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Showing papers on "AC power published in 2013"


Journal ArticleDOI
TL;DR: In this article, the authors review the current status and implementation impact of V2G/grid-to-vehicle (G2V) technologies on distributed systems, requirements, benefits, challenges, and strategies for VUE interfaces of both individual vehicles and fleets.
Abstract: Plug-in vehicles can behave either as loads or as a distributed energy and power resource in a concept known as vehicle-to-grid (V2G) connection. This paper reviews the current status and implementation impact of V2G/grid-to-vehicle (G2V) technologies on distributed systems, requirements, benefits, challenges, and strategies for V2G interfaces of both individual vehicles and fleets. The V2G concept can improve the performance of the electricity grid in areas such as efficiency, stability, and reliability. A V2G-capable vehicle offers reactive power support, active power regulation, tracking of variable renewable energy sources, load balancing, and current harmonic filtering. These technologies can enable ancillary services, such as voltage and frequency control and spinning reserve. Costs of V2G include battery degradation, the need for intensive communication between the vehicles and the grid, effects on grid distribution equipment, infrastructure changes, and social, political, cultural, and technical obstacles. Although V2G operation can reduce the lifetime of vehicle batteries, it is projected to become economical for vehicle owners and grid operators. Components and unidirectional/bidirectional power flow technologies of V2G systems, individual and aggregated structures, and charging/recharging frequency and strategies (uncoordinated/coordinated smart) are addressed. Three elements are required for successful V2G operation: power connection to the grid, control and communication between vehicles and the grid operator, and on-board/off-board intelligent metering. Success of the V2G concept depends on standardization of requirements and infrastructure decisions, battery technology, and efficient and smart scheduling of limited fast-charge infrastructure. A charging/discharging infrastructure must be deployed. Economic benefits of V2G technologies depend on vehicle aggregation and charging/recharging frequency and strategies. The benefits will receive increased attention from grid operators and vehicle owners in the future.

788 citations


Journal ArticleDOI
TL;DR: An improved droop controller is proposed to achieve accurate proportional load sharing without meeting these two requirements and to reduce the load voltage drop due to the load effect and the droop effect.
Abstract: In this paper, the inherent limitations of the conventional droop control scheme are revealed. It has been proven that parallel-operated inverters should have the same per-unit impedance in order for them to share the load accurately in proportion to their power ratings when the conventional droop control scheme is adopted. The droop controllers should also generate the same voltage set-point for the inverters. Both conditions are difficult to meet in practice, which results in errors in proportional load sharing. An improved droop controller is then proposed to achieve accurate proportional load sharing without meeting these two requirements and to reduce the load voltage drop due to the load effect and the droop effect. The load voltage can be maintained within the desired range around the rated value. The strategy is robust against numerical errors, disturbances, noises, feeder impedance, parameter drifts and component mismatches. The only sharing error, which is quantified in this paper, comes from the error in measuring the load voltage. When there are errors in the voltage measured, a fundamental tradeoff between the voltage drop and the sharing accuracy appears. It has also been explained that, in order to avoid errors in power sharing, the global settings of the rated voltage and frequency should be accurate. Experimental results are provided to verify the analysis and design.

777 citations


Journal ArticleDOI
TL;DR: An improved analytical (IA) method based on IA expressions to calculate the optimal size of four different DG types and a methodology to identify the best location for DG allocation is proposed, and a technique to get the optimal power factor is presented for DG capable of delivering real and reactive power.
Abstract: This paper investigates the problem of multiple distributed generator (DG units) placement to achieve a high loss reduction in large-scale primary distribution networks. An improved analytical (IA) method is proposed in this paper. This method is based on IA expressions to calculate the optimal size of four different DG types and a methodology to identify the best location for DG allocation. A technique to get the optimal power factor is presented for DG capable of delivering real and reactive power. Moreover, loss sensitivity factor (LSF) and exhaustive load flow (ELF) methods are also introduced. IA method was tested and validated on three distribution test systems with varying sizes and complexity. Results show that IA method is effective as compared with LSF and ELF solutions. Some interesting results are also discussed in this paper.

689 citations


Journal ArticleDOI
TL;DR: The modeling, control design, and stability analysis of parallel-connected three-phase VSIs are derived and a hierarchical control scheme for the paralleled VSI system control architecture is developed.
Abstract: Power-electronics-based microgrids (MGs) consist of a number of voltage source inverters (VSIs) operating in parallel. In this paper, the modeling, control design, and stability analysis of parallel-connected three-phase VSIs are derived. The proposed voltage and current inner control loops and the mathematical models of the VSIs are based on the stationary reference frame. A hierarchical control scheme for the paralleled VSI system is developed comprising two levels. The primary control includes the droop method and the virtual impedance loops, in order to share active and reactive powers. The secondary control restores the frequency and amplitude deviations produced by the primary control. Also, a synchronization algorithm is presented in order to connect the MG to the grid. Experimental results are provided to validate the performance and robustness of the parallel VSI system control architecture.

610 citations


Journal ArticleDOI
TL;DR: In this article, a double-layer coordinated control approach for microgrid energy management is proposed, which consists of two layers: the schedule layer and the dispatch layer, which provides power of controllable units based on real-time data.
Abstract: There are two operation modes of microgrids: grid-connected mode and stand-alone mode. Normally, a microgrid will be connected to the main grid for the majority of time, i.e., operates in the grid-connected mode. In the stand-alone mode, a microgrid is isolated from the main grid; the highest priority for microgrids is to keep a reliable power supply to customers instead of economic benefits. So, the objectives and energy management strategies are different in two modes. In this paper, a novel double-layer coordinated control approach for microgrid energy management is proposed, which consists of two layers: the schedule layer and the dispatch layer. The schedule layer obtains an economic operation scheme based on forecasting data, while the dispatch layer provides power of controllable units based on real-time data. Errors between the forecasting and real-time data are resolved through coordination control of the two layers by reserving adequate active power in the schedule layer, then allocating that reserve in the dispatch layer to deal with the indeterminacy of uncontrollable units. A typical-structure microgrid is studied as an example, and simulation results are presented to demonstrate the performance of the proposed double-layer coordination control method in both grid-connected mode and stand-alone mode.

537 citations


Journal ArticleDOI
TL;DR: In this paper, the authors describe the implementation of a voltage control loop within PV inverters that maintains the voltage within acceptable bounds by absorbing or supplying reactive power, which can be considered to be a form of distributed Volt/VAr control.
Abstract: A major technical obstacle for rooftop photovoltaics (PV) integration into existing distribution systems is the voltage rise due to the reverse power flow from the distributed PV sources. This paper describes the implementation of a voltage control loop within PV inverters that maintains the voltage within acceptable bounds by absorbing or supplying reactive power. In principle, this can be considered to be a form of distributed Volt/VAr control, which is conventionally performed by coordinated control of capacitor banks and transformer tap changers. Comprehensive simulation studies on detailed feeder models are used to demonstrate that the proposed control scheme will mitigate voltage rises.

410 citations


Journal ArticleDOI
TL;DR: In this article, the main design objective of photovoltaic (PV) systems has been, for a long time, to extract the maximum power from the PV array and inject it into the ac grid.
Abstract: The main design objective of photovoltaic (PV) systems has been, for a long time, to extract the maximum power from the PV array and inject it into the ac grid. Therefore, the maximum power point tracking (MPPT) of a uniformly irradiated PV array and the maximization of the conversion efficiency have been the main design issues. However, when the PV plant is connected to the grid, special attention has to be paid to the reliability of the system, the power quality, and the implementation of protection and grid synchronization functions. Modern power plants are required to maximize their energy production, requiring suitable control strategies to solve the problems related to the partial shading phenomena and different orientation of the PV modules toward the sun. Moreover, the new policy concerning the injection of reactive power into the grid makes the development of suitable topologies and control algorithms mandatory. A general view of actual solutions for applications of the PV energy systems is presented. This article covers several important issues, including the most reliable models used for simulation, which are useful in the design of control systems, and the MPPT function, particularly in distributed applications. The main topologies used in the PV power processing system and, finally, grid connection aspects are discussed, with emphasis on synchronization, protections, and integration.

406 citations


Journal ArticleDOI
TL;DR: In this article, a droop control method with a restoration mechanism is proposed to improve reactive power sharing in a microgrid, and its operation principle and control method are explained and analyzed.
Abstract: Microgrid is widely accepted as an effective mean of integrating various distributed energy resources (DERs) through their interface converters to provide electric power of high quality and reliability. These distributed resources interface converters can operate in an autonomous fashion without any communication for enhanced system reliability and reduced complexity. Conventionally, the real power-frequency droop control and the reactive power-voltage magnitude droop are adopted as the decentralized control strategies in these DERs interface converters for the autonomous power sharing operations. However, the reactive power sharing of $Q\hbox{--}V$ droop control often deteriorates due to its dependence on the line impedances. In this paper, a $Q\hbox{--}\dot{V}$ droop control method with $\dot{V}$ restoration mechanism is proposed to improve reactive power sharing. Its operation principle and control method are explained and analyzed. Simulation and experimental results are presented to validate the effectiveness of the proposed method.

356 citations


Journal ArticleDOI
TL;DR: The particle swarm optimization (PSO) technique has been used to solve the optimal placement of DGs and the optimal power factor for DG supplying, both real and reactive power, has been obtained.

322 citations


Journal ArticleDOI
TL;DR: In this paper, an appropriate control scheme is developed for controlling the interlinking converter to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources.
Abstract: The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence.

321 citations


Journal ArticleDOI
TL;DR: A stationary-frame control method for voltage unbalance compensation in an islanded microgrid is proposed, based on the proper control of DGs interface converters, which demonstrates the effectiveness of the proposed method in the compensation of voltage un balance.
Abstract: Recently, there has been an increasing interest in using distributed generators (DGs) not only to inject power into the grid but also to enhance the power quality. In this paper, a stationary-frame control method for voltage unbalance compensation in an islanded microgrid is proposed. This method is based on the proper control of DGs interface converters. The DGs are properly controlled to autonomously compensate for voltage unbalance while sharing the compensation effort and also active and reactive powers. The control system of the DGs mainly consists of active and reactive power droop controllers, a virtual impedance loop, voltage and current controllers, and an unbalance compensator. The design approach of the control system is discussed in detail, and simulation and experimental results are presented. The results demonstrate the effectiveness of the proposed method in the compensation of voltage unbalance.

Journal ArticleDOI
TL;DR: The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults.
Abstract: Ancillary services for distributed generation (DG) systems become a challenging issue to smartly integrate renewable-energy sources into the grid. Voltage control is one of these ancillary services which can ride through and support the voltage under grid faults. Grid codes from the transmission system operators describe the behavior of the energy source, regulating voltage limits and reactive power injection to remain connected and support the grid under fault. On the basis that different kinds of voltage sags require different voltage support strategies, a flexible control scheme for three-phase grid-connected inverters is proposed. In three-phase balanced voltage sags, the inverter should inject reactive power in order to raise the voltage in all phases. In one- or two-phase faults, the main concern of the DG inverter is to equalize voltages by reducing the negative symmetric sequence and clear the phase jump. Due to system limitations, a balance between these two extreme policies is mandatory. Thus, over- and undervoltage can be avoided, and the proposed control scheme prevents disconnection while achieving the desired voltage support service. The main contribution of this work is the introduction of a control algorithm for reference current generation that provides flexible voltage support under grid faults. Two different voltage sags have been experimentally tested to illustrate the behavior of the proposed voltage support control scheme.

Journal ArticleDOI
TL;DR: Simulation and experimental results show that the proposed APF scheme has good power decoupling performance and is more suited for high-power applications where switching frequency is limited.
Abstract: Single-phase pulsewidth modulation rectifiers suffer from ripple power pulsating at twice the line frequency. The ripple power is usually filtered by a bulky capacitor bank or an LC branch, resulting in lower power density. The alternative way is active power decoupling, which uses an active circuit to direct the pulsating power into another energy-storage component. The main dc-link filter capacitor can, therefore, be reduced substantially. This paper proposed a new scheme of active power decoupling. The circuit consists of a third leg, an energy-storage capacitor and a smoothing inductor. The topology combined the advantages of high energy-storage efficiency and low requirement on control bandwidth. Both the pulsating power from the ac source and the reactive power of the smoothing inductors are taken into consideration when deriving the power decoupling scheme. The active power filter's (APF) capacitor voltage control system consists of inner loop pole-placement control and outer loop proportional-resonant control. To enhance the steady-state performance, the capacitor voltage reference is modified in a closed-loop manner. Simulation and experimental results show that the proposed APF scheme has good power decoupling performance and is more suited for high-power applications where switching frequency is limited.

Journal ArticleDOI
TL;DR: In this paper, a generic three-phase power flow algorithm is formulated for islanded microgrids, where the features of distribution systems, i.e., threephase feeder models, unbalanced loads and load models have been taken in consideration.
Abstract: A new formulation is required to provide a proper power flow analysis in islanded microgrids taking into consideration their special philosophy of operation. In this paper, a novel and generic three-phase power flow algorithm is formulated for islanded microgrids. The algorithm is novel since it adapts the real characteristics of the islanded microgrid operation; i.e., 1) some of the distributed generation (DG) units are controlled using the droop control methods and their generated active and reactive power are dependent on the power flow variables; 2) the steady-state system frequency is considered as one of the power flow variables. The proposed algorithm is generic, where the features of distribution systems, i.e., three-phase feeder models, unbalanced loads and load models have been taken in consideration. Further, all possible operation modes of DG units (droop, PV, or PQ) have been considered. The problem has been formulated as a set of nonlinear equations. A globally convergent Newton-trust region method has been proposed to solve this set of nonlinear equations. The proposed algorithm is a helpful tool to perform accurate steady state studies of the islanded microgrid. Different case studies have been carried out to test the effectiveness and the robustness of the proposed algorithm.

Journal ArticleDOI
TL;DR: In this article, the authors propose an architecture for voltage regulation in distribution networks that relies on controlling reactive power injections provided by distributed energy resources (DERs), where a local controller on each bus monitors the bus voltage and, whenever there is a voltage violation, it uses locally available information to estimate the amount of reactive power that needs to be injected into the bus in order to correct the violation.
Abstract: In this paper, we propose an architecture for voltage regulation in distribution networks that relies on controlling reactive power injections provided by distributed energy resources (DERs). A local controller on each bus of the network monitors the bus voltage and, whenever there is a voltage violation, it uses locally available information to estimate the amount of reactive power that needs to be injected into the bus in order to correct the violation. If the DERs connected to the bus can collectively provide the reactive power estimated by the local controller, they are instructed to do so. Otherwise, the local controller initiates a request for additional reactive power support from other controllers at neighboring buses through a distributed algorithm that relies on a local exchange of information among neighboring controllers. We show that the proposed architecture helps prevent voltage violations and shapes the voltage profile in radial distribution networks, even in the presence of considerable penetration of variable generation and loads. We present several case studies involving 8-, 13-, and 123-bus distribution systems to illustrate the operation of the architecture.

Proceedings ArticleDOI
Jiangchao Qin1
21 Jul 2013
TL;DR: In this article, a model predictive control (MPC) strategy is proposed to eliminate the circulating currents and carry out the voltage balancing task of an MMC-based back-to-back HVDC system.
Abstract: The modular multilevel converter (MMC) is one of the most potential converter topologies for high-power/voltage systems, specifically for high-voltage direct current (HVDC). One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents of converter arms while the capacitor voltages are maintained balanced. This paper proposes a model predictive control (MPC) strategy that takes the advantage of a cost function minimization technique to eliminate the circulating currents and carry out the voltage balancing task of an MMC-based back-to-back HVDC system. A discrete-time mathematical model of the system is derived and a predictive model corresponding to the discrete-time model is developed. The predictive model is used to select the best switching states of each MMC unit based on evaluation and minimization a defined cost function associated with the control objectives of MMC units and the overall HVDC system. The proposed predictive control strategy: 1) enables control of real and reactive power of the HVDC system; 2) achieves capacitor voltage balancing of the MMC units; and 3) mitigates the circulating currents of the MMC units. Performance of the proposed MPC-based strategy for a five-level back-to-back MMC-HVDC is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate a satisfactory response of the MMC-HVDC station operating based on the proposed MPC strategy, under various conditions.

Journal ArticleDOI
TL;DR: In this article, a new constrained multi-objective Particle Swarm Optimization (PSO) based wind Turbine Generation Unit (WTGU) and photovoltaic (PV) array placement approach for power loss reduction and voltage stability improvement of radial distribution system is proposed.

Journal ArticleDOI
TL;DR: In this article, the authors consider the problem of optimal reactive power compensation for the minimization of power distribution losses in a smart microgrid and propose an approximate model for the power distribution network, which allows them to cast the problem into the class of convex quadratic, linearly constrained optimization problems.
Abstract: We consider the problem of optimal reactive power compensation for the minimization of power distribution losses in a smart microgrid. We first propose an approximate model for the power distribution network, which allows us to cast the problem into the class of convex quadratic, linearly constrained, optimization problems. We then consider the specific problem of commanding the microgenerators connected to a microgrid, in order to achieve the optimal injection of reactive power. For this task, we design a randomized, leader-less, gossip-like optimization algorithm. We show how a distributed approach is possible, where microgenerators need to have only a partial knowledge of the problem parameters and of the state, and can perform only local measurements. For the proposed algorithm, we provide conditions for convergence together with an analytic characterization of the convergence speed. The analysis shows that, in radial networks, the best performance is achieved when we command cooperation among microgenerators that are neighbors in the electric topology. Numerical simulations are included to validate both the proposed model and the analytic results about the performance of the proposed algorithm.

Journal ArticleDOI
TL;DR: In this paper, an Artificial Bee Colony (ABC) algorithm is employed as the main optimizer for optimal adjustments of the power system control variables of the OPF problem, which involves both continuous and discrete variables.

Journal ArticleDOI
TL;DR: This work forms a constrained optimization that aims to minimize power losses subject to finite inverter capacity and upper and lower voltage limits at all nodes in the circuit and explores protocols based on the dual-ascent method and on the alternating direction method of multipliers (ADMMs), finding that the ADMM protocol performs significantly better.
Abstract: We formulate the control of reactive power generation by photovoltaic inverters in a power distribution circuit as a constrained optimization that aims to minimize reactive power losses subject to finite inverter capacity and upper and lower voltage limits at all nodes in the circuit. When voltage variations along the circuit are small and losses of both real and reactive powers are small compared to the respective flows, the resulting optimization problem is convex. Moreover, the cost function is separable enabling a distributed, on-line implementation with node-local computations using only local measurements augmented with limited information from the neighboring nodes communicated over cyber channels. Such an approach lies between the fully centralized and local policy approaches previously considered. We explore protocols based on the dual ascent method and on the Alternating Direction Method of Multipliers (ADMM) and find that the ADMM protocol performs significantly better.

Journal ArticleDOI
TL;DR: In this paper, the authors present a summary of the available single-phase ac-dc topologies used for EV/PHEV, level-1 and -2 on-board charging and for providing reactive power support to the utility grid.
Abstract: This paper presents a summary of the available single-phase ac-dc topologies used for EV/PHEV, level-1 and -2 on-board charging and for providing reactive power support to the utility grid. It presents the design motives of single-phase on-board chargers in detail and makes a classification of the chargers based on their future vehicle-to-grid usage. The pros and cons of each different ac-dc topology are discussed to shed light on their suitability for reactive power support. This paper also presents and analyzes the differences between charging-only operation and capacitive reactive power operation that results in increased demand from the dc-link capacitor (more charge/discharge cycles and increased second harmonic ripple current). Moreover, battery state of charge is spared from losses during reactive power operation, but converter output power must be limited below its rated power rating to have the same stress on the dc-link capacitor.

Journal ArticleDOI
TL;DR: A systematic and optimized approach for designing microgrids taking into account system reliability- and supply-security-related aspects is presented, and the effect of optimization coefficients on the design and the robustness of the algorithm are investigated using sensitivity studies.
Abstract: Microgrids are known as clusters of distributed energy resources serving a group of distributed loads in grid-connected and isolated grid modes. Nowadays, the concept of microgrids has become a key subject in the smart grid area, demanding a systematic procedure for their optimal construction. According to the IEEE Std 1547.4, large distribution systems can be clustered into a number of microgrids to facilitate powerful control and operation infrastructure in future distribution systems. However, clustering large systems into a set of microgrids with high reliability and security is not reported in current literature. To fill-out this gap, this paper presents a systematic and optimized approach for designing microgrids taking into account system reliability- and supply-security-related aspects. The optimum design considers sustained and temporary faults, for system reliability via a combined probabilistic reliability index, and real and reactive power balance, for supply security. The loads are assumed to be variable and different distributed generation (DG) technologies are considered. Conceptual design, problem formulation and solution algorithms are presented in this paper. The well-known PG&E 69-bus distribution system is selected as the test system. The effect of optimization coefficients on the design and the robustness of the algorithm are investigated using sensitivity studies.

Journal ArticleDOI
TL;DR: In this article, a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells is proposed for high-voltage dc transmission systems, which offers the operational flexibility of VSC-based systems in terms of active and reactive power control.
Abstract: This paper proposes a new breed of high-voltage dc (HVDC) transmission systems based on a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells. The proposed HVDC system offers the operational flexibility of VSC-based systems in terms of active and reactive power control, black-start capability, in addition to improved ac fault ride-through capability and the unique feature of current-limiting capability during dc side faults. Additionally, it offers features such as smaller footprint and a larger active and reactive power capability curve than existing VSC-based HVDC systems, including those using modular multilevel converters. To illustrate the feasibility of the proposed HVDC system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to ac and dc side faults.

Journal ArticleDOI
TL;DR: In this article, a new control strategy that enables photovoltaic systems to adjust the active power outputs and provide frequency regulation to power systems is proposed. But the focus of this paper is to develop a new controller that enables PVs to adjust active power output to provide ancillary services.
Abstract: To maximize the revenue from selling energy, photovoltaic systems (PVs) in general operate in the so-called maximum power point tracking mode. However, the increasing penetration of renewable energy sources in power systems has motivated the design of innovative control to provide ancillary services. The focus of this paper is to develop a new control strategy that enables PVs to adjust the active power outputs and provide frequency regulation to power systems. In this strategy, two different modes are designed: 1) the frequency droop control mode for PVs to provide primary frequency support to power systems, and 2) the emergency control mode to prevent system frequency collapse and, therefore, to prevent too much generation tripping after fault. Based on a detailed PV dynamic model, simulation results show the effectiveness of the proposed control strategy in improving the frequency stability.

Journal ArticleDOI
TL;DR: In this paper, a low voltage ride through (LVRT) control scheme is proposed to enhance the ability of reactive power support of a doubly fed induction generator (DFIG) based wind turbine during serious voltage dips.
Abstract: The paper presents a new control strategy to enhance the ability of reactive power support of a doubly fed induction generator (DFIG) based wind turbine during serious voltage dips. The proposed strategy is an advanced low voltage ride through (LVRT) control scheme, with which a part of the captured wind energy during grid faults is stored temporarily in the rotor's inertia energy and the remaining energy is available to the grid while the DC-link voltage and rotor current are kept below the dangerous levels. After grid fault clearance, the control strategy ensures smooth release of the rotor's excessive inertia energy into the grid. Based on these designs, the DFIG's reactive power capacity on the stator and the grid side converter is handled carefully to satisfy the new grid code requirements strictly. Simulation studies are presented and discussed.

Proceedings ArticleDOI
01 Dec 2013
TL;DR: In this article, the authors consider a class of local volt/var control schemes where the control decision on the reactive power at a bus depends only on the local bus voltage and show that the dynamical system has a unique equilibrium by interpreting the dynamics as a distributed algorithm for solving a certain convex optimization problem whose unique optimal point is the system equilibrium.
Abstract: We consider a class of local volt/var control schemes where the control decision on the reactive power at a bus depends only on the local bus voltage. These local algorithms form a feedback dynamical system and collectively determine the bus voltages of a power network. We show that the dynamical system has a unique equilibrium by interpreting the dynamics as a distributed algorithm for solving a certain convex optimization problem whose unique optimal point is the system equilibrium. Moreover, the objective function serves as a Lyapunov function implying global asymptotic stability of the equilibrium. The optimization based model does not only provide a way to characterize the equilibrium, but also suggests a principled way to engineer the control. We apply the results to study the parameter setting for the inverter-based volt/var control in the proposed IEEE 1547.8 standard.

Journal ArticleDOI
TL;DR: In this article, a centralized, adaptive load shedding algorithm, which uses both voltage and frequency information provided by phasor measurement units (PMUs), is proposed, and the main contribution of the new method is the consideration of reactive power together with active power in the load shedding strategy.
Abstract: Under frequency load shedding (UFLS) and under voltage load shedding (UVLS) are attracting more attention, as large disturbances occur more frequently than in the past. Usually, these two schemes work independently from each other, and are not designed in an integrated way to exploit their combined effect on load shedding. Besides, reactive power is seldom considered in the load shedding process. To fill this gap, we propose in this paper a new centralized, adaptive load shedding algorithm, which uses both voltage and frequency information provided by phasor measurement units (PMUs). The main contribution of the new method is the consideration of reactive power together with active power in the load shedding strategy. Therefore, this method addresses the combined voltage and frequency stability issues better than the independent approaches. The new method is tested on the IEEE 39-Bus system, in order to compare it with other methods. Simulation results show that, after large disturbance, this method can bring the system back to a new stable steady state that is better from the point of view of frequency and voltage stability, and loadability.

01 Oct 2013
TL;DR: In this paper, the authors proposed a method to identify unidirectional flows with a preferred flow direction and applied it on a casestudy for the European super grid.
Abstract: The construction of the North Sea Super Grid is the major step towards meetingthe future demand for electric power transmission in northern Europe This gridwill likely also extend onshore towards the load centres, and eventually form theEuropean Super GridLarge-scale electric power generation at remote locations will lead tosignificant long distance power flows with a preferred flow direction A methodto identify these unidirectional flows has been developed and applied on a casestudy, indicating the importance to consider unidirectional flows when designinga super gridVoltage source converter based HVDC appears to be the best technicalsolution for the implementation of long distance transmission in such an offshoresuper grid AC technology appears to be the most convenient choice for offshorenodes, but DC might also gain importance in this field, if reliable and affordableDC protection systems become available A meshed DC grid offers significantadvantages towards a solution with many independent point-to-point HVDClinks, but also here protection is an unsolved issue that has to be overcome firstReliability assessment of HVDC-based super grids is still very difficult,because operational experience with new technologies like the modularmultilevel converter is limited This leads to a lack of data to calculate thefailure probabilitiesA test system with a DC grid and the connected AC grids has been developedto serve as a common reference for a variety of DC grid studiesUnlike classical AC grids, DC grids will be dominated by power electronicsand the system behaviour will be determined to a large extent by the controllersof those power electronic systems Large-scale implementation of powerelectronics with inappropriate control design has led to problems in AC systemsbefore Photovoltaic generation systems in Germany are a good example forthisA simplified AC frequency model has been developed to assess how powerelectronic systems influence the grid frequency This model has been used to simulate how photovoltaic generation systems in Germany can endanger systemstability A ‘grid-friendly’ charging controller for plug-in electric vehicles withbattery storage has been developed, and simulations have indicated that thiscontrol can contribute significantly to system stabilityEven though AC and DC grids have some significant differences, some ofthe general concepts and lessons regarding balancing are true for both, andtomorrow’s DC grids can learn from today’s AC challengesThe balance in a DC grid should be defined as a current balance rather thanan active power balance (as it is used in AC grids), and the voltage can serve asa balance indicator, similar to AC frequency in AC grids The control base forcontrolling the voltage should also be current instead of active power, leadingto linear system behaviour and a linear control taskHVDC converter control methods can be regarded as cases of droop controlwith one or more linear segments in the characteristic control curve Withinone linear segment of the control curve, a HVDC converter can be representedby the Thevenin or Norton equivalent circuitTo unify a variety of proposed control concepts, Undead-band droop controlhas been proposed as a general piece-wise linear voltage control, which includesall other proposed methods as special implementations of undead-band droopcontrol This concept could also be applied for other tasks than DC voltagecontrol like AC frequency control

Journal ArticleDOI
TL;DR: A power-efficient wireless stimulating system for a head-mounted deep brain stimulator (DBS) is presented, which increases the stimulation efficiency up to 30% higher than a fixed supply voltage and achieves high AC-DC power conversion efficiency (PCE) through active synchronous switching.
Abstract: A power-efficient wireless stimulating system for a head-mounted deep brain stimulator (DBS) is presented. A new adaptive rectifier generates a variable DC supply voltage from a constant AC power carrier utilizing phase control feedback, while achieving high AC-DC power conversion efficiency (PCE) through active synchronous switching. A current-controlled stimulator adopts closed-loop supply control to automatically adjust the stimulation compliance voltage by detecting stimulation site potentials through a voltage readout channel, and improve the stimulation efficiency. The stimulator also utilizes closed-loop active charge balancing to maintain the residual charge at each site within a safe limit, while receiving the stimulation parameters wirelessly from the amplitude-shift-keyed power carrier. A 4-ch wireless stimulating system prototype was fabricated in a 0.5-μm 3M2P standard CMOS process, occupying 2.25 mm2. With 5 V peak AC input at 2 MHz, the adaptive rectifier provides an adjustable DC output between 2.5 V and 4.6 V at 2.8 mA loading, resulting in measured PCE of 72 ~ 87%. The adaptive supply control increases the stimulation efficiency up to 30% higher than a fixed supply voltage to 58 ~ 68%. The prototype wireless stimulating system was verified in vitro.

Journal ArticleDOI
TL;DR: This paper proposes a power sharing based control strategy which incorporates the information of the total real and reactive power generation of all DG units and evaluates the performance of the proposed control strategy based on small-signal stability analysis.
Abstract: Decentralized inverter control is essential in distributed generation (DG) microgrids for low deployment/operation cost and high reliability. However, decentralized inverter control suffers from a limited system stability mainly because of the lack of communications among different inverters. In this paper, we investigate stability enhancement of the droop based decentralized inverter control in microgrids. Specifically, we propose a power sharing based control strategy which incorporates the information of the total real and reactive power generation of all DG units. The information is acquired by a wireless network (such as a WiFi, ZigBee, and/or cellular communication network) in a decentralized manner. Based on the desired power sharing of each DG unit and the acquired information of total generation, additional control terms are added to the traditional droop controller. We evaluate the performance of the proposed control strategy based on small-signal stability analysis. As timely communication may not be established for a microgrid with low-cost wireless communication devices, two kinds of analytical models are developed with respect to negligible and nonnegligible communication delays, respectively. Extensive numerical results are presented to demonstrate the system stability under the proposed control strategy with respect to different.