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Synchronverters are inverters that mimic the behavior of synchronous generators. In this paper, we propose five modifications to the synchronverter algorithm from the paper “Synchronverters: Inverters that mimic synchronous generators,” (IEEE Trans. Ind. Electron., vol. 58, no. 4, pp. 1259–1267, Apr. 2011), to improve its stability and performance. These modifications are implemented in software and do not require any changes in the inverter hardware. The first two modifications concern the control of the virtual field current in the synchronverter so that it is more robust to faults. We prove the stability of the grid-connected synchronverter with this improved field current controller. The third modification is to increase the effective size of the filter inductors virtually. This is motivated using results from the stability analysis of a constant field current synchronous generator connected to an ac power grid and also by practical considerations. Simulations and experiments show that this leads to a much better response to changes in grid frequency, voltage, or to imbalance in the grid. The fourth modification is to change the formula for the (virtual) nominal active mechanical torque to take into account the (virtual) losses in the output impedance of the converter. This makes the tracking of the desired active power much more accurate. The fifth modification is to introduce virtual capacitors in series with the inverter outputs to filter spurious dc components from the current supplied to the grid. [ABSTRACT FROM PUBLISHER]

Multilevel converters have gained popularity in both medium voltage and low voltage applications. To find out the connections between various multilevel voltage-source converter topologies and to reveal how to obtain new topologies, this paper has presented four methods to derive multilevel converter topologies. Many existing topologies as well as new topologies can be derived with the methods presented in this paper. The fundamental characteristics of the multilevel converters which determine their usability such as dc-link neutral point voltage balancing and flying capacitor voltage control are also investigated in this paper with a mathematical model and an example. It is expected that more new topologies will be invented based on the work in this paper for emerging applications. [ABSTRACT FROM AUTHOR]

In this paper, a new soft switching flyback dc–dc converter with a simple zero-voltage and zero-current switching (ZVZCS) cell is proposed. The auxiliary resonant cell consists of one switch, one diode, one inductor, and two capacitors. It provides ZVZCS not only for the main switch but also for the auxiliary switch at both turn-on and turn-off times. In addition, all the auxiliary diodes and body diodes of the switches operate under full ZVZCS except the output diode which turns off only with zero current switching. This soft switching technique makes the proposed converter have less switching losses which leads the converter to operate in higher frequencies and efficiency. Another outstanding feature of the proposed converter is using the auxiliary resonant cell out of the main power path to have lower voltage stress on the switches. In this paper, pulse width modulation (PWM) is employed to control the main switch. A detailed mode analysis of the proposed converter operation is presented along with design procedure. Finally, the accuracy performance of the converter is verified through a 60 W experimental prototype results. [ABSTRACT FROM AUTHOR]

In this paper, an electrolytic capacitor-less ac–dc light-emitting diode (LED) driver, consisting of a power factor correction (PFC) converter and a buck/boost bidirectional converter, is investigated. The buck/boost bidirectional converter is connected in parallel with the output of the PFC converter and serves to absorb the second harmonic current in the PFC output current, leaving only a dc component to drive the LEDs. This paper proposes an adaptive voltage control scheme for the storage capacitor in the buck/boost bidirectional converter to make the storage capacitor voltage adaptively decrease as the load becomes lighter. Hence, the power losses of the buck/boost bidirectional converter could be reduced at light load. Experimental results are provided to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM PUBLISHER]

In this paper, a new topology for half-bridge Z-source inverter is proposed. The proposed topology has only one impedance network. Unlike to the conventional half-bridge inverter, the proposed topology can provide zero voltage level at the output. It also increases output voltage level and stabilizes it in the desired value. Capacitor voltage stress in the proposed topology is low, and, therefore, nominal voltage of capacitor and cost decreases. In this paper, the steady-state analysis of the proposed inverter in two new operations which are named synchronous operation of diodes and asynchronous operation of diodes is conducted based on mathematics calculations. A method to obtain high-voltage gains by cascading the Z-network and combining middle inductors is presented that leads to cost, size, and weight reduction. Comparison among the proposed converter with conventional ones shows its excellent performance. The experimental results have good agreement with analytical analysis for the proposed topology. [ABSTRACT FROM PUBLISHER]

A quasi-Z-source modular cascaded converter (qZS-MCC) is proposed for dc integration of high-power photovoltaic (PV) systems. The qZS-MCC comprises series-connected front-end isolated qZS half-bridge (HB) dc–dc converter submodules (SMs). With the front-end isolation, the qZS-MCC achieves high-voltage dc capability, while maintaining modularity and PV panel grounded. The post-stage qZS-HB handles the PV voltage and power flows, dc-link voltage balance, and output-series power integration. Whereas, the front-end isolation converters of all SMs perform a constant duty cycle, lowing the control complexity. There is no double-line-frequency power flowing through the dc-side PV panels, qZS inductors, and qZS capacitors in the qZS-MCC, so small qZS impedance is possible compared to the existing qZS cascaded multilevel inverter. The configuration, operating principle, power loss evaluation, and passive components design of the proposed system are investigated in this part of the paper. The system control, modeling, and corresponding verifications are stated in Part II of this paper. [ABSTRACT FROM PUBLISHER]

To improve the power factor and reliability for medium/high-power supply, a three-phase single-stage ac–dc converter based on magnetic-combination transformer with power factor correction (PFC) function is presented in this paper. According to the three-phase instantaneous power balance principle and the concept of the magnetic-flux superposition, a novel magnetic-combination transformer is proposed, which can make three-phase symmetrically and sinusoidally fluctuant power add to be constant. Therefore, the induced voltage of the secondary winding is constant. To realize sinusoidally fluctuant power in the input stage, dc-link filter capacitance can be reduced to a several-microfarad level. Single-stage PFC function, no electrolytic capacitor, and constant output voltage can be realized in the converter. The principle and the structure of the novel magnetic-combination transformer are analyzed in detail in this paper. Simulation and experimental results are presented to verify the feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

Zero-common-mode-voltage (CMV) operation of multilevel inverters results in reduced dc-bus utilization and reduced linear modulation range. In this paper, a method to increase the linear modulation range for zero-CMV operation without increasing the dc-bus voltage using a cascaded multilevel inverter with a single dc supply is presented. Using this method, the peak fundamental output voltage can be increased from 0.499 to 0.637  \textV\text{dc} with zero CMV inside the linear modulation range. Also, various pulse width modulation (PWM) switching sequences are analyzed in this paper, and the PWM sequence that gives minimum current ripple is used for the zero-CMV operation of the inverter. The inverter topology with single dc supply is realized by cascading a two-level inverter with two floating-capacitor-fed full-bridge modules. Simulation and experimental results for steady-state and dynamic operating conditions are presented to validate the proposed method. [ABSTRACT FROM AUTHOR]

To interconnect low-voltage solar photovoltaics (PV) with dc system, a dc–dc boost converter is required. To minimize the switching frequency oscillations in solar PV voltage, aluminum electrolytic capacitors (AECs) are connected between solar PV and the converter. Operational life of AECs depends on electrical and environmental parameters and is less than that of solar PV panels. Equivalent series resistance (ESR) of AEC increases with life, thereby increasing the voltage ripple across PV terminals. This results in less power extraction from solar PV. This paper proposes a method for health monitoring of AEC using the parameters measured for maximum power point tracking (MPPT) (PV voltage and current). The proposed technique is applicable for both continuous conduction mode and discontinuous conduction mode of operation of the converter. Effect of temperature on ESR is incorporated for accurate determination of the health of capacitor. Key advantage is that no additional current or voltage sensor is required to implement this technique, thereby offering a low-cost solution. Further, the proposed technique does not require significant processing capabilities and is implemented in the same digital controller used for MPPT. Detailed simulation studies are carried out and results are included in this paper. A laboratory prototype of dc–dc boost converter is developed for experimentation. Results are in agreement with that obtained from simulation studies. [ABSTRACT FROM AUTHOR]

The large number of semiconductors typically implemented in multilevel converters generates large stray inductances in their commutation loops. As a consequence, high-voltage overshoots appear in the semiconductors during commutation which may limit the operation of the converter. The paper analyzes these overvoltages in the five-level active neutral-point-clamped (5L­ANPC) converter, one of the most popular five level converters and employed by ABB in its ACS­2000 drive. Experimental results show overvoltages almost three times higher than those corresponding to a two-level converter. In order to overcome this problem, the paper presents a new 5L­ANPC with the same characteristics (number of semiconductors, output waveform, switching frequency, efficiency, etc.), but with smaller overvoltages. This is achieved by adding some small capacitors to the busbar that behave like lossless snubbers but with a much simpler implementation. The experimental results illustrate the working principle of these capacitors and demonstrate the reduction of the overvoltages. As a result, the new converter can reach a higher output current or speed-up the commutations reducing, therefore, the losses and increasing the efficiency compared with the previous 5L­ANPC. [ABSTRACT FROM AUTHOR]

In this paper, a new structure for symmetric and cascade multilevel converter is presented. This structure requires the least power electronic components, gate driver circuits, power diode, and dc voltage source. The number of output levels is high. Reduction of the number of power electronic switches against voltage levels is an important factor in multilevel converters due to safety in switches, circuit size, cost, installation area, and complexity in control. To maximize the number of output level in the proposed optimized structure in order to attain different goals, such as reduction of stress on switches, the result of comparing the proposed structure and other structures present. The proposed method is robust against the switching faults in multilevel converter for dc source and the equipment. The simulation and experimental results presented in this paper show the performance of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents a model predictive control technique applied to a dual-active bridge inverter where one of the bridges is floating. The proposed floating bridge topology eliminates the need for isolation transformer in a dual inverter system and therefore reduces the size, weight, and losses in the system. To achieve multilevel output voltage waveforms, the floating inverter dc-link capacitor is charged to the half of the main dc-link voltage. A finite-set model predictive control technique is used to control the load current of the converter as well as the floating capacitor voltage. Model predictive control does not require any switching sequence design or complex switching time calculations as used for space vector modulation; thus, the technique has some advantages in this application. A detailed analysis of the converter as well as the predictive control strategy is given in this paper. Simulation and experimental results to validate the approach are also presented. [ABSTRACT FROM PUBLISHER]

The magnetic resonant wireless power transfer (WPT) system with a class E inverter has wide application in mid-range power transmission, especially for several megahertz scenario. However, it is cumbersome for the existing modeling and analysis methods to obtain the analytical solution to the system variables, which limits the evaluation of system characteristic. This paper proposes a nonlinear mathematical model for the WPT system with a class E inverter. The equivalent small parameter method (ESPM) is first adopted to solve the proposed six-order nonlinear model, and symbolic-form steady-state periodic solutions are yielded. During the whole solving process, only three iteration steps are needed for the proposed method. Compared with other existing modeling and analysis methods, the symbolic periodic results acquired by the ESPM exhibit sufficient accuracy by virtue of the combination of more higher harmonics. Meanwhile, the calculation complexity is simplified remarkably. Simulation and available experiments validate the effectiveness and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a generic framework for a rectifier to continuously take part in the regulation of grid voltage and frequency is proposed. It can automatically change the power consumed to support the grid, without affecting the normal operation of the load. Such a rectifier has a built-in storage port, in addition to the normal ac and dc ports. The flexibility required by the ac port to support the grid is provided by the storage port without affecting the dc port. The grid support of the ac port is achieved through the recently proposed universal droop controller (UDC). A dc-bus voltage controller cascaded to the UDC is proposed to regulate the dc-bus voltage of the storage port within a wide range so as to provide the flexibility needed, while the dc-port voltage is maintained constant. An illustrative example is presented with the recently proposed $\theta$ -converter, which consists of three ports with only four switches. Experimental results are provided to verify the capability of the rectifier to regulate the grid voltage and frequency without affecting the load. [ABSTRACT FROM AUTHOR]

The forced commutated controlled series capacitor (FCSC) rectifier has been recently proposed to power stand-alone variable-voltage variable-frequency generation systems, such as the full authority digital electronic control (FADEC) system, in more electric aircraft. The employment of this rectifier helps to meet increasing power demand requirements while maintaining low current harmonic levels without the need for bulky filters. The FCSC rectifier is controlled to maintain resonance with line frequency variation by inserting variable capacitive reactance in series with the variable-frequency generator's inductive reactance. Unity power factor operation with nearly pure sinusoidal converter input currents is thus achieved, for better utilization of the generator. The FCSC rectifier operation with a variable dc on-board load is analyzed in this paper, including a description of the principle modes of operation of the circuit. The enhancement in circuit's output voltage and output power achieved by replacing the conventional diode bridge rectifier with the FCSC rectifier is also demonstrated. Both rectifiers are analyzed numerically using Saber/Synopsys over a wide range of operating voltages and frequencies. Results are experimentally verified using 1-kW laboratory test rig. [ABSTRACT FROM AUTHOR]

This paper proposes a capacitor voltage regulation method for the dual converter with a floating bridge for aerospace applications. This topology has previously been reported, but with a constrained voltage utilization factor due to the need for capacitor voltage regulations. In this paper, the effect of switching states on the voltage variation of capacitor is quantitatively modeled and an enhanced space vector modulation scheme with current feedback is proposed to achieve an active control of the floating capacitor voltages. This proposed method also allows further exploitation and utilization of converter voltage. The relationship between the allowed modulation index of dual converter and load power factor is obtained and expressed using a fitted polynomial equation. The advantages of the proposed method include boosted voltage utilization and superior performance in term of capacitor voltage balance. These advantages have been proven through simulation and experimental results on RL loads as well as with an open-end winding induction motor. The proposed modulation scheme can boost the converter voltage utilization by at least 10% while achieving full four-level operation. More importantly, the higher available voltage allows extending the constant torque region of the motor, the further beginning of field weakening operation could be postponed. [ABSTRACT FROM AUTHOR]

Direct modulation principle is widely used in modular multilevel converters (MMCs) for its simplicity and natural stability. However, it results in strong active and reactive power coupling. This paper carefully analyzes and identifies four power coupling paths and two power influence factors with the direct modulation principle. Full power decoupling of the MMC can be realized by cutting off the derived coupling paths and compensating the influence factors. However, this will further deteriorate the already poor damping of the MMC and significantly increase dc-bus current fluctuations. To solve this problem, the sources resulting in poor damping are identified and then a new closed-loop dc-bus current controller is incorporated into the control system. Finally, the proposed power decoupling strategy with the newly designed current controller realizes completely decoupled power regulation, avoids overload operation, and obviously improves the dynamic performance of the MMC. Furthermore, it increases the stability of the MMC system under all possible operation conditions. The proposed control strategy is confirmed by experimental results. [ABSTRACT FROM AUTHOR]

This paper proposes a dual-function dynamic voltage restorer (DFDVR) with fault current-limiting capability. The new topology can operate under different modes: the short-circuit fault current-limiting mode and the compensation mode for voltage fluctuation. Different functions of the DFDVR are achieved by controlling a brake chopper circuit and antiparallel thyristors. During fault conditions, the antiparallel thyristors are deactivated to protect the dc-side capacitor, and the duration of the on- and off-periods of the semiconductor switch in the brake chopper circuit is controlled to adjust fault current amplitudes. During voltage sags, the antiparallel thyristors are activated, and the brake chopper circuit is turned off by controlling the gate turn-off thyristor in the brake chopper circuit, thus making the DFDVR similar to a conventional DVR. The control scheme and optimal parameter selection of the DFDVR are provided. The design methodology of the DFDVR was verified through simulations performed using the PSCAD/EMTDC software. The experimental results prove the capability and effectiveness of the DFDVR topology. [ABSTRACT FROM AUTHOR]

Recently, a star-channel modular multilevel converter (MMC) was proposed for zero/low-fundamental-frequency operation. However, it is companied by two potential issues in practices: the star-channel branch suffers from submodule capacitor voltage deviations and the entire-speed-range operation for motor drive is not confirmed. This paper investigates the control degrees of freedom, and proposes control algorithms for star-channel branch and main arms. These control algorithms cascaded with advanced motor controls form the overall control scheme for drive system. Simulation and experimental results show that the star-channel MMC along with the proposed algorithms successfully maintain the SM capacitor voltages, and achieves the variable speed operation from standstill to rated speed. [ABSTRACT FROM AUTHOR]

The interleaved bidirectional buck–boost converter dual active half-bridge (IBBC-DAHB) dc–dc converter consists of two converters IBBC and DAHB and a coupled inductor. The IBBC-DAHB converter has significant features such as low ripple current at the low-voltage side, low current stress on the input inductors, voltage stress across the switches less than the high-voltage side, high voltage gain, and zero-voltage soft switching. The converter offers high efficiency using phase-shift modulation when it operates at a nominal design operating point. However, the efficiency decreases due to high circulating current, high current stress, and limited soft-switching range when the source and load voltages deviate from their nominal values. This paper employs both duty cycle and phase-shift modulation methods to improve the above mentioned shortcomings using voltage matching for the DAHB converter. The proposed converter is analyzed and a 500-W experimental prototype with 40–60 V input voltage and 400 V output voltage under 100 kHz switching frequency is implemented to verify the theoretical results. The maximum efficiency of 97.15% is achieved at 60 V and 400 W. A flat efficiency characteristic for the converter is derived by the proposed control method, where the efficiency at 20% of nominal load is 91% and much higher than the phase-shift control, which is 70.8%. [ABSTRACT FROM AUTHOR]

This paper presents a generalized topology for multilevel converters based on flying capacitor multicell structure. The proposed topology consists of two modular high-frequency (HF) and low-frequency (LF) sections or cells. The HF cells consist of flying capacitors and semiconductor switches operating at carrier frequency. The LF cells consist of dc sources and semiconductor switches operating at line frequency and its multiples. The proposed topology can be implemented with arbitrary number HF and LF cells. An important advantage of the proposed topology is its reduced components when compared to other multilevel converter topologies in the same family. Another interesting feature is the relatively low blocking voltage requirement for HF switches, making it an attractive choice for high-power applications. A modulating algorithm is devised to generate the gating pulses for the HF and LF cells using phase-shifted carrier-based pulsewidth modulation. As a particular case, the operation of a 25-level topology is explained by the help of illustrating waveforms. A comprehensive comparative study is presented to illustrate the advantages of the proposed topology in terms of the number of components and other performance criteria. Various simulations are carried out to verify the operation of the proposed topology at different operating conditions. The theoretical analysis is experimentally verified using a scaled-down laboratory setup. [ABSTRACT FROM AUTHOR]

A trinary multilevel converter uses a few heterogeneous H-bridge submodules (SMs) to synthesize an output voltage with multiple levels. Two major limitations of this converter are that it needs multiple dc sources and it cannot operate under pulsewidth modulation (PWM). This paper addresses these limitations by adding a low-voltage H-bridge SM to this converter and proposing a new switching algorithm. The resulting PWM-based asymmetric cascaded H-bridge converter uses only one dc source and synthesizes a high-quality output voltage. This improved converter can be a potential choice for real and reactive power support in power system applications. This paper also proposes methods for stability analysis and design of this converter. Experimental case studies evaluate the performance of the proposed converter for both standalone and grid-connected modes of operation. [ABSTRACT FROM AUTHOR]

Due to the nonlinear characteristics of a photovoltaic (PV) array, its regulation is highly dependent on the operating point. Focusing on a dc–dc boost converter, this paper first shows how the PV voltage and inductor current controls are affected by the PV array. It then proposes to emulate an impedance virtually connected to the PV array, making it possible to greatly improve the control robustness. Thanks to the proposed strategy, the crossover frequency variation for the whole operating range is reduced from 42 times for the traditional control to 3.5 times when emulating parallel resistance or to 1.4 times when emulating series and parallel resistances, all with simple implementation. Experimental results with a commercial PV inverter and a 4-kWp PV array validate the theoretical analysis and demonstrate the superior performance of the proposed control. [ABSTRACT FROM AUTHOR]

This paper investigates robust control strategies for wind energy conversion systems with variable-speed permanent magnet synchronous generators, which are integrated into the grid to provide reliable, secure, and efficient electrical power. A three-phase grid-side converter without a grid transformer is connected to the grid using an LCL filter with low resistive losses. In these working conditions, an instantaneous power proportional integral (PI) controller for the outer voltage-loop in the grid-side converter is used to regulate the DC-link voltage and generate the required currents for the inner current-loop in the grid-side converter. Two integral-type terminal sliding-mode controllers are proposed to control the active and reactive powers exchanged between the converter and the grid. The switching signals in the controllers are softened to attenuate chattering. The time-varying gains in the controllers are constructed to reduce control energy wastage and avoid overestimation of the boundaries of system uncertainties. Simulated and experimental results validate the proposed method. [ABSTRACT FROM AUTHOR]

A 1-MHz-modified single-ended primary-inductor converter (SEPIC) circuit is presented in this paper as an LED driver for automotive headlights. On the basis of traditional SEPIC, only a diode and two capacitors are added; therefore, the circuit structure is relatively simple and the volume is relatively small. High step-up capability is obtained with moderate duty cycle. Compared with traditional step-up circuit such as boost, low-voltage stress is gained which is beneficial to device selection. Zero-voltage switching is achieved at the same time to reduce switching loss. Planar inductors are used with the advantages of low profile and good thermal characteristic. Operational principle and parameters design process are introduced in detail. A 36-W prototype has been designed to demonstrate theoretical analysis. Obtained efficiency was up to 90.91% at full load. [ABSTRACT FROM AUTHOR]

In this paper, the concept of an Optiverter is proposed as a novel class of photovoltaic (PV) module level power electronics systems. Functionally, the Optiverter is a hybrid technology that combines the ultrawide maximum power point tracking (MPPT) voltage window of the PV power optimizers with the direct ac connectivity and inherent safety of the PV microinverters. Thanks to the advanced multimode control with variable dc-link and the shade-tolerant MPPT algorithm, the proposed Optiverter ensures efficient energy harvest from the PV module in different shading scenarios. To justify the superiority of the concept, the performance of a 300 W prototype of the PV Optiverter is experimentally compared to that of the industrial microinverters in different operation conditions, including an extreme case with opaque shading of two out of three substrings of the PV module. [ABSTRACT FROM AUTHOR]

Soft-switching inverters can achieve high efficiencies and low switchdv/dtanddi/dt, but often impose additional stress on the switches due to their resonant behavior. Multicell converters, on the other hand, can process high voltages by arranging the switches to share the stress. This paper proposes a buck–boost multicell soft-switching inverter with inherent cell voltage balancing. Both the current and voltage stresses are shared by the cells. Soft-switching conditions are achieved for all switching transitions without any need for auxiliary circuitry or clamps. A multiple-winding transformer transfers energy between the terminals and provides galvanic isolation. An analytical study and experimental results on a 1200-W prototype verify the operation of the inverter. [ABSTRACT FROM AUTHOR]

In this paper, a novel buck–boost converter with low electric stress on components and negative output voltage is proposed. The operational principle, steady-state analysis, and small-signal modeling of the proposed buck–boost converter in continuous conduction mode are presented. Comparisons among another five buck–boost converters and the proposed buck–boost converter are presented. It is found that the proposed converter's voltage gain is smaller than the other converters’ in step-down mode. Also, based upon the comparisons among the same kind and same number of components, the voltage and current stresses on the power switch of the proposed buck–boost converter are less than or equal to those of the comparative converters, and the voltage stress on the charge pump capacitor and the switching device power rating of the proposed buck–boost converter are always lower than those of other comparative converters. The efficiency of the proposed buck–boost converter is highest among those converters having the same number of elements. These advantages make component selection for the proposed converter much easier, and it can be used directly in applications needing a negative voltage source. Finally, simulation results and circuit experiments are given for validating the effectiveness of the proposed buck–boost converter. [ABSTRACT FROM AUTHOR]

For dc–ac modular multilevel converters (MMCs) in double-star configuration, this paper proposes a fast closed-loop control method based on optimized pulse patterns (OPPs). Thanks to the direct manipulation of the OPP switching instants, OPPs with discontinuous switching angles can be used, and the intrinsic cell capacitor voltage ripple is compensated for to preserve the OPPs’ superior harmonic performance. This model predictive pulse pattern controller is combined with a fast acting dead-beat circulating current controller and linked to an upper layer voltage balancing control scheme. The proposed control and modulation method is particularly suitable for medium voltage MMCs with a low number of cells. Despite operation at very low switching frequencies close to and including the fundamental frequency, a superior harmonic performance can be achieved. [ABSTRACT FROM AUTHOR]

This paper develops a modulation based fault-tolerant (FT) strategy for restoring the operation of three-phase modular multilevel converters (MMCs) with faulty switches. This FT strategy is based on a proposed modified space vector modulation (SVM) technique that generates balanced line-to-line (line) voltages even in the case of a fault occurrence. In the postfault operation, the proposed strategy is able to restore the fundamental amplitude of the line to the neutral (load) voltages to that of the normal operation condition with a slightly increased voltage stress over the switches in the faulty phase. In this paper, first a brief background about MMCs and SVM technique is provided. Then, the proposed FT strategy and the modified SVM technique are presented. Finally, several simulation and experimental results are provided to validate operation of the proposed strategy. [ABSTRACT FROM AUTHOR]

Generally, the small-signal model (SSM) of a modular multilevel converter (MMC) needs to consider the dynamics of equivalent arm capacitors, which inevitably increases the model order. In view of this, this paper proposes a general order-reducing approach based on balanced realization theory to reduce the SSM order of the MMC. In particular, two reduced-order models of an MMC-based HVdc grid are proposed with respect to different research objectives. To determine the potential minimum order of single MMC or MMC-HVdc grid, the criteria including singular values (SVs) of the frequency response, dynamic response in the time domain, and the largest absolute error of SVs are fully investigated. The simulation results of a four-terminal MMC-HVdc grid show that reduced-order models of the MMC and the HVdc grid can precisely predict both the stability and instability while the dynamic characteristics are retained. [ABSTRACT FROM AUTHOR]

Traditional control strategy for a power decoupling circuit typically involves a dedicate ripple power control loop, which makes it hard to achieve satisfied transient performance. Meanwhile, traditional single-phase ac–dc converters achieve active power decoupling function with extra power switches, which inevitably increased the cost of the system. In this paper, a controller based on automatic power decoupling concept is proposed for a dependent power decoupling circuit with no extra switches. Compared with the existing methods, the system controlled by the proposed controller has superior transient performance with simplified controller's structure. The controller parameter's design procedures will also be illustrated step by step with the help of the bode diagram. In comparison with traditional control methods, simulations and experiments verify the feasibility of the proposed control methods on the dependent power decoupling circuit. [ABSTRACT FROM AUTHOR]

In this paper, a quantitative evaluation of the flux-switching permanent magnet (FSPM) machine system reliability is carried out based on the Markov model. The evaluation model of the permanent magnet machine system consists of six main functional components and their dominant failure modes are counted. The performance of the FSPM machine system under each fault is simulated and compared with the reliability criteria to judge the reliability state of the machine system. The Markov model is used to calculate the FSPM machine system reliability. The influence of machine phase number, stator core topology, and hybrid excitation method on the machine system reliability is investigated. The experiments are presented to validate the accuracy of fault simulation. The reliability analysis results lay the foundation for the reliability design of an FSPM machine. [ABSTRACT FROM AUTHOR]

Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC)-based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopole MMCs are first studied. Upper arm overvoltages and ac-side nonzero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings, which will enable the operation of grid-side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults. [ABSTRACT FROM AUTHOR]

A nonisolated dc–dc converter with chain links of submodules (SMs) is proposed in this paper. The converter can perform as a dc transformer with a low step-up ratio. To demonstrate the step-up dc–dc conversion, the converter is operated in a resonant conversion mode, achieving soft switching and inherent balancing of the SM capacitor voltages. The circuit configuration is presented and the operation principle is analyzed in this paper. Experimental results based on a downscaled prototype validate the performance of the proposed converter as a low step-up ratio dc transformer. The converter also exhibits a good linearity covering a broad range. [ABSTRACT FROM AUTHOR]

The circulating current of modular multilevel converter (MMC) has a great impact on the power losses, the submodule (SM) capacitor voltage ripples, and the rating of power devices. Existing analysis of circulating current has considered the capacitor voltage and the modulation signal. In this paper, the effect of semiconductor devices on the circulating current is further presented. In order to realize the different circulating current injection targets under different conditions, a unified online calculation scheme by using the instantaneous information of MMC is proposed in this paper to generate the circulating current injection references. It avoids detecting the amplitude and phase of the output current or using an extensive lookup table. Besides, a mathematical analysis model is built and it reveals the impact on the capacitor voltage ripple and the rms value of arm current with different injection references, thus guide the determination of the injection ratio. Experimental results based on a three-phase MMC prototype with six SMs per arm are presented to demonstrate the analysis of circulating current and the proposed circulating current injection references generating method is verified by different circulating current control targets with different loads. [ABSTRACT FROM AUTHOR]

Among the various topologies proposed in this literature, the modular multilevel converter (MMC) is considered the next generation of converters for medium- and high-voltage applications. This paper presents a comparison between two variants of MMCs for STATCOM applications: The double-star chopper cell-MMC and the single-delta bridge cell-MMC. These converter topologies are compared and benchmarked during positive and negative sequence compensation. The comparisons are supported by analytical results regarding minimum effective dc-link voltage, number of cells, current rating, energy storage requirements, and operation during unbalanced conditions. The dynamic behavior, power losses, and cost of both solutions are evaluated through simulation model of a 15 mega volt-ampère (MVA) MMC STATCOM. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a promising topology for high-power drive applications. However, large voltage fluctuations can be produced in the floating capacitors when the machine operates with high stator currents at low rotational speed. To compensate these oscillations, relatively large mitigation currents are required to keep the capacitor voltages within an acceptable range. In this paper, a new integrated control scheme is discussed to regulate the voltage fluctuations. The strategy is based on closed-loop vector-control of the voltage fluctuations, maintaining them inside a predefined threshold. The proposed control system is also augmented using flux-weakening operation of the machine at low rotational speeds. An experimental prototype composed of 18 power cells, feeding a vector-controlled induction machine in the whole speed range, is used to validate the effectiveness and feasibility of the proposed control strategies. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is very attractive in a high-voltage dc (HVdc) transmission system due to its high modularity and scalability. It is also a promising candidate for medium-voltage applications. An MMC has many power devices that lead to a relatively low reliability based only on a direct failure rate calculation. The reliability of an MMC can be improved by redundant designs. Lots of redundant designs and strategies have been proposed in recent years, while their reliability improvements are hardly quantified in existing literature. In practice, reliability improvement with constraint costs is a challenge. In view of these, this paper proposes a reliability and cost assessment scheme for MMC redundant designs and strategies. Reliability and cost are quantified for comparison purposes. A medium-voltage application is used to demonstrate the reliability and cost assessment scheme. The reliability analysis is based on a combinatorial model and the Poisson process model. The cost analysis focuses on capital cost and power loss cost. The proposed reliability and cost analysis scheme provides a guideline for MMC redundant design and redundant strategy selection. [ABSTRACT FROM AUTHOR]

This paper describes a high-performance neutral point voltage balancing technique for a neutral point clamped (NPC) converter. Conventional neutral point voltage balancing methods do not function well under low power factor, low pulse ratio, and near-unity modulation index operation conditions. These conditions are essentially dominant operation conditions for aircraft starter/generator systems. This paper introduces an alternative space vector modulation technique for three-level NPC converters in an aircraft starter generator system. The selection of voltage space vectors is optimized for high modulation index and low power factor operation. Disturbances caused by a low pulse ratio are also compensated. The proposed method maintains neutral point voltage balance and ripple minimization over the full range of operating conditions. This paper also provides a detailed analysis of the sources of neutral point voltage imbalances and ripples in high-speed drives with deep flux weakening. Simulation results obtained from a Simulink/PLECS model and experimental results obtained from a 45 kVA, 32 krpm aircraft starter generator test rig prove that the proposed method eliminates the neutral point voltage imbalance and significantly reduces the neutral point voltage ripple. [ABSTRACT FROM AUTHOR]

The dc-link capacitors are important components that tend to be one of the weakest part of power converters. Thus, it is necessary to monitor the aging parameters of dc-link capacitors in valuable power electronic systems. This paper proposes a simple yet effective variable electrical network (VEN) condition monitoring method for dc-link capacitors in three-phase pulsewidth modulation (PWM) ac–dc–ac power converters. The capacitance (C) and equivalent series resistance of dc-link capacitors are estimated through the designed VEN unit during the discharging process. This VEN method does not need the current sensor or injecting signals into the control loop. The above property is favorable for the hardware and controller design. Besides, the designed monitoring circuit can be an external unit or a built-in unit which offers a flexible solution for electrical devices. When the designed VEN is used as an external unit, neither the hardware design nor the programs in the controller needs to change, which is practical and economic for electrical devices in use. Experiments are carried out in a 55 kW ac–dc–ac inverter with different capacitors. The experiment results prove the method to be effective and accurate. [ABSTRACT FROM AUTHOR]

In previous research, the variable conductance of a parallel active power filter has been used for antiparallel resonance with parameter variations in power systems. Its disadvantages are many sensors, complicated harmonic detection, and complexity of the algorithm for successful accomplishment in antiparallel resonance. In order to eliminate such drawbacks, this paper presents the parallel active power filter based on source current detection for antiparallel resonance with robustness to parameter variations for power factor correction using capacitor banks. The proposed method employs only single harmonic detector and an open-loop shaping design for robustness to variations of source impedance and a sudden change of capacitor bank values for power factor correction. The proposed antiparallel resonance controller can be readily applied to both large-scale and laboratory-scale power systems without any change. The large-scale simulation and the laboratory-scale experiment results confirm the validity, the accuracy, and good performance for antiparallel resonance, harmonic suppression, and robustness to parameter variations in power systems. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become the most promising converter topology for medium- and high-power applications. In this paper, a sliding mode control (SMC) based method is proposed for the MMC. Based on analysis of the system dynamics under the proposed control method, relations among control parameters and their validity conditions are obtained, which provide a guidance for systematic controller design. Compared with the conventional proportional-integral based method, the proposed SMC-based method provides comparable steady state performance and faster dynamic responses, without compromising the computational effort or requirement for precise system model. Simulation studies in the PSCAD/EMTDC software environment are carried out to validate the performance and effectiveness of the proposed control method in the context of an HVdc transmission system. A laboratory prototype is also developed to experimentally validate the proposed control method. [ABSTRACT FROM AUTHOR]

This paper proposes a new dc–ac converter with a reduced number of semiconductor switches and gate drive and control circuit components. A salient feature of this inverter is its pure sinusoidal line-to-line voltages with no need for output filter. Also, the proposed inverter improves the voltage utilization factor of the input dc supply compared to the four-switch three-phase inverter (first best topology in the literature from the number of switches point of view) and standard six-switch inverter with sine pulsewidth modulation. The proposed inverter is capable of operating with a wide range of output voltages from zero to the full value of the dc input voltage by appropriately altering the instantaneous duty cycle. A three-phase universal inverter with only three power switch has been proposed in this paper for the first time ever, to our knowledge. For the purpose of providing a good compromise between fast transient response and stability, a model predictive controller is proposed. All the design expressions have been derived. A comparison with other dc–ac converters is given to show the merits of the proposed converter. Finally, satisfactory circuit operation is confirmed by experimental results from a laboratory prototype. [ABSTRACT FROM AUTHOR]

For the effective operation of a dynamic voltage restorer (DVR), a control strategy plays a significant role. This paper presents an enhanced control strategy for DVR using dual role cascaded delay signal cancellation (CDSC)-based dual vector control (DVC) under unbalanced and distorted grid conditions. Based on the numerical analysis, it is found that the CDSC prefilter is a promising solution when grid voltage is distorted by symmetric, asymmetric harmonics, and unbalanced sag. Mainly, the CDSC prefilter extracts instantaneous symmetrical components of the grid voltage required for voltage sags detection and generation of fundamental component of reference voltage for the DVR. A CDSC-based DVC algorithm with inductor current and capacitor voltage feedback is implemented in a synchronous reference frame, which tracks the fundamental DVR reference voltages. An extractor based on the modified CDSC strategy is designed to extract harmonics from load voltage during distorted grid conditions. These extracted harmonic components (nonfundamental) are added in phase opposition with fundamental component and fed to pulse width modulation block to generate reference voltages. Experimental studies are conducted on scaled down (100 V, 0.5 kVA) laboratory prototype DVR to verify the effectiveness of the proposed control algorithm under unbalanced and distorted grid conditions. [ABSTRACT FROM AUTHOR]

This paper investigates dc-link capacitor optimization for three-phase photovoltaic-based microinverters. This concept minimizes the storage capacitance by allowing greater voltage ripple on the dc link. Therefore, the microinverter reliability can be significantly increased by replacing electrolytic capacitors with film capacitors. However, this intentionally increased voltage ripple can introduce harmonic distortion on the output current of the inverter stage if it is not mitigated by the dc-link voltage controller. For this purpose, a robust and accurate dc-link voltage control is proposed to filter this ripple while regulating the dc-link voltage without using any additional circuit components. The experimental results on a 400-W three-phase half-bridge microinverter prototype validate the theoretical analysis of the dc-link capacitor optimization and show that a significant reduction of the dc-link capacitor requirement can be achieved. The proposed high-accuracy dc-link voltage controller is also implemented on this prototype to demonstrate very low harmonic distortion of the inverter output current while tightly regulating the dc-link voltage even during transients. [ABSTRACT FROM AUTHOR]

In this paper, an integrated double buck–boost (IDBB) converter is proposed as a high-power-factor offline power supply for power-LED lamps. The IDBB converter features just one controlled switch and two inductors and is able to supply a solid-state lamp from the mains, providing high power factor and good efficiency. In this paper, the IDBB converter is analyzed, and a design methodology is proposed. It is demonstrated that, with a careful design of the converter, the filter capacitances can be made small enough so that film capacitors may be used. In this way, the converter mean time between failures can be made as high as that of the solid-state lamp. A design example for a 70-W converter supplied from a 230 V/50 Hz mains for street lighting applications is shown. Finally, experimental results from a laboratory prototype are also presented. [ABSTRACT FROM AUTHOR]

This paper proposes a novel family of cascade three-level (TL) ac–ac direct converters based on ac switch cells, which transfer unsteady high ac voltage with distortion into regulated sinusoidal voltage with low total harmonic distortion (THD). The topological family includes buck TL–boost, buck–boost TL, and buck TL–boost TL modes. In order to achieve a reliable TL ac–ac conversion, a double transient voltage feedback control strategy of the output voltage and the voltage across the flying capacitor is introduced in this paper. A 500-VA 220-V \pm10% 50-Hz ac/220-V 50-Hz ac prototype is presented with the experimental results to prove that the converters have four improved advantages simultaneously, including lower voltage across power switches, bidirectional power flow, low THD of output voltage, and higher input power factor. [ABSTRACT FROM AUTHOR]

This paper proposes a CLL resonant dc–dc converter as an option for offline applications. This topology can achieve zero-voltage switching from zero load to a full load and zero-current switching for output rectifiers and makes the implementation of a secondary rectifier easy. This paper also presents a novel methodology for designing CLL resonant converters based on efficiency and holdup time requirements. An optimal transformer structure is proposed, which uses a current-type synchronous rectifier (SR) drive scheme. An 800-kHz 250-W CLL resonant converter prototype is built to verify the proposed circuit, design method, transformer structure, and SR drive scheme. [ABSTRACT FROM PUBLISHER]

Resonant power conversion provides high-efficiency and high-power-density power converters that have boosted the performance of many applications, including induction heating (IH). One of the main challenges of such converters is the fine tuning and operation monitoring of the resonant tank, especially when it is highly variable. This paper proposes an field-programmable gate array (FPGA)-based load identification technique based on measuring the resonant capacitor voltage and calculating its spectral components. This approach proposes an accurate and cost-effective implementation, since the sensing, conditioning, and analog-to-digital conversion systems can be significantly simplified to a single low-harmonic-content signal. The proposed approach has been tested using a wide range of IH loads and operating conditions, proving the feasibility of this proposal. [ABSTRACT FROM AUTHOR]