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The railway power conditioner-based energy storage system (RPC-based ESS) is a promising technology to improve the regenerative braking energy (RBE) utilization and power quality of ac direct-fed (25 kV) and AT-fed (2 × 25 kV) railway power systems. However, despite its benefits from the technical perspective, economic viability must be concerned in industrial applications. The capacity is one of the critical issues affecting ESS performance in technical and economic aspects. Therefore, this paper presents a comprehensive approach to designing the capacity for the RPC-based ESS by considering multiple objectives, including the RBE utilization rate, power quality standard, and net lifetime benefit. The proposed approach develops a technical-economic model for the RPC-based ESS based on variables such as the system capacity for RBE utilization and power quality improvement, lifetime cost, and electric charge-based benefits. Moreover, the master-slave control strategy is employed to control the RPC-based ESS to implement the capacity design objectives. Finally, the correctness and effectiveness of the proposed method are validated by sufficient tests based on field load data of a Chinese traction substation. [ABSTRACT FROM AUTHOR]

Three-port converters with high voltage gain are desirable solutions for integrating renewable energy and energy storage devices into high voltage dc bus. A high gain three-port converter with soft switching is proposed in this paper, which can realize zero voltage switching for all switches and zero current switching for all diodes in various operating modes. Single coupled inductor is used to achieve high voltage gain and to reduce the voltage stress of switches so that switches with low on-resistance can be selected to reduce the conduction loss. In addition, the advantages of fewer components, higher voltage gain, and very low switch voltage stresses make the proposed converter more suitable for application in renewable energy systems than similar solutions. Various operating modes, performance analysis, design considerations, efficiency analysis, and control method of the proposed converter are discussed. A laboratory prototype with 30 V renewable energy source, 48 V energy storage device and 400 V output is designed to verify the performance of the proposed three-port converter. [ABSTRACT FROM AUTHOR]

In this article, the magnetic energy harvester (MEH) based on the current transformer is an innovative method to provide a potential solution for the power supply of sensor networks. Due to the current fluctuation and nonlinearity of the core, the harvester may produce insufficient power at low primary currents, while the core saturates at high primary currents. Aiming at resolving this problem, a novel method is proposed to improve the performance of MEH. In this method, a capacitor and two switches are used to store energy and modulate the core saturation. The performed analyses demonstrate that the proposed method enhances the harvested power under different primary current and load conditions. This is especially more pronounced at low primary currents and loads. It is found that for a low primary current of 2 Arms at 50 Hz, the harvested power increases by 206%. Moreover, applying this method can also reduce the load voltage by controlling the alternating on–off of the switches, thereby protecting the harvester in case of high primary currents. The theoretical calculation, circuit simulation, and experimental results demonstrate the effectiveness of this method. [ABSTRACT FROM AUTHOR]

A series of three-port forward converters with a compact structure and extended duty cycle range is presented for the standalone renewable energy system. Bidirectional power-flow port embedding and switch device sharing can be used to derive these converters from conventional two-switches forward converters. In comparison to the conventional two-switch forward converter, only one switch and one diode are added to build an energy storage port with bidirectional power flow characteristics, which has the benefits of compact structure and low cost. Meanwhile, the embedding of the energy storage port provides an additional demagnetization loop for the magnetic inductor, which extends the duty cycle range of the forward converter so that the proposed converter no longer requires a duty cycle of less than 0.5 as in the case of conventional forward converters. The proposed converters also offer the advantages of fewer components, easy control, and independent control of power flow between multiple ports when compared to similar solutions. Finally, an experimental setup with a photovoltaic battery as an application is developed, and the experimental results verify the effectiveness and flexibility of the proposed topology and control method. [ABSTRACT FROM AUTHOR]

This article presents a multiobjective control scheme for a bidirectional interlinking converter (ILC) of renewable energy based ac–dc hybrid microgrid. The prime focus of the power management algorithm is to feed the ac loads, and dc loads from ac source, and photovoltaic battery energy storage (BES), respectively. The second objective is to utilize the solar power to maximum capacity, by feeding it to dc load, BES, and ac load, in the given order of priority. The ac and dc subgrids operate independently, while supporting each other in case of overloading/underloading, thus reducing the amount of power reserve required in each, and eliminating the need of dump loads. The ANFIS-MN-based power quality control is integrated in order to ensure the total harmonics distortion of ac source current remains below 5% as per IEEE 519 standard, irrespective of the nature of load connected. In addition, the ILC control prevents any negative sequence currents from entering the ac source, even if there is any open circuit fault leading to unbalanced loading on the three phases. The performance of ANFIS-MN-based ILC control is demonstrated to be superior when compared with the conventional and contemporary techniques under different scenarios. [ABSTRACT FROM AUTHOR]

The rapid development of Internet-of-Things in smart grid has enabled millions of grid-connected distributed controllable resources (DCR; e.g., electric vehicles, controllable loads) to provide service to the grid, such as frequency regulation and demand response. The integration of these DCRs may become a large virtual power plant network with various characteristics. This poses great challenges from both control and management perspectives, e.g., computation/communication burden, optimization complexity, scalability limitation, prosumer privacy, etc. In this article, we propose an effective autonomous incentive-based DCR control and management framework to integrate a large amount of DCRs to provide grid services, which simultaneously provides accurate active power adjustment to the grid, optimizes DCR allocations, and maximizes the profits for all prosumers and system operators. A model-free deep deterministic policy gradient-based method is designed to find the optimal incentives in a continuous action space to encourage prosumers to adjust their power consumptions. The method is implemented in a consortium open-source blockchain platform, Hyperledger Fabric, which facilitates controls and transaction management. To demonstrate the effectiveness of the framework, extensive experimental studies are conducted using real-world data. [ABSTRACT FROM AUTHOR]

In a battery energy storage system (BESS) based on the cascaded H-bridge (CHB) converter or modular multilevel converter (MMC), the active power distribution among the submodules (SMs) can be highly unbalanced, especially when batteries with different power ratings are used. Meanwhile, there are certain limits for the unbalanced power distribution among the SMs. It is a challenge to determine whether the active power distribution is viable for cascaded multilevel converters with unequal capacitor voltages because the complexity grows exponentially as the number of SMs increases. This article proposes a generalized analytic solution to this problem, which reduces the complexity significantly and is readily applicable to converters with large number of SMs. The validity of this method is proved theoretically and verified with simulation and experimental results on both the MMC and CHB converter. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the grid connection of offshore wind farms through modular multilevel converter–high voltage direct current (MMC-HVdc) transmission system has brought low inertia issue. It is an urgent requirement for offshore wind farms to compensate for the inertia. After analysis and evaluation in this article, it is found that integrating an ultracapacitor energy storage system (UESS) into an MMC has an advantage comprehensively considering cost, size, and dynamic. Based on the evaluation, this article proposes an approach to inertial compensation of HVdc offshore wind farms by MMC-UESS integration. An ultracapacitor is selected as an energy storage element (ESE) for its high power density since the inertial response is a short-term response and it is hoped that added ESE does not significantly increase the size of an MMC submodule. Batteries with high energy density but low power density are not suitable for such applications. The optimal operating voltage of a UESS and the control scheme between MMC loops and inertial emulation are proposed in this article. Experimental results from real-time simulation and a downscaled prototype are presented to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A typical flywheel energy storage system (FESS) includes an electrical machine, a flywheel, and magnetic bearings, which are independent of each other. Therefore, the structure of FESS is complicated, which leads to the problems of high cost and low integration. This article presents a novel stator PM excitation solid rotor machine (SPE-SRM) for FESS, which integrates an electrical machine, flywheels, and a magnetic bearing. The design method of the SPE-SRM is presented specifically. First, the structure and operation principle of the SPE-SRM are illustrated. Second, the design and optimization of the flywheel-rotor are presented with considering of power, energy storage, and mechanical strength. Third, the magnetic equivalent circuit of the SPE-SRM is established to simplify the design. Besides, the armature reaction and leakage flux factor are investigated. Fourth, the performance indexes, including torque, suspension force, no-load back electromotive force (back-EMF), losses, and temperature rise are fully analyzed by the finite element analysis. Finally, a 22 kW prototype is manufactured and tested. The rationality of the concept and design is validated by experiments. The SPE-SRM has the merits of high integration, robust structure, and low cost, so it has the potential for wide application in FESS. [ABSTRACT FROM AUTHOR]

This article proposes a two-stage power converter with novel control methods for pulsed load applications. The first stage is an isolated converter that transfers only average power to the second stage, which dramatically reduces the input filter size and components’ current rating. The second stage is a buck converter designed for a fast response during pulsed load transients. A flexible intermediate voltage is implemented to reduce the size of the midpoint energy storage capacitor that is responsible for compensating the instantaneous power difference between the two stages. A novel digital input feed-forward controller is proposed for the second stage to eliminate the poor line regulation issue caused by the flexible intermediate voltage. To verify the effectiveness of the two-stage power supply and the control methods, a full-scale 800 W (average)/4 kW (peak) converter prototype is built and tested. The results prove the feasibility of the proposed topology as well as control methods and demonstrate the advantages over traditional concepts. [ABSTRACT FROM AUTHOR]

The system stability and injection of maximum active power to the grid are among the most important technical issues in cascaded H-bridge (CHB) based photovoltaic (PV) systems, especially when the irradiation of PV arrays connected to CHB cells are not uniform. In other words, the unequal irradiation of PV arrays leads to the unbalanced output power of H-bridge cells, which causes overmodulation and increased grid current total harmonic distortion in the CHB inverter. Conventional control methods cannot maintain the stability of a grid-connected CHB inverter under asymmetric operating conditions. So, this article proposes a new CHB-based structure with reserve batteries and a novel modulation technique to stabilize the system in heavy mismatching conditions and avoid overmodulation. Furthermore, the number of switches and series batteries is reduced, and the number of output voltage levels is increased compared to the conventional PV-CHB system with energy storage. The validity of the proposed solution is verified by simulation in MATLAB/Simulink, and then the experimental results are presented to confirm the theoretical and simulation results. [ABSTRACT FROM AUTHOR]

Cascaded split-source inverter (CSSI) is a new single-stage modular multilevel topology. Each cell of this converter converts dc to ac power in the buck or boost operation mode without any additional power switch. This article develops a design method based on a detailed model for CSSI. This model shows that energy storage elements experience double-fundamental frequency ripples besides high-frequency ones. It accurately calculates voltage gains and capacitor voltage and inductor current ripples. On the other hand, common carrier-based multilevel modulations have been modified in terms of both reference and carrier signals to control the inverter under symmetric and asymmetric conditions. With proposed simple modulations, the cell inductor is charged with the constant duty cycle. Also, the reverse recovery of diodes occurs in the minimum level of input current once per switching cycle and output power is distributed equally between cells. These characteristics reduce power losses, the input inductance of each cell, and the complexity of the system model. By phase-shifted pulsewidth modulation (PWM), the output voltage harmonics of three-cell CSSI are centralized around multiples of the triple switching frequency. In this method, high-frequency ripples on the network impedance are one-third of those in level-shifted PWM. Also, CSSI has been compared with cascaded multilevel inverters with voltage boosting ability and other SSI-based multilevel topologies. Validity of operation principles, mathematical relations, and modulation schemes are verified through simulation and experimental results. [ABSTRACT FROM AUTHOR]

In islanding microgrids, supercapacitors (SCs) are used to compensate the transient power fluctuation caused by sudden variations of load demand and generation power to keep the output voltage stable and reduce the stress in batteries. However, SC current in dynamic response leads to transient power loss on power electronic converters, and it would cause an additional voltage ripple. To smoothen the voltage fluctuation, a dual-layer model predictive control (MPC) method is proposed in this article to control the charging/discharging behaviors efficiently. The dynamic power loss can be predicted by the predicted current and duty ratio in the primary layer MPC. The predicted dynamic power loss is one of the state variables in the secondary layer MPC to generate the more optimal power reference for the primary layer MPC, which can compensate the voltage ripple caused by the dynamic power loss and reduce the dynamic error and settling time. The proposed method is validated by simulation and hardware experimental results, demonstrating significant improvements than other methods. [ABSTRACT FROM AUTHOR]

State-of-health (SOH) is crucial to the maintenance of various kinds of energy storage systems, including power batteries. Relevant research articles are mostly based on battery external information, such as current, voltage, and temperature, which are susceptible to fluctuation and ultimately affects the SOH estimation accuracy. In this article, to solve these problems, a fast impedance calculation-based battery SOH estimation method for lithium-ion battery is proposed from the perspective of electrochemical impedance spectroscopy (EIS). The relationship between EIS and state of charge and that between EIS and degraded capacity is first studied by experimental tests. Some impedance features called health factors effectively indicating battery aging states are selected. Second, an improved fast Fourier transform (FFT) utilizing the conversion relationship between the real and complex signals is proposed to realize online fast EIS acquisition. Compared with ordinary FFT, such treatments reduce computational complexity. Then, the SOH evaluation model is built by the extreme learning machine with regularization mechanism, further reducing the computational burden. The relationship between the health factors and aging capacity of batteries is established. Finally, an experimental bench is established. The results indicate that the estimated SOH can be obtained within 35 s for a four-cell series-connected battery pack and the estimation errors are less than 2%. [ABSTRACT FROM AUTHOR]

In this article, an interleaved coupled-inductor (CI) based bidirectional dc–dc converter (BDC) is proposed with a higher voltage conversion ratio (VCR). In this proposed interleave CI-based BDC (ICI-BDC), the CIs can operate as both a filter inductor and a transformer simultaneously, so that the power density is improved. Moreover, as a transformer, the turns ratio of CIs is used to improve the VCR with a voltage stress reduction purpose. Furthermore, the interleaved technique can well achieve the low current ripple and reduced current stress in this topology. To control ICI-BDC, the pulsewidth modulation plus phase-shift (PPS) control is employed, where the duty cycle is used to match the two side voltages, and the phase-shift angle is used to control the amount and direction of power flow. Uniquely, a difference value is set between the two side duty cycles, and the duty cycle of the high voltage side is smaller than that of the low voltage side by a fixed value. In this proposed control method, all the switches can achieve soft-switching even at light load condition, which can improve transmission efficiency. Experimental results obtained from a 1-kW prototype show a nearly high efficiency under all loading conditions, validating the viability of the proposed ICI-BDC topology and control method. [ABSTRACT FROM AUTHOR]

Hybrid energy storage microgrid represents the future direction of distributed energy development. However, under the influence of interference, the dynamic performance of the output side interface will be severely deteriorated. In this article, a linear active disturbance rejection controller with reference differential feedforward and cascade linear extended state observer (ADRC-RDCO) is proposed to deal with unknown modeling error and the unknown dynamics of the interface converter. In this strategy, a cascaded linear extended state observer is introduced to cope with the steady-state offset in the disturbance estimation error, so as to implement the accurate reconstruction of the disturbance state. Furthermore, a control law with RDF is applied to offset a portion of the tracking error of the given signal to achieve the purpose of optimizing the tracking accuracy of the given signal. Then, the tracking performance and stability of ADRC-RDCO are analyzed theoretically. On the basis of discussing the relationship between dynamic characteristics and control parameters, a parameter configuration method is proposed. Finally, the proposed ADRC-RDCO technology was verified on the 40 kW microgrid platform. [ABSTRACT FROM AUTHOR]

In the operation of cascaded H-bridge converters and modular multilevel converters with energy storage or renewable power resources, unbalanced active power distribution among the submodules (SMs) is unavoidable. Depending on the operating conditions, there are certain upper and lower limits on the active power that can be processed by a single SM or a subset of SMs. The control system needs to restrict the SM power references to these limits, hence, accurate knowledge of the power limits is important. In existing methods to derive the power limits, the SM capacitor voltages are assumed to have negligible ripples, whereas in practice the ripples can be considerable. This article analyzes the effect of capacitor voltage ripples on the SM active power control limits and highlights the importance of considering the ripple effect. A methodology is proposed to accurately incorporate capacitor voltage ripples in the derivation of SM active power control limits. Simulation and experimental results are provided to evaluate the effectiveness of the proposed methodology. [ABSTRACT FROM AUTHOR]

This article presents an improved Gaussian filter with phase-locked loop based grid monitoring and seamless transfer control for solar photovoltaic-battery energy storage (PV-BES) based microgrid with improved power quality. The PV-BES microgrid comprises of a PV array and a battery, interfaced to the utility through a voltage source converter, and performs multiple tasks. In grid-connected mode, the improved Gaussian filter with quadrature signal generator-based second-order generalized integrator based prefiltering provides solar PV power to the nonlinear loads and the utility grid, along with providing reactive power compensation and power quality improvement at the point of common coupling. To regulate the power absorbed or discharged by the BES, a bidirectional converter is used. The PV feed-forward term enables PV power injection and also improves the dynamic performance of the system. On grid outage, the controller transits the PV-BES microgrid to standalone mode of operation ensuring uninterrupted supply to the load. The effectiveness of the controller is tested in various scenarios and is verified through experimentation. The controller has satisfied the specifications set by the IEEE 519-2014 power quality standard and IEEE 1547-2018 (revised) standard for interconnection and interoperability of distributed energy resources. [ABSTRACT FROM AUTHOR]

In order to solve the problems of existing three-port isolated dc-dc converter such as power decoupling and the nonzero instantaneous power of the standby port, a three-port isolated bidirectional current-fed series-resonant dc-dc converter is proposed in this article. The operational principle and general transfer power expressions of various ports are analyzed with the duty-cycle plus phase-shift control. Then, the power decoupling at the topology level is realized by setting the switching frequency equal to the resonant frequency of the resonant tank on the high-voltage side; thus, the transfer power of the two ports on the low-voltage sides can be controlled independently. Moreover, it is proved that the proposed converter realizes the zero instantaneous power of the standby port. On this basis, an optimized modulation strategy by taking the minimum root mean square value of the resonant current as the objective function is presented. The exact relationship between the desired transfer power and various phase shifts is accordingly obtained. The detailed experimental results verify the correctness of the proposed converter and corresponding modulation strategy. [ABSTRACT FROM AUTHOR]

This article aims to develop a solar- battery energy storage (BES) based system, which ensures an uninterruptable supply to loads irrespective of availability of the grid. This system comprises of a solar photovoltaic (PV) array, a BES, the grid and local residential loads. A new control is implemented such that the active power demand of residential loads, is fed from the PV array, a BES unit and the utility grid. In this system, the power control operates in different power modes, which delivers the benefits to the end users with an integration of BES and an excess of PV array power, which is sold back to the grid. For this, an effective control logic is developed for the grid tied voltage source converter. Moreover, this system deals with the issue of an integrating power quality enhancement along with the power generation from the solar PV source. The cascaded delayed signal cancellation based phase locked loop is implemented for grid synchronization during the grid voltage distortion. The developed control is easily implemented in a real time controller (dSPACE-1202). Test results validate the performance of the implemented control in different operating conditions such as varying solar power generation, load variations and unavailability of the grid. [ABSTRACT FROM AUTHOR]

This article presents a high gain pure sine- wave inverter based on the full-bridge dc–ac high-frequency link cycloconverter topology for telecom or general-purpose applications. The improved quasi-resonant modulation method allows reduction of ringing and turn-off losses of the dc-side switches. This is achieved with minimal energy circulation and requires no multi-mode operation or extra auxiliary clamping circuits. The soft switching can be provided even if relatively large lossless snubber capacitors are connected across the input side transistors. Moreover, two of the switches at the ac side operate at fundamental frequency, while the rest feature zero current turn-off. A 48 VDC to 230 VAC, 1.2 kW sine-wave inverter prototype was developed to verify the proposed concept. [ABSTRACT FROM AUTHOR]

The grid-connected quasi-single-stage converter (QSSC) provides a direct power flow path from low-voltage (LV) energy storage systems (ESS) to ac–dc converters, resulting in reduced power conversion stage for high-efficiency ESS integration. However, due to the dc-link voltage ratio between the two ports varies with the ESS state of charge, the voltage vectors in the ac side are unevenly distributed in the QSSC. This nonlinearity brings some challenges in control and modulation design for the ac–dc part of QSSC. In this article, a virtual two-level converter model considering the two ports as a single dc source is proposed, and model predictive power control (MPPC) based on the virtual two-level model is proposed to enhance the grid current performance. The duty cycle of each phase is calculated based on the active and reactive power tracking error minimization in the two-level converter frame, and the duty cycle for each switching pair is obtained by minimizing the power processed by the dc–dc converter. In this way, the complicated implementation of MPPC for a three-level converter is avoided, and the controller design of the QSSC can be simplified. The effectiveness of the proposed virtual two-level converter model and MPPC method are verified with experimental results. [ABSTRACT FROM AUTHOR]

This article proposes an optimal current control technique with switching event minimization for grid-interactive cascaded multilevel inverters (CMI) interfaced with battery energy storage sources. The proposed control scheme enables power-balancing functionality of battery cells, realizing optimal smart operation of CMI. Model predictive control (MPC) is known as a potential approach for multiobjective control schemes in single-loop manner for power electronics interfaces. However, MPC schemes are suffering from high computational burden that is magnified in topologies like the CMI, which have a substantial number of redundant control actions. The proposed control scheme utilizes a dynamic lookup matrix as an internal optimizer tool. The redundant switching sequences are cycled to equalize the power drawn from the independent battery energy storage sources while achieving a minimum energy control. The theoretical analysis and experimental case studies verify robustness and computational efficiency of the proposed multicriteria optimal controller with similar objective tracking when compared to finite-set MPC. [ABSTRACT FROM AUTHOR]

The use of dc shipboard power distribution networks has been shown to offer several advantages over their ac counterparts. The reduction of the size and number of system components, the increased simplicity of operation and fuel efficiency of the system, and convenient integration of energy storage systems offer significant economical benefits to the ship operator. Solid-state bus-tie switches are widely regarded as being an indispensable component in dc shipboard power distribution networks, for their role as first line of defense against faults. Nevertheless, these devices are not limited exclusively to protection, but can provide additional functionality thanks to their design. This article shows how a solid-state bus-tie switch can be employed for soft-starting an unenergized section of a shipboard power distribution network, eliminating the need for additional components aimed specifically at this task. A technique is suggested and its performance is experimentally verified in a laboratory scaled-down model of a dc shipboard network. The additional functionality is achieved exclusively on a control basis, without modification of the switch topology initially intended purely for system protection. [ABSTRACT FROM AUTHOR]

An integrated boost-LC-resonant-flyback multimode converter with a wide voltage gain is proposed in this article. With three different pulsewidth modulation strategies, the proposed converter can realize three modes including high-, medium- and low-voltage-gain (LG), by multiplexing inductors and power switches. In the high-voltage-gain mode, it has better load characteristics than the conventional LLC converter through an extra energy storage stage. In the medium-voltage-gain mode, the working condition is similar to that of the conventional LLC converter, but the issues of current reflow and circulation can be addressed than reducing the conduction loss. In the LG mode, the proposed converter works as a flyback converter, and it has a higher conversion efficiency and simpler operation state than the conventional LLC converter. Additionally, all power switches can realize zero-current switching over a wide load range. The operating principle, voltage gain, design methodology, and control scheme are fully presented in this article. Finally, a laboratory prototype with 110 V input voltage, 0–5 A output current, and 24–165 V output voltage was conducted and verified the proposed solution. [ABSTRACT FROM AUTHOR]

Voltage sag and swell affect the normal operation of loads as well as distributed generating sources and reduce the overall reliability of the system. For improving the voltage at point of common coupling, different grid code requires control of active and reactive powers through power electronic solutions. In this article, a smart power converter (SPC) is introduced in the distribution grid, and different from the existing voltage support solutions, this article utilizes control and coordination capabilities of SPC to provide medium voltage (MV) grid voltage support during voltage sag/swell. The proposed control scheme aims to improve the voltage support capability of SPC in the MV grid by the coordinated operation of SPC MV converter, low voltage converter, and battery energy storage system (BESS). For achieving the same voltage support capability, the proposed method requires lower rated BESS as compared to existing solutions. Simulation and experimental results show the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) feature an established topology for the highest voltage and power levels. For lower levels, MMCs find a wide range of applications in generation, energy storage, and electric motor drive. However, the large quantity of expensive voltage sensors as well as high data-processing requirements for the module voltage balancing remains an obstacle. Additionally, the stark cost reductions of power electronic semiconductors have reduced their share in the total cost and rendered initially uneconomical module topologies appealing in low and medium voltages. In this article, we proposed an MMC topology with an active clamping path that balances the voltage of the modules in an arm. The proposed structure can achieve sensorless voltage balancing with a minimum control effort. The sensorless operation has the added benefit of reducing the communication as well as computation demand between the modules and the central controller. As compared with the state-of-the-art voltage balancing schemes, the proposed solution achieves a balanced system operation with less power loss, fewer components, and without any measurement. The presented analysis as well as the simulation and experimental results verify the benefits of the proposed topology. The results demonstrate that even in the face of a severe imbalance, all the module voltages are maintained within tight boundaries. [ABSTRACT FROM AUTHOR]

Energy storage is an emerging technology that can enable the transition toward renewable-energy-based distributed generation, reducing peak power demand and the time difference between production and use. The energy storage could be implemented both at grid level (concentrated) or at user level (distributed). Chemical batteries represent the de facto standard of storage systems for performance and maturity; however, batteries feature a quite large environmental footprint and use precious raw materials. Mechanical storage technologies could represent a viable alternative to chemical batteries, because of their reduced impacts on the environment and on raw materials. This article presents the design of a motor/generator for a flywheel energy storage at household level. Three reference machines were compared by means of finite element analysis: a traditional iron-core surface permanent-magnet (SPM) synchronous machine, a synchronous reluctance machine (SynchRel), and an ironless SPM synchronous machine. Simulation shows that the ironless machine is good candidate for distributed energy storage, because of its high efficiency, high discharge duration, and low losses. A prototype ironless machine was designed and manufactured. Experiments confirm the simulation results. [ABSTRACT FROM AUTHOR]

In microgrids, intermittency of renewable energy sources (RES) and uncertain state-of-charge (SoC) of energy storage systems (ESS) can cause power deficiency to some distributed generation units (DGs). In this case, DGs with power deficiency may not meet the power demand, resulting in voltage collapse or frequency divergence. Unfortunately, this is seldom considered in inverter control design in existing literature. Thus, in-depth investigation into the microgrid performance under renewable energy resource fluctuations and appropriate control methods are urgently needed. In this article, an adaptive droop and adaptive virtual impedance control strategy is proposed. Unlike conventional droop control where the droop coefficients are fixed by assuming the DGs can always meet the load demand, the droop coefficients here are adjusted according to actual solar PV power output. In this way, proper power sharing among DGs can be achieved under renewable energy variation. Furthermore, the impact of varying DG capacities on system stability is mathematically investigated. An adaptive virtual impedance is then incorporated into the adaptive droop method to deal with the system instability caused by renewable energy variations. The proposed strategy is analyzed theoretically and validated in MATLAB/Simulink simulation and laboratory experiments. The results demonstrate the advantages of the proposed method over conventional approaches under various scenarios. [ABSTRACT FROM AUTHOR]

A multilevel topology for photovoltaic (PV) systems with integrated energy storage (ES) is presented in this article. Both PV and ES power cells are connected in series to form a dc link, which is then connected to an H-bridge to convert the dc voltage to an ac one. The main advantage of the proposed converter compared to the cascaded-H-bridge (CHB) converter, as well as compared to the available multilevel topologies, is that fewer semiconductor devices are needed here. As the output voltage levels increase, more switches are saved, which results in a more efficient, cheaper, and smaller converter. So far, there is still no modulation strategy that is designed particularly for PV-fed multilevel converters with built-in ES. The standard modulations are impractical for such an application since they suffer from deficiencies, such as polluted output signals—thus, requiring larger output filter—and overmodulation. A modified modulation strategy for PV+ES multilevel inverters is, therefore, introduced in this article. The proposal has been simulated and experimentally validated to evaluate its effectiveness, where it has been shown that the proposed topology is not exclusively feasible, but also suffers from less conduction and switching loss, achieving higher efficiency with respect to its counterpart CHB. [ABSTRACT FROM AUTHOR]

In this article, a modularized charge equalizer is proposed. In the proposed topology, N series cells are divided into K modules and each module consists of J series cells and its multiplexer switches. A multioutput transformer is used as temporary energy storage. Each of the transformer outputs is connected to one of the K modules and targets the cells in its module. In this way, at the same time K cells are targeted simultaneously and the speed of balancing has increased. In the proposed circuit, energy can be transferred by string to cell, cell to string, and cell to cell methods. In addition, it can be transferred between modules and equalize them. Therefore, the proposed topology is very flexible and can circumvent the limitations of each method and use the appropriate method in the right place. Also, a step-by-step method for command the multiplex switches is proposed. To illustrate the correctness of the circuit operation, a prototype is implemented to balance the charge of four lithium-ion battery cells. The experimental results confirm the correctness of the proposed circuit operation. [ABSTRACT FROM AUTHOR]