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This paper presents the design, implementation, and performance testing of integral sliding mode control (ISMC) for a permanent magnet synchronous generator based wind turbine. More specifically, ISMC is combined with a feedback linearization (FBL) technique to recover its transient performance under model uncertainty and external disturbances. In this paper, we simplified the integral sliding surface structure to make it more convenient for real-time implementation. It turns out that the resulting integral sliding surface has a PI-like structure plus an additional part that has the role of preserving the nominal transient performance of the FBL. This key feature of the proposed PI-type sliding surface (PI-type SS) cannot be achieved under the conventional PI-type SS. Simulation and experimental tests are carried out to show the effectiveness of the proposed approach. The results revealed that the proposed PI-type SS is able to remove the steady-state error and retain the nominal transient performance of the FBL despite the presence of model uncertainty. [ABSTRACT FROM AUTHOR]

An analytical steady-state model of the modular multilevel converter (MMC) can be used for component sizing and assessment of the impact of different parameters on the MMC performance. Thus, this paper proposes an enhanced steady-state model for the MMC, which enables one to calculate the amplitudes of the harmonic components of MMC arm currents, submodule capacitor voltages, and arm voltages. In addition, the proposed model allows for the calculation of the amplitude and phase angle of the modulating signal, and it also presents a criterion for resonance of the arm currents. This is achieved by means of a polynomial function of the modulation index, and through checking the stability criteria established by the proposed model. The capability of formulating the modulating signal enables a more accurate calculation of the steady-state amplitudes of the variables, as compared to the previously published models. This paper also presents an accurate method of submodule capacitor sizing, based on the proposed steady-state model. The proposed method gives the exact value of the submodule capacitance (i.e., the capacitance that results in dc voltage fluctuations within a desired range) for a prespecified submodule capacitor voltage variation and a given circulating current amplitude. The stability criterion given by the proposed steady-state model, used for sizing the arm inductance, and the submodule capacitor voltage variation given by the proposed capacitor sizing method, used for sizing the submodule capacitor, allows for the determining of the operating limits of the MMC in terms of its parameters. Extensive simulation results are presented to demonstrate the efficacy of the proposed steady-state model and the capacitor sizing method. Further, the results obtained from the proposed capacitor sizing method are compared with experimental results from the literature. [ABSTRACT FROM AUTHOR]

This paper presents a single-stage nonisolated switched capacitor ac–dc converter for power-factor correction (PFC). This converter features reduced passive components, single switch, transformerless high voltage gain, and improves the power factor, reduces total harmonic distortion, and voltage stress across the switch. The peak current mode with a dynamic resonant perturbation scheme compensates the subharmonics that exist at zero crossing of the source current is implemented for PFC. In this paper, the principle of operation, large signal modeling of the circuit, and control method for the proposed PFC converter are provided. [ABSTRACT FROM PUBLISHER]

A fast output voltage equalization control strategy applied to 10-kV/100-kW high power density photovoltaic dc–dc converter system is proposed in this paper. The converter modules in the system have two dynamic priority factor loops for fast response with an inconsistent voltage between modules and a soft start-up circuit to precharge output capacitance for solving inrush current problem in boost circuit. The proposed strategy can realize fast voltage equalization control under inconsistent leakage inductance and parasitic resistance parameters condition between modules compared to the traditional unified duty cycle control strategy. The fast equalization performance is verified by 10-kV/100-kW photovoltaic dc–dc converter system. The converter system adopts an isolated boost full-bridge topology and input parallel output series structure, which meets high conversion ratio and high-power density requirements. [ABSTRACT FROM AUTHOR]

The term Virtual Synchronous Machine (VSM) commonly refers to the emulation of synchronous machines operation by using power electronics converters. However, a wider application of this control technique is still pending on further refinement of its design aspects and digital implementation. In this paper, a systematic approach to design the VSM parameters is introduced. First, a simplified third-order small-signal model of a VSM based on the conventional model of synchronous generators is developed. Unlike the existing works published in literature, the proposed model takes into account the effects of inductive and resistive weak grids. The model is then used to design the controller parameters by appropriately placing the poles of the closed-loop system. VSM robustness is studied for weak grids dominated by either inductance or resistance, and the aspects of digital control implementation are addressed. All the control system features and improvements were tested on a 15 kW prototype of a battery-supported VSM connected to a weak grid. [ABSTRACT FROM AUTHOR]

The back-to-back (BTB) converter is one of the most popular converter topologies for the control of electrical machines, power transmission systems, and power quality applications. The conventional cascade control structure is commonly used due to its simple design procedure and reliable operation. However, the bandwidth of the dc-link voltage controller has to be limited in order to avoid instability issues and be restrictive in high-performance applications. This paper presents a differential- and common-current (power)-based state-feedback control for BTB converters. This controller features a fast control of active and reactive powers, and a stiff regulation of the dc-link voltage. A theoretical analysis of the proposed controller along with a strategy for current limiting is presented. Its robustness was tested against variations of the filter parameters, the dc capacitance, the grid inductance, and the grid voltage values. The controller was experimentally validated under nominal operation, voltage sags, and connected to a weak grid by using a 15-kVA BTB converter in a laboratory test network. [ABSTRACT FROM AUTHOR]

This paper presents a new five-level T-type nested neutral point clamped (T-NNPC) converter. The proposed five-level T-NNPC topology is very attractive for medium-voltage applications as it can work in a wide range of voltages without the need of devices in series, and fewer components compared to the other five-level topologies. A model predictive control (MPC) strategy is also developed for the proposed converter to control the output currents at different output frequencies, and control flying capacitor voltages. A discrete-time model of the converter is developed and the control objectives that are the output currents and flying capacitor voltages are defined regarding switching states. A cost function is defined to minimize the deviation of the predicted values of the control objectives from their desired values. During each sampling time, the best switching state is selected and applied to the converter. The performance of the proposed five-level T-NNPC and the developed MPC strategy is studied in the MATLAB/Simulink environment, and the feasibility of the converter is evaluated experimentally. [ABSTRACT FROM AUTHOR]

Two-stage single-phase inverter system has found many applications in distributed generators and grid-connected systems. However, the existence of double-line frequency ripple on the dc-bus voltage due to single-phase inverter's operation has given rise to some design and reliability issues. Some of these are the degradation in the quality of inverter's output voltage and shortening of dc-bus capacitor's lifetime. The problem is rooted in the controller design of the front-end dc–dc converter, which is often designed based on a simplified mathematical model of the dc–dc converter that fails to account for some complex phenomena, such as system's nonlinearities and circuit parameter variations due to temperature and aging effects. In this paper, a disturbance-observer-based control is proposed where these unmodeled effects are treated in the form of lumped disturbance and compensated for in a feed-forward manner. The major advantage of this control solution is that only minimal plant information is needed for controller design and that a decoupled composite controller can be realized, where the feedback compensator is designed to meet certain stability criteria while the disturbance-observer-based feed-forward compensator is designed to handle fast transient events. A prototype of a dual-active-bridge (DAB) dc–dc converter implemented with disturbance-observer-based control is constructed to verify its effectiveness. Experimental results show that the dynamic response of the DAB converter is significantly improved and the double-line frequency ripple on the dc-bus voltage is effectively suppressed by the proposed control solution. [ABSTRACT FROM AUTHOR]

This paper presents an approach for dealing with the control of five-level, diode-clamped rectifiers. The control of such converters is challenging, as voltage balancing among capacitors is not a trivial task. Some of the existing approaches that cope with this problem use specific modifications of one of the traditional modulation techniques, such as using redundant vectors in space vector modulation (SVM). The main feature of the proposed technique is that part of the modulation is considered in the system equations and, in this way, the voltage balance can be solved designing a specific controller for this problem. As a result, several levels are used within a switching period. Furthermore, it is shown that the proposed approach hardly affects the control of active and reactive powers and total dc-link voltage in such a way that the well-known direct power control can be applied. As a consequence, the resultant modulation stage is simpler than other techniques based on, e.g., SVM. The effectiveness and good performance of the system under the proposed control approach are validated by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Conventionally, standard proportional and integral (PI) controllers with constant PI gains are commonly used for the dc-link voltage control of single-phase grid-connected converters (GCCs). For such controllers, the selection of the PI gains will lead to a tradeoff between two control objectives: 1) the reduction of the dc-link voltage fluctuations caused by random swings of the active power drawn by the single-phase GCC; and 2) the reduction of the grid current harmonics mainly caused by the 2f oscillation of the active power in single-phase applications. To solve this tradeoff, this paper presents a systematic approach for the design of an adaptive PI controller for the dc-link voltage control of single-phase GCCs. The proposed design approach is simple and it provides a convenient method to properly determine the adaptive PI controller parameters. Representative simulation and experimental results are presented and discussed in order to show the effectiveness of the proposed dc-link voltage controller. [ABSTRACT FROM AUTHOR]

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission systems have been employed widely in recent years. However, connecting a VSC-HVDC link to a very weak grid (a high-impedance grid) is challenging. A vector-controlled VSC is incapable of injecting/absorbing its maximum theoretical active power in such grids. A simple yet effective control system for a standard vector-controlled VSC in a very weak grid condition has not been reported in the literature. This paper, benefiting from a comprehensive small-signal model, presents a detailed analysis of the VSC dynamics and shows how the assumptions made for designing VSC regulators in strong grids are no longer valid in very weak grids. The paper then proposes and compares two straightforward solutions: retuning the control parameters and using an artificial bus for converter-grid synchronization. Both methods enable the VSC to operate at the maximum theoretical active power at a very weak grid condition (i.e., at unity short-circuit ratio) by minimal modification in the widely accepted vector control method. The advantages and disadvantages of each method are discussed. The analytical results are verified by detailed nonlinear time-domain simulation results. [ABSTRACT FROM AUTHOR]

This paper proposes a new approach to optimally design LLC converters. The core of the proposed approach is an accurate algorithm that can find all the possible designs satisfying the peak-gain requirement. Designers can conveniently evaluate several design results and find the optimal one for their respective applications. Part II of this paper discusses design considerations and procedure using the proposed algorithm. Case studies reveal different design tradeoffs in different application scenarios. A 384-V nominal 300-V minimum input voltage range 12-V/50-A output LLC converter prototype with 97.3% peak efficiency is built to verify the proposed design algorithm. [ABSTRACT FROM AUTHOR]

Ensuring electrification of remote locations continues to be a major challenge for power engineers. To deal with the effect of intermittent nature of wind, this paper presents the design and implementation of a composite sliding mode controller (CSMC) for a battery energy storage (BES) supported solar photovoltaic (PV)–wind hybrid system in a remote location. This control technique comprises of a soft-switching sliding-mode observer (SS-SMO) and a nonsingular terminal sliding mode controller (NTSMC). The SS-SMO is used to observe the disturbances, whereas the NTSMC is used as a speed controller. The chattering problem caused by the conventional sliding mode controller is alleviated by replacing the conventional signum switching function with the smooth hyperbolic tangent function in disturbance observer loop. The fast and finite time convergence NTSMC based speed controller along with the SS-SMO based disturbance rejection unit, serves the benefits of CSMC. This technique exhibits robustness against model uncertainties and external disturbances. Moreover, the complexity of the system is reduced by replacing the mechanical speed and position sensors with parameter estimation. A double-stage configuration using a dc/dc boost converter is adopted for a PV system. A comparative analysis is presented between the proposed and conventional techniques. A prototype of the hybrid system is developed in the laboratory with permanent magnet synchronous generator. The CSMC-based controller with disturbance rejection ability is implemented to harvest peak wind power. A perturb and observe maximum power point tracking technique is adopted to harvest peak solar power. A voltage control technique is adopted to maintain the voltage at the point of common coupling. [ABSTRACT FROM AUTHOR]

This paper presents a novel oversampling deadbeat current control approach for permanent magnet synchronous motor drives capable of operating at a controller sampling frequency multiple of the power converter switching frequency. Model-based controllers suffer from heavy computational demand and performance degradation due to parameter uncertainties. The proposed controller concurrently with field-programmable gate array implementation permits to achieve a constant switching frequency and an optimal current ripple along with a high current-loop bandwidth and robust behavior to parameter variation. A disturbance observer has been added to the proposed controller in order to compensate for the converter voltage distortions. The proposed control strategy is tested through both simulations and experiments. [ABSTRACT FROM AUTHOR]

The five-level nested neutral-point-clamped (5L-NNPC) converter is one of the most promising topologies for medium-voltage (2.3–7.2 kV) high-power applications such as medium-voltage drives, wind energy conversion systems, and grid-connected systems. The 5L-NNPC requires a fewer number of switching devices, freewheeling and clamping diodes, and flying capacitors compared to the existing five-level multilevel converters. In the 5L-NNPC topology, each flying capacitor voltage is regulated at one-fourth of the dc-bus voltage to obtain the five-level operation. Due to the lack of redundant switching states, it is difficult to control the flying capacitor voltages by using the pulse-width-modulation-based classical control methods. This paper proposes a model-predictive current control (MPCC) approach to control the flying capacitor voltages along with the output currents of the 5L-NNPC converter. The discrete-time model of 5L-NNPC is developed to implement the MPCC scheme. The simulation and experimental studies are conducted to verify the dynamic and steady-state performance of 5L-NNPC with the MPCC scheme. The performance of the proposed MPCC approach is compared with the conventional space-vector-modulation-based voltage-balancing approach. Furthermore, the flying capacitor voltage control capability of MPCC is verified at different load power factors. [ABSTRACT FROM AUTHOR]

In this paper, two control strategies for voltage balancing in a modular serial parallel resonant converter modulator system, which is used in a high-voltage-pulsed power application, are presented and verified by simulations and measurements. The system is based on two series parallel resonant converter modules, forming an input series output parallel stack. To obtain the given output voltage pulse of 115 kV, nine of these input series output parallel stacks are connected in parallel at the input and in series at the output, forming an input parallel output series system. A robust input voltage balancing by an auxiliary circuit and an output voltage balancing by control is introduced and proven by measurements. In addition, an alternative approach, where the input and output voltage balancing is achieved purely by control, is given and verified by simulations. For designing the control of this system, a large and a small signal model is derived and the influence of component tolerances is investigated. [ABSTRACT FROM AUTHOR]

This paper presents a model-predictive sliding-mode control (MPSMC) scheme for a three-phase ac/dc converter to achieve better stability and dynamic performances. In the conventional model-predictive control method, a proportional–integral (PI) controller is used to generate the active power reference. This traditional model-predictive PI control (MPPIC) scheme, however, produces a large overshoot/undershoot, a long settling time, and a large steady-state error under disturbances. To overcome these deficiencies, a sliding-mode controller is employed to replace the PI controller. Since the control law and the controller are designed based on the system model, the proposed MPSMC scheme can reduce the effects of unexpected disturbances, such as the output voltage demand and the resistance load variations. Both methods have been simulated in MATLAB/Simulink during various disturbances. Compared with the performances of MPPIC, the results obtained from MPSMC show that the settling time of the dc voltage can be minimized by about 91%, and the overshoot can be eliminated from 9.13% during the steady-state progress. The active and reactive power from MPSMC can also be controlled to the desired values, respectively, with a much smaller overshoot/undershoot and a faster response speed. Similar dynamic improvements can be achieved with MPSMC when the dc voltage demand varies. The simulation results are validated by experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a new predictive power-factor-correction (PFC) controller for series-interleaved three-level boost (TLB) converters. Compared to the state-of-the-art TLB PFC controllers, where a two-cycle prediction and a detection of an operating region are necessary, the proposed controller achieves a low total harmonic distortion of the input current by using a single equation to predict the input current in all operating regions of the converter, in just one operating cycle. The average current control is achieved by sampling at the peak of the triangular carrier. The proposed PFC controller significantly reduces the distortion of the input ac current near the zero-crossing points, resulting in a low total harmonic distortion of the input current. The operation of the proposed controller was evaluated, and its stability and robustness to parameter changes was confirmed analytically. The controller operating principles were verified in simulations and validated by experiments on a medium-voltage 50-kW converter prototype. [ABSTRACT FROM AUTHOR]

In this paper, the power-angle synchronization control of a grid-tied bidirectional dc–ac converter is investigated for low-voltage grids. The power flow equations for the low-voltage grid are analyzed and compensators are designed to ensure the decoupled control of active and reactive power. The proposed control system operates without the need for a phase-locked loop during balanced and unbalanced grid conditions. It is shown that during balanced conditions the compensators ensure stable operation controlling the desired power flow to and from the grid. It is also demonstrated that the compensators are immune to grid fluctuations and to a large extent can cater grid unbalanced conditions. Experimental results verify the performance of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper investigates power shunt filter topologies composed of three standard three-leg converters applied in either three or four-wire system. Compared with the usual three-phase shunt filter, the proposed ones allow us to reduce dc-link voltages, harmonic distortion or/and the semiconductor losses, and fault tolerant operation. Suitable control strategies to adjust filter and circulating currents, as well as the three dc-link voltages are presented. Two pulse-width modulation techniques are developed for the proposed topologies in order to achieve the best voltage-pulse pattern. Comparisons between these techniques are carried out in this paper. Simulation and experimental results are presented to validate the theoretical approach. [ABSTRACT FROM PUBLISHER]

This paper presents a dc capacitor voltage balancing control method for the star-connected cascaded H-bridge pulsewidth modulation converter in the static synchronous compensator applications. With the zero-sequence voltage injection as a basis for dc capacitors voltage balancing, this paper investigates the detailed power flow of the converter as a whole and within individual modules under unbalanced grid voltages and then proposes a method to control the reactive power output and the dc capacitors voltage by precisely managing the power flow. This approach enhances the low-voltage ride-through capability, which is very critical as the penetration of distributed energy resources grows rapidly. The proposed control method is verified by a scaled-down prototype in the laboratory. [ABSTRACT FROM PUBLISHER]

This paper presents a robust input–output linearization controller as maximum power point tracking (MPPT) technique in a photovoltaic (PV) buck dc–dc converter with applications to dc microgrids, solar vehicles, or stand-alone systems. Due to the control structure proposed in this paper, the MPPT control system is able to track very fast irradiance changes. Meanwhile, the internal stability of the overall closed-loop system is guaranteed for different load scenarios. A sector condition is only required for the load current, which is satisfied for most of the current PV applications. In turn, this condition implies the robustness against oscillations in the dc bus voltage. Finally, the MPPT control system is validated through experimental results, where the closed-loop performance is evaluated under abrupt irradiance and set-point changes, parametric uncertainty, and dc bus load variations. [ABSTRACT FROM PUBLISHER]

In this paper, a novel modular cascaded multi-level converter with multiwinding high-frequency (MWHF) transformer is proposed for medium- or high-voltage applications. In the proposed converter, a cascaded H-bridge rectifier (CHBR) is connected directly with the input ac source while the traditional step-down transformer is no longer necessary. Then, an isolated dc-dc converter composed by a group of H-bridge converters and a MWHF transformer with high power density is used to isolate the dc buses produced by the CHBR. The equivalent circuit and mathematical model of the MWHF transformer and the high frequency (HF) converter are obtained firstly in this paper. Then, the voltage stability under unbalanced loads is analyzed and the naturally balance ability is verified. To accelerate the dc bus voltage balancing process, a voltage balance control algorithm based on energy exchange between different transformer windings is proposed that is realized by accomplishing the phase shift adjustment of the terminal voltages on different windings. Simulations and experiments are implemented to verify the performance of the proposed converter and voltage balance algorithm. [ABSTRACT FROM AUTHOR]

This paper presents a converter-based permanent magnet synchronous generator (PMSG) emulator as a testbed for designing, analyzing, and testing of the generator's power electronic interface and its control system. The PMSG model is formulated in a real-time digital simulator. A voltage type ideal transformer model combined with a virtual impedance is presented as an interface algorithm. The design procedure and implementation of the virtual impedance in a simulation platform is discussed. A six-switch voltage source converter is used as a power amplifier to mimic the behavior of the PMSG supplying linear and nonlinear loads. A proportional–integral plus resonant controller is proposed as a voltage loop controller for tracking a distorted output voltage reference signal. The accuracy of the proposed emulator is investigated for the fundamental and low-order voltage harmonics. Technical challenges of the PMSG emulator are considered and proper solutions are suggested. The performance of the proposed emulator is compared with an actual PMSG. [ABSTRACT FROM AUTHOR]

In this paper, the design and development of a novel interleaved tri-state boost converter (ITBC), which produces lower ripple and exhibits better dynamic response, is discussed. Boost converters are frequently connected in parallel and operate in an interleaving mode for the reduction of ripple content in source current and in output voltage. In this way, interleaved boost converter (IBC) is conceived, which improves the power handling capabilities and increases the overall system rating. It also has the advantage of reduction of the ripple content in source current and output voltage, but when control-to-output transfer function of IBC is derived under continuous conduction mode of operation, then a right-half-plane (RHP) zero appears in the transfer function. Due to the presence of RHP zero, IBC has nonminimum phase problem, which deteriorates the dynamic performance. The tri-state boost converter (TBC) is the best choice for RHP zero elimination, but due to the extra freewheeling mode, ripple content will also be increased. The proposed converter is a parallel combination of two TBC and operates in an interleaving mode. Therefore, the proposed converter has both of the advantages of TBC and IBC. The performance analyses of ITBC, TBC, and IBC have been studied based on simulation and experimental results. From the comparative analysis, it is observed that ITBC is performed better than other two converters. The ripple comparisons between three converters have also been done. It is found that the ripple content in ITBC is slightly greater than IBC but is less than TBC. [ABSTRACT FROM PUBLISHER]

This study adopts the method of third-order harmonic voltage injection (THVI) to move the peak value of a reference voltage toward the high capacitor voltage and increase the utilization rate of the capacitor voltage. This method can lower the cost or enlarge the capacity of a modular multilevel converter (MMC) when applied in high-voltage direct-current transmission. In this paper, the principle of THVI is introduced. Two calculation methods for modulation waves are studied, and the mathematical expressions of the capacitor voltage under all operating conditions are deduced. Thus, the optimal magnitude and phase of the third-order harmonic voltage are determined under all operating conditions. The mathematical expressions of the optimal magnitude and phase of the third-order harmonic voltage are obtained, and their online calculation burden is reduced. Simulation and experimental results show that the proposed method of THVI is feasible and effective. [ABSTRACT FROM PUBLISHER]

When investigating voltage and current stresses in critical main circuit components during faults of the converter, a detailed equivalent model capable of representing the balancing control strategies of the capacitor voltages on the submodule level, along with blocking and delocking, is always necessary. Among previously proposed equivalent models of the modular multilevel converter (MMC), only submodule averaged models (SAMs) can capture interested inner dynamics inside each arm. However, the simulation accuracy of SAMs is not always satisfactory, especially when the time step is larger than 10 $\mu$s. In order to further improve the simulation accuracy with guaranteed simulation efficiency, the shifted frequency modeling of the half- and full-bridge hybrid MMC is proposed in this paper. Therein, each submodule is represented by Thévenin equivalents derived by submodule dynamic phasors. The arm of the MMC is represented by Norton equivalents to guarantee the efficiency, considering both normal and dc-blocking conditions. The effectiveness of the proposed model in terms of accuracy and efficiency is validated by simulating an MMC-based HVdc transmission. [ABSTRACT FROM AUTHOR]

Switched-capacitor (SC) converter is a type of variable structure systems. SC converters are typically equipped with open-loop control or some linear feedback control methods. However, these approaches provide slow dynamic response speed, small operation region, and large voltage ripple. Some new nonlinear control methods have been proposed in the literature to overcome these barriers. However, such control methods are usually proposed for applications involving low-power SC converters due to the limitations of them. This paper presents a nonlinear constant on-time variable frequency one-cycle control (CVFOCC) strategy based on dynamic capacitor ampere-second balance transient calculation (CASBTC) modeling method and one-cycle control (OCC) technique for high-power SC converters. This method successfully addresses the regulation issue of high-power SC converters. A high-power 3X Two-switch Boosting Switched-Capacitor converter (TBSC) is employed as an example topology. Simulation and experimental results both demonstrate that CVFOCC method could significantly improve the dynamic response speed and achieve excellent line and load regulation for the high-power SC converter in wide operation range. [ABSTRACT FROM AUTHOR]

A fractional-order capacitor (FOC) plays a vital role in fractional-order circuits. However, the FOC with order $1< \alpha < 2$ based on existing manufacturing schemes can be only operated in milliwatt power level. Thus, the applications of an FOC in power conversion are limited. To solve the problem, this paper proposes a novel method for realizing a high-power FOC with order $1< \alpha < 2$, in which a power converter is applied to emulate the current–voltage characteristic of an ideal FOC, so the rated power of the FOC can be increased greatly and depended on the power converter. The corresponding topology and its control strategy are provided. The available experiments verify the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

LLC resonant converter is a nonlinear system, limiting the use of typical linear control methods. This paper proposed a new nonlinear control strategy, using load feedback linearization for an LLC resonant converter. Compared with the conventional PI controllers, the proposed feedback linearized control strategy can achieve better performance with elimination of the nonlinear characteristics. The LLC resonant converter's dynamic model is built based on fundamental harmonic approximation using extended describing function. By assuming the dynamics of resonant network is much faster than the output voltage and controller, the LLC resonant converter's model is simplified from seven-order state equations to two-order ones. Then, the feedback linearized control strategy is presented. A double loop PI controller is designed to regulate the modulation voltage. The switching frequency can be calculated as a function of the load, input voltage, and modulation voltage. Finally, a 200 W laboratory prototype is built to verify the proposed control scheme. The settling time of the LLC resonant converter is reduced from 38.8 to 20.4 ms under the positive load step using the proposed controller. Experimental results prove the superiority of the proposed feedback linearized controller over the conventional PI controller. [ABSTRACT FROM PUBLISHER]

Recently, a modular multilevel converter (MMC) has become one of the most popular converters in medium/high-power applications. In this paper, a new technique is presented to decrease the computational time of the MMC by using switching state grouping for the predictive controller. The proposed technique has the ability to control the output current, minimize the circulating currents, and balance all capacitor voltages, simultaneously. For a predefined number of MMC submodules, the number available switching states is known. The proposed technique uses two stages of predictive modules. In the first stage, the switching states of the MMC are divided equally into a number of groups, controlled simultaneously by the same number of predictive modules. The optimal number of the switching state groups in the first stage is obtained. In the second stage, only one predictive module is used to obtain the optimal switching state of the MMC without any other complexities or additional sorting controllers. The effectiveness of the proposed control technique has been verified by using MATLAB/SIMULINK software considering steady state and dynamic analysis. The results of the proposed technique are compared with the conventional one in terms of computation time, verifying better performance of the proposed method. [ABSTRACT FROM PUBLISHER]

Recently, riding through grid faults and supporting the grid voltage by using grid-connected converters (GCCs) have become major requirements reflected in the grid codes. This paper presents a novel reference current generation scheme with the ability to support the grid voltage by injecting a proper set of positive/negative active/reactive currents by using four controlling parameters. Analytical expressions are proposed to obtain the optimal values of these parameters under any grid voltage condition. The optimal performances can be obtained by achieving the following objectives: first, compliance with the phase voltage limits, second, maximized active and reactive power delivery, third, minimized fault currents, and fourth reduced oscillations on the active and reactive powers. These optimal behaviors bring significant advantages to emerging GCCs, such as increasing the efficiency, lowering the dc-link ripples, improving ac system stability, and avoiding equipment tripping. Simulation and experimental results verify the analytical results and the proposed expressions. [ABSTRACT FROM PUBLISHER]

Grid-connected power conversion systems for renewable energy sources must fulfill several requirements, e.g., the high efficiency, the reduced cost and complexity, and, quite often, the boost capabilities that is usually achieved using a front-end dc–dc boost converter before the inversion stage, leading to a two-stage architecture. Meanwhile, single-stage power conversion systems, which perform the boosting operation within the inversion one, offer some potential advantages, in terms of reducing the complexity and the volume of the whole system. Among several proposed options, the split-source inverter (SSI) has been recently proposed by Abdelhakim et al. as an alternative option with some interesting features to the commonly used Z-source inverter. Taking into account that the SSI is controlled by a single parameter, i.e., its dc and ac sides are controlled by the modulation index, it is of paramount importance to investigate its control scheme in grid-connected mode, which has never been studied yet. Hence, this paper models the SSI dc side and proposes a modified modulation scheme combined with the commonly used synchronous reference frame control technique to achieve a decoupled control scheme of the SSI in grid-connected mode, i.e., the dc and the ac sides of the SSI can be controlled independently, which is convenient for many applications. The introduced control scheme is analyzed and simulated using a MATLAB/Simulink model, where a reduced scale 1-kVA grid-connected SSI is designed and simulated for the sake of experimental validation. Finally, the designed system is implemented experimentally to validate and verify the reported analysis and simulations. [ABSTRACT FROM PUBLISHER]

The increasing penetration of distributed and renewable generation presents power systems with a challenge of accommodating multiple inverter-based generators. Distributed generators (DGs), capable of operating in the islanded mode, are often installed in microgrids. However, a control system for operating multiple DGs in the islanded mode using voltage-sourced converters is a real challenge under uncertainties in load parameters, unbalanced phase conditions, and transients. This paper proposes a novel approach to decentralized control of multiple DGs. This approach is based on a combination of a voltage controller using model predictive control and a fast current controller using a discrete-time sliding-mode controller for limiting the inverter currents under overload conditions. Performance of the proposed control system is compared with a robust servomechanism program controller. The proposed control strategy provides fast tracking, robustness, and fast transient recovery in voltage and current under: 1) short-circuit conditions; 2) load transients; and 3) unbalanced loads. Various scenarios including reference signal tracking and robustness against the load perturbations are considered. Results are presented and discussed. [ABSTRACT FROM PUBLISHER]

This paper presents a flying-capacitor modular multilevel converter (FC-MMC) based on series-connected submodules. It is intended for completely improving the performance of a medium-voltage motor drive system in the entire speed range especially at zero/low speed under rated torque condition. The proposed FC-MMC circuit is characterized by the cross connection of upper and lower arm middle taps through a flying capacitor in per phase leg. By properly controlling the ac current flowing through the flying capacitor, the power balance between upper and lower arms is achieved, leading to very small voltage ripples on submodule dc capacitors in the entire speed range from standstill to rated speed even under the rated torque condition. Meanwhile, no common-mode voltage is injected. Simulation results obtained from a 4160-V 1-MW model show that the proposed FC-MMC along with the proposed control method performances satisfactorily in dynamic and static state even when operated at zero/low speed. Experiments on a downscaled prototype also prove the effectiveness of the proposal. [ABSTRACT FROM AUTHOR]

This paper develops an appropriate predictive current controller (PCC) for fast torque monitoring and high-performance operation of a permanent-magnet synchronous motor (PMSM) drive involving constant switching frequency. The proposed controller is based on a convenient combination of deadbeat and direct predictive control (DPC) techniques. Its response is compared to that of a typical proportional–integral (PI) current controller in field-oriented control (FOC) strategy. The analysis illustrated improved transient characteristics concerning faster speed changes as well as reduced current and torque ripple while it enables additional control criteria inclusion. The simulated results have been validated by measurements on a test bench. [ABSTRACT FROM PUBLISHER]

A fast and robust predictive current controller for reduction of flicker caused by dc electric arc furnaces (EAFs) supplied by thyristor converters is presented in this paper. During normal EAF operation, large active- and reactive-power variations can occur at the point of furnace connection, causing unwanted variations of network voltage and correspondingly a flicker phenomenon. The main cause of this phenomenon are electrode short-circuits with large current swings, which occur during the so-called bore-in period of the furnace cycle. The proposed controller reduces the flicker caused mainly by reactive-power variations during the electrode short-circuits by providing fast and robust predictive control of the arc current without relying on the accurate arc model. The obtained simulation and experimental results demonstrate the ability of the presented current controller to effectively compensate for the electrode short-circuits, thereby enabling a reduction of the unwanted fluctuations of the reactive power, which are the main cause of the induced flicker. The simulation results show that the proposed controller can reduce the induced flicker by up to 50%, compared to the traditionally used system with PI current controller. Also, the controller is robust to the asymmetries and the abrupt changes of the network voltage. [ABSTRACT FROM PUBLISHER]

In recent years, modular multilevel converters (MMCs) are very popular in medium/high-voltage motor drive systems and high-voltage direct-current (HVDC) applications. The conventional model predictive control (MPC) strategies for the MMC are not practical due to their substantial calculation requirements, especially under high number of voltage levels. To solve this problem, a fast MPC strategy combined with a submodule (SM)-voltage sorting balancing method is proposed based on the discrete mathematical model derived for the MMC in this paper. By optimizing the implementation of the control objectives and simplifying the rolling optimization, the proposed strategy is simpler to expand to a system with larger number of SMs, with the amount of calculation reduced and the advantages of the conventional MPC algorithm reserved. In addition, the proposed control can minimize the $dv/dt$ of the output voltages. Both steady-state and transient performances are evaluated by a down-scaled three-phase MMC prototype under various experimental conditions, which validates the proposed fast MPC strategy. [ABSTRACT FROM PUBLISHER]

This paper proposed a modulation scheme for the dual-active-bridge (DAB) converter to reduce rms current in wide-range operating conditions. The operating principle of the proposed fundamental duty modulation (FDM) is formulated based on the fundamental component analysis of the DAB converter. By modulating the PWM signals in the fundamental component domain, the optimal operation is implemented with a simple controller structure not requiring an operating mode classification, offline calculation, or current information. Operating characteristics including rms current level and ZVS characteristics are analyzed to compare loss breakdowns of the proposed scheme to those of recent related works. The proposed FDM achieves high efficiency under wide operation conditions due to reduced conduction level and wide ZVS range. Experimental results are obtained under various voltage gain and load conditions to confirm the operation of the proposed modulation scheme. A thorough experimental comparison with other sophisticated modulation schemes has verified the efficiency improvement of FDM. [ABSTRACT FROM AUTHOR]

Modular multilevel converters have several attractive features such as a modular structure, high voltage capability and scalability, excellent quality of the output voltage waveforms, as well as, low expense for redundancy. However, the main drawback of this converter is the complexity of the control system due to the multiple control objectives which include the input, output, circulating current and the capacitor voltages. Several control schemes for the capacitor voltages have been proposed, being usually divided into the control of the average voltage and the balancing of the capacitor voltages among the submodules and among the arms. These control schemes can effectively control the capacitor voltage when the reference is the same for all arms, but they cannot manage arbitrary voltage references when each arm capacitor voltage must be controlled independently as required, for example, to achieve maximum power point tracking in photovoltaic applications. This paper proposes a control strategy for a modular multilevel converter capable to manage the average capacitor voltage of each arm independently. The proposed scheme is based on a decomposition of the energy of the arms in different components, according to theirs symmetries. Results validating the performance of the proposed control scheme are shown and the impact on the output current is also discussed. [ABSTRACT FROM PUBLISHER]

New grid codes demand the wind turbine systems to ride through recurring grid faults. In this paper, the performance of the doubly Ffed induction generator (DFIG) wind turbine system under recurring symmetrical grid faults is analyzed. The mathematical model of the DFIG under recurring symmetrical grid faults is established. The analysis is based on the DFIG wind turbine system with the typical low-voltage ride-through strategy—with rotor-side crowbar. The stator natural flux produced by the voltage recovery after the first grid fault may be superposed on the stator natural flux produced by the second grid fault, so that the transient rotor and stator current and torque fluctuations under the second grid fault may be influenced by the characteristic of the first grid fault, including the voltage dips level and the grid fault angle, as well as the duration between two faults. The mathematical model of the DFIG under recurring grid faults is verified by simulations on a 1.5-MW DFIG wind turbine system model and experiments on a 30-kW reduced scale DFIG test system. [ABSTRACT FROM AUTHOR]

The inherent double line ripple power in single-phase systems is adverse to the performance of power electronics converters, e.g., limited lifetime due to the requirement of large electrolytic capacitors and low voltage control bandwidth due to harmonic disturbance. In this paper, an active converter topology based on a symmetrical half-bridge circuit is proposed to decouple the ripple power so that balanced instantaneous power flow is assured between source and load, and the required dc-link capacitance can be dramatically reduced. For proper closed-loop regulation, the small signal modeling of the proposed system is presented, and a dual voltage control strategy is then proposed, which comprises one voltage loop implemented in the synchronous reference frame for active power balancing, and another one implemented in the stationary reference frame for ripple power compensation. Special attention is given to the bandwidth of voltage control loops because the variation of dc-link voltage should be kept within an acceptable range during load transients. This is particularly important for systems with reduced dc-link capacitance because they are of lower energy capacity, and the dc-link voltage is therefore very sensitive to step load changes. Comprehensive simulation and experimental results are presented to show the effectiveness of the proposed circuit and control algorithm. [ABSTRACT FROM AUTHOR]

A new discrete-time state feedback controller is presented, which allows high-bandwidth voltage control of a buck converter for any load condition, whether it operates in discontinuous conduction mode (DCM), continuous conduction mode (CCM), or at the boundary of these regions. This makes the buck converter applicable for a wide range of applications. For the control design process, two large-signal models, which represent the buck converter's discrete time dynamics in CCM and DCM, are developed. A simple proportional-integral regulator is used for the voltage control of the converter. The operation mode is detected and the voltage controller is connected in cascade to a current controller in CCM or to a nonlinear state feedback decoupling structure in DCM. In this paper, the modeling and design of the proposed control topology are introduced and its performance is demonstrated in simulation and experiment. [ABSTRACT FROM PUBLISHER]

Modular multilevel converters offer several benefits, such as high scalability and power quality, which are particularly advantageous for high-voltage dc transmission systems. However, the control of such converters is very challenging due to the number of control objectives to be achieved simultaneously. The input and output currents must be controlled at their own references, and these references must be properly generated in order to keep the average capacitor voltage constant. The circulating current can be controlled either to minimize losses or to reduce capacitor voltage ripple. Additionally, the capacitor voltage must be kept balanced among cells during the converter operation. In this paper, a model with four independent dynamical components of the arm currents, which also considers the effect of ac and dc systems, is proposed. The proposed model facilitates the dynamical analysis of currents and simplifies the design and implementation of current controllers. Analysis of the proposed current model and experimental results, which confirm the performance of the designed controllers, is shown. [ABSTRACT FROM PUBLISHER]

Solar photovoltaic (PV)-based stand-alone systems have evolved as a promising solution to the issue of electrification in areas where the grid is not available. The major challenges in designing such systems are as follows: 1) extraction of maximum power from the PV array; 2) protection of the battery from overcharge and overdischarge; 3) dc to ac conversion; and 4) provision for adequate voltage boosting. As multiple objectives are required to be satisfied, the existing schemes for stand-alone systems require a minimum of three converter stages, leading to considerable reduction in the reliability and efficiency of the system. In order to address this issue, a two-stage stand-alone scheme consisting of a novel transformer-coupled dual-input converter (TCDIC) followed by a conventional full-bridge inverter is proposed in this paper. The proposed TCDIC can realize maximum power point tracking and battery charge control while maintaining the proper voltage level at the load terminal. The small signal mathematical model of the TCDIC is derived. A suitable control strategy for the proposed TCDIC is devised. The operation of the scheme is verified by performing detailed simulation studies. A laboratory prototype of the scheme is developed. Detailed experimental validation of the scheme utilizing the laboratory prototype is carried out to confirm the viability of the scheme. [ABSTRACT FROM PUBLISHER]

To fully utilize second-life batteries on the grid system, a hybrid battery scheme needs to be considered for several reasons: the uncertainty over using a single source supply chain for second-life batteries, the differences in evolving battery chemistry and battery configuration by different suppliers to strive for greater power levels, and the uncertainty of degradation within a second-life battery. Therefore, these hybrid battery systems could have widely different module voltage, capacity, and initial state of charge and state of health. In order to suitably integrate and control these widely different batteries, a suitable multimodular converter topology and an associated control structure are required. This paper addresses these issues proposing a modular boost-multilevel buck converter based topology to integrate these hybrid second-life batteries to a grid-tie inverter. Thereafter, a suitable module-based distributed control architecture is introduced to independently utilize each converter module according to its characteristics. The proposed converter and control architecture are found to be flexible enough to integrate widely different batteries to an inverter dc link. Modeling, analysis, and experimental validation are performed on a single-phase modular hybrid battery energy storage system prototype to understand the operation of the control strategy with different hybrid battery configurations. [ABSTRACT FROM PUBLISHER]

This paper presents the application of the Active Disturbance Rejection Control Technique (ADRCT) to improve the performance of a Flywheel Energy Storage System (FESS). The FESS is designed for the DC MicroGrid (MG) application. It mainly consists of a flywheel with a coaxial BrushLess DC (BLDC) machine, a three-phase full-bridge circuit, and a bidirectional Buck–Boost converter. A model-independent controller based on the ADRCT, which can estimate and compensate model uncertainties and unknown disturbances in real time, is designed for the bidirectional Buck–Boost converter control in the FESS. Simulations and experiments are conducted in both charge and discharge mode to verify the performance of the proposed controller by comparison with a traditional double loop PI controller. Results show that the new controller is more robust and more adaptive. It has a better anti-disturbance capability and a higher dynamic performance than the PI controller. Moreover, to adapt to different applications and operating conditions, the charging process of the FESS is further improved. Strategies on maximum torque control and power limiting control are developed and realized on the experimental platform. [ABSTRACT FROM PUBLISHER]