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An oil-paper insulation system which will withstand DC voltage under normal conditions is a key component of converter transformers as well as high voltage direct current (HVDC) bushings. When the system changes its state, operating overvoltage appears. This kind of overvoltage will consequently affect the application of oilimmersed facilities and the safety of entire grid. Therefore, this paper studies the charge coupling behavior of double-layer oil-paper insulation under DC and pulse voltages. From the research of different pulse voltage amplitudes, pulse numbers and polarity, conclusions can be drawn that, with the increase of the pulse voltage amplitudes, DC and pulse voltages in the same polarity cause the absolute value of the initial surface potential to decrease and then rise; while in different polarities, the charge with the same polarity as the pulse voltage increases. Furthermore, for the same pulse voltage, the absolute initial value of the DC voltage applied in the same polarity is higher than that in different polarities. Considering the pulse number effect, it is obvious that the initial value increases with the increase in the pulse number, and then reaches saturation. Also, with the increase in pulse number, the decay rate decreases and thus the charge is slowly dissipated. From the tdV/dt and the decay time curves, the peak value increases with the increase in the pulse number, while the characteristic time varies by the polarity of the DC and pulse voltages. [ABSTRACT FROM AUTHOR]

Negative-sequence current is required to be intentionally injected to support the power grid or compensate the imbalanced load in asymmetrical faults utilizing STATCOM. However, the extreme compensation range of negative-sequence current injection becomes complicated when grid voltage becomes asymmetrical. This article presents a novel off-line strategy for negative-sequence current compensation under two typical kinds of asymmetrical grid faults — single-phase voltage drop and double-phase voltage drop, with the following two benefits simultaneously: maximizing negative current compensation capacity; avoiding iterations and reducing computation burdens. In this paper, the mechanism of DC voltage balancing among three phases in STATCOM is analyzed and a new identification method of negative-sequence current compensation range is explored out. To simplify the calculation, this paper takes the ratio of negative-sequence reactive (Iq-) and active (Id-) current as a variable (s). In this way, the relationship between the extreme compensation range, asymmetrical voltage sag depth of power grid h is drawn with MATLAB. Different from the previous iterative method, the off-line identification method is applied to avoid huge iterations. By fitting the curve, the precise ranges of negative-sequence current compensation under different asymmetrical grid faults are approximated as simple polynomials, thus significantly reducing the computation burdens. Simulation and experimental results verify the practicability of the proposed compensation method. [ABSTRACT FROM AUTHOR]

Voltage-controlled three-phase three-leg inverters are widely applied in various occasions. The inverters are generally switched to the current-controlled mode to limit the fault currents when short-circuit faults happen. Under symmetrical fault, the inverter can be controlled as a symmetrical, three-phase current source. However, the voltage limiting will happen under asymmetrical fault. As a result, the fault phase currents will be distorted and cannot be completely controlled. Considering that the voltage limiting is mainly caused by large load impedance of healthy phase under asymmetrical fault, the voltage limiting is avoided by paralleling the virtual impedance with the output filter capacitor in this paper. As a result, the current limiting loop is not broken and the fault phase currents can be effectively controlled. The implementation of virtual impedance in control system and the influence of virtual impedance on the stability of system are also analyzed in detail in this paper. The theoretical results are validated by experiments. [ABSTRACT FROM AUTHOR]

Electrical equipment of the floating nuclear power plant operates in a long-term vibration environment. Ground faults occur frequently and are often accompanied by arc, a reliable arc suppression algorithm for the generator stator winding is necessary. In this paper, an active arc suppression method is proposed by connecting a controllable voltage source to the generator neutral point. Through regulating the neutral point voltage to be equal in amplitude and opposite in phase to the fault potential, the fault point voltage can be clamped to a low level to achieve arc suppression. Since the generator's phase potential contains certain harmonic components, the controllable voltage source includes fundamental and third-order harmonic components. To accurately calculate the output voltage, online calculation methods for the fault location, fault potential and grounding capacitance are proposed. The accuracy of the fault location method is verified by simulation and dynamic test. And simulation results based on PSCAD/EMTDC software show that the proposed method can achieve effective arc suppression at different ground fault conditions. [ABSTRACT FROM AUTHOR]

This paper presents a novel hierarchical cooperative control strategy to solve the problems of unbalanced State of Charge (SoC), unreasonable load current sharing, and unstable bus voltage for battery storage system (BSS) in islanded DC microgrid. In the communication layer, a neighbor-to-neighbor communication network architecture is constructed in which each battery storage unit (BSU) only communicates information with neighboring nodes, and a multi-agent consensus algorithm is used to iteratively estimate the relevant average global variables. In the main control layer, the droop coefficient is dynamically adjusted by constructing a fast SoC convergence function to ensure that the load current is reasonably distributed based on the rated capacity of each BSU. In the secondary control layer, a virtual voltage drop equalization controller and an average bus voltage compensation controller are designed to eliminate the impact of line impedance mismatch and compensate the average bus voltage. Furthermore, an optimized factor is designed to accelerate the later stage of SoC convergence. Finally, a DC microgrid experimental platform based on StarSim HIL is built, and the results demonstrate that the proposed strategy can achieve the control objectives of fast SoC balance, proportional load current sharing, and bus voltage stability. [ABSTRACT FROM AUTHOR]

This paper presents a novel distributed voltage control method for multi-terminal direct current (MTDC) systems based on the average consensus algorithm. The proposed method combines the idea of secondary integral voltage control and regulator synchronization problem. It realizes adjustable tradeoff between two conflicting control objectives, that is, voltage regulation and power sharing. The proposed control algorithm only needs a low-bandwidth communication between two adjacent voltage source converters (VSCs) and local information to work effectively. Moreover, regarding the system dynamic response, a parameter tuning method for the proposed controller is provided to achieve the fastest convergence of the distributed control algorithm. This paper also develops a small-signal dynamic model for VSC-MTDC systems with distributed controllers to evaluate the system dynamic performance. The proposed method is validated by time-domain simulation of a 4-terminal VSC-MTDC system. The simulation results show that flexible tradeoff between voltage regulation and power sharing can be achieved under necessary constraints on the DC bus voltages and power sharing ratio. [ABSTRACT FROM AUTHOR]

This paper emphasizes the importance of including unbalanced distribution systems for voltage stability studies in power systems. The paper aims to: discuss the various simulation methods for power system analysis; highlight the need for modeling unbalanced distribution system for accurate load margin assessment; demonstrate the influence of net-load unbalance (NLU) on voltage stability margin (VSM). We also share a T&D co-simulation interface with commercial power system solvers. The distribution system is evolving rapidly with high proliferation of distributed energy resources (DERs); these are not guaranteed to proliferate in a balanced manner and uncertainty resulting due to these DERs is well acknowledged. These uncertainties cannot be captured or visualized without representing the distribution system in detail along with the transmission system. We show the impact of proliferation of DERs in various 3-phase proportions on voltage stability margin through T&D co-simulation. We also study the impact of volt/VAR control on voltage stability margin. This analysis is only possible by representing the distribution system in detail through T&D co-simulation. Higher percentage of net-load unbalance (NLU) in distribution system aggravates the voltage stability margin of the distribution system, which can further negatively influence the overall voltage stability margin of the system. [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]

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]

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]

Operating in the high-speed range is necessary for high-performance permanent-magnet synchronous motor (PMSM) drives. However, due to the back electromotive force effect, the PMSM is approaching the voltage limit at field decreasing scope. This paper presents a new flux-weakening scheme along with an improved vector control strategy to alleviate the influence of this problem. Control parameters of the anti-windup proportional and integral (AWPI) controller are optimized off-line in relying on an adaptive velocity particle swarm optimization (AVPSO) algorithm. The AVPSO algorithm considers the summation of AWPI measurement error which is the objective function of the optimization problem without knowing the transfer function of the plant. Hence, the tuned flux-weakening controller with a filter is used to set the flux level without saturating the current controllers. Meanwhile, the other controllers of inner and outer loops award a great dynamic and steady-state performance for the PMSM. In the proposed scheme, the flux-weakening control is not dependent on machine parameters that adapts the flux level automatically and provide a fast transition between the constant torque region and the field-weakening region. Effectiveness and advantages of the proposed scheme are presented in this paper through both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Variability in power generation at multiple consumer-owned renewable energy sources can cause random voltage fluctuations, which may result in voltage violations at multiple nodes in a power distribution system. Traditional methods of voltage control are ineffective to address fast voltage variations. New dynamic methods of voltage control using end-user devices can significantly benefit from the knowledge of the dominant influencer of voltage fluctuation (DIVF). This paper introduces information theoretic metrics to identify DIVF for every node in the power distribution system. A node can be DIVF due to its location in the distribution system and its capacity to generate and change its power. Use of traditional power flow algorithms to identify the DIVF is computationally complex, which limits their use in real-time applications. In this paper, information theoretic indicators of the DIVF are computed analytically in a computationally efficient manner, and their effectiveness to identify the DIVF is tested using the IEEE 69 node test system. From simulations, it is shown that differential entropy, Kullback–Leibler distance, and Frechet distance are effective indicators of the DIVF and can successfully identify the most influential nodes providing the foundation for intelligent voltage control. [ABSTRACT FROM AUTHOR]

This paper presents a novel seven-level inverter topology for medium-voltage high-power applications. It consists of eight active switches and two inner flying capacitor (FC) units forming a similar structure as in a conventional active neutral-point-clamped (ANPC) inverter. This unique arrangement reduces the number of active and passive components. A simple modulation technique reduces cost and complexity in the control system design without compromising reactive power capability. In addition, compared to major conventional seven-level inverter topologies, such as the neutral point clamped, FC, cascaded H-bridge, and ANPC topologies, the new topology reduces the dc-link voltage requirement by 50%. This recued dc-link voltage makes the new topology appealing for various industrial applications. Experimental results from a 2.2-kVA prototype are presented to support the theoretical analysis presented in this paper. The prototype demonstrates a conversion efficiency of around 97.2% ± 1% for a wide load range. [ABSTRACT FROM AUTHOR]

This paper presents a neutral-point deviation and ripple compensating control method applied to a three-level neutral-point-clamped (NPC) converter. The neutral-point deviation and its harmonic components are analyzed with a focus on the average current flowing through the neutral-point of the dc link. This paper also proposes a control scheme compensating for the neutral-point deviation and dominant harmonic components under generalized unbalanced grid operating conditions. The positive and negative sequence components of the switching functions and ac input currents are employed to accurately explain the behavior of three-level NPC converter. Simulation and experimental results are presented to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents the design and implementation of a novel low-cost speed sensorless control technique for a permanent magnet assisted synchronous reluctance motor (PMa-SynRM). A robust adaptive speed (RAS) observer is designed for estimating the rotor speed and position, respectively. The RAS observer estimates the PMa-SynRM speed and position from the back electromotive force space-vector determination without voltage sensors by using the reference voltages issued from the current controllers instead of the actual ones. The novelty of the proposed RAS estimation technique is the adaptation of the feedback error of the actual values. Thus, the proposed observer structure promises a higher degree of robustness against PMa-SynRM parameter changes in the sensitivity analysis presented in this paper. The dynamic model and experimental realizations of the proposed control technique are introduced. A 6-kW PMa-SynRM drive test setup is constructed including a dSPACE 1104 board as the control heart of the proposed system. The hardware-in-the-loop Typhoon HIL 402 device is used to experimentally emulate the PMa-SynRM and the inverter connected to a dSPACE MicroLabBox. The proposed RAS observer robustness is evaluated using a HIL model and experimental results under different conditions. A comparative experimental analysis between the proposed RAS and Luenberger observers has been performed. [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]

The voltage-balance mechanism is a required ability to simplify the control scheme for modular input-series and output-series ($\text{ISOS}$) connection, in spite of the fairly large number of studies, such mechanism has not been addressed clearly and satisfactory. This paper proposes a methodology, based on the converter output characteristics, to standardize the analysis of voltage-balance mechanism in steady state, which allows understanding the behavior of voltage sharing among the modules in steady state, in the presence of mismatched parameters for $\text{ISOS}$ connection of modular converters. In addition, the $\text{ISOS}$ connected modular two-switch flyback converter is presented. The intrinsic voltage-balance mechanism, even operating in continuous conduction mode, make this converter feasible, as an alternative to the predecessors modular converters. The operation and analysis of the proposed connection were corroborated, based on the experiment carried out on a laboratory prototype with three modules: ${\text{1}}, {\text{200 V}}_{\text{dc}}$ total input voltage, ${\text{1.5 kW}}$ rated power, and $\text{50 kHz}$ switching frequency. This paper is accompanied by a video demonstrating the operation, in steady state, of the laboratory prototype. [ABSTRACT FROM AUTHOR]

This paper describes a dc-link voltage control method of a single-phase back-to-back cascaded H-bridge inverter (SBCI) for a medium-voltage motor drive system. The main advantage of the SBCI topology over the conventional regenerative cascaded H-bridge topology with a three-phase active front-end (AFE) is a simple system structure, which is composed of an input transformer, a power cell, a current sensor, etc. However, the challenging points of the SBCI are larger voltage ripple in the dc-link capacitor and imbalance of dc-link voltages of each phase. The asymmetric dc-link voltage of each power cells could cause unstable operation such as over-modulation due to the lack of the dc-link voltage of a particular phase and result in over-voltage or under-voltage faults. In this paper, the control strategy of the dc-link voltage for the SBCI that uses the negative-sequence voltage of the converter is described. The proposed control method is verified with a computer simulation whose target is a 6.6-kV–1.25-MW medium-voltage drive system. Also, through the experimental setup with the prototype SBCI whose power rating is 16.2 kVA, the dc-link voltage of each AFE has been controlled within a 0.5% error of its reference value at the full load. [ABSTRACT FROM AUTHOR]

High-frequency overvoltages are most dangerous for the insulation of transformer coils. It is because high-frequency pulse voltage creates an uneven distribution of voltage across the windings. The coil's first turns receive a disproportionately high voltage drop, leading to the inter-turn circuit. The paper analyzes the types of lightning pulses and proposes a new suppression method based on the anomalous skin effect in a multilayer material and the design in the form of a coil. The developed device is simulated, and the results of overvoltage monitoring at the substation with the installed high-frequency overvoltage suppression device are analyzed. [ABSTRACT FROM AUTHOR]

In practical power systems, it is still very challenging to figure out a cost-effective undervoltage load shedding (UVLS) scheme that can reliably and adaptively react to the short-term voltage stability (SVS) problem. Faced with this challenge, this paper develops an intelligent data-driven predictive UVLS scheme for online SVS enhancement. Inspired by valuable ideas in model predictive control and supplementary excitation control in power systems, a novel deep feedback learning machine (DFLM) is designed to precisely predict future voltage violations after UVLS execution. With the help of the DFLM, the UVLS scheme is aware of potential effects of various candidate UVLS actions. Owing to this desirable nature, it can adaptively respond to diverse SVS conditions and optimize UVLS decisions in a non-iterative way. Further, two well-designed strategies, i.e., stepwise constraint relaxation and incremental DFLM adaptation, are introduced to enhance the scheme's reliability and adaptability during online application. Numerical test results on the Nordic test system and the realistic North GZ Power Grid in China showcase the excellent performances of the proposed scheme. [ABSTRACT FROM AUTHOR]

The impedance analysis has been proposed and proved to be an effective way to analyze the grid-connected stability of Voltage Source Converter (VSC) based energy conversion systems. Most of the existing impedance analyzes are discussed based on vector oriented control, while there are few discussions about the impedance analysis of Direct Power Control (DPC). In this paper, an impedance modeling approach of VSC using the DPC is firstly proposed and analyzed, which fills the gap of impedance analysis of the DPC based converters and provides a basis for studying the corresponding grid-integration stability problems. Since there is no need for PLL and Park transformation, the impedance matrix is built directly using the $\alpha \beta$ reference frame and features symmetrical characteristics. The proposed impedance matrix can be transformed into a positive-sequence impedance and analyzed by using SISO Nyquist criteria. The stability of a weak-grid connected VSC using DPC is analyzed in this article. The main factors affecting system stability, including the grid and control parameters, are considered in the analysis. The effectiveness of the proposed method is demonstrated by the simulations carried out in the Matlab/Simulink Simscape Power Systems as well as a hardware-in-loop system. [ABSTRACT FROM AUTHOR]

This paper proposes advanced control strategies to enhance fault ride-through capability of modular multilevel converter-based high voltage direct current (MMC-HVDC) connected doubly-fed induction generator (DFIG)-based wind farms. A stepwise voltage drop control is proposed for the sending-end MMC to suppress dc line voltage fluctuations by demagnetizing DFIGs. Meanwhile, a modified transient current control is proposed for the DFIG, where a voltage-dependent active current control is integrated into the rotor-side converter to improve synchronization stability of the converter, and a feedforward transient stator current control structure is designed to eliminate dc component in the stator current. The proposed control strategies are validated by comparing with three typical methods through time-domain simulations. [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]

The photovoltaic (PV) output voltage varies over a wide range depending on operating conditions. Thus, the PV-connected converters should be capable of handling a wide input voltage range while maintaining high efficiencies. This paper proposes a new series resonant dc–dc converter for PV microinverter applications. Compared with the conventional series resonant converter, a dual-mode rectifier is configured on the secondary side, which enables a twofold voltage gain range for the proposed converter with a fixed-frequency phase-shift modulation scheme. The zero-voltage switching turn-on and zero-current switching turn-off can be achieved for active switches and diodes, thereby, minimizing the switching losses. Moreover, a variable dc-link voltage control scheme is introduced to the proposed converter, leading to a further efficiency improvement and input-voltage-range extension. The operation principle and essential characteristics (e.g., voltage gain, soft-switching, and root-mean-square current) of the proposed converter are detailed in this paper, and the power loss modeling and design optimization of components are also presented. A 1-MHz 250-W converter prototype with an input voltage range of 17–43 V is built and tested to verify the feasibility of the proposed converter. [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]

Phase-to-neutral voltage or neutral-to-virtual neutral voltage zero-crossing points (ZCPs) detection method is usually used for sensorless brushless dc motor commutation control. Unfortunately, neither of them can be realized in lower speed range. In this paper, a simple commutation point detection method is proposed based on detecting inactive phase terminal to dc-link midpoint voltage. It eliminates the requirement of neutral wire or virtual neutral voltage and provides an amplified version of back electromotive force at the ZCPs which makes the lower speed range detection possible. As the speed increases, commutation point error is enlarged due to the low-pass filter. Utilizing the symmetry of the terminal to midpoint voltage, the phase error can be corrected. However, due to the nonlinear relationship between the detected voltage difference and phase error, it is difficult to regulate the error fast and robustly. Therefore, a novel phase regulator based on fuzzy neural network is proposed in this paper with simple structure and learning ability. The validity of the proposed ZCPs detection method and commutation instant shift correction method are verified through experimental results. [ABSTRACT FROM AUTHOR]

The planned China fusion engineering test reactor (CFETR) needs one or more negative-ion-based neutral beam injectors (N-NBIs) with a beam energy of 500 or even 1000 keV. In order to prepare for CFETR, a prototype of CFETR N-NBI is being designed. The prototype needs an acceleration grid power supply (AGPS) rated at 200 kV/25 A with a maximum pulselength reaches to 3600 s. The AGPS adopted a single-stage inverter-type high-voltage power supply (HVPS). The output voltage ripple is an important specification of the AGPS, which can affect the efficiency and stability of N-NBI system by influencing the beam divergence. However, the conventional methods cannot be used to limit the output voltage ripple of the inverter-type HVPS used as AGPS of CFETR N-NBI prototype. This paper simplified the AGPS to an insulated three-phase three-level (TPTL) dc–dc converter connected with a dual dc link. The working process of the TPTL dc–dc converter with duty-cycle modulation has been analyzed, and two main continuous conduction modes are described. The influence of key parameters such as duty cycle of inverter, leakage inductance of transformer, and output current on output voltage ripple is analyzed. Based on the analysis results, a so-called combined control strategy was proposed to keep the output voltage ripple at a level. The proposed strategy is suitable for the cases where the object output voltage is below 175 kV. Some simulations were carried out by using MATLAB/Simulink. The results showed that the analysis in this paper is reliable and the proposed control strategy has a good performance. [ABSTRACT FROM AUTHOR]

This paper presents a practical strategy for utilizing the submodule (SM) redundancy of a modular multilevel converter (MMC) for its fault tolerance. This strategy provides a systematic framework for balancing the tradeoff between two conventional methods for using the active redundancy and, thus, achieves operational flexibility. One of the existing methods improves SM reliability owing to less voltage stress on the SM components by employing all of the SMs to form the ac or dc voltages (voltage-sharing mode). The other avoids transients by keeping the average SM voltage constant at the cost of slightly increased stress on the SM components (fixed-level mode), which, however, can be controlled to provide the grid-adaptive operation by reserving the energy of the SMs not in service. We, thus, develop a new redundancy management scheme by integrating these two methods and exploiting their technical benefits to meet the PQ requirements and MMC control performance. This research provides a theoretical basis and a technical guide to determining the number of SMs, which can further increase the voltage steps as per the MMC and grid conditions. This paper also connects the remaining PQ capability of the MMC at a particular operating point with the SM redundancy concept by defining a potential redundancy, especially useful when the physical redundancy is exhausted. The theoretical findings and efficacy of the proposed strategy are validated through PSCAD/EMTDC time-domain simulations followed by experiments using a nine-level single-phase MMC system. [ABSTRACT FROM AUTHOR]

This paper introduces an averaging control strategy of the phase-unit losses of modular multilevel converters (MMCs). The quantitative analysis of the MMC phase-unit loss distribution under unbalanced ac conditions is carried out by investigating the individual component of the MMC arm currents. It shows that the unbalanced arm currents will result in unbalanced phase-unit losses. To address this issue, this paper proposes a predictive closed-loop-control-based zero-sequence voltage injection method that results in much averaged loss distribution under unbalanced ac conditions. The essence of the new method is that it adjusts the three-phase sinusoidal references so that the unbalanced arm currents are fully utilized to average phase-unit losses. This paper shows the effectiveness of the new approach by electromagnetic transient simulations on PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

Grid-interactive inverters allow distributed generation units to provide various ancillary services in microgrids. As the linear modulation region of inverters is restricted by the dc-bus voltage, providing ancillary services may drive the fundamental positive-sequence inverter voltage to the overmodulation region. Therefore, to operate in the linear modulation region the maximum active- and reactive-power of the inverter may have to be reduced unless pulsewidth modulation (PWM) references are adjusted. In this paper, an atypical PWM method is proposed for maximizing dc-bus utilization of grid-interactive two-level voltage source inverters (2L-VSI) providing ancillary services. Two important ancillary services, i.e., negative-sequence compensation and harmonic compensation are considered in this paper. The proposed method increases the maximum active- and reactive-power that a 2L-VSI can deliver when providing ancillary services by injecting a common-mode component in the references computed based on instantaneous reference magnitudes. The validity of the proposed technique is verified through simulation, as well as experimental data obtained using a 208-V three phase grid-connected 2L-VSI. [ABSTRACT FROM AUTHOR]

Power management integrated circuits (PMICs) with on-chip compensation are widely used to power multiple loads in mobile devices with increased power density. However, there are two issues for on-chip compensated PMICs. First, on-chip compensation reconfigurability is required to obtain the appropriate response in various passive components. Second, a converter for a processor requires adaptive voltage positioning (AVP) to reduce the output capacitor size. In this paper, a novel reconfigurable on-chip compensated current-mode buck converter with AVP is proposed to solve the aforementioned issues without requiring high-speed and high-resolution analog-to-digital converter (ADC). A reconfigurable accurate load line control scheme and on-chip compensation are proposed to achieve an accurate load line and AVP in various load lines and passive components. The control scheme, small-signal model, and circuit implementation are illustrated in this paper. The proposed buck converter was implemented into an integrated circuit to verify the analysis. [ABSTRACT FROM AUTHOR]

This paper presents the digital implementation of a boundary controller with unipolar switching characteristic for single-phase voltage source full-bridge inverters. This paper expands the application of a second-order switching surface-based control method to unipolar switching of single-phase voltage source inverters (VSIs) using a finite-state machine. The finite-state machine has been formulated considering four different states of the inverter; positive,zero1, negative, andzero2. The second-order boundary control governs the current state of the system and provides proper switching action to keep the system within the desired reference. The control law is implemented digitally in F28m35x digital control card. A full-bridge inverter topology is used to achieve the three-level voltage switching. Various simulations and experiments were performed in a 550 VA, 120 V, 60 Hz VSI with a digitally implemented controller to verify the theoretical predictions. A high-quality voltage output was obtained for various loading conditions. The transient performance of the controller was investigated using a reference and load changes. A comparison of the implementation was made with the existing classical controllers to verify the fast-dynamic response of the system. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive optimization control scheme to improve efficiency and dynamic response of dual-active-bridge (DAB) dc–dc converters. Unified-phase-shift (UPS) control is widely used to increase the efficiency of DAB dc–dc converters by minimizing the peak current, but dynamic performance of the converters needs to be further enhanced. In this paper, to gain superior dynamic performance of DAB dc–dc converters, an equivalent power-balancing (PB) model is employed, which is capable of predicting dynamic behavior of converter output voltages due to input voltage fluctuation and load disturbance. And then, combining the UPS control and the PB control, a comprehensive UPS and PB (UPS-PB) scheme is proposed to improve the efficiency and dynamic performance simultaneously. This work also includes the detailed inductance parameter sensitivity of the proposed UPS-PB scheme and a zonal voltage control strategy to further improve dynamic responses of the output voltage under the start-up process or a large step change of the output voltage reference. Moreover, the variant of UPS-PB scheme for constant power load is analyzed in the experimental part. Finally, experimental results have verified the excellent performance of the proposed UPS-PB scheme and correctness of theoretical analysis in this work. [ABSTRACT FROM AUTHOR]

Power converters and electric motor drives when tightly regulated behave as constant power loads. These loads are different from resistive loads and have destabilizing negative impedance characteristics, which impact a system's stability. A boost converter is intrinsically nonlinear and is a nonminimum phase system at the output voltage with respect to the control input. The linear approximation of this boost converter loaded with a constant power load has a zero and poles in the right half of the $s$ -plane, making the system unstable and very difficult to control. Control techniques that employ some form of system inversion cannot be implemented for a nonminimum phase system. This paper describes a technique to modify the nonminimum phase boost converter to a minimum phase for a constant power load, further implementing the input–output linearization technique to stabilize the system. This paper also provides a methodological analysis of the problem followed by the proposed solution. Furthermore, it verifies the analysis of the proposed solution through simulation and experimental results. [ABSTRACT FROM AUTHOR]

Due to the anticipated proliferation of HVDC links up to and beyond 2020 into transmission systems, it is expected that in some locations, two or more HVDC links will be installed with a short electrical distance between them and will form a multiinfeed (MI)-HVDC scenario for the system. The control and operation of MI-HVDC systems are of particular concern for weak grids such as that likely to be found in the North of Scotland network. It is shown that the ac voltage regulation in the VSC-HVDC link may adversely affect the dynamic performance of the host weak ac system under the MI-HVDC scenario. To overcome this identified limitation associated with the ac voltage controller in the VSC-HVDC link, a new controller modification conforming to the grid code is proposed in this paper. Furthermore, this paper presents a robust control design method for ac voltage control using the fractional order (FO) control design theory. The influences of the proposed FO-based ac voltage controller are assessed alongside various voltage controllers for the representative North Scotland system. It is shown that the FO-based ac voltage controller has favorable effects on the dynamic behavior of weak ac systems with MI-HVDC. [ABSTRACT FROM AUTHOR]

In a hybrid ac/dc grid, the ac-side unbalance voltage introduces adverse effects on dc grids and interfacing converters (IFC) (active power oscillations, dc-link voltage oscillations, and IFC peak current increase at the same average power production). For parallel IFCs, these adverse effects can be easily aggregated. In this paper, two new control strategies are proposed for parallel IFCs to improve both ac and dc subgrids power quality. The proposed control strategies focus on canceling active power oscillations of parallel IFCs that provide oscillation-free dc link and sharing collective peak current of parallel IFCs. In the first proposed control strategy, IFCs’ power coefficients are controlled by solving a set of nonlinear equations, and this method is called as coefficient-based strategy, whereas, in the second proposed control strategy, peak currents of IFCs are controlled directly through the derived relationship of IFCs' peak currents under zero power oscillation in this paper, and this method is named as peak current based strategy. This peak current based strategy features much simplified calculation and could be easy to implement. To achieve control objectives, thorough study on parallel IFCs' peak currents is conducted. Based on the study, it is shown that the collective peak current of all IFCs is a constant under zero total active power oscillation, and therefore, keeping all IFCs' peak currents in the same phase and in-phase with collective peak current optimizes the utilization range of parallel IFCs. The numerical examples and experimental results are provided to verify the validity of the proposed control strategies under different operating conditions. [ABSTRACT FROM AUTHOR]

Inductive power transfer systems deployed in material handling have traditionally used parallel-tuned (current-sourced) boost regulators to independently control and regulate the load on each secondary. This paper evaluates the current doubler as an alternative parallel-tuned secondary controller within this application to better match the current and voltage requirements in the secondary resonant tuning network in high-power applications. This enables designs that enable the secondary magnetic pickup to achieve a smaller profile and fit within typical space constraints of material-handling vehicles. This paper presents the design approach needed for this regulator and compares its operation and performance against a traditional boost controller using both the simulation and the measurement of two built 5-kW secondary regulators. The current doubler naturally achieves a lower output ripple due to its interleaved switching and is found to have a comparable efficiency to the boost controller but requires slightly more components. [ABSTRACT FROM AUTHOR]

This paper proposes a split-capacitor four-wire current source inverter, which is the dual of the split-capacitor four-wire voltage source inverter. Since the midpoint of the dc link is tied to the neutral point of ac filter capacitors, the common-mode voltage (CMV) is reduced significantly. Consequently, the leakage current issue is effectively addressed. The proposed circuit is cost-effective as no extra switch is added. This paper, first, establishes the equivalent common-mode circuit of the proposed inverter. The impact of the neutral line inductance on CMV is also analyzed. Then, a specific modulation is introduced to balance the dc-link voltages/currents. To achieve good input/output performance, a nonlinear control method is developed based on time-domain models. Finally, all the proposed methods and related theoretical analysis are verified by simulations and experimental results. [ABSTRACT FROM AUTHOR]

To uncover the mechanism by which voltage-dependent loads (VDLs) influence electromechanical dynamics, this paper mathematically deduces the damping torque induced by VDLs based on a single machine infinite bus system. Moreover, by continuously regulating the terminal voltage of a typical large-scale VDL, i.e., the aluminum electrolysis load, this paper proposes a novel load damping control architecture to dynamically modulate the power consumption of controlled loads so as to eliminate interarea oscillations in power systems. Specifically, the decentralized load damping controllers (LDCs) used in this architecture are designed by a proposed multistage mixed ${{{H}}_{2}} / {{{H}}_\infty }$ control approach to guarantee robustness against uncertainties (e.g., tie-line outages and wind generation fluctuations) as well as to ensure reasonable control efforts of controlled VDLs. Simulation results on the modified New England and New York interconnected system validate the method for analysis of VDLs’ effects on interarea oscillations and show the proposed load damping control strategy can satisfactorily damp interarea oscillations over multiple operating points. [ABSTRACT FROM AUTHOR]

A multifunctional grid-connected inverter (MFGCI) has been a promising solution to mitigate power quality problems in distributed generation system and microgrid applications. However, the capability of a conventional MFGCI is limited when compensating a voltage-based power quality problem. In this paper, a new configuration of MFGCI is proposed to improve both voltage-based and current-based power quality issues and is named shunt-series-switched MFGCI (SSS-MFGCI). The SSS-MFGCI can be connected in series or parallel to the grid by implementing bidirectional switches to provide different modes of operation depending on various grid disturbances. The required compensating current and voltage are analytically derived considering the grid and load conditions for voltage regulation. This paper also presents the compensation strategies, control scheme, and transition scheme between different modes of the proposed SSS-MFGCI. Simulation results to prove the concept are also included for unbalanced sag and swell cases. [ABSTRACT FROM AUTHOR]

The active power distribution network operation with the integration of various renewable distributed generations (DGs), storage units, and dynamic loads are attracting a lot of interest due to substantial benefits in maximizing the penetration of available renewable energy resources (RES). However, the risk of voltage violation in the active distribution system (ADS) is exacerbated due to increasing penetration of DGs. It has become a bottleneck to maximize the DG penetration into the system. This bottleneck can be handled by effective voltage regulation and reactive power support into ADS. Nowadays, power electronic devices are frequently used for voltage and reactive power support over the conventional approaches. In this paper, an approach to maximize the penetration of RES, such as solar, wind, etc., into ADS by maximum utilization of existing solar photovoltaic inverters (SPVI) in the system is presented. Optimal utilization of SPVI may enhance the voltage stability of the ADS and wind power penetration into the system. The proposed SPVI control approaches are depended on active power generation of solar photovoltaic (SPVG), rating and capability of SPVI, and required reactive power in the ADS. In this paper, the problem of maximum penetration of RES is formulated as an optimal power flow embedded nonlinear optimization problem subject to the physical limitations of the SPVI. The results of the analysis performed on an IEEE 33 bus system show that the proposed SPVI control strategies can improve the voltage profile of the ADS with minimization of curtailment of wind power generation. [ABSTRACT FROM AUTHOR]

Grid-tied inverters and associated control techniques have gained importance in the domain of distributed generation. Different methods of control have been compared in the literature depending upon the ability to meet total harmonic distortion limits, damping offered to resonant oscillations, and stable operation. Multiple-loop methods compared to their single-loop counterparts have proved to be highly effective for meeting the grid regulations. In the literature, while single-loop and two-loop control systems for LCL filter have been assessed, not much study on three-loop control (TLC) has been done, possibly due to the hardware complexity and cost imparted by the increase in the number of sensors. In this paper, a TLC structure for grid connected inverters is analyzed for stability and parametric variations. Robustness of TLC is compared with single-loop grid current feedback control and a two-loop control structure. An approach to estimate the state variables is proposed in this paper, which cuts down the number of sensors and cost of implementation. The performance of the system is analyzed through simulations and results are validated in a hardware setup, under dynamic and steady-state conditions, thereby showing that this is a promising approach. [ABSTRACT FROM AUTHOR]