Search

Severe geomagnetic disturbances (GMDs) increase the magnitude of the electric field on the Earth’s surface (E-field) and drive geomagnetically-induced currents (GICs) along the transmission lines in electric grids. These additional currents can pose severe risks, such as current distortions, transformer saturation and increased reactive power losses, each of which can lead to system unreliability. Several mitigation actions (e.g., changing grid topology) exist that can reduce the harmful GIC effects on the grids. Making such decisions can be challenging, however, because the magnitude and direction of the E-field are uncertain and non-stationary. In this paper, we model uncertain E-fields using the distributionally robust optimization (DRO) approach that determines optimal transmission grid operations such that the worst-case expectation of the system cost is minimized. We also capture the effect of GICs on the nonlinear AC power flow equations. For solution approaches, we develop an accelerated column-and-constraint generation (CCG) algorithm by exploiting a special structure of the support set of uncertain parameters representing the E-field. Extensive numerical experiments based on “epri-21” and “uiuc-150” systems, designed for GMD studies, demonstrate (i) the computational performance of the accelerated CCG algorithm, (ii) the superior performance of distributionally robust grid operations that satisfy nonlinear, nonconvex AC power flow equations and GIC constraints, in comparison with standard stochastic programming-based methods during the out-of-sample testing. [ABSTRACT FROM AUTHOR]

This paper presents a combined control scheme that consists of two complementary control strategies, namely hierarchical control and autonomous control to handle contingencies in a multi-terminal high voltage direct current (MTDC) system. The proposed hierarchical control considers three optimal targets, i.e., equal or proportional power sharing, minimizations of transmission loss and total DC voltage variation. The primary control layer implements the droop control for voltage source converters (VSCs). While in the secondary control layer, voltage reference setpoints of the VSCs in droop control scheme are calculated by the proposed optimal power flow method with the equal or proportional power sharing as equality constraints. As the hierarchical control relies on communication system, the autonomous control for power sharing can be activated during communication system malfunctioning or rapid power variation caused by renewable energy sources. The combined control strategy can greatly increase the reliability of the MTDC system. The simulation results of a five-terminal MTDC network conducted in the MATLAB/Simulink software verify the performance of the proposed combined control strategy. [ABSTRACT FROM AUTHOR]

VSC-HVDC systems are being increasingly employed in the power systems. The recently installed HVDC systems have a power capacity similar to traditional power plants. Hence, they are expected to have a similar behaviour as traditional synchronous generators during faults in AC grid, within their limits of course. Recent grid codes require HVDC converter stations to incorporate fault ride-through (FRT) capabilities in order to avoid HVDC converter station disconnection from AC grid for certain fault characteristics. In this paper, two FRT mechanisms are suggested for the two converter stations of an HVDC system. One FRT mechanism is added to the DC voltage control loop of the master converter station, while the other FRT mechanism is added to the active power control loop of the slave converter station. The objective is to ensure the stable operation of the HVDC system during faults that may occur in AC grids located on both sides of the HVDC system. The performance and stability of the suggested FRT mechanisms are tested considering the pre-fault power flow direction and all possible types of balanced and unbalanced faults. Simulation results confirm the effectiveness of the FRT mechanisms and revealed the critical modes during FRT operation. [ABSTRACT FROM AUTHOR]

The synchronization requirement is a bottleneck of many distributed optimization algorithms, particularly for solving problems with computationally heterogeneous subproblems and during the occurrence of communication failure/delay. This paper presents a double-loop learning-aided asynchronous alternating direction method of multipliers (LA-ADMM) that has information prediction capability and handles a considerable level of asynchrony between subproblems. A momentum-extrapolation prediction-correction technique is developed to enable subproblems to predict their neighbors missing shared variable information instead of using the latest received values. An online streaming-based anomaly classification is designed to observe the performance of predicted data and control Lagrange multipliers update over the course of iterations. The proposed LA-ADMM reduces under-utilization of computation resources, especially if subproblems are computationally heterogeneous. This algorithm also enhances distributed optimization robustness against communication failure/delay that may result in a considerable level of asynchrony between subproblems. LA-ADMM is applied to solve the optimal power flow problem for several test systems. Promising results are obtained as compared to the classical synchronous ADMM and asynchronous ADMM without the anomaly switch control. [ABSTRACT FROM AUTHOR]

In the face of increasing natural disasters and an aging grid, utilities need to optimally choose investments to the existing infrastructure to promote resiliency. This paper presents a new investment decision optimization model to minimize unserved load over the recovery time and improve grid resilience to extreme weather event scenarios. Our optimization model includes a network power flow model which decides generator status and generator dispatch, optimal transmission switching (OTS) during the multi-time period recovery process, and an investment decision model subject to a given budget. Investment decisions include the hardening of transmission lines, generators, and substations. Our model uses a second order cone programming (SOCP) relaxation of the AC power flow model and is compared to the classic DC power flow approximation. A case study is provided on the 73-bus RTS-GMLC test system for various investment budgets and multiple hurricane scenarios to highlight the difference in optimal investment decisions between the SOCP model and the DC model, and demonstrate the advantages of OTS in resiliency settings. Results indicate that the network models yield different optimal investments, unit commitment, and OTS decisions, and an AC feasibility study indicates our SOCP resiliency model is more accurate than the DC model. [ABSTRACT FROM AUTHOR]

Line-switching operations introduce several concerns to microgrid operators since they trigger reliability and power quality issues in addition to increasing the wear and tear cost of switching devices. This paper proposes a novel methodology for achieving seamless line-switching in low-inertia islanded microgrids. The proposed methodology aims at mitigating the detrimental line-switching impacts by minimizing the power flow in the switched lines at the moments of executing the switching operations. The minimization of the power flow is realized by optimizing the droop control parameters of the DGs during the transition from one network topology to another. Besides, during executing a series of line-switching operations, the sequence of switching is also optimized to minimize the total switched power. While optimizing the line-switching execution sequence, the transitional droops are optimized for each line-switching operation within that sequence. A dynamic mixed-integer nonlinear programming problem is formulated to find both the optimized dynamic transitional droops and line-switching sequence simultaneously. Simulation results demonstrate the effectiveness of the proposed seamless line-switching methodology where the switched power can be reduced by more than 80% for a single line-switching operation and 54% considering a series of four switch exchanges in addition to achieving several instances of zero-current switching. As a result, a significant decrease in the peak of voltage transients has been recorded for some cases. [ABSTRACT FROM AUTHOR]

In this paper, we propose a topology-cognizant optimal power flow (OPF) paradigm with additional safety constraints for multi-terminal direct current (MTDC) grids. The resulting formulation is concerned with the optimization of controller set-points, i.e., voltage and power levels at each bus, and the switching status of transmission lines that collectively referred to as grid topology. A pair of additional safety constraints are integrated into the problem formulation to prevent voltage violations caused by power fluctuations in between two controller set-point updates. Searching for a grid topology that offers more efficient operation leads to a mixed-integer nonlinear program (MINLP) which is computationally challenging due to: i) Non-convex power flow equations, (ii) Non-convex converter loss equations, and iii) Binary variables accounting for the operational status of transmission lines. Non-convexities of power flows and converter loss equations are tackled by means of a mixed-integer second-order cone programming (MISOCP) relaxation, while the optimal switching status of transmission lines are determined via a branch-and-bound search. Numerical results for the modified IEEE 14, 30, and 57-bus systems are used to verify the merits of the proposed method. Furthermore, this method is experimentally validated using the CIGRE B4 DC grid benchmark in a real-time hardware-in-the-loop platform. [ABSTRACT FROM AUTHOR]

This paper presents and evaluates a discrete-event control-based Supervisory Controller (SC) for a grid-connected, battery-enabled, DC Fast-Charging (DCFC) station for Electric Vehicles’ (EVs). The SC enhances operational resiliency of the DCFC station and its host grid by decoupling the dynamics and limiting the steady-state power between the station and its host grid. The charging station includes multiple DCFC units, a Battery Energy Storage System (BESS), a solar-PV unit, and a grid-interface Voltage-Sourced Converter (VSC). The SC (i) limits the station-imported power from the grid based on the grid-imposed requirements, (ii) dynamically dispatches power flow among the grid, PV unit, DCFC units, and BESS to meet charging needs of EVs and when applicable charges BESS or exports power to the grid, (iii) protects the BESS by enforcing its state of charge upper and lower limits and the maximum charge/discharge rates, and (iv) enables the VSC to provide grid ancillary services, if required. Simulation results in PLECS software platform are given to assess the dynamic performance of the proposed SC in response to various discrete events/transitions. The SC algorithm is implemented on a Programmable Logic Controller (PLC) and verified based on real-time Hardware-In-the-Loop approach using an OPAL-RT platform. [ABSTRACT FROM AUTHOR]

These days, constant on-time current mode (COTCM) control scheme is widely used in the voltage regulator (VR) controllers because it has a higher light-load efficiency and a higher BW design capability while maintaining a simpler compensation requirement. One issue plaguing the COTCM control is its slow transient response which is caused by its fixed TON operation. During the heavy-load step-up transient, the duty cycle becomes saturated and the inductor current increment becomes limited by TON and the minimum off time (TOFF$\_$ MIN) ratio, which can create a large undershoot at load step up. On the other hand, in the load step-down case, if the load step down occurs at the beginning of TON, a large overshoot can be created at the output. To solve this issue, this paper presents a method designed to increase the TON at load step up and then very quickly, decrease at load step down in order to reduce the undershoot and overshoot at output or otherwise save the output capacitor. In this proposed method, the increase or decrease of T ON is proportional to the output change which eliminates the chance of any overcorrection or ring-back problem unlike the methods presented in prior forums. This feature enables the control to work seamlessly in a high-frequency load repetitive case in VR applications. Since this TON change occurs only in the transient period when duty cycle is saturated, it does not affect the small-signal property of the COT control. Moreover, the proposed methods are very much compatible with the state-of-the-art single and multiphase COT control structures. Simulation and test results in both single and multiphase operations are also presented in the paper to verify the proposed concept. [ABSTRACT FROM PUBLISHER]

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CB's design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved. [ABSTRACT FROM PUBLISHER]

DC electric springs (DCES) are emerging technologies for the (i) regulation of mains voltage against the intermittent renewable generations and (ii) harmonic cancellation in dc microgrids. When conventional converter topologies (e.g., half-bridge or full-bridge converter) are adopted as DCES, the battery storage of the DCES has to process both the dc power and the ac harmonic power. The pulsating ac power can severely reduce the lifetime of the battery. To address this issue, a hybrid-DCES (H-DCES) is proposed in this paper to perform (i) and (ii) in a decoupled manner. With a modified topology and control method, the H-DCES can divert the ac current to the ground and retains the function of manipulating noncritical load for dc voltage regulation. The immediate benefits of this H-DCES are the reduction of storage capacity and a prolonged lifetime of the battery. Both the operating principle and the mathematical model of the proposed H-DCES are analyzed in this paper. A prototype of the H-DCES is practically tested in a 48-V dc grid. The experimental results show that the H-DCES can realize the decoupled operation of dc voltage regulation and harmonic cancellation. Simulation studies further demonstrate that the H-DCES requires less storage capacity than its counterparts. [ABSTRACT FROM AUTHOR]

This paper proposes a multilevel series compensator (MSC) to deal with voltage sags/swells, harmonic compensation, or reactive power compensation. Such a device can be considered as a dynamic voltage restorer or a series active power filter (Series-APF). The MSC can improve the power quality of loads located in stiff systems. The configuration is based on three-phase bridge (TPB) converters connected by means of cascaded single-phase transformers. This arrangement permits the use of a single dc-link. A generalization for $K$ -stages in which $K$ -transformers are coupled with $K$-TPB converters is presented. The topology permits generating a high number of levels in the voltage waveforms with a low number of power switches in comparison with a classic topology. The multilevel waveforms are generated by the converters through a suitable pulsewidth modulation (PWM) strategy that takes into consideration the transformer turns ratios. Modularity and simple maintenance make the proposed MSC an attractive solution compared with some conventional configurations. Model, PWM strategy, and overall control are discussed in this paper. Simulation and experimental results are presented as well. [ABSTRACT FROM PUBLISHER]

To reduce current stress and improve efficiency of dual active bridge (DAB) dc–dc converters, various control schemes have been proposed in recent decades. Most control schemes for directly minimizing power losses from power loss modeling analysis and optimization aspect of the adopted converter are too difficult and complicated to implement in real-time digital microcontrollers. Thus, this paper focuses on a simple solution to reduce current stress and improve the efficiency of the adopted DAB converter. However, traditional current-stress-optimized (CSO) schemes have some drawbacks, such as inductance dependency and an additional load-current sensor. In this paper, a simple CSO scheme with a unified phase-shift (UPS) control, which can be equivalent to the existing conventional phase-shift controls, is proposed for DAB dc–dc converters to realize current stress optimization. The simple CSO scheme can overcome those drawbacks of traditional CSO schemes, gain the minimum current stress, and improve efficiency. Then, a comparison of single-phase-shift (SPS) control, simple CSO scheme with dual-phase-shift (CSO-DPS) control, simple CSO scheme with extended-phase-shift (CSO-EPS) control, and simple CSO scheme with UPS (CSO-UPS) control is analyzed in detail. Finally, experimental results verify the excellent performance of the proposed CSO-UPS control scheme and the correctness of theoretical analysis. [ABSTRACT FROM AUTHOR]

In order to provide the ancillary service for smart grid, this paper proposes a modelling and control protocol design approach for the aggregation of heterogeneous thermostatically controlled loads (TCLs). A 2-D state bin is proposed to model the second-order TCL dynamics in a population model. Detailed procedure of calculating the transition probability in the system matrix is provided. In the controller design, a model predictive control (MPC) scheme is proposed to obtain the optimal control actions along the prediction horizon. In addition, implementation of the control signal for adjusting TCLs' statuses are also investigated with practical situations considered. Simulation results reveal the feasibility and efficacy of the proposed modelling and control approach when applied on a large population of TCLs. Some factors that may affect the service performance are also discussed in this paper. [ABSTRACT FROM AUTHOR]

The development of a large-scale high-voltage direct current (HVDC) power grid requires a reliable, fast, and low-loss circuit breaker. The load commutation switch (LCS) is an essential part of ABB's 1200-MW hybrid HVDC breaker concept, which builds up a low-loss conducting path for the load current. The technical requirements for the LCS are expressed in this paper by studying the operation principle of the hybrid HVDC breaker. The voltage stress over the LCS is determined based on a dc grid with 320 and 2 kA rated voltage and current. A system model of the hybrid HVDC breaker is developed in PSCAD/EMTDC to study the design criteria for snubber circuit and arrester blocks. It is observed that conventional snubber circuits are not suitable for a bidirectional LCS as the current of snubber capacitors prevent the fast interruption action. A modified snubber circuit is proposed in this paper along with two more alternatives for the LCS to overcome this problem. Moreover, the power loss model for a semiconductor device is discussed in this paper based on the 4.5-kV StakPak IGBT. The model is used to calculate the conduction power losses for different LCS topologies. Ultimately, a matrix of 3 $\times$ 3 IGBT modules is selected to provide a reliable LCS design which can handle several internal fault cases with no interruption of operation. A full-scale prototype has been constructed and tested in ABB HVDC Center, Ludvika, Sweden. The experimental test results are also included in the paper in order to verify the calculation and simulation study. [ABSTRACT FROM AUTHOR]

In this paper, a 2.5-kW 330-410-V/14-V, 250-kHz dc/dc converter prototype is developed targeted for electric vehicle/hybrid vehicle applications. Benefiting from numerous advantages brought by the LLC resonant topology, this converter is able to perform high efficiency, high power density, and low EMI. To arrange high-output current, this paper proposes a parallel-connected LLC structure with developed novel double-loop control to realize an equal current distribution and an overall efficiency improvement. Considering the LLC cell's dimensioning, this paper establishes a more precise model by taking the secondary leakage inductance into consideration. System amelioration and design considerations of the developed LLC are also presented in this paper. A special transformer is presented, and various types of power losses are quantified to improve its efficiency. This converter also implements synchronous rectification, power semiconductor module, and an air-cooling system. The power conversion performance of this prototype is measured and the developed prototype attains a peak efficiency of 95% and efficiency is higher than 94% from 500 W to 2 kW, with a power density of 1 W/cm3. Finally, the EMC results of this prototype are also measured and presented. [ABSTRACT FROM AUTHOR]

Pulse width modulation (PWM) converters generate switching common-mode voltages (CMVs) across the load terminals. These voltages cause common-mode currents, leading to bearing failure in motor loads and electromagnetic interference problems. This paper presents a generalized carrier-based PWM technique for open-end winding motor drives that completely eliminates switching CMV. The proposed method is applicable to both dual two-level voltage source inverter and dual matrix converter-based open-end winding drives. Detailed analysis shows that the carrier-based method requires significantly less computation compared to the corresponding space vector implementation. This paper also outlines the relationship between the two implementations. The carrier-based method is shown to achieve superior performance in terms of resource requirements and execution time when implemented on a field-programmable gate array-based real-time control platform. Simulation and experimental results have been presented to validate the proposed method. [ABSTRACT FROM AUTHOR]

The parameters of electrical energy such as voltage amplitude are very important, particularly from the viewpoint of the final consumer and sensitive loads connected to the grid. The dynamic states in the power grid—deep voltage sags and swells—might cause faults and defects in sensitive loads. This paper deals with a three-phase hybrid transformer (HT) without dc energy storage to compensate voltage sags and swells and to protect sensitive loads against the rapid and extensive changes in supply voltage amplitude. The analyzed HT contains two main units: the first one is the conventional electromagnetic transformer, realizing an electromagnetic coupling, and the second one is the buck–boost matrix-reactance chopper, realizing an electrical coupling in the HT unit. In the presented solution, output voltage is transformed in two ways—electromagnetically and electrically. This paper presents an operational description, the theoretical analysis, and the experimental test results from a 2-kVA laboratory model. On the basis of the authors' research, it can be stated that the HT makes it possible to compensate deep voltage sags (deeper than 50% of nominal source voltage) and overvoltages (up to 140% of nominal source voltage) while maintaining good dynamic properties. The main advantages of the proposed solution, in comparison to other conventional solutions, are the ability to control the output voltage in the range of 0.66–3.5 US, good dynamics (transient state during source voltage uS amplitude change is shorter than 10 ms), and galvanic separation between source and load (such as in the case of the conventional electromagnetic transformer). [ABSTRACT FROM PUBLISHER]

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on–off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils. [ABSTRACT FROM AUTHOR]

Shunt active power filter (SAPF) is the preeminent solution against nonlinear loads, current harmonics, and power quality problems. APF topologies for harmonic compensation use numerous high-power rating components and are therefore disadvantageous. Hybrid topologies combining low-power rating APF with passive filters are used to reduce the power rating of voltage source inverter (VSI). Hybrid APF topologies for high-power rating systems use a transformer with large numbers of passive components. In this paper, a novel four-switch two-leg VSI topology for a three-phase SAPF is proposed for reducing the system cost and size. The proposed topology comprises a two-arm bridge structure, four switches, coupling inductors, and sets of LC PFs. The third leg of the three-phase VSI is removed by eliminating the set of power switching devices, thereby directly connecting the phase with the negative terminals of the dc-link capacitor. The proposed topology enhances the harmonic compensation capability and provides complete reactive power compensation compared with conventional APF topologies. The new experimental prototype is tested in the laboratory to verify the results in terms of total harmonic distortion, balanced supply current, and harmonic compensation, following the IEEE-519 standard. [ABSTRACT FROM AUTHOR]

Transmission grid topology control makes power systems more flexible and efficient. However, a line switching action itself may cause transient instability since a large flow disturbance may be involved. In this paper, a critical switching flow (CSF) concept is developed and an associated preventive stabilizing redispatch scheme is proposed to provide control guidelines for line switching actions to prevent instability and enhance transient stability for the system. The CSF is calculated in the day-ahead stage using a two-stage Monte Carlo algorithm, in which the uncertainties of generations and loads are considered and the worst case in the system is identified statistically. The obtained information is then used in the proposed scheme in the operation stage. Selected generators are dispatched before a switching action to ensure system stability. The IEEE reliability test system, the modified IEEE-118 bus system, and the Texas 2000 system are used to demonstrate the effectiveness and scalability of the proposed algorithm and scheme. [ABSTRACT FROM PUBLISHER]

In this paper, a practical fault diagnosis algorithm is presented to realize multiple open-circuit fault diagnosis for photovoltaic (PV) inverters. By the feature analysis for the output currents of normal and fault states, a fault diagnosis algorithm is developed, which is composed of multistate data processing (MSDP) block, subsection fluctuation analysis (SSFA) block, and the artificial neural network (ANN) block. First, the MSDP block is used to distinguish the different feature data and to adopt the data processing scheme, which improves the smoothness of the main data, retains the main fault features, and removes the influence of load change. Second, the SSFA block is used to extract the data feature, which can be used to accurately distinguish the different states of any switch. Finally, ANN is used by combining with the proposed MSDP and SSFA to implement intelligent classification, in which the dependency and the number of thresholds can be reduced. Comparing with the existing fault classification algorithms, the proposed algorithm is simple and stable to realize the multiple switches fault diagnosis for PV inverters. It does not require additional hardware equipment, and it can reduce the complexity of design and realization. Finally, the effectiveness of the fault diagnosis algorithm is verified by the experimental results. [ABSTRACT FROM AUTHOR]

A new zero-voltage switching full-bridge dc–dc converter for battery charging is proposed in this paper. The proposed isolated dc–dc converter is used for the dc–dc conversion stage of the electric vehicle charger. The primary switches in dc–dc converter turn-on at zero voltage over the battery-charging range with the help of passive auxiliary circuit. The diode clamping circuit on the primary side minimizes the severity of voltage spikes across the secondary rectifier diodes, which are commonly present in conventional full-bridge dc–dc converters. The main switches are controlled with an asymmetrical pulse-width modulation (APWM) technique resulting in higher efficiency. APWM reduces the current stress of the main switches and the circulating losses compared with the conventional phase-shift modulation method by controlling the auxiliary inductor current over the entire operating range of the proposed converter. The steady-state analysis of auxiliary circuit and its design considerations are discussed in detail. A 100-kHz 1.2-kW full-bridge dc–dc converter prototype is developed. The experimental results are presented to validate the analysis and efficiency of the proposed converter. [ABSTRACT FROM PUBLISHER]

This paper presents a multiobjective real-time controller for a modular multilevel converter capable of parallel module connectivity, the so-called modular multilevel series parallel converter (MMSPC). The MMSPC topology allows the batteries to be dynamically rewired in various series–parallel configurations, generating a wide range of output voltage levels. The novel control method parallelizes the modules to balance their voltages without the need for individual module voltage monitoring. Additionally, the controller optimizes across the large number of feasible system configurations to minimize switching and conduction losses. Finally, the controller efficiently encodes the system configuration with module interconnection states rather than the module switch states, which substantially simplifies control. Furthermore, this work experimentally validates the MMSPC topology and concept. In the prototype, the parallel mode reduced the system losses at 5 kW output power by 18% and 24% for load power factors of 1.0 and 0.8, respectively. Sensorless balancing via parallelization maintained well-matched module voltages (standard deviation = 0.045 V) over a 5-h battery discharge with highly variable load current. The reduced conduction losses and simple balancing capability of the MMSPC can enable new applications at medium and low voltages that benefit from its high-quality output, elimination of filtering magnetics, fast response, and modularity. [ABSTRACT FROM AUTHOR]

Traditional load regulation methods for a resonant converter mainly rely on frequency modulation. It is always a tradeoff between the design of the resonant network and the range of load. Especially for wireless power transfer (WPT) systems, the resonant network usually has a high quality factor. Small variation on frequency leads to huge drop in gain and efficiency. Due to this problem, many WPT systems are unregulated and they need one or two more front-end stages to regulate the dc bus voltage and perform power factor correction (PFC). In order to lower the cost and complexity of two- or three-stages structure, a single-stage solution with a silicon carbide (SiC) based Z-source resonant converter (ZSRC) was recently proposed. The Z-source network provides high reliability as being immune to shoot-through problems. Additionally, a ZSRC can boost the dc bus voltage while the traditional voltage-source inverter can only produce a lower voltage. However, the load regulation of this new topology has not been addressed. Two effective load regulation methods with constant frequency are presented for this SiC-based ZSRC specifically. Operation principle of the two load regulation methods are described in this paper. Experimental results based on a 200-W scale-down prototype with a full-bridge series resonant dc–dc converter are presented to illustrate the mechanism of these two methods. [ABSTRACT FROM AUTHOR]

High-voltage (HV) pulses are used in pulsed electric field (PEF) applications to provide an effective electroporation process, a process in which harmful microorganisms are disinfected when subjected to a PEF. Depending on the PEF application, different HV pulse specifications are required such as the pulse-waveform shape, the voltage magnitude, the pulse duration, and the pulse repetition rate. In this paper, a generic pulse-waveform generator (GPG) is proposed, and the GPG topology is based on half-bridge modular multilevel converter (HB-MMC) cells. The GPG topology is formed of four identical arms of series-connected HB-MMC cells forming an H-bridge. Unlike the conventional HB-MMC-based converters in HVdc transmission, the GPG load power flow is not continuous which leads to smaller size cell capacitors utilization; hence, smaller footprint of the GPG is achieved. The GPG topology flexibility allows the controller software to generate a basic multilevel waveform which can be manipulated to generate the commonly used PEF pulse waveforms. Therefore, the proposed topology offers modularity, redundancy, and scalability. The viability of the proposed GPG converter is validated by MATLAB/Simulink simulation and experimentation. [ABSTRACT FROM PUBLISHER]

The effect of bus-bar splitting on the static stability limit is studied in this paper. A novel online bus-bar splitting methodology is developed to increase the static stability limit of a look-ahead (i.e., short term, say, 30 minutes ahead) power system. The proposed methodology, designed for online application, is based on a strategy consisting of screening, ranking, and decision stages. A tailored model of bus-bar splitting is developed to accurately model bus-bar splitting scenarios. The effectiveness of the proposed online bus-bar splitting methodology is evaluated on the IEEE 118-bus, 1648-bus, and 3120-bus power systems. Results indicate that significant load margin increases can be achieved by bus-bar splitting. [ABSTRACT FROM PUBLISHER]

An increasing number of power-electronic-based distributed generation systems and loads generate not only characteristic harmonics but also unexpected harmonics. Several methods, such as impedance-based analysis, which are derived from the conventional average model, are introduced to perform research about the harmonic interaction. However, it is found that the linear-time-invariant-based model analysis makes it difficult to analyze these phenomena because of the time-varying properties of the power-electronic-based systems. This paper investigates a grid-connected converter by using the harmonic state-space (HSS) small-signal model, which is based on a linear time-varying periodically theory. The proposed model can include the switching behavior of the model, where it makes the model possible to analyze how harmonics are transferred into both the ac-side and dc-side circuits. Furthermore, a harmonic matrix of the grid-connected converter is developed to analyze the harmonic interaction at the steady-state behavior. Besides, the frequency-domain results are compared with time-domain simulation results by using the HSS modeling to verify the theoretical analysis. Experimental results are finally discussed to verify the proposed model and study. [ABSTRACT FROM PUBLISHER]

In this paper, a new common inductor current sharing method is proposed for a multiphase LLC resonant converter for high-power applications. Automatic current sharing is achieved by using a common resonant inductor connecting the resonant inductors in each LLC phase in parallel. The current sharing performance of the proposed method is evaluated under first harmonic approximation (FHA) assumption. The proposed method can automatically share the primary resonant current and the load current for all phases without any additional circuit and control strategy. A 600-W two-phase LLC converter prototype based on the proposed method is built to verify the feasibility. Excellent current sharing performance (less than 0.5% current sharing error at full load) has been achieved. [ABSTRACT FROM PUBLISHER]

An interline dynamic voltage restorer (IDVR) is invariably employed in distribution systems to mitigate voltage sag/swell problems. An IDVR merely consists of several dynamic voltage restorers (DVRs) sharing a common dc link connecting independent feeders to secure electric power to critical loads. While one of the DVRs compensates for the local voltage sag in its feeder, the other DVRs replenish the common dc-link voltage. For normal voltage levels, the DVRs should be bypassed. Instead of bypassing the DVRs in normal conditions, this paper proposes operating the DVRs, if needed, to improve the displacement factor (DF) of one of the involved feeders. DF improvement can be achieved via active and reactive power exchange (PQ sharing) between different feeders. To successfully apply this concept, several constraints are addressed throughout the paper. Simulation and experimental results elucidate and substantiate the proposed concept. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to achieve both tight dc voltage regulation and direct power factor control by applying general direct space vector modulation (G-SVM) of matrix converter theory to a three-phase ac–dc matrix rectifier. The reduced form of G-SVM and the derivation of the three-phase ac–dc matrix rectifier from a three-phase ac–ac matrix converter are described and theoretically justified. The proposed matrix converter features intrinsic buck conversion, tight output voltage regulation via modulation, inherent capability of four-quadrant operation, elimination of the bulky storage component on the dc bus, and sinusoidal input current. Moreover, the reduced G-SVM-controlled three-phase ac–dc matrix rectifier can achieve leading, lagging, or unity input power factor by simple and direct control of the input current displacement angle. This significantly benefits the potential application of the reduced G-SVM-controlled three-phase matrix rectifier to automotive, aircraft, ship, and other three-phase standby dc supplies. In these applications, the unity power factor control is indispensable when the dc load widely changes. Simulated and experimental verification for this strategy has been presented and discussed in this paper. [ABSTRACT FROM PUBLISHER]

This paper presents a method of mitigating the transient overshoots of dc-dc converters undergoing large load disturbances. The method involves a small power auxiliary circuit which exists as an energy buffer to provide a smooth absorption and release of excess energy from and to the main dc-dc converter during transients. In the method, the regulation of the current magnitude of the converter and energy storage of the auxiliary circuit are guaranteed by the relevant control schemes. The cost and size of the auxiliary circuit will be relatively small as it only operates intermittently. In this paper, the voltage deviation analysis and the auxiliary circuit design guideline are provided. Experimental results validating the approach are presented. [ABSTRACT FROM AUTHOR]

This paper presents a new control method, in which a distributed generator (DG) actively participates in steady-state voltage control, together with an under-load tap changer (ULTC) and shunt capacitors (Sh.Cs). In the conventional DG control method, the integration of DGs into a distribution power system increases the number of switching operations of the ULTC and the Sh.Cs. To solve this problem, this paper proposes that the DG output voltage be dispatched cooperatively with the operation of the ULTC and the Sh.Cs, based on load forecasts for one day in advance. The objective of the proposed method is to decrease the number of switching device operations, as well as to reduce the power loss in the distribution lines, while maintaining the grid voltage within the allowed range. The proposed method is designed and implemented with Matlab, using two different dynamic programming algorithms for a dispatchable and a nondispatchable DG, respectively. Simulation studies demonstrate that the objective can be achieved under various grid conditions, determined by factors such as the DG output power characteristics, the location of the DG-connected bus on the feeder, and the load profile of the feeder containing the DG. [ABSTRACT FROM PUBLISHER]

This paper presents a generalized pulse width modulation (PWM)-based control algorithm for multiphase neutral-point-clamped (NPC) converters. The proposed algorithm provides a zero sequence to be added to the reference voltages that contributes to improve the performance of the converter by: 1) Regulating the neutral-point (NP) current to eliminate/attenuate the low-frequency NP voltage ripples; 2) reducing the switching losses of the power semiconductors; and 3) maximizing the range of modulation indices for linear operation mode. The control method is formulated following a carrier-based PWM approach. Hence, dealing with complex space-vector diagrams to solve the modulation problem for multiphase converters is avoided. The recursive approach means that it can be easily extended to n-phase converters without increasing the complexity and computational burden, making it especially attractive for digital implementation. The proposed method allows regulating the NP voltage without the need for external controllers; therefore, no parameter tuning is required. The algorithm has been tested in a four-leg NPC converter prototype performing as a three- and four-phase system and operating with balanced and unbalanced loads. [ABSTRACT FROM AUTHOR]

The performance of a novel three-phase four-leg fault-tolerant T-type inverter topology is introduced in this paper. This inverter topology provides a fault-tolerant solution to any open-circuit and certain short-circuit switching faults in the power devices. During any of the fault-tolerant operation modes for these device faults, there is no derating required in the inverter output voltage or output power. In addition, overload capability is increased in this new T-type inverter compared to that in the conventional three-level T-type inverter. Such increase in inverter overload capability is due to the utilization of the redundant leg for overload current sharing with other main phase legs under healthy condition. Moreover, if the redundant phase leg is composed of silicon carbide metal-oxide-semiconductor field-effect transistors, quasi-zero-voltage switching, and zero-current switching of the silicon insulated-gate bipolar transistors (IGBTs) in the conventional main phase legs can be achieved at certain switching states, which can significantly relieve the thermal stress on the outer IGBTs and improve the whole inverter efficiency. Simulation and experimental results are given to verify the efficacy and merits of this high-performance fault-tolerant T-type inverter topology. [ABSTRACT FROM PUBLISHER]

To transmit constant powers particularly at light load, the isolated dual active bridge (DAB) with conventional dual-phase-shift (DPS) control exhibits large variations of currents, losses, and efficiencies if there is a small change of outer phase shift ratio, which is caused by the change of command to adjust power transmission. This is mainly because of the narrow operation region of outer phase shift ratio if the conventional DPS modulation is used. To solve this problem, a novel DPS control with bidirectional inner phase shifts for the DAB is developed to reduce the current surge in the high-frequency transformer, which can stabilize the output power with high efficiency. From the analysis of operation modes and power characteristics of the proposed DPS control, it is shown that due to wider operation region of outer phase shift ratios compared with the conventional DPS control in which the inner phase shifts of both the primary and secondary H-bridges are in the same direction, lower surge current can be obtained in the change of phase shift command, which results in stable power transmission particularly at light load without scarifying the efficiency. Moreover, the DAB of using the proposed DPS control can transmit bidirectional energy with the inner and outer shift ratios of 0–1. For experimental verifications, the test results are also presented in this paper. [ABSTRACT FROM AUTHOR]

Multiphase buck converters are widely used to power up low-voltage high-current loads, such as microprocessors, battery chargers, hybrid vehicles, among others. For this converter, topology is necessary to guarantee a correct distribution of the total output current among phases in order to ensure proportionality in electrical losses and thermal stresses. So, a current sharing scheme is a desirable part in a multiphase buck converter. Most of the proposed current distribution schemes include sensing circuitry and a current loop for each phase. Nevertheless, by using sensors, control complexity and assembly costs increases proportionally to the number of phases. This paper presents a simple sensorless current sharing algorithm for multiphase buck converters. The proposed technique is based on simple one-time tests by turning-on each phase in order to estimate the parasitic resistance for the turned-on phase. Once the parasitic resistances are known, the duty cycles, which allow for adequate current sharing among phases, are calculated. An extra control algorithm ensures that the compensated duty cycle is applied at the switching node. This extra control loop does not require analog-to-digital converters. Analysis and experimental results of proposed control technique are shown using a five-phase dc–dc buck converter. The proposed algorithm exhibits an adequate distribution of the load current between phases. [ABSTRACT FROM AUTHOR]

This paper presents the high-performance quasi-Z-source series resonant dc–dc converter as a candidate topology for the photovoltaic module-level power electronics applications. The converter features a wide input voltage and load regulation range thanks to the multimode operation, i.e., when the shoot-through pulse width modulation and phase-shift modulation are combined in a single switching stage to realize the boost and buck operating modes, respectively. Our experiments confirmed that the proposed converter is capable of ensuring ripple-free 400 V output voltage within the sixfold variation of the input voltage (from 10 to 60 V). The converter prototype assembled achieved a maximum efficiency of 97.4%, which includes the auxiliary power and control system losses. [ABSTRACT FROM AUTHOR]

Burst mode control techniques have been studied for LLC resonant dc–dc converters to improve power conversion efficiencies at light load conditions. However, there are not many studies about how to detect the load conditions at burst mode not even at normal mode, at the primary side of LLC resonant dc–dc converters. In this paper, we propose and implement a lossless load detection technique for normal and burst modes by a novel average input current sensing method at the primary side of LLC resonant converters. Detailed theoretical analysis at normal and burst modes is presented and a load detection circuit is proposed with the consideration of integrated circuits. To verify the analysis, an LLC resonant offline converter with 400 V input and 16 V/10 A output is simulated using PSIM and a prototype board is built. The experimental results show that the proposed load detection technique is valid and practical. As a result, an automatic burst mode control for LLC resonant converters at light load conditions is realized with the proposed average input current sensing method. [ABSTRACT FROM AUTHOR]

Exploring high-performance controller for buck converter is challenging since it can be easily affected by converter model accuracy. In this paper, a novel nonaveraged current discrete-time (NCD) model is proposed, in which inductor current is expressed as time-varying equations during switch-on and switch-off states. It achieves higher accuracy than the conventional averaged model at high-frequency range, thus can be used to optimize high-speed controller design. Based on the NCD model, a multiloop minimum switching cycle (MMSC) control strategy, composed of output feedback (OF), line feed forward (LFF), and reference feed forward (RFF) loops, is proposed and tuned for buck converter operating in continuous conduction mode. Mutual influences among three loops are considered and eliminated by specifically designed LFF and RFF compensations, which adapt the OF compensation. With consideration of sampling and calculation delays, relationship between transient switching cycles and geometric center of controller poles is discovered from a calculated output voltage error series. Furthermore, theoretical minimum switching cycles are calculated by moving the center inside the unit cycle of complex plane, which ensures system stability. Moreover, load/line transient response and reference tracking time are simultaneously optimized to the minimum switching cycles. Effectiveness of the controller is proved by converter closed-loop pole/zero plots, transient response simulations, and experiments. [ABSTRACT FROM AUTHOR]

In this paper, the near-state pulse width modulation (NSPWM), adapted to be used in dual-voltage source inverter (VSI) fed open-end load, is introduced primarily aiming to appreciably minimize the voltage total harmonic distortion (THD). Within the framework of the proposed method, the active time of pulses at the second inverter is relatively adjusted within switching interval (compared to the first VSI). The phase voltage harmonics are formulated for dual-inverter modulated by NSPWM; then, optimal adjustment is identified via a three-dimensional curve of phase voltage THD versus modulation index (MI) and phase angle displacement (PAD). Due to the limitation of MI in NSPWM, the desired output voltage is synthesized with modifying not only MI but also PAD between references of two VSIs. Furthermore, NSPWM inherently takes advantage of better efficiency compared to conventional space vector modulation due to switching only two phases within an interval (by clamping one phase to positive/negative dc-rail). The dual-VSI supplied by two isolated dc sources is assembled in the laboratory to experimentally evaluate the THD reduction feature of the proposed method; also, the simulation results obtained by means of a MATLAB/Simulink environment show close agreement with experimental data. [ABSTRACT FROM PUBLISHER]

Due to an increasing in-vehicle electric load demand, a new 48-V electric system architecture proposed by German premier car manufacturers has been gaining increasing attention recently for electrified vehicles. In this paper, a multiple-voltage bus converter for the 48-V and existing 12-V dual supply system, providing additional flexible dc bus voltages, is proposed. The proposed converter can provide a dual flexible dc voltage bus and an additional dependent voltage bus while simultaneously managing the 12-V supply net using only one single-leg switch pole and a diode. The additional dc voltage buses will provide more options to accommodate variable electric load components in a vehicle, improving their power density. Due to the multiple control objectives, the single-leg pole requires a multimode switching strategy to regulate the additional bus voltages. To improve the onboard vehicle system reliability and performance, an impedance network structure is implemented in the converter to obtain a dead-time-free pulsewidth modulation (PWM). Simulation and experimental results are presented to verify the feasibility and effectiveness of the proposed structure and control method. [ABSTRACT FROM PUBLISHER]

This paper proposes a new power conversion scheme for paralleled server power supplies. The snubber capacitor voltage is utilized for the secondary voltage source, from which bidirectional buck converter provides output power to the load under a very light-load condition. To increase the energy of the secondary voltage source, an additional voltage bus is connected between the snubber capacitors from each power supply. The main advantage of the proposed scheme is that high efficiency can be achieved especially under a very light-load condition because of the low switching and core loss achieved by using the buck converter instead of the conventional structure composed of a primary inverter, an isolation transformer, and a secondary rectifier. Furthermore, the buck converter is integrated into the secondary rectifier circuits, so additional components are minimized. The validity of the proposed converter is confirmed by the experimental results from two 12-V/750-W prototype modules. [ABSTRACT FROM PUBLISHER]

Linear small-signal models of a dc–dc converter often ignore switching dynamics; thus such models are insufficient to fully explore the performance objective under large signal transients. Linear/nonlinear hybrid controllers are promising alternatives; however, they require structurally different hardware resources along with extra antiwindup arrangements. This paper proposes a geometric tuning method in a digitally current-mode-controlled buck converter under continuous-conduction mode. This considers a proportional-integral voltage controller along with a load current feedforward in the digital domain, while the inductor current has a traditional analog implementation. This resembles a first-order switching surface with near load-invariant regulation; thus a (fixed) small integral gain is sufficient to minimize the steady-state error. Using phase-plane geometry, the objective is to tune the controller gain in a way to achieve proximate time optimal recovery using a fixed-frequency pulse-width modulator and also to retain the large-signal stability. The effects of parameter variation and finite sampling are analyzed. The proposed tuning is implemented using an FPGA device. [ABSTRACT FROM AUTHOR]

This paper presents power and frequency tuning loops based on fundamental harmonic operation (FHO) and third harmonic operation (THO) for inductive power transfer (IPT) systems. THO is an effective solution for applications requiring high-frequency power supply with low voltage gains, e.g., low-voltage contactless battery chargers. Moreover, it can reduce the switching losses and increase the net efficiency of IPT systems in light-load conditions. Compared to conventional methods, such as phase-locked-loop (PLL) circuits, the proposed methods are less affected by misalignments and instantly track the resonant frequency. Hence, they can be utilized in self-switched capacitor and variable coupling applications. In addition, the new tuning systems can switch between THO and FHO modes for different loadings, voltage gains, and couplings conditions, e.g., dynamic powering of different moving pickups. To show the performance of the new tuning loops, laboratory prototypes with switching frequency of 20–50 kHz are implemented based on half-bridge and full-bridge inverters. [ABSTRACT FROM AUTHOR]

Three-phase inverters with an output LC filter are commonly used to provide sinusoidal voltages with low-harmonic distortion. Constraint on peak filter current is often desirable to protect the components from potential damage. In case of constraints, implicit and explicit model-predictive control (MPC) are some of the feasible controller options. The conventional implicit MPC requires a large number of computations, whereas explicit MPC cannot directly incorporate real-time changes in model parameters, while still being computationally expensive than some of the other control schemes, e.g., hysteresis, dead beat control, etc. In this paper, we propose a new approach to solve the optimization problem in implicit MPC that has a computational complexity approximately five times less than that of explicit MPC. We have been able to achieve lower computational requirements by exploiting the inverter model and the structure of constraints. [ABSTRACT FROM AUTHOR]

Linear controllers and small-signal modeling techniques have been used for over 40 years to control power electronic converters around the equilibrium operating point. Outside the quiescent area, the shortcomings of linear controllers are well known and include uncertain large-signal transient behavior and sluggish dynamic/recovery response. In particular, resonant converters have inherent large-signal behavior, making the transient performance of small-signal linear controllers poor. This paper develops a geometric controller based on large-signal modeling to achieve enhanced dynamic response of the series resonant converter (SRC). The proposed modeling strategy provides detailed insight into the behavior of the SRC and creates an analytical framework to successfully perform geometric control. A new geometric control law is mathematically derived using average circular trajectories producing accurate and fast dynamics during startup, sudden load or reference changes. In order to validate the theoretical analysis, experimental and simulation results of an SRC are presented and compared to those of a linear and a nonlinear controller. The dynamic response is, on average, over 100 times faster than that of the designed linear controller, while overshoot is virtually eliminated. [ABSTRACT FROM AUTHOR]

Although the dc/dc converter as a common power supply is compatible with the electromagnetic compatibility standard, it may still bring the peak interference generated from one nonlinear load to the other loads. In this paper, a novel filter is designed in a low-voltage buck converter to suppress some extra peak harmonics generated from a nonlinear load, i.e., a pulse width modulation switching circuit. This filter is a variable capacitance filter (VCF) whose active devices are working in the linear areas. The capacitance of the filter is controlled to vary with the change of the switching load's carrier-frequency in the scheme. The impedance of the VCF can not only be matched with the switching load, but also avoid the mismatch effect from the parameter drift caused by the aging of the components. Thus, the VCF can suppress the introductive peak harmonics from the switching load at its carrier-frequency, especially the peak harmonics in the carrier-frequency region when the carrier-frequency modulation is used as the control strategy in the switching load. The feasibility and validity of the VCF are proved by simulations and experiments in a low-voltage buck converter, in which a boost circuit is working as one of its loads. [ABSTRACT FROM AUTHOR]

In this paper, a resonant voltage divider (RVD) is presented with the fixed switching frequency and used as a bus converter. The proposed RVD without any control loop embedded can reach very high efficiency from the light load to the rated load, especially for the light load. In addition, the bidirectional operation of the RVD is taken into consideration, so the RVD not only can transfer energy to the voltage-regulated module (VRM) but also can reversely transfer the energy to the dc bus during the download transient period of the VRM. However, the RVD has a serious drawback that a large surge current at startup causes components to be saturated or destroyed. Consequently, how to reduce such a current based on the proposed variable switching frequency at startup is also taken into account. In this paper, the bidirectional operation of the RVD and how to suppress the startup surge current are illustrated in detail, along with some simulated and experimental results to demonstrate its effectiveness. [ABSTRACT FROM AUTHOR]