Output Filter Research Articles

A High-Gain Multilevel dc–dc Converter for Interfacing Electric Vehicle Battery and Inverter

An isolated multilevel bidirectional dc–dc converter is presented in this article to interface the low-voltage (LV) battery and the high-voltage (HV) propulsion inverter in electric vehicles. The proposed topology achieves a HV step-up ratio in the motoring mode by reconfiguring two split capacitor half-bridge circuits in series/parallel through the symmetrical modulation of a voltage bidirectional, two quadrant switch. This reconfiguration generates a high-frequency multilevel voltage output at the dc-bus side of the inverter, which reduces the output filter volume. The dc-bus voltage is regulated by the duty cycle modulation of the intermediary switch in the motoring mode, simplifying the sensing and control. During regenerative braking, the converter is operated with phase shift modulation between the HV half bridges and the LV full bridge to regulate the battery voltage. The presented modulation strategies achieve zero voltage switching for the full-bridge devices over a wide load range. A comprehensive analysis of the proposed multilevel converter in motoring and braking modes along with the design constraints and soft switching boundary conditions is presented in this article. Detailed experimental results on a 1.5-kW laboratory prototype are presented to verify the design approach and illustrate the converter performance.

Leakage-Ground Currents Compensation in a Transformerless HB-NPC Topology Using a DC-Link-Tied LC Filter for Photovoltaic Applications

In this article, a solution to mitigate the leakage-ground currents (LGCs) in a grid-tied H-bridge neutral-point-clamped inverter is presented. This solution consists in splitting the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter capacitor and connecting each capacitor to a dc-link terminal. The proposal is referred to as the dc-link-tied <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> output filter. Out of this hardware modification, it is not necessary to increase the number of switches nor design a new modulation scheme. Furthermore, a formal analysis is provided based on the common-mode (CM) model describing the role of this particular connection of the output <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter on the minimization of the LGC. In addition, a closed-loop control strategy has been implemented to allow testing the prototype connected to the electrical grid. The mitigation of the LGCs has been evaluated under this condition as well. To validate the proposed circuit modification, numerical simulations and experimental tests have been performed in a 1-kW laboratory prototype of the H-bridge neutral-point-clamped inverter with the dc-link-tied <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter solution.

Modeling and Analysis of Multiple Inverters With Dual-Loop-Based Virtual Oscillator Control

Virtual oscillator control (VOC) is an emerging decentralized control technique for grid-forming inverter applications. In contrast to conventional phasor-based droop control or virtual synchronous machine control, VOC is a time-domain controller designed to emulate the dynamics of a nonlinear oscillator. VOC is a current-controlled voltage source, lacking the ability to regulate the inverter terminal voltage, which will result in significant voltage deviations caused by dead-time effects, nonideal semiconductor devices, and output filter voltage drops. In this article, a voltage and current dual-loop control structure augments the VOC to compensate for these voltage deviations and regulate the inverter output variables directly. A complete small-signal model for a multiple inverter-based microgrids with the proposed control structure is presented in order to assess system stability using eigenvalue and participation factor analysis. Analytical results show that the parameter related to the frequency regulation and the integral gain of the voltage controller affect the location of the system’s dominant modes significantly. The stability margin is determined by modifying these control parameters. Experimental results on a laboratory test microgrid verify the predication from the small-signal analysis and time-domain simulations.

An Observer-Based Current Sensor-Less Control Scheme for Grid-Following Converters

Distributed energy resources based on grid-following inverters are the dominant part of future modern power systems. To achieve a higher performance, the reliability enhancements and cost reductions of such inverters are the most important demands. In this respect, the sensors are one of the elements which greatly determine the cost and reliability of the inverters, and minimizing the number of sensors may achieve both lower costs and higher reliability. In addition, sensor data are prone to cyber-attacks, and sensor-less control techniques would improve the cyber-physical tolerance capabilities of the system. This paper proposes a grid-side current sensor-less control technique for grid-following inverters. In the proposed scheme, an observer is suggested which estimates the grid-side currents by processing the measured DC-link voltage. The estimated grid-side currents are fed to the control loops of the inverter. To verify the effectiveness of the proposed scheme, the stability of the overall control structure is analyzed, including the nonlinear dynamics of the DC-link voltage in the various operating points, using eigen value analysis and time domain simulations. The results demonstrate that the proposed scheme preserves the proper stability margin and performance of the GF-VSC, even in the presence of uncertainty in output filter inductance.

Model-Free Predictive Control of Grid-Forming Inverters With $LCL$ Filters

With the increasing penetration of distributed energy resources into islanded microgrids, a grid-forming inverter (GFI) has become the key element interfacing renewable energy sources. Usually, the GFI is employed with an output filter to minimize the harmonic content achieving high-quality output voltage regulation. To this end, model-predictive control (MPC) has been widely proposed to control the output voltage of GFI systems due to the high-quality performance, fast control response, and straightforward handling of constraints. However, an accurate model of the system is required for the conventional MPC to avoid a suboptimal performance under uncertainties. To overcome this known drawback, a novel model-free predictive control is proposed in this article. Consequently, the output voltage of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LCL$</tex-math></inline-formula> -filtered GFI is regulated without the knowledge of the physical model.

A 380 V/50 kVar SiC-SVG Achieving a Power Density of 1.652 kVar/L With the Optimization of Heatsink and Output Filter Volume

With the rapid development of new energy vehicles, 5G facilities, electric aircraft, and new energy power generation, the high-power-density inverters are developed rapidly. A comprehensive optimization method combining heatsink and inductance filter was proposed in this article. Based on accurate thermal modeling and magnetic modeling, the method proposed chose an optimized switching frequency to minimize the volume of the inverters by compromising the design of the heatsink and inductor volume. Among them, a model of multichip forced air-cooling heatsink was proposed to reduce the size of the heatsink, and the comprehensive thermal resistance error of the proposed model is within 2%. The structure of the grid-connected filter and the inductor model is analyzed showing that the single inductor L-filter has a smaller size and more stability than the LCL-filter at a high switching frequency. The comprehensive optimization method proposed was implemented by 380-V/50-kVar SiC-static var generator (SVG) with the 50-kHz switching frequency, and its topology is a two-level inverter with a single inductor filter. This SiC-SVG reached a volume of 30.2 L (including all components required for mainstream products), which is 50% lower than insulated gate bipolar translator (IGBT)-SVGs. Its efficiency is 98.5% at full load, which is 1.5% higher than IGBT-SVGs. The experiments and temperature rising tests at full load proved the feasibility of the comprehensive optimization method.

A Voltage Spike Suppression Strategy Based on De-Re-Coupling Idea for the Three-Phase High-Frequency Isolated Matrix-Type Inverter

In order to solve the problem of high voltage spikes on the secondary side of the high-frequency transformer when the three-phase high-frequency isolated matrix inverter (HFIMI) operates under the conventional modulation strategies, a new modulation strategy is proposed without introducing an auxiliary circuit. In the proposed scheme, the H-bridge inverter adopts phase-shift control. The matrix converter (MC) adopts the voltage-type de- and recoupling idea, and the decoupled positive and negative group inverters are, respectively, applied with a modified Space vector pulse width modulation (SVPWM) strategy to operate synchronously with the H-bridge inverter. When the H-bridge inverter is in dead-zone mode, the switching tubes of MC are all turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> to provide continuous flow paths for leakage inductance and output filter inductance current and suppress high voltage spikes. The operating mode of the three-phase HFIMI under the proposed modulation strategy is analyzed in detail, the realization conditions for soft switching are designed, and the soft-switching ranges are also discussed. The feasibility and validity of the voltage spike suppression strategy are verified by building a 3-kW principle prototype. The experimental results show that the voltage spikes are effectively suppressed, all switches achieve zero-voltage soft-switching, and the peak efficiency of the three-phase HFIMI can reach 95.2%.

A phase‐shift modulated series resonant converter achieving zero‐voltage and zero‐current switching for wide output voltage range battery charging applications

In this article, a phase-shift modulated zero-voltage and zero-current switching (ZVZCS) series resonant converter (SRC) was proposed for constant current (CC) and constant voltage (CV) charging operating at the series resonant frequency. Wide output voltage range is achieved by adjusting the duty cycle of the primary inverter for CC charging and unity gain is for CV charging at full duty cycle. By tuning resonant tank parameters, discontinuous conduction mode (DCM) and boundary conduction mode (BCM) are obtained in CC charging and CV charging, respectively, which ensures zero-current switching (ZCS) of rectifier diodes and nearly ZCS of leading leg in whole charging profile. Meanwhile, a constant triangular current provided by a novel auxiliary circuit is used to implement zero-voltage switching (ZVS) for all primary MOSFETs with a reduced conduction loss. The auxiliary current is independent of duty cycle and load variation. Compared with the conventional ZVZCS phase-shift full-bridge converter with capacitive output filter, smaller conduction loss and turn-off loss can be obtained in battery charging profile. Detailed circuit operation principles and model analysis are presented. A 100 kHz/1 kW prototype is built to verify the performance.

Actual Sensing Sensitivity and SNR Measurement of Optical Tweezers Based on Coulomb Force Input

Sensing sensitivity is the key performance of optical tweezers. By adjusting the frequency and magnitude of an applied Coulomb force as an input of optical tweezers, we directly measured the sensitivity and signal-to-noise ratio (SNR) of a system and indirectly calculated the actual noise magnitude. Combined with an output filter, the relationship between the SNR and bandwidths was studied. We established the simulation model of a system using Simulink and simulated the relationship between the SNR and magnitude of the input forces and filter bandwidths. In addition, we built an experimental system to determine the relationship between the SNR and the magnitude of the input forces and filter bandwidths. The actual minimum detectable force was measured as 1.8275×10−17 N at a 1 Hz bandwidth. The experimental results were correlated with the simulation and theoretical results, confirming the effectiveness of the proposed method and demonstrating the high sensitivity of vacuum optical tweezers as mechanical sensors. We proposed a novel method of calibration and measurement of system sensing parameters by applying an actual force that was more direct and precise than the theoretical calculation method that requires accurate fitting parameters, such as the particle radius and density. This method can be employed to analyze the system noise and phase characteristics to confirm and improve the real performance of the system.

A voltage type single-stage multi-input distributed generation system

To solve the problems of unstable and discontinuous power supply resulted by single input power generation, like photovoltaic(PV) and wind energy changing with climatic conditions, it&apos;s significant to research multiple new energy combined power generation systems. A voltage type single-stage multi-input distributed generation system(PGS) is proposed and studied in this paper. The system&apos;s circuit topology, control strategy and derivation of multiple duty cycles are researched thoroughly. This system consists of multiple input sources and their corresponding filter capacitors, parallel time-sharing selector switches, inverter bridge, output filter element and line-frequency(LF) transformers. In the control strategy, the maximum power point tracking (MPPT) is used in the outer loop, and grid-connected current (GCC) tracking is used in the inner loop. The duty cycle of each power source is deduced under this control strategy. The problems of voltage spike and DC bias are solved. A 3kW experimental prototype is designed, and the results are favorable, which proves that the system has practical significance.

On the Use of Nonlinear Inductor to Enhance the Stability of DC Distribution Networks

Typical dc distribution networks contain source-interface dc–dc converters for feeding constant power loads (CPLs). Instability may arise if the interaction between the output impedance of the converters and the input impedance of the CPLs does not satisfy stability criteria. A simple technique that aims to deal with such issue is proposed. Instead of shaping the network characteristics with sophisticated control laws or introducing passive or active damping, the proposed idea is based on using an output filter with a nonlinear inductor in the converter to enhance small-signal and large-signal stabilities. First, the eigenvalues of the Jacobian at the equilibrium points can be maintained in the left-half plane. Second, the system state can always be reverted to the target operating point after large-signal perturbation. Furthermore, the physical size of the nonlinear inductor is comparable with that of the fixed-value inductor in the output filter without sacrificing ripple current requirement. The proposed method has been evaluated on a downscaled system with the power level of 80 W and bus voltage of 24 V. The experimental results are in good agreement with the theoretical predictions.

Zero-Voltage Switching Full-Bridge Converter With Reduced Filter Requirement and Wide ZVS Range for Variable Output Application

In this article, a novel zero-voltage switching (ZVS) full-bridge converter is proposed to overcome the disadvantages of traditional phase-shifted full-bridge (PSFB) converter. In the proposed converter, a half-bridge (HB) converter with center-tap rectifier (CTR) is integrated into the traditional PSFB converter by sharing the lagging-leg switches and output filter. The HB converter operates with full duty cycle to ensure continuous energy transmission and the output voltage is regulated by adjusting the phase-shifted time of full-bridge (FB) converter. The proposed converter has many advantages such as reduced filter requirement, wide ZVS range and low primary circulating current. In addition, the rectifier in the proposed converter is composed of a full-bridge rectifier section and a CTR section. This feature makes the proposed converter well appropriate for the medium output voltage applications. Key operation principle and characteristics are fully discussed in this article. The effectiveness of proposed converter is verified using a 1.2 kW prototype circuit with 280 V input voltage and 60–120 V output voltage.

Harmonic Analysis of Droop-Based Islanded Microgrids

Islanded microgrids have specific features that should be considered in their harmonic analysis. First, microgrids&#x2019; distributed generators (DGs) are typically droop-based, and accordingly, the steady-state frequency is different from the nominal one. This affects the harmonics produced by DGs and nonlinear loads. Second, there is no slack bus, which affects the distribution of the fundamental currents, and thus, the harmonics. Third, DGs usually have voltage source converters (VSCs) as an interface for energy transfer to microgrids. The control structures and output filters of DGs affect the voltage and current distortions, and hence, they should be considered in the harmonic analysis. This paper introduces a sequential algorithm for the harmonic analysis of droop-based islanded microgrids that considers these features. For being accurate and yet computationally efficient, DGs&#x2019; cascaded control loops are modeled in the proposed algorithm through their transfer functions. Due to its quadratic convergence, a Newton-based approach is proposed for modeling nonlinear loads that relies on an analytical Jacobian matrix to further speed up its convergence. The meritorious advantage of the proposed approach is the consideration of the mutual coupling between different harmonics. The effectiveness of the proposed algorithm is confirmed through comparative evaluations with time-domain simulations.

Modified W-type configuration for a single-phase reduced parts count 81-level inverter.

The technology of modern power systems is revolutionizing as renewable energy sources are being integrated with electric power grids. In the form of inverters, power electronic converters are becoming an integral part of power systems due to their massive demand for grid integration of photovoltaic (PV) systems. Existing multilevel inverter topologies either require an output filter to get a sinusoidal voltage or generate a higher number of output voltage levels at the expense of many hardware resources. This paper presents a new single-phase 81-level inverter configuration with the name given as 'Modified W-Type Multilevel Inverter'. The proposed inverter configuration uses only eighteen power switches and four DC voltage sources to generate an 81-level output voltage approaching a sinusoidal waveform without an output filter. The general design equations are developed to calculate the number of switches, the number of voltage levels, and the number of DC sources for the proposed configuration. Loss and efficiency analysis is carried out that verifies a good practical efficiency of the proposed inverter configuration during the dynamic operation. A comparative analysis with the existing MLI topologies is also carried out that validates the effectiveness and novelty in reducing parts count and higher number of voltage levels. The proposed topology offers 1.04% total harmonic distortion of the output voltage which is within the benchmarks specified for grid integration without any filter requirements. The proposed inverter configuration is simulated in MATLAB/Simulink, and the results are validated by the design and development of a hardware prototype.

Single-Stage Multi-Input Buck Type High-Frequency Link's Inverters With Series and Simultaneous Power Supply

A class of single-stage multi-input Buck type high-frequency link's inverters with series and simultaneous power supply are proposed in this article, and the key technologies such as circuit structures and topological family, energy management control strategy (EMCS), power supply modes, steady-state principle characteristics are studied in depth, from which some important conclusions are obtained. The circuit structure is composed of a multi-input-single-output high-frequency inverter, high-frequency transformer, cycloconverter, and output filter in sequence. The EMCS is to generate bipolar tri-state multilevel sinusoidal pulse width modulation (SPWM) waves by phase shifting between right and left arms of the high-frequency inverting bridge, and the cycloconverter demodulates it into a unipolar tristate multilevel SPWM wave and performs commutation when the unipolar tristate multilevel SPWM wave is zero. In addition, the EMCS realizes the, smooth switching of different power supply modes and the stability of output voltage. 1 kVA multi-input inverter prototype is designed and developed, which has the advantages of single-stage power conversion, high-frequency electrical isolation, small volume and weight, wide duty cycle regulation range, high conversion efficiency, high output voltage waveform quality and strong load adaptability. It provides a kind of effective method for a variety of new energy joint power supply.

Single-Phase Rectifier With Reduced Common-Mode Current, Auto-PFC, and Power Decoupling Ability

Transformer and electrolytic capacitors (Ecaps) in light-emitting diode (LED) drivers lead to increased cost and shortened lifespan. Therefore, eliminating the transformer and Ecaps of the LED driver can not only achieve a lightweight design but also extend the service life. In this article, a novel single-phase transformerless capacitorless power factor correction rectifier is proposed. The proposed topology shares a common ground with the grid and the load. Thus, the reduced common-mode current is obtained, and the demand for an isolation transformer is eliminated. In addition, this article presents a unidirectional active buffer circuit to isolate the pulsating power, and the capacitor of the buffer circuit is multiplexed as an output filter. Compared with similar works, a lower voltage on the decoupling capacitor, fewer power components, and a simple control unit are obtained. Then, the operation principle and switching modes as well as the parameter design and implementation of the control strategy are elaborated systematically. Finally, an 80-W experimental prototype is built, and the experimental results confirm the feasibility of the proposed rectifier by reaching the efficiency up to 97.1% and the power factor to 0.984 at a wide range of operating voltage.

Modeling and Analysis of the Harmonic Interaction between Grid-Connected Inverter Clusters and the Utility Grid

The virtual synchronous generator (VSG) is a promising technology for future utility grids, since it can mimic the output characteristics of a synchronous generator, which provides the necessary inertia to a utility grid. However, the large-scale application of VSGs is limited due to the harmonic interaction between VSGs and the utility grid. Therefore, in order to investigate the stability issue as well as improve the practical application for large-scale power stations, the harmonic interaction mechanism between the VSG cluster and the utility grid is addressed. Firstly, the output impedance model of a single VSG is established, and it is found that the resonance frequency is related to parameters including the output filter, controller, and grid impedance. On this basis, the capacitor current control for a grid-connected inverter based on a VSG is proposed to enhance the resonance suppression. Furthermore, the output impedance of the VSG cluster is established, which reveals the harmonic interaction characteristics between the VSG cluster and the utility grid. In addition, in order to suppress the resonance and improve the stability, an inner-loop control strategy of VSG is introduced. Finally, the simulation and experimental results verified the correctness of the established modeling and analysis of the harmonic interaction between the clustered VSGs and the utility grid. The results show that the proposed impedance model is correct and can predict the resonant point accuracy (which is around 2.3 kHz in the simulation and experimental cases). The total harmonic distortion (THD) can be reduced to 3.2% which meets the requirements of IEEE standard 519.

CONTROL STRATEGY OF PHOTOVOLTAIC GENERATION INVERTER GRID-CONNECTED OPERATING AND HARMONIC ELIMINATION HYBRID SYSTEM

This paper proposes a three-phase three-wire photovoltaic generation inverter grid-connected operating and harmonic elimination hybrid system. The hybrid system mainly consists of photovoltaic array battery, photovoltaic output filter, three-phase voltage-type inverter, inverter output filter and passive filters. Based on working principle and working characteristics of the proposed hybrid system, the composite control strategy about active power, reactive power and harmonic suppression is proposed. The composite control strategy mainly consists of a single closed-loop control slip of active power and reactive power, double closed-loop control slip of harmonics. Simulation results show the correctly of this paper’s contents, the hybrid system have an effective to improve power factor, supply active power for loads and suppress Key words - Micro grid; harmonic restraint; active power control; reactive power control; photovoltaic generation.

A simple architecture for high performance class-D audio amplifier with novel RC network as negative feedback loop

&lt;span&gt;In this work, we have designed and simulated a proposed architecture and good performances single-ended class-D audio amplifier (CDA). The proposed circuit is build using pulse-width modulation (PWM). We have proposed a novel RC network as negative feedback loop attached to a second-order integrator to get free of high-frequency carrier ripples and to reach improved output accuracy, thus improved performance and efficiency. This proposed negative feedback loop extremely reduces total harmonic distortion plus noise (THD+N). We have used a passive second-order butterworth output filter. This circuit fulfills a power supply rejection ratio (PSRR) of 75 dB, a THD+N of 0.004%, 3 times less than attained by usual feedback loop. The proposed circuit with quiescent current of 0.02 mA, signal to noise ratio (SNR) of 89 dB, and peak efficiency of 90% is running at 400 kHz switching frequency. The results of simulation demonstrate that this circuit significantly keeps up with the performance of other circuits but taking advantage of its simple architecture.&lt;/span&gt;

Novel Concept of Solar Converter With Universal Applicability for DC and AC Microgrids

This article presents a novel concept of a universal solar converter suitable for application in both in the dc or single-phase ac grids using the same terminals. The idea lies in the utilization of the same semiconductors in the dc–dc and in the dc–ac configuration, resulting in minimal redundancy. Possible semiconductor stages are considered. The particular attention is focused on the output filter design along with proper protection circuit selection for dc and ac grids. The design example and comparative analysis between dc–dc, dc–ac, and universal solutions are given. The experimental prototype of the universal solar converter that is rated for 3.6 kVA power in the ac mode and 5 kW in the dc mode is presented. The experimental results demonstrate the ability of operation in ac or dc grids with main correspondent modes. Possible fields of application along with main benefits are addressed in conclusions.