Showing papers in "IEEE Transactions on Power Delivery in 2009"

Protection of Low-Voltage DC Microgrids

Abstract: In this paper, a low-voltage (LV) DC microgrid protection system design is proposed. The LV DC microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV DC microgrid protection system, knowledge from existing DC power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during DC faults. In contrast, an LV DC microgrid must be connected to an AC grid through converters with bidirectional power flow and, therefore, a different protection-system design is needed. In this paper, the operating principles and technical data of LV DC protection devices, both available and in the research stage, are presented. Furthermore, different fault-detection and grounding methods are discussed. The influence of the selected protection devices and grounding method on an LV DC microgrid is studied through simulations. The results show that it is possible to use available devices to protect such a system. Problems may arise with high-impedance ground faults which can be difficult to detect.

Distributed Generation Interface to the CERTS Microgrid

Abstract: This paper focuses on the energy storage system and the power electronic interface included in microsources of the CERTS microgrid. CERTS stands for the Consortium for Electric Reliability Technology Solutions. The consortium was formed in 1999 to research, develop, and disseminate new methods, tools, and technologies to protect and enhance the reliability of the U.S. electric power system and efficiency of competitive electricity markets. To provide the plug-and-play feature and the power quality requirements of the CERTS microgrid, all microsources regardless of their prime mover type must have a unified dynamic performance. This necessitates attaching an energy storage module to some or all of the microsources. The storage module is attached to the prime mover through a power electronic interface that couples the microsource to the microgrid. Details of the energy storage module, the power electronic interface and the corresponding controls are described. Performance of an example microsource, which includes a synchronous generator, a storage module and an electronic interface, is studied. Dynamic performance of the example microsource when operating in the CERTS microgrid is evaluated based on digital time-domain simulations in the EMTP-RV software environment. Effectiveness of the storage module, the electronic interface and the corresponding controls in enhancing the microsource performance is verified.

A Control Methodology and Characterization of Dynamics for a Photovoltaic (PV) System Interfaced With a Distribution Network

Abstract: This paper proposes a control strategy for a single-stage, three-phase, photovoltaic (PV) system that is connected to a distribution network. The control is based on an inner current-control loop and an outer DC-link voltage regulator. The current-control mechanism decouples the PV system dynamics from those of the network and the loads. The DC-link voltage-control scheme enables control and maximization of the real power output. Proper feedforward actions are proposed for the current-control loop to make its dynamics independent of those of the rest of the system. Further, a feedforward compensation mechanism is proposed for the DC-link voltage-control loop, to make the PV system dynamics immune to the PV array nonlinear characteristic. This, in turn, permits the design and optimization of the PV system controllers for a wide range of operating conditions. A modal/sensitivity analysis is also conducted on a linearized model of the overall system, to characterize dynamic properties of the system, to evaluate robustness of the controllers, and to identify the nature of interactions between the PV system and the network/loads. The results of the modal analysis confirm that under the proposed control strategy, dynamics of the PV system are decoupled from those of the distribution network and, therefore, the PV system does not destabilize the distribution network. It is also shown that the PV system dynamics are not influenced by those of the network (i.e., the PV system maintains its stability and dynamic properties despite major variations in the line length, line X/R ratio, load type, and load distance from the PV system).

Placement of Synchronized Measurements for Power System Observability

Abstract: This paper presents a method for the use of synchronized measurements for complete observability of a power system. The placement of phasor measurement units (PMUs), utilizing time-synchronized measurements of voltage and current phasors, is studied in this paper. An integer quadratic programming approach is used to minimize the total number of PMUs required, and to maximize the measurement redundancy at the power system buses. Existing conventional measurements can also be accommodated in the proposed PMU placement method. Complete observability of the system is ensured under normal operating conditions as well as under the outage of a single transmission line or a single PMU. Simulation results on the IEEE 14-bus, 30-bus, 57-bus, and 118-bus test systems as well as on a 298-bus test system are presented in this paper.

Considering Different Network Topologies in Optimal Overcurrent Relay Coordination Using a Hybrid GA

Abstract: The directional overcurrent relays (DOCRs) coordination problem is usually studied based on a fixed network topology in an interconnected power system, and is formulated as an optimization problem. In practice, the system may be operated in different topologies due to outage of the transmission lines, transformers, and generating units. There are some situations for which the changes in the network topology of a system could cause the protective system to operate without selectivity. The aim of this paper is to study DOCRs coordination considering the effects of the different network topologies in the optimization problem. Corresponding to each network topology, a large number of coordination constraints should be taken into account in the problem formulation. In this situation, in addition to nonlinearity and nonconvexity, the optimization problem experiences many coordination constraints. The genetic algorithm (GA) is selected as a powerful tool in solving this complex and nonconvex optimization problem. In this paper, in order to improve the convergence of the GA, a new hybrid method is introduced. The results show a robust and optimal solution can be efficiently obtained by implementing the proposed hybrid GA method.

Optimal Placement of Phasor Measurement Units Using Immunity Genetic Algorithm

Abstract: This paper investigates the application of immunity genetic algorithm (IGA) for the problem of optimal placement of phasor measurement units (PMUs) in an electric power network. The problem is to determine the placement sites of the minimal set of PMUs which makes the system observable. Incorporating immune operator in the canonical genetic algorithm (GA), on the condition of preserving GA's advantages, utilizes some characteristics and knowledge of the problems for restraining the degenerative phenomena during evolution, so as to improve the algorithm efficiency. This type of prior knowledge about some parts of optimal solution exists in the PMU placement problem. So, the IGA is adopted in this paper to solve the problem. Also, a new effect which is preventing from familial reproduction is studied which shows an increase in converging speed. The effectiveness of the proposed method is verified via IEEE standard systems and a realistic large-scale power system.

Synchronized Phasor and Frequency Measurement Under Transient Conditions

Abstract: Synchronized phasor measurements are becoming an important element of wide area measurement systems used in advanced power system monitoring, protection, and control applications. The recently issued revised standard C37.118 for synchrophasors has facilitated interoperability of phasor measurement units (PMUs) from different manufacturers. This standard defines performance for compliance when the input signals are in steady state. The performance of PMUs under transient conditions is not considered by the standard at this time, although clearly PMUs will be subjected to inputs under transient conditions. This paper is an attempt to provide the authors' views on how one may approach the question of standardizing PMU response under transient conditions.

A Hybrid Islanding Detection Technique Using Average Rate of Voltage Change and Real Power Shift

Abstract: The mainly used islanding detection techniques may be classified as active and passive techniques. Passive techniques don't perturb the system but they have larger non-detection zones, whereas active techniques have smaller non-detection zones but they perturb the system. In this paper, a new hybrid technique is proposed to solve this problem. An average rate of voltage change (passive technique) has been used to initiate a real power shift (active technique), which changes the real power of distributed generation (DG), when the passive technique cannot have a clear discrimination between islanding and other events in the system. Simulation results show that the proposed method is effective in detecting islanding of distribution systems with DGs.

Extended Fault-Location Formulation for Power Distribution Systems

Abstract: In this paper, an extended impedance-based fault-location formulation for generalized distribution systems is presented. The majority of distribution feeders are characterized by having several laterals, nonsymmetrical lines, highly unbalanced operation, and time-varying loads. These characteristics compromise traditional fault-location methods performance. The proposed method uses only local voltages and currents as input data. The current load profile is obtained through these measurements. The formulation considers load variation effects and different fault types. Results are obtained from numerical simulations by using a real distribution system from the Electrical Energy Distribution State Company of Rio Grande do Sul (CEEE-D), Southern Brazil. Comparative results show the technique robustness with respect to fault type and traditional fault-location problems, such as fault distance, resistance, inception angle, and load variation. The formulation was implemented as embedded software and is currently used at CEEE-D's distribution operation center.

Bidirectional, High-Power DC Transformer

Abstract: This paper studies a bidirectional dc-dc converter concept which is capable of achieving very high stepping ratios with MW level power transfers. The converter can find potential application in connecting high-power dc sources, interfacing to high-voltage dc transmission or to flexible ac transmission system elements. The converter is based on two resonant circuits which share a common ac capacitor. The topology is simple and utilizes thyristors with potentially all soft switchings. Complete analytical modelling is presented which enables systematic design procedure for the converter. The detailed digital simulation on PSCAD platform confirms satisfactory operation on a 5-MW test system, which connects 4-kV dc source to an 80-kV high voltage dc grid. The converter shows good responses to rapid changes in power magnitude/direction and it is concluded that robustness to terminal voltage disturbances is excellent. Since the highest-power phase-control thyristors are employed, the converter can potentially be used at much higher power levels. The passive components are of reasonable size.

Ant Colony Optimization-Based Method for Placement of Sectionalizing Switches in Distribution Networks Using a Fuzzy Multiobjective Approach

Abstract: This paper proposes a methodology for placement of sectionalizing switches in distribution networks in the presence of distributed generation sources. The multiobjective considerations are handled using a fuzzy approach. The primary objective is reliability improvement with consideration of economic aspects. Thus, the objectives are defined as reliability improvement and minimization of the cost of sectionalizing switches. A fuzzy membership function is defined for each term in the objective function according to relevant conditions. Some considerations incorporated in the proposed model are relocation of existing switches and operating constraints on distribution networks and distributed generation (DG) sources during post-fault service restoration. The ant colony optimization (ACO) algorithm is adopted to solve the fuzzy multiobjective problem efficiently. The performance of the proposed approach is assessed and illustrated by various case studies on a test distribution system and also a real distribution network.

Hybrid Traveling Wave/Boundary Protection for Monopolar HVDC Line

Abstract: A novel hybrid protection algorithm, based on traveling wave protection principle and boundary protection principle for a monopolar HVDC line is proposed. Stationary wavelet transform (SWT) is adopted in the traveling wave protection to process the dc signal and then wavelet modulus maxima are used to further represent the useful traveling wave signal. The boundary protection principle based on SWT is used jointly with traveling wave protection to distinguish internal faults from external faults. The effect of border distortion, noise, high ground fault resistance, close-up faults, transients caused by lightning strokes and different dc line terminations are considered in the paper.

Modeling and Stability Analysis of a DFIG-Based Wind-Power Generator Interfaced With a Series-Compensated Line

Abstract: This paper deals with modeling and stability analysis of a doubly-fed induction generator (DFIG)-based wind-power unit that is interfaced with the grid via a series-compensated transmission line. A detailed mathematical model is developed in this paper that takes into account dynamics of the flux observer, phase-locked loop (PLL), controllers of the power-electronic converter, and wind turbine. Using the model and based on eigenvalue/participation-factor analysis, the system and controller parameters that substantially influence the system stability have been identified. The developed model is validated through a comprehensive set of simulation studies in the Matlab/Simulink and PSCAD/EMTDC software environments.

A Versatile Control Scheme for a Dynamic Voltage Restorer for Power-Quality Improvement

Abstract: This paper presents a control system based on a repetitive controller to compensate for key power-quality disturbances, namely voltage sags, harmonic voltages, and voltage imbalances, using a dynamic voltage restorer (DVR). The control scheme deals with all three disturbances simultaneously within a bandwidth. The control structure is quite simple and yet very robust; it contains a feedforward term to improve the transient response and a feedback term to enable zero error in steady state. The well-developed graphical facilities available in PSCAD/EMTDC are used to carry out all modeling aspects of the repetitive controller and test system. Simulation results show that the control approach performs very effectively and yields excellent voltage regulation.

Distribution Systems Reconfiguration Based on OPF Using Benders Decomposition

Abstract: This paper presents a new and efficient methodology for distribution network reconfiguration integrated with optimal power flow (OPF) based on a Benders decomposition approach. The objective minimizes power losses, load balancing among feeders, and is subject to constraints: capacity limit of branches, minimum and maximum power limits of substations or distributed generators, minimum deviation of bus voltages, and radial optimal operation of networks. A specific approach of the generalized Benders decomposition algorithm is applied to solve the problem. The formulation can be embedded under two stages: the first one is the master problem and is formulated as a mixed integer nonlinear programming problem. This stage determines the radial topology of the distribution network. The second stage is the slave problem and is formulated as a nonlinear programming problem. This stage is used to determine the feasibility of the Master problem solution by means of an OPF and provides information to formulate the linear Benders cuts that connect both problems. The model is programmed in the general algebraic modeling system. The effectiveness of the proposal is demonstrated through three examples extracted from the literature.

Impulse Efficiency of Ground Electrodes

Abstract: The lightning current waveform has a major influence on the dynamic performance of ground electrodes. While high lightning current intensity improves the dynamic grounding performance due to ionization of the soil, very fast fronted pulses might worsen the performance in case of inductive behavior. The previous analysis has often been based on quasistatic approximation that is not applicable to very fast fronted pulses. To extend the analysis to fast fronted pulses in this paper, the full-wave analysis method based on the rigorous electromagnetic-field theory approach is used. In addition, realistic lightning current waveforms are applied, which reproduce the observed concave rising portion of typical recorded lightning current pulses. Based on the simulation results, new empirical formulas applicable for slow and very fast fronted lightning current pulses are proposed. The effects of the ionization of the soil are disregarded; therefore, the new formulas are applicable for a conservative estimate of the upper bound of the impulse impedance of ground electrodes.

Simulation Tools for Electromagnetic Transients in Power Systems: Overview and Challenges

Abstract: This paper presents an overview on available tools and methods for the simulation of electromagnetic transients in power systems. Both off-line and real-time simulation tools are presented and discussed. The first objective is to give the reader an overview on the modeling and simulation capabilities in currently available state-of-the-art tools. The second objective is to provide perspectives on research topics and needed enhancements.

Empirical-Mode Decomposition With Hilbert Transform for Power-Quality Assessment

Abstract: The aim of this paper is to develop a method based on combination of empirical-mode decomposition (EMD) and Hilbert transform for assessment of power quality events. A distorted waveform can be conceived as superimposition of various oscillating modes and EMD is used to separate out these intrinsic modes known as intrinsic mode functions (IMF). Hilbert transform is applied to first three IMF to obtain instantaneous amplitude and phase which are then used for constructing feature vector. The work evaluates the detection capability of the methodolpogy and a comparison with S-transform is made to show the superiority of the technique in detecting the PQ disturbance like voltage spike and notch. A probabilistic neural network is used as a mapping function for identifying the various disturbance classes. Results show a better classification accuracy of the methodology.

Implications for Distribution Networks of High Penetration of Compact Fluorescent Lamps

Abstract: The desire to reduce electrical loading by using energy efficient lighting has resulted in a high level of interest in replacing conventional incandescent lamps with compact fluorescent lamps (CFL). This has resulted in the New Zealand Government, through the Electricity Commission of New Zealand, running campaigns to install five CFLs in every home by, subsidizing the cost of suitable CFLs. CFLs are however, a nonlinear load hence inject harmonics into the electrical network. The CFL use electronic ballasts and the design of these have an enormous impact on the electrical performance of the CFL. In the past, the harmonics injected into the network by CFLs has been ignored as each is very small as the typically CFL is only 20 Watts. However, if widespread adoption of CFLs occurs, the combined effect of all these small sources can be just as detrimental as one large source, and is even harder to mitigate due to their distributed nature. This paper presents the results of a study to quantify the effect widespread adoption of CFLs will have on a typical distribution network. The two aspects investigated were harmonic distortion and system losses. To enable 28 800 homes to be represented a methodology for modelling a large number of distributed loads using Norton equivalents was developed and applied. In order to assess losses, a fundamental frequency power-flow was also incorporated.

Transformer Design and Optimization: A Literature Survey

Abstract: With the fast-paced changing technologies in the power industry, new references addressing new technologies are coming to the market. Based on this fact, there is an urgent need to keep track of international experiences and activities taking place in the field of modern transformer design. The complexity of transformer design demands reliable and rigorous solution methods. A survey of current research reveals the continued interest in application of advanced techniques for transformer design optimization. This paper conducts a literature survey and reveals general backgrounds of research and developments in the field of transformer design and optimization for the past 35 years, based on more than 420 published articles, 50 transformer books, and 65 standards.

Soft Computing Techniques for the Control of an Active Power Filter

Abstract: Nonmodel-based controllers have been explored for the control of a shunt active power filter (APF) designed for harmonic and reactive current mitigation. In this paper, three soft computing techniques viz; fuzzy logic, neural network, and genetic algorithm are used to design alternative control schemes for switching the APF. The models for these control schemes are designed and simulated in MATLAB. A comparative study of the results obtained using these artificial-intelligence-based schemes is presented.

A New Proposal for Power Quality and Custom Power Improvement: OPEN UPQC

Abstract: Currently, the quality of supplied power is important to several customers Power quality (PQ) is a service and many customers are ready to pay for it In the future, distribution system operators could decide, or could be obliged by authorities, to supply their customers with different PQ levels and at different prices A new device that can fulfill this role is the OPEN unified power-quality conditioner (UPQC), composed of a power-electronic series steady-state performancemain unit installed in the medium-voltage/low-voltage (LV) substation, along with several power-electronic shunt units connected close to the end users The series and parallel units do not have a common dc link, so their control strategies are different than traditional UPQC control techniques This device can achieve general improvement in PQ, reducing the most common disturbances for all customers that are supplied by the mains (PQ) by using only the series unit Additional increments in PQ (ie, mains power interruptions), can be provided to the customers who need it (custom power) by the shunt units Therefore, this new solution combines an improvement in PQ for all end users, with a cost reduction for those that need high quality power The proposed solution has been analyzed and described, and a model of a 400-kVA LV grid is considered a test network to evaluate the steady-state performance and functioning limits The results obtained under steady-state conditions justify the configuration chosen and good device performance

RF-Based Partial Discharge Early Warning System for Air-Insulated Substations

Abstract: Partial discharges (PDs) generate wideband radio-frequency interference which can be used for noninvasive monitoring of discharges. This paper presents a novel method based on this principle for PD monitoring of substations. The significant advantage of this method lies in the ability to detect PD sources in energized equipment anywhere within a substation compound during normal operating conditions. The results obtained from the prototypes installed in the U.K. and U.S. substations are reported. Results include correlation with apparent charge and daily recordings obtained before, during, and after the failure of a 132-kV current transformer and 69-kV voltage transformer.

Improved Fault Location on Distribution Feeders Based on Matching During-Fault Voltage Sags

Abstract: A fault location algorithm for distribution feeders based on matching during-fault voltage sags is presented. The basic principle of the algorithm is based on the fact that when a fault occurs on the feeder, voltage sags propagate presenting different characteristics for each feeder node. Knowing the voltage sag characteristics, it is possible to locate the faulty node or the faulty area of the feeder. This voltage-based approach ensures the efficiency and robustness of the algorithm, which provides suitable and accurate results. An overhead, three-phase, three-wire, 13.8 kV, 134-node, real-life feeder model is used to evaluate the algorithm. Test results show that the algorithm is quite suitable due to not only the faulted point being located, but also a faulted area being identified providing important information to direct the maintenance crew seeking to repair any fault inflicted damage.

Game-Theoretic Control of Small-Scale Power Systems

Abstract: A power system is a collection of individual components that compete for system resources. This paper presents a game theoretic approach to the control decision process of individual sources and loads in small-scale and dc power systems. Framing the power system as a game between players facilitates the definition of individual objectives, which adds modularity and adaptability. The proposed methodology enhances the reliability and robustness of the system by avoiding the need for a central or supervisory control. It is also a way to integrate and combine supply and demand side management into a single approach. Examples are presented that use a simple nine bus dc power system to demonstrate the proposed method for various scenarios and player formulations.

Adaptive Current Differential Protection Schemes for Transmission-Line Protection

Abstract: Throughout the history of power system protection, researchers have strived to increase sensitivity and speed of apparatus protection systems without compromising security. With the significant technological advances in wide-area measurement systems, for transmission system protection, current differential protection scheme outscores alternatives like overcurrent and distance protection schemes. Therefore, in this paper, we address this challenge by proposing a methodology for adaptive control of the restraining region in a current differential plane. First an error analysis of conventional phasor approach for current differential protection is provided using the concept of dynamic phasor. Subsequently, we extend the methodology for protection of series compensated transmission lines. Finally, we also evaluate the speed versus accuracy conflict using phasorlets. Electromagnetic Transient Program simulations are used to substantiate the claims. The results demonstrate the utility of the proposed approach.

Fault Classification and Faulted-Phase Selection Based on the Initial Current Traveling Wave

Abstract: Accurate fault classification and swift faulted-phase selection are the bases of protection for transmission systems. This paper presents the algorithm of fault classification and faulted-phase selection based on the initial current traveling wave. The characteristics of various faults are investigated on the basis of the Karenbauer transform. The criteria of fault classification and faulted-phase selection are put forward. In order to extract the traveling wave from postfault signals and to construct the algorithm, wavelet transform technology is adopted. The simulations based on the electromagnetic transients program have been done to verify the performance of the proposed algorithm.

Pantograph Arcing in Electrified Railways—Mechanism and Influence of Various Parameters—Part II: With AC Traction Power Supply

Abstract: Pantograph arcing is a common phenomenon in electrified railway systems. This is also a source of broadband-conducted and radiated electromagnetic interference (EMI) for vehicle as well as traction power and signaling systems. In this paper and the companion paper, experimental analyses of pantograph arcing phenomena are presented for dc and ac feeding systems, respectively. Influences of different parameters on dc traction system, such as supply-voltage polarity, relative motion between pantograph and overhead contact wire, namely, forward motion along the track (longitudinal), and lateral sliding motion of the pantograph (zigzag) are presented here. From the voltage and current waveforms of the test runs, it is shown that pantograph arcing is a polarity-dependent phenomenon. For the positive-fed overhead traction system, where pantograph is cathode, the supply interruptions due to zigzag motion are comparatively less compared to negatively fed traction systems. As a result, the transients, due to pantograph arcing, are more frequent in negatively fed traction systems. It is found that the arc root movement along the electrode surfaces (pantograph-contact wire) is governed by the relative motion and polarity of the electrodes. The analyses presented in this paper also form a foundation to understand the pantograph arcing process and the corresponding influential parameters with the ac supply presented in the companion paper. The findings presented in this paper could be beneficial for coming up with appropriate mitigation techniques from the EMI due to pantograph arcing in dc-fed traction systems.

Space Vector Method for Voltage Dips and Swells Analysis

Abstract: A new method for voltage dips and swells analysis is presented in this paper. This method is based on the space vector representation in the complex plane and the zero-sequence voltage. Indeed, in the case of nonfaulted system voltages, the space vector follows a circle in the complex plane with a radius equal to the nominal voltage. It follows the same shape for balanced dips, but with a smaller radius. For unbalanced dips, this shape becomes an ellipse with parameters depending on the phase(s) in drop, dip magnitude and phase angle shift. For swells the space vector shape is not modified, though the zero-sequence voltage presents significant changes in its phase and magnitude and can be used for swells analysis. The changes in the space vector and the zero-sequence voltage are used to determine the dip/swell time occurrence, to classify and finally characterize the measured power-quality disturbance. Algorithms are developed for each step of this automatic voltage dips and swells analysis (segmentation, classification, and characterization) and are validated on real measurement data.

Fourier Transform-Based Modified Phasor Estimation Method Immune to the Effect of the DC Offsets

Abstract: This paper proposes a Fourier transform-based modified phasor estimation method to eliminate the adverse influence of the exponentially decaying dc offsets when discrete Fourier transform (DFT) is used to calculate the phasor of the fundamental frequency component in a relaying signal. By subtracting the result of odd-sample-set DFT from the result of even-sample-set DFT, the information of dc offsets can be obtained. Two dc offsets in a secondary relaying signal are treated as one dc offset which is piecewise approximated in one cycle data window. The effect of the dc offsets can be eliminated by the approximated dc offset. The performance of the proposed algorithm is evaluated by using computer-simulated signals and Electromagnetic Transients Program-generated signals. The algorithm is also tested on a hardware board with TMS320C32 microprocessor. The evaluation results indicate that the proposed algorithm can estimate the accurate phasor of the fundamental frequency component regardless of not only the primary decaying dc offset but also the secondary decaying dc offset caused by CT circuit itself including its burden.