Showing papers on "Converters published in 2017"
TL;DR: In this paper, the authors comprehensively review and classify various step-up dc-dc converters based on their characteristics and voltage-boosting techniques, and discuss the advantages and disadvantages of these voltage boosting techniques and associated converters.
Abstract: DC–DC converters with voltage boost capability are widely used in a large number of power conversion applications, from fraction-of-volt to tens of thousands of volts at power levels from milliwatts to megawatts. The literature has reported on various voltage-boosting techniques, in which fundamental energy storing elements (inductors and capacitors) and/or transformers in conjunction with switch(es) and diode(s) are utilized in the circuit. These techniques include switched capacitor (charge pump), voltage multiplier, switched inductor/voltage lift, magnetic coupling, and multistage/-level, and each has its own merits and demerits depending on application, in terms of cost, complexity, power density, reliability, and efficiency. To meet the growing demand for such applications, new power converter topologies that use the above voltage-boosting techniques, as well as some active and passive components, are continuously being proposed. The permutations and combinations of the various voltage-boosting techniques with additional components in a circuit allow for numerous new topologies and configurations, which are often confusing and difficult to follow. Therefore, to present a clear picture on the general law and framework of the development of next-generation step-up dc–dc converters, this paper aims to comprehensively review and classify various step-up dc–dc converters based on their characteristics and voltage-boosting techniques. In addition, the advantages and disadvantages of these voltage-boosting techniques and associated converters are discussed in detail. Finally, broad applications of dc–dc converters are presented and summarized with comparative study of different voltage-boosting techniques.
1,230 citations
14 Apr 2017
TL;DR: Six family members of the modular multilevel cascade converters, intended for grid-tied applications and medium-voltage high-power motor drives are paid much attention.
Abstract: This paper provides a chronological overview of the topology for multilevel converters, and discusses their different terminology usages and characteristics. The multilevel converters include three-level neutral-point-clamped (NPC) and neutral-point-piloted (NPP) inverters, three-level and four-level flying-capacitor (FLC) inverters, and a family of modular multilevel cascade converters. Some have already been put into commercial use, some have been on a research and development stage, and others have been on an academic research stage. This paper pays much attention to six family members of the modular multilevel cascade converters, intended for grid-tied applications and medium-voltage high-power motor drives.
380 citations
15 Aug 2017
TL;DR: The operation of multilevel converters is addressed focusing on control and modulation techniques for different well-known applications as an extension of conventional methods for two-level voltage-source converters which are still the mainstream solution for most cases.
Abstract: In the last decades, multilevel converters have been developed usually for medium-voltage high-power applications. They have become a mature solution for the increasing power demand of multiple applications such as renewable energy systems, power quality improvement, and motor drives. In this paper, the operation of multilevel converters is addressed focusing on control and modulation techniques for different well-known applications. The new developments are presented as an extension of conventional methods for two-level voltage-source converters which are still the mainstream solution for most cases.
331 citations
01 Mar 2017
TL;DR: In this article, full-bridge CLLC, dual active bridge (DAB), and half-bridge DAB dc-dc converters are evaluated and compared for their suitability for EV chargers.
Abstract: Isolated dc–dc converters with galvanic isolation are commonly used in electric vehicle (EV) battery chargers. These converters interface between a dc voltage link, which is usually the output of a power factor correction stage, and an energy storage unit. CLLC and dual active bridge (DAB) dc–dc converters can achieve high power density, high-energy efficiency, wide gain range, galvanic isolation, and bidirectional power flow, and therefore, have potential applications as dc–dc converters for bidirectional EV charging systems. In this paper, full-bridge CLLC, half-bridge CLLC, full-bridge DAB, and half-bridge DAB dc–dc converters are evaluated and compared for their suitability for EV chargers. All the converters are designed with optimal soft-switching features. The operating principles, design methodologies, and design considerations are presented. Prototypes of the converters with power rating of 1 kW are designed and developed. The prototypes interface a 500 V dc link and a 200–420 V load, which is common for EV applications. The performances of the circuits are analyzed and a comprehensive comparison is conducted.
320 citations
TL;DR: In this paper, the authors present the development of sequence impedance models for modular multilevel converters (MMC) and validate these models by point-by-point simulation of detailed converter circuit models.
Abstract: This paper presents the development of sequence impedance models for modular multilevel converters (MMC). The intended applications of the models include stability and resonance analysis of high-voltage dc transmission, static synchronous compensation, and other systems that use MMC. The basis of the modeling method is harmonic linearization that has been applied to other types of converters, but a new formulation is presented to allow the inclusion of harmonic effects. This generalization, called multiharmonic linearization, is necessary for MMC because of the significant second harmonics present in the arm currents, capacitor voltages, and control signals. To accommodate multiple harmonics in the linearization process, a matrix formulation is introduced and used to model both the converter power stage and its control. The developed sequence impedance models are validated by point-by-point simulation of detailed converter circuit models, and used to understand the effects of harmonics and control on MMC frequency-domain characteristics.
187 citations
TL;DR: In this article, a fully-active hybrid energy storage system, which uses two bi-directional DC/DC converters to decouple supercapacitor and battery pack from the DC bus, is proposed based on a 5th-order averaged model.
173 citations
TL;DR: In this article, a family of higher order compensation circuits for IPT converters that achieves any desired constant-voltage or constant-current (CC) output with near zero reactive power and soft switching was proposed.
Abstract: Compensation is crucial for improving performance of inductive-power-transfer (IPT) converters. With proper compensation at some specific frequencies, an IPT converter can achieve load-independent constant output voltage or current, near zero reactive power, and soft switching of power switches simultaneously, resulting in simplified control circuitry, reduced component ratings, and improved power conversion efficiency. However, constant output voltage or current depends significantly on parameters of the transformer, which is often space constrained, making the converter design hard to optimize. To free the design from the constraints imposed by the transformer parameters, this paper proposes a family of higher order compensation circuits for IPT converters that achieves any desired constant-voltage or constant-current (CC) output with near zero reactive power and soft switching. Detailed derivation of the compensation method is given for the desired transfer function not constrained by transformer parameters. Prototypes of CC IPT configurations based on a single transformer are constructed to verify the analysis with three different output specifications.
165 citations
TL;DR: In this paper, the authors present a literature overview of power decoupling in single-phase applications and present the best reference on this topic, which can be implemented as series or parallel with respect to the ac, dc or link side.
Abstract: This paper presents a literature overview of all techniques proposed until the submission of this paper in terms of mitigating power oscillation in single-phase applications. This pulsating energy is the major factor for increasing the size of passive components and power losses in the converter and can be responsible for losses or malfunctioning of the dc sources. Reduction of power ripple at twice the fundamental frequency is one of the key elements to increase power converter density without lack of dc stiffness. Pulsation reduction is achieved by incorporating control techniques or auxiliary circuitries with energy storage capability in reactive elements to avoid this oscillating power to propagate through the converter, creating what is called as single-phase power decoupling. The topologies are divided as: rectifiers, inverters, and bidirectional. Among them, it is possible to classify as isolated and nonisolated converters. The energy storage method may be classify as: capacitive and inductive. For the power decoupling technique, it is convenient to divide as control and topology. The power decoupling technique may be implemented as series or parallel with respect to the ac, dc or link side. This paper represents the best reference on this topic.
165 citations
TL;DR: In this paper, a virtual synchronous control (ViSynC) is proposed for VSCs, which utilizes the dynamics of the dc-link capacitor to realize self-synchronization.
Abstract: Voltage-source converters (VSCs) are widely used in renewable energy sources as the grid interface, e.g., wind turbine generators and photovoltaics. These VSCs control the dc-link capacitor voltage and the reactive power output to track the reference values, which generally apply phase-locked loop (PLL) for grid synchronization. However, the dynamic performance of the conventional PLL can be deteriorated when the VSC is integrated into weak grids, which may even cause instability of the VSC. In this paper, a virtual synchronous control (ViSynC) is proposed for VSCs, which utilizes the dynamics of the dc-link capacitor to realize self-synchronization. Grid synchronization mechanism of the ViSynC-based VSC is particularly analyzed in this paper. The ViSynC-based VSC can provide inertial responses to the grid, and has the advantage that it can operate normally under weak grid conditions without any modification of the grid synchronization unit. Furthermore, virtual impedance and Q–V droop control can be easily implemented in the control structure of the ViSynC. Simulations based on MATLAB/ Simulink and hardware-in-the-loop real-time simulations based on RT-LAB verify the effectiveness of the proposed ViSynC.
153 citations
TL;DR: In this article, the authors investigated the problem of high-parasitic capacitances produced by large overlapping layers in planar transformers (PTs) and proposed mitigation strategies to improve the performance of $LLC$ converters with PTs.
Abstract: The use of $LLC$ resonant converters has gained popularity in multiple applications that require high conversion efficiency and galvanic isolation. In particular, many applications like portable devices, flat TVs, and electric vehicle battery chargers require demanding slim-profile packaging and enforce the use of planar transformers (PTs) with low-height, low leakage inductance, excellent thermal characteristics, and manufacturing simplicity. The main challenge in successfully designing $LLC$ converters with PT resides in controlling high-parasitic capacitances produced by large overlapping layers in PT windings. When the parasitic capacitances are not controlled, they severely impair the converters’ performance and regulation, and limit the application of PTs in high-frequency $LLC$ converters. This paper characterizes the PT capacitance issue in detail and proposes mitigation strategies to improve the performance of $LLC$ converters with PTs. A systematic analysis is performed, and six PT winding layouts are introduced and benchmarked with a traditional design. As a result of the investigation, an optimized structure is obtained, which minimizes both the interwinding capacitance and ac resistance, while improving the regulation performance of $LLC$ converters. Experimental measurements are presented and show a significant reduction of parasitic capacitance by up to 21.2 intra- and 16.6 interwinding capacitances, without compromising resistance. This substantial capacitance reduction has a tangible effect on the regulation performance of $LLC$ resonant converters. Experimental results of the proposed PT structure in a 1.2 kW $LLC$ resonant converter show a reduction in common-mode noise, extended output voltage regulation, and improved overall efficiency of the converter.
141 citations
TL;DR: It is demonstrated that non-isolated dc-dc converters based on the conventional Boost converter can be treated as a unique generalized converter, called Boost Converter with Gain Cell (BCGC).
Abstract: High step-up conversion is an indispensable feature for the power processing of low voltage renewable sources in grid-connected systems. Motivated by this necessity, this paper presents a study on non-isolated dc-dc converters based on the conventional Boost converter that can provide such feature with high efficiency. By the topological variation and gain cell concepts, it is demonstrated that these converters can be treated as a unique generalized converter, called Boost Converter with Gain Cell (BCGC). The operating principle, the key waveforms and the components stresses of the BGCG are analyzed for the continuous-conduction mode, independently of the employed gain cell. A methodology to create the gain cells is developed from the combination of coupled inductors and voltage multiplier techniques. In order to verify the realized analysis, a 150 W prototype concerning to the proposed generalized converter and able to operate with several different gain cells is developed for the comparison between theoretical and experimental static gain results.
TL;DR: This paper presents a new family of hybrid Z-source boost dc-DC converters intended for photovoltaic applications, where the high step-up dc-dc converters are demanded to boost the low-source voltages to a predefined grid voltage.
Abstract: This paper presents a new family of hybrid Z-source boost dc–dc converters intended for photovoltaic applications, where the high step-up dc–dc converters are demanded to boost the low-source voltages to a predefined grid voltage. Because the boost capabilities of the traditional Z-source networks are limited, the proposed converters are composed of combine traditional Z-source networks in different ways to enhance the boost abilities of the traditional Z-source networks. The new version of the proposed Z-source converters is termed as hybrid Z-source boost dc–dc converters to satisfy the traditional benefits of Z-source networks with stronger voltage boost abilities which can also be applied to dc–ac, ac–ac, and ac–dc power conversions. The performances of the proposed converters are compared with other Z-source networks behaviors. The simulation and experimental results of the proposed converters are validated at different operating conditions.
TL;DR: In this article, a hybrid MMC topology is introduced, which significantly reduces the voltage ripple of capacitors, particularly at low motor speeds, and this topology does not introduce any motor common-mode voltage; as a result, there are no insulation and bearing current problems.
Abstract: Modular multilevel converters (MMC) have revolutionized the voltage-sourced converter-based high-voltage direct current transmission, but not yet got widespread application in medium-voltage variable-speed motor drives, because of the large capacitor voltage ripples at low motor speeds. In this paper, a novel hybrid MMC topology is introduced, which significantly reduces the voltage ripple of capacitors, particularly at low motor speeds. Moreover, this topology does not introduce any motor common-mode voltage; as a result, there are no insulation and bearing current problems. Additionally, the current stress can remain at rated value throughout the whole speed range; thus, no device needs to be oversized and converter efficiency can be ensured. Operating principle of this hybrid topology is explained, and control schemes are also developed. Validity and performance of the proposed topology are verified by simulation and experimental results.
TL;DR: In this article, the performance of modular multilevel converters with integrated battery cells when used as traction drives for battery electric vehicles was evaluated. But the performance was not compared with a traditional two-level converter.
Abstract: This paper evaluates the performance of modular multilevel converters with integrated battery cells when used as traction drives for battery electric vehicles. In this topology, individual battery cells are connected to the dc link of the converter submodules, allowing the highest flexibility for the discharge and recharge. The traditional battery management system of battery electric vehicles is replaced by the control of the converter, which individually balances all the cells. The performance of the converter as a traction drive is assessed in terms of torque–speed characteristic and power loss for the full frequency range, including field weakening. Conduction and switching losses for the modular multilevel converter are calculated using a simplified model, based on the datasheet of power devices. The performance of the modular multilevel converter is then compared with a traditional two-level converter. The loss model of the modular multilevel converter is finally validated by experimental tests on a small-scale prototype of traction drive.
TL;DR: In this paper, the authors proposed a switched-boost dc-dc converter for high stepup voltage conversion applications, such as renewable energy power generation, uninterruptible power supply, and automobile high-intensity discharge headlamps.
Abstract: This paper proposes a family of switched-boost dc–dc converters for the high stepup voltage conversion applications, such as renewable energy power generation, uninterruptible power supply, and automobile high-intensity discharge headlamps. Compared with other dc–dc converters, the proposed switched-boost converter, which combines the traditional switched-boost network with the switched-capacitor/switched-inductor cells, has the following features: higher output voltage gain, a fewer passive components such as inductors and capacitors, and lower voltage stress across the output diode and power switches. Another advantage of the proposed topology is its expandability. If a higher voltage conversion ratio is required, additional cells can be easily cascaded by adding one inductor and three diodes. The structure, operating principle analysis, parameter design, and comparison with other dc–dc converters are also analyzed. Finally, both simulations and experimental results are presented to verify the effectiveness of the proposed converter.
TL;DR: In this article, a model predictive control (MPC) strategy that combines finite-control-set MPC with selective harmonic elimination (SHE) modulation pattern in its formulation is proposed to govern multilevel power converters.
Abstract: In this study, a model predictive control (MPC) strategy that combines finite-control-set MPC with selective harmonic elimination (SHE) modulation pattern in its formulation is proposed to govern multilevel power converters. Based on a desired operating point for the system state (converter current reference), an associated predefined SHE voltage pattern is obtained as a required steady-state control input reference. Then, the cost function is formulated with the inclusion of both system state and control input references. According with the proposed reference and cost function formulation, the predictive controller prefers to track the converter output current reference in transients, while preserving the SHE voltage pattern in steady state. Hence, as evidenced by experimental results, a fast dynamic response is obtained throughout transients while a predefined voltage and current spectrum with low switching frequency is achieved in steady state.
TL;DR: In this article, identical modular magnetic-links are proposed for high-power transmission and isolation between the low and the high voltage sides, which shows better frequency spectra as well as reduced switching loss.
Abstract: The high-frequency common magnetic-link made of amorphous material, as a replacement for common dc-link, has been gaining considerable interest for the development of solar photovoltaic medium-voltage converters. Even though the common magnetic-link can almost maintain identical voltages at the secondary terminals, the power conversion system loses its modularity. Moreover, the development of high-capacity high-frequency inverter and power limit of the common magnetic-link due to leakage inductance are the main challenging issues. In this regard, a new concept of identical modular magnetic-links is proposed for high-power transmission and isolation between the low and the high voltage sides. Third harmonic injected sixty degree bus clamping pulse width modulation and third harmonic injected thirty degree bus clamping pulse width modulation techniques are proposed which show better frequency spectra as well as reduced switching loss. In this paper, precise loss estimation method is used to calculate switching and conduction losses of a modular multilevel cascaded converter. To ensure the feasibility of the new concepts, a reduced size of 5 kVA rating, three-phase, five-level, 1.2 kV converter is designed with two 2.5 kVA identical high-frequency magnetic-links using Metglas magnetic alloy-based cores.
TL;DR: A hybrid control method combining pulse frequency modulation (PFM) and pulse width modulation is proposed to overcome the limited frequency resolution issue and improve voltage regulation performance for LLC resonant converters.
Abstract: High switching frequency is an effective method to improve power density for LLC resonant converters. However, conventional digital controllers, such as general-purpose digital signal processors and microprocessors, have limited frequency resolution, which induces high primary- and secondary-side current variation and leads to poor output voltage regulation. In this paper, a hybrid control method combining pulse frequency modulation (PFM) and pulse width modulation is proposed to overcome the limited frequency resolution issue. The proposed hybrid control method focuses on steady-state operation, and its operating principles are introduced and analyzed. In addition, the proper magnetizing inductance and dead time duration are derived to ensure that the power mosfet s achieve zero voltage switching with the proposed control method. The improved voltage regulation performance is compared with the conventional PFM control and verified through simulation and experimental results using a 240 W prototype converter operating at a switching frequency of 1 MHz.
TL;DR: In this article, a perturb-and-observve LPPT method is presented that enables the DPP converters to maintain optimal operating conditions, while reducing the total power loss and converter stress.
Abstract: Differential power processing (DPP) systems are a promising architecture for future photovoltaic (PV) power systems that achieve high system efficiency through processing a faction of the full PV power, while achieving distributed local maximum power point tracking (MPPT). In the PV-to-bus DPP architecture, the power processed through the DPP converters depends on the string current, which must be controlled to minimize the power processed through the DPP converters. A real-time least power point tracking (LPPT) method is proposed to minimize power stress on PV DPP converters. Mathematical analysis shows the uniqueness of the least power point for the total power processed through the system. The perturb-and-observe LPPT method is presented that enables the DPP converters to maintain optimal operating conditions, while reducing the total power loss and converter stress. This work validates through simulation and experimentation that LPPT in the string-level converter successfully operates with MPPT in the DPP converters to maximize output power for the PV-to-bus architecture. Hardware prototypes were developed and tested at 140 and 300 W, and the LPPT control algorithm showed effective operation under steady-state operation and an irradiance step change. Peak system efficiency achieved with a 140-W prototype DPP system employing LPPT is 95.7%.
TL;DR: Not only chattering problem is reduced significantly, but also the robustness, regulation, and dynamic response of the SMC system are not affected, and the derivation and implementation of the proposed SMC is so simple with less current sensors and without high computational burden.
Abstract: In three-phase transformerless uninterruptible power supply applications, a four-leg inverter is an excellent solution to handle the full unbalanced loading condition. A sliding-mode control (SMC) system is well known for robustness, simple implementation, high stability, and suitable solution for variable structure systems like power converters. However, it suffers from the chattering problem, variable switching frequency, and high electromagnetic compatibility noises. In this paper, a high performance SMC system is proposed to control the output voltage of the four-leg inverter with fixed switching frequency. In the proposed SMC system, as a considerable achievement, not only chattering problem is reduced significantly, but also the robustness, regulation, and dynamic response of the SMC system are not affected. Also, the derivation and implementation of the proposed SMC is so simple with less current sensors and without high computational burden. Furthermore, fixed switching frequency offers remarkable simplicity of the converter and filter design. Several experimental results on a 3-kW test bench verify the superior performance of the proposed SMC system according to the IEC62040-3 standard.
TL;DR: In this paper, an energy storage management (ESM) system based on fuzzy logic has been proposed and its performance with a proportional-integral (PI) control based ESM system is compared.
Abstract: In this paper, for supporting the medium voltage dc (MVDC) shipboard power system, an energy storage management (ESM) system based on fuzzy logic (FL) has been proposed and its performance with a proportional-integral (PI) control based ESM system is compared. In order to support the peak demand and pulsed load, a hybrid energy storage system incorporating high energy density storage (battery) and high power density storage (supercapacitor) is proposed. For energy transfer among the energy storages and the MVDC system, bidirectional dc–dc converters with dual active bridge (DAB) configuration are used. With the change of the bus voltage and load power demand, the ESM systems provide instantaneous reference powers for charging or discharging of the battery and supercapacitor. The reference powers for the battery and supercapacitor are sent to the respective controllers of the DAB converters. Two power sharing strategies are designed to share power among multiple energy storages. The MVDC shipboard power system with the generators, loads, battery, and supercapacitor with DAB converters are modeled in SimPowerSystems. Simulation results are used to make a comparison of performances of the FL and PI controller based ESM systems. Finally, controller hardware-in-the-loop based experimental results are added to demonstrate the effectiveness of the controller.
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TL;DR: A novel grid-forming converter control strategy which dwells upon the main characteristic of a SM: the presence of an internal rotating magnetic field is proposed, and a virtual oscillator is augmented whose frequency is driven by the DC-side voltage measurement and which sets the converter pulse-width-modulation signal.
Abstract: We consider the problem of grid-forming control of power converters in low-inertia power systems. Starting from an average-switch three-phase inverter model, we draw parallels to a synchronous machine (SM) model and propose a novel grid-forming converter control strategy which dwells upon the main characteristic of a SM: the presence of an internal rotating magnetic field. In particular, we augment the converter system with a virtual oscillator whose frequency is driven by the DC-side voltage measurement and which sets the converter pulse-width-modulation signal, thereby achieving exact matching between the converter in closed-loop and the SM dynamics. We then provide a sufficient condition assuring existence, uniqueness, and global asymptotic stability of equilibria in a coordinate frame attached to the virtual oscillator angle. By actuating the DC-side input of the converter we are able to enforce this sufficient condition. In the same setting, we highlight strict incremental passivity, droop, and power-sharing properties of the proposed framework, which are compatible with conventional requirements of power system operation. We subsequently adopt disturbance decoupling techniques to design additional control loops that regulate the DC-side voltage, as well as AC-side frequency and amplitude, while in the end validating them with numerical experiments.
TL;DR: The most significant factor investigated in the work presented in this study is the viability of VSM algorithms during the kind of abnormal operation that might raise instability issues with respect to practical discrete time operation.
Abstract: The substantial potential for the integration of renewable energy into power systems using power electronics converters might result in stability issues because of a lack of inertia. For this reason, this study introduces the concept of a virtual synchronous machine (VSM) control algorithm that emulates the properties of traditional synchronous machines. The literature includes references to several differently structured control algorithms. However, synchronous machine inertia and damping characteristics must be mimicked, which makes the cost and simplicity of implementation important from an economic perspective. This study presents a comprehensive comparison of VSM control algorithms. The most significant factor investigated in the work presented in this study is the viability of VSM algorithms during the kind of abnormal operation that might raise instability issues with respect to practical discrete time operation. The test system used in this study, which was simulated in a PSCAD/EMTDC environment, consisted of simulated voltage source converters based on a fully detailed switching model with two AC voltage levels. The results indicate a significant outcome that can facilitate a determination of the most effective VSM control algorithm.
TL;DR: In this article, a three-level space vector modulation (SVM) scheme was proposed for a system with two paralleled voltage-source converters (VSCs) with common-mode inductor (CMI) or single-phase inductors.
Abstract: For high-power applications, paralleling converters is a popular approach to increase the power capacity of the system. Circulating current has been a major concern for the implementation of paralleled converters. This paper proposes a three-level space vector modulation (SVM) scheme for a system with two paralleled voltage-source converters (VSCs) with common-mode inductor (CMI) or single-phase inductors. The proposed scheme aims to reduce the zero-sequence circulating current (ZSCC) and the magnitude of common-mode voltage (CMV) of the system simultaneously. The ZSCC patterns with respect to modulation schemes are first analyzed to provide a clear understanding of the generation of ZSCC. Based on the analysis, the proposed three-level modulation scheme is introduced. Furthermore, performance regarding the ZSCC peak value, impact on the common-mode current (CMC), CMI scaling analysis, and switching losses are analyzed and compared with the existing methods. The proposed method has been verified in both simulation and experiment.
TL;DR: The aim is to enhance the power quality at the point-of-common-coupling of a microgrid, improve voltage profile through the lines, and reduce the overall distribution losses.
Abstract: This paper proposes an effective technique to control the power flow among different phases of a three-phase four-wire distribution power system by means of single-phase converters arbitrarily connected among the phases. The aim is to enhance the power quality at the point-of-common-coupling of a microgrid, improve voltage profile through the lines, and reduce the overall distribution losses. The technique is based on a master/slave organization where the distributed single-phase converters act as slave units driven by a centralized master controller. Active, reactive, and unbalance power terms are processed by the master controller and shared proportionally among distributed energy resources to achieve the compensation target at the point-of-common-coupling. The proposed control technique is evaluated in simulation considering the model of a real urban power distribution grid under non-sinusoidal and asymmetrical voltage conditions. The main results, concerning both steady-state and transient conditions, are finally reported and discussed.
TL;DR: In this article, the authors investigated parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposed a novel control strategy to enhance the active power transfer capability with zero active power oscillation.
Abstract: Today, interests on hybrid ac/dc microgrids, which contain the advantages of both ac and dc microgrids, are growing rapidly. In the hybrid ac/dc microgrid, the parallel-operated ac/dc bidirectional interfacing converters (IFCs) are increasingly used for large capacity renewable energy sources or as the interlinking converters between the ac and dc subsystems. When unbalanced grid faults occur, the active power transferred by the parallel-operated IFCs must be kept constant and oscillation-free to stabilize the dc bus voltage. However, under conventional control strategies in unbalanced grid conditions, the active power transfer capability of IFCs is affected due to the converters’ current rating limitations. Moreover, unbalanced voltage adverse effects on IFCs (such as output power oscillations, dc-link ripples, and output current enhancement) could be amplified by the number of parallel converters. Therefore, this paper investigates parallel operation of IFCs in hybrid ac/dc microgrids under unbalanced ac grid conditions and proposes a novel control strategy to enhance the active power transfer capability with zero active power oscillation. The proposed control strategy employs a new current sharing method which introduces adjustable current reference coefficients for parallel IFCs. In the proposed control strategy, only one IFC, named as redundant IFC, needs to be designed and installed with higher current rating to ensure the constant and oscillation-free output active power of parallel IFCs. Simulation and experimental results verify the feasibility and effectiveness of the proposed control strategy.
TL;DR: In this article, a very simple geometric structure whose branches can be filled by inductors, capacitors, diodes, short-circuits, or opencircuits is proposed.
Abstract: A very simple geometric structure whose branches can be filled by inductors, capacitors, diodes, short-circuits, or open-circuits is proposed. It serves for generating large dc gain-purposed switching cells by making different choices of the type of component on each branch. The switching cells are integrated in basic converters. It is shown that almost all the high dc gain nonisolated converters based on switched-capacitor-inductor cells proposed in the last years, regardless of their complexity, can be derived through this method. From the same geometric structure, new high dc gain boosting converters can be derived in a systematic manner. The available and the new converters in each class as defined by the number of reactive components are compared in terms of their performance: dc gain, semiconductor elements count, voltage and current stress on transistors and diodes, character of the input current, easiness of the transistor driving, and easiness of the control as determined by common/uncommon line-load ground, power stage efficiency. This comparison allows us to choose the optimal solution for each specific application in conjunction with the green sources of energy, multisource microgrids, electric vehicles, data and communications systems, and so on. The geometric structure is generalized in different ways, allowing for the development of ultrahigh dc gain converters. One of the proposed generalized ultrahigh dc gain converters is fully analyzed and built in the laboratory, with the experimental results verifying the theoretical analysis.
TL;DR: An improved FCS-MPC algorithm with fast computation and fixed switching frequency is proposed in this paper for two-level three-phase inverters, which greatly improves the computation efficiency and the delay due to digital implementation is effectively compensated in the proposed algorithm.
Abstract: Finite control set-model predictive control (FCS-MPC) has been used in power converters due to its advantages, such as fast dynamics, multi-objective control, and easy implement. However, due to variable switching frequency, the harmonics of inverter output current spread in a wide range of frequency. Furthermore, a large amount of computation is required for the implementation of the traditional FCS-MPC method. Here, an improved FCS-MPC algorithm with fast computation and fixed switching frequency is proposed in this paper for two-level three-phase inverters. First, according to the principle of deadbeat control, the inverter voltage vector reference can be constructed. Then, the operation durations and sequences of different voltage vectors are determined according to the location of the inverter voltage vector reference and the cost functions of different voltage vectors. In this algorithm, the operation durations of different voltage vectors are arranged inversely proportional to their cost functions. Compared with the conventional fixed switching frequency FCS-MPC control, the number of sectors involved in the FCS-MPC calculation can be reduced from 6 to 1, which greatly improves the computation efficiency. Moreover, the delay due to digital implementation is effectively compensated in the proposed algorithm. Finally, experimental tests are carried out to verify the advantages of the proposed method in terms of both steady-state and dynamic performance.
TL;DR: The proposed control strategies enable decoupled operation of the converter, provide maximum power point tracking capability at the generator side, grid code compliance at the grid side (including low-voltage ride-through control) and good steady state and dynamic performance for balancing the capacitor voltages in all the clusters.
Abstract: The nominal power of single wind energy conversion systems (WECS) has been steadily increasing, now reaching power ratings close to 10 MW. In the power conversion stage, medium-voltage power converters are replacing the conventional low-voltage back-to-back topology. Modular multilevel converters have appeared as a promising solution for multi-MW WECSs, due to their modularity and the capability to reach high nominal voltages. This paper discusses the application of the modular multilevel matrix converter to drive multi-MW WECSs. The modeling and control systems required for this application are extensively analyzed and discussed in this paper. The proposed control strategies enable decoupled operation of the converter, provide maximum power point tracking capability at the generator side, grid code compliance at the grid side (including low-voltage ride-through control) and good steady state and dynamic performance for balancing the capacitor voltages in all the clusters. Finally, the effectiveness of the proposed control strategy is validated using simulation and through experimental results obtained with a 27-power-cell prototype.
TL;DR: In this article, the authors proposed a battery charging system that integrates a non-isolated on-board charger and low-voltage dc-dc converters by sharing the semiconductor devices and mechanical elements.
Abstract: Power conversion systems for electric vehicles (EVs) have been researched to improve power density and efficiency at low cost. To satisfy these needs for EVs, this paper proposes a novel battery charging system that integrates a nonisolated on-board charger (OBC) and low-voltage dc–dc converters (LDCs) by sharing the semiconductor devices and mechanical elements. Thus, the volume of LDCs is reduced dramatically compared with a conventional nonintegrated charging system. The proposed integrated system is configured based on a driving condition that is derived from the analysis of vehicle operating modes. In order to improve system's performance, an asynchronous control algorithm is applied to control the OBC optimally. In the LDC system, two LLC resonant converters are composed by sharing a transformer and secondary-side components. To increase the efficiency of each LDC, which is operated in the wide input and output voltage range, a duty and frequency control algorithm is proposed. The theoretical analysis, operating strategy, and experimental results on a 6.6-kW OBC and 1.9-kW LDC are presented to evaluate the performance of the proposed system; the total volume of LDCs is 1.87 L, and peak efficiencies of OBC and LDC are 97.3% and 93.13%, respectively. Moreover, a comparative analysis is presented to evaluate the performance of the proposed system.