Showing papers in "IEEE Transactions on Power Electronics in 2010"

Low-Voltage Bipolar-Type DC Microgrid for Super High Quality Distribution

Abstract: Microgrid is one of the new conceptual power systems for smooth installation of many distributed generations (DGs). While most of the microgrids adopt ac distribution as well as conventional power systems, dc microgrids are proposed and researched for the good connection with dc output type sources such as photovoltaic (PV) system, fuel cell, and secondary battery. Moreover, if loads in the system are supplied with dc power, the conversion losses from sources to loads are reduced compared with ac microgrid. As one of the dc microgrids, we propose “low-voltage bipolar-type dc microgrid,” which can supply super high quality power with three-wire dc distribution line. In this paper, one system for a residential complex is presented as an instance of the dc microgrid. In this system, each house has a cogeneration system (CGS) such as gas engine and fuel cell. The output electric power is shared among the houses, and the total power can be controlled by changing the running number of CGSs. Super capacitors are chosen as main energy storage. To confirm the fundamental characteristics and system operations, we experimented with a laboratory scale system. The results showed that the proposed system could supply high-quality power under several conditions.

Condition Monitoring for Device Reliability in Power Electronic Converters: A Review

Abstract: Condition monitoring (CM) has already been proven to be a cost effective means of enhancing reliability and improving customer service in power equipment, such as transformers and rotating electrical machinery. CM for power semiconductor devices in power electronic converters is at a more embryonic stage; however, as progress is made in understanding semiconductor device failure modes, appropriate sensor technologies, and signal processing techniques, this situation will rapidly improve. This technical review is carried out with the aim of describing the current state of the art in CM research for power electronics. Reliability models for power electronics, including dominant failure mechanisms of devices are described first. This is followed by a description of recently proposed CM techniques. The benefits and limitations of these techniques are then discussed. It is intended that this review will provide the basis for future developments in power electronics CM.

A Medium-Voltage Motor Drive With a Modular Multilevel PWM Inverter

Abstract: This paper describes the control and operating performance of a modular multilevel PWM inverter for a transformerless medium-voltage motor drive. The inverter is prominent in the modular arm structure consisting of a cascaded stack of multiple bidirectional chopper-cells. The dominant ac-voltage fluctuation with the same frequency as the motor (inverter) frequency occurs across the dc capacitor of each chopper-cell. The magnitude of the voltage fluctuation is inversely proportional to the motor frequency. This paper achieves theoretical analysis on the voltage fluctuation, leading to system design. A downscaled model rated at 400 V and 15 kW is designed and built up to confirm the validity and effectiveness of the nine-level (17-level in line-to-line) PWM inverter for a medium-voltage motor drive.

Switched Inductor Z-Source Inverter

Abstract: On the basis of the classical Z-source inverter, this paper presents a developed impedance-type power inverter that is termed the switched inductor (SL) Z-source inverter. To enlarge voltage adjustability, the proposed inverter employs a unique SL impedance network to couple the main circuit and the power source. Compared with the classical Z-source inverter, the proposed inverter increases the voltage boost inversion ability significantly. Only a very short shoot-through zero state is required to obtain high voltage conversion ratios, which is beneficial for improving the output power quality of the main circuit. In addition, the voltage buck inversion ability is also provided in the proposed inverter for those applications that need low ac voltages. Similar to the classical Z-source inverter, the proposed concepts of SL Z-source inverter can be applied to various applications of dc-ac, ac-ac, dc-dc, and ac-dc power conversion. A detailed topology analysis and a generalized discussion are given. Both simulation and experimental results verify the analytical results.

Effects of Discretization Methods on the Performance of Resonant Controllers

Abstract: Resonant controllers have gained significant importance in recent years in multiple applications. Because of their high selectivity, their performance is very dependent on the accuracy of the resonant frequency. An exhaustive study about different discrete-time implementations is contributed in this paper. Some methods, such as the popular ones based on two integrators, cause that the resonant peaks differ from expected. Such inaccuracies result in significant loss of performance, especially for tracking high-frequency signals, since infinite gain at the expected frequency is not achieved, and therefore, zero steady-state error is not assured. Other discretization techniques are demonstrated to be more reliable. The effect on zeros is also analyzed, establishing the influence of each method on the stability. Finally, the study is extended to the discretization of the schemes with delay compensation, which is also proved to be of great importance in relation with their performance. A single-phase active power filter laboratory prototype has been implemented and tested. Experimental results provide a real-time comparison among discretization strategies, which validate the theoretical analysis. The optimum discrete-time implementation alternatives are assessed and summarized.

Cooperative Control Strategy of Energy Storage System and Microsources for Stabilizing the Microgrid during Islanded Operation

Abstract: In this paper, the cooperative control strategy of microsources and the energy storage system (ESS) during islanded operation is presented and evaluated by a simulation and experiment. The ESS handles the frequency and the voltage as a primary control. And then, the secondary control in microgrid management system returns the current power output of the ESS into zero. The test results show that the proposed cooperative control strategy can regulate the frequency and the voltage, and the secondary control action can contribute to improve the control capability.

Design and Analysis of a Grid-Connected Photovoltaic Power System

Abstract: A grid-connected photovoltaic (PV) power system with high voltage gain is proposed, and the steady-state model analysis and the control strategy of the system are presented in this paper. For a typical PV array, the output voltage is relatively low, and a high voltage gain is obligatory to realize the grid-connected function. The proposed PV system employs a ZVT-interleaved boost converter with winding-coupled inductors and active-clamp circuits as the first power-processing stage, which can boost a low voltage of the PV array up to a high dc-bus voltage. Accordingly, an accurate steady-state model is obtained and verified by the simulation and experimental results, and a full-bridge inverter with bidirectional power flow is used as the second power-processing stage, which can stabilize the dc-bus voltage and shape the output current. Two compensation units are added to perform in the system control loops to achieve the low total harmonic distortion and fast dynamic response of the output current. Furthermore, a simple maximum-power-point-tracking method based on power balance is applied in the PV system to reduce the system complexity and cost with a high performance. At last, a 2-kW prototype has been built and tested to verify the theoretical analysis of the paper.

A Technique for Estimating the State of Health of Lithium Batteries Through a Dual-Sliding-Mode Observer

Abstract: A new state-of-health (SOH) estimation method for lithium batteries that uses a dual-sliding-mode observer is presented. The capacity fade and resistance deterioration were estimated by the dual-sliding-mode observer. The dual-sliding-mode observer consists of a fast-paced time-varying observer and a slow-paced time-varying observer. The fast-paced time-varying observer estimates parameters, such as the state of charge and terminal voltage, and polarization effects. The slow-paced time-varying observer estimates the SOH in terms of the capacity fade and resistance deterioration. The convergence of the proposed observer was proved by the Lyapunov equation. The structure of the proposed system is simple and easy to implement and shows robust control properties against modeling errors and temperature variations. The test results show that the proposed observer system has superior tracking performance than previous ampere-counting method.

Analysis and Design of High-Frequency Isolated Dual-Bridge Series Resonant DC/DC Converter

Abstract: Bidirectional dual-bridge dc/dc converter with high frequency isolation is gaining more attentions in renewable energy system due to small size and high-power density. In this paper, a dual-bridge series resonant dc/dc converter is analyzed with two simple modified ac equivalent circuit analysis methods for both voltage source load and resistive load. In both methods, only fundamental components of voltages and currents are considered. All the switches may work in either zero-voltage-switching or zero-current-switching for a wide variation of voltage gain, which is important in renewable energy generation. It is also shown in the second method that the load side circuit could be represented with an equivalent impedance. The polarity of cosine value of this equivalent impedance angle reveals the power flow direction. The analysis is verified with computer simulation results. Experimental data based on a 200 W prototype circuit is included for validation purpose.

Rotor Voltage Dynamics in the Doubly Fed Induction Generator During Grid Faults

Abstract: This paper presents a new control strategy for the rotor-side converter (RSC) of wind turbines (WTs) based on doubly fed induction generators (DFIG) that intends to improve its low-voltage ride through capability. The main objective of this work is to design an algorithm that would enable the system to control the initial overcurrents that appear in the generator during voltage sags, which can damage the RSC, without tripping it. As a difference with classical solutions, based on the installation of crowbar circuits, this operation mode permits to keep the inverter connected to the generator, something that would permit the injection of power to the grid during the fault, as the new grid codes demand. A theoretical study of the dynamical behavior of the rotor voltage is also developed, in order to show that the voltage at the rotor terminals required for the control strategy implementation remains under controllable limits. In order to validate the proposed control system simulation, results have been collected using PSCAD/EMTDC and experimental tests have been carried out in a scaled prototype.

Modeling and Control of Three-Port DC/DC Converter Interface for Satellite Applications

Abstract: This paper presents the control strategy and power management for an integrated three-port converter, which interfaces one solar input port, one bidirectional battery port, and an isolated output port. Multimode operations and multiloop designs are vital for such multiport converters. However, control design is difficult for a multiport converter to achieve multifunctional power management because of various cross-coupled control loops. Since there are various modes of operation, it is challenging to define different modes and to further implement autonomous mode transition based on the energy state of the three power ports. A competitive method is used to realize smooth and seamless mode transition. Multiport converter has plenty of interacting control loops due to integrated power trains. It is difficult to design close-loop controls without proper decoupling method. A detailed approach is provided utilizing state-space averaging method to obtain the converter model under different modes of operation, and then a decoupling network is introduced to allow separate controller designs. Simulation and experimental results verify the converter control design and power management during various operational modes.

Protection Principles for Future Microgrids

Abstract: Realization of future low-voltage (LV) microgrids requires that all technical issues, such as power and energy balance, power quality and protection, are solved. One of the most crucial one is the protection of LV microgrid during normal and island operation. In this paper, protection issues of LV microgrids are presented and extensions to the novel LV-microgrid-protection concept has been developed based on simulations with PSCAD simulation software. Essential in the future-protection concept for LV microgrids will be the utilization of high speed, standard, e.g., IEC-61850-based communication to achieve fast, selective, and reliable operation protection.

Design of Parallel Inverters for Smooth Mode Transfer Microgrid Applications

Abstract: In this paper, smooth mode transfers and accurate current sharing are performed in a multi-inverter-based microgrid system by the designed system level controls with control area network communication. Controllers of individual inverters within the microgrid in both grid-tie and islanding modes are also designed to ensure high-quality output waveforms. The mode transfer tests are conducted with an inverter-simulated grid to define the proper transfer procedures. Experimental results show that the inverters can provide stable outputs in different basic microgrid operation modes. With the designed current sharing scheme, the output current is equally shared among paralleled inverters without noticeable circulating current. Both the simulation and experimental results of mode transfer show that the multi-inverter-based microgrid system is able to smoothly switch between the grid-tie and islanding modes to guarantee an uninterrupted power supply to the critical loads within the microgrid.

Predictive Direct Power Control of Three-Phase Pulsewidth Modulation (PWM) Rectifier Using Space-Vector Modulation (SVM)

Abstract: In this paper, we present a direct power control (DPC) of three-phase pulsewidth modulation rectifier with constant switching frequency using space-vector modulation (SVM). The developed DPC scheme is based on the predictive control strategy to achieve direct control of instantaneous active and reactive power of the converter. For this purpose, at the beginning of each switching period, the required rectifier average voltage vector allowing the cancellation of active and reactive power tracking errors, at the end of the switching period, is calculated by means of predictive control algorithm in the sense of deadbeat control. The main advantages of the proposed control, compared to the works published in this subject, are that no need to use predefined switching table and voltage vector or virtual flux position, PI-based active and reactive power control loops are not necessary and constant-switching frequency. The proposed predictive direct power control was tested both in simulations and experimentally and compared with DPC using switching table. Results have proved excellent performance, and verify the validity of the proposed DPC scheme, which is much better than conventional DPC using switching table.

The Balance of Renewable Sources and User Demands in Grids: Power Electronics for Modular Battery Energy Storage Systems

Abstract: The continuously growing amount of renewable sources starts compromising the stability of electrical grids. Contradictory to fossil fuel power plants, energy production of wind and photovoltaic (PV) energy is fluctuating. Although predictions have significantly improved, an outage of multi-MW offshore wind farms poses a challenging problem. One solution could be the integration of storage systems in the grid. After a short overview, this paper focuses on two exemplary battery storage systems, including the required power electronics. The grid integration, as well as the optimal usage of volatile energy reserves, is presented for a 5- kW PV system for home application, as well as for a 100- MW medium-voltage system, intended for wind farm usage. The efficiency and cost of topologies are investigated as a key parameter for large-scale integration of renewable power at medium- and low-voltage.

Extended-Boost $Z$ -Source Inverters

Abstract: The Z-source inverter has gained popularity as a single-stage buck-boost inverter topology among many researchers. However, its boosting capability could be limited, and therefore, it may not be suitable for some applications requiring very high boost demand of cascading other dc-dc boost converters. This could lose the efficiency and demand more sensing for controlling the added new stages. This paper is proposing a new family of extended-boost quasi Z -source inverter (ZSI) to fill the research gap left in the development of ZSI. These new topologies can be operated with same modulation methods that were developed for original ZSI. Also, they have the same number of active switches as original ZSI preserving the single-stage nature of ZSI. Proposed topologies are analyzed in the steady state and their performances are validated using simulated results obtained in MATLAB/Simulink. Furthermore, they are experimentally validated with results obtained from a prototype developed in the laboratory.

A Method of Reducing the Peak-to-Average Ratio of LED Current for Electrolytic Capacitor-Less AC–DC Drivers

Abstract: This paper proposes a concept of electrolytic capacitor-less light-emitting diode (LED) driver, which converts the commercial ac voltage to a pulsating current with twice the line frequency driving high-brightness LEDs. As no electrolytic capacitor is used, this driver possesses the unique advantage of long lifetime to match with that of LEDs. A method of injecting the third and fifth harmonics into the input current to reduce the peak-to-average ratio of the output current is also proposed. While ensuring that the input power factor is higher than 0.9 to meet regulation standards such as ENERGY STAR, the proposed method allows the peak-to-average ratio of the output current to be reduced to 1.34 theoretically, which is beneficial for the safe operation of the LEDs. As an example, a flyback-based electrolytic capacitor-less LED driver is proposed, and its operation is analyzed. In order to inject the third and fifth harmonics into the input current, the function of the duty cycle in a half-line cycle is derived. It is then simplified to a fitting function, which can be easily implemented with the input voltage sensing. A 25 V, 0.35 A output prototype is built and tested in the laboratory, and the experimental results are presented to verify the effectiveness of the electrolytic capacitor-less LED driver and its control method.

Maximum Power Point Tracking for Photovoltaic Optimization Using Ripple-Based Extremum Seeking Control

Abstract: This study develops a maximum power point tracking algorithm that optimizes solar array performance and adapts to rapidly varying irradiance conditions. In particular, a novel extremum seeking (ES) controller that utilizes the natural inverter ripple is designed and tested on a simulated solar array with a grid-tied inverter. The new algorithm is benchmarked against the perturb and observe (PO) method using high-variance irradiance data gathered on a rooftop array experiment in Princeton, NJ. The ES controller achieves efficiencies exceeding 99% with transient rise-time to the maximum power point of less than 0.1 s. It is shown that voltage control is more stable than current control and allows for accurate tracking of faster irradiance transients. The limitations of current control are demonstrated in an example. Finally, the effect of capacitor size on the performance of ripple-based ES control is investigated.

Full Feedforward of Grid Voltage for Grid-Connected Inverter With LCL Filter to Suppress Current Distortion Due to Grid Voltage Harmonics

Abstract: The grid-connected inverter with an LCL filter has the ability of attenuating the high-frequency current harmonics. However, the current distortion caused by harmonics in the grid voltage is difficult to be eliminated. Increasing the loop gain can reduce the current distortion, but this approach is compromised by the system stability requirement. Without increasing the loop gain, applying feedforward of the grid voltage can suppress the effect of grid voltage harmonics. This paper proposes the feedforward function of the grid voltage for the grid-connected inverter with an LCL filter. Specifically, the proposed feedforward function involves proportional, derivative, and second derivative of the grid voltage, and can be simplified according to the dominant harmonics in the grid voltage. The proposed feedforward scheme can effectively suppress the current distortion arising from the grid voltage harmonics, and the steady-state error of the injected current can be substantially reduced even if a conventional proportional and integral regulator is applied. A 6-kW experimental prototype has been tested to verify the effectiveness of the proposed feedforward scheme.

Conducted EMI Mitigation Techniques for Switch-Mode Power Converters: A Survey

Abstract: Several techniques to mitigate conducted electromagnetic interference (EMI) in switch-mode power supplies (SMPS) have been reported in literature. Of these, this paper reviews those techniques that are primarily meant for ac-dc and dc-dc power converters. The techniques are broadly classified based on two criteria-1) reduction of the noise after generation and 2) reduction of the noise at the generation stage itself. A detailed classification and review of various noise mitigation techniques currently available in literature are presented. It is believed that the classification and review of the conducted EMI mitigation techniques presented in this paper would be useful to SMPS researchers and designers.

Decentralized Parallel Operation of Inverters Sharing Unbalanced and Nonlinear Loads

Abstract: In this paper, a wireless control strategy for parallel operation of three-phase four-wire inverters is proposed. A generalized situation is considered where the inverters are of unequal power ratings and the loads are nonlinear and unbalanced in nature. The proposed control algorithm exploits the potential of sinusoidal domain proportional+multiresonant controller (in the inner voltage regulation loop) to make the system suitable for nonlinear and unbalanced loads with a simple and generalized structure of virtual output-impedance loop. The decentralized operation is achieved by using three-phase P/Q droop characteristics. The overall control algorithm helps to limit the harmonic contents and the degree of unbalance in the output-voltage waveform and to achieve excellent power-sharing accuracy in spite of mismatch in the inverter output impedances. Moreover, a synchronized turn on with consequent change over to the droop mode is applied for the new incoming unit in order to limit the circulating current completely. The simulation and experimental results from -1 kVA and -0.5 kVA paralleled units validate the effectiveness of the scheme.

Synthesis of Multiple-Input DC/DC Converters

Abstract: Hybrid power systems continuously deliver power to the load from several renewable energy sources. For such systems, the use of a multiple-input converter (MIC) has the advantage of simpler circuit structure and lower cost, compared to the use of several single-input converters. By decomposing converters into basic cells, namely, pulsating source cells and output filters, a set of basic rules for generating multiple-input converter topologies is proposed. Specifically, two families of multiple-input converters are systematically generated. In the first family of MICs, all the input sources can power the load simultaneously or individually. In the second family, only one power source is allowed to transfer energy to the load at a time. Furthermore, some isolated MICs are simplified for reducing the complexity of the circuit configuration.

A Generalized Computational Method to Determine Stability of a Multi-inverter Microgrid

Abstract: Microgrid-containing parallel-connected inverters, where each inverter is controlled by decentralized active power/voltage frequency and reactive power/voltage magnitude droop control laws results in flexible and expandable systems. These systems have been known to have stability problems for large values of active power/voltage frequency droop control gain. However, so far the stability analysis of multi-inverter systems has always been performed in a computationally intensive manner by considering the entire microgrid. In a practical microgrid, where the number of inverters may be large or the capacity of the units may differ, it becomes essential to develop a method by which stability can be examined without much computational burden. The system of differential algebraic equations has been simplified using justifiable assumptions to result in a final expression that allows the stability of the microgrid to be examined separately with respect to the droop control laws of each inverter transformed into an equivalent network. Moreover, the procedure allows taking into consideration the R/X ratio of the interconnecting cables. Analysis of final expressions validate the stability results reported in literature. Experimental results on hardware show the stable operation of the microgrid.

Seamless Transfer of Single-Phase Grid-Interactive Inverters Between Grid-Connected and Stand-Alone Modes

Abstract: This paper presents a novel seamless transfer of single-phase grid-interactive inverters between grid-connected and stand-alone modes. The grid-connected inverter should operate in grid-tied and off-grid modes in order to provide power to the emergency load during system outages. However, the grid current controller and the output voltage controller are switched between the two modes, so the outputs of both controllers may not be equal during the transfer instant, which will cause the current or voltage spikes during the switching process. The transfer between the two controllers does not exist in the proposed method. In grid-tied mode, the voltage controller is used for compensating the filter capacitor current, and the current controller is used to control the grid current. In stand-alone mode, the voltage controller is used to regulate the output voltage, whereas the output of the current controller is zero. With the proposed control method, the seamless transfer can be achieved between both modes, even in polluted grid voltage. The principle and realization conditions of the control methods at both modes are analyzed. The detailed process of the seamless transfer between the two modes is illustrated. Finally, the simulation and experimental results verify the theoretical analysis.

Direct Power Control of Doubly-Fed-Induction-Generator-Based Wind Turbines Under Unbalanced Grid Voltage

Abstract: In this paper, the behavior of a doubly fed induction generator (DFIG) is studied under unbalanced grid voltage conditions. It is shown that if no special control efforts are employed, the behavior of the generator is deteriorated, basically due to two reasons: electromagnetic torque oscillations and nonsinusoidal current exchange with the grid. These phenomena are first analyzed theoretically as a function of the stator active and reactive instantaneous power exchange by the stator of the DFIG and the grid-side converter (GSC). This analysis provides the main ideas for generation of the active and reactive power references for the rotor-side converter (RSC) and the GSC, controlled by means of direct power control techniques. Therefore, this paper proposes a new algorithm that generates the RSC power references, without the necessity of a sequence component extraction, in order to eliminate torque oscillations and achieve sinusoidal stator currents exchange. On the contrary, the GSC power references are provided by means of voltage and current sequence extraction. Finally, simulation and experimental results successfully validate the proposed power reference generation methods.

Novel STATCOM Controller for Mitigating SSR and Damping Power System Oscillations in a Series Compensated Wind Park

Abstract: This paper addresses implementation issues associated with a novel damping control algorithm for a STATCOM in a series compensated wind park for mitigating SSR and damping power system oscillations. The IEEE first benchmark model on SSR is adopted with integrating aggregated self-excited induction generator-based wind turbine to perform the studies. The potential occurrence and mitigation of the SSR caused by induction generator effects as well as torsional interactions, in a series compensated wind park, are investigated. The auxiliary subsynchronous damping control loop for the STATCOM based on a novel design procedure of nonlinear optimization is developed to meet the damping torque in the range of critical torsional frequencies. The intelligent shaft monitor (ISM) scheme with synthesized special indicator signals is developed and examined in the STATCOM control structure. The performances of the controllers are tested in steady-state operation and in response to system contingencies, taking into account the impact of SCRs. Simulation results are presented to demonstrate the capability of the controllers for mitigating the SSR, damping the power system oscillation, and enhancing the transient stability margin in response to different SCRs.

PI State Space Current Control of Grid-Connected PWM Converters With LCL Filters

Abstract: Design and analysis of PI state space control for grid-connected pulsewidth modulation (PWM) converters with LCL filters based on pole placement approach is addressed. State space control offers almost full controllability of system dynamic. However, pole placement design is difficult and usually requires much experience. In this paper, a suitable pole placement strategy is proposed, which ensures fulfilling the requirements, which are commonly specified with respect to rise time, overshoot, and proper resonance damping. Controller parameter expressions are derived in terms of system parameters and specified poles and zeros. Hence, straightforward controller tuning for a particular system setting is possible. Performance is analyzed by means of transfer function-based calculations, simulations with MATLAB, and experimental tests. Dynamic performance and robustness against grid impedance variations are addressed as well as harmonic rejection capability and other practical issues.

Interleaved Converter With Voltage Multiplier Cell for High Step-Up and High-Efficiency Conversion

Abstract: A novel interleaved high step-up converter with voltage multiplier cell is proposed in this paper to avoid the extremely narrow turn-off period and to reduce the current ripple, which flows through the power devices compared with the conventional interleaved boost converter in high step-up applications. Interleaved structure is employed in the input side to distribute the input current, and the voltage multiplier cell is adopted in the output side to achieve a high step-up gain. The voltage multiplier cell is composed of the secondary windings of the coupled inductors, a series capacitor, and two diodes. Furthermore, the switch voltage stress is reduced due to the transformer function of the coupled inductors, which makes low-voltage-rated MOSFETs available to reduce the conduction losses. Moreover, zero-current-switching turn- on soft-switching performance is realized to reduce the switching losses. In addition, the output diode turn-off current falling rate is controlled by the leakage inductance of the coupled inductors, which alleviates the diode reverse recovery problem. Additional active device is not required in the proposed converter, which makes the presented circuit easy to design and control. Finally, a 1-kW 40-V-input 380-V-output prototype operating at 100 kHz switching frequency is built and tested to verify the effectiveness of the presented converter.

Virtual Impedance Loop for Droop-Controlled Single-Phase Parallel Inverters Using a Second-Order General-Integrator Scheme

Abstract: This paper explores the impact of the output impedance on the active and reactive power flows between parallelized inverters operating with the droop method. In these systems, a virtual output impedance is usually added to the control loop of each inverter to improve the reactive power sharing, regardless of line-impedance unbalances and the sharing of nonlinear loads. The virtual impedance is usually implemented as the time derivative of the inverter output current, which makes the system highly sensitive to the output current noise and to nonlinear loads with high slew rate. To solve this, a second-order general-integrator (SOGI) scheme is proposed to implement the virtual impedance, which is less sensitive to the output current noise, avoids to perform the time derivative function, achieves better output-voltage total harmonic distortion, and enhances the sharing of nonlinear loads. Experimental results with two 2-kVA inverter systems under linear and nonlinear loads are provided to validate this approach.

An Integrated Four-Port DC/DC Converter for Renewable Energy Applications

Abstract: This paper proposes a novel converter topology that interfaces four power ports: two sources, one bidirectional storage port, and one isolated load port. The proposed four-port dc/dc converter is derived by simply adding two switches and two diodes to the traditional half-bridge topology. Zero-voltage switching is realized for all four main switches. Three of the four ports can be tightly regulated by adjusting their independent duty-cycle values, while the fourth port is left unregulated to maintain the power balance for the system. Circuit analysis and design considerations are presented; the dynamic modeling and close-loop design guidance are given as well. Experimental results verify the proposed topology and confirm its ability to achieve tight independent control over three power-processing paths. This topology promises significant savings in component count and losses for renewable energy power-harvesting systems.