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This article investigates behavior of flying capacitor multilevel (FCML) converters in single-phase buck-type power factor correction (PFC) applications. Recent developments in FCML converters using gallium nitride (GaN) transistors are leveraged to improve power density of a single-phase 240 \text{V}_{\text{RMS}} ac to direct 48 V dc conversion stage in data center power delivery applications. Here, we experimentally demonstrate this concept in a digitally controlled six-level FCML converter hardware prototype. The experimental prototype can deliver 4.5 A average output current at 48 V, resulting in 216 W output power. A key contribution of this article is experimental demonstration of an FCML buck converter in a single-phase PFC application where the flying capacitor voltages follow fractions of the rectified input voltage by swinging at twice-line frequency.

This article analyzes the power transfer limit (Pmax) of wind power plants (WPPs) under the weak grid condition. It is pointed out that the impact of different grid parameters and grid configurations on Pmax cannot be fully reflected by the short-circuit ratio (SCR), but can be readily captured by voltage sensitivity (∂Q/∂V). Hence, ∂Q/∂V turns out to be a more appropriate metric for the assessment of Pmax compared with the SCR. Based on this insight, the voltage control of the WPP is suggested to be implemented at the bus with the highest ∂Q/∂V to maximize the power transfer capability. Moreover, a dynamic voltage control is further added at the turbine level to improve the disturbance rejection capability of the WPP as well as its dynamic robustness against grid strength variations. Finally, simulation and field test results are given to demonstrate the theoretical analysis and the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

In this article, a new inverter unit is proposed. It is a compact switched-capacitor-based inverter, which is combined of four switches, two diodes, and four capacitors. Since this inverter unit is a mid-point-clamped inverter, it can mitigate the capacitive leakage current in many applications. One of the main applications of the proposed inverter is in grid-tied photovoltaic systems, where the common-mode current brings many challenges. Other than the mentioned feature, the proposed inverter includes the following advantages: 1) spontaneous balancing of the voltage across the capacitors without needing any control system or auxiliary circuits; 2) capability of full reactive power transfer; 3) low voltage stress on the capacitors compared to common three-level inverter topologies (NPC, T-type-NPC, FC, etc.); and 4) symmetric switching sequence, which causes a continuous input current. The feasibility and versatility of the proposed inverter unit is validated by experimental tests, which are conducted by using a lab-scale prototype. [ABSTRACT FROM AUTHOR]

Accurate synchronization and harmonics control are keys to achieve a distortionless zero-cross of currents in active unity power factor rectifiers. This article proposes a method to achieve optimal operation: 1) by settling a synchronization path that feeds the phase-locked loop with the output voltage reference that sets the theoretical unity power factor operation, and 2) low-order harmonics control with zero-cross driven antiwindup to reduce distortion further. Controller design is detailed and supported by theoretical analysis; the key performances are proved by experimental verification. [ABSTRACT FROM AUTHOR]

The lack of uniform production of panels in photovoltaic (PV) systems and snowball effects in fuel cell (FC) systems led us to use a separate dc–dc converter. In addition, in the PV and FC systems, the output voltage is low. Hence, dc–dc boost converters are commonly used in a series connection. Each small unit of a PV or FC operates independently. Therefore, another problem arises in the system that uses the series connection of the converter's output. Owing to the unequal electricity production of the cells, a voltage imbalance occurs at the output terminals. To create voltage balance, a modular structure grounded on a three-level boost converter was investigated. The main objective of this article is to design the control of a modular topology, allowing perfect control of the dc-bus voltage, even with mismatched input powers. The proposed control strategy ensures dynamical properties independent of the operating point, voltage balance, and robust control of the dc output voltage with respect to load perturbations. To ensure the balance of voltages and the control of currents, a sliding mode controller based on indirect synthesis was used. To control the dc–bus voltage, an energy controller is proposed that ensures dynamical performance independent of the operating conditions. The performance in terms of tracking and regulation, as well as its effect on the voltage balance of the output capacitors, was validated through simulations and experimental results. A comparison with a classical proportional integral (PI) approach for controlling the dc-bus voltage was performed. [ABSTRACT FROM AUTHOR]

This article proposes a new open-circuit (OC) fault diagnosis method for power switches in three-level neutral-point-clamped (NPC) rectifiers. This method is based on fault assumption by introducing a new voltage deviation for diagnosis, which is the difference between the switching-state-based voltage and the fault-assumed-based voltage, where the fault-assumed-based voltage is the estimated pole voltage when all inner switches are assumed to be in the OC state. After average and sum operations, diagnostic variables are developed from three-phase voltage deviations without extra hardware and precise modeling. Since the fault characteristics of each switch reflected by diagnostic variables are not changed by other concurrent OC faults, multiple-switch faults can be diagnosed in the same way as single-switch faults. As a result, the proposed method can detect both single-switch and multiple-switch faults without complex diagnostic rules and procedures. Besides, adaptive thresholds are designed by considering voltage imbalance, current amplitude, and power factor. Owing to the self-adjusting characteristics of thresholds, the proposed method provides reliable diagnostic results in various operating states of NPC rectifiers and has good robustness against transient changes. Experiments are performed to confirm its effectiveness and robustness. [ABSTRACT FROM AUTHOR]

The ripple effect phenomenon wherein the capacitor ripple voltage generates the fundamental-frequency output ac voltage of a cascaded H-bridge-based static synchronous compensator (CHB-StatCom) is revealed and investigated in this work. First, an observation method of the ripple effect for the CHB-StatCom is presented. Then, the reason why CHB-StatCom has a ripple effect is explained. Subsequently, the accurate mathematical model of the CHB-StatCom considering ripple effect is reformulated. Based on the remodeled mathematical model, the cognition inaccuracy of the conventional design of a CHB-StatCom is analyzed, and a new cost-majorization-oriented (CMO) design principle is proposed to optimize the cost of CHB-StatCom. The proposed CMO design needs a lower dc-link voltage to generate the same maximum fundamental-frequency output ac voltage compared with the conventional design, thereby reducing the cost of CHB-StatCom. The simulation and experimental results validate the correctness and effectiveness of the analysis. [ABSTRACT FROM AUTHOR]

New emerging power inverter topologies are aiming at high-power density and efficiency with reliable performance. The recently proposed family of single-phase single-stage buck–boost inverters with output unfolding circuits have promising features for application in different fields. Nevertheless, the incapacity of injecting reactive power by the classical unfolding inverters has been assumed as its main drawback. Thus, our focus is on the reactive power injection capability of the single-stage buck–boost inverters with unfolding circuits. This attribute enables the converter to provide some extra ancillary services for friendly integration of the renewable energy resources and energy storage systems, providing support to the smart grid/microgrid, thus contributing to its power quality welfare. By means of a comprehensive analysis and implementation of proper closed-loop model predictive control, together with an adequate selection of the output capacitor, the single-inductor unfolding buck–boost inverter is capable of delivering reactive power to the utility grid. We used the simulation and experimental tests to validate the theoretical aspects. [ABSTRACT FROM AUTHOR]

The parallel-hybrid converter (PHC) based high-voltage dc (HVdc) plant (PHC-HVdc) mandates the injection of additional harmonic-voltage components into its chain-links (CLs) to regulate its output voltage/modulation index (MI). However, these harmonic voltages are restricted to have limited magnitude and their zero crossings must coincide with the respective output voltages of converter. Hence, the operating range of MI for the PHC-HVdc has narrow limits. This article explores the reactive power compensating capabilities of the PHC based static compensator (PHC-STATCOM), which is relieved from the fixed dc source and is free to operate at any MI without requiring any additional efforts. Two degrees of freedom inherent with PHC-STATCOM are discovered, which proves that PHC is utilized to its maximum when applied for reactive compensation. The capacitor voltages of the PHC-STATCOM belonging to different CLs divert from desired value under unbalanced grid/load conditions. By the virtue of degrees of freedom associated with PHC-STATCOM, a capacitor voltage control technique is proposed using fundamental-frequency zero-sequence voltage (FFZSV). The generalized expression of the reference FFZSV is derived, which regulates the capacitor voltages. The loci planes indicating the maximum range of load unbalance that the PHC-STATCOM can compensate with specific grid voltage conditions is obtained and plotted. The working principles and proposed capacitor control are verified using experimental studies. A detailed comparative study is presented to substantiate the advantages of the proposed topology. [ABSTRACT FROM AUTHOR]

Due to the high-performance requirements of power converters in industrial applications, sliding-mode control has drawn extensive research attention for its insensitivity against external disturbances. In order to ensure this advantage while suppressing the chattering problem caused by discontinuous switching functions, in this article, a fuzzy sliding-mode control (FSMC) strategy consisting of three control loops is proposed for three-phase three-level neutral-point-clamped active front-end rectifiers. In the power tracking loop, an improved super-twisting algorithm is proposed to enforce the tracking errors of active power and reactive power to bounded regions in finite time and guarantee the transient performance. Next, in the dc-link voltage regulation loop, the regulation error of dc-link voltage converges to a bounded region with a super-twisting extended state observer-based FSMC strategy. A simple but efficient proportional-integral controller is employed in dc-link voltage balancing loop to reduce the voltage difference between two dc-link capacitors. The proposed controller is characterized by attenuated chattering phenomenon and improved the dc-link voltage regulation performance, disturbance rejection ability, and power tracking performance. Experimental results are provided to verify the effectiveness and superiority of the proposed strategy. [ABSTRACT FROM AUTHOR]

This article presents a model reference adaptive system (MRAS) speed observer based on the power winding (PW) active power and reactive power for dual-cage rotor brushless doubly fed induction generator (DCR-BDFIG). The mathematical relationship between the PW current and control winding (CW) current is derived first. Then, to reduce costs and to improve the reliability, a PW power-MRAS-based speed observer is designed with the stability analyzed. In the proposed speed observer, the PW power is selected as the reference model since it is relatively readily available and smooth due to the clamped PW voltage under grid-connected condition. In addition, only the CW current is required to be measured for the estimated PW power of the adaptive model, being simple and easy implemented. Finally, combining the proposed speed observer and the grid-connected power control system, a complete speed sensorless control system for grid-connected DCR-BDFIG is designed and implemented to regulate the active power and reactive power independently. The detailed simulation and experiments are carried out, showing that the estimated rotor speed can track its real values with minor steady-state errors and fast transient responses. Both simulation and experiments verify the effectiveness of the proposed speed sensorless control method for DCR-BDFIG. [ABSTRACT FROM AUTHOR]

Bidirectional wireless power transfer greatly improves the reliability of power supply. However, wireless communication between the primary and pickup controllers is usually required to transmit load information, which has time delay and is easily interfered by electromagnetic fields. Existing solutions to avoid the use of communication can be divided into three categories: additional dc/dc circuit, primary side bridge control, and secondary side bridge control. Compared with the other two methods, the secondary side bridge control has low loss and good robustness. But, it usually requires phase-locked loop (PLL) to eliminate the frequency error between primary and secondary side bridges, which increases the complexity. In this article, a constant current control strategy without communication is proposed. It is based on the secondary side bridge control and the switching sequence of the secondary side bridge is periodically synchronized according to the phase of the resonant current. It works robustly without the need for additional hardware such as PLL or Multiplier. The harmonic sensitivity analysis, parameter optimization design and stability analysis are carried out to improve the control efficiency. Experimental results based on a 1 kW prototype with dual LCC resonant network are presented to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Wind turbines (WTs) are prone to lose the synchronization during severe grid faults due to the absence of equilibrium point. To cope with the issue, this letter proposes a power compensation control for doubly fed induction generator based WT to enhance the transient synchronization stability from the new perspective of active power balance. In the method, the active power of WT is controlled to compensate the power loss on the network. Compared with the existing methods, the proposed method has more widely application scenarios because it avoids using the frequency error as control signal. Experimental tests are performed to validate proposed method. [ABSTRACT FROM AUTHOR]

The optimal design of a microgrid’s primary control’s inner loops is a severe challenge in high-bandwidth (BW) applications. Mostly, nonoptimal controllers are given as a sublevel design for the current and voltage inner loops. Unlike conventional methods, such as proportional resonant, proportional-integral, and finite-set model predictive control, in this article, an optimal integrated inner controller is proposed based on a linear quadratic tracking (LQT) methodology. In the proposed method, a model-based optimal LQT controls the microgrid’s voltage and current simultaneously. By employing this method, a performance index, which is a system energy indicator is minimized, and the BW of the controller is systematically adjusted through the employed weighting matrices in the performance index. Besides, a current limiting mechanism is provided for the proposed method, which discriminates its applicability for practical purposes. Unlike the conventional current limiting algorithms, which limit the sine-waveform of the current, the proposed method limits the amplitude of the current and causes less current and voltage distortions during over-current situations. The simulation and experimental results reveal the superiority of the proposed method. [ABSTRACT FROM AUTHOR]

Hybrid modular multilevel rectifier (HMMR) is a type of unidirectional modular voltage source converter (VSC) that has been proposed for the high-voltage (HV) dc transmission applications. Compared to the modular multilevel converter (MMC), up to 50% of the submodules (SMs) and capacitors can be replaced by the simple HV line-frequency diode stacks at unity power factor (PF). The nonunity PF operation is also favored for the rectifier station to support the grid. Therefore, this article proposes two new current commutation methods for HMMR to reduce the SM number at nonunity PF. In addition, an average model is given to reveal the inherent power flow relationship of HMMR. It is found that part of the active power is delivered from ac to dc side through the diode directly, which reduces the energy storage requirement. The performance of HMMR is also evaluated and compared with MMC. Both methods can successfully decrease the 30% low voltage devices. Method 1 can reduce 32% capacitor and method 2 achieves 52% reduction. The feasibility of the proposed converter is verified by the simulation results from a 200-kV 200 MVA HVdc model. Finally, a scale-down 400-V HMMR prototype is developed to validate the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Hybrid and resonant switched-capacitor (SC) convertersenable efficient utilization of both active and passive components, and have the potential to achieve higher efficiency and higher power density than conventional SC and magnetic-based converters. One or more added inductors offer an additional degree of freedom in the design space. It is of great interest to understand the tradeoffsbetween capacitor and inductor size and volume allocation. In this article, we analyze the reactive energy processed by the passive components and use it to calculate the total passive component volume. It is shown that the total passive component volume of resonant SC (ReSC) converters can be expressed as a function of flying capacitor voltage ripple, and the optimized capacitor voltage ripple that minimizes the total volume is dependent on topology specific parameters and the relative energy density ratio between capacitors and inductors. Moreover, we also demonstrate through theoretical analysis and experimentation that ReSC converters use significantly less passive component volume than conventional SC and buck converters for the same amount of power converted. Next, to compare different ReSC topologies, a normalized passive volume parameter is proposed for simple and fair comparison. This can be used along with a normalized switch stress parameter (based on switch VA ratings) to create a framework to showcase the relative performance of different topologies. This framework can be used to visualize and compare the passive and active component utilization among different topologies. Additionally, the proposed reactive power analysis is extended to hybrid converters with regulation capability. [ABSTRACT FROM AUTHOR]

Aiming at the temperature rise and thermal control issues of power devices in H-bridge converters under conventional pulsewidth modulation (PWM) methods, a novel PWM method of H-bridge converter with temperature rise balancing and minimization based on periodically alternating employment of power devices is proposed. In the proposed method, two novel operation mechanisms of power devices are alternately applied into the eight regions with the identical and opposite polarities of reference voltage and input current, which are composed of two fundamental frequency periods. All active switches and antiparallel diodes in H-bridge converters are made to work alternately in these eight regions, each of them is designed to keep in on-state in one region, operate in switching state in another two regions, and inactive in the left other five regions. This way, the power dissipation and temperature rise of all the devices are balanced and minimized, and H-bridge converters no longer suffer from the dead-time effect on current quality and dc-bus voltage utilization. The implementation strategy of the proposed modulation method is discussed in detail. Simulation and experimental results are presented that verify the effectiveness of the proposed method and the corresponding implementation strategy at different operating points. [ABSTRACT FROM AUTHOR]

A charging strategy that transitions from constant current (CC) mode to constant voltage (CV) mode is typical for lithium batteries. This presents design challenges for inductive power transfer (IPT) chargers to achieve high efficiency. In this article, a multiband frequency tracking control is proposed to match CC and CV modes and to mitigate misalignment. As in prior work, two load-independent frequencies fCC and fCV are used for respective CC and CV outputs. Unity power factor can be achieved when fully aligned. For misalignment cases, two narrow frequency bands around fCC are selected for control to deliver target output currents and limit reactive power. The duty ratio of the inverter is controlled to set target output voltages. A single-stage IPT system with a series LCC topology is analyzed. The proposed control has advantages of narrow operating frequency ranges, high misalignment tolerance, direct design procedures, and high efficiency in both CC and CV modes. The method is verified on a 4.4 kW prototype with a 150 mm air gap. Experimental results show that the target CC and CV charging profile can be achieved with up to 120 mm of lateral misalignment. When fully aligned, the maximum system conversion efficiency reaches 97.5%. It stays above 95.5% and 93.3% over the entire CC and CV mode ranges. [ABSTRACT FROM AUTHOR]

This article presents a control strategy for the single-stage dual active bridge (DAB) ac-dc converters. To reduce the conduction loss and achieve zero voltage switching (ZVS), the global optimal working modes are reviewed. Based on the global optimal working modes, the input current of the DAB ac-dc converter is analyzed for the sinusoidal input voltage. The control algorithm of the DAB ac-dc converter is proposed to achieve the unit power factor with the minimum current stress and ZVS. The range of the working modes is analyzed to meet the wide load power. The working mode is varied with the line-frequency angle and load power, which guarantees that the DAB ac-dc converter works in the optimal working modes. The conduction loss in the converter with the variation of the series inductance and the turns ratio of the transformer is analyzed. The series inductance and the turns ratio of the transformer are designed to further improve the conduction loss. The proposed control strategy is applied to a 1 kW DAB ac-dc converter with 220 V/50 Hz input voltage and 200 V output voltage. The experimental results demonstrate the working mode, the soft switching performance, fast dynamic response, high efficiency, and high power quality. [ABSTRACT FROM AUTHOR]

A field weakening controller is presented that extends the speed range of a dual inverter drive using a floating capacitor bridge. The controller utilizes the stator current reference frame to decouple the motor demand into active and reactive components. This is used so that the main bridge supplies the real power and the floating bridge supplies the reactive power until the floating bridge reaches its maximum voltage. The main bridge is then used to supply some of the reactive demand, which in turn adds additional voltage boost that is used to extend the drive’s constant power region, improve the motor speed acceleration performance, output power, and torque. It is found that the presented controller can increase the drive speed extension ratio to 9.2 times the base speed compared to 5 when always operating the main bridge at unity power factor. The motor maximum fundamental voltage is 1.82 p.u when compared to that of a single inverter drive. This controller performance is demonstrated experimentally using both transient and steady-state analysis. The controller is also compared with other two field weakening controllers using the same regulator tunings and experimental setup. [ABSTRACT FROM AUTHOR]

This article proposes a grid-forming based decoupling approach, which effectively mitigates the dynamic interactions between active and reactive power, especially suitable for system stabilization in ultrastrong grids with short-circuit ratio over 30. The physical coupling behaviors are first revealed by analytical studies, where two potential resonances, respectively, located in 40–50 Hz and 0–10 Hz are discovered with mathematical proofs. Based on the results, the decoupling controller is then designed with transfer function approximation for practical usage. The control method not only suppresses the couplings but also cancels two pairs of conjugate poles through decoupling paths. Thanks to the cancellation, the stability margin is remarkably increased in stiff grids, and therefore, a significant bandwidth boost on the power synchronization control can be realized if desired. The experimental results are finally performed to verify the proposed decoupling method. [ABSTRACT FROM AUTHOR]

This article presents a generalized simplified virtual vector pulsewidth modulation (SVVPWM) to attain the capacitor voltage balancing of multilevel diode-clamped converter (DCC) for all operational conditions, which is not possible by using conventional space vector pulsewidth modulation (PWM) and carrier-based PWM methods. Virtual vector PWM is a solution presented in the literature to sort out this issue but it becomes more complex for higher level converters. The proposed SVVPWM method is simple and easy to implement for any level converter because it converts the space vector (SV) diagram of any level converter to three-level SV diagram. The SVVPWM assures the natural voltage balancing under steady states and ideal operations. Under dynamic operations, a generalized equation that describes the influence of neutral point currents on dc-link capacitor's voltage has been derived and a general decoupled voltage balancing control for n-level DCC is presented. The proposed modulation method is implemented on five-level and seven-level DCCs to authenticate the effectiveness of decoupled voltage balancing control. Simulation and experimental results demonstrate that the proposed modulation strategy is easy to implement and effectively controls the balance of the capacitor voltages for the whole range of modulation index and load power factor. [ABSTRACT FROM AUTHOR]

Three-phase grids imbalance causes serious alternating input current distortion in three-phase pulsewidth modulation (PWM) rectifiers where the conventional control scheme is employed. In this article, a generalized Clarke transformation is proposed to convert three-phase unbalanced sinusoidal quantities to two orthogonal sinusoidal quantities with equal amplitudes. Then, these two orthogonal sinusoidal quantities could be converted to two direct quantities by the Park transformation. Furthermore, based on the generalized Clarke transformation, an enhanced dual-loop control (EDLC) scheme is proposed to control the three-phase PWM rectifier in the synchronous rotating frame. Finally, a 5-kW Vienna-type rectifier prototype was built to verify the proposed generalized Clarke transformation and the proposed EDLC control scheme. Experimental results show that, by using the proposed EDLC scheme, the low current total harmonic distortion is achieved even both the amplitude and phase of the utility grid voltages are unbalanced. [ABSTRACT FROM AUTHOR]

This article presents a novel modulated model-predictive control (MMPC) scheme for modular multilevel cascaded converter-based static synchronous compensator (STATCOMs) (MMCC-STATCOMs) to compensate unbalanced load current and regulate reactive power flow. By adding a common-mode voltage (CMV) to the phase-voltages of the star-connected MMCC current model, the method allows natural injection of a nonsinusoidal voltage to the neutral point of the converter, hence achieving interphase cluster voltage balance. Moreover, the imposed CMV is shown to extend the operating ranges of MMCC-STATCOMs when used for negative sequence current compensation. The proposed MMPC method incorporates a modified branch and bound algorithm to optimize the per-phase switch duty ratios. It is shown to be computationally more efficient compared to model-predictive control schemes using the optimal voltage level method combined with voltage sorting schemes. Experimental results with different weighting factors confirm the effectiveness of this control scheme and compared favorably with the conventional scheme of injecting only a sinusoidal zero-sequence voltage. [ABSTRACT FROM AUTHOR]

This article proposes a novel generalized predictive direct power control (PDPC) with a constant switching frequency for a multilevel four-leg (4L) grid connected converter (GCC) diode clamped converter (DCC). A new predictive power model for 4L-GCC is developed in which the proposed PDPC can operate with a constant switching frequency without any modulation stage. During each switching period, four appropriate switching vectors are selected and applied using precalculated switching times. Moreover, these appropriate switching vectors are selected only from one sector, and a new proposed approach to calculate the switching times is proposed and generalized for any level of 4L-DCC. The proposed PDPC for the multilevel 4L-GCC can ensure better active and reactive powers control, lower grid current harmonics, better compensation of grid neutral current and reactive power, and accurate balancing of the dc capacitor voltages. The presented results demonstrate the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

The modular multilevel railway power conditioner (MM-RPC) can effectively improve the power quality of the railway power grid. However, since the operating conditions of MM-RPC frequently vary with the traction load, the junction temperatures of semiconductor devices fluctuate very drastically. Severe junction temperature swings can easily affect the service lifetime and reliability of the MM-RPC. In this article, a power hysteresis based on incomplete compensation control strategy is proposed. The proposed control can effectively reduce junction temperature fluctuations under complex mission profiles and increase the overall life of the system while meeting the power quality requirements. Furthermore, the effectiveness of the proposed control is verified in MATLAB/PLECS electrothermal cosimulation under a section of the load curve of the Beijing–Shanghai high-speed railway traction substation. Finally, the experimental thermal platform is built to validate the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

The aim of this article is to analyze the parameter stability and power transfer limits of active distribution networks with power converters and load tap changers by using bifurcation theory.First, the nonlinear model of a representative distribution grid with mixed loads and power converters is derived. Frequently neglected effects of converter capacity limits and transformers with load tap changers are considered, and for this purpose, static and dynamic saturations are modeled by using approximations based on smooth functions. In addition, a solution based on an antiwindup scheme is provided to model load tap changers without numerical convergence problems. These methods allow their integration to the model in a comprehensive way. Results show the impact of converter parameters and its limits on system stability as well as the impact of weak grid conditions. Moreover, they show that these effects can destabilize the system under, seemingly, safe operating conditions. The results indicate that reactive power support from converters can improve stability margins and the performed analysis can help in the design of the converter control parameters. Experimental results obtained from the laboratory environment comprising two distributed generators and a grid emulator are used for the validation of the contributions of this article. [ABSTRACT FROM AUTHOR]

This letter proposes a novel step-up/down series-connected partial power converter (S-PPC) for string photovoltaic (PV) applications. The proposed bidirectional two-switch forward (BFwd) dc–dc S-PPC performs partial power processing, reducing component stresses and losses. The PV active power is unidirectional, but the BFwd alone operates with bidirectional active power flow; with bidirectional current at the input and bipolar voltage at the output, characterizing two distinct modes of operation. To validate the analysis, a 950-W system prototype of the novel BFwd S-PPC was implemented experimentally for a realistic PV application example, where the transformer turns ratio was designed to decrease the nonactive power processed, resulting in a high power factor ($ >\text{0.75}$). The prototype reached a rated/peak system efficiency of 98.68%/99.48%. Compared with previously published step-up/down S-PPCs, the BFwd S-PPC employs fewer active switches (and gate drivers), which reduces its complexity and cost; but moreover, increases its performance. In fact, the proposed BFwd S-PPC presents higher efficiencies than most step-up/down S-PPCs in the literature. [ABSTRACT FROM AUTHOR]

Electric spring (ES), which has been widely discussed in recent years, is a new demand response method. By adjusting the power of noncritical load (NCL), ES can adaptively improve power quality of the grid containing a high proportion of renewable energy sources. Most of the existing ESs is connected in series with NCLs, but this structure limits the compensation power range of ES. In this article, a new ES topology based on LCL filter is proposed, where NCL is paralleled with the capacitor branch as the shunt damper. Compared with other types of ES, the proposed LCL-ES has the advantages which can extend the adjustable power range, improve the voltage regulation performance, and suppress the inherent resonance spikes effectively. Finally, the feasibility of LCL-ES has been verified by simulation and experimental results. [ABSTRACT FROM AUTHOR]

This article proposes an improved low-complexity model predictive direct power control (LC-MPDPC) along with reduced power ripples. Different from those MPDPC methods under balanced grid voltage conditions, both the design and implication of MPDPC under unbalanced grid conditions are further studied. In the proposed MPDPC, to reduce the computational burden of the controller, an improved LC-MPDPC based on the extended reactive pq theory is proposed. Only one prediction is needed to select the next voltage vector. Besides, the defined cost function under balanced grid conditions cannot obtain the minimized value if the unbalanced grid conditions occur. Accordingly, a new power error is defined, which is described analytically in the optimal model. With the proposed LC-MPDPC, a negative conjugate of the new defined complex power is selected as the control variable, and an improved modulation for the LC-MPDPC is presented to ensure the best voltage and calculate the optimal duration under unbalanced grid conditions. Compared with prior MPDPC, the proposed LC-MPDPC obtains much lower power ripples and grid current total harmonic distortion, and further, it offers a fast dynamic response and robustness. The effectiveness of the proposed LC-MPDPC is evaluated and validated by the experimental results. [ABSTRACT FROM AUTHOR]

Thermal stress because of mean junction temperature ($T_{{{jm}}}$) and junction temperature swing ($\Delta T_{{j}}$) reduces the lifetime of the power electronic (PE) converters. It is beneficial to distribute the stress evenly among the converters operating in parallel in a system like microgrid to improve their lifetime. The existing techniques usually only distribute stress due to $T_{{{jm}}}$. A few techniques are based on the accumulated damage in PE converters do not necessarily mitigate the effect of $\Delta T_{{j}}$. Moreover, most of the the existing techniques are not applicable for nondispatchable energy source like PV. The intermittent nature of active power generation profile of the PV inverter further reduces the lifetime of the PE converters operating in an islanded microgrid. To address this issue, a temperature droop-based dynamic reactive power sharing (TDDRPS) technique is proposed for an islanded ac microgrid to distribute the thermal stress due to both $T_{{{jm}}}$ and $\Delta T_{{j}}$. The stability of the proposed TDDRPS technique is mathematically analyzed. Finally, the proposed technique is validated through simulation and experimentation on a scaled down laboratory prototype of microgrid. [ABSTRACT FROM AUTHOR]

Transmission system operators (TSOs) are increasingly requesting frequency support, reactive power compensation, and grid-forming capability to power plants interfaced by power converters. In these plants, a large number of converters is operated in parallel and all of them should be adequately coordinated to provide these services. For that purpose, the virtual synchronous machine (VSM) control technique is commonly adopted. However, the dynamic characteristics of the whole plant depend on the number of converters connected, their size, control parameters, and electrical interconnections. This makes it difficult to know the aggregated dynamic characteristic of the plant and how each VSM contributes to the plant response. In this article, the operation of VSMs in parallel configuration for island and grid-connected applications is studied. The developments make it possible to adapt the VSMs parameters so that the grid sees the power plant as an equivalent generator whose low-frequency characteristics do not vary with the number of converters in operation. The developments were validated in a laboratory consisting of four 15 kVA converters operating as VSMs, and a grid emulator. [ABSTRACT FROM AUTHOR]

This article proposes a novel power flow tracing method based on power electronic signaling for peer-to-peer (P2P) electricity trading in dc microgrids. It employs a superimposed low-frequency sinusoidal carrier to trace dc power flows according to the bus port impedance characteristics of power converters. In order to support fair and accurate P2P trading, source-to-load power signaling (S2LPS) and source-to-source power verification (S2SPV) methods are presented. Through S2LPS, the P2P dc power flow from a specific distributed source (DS) to a power load (PL) is determined by detecting the carrier's active power at the PLs bus port. The same carrier is used to check the actual output power of the DS by S2SPV. By implementing S2LPS and S2SPV, the power flows of the system are traced, recorded, and verified by each DS and PL. Accurate power flow tracing is achieved on the physical layer, providing reliable data for P2P electricity trading. The principles of the proposed method are deduced in detail. Furthermore, the modifications to the control loops of DSs and PLs are depicted for implementations in dc microgrids. Finally, a 2.5 kW experimental platform is built to validate the correctness and feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

This article presents a finite-set model predictive control (FS-MPC) applied to the shunt active power filters (SAPF) based on three-phase inverters connected in parallel sharing the same dc-link. The discrete-time model of the system is used to predict the future value of the grid, circulation, and offset currents. The presence of circulation and offset currents occurs due to the connection of the two inverters in parallel sharing the same dc-link. There are 64 switching state vectors for SAPF and, in order to reduce the burden of calculation, only 30 switching states are chosen and applied, however keeping the advantages of the FS-MPC algorithm. The control strategy ensures the sinusoidal shape of the grid current, high power factor, and circulating and offset currents suppression. The FS-MPC has its performance compared to the pulsewidth modulation (PWM) strategy considering the SAPF with two parallel inverters, the conventional SAPF and the neutral-point-clamped SAPF. These comparisons include harmonic distortion, power semiconductor losses, and analysis of dc-link capacitor losses. As a multilevel topology, the SAPF with two parallel inverters using FS-MPC present competitive efficiency and can be applied with a good performance in industrial and residential applications. Simulation results and a laboratory-scale experimental platform is used for corroborating the proposal. [ABSTRACT FROM AUTHOR]

In this article, a novel cascade model-predictive control for a three-phase unity power factor rectifier is presented. Unlike the traditional control techniques for rectifiers, the proposed method uses a cascade dual-model-predictive control for the inner and outer control loops. The three-phase currents of the rectifier are controlled with the inner control loop, while the desired output voltage is achieved with the outer control loop. In this proposal, the output power is used as a feedforward to enhance the transient dynamics of the output dc voltage. Several advantages can be highlighted such as a fixed switching frequency, and output voltage variations are avoided when a sudden change in the load or a voltage sag appears. The overall control proposal has been fully integrated into a digital signal processor. Selected experimental results are introduced to validate the theoretical contributions of this article. [ABSTRACT FROM AUTHOR]

A novel carrier-based discontinuous pulsewidth modulation (CB_DPWM) strategy for T-type three-level converter is proposed in this article. Because one significant advantage of the proposed CB_DPWM is the reduction of common mode voltage (CMV), it is named as RCMV_CB_DPWM hereafter. It also can reduce switching loss and remove dc offset from the neutral point voltage (NPV) simultaneously. RCMV_CB_DPWM has two design freedoms, one is modulation wave and the other is carrier wave. By injecting zero sequence voltage properly, the corresponding clamping mode can be acquired. Moreover, reduced CMV can be achieved by arranging carrier wave flexibly. The integrity of RCMV_CB_DPWM is analyzed as well as its NPV self-equilibrium ability, and an active NPV control method is then introduced. Moreover, its realization is discussed briefly. Finally, the experiments are carried out to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Grid-forming converters, taking droop-controlled converters and virtual synchronous generators (VSGs) as typical examples, have shown great promise as the interfaces between renewable energy sources and the future power-electronics-based power system. However, the coupling between active and reactive power will adversely affect the dynamic performance and stability of grid-forming converters. In order to facilitate quantitatively analyzing and assessing the power coupling characteristics of various types of grid-forming strategies, this article develops a unified dynamic power coupling model based on a first amplification coefficient array and a relative gain array in the frequency domain. Then, by using the established unified model, a comprehensive analysis of dynamic power coupling properties for typical VSG control and droop control is presented, which reveals that droop control is helpful in weakening power coupling while VSG control is prone to amplify the coupling. Moreover, the influences of the pivotal control parameters in droop and VSG controllers on power coupling characteristics are discussed in detail, which can provide novel supplementary guidelines for parameter design of droop and VSG controllers. Finally, the experimental results verify the correctness of theoretical modeling and analysis. [ABSTRACT FROM AUTHOR]

Soft-switching technique contributes to higher conversion efficiency and switching frequency for transformerless grid-connected inverters (TLIs), which is beneficial to improve power density and reduce cost. However, existing soft-switching TLIs can only operate with unity power factor; as a consequence, they could not supply reactive power required by newly technical standards. In order to solve this issue, a novel zero-voltage-transition (ZVT) scheme has been proposed in this article, and two resonant tanks with a compact structure are applied in H6-type topology. The derived ZVT inverter enables high-frequency switches of both positive and negative power regions to be turned on and off under zero-voltage conditions, as well as all auxiliary switches can be turned on with zero-current switching. Meanwhile, unipolar differential-mode voltage and constant common-mode voltage characteristics are also maintained. Finally, comprehensive experiment results from a 100-kHz/1-kW prototype are provided to verify the correctness of the proposed inverter. [ABSTRACT FROM AUTHOR]

The Vienna rectifier can produce three voltage levels, however only the connection to the neutral-point is fully controllable using the bidirectional switches. When the neutral-point voltage is not imposed in a phase-leg, the polarity of the pulses generated depends on the diode that is conducting in that phase-leg, which is defined by the current direction. As a result, the voltage pulses generated can go in the opposite direction to what is demanded by the controller, and the current becomes distorted. This can be mitigated by injecting a particular zero sequence so that the reference voltage is clamped to the neutral-point during the intervals, where the current of a phase-leg has the opposite sign than its reference voltage. However, the converter is at risk of entering into overmodulation when operating with high modulation indices. In this article, a hybrid modulation strategy is introduced where a calculated minimal amount of reactive current depending on the operating conditions is injected to avoid overmodulation. The reduction of reactive current required compared to the state-of-the-art solutions is up to 100% in certain operating conditions. With the proposed method, the Vienna rectifier can operate in a wide range of power factors without compromising the quality of the source currents. The concept is verified by simulation and in a 1-kW Vienna rectifier experimental setup. [ABSTRACT FROM AUTHOR]

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage and high-power direct ac-to-ac power conversion applications. Fault tolerance ability is one of the advantages of the M3C. To further enhance the reliability of the M3C, this article proposes a novel branch current configuration method for branch fault conditions, which is available either one or two branches are failed. By deriving basic branch current configurations and analyzing branch dc power equations under branch fault conditions, feasible branch current configurations can be directly derived. In terms of minimizing the maximum peak branch current, the derived configuration is also the optimal one for the single branch fault condition. Compared with the existing method, the proposed method does not need to solve configuration coefficients of branch currents offline, which is automatically adaptive to different load conditions. An M3C prototype with three submodules each branch is built, and experimental results are presented to validate the proposed branch fault tolerance method. [ABSTRACT FROM AUTHOR]

The traditional neutral point clamped (NPC) inverter has short-circuit problem. The risk of short-circuit can be decreased by using dead-time in the switching signals. However, the dead-time decreases the achievable output voltage and causes distortion in the waveforms. To overcome the short-circuit problem, dual-buck NPC (DB-NPC) and split-inductor NPC (SI-NPC) inverters have been researched. However, the voltage stress of the two external diodes in the DB-NPC inverter is higher. On the other hand, the SI-NPC inverter has a problem of generating huge voltage spikes in the dead-time, which can destroy the semiconductor devices. In addition, the SI-NPC inverter cannot provide reactive power. This article presents a family of NPC inverters consisting of single-phase, three-phase, and cascaded inverters. The proposed inverters have no short-circuit and dead-time issues, therefore no high voltage and current spikes are caused. Also, the dead-time in the switching signals can be minimized. As a result, the magnitude of the output waveforms can be increased, and quality can be improved. Unlike the DB-NPC inverter, the voltage stress of all the semiconductor in the proposed inverter is lower, and unlike the SI-NPC inverter the proposed inverter provides reactive power. In this article, the proposed three-level NPC inverter is analyzed, designed, and tested. The voltage stress of the semiconductor devices in the proposed inverter is half of the source voltage, whereas in the conventional DB-NPC inverter the voltage stress of the two external diodes is the source voltage. In addition to the aforementioned benefits, the proposed cascaded inverter reduces the total number of inductors. To verify the analysis, detailed simulation, and experimental results of the proposed three-level inverter with input voltage 640 V, output power 1.2 kW, and output voltage 220 Vrms are provided. [ABSTRACT FROM AUTHOR]

As one of the approaches for a grid-sustaining inverter, the inverter should cover not only grid-connected (GC) mode but also stand-alone (SA) mode for power supply to local loads; therefore, there are separate control loops for each mode. In order for an uninterruptible power supply to the local load, it should be seamless to change from GC mode to SA mode. This carries two types of issues. One is a transient state caused by switching the control loops. The other is an uncontrolled state during the time interval between a grid failure and its detection, called clearing time. These can lead to unstable voltage for the local load. Hence, a smooth and autonomous mode switching method is required even if the fault detection is late. Existing mode transfer methods considering both the issues have been accompanied by a slow dynamic response, additional sensors, restriction of adopting filter, or controller complexity due to third-order plant. To overcome them, in this article, a control scheme realized by a unified control loop is proposed for smooth and autonomous mode switching with a novel antiderailing control. Furthermore, the proposed control scheme can achieve a high bandwidth for the output power control in GC mode because it is based on controlling the current flowing the inverter-side inductor, and both filter types, LC and LCL, can be adopted. To validate the proposed concept, simulations and experiments were conducted. The results show that the proposed control scheme can be used to achieve autonomous and smooth mode transition even under reactive power reference and reactive load conditions. [ABSTRACT FROM AUTHOR]

LCC resonant converters have been widely adopted to achieve high power density and high efficiency. Due to high switching frequency and large reactive power in the resonant tank, the efficiency of LCC converters at light load decreases dramatically. Burst mode, which is commonly used in dc–dc converters to improve efficiency at light load, has not been applied to LCC resonant converters due to the oscillation of resonant tank during the burst off-time of burst mode. In this article, a simplified state model, which has only two state variables that can easily describe its state trajectory in the state plane, is proposed for LCC resonant converters with a capacitive output filter. Then, optimal state trajectory control of LCC converters for burst mode is proposed based on the state model, which can eliminate the resonance during the burst off-time. In addition, the transition from start-up mode to burst mode is proposed based on state trajectory control, which can complete this transition in one switching cycle. Finally, the proposed optimal trajectory control in burst mode is verified by an LCC converter prototype. [ABSTRACT FROM AUTHOR]

Inductive power transfer (IPT) systems with multiple output voltages are potential to supply a variety of loads simultaneously. This article proposes an IPT system with multiple semiactive rectifier cells (SARCs) connected in series at the receiver (Rx) side. These SARCs are independent of each other and their output voltages can be regulated individually with single receiver coil, which reduces the cost and volume. The proposed IPT configuration has the advantages of high flexibility, simple control, and no cross regulation. Considering that the SARC may cause additional reactance and lead to detuning of the Rx side, variable inductor is introduced to ensure the full tuning condition. Thus, the proposed IPT system can maintain zero phase angle operation with unity power factor in the whole load range, minimizing the reactive power losses. In addition, both primary inverter and the SARCs can achieve zero voltage switching. To verify the feasibility and validity of the proposed IPT system, an experimental prototype with two outputs of 640 W/150 V and 400 W/120 V is built. [ABSTRACT FROM AUTHOR]

Voltage fluctuation suppression and reactive power proportional sharing in islanded microgrids (MGs) are often difficult to maintain simultaneously. Hence, a dynamical reconfigurable master–slave control architecture is proposed herein. When a load fluctuates, the leader will be dynamically selected through specific features (such as the output power, output voltage, and power capacity); subsequently, the leader sends his information to all other distributed generators (DGs) as a unified signal in the MG. Meanwhile, the other DGs adjust their local output based on the unified reference to achieve accurate power sharing in each DG. Moreover, system voltage recovery can be realized simultaneously since each DG contains closed-loop voltage control. In addition, for the automatic selection of the leader, a dynamically selected method based on a weight-voting-based differential delay method is proposed herein; the method realizes the unicity and automatic selection of the leader during communication failure. Additionally, the stability of the proposed method is proven mathematically. The results show that the proposed method is stable for any communication period and DG quantity. Furthermore, the communication delay and communication failure are derived mathematically. The results show that the proposed method demonstrates robustness against communication delay. Finally, experimental results verify the effectiveness of the proposed method under communication delays, failures, and power failures. [ABSTRACT FROM AUTHOR]

DC-link dynamic is one of the most important issues for current source converters (CSC). In practice, the dc-link voltage suffers from the slow dynamic due to grid disturbances, which will affect the quality of output waveform and damage the converter. Many solutions have been presented to cope with it, but none of them achieve the zero dynamic dc-link voltage control. In order to fill the technical gap, a novel solution is proposed in this article. First, the impact of grid disturbances on the CSC is discussed with instantaneous power theory and small-signal model analysis. Second, a new solution is presented to eliminate the adverse impacts of grid disturbances, so as to achieve the zero dynamic dc-link control concept. Meanwhile, the power quality of grid current is improved. Finally, the experiments for the proposed solution under grid disturbances are carried out with TMS320F28335 DSP + XC3400 FPGA digital control hardware platform. The experimental results verify the effectiveness of the proposed solution. [ABSTRACT FROM AUTHOR]

A typical battery charging process consists of a constant-current (CC) charging phase which is followed and completed by a constant-voltage charging phase. Moreover, replacing the CC charging by constant-power (CP) charging can eliminate thermal problems and enhance the cycle life of the battery. This work aims to maximize the system efficiency of a single-stage inductive power transfer (IPT) charger by minimizing the overall losses using a CP charging scheme. The single-stage CP IPT charger employs series-series compensation and adopts an active rectifier on the secondary side. Based on a time-domain model, the conditions of zero voltage switching (ZVS) and minimum circulating reactive power are derived. Then, the power losses in the magnetic coupler, inverter and active rectifier are analyzed and optimized under CP output condition. Combining the conditions of ZVS, minimum circulating reactive power, and minimum overall losses, we propose a novel optimal control strategy to maintain CP output and maximum efficiency throughout the charging process. In addition, the proportional integral controller is not needed. Finally, a 120-W experimental prototype is built to verify the performance of the proposed control strategy. Experimental results demonstrate high precision CP output and an efficiency of around 87.5 $\%$ for the proposed single-stage inductive power transfer battery charger. [ABSTRACT FROM AUTHOR]

Wireless power transfer (WPT) is more convenient and safer than conductive charging for power consumer electronics, biomedical devices, transportation systems, etc. Inductive power transfer is the most widely studied and commercialized WPT technique; however, capacitive power transfer (CPT) is becoming an attractive alternative, offering better misalignment tolerance and lower cost and weight. The electrical-field-resonance-based six-plate coupler system is one of the most typical configurations for high-performance CPT systems, but the associated large number of circuit parameters is always a critical issue for system design. In this article, a parameter optimization method is proposed for this topology. The ratio of the reactive power in the compensation network to the system transferred power is set as the main optimization goal. To solve the high-order optimization problem, a two-stage method is proposed to significantly reduce the optimization complexity while providing the optimized parameters of the whole system. To verify the effectiveness of this method, a 3-kW, 1-MHz CPT system with a 16-pF coupling capacitor is built. Both the simulation and experimental results show that the optimized parameters effectively improve the system efficiency, experimentally achieving 95.7% dc–dc overall efficiency under a 100-mm gap distance at the rated power. [ABSTRACT FROM AUTHOR]

In this article, a novel hybrid voltage/current control scheme with low-communication burden is proposed for series-type inverters in a decentralized manner. All the inverter units are controlled by their individual local controllers without a real-time central global-communication. Among them, a few inverters serve as the current controlled inverters (CCIs) to regulate the grid current, and the rest inverters work as the voltage controlled inverters (VCIs). Under this control architecture, five main control targets are achieved: 1) frequency self-synchronization of each VCI inverter based on local current phase-angle signals; 2) fault-tolerant capability of all inverters without one-to-all-failure; 3) high disturbance-rejection-capability to grid variations; 4) adjustable grid power factor; and 5) independent power control for inverters with unbalanced power sources. As the central high-bandwidth communication is avoided, it has the features of decreased cost, increased scalability, and reduced geographic restrictions of inverters. Compared with conventional decentralized approaches, only a few CCI units acquire the grid phase-angle signal to achieve grid synchronization by communication, and the most VCI units are local self-synchronization via the local line-current-phase signal rather than real-time grid-voltage-phase communication signal. Thus, the proposed control can be realized with a very low-communication burden, which is more cost-effective and higher reliability in terms of communication faults. The feasibility of the proposed method is verified by experimental results of eight different cases, such as source power change, large power gap, grid voltage/frequency deviation, grid harmonics, and one-fault redundancy. [ABSTRACT FROM AUTHOR]

Load commutated inverter (LCI)-based drives are quite popular in high-power, medium-voltage (MV) applications. However, the limited torque capability during starting and low-speed operation is a major drawback of these drives. This is due to the small back electromotive force (EMF) in the drive motor at low speeds, which is usually insufficient to commutate the silicon-controlled rectifiers in the LCI. A new technique to overcome this drawback for an LCI-fed active-reactive induction motor (ARIM) drive is proposed in this article. The ARIM is a dual-stator induction machine reported in literature, with one winding rated for MV and the other rated for low voltage (LV). The MV winding is fed with active power from the LCI, while the LV winding is supplied with reactive power from a lower rated LV voltage source inverter (VSI). In the proposed method, the VSI is used to apply a preselected voltage vector for a short time during each turn-off instant, such that a commutation voltage of sufficient magnitude is induced in the MV winding, providing safe commutation irrespective of the magnitude of the back EMF. Thus, the drawback of the low torque capability at starting and low speeds is overcome in the proposed method, which is also experimentally validated using a 75-kW, 3.3-kV prototype. [ABSTRACT FROM AUTHOR]