Search

Exponentially decaying direct current (EDDC) components in the faulted current of a power system can result in an imprecise estimation of the fundamental frequency component. Because of sources such as the faulted transmission line, the current transformer (CT), and the relay's internal CT, current signals tend to include multiple EDDC components. This paper presents an accurate model of the faulted current, including a new term of EDDC components that belongs to the relay's internal CT along with the transmission line and the CT response. Moreover, this paper proposes an innovative algorithm to eliminate multiple EDDC components. The feasibility and performance of the proposed algorithm were validated using several static, dynamic, and experimental tests. The static test was carried out using MathWorks' MATLAB® software. For dynamic validation, the transient data of a power system (simulated in the commercial PSCAD© software) were employed to test the performance of the propounded method. The experimental test was performed on a distance protective relay manufactured by VEBKO AMIRKABIR Co. The main advantages of this method include the practical feasibility resulting from its simple structure, the ability to effectively suppress the adverse influence of multiple EDDC components (including the response of the transmission line, the CT, and the relay's internal CT), and the accuracy in estimating the fundamental frequency component. [ABSTRACT FROM AUTHOR]

In addition to the dynamic nature of microgrids, uncertainty in the proper operation of protection system and communication links are other challenges affecting the protection coordination of these networks. Therefore, in this paper, a new protection coordination plan based on decision tree for considering uncertainties in the topology of microgrid, protection system, and communication links is presented. The proposed method allows the adaptive protection to make global decisions and adopt the best strategy to clear faults depending on considered uncertainties. Since circuit breakers are the most prone to failure equipment in the protection system due to fault-caused stress, this paper models uncertainty in the protection system with uncertainty in the performance of circuit breakers. In order to consider uncertainty in circuit breakers and communication links, their probability of correct operation are not considered fixed but variable, respectively, proportional to the fault current flowing through the circuit breakers and the latency of communication links. The proposed plan was tested on a sample microgrid in DIgSILENT Power Factory. Results prove that using the proposed method, adaptive protection can establish an optimal sequence of strategies so that with the failure of each strategy, the best backup strategy is replaced given the uncertainties. [ABSTRACT FROM AUTHOR]

To address the inaccurate load demand sharing problems among parallel inverter-interfaced voltage-controlled distributed generation (DG) units in islanded microgrids (MGs) with different DG power ratings and mismatched feeder impedances, an enhanced voltage control scheme based on the active compensation of circulating voltage drops is proposed in this paper. Using the proposed strategy, reactive power and harmonic currents are shared accurately and proportionally without knowledge of the feeder impedances. Since the proposed local controller consists of two well-separated fundamental and harmonic voltage control branches, the reactive power and harmonic currents can be independently shared without having a remarkable effect on the amplitude or quality of the DGs voltage, even if nonlinear (harmonic) loads are directly connected at the output terminals of the units. In addition, accurate load sharing can also be attained when the plug-and-play performance of DGs and various loading conditions are applied to MGs. The effects of communication failures and latency on the performance of the proposed strategy are also explored. The design process of the proposed control system is presented in detail and comprehensive simulation studies on a three-phase MG are provided to validate the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

Transmission Expansion Planning (TEP) involves determining if and how transmission lines should be added to the power grid so that the operational and investment costs are minimized. TEP is a major issue in smart grid development, where demand response resources affect short- and long-term power system decisions, and these in turn, affect TEP. First, this paper discusses the effects of demand response programs on reducing the final costs of a system in TEP. Then, the TEP problem is solved using a Teaching Learning Based Optimization (TLBO) algorithm taking into consideration power generation costs, power loss, and line construction costs. Simulation results show the optimal effect of demand response programs on postponing the additional cost of investments for supplying peak load. [ABSTRACT FROM AUTHOR]

Microgrid is a distribution system composed of a set of micro-generators which are added to a network. One serious challenge facing a microgrid network is designing a proper protection scheme. This is because the fault current in a microgrid is constantly changing due to the presence of distributed generation (DG) resources at all levels of the distribution network and to the fact that they can operate in two islanding and normal modes. This makes the conventional methods inappropriate for microgrid protection. That is, new schemes should be developed for this purpose. This paper is a summary of studies recently carried out in the field of microgrid protection. Along this line, the structure and topology of microgrids are reviewed first. Afterwards, the protective challenges facing DG-equipped distribution networks are discussed. Then, the innovative methods proposed to solve these problems are analyzed. These methods can be divided up into six main categories: changing settings of protective devices at presence of DG units, disconnecting pertinent DG units when faults occur, creating a balance among different DG technologies, using fault current limiter, using smart transformers, and adaptive protection. In so doing, we consider the factors involved in selecting each method. The factors identified are microgrid type and topology, DG type, communication type and delay time, method of fault detection and analysis, relay type, fault type, method of grounding, and use of smart transformers in microgrid. It is hoped that this work will be useful to the researchers in the field of microgrid protection in finding relevant references and designing appropriate methods. [ABSTRACT FROM AUTHOR]

SUMMARY Useful lifetime of power cables and related aging failure rates mostly depends on thermal stresses that they experience because of the loss-resulted heat inside the cable in normal or abnormal currents of the network. Knowing the real values of the cable aging failure rate is a key point, and the network operators should enable to estimate the related aging failure rates of their network cables. The already provided methods for cable transient temperature estimation require several different inputs as well as cable installation configuration data that are difficult to collect for operators. Hence, in this article, an artificial neural network-based approach is applied for estimation a cable maximum temperature, which serves a certain daily load curve. The artificial neural network only requires four inputs that are easy to provide. An experimental equation is then used for estimating the cable minimum temperature, and finally, a three-level temperature curve is formed for cable aging failure rate estimation. The life fractions lost during each level of the predicted temperature curve are evaluated by resorting to an already existing combined electrothermal life model held for cable insulation. This method uses the life model and the probabilistic failure model to predict the failure rate of power cables for a future period. The results show that failure rate estimations are in good accordance with exact results, remarking that the estimated three-level stepwise curve of cable temperature is a good approximation of cable thermal transients during cyclic loads. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Among the several components of distribution systems, protection devices present a fundamental importance, since they aim at keeping the physical integrity not only of the system equipment, but also of the electricians team and the population in general. One of the protective devices playing a vital role in overhead distribution lines is fused cutout. At the era of privatized utilities, the protection devices should be allocated and coordinated optimally to reduce capital investments and the system outage costs. Amid this situation, the mentioned type of the protection device (fuse cutout) has not been studied for economical allocation up to now. In this paper, responding to this need, an accurate reliability model of fuse cutout containing hidden failures is figured out in the shape of a new Markov model. This model is used for economical allocation of the fuse cutouts. On the basis of the proposed model, a methodology for economic allocation of fuse cutouts is presented. This methodology involves the worth of energy not supplied (ENS) of the network and makes a balance between the cost of fuse cutout installation and the benefit of ENS decrease because of the minimizing the faulty zone by using more fuse cutouts. The methodology is tested on a sample distribution network as well as on IEEE 6-bus distribution test system (RBTS). Moreover, its capability on decision making about the fuse cutout placement and its simplicity for implementing on MV overhead lines are displayed. [Copyright &y& Elsevier]

This paper proposes a semi-analytical method to obtain the equilibrium bidding strategies for generation and demand units in a combined oligopoly and oligopsony wholesale electricity market. Such market structure is the outcome of the increasing deployment of demand response programs that facilitate active participation of demand-side players in the price-setting process. In this analysis, the concept of supply function equilibrium (SFE) is used to investigate the oligopolistic competition among generation units. The SFE model is extended and the demand function equilibrium (DFE) is obtained to study the oligopsonistic competition among demand units. The economic behavior of a market participant is formulated as a bi-level programming (BLP) problem. The imperfect competition among generation units, as well as among demand units, are modeled as a non-cooperative game. Next, a direct method is developed to calculate all candidate equilibriums of the market, and the locational marginal prices (LMPs) in terms of the bidding strategies of the market participants. The BLP problem is solved by obtaining the coordinated Pareto-dominant Nash equilibrium of the market participants’ non-cooperative games. Finally, the proposed analysis is examined in case studies. Accordingly, we report insightful observations with respect to the impact of the changes in the new market structure, at firm-level and market-level, such as in terms of mitigating market power of generation units, the market clearing prices and quantities, surplus for generation units and demand units, and potential impact on market efficiency. [ABSTRACT FROM AUTHOR]

In interconnected networks, it is impossible to satisfy all coordination constraints. Some methods have been proposed in the literature to determine the minimum break point set (MBPS) before the coordination procedure. In this paper, a new method is presented which coordinates overcurrent relays and simultaneously determines minimum unsatisfied constraints as broken constraints (BCs). For the implementation of this method, a new objective function (OF) is defined that also includes constraints with various weightings. These weightings determine the priority of constraints for being satisfied which are obtained by means of some expert rules. The expert rules of previous papers are modified and weighted to determine the priority of constraints. The genetic algorithm is used to optimize the OF that has an internal feedback for determining the minimum BCs considering coordination criterion. The effectiveness of the proposed technique is demonstrated by comparing the results of two sample networks with those obtained by using the recently developed techniques. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive approach to probabilistic characterization of aggregated load pattern of low-voltage consumers. Different from previous methods, consumer load patterns obtained from load survey are converted to empirical cumulative density functions. The functions are then used to address stochastic nature of load pattern in any given point of distribution network. The proposed approach is adopted to investigate the effect of load models on the characterization of aggregated load patterns. In addition, a goodness-of-fit analysis has been carried out to show that the load models can significantly affect the accuracy of aggregated load modeling. It is demonstrated that the constant power (real and reactive) load model which is normally adopted in most distribution network management studies may lead to misleading results compared to the actual network. Case studies are presented and discussed with reference to a real distribution network. The results verify that the proposed method is accurate and flexible, and the voltage-dependent load model is the most promising solution for the aggregated load modeling. [ABSTRACT FROM PUBLISHER]

This paper presents a methodology to quantify the benefit of the distribution transformer life extension brought about by customer-owned distributed-generation (DG) units. The proposed methodology is applied to a realistic distribution system with various scenarios, including different penetration levels and combinations of the DG technologies. A number of realistic distribution transformers are then used to demonstrate the results. The results show the distribution transformer loss-of-life variation, as a function of the DG penetration level, presenting a monotonically decreasing characteristic which saturates after a certain penetration level. Regarding DG technologies, it should be noted that remarkable reduction is achieved in loss-of-life rates of the examined transformers for all of the proposed scenarios. However, the extent of the reduction largely depends on the production pattern of the DG unit. As revealed by the results, the microturbine emerges as the most promising technology in the life extension of distribution transformers. [ABSTRACT FROM AUTHOR]

Abstract: Protection system failure is the main cause of cascading outages. It has been shown that the hidden failures lead to cascading outages. However, the effects of the coordination method as well as the hidden failure of the protection systems have not been investigated yet. In this paper, an algorithm based on sequential Monte Carlo simulation is proposed to find the reliability indices for different overcurrent relay coordination methods. The proposed algorithm is applied to the results of different relay coordination methods of a real network. The indices SAIFI, CAIDI, CAIFI and ENS for the results of different relay coordination methods are obtained strictly based on the contribution of the protection failure issues. Then, a comparison is made between the results of the different relay coordination methods in order to show how the coordination methods affect the reliability indices. [Copyright &y& Elsevier]

Abstract: For optimal co-ordination of overcurrent relays, linear programming techniques such as simplex, two-phase simplex and dual simplex are used. Another way of optimal coordination program is using artificial intelligent system such as genetic algorithm (GA). In this paper, a powerful optimal coordination method based on GA is introduced. The objective function (OF) is developed to solve the problems of miscoordination and continuous or discrete time setting multiplier (TSM) or time dial setting (TDS). In other words; the novelty of the paper is the modification of the existing objective function of GA, by introducing a new parameter and adding a new term to OF, to handle miscoordination problems both for continues and discrete TSM or TDS. The method is applied to two different power system networks and from the obtained results it is revealed that the new method is efficient, accurate and flexible. [Copyright &y& Elsevier]

Abstract: Interconnected power systems are multi-loop structured. In such networks, the determination of settings for all overcurrent relays can be carried out in different forms and may be quite complicated. The main problem for coordination is the determination of starting points, i.e. the location of starting relays in the procedure for settings, which is referred to as break points. In this paper, a powerful approach based on expert system is applied. The rules of the expert system include network configuration, protection systems, fault levels, etc. The method is applied to two networks with different configurations, pilot protection and other protection systems. From the obtained results, it is reviled that the new method is efficient, accurate, comprehensive and more optimal than the previously used graph theory. [Copyright &y& Elsevier]

Abstract: This paper presents a method to reduce the number of constraints and detect those that make the optimization of overcurrent relay settings to be infeasible. By reducing the number of constraints, the feasibility of the optimization is increased and the run-time of the program is decreased. It should be noted that, in the optimal coordination methods, the operating times of the relays are minimized in the frame of the objective function subjects to the so-called coordination constraints. However, a new method for optimum coordination of protective relays based on only constraints has been developed by authors of the paper. The pre-processing reduction constraints method introduced in this paper can be used for any optimal coordination of overcurrent relays. The new method is supported by results obtained from a typical test network and a real power system network. [Copyright &y& Elsevier]

Accurate models of overcurrent (OC) with inverse time relay characteristics play an important role for the coordination of power system protection schemes. This paper proposes a new method for modelling OC relays curves. The model is based on fuzzy logic and artificial neural network. The feedforward multilayer perceptron neural network is used to calculate the operating times of OC relays for various time dial settings (TDS) or time multiplier settings (TMS). The new model is more accurate than traditional models. The model has been validated by comparing the results obtained from the new method with linear and nonlinear analytical models. [Copyright &y& Elsevier]

In this paper, a new method for optimum coordination of overcurrent relays is proposed. The proposed method is based on only constraints. Minimization is inherently included by setting the time dials to minimum and increasing their values gradually. Configuration changes of the network is taken into account. The method does not need any initial solution. It can consider both linear and nonlinear relay characteristics models. The optimal settings of overcurrent relays for 8-Bus and England Norweb networks are obtained and comparison between the new method and existing methods is made. [ABSTRACT FROM AUTHOR]

Turbo-expander systems have long been used instead of regulators, but they have recently received attention as a driving medium for power electrical generators. These systems typically replace the regulator valves that reduce the gas pressure in gas distribution networks. As a generator, however, its turbine generates mechanical power that can be considered as a new source for dispersed generation. To study the dynamics of turbo-expander systems, we first introduce a detailed model that incorporates components such as the preheater, reheater, nozzle angle, and the turbine. This model is validated using empirical data collected from thermo-dynamic analysis. To be specific, we also investigated the effects of using a variable-angle nozzle versus a fixed-angle nozzle turbine. As an application in distribution networks, we looked into the use of turbo-expander driven generators as highly efficient components. The dynamic behavior and transient stability of such generators are thoroughly analyzed for disturbances in either mechanical or electrical parts using the proposed model. [ABSTRACT FROM PUBLISHER]

• The simplicity of the algorithm. • A simple and efficient trip zone. • Independency of the trip zone on the line length or its losses. • Fault classification capability. • Operation during symmetric and asymmetric power swings. Wide area protection (WAP) utilizes multiple information for faults identification. The use of several information sources leads to fault detection in a wider range and fault discrimination from other network conditions. But these methods are usually complex and require a lot of calculations. A novel WAP algorithm is proposed in this paper. The method is based on comparing active and reactive power flow directions on both sides of the related line. Active and reactive power flow directions are used for the detection of low- and high-impedance faults respectively. The polarity of active power fluctuations on both sides of the line is adopted to identify very high-impedance faults. The suggested approach benefits low computational burden and can detect and classify various faults, in addition to discriminating faulty and stress conditions. The simulation results by scientific and powerful software on the standard IEEE 39-bus system represent the capability of the proposed method. [ABSTRACT FROM AUTHOR]

Transmission loss has a considerable effect in overall power generation. For fairly distributing the charge of losses to generators and consumers in a deregulated power system, the allocation of this loss is very important. Game-theoretic methods seem fairer for share determination of each participant of a coalition with no discrimination. In this paper, the active and reactive power transmission losses are allocated to bilateral transactions simultaneously through load ow calculations and cooperative game theory solutions. The loss allocation problem and each bilateral transaction are treated as a game and a player of the game, correspondingly. Two game theory-based approaches, the Shapley value and the τ-value, are surveyed. The former is the most relevant game theory allocation method, while the latter is a novel approach. The in uences of all loss allocation game players and bilateral bargains on transmission loss are considered. These two proposed methods are applied to a simple 6-bus network and the modified IEEE 57-bus test system. In the 6-bus network positive MVA loss allocations and in the IEEE 57-bus system negative MVA loss allocations are studied. Finally, the results of allocation procedures are compared to each other. [ABSTRACT FROM AUTHOR]

The use of wind energy is increasing due to rapid increase in energy consumption and rapid deployment of wind energy. Among different types of wind turbine generators, doubly fed induction generators are most widely used because of their low cost and capability to work in different wind speeds. On the other hand, doubly fed induction generators (DFIGs) are very sensitive to voltage drop. If a fault occurs in the network the voltage of DFIG terminal drops. So the rotor current increases and this may lead to damage DFIG converters. So, a protection system must be used to protect DFIG converters. One of the common schemes used to protect DFIGs is crowbar protection. In this paper, the effects of static volt ampere reactive compensator (SVC) on DFIG operation during voltage drop and crowbar protection are investigated in detail. Results show that by using SVC the voltage drop of DFIG terminal during grid faults decreases. Therefore, the operation time of crowbar decreases and the fault ride through capability of DFIG increases. [ABSTRACT FROM AUTHOR]

In this paper, total harmonic distortion (THD) minimization of the output voltage of multilevel inverters is discussed. An efficient approach in reducing the harmonic contents of the inverter's output voltage is THD minimization. In multilevel inverters with a fundamental frequency switching strategy (each switch turning on and off once per output cycle), the switching angles can be selected so that the output THD is minimized (such as the so-called optimal-minimization-of-THD strategy). To obtain the optimum switching angles, an optimization algorithm is applied to the output-voltage THD. In three-phase multilevel inverters, the optimization algorithm is commonly applied to the phase voltage of the inverter. This results in the minimum THD in phase voltage but not necessarily in the line-to-line minimum THD, whereas in three-phase applications, the line-voltage harmonics are of the main concern from the load point of view. In this paper, using the genetic algorithm, a THD minimization process is directly applied to the line-to-line voltage of the inverter. This paper is based on a seven-level inverter. To verify the simulation results, a seven-level-cascaded-H-bridge-inverter-based hardware prototype, including an ATMEGA32 AVR microcontroller, has been implemented. Both simulation and experimental results indicate superiority of this approach over the commonly used phase-voltage THD minimization approach. [ABSTRACT FROM AUTHOR]