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The dual-active bridge (DAB) topology is commonly preferred in bidirectional applications due to several attractive features, including auto-adjust of power flow, galvanic insulation, wide voltage gain, and zero voltage switching (ZVS) capability over some power ranges. However, the efficiency of the converter drops at light loads because the ZVS range is directly dependent on the circulating current. Assuming that the processed power is variable, the DAB converter's design must find a compromise between extending ZVS ranges and reducing the reactive power processing to ensure higher efficiency. Aiming this compromise, many papers propose hybrid approaches in which the modulation strategy is selected according to the processed power. However, this is not a simple solution because it demands multivariable control and offline optimizations. This paper proposes a modulation strategy to ensure ZVS and reduce the circulating current for the DAB converters. While the usual phase-shift modulation provides ZVS operation in a range of 40% to 100% of rated power, the proposed asymmetrical pulse-width modulation can obtain ZVS operation in a range of 2% to 100% of rated power. Experimental results demonstrated that the proposed strategy improves the converter efficiency for all power ranges, especially at light loads. [ABSTRACT FROM AUTHOR]

The bidirectional CLLC resonant converter has distinguished potential in battery chargers and energy storage systems for its advantages in soft switching and bidirectional power flow capability. However, traditional CLLC converters generally adopt symmetrical design to maintain bidirectional symmetrical characteristics, which means the secondary LC network is designed to be equal to the primary LC network after reflection. The symmetrical design is only suitable for voltage grade matching scenarios where a wide voltage range is not required, such as CLLC-type dc transformers. Whereas in the field of bidirectional chargers, due to the wide voltage range of battery, there are significant differences in the characteristics required between charging and discharging mode, which makes symmetrical design no longer applicable. To cope with this issue, this paper proposes a novel design methodology of CLLC based on parameter equivalent and time domain model. With the parameter equivalent principle, the CLLC resonant tank with arbitrary parameters is investigated to satisfy the requirements of wide voltage range for bidirectional charger application. Compared with the symmetrical design, the proposed method can meet the requirements of bidirectional gain within preset frequency range and guarantee the achievement of zero-voltage switching under required load conditions with the minimum reactive power. In addition, the area product capacity of the magnetic part of the CLLC resonant tank is minimized based on the parameter equivalent principle. Finally, experiments have been performed on a 1 kW prototype to confirm the validity and feasibility of the proposed design methodology. [ABSTRACT FROM AUTHOR]

Cascaded H-bridge (CHB) converter technology is actually the most popular solution for grid-connected converters in both medium- and high-voltage applications. This paper introduces a new control of a single-phase CHB active rectifier. The control uses a direct grid current control based on adaptive resonant controllers. The resonant controllers are adapting to slow fluctuation of ac grid frequency. The challenging problem of CHB converters is dc-link capacitor voltage balancing especially under unbalanced load conditions. This paper presents a reliable voltage-balancing technique utilizing energy estimation of particular dc-link capacitor banks and simple prediction of the changes of their energies. A nonlinear behavior of the converter caused by power semiconductors voltage drops and dead-time effects is minimized by auxiliary adaptive resonant controllers. The resulting control has very good performance, including the operation under unbalanced load conditions, and achieves low total harmonic distortion of converter grid current. The theoretical results were verified by experiments made on the developed CHB active rectifier prototype with three power cells in cascade. [ABSTRACT FROM AUTHOR]

A new modulated carrier control (MCC) method is proposed in this paper for a power factor correction (PFC) boost converter, which provides high power factor (PF) and low total harmonic distortion (THD) in wide input voltage and load range. The proposed MCC method employs a zero current duration (ZCD) demodulator that detects ZCD in each switching cycle to estimate current conduction duration of the boost inductor. Using the estimated signal of the ZCD demodulator, the proposed controller generates a compensated duty signal with a PFC converter to properly operate in continuous conduction mode (CCM) or discontinuous conduction mode (DCM). Unlike the conventional MCC PFC converter where the line current is distorted in DCM resulting in poor PF and high THD, the proposed MCC method can provide higher PFC performance regardless of CCM and DCM while maintaining the simple control loop of conventional MCC control method that does not require inner current loop and using relatively small inductance. In this paper, the operational principle of the proposed method has been discussed. To verify the proposed method, it has been tested on a 400-W PFC boost converter. [ABSTRACT FROM AUTHOR]

In this paper, two resistive matching networks are proposed, which can maintain the resistive transferring property under different load conditions. The high-frequency converter based on the resistive network can adequately achieve the soft-switching characteristics within a large load variation range, which helps to reduce switching losses and improve system efficiency. At the same time, a detailed design method for planar spiral inductor is described. In addition, the quality factor characteristics of different winding structures are analyzed. A 20-MHz high-frequency dc/dc converter based on the T-type matching network and circular spiral inductor is designed in this paper. The experimental results verify the advantages of the proposed resistive matching network and spiral inductor. [ABSTRACT FROM AUTHOR]

An optimal dual-phase-shift control strategy of an isolated buck–boost converter is proposed in this paper. The theoretical analysis of dual-phase-shift control strategy is complemented, and the three-dimensional graphs of the output current versus two control variables are given. The optimal operating point is determined by the peak current of a clamped inductor and is shown as control trajectories in the three-dimensional graphs. Moreover, with the optimal dual-phase-shift control strategy, low current value, soft-switching operation, wide input voltage range regulation, whole load range regulation, and high-efficiency performance can be achieved. Furthermore, parameter design method deriving from the minimum peak current of the clamped inductor is given. At the end of this paper, a 1000 W-output prototype is fabricated to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

These days, constant on-time current mode (COTCM) control scheme is widely used in the voltage regulator (VR) controllers because it has a higher light-load efficiency and a higher BW design capability while maintaining a simpler compensation requirement. One issue plaguing the COTCM control is its slow transient response which is caused by its fixed TON operation. During the heavy-load step-up transient, the duty cycle becomes saturated and the inductor current increment becomes limited by TON and the minimum off time (TOFF$\_$ MIN) ratio, which can create a large undershoot at load step up. On the other hand, in the load step-down case, if the load step down occurs at the beginning of TON, a large overshoot can be created at the output. To solve this issue, this paper presents a method designed to increase the TON at load step up and then very quickly, decrease at load step down in order to reduce the undershoot and overshoot at output or otherwise save the output capacitor. In this proposed method, the increase or decrease of T ON is proportional to the output change which eliminates the chance of any overcorrection or ring-back problem unlike the methods presented in prior forums. This feature enables the control to work seamlessly in a high-frequency load repetitive case in VR applications. Since this TON change occurs only in the transient period when duty cycle is saturated, it does not affect the small-signal property of the COT control. Moreover, the proposed methods are very much compatible with the state-of-the-art single and multiphase COT control structures. Simulation and test results in both single and multiphase operations are also presented in the paper to verify the proposed concept. [ABSTRACT FROM PUBLISHER]

It is mandatory to keep the total harmonic distortion in the output of most inverters below the limits prescribed by different international standards. The harmonic distortions caused due to nonlinear loads may not get corrected by typical magnitude controller, like PID controller. Repetitive controller is more effective in such situations. Often, repetitive controller is used as add-on block to the conventional magnitude controller. Design of repetitive controller can be easily implemented on digital platform. Rapid change in phase and magnitude of inverter-filter's transfer function near corner frequency causes instability problems in repetitive controller. Various techniques have been proposed by researchers to overcome this problem. Most of the reported techniques rely on digital compensation. However, it is hard to find a digital compensator, which simultaneously meets the phase and gain requirements at moderate computational effort. Here, a new output voltage feedback scheme has been proposed, which simplifies the requirement of digital compensation. The proposed scheme requires an extra hardware but at negligible added cost. In spite of this extra hardware, the proposed circuit turns out to be more energy efficient in comparison to few popular schemes, reported in literature, where some resistance is introduced in the inverter's filter for better stability of controller. In addition to the proposed modification the paper presents a simplified explanation of the various factors affecting stability of repetitive controller. The paper also discusses a novel experimental method for verifying the efficacy of repetitive controller in suppressing distortion of any particular harmonic order. [ABSTRACT FROM PUBLISHER]

DC electric springs (DCES) are emerging technologies for the (i) regulation of mains voltage against the intermittent renewable generations and (ii) harmonic cancellation in dc microgrids. When conventional converter topologies (e.g., half-bridge or full-bridge converter) are adopted as DCES, the battery storage of the DCES has to process both the dc power and the ac harmonic power. The pulsating ac power can severely reduce the lifetime of the battery. To address this issue, a hybrid-DCES (H-DCES) is proposed in this paper to perform (i) and (ii) in a decoupled manner. With a modified topology and control method, the H-DCES can divert the ac current to the ground and retains the function of manipulating noncritical load for dc voltage regulation. The immediate benefits of this H-DCES are the reduction of storage capacity and a prolonged lifetime of the battery. Both the operating principle and the mathematical model of the proposed H-DCES are analyzed in this paper. A prototype of the H-DCES is practically tested in a 48-V dc grid. The experimental results show that the H-DCES can realize the decoupled operation of dc voltage regulation and harmonic cancellation. Simulation studies further demonstrate that the H-DCES requires less storage capacity than its counterparts. [ABSTRACT FROM AUTHOR]

This paper presents an adaptive power management circuit, which maximizes the energy transfer from the energy harvester to wireless sensor nodes in real-world applications. Low power consumption techniques were adopted in the power management circuit to maximize the delivery of the harvested energy to the load instead of being consumed by the circuit. The presented circuit incorporates an analogue control circuit (ACC) for maximum power transfer from the energy harvester to the storage capacitor and an energy-aware interface (EAI) for controlling the energy flow from the storage capacitor to the load. To evaluate the performance of the presented circuit, piezoelectric energy harvesting was used as a studied case. The piezoelectric energy harvester (PEH) was mechanically excited at different strain loadings and frequencies. The experimental results show the circuit can self-start and powered directly by the PEH since the EAI and ACC have low power consumption in the range of microwatts. The circuit is adaptive to energy harvesters of varying output and various electrical loads, with a peak efficiency of 76.18% in transferring the harvested energy from the PEH to the storage capacitor. More than 96% of the energy released from the storage capacitor is effectively transferred to the electrical load. [ABSTRACT FROM AUTHOR]

This paper proposes a control strategy for a three-phase three-wire thyristor-controlled LC -coupling hybrid active power filter (TCLC-HAPF), which can balance active power and compensate reactive power and harmonic currents under unbalanced loading. Compared with TCLC-HAPF with conventional control strategy, active power filters and hybrid active power filters which either fail to perform satisfactory compensation or require high-rating active inverter part for unbalanced compensation, a control strategy was proposed for TCLC-HAPF to operate with a small rating active inverter part for a variety of loads with satisfactory performance. The control idea is to provide different firing angles for each phase of the thyristor-controlled LC-coupling part (TCLC) to balance active power and compensate reactive power, while the active inverter part aims to compensate harmonic currents. First, the required different TCLC impedances are deduced. Then, independent firing angles referenced to the phase angle of voltage across TCLC are calculated. After angle transformations, final firing angles referenced to phase angle of load voltages are obtained. In this paper, a novel controller for TCLC-HAPF under unbalanced loading is proposed. Simulation and experimental results are provided to verify the effectiveness of the proposed controller in comparison with a state-of-the-art controller. [ABSTRACT FROM PUBLISHER]

To reduce current stress and improve efficiency of dual active bridge (DAB) dc–dc converters, various control schemes have been proposed in recent decades. Most control schemes for directly minimizing power losses from power loss modeling analysis and optimization aspect of the adopted converter are too difficult and complicated to implement in real-time digital microcontrollers. Thus, this paper focuses on a simple solution to reduce current stress and improve the efficiency of the adopted DAB converter. However, traditional current-stress-optimized (CSO) schemes have some drawbacks, such as inductance dependency and an additional load-current sensor. In this paper, a simple CSO scheme with a unified phase-shift (UPS) control, which can be equivalent to the existing conventional phase-shift controls, is proposed for DAB dc–dc converters to realize current stress optimization. The simple CSO scheme can overcome those drawbacks of traditional CSO schemes, gain the minimum current stress, and improve efficiency. Then, a comparison of single-phase-shift (SPS) control, simple CSO scheme with dual-phase-shift (CSO-DPS) control, simple CSO scheme with extended-phase-shift (CSO-EPS) control, and simple CSO scheme with UPS (CSO-UPS) control is analyzed in detail. Finally, experimental results verify the excellent performance of the proposed CSO-UPS control scheme and the correctness of theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper presents a new hybrid passive power device, denoted as the hybrid transformer, which is an improved alternative to the classical two-winding high-frequency transformer (100's kHz–MHz). The hybrid transformer is comprised of a coupled inductor topology, that provides load–source matching, denoted as the mutual branch topology, and a dielectric isolation device denoted as the multilayer capacitive isolation device. The main focus of this paper is on the design and operation of the mutual branch topology. Simulation results are shown to highlight the improved efficiency of the hybrid transformer over a conventional high-frequency transformer. Finally, the efficiency of the hybrid transformer is compared with that of a classical two-winding transformer using the same resonant converter, highlighting the improved efficiency of the hybrid transformer (up to 10% improved efficiency for light loads). [ABSTRACT FROM AUTHOR]

This paper proposes a new control method of three-phase active power filters for reducing the dc capacitor voltage fluctuations in transient load change. The proposed control method introduces a new \boldsymbolk-step compensator, which maintains the mean active power flowing into the dc capacitor at zero every \boldsymbol1/(k-1) ac line period. Therefore, the compensator can suppress the transient voltage fluctuations across the dc capacitor even when a quick load change occurs. This paper theoretically reveals the relationship between the step number \boldsymbolk and the reduction effect of the dc voltage fluctuations. The experimental setup introducing the proposed seven-step compensator exhibits a good suppression effect of the dc voltage fluctuations and makes it possible to reduce the required capacitance value of the dc capacitor by a factor of seven. [ABSTRACT FROM AUTHOR]

The more electric aircraft concept aims at increasing the amount and the power of the electrical subsystems on the next generation of aircraft. One of the major challenges in this framework is that the electric power is generated by means of electrical generators connected to the jet turbine shaft. In order to satisfy the peak power demand and overload conditions, the generators are oversized, increasing weight and total cost of operation. Possible solutions to this problem is to decrease the size of the generator and to provide the peak power with storage systems. As an alternative, load disconnection during peak demand (load-shedding) can be enforced with solid-state switches. This paper presents a different approach, i.e., by modifying the dc voltage of the distribution, the load can be controlled. In order to achieve this objective, a precise on-line identification algorithm is proposed. Theoretical analysis is performed, and simulation results on a study-case distribution system are reported, highlighting how the proposed load identification and control method effectively manages a power curtailment condition without load-shedding or storage systems. Power hardware in the loop simulations on a dc microgrid with a multiport power converter confirm the simulation analysis. [ABSTRACT FROM AUTHOR]

Smart loads associated with electric springs (ES) have been used for fast demand-side management for smart grid. While simplified dynamic ES models have been used for power system simulation, these models do not include the dynamics of the power electronic circuits and control of the ES. This paper presents a dynamic and modular ES model that can incorporate the controller design and the dynamics of the power electronic circuits. Based on experimental measurements, the order of this dynamic model has been reduced so that the model suits both circuit and system simulations. The model is demonstrated with the radial-chordal decomposition controller for both voltage and frequency regulation. The modular approach allows the circuit and controller of the ES model and the load module to be combined in the d–q frame. Experimental results based on single and multiple smart loads setup are provided to verify the results obtained from the model simulation. Then, the ES model is incorporated into power system simulations including an IEEE 13 node power system and a three-phase balanced microgrid system. [ABSTRACT FROM AUTHOR]

The transient performance of power semiconductor devices relates directly to their available power rating, reliability, and operating lifetime. This paper examines the transient thermal performance of liquid-cooled, silicon carbide power devices subjected to different unsteady electrical loads. The first part uses infrared thermography to examine an observed asymmetrical device thermal time constant when subjected to step-change increases and decreases in current. A theoretical analysis connects this behavior to the dependence of mosfet on-resistance on junction temperature. It also identifies three time constants that characterize junction transient response, one each for the die, the module, and the cold plate. The second part extends the transient thermal evaluation to half-sine wave periodic excitations that emulate real-application operating conditions. These experiments show that thermal ripple increases with increasing excitation amplitude but decreases with increasing excitation frequency. They also connect the observed thermal response to the time constants inferred from the step-change experiments. Both parts of the study show the importance of considering transient loads when designing power electronics cooling systems and the role that electrical properties play in determining unsteady thermal response. [ABSTRACT FROM AUTHOR]

Traditionally, switched-capacitor (SC) converters have suffered from high transient currents, which limit both the efficiency and power density of such converters. Soft-charging operation can be employed to eliminate the current transients and greatly improve the power density of SC converters. In this approach, a second-stage magnetic converter is cascaded with the SC stage to act as a controlled current load. Another approach is to use resonant SC converters with zero-current switching. This paper shows that resonant and soft-charging operations of SC converters are closely related, and a technique will be proposed, which achieves either operation by adding a single inductor to existing SC topologies. In addition, since most preexisting resonant or soft-charging SC converters were devised in an ad-hoc manner, this paper formulates an analytical method that can determine whether an existing conventional SC converter topology is compatible with the proposed approach. A number of common SC topologies are analyzed, including Dickson, series-parallel, ladder, Fibonacci, and doubler configurations. Through comparison to simulated results, as well as experimental work, the proposed method is validated and a family of high-performance SC converters is obtained. [ABSTRACT FROM AUTHOR]

Passive or active current balancing circuits are usually used to mitigate current imbalance in driving multiple light-emitting-diode (LED) strings. Passive current balancing schemes adopting capacitors with high reliability, small size and low cost are very popular in many applications. However, the high reactive power of the capacitive balancing scheme with variable frequency control will bring high power stress on the VA rating of the main switches which drive this passive current balancing circuit and decrease the overall efficiency. Fixed frequency control does not permit zero-voltage switching (ZVS) under load variations. Hence, this paper proposes a current-source-output LED driver based on LCLC resonant circuit to provide a constant output current regardless of variations in LED parameters. In the LCLC circuit, the number of additional capacitors is scalable with the number of LED strings for current balancing. Moreover, the input impedance of the improved LCLC circuit is designed to be resistive at the operating frequency to minimize reactive power. The conventional duty cycle control can easily incorporate ZVS. The analysis, implementation and verification are detailed in this paper. [ABSTRACT FROM AUTHOR]

Small-signal stability is of great concern for electrical power systems with a large number of regulated power converters. In the case of dc systems, stability can be predicted by examining the locus described by the ratio of the source and load impedances in the complex plane per the Nyquist stability criterion. For balanced three-phase ac systems the same impedance-based method applies, for which this paper uses impedances in the synchronous rotating reference (d-q) frame. Small-signal stability can be determined by applying the generalized Nyquist stability criterion (GNC). This approach relies on the actual measurement of these impedances, which up to now has severely hindered its applicability. Addressing this shortcoming, this paper investigates the small-signal stability of a three-phase ac system using measured d-q frame impedances. The results obtained show how the stability at the ac interface can be easily and readily predicted using the measured impedances and the GNC, thus illustrating the practicality of the approach, and validating the use of ac impedances as a valuable dynamic analysis tool for ac system integration, in perfect dualism with the dc case. [ABSTRACT FROM AUTHOR]

The development of a large-scale high-voltage direct current (HVDC) power grid requires a reliable, fast, and low-loss circuit breaker. The load commutation switch (LCS) is an essential part of ABB's 1200-MW hybrid HVDC breaker concept, which builds up a low-loss conducting path for the load current. The technical requirements for the LCS are expressed in this paper by studying the operation principle of the hybrid HVDC breaker. The voltage stress over the LCS is determined based on a dc grid with 320 and 2 kA rated voltage and current. A system model of the hybrid HVDC breaker is developed in PSCAD/EMTDC to study the design criteria for snubber circuit and arrester blocks. It is observed that conventional snubber circuits are not suitable for a bidirectional LCS as the current of snubber capacitors prevent the fast interruption action. A modified snubber circuit is proposed in this paper along with two more alternatives for the LCS to overcome this problem. Moreover, the power loss model for a semiconductor device is discussed in this paper based on the 4.5-kV StakPak IGBT. The model is used to calculate the conduction power losses for different LCS topologies. Ultimately, a matrix of 3 $\times$ 3 IGBT modules is selected to provide a reliable LCS design which can handle several internal fault cases with no interruption of operation. A full-scale prototype has been constructed and tested in ABB HVDC Center, Ludvika, Sweden. The experimental test results are also included in the paper in order to verify the calculation and simulation study. [ABSTRACT FROM AUTHOR]

Facing the challenges of power quality problems and excessive neutral sections of conventional two-phase electric railway supply system, the way of adopting railway static power conditioner (RPC)-based single-phase supply system is a feasible solution. To enhance the cost-efficiency and reliability of RPC, a novel compensating system named asymmetric double LC-coupled railway power flow conditioner (ALC-RPFC) is proposed in this paper. The study indicates that compared with the conventional RPC, the proposed ALC-RPFC has an effective heavy-load compensating ability with lower power rating, which is benefit to enhance converter's operating efficiency and reliability. Besides, a design method for LC coupling branches mentioned in this paper is suitable for fluctuated railway loads, and is useful for designers of industrial applications as well. Finally, the good heavy-load compensating performance and power capacity decreasing ability of the ALC-RPFC are validated based on simulation and experiment results. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid phase-shift-controlled three-level (TL) and LLC dc-dc converter. The TL dc-dc converter and LLC dc-dc converter have their own transformers. Compared with conventional half-bridge TL dc-dc converters, the proposed one has no additional switch at the primary side of the transformer, where the TL converter shares the lagging switches with the LLC converter. At the secondary side of the transformers, the TL and LLC converters are connected by an active switch. With the aid of the LLC converter, the zero voltage switching (ZVS) of the lagging switches can be achieved easily even under light load conditions. Wide ZVS range for all the switches can be ensured. Both the circulating current at the primary side and the output filter inductance are reduced. Furthermore, the efficiency of the converter is improved dramatically. The features of the proposed converter are analyzed, and the design guidelines are given in the paper. Finally, the performance of the converter is verified by a 1-kW experimental prototype. [ABSTRACT FROM AUTHOR]

This paper studies the inherent relationship between two widely used pulse width modulation (PM) methods for multilevel converters, i.e., the space vector modulation (SVM) method, also called space vector pulse width modulation, and the nearest level modulation method. The nearest-level modulation method directly controls the voltage of each phase, while the SVM method simultaneously deals with all phases. It is demonstrated in this paper that the two modulation methods are functionally equivalent: with proper common-mode voltage injections, the nearest-level modulation method is equivalent to the SVM method; by selecting the appropriate redundant switching sequences and the corresponding duty cycles, the SVM method is equivalent to the nearest-level modulation method. Nevertheless, the SVM method can conveniently provide more flexibility of optimizing the switching patterns, without the need of designing sophisticated common-mode voltages. An efficient and flexible modulation method for any multiphase multilevel converter is, therefore, proposed, which combines the advantages of the nearest-level modulation and the SVM methods, i.e., both with less computational burden and high flexibility of optimizing the output waveforms. Simulation and experimental results validate the analysis. [ABSTRACT FROM AUTHOR]

In this paper, a 2.5-kW 330-410-V/14-V, 250-kHz dc/dc converter prototype is developed targeted for electric vehicle/hybrid vehicle applications. Benefiting from numerous advantages brought by the LLC resonant topology, this converter is able to perform high efficiency, high power density, and low EMI. To arrange high-output current, this paper proposes a parallel-connected LLC structure with developed novel double-loop control to realize an equal current distribution and an overall efficiency improvement. Considering the LLC cell's dimensioning, this paper establishes a more precise model by taking the secondary leakage inductance into consideration. System amelioration and design considerations of the developed LLC are also presented in this paper. A special transformer is presented, and various types of power losses are quantified to improve its efficiency. This converter also implements synchronous rectification, power semiconductor module, and an air-cooling system. The power conversion performance of this prototype is measured and the developed prototype attains a peak efficiency of 95% and efficiency is higher than 94% from 500 W to 2 kW, with a power density of 1 W/cm3. Finally, the EMC results of this prototype are also measured and presented. [ABSTRACT FROM AUTHOR]

Recent years have witnessed a continuing increase in the wind turbine (WT) size and penetration level into the grid, revealing critical challenges to sustainable market development and urgent demands of low voltage ride-through, load reduction, and optimal operation. This paper presents an overview on grid-friendly WTs and relevant technologies for control and monitoring. The development trends are discussed as well to explore potential approaches for optimizing WT systems. The main body contains three parts: the “grid-friendly” criterion and a prototype, the overview and suggestions for WT controller design concerning load reduction and power optimization, and suggested solutions for wind power monitoring systems. This paper is expected to be a reference for extending the knowledge of grid-friendly WTs and optimizing the performance of future large-scale wind power systems. [ABSTRACT FROM AUTHOR]

Pulse width modulation (PWM) converters generate switching common-mode voltages (CMVs) across the load terminals. These voltages cause common-mode currents, leading to bearing failure in motor loads and electromagnetic interference problems. This paper presents a generalized carrier-based PWM technique for open-end winding motor drives that completely eliminates switching CMV. The proposed method is applicable to both dual two-level voltage source inverter and dual matrix converter-based open-end winding drives. Detailed analysis shows that the carrier-based method requires significantly less computation compared to the corresponding space vector implementation. This paper also outlines the relationship between the two implementations. The carrier-based method is shown to achieve superior performance in terms of resource requirements and execution time when implemented on a field-programmable gate array-based real-time control platform. Simulation and experimental results have been presented to validate the proposed method. [ABSTRACT FROM AUTHOR]

Frequency control in autonomous microgrids (MG) with high penetration of renewable energy sources represents a great concern to ensure the system stability. In this regard, this paper presents an enhanced control method for battery energy storage systems (BESS) to support the frequency of MG and with the ability of disconnecting from the MG to supplying in the island mode a local consumer. A frequency controller, combining a conventional droop control with an inertia emulation function, governs the BESS active power transfer during the primary frequency control level. The BESS may also provide voltage support in the point of common coupling with the MG. Moreover, the proposed BESS may compensate, partially or totally, the power absorbed by the local loads in order to improve the MG frequency response. When the MG power quality worsens below a certain level, in terms of voltage and frequency, the BESS detaches from the MG and continues to operate islanded. The reconnection is accomplished following a smoothly resynchronization of the local voltage with the MG, without disturbing the local loads supply. Additionally, this paper also discusses about the aspects related to the BESS management and its integration within the proposed system. The simulation and experimental results assess the feasibility of the proposed control solutions. [ABSTRACT FROM AUTHOR]

In the renewable hybrid power system, multiple-input converters (MICs) serve as the interface of several sources with a load, and provide the energy to the load simultaneously or individually, which can optimize the utilization of sources, simplify the system configuration, and reduce the overall cost. In the applications with isolated requirement, available MICs usually have complex configuration and numerous transformer windings. In this paper, a family of asymmetrical half-bridge double-input converters (DICs) adopting pulsating voltage source cells (PVSCs) for low power applications is proposed. Compared with available isolated DICs, the proposed converters have advantages of simple architecture, soft-switching realization, and high conversion efficiency. In this paper, the derivation and feasible control strategy for the proposed converters are illustrated. Specifically, the operation principle of DIC composed of two buck PVSCs is analyzed. Finally, theoretical analysis is validated by the experimental results from a 240 W prototype. [ABSTRACT FROM AUTHOR]

In classical inverter control, load current (disturbance) decoupling control is often used to improve the control performance. Yet, in a recent state-space control structure for multiresonant controllers, the load current decoupling technique is less studied. Moreover, in the classical control, the load current decoupling only has one control degree of freedom. Therefore, in this paper, a load current decoupling control combed linear quadratic (LQ) regulation is investigated, the proposed control structure that manipulates α and β axes as a unified system is adopted to increase the control degree of freedom. The LQ control synthesizes the state feedback control law including gains of the resonant controllers. Two general criteria, namely H\infty norm and zero dynamic, are proposed as the design guideline for the load current decoupling control. The effectiveness and robustness of the proposed approach are validated through MATLAB/Simulink simulations and experiments with a 5-kVA testbed. The results prove that some favorable performances, for example, the fast recovering time, the small voltage drop, and the low total harmonic distortion, are achieved by the proposed approach compared to the classical dual-loop proportional-resonant (PR) control with load current decoupling and the optimal H\infty control. [ABSTRACT FROM AUTHOR]

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid energy storage system (ESS) that integrates an ESS with an online uninterruptible power supply (UPS). The power conversion system cost of the proposed hybrid ESS-UPS is reduced, and battery utilization is increased by using the energy of the integrated battery for both demand management and emergency power supply. Further, unlike the conventional online UPS, the proposed hybrid ESS-UPS is capable of supplying emergency power to two types of critical loads, voltage-frequency independent (VFI) load and voltage-frequency dependent (VFD) load. A seamless mode transfer algorithm of the ac–dc converter is proposed for supplying uninterruptible power to the VFD load as well. In addition, an autonomous and seamless mode transfer algorithm of the bidirectional dc–dc converter is proposed to minimize the transient across the dc link that could affect the power quality of the VFD load and the VFI load at the mode transition. To validate the proposed control scheme, experimental results from a 5-kW prototype are provided. [ABSTRACT FROM AUTHOR]

Shunt active power filter (SAPF) is the preeminent solution against nonlinear loads, current harmonics, and power quality problems. APF topologies for harmonic compensation use numerous high-power rating components and are therefore disadvantageous. Hybrid topologies combining low-power rating APF with passive filters are used to reduce the power rating of voltage source inverter (VSI). Hybrid APF topologies for high-power rating systems use a transformer with large numbers of passive components. In this paper, a novel four-switch two-leg VSI topology for a three-phase SAPF is proposed for reducing the system cost and size. The proposed topology comprises a two-arm bridge structure, four switches, coupling inductors, and sets of LC PFs. The third leg of the three-phase VSI is removed by eliminating the set of power switching devices, thereby directly connecting the phase with the negative terminals of the dc-link capacitor. The proposed topology enhances the harmonic compensation capability and provides complete reactive power compensation compared with conventional APF topologies. The new experimental prototype is tested in the laboratory to verify the results in terms of total harmonic distortion, balanced supply current, and harmonic compensation, following the IEEE-519 standard. [ABSTRACT FROM AUTHOR]

In this paper, a comprehensive control scheme for a delta-connected cascaded H-bridge (CHB) converter-based static synchronous compensator (STATCOM) is presented, especially focusing on improving dynamic performance by novel feedforward control method. The method can conspicuously improve the dynamics of zero-sequence circulating current regulation of delta-connected CHB STATCOM especially under grid fault condition as well as load unbalance without excessive dc cell capacitor voltage fluctuation. The validity of the proposed method is demonstrated by mathematical proof. Finally, the full-scaled simulation results and the down-scaled experimental results verify that stable operation is guaranteed for both emulated grid and load unbalance conditions. [ABSTRACT FROM AUTHOR]

Inductive power transfer (IPT) converters are resonant converters that attain optimal energy efficiencies for a certain load range. To achieve maximum efficiency, it is common to cascade the IPT converter with front-side and load-side dc/dc converters. The two dc/dc converters are normally controlled cooperatively for the requirements of output regulation and maximum efficiency tracking using a control technique based on perturbation and observation, which is inevitably slow in response. In this paper, a decoupled control technique is developed. The load-side dc/dc converter is solely responsible for output regulation, while the front-side converter is responsible for impedance-matching of the IPT converter by controlling its input-to-output voltage ratio. The controls are linear and therefore fast. DC and small-signal transfer functions are derived for designing the control parameters. The performances of fast regulation and high efficiency of the IPT converter system are verified using a prototype system. [ABSTRACT FROM AUTHOR]

Most existing wind turbine generation systems (WTGS) are based on double-fed induction generator (DFIG) and direct-drive synchronous generator (DDSG). In this paper, we investigate the modeling and controller design of a novel front-end speed regulation (FESR) wind turbine with differential variable ratio gearbox (VRG). For this wind turbine, a surface mount permanent magnet synchronous motor (SPMSM) unit is embedded to maintain the constant (i.e., synchronous) speed of the generator shaft over wide range of wind speed, which allows the conventional synchronous generator to be used to construct a grid-friendly WTGS. Compared with DFIG and DDSG, the novel FESR does not use converters for grid-connecting but adopt SPMSM to ensure the synchronous speed. A model-independent speed control algorithm for maximum power point tracking (MPPT) is developed based on a neuro-adaptive backstepping approach. Theoretical analysis and numerical simulations show that the proposed control scheme ensures more precise motor speed tracking in the presence of parameter uncertainties and external load disturbances. [ABSTRACT FROM PUBLISHER]

Battery energy storage systems are often adopted to buffer the difference between the intermittent solar power and the load demand in power grids. The costs of such photovoltaic (PV) battery systems increase as the required energy storage increases. In this paper, a new configuration comprising the PV panels, a series dc electric spring (series ES) and a noncritical load is proposed to reduce the battery storage capacity of dc microgrids that have substantial PV installations. This arrangement forms a PV-embedded series dc ES (PVES). An optimization method considering the minimization of electricity bills of the dc microgrids is included to size the storage capacity and to determine the rating of the PV that are connected to the series ES. Experiments on a 48-V isolated dc grid and simulations on a 400-kVA grid-connected dc microgrid have been conducted to verify the storage reduction feature of the PVES. Both sets of results show that the PVES can tackle the intermittency of the solar power with a smaller storage capacity than that typically required in dc grids with PV installations. [ABSTRACT FROM AUTHOR]

To facilitate the connection of various renewable ac and dc sources and loads to power systems, the hybrid ac/dc microgrid with multiple subgrids has been grasping more and more attention, in which the rated ac frequencies and dc voltages in different ac subgrids and dc subgrids can diversify for various applications. As the interaction among these subgrids is highly complicated, the stable and efficient operation of the hybrid microgrid is very challenging. Focusing on this problem, this paper proposes a decentralized multi-time scale power control method to make the interacted subgrids operate in coordination and support each other when the power of the system is fluctuated. The proposed control method stabilizes the power fluctuation in three time scales, of which the first two are conducted at the subgrid level and the last is conducted at the system level. In the first time scale, the ac frequency or dc voltage is changed to regulate the load power through the droop control. Further, in the second time scale, the output power of distributed generations is changed according to the deviations of ac frequency or dc voltage. Through these two coordinated control strategies, the power fluctuation is primarily stabilized. In the third time scale, the ac frequencies and dc voltages of all the subgrids are changed according to a designed coordinated power control strategy with consideration of the capacities and load types of each subgrids, which can make the subgrids support each other. Through this coordinated control strategy, the proper power interaction among different subgrids can be realized and the power fluctuation is further stabilized with the support of other subgrids. The effectiveness of the proposed power control method is verified by the real-time hardware-in-loop tests. [ABSTRACT FROM PUBLISHER]

Voltage-source converter (VSC)-based multiterminal dc grids are receiving widespread acceptance as an enabling technology to integrate renewable energy sources, energy storage units, and modern dc-type loads into existing ac grids. Droop control is a common power-sharing strategy to facilitate autonomous power sharing among different terminals in dc grids. However, the dynamics and stability of a gird-connected VSC with dc power sharing droop control can be affected by several important factors that are not addressed in the current literature. Important among these are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed from the dc side the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. To overcome these difficulties, this paper presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in multiterminal dc grids applications. A detailed dynamic model that considers the droop controller dynamics and the impact of the effective dc-side load parameters and disturbances imposed on the VSC is developed. Then, a robust controller that preserves system stability and robust performance is developed. Theoretical analyses, including the MIMO dynamic analysis of a multiterminal dc grid with the proposed controller, as well as simulation and experimental results are provided to show the effectiveness of the proposed controller. [ABSTRACT FROM PUBLISHER]

Taking into account extension of power systems, their interconnections, and increment of load demand, power capacity of networks is increased and the impedance of sources is decreased. Such variations in power systems resulted to occurring high ratings of fault currents. Fault current limiter (FCL) can be practically utilized in order to interrupt fault currents to a specified amount without requirement to upgrading protection devices. In this paper, a novel simple structure for nonsuperconducting FCL (NSFCL), which is called capacitor-based FCL (CBFCL), is proposed for limiting the high ratings of fault currents in power networks. The proposed NSFCL circuit is based on transferring electrical energy to a capacitor during fault occurrence, which can be used after fault removal. Simulation of the proposed NSFCL structure is done and the results are provided and analyzed. Additionally, the operation of the circuit is experimentally studied, and the similarity between the simulation results and the experimental results is validated. The provided simulation and experimental results ensure the practicability and possibility of employment of the proposed CBFCL. [ABSTRACT FROM PUBLISHER]

Microgrids are energy systems that can work independently from the main grid in a stable and self-sustainable way. They rely on energy management systems to schedule optimally the distributed energy resources. Conventionally, the main research in this field is focused on scheduling problems applicable for specific case studies rather than in generic architectures that can deal with the uncertainties of the renewable energy sources. This paper contributes a design and experimental validation of an adaptable energy management system implemented in an online scheme, as well as an evaluation framework to quantitatively assess the enhancement attained by different online energy management strategies. The proposed architecture allows the interaction of measurement, forecasting and optimization modules, in which a generic generation-side mathematical problem is modeled, aiming to minimize operating costs and load disconnections. The whole energy management system has been tested experimentally in a test bench under both grid-connected and islanded mode. Also, its performance has been proved considering severe mismatches in forecast generation and load. Several experimental results have demonstrated the effectiveness of the proposed EMS, assessed by the corresponding average gap with respect to a selected benchmark strategy and ideal boundaries of the best and worst known solutions. [ABSTRACT FROM PUBLISHER]

An active power filter (APF) plays an important role in compensating the harmonic component of a nonlinear load current. In this paper, deadbeat control (DBC), which has high control performance and fast dynamic response, is used as a current controller for an APF. An improved DBC (Imp_DBC) is introduced to suppress the adverse effect of the current sampling error and to enhance the interference rejection capability of the control system. The characteristics of the conventional DBC and Imp_DBC are analyzed using transfer functions in the discrete-time domain. The robustness of the system is improved with the inductance variation. Specifically, aiming at the dramatic change of the harmonic component of the nonlinear load current, a current-tracking error compensation method is proposed. The phenomena of the upper co-nvex and lower concave in grid current are eliminated. Finally, the experimental setup of three-phase three-line APF is built, and the effectiveness of this method is verified by experiments. [ABSTRACT FROM PUBLISHER]

A “one-and-a-half stage” forward-flyback converter for electrolytic capacitor-less light-emitting diode (LED) driver with high power factor (PF), high efficiency, low output ripple current, and long lifetime has been proposed and studied in this paper. The basic topology of the proposed topology is a single-switch forward-flyback converter for achieving high PF. In addition, a buck converter is inserted between the forward subconverter and the load for creating two paralleled power transfer paths. The most of input energy directly reaches the load through flyback subconverter, and only about 1/4 of total energy is transferred to the load through forward subconverter and buck converter. Therefore, the proposed topology is a “one-and-a-half stage” converter and can achieve higher efficiency than the traditional two-stage topologies. At the same time, power decoupling can be realized and the electrolytic capacitor can be eliminated. Optimal control scheme, detailed analysis, and design considerations for this improved converter are presented. Finally, an experimental prototype for 28 V/700 mA LED driver was built up to verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid rectifier consisting of a 12-pulse rectifier in parallel with a Vienna rectifier. In the proposed structure, the 12-pulse rectifier supplies the bulk power to the load, whereas the Vienna rectifier shapes the input current to reduce its harmonic distortion. The unidirectional power flow of the Vienna rectifier results in undesirable distortion around the current zero crossing, which deteriorates current shaping in this region. To overcome this problem, besides current shaping, the Vienna rectifier is forced to participate in the active power. In this regard, the share of output power in each rectifier module must be calculated. To shape the input current, the reference current for the Vienna rectifier is generated using instantaneous power theory, and current tracking is carried out using the finite control set model predictive control. A detailed analysis of how to select the output power sharing ratio based on system losses and input current distortion is presented. The theoretical analysis is verified by using simulation and experimental results obtained from a 1 kW laboratory setup. [ABSTRACT FROM AUTHOR]

This paper introduces a series compensation philosophy to improve the voltage property of loads using three-phase voltage ellipse parameters intended for optimal utilization of a dynamic voltage restorer. The proposed approach relies on the instantaneous magnitude of voltage signals and functions by inserting a virtual equivalent impedance in series with the distribution feeder during voltage sags, compensating for the difference between faulty and nominal voltages. In addition, the definite mathematical derivations using iterative processes are properly used as an identification mark of the resultant rotating vector tracing an ellipse. The ellipse parameters including major axis, minor axis, and inclination angle are utilized to develop the proposed algorithm and hence, a set of generalized VA loading formulations calculating an optimal sizing with the minimum possible rating are presented. The uniqueness of dual VSCs connected with common dc-side is an attempt to formalize the topological structure keeping a higher level of compensation accuracy. A novel design and the corresponding algorithm are proposed and all possible scenarios concerning sag depth and phase jump are taken into consideration. MATLAB-based simulation results are discussed in detail to support the concept and the test is also performed to verify the effectiveness through real-time experimental laboratory. [ABSTRACT FROM AUTHOR]

Power electronic systems are prone to instability. The problem, generally attributed to the constant power load (CPL) behavior of their power electronic controlled loads, can become more acute when the systems are subject to parametric uncertainties. The structured singular value (SSV) based $\mu$ method has proven to be a reliable approach for assessing the stability robustness of such uncertain systems. Despite its numerous benefits, the $\mu$ method is not often applied to electrical power systems (EPS) with multiple uncertainties. This may be due to the mathematical complexity underlying the $\mu$ theory. This paper aims to make the $\mu$ approach more application-friendly by providing clearer insights into the meaning and usefulness of the robust stability measure $\mu$ for EPS with multiple parametric uncertainties. This is achieved by presenting a methodology for translating $\mu$ analysis results from the frequency domain to the more perceivable uncertain parameters domain. The method directly demonstrates dependences of system stability on uncertain system parameters. Further, it clearly identifies robust stability domains as subsets of the much wider stability domains. The work is based on a representative EPS connected to an ideal CPL. $\mu$ analysis predictions are evaluated and validated against analytical results, for example, CPL system. [ABSTRACT FROM PUBLISHER]

LLC resonant converters suffer from a startup inrush current that may push power switches beyond the safe operating area. The conventional method of limiting the startup inrush current is to adopt the frequency decrement method; however, doing so, results in the overdesign of magnetic components and the gate driver circuit, sluggish startup response, and circulating current in light and no-load conditions. To tackle these problems, this paper proposes a nonlinear controller called the average geometric controller (AGC), offering low-cost implementation requirements. The proposed controller uses the advantages of a new LLC average large-signal model, in order to analyze the large-signal nature of the converter. Studying the average nature of LLC converters enables the use of low-bandwidth sensors, and lower sampling rates, while improving the system performance. In addition to limiting the startup inrush current without employing a very high switching frequency, the proposed AGC provides an extreme dynamic startup response for LLC converters, with near zero voltage overshoot. In order to validate the theoretical analysis, experimental results of an LLC converter are presented using the proposed controller. Comparative experimental analysis is performed with a conventional method for limiting inrush current. Moreover, the accuracy of the average large-signal model is experimentally validated. [ABSTRACT FROM PUBLISHER]

For maximizing the power transfer in an inductive power transfer system, the operating frequency of the system is normally designed to be the same as the resonant frequency of the secondary power pick-up. But the load deviation reflected from secondary side to primary side will result in the frequency mismatch issue. Therefore, the contactless load detection technique is significant to ensure the power transfer efficiency and capacity. The emerging technique taking advantages of the amplitude decay rate in a transient process is promising. The robustness and practicability can get enhanced and the complicated steady circuit model is avoided. However, the accuracy for the small resistance application is still a challenge even with sophisticated compensation algorithms. This paper classified the system detectable region through the root locus method. The transient process in different subregions is excavated mathematically. It is revealed that the variation of the system from the second order to fourth order is the key issue for the detection error in the small load resistance application. The analysis brings out a novel automatic contactless load detection method to extend the detection range. Both simulation and experimental results verified the effectiveness of the proposed theory and detection method. [ABSTRACT FROM AUTHOR]

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

Traditional load regulation methods for a resonant converter mainly rely on frequency modulation. It is always a tradeoff between the design of the resonant network and the range of load. Especially for wireless power transfer (WPT) systems, the resonant network usually has a high quality factor. Small variation on frequency leads to huge drop in gain and efficiency. Due to this problem, many WPT systems are unregulated and they need one or two more front-end stages to regulate the dc bus voltage and perform power factor correction (PFC). In order to lower the cost and complexity of two- or three-stages structure, a single-stage solution with a silicon carbide (SiC) based Z-source resonant converter (ZSRC) was recently proposed. The Z-source network provides high reliability as being immune to shoot-through problems. Additionally, a ZSRC can boost the dc bus voltage while the traditional voltage-source inverter can only produce a lower voltage. However, the load regulation of this new topology has not been addressed. Two effective load regulation methods with constant frequency are presented for this SiC-based ZSRC specifically. Operation principle of the two load regulation methods are described in this paper. Experimental results based on a 200-W scale-down prototype with a full-bridge series resonant dc–dc converter are presented to illustrate the mechanism of these two methods. [ABSTRACT FROM AUTHOR]