Showing papers on "Three-phase published in 2019"

A Bidirectional Soft-Switched DAB-Based Single-Stage Three-Phase AC–DC Converter for V2G Application

Abstract: In vehicle-to-grid applications, the battery charger of the electric vehicle (EV) needs to have a bidirectional power flow capability. Galvanic isolation is necessary for safety. An ac–dc bidirectional power converter with high-frequency isolation results in high power density, a key requirement for an on-board charger of an EV. Dual-active-bridge (DAB) converters are preferred in medium power and high voltage isolated dc–dc converters due to high power density and better efficiency. This paper presents a DAB-based three-phase ac–dc isolated converter with a novel modulation strategy that results in: 1) single-stage power conversion with no electrolytic capacitor, improving the reliability and power density; 2) open-loop power factor correction; 3) soft-switching of all semiconductor devices; and 4) a simple linear relationship between the control variable and the transferred active power. This paper presents a detailed analysis of the proposed operation, along with simulation results and experimental verification.

High-Frequency Three-Phase Interleaved LLC Resonant Converter With GaN Devices and Integrated Planar Magnetics

Abstract: The LLC converter is deemed the most widely used topology as dc/dc converter in server and telecom applications. To increase the output power and reduce the input and output current ripples, three-phase interleaved LLC converter is becoming more and more popular. It has been demonstrated that three interleaved LLC converter can achieve further efficiency improvement at the 3-kW power level. However, the magnetic components for multiphase LLC converter are complex, bulky, and difficult to manufacture in a cost-effective manner. In this paper, a high-frequency gallium nitride (GaN)-based three-phase LLC converter is employed to address these aforementioned challenges. With GaN operating at 1 MHz, all magnetic components, namely, three inductors and three transformers, can be integrated into one common structure while all magnetic windings implemented in a compact four-layer PCB with 3-oz copper. The proposed structure can be easily manufactured cost-effectively in high quality. Furthermore, shielding techniques for full-bridge secondary have been investigated, and additional two-layer shielding has been integrated to reduce common-mode noise. A 1-MHz 3-kW 400 V/48 V three-phase LLC converter is demonstrated, and the peak efficiency of 97.7% and power density of 600 W/in3 (37 kW/L) are achieved.

Critical-Mode-Based Soft-Switching Modulation for High-Frequency Three-Phase Bidirectional AC–DC Converters

Abstract: In this paper, a novel critical-conduction-mode-based modulation is proposed for three-phase bidirectional ac–dc converters. With this modulation, the switching frequency variation range shrinks, zero-voltage-switch soft switching is achieved, and the switching-related loss is reduced, which is especially beneficial for systems operating above hundreds of kHz high switching frequency with wide-band-gap power semiconductor devices to achieve both high power density and high efficiency. A 25 kW silicon carbide based high-frequency three-phase bidirectional ac–dc converter prototype is designed to achieve a power density of 127 ${\text{W/in}}^{3}$ , which is at least five times higher than commercial products. All the control functions are digitally implemented with one low-cost microcontroller, and the aforementioned benefits are experimentally verified on this prototype under both inverter mode and rectifier mode operations. With the proposed soft-switching modulation, the tested peak efficiency is close to 99.0% for this prototype even at above 300 kHz high switching frequency operation.

Dual Three-Phase PMSM Torque Modeling and Maximum Torque per Peak Current Control Through Optimized Harmonic Current Injection

Abstract: Vector space decomposition (VSD) model is widely used for dual three-phase permanent magnet synchronous machine (dual-PMSM) control, in which two direct-quadrature (DQ) frames, DQ1 and DQ2, are introduced to facilitate the controller design. Existing studies show that harmonic current injection in DQ2 frame can increase the output torque for a given peak phase current, which is referred as maximum torque per peak current (MTPPC) control. However, the injected harmonic current will induce a small dc torque and the harmonic torque. This paper first proposes a comprehensive torque model considering the harmonics in PM flux linkages, inductances and stator currents to investigate the induced torque components, which are neglected in existing approaches. These torque components are then considered in the harmonic current design to improve the MTPPC control performance. The harmonic current design results in a multiobjective optimization problem, and genetic algorithm (GA) is employed to optimize the harmonic current to maximize the output torque with minimal torque harmonic. Compared with existing approaches, the proposed approach is applicable to both surface-mounted and interior dual-PMSMs. Experimental investigations on a laboratory interior dual-PMSM show that the output torque of the test motor can be increased by more than ten percent with a negligible increase in torque ripple.

SVPWM Strategy Based on the Hysteresis Controller of Zero-Sequence Current for Three-Phase Open-End Winding PMSM

Abstract: Owing to high driving power, good controllability, and strong fault-tolerant ability, a three-phase open-end winding permanent magnet synchronous motor driven by a dual inverter with common dc bus is considered as a competitive solution However, a path for zero-sequence current is provided in this drive topology Thus, the system loss would be increased, and the maximum output power is limited In order to suppress the zero-sequence current, a novel space vector pulsewidth modulation (SVPWM) strategy based on the hysteresis controller of zero-sequence current is proposed in this paper The voltage vectors, whose polarity of zero-sequence voltages (ZSVs) is opposite with the polarity of zero-sequence current, are selected as the basic voltage vectors of SVPWM for fundamental plane Meanwhile, the linear modulation range is analyzed According to the analysis results, the proposed strategy has the advantage of wide linear modulation range of ZSV under the condition of high fundamental modulation ratio The experimental results show that the zero-sequence current is effectively suppressed by the proposed modulation strategy Furthermore, the comparative evaluation of the proposed modulation strategy, the conventional SVPWM, the SVPWM with zero ZSVs, and the zero vector redistribution SVPWM with zero-sequence current proportional-resonant controller is conducted to demonstrate the superiority of the proposed strategy

An Output Ripple-Free Fast Charger for Electric Vehicles Based on Grid-Tied Modular Three-Phase Interleaved Converters

Abstract: An off-board dc fast battery charger for electric vehicles (EVs) with an original control strategy aimed to provide ripple-free output current in the typical EV batteries voltage range is presented in this article. The proposed configuration is based on modular three-phase interleaved converters and supplied by the low-voltage ac grid. The ac/dc interleaved three-phase active rectifier is composed of three standard two-level three-phase converter modules with a possibility to slightly adjust the dc-link voltage level in order to null the output current ripple. A modular interleaved dc/dc converter, formed by the same three-phase converter modules connected in parallel, is used as an interface between the dc link and the battery. The use of low-cost, standard and industry-recognized three-phase power modules for high-power fast EV charging stations enables the reduction of capital and maintenance costs of the charging facilities. The effect of coupling on the individual input/output inductors and total input/output current ripples has been investigated as well, considering both possible coupling implementations, i.e., inverse and direct coupling. Numerical simulations are reported to confirm the feasibility and the effectiveness of the whole EV fast charging configuration, including the proposed control strategy aimed to null the ripple of the output current. Experimental results are provided by a reduced scale prototype of the output stage to verify the ripple-free output current operation capability.

Bidirectional Three-Phase DC–AC Converter With Embedded DC–DC Converter and Carrier-Based PWM Strategy for Wide Voltage Range Applications

Abstract: A bidirectional three-phase dc–ac converter with embedded dc–dc converter for bidirectional storage interface is proposed in this paper. With the help of the embedded dc–dc converter, the voltage of the storage battery can vary in a wide range. To minimize the power rating and power losses of the embedded dc–dc converter, a simple carrier-based pulsewidth modulation (PWM) strategy with zero-sequence injection is proposed. By adopting the proposed PWM strategy, only a small ratio of total power needs to be processed by the embedded dc–dc converter, while most of the power is only processed by the three-phase dc–ac stage within a single conversion stage. As a result, quasi single-stage power conversion is achieved to improve the conversion efficiency of the overall dc–ac power system. Principles, characteristics, and implementations of the proposed three-phase dc–ac converter and its PWM strategy are analyzed in detail. The feasibility and effectiveness of the proposed solutions are verified with experimental results.

All-silicon 99.35% efficient three-phase seven-level hybrid neutral point clamped/flying capacitor inverter

Abstract: With the increasing use of photovoltaic systems, a large demand for efficient, power-dense and lightweight grid-interface inverters is arising. Accordingly, new concepts like multi-level converters, which are able to reduce the converter losses while still keeping a low construction volume, have to be investigated. The hybrid seven-level topology analyzed in this paper comprises an active neutral point clamped stage, followed by a flying capacitor stage. Compared to a pure flying capacitor converter, the combination of these two stages allows to save more than half of the capacitor volume, while still having the same requirement for the output filter stage, and hence, the same output filter volume. Moreover, the topology employs low-voltage devices and ensures low conduction and switching losses, resulting in a higher efficiency. The principle of operation of the system is briefly reviewed, and based on a detailed component modeling, an efficiency vs. power density optimization is carried out, for which switching loss measurements of state-of-the-art 200 V semiconductors are performed. From the optimization, a high-efficiency design is selected and the practical hardware realization is discussed. The simulation and optimization results are then verified by realizing an all-silicon 99.35% efficient three-phase seven-level system, featuring a volumetric power density of 3.4 kW/dm 3 (55.9 W/in 3 ), a gravimetric power density of 3.2 kW/kg, and fulfilling CISPR Class A EMI requirements. Finally, it is shown that an all-silicon realization with next generation silicon switches can achieve 99.5% efficiency with the same hardware, and 99.6% with commercial state-of-the-art GaN switches.

99.3% Efficient Three-Phase Buck-Type All-SiC SWISS Rectifier for DC Distribution Systems

Abstract: DC power distribution systems for data centers, industrial applications, and residential areas are expected to provide higher efficiency, higher reliability, and lower cost compared to ac systems and have been an important research topic in recent years. In these applications, an efficient power factor correction (PFC) rectifier, supplying the dc distribution bus from the conventional three-phase ac mains, is typically required. This paper analyzes the three-phase, buck-type, unity power factor SWISS Rectifier for the realization of an ultrahigh-efficiency PFC rectifier stage with a 400-V rms line-to-line ac input voltage and a 400-V dc output voltage. It is shown that the mains current total harmonic distortion of the rectifier can be improved significantly by interleaving two converter output stages. Furthermore, the dc output filter is implemented using a current-compensated integrated common-mode coupled inductor, which ensures equal current sharing between the interleaved half bridges and provides common-mode electromagnetic interference (EMI) filter inductance. Based on a theoretical analysis of the coupled inductor's magnetic properties, the necessary equations and the design procedure for selecting semiconductors, magnetic cores, the number of turns, and the EMI filter are discussed. Based on these results, an ultrahigh-efficient 8-kW 4-kW $\cdot$ dm $^{-3}$ (66-W $\cdot$ in $^{-3}$ ) laboratory-scale prototype converter using 1.2-kV SiC MOSFETs is designed. Measurements taken on the prototype confirm a full power efficiency of $\text{{99.16}{\%}}$ and a peak efficiency of $\text{{99.26}{\%,}}$ as well as the compliance to CISPR 11 Class B conducted emission limits.

Two-Segment Three-Phase PMSM Drive With Carrier Phase-Shift PWM for Torque Ripple and Vibration Reduction

Abstract: This paper presents a two-segment motor system powered by two inverters with phase-shift pulsewidth modulation (PWM) to fulfill the purpose of low-torque ripple and vibration noise. Different from conventional multi-inverter structure, the inverters for this type of machine are electrically isolated between each other. First, the concept of multisegment machine is presented and the basic operation principle is analyzed with finite-element analysis (FEA) method. Then vibration caused by the switching device in machine is investigated and the control model of a two-segment motor is developed. Further, the principle of carrier phase-shift PWM is illustrated with the vibration analysis. Carrier phase shift can generate interleaved torque and force on the rotor for the two groups of windings in the machine and cancel each other for certain harmonics. Then simulation of control strategy for this type of machine has been done in MATLAB. Finally, based on the concept and principles mentioned before, a two-segment motor with its drive system is fabricated. Both the test of torque ripple and the vibration of the motor have been significantly improved. Compared with a normal three-phase motor driven by a paralleled inverter with interleaving, the proposed approach is with comparable vibration reduction but without using extra coupled inductors.

Leakage Current Reduction of Three-Phase Z -Source Three-Level Four-Leg Inverter for Transformerless PV System

Abstract: Leakage current reduction is one of the important issues for transformerless photovoltaic (PV) systems. Many interesting solutions have been reported to reduce the leakage current for three-phase PV inverters. However, most of them are limited to two-level inverters. Moreover, there is a potential risk of the overcurrent phenomenon. In order to solve the problem, a Z -source three-level four-leg inverter with a new modulation strategy is proposed in this paper. First, the mathematical modeling of the three-level four-leg Z -source inverter for leakage current reduction is established for the first time. Second, a new carrier-based modulation strategy is proposed by utilizing the effective large, medium, small, and zero vectors, instead of the invalid vectors, to achieve the constant common-mode voltage, as well as the leakage current suppression. Finally, the proposed solution is carried out on the Texas Instruments TMS320F28335 DSP + Xilinx XC3400 field-programmable gate array digital control hardware platform. The experimental results verify the effectiveness of the proposed solution.

Three-Phase PLL for Grid-Connected Power Converters Under Both Amplitude and Phase Unbalanced Conditions

Abstract: This paper proposes a technique for accurate phase estimation of distorted three-phase grid voltages including unbalanced amplitudes and/or phase angles. An adaptive cascaded delayed signal cancellation (CDSC) strategy is used for the generation of the amplitude balanced three-phase voltages. The CDSC strategy also eliminates both odd and even harmonics from the input voltage. An algorithm is also reported for removing the phase angle deviations from the three-phase voltages including unbalanced phase angles. Finally, a phase-locked loop (PLL) is applied to estimate the phase angle of the reference phase voltage. It requires only one PLL to estimate three phase angles of the thee-phase voltages, respectively, suffering from the unbalanced amplitudes and phase angles. It is also immune to harmonic distortions at changeable frequency environment. When compared to the CDSC-PLL and dq CDSC-PLL techniques reported in the technical literature, it can provide improved phase estimation under amplitude and phase unbalanced condition. Simulated and experimental results of the technique are acquired from the MATLAB/Simulink and dSPACE1104 control board platform, respectively, for a number of case studies observed in the grid voltage.

An SiC MOSFET Based Three-Phase ZVS Inverter Employing Variable Switching Frequency Space Vector PWM Control

Abstract: In this paper, a variable switching frequency space vector pulsewidth modulation control is proposed. It is used to achieve zero voltage switching (ZVS) for a three-phase grid-connected voltage source inverter with unity power factor. A wide range of ZVS can be realized without any additional sensors, auxiliary circuits, or current zero crossing detection circuits. The switching frequency can be easily calculated by a digital controller. The frequency variation range in a line cycle is only about 1.5 times at any specific load. An LCL filter is used to attenuate the high current ripples at the inverter side, and active damping is adopted to avoid the resonance and reduce the filter power loss. The switching loss can be significantly reduced by using silicon carbide mosfet s so that the conversion efficiency is high. The power density can also be improved due to the high switching frequency and the low inductance value of the filter. The operating principle, design considerations, and loss analyses are discussed in detail. A 3.5-kW simulation and experimental prototype interfacing a 350–400-V dc with a three-phase 110-V ac grid is developed to verify the performance of the proposed control strategy.

An Integrated Method for Three-Phase AC Excitation and High-Frequency Voltage Signal Injection for Sensorless Starting of Aircraft Starter/Generator

Abstract: This paper proposes an integrated method of three-phase ac excitation and high-frequency voltage signal injection (HFVSI) for sensorless controlled starting of brushless synchronous machines (BSM) used as starter/generator in variable frequency ac power systems of civil aircraft. Fixed 400 Hz of the three-phase ac power is adopted both for the ac excitation and HFVSI in the initial starting process to eliminate the bulky rotor position sensor for BSM. The resulting 6th sequence harmonic voltage determined by the rotating rectifier is utilized as the HFVSI into the field-winding of main generator without any extra high-frequency signal injection. The rotor position is estimated by the high-frequency response signals extracted from the armature windings of main generator. Due to the nonlinear rotating rectifier linked in the HFSI chain, a novel frequency-insensitive asynchronous demodulation strategy is proposed in this paper for rotor position estimation. Furthermore, the initial rotor position detection is calibrated by polarity decision of the induced currents of the armature windings within the establishment procedure of field current of the main generator at standstill. The effectiveness of the ac excitation and feasibility of rotor position estimation for sensorless starting control of BSM are validated by the simulation and experimental results.

Unbalanced Three-Phase $LLC$ Resonant Converters: Analysis and Trigonometric Current Balancing

Abstract: Three-phase $LLC$ resonant converters can handle very high power levels beyond the capabilities of half-bridge and full-bridge $LLC$ topologies. Among other characteristics, three-phase $LLC$ structures reduce output current ripple (small output filter), enable parallel power processing (low peak current), and provide good thermal distribution. However, all these key advantages can be severely compromised due to passive components tolerances, leading to undesired current balance issues in three-phase $LLC$ resonant converters. Tolerances in resonant tank passive components are inevitable and lead to unequal peak currents between phases, uneven temperature distribution, and large output current ripple. This paper investigates the imbalances in three-phase $LLC$ converters and proposes a novel trigonometric current balancing (TCB) technique using phasor analysis. In this strategy, the required input voltage phase angles are calculated to achieve balanced phase currents, even under severe unbalanced conditions. In some cases, the output filter current ripple is reduced to less than half. The methodology is verified with a 3-kW experimental prototype, which validates the analytical framework and effectiveness of TCB.

A DSOGI-FLL-Based Dead-Time Elimination PWM for Three-Phase Power Converters

Abstract: The dead-time elimination pulsewidth modulation (PWM) enables the drive pulses of the upper and the lower switching devices to alternate according to the current polarity, thus abandoning the dead time and essentially avoiding the dead-time effect in power converters. However, the current zero crossing will be distorted by the current jump generated by the drive-pulse alternation, and the current zero-crossing distortion will be further intensified if errors exist in the detected current polarity. Thanks to the noise-attenuation and frequency-adaptability characteristics of the double second-order generalized integrator frequency-locked loop (DSOGI-FLL), it can obtain accurate current polarities even in harmonic and unbalanced conditions. A DSOGI-FLL-based dead-time elimination PWM is, therefore, proposed in this paper, and several improvements are made to minimize the current zero-crossing distortion. An underlap period is added when alternating the upper and lower drive pulses to smooth the current jump at zero crossing, and a delay compensation term is inserted in the DSOGI-FLL to compensate both the current measurement delay and the control delay. The effectiveness of the proposed DSOGI-FLL-based dead-time elimination PWM has been validated by experiments, respectively, in the unbalanced-current, power-change, and frequency-variation conditions with an RL load, as well as in a grid-connected converter.

Dynamic Modeling of Dual Three-Phase IPMSM Drives With Different Neutral Configurations

Abstract: Without prior knowledge of the geometric and design data of the employed asymmetrical dual three-phase interior permanent magnet synchronous machine (ADT-IPMSM), this paper proposes effective fundamental and enhanced harmonic models along with parameter identification for the different subspaces and with different neutral point configurations. The cross coupling between the coordinates of the different subspaces is also taken into account. The proposed method is based on simple experimental tests that can be applied to any ADT-IPMSMs. The performed computer simulations coincide to a high extent with experimental validations on a 2.5-kW ADT-IPMSM prototype.

Active and Reactive Power Injection Strategies for Three-Phase Four-Wire Inverters During Symmetrical/Asymmetrical Voltage Sags

Abstract: Electric grid codes are expected to change in near future to accommodate an increased number of distributed generation units in the distribution power system without impairing its power quality. It is desired that the generators remain connected during voltage sags and provide ancillary services, such as voltage and reactive power control, ensuring the operational stability of the power system. This paper explores how the existing strategies for active and reactive power injection impact the operation of grid-tied inverters in terms of required power, current flowing, and reduction of active power delivery during the voltage sags. Such inputs are relevant to properly size converters for operation under fault events. In addition, this paper contributes to devise: 1) constant peak current control, 2) constant active current control, and 3) constant average active power control strategies for three-phase four-wire grid-tied inverters considering the natural (abc ) reference frame. The design and implementation of the investigated power injection strategies are discussed, and their effectiveness and technical viability are analyzed through dynamic computational simulations under symmetrical/asymmetrical voltage sags, and variation of the short-circuit ratio.

Comparison of Virtual Oscillator and Droop Controlled Islanded Three-Phase Microgrids

Abstract: This paper compares transient responses of virtual oscillator control (VOC) to the conventional droop control method, under various inverter terminal voltage amplitude and frequency regulations. The system of interest is an islanded microgrid of three three-phase inverters sharing a resistive load. By letting the two controllers have similar steady-state droop characteristics, the virtual oscillator and droop controlled systems are compared in a small-signal framework with the aid of eigenvalue analyses. The analytical results are verified by simulating the two systems in MATLAB/Simulink. In the simulations, the third inverter in each system is connected to share the load when the other two inverters are already synchronized. Transients are compared by examining synchronization time after the third inverter is connected. The key finding is VOC outperforms droop control in terms of time responses when the terminal voltage frequency regulation range is allowed to be wide; otherwise, the droop control responds faster than the VOC technique. The same conclusions are found for two other load types: constant power load and nonlinear load. In addition, transient energy losses are investigated, which shows positive correlation with the time response.

A Multilevel DC to Three-Phase AC Architecture for Photovoltaic Power Plants

Abstract: This paper presents a photovoltaic (PV) inverter architecture composed of stackable dc to three-phase ac converter blocks. Several such blocks, each containing a converter power stage and controls, are connected in series on their ac sides to obtain transformerless medium-voltage ac interfaces for PV power plants. The series-connected structure is made possible by a quadruple active bridge dc–dc converter that provides isolation between the PV input and each of the three ac-side phases within each block. Furthermore, since incoming PV power is transferred as constant balanced three-phase ac power, instantaneous input–output power balance bypasses the need for bulk energy storage. To streamline implementation and maximize system scalability and resilience, decentralized block-level controllers accomplish dc-link voltage regulation, maximum power point tracking, and ac-side power sharing without centralized means. The proposed architecture is validated by simulations of a PV string to medium-voltage ac system consisting of six blocks and on a proof-of-concept hardware prototype that consists of three cascaded converter blocks.

Optimized Modulation and Dynamic Control of a Three-Phase Dual Active Bridge Converter With Variable Duty Cycles

Abstract: The three-phase dual active bridge (3p-DAB) converter is a promising topology for high power dc–dc conversion due to advantages of bidirectional power flow, inherent soft-switching capability, and reduced filter volume. This paper presents comprehensive analysis of the duty cycle control (DCC) for optimizing the performance of the 3p-DAB. Based on DCC, an optimized modulation strategy is proposed to minimize the conduction losses of the 3p-DAB in the whole load range. The proposed modulation strategy extends the soft-switching range of the 3p-DAB with large voltage variations simultaneously. It is established through loss analysis that the proposed modulation strategy boosts the efficiency of the 3p-DAB, especially at low loads. When the duty cycles change fast as a result of the abruptly changed transmission power, the transformer currents can become unbalanced, leading to the magnetic bias and oscillations in dc currents. This paper further proposes a fast transient current control (FTCC) method for the 3p-DAB with variable duty cycles. The FTCC enables the converter to transfer from one steady state to another within about one-third switching period, hence balancing the transformer currents rapidly and avoiding oscillations in dc currents. Finally, experimental results verify the outstanding performance of the proposed modulation strategy and FTCC method.

A Modular Multilevel Converter With Ripple-Power Decoupling Channels for Three-Phase MV Adjustable-Speed Drives

Abstract: This paper presents a drive system based on a modular multilevel converter (MMC) with high-frequency magnetic channels between adjacent-arm submodules (SMs), suitable for medium-voltage, high-power three-phase variable-speed machines. The configuration employs chains of dual half-bridge (DHB) modules linking adjacent SMs of three-phase symmetrical arms. The DHB modules are operating as power channels enabling energy exchange to restore the power imbalance among the SM capacitors. This allows arms’ ripple-powers to be entirely decoupled through bidirectional power transfer between adjacent-arm SMs, resulting in a near ripple-free SM capacitor voltage profile. Therefore, the MMC common problem of wide voltage fluctuation across SM capacitors is comprehensively solved, independent of the operating frequency. Additionally, a significant reduction in the sizing requirement of SM capacitance is achieved. The configuration is able to drive multimegawatt machines from standstill to the rated speed at the rated torque operating condition. The operating principle of the proposed MMC configuration is explained and necessary mathematical analysis is derived. Features and viability of the proposed drive system are verified through simulation and experimentation.

Optimized Design of the Neutral Inductor and Filter Inductors in Three-Phase Four-Wire Inverter With Split DC-Link Capacitors

Abstract: The three-phase four-wire inverter with split dc-link capacitors can supply unbalanced loads. For the purpose of reducing the filter inductors, a neutral inductor could be introduced into the neutral line. This paper analyzes the operation principle of the three-phase four-wire inverter with split dc-link capacitors when a neutral inductor is introduced. It is illustrated that the neutral inductor can reduce the zero-sequence switching harmonics in the voltages between the phase-leg midpoints and the output neutral point, thus the filter inductors can be reduced. The optimized design of the neutral inductor and filter inductors is proposed with the considerations of the inductor current ripple, the ability of supplying unbalanced loads, and the total energy stored in the inductors. The equivalent circuits of the three-phase four-wire inverter with split dc-link capacitors and neutral inductor is derived in the α–β–0 frame, and the zero-axis voltage regulator is modified to suppress the third-order harmonic in the zero-sequence current caused by the deadtime of the drive signals for the power switches. Finally, the experimental results from a 9-kW prototype are provided to prove the effectiveness of the proposed optimized design of the neutral inductor and filter inductors and the control strategy of the inverter.

Modular Full-Bridge Converter for Three-Phase Switched Reluctance Motors With Integrated Fault-Tolerance Capability

Abstract: In this paper, a novel modular full-bridge converter is proposed for three-phase switched reluctance motors (SRMs), which provides an advanced fault-tolerance solution for both the open- and short-circuit faults in switching devices. The proposed converter is composed of two standard six-pack switch modules, providing great benefits for industrial massive production due to the modular topology. Under normal conditions, bidirectional current excitation is developed in the proposed converter, where all the switching devices are actively involved, which makes full use of the switches and averages the heat dissipation for the heat sink design. When a switch fault occurs, the fault signature can be extracted by detecting the characteristics of phase currents and operation modes, and effective fault-tolerance strategies are further applied to achieve the fault-tolerance operation accordingly. Under switch open-circuit faults, the faulty phase is adjusted to unidirectional current excitation without degrading the motor performance. Under switch short-circuit faults, the motor can still work steadily with a single six-switch inverter module by easily reconstructing the proposed converter, without losing any phase windings. The simulation and experiments on a three-phase 12/8 SRM are carried out to validate the feasibility of the proposed converter and control techniques.

Multi-Domain Design Optimization of d v /d t Filter for SiC-Based Three-Phase Inverters in High-Frequency Motor-Drive Applications

Abstract: High slew rate of the line voltage $(\boldsymbol {dv}/\boldsymbol {dt})$ has been a concern for power inverters based on the emerging wide bandgap switching devices, such as silicon carbide (SiC) mosfets . Particularly, for SiC-based power inverters feeding electric machines interconnected with long cables, there could be more severe insulation stress on the stator windings in electric machines, due to the higher d v /d t output from the SiC inverters. Compared to the conventional lower voltage electric transportation applications (e.g., 270 V dc), higher dc bus voltage of 500–600 V can dramatically reduce cable weight and systematic copper losses, hence improve the power density and efficiency of power converters. However, high dc bus voltage further increases the $\boldsymbol {dv}/\boldsymbol {dt}$ level in the converter ac line voltages. Driven by the necessity of developing SiC inverters with 500 V dc bus in electric transportation applications while attenuating the $\boldsymbol {dv}/\boldsymbol {dt}$ stress to a low level, this paper presents a multi-domain design approach for d v /d t filters that comprehensively considers the constraints in electrical, magnetic, and thermal domains. Experimental results based on a 75 kW SiC inverter are provided to verify the efficacy of this design approach.

A Hybrid Modular Multilevel Converter for Solar Power Integration

Abstract: A hybrid modular multilevel converter (H-MMC) for grid-connected photovoltaic (PV) system is presented in this paper. The array of PV panels is connected in each submodule of the converter. The submodules are connected in series to form the dc link for each phase. The dc-link voltage is unipolar and multilevel in nature, which is converted to ac voltage by a low frequency converter. This topology can be applicable at high power by increasing number of submodules per phase. The H-MMC has many advantages compared to conventional MMC, e.g., more voltage levels, less capacitor count, no circulating current, higher efficiency, and less foot print. A global maximum power point tracking (MPPT) algorithm is proposed for this converter, which requires the voltage and current signals of only one submodule. The limitations of global MPPT under partial shading are overcome through distributed MPPT. A least-MPPT algorithm is presented to make the three phase powers balanced under nonuniform partial shading among the phases. The usefulness of this converter configuration with two different MPPT algorithms (global and distributed) is investigated through PSCAD/EMTDC simulation. A single-phase version of the converter with global and distributed MPPT is experimentally validated considering partial shading condition. The results show the usefulness of H-MMC and associated control for PV integration.

Model-Based Current Control Strategy With Virtual Time Constant for Improved Dynamic Response of Three-Phase Grid-Connected VSI

Abstract: In this paper, a model-based current control strategy with virtual time constant is proposed for three-phase grid-connected LCL -filtered voltage source inverters. The proposed control strategy is based on controlling the inverter currents in the rotating dq frame by using current-oriented proportional-integral (PI) controllers rather than voltage-oriented PI. The PI controllers determine the inverter current references in the d - and q -axes by regulating the grid current. It is shown that the proposed strategy decouples the inverter current from other variables provided that the inverter-side inductance and its resistance values used in the control variable match the actual values in the system. In addition, the virtual time constant is introduced in the control variables to offer flexibility for adjusting the inverter current dynamics as desired. Moreover, the integral gain of PI controller has the ability to keep the LCL -resonance peak below 0 dB. Unlike the existing methods, the proposed strategy does not require a dedicated active damping. Computer simulations and experimental studies show that the proposed control strategy exhibits a good performance in achieving fast dynamic response and sinusoidal grid current with low THD under balanced, unbalanced, and distorted grid conditions.

Application of artificial neural networks for transistor open-circuit fault diagnosis in three-phase rectifiers

Abstract: This study deals with the transistor open-circuit fault diagnosis technique based on the grid current processing. In accordance with the proposed method, in the first stage, the defect of the power electronics converter is recognised. For this purpose, the zero current periods are registered in each converter phase circuits. The faulty transistors are identified calculating the average values of differences between predicted and measured phase currents. The novelty of the presented technique is an application of a neural network for the grid current prediction in the active rectifier. In fact, the transistor open-circuit faults do not affect the predicted grid currents immediately as soon as the transistor defects happen. Therefore, the differences between the predicted currents and the measured ones increase which are used for the faulty transistors identification. In the comparison to the switch open-circuit fault diagnostic techniques, which are known from the scientific literature survey, the method presented in this study is insensitive to load changes no matter a direction of the energy flow in the power conversion system.

Novel High Gain, High Efficiency DC–DC Converter Suitable for Solar PV Module Integration With Three-Phase Grid Tied Inverters

Abstract: This paper proposes a novel configuration for high gain, high efficiency dc–dc converter comprising a single switch, two intermediate capacitors, and a coupled inductor for low voltage solar PV module fed applications. The high voltage gain is achieved by charging the intermediate capacitors through the coupled inductor in parallel and discharging in series. In a two winding coupled inductor, considered in the presented work, maximum two intermediate capacitors can be integrated with the secondary winding. A passive lossless clamped circuit is also provided in the converter, which recovers the leakage energy to improve the efficiency and alleviate large voltage spike. The structure of the circuit is such that the power device voltage stress is reduced thereby increasing the efficiency. The maximum power point tracking at various irradiation levels is implemented in the proposed converter. Laboratory prototype of a 300-W system with 30–45-V input and 700-V output was built to validate the theoretical claims. All the detailed analysis, simulation, and experimental waveforms are presented. A maximum converter efficiency of around 95% is achieved.