice-fundamental-frequency capacitor voltage ripple, and current total harmonic distortion, for different phase-shift angles of the DM carrier signal. Besides, the feasibility of the proposed DM in dealing with a grid voltage sag is also discussed.

Publisher Copyright: © 2022 IEEE.

In this paper, the effect of min-max zero-sequence voltage (ZSV) injection on the maximum average neutral-point (NP) current control limits of a neutral-point-clamped (NPC) converter is analyzed. These current limits determine the stable operating range of the NPC converter in applications that impose a nonzero average NP current, like- NPC converters feeding multiterminal dc loads, back-to-back-connected NPC converter-based wind and high voltage dc transmission systems, etc. Previously, a dual-mode modulation technique was implemented in the literature which provided the maximum average NP current limits without considering the ZSV effect. As the ZSV injection affects the NP duty cycles, and in turn the NP current, it is necessary to analyze and quantify its effect. In this paper, the effect of ZSV on the current limits is analytically calculated. Simulation results are obtained in MATLAB/Simulink environment to validate the derived analytical equations.

Photovoltaic (PV) grid support functionality (frequency support) can be accomplished with the aid of flexible power point tracking (FPPT) technology. However, the convergence rate of FPPT algorithms needs to be fast, to provide grid support under transients. To fulfil this purpose, a Newton’s method-based (NM-B) FPPT algorithm is proposed in this paper that offers quadratic convergence, which is a higher convergence rate compared to the existing methods. Moreover, the search algorithm requires only a single initialization point, thus eliminating the difficulty of setting up the search boundary as in the case of the binary search-based and secant method-based (SM-B) FPPT algorithms. Simulation validation is performed for the proposed NM-B FPPT algorithm and the results are compared with the SM-B FPPT algorithm.

In a modular multilevel converter (MMC) based battery energy storage system (BESS), a fault tolerant design ensures uninterrupted operation of the MMC when a given number of submodules (SMs) have faulty batteries or no batteries. This paper quantitatively investigates the fault tolerance improvement in MMC-based BESSs with different numbers of redundant SMs, which is commonly practiced to improve the fault tolerance in modular converter topologies. Simulation results are obtained to verify the proposed analysis. The presented analysis provides guidelines for designers to choose an appropriate number of redundant SMs to achieve a desired fault tolerance.

Frequency support from photovoltaic (PV) power plants is required with new grid codes. To provide frequency support, the PV system needs to regulate the PV power in such a way that a predefined amount power reserve is kept in the PV system. Accordingly, a dynamic flexible power point tracking algorithm (FPPT) for grid-connected PV power plants is introduced in this paper. The proposed algorithm regulates the average PV power by continuously sweeping some portion of the PV curve that includes the given power reference point. The proposed strategy is different from traditional static FPPT techniques as it ensures a predefined amount of power reserve in the PV system. This power reserve provides the flexibility of increasing the power injected into the grid that can be withdrawn with a fast dynamic. Compared to the traditional solutions, in the proposed algorithm, the PV injects a constant power into the grid without any fluctuations. Effectiveness of the proposed technique in providing frequency support in a two-stage grid-connected PV plan is demonstrated by simulation results.

A precisely parameterized battery model is the prerequisite of the model-based management of lithium-ion battery. However, the unexpected sensing of noises may discount the identification of model parameters in practical applications. This article focuses on the noise effect compensation and online parameter identification for the widely used equivalent circuit model. A novel degree of freedom (DOF) eliminator is proposed and combined with the Frisch scheme in a recursive fashion, for the first time, to coestimate the noise statistics and unbiased model parameters. A computationally tractable numerical solver is further proposed for the DOF eliminator to improve the real-time performance. Simulations and experiments are performed to validate the proposed method from theoretical to practical perspective. Results show that the proposed method can effectively mitigate the noise-induced identification biases and outperform the existing methods in terms of the accuracy and the robustness to noise corruption.

The neutral-point-clamped (NPC) converter employed in applications like bipolar dc-bus, back-to-back converters, power compensator, etc., requires a nonzero average neutral-point (NP) current injection under certain operating conditions. In order to better implement the average NP current injection, this paper studies the maximum and minimum average NP current that can be injected with an NPC converter. For this study, a three-level modulation technique is employed in which one of the three phases is selected to switch between the positive dc-rail and the negative dc-rail while the other two phases switch in two consecutive voltage levels. The maximum and minimum average NP current injection limits are analyzed under different loading conditions. The presented study also evaluates the effects of modulation index and operating power factor on the limits.

A coupled-inductor-based bidirectional Z-source breaker topology is introduced in this paper, which has the ability to detect and isolate the faulty section of the dc power system. Compared to existing topologies, the proposed topology utilizes a lower number of components and the coupled inductors reduce the footprint of the breaker. Analytical design equations are derived for the sizing of inductors and capacitor. Fault condition is analyzed to compute the essential criteria for the breaker to operate. Operation under step load and fault conditions are validated by simulation in both SPICE and MATLAB/Simulink environment.

The Vienna rectifier is an attractive converter solution due to the three-level voltage generation and its simple structure. When the Vienna rectifier operates with nonunity power factor, the reference voltage and the input current have different signs during some intervals around the current zero crossings. This creates low-frequency distortion in the current waveforms. One of the preferable methods to reduce this distortion is the zero sequence injection which, however, risks the converter entering into overmodulation. This paper analyses the above distortion and introduces the operation of the Vienna rectifier in two modes, which includes injecting a proper zero sequence and reactive power compensation. This allows the converter to operate in a wide range of power factors without constraining the modulation index. The required reactive current is obtained analytically from the instantaneous values of the converter at any operating point.

This paper proposes a horizon-one constrained model predictive control (MPC) with integral action for dc-dc H-bridge converters. An integral-action MPC law is analytically derived. The proposed controller takes into account the bilinear dynamics of the converter both in the control law derivation and constraints modeling. In addition, the proposed cost function does not presume the knowledge of the equilibrium point. In this paper, a three-level H-bridge converter for regulation purposes is considered to illustrate the proposed approach, though it is applicable to other dc-dc converter topologies as well. The paper compares the performance of the proposed analytical MPC law and the solution provided by an iterative optimisation algorithm. Simulation results are provided to verify the theoretical predictions, that is, the control law rejects constant disturbances while satisfying the designed constraints.

The frequency response support has been added to the new standards and grid codes for the grid-connected photovoltaic (PV) systems, to retain the reliability and quality of the power system under the high penetration of renewable energy resources. This paper proposes an algorithm for the control of cascaded H-bridge (CHB)-based PV systems to achieve frequency response for them. The required power reference, based on the grid operation condition, is distributed among the sub-modules (SMs) of the CHB converter considering the available power from each SM and operational constraints of the system. The details of the proposed control algorithm are provided, while its effectiveness is evaluated on a 1.5-MW PV simulation setup, connected directly to a 6.6-kV grid.

Low losses are a major advantage of the modular multilevel converter (MMC) in high-power applications. Losses on the MMC depend on a number of parameters such as switching frequency, voltage balancing algorithm, the topology of the sub-module (SM). This paper investigates the impact of different power factors in the losses of different MMC SMs with a focus on HVDC applications. Considering 27 different SM topologies, we demonstrate that the operation mode of the converter (inverter / rectifier) and the power factor affect not only the average losses but also the minimum and maximum losses within a period due to the voltage balancing requirements.

In this paper, a cascaded H-bridge low capacitance StatCom with modular capacitors is proposed. The low capacitance StatCom benefits from having large capacitor voltage ripple to reduce losses and improve harmonic performance. The proposed LC-StatCom can operate with large capacitor voltage ripple even in light loading conditions by keeping redundant capacitor modules in standby mode when the current is low. Effectiveness of the proposed system is confirmed using a simulated 8.5-MVA StatCom system.

Flexible power point tracking (FPPT) is control of active power generated by grid-connected photovoltaic power plants (GCPVPPs) to provide various grid-support functionalities, such as frequency response and voltage support. Fast transient response is required during grid voltage and frequency variations. An algorithm, based on dynamic voltage reference design and model predictive control for the dc-dc converter in GCPVPPs is introduced in this paper. The main novelty of the proposed algorithm is fast dynamic response based on the maximum capacity of the system. This feature enables fast FPPT operation in GCPVPPs. The proposed control algorithm results in several features for the FPPT operation, including: higher bandwidth, low power oscillations during steady-state, flexibility for adding constraints based on the operation conditions, and ease of implementation without requiring any complex control parameters tuning and design procedure. The transient performance of the proposed algorithm is evaluated under conditions of voltage sag and frequency deviation. Experimental results are also provided to demonstrate the effectiveness of the proposed algorithm when subjected to rapid changes in PV reference power.

This paper proposes a multi-input linear time-variant control law based on the incremental passivity theory for cascaded H-bridge (CHB) low-capacitance static compensators (LC-StatComs). The proposed control law guarantees global asymptotic stability of the system and controls the capacitor voltages and inductor current without assuming fast dynamics in the current and slow dynamics in voltages. This is particularly important for LC-StatCom applications as the control of the inductor current and capacitor voltages are inherently coupled due to the reduced capacitance. After introducing the incremental model of the converter, a passivity-based control that ensures the global stability is derived. Simulation results on a seven-level 1-kVA CHB LC-StatCom are shown to corroborate its excellent performance in steady-state and during transients.

DC marine vessels with medium-voltage compact dc power systems are dominated by a significant amount of active loads and a finite number of generation sources. In such scenarios, the network configuration of the dc power system is expected to get dynamically altered to fulfill the required generation and load demands for the desired marine mission. Such varying network configurations make the transient responses significantly different from the conventional ac grids and the prospective dc grids. In this regard, this paper performs systematic transient studies to devise fault management strategies for the dc marine vessels. A platform supply vessel (PSV) is taken as an example of the marine vessel, due to its complex operating scenarios and wider applicability in the marine industry. Pole-to-pole short-circuit faults are considered owing to its severity. A novel current-only directional protection for the dc PSV is proposed based on the directional zonal interlocking and short-time Fourier transform. The efficacy of the proposed method is substantiated by confirming against a range of fault impedances initiated at the generator terminals, load terminals, lines, and buses of the dc PSV. All the analyses are conducted in the real-time simulation model of the dc PSV.

In this paper, application of thyristor switched reactors as a filtering inductor for the cascaded H-bridge low-capacitance static compensator (LC-StatCom) is demonstrated. Replacing the fixed-value filter inductor with thyristor switched reactors reduces the overall energy storage capacity and voltage rating of the LC-StatCom. The proposed concept takes advantage of LC-StatCom's ability to synthesize high quality ac voltage to reduce the overall energy storage requirement of the filter inductance. Using modular filter inductor concept adds an additional degree of freedom to control the maximum voltage drop on the filter inductor by changing the inductance value. This additional capability decreases the overall voltage rating of the converter. The effects of adding the modular inductor unit on the I-V characteristic of the LC-StatCom are evaluated. The effectiveness of the proposed solution is demonstrated by simulation results on an 11-level 3 MVA LC-StatCom.

Propulsors with outboard drives (PODs) are propeller propulsion units situated under the hull of a marine vessel. They contain an electric motor which is directly connected to the propeller. With the absence of the gearbox, higher efficiency, lower noise and vibration can be achieved. Presently, the wound rotor synchronous motors dominate a large share of the podded propulsion market due to their high efficiency. In this paper, the design process of a 5-MW salient pole wound field synchronous motor for application in POD marine propulsion is presented. The paper contains preliminary design evaluation to describe motor specifications and the main steps of electromagnetic design. Prediction of motor performances are also presented and discussed.

The negative- and zero-sequence currents of a modular multilevel converter (MMC) under imbalanced ac-grid conditions increase the maximum voltage of the submodule (SM) capacitors, which should be kept below a maximum voltage limit. This paper investigates the impact of offset pulse-width modulation (OPWM) on the capacitor voltage of the MMC under imbalanced ac-grid conditions. It is demonstrated that OPWM increases the maximum operating range of the MMC for a given set of restrictions, as it leads to a reduction of capacitor voltage by utilizing the total number of the SMs in the arms. Simulation results from an 800-MVA MMC-based HVDC transmission system demonstrate the effect of OPWM in the operation of the converters and the efficacy of the method compared to standard modulation methods.

Integrated power systems (IPSs) with medium voltage direct current (MVDC) distribution are gaining importance in civil and defense marine vessels as they promise to provide cleaner, more reliable operation of vessels along with reduced fuel consumption. The integration of battery energy storage systems (BESSs) in the MVDC distribution system enables peak shaving of generators, optimal scheduling of generators, and near instantaneous power reserve. In this paper, an isolated bipolar dc-dc converter is presented which interfaces the BESS with the bipolar dc distribution bus. The proposed converter uses high frequency transformer isolation which helps in protecting the BESS against rapid discharge in the event of short circuit faults on the dc bus. Detailed steady-state analysis of the proposed converter is presented in this paper. A closed loop control system is developed in the MATLAB®/Simulink simulation platform which regulates the output of the isolated bipolar converter to its reference value. The dynamic performance of the converter is demonstrated with load variations on both the positive and negative buses. The battery discharge limiting capability is also verified.

This paper proposes a four-phase mutually coupled switched reluctance machine (MCSRM) arrangement with single pulse current control strategy, targeting performance improvements of both the machine and the power converter. Previously published works on MCSRM have focused only on machine performance while neglecting the power converters. Designs and comparative studies have been carried out among conventional switched reluctance machine (CSRM), full pitch MCSRM, short pitch MCSRM and single pulse MCSRM with practical specifications. Coupled finite element analysis and circuit simulations have been employed to evaluate performance indices including torque ripple, machine iron/copper loss, power factor, winding current density, power converter losses and power densities. The machine-converter system level investigations have confirmed that in the proposed four-phase MCSRM under single pulse control, improved machine performances can be obtained with reduced power converter loss.

This paper develops an alternate arm converter (AAC) high-voltage direct current (HVDC) terminal model that is compatible with the existing benchmark models of modular multilevel converter (MMC) based HVDC and dc-grid benchmark models. The developed model is a first step towards an AAC-based point-to-point transmission system and the future development of multi-converter, multiterminal benchmark models that facilitate system level studies with multiple topologies. The topology, configuration, design considerations and parameters of the model are derived in order to provide a direct equivalence with the MMC-based system. Detailed simulation studies of the multilevel converter demonstrate the operation and performance of the AAC-based benchmark model and the associated control functions.

Selective harmonic elimination (SHE) - pulse with modulation (PWM), when applied to multilevel converters (MCs), can be optimized in case a half-wave (HW) symmetry is implemented. This kind of symmetry is able to provide a wide variety of solutions. This paper proposes a novel method, based on optimization algorithms, which provides a universal formulation of the SHE-PWM problem with HW symmetry, in such a way that different equation systems are not required to find a solution when different switching patterns are considered. In this way, the proposed technique is able to search simultaneously both the switching patterns and their associated firing angles that solve the SHE-PWM problem, without using predefined waveforms. Therefore, the search process is significantly simplified and the flexibility to find solutions with different waveforms, throughout the reference modulation index, ma, range, is increased. Different sets of solutions provided by this method have been presented and simulation results, obtained from a five-level modular multilevel converter (MMC), have proved the validity of the new proposed technique.

Tolerance and component aging can cause significant differences in the capacitance values of the submodules (SMs) in a modular multilevel converter (MMC). Depending on the modulation technique, capacitance mismatches may produce uneven switching transitions of the SMs, hence imbalances in the power losses that can lead to reliability problems. In this paper, a new algorithm that helps to achieve evenly distributed switching and conduction losses within the converter SMs is presented. The proposed algorithm is based on a modification of the common voltage balancing algorithms, balancing a weighted function of voltage and losses. Even distribution of power losses is achieved at the cost of slightly increasing the capacitor voltage ripples. The effectiveness of the strategy has been demonstrated by simulation results of a high-power grid-connected MMC.

Due to the high penetration of the installed distributed generation units in the power system, the injection of reactive power is required for the medium-scale and large-scale grid-connected photovoltaic power plants (PVPPs). Because of the current limitation of the grid-connected inverter, the injected active power should be reduced during voltage sags. In order to obtain a constant dc-link voltage in a multi-string PVPP, the extracted power from PV strings should be equal to the injected power to the grid in all operating conditions (excluding power losses). Therefore, the extracted power of PV strings should be reduced during voltage sags. In this paper, an algorithm is proposed for determining the reference voltage of the PV string which results in a reduction of the output power to a certain amount. The proposed algorithm calculates the reference voltage for the dc/dc converter controller, based on the characteristics of the power-voltage curve of the PV string and therefore, no modification is required in the the controller of the dc/dc converter. Simulation results on a 50-kW PV string verified the effectiveness of the proposed algorithm in reducing the power from PV strings under different irradiation and reference power values.

The alternate arm converter (AAC) is a multilevel converter of the same family as the modular multilevel converter (MMC). Contrary to the MMC, the AAC offers dc-fault tolerant capabilities but requires complex submodule (SM) capacitor voltage/energy regulation and circulating current control. Such control is also limited due to the small overlap period where both arms conduct and energy between the arms can be exchanged. This paper develops a double-band hysteresis-based circulating current control strategy by introducing an asymmetric overlap period control of the director switches (DSs). The proposed controller ensures zero-current switching operation of the DSs, and increases the flexibility of SM capacitor voltage regulation owing to the asymmetric overlap period. It improves the performance of current control and energy regulation for the AAC at both near sweet-spot and non sweet-spot operation without disturbing the output voltages and currents. The performance and effectiveness of the proposed controller are illustrated through simulations on MATLAB-Simulink and PLECS.

Research on dc-fault tolerant multilevel converters has gained noticeable attention over recent years. The alternate arm converter (AAC) is one of such emerging multilevel converter topologies, and a hybrid topology of the two-level converter and the modular multilevel converter (MMC). Bipolar sub-modules (SMs) that can produce both positive and negative voltages are the building blocks of the AAC. This paper analyses the operation of an AAC with the full-bridge SM (FB-SM) and the cross-connected SM (CC-SM). The conduction and switching losses of the two SM configurations are evaluated and compared, in order to identify the suitability of CC-SM for AACs and its performance compared to the FB-SM. The CC-SM with identical semiconductor devices has reduced losses compared to the CC-SM with higher rated devices in the cross-connected path. It is concluded that the CC-SM does not offer advantages in the losses, construction, and application to the AAC, compared to FB-SM.

Control of the circulating current in the modular multilevel converter (MMC) is one of the major tasks for the proper operation of the converter topology. This paper develops a controller for the circulating current of the MMC that utilises the redundant voltage levels in 2N+1 modulated MMCs in order to regulate the current to its reference. The main advantages of the approach are the elimination of control loops that generate the voltages for the control of the circulating current, simple implementation and very fast dynamic performance. The application of the control is also independent of the circulating current reference. The simplicity and effectiveness of the proposed controller is illustrated through simulations and experimental results.

In this paper, a new approach to the discontinuous modulation technique for the operation of the modular multilevel converter (MMC) is presented. Discontinuous modulation is based on adding a zero-sequence to the original modulation signals so that each MMC arm is clamped to the upper or lower terminals of the dc-link bus during some intervals. In combination with a circulating current control, the original discontinuous modulation can reduce the capacitor voltage ripple amplitudes and the switching power losses. However, additional submodules (SMs) are required to control the circulating current. This new approach presents a clamping algorithm that eliminates the requirement of additional SMs. As a result, the conduction losses are reduced while the capacitor voltage ripples are maintained low. Simulation and experimental results on a silicon-carbide-based MMC are reported and compared against the original discontinuous modulation and a conventional carrier-based pulse-width modulation.

The application and research interest over utility-scale battery energy storage systems have grown significantly over the last years. To achieve utility-scale, series and/or parallel connection of many battery packs with power rating of a few hundreds kilowatts each is the most viable technology at present. The major challenge regarding the connection of battery packs is charging imbalance. This paper proposes a solution using reconfigurable selection panels and series-connected dual-active-bridges to connect many battery packs directly to a medium voltage level bus. The proposed control algorithm facilitates optimal battery operation by prioritizing the packs to be used based on different operation conditions, so that rest time for each battery pack is guaranteed. It also optimizes the charging/discharging processes of the packs to extend the life of the batteries. Simulation results using MATLAB/Simulink and PLECS confirm the effectiveness of the proposed solution.

This paper presents a voltage balancing method for stacked multicell converters based on phase disposition pulse-width modulation. This method is based on minimizing a cost function to determine the optimum redundant state for capacitor voltage balance for each particular voltage level. The robustness of the proposed voltage balancing method is verified against static and dynamic unbalanced load conditions. Furthermore, a significant reduction in the switching frequencies of the power devices is achieved by using sawtooth carriers instead of standard triangular carriers without affecting the voltage balancing capability.

This paper proposes a voltage balancing method for a seven-level stacked multicell converter (SMC) based on phase disposition pulse-width modulation (PD-PWM) using reduced switching transitions. This method is called optimal-transition voltage balancing method. The selection of the optimal transition sequence is performed by minimizing a cost function and the transitions that would result in more switchings of the converter semiconductor devices are avoided. The simulation results show a significant reduction of the average switching frequency as compared to the use of the optimal-state voltage balancing method, while maintaining the balance of the FC voltages. Moreover, the proposed PD-PWM voltage balancing method is robust to unbalanced linear loads, non-linear loads and transients.

This paper proposes an active voltage balancing method for stacked multicell converters (SMC) using phase-shifted pulse-width modulation, which is easy to implement and extend to high number of levels. The proposed method balances the voltages of the capacitors by modifying the duty cycle of each switch of the SMC using a proportional controller. The crossed effect between capacitor currents and duty cycles is considered and is used for optimal capacitor voltage balance. The performance of the proposed voltage balancing method is verified by simulation for different operating conditions, such as unbalanced linear loads, non-linear loads and load transients.

The life expectancy and long term reliability of grid-connected three-phase photovoltaic (PV) inverters can be increased by replacing the conventional electrolytic film capacitors by metallized polypropylene film capacitors. This paper presents a detailed evaluation of a three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor-one. The minimum dc bus capacitance leads to larger voltage ripples. However, such ripples were found to be within acceptable limits to run the inverter satisfactorily. Simulation results are presented for a 15-kW grid-connected inverter at nominal voltage of 700V dc and experimental results are provided for a 3.0-kW system at a nominal voltage of 400V dc, built in the laboratory.

Interleaved four-quadrant, single-phase rectifiers are widely used in traction applications as part of an AC-DC-AC conversion system. This paper presents selective harmonic elimination pulse-width modulation (SHE-PWM) patterns for single-phase interleaved PWM rectifiers based on both H-bridges and full-bridge NPCs (H-NPCs). SHE-PWM waveforms are defined so that the low-order harmonics of the combined voltage and input currents on the primary of the supply transformer are eliminated. The formulation of the problem and the acquisition of solutions becomes more challenging as the number of levels in the voltage waveform increases. Solutions based on reduced eliminated harmonics are proposed in order to increase the number of solutions as well as their continuity. It is shown that the proposed formulation improves the available solution space with minimal effect to the output. The theoretical analysis are verified through simulation studies.

This paper proposes a PWM based modulation strategy particularly useful for three-level multi-phase Neutral-Point-Clamped Converters. It achieves similar results to those obtained with the space vector modulation but allowing a very fast and simple implementation in a digital controller. The proposed algorithm is able to maintain the neutral-point voltage under control by the addition of a zero-sequence voltage component to the modulation signals. A simple and efficient methodology to calculate the most appropriate zero-sequence component is described. In addition the modulation strategy is completely general, regardless the number of phases of the converter, and therefore it is easy extendible to n-phase converters.

In this paper the control of a single-stage grid-connected photovoltaic power plant (GCPPP) is developed to address the issue of inverter disconnection under various grid faults. There are three main reasons for inverter disconnection which are (i) excessive dc-link voltage, (ii) excessive ac currents and (iii) loss of grid-voltage synchronization. The control of the inverter incorporates reactive power support in the case of voltage sags based on grid code requirements to ride-through the faults and support the grid voltages. Accordingly, another requirement of the grid codes is to control the reactive current injection under unbalanced voltage sags so that the voltages in the non-faulty phases do not exceed the specified limitations. All these issues are discussed in this paper for a case study of 1-MVA GCPPP using MATLAB/Simulink. The results illustrate the capability of the developed GCPPP to ride-through different types of faults occurred on the grid side.

The voltage ripple of the capacitors of the modular multilevel converter (MMC) increases when the converter output frequency decreases. This occurs in motor drive applications at low motor speeds, where the output frequency is reduced proportionally to the converter modulation index. In this paper, a discontinuous modulation technique for reducing the amplitude of the capacitor voltage ripples and the switching losses, especially at low modulation indices, is presented. The technique is based on adding a zero-sequence to the original modulation signals, clamping the MMC arms to the upper or lower rails of the fictitious dc-link bus. A current control strategy suitable for this modulation strategy is also proposed. Simulation results under various operating points are reported along with evaluation and comparison results against the conventional carrier-based pulse-width space-vector modulation.

The flying capacitor (FC) multilevel converter requires a capacitor voltage balancing mechanism for proper operation. This paper proposes a voltage balancing scheme for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM) using transitions that produce the minimum number of switching events. This strategy will be called optimal-transition voltage balancing (OTVB) scheme. Since multiple optimum switching transitions between two consecutive voltage levels may be available, such a redundancy is used to regulate the FC voltages at their desired levels. Those transitions that produce more switching events are avoided. The rest of transitions are evaluated by means of a cost function and the one that produces the minimum value is selected. Simulation results show a significant reduction of the average switching frequency as compared to the use of the optimal-state voltage balancing (OSVB) scheme, while maintaining the balance of the FC voltages. Moreover, the proposed PD-PWM voltage balancing strategy is robust to static and dynamic load conditions.

The flying capacitor (FC) multilevel converter has attracted a great deal of interest in the recent years because of its easier extension to a higher number of levels (n>;3), as compared to its counterpart, the diode-clamped converter (DCC). The main focus of this paper is to develop a voltage balancing scheme of FCs for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM). Since there are multiple states that produce the same output voltage at the leg of the converter, such a redundancy is used to regulate the FC voltages at their desired levels. The selection of the optimal states is performed by minimizing a cost function. A drawback observed when using standard symmetrical triangular carriers for the PD-PWM, is the additional switching events that are produced due to transitions within the same voltage level. Nevertheless, this fact can be avoided by using sawtooth carrier waveforms instead. Simulation results verify the robustness of the proposed voltage balancing scheme against static and dynamic load conditions. Moreover, using sawtooth carriers a significant reduction of the switching frequency is achieved as compared to the use of standard triangle carriers while maintaining the FC voltage balanced.

The modular multilevel converter (MMC) is one of the most potential converter topologies for medium/high power/voltage applications. One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents within the legs. Circulating currents, if not properly controlled, increase the amplitude of capacitor voltage variations, rating values of the converter components and converter losses. This paper proposes a closed-loop circulating current control strategy for an MMC to specifically minimize the amplitude of capacitor voltage variations. The proposed strategy is based on adding an offset signal to the modulating signal of each arm. To minimize the amplitude of the capacitor voltage oscillations, an optimal circulating current component is determined and used as a reference signal for the current control of each MMC leg. Performance of the proposed control strategy is evaluated based on simulation studies in the MATLAB/Simulink environment. The reported study results demonstrate effectiveness of the proposed strategy to reduce the amplitude of the capacitor voltage oscillations.