Showing papers in "IEEE Transactions on Power Delivery in 2012"
TL;DR: In this article, the authors present a novel average value model (AVM) for efficient and accurate representation of a detailed MMC-HVDC system and develop a detailed 401-level model for validating the AVM and studies the performance of both models when integrated into a large 400kV transmission system in Europe.
Abstract: Voltage-source-converter (VSC) technologies present a bright opportunity in a variety of fields within the power system industry. New modular multilevel converters (MMCs) are expected to supersede two- and three-level VSC-based technologies for HVDC applications due to their recognized advantages in terms of scalability, performance, and efficiency. The computational burden introduced by detailed modeling of MMC-HVDC systems in electromagnetic-transients (EMT)-type programs complicates the study of transients especially when these systems are integrated into a large network. This paper presents a novel average-value model (AVM) for efficient and accurate representation of a detailed MMC-HVDC system. It also develops a detailed 401-level MMC-HVDC model for validating the AVM and studies the performance of both models when integrated into a large 400-kV transmission system in Europe. The results show that the AVM is significantly more efficient while maintaining its accuracy for the dynamic response of the overall system.
499 citations
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.
372 citations
TL;DR: In this article, the coordination of directional overcurrent relays is formulated as a mixed-integer nonlinear programming problem and is then solved by a new seeker optimization technique based on the act of human searching, in which the search direction and step length are determined in an adaptive way.
Abstract: Coordination of directional overcurrent relays in a multiloop subtransmission or distribution network is formulated as an optimization problem. In this paper, the coordination of directional overcurrent relays is formulated as a mixed-integer nonlinear programming problem and is then solved by a new seeker optimization technique. Based on the act of human searching, in the proposed seeker technique, the search direction and step length are determined in an adaptive way. The proposed method is implemented in three different test cases. The results are compared with previously proposed analytic and evolutionary approaches.
270 citations
TL;DR: In this article, a power distribution system reconfiguration methodology considering the reliability and the power loss is developed in order to evaluate the reliability at the load points, and an algorithm for finding the minimal cut sets is utilized to find the minimal set of components appearing between the feeder and any particular load point.
Abstract: A power distribution system reconfiguration methodology considering the reliability and the power loss is developed in this paper. Probabilistic reliability models are used in order to evaluate the reliability at the load points. An algorithm for finding the minimal cut sets is utilized to find the minimal set of components appearing between the feeder and any particular load point. The optimal status of the switches in order to maximize the reliability and minimize the real power loss is found by a binary particle swarm optimization-based search algorithm. The effectiveness of the proposed methodology is demonstrated on a 33-bus and a 123-bus radial distribution system.
263 citations
TL;DR: In this article, the authors present fundamental concepts of a central power management system (PMS) and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units.
Abstract: This paper presents fundamental concepts of a central power-management system (PMS) and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units. The DER units are interfaced to the utility grid through voltage-sourced converters (VSCs). The frequency of each DER unit is specified by its independent internal oscillator and all oscillators are synchronized by a common time-reference signal received from a global positioning system. The PMS specifies the voltage set points for the local controllers. A linear, time-invariant, multivariable, robust, decentralized, servomechanism control system is designed to track the set points. Each control agent guarantees fast tracking, zero steady-state error, and robust performance despite uncertainties of the microgrid parameter, topology, and the operating point. The theoretical concept of the proposed control strategy, including the existence conditions, design of the controller, robust stability analysis of the closed-loop system, time-delay tolerance, tolerance to high-frequency effects and its gain-margins, are presented in this Part I paper. Part II reports on the performance of the control strategy based on digital time-domain simulation and hardware-in-the-loop case studies.
252 citations
TL;DR: In this article, a dual sorting algorithm is proposed for reducing the switching commutations of each power module as well as for voltage balancing control in modular multilevel converter (MMC) topology.
Abstract: This paper presents design and control methods for fault-tolerant operations with redundant converter modules, one of the most prominent features in modular multilevel converter (MMC) topology. In fully implementing MMC functionalities, a nearest level control is applied as a low-switching modulation method. A dual sorting algorithm is newly proposed for effectively reducing the switching commutations of each power module as well as for voltage balancing control. Built upon these primary MMC topological and control features, its redundant operation is comprehensively investigated for fail-safe energy transfer. In particular, a novel spare process is proposed to handle an emergency situation when the number of faulty power modules exceeds the module redundancy. Since topological redundancy may cause the switching commutations of power modules in an arm to be unevenly distributed, a practical and effective mitigation measure is incorporated to keep the energy balance while avoiding the undesired switching stresses. Rigorous simulation studies for MMC and its application for high-voltage direct current are performed to demonstrate the validity and effectiveness of the proposed spare process under normal and emergency conditions.
250 citations
TL;DR: In this paper, a supplementary dc voltage ripple suppressing controller (DCVRSC) is proposed to eliminate the second-order harmonic in the dc voltage of the MMC-HVDC system.
Abstract: There are second-order harmonics in the dc voltage and current when the MMC-HVDC system is under unbalanced grid conditions, even if the negative-sequence current controller is employed. This paper presents a supplementary dc voltage ripple suppressing controller (DCVRSC) to eliminate the second-order harmonic in the dc voltage of the MMC-HVDC system. The instantaneous power of the converter arm and phase unit indicates that there are zero-sequence double-line frequency components in the three-phase unit voltages when the ac system is under an unbalanced fault. Since the zero-sequence components cannot be offset by each other, they lead to the second-order harmonic in the dc voltage and dc current. The DCVRSC is developed to compensate the zero-sequence components in three-phase unit voltages. Simulation results based on a detailed PSCAD/EMTDC model prove that the DCVRSC can eliminate the second-order harmonic in the dc voltage. Meanwhile, the ac currents are kept balanced under the unbalanced fault conditions.
228 citations
TL;DR: In this article, the authors proposed a fault classification and faulty-pole selection based on zero and positive-sequence backward traveling waves, and an integrated traveling wave-based protection scheme was proposed.
Abstract: Explicit fault analysis is the basis of protection for the bipolar HVDC Line. The characteristics of the initial values of traveling waves under various internal fault conditions are investigated on the basis of the symmetrical component analysis. The criteria of fault classification and faulty-pole selection are put forward based on the zero- and positive-sequence backward traveling waves, and an integrated traveling wave-based protection scheme is proposed. The simulations based on real-time digital simulation show that the proposed scheme can detect faults rapidly, determine the fault type effectively, and select the faulty pole correctly.
227 citations
TL;DR: In this article, an optimization procedure that enables the optimal dispatching of distributed generators and storage systems in a medium-voltage islanded microgrid is presented, where the optimization goal is to minimize the overall microgrid operating cost and the pollutants emission of the programmable generators, assuming that all of the power made available by the renewable generators is either directly injected into the network or stored in order to be subsequently delivered according to the proposed storage units' management strategy.
Abstract: This paper presents an optimization procedure that enables the optimal dispatching of distributed generators and storage systems in a medium-voltage islanded microgrid. The network is assumed to be supplied by programmable (dispatchable) and nonprogrammable generators (i.e. nondispatchable, such as renewable energy sources-based units). The optimization goal is to minimize the overall microgrid operating cost and the pollutants emission of the programmable generators, assuming that all of the power made available by the renewable generators (photovoltaic and wind systems) is either directly injected into the network or stored in order to be subsequently delivered according to the proposed storage units' management strategy. The optimization is carried out by a niching evolutionary algorithm (NEA) that is able to find multiple optima and the variation of the objective function in their neighborhood. NEAs allow overcoming the performance of standard algorithms used for optimal power-flow calculations in power systems by avoiding falling into local optima. The optimization procedure is performed on a test microgrid and verified by computer simulations. The numerical results show that the solutions can always improve the microgrid performances irrespective of the network operating conditions in all of the considered cases.
223 citations
TL;DR: In this article, a stochastic multiobjective framework for daily volt/var control (VVC), including hydroturbine, fuel cell, wind turbine, and photovoltaic powerplants, is proposed to minimize the electrical losses, voltage deviations, total electrical energy costs, and total emissions of renewable energy sources and grid.
Abstract: This paper proposes a stochastic multiobjective framework for daily volt/var control (VVC), including hydroturbine, fuel cell, wind turbine, and photovoltaic powerplants The multiple objectives of the VVC problem to be minimized are the electrical energy losses, voltage deviations, total electrical energy costs, and total emissions of renewable energy sources and grid For this purpose, the uncertainty related to hourly load, wind power, and solar irradiance forecasts are modeled in a scenario-based stochastic framework A roulette wheel mechanism based on the probability distribution functions of these random variables is considered to generate the scenarios Consequently, the stochastic multiobjective VVC (SMVVC) problem is converted to a series of equivalent deterministic scenarios Furthermore, an Evolutionary Algorithm using the Modified Teaching-Learning-Algorithm (MTLA) is proposed to solve the SMVVC in the form of a mixed-integer nonlinear programming problem In the proposed algorithm, a new mutation method is taken into account in order to enhance the global searching ability and mitigate the premature convergence to local minima Finally, two distribution test feeders are considered as case studies to demonstrate the effectiveness of the proposed SMVVC
215 citations
TL;DR: In this paper, an experimental methodology was applied to determine the frequency dependence of the soil resistivity and permittivity under field conditions, and simplified expressions were proposed to predict this frequency dependence.
Abstract: An experimental methodology was applied to determine the frequency dependence of the soil resistivity and permittivity under field conditions. A large number of soils of low-frequency resistivity ranging from 50 to 9100 .m were tested and showed strong variation of both parameters in the Hz frequency interval. Simplified expressions were proposed to predict this frequency dependence. The response of grounding electrodes subjected to lightning currents was simulated using an electromagnetic model under the assumption of variation of soil parameters given by such expressions and obtained from measurements. The results were very similar, though quite different from those obtained under the assumption of constant values for soil resistivity and permittivity.
TL;DR: In this article, a fuzzy-c-means (FCM) is employed to cluster the operation states for system load, wind-turbine generations (WTG), and photovoltaic (PV) in 8760h.
Abstract: Owing to the Kyoto Protocol and the growing depletion of natural resources, renewable energies have attracted much attention. This paper considers 25-kW wind-turbine generator, 5-kW PV and 30-kW diesel generator as unit sizes for generation planning in a stand-alone power system. The investment cost (installation and unit costs) and fuel cost are minimized while retaining the reliability requirement and CO emission limit. First, the fuzzy-c-means (FCM) is employed to cluster the operation states for system load, wind-turbine generations (WTG), and PV in 8760 h. Then, the Markov models for the system load, WTG, and photovoltaic (PV) are established. The Markov models are embedded into the genetic algorithm to determine the optimal sizes for WTG, PV, and the diesel generator. The simulation results reveal that computation time can be reduced greatly while optimality can be still retained, compared with the traditional method using chronological data.
TL;DR: In this article, the authors present a comprehensive analysis of the possible impacts of different penetration levels of distributed generation on voltage profiles in low-voltage secondary distribution networks, and determine the maximum amount of DG that secondary networks can withstand without exhibiting undervoltage and overvoltage problems or unexpected load disconnections.
Abstract: This paper presents a comprehensive analysis of the possible impacts of different penetration levels of distributed generation (DG) on voltage profiles in low-voltage secondary distribution networks. Detailed models of all system components are utilized in a study that performs hundreds of time-domain simulations of large networked distribution systems using the Electromagnetic Transients Program (EMTP). DGs are allocated in a probabilistic fashion to account for the uncertainties of future installations. The main contribution of this paper is the determination of the maximum amount of DG that secondary distribution networks can withstand without exhibiting undervoltage and overvoltage problems or unexpected load disconnections. This information is important for network planning engineers to facilitate the extension of the maximum penetration limit. The results show that depending on the location, type, and size of the installed DGs, small amounts of DG may cause overvoltage problems. However, large amounts of DG may not cause any voltage problems when properly selected.
TL;DR: In this article, an optimal power flow (OPF) is proposed to minimize capital costs for network upgrading, operation and maintenance costs, and the cost of losses for handling the load growth for the planning horizon.
Abstract: Deregulation in the power system industry and invention of new technologies for producing electrical energy have led to innovations in distribution system planning (DSP). Distributed generation (DG) is one of the most attractive technologies that brings different kinds of advantages to a wide range of entities, engaged in power systems. In this paper, a new model for considering DGs in the DSP problem is presented. In this model, an optimal power flow (OPF) is proposed to minimize capital costs for network upgrading, operation and maintenance costs, and the cost of losses for handling the load growth for the planning horizon. The term “dynamic” is used to refer to the planning over a specific period so that dynamic distribution system planning is, in fact, proposed. Besides, a modified genetic algorithm is used to find the optimal topology solution. The effectiveness of this method is demonstrated through examination on a radial distribution network.
TL;DR: In this paper, the impact of different control schemes of the inverter-based DG and microgrid load types on the microgrid stability subsequent to fault-forced islanding is investigated.
Abstract: With the growing deployment of microgrids, it has become urgent to investigate the microgrid behavior during transient faults and subsequent islanding conditions. The load type and the manner in which distributed generations (DGs) are controlled can have substantial impacts on the dynamic performance of microgrids. In this paper, impacts of different control schemes of the inverter-based DG and microgrid load types on the microgrid stability subsequent to fault-forced islanding are investigated. A microgrid model, simulated on Matlab/Simulink software, is analyzed including a mix of synchronous and inverter-based DG and a combination of passive RLC and induction motor (IM) loads. Simulation results show that in the presence of IM loads, the microgrid may lose its stable operation even if the fault is isolated within a typical clearing time. The critical clearing time of a microgrid is highly dependent on the microgrid control strategy, DG interface control, and load type. Induction motor loads can prove problematical to microgrid transient stability, particularly in situations in which the voltage dip can cause the induction motor to “pull out”.
TL;DR: In this paper, a novel online technique is introduced to detect the internal faults within a power transformer by constructing the voltage-current (V - I) locus diagram to provide a current state of the transformer.
Abstract: Frequency-response analysis (FRA) has been growing in popularity in recent times as a tool to detect mechanical deformation within power transformers. To conduct the test, the transformer has to be taken out of service which may cause interruption to the electricity grid. Moreover, because FRA relies on graphical analysis, it calls for an expert to analyze the results. As so far, there is no standard code for FRA interpretation worldwide. In this paper, a novel online technique is introduced to detect the internal faults within a power transformer by constructing the voltage-current (V - I) locus diagram to provide a current state of the transformer. The technique does not call for any special equipment as it uses the existing metering devices attached to any power transformer to monitor the input voltage, output voltage, and the input current at the power frequency and, hence, online monitoring can be realized. Various types of faults have been simulated to assess its impact on the proposed locus. A Matlab code based on digital image processing is developed to calculate any deviation of the V - I locus with respect to the reference one and to identify the type of fault. The proposed technique is easy to be implemented and automated so that the requirement for expert personnel can be eliminated.
TL;DR: In this paper, a new transient harmonic current protection scheme is proposed, which can not only identify internal faults and external faults correctly and quickly, but also can respond to high ground resistance faults.
Abstract: Based on the boundary characteristic of a dc transmission line, the response of transient harmonic current in the line-commutated converter HVDC transmission system is analyzed under various fault conditions in this paper. A new transient harmonic current protection scheme is proposed. The discrete Fourier transform is used to extract transient harmonic currents at both terminals of the dc transmission line and the type of fault can be identified by the transient harmonic currents. Two main factors including fault resistance and fault location that affect performance of the protection are also discussed in this study. The test system is modeled based on the CIGRE benchmark including distributed parameters of the dc transmission line. Comprehensive test studies show that the performance of the transient harmonic current protection scheme is encouraging. It can not only identify internal faults and external faults correctly and quickly, but also can respond to high ground resistance faults.
TL;DR: In this article, the theoretical analogy between conventional grid control by means of synchronous generators and the control of converter-interfaced distributed generation (CIDG) units in microgrids is studied.
Abstract: For islanded microgrids, droop-based control methods are often used to achieve a reliable energy supply. However, in case of resistive microgrids, these control strategies can be rather different to what conventional grid control is accustomed to. Therefore, in this paper, the theoretical analogy between conventional grid control by means of synchronous generators (SGs) and the control of converter-interfaced distributed generation (CIDG) units in microgrids is studied. The conventional grid control is based on the frequency as a global parameter showing differences between mechanical power and ac power. The SGs act on changes of frequency through their P/f droop controller, without interunit communication. For CIDG units, a difference between dc-side power and ac-side power is visible in the dc-link voltage of each unit. Opposed to grid frequency, this is not a global parameter. Thus, in order to make a theoretical analogy, a global measure of the dc-link voltages is required. A control strategy based on this global voltage is presented and the analogy with the conventional grid control is studied, with the emphasis on the need for interunit communication to achieve this analogy. A known control strategy in resistive microgrids, called the voltage-based droop control for CIDG units, approximates this analogy closely, but avoids interunit communication. Therefore, this control strategy is straightforward for implementation since it is close to what control engineers are used to. Also, it has some specific advantages for the integration of renewables in the network.
TL;DR: In this paper, the impact of a voltage source converter-based HVDC (VSC-HVDC) infeed on a line-commutated converter based HVDC infeed (LCC-HVDC) at the same busbar is investigated.
Abstract: The impact of a voltage source converter-based HVDC (VSC-HVDC) infeed on a line-commutated converter-based HVDC infeed (LCC-HVDC) at the same busbar is investigated. Steady-state aspects such as the maximum available power (MAP) as well as transient aspects such as temporary overvoltage (TOV) and commutation failure immunity index are considered in the analysis. The results show that VSC-HVDC improves the maximum available power, reduces TOV, and makes the LCC-HVDC less susceptible to commutation failure. The resulting improvement in performance of the LCC-HVDC is quantified by a proposed “apparent increase in short circuit ratio” (AISCR) index. AISCR is a measure of the increase in short-circuit ratio for the LCC-HVDC that would be necessary in the absence of the VSC-HVDC link to obtain similar performance. The analytical results are validated by a detailed electromagnetic transient simulation.
TL;DR: In this paper, a brief review of techniques available for reducing large systems to smaller equivalents is presented, which is divided into high frequency equivalents, low frequency equivalents and wideband equivalents.
Abstract: This paper presents a brief review of techniques available for reducing large systems to smaller equivalents This paper is divided into high frequency equivalents, low frequency equivalents, and wideband equivalents Numerical examples are presented to demonstrate selected methods of high frequency equivalencing
TL;DR: In this article, a new optimization approach for distribution automation in terms of automated and remotely controlled sectionalizing switch placement is introduced, where mixed-integer linear programming (MILP) is utilized to model the problem.
Abstract: Automation is acknowledged by distribution utilities as a successful investment strategy to enhance reliability and operation efficiency However, practical approaches that can handle the complex decision-making process faced by decision makers to justify the long-term financial effects of distribution automation have remained scarce An automated and remote-controlled sectionalizing switch play a fundamental role in an automated distribution network This paper introduces a new optimization approach for distribution automation in terms of automated and remotely controlled sectionalizing switch placement Mixed-integer linear programming (MILP) is utilized to model the problem The proposed model can be solved with large-scale commercial solvers in a computationally efficient manner The proposed sectionalizing switch placement problem considers customer outage costs in conjunction with sectionalizing switch capital investment, installation, as well as annual operation and maintenance costs The effectiveness of the proposed approach is tested on a reliability test system and a typical real size system The presented results indicate the accuracy and efficiency of the proposed method
TL;DR: In this paper, a robust coordination approach for the controller design of multiple highvoltage direct-current (HVDC) and flexible ac transmission systems (FACTS) wide-area controls (WACs) is presented and has the aim of stabilizing multiple interarea oscillation modes in large-scale power systems.
Abstract: A robust coordination approach for the controller design of multiple high-voltage direct-current (HVDC) and flexible ac transmission systems (FACTS) wide-area controls (WACs) is presented in this paper and has the aim of stabilizing multiple interarea oscillation modes in large-scale power systems. The suitable wide-area control signals, which are given to HVDC and FACTS wide-area controllers, respectively, are chosen from a large number of candidate items. Then, a sequential robust design approach is planned for the wide-area controller coordination of HVDC and FACTS devices. This approach is based on the robust control theory and is formulated as a standard problem of multiobjective mixed H2/H∞ output-feedback control with regional pole placement constraints. The linear matrix inequality (LMI) theory is applied to solve such a robust control problem. A case study on the 16-machine 5-area system, which is modified with one HVDC interconnected transmission, one shunt-FACTS device (SVC), and one series-FACTS device (TCSC), is performed to validate the robust performance in terms of multiple oscillations damping under various operating conditions.
TL;DR: In this paper, the authors present a test case for validating results from GIC modeling software, which is based on a real GIC network with different voltage levels, two-and three-winding transformers and autotransformers, multiple transmission lines in the same corridorn and GIC blocking devices.
Abstract: Geomagnetically induced currents (GICs) in power systems can be attributed to problems ranging from transformer overheating, misoperation of protective relays, and voltage instability. The assessment of the geomagnetic hazard to power systems requires accurate modeling of the GICs that are expected to occur. However, to date, there are no publicly available test cases to validate software programs used to compute GIC. The following paper presents a hypothetical network that can be used as a test case for validating results from GIC modeling software. The network contains many features found in real networks such as: different voltage levels, two- and three-winding transformers and autotransformers, multiple transmission lines in the same corridorn and GIC blocking devices. GIC is calculated in the network for two geoelectric field scenarios: a 1 V/km uniform Northward electric field and a 1 V/km uniform Eastward electric field. Detailed simulation results and corresponding input data are provided for each of the two scenarios. Simulation results that are provided have been validated using four independent GIC modeling programs.
TL;DR: In this paper, the authors proposed a fault detection algorithm based on the traveling-wave principle, which relies on the fault-generated surge arrival times at two ends of the dc line as inputs.
Abstract: This paper presents a novel algorithm to determine the location of dc line faults in an HVDC system with a mixed transmission media consisting of overhead lines and cables, using only the measurements taken at the rectifier and inverter ends of the composite transmission line. The algorithm relies on the traveling-wave principle, and requires the fault-generated surge arrival times at two ends of the dc line as inputs. With accurate surge arrival times obtained from time-synchronized measurements, the proposed algorithm can accurately predict the faulty segment as well as the exact fault location. Continuous wavelet transform coefficients of the input signal are used to determine the precise time of arrival of traveling waves at the dc line terminals. Two possible input signals-the dc voltage measured at the converter terminal and the current through the surge capacitors connected at the dc line end-are examined and both signals are found to be equally effective for detecting the traveling-wave arrival times. Performance of the proposed fault-location scheme is analyzed through detailed simulations carried out using the electromagnetic transient simulation software PSCAD. The impact of measurement noise on the fault-location accuracy is also studied in this paper.
TL;DR: In this paper, a new method for calculating a health index for transformers (for operating voltage 69 kV or less) based on diagnostic tests is presented, which relies on the use of furan analysis, dissolved gas analysis, and other oil analysis results as a means of calculating the health index using fuzzy set theory.
Abstract: This paper presents a new method for calculating a health index for transformers (for operating voltage 69 kV or less) based on diagnostic tests. The method relies on the use of furan analysis, dissolved gas analysis, and other oil analysis results as a means of calculating the health index using fuzzy set theory. Real field data for 90 working transformers were used to test the proposed method. The results were compared with the results calculated for the same set of transformers by an experienced asset management and health assessment consulting company. The comparison shows that the results are highly reliable.
TL;DR: In this paper, an energy storage system (ESS) was proposed for mitigating voltage unbalance as well as improving the efficiency of the network, where a power system simulation tool, namely PSCAD, was used to model two generic low-voltage networks, BIPV systems and an ESS in order to simulate the performance of the networks with various levels of PV penetrations.
Abstract: The growth of building integrated photovoltaic (BIPV) systems in low-voltage (LV) networks has the potential to raise several technical issues, including voltage unbalance and distribution system efficiency. This paper proposes an energy storage system (ESS) for mitigating voltage unbalance as well as improving the efficiency of the network. In the study, a power system simulation tool, namely PSCAD, is used to model two generic LV networks, BIPV systems and an ESS in order to simulate the performance of the networks with various levels of BIPV penetrations. A control algorithm is developed and implemented in the energy storage model in order to study the ability of the ESS to mitigate network voltage unbalance and reduce losses. Experimental studies are carried out in the experimental small-scale energy zone to investigate the effectiveness of the energy storage system under various levels of PV penetration and load conditions. The simulation and experimental studies carried out clearly show the effectiveness of the ESS in reducing the voltage unbalance factor and improving the efficiency of the two networks considered.
TL;DR: In this paper, the authors proposed an enhanced control strategy for electronically coupled distributed energy resources that improves the performance of the host microgrid under network faults and transient disturbances, which does not require controller mode switchings and enables the EH resources to ride through network faults, irrespective of whether they take place within the host EH or impact the upstream grid.
Abstract: This paper proposes an enhanced control strategy for electronically coupled distributed energy resources that improves the performance of the host microgrid under network faults and transient disturbances. The proposed control strategy does not require controller mode switchings and enables the electronically coupled distributed energy resources to ride through network faults, irrespective of whether they take place within the host microgrid or impact the upstream grid. Moreover, the proposed control ensures acceptable power quality for the duration of the faults, which is an important feature for protection against certain classes of faults, as well as for sensitive loads. Further, the paper proposes a supplementary control loop that improves the microgrid post-fault recovery. The effectiveness of the proposed control strategy is demonstrated through a comprehensive set of simulation studies, conducted in the PSCAD/EMTDC software environment.
TL;DR: In this article, a two-stage fault-location optimization model is proposed, along with defining a matching degree index, where the first stage is the fault region identification stage, and the second stage is used to identify the exact fault line and fault distance.
Abstract: This paper presents a general fault-location method for large transmission networks which uses phasor measurement unit (PMU) voltage measurements where the injected current at a fault point can be calculated by using the voltage change and its relevant transfer impedance on any bus. A two-stage fault-location optimization model is proposed, along with defining a matching degree index. The first stage is the fault region identification stage, which uses the matching degree index to determine the suspicious fault region in order to reduce the search area. The second stage is used to identify the exact fault line and fault distance. A method to determine optimal PMU placement is also proposed in this paper. Case studies verify that the proposed fault-location algorithm and optimal PMU placement scheme can locate faults in large transmission networks quickly and accurately without requiring fault-type classification or fault phase selection.
TL;DR: In this article, the authors developed appropriate models of flexible ac transmission systems (FACTS) shunt-series controllers for multiobjective optimization and also presented a multi-objective approach to find the optimal location of FACTS shunt series controllers.
Abstract: This paper develops appropriate models of flexible ac transmission systems (FACTS) shunt-series controllers for multiobjective optimization and also presents a multiobjective optimization methodology to find the optimal location of FACTS shunt-series controllers. The objective functions are the total fuel cost, power losses, and system loadability with and without minimum cost of FACTS installation. The e-constraint approach is implemented for the multiobjective mathematical programming (MMP) formulation, including the FACTS shunt-series controllers (i.e., phase-shifting transformer (PST), hybrid flow controller (HFC), and unified power-flow controller (UPFC)). Simulation results are presented for the IEEE 14-bus system. The optimization method is numerically solved using Matlab and general algebraic modeling system (GAMS) software environments. The solution procedure uses nonlinear programming (NLP) and mixed-integer nonlinear programming (MINLP) to solve the optimal location and setting of FACTS incorporated in the optimal power-flow problem considering these objective functions and improving the power system operation. Furthermore, the results demonstrate that the HFC is outperformed by PST and UPFC from the analytical and technical point of views.
TL;DR: In this paper, the authors investigated whether the grid voltage is actually a disadvantage of the active power/voltage (P/V) droop controllers and showed that with P/V droop control, the DG units that are located electrically far from the load centers automatically deliver a lower share of the power.
Abstract: Microgrids are receiving an increasing interest to integrate the growing share of distributed-generation (DG) units in the electrical network. For the islanded operation of the microgrid, several control strategies for the primary control have been developed to ensure stable microgrid operation. In low-voltage (LV) microgrids, active power/voltage (P/V) droop controllers are gaining attention as they take the resistive nature of the network lines and the lack of directly coupled rotating inertia into account. However, a problem often cited with these droop controllers is that the grid voltage is not a global parameter. This can influence the power sharing between different units. In this paper, it is investigated whether this is actually a disadvantage of the control strategy. It is shown that with P/V droop control, the DG units that are located electrically far from the load centers automatically deliver a lower share of the power. This automatic power-sharing modification can lead to decreased line losses; therefore, there is overall better efficiency compared to the methods that focus on perfect power sharing. In this paper, the P/V and P/f droop control strategies are compared with respect to this power-sharing modification and the line losses.