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Microinverters for photovoltaic (PV) energy conversion typically encounter challenges, including the requirement of a voltage boost and suppression of the high-frequency leakage current. These challenges are usually not met simultaneously if only a standard full (H)-bridge is used. Such limitation is, however, not applicable to the proposed dual-boost H-bridge inverter, which uses only four switches and a unique dual-boost circuit for voltage boosting and forming a common ground. The latter stops PV terminal voltages from changing rapidly, and hence, no high-frequency capacitive current leaking to the ground even without using extra switches. Additionally, the proposed inverter can operate with either one or two PV sources whose terminal voltages need not be balanced. One of the source voltages can, in fact, fall to zero (due to shading) without affecting the continuous operation of the inverter, whose modulation has been formulated to keep its conduction losses low at a steady dc-link voltage. The dc-link capacitor across the H-bridge meanwhile decouples the pulsating output power, which in practice, has always challenged the design of single-phase microinverters. These intended performances have eventually been proven by the simulation and experimental results of a 500-W prototype. [ABSTRACT FROM AUTHOR]

Multilevel inverters in renewable applications usually require a high-voltage dc link fed by a front-end dc--dc boost stage. Such a two-stage power conversion, however, increases the switch count, leading to higher costs and power losses. To lower the number of switches, this letter proposes a six-switch inverter, capable of generating a seven-level voltage and a triple-voltage boost. Both features are achieved with only four switches operating at a high frequency while clamped by low-voltage capacitors. The remaining two switches must, however, endure higher voltage stresses but fortunately operate at a much lower frequency. The overall switching losses of the inverter are, thus, significantly reduced. Besides, the inverter shares a common ground between its dc input and ac output, which theoretically helps to eliminate the leakage current, if powered by photovoltaic sources. The operating principles of the inverter have been described and verified through experiments with an 800-W prototype. [ABSTRACT FROM AUTHOR]

Instead of bulky electrolytic capacitors, active power decoupling circuit can be introduced to a single-phase converter for diverting second harmonic ripple away from its dc source or load. One possible circuit consists of a half-bridge and two capacitors in series for forming a dc-split capacitor, instead of the usual single dc-link capacitor bank. Methods for regulating this power decoupler have earlier been developed, but almost always with equal capacitances assumed for forming the dc-split capacitor, even though it is not realistic in practice. The assumption should, hence, be evaluated more thoroughly, especially when it is shown in the paper that even a slight mismatch can render the power decoupling scheme ineffective and the IEEE 1547 standard to be breached. A more robust compensation scheme is, thus, needed for the dc-split capacitor circuit, as proposed and tested experimentally in the paper. [ABSTRACT FROM AUTHOR]

This paper analyzes the stability of digitally voltage-controlled voltage-source inverters (VSIs) with the linear voltage regulators. It is revealed that the phase lags, caused by using the resonant controller and the time delay of a digital control system, can stabilize the single-loop voltage control without damping of the LC -filter resonance. The stability region for the digital single-loop resonant voltage control is then identified, considering the effects of different discretization methods for the resonant controller. An enhanced voltage control approach with a widened stability region is subsequently proposed, and a step-by-step design method of the proposed controller is developed based on the root contours in the discrete z-domain. Simulations and experimental tests of a 400-Hz VSI system validate the stability analysis and the performance of the proposed control approach. [ABSTRACT FROM PUBLISHER]

Droop control has commonly been used with distributed generators for relating their terminal parameters with power generation. The generators have also been assumed to have enough capacities for supplying the required power. This is, however, not always true, especially with renewable sources with no or insufficient storage for cushioning climatic changes. In addition, most droop-controlled literatures have assumed a single dc–ac inverter with its input dc source fixed. Front-end dc–dc converter added to a two-stage photovoltaic (PV) system has, therefore, usually been ignored. To address these unresolved issues, an improved droop scheme for a two-stage PV system has been developed in this paper. The developed scheme uses the same control structure in both grid connected and islanded modes, which together with properly tuned synchronizers, allows mode transfer to be seamlessly triggered. Moreover, the proposed scheme adapts well with internal PV and external grid fluctuations, and is, hence, more precise with its tracking, as compared with the traditional droop scheme. Simulation and experimental results have verified these expectations, and, hence, the effectiveness of the proposed scheme. [ABSTRACT FROM PUBLISHER]

A two-stage cascaded converter is formed by connecting two subconverters in series. Output impedance of one subconverter will, therefore, interact with input impedance of the other subconverter. Such interaction may affect the system dynamics and stability, especially when one subconverter is constant-power-controlled, and hence, has different impedance characteristics when its power reverses. To lessen such constant-power effects, a control scheme that can coordinate impedance behaviors of the two subconverters is proposed. The idea is to reshape the low-frequency negative impedance of the constant-power subconverter to a positive resistance needed for neutralizing stability differences between the two directions of power flow. A smooth power reversal can thus be ensured, as verified through simulations and experiments performed with a scaled-down 400-W laboratory prototype. [ABSTRACT FROM PUBLISHER]

The middle capacitor voltage of an LCL-filter, if fed back for synchronization, can be used for active damping. An extra sensor for measuring the capacitor current is then avoided. Relating the capacitor voltage to existing popular damping techniques designed with capacitor current feedback would however demand a noise-sensitive derivative term. Digital implementation of this derivative term is generally a challenge with many methods presently developed for resolving it. These methods are however still facing drawbacks, which have comprehensively been explained in this paper. Two derivatives are then proposed, based on either second-order or nonideal generalized integrator. Performances of these derivatives have been found to match the ideal “s” function closely. Active damping based on capacitor voltage feedback can therefore be realized accurately. Experimental results presented have verified the effectiveness of the proposed derivative, which can similarly be used with other applications, where differentiation is needed. [ABSTRACT FROM AUTHOR]

Modulation schemes can significantly change the performance of a converter in the steady state. They have, thus, been extensively studied, but directed more at three-phase topologies, where triplen offsets are added. Single-phase systems, including ac–dc–ac, are less widely pursued, even though various topologies have since been proposed. The intention of this paper is, thus, to study two promising single-phase ac–dc–ac converters using reduced number of switches, and subsequently proposes modulation schemes for them. The proposed schemes help the converters to achieve better thermal spread among their switches, while keeping their dc-link voltages low. Single points of failure are thus minimized, allowing the converters to have longer lifetimes. Simulation and experimental results have demonstrated the intended performances, hence verifying the analyses presented in this paper. [ABSTRACT FROM PUBLISHER]

Cascaded converter is formed by connecting two subconverters together, sharing a common intermediate dc-link voltage. Regulation of this dc-link voltage is frequently realized with a proportional-integral (PI) controller, whose high gain at dc helps to force a zero steady-state tracking error. Such precise tracking is, however, at the expense of increasing the system type, caused by the extra pole at the origin introduced by the PI controller. The overall system may, hence, be tougher to control. To reduce the system type while preserving precise dc-link voltage tracking, this paper proposes a coordinated control scheme for the cascaded converter, which uses only a proportional dc-link voltage regulator. The resulting converter is thus dynamically faster, and when compared with the conventional PI-controlled converter, it is less affected by impedance interaction between its two subconverters. The proposed scheme can be used with either unidirectional or bidirectional power flow, and has been verified by simulation and experimental results presented in this paper. [ABSTRACT FROM PUBLISHER]

This paper presents two linear power sharing schemes for distributed generators (DGs). The purpose is to reduce the total generation cost (TGC) of the considered autonomous microgrid. The saving in cost is realized by tuning the DG droop gradients in accordance to their respective maximum or mean generation costs. Compared with the existing nonlinear cost-based schemes, the proposed schemes use linear droop functions, which are easier to tune and implement in practice for a greater reduction in TGC. The proposed schemes have been tested experimentally with an example microgrid powered first by dispatchable DGs only, and then by a combination of dispatchable and nondispatchable DGs. Experimental results obtained have demonstrated the anticipated outcomes under different operating conditions. [ABSTRACT FROM AUTHOR]

The B6 converter uses six switches divided equally among three phase legs. It has been commonly used as a three-phase rectifier or inverter, mostly under balanced conditions. Three-phase conversion, however, is not the only area where the B6 converter has been used. The same topology has been tried for single-phase power decoupling and single-phase ac–dc–ac conversion, where comparably lesser switches are needed. Although the basic modulation principle is still reliant on three modulating references for the three phase legs, requirements imposed on the modulating references are different and usually asymmetrical. How these asymmetrical references should be formulated to meet various performance specifications of a single-phase B6 converter is the theme of this paper. Simulation and experimental results have been obtained to verify the modulation schemes proposed. [ABSTRACT FROM AUTHOR]

As the modular multi-level converter is now developing rapidly, especially in high power applications, and the large amount of sub-modules in the modular multi-level converter will increase the probability of failures, fault detection and diagnosis of the sub-modules are of importance for continuous operation and post-fault maintenance. In this article, a fault diagnosis technique is proposed for the short circuit fault in a modular multi-level converter sub-module using the wavelet transform and adaptive neuro fuzzy inference system. The fault features are extracted from output phase voltage by employing wavelet transform under different fault conditions. Then the fuzzy logic rules are automatically trained based on the fuzzified fault features to diagnose the different faults. Neither additional sensor nor the capacitor voltages are needed in the proposed method. The high accuracy, good generalization, as well as the time-saving characteristic is verified by conducting a comparison with the neural network method. [ABSTRACT FROM AUTHOR]

Intertied ac–ac microgrids offer many potential benefits like flexible frequency and voltage selection, reserve sharing and enhanced supply reliability. Operating them is however challenging due to their larger size and presence of more power converters with different responsibilities. These complexities can further be amplified when reliance on fast communication links is not permitted. To address these concerns, a new autonomous droop scheme is proposed here for managing intertied microgrids in either the islanded or grid-connected mode. The scheme prioritizes various power generation/transfer responsibilities among power converters found in the intertied microgrids. The objective is to smoothly maximize the overall power handling capability of the intertied microgrids without relying on fast communication links. The proposed scheme has been tested experimentally in the laboratory. [ABSTRACT FROM AUTHOR]

Droop schemes have traditionally been applied to the control of parallel synchronous generators in power systems. It has subsequently been brought over to the control of distributed generators (DGs) in microgrids with the same retained objective of proportional power sharing based on ratings. This objective might, however, not suit microgrids well since DGs are usually of different types, unlike synchronous generators. Other factors like cost, efficiency, and emission penalty of each DG at different loading must be considered since they contribute directly to the total generation cost (TGC) of the microgrid. To reduce this TGC without relying on fast communication links, an autonomous droop scheme is proposed here, whose resulting power sharing is decided by the individual DG generation costs. Comparing it with the traditional scheme, the proposed scheme retains its simplicity and it is hence more likely to be accepted by the industry. The reduction in TGC has been verified by experiment. [ABSTRACT FROM PUBLISHER]

Although many droop schemes have been proposed for distributed generator (DG) control in a microgrid, they mainly focus on achieving proportional power sharing based on the DG kVA ratings. Other factors like generation costs, efficiencies, and emission penalties at different loads have not been considered even though they are different for different types of DGs. This letter thus proposes an alternative droop scheme, which can better distinguish the different operating characteristics and objectives of the DGs grouped together in a weighted droop expression. The power sharing arrived in the steady state will meet the intended objectives, as demonstrated in the experiment with a defined objective of minimizing the total microgrid operating cost. [ABSTRACT FROM AUTHOR]

Impedance-source inverters are inverters with voltage-buck–boost capability that cannot be achieved by the traditional inverters. Their boost capability is introduced by shorting their phase legs without causing damages. Impedance-source inverters are therefore less prone to false triggering caused by electromagnetic interference. Present impedance-source inverters are, however, burdened by their low modulation ratio at high input-to-output gain. Such low modulation usually leads to high-voltage stresses across the components and poor spectral performance at the inverter output. To avoid these problems, inverters with coupled transformers have been introduced, but they usually lead to high turns ratio, and hence many winding turns, at high gain. An alternative would then be the asymmetrical \Gamma-source inverters proposed in this paper, whose gain is raised by lowering their turns ratio toward unity. The input current drawn by the proposed inverters is smoother and, hence, more adaptable by the source. Theories and experimental results have been presented in this paper for validating the concepts proposed. [ABSTRACT FROM AUTHOR]

This paper proposes a multiple-input versatile matrix converter (VMC) for integrating hybrid ac/dc energy resources and storages to the power grid. The VMC is developed from the traditional indirect matrix converter but operates in the reverse-boost mode rather than in the forward-buck mode. The reverse-boost mode is more relevant here since most renewable sources and energy storages have lower voltages than the grid. The eventual VMC developed uses an alternative nine-switch converter, rather than usual six-switch voltage-source converter, for providing six input terminals in total. One three-phase ac source and three dc sources, or other source combinations, can therefore be connected to the VMC. Powers from these sources are channeled to the three-phase utility grid through the VMC's current-source inverter. Their proper dispatches are guaranteed by the proposed control and modulation schemes, which also help maintain near-sinusoidal input and output current waveforms. Mathematical proofs, simulation, and experimental results have shown that the VMC can indeed operate as intended. [ABSTRACT FROM PUBLISHER]

Voltage-type Γ-Z-source inverters are proposed in this letter. They use a unique Γ-shaped impedance network for boosting their output voltage in addition to their usual voltage-buck behavior. Comparing them with other topologies, the proposed inverters use lesser components and a coupled transformer for producing the high-gain and modulation ratio simultaneously. The obtained gain can be tuned by varying the turns ratio \gamma \Gamma Z of the transformer within the narrow range of 1 < \gamma \Gamma Z \le 2. This leads to lesser winding turns at high gain, as compared to other related topologies. Experimental testing has already proven the validity of the proposed inverters. [ABSTRACT FROM AUTHOR]

Microgrids are small grids formed by clustering modern generating sources, storage systems, and loads together. Being independent, the formed microgrids can, in principle, operate at their own preferred voltages and frequencies. Tying them to the mains grid or another microgrid would therefore require some interlinking power converters, whose control should preferably be autonomous without depending on fast communication links. Contributing to this theme of research, a distributed power management scheme has been proposed in this paper for interlinking two or more independent microgrids operating at different voltages and frequencies. The proposed scheme allows sources in the microgrids to concentrate more on active power harnessing, while the interlinking converters focus more on meeting the load reactive demand. If necessary, backup active power from an underloaded microgrid can also be transferred to an overloaded microgrid, allowing it to supply loads in excess of its rated capacity. The performance of the proposed scheme has already been tested in experiment. [ABSTRACT FROM AUTHOR]

Z-source inverters are a new class of inverters proposed with output voltage or current buck–boost ability. Despite their general attractiveness, there are some present limitations faced by existing Z-source inverters, most of which are linked to their requirement for low modulation ratio at high input-to-output gain and the presence of an impedance network. The former means a high dc-link voltage, which can stress the semiconductor switches unnecessarily. The latter leads to increases in cost and size, which similarly are undesirable. To lessen these concerns, an interesting approach is to use magnetically coupled transformers or inductors to raise the gain and modulation ratio simultaneously, while reducing the number of passive components needed. A study of the approach is now presented to show how various existing magnetically coupled inverters can be derived by applying a generic methodology. The same methodology is then applied to develop more magnetically coupled Z-source inverters with advantages that have not been identified in the literature. These findings have already been proven in experiments. [ABSTRACT FROM AUTHOR]

In this paper, a number of alternate-cascaded switched-inductor and tapped-inductor networks have been proposed for Z-source inverters. The resulting topologies have enhanced voltage-boost capability while retaining the usual voltage-buck flexibility of a conventional voltage-source inverter. The enhanced capability is achieved by using lower rated components that can more readily be found. These components are assembled without direct series connection and hence avoid unbalanced voltage sharing problems and losses linked to balancing resistive circuits. The component count is, however, high, meaning that the proposed inverters should only be considered when a design requires multiple lower rated components rather than a few higher rated ones. Analysis, simulation, and experimental results have already validated the concepts discussed. [ABSTRACT FROM PUBLISHER]

The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence. [ABSTRACT FROM PUBLISHER]

This paper summarizes the main problems and solutions of power quality in microgrids, distributed-energy-storage systems, and ac/dc hybrid microgrids. First, the power quality enhancement of grid-interactive microgrids is presented. Then, the cooperative control for enhance voltage harmonics and unbalances in microgrids is reviewed. Afterward, the use of static synchronous compensator (STATCOM) in grid-connected microgrids is introduced in order to improve voltage sags/swells and unbalances. Finally, the coordinated control of distributed storage systems and ac/dc hybrid microgrids is explained. [ABSTRACT FROM PUBLISHER]

Distributed generation systems usually include local energy sources, storages, and loads. Almost always, these entities have their own power converters for grid interfacing and energy processing. Having individual converters has advantages like more flexible individual control and simpler design but does not encourage functionality merging. Reduction of semiconductors to arrive at a more compact integrated design is thus not possible. Addressing this concern, a number of integrated energy generation systems that use 25% lesser semiconductors are proposed. The systems can be single or three phase depending on the types of sources, storages, and loads assembled. They can operate in the grid-tied or stand-alone mode with no compromise in performances expected, when compared with other solutions using more switches. For verifying the practicalities of the proposed integrated systems, experimental testing has already been performed on two example systems assembled in the laboratory. [ABSTRACT FROM PUBLISHER]

Modern distributed sources and loads can either be ac or dc. It is thus possible to have ac and dc subgrids intertied to form hybrid microgrids with lesser power conversion, and hence higher efficiency. However, operation of hybrid microgrids to gain these advantages is not trivial, and is so far limited. It is, therefore, the theme here to develop a control scheme for the hybrid microgrids so that they can operate efficiently with progressive energy flow tuning and no requirement for fast communication links. A plug-in scheme is also proposed for progressive and selective charging/discharging of storages, which are likely to be present in the hybrid microgrids for energy smoothing and ride-through purposes. The overall scheme has already been verified in experiment. [ABSTRACT FROM AUTHOR]

Indirect matrix converter (IMC) is an alternative for ac/ac energy conversion, usually operated with a voltage stepped-down gain of only 0.866. For applications like distribution generation where voltage-boost functionality is required, the traditional style of operating the IMC is therefore not appropriate. Like most power converters, the operation of the IMC can surely be reversed to produce a boosted gain, but so far its relevant control principles have not been discussed. These challenges are now addressed in this paper with distributed generation suggested as a potential application. Simulation and experimental results for validating various performance aspects of the proposed control schemes can be found in a later section of this paper. [ABSTRACT FROM AUTHOR]

Inverters with high-output voltage gain usually face the problem of high-input current flowing through their components. The problem might further be exaggerated if the inverters use high-frequency magnetic devices like transformers or coupled inductors. Leakage inductances of these devices must strictly be small to prevent overvoltages caused by switching of their winding currents. To avoid these related problems, cascaded trans-Z-source inverters are proposed. They use multiple magnetic cells in an alternately cascading pattern rather than a single magnetic cell with large turns ratio. Simulation and experimental results have shown that the multicell inverters can produce the same high-voltage gain, while keeping currents and voltages of the components low. The inverters can also step down their output voltages like a traditional voltage-source inverter without compromising waveform quality. [ABSTRACT FROM AUTHOR]

Traditional voltage-source inverter is limited by its only voltage step-down operation, while current-source inverter is limited by its only current step-down mode. In order to add an extra boosting flexibility while keeping the number of active semiconductors unchanged, voltage-type and current-type Z-source inverters were earlier proposed. These new classes of inverters are generally more robust and less sensitive to electromagnetic noises. However, their boosting capabilities are somehow compromised by high component stresses and poorer spectral performances caused by low modulation ratios. Their boosting gains are, therefore, limited in practice. To overcome these shortcomings, the generalized switched-inductor and switched-capacitor Z-source inverters are proposed, whose extra boosting abilities and other advantages have already been verified in simulation and experiment. [ABSTRACT FROM AUTHOR]

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

In this paper, a modified one-cycle-control (OCC) scheme is proposed for regulating three-phase pulsewidth-modulated (PWM) rectifiers operating under unbalanced and distorted supply conditions. Mathematical analysis is first presented to show the dominant harmonic-voltage components appearing across the dc link of the PWM rectifier when it is controlled by an existing OCC scheme. Such dc-link harmonics when produced may aggravate harmonic currents flowing into the input terminals of the three-phase rectifier. To address these interrelated unbalance and harmonic concerns, the concept of reconstructing the appropriate reference signals in the OCC control core is introduced, before modifications are recommended for altering the key control equation. The improved OCC controller eventually developed is proven to exhibit minimal input-current distortion and a smoother dc-link voltage without using large capacitance. Other advantages exhibited by the traditional OCC scheme, such as no phase-locked loop, no frame transformation, and constant switching frequency, are also retained by the improved controller, whose theoretical findings are fully verified by experimental results obtained from a 1.2-kW three-phase PWM rectifier built in the laboratory. [ABSTRACT FROM PUBLISHER]

An indirect matrix converter is an alternative "all-semiconductor" energy processor proposed recently for converting an ac source with fixed magnitude and frequency to a variable voltage and frequency supply that can meet the requirements of a particular industry application. In principle, an indirect matrix converter is constructed by connecting a front-end bidirectional rectification stage to a conventional two-level inversion stage with no bulky capacitive or inductive energy storage connected to their intermediate dc link. Through the proper modulation and compensation of the resulting converter, sinusoidal input current and output voltage can be achieved with minimized rectification switching loss, rendering the indirect matrix converter as a competitive choice for interfacing the utility grid to, e.g., defense facilities that require a different frequency supply. As an improvement, an indirect matrix converter assembled using a three-level energy, processing stage has briefly been mentioned in the literature, but has neither been tested in simulation nor experimentally because its operational principles, pulsewidth-modulation (PWM) control, and index compensation have not yet been discussed in the existing literature. Addressing the previously described issues, this paper focuses on the operational mode analysis of a three-level indirect matrix converter implemented using a neutral-point-clamped inversion stage and the design of a number of PWM and modulation ratio compensation schemes for controlling the converter with improved waveform quality. The performances and practicalities of the designed schemes are verified in simulation and experimentally using an implemented laboratory prototype with some representative results captured and presented in this paper. [ABSTRACT FROM AUTHOR]

This paper presents the development of two three-level cascaded Z-source inverters, whose output voltage can be stepped down or up unlike a traditional buck three-level inverter. The proposed inverters are designed using two three-phase voltage-source inverter bridges, supplied by two uniquely designed Z-source impedance networks. These three-phase bridges can either be cascaded at their dc sides to form a dc-link-cascaded Z-source inverter or at their ac outputs using single-phase transformers to form a dual Z-source inverter. The dc-link-cascaded inverter has the advantages of not using any clamping diodes and transformers, but does not have redundant switching states within a phase leg for equalizing switching losses among the power devices. This constraint limits the modulation options for the dc-link-cascaded inverter, and indeed, it can only be controlled using the modified carrier disposition technique with appropriate "Z-source shoot-through" states inserted for achieving balanced voltage boosting and optimal "nearest-three-vectors" switching. On the other hand, the dual Z-source inverter with transformer isolation can be controlled using different modulation approaches due to the presence of redundant switching states within a phase leg. Particularly, using a modified phase-shifted-carrier (PSC) scheme with shoot-through states inserted, it is shown that the dual inverter can be implemented using only a single Z-source network, while still achieving the correct volt-sec average and switching loss equalization. This represents a significant reduction in cost, and can more than compensate for the slightly degraded spectral characteristics of the PSC scheme. To verify the theoretical concepts discussed, experimental testing has been performed with the captured results presented in a later section of the paper. [ABSTRACT FROM AUTHOR]

The newly proposed Z-Source inverter has been proven in the literature to exhibit both steady-state voltage buck and boost capabilities using a unique LC impedance network coupled between the power source and converter circuit. This paper now presents transient modeling and analysis of a voltage-type Z-source inverter. These aspects are found to be challenging and they need to be carefully investigated before attempting to design advanced control algorithms for controlling the Z-source inverter. Through detailed analysis, the paper identifies several phenomena on the dc and ac-sides of the inverter, which would result in the inverter having a non-minimum-phase transient response. The dc-side phenomenon is associated with the Z-source impedance network, which is shown through small-signal and signal-flow-graph analyses to be having a right-half-plane zero in its control-to-output transfer function. Also, the ac-side phenomenon is shown through space vector analysis to depend on the time intervals of inverter states used for reconstructing the desired inverter output voltage. Based on the ac vectorial analysis, a method for improving the inverter transient response is also presented. Last, simulation results obtained using a switching-functional model and experimental results obtained using a laboratory prototype are presented for validating the described theoretical concepts. [ABSTRACT FROM AUTHOR]

Multioop control strategies have commonly been used to control power inverters of both the voltage-source and current-source topologies for power conversion applications including uninterruptible power supplies and utility interfaces for distributed power generation. However, these control strategies tend to be developed and comparatively evaluated for a particular application, with strategies for other applications presented as independent new developments. This paper presents a generalized analysis of different multiloop control approaches using alternative feedback control variables for several popularly adopted system configurations, highlighting similarities and identifying a generalized optimal control-variable selection criterion that is applicable across most multiloop-controlled inverter systems. The generality of the presented optimal variable selection criterion has been verified through the close similarities between the time-domain waveforms of the different inverter systems simulated in MATLAB Simulink and implemented experimentally in the lab- oratory. [ABSTRACT FROM AUTHOR]

Cascaded multilevel inverters can be implemented through the series connection of single-phase modular power bridges. This paper presents details on how these bridges should be implemented and operated to synchronize their pulse-width-modulation (PWM) carriers, fundamental references and sampling instances to implement a network-controlled cascaded inverter with distributed PWM computation and overall optimal system performance. The paper begins by detailing the development and control of an integrated power bridge, designed with its own digital signal processor and associated control circuitry. Details describing the networked control algorithm and signal protocol needed for synchronizing the multiple power bridges through a dynamically fast data communication network, are then presented to achieve optimum harmonic cancellation and reduced common-mode voltage. The practicality and performance of the presented modular implementation concepts have been confirmed through the close match between simulation and experimental results obtained using a modular cascaded five-level inverter prototype. [ABSTRACT FROM AUTHOR]

On many electrified railway systems with single-phase 25-kV industrial frequency supply, the power quality can be particularly poor when conventional thyristor based locomotives are operating, and this constrains the amount of power that can be delivered to the locomotives. This paper presents a hybrid shunt compensation system consisting of alternatively a cascaded or a reduced topology multilevel active power filter, and a low rating passive damping filter. The active power filter is controlled by a novel hysteresis current regulation strategy, and both mitigates low-order voltage harmonic distortion along the feeder and provides root mean squared voltage support. The passive filter damps harmonic resonances that are typical in such 25-kV traction systems. The results show that the filter system can significantly increase traction system power transfer capacity with only a relatively small capital investment, allowing older thyristor based locomotives and increased traffic levels to be supported without necessarily requiring a complete system upgrade. Simulation and experimental results are included showing the performance of the filter for both steady state and transient conditions. [ABSTRACT FROM AUTHOR]

Voltage boosting three-level inverters may present a monotonically increasing or decreasing voltage gain when enlarging the pulsewidth modulation (PWM) ratio. To produce a flexible voltage gain, this article proposes a novel three-phase three-level PWM inverter by cascading a traditional two-level three-leg (B6) inverter with half-bridges. The proposed inverter is supplied by an inductive dc-link consisting of a dc source in series with an inductor that is charged by shooting through the half-bridges in the B6 structure. The voltages across the cascaded half-bridges are subsequently boosted, and the output voltage is stepped up due to the cascaded connection. A moderate voltage gain is achieved with proper selections of two modulation variables while maximizing the PWM ratio. In particular, when the two modulation variables are equal, a constant voltage boosting gain of two is maintained irrespective of the PWM ratio. Additionally, low voltages are induced on the components, which facilitates the use of low-voltage devices. The operating principle of the proposed inverter with a modified third-harmonic-injection PWM scheme is presented, followed with theoretical analysis, comparison, and design considerations. Experimental tests on a 1.5-kVA inverter prototype verify its feasibility. [ABSTRACT FROM AUTHOR]

Switched-capacitor-based multilevel inverters for boost-type dc–ac power conversions usually exhibit a trade-off between the switch count and switch-voltage rating, i.e., a reduction of one necessitating an increase of the other. Such a dilemma is well addressed in this article by proposing a novel single-phase nine-level inverter based on a switched-capacitor network with a single switch. The proposed inverter then reduces the number of switches while generating a boosted dc-link voltage. A unique six-switch full-bridge cooperating with a low-frequency half-bridge further steps-up the output voltage with a quadruple gain. The voltage stresses on the power devices are, however, maintained low even under the boosted high output voltage, as all the switches/diodes can be clamped to any of the low-voltage capacitors. Consequently, low-voltage power devices can be utilized, reducing the overall power loss. Detailed theoretical analysis, calculations, and design considerations of the proposed inverter are provided. Comparisons with the prior-art inverters illustrate its advantages. Simulations and experimental tests on a 1-kVA inverter prototype verify the above-claimed benefits. [ABSTRACT FROM AUTHOR]

This article presents a direct carrier-based modulation scheme to mitigate the dc-link current ripple for a current source converter (CSC). Excessive dc-link current ripple increases system losses, causes current distortion, and leads to more electromagnetic interference, so, it should be mitigated. Compared with increasing dc-link inductor or switching frequency, it is more attractive to mitigate dc-link current ripple with modulation schemes. However, existing modulation schemes suffer from either high dc-link current ripple or shrink of modulation index range. To address this issue, this article analyzes the mechanism of mitigating the dc-link current ripple and proposes a new modulation scheme for a CSC. By employing two kinds of non-nearest three vectors to synthesize the reference current, the proposed scheme successfully extends the index range from [ $\sqrt 3 /3$ , 1] to. For easier implementation, its carrier-based modulation, which can automatically select the desired vectors and last predetermined dwell times, is also presented. Theoretical analyses show that the proposed scheme can mitigate the dc-link current ripple over the full index range for a CSC. The effectiveness of the proposed modulation scheme is verified experimentally with a CSC operating as a rectifier. [ABSTRACT FROM AUTHOR]

Latest wide-bandgap power devices are switching progressively faster compared with existing silicon devices. Their accurate current measurements for either control or protection have therefore become tougher. One method that can fulfill the requirements is to use a Rogowski coil and its accompanied electronics to form a Rogowski current sensor with high bandwidth, small volume, low cost, and ease of integration. This article therefore aims to review various aspects of the Rogowski current sensor from its history, measuring principles to modern power-electronic applications. The applications have, in turn, motivated a progression from traditional helical to recent miniaturized printed circuit board implementation, in order to improve the overall power density. This progression has similarly been reviewed, together with its various design aspects applied to the winding, integrator, shielding, and parameters of the Rogowski current sensor. Future challenges and directions are then summarized, whose ultimate goal is to maximize accuracy over a wide bandwidth without being affected by radiated and near-field-coupling interferences. [ABSTRACT FROM AUTHOR]

A three-winding coupled inductor, arranged as a Y-source network, has been used for voltage boosting with a standard inverter bridge and no other active switches. However, its maximum modulation and boost duty ratios are correlated with an increase of one maximum necessitating the reduction of the other. Its voltage quality and dc-bus utilization will hence deteriorate greatly at high gain. Moreover, in the presence of nonnegligible leakage, the Y-source network experiences large voltage spikes caused by unintentionally abrupt inductive current changes. Its coupling coefficient must hence be very close to unity or a snubber and always be added. To overcome these limitations, a split-Y-source inverter has been proposed, whose maximum modulation and boost duty ratios can be increased simultaneously, while using only a single-inverter bridge. It is also not burdened by inrush currents and voltage spikes and hence does not impose strict requirements on coupling and snubber. These advantages are in addition to the increased freedom of tuning two turns ratios to keep the total number of turns small, while retaining a high-voltage boost. Theoretical analyses, simulations, and experimental results have successfully verified the performance of the proposed inverter. [ABSTRACT FROM AUTHOR]

In a three-phase three-level grid-connected inverter, the ac output current ripple actually fluctuates four times per switching period with each fluctuation generating some core loss. However, only the two larger fluctuations are considered in most loss estimation methods, resulting in a smaller core loss evaluation. In this letter, an alternative method has, hence, been proposed, beginning with the deduction of fundamental and ripple equivalent circuits from the inverter. These circuits permit current ripple to be calculated, from which its four fluctuations are all considered for forming the dual current-ripple envelopes (DCREs). These DCREs, together with the fundamental current and a selected core loss model, can then be solved to give the estimated fundamental and ripple core loss values. The nonlinear characteristic of the filter inductor has also been considered in the calculation of the DCREs and the core loss. The estimated values have eventually been verified through testing with a 10-kW three-phase T-type three-level grid-connected inverter. [ABSTRACT FROM AUTHOR]

A Dual active bridge (DAB) converter can achieve a wide high-efficiency range when its input and output voltages are equal, assuming a 1:1 turns ratio for its isolation transformer. If its input or output voltage is doubled, efficiency of the DAB will drop significantly, because of the introduction of the hard switching and high circulating power. Thus, a new modulation scheme has been proposed, whose main idea is to introduce a voltage offset across the dc blocking capacitor connected in series with the transformer. Operational principle of the proposed modulation has been introduced, before analyzing its soft-switching area and circulating power mathematically. The final modulation scheme is not difficult to implement, but can help the DAB achieve soft switching, low circulating power, and thereby high efficiency, even with its input or output voltage doubled. These features have been verified by experimental results obtained with a 1.2-kW prototype. [ABSTRACT FROM PUBLISHER]

Low switching frequency is always used in an electric rail traction induction motor control system, in order to reduce switching losses and increase system reliability. But under low switching frequency, the conventional single pulse width modulation (PWM) technique for the whole speed region will produce a large number of harmonic components, which in turn cause motor heating, torque ripple, and other adverse effects. In this paper, a hybrid PWM technique formed by merging carrier-based asynchronous and synchronous modulation and optimal synchronous modulation is proposed to achieve low losses and total harmonics based on very low switching frequency. A simple low-cost and low-power digital signal processor (DSP) and field programmable gate array (FPGA) hybrid hardware structure is then adopted for implementing the technique, which fully utilizes advantages and resources of each processor. Total harmonic distortion of the system can hence be reduced by operating with the high-precision hybrid PWM, while not compromising the multirate and multitask processing requirements of traction control. Detailed implementations in both DSP and FPGA with high accuracy, low usage of resources, and computational burden are then described, focusing particularly at the smooth transitions among different PWM schemes. Simulations and experiments are eventually performed with results captured for validating the presented hybrid PWM technique. [ABSTRACT FROM PUBLISHER]

A main design constraint of the digitally controlled power electronics converters is the time delay of control systems, which may lead to the reduced control loop bandwidth and even unstable dynamics. Numerous time-delay compensation methods have been developed, of which the model-free schemes are independent to model accuracy whereas the model-based alternatives are sensitive to system modeling. This paper first presents a graphical illustration of four model-free delay compensation techniques, where their principles and performances are intuitively elaborated and compared by means of the impulse area equivalence method. An improved time-delay compensation approach is then proposed based on additional area insertion, which provides a wider frequency of phase compensation. Finally, the dynamic response of an LCL-filtered grid-connected inverter was tested with the studied delay compensation methods. Simulations and experimental test results validate the effectiveness of the graphical comparisons and the proposed approach. [ABSTRACT FROM PUBLISHER]

Switching harmonic attenuation has always been challenging for inverters used in high-power conversion applications, where ratio of switching to fundamental frequency is low. Addition of multiple LC-trap filters is no doubt a feasible cost-effective method, which has increasingly been used, but generally susceptible to filter parameter variations and harmonic resonances. This paper hence presents an alternative active trap filter (ATF), based on a series-LC-filtered inverter, for attenuating switching harmonics in a flexible, while yet not cost burdensome, approach. A direct impedance synthesis method has also been proposed for the ATF to better enforce its active switching harmonic bypassing ability. Compared with conventional schemes for controlling active power filters, the proposed method is more readily implemented, since it requires neither current reference generation nor high-bandwidth current control loop. Moreover, the use of a series LC filter at its ac side helps the ATF to reduce its inverter voltage and power ratings. Compensated frequency range of the ATF can hence be enlarged by using a comparably higher switching frequency and a proper step-by-step design procedure to be presented in this paper. Simulation and experimental results have confirmed the design procedures, and hence expected performance of the ATF. [ABSTRACT FROM PUBLISHER]

This paper addresses the harmonic stability resulting from the current-control interactions of the multiparalleled, LCL-filtered voltage-source converters. First, an alternative impedance model is proposed for the single-loop current control. The control output admittance of the converter is decomposed into a passive filter output admittance in series with an active admittance, which is dependent on the current controller and the time delay. The frequency-domain passivity theory is then applied to the active admittance for system stability analysis. It reveals that the stability region of the single-loop grid current control is not only dependent on the time delay, but affected also by the resonance frequency of the converter-side filter inductor and filter capacitor. Further on, the damping injection based on the discrete derivative controller is proposed to enhance the passivity of individual converters and thereby stabilizing the paralleled converters. Finally, simulation studies and laboratory tests validate the effectiveness of theoretical analysis and controller design. [ABSTRACT FROM AUTHOR]

Accurate flux estimation is essential for the implementation of a high-performance ac motor drive. However, it still faces some problems, which can better be projected by analyzing performances of existing flux estimators, implemented with either a pure integrator or a low-pass filter (LPF). To solve the problems, an alternative flux estimator, implemented with a single second-order generalized integrator (SOGI) and a frequency-locked loop (FLL), is discussed for induction motor drives. The SOGI block included in this algorithm works for integrating the back-electromotive force, which unlike the pure integrator and LPF, does not experience saturation and significant dc offsets caused by different initial conditions. The single-SOGI-FLL estimator does not need additional magnitude and phase compensation, while its performance may deteriorate at low speed, caused by the inverse proportional relationship between its estimated flux and the frequency. A multi-SOGI-FLL flux estimator is, thus, proposed for uncompromised attenuation of dc and harmonic errors even under low-speed condition. Excellent flux estimation can, hence, be offered over the full-speed range, as proven through theoretical studies and experiments. [ABSTRACT FROM AUTHOR]

This paper proposes a second-order-generalized-integrator (SOGI)-based time-delay compensation method for extending the stable region of a dual-loop grid-current-feedback (GCF) control system. According to the analysis, stable region of the dual-loop system should be designed below a certain critical frequency, before time-delay compensation method can be applied. To always meet the requirement, relationship between single-loop converter-current-feedback and dual-loop GCF control is clarified, before a robust inner-loop gain for the dual-loop GCF scheme is determined. Enforcing this gain allows the converter to remain in its stable region, regardless of how its LCL -filter parameters and grid impedance vary. The SOGI-based delay compensation method can then be applied for widening the stable region of the dual-loop GCF scheme, as proven through s-domain Bode diagrams and z -domain root loci. These theoretical proofs are eventually validated by experimental results obtained in the laboratory. [ABSTRACT FROM AUTHOR]

This letter investigates the resonance characteristics and stability problem caused by the interactions of multiparallel LCL-filtered inverters. Compared to single grid-connected inverter, the multiinverter system presents a more challenging resonance issue, where the inverter interactions may excite multiple resonances at various frequencies. This letter proposes a modeling and analysis method based on the current separation scheme. It reveals that an interactive resonant current that circulates between the paralleled three-phase inverters may arise, depending on the current distribution and the system resonance characteristics. This is different from the common resonant current injected into the grid and has yet been discussed before. The analysis with the root loci in the discrete z-domain are carried out with two stability constraints identified, which correspond to the conditions of triggering the interactive resonant current and common resonant current, respectively. Simulations and experimental results verify the theoretical analysis. [ABSTRACT FROM PUBLISHER]