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A phase-locked loop (PLL) is commonly used in grid-connected power converter for synchronization. A single input single output (SISO) linear model of PLL with the phase angle of the point of common coupling (PCC) voltage as input and the estimated phase angle as the output is generally used in the PLL analysis and design. However, an undesirable coupling between the magnitude and phase of the input voltage is present when a filtering stage is incorporated before the control loop, which a SISO model cannot capture, and could result in a significant disturbance in the PLL estimated phase during grid faults. This paper proposes a generalized multi-input multi-output (MIMO) linear model that captures the complete PLL dynamics during symmetrical faults for a PLL equipped with any prefilter. Furthermore, a compensation method that decouples the magnitude dynamics of the input voltage from the phase dynamics is proposed in this paper. The proposed compensation improves the PLL's phase tracking performance by ensuring that the prefiltered PLL acts only on the PCC voltage phase changes. A vital application of the proposed compensation method for PLL is during a grid fault case, wherein the converters are expected to contribute to the fault current, which requires an accurate measurement of the PCC voltage phase. The compensation method's effectiveness is demonstrated using power hardware in the loop simulation of a hardware voltage source converter interfaced to a distribution system simulated in real-time. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive study on current loop auto-tuning for self-commissioning of sensorless interior permanent magnet synchronous machine (IPMSM) drives. Well-tuned current controllers are essential for both high-performance operation and self-commissioning procedure. However, current controller design parameters, i.e., $L_{d}$ and $L_{q}$ , are not known at the beginning of the self-commissioning procedure where their estimation also has unique challenges. For example, an initial rotor position to determine the actual d- and q-axes is not known. Also, feedback controllers cannot be used in the estimation because they are not tuned yet. As a solution, this paper proposes a method to estimate the spatial inductance map by spatially scanning the motor via sinusoidal voltage injection in a controlled manner. Then it uses the estimated inductance values to tune the current controllers. The proposed method identifies $L_{d}$ and $L_{q}$ values using open-loop voltage injection without needing actual d- and q-axis positions. The whole procedure takes around a second. Practical considerations such as an automatic selection of the injection voltage, and digital control and dead-time effects are carefully addressed. The findings are experimentally verified on a 3-phase IPMSM drive. Furthermore, its applicability to different motor types is also demonstrated. [ABSTRACT FROM AUTHOR]

This paper deals with the high accurate current set-points solution for interior permanent-magnet synchronous motors (IPMSM) in wide-speed range applications. Considering voltage and current constraints, the operating regions can be divided into maximum torque per ampere, maximum current, field weakening, and maximum torque per voltage regions, which requires solving different nonlinear functions in real time to obtain optimal current set-points. Traditional methods including curve-fitting methods and polynomial approximation (PA) methods are not easy to obtain these solutions, especially involving magnetic saturation problems. In this paper, Newton–Raphson algorithm for improving the control accuracy of the current set-points is proposed. Meanwhile, parameters influence including magnetic saturation and resistive voltage drop is fully investigated. Compared with PA method, the proposed method is able to converge to accurate solutions in few numbers of iterations with reduced execution time, which can be easily implemented on an off-the-shelf digital signal processor. Both simulation results and experimental results on an 8-kW IPMSM rig are conducted showing good agreement with the expected results. [ABSTRACT FROM AUTHOR]

Series-connected-based offshore wind farms can eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the dc voltage of the converters should be changed within a large range to control the dc-link voltage (grid side) and maximize wind power acquisition (generator side). In this paper, the full-bridge-based modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The dc voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The dc current control and capacitor voltage balance control are proposed in this paper, with which the FB-MMC can operate normally in the series-connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designed in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation and experiment results verify the feasibility of the proposed configuration and the corresponding control algorithm. [ABSTRACT FROM AUTHOR]

A novel high conversion ratio bidirectional C-dual transformers-LC (CDT-LC) resonant dc–dc converter is proposed in this paper. Based on the traditional LLC, an auxiliary transformer and an extra resonant capacitance are employed in the CDT-LC structure. It not only harvests high efficiency, but exhibits a good voltage gain feature. Especially for the case of light load, CDT-LC still harvests a high efficiency, compared with traditional converters. Also, it can achieve a fast gain reduction when the switching frequency is higher than resonant frequency. In this paper, the operating principles of the converter are analyzed in both bidirectional operation modes. In addition, its dual-transformers structure feature and voltage gain characteristics are discussed in details. Furthermore, the parameter design method is provided. Moreover, the loss breakdown for the converter is given in details. Finally, a 2.5-kW prototype is established to verify the performance of the CDT-LC converter, and its maximum efficiency reaches 97%. [ABSTRACT FROM AUTHOR]

With the proliferation of distributed generation in distribution networks, quickly and smoothly converting to a state of regional autonomy in the case of distribution network failure is one of the main challenges for the stable operation of a multimicrogrid (MMG). To address the aforementioned problem, this paper proposes coordinated control of the passive transition from grid-connected to islanded operation for a three/single-phase hybrid MMG. Under the premise of fully considering the constraints on the tie-line transfer power undertaken by the master power supply (MPS), the following two scenarios regarding safety and emergency are formulated: 1) when the output power of the MPS is in the safe range, the proposed strategy coordinates the energy storage (ES) in each submicrogrid (SMG) to achieve power balance; 2) when the output power is in the emergency range, this paper proposes a three-phase balance method based on a bubble sort to realize the combination optimization of the single-phase source-load-storage (SLS) and a fast power regulation method based on implicit enumeration for the SLS to reduce the tie-line transfer power, which could prevent failure of the mode transition from power overload. The test results verify the effectiveness and feasibility of the proposed coordination strategy. [ABSTRACT FROM AUTHOR]

This paper presents a model predictive control for a six-phase permanent magnet synchronous machine (PMSM) with a reduced-dimension cost function. Only the variables in the harmonic subspace are included into the cost function, while the energy conversion related variables are excluded. This is achieved by using the deadbeat direct torque and flux control method to obtain a reference voltage vector and then to track the torque and stator flux properly in the energy conversion related subspace. Then, according to the position of the reference vector, the appropriate prediction vectors can be determined. Subsequently, a reduced-dimension cost function including only the harmonic constraints is defined to evaluate the feasible prediction vectors. In this way, the predictive model and the cost function are simplified without redundant constraints. Meanwhile, the torque and flux can be regulated satisfactorily, and the harmonic currents are suppressed effectively. Finally, experimental results of the proposed method and the conventional model predictive torque control are presented in this paper to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

This paper briefly reviews the common-mode voltage (CMV) and neutral point (NP) voltage for neutral point clamped three-level inverter (NPC TLI). Then, the space vector pulsewidth modulation (SVPWM) and traditional virtual SVPWM (VSVPWM) are discussed in terms of these two issues, revealing the drawbacks in reducing CMV or eliminating NP voltage oscillation. A novel virtual space vector modulation, named as RCMV_VSVPWM, is proposed in this paper to reduce CMV and eliminate NP voltage oscillation for NPC TLI. By selecting vectors with lower CMVs, a set of novel virtual voltage vectors are generated. The highlight of the method is zero average NP current in one control cycle and lower CMV. Furthermore, the active NP voltage control suitable for RCMV_VSVPWM is presented and evaluated. The corresponding experimental results are given, which are well-consistent with theoretical analysis. [ABSTRACT FROM AUTHOR]

Compared with the conventional two-level dual-active-bridge (DAB) dc–dc converter, the three-level DAB converter could operate with lower voltage rated switching devices and provide more control degrees of freedom, which enhances the control flexibility. This paper proposes a modulation strategy with five control degrees of freedom. In the previous work with multiple control degrees, the optimization strategies for efficiency or inductor current minimization are investigated; however, these strategies all need detailed mode analysis and complex mathematical optimization tools to obtain the optimal duty cycles. In this paper, an optimization strategy for the root-mean-square value of the inductor current is proposed. This method extracts the fundamental and third-harmonic components from the high-frequency voltages on both sides and adjusts the amplitude of the bridge voltages by the vector diagram to obtain the optimal duty cycles. The inductor current can be reduced significantly at light loads with the proposed strategy, so the efficiency of the converter can be improved. Moreover, the optimization process does not need the detailed operation analysis of the converter, nor does it require the complex mathematical optimization tools. Finally, the performance of the three-level DAB and the optimization strategy are verified by the experimental results. [ABSTRACT FROM AUTHOR]

This paper improves the position sensorless drive by sensing actual machine phase voltages. A high-bandwidth phase voltage measurement is developed for pulsewidth modulation (PWM) voltage inverters. On the basis, the actual phase voltage is obtained based on the digital integration of PWM voltage using the capture modulator in existing drive microcontrollers (MCU's). Comparing to existing phase voltage measurement, no separated A/D converter and communication hardware are required because PWM pulses are directly measured using MCUs. However, for standard machines without neutral points, only line-to-line ac PWM voltages can be measured for the phase voltage reconstruction. Since the capture based on transistor-transistor logic (TTL) logics receives only digital signals, a preprocess circuit to convert ac PWM line voltages to equivalent digital signals is proposed. This paper clearly explains the voltage sensing hardware using MCU capture modulator. According to experimental results, a 150-MHz sampling rate for phase voltage measurement is achieved based on the proposed capture-based voltage measurement. Although the physical limitation of back electromotive force estimation still appears, the proposed phase voltage measurement substantially enhances the sensorless drive performance at low speed. [ABSTRACT FROM AUTHOR]

A space vector modulation (SVM) technique of a dual-inverter system for an open-end winding motor drive is described in this paper, where one inverter has a battery power source and the other has an only capacitor across the dc bus. The SVM must be achieved to operate the motor with field-oriented control and simultaneously to control the capacitor voltage at a constant value by using redundant switching states of the dual-inverter system. The control of the capacitor voltage is carried out by selecting a charging or a discharging mode in each redundant switching state, taking the instantaneous motor power factor into account. In addition, it is also required to reduce the error voltage pulses, which are generated in output multilevel voltage waveforms during the dead time. The compensation method of the existing dead-time scheme and the improved SVM sequence to reduce the error voltage vectors are proposed in this paper. The proposed methods are examined through several experimental tests and are confirmed to generate superior output voltage waveforms from the viewpoint of the measured total harmonic distortion and dv/dt. [ABSTRACT FROM AUTHOR]

Solid state transformer (SST) integrated hybrid dc–ac microgrid (HMG) has wide operating capabilities in presence of dc–ac loads, renewable energy resources, and energy storage. This paper proposes control strategies for localized control of zonal hybrid microgrid, enabled by the SST and centralized energy storage devices (ESD) integrated using dual active bridge (DAB) converter. The control of DAB dc–dc converter is modified for zero circulating power flow (ZCPF) operation in both SST and ESD. Power management and coordination control used in this paper ensures the power supply reliability of the zonal HMG integrated through the SST. Microgrid system, along with its proposed controllers, provides a stabilized low-voltage dc and ac network ports, in both grid feeding and forming modes. The performance of the configured HMG system is evaluated under various power management scenarios using the MATLAB software platform. Further, the SST architecture, with its proposed controller, is tested on FPGA-based hardware platform. [ABSTRACT FROM AUTHOR]

This paper introduces a novel leaky least logarithmic absolute difference (LLLAD)-based control algorithm and a learning-based incremental conductance (LIC) maximum power point tracking algorithm for a grid-integrated solar photovoltaic (PV) system. Here, a three-phase topology of the grid-integrated PV system is implemented, with the nonlinear/linear loads. The proposed LIC technique is an improved form of an incremental conductance (InC) algorithm, where inherent problems of the traditional InC technique, such as steady-state oscillations, slow dynamic responses, and fixed-step-size issues, are successfully mitigated. The prime objective of the proposed LLLAD control is to meet the active power requirement of the loads from the generated solar PV power, and after satisfying the load demand, the excess power is supplied to the grid. However, when the generated solar power is less than the load demand, then LLLAD meets the load by taking extra required power from the grid. During these power management processes, on the grid side, the power quality is maintained. During daytime, the proposed control technique provides load balancing, power factor correction, and harmonic filtering. Moreover, when solar irradiation is zero, then the dc-link capacitor and a voltage-source converter act as a distribution static compensator, which enhances the utilization factor of the system. The proposed techniques are modeled, and their performances are verified experimentally on a developed prototype in solar insolation variation conditions, unbalanced loading, and in different grid disturbances such as over- and undervoltage, phase imbalance, harmonics distortion in the grid voltage, etc. Test results have met the objectives of the proposed paper, and parameters are under the permissible limit, according to the IEEE-519 standard. [ABSTRACT FROM AUTHOR]

This paper presents a Tornambe controller (TC) for a two-degree-of-freedom (DOF) parallel mechanism of pneumatic muscle actuators (PMAs) based on tracking differentiators (TDs). A dynamic nonlinear system is established for the 2-DOF parallel mechanism of PMAs. TC is designed for improving control performance of the dynamic nonlinear system. The TDs are utilized to obtain high order derivatives of tracking errors, which are required by TC. Moreover, stability analysis of the dynamic nonlinear system is shown by a Lyapunov approach. Finally, experimental results are shown to verify the effectiveness of the proposed control method in this paper. [ABSTRACT FROM AUTHOR]

This paper proposes an improved coordinated control of standalone brushless doubly fed induction generator (BDFIG) supplying nonlinear loads. Due to the existence of nonlinear loads, the voltage at the point of common coupling becomes distorted with a large number of low-order harmonics, which severely degrades the performance of power generation systems. In order to eliminate the included low-order harmonics, a coordinated control method of harmonic mitigation has been presented in this paper for the control of both machine side and load side converters. Then, a weight factor is introduced to the proposed control method for distributing the contribution of harmonic mitigation between the two converters. Comprehensive experimental results on a 30 kVA BDFIG are presented to verify the proposed control method. [ABSTRACT FROM AUTHOR]

The voltage source converter based high voltage direct current transmission system with the true bipolar wiring mode features higher flexibility and reliability, yet it cannot be without a proper and effective coordinated control strategy, which is the key to the safe and reliable operation of the system as well as the effective and full utilization of renewable energy on a large scale. A step further from the conventional droop control strategy, the coordinated control strategy based on operating point optimization is proposed in this paper. This paper first illustrates and examines the coordinated control method at the system level in view of special wiring and control modes; it then looks into the method of operating point optimization seeking to achieve both safe and cost-effective operation; and the paper finally demonstrates the effectiveness of the coordinated control strategy verified by MATLAB and PSCAD simulations from the perspective of system operating point deviation, security, and cost-effectiveness. [ABSTRACT FROM AUTHOR]

In this paper, a single-inductor dual-output (SIDO) dc–dc converter is proposed. The significant feature of the proposed converter is feeding a constant power load in low-voltage side (LVS) port, where even feeding a variable load at LVS does not have any cross regulation to high-voltage side port with the desired approximation. Magnificently, both merits are obtained in open-loop manner, which is an outstanding characteristic in comparison with that of the existed converters. In this paper, soft commutation and soft switching of switches and diodes are investigated along with the aforementioned main novelty. Moreover, detailed theoretical analyses of the proposed SIDO dc–dc converter in continuous conduction mode are presented besides the comparisons with some other structures. Finally, theoretical analyses are verified through a 1500 W experimental prototype results. [ABSTRACT FROM AUTHOR]

This paper introduces modeling and control of a cascaded flying capacitor bridge (CFCB) multilevel converter for a solid-state transformer. The finite control set-model predictive control (FCS-MPC) is easy and simple to implement for many applications. However, the computational complexity increases with the increase in the number of switches. Furthermore, as the number of control variables increases, the weighting factors also increase. Thus, it is difficult to apply to a system with a large number of switches and variables to be controlled similar to a CFCB multilevel converter. In this paper, the proposed method divides variables such as current, dc-link voltage, and flying capacitor voltage into layers. The proposed method reduces the number of states to be considered in the control, which shortens the computational time and simplifies expansion of the flying capacitor bridge. In addition, since weighting factors are not used, there is no tradeoff between current quality and voltage quality, and there is no difficulty in selecting weighting factors. Algorithms that include a method for compensating for the distortion caused by the delay of the digital control system are also described. The effectiveness of the proposed method is verified via experiments using a two-cell CFCB multilevel converter. [ABSTRACT FROM AUTHOR]

This paper deals with the innovative “variable-voltage” bus concept for future more-electric aircraft platforms. Using new functionalities and opportunities offered by innovative sources actively controlled by power electronics, there is an opportunity for a significant increase of their output power (within powertrain capabilities, machine thermal limits, and power converter maximum current) to supply increased load demands under certain conditions. This paper investigates applications of this concept to satisfy power demands for the wing ice-protection system for business jet. The study includes a detailed study into the control challenges of the proposed approach and suggests corresponding theoretical solutions. Furthermore, the controller design aspects are explored with considerations to achieve stable operation. The concept is tested in simulation environment and validated through experimental studies. This paper might be of interest for the researchers and engineers focused on innovative solutions for future aircraft platforms. [ABSTRACT FROM AUTHOR]

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

The extra-LC auxiliary-resonant-snubber-based converter (ELCARSC), as a soft-switching converter, has the advantages of low switching loss, low electromagnetic interference noise, and low-output distortion caused by the dead-time effect. However, the dead-time effect is still the main problem in a high-precision application. Besides, the traditional control and compensation methods rely on the accuracy of the circuit parameters, especially the junction capacitance, thus resulting in compensation error. Therefore, this paper proposed a novel high-precision control method through modulating the initial resonant current to eliminate the dead-time effect of ELCARSC. The method does not need complicated online calculation and reduces the dependence on the accuracy of the circuit parameters, especially the influence of the junction capacitances of the switches. In this paper, the dead-time effect of the ELCARSC is analyzed. The output voltage error with the conventional variable-timing control method is analyzed. Subsequently, the proposed modulated initial resonant current control is introduced. Finally, a prototype was developed that verified the effectiveness of the control method. The quality of the output current and voltage is improved by utilizing the proposed control method. However, the peak current of the auxiliary switch is increased. [ABSTRACT FROM AUTHOR]

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

This paper presents a multifunctional control system for interlinking converters interfacing microgrids and ac mains. Applicable standards pose power quality and grid-support function requirements for converters that interface energy resources of dc nature with an ac utility grid. At the same time, the forthcoming smart grid scenario calls for additional functionalities, enabling the effective integration of hybrid (i.e., dc or ac) microgrids, that are applicable to both grid-connected and islanded modes of operation. This requires novel interlinking converter controllers with extended flexibility. The control system described in this paper fulfills both connected and islanded operation needs and provides seamless transitions between the two modes. Such valuable functionalities are attained owing to an underlying, large-bandwidth, triple-loop control organization. The target converter is meant to be connected to a local dc subgrid at the dc side and to a local ac subgrid at the ac side. The controller manages the connection of the ac-side subgrid with the utility grid, ensuring a fast response control of the ac grid current. Therefore, it allows interfacing of renewable sources and loads, aggregated in dc and ac nanogrids or microgrids, with smart power systems, providing enhanced power quality, stability, and flexibility of operation. [ABSTRACT FROM AUTHOR]

This paper presents research results of voltage gain number control methods of a power electronic resonant zero-current switching dc–dc switched-capacitor (SC) voltage multiplier (SCVM) in a topology with fault tolerance capability (FSCVM). The converters operated under the basic switching patterns are constant-voltage-gain dc–dc series-parallel resonant SC converters. However, this paper presents a method for the output voltage regulation by a special switching strategy. A variable voltage gain is made possible by selecting the number of active switching cells, using an appropriate control method. This creates a set of voltage ratio ranges. In addition, the proposed topology and method of control enable a fault-tolerant operation of the FSCVM under various failures of the device. The paper presents an analysis of the concept, simulation results and an experimental verification in a three-cell FSCVM dc–dc resonant boost converter operating at a 200-watt charge. Issues regarding the selection of components related to the problem of inrush currents that can occur during the voltage ratio increase are also analyzed in this paper, and the design for low inrush currents is demonstrated. [ABSTRACT FROM AUTHOR]

This paper presents a decoupled power control strategy for a modular multilevel converter (MMC)-based hybrid ac–dc grid integrated with a hybrid energy storage system. This system can mitigate the active power fluctuations caused by intermittent renewable generation and also realize reactive power compensation as required by voltage regulation. The proposed hybrid ac–dc system has a novel configuration in which batteries and supercapacitors are distributed into the upper arm and the lower arm of the MMC, respectively. Therefore, it is possible to design fully decoupled power control and simultaneously optimize the performances of batteries and supercapacitors. First, the system modeling and operational principles of the MMC are briefly introduced in this paper. On the basis of these analyses, the power decoupling between the upper arm and the lower arm is presented to realize different control objectives. Moreover, battery state-of-charge balancing control and supercapacitor voltage control are also presented, which are crucial for ensuring the normal operation of these energy storage components and the proposed MMC-based hybrid ac–dc grid. Finally, the proposed system and its control strategies are verified by laboratory experimental results. [ABSTRACT FROM AUTHOR]

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

Standard pulsewidth modulation (PWM) schemes for modular multilevel converters (MMC) have neglected deviations of the module voltages from the mean module voltage of an MMC arm as well as the dynamic change in the module voltages since their last measurement and during the switching period. This induces an output voltage error. Large output voltage errors disturb the higher level closed-loop control of the MMC, such that control targets cannot be met anymore. The output voltage error can be reduced using advanced PWM methods that predict and include the dynamic change of the individual module voltages during a switching period in the calculations for the duty cycle. This paper shows measurement results obtained from an experimental medium-voltage prototype, proving that advanced methods lead to much lower output voltage errors than the standard methods, but cannot fulfill the expectations raised by the simulations documented in the literature. In order to overcome the limitations of the old method, a new method that includes the semiconductor voltage drop and parasitic resistances is introduced and evaluated with measurements in this paper. Furthermore, simulation results showing the influence of the considered modulation methods on the closed-loop current control performance are presented. [ABSTRACT FROM AUTHOR]

This paper presents two capacitor voltage balancing (CVB) strategies for modular multilevel converter (MMC) applications. Both balancing schemes are based on model predictive control and are designed to efficiently solve a constrained optimal control problem, where the predicted capacitor voltage errors are included in the cost function with the demanded output voltage of a cluster being forced through an equality constraint. The first method proposed in this paper computes specific modulation indexes for each module using the explicit solution of a relaxed version of the original optimization problem. The second approach proposed in this paper reduces the complexity of the original problem by linearizing the objective function and using an optimal sorting network based on a greedy algorithm to solve this approximation. Considering the structures of both solution approaches, they are integrated into modulation schemes based on phase-shifted and level-shifted pulsewidth modulation algorithms, respectively. Experimental results obtained from a nine-cell single-phase MMC prototype demonstrate the good performance achieved with the proposed methodologies, as well as the implementation simplicity offered by the proposed CVB algorithms. [ABSTRACT FROM AUTHOR]

CLLC-type dc transformer (CLLC-DCT) is very popular in the hybrid ac–dc microgrid thanks to its high-power density advantage and good bidirectional power transfer capacity. In the hybrid ac/dc microgrid, the open-loop control is always utilized by the CLLC-DCT to cooperate with the bidirectional interlinking converter to realize the power and voltage conversion between the ac and dc bus. This paper first studies the circuit parameters design of the open-loop controlled CLLC-DCT with consideration of such a realistic problem: The real inductors/capacitors values are actually different with their theoretically designed values due to the operation power and temperature variation. To solve this problem, a robust circuit parameters design scheme is proposed for the CLLC-DCT in this paper. With the proposed scheme, the designed CLLC-DCT exhibits good power transmission and voltage regulation ability in the hybrid ac/dc microgrid even when its actual inductors/capacitors values vary with the practical power and temperature. The robust design method is experimentally verified in a hybrid ac/dc microgrid prototype. [ABSTRACT FROM AUTHOR]

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

Increasing penetration level of distributed generation (DG) units necessitates their participation in supporting power system stability and providing ancillary services. This paper highlights the necessity of supporting the connection voltage by these units under short-term unbalanced voltage sags. To address this, a new regulation scheme, named asymmetrical ride-through (ART), is proposed in this paper. The proposed scheme enforces DG units to properly regulate the voltage within the dynamic limits for three important voltage parameters: positive sequence, negative sequence, and phase voltage magnitude. The main advantages of applying the ART scheme are avoiding unnecessary outages due to temporary unbalanced faults and enhancing the grid stability. As the second contribution, an advanced dynamic voltage support method is also proposed to accurately address the specifications determined in the ART scheme. The successful results of the ART regulation scheme and the proposed dynamic voltage support method are verified using simulation and experimental test cases. [ABSTRACT FROM AUTHOR]

In a sensorless vector control method, at low speeds and self-commissioning for AC motors, it is necessary to model dead time, rise/fall time, parasitic capacitors, and semiconductors’ voltage drop accurately. In this paper, voltage drop in each phase due to the nonlinear behavior of the inverter from the control system's viewpoint is precisely modeled by using two lookup tables. These lookup tables make the proposed model robust against the dc-link voltage and carrier frequency variations in the entire current range. In conventional methods at standstill, the measured voltage in the dc current test contains the nonlinearity of two phases. Therefore, to obtain the nonlinear voltage of each inverter phase, it is assumed that the values of inverter nonlinearity voltage at both high current and low current are equal. This simplification leads to distortion in measured voltage in the low currents. However, in this paper, the voltage drop equation of each phase is solved by numerical method that is realizable in conventional processors. The results obtained in actual voltage measurement show the superiority of the proposed method over traditional methods in the low currents. The proposed method is verified by using a voltage source inverter feeding a 2.2-kW three-phase induction motor. [ABSTRACT FROM AUTHOR]

Aiming at effective power management in microgrids with high penetration of renewable energy sources (RESs), this paper proposes a simple power control for the so-called second-generation, single-phase electric springs (ES-2), which overcomes the shortcomings of the existing ES control methods. By the proposed control, the unpredictable power generated from RESs is divided into two parts, i.e., the one absorbed by the ES-2 that still varies and the other injected into the grid that is controllable by a simple and accurate signal manipulation that works both at steady-state and during RES transients. It is believed that such a control is suitable for the distributed power generation, especially at domestic homes. In this paper, the proposed control is supported by a theoretical background. Its effectiveness is at first validated by simulations and then by experiments. To this purpose, a typical RES application is considered, and an experimental setup is arranged, built up around an ES-2 implementing the proposed control. Testing of the setup is carried out in three steps and proves not only the smooth operation of the ES-2 itself, but also its capability in running the application properly. [ABSTRACT FROM PUBLISHER]

Essential imbalance and fluctuation of the mobile train load draw some problems, e.g., three-phase unbalance, reactive power, voltage and load fluctuation, harmonics, etc. in both public and train supply networks (TSNs). Railway power flow controller (RPFC)'s good performance on dealing with those problems has been convinced in practice these years. However, as the primary task in commercialization, how to qualitatively determine RPFC's design capacity with high cost efficiency and how to comprehensively assess its performance in the design stage have perplexed engineers for many years. To solve those problems in both planning and real-time executing processes, this paper focuses on an RPFC design capacity optimization strategy for grid-connection compatibility and operating performance improvement in terms of both the utility and the TSN. The strategy not only makes a railway substation satisfy the grid-connection standards with high calculating efficiency, but also renders the RPFC to have the minimum design capacity, the excellent feeder voltage control ability, the good fixed capacitor installing prospect, and the high operating efficiency on the targets of reducing the investment and enhancing the transportation ability. The generic models, the industry-oriented design and the quantitative analysis, discussed in this paper give an insight for those concerned based on a balance transformer. Future RPFC projects on serviced railway power systems will benefit from the proposed methodology. [ABSTRACT FROM PUBLISHER]

In this paper, the model predictive current control is introduced in the semicontrolled open-winding permanent magnet synchronous motor (OW-PMSM) generation system with a common dc bus to suppress the zero-sequence current for the first time. Particularly, the basic voltage vectors of the semicontrolled OW-PMSM, which are synthesized by the three-phase full-controlled converter and the three-phase uncontrollable diode bridge, is analyzed and their influences on the zero-sequence current are investigated for the first time in this paper. Then the current-error-based cost function that includes dq-axis current error and zero-sequence current error is designed to evaluate all the voltage vectors. Moreover, to further improve the steady-state control performance of the whole generation system, a zero-sequence current suppression method based on the uncontrollable side voltage vector adjustment, which has the advantage of low computation burden, is developed. In this method, two voltage vectors of the controllable converter are applied during one control period. Based on the distribution character of the basic voltage vector in the semicontrollable OW-PMSM generation system, the first voltage vector is selected according to the three-phase current direction to adjust equivalent vector of the uncontrollable diode bridge, whereas for the second voltage vector, the selection method based deadbeat voltage vector sector is designed to reduce the range of the candidate voltage vector. In addition, the vector durations are also computed. Finally, simulation and experiment are carried out to validate the proposed control approaches. [ABSTRACT FROM PUBLISHER]

This paper presents a uniform control strategy for the bidirectional ac/dc interlinking converter (BIC) in hierarchical controlled hybrid microgrid. The uniform control strategy unifies different control structures for multifunctional BIC in terms of power management, ac and dc voltage support. With the unified control structure, various triggering mechanisms for BIC mode-switch are not compulsory in response to the relevant scenario changes in the power network. Therefore, the negative consequences, such as unsmooth transition and system collapse due to the inaccurate or slower mode switch, can be avoided. Moreover, the uniform control strategy is also applicable for the hierarchical controlled hybrid microgrid, whose control architecture is typically comprised of the centralized and decentralized levels. This enables the communication fault ride-through capability and the impact from the communication failures can thus be mitigated. To sum up the above two features, the proposed control strategy is able to provide a smooth operation for the BIC without control mode switch in the event of unintentional scenario changes in power or communication network. To verify the validity of the proposed uniform control strategy, the controller hardware-in-the-loop experimental results have been provided in this paper. [ABSTRACT FROM PUBLISHER]

The steady-state performance of modular multilevel converter (MMC) can be optimized by the following methods: circulating current injection, third-harmonic signal injection, and average capacitor voltage control. However, most of the papers, due to the deficiencies of their MMC models, are limited to the study of adopting just one method and only optimizing a single-aspect of the performance. Thus, this article proposes a general MMC model for the comprehensive study of the steady-state performance optimization. The general model is built based on an equilibrium equation between the internal quantities and the external quantities of the MMC. This general model is capable of analyzing the optimization where the optimizing methods are combined and used to improve the multiaspect performance of the MMC. In addition, more influence factors are taken into account; hence, the calculation error can be smaller than 1% and even the minor effect of each optimizing method can be reflected. The proposed model is compared with traditional models; and it is found that both dc and fundamental components in modulation signals will be affected by the injected circulating current, rather than the constant values in the traditional models. Finally, the proposed MMC model is verified by both simulation and experiment. [ABSTRACT FROM AUTHOR]

The second harmonic current compensator (SHCC) can be added into the single-phase converters for compensating the second harmonic current (SHC) and thus removing the undesired electrolytic capacitor. In this paper, a virtual impedance, which is in the form of a capacitor and a resistor connected in parallel, is introduced to be in parallel at the bus-side port of the SHCC for effectively compensating the SHC while guaranteeing the system stability. This virtual parallel impedance is realized by feeding forward the bus-side port voltage of the SHCC, and thus the SHCC can be modular designed. The closed-loop parameter design of the SHCC is also presented. A 3.3-kW prototype of a two-stage single-phase power factor correction converter is built and tested in the laboratory, and the experimental results verify the effectiveness and feasibility of the proposed control method. [ABSTRACT FROM AUTHOR]

This paper presents an improved saliency-based position sensorless drive of an interior permanent-magnet synchronous machine (IPMSM) with a single current sensor, used for low cost applications. The phase current, reconstructed from a dc-link current, shows a reconstruction error resulted from the difference of the sampling point from that of a full current sensor drive. This reconstruction error results in degradation of the overall sensorless control performance. In this paper, we analyze the impact of the reconstruction error depending on the injection frequency for a high frequency voltage signal injection sensorless control. Based on this analysis, we propose an improved position sensorless drive that employs a new current reconstruction method. In order to minimize the reconstruction error, the proposed current reconstruction method predicts the dq-axis currents with the same sampling point as a full current sensor drive. Several experimental results are provided to verify the analysis of the reconstruction error and the proposed method's improved sensorless control performances. [ABSTRACT FROM PUBLISHER]

This paper analyzes the performances of different carrier phase-shifting pulse width modulation (PWM) techniques to be used with a multilevel cascaded H-bridge converter in case of unbalanced operational conditions. In fact, in many practical applications, the ideal condition of equal dc voltages and equal reference signals for each H-bridge cannot be achieved. In such conditions, the conventional carrier phase-shifting PWM technique loses its harmonic canceling capabilities and then the multilevel ac voltage harmonic quality is deeply affected. To overcome this limit of the original technique, different variations have been proposed. All of them still rely on the carrier phase-shifting concept and propose to use a different value of the shifting angle for each carrier (unlike the original technique) whenever unbalanced operational conditions occur. In this paper, the three main solutions proposed over the last years to extend the capabilities of the carrier phase-shifting PWM technique are compared. The analysis is focused on a three-cell cascaded H-bridge converter. Simulation and experimental results are presented. [ABSTRACT FROM PUBLISHER]

Tap changers are commonly used to maintain the limits in medium-voltage lines. Initially based on electromechanical switches, efforts have been made since the 1990's to reduce maintenance expenses with thyristor-based versions. Among the methods to limit the short-circuiting current in commutating taps, the multiwinding coil has demonstrated, during decades of use in the low-voltage field, to be reliable and cheap. Two previous papers showed that multiwinding changers can be applied to medium-voltage lines up to 34.5 kV, 15 MVA with no parallel or series thyristor association. This paper proposes a new topology, based on a double array of tap switches per phase, allowing for step-up the power from 15 to 36 MVA and insertion in any point of the line with no connection to the head transformer. A comparative cost analysis gives technical and economical results for a typical 10 MVA, 34.5 kV stabilizer demonstrating the advantages of the proposed circuit over the electronic single array topology and the classic mechanical solutions. [ABSTRACT FROM PUBLISHER]

This paper analyzes the combination in a cascaded H-bridge multilevel inverter (CHBI) based on transformers with the different turn ratios for increasing the voltage level and proposes the switching method for achieving the output voltage distribution among H-bridge cells (HBCs). The transformers used in this paper are connected to the output of the respective HBCs, and the secondary sides of all the transformers are connected in series for generating the final output voltage. Only one of the transformers, in particular, has a different turn ratio for increasing the output voltage level. In this paper, the possible turn ratio of the special transformer with a different turn ratio is discussed in detail, and a switching method based on the level-shifted switching method for the topology used in this paper is proposed. To verify the effectiveness of the proposed method, a three-phase 21-level CHBI is experimentally tested. [ABSTRACT FROM PUBLISHER]

In applications that demand single-input dual-output (SIDO) or dual-input single-output (DISO) dc–dc converters, employing two separate single-input single-output (SISO) converters is a solution. However, the number of components is doubled, resulting in high overall cost. In order to reduce costs, this paper proposes a novel topology synthesis methodology, with which a variety of SIDO and DISO dc–dc converters with reduced components can be derived. The principle of topology synthesis states that integrated SIDO and DISO dc–dc converters can be easily developed from conventional SISO converters by replacing a diode with a basic cell inclusive of additional input/output port. The principle is effective for many SISO dc–dc converters, and as an example, topology synthesis based on buck, boost, buck–boost, Cuk, sepic, and zeta SISO converters is performed in this paper. In order to achieve better understanding of the proposed converters, the integrated SIDO Cuk converter is specifically analyzed and experimentally verified. In comparison with the conventional scheme of two separate SISO Cuk converters, good cross regulation is retained while the number of diodes, inductors, and capacitors is reduced in the proposed SIDO Cuk converter. In addition, zero-voltage-switching operation of one switch is achieved, contributing to lower switching losses. Finally, a prototype circuit with 48-V input and 156 V/1 A, 24 V/4 A outputs is built to validate the theoretical analysis. [ABSTRACT FROM PUBLISHER]

To improve light load efficiency, a three-phase photovoltaic (PV)-based microinverter may operate in single-phase, two-phase or three-phase mode depending on the available input power. Therefore, its dc-link capacitor may be subjected to large peak-to-peak voltage ripple when operating in phase skipping mode. This paper determines the minimum value of the dc-link capacitor by allowing the maximum acceptable voltage ripple on the dc link. This allows for the use of film capacitors instead of electrolytic capacitors in order to improve the microinverter reliability. Unfortunately, higher dc-link voltage ripple introduces harmonic distortion on the inverter output current waveform if it is not compensated for by the dc-link voltage controller. This paper proposes a simple synchronous control method to regulate the dc-link voltage with the presence of large voltage ripple. The dc-link voltage controller is synchronized with the phase-locked loop and samples the average value of dc-link voltage without introducing unwanted harmonics into the voltage sense path. Experimental results were obtained from a 300-W three-phase half-bridge microinverter prototype to verify the proposed dc-link voltage control performance. Inverter output current total harmonic distortion of 1.6% was achieved even though the dc-link peak-to-peak voltage ripple reached as high as 87 V with 219-V split dc-link voltage. Dynamic response is fast enough to easily track the most extreme ramp change in input PV power. [ABSTRACT FROM PUBLISHER]

This paper presents a new modulation and control strategies for the high-frequency link matrix converter (HFLMC). The proposed method aims to achieve controllable power factor in the grid interface as well as voltage and current regulation for a battery energy storage device. The matrix converter (MC) is a key element of the system, since it performs a direct ac to ac conversion between the grid and the power transformer, dispensing the traditional dc-link capacitors. Therefore, the circuit volume and weight are reduced and a longer service life is expected when compared with the existing technical solutions. A prototype was built to validate the mathematical analysis and the simulation results. Experimental tests developed in this paper show the capability of controling the grid currents in the synchronous reference frame in order to provide grid services. Simultaneously, the battery current is well regulated with small ripple, which makes this converter ideal for battery charging of electric vehicles and energy storage applications. [ABSTRACT FROM AUTHOR]

Digital current controllers have the key impact on the performance of grid-side converters and ac drives. The voltage disturbances are commonly suppressed by enhancing the controller with an inner active resistance feedback. In cases where the switching noise and parasitic oscillations introduce sampling errors, conventional sampling is replaced by the oversampling-based error-free feedback acquisition which derives the average of the measured currents over the past switching period. The time delay introduced into the feedback path creates difficulties in designing the current controller with the active resistance. In this paper, we introduce a novel structure of the current controller, which includes the error-free sampling and the active resistance feedback. Devised structure improves the disturbance rejection by extending the range of permissible values of the active resistance. Controller structure is based on the internal model principles, and it maintains the input step response unaffected. This paper comprises analytical design, the gain setting procedure, computer simulation, and experimental results obtained from an experimental setup with a three-phase inverter, digital controller, and a permanent-magnet synchronous motor. [ABSTRACT FROM PUBLISHER]

The stability of the grid-connected inverters adopted in distributed energy resources (DERs) highly depends on the characteristics of the grid impedance. Hence, various active damping methods and adaptive control algorithms have been proposed for the control of the grid inverters. Differently from those existing solutions, this paper proposes to use a smart transformer (ST) to shape the grid impedance to interact with the controllers of local inverters, aiming at improving the stability of the local controllers as well as the overall grid. A main advantage of this solution is that it offers a pervasive service to all the available DERs, reducing the cost and design effort of the local controllers. Three active damping methods are presented in this paper and the implementation issues in the ST LV-side voltage controller are given. Moreover, the design criteria of the active damping methods considering the control performance of ST and the stability requirements of local DERs are presented. Analysis and experiments are carried out to verify the effectiveness of the proposed solution. [ABSTRACT FROM PUBLISHER]

Rising demand for distributed generation based on renewable energy sources (RES) has led to several issues in the operation of utility grids. The microgrid is a promising solution to solve these problems. A dedicated energy storage system could contribute to a better integration of RES into the microgrid by smoothing the renewable resource's intermittency, improving the quality of the injected power and enabling additional services like voltage and frequency regulation. However, due to energy/power technological limitations, it is often necessary to use hybrid energy storage systems (HESS). In this paper, a second-order sliding mode controller is proposed for the power flow control of a HESS, using a four-leg three-level neutral-point-clamped (4-Leg 3L-NPC) inverter as the only interface between the RES/HESS and the microgrid. A 3-D space vector modulation and a sequence-decomposition-based ac-side control allow the inverter to work in unbalanced load conditions while maintaining a balanced ac voltage at the point of common coupling. DC current harmonics caused by unbalanced load and the NPC floating middle point voltage, together with the power division limits, are carefully addressed in this paper. The effectiveness of the proposed technique for the HESS power flow control is compared to a classical PI control scheme and is proven through simulations and experimentally using a 4-Leg 3L-NPC prototype on a test bench. [ABSTRACT FROM PUBLISHER]

This paper presents three unidirectional step-up ac–dc converter topologies with voltage multiplication on their outputs. The first topology is the hybrid boost rectifier and the other two are novel bridgeless structures, all based on the integration of boost rectifier and the ladder switched-capacitor (SC) cell. The new hybrid structures provide high voltage gain with low voltage stress across the switches, high-power factor, and output voltage regulation. This paper presents the principle of operation, steady-state and dynamic analysis, design methodology, and experimental verification. A different approach is also presented for the design methodology of the SC cell and for the analysis of its influence on the dynamic model. Three 1000-W prototypes with a 220-V input voltage, 800-V output voltage, and 100-kHz switching frequency were designed to corroborate the theoretical study. The best efficiency was 97.1$\%$ in the proposed bridgeless versions and the voltage stress across the switches stands around 400 V. Furthermore, a comparison between the proposed structures and previous topologies is also presented. [ABSTRACT FROM PUBLISHER]

This paper presents a method for small-signal dynamic studies of power systems including utility-scale PV energy systems considering the effects of dc-side components. Existing methods commonly model a PV energy system as a dc source connected to a grid-tied inverter. This paper shows that the dc components of a PV energy system can also contribute in the dynamics of the power system in the range of small-signal studies. The paper first develops the model of a PV energy system by merging the submodels of dc- and ac-side components. Then, a method to obtain the augmented model of the power system including a PV energy system will be explained using a study system. The study system consists of a PV energy system in parallel with a synchronous generator connected to a grid. Then, based on eigenvalue analysis, verified by time-domain simulations, it is shown that the dc-link capacitor, front-end converter, and its controller can introduce oscillatory modes in subsynchronous range of frequencies. Furthermore, it is shown that the damping of the sensitive modes can significantly vary with respect to the parameters of the dc-side components. [ABSTRACT FROM PUBLISHER]