Showing papers on "Voltage regulation published in 2015"
TL;DR: In this paper, a cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids, where the conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator.
Abstract: A cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids. The conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator. A noise-resilient voltage observer is introduced that uses neighbors’ data to estimate the average voltage across the microgrid. The voltage regulator processes this estimation and generates a voltage correction term to adjust the local voltage set point. This adjustment maintains the microgrid voltage level as desired by the tertiary control. The current regulator compares the local per-unit current of each converter with the neighbors’ and, accordingly, provides a second voltage correction term to synchronize per-unit currents and, thus, provide proportional load sharing. The proposed controller precisely handles the transmission line impedances. The controller on each converter communicates with only its neighbor converters on a communication graph. The graph is a sparse network of communication links spanned across the microgrid to facilitate data exchange. The global dynamic model of the microgrid is derived, and design guidelines are provided to tune the system's dynamic response. A low-voltage dc microgrid prototype is set up, where the controller performance, noise resiliency, link-failure resiliency, and the plug-and-play capability features are successfully verified.
715 citations
TL;DR: In this paper, a distributed controller for secondary frequency and voltage control in islanded microgrids is proposed, which uses localized information and nearest-neighbor communication to collectively perform secondary control actions.
Abstract: In this paper, we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive tradeoff between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances, or loads. The distributed architecture allows for flexibility and redundancy, eliminating the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.
600 citations
TL;DR: In this paper, a figure of merit called droop index (DI) is introduced in order to improve the performance of dc microgrid, which is a function of normalized current sharing difference and losses in the output side of the converters.
Abstract: This paper addresses load current sharing and cir- culating current issues of parallel-connected dc-dc converters in low-voltage dc microgrid. Droop control is the popular technique for load current sharing in dc microgrid. The main drawbacks of the conventional droop method are poor current sharing and drop in dcgrid voltage due tothe droop action. Circulating current issue will also arise due to mismatch in the converters output voltages. In this work, a figure of merit called droop index (DI) is introduced in order to improve the performance of dc microgrid, which is a function of normalized current sharing difference and losses in the output side of the converters. This proposed adaptive droop con- trol method minimizes the circulating current and current sharing difference between the converters based on instantaneous virtual resistance Rdroop .U singRdroop shifting, the proposed method also eliminates the tradeoff between current sharing difference and voltage regulation. The detailed analysis and design procedure are explained for two dc-dc boost converters connected in paral- lel. The effectiveness of the proposed method is verified by detailed simulation and experimental studies.
343 citations
TL;DR: This paper provides sufficient conditions under which the optimization problem can be solved via its convex relaxation, and demonstrates the operation of the algorithm, including its robustness against communication link failures, through several case studies involving 5-, 34-, and 123-bus power distribution systems.
Abstract: This paper addresses the problem of voltage regulation in power distribution networks with deep-penetration of distributed energy resources, e.g., renewable-based generation, and storage-capable loads such as plug-in hybrid electric vehicles. We cast the problem as an optimization program, where the objective is to minimize the losses in the network subject to constraints on bus voltage magnitudes, limits on active and reactive power injections, transmission line thermal limits and losses. We provide sufficient conditions under which the optimization problem can be solved via its convex relaxation. Using data from existing networks, we show that these sufficient conditions are expected to be satisfied by most networks. We also provide an efficient distributed algorithm to solve the problem. The algorithm adheres to a communication topology described by a graph that is the same as the graph that describes the electrical network topology. We illustrate the operation of the algorithm, including its robustness against communication link failures, through several case studies involving 5-, 34-, and 123-bus power distribution systems.
314 citations
TL;DR: In this article, a small-signal model of voltage source converter (VSC) is developed to investigate the stability of dc-link voltage control with varying operating conditions, such as grid strength, operating point, and control loops' interactions on the performance of VSC.
Abstract: In this paper, a small-signal model of voltage source converter (VSC) is developed to investigate the stability of dc-link voltage control. This model contributes to representing the dc-link voltage dynamics characteristics of VSC integrated to weak grid. Effects of grid strength, operating point, and control loops’ interactions on the performance of VSC are taken into consideration. Based on the proposed small-signal model, eigenvalue analysis is employed to study the stability of dc-link voltage control with varying operating conditions. Analysis results show that control loops’ interactions introduce a partial positive feedback to the dc-link voltage control in weak grid. Furthermore, the effect of control loops’ interactions on dc-link voltage control stability reaches largest when the bandwidth of the phase-locked loop is close to that of the dc-link voltage control. Time-domain simulations and experiments were conducted to validate the analysis.
264 citations
TL;DR: In this paper, the consequences of droop implementation on the voltage stability of dc power systems, whose loads are active and nonlinear, e.g., constant power loads, are shown.
Abstract: The stability of dc microgrids (MG s ) depends on the control strategy adopted for each mode of operation. In an islanded operation mode, droop control is the basic method for bus voltage stabilization when there is no communication among the sources. In this paper, it is shown the consequences of droop implementation on the voltage stability of dc power systems, whose loads are active and nonlinear, e.g., constant power loads. The set of parallel sources and their corresponding transmission lines are modeled by an ideal voltage source in series with an equivalent resistance and inductance. This approximate model allows performing a nonlinear stability analysis to predict the system qualitative behavior due to the reduced number of differential equations. Additionally, nonlinear analysis provides analytical stability conditions as a function of the model parameters and it leads to a design guideline to build reliable (MG s ) based on safe operating regions.
260 citations
TL;DR: Convergence to the configuration of minimum losses and feasible voltages is proved analytically for both a synchronous and an asynchronous version of the algorithm, where agents update their state independently one from the other.
Abstract: We consider the problem of exploiting the microgenerators dispersed in the power distribution network in order to provide distributed reactive power compensation for power losses minimization and voltage regulation. In the proposed strategy, microgenerators are smart agents that can measure their phasorial voltage, share these data with the other agents on a cyber layer, and adjust the amount of reactive power injected into the grid, according to a feedback control law that descends from duality-based methods applied to the optimal reactive power flow problem. Convergence to the configuration of minimum losses and feasible voltages is proved analytically for both a synchronous and an asynchronous version of the algorithm, where agents update their state independently one from the other. Simulations are provided in order to illustrate the performance and the robustness of the algorithm, and the innovative feedback nature of such strategy is discussed.
247 citations
TL;DR: Two novel backscattering uplink techniques are proposed for fast and energy-efficient data feedback for general data transmission using Manchester code and for fast duty cycle feedback to cater for fast load-transient responses.
Abstract: A 13.56 MHz wireless power transfer system with a 1X/2X reconfigurable resonant regulating (R $^3$ ) rectifier and wireless power control for biomedical implants is presented. Output voltage regulation is achieved through two mechanisms: 1) a local PWM loop at the secondary side controls the duty cycle of mode-switching of the rectifier between the 1X and 2X modes; and 2) a global control loop obtains the mode-switching information from the secondary side and send it back to the primary side through the wireless channel and adjusts the transmitter power of the primary coil to adapt to load and coupling variations. Two novel backscattering uplink techniques are proposed for fast and energy-efficient data feedback. The first is for general data transmission using Manchester code; and the second is for fast duty cycle feedback to cater for fast load-transient responses. Stability analysis of the entire system with the two control loops is also presented. The primary transmitter and the secondary R $^3$ rectifier are fabricated in 0.35 µm CMOS process with the digital control circuits implemented using FPGA. The measured maximum received power and receiver efficiency are 102 mW and 92.6%, respectively. For load transients, the overshoot and the undershoot are approximately 110 mV and the settling times are less than 130 µs.
247 citations
TL;DR: In this article, an improved droop control method was proposed to improve the reactive power sharing accuracy, which mainly includes two important operations: error reduction operation and voltage recovery operation, which is activated by the low-bandwidth synchronization signals.
Abstract: For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this paper proposes an improved droop control method. The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.
245 citations
TL;DR: H hierarchical control of HESS, composed of both centralized and distributed control, is proposed to enhance system reliability and a laboratory-scale dc microgrid is developed to verify the proposed hierarchical control.
Abstract: Hybridization of energy storages (ESs) with different characteristics takes advantages of all ESs. Centralized control with high-/low-pass filter (LPF) for system net power decomposition and ESs' power dispatch is usually implemented in hybrid ES system (HESS). In this paper, hierarchical control of HESS, composed of both centralized and distributed control, is proposed to enhance system reliability. The conventional HESS centralized control is refined with implementations of online iteration, secondary voltage regulation, and autonomous state-of-charge (SoC) recovery. ESs' power references are iteratively generated to maximize the utilization of ES ramp rates and power capacities. Secondary voltage regulation and autonomous SoC recovery are applied to minimize bus voltage deviation and limit slack terminal SoC variation, respectively. In case of communication failure, a novel algorithm for HESS distributed control is proposed to retain system operation. Bus voltage is regarded as the global indicator for system power balance, and droop relationships are imposed for ES control. System net power decomposition and ESs' power dispatch are realized with localized LPFs. SoC recovery in distributed control is implemented by tuning the threshold voltage of a slack terminal. A laboratory-scale dc microgrid is developed to verify the proposed hierarchical control of HESS.
TL;DR: New distributed controllers for secondary frequency and voltage control in islanded microgrids Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions.
Abstract: In this work we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive trade-off between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances or loads. The distributed architecture allows for flexibility and redundancy, and eliminates the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.
TL;DR: In this paper, a generalized voltage droop (GVD) control strategy for dc voltage control and power sharing in voltage source converter (VSC)-based multiterminal dc (MTDC) grids is proposed.
Abstract: This paper proposes a generalized voltage droop (GVD) control strategy for dc voltage control and power sharing in voltage source converter (VSC)-based multiterminal dc (MTDC) grids The proposed GVD control is implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid, and constitutes an alternative to the conventional voltage droop characteristics of voltage-regulating VSC stations, providing higher flexibility and, thus, controllability to these networks As a difference with other methods, the proposed GVD control strategy can be operated in three different control modes, including conventional voltage droop control, fixed active power control, and fixed dc voltage control, by adjusting the GVD characteristics of the voltage-regulating converters Such adjustment is carried out in the secondary layer of the hierarchical control structure The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability The simulation results, obtained by employing a CIGRE B4 dc grid test system, demonstrate the efficiency of the proposed approach and its flexibility in active power sharing and power control as well as voltage control In these analysis, it will be also shown how the transitions between the operating modes of the GVD control does not give rise to active power oscillations in the MTDC grids
TL;DR: In this paper, a decoupled active and reactive power control strategy was proposed to enhance system operation performance in large-scale grid-connected photovoltaic (PV) systems.
Abstract: Large-scale grid-connected photovoltaic (PV) systems significantly contribute to worldwide renewable energy growth and penetration, which has inspired the application of cascaded modular multilevel converters due to their unique features such as modular structures, enhanced energy harvesting capability, scalability and so on However, power distribution and control in the cascaded PV system faces tough challenge on output voltage overmodulation when considering the varied and nonuniform solar energy on segmented PV arrays This paper addresses this issue and proposes a decoupled active and reactive power control strategy to enhance system operation performance The relationship between output voltage components of each module and power generation is analyzed with the help of a newly derived vector diagram which illustrates the proposed power distribution principle On top of this, an effective control system including active and reactive components extraction, voltage distribution and synthesization, is developed to achieve independent active and reactive power distribution and mitigate the aforementioned issue Finally, a 3-MW, 12-kV PV system with the proposed control strategy is modeled and simulated in MATLAB and PSIM cosimulation platform A downscaled PV system including two cascaded 5-kW converters with proposed control strategy is also implemented in the laboratory Simulation and experimental results are provided to demonstrate the effectiveness of the proposed control strategy for large-scale grid-connected cascaded PV systems
TL;DR: In this article, a stochastic reactive power compensation scheme was developed to account for the increasing time-variability of distributed generation and demand, and an online reactive control scheme was devised to track variations in solar generation and household demand.
Abstract: Distribution microgrids are being challenged by reverse power flows and voltage fluctuations due to renewable generation, demand response, and electric vehicles Advances in photovoltaic (PV) inverters offer new opportunities for reactive power management provided PV owners have the right investment incentives In this context, reactive power compensation is considered here as an ancillary service Accounting for the increasing time-variability of distributed generation and demand, a stochastic reactive power compensation scheme is developed Given uncertain active power injections, an online reactive control scheme is devised This scheme is distribution-free and relies solely on power injection data Reactive injections are updated using the Lagrange multipliers of a second-order cone program Numerical tests on an industrial 47-bus microgrid and the residential IEEE 123-bus feeder corroborate the reactive power management efficiency of the novel stochastic scheme over its deterministic alternative, as well as its capability to track variations in solar generation and household demand
TL;DR: This paper proposes an algorithm for coordinated control of the distributed generators integrated to a dc microgrid (DCMG), in islanded and grid connected modes of operation, and a control strategy uses a combination of the feed-back and feed-forward control loops.
Abstract: This paper proposes an algorithm for coordinated control of the distributed generators integrated to a dc microgrid (DCMG), in islanded and grid connected modes of operation The proposed DCMG connects various types of nonconventional energy sources, storage system to the dc, and three-phase, as well as single-phase ac loads A control strategy for three-phase voltage source inverter to integrate the three-phase load, as well as utility grid into the DCMG, under various operating scenarios, has also been proposed The proposed control strategy uses a combination of the feed-back and feed-forward control loops Dual proportional integral controllers for ac voltage regulation and inner current control have been suggested in two rotating direct- and quadrature- axis synchronous reference frames for controlling the respective positive and negative sequence components Simulations are carried out to verify the robustness of the proposed algorithm and control strategy under different operating conditions including fault scenario and its effectiveness in maintaining the dc voltage of the microgrid
TL;DR: A ripple eliminator, which is a bidirectional buck-boost converter terminated with an auxiliary capacitor, is adopted to replace bulky capacitors in dc systems, and the total capacitance required can be much smaller than the originally needed.
Abstract: Bulky electrolytic capacitors, which are often needed in dc systems to filter out voltage ripples, considerably reduce power density and system reliability. In this paper, a ripple eliminator, which is a bidirectional buck–boost converter terminated with an auxiliary capacitor, is adopted to replace bulky capacitors in dc systems. The voltage ripples on the terminals (i.e., the dc bus) can be transferred to the auxiliary capacitor, and the ripples on the auxiliary capacitor can vary in a wide range. Moreover, the average voltage of the auxiliary capacitor can be controlled either lower or higher than the dc-bus voltage, which offers a wide operational range for the ripple eliminator and also the possibility of further reducing the auxiliary capacitance. Hence, the total capacitance required can be much smaller than the originally needed. After proposing a control strategy to transfer the voltage ripples to the auxiliary capacitor, three control strategies are proposed to regulate the auxiliary-capacitor voltage to maintain proper operation. Intensive experimental results are presented to demonstrate the performance.
TL;DR: In this paper, a robust controller design for output voltage regulation in a quadratic boost converter with high dc gain is discussed, where an inner loop based on sliding-mode control whose sliding surface is defined for the input inductor current is modified by a proportional integral compensator in an outer loop that operates over the output voltage error.
Abstract: A robust controller design to obtain output voltage regulation in a quadratic boost converter with high dc gain is discussed in this paper. The proposed controller has an inner loop based on sliding-mode control whose sliding surface is defined for the input inductor current. The current reference value of the sliding surface is modified by a proportional-integral compensator in an outer loop that operates over the output voltage error. The stability of the two-loop controller is proved by using the Routh-Hurwitz criterion, which determines a region in the K p -K i plane, where the closed-loop system is always stable. The analysis of the sliding-mode-based control loop is performed by means of the equivalent control method, while the outer loop compensator is derived by means of the Nyquist-based Robust Loop Shaping approach with the M-constrained Integral Gain Maximization technique. Robustness is analyzed in depth taking into account the parameter variation related with the operation of the converter in different equilibrium points. Simulations and experimental results are presented to validate the approach for a 20-100-W quadratic boost converter stepping-up a low dc voltage (15-25-V dc) to a 400-V dc level.
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TL;DR: In this article, a local reactive power (VAR) control framework is developed that can fast respond to voltage mismatch and address the robustness issues of (de-)centralized approaches against communication delays and noises.
Abstract: High penetration of distributed energy resources presents several challenges and opportunities for voltage regulation in power distribution systems. A local reactive power (VAR) control framework will be developed that can fast respond to voltage mismatch and address the robustness issues of (de-)centralized approaches against communication delays and noises. Using local bus voltage measurements, the proposed gradient-projection based schemes explicitly account for the VAR limit of every bus, and are proven convergent to a surrogate centralized problem with proper parameter choices. This optimality result quantifies the capability of local VAR control without requiring any real-time communications. The proposed framework and analysis generalize earlier results on the droop VAR control design, which may suffer from under-utilization of VAR resources in order to ensure stability. Numerical tests have demonstrated the validity of our analytical results and the effectiveness of proposed approaches implemented on realistic three-phase systems.
TL;DR: The proposed distributed secondary voltage control utilizes an average voltage sharing scheme to compensate the voltage deviation caused by the droop control to perform precise terminal voltage regulation and enhance the system reliability against system failures.
Abstract: This paper introduced a decentralized voltage control strategy for dc microgrids that is based on the droop method. The proposed distributed secondary voltage control utilizes an average voltage sharing scheme to compensate the voltage deviation caused by the droop control. Through nonexplicit communication, the proposed control strategy can perform precise terminal voltage regulation and enhance the system reliability against system failures. The distributed voltage compensators that resemble a centralized secondary voltage controller are implemented with the bi-proper anti-wind-up design method to solve the integration issues that necessarily lead to the saturation of the controller output efforts. The proposed concept of pilot bus voltage regulation shows the possibility of managing the terminal voltage without centralized structure. Moreover, the network dynamics are illustrated with a focus on cable resonance mode based on the eigenvalue analysis and small-signal modeling; analytical explanations with the development of equivalent circuits give a clear picture regarding how the controller parameters and droop gains affect the system damping performance. The proposed derivative droop control has been demonstrated to damp the oscillation and to improve the system stability during transients. Finally, the effectiveness and feasibility of the proposed control strategy are validated by both simulation and experimental evaluation.
TL;DR: In this paper, the authors explore the capability of using vehicle-to-grid (V2G) electric vehicles (EVs) to join distribution system voltage management, and to collaborate with online load tap changing (OLTC) transformers, voltage regulators (VRs), or shunt capacitors.
Abstract: Distributed solar generation has the potential to reach high penetration levels in distribution systems. However, its integration reshapes distribution system power flows and causes rapid-fluctuations in system statuses. The facts challenge major voltage management approaches, nowadays, such as using online load tap changing (OLTC) transformers, voltage regulators (VRs), or shunt capacitors. In this paper, we explore the capability of using vehicle-to-grid (V2G) electric vehicles (EVs) to join distribution system voltage management, and to collaborate with OLTCs to mitigate the voltage problems caused by distribution solar generations. A two-stage control method is proposed for this purpose. The first stage controls the making of rolling schedules for EV charging and OLTC tap positions, while the second controls the EVs to resist the solar generation fluctuation to maintain voltage profiles. A case system with simultaneous overvoltage/undervoltage risks is designed to test the effectiveness of the proposed method. The results demonstrate that both the over/undervoltage risks are mitigated.
TL;DR: Results from the case studies show that with appropriate setting and operating strategy, ES can mitigate the voltage and frequency fluctuation caused by wind speed fluctuation, load fluctuated, and generator tripping wherever it is installed in the microgrid.
Abstract: Voltage and frequency fluctuation associated with renewable integration have been well identified by power system operators and planners. At the microgrid level, a novel device for the implementation of dynamic load response, which is known as the electric springs (ES), has been developed for mitigating both active and reactive power imbalances. In this paper, a comprehensive control strategy is proposed for ES to participate in both voltage and frequency response control. It adopts the phase angle and amplitude control which respectively adjust the active power and the reactive power of the system. The proposed control strategy is validated using a model established with power system computer aided design/electro-magnetic transient in dc system. Results from the case studies show that with appropriate setting and operating strategy, ES can mitigate the voltage and frequency fluctuation caused by wind speed fluctuation, load fluctuation, and generator tripping wherever it is installed in the microgrid.
TL;DR: The possibility of leveraging the data provided by smart meters to understand the load characteristics is studied and a deterministic framework is proposed that formulates the VVO problem as a mixed-integer quadratically constrained programming problem, which is solved efficiently using advanced branch-and-cut techniques.
Abstract: The possibility of leveraging the data provided by smart meters to understand the load characteristics is studied in this paper. The loads are modeled as voltage-dependent elements to increase the accuracy of volt-VAR optimization (VVO) techniques for distribution systems. VVO techniques are part of the distribution management system and may be used for purposes such as loss reduction, voltage profile improvement, and conservation voltage reduction. A deterministic framework is proposed that formulates the VVO problem as a mixed-integer quadratically constrained programming problem, which is solved efficiently using advanced branch-and-cut techniques. The proposed framework is capable of optimally controlling capacitor banks, voltage regulators, and under-load tap changers (ULTCs) for day-ahead operation planning. The results indicate that loss reductions of up to 40% and a total demand reduction of up to 4.8% are achievable under some loading conditions in a radial test system. The effect of the load voltage dependence is also demonstrated through analytical simulations.
TL;DR: In this paper, a comprehensive overview of the power converter interfaces potentially favorable for the automotive, railways, aircrafts and small stationary domains is presented, and the importance of choosing the right power converter architecture and related technology is highlighted.
TL;DR: A new decentralized control scheme for islanded microgrids (ImGs) composed by the interconnection of distributed generation units (DGUs) that enables plug-and-play operations: when a DGU is plugged in or out, only DGUs physically connected to it have to retune their local controllers.
Abstract: In this paper we propose a new decentralized control scheme for Islanded microGrids (ImGs) composed by the interconnection of Distributed Generation Units (DGUs). Local controllers regulate voltage and frequency at the Point of Common Coupling (PCC) of each DGU and they are able to guarantee stability of the overall ImG. The control design procedure is decentralized, since, besides two global scalar quantities, the synthesis of a local controller uses only information on the corresponding DGU and lines connected to it. Most important, our design procedure enables Plug-and-Play (PnP) operations: when a DGU is plugged in or out, only DGUs physically connected to it have to retune their local controllers. We study the performance of the proposed controllers simulating different scenarios in MatLab/Simulink and using indexes proposed in IEEE standards.
TL;DR: A strategy that employs an adaptive voltage droop control to achieve accurate reactive power sharing is investigated, and the effectiveness of the proposed strategy is demonstrated on a 1.2 kVA prototype microgrid.
Abstract: In this paper, a strategy that employs an adaptive voltage droop control to achieve accurate reactive power sharing is investigated. Instead of controlling the output voltage of the inverter directly, the voltage droop slope is tuned to compensate for the mismatch in the voltage drops across feeders by using communication links. If the communication channel is disrupted, the controller will operate with the last tuned droop coefficient, which is shown to still outperform the controller with the initial fixed droop coefficient. Also, the net control action of the adaptive droop terms is demonstrated to have a negligible effect on the microgrid bus voltage. Since communication is not used within the tuning control loop, the strategy is inherently immune to delays in communication links. A small-signal model of the proposed controller is presented, and the effectiveness of the proposed strategy is demonstrated on a 1.2 kVA prototype microgrid.
TL;DR: The proposed method for the explicit control of the grid status is based on a common abstract model characterized by the main property of being composable, which means subsystems can be aggre-gated into virtual devices that hide their internal complexity.
TL;DR: In this article, a current-fed LLC resonant converter that is able to achieve high efficiency over a wide input voltage range is derived by integrating a two-phase interleaved boost circuit and a full-bridge LLC circuit together by virtue of sharing the same fullbridge switching unit.
Abstract: This paper proposes a current-fed LLC resonant converter that is able to achieve high efficiency over a wide input voltage range. It is derived by integrating a two-phase interleaved boost circuit and a full-bridge LLC circuit together by virtue of sharing the same full-bridge switching unit. Compared with conventional full-bridge LLC converter, the gain characteristic is improved in terms of both gain range and optimal operation area, fixed-frequency pulsewidth-modulated (PWM) control is employed to achieve output voltage regulation, and the input current ripple is minimized as well. The voltage across the turned-off primary-side switch can be always clamped by the bus voltage, reducing the switch voltage stress. Besides, its other distinct features, such as single-stage configuration, and soft switching for all switches also contribute to high power conversion efficiency. The operation principles are presented, and then the main characteristics regarding gain, input current ripple, and zero-voltage switching (ZVS) considering the nonlinear output capacitance of MOSFET are investigated and compared with conventional solutions. Also, the design procedure for some key parameters is presented, and two kinds of interleaved boost integrated resonant converter topologies are generalized. Finally, experimental results of a converter prototype with 120–240 V input and 24 V/25 A output verify all considerations.
TL;DR: In this paper, a three-port dc-dc converter for stand-alone PV systems, based on an improved Flyback-Forward topology, is presented. And a comprehensive modulation strategy utilizing both PWM and phase-shifted control that satisfies the requirement of PV power systems to achieve MPPT and output voltage regulation is proposed.
Abstract: System efficiency and cost effectiveness are of critical importance for photovoltaic (PV) systems. This paper addresses the two issues by developing a novel three-port dc–dc converter for stand-alone PV systems, based on an improved Flyback-Forward topology. It provides a compact single-unit solution with a combined feature of optimized maximum power point tracking (MPPT), high step-up ratio, galvanic isolation, and multiple operating modes for domestic and aerospace applications. A theoretical analysis is conducted to analyze the operating modes followed by simulation and experimental work. This paper is focused on a comprehensive modulation strategy utilizing both PWM and phase-shifted control that satisfies the requirement of PV power systems to achieve MPPT and output voltage regulation. A 250-W converter was designed and prototyped to provide experimental verification in term of system integration and high conversion efficiency.
TL;DR: In this article, the authors proposed a model predictive control (RCMV-MPC) algorithm to reduce the common-mode voltage of three-phase voltage source inverters (VSIs).
Abstract: In this paper, we propose model predictive control methods to reduce the common-mode voltage of three-phase voltage source inverters (VSIs). In the reduced common-mode voltage-model predictive control (RCMV-MPC) methods proposed in this paper, only nonzero voltage vectors are utilized to reduce the common-mode voltage as well as to control the load currents. In addition, two nonzero voltage vectors are selected from the cost function at every sampling period, instead of using only one optimal vector during one sampling period. The two selected nonzero vectors are distributed in one sampling period in such a way as to minimize the error between the measured load current and the reference. Without utilizing the zero vectors, the common-mode voltage controlled by the proposed RCMV-MPC algorithms can be restricted within ± V dc/6. Furthermore, application of the two nonzero vectors with optimal time sharing between them can yield satisfactory load current ripple performance without using the zero vectors. Thus, the proposed RCMV-MPC methods can reduce the common-mode voltage as well as control the load currents with fast transient response and satisfactory load current ripple performance compared with the conventional model predictive control method. Simulation and experimental results are included to verify the effectiveness of the proposed RCMV-MPC methods.