Showing papers in "IEEE Power & Energy Magazine in 2001"
TL;DR: In this article, the authors present a comprehensive summary of the causes and effects of voltage unbalance and discuss related standards, definitions, and mitigation techniques, as well as the resulting adverse effects on the system and on equipment such as induction motors and power electronic converters and drives.
Abstract: This paper endeavors to present a comprehensive summary of the causes and effects of voltage unbalance and to discuss related standards, definitions, and mitigation techniques. Several causes of voltage unbalance on the power system and in industrial facilities are presented as well as the resulting adverse effects on the system and on equipment such as induction motors and power electronic converters and drives. Standards addressing voltage unbalance are discussed and clarified, and several mitigation techniques are suggested to correct voltage unbalance problems. This paper makes apparent the importance of identifying potential unbalance problems for the benefit of both the utility and customer.
656 citations
TL;DR: In this article, the authors used coupled magnetic circuit simulation results for a four pole, three phase induction motor with 44, 43, and 42 rotor slots under healthy, static, dynamic and mixed eccentricity conditions.
Abstract: Detection of rotor slot and other eccentricity related harmonics in the line current of a three phase induction motor is important both from the viewpoint of sensorless speed estimation as well as eccentricity related fault detection. However, it is now clear that not all three phase induction motors are capable of generating such harmonics in the line current. Recent research has shown that the presence of these harmonics is primarily dependent on the number of rotor slots and the number of fundamental pole pairs of the machine. While the number of fundamental pole pairs of a three phase induction motor usually is within one to four (higher pole pairs are generally avoided due to increased magnetizing current), the number of rotor slots can vary widely. The present paper investigates this phenomenon further and obtains a hitherto nebulous theoretical basis for the experimentally verified results. Detailed coupled magnetic circuit simulation results are presented for a four pole, three phase induction motor with 44, 43, and 42 rotor slots under healthy, static, dynamic and mixed eccentricity conditions. The simulation is flexible enough to accommodate other pole numbers also. These simulations are helpful in quantifying the predicted harmonics under different combinations of load, pole pair numbers, rotor slots and eccentricity conditions, thus making the problem easier for drive designers or diagnostic tools' developers. Data from three different induction machines, namely, a 4 pole, 44 bar, 3 hp, a 4 pole, 28 bar, 3 hp and a 2 pole, 39 bar, 100 hp motor have been used to verify the results experimentally. The simulation and the experimental results clearly validate the theoretical findings put forward in this paper.
392 citations
TL;DR: In this article, appropriate modeling and simulation techniques are discussed for studying the voltage fluctuation and harmonic distortion in a network to which variable speed wind turbines are connected, which can be minimized with the proposed power electronics interface and control system while the wind energy conversion system captures the maximum power from the wind as wind speed varies.
Abstract: Grid connection of renewable energy sources is essential if they are to be effectively exploited, but grid connection brings problems of voltage fluctuation and harmonic distortion. In this paper, appropriate modeling and simulation techniques are discussed for studying the voltage fluctuation and harmonic distortion in a network to which variable speed wind turbines are connected. Case studies on a distribution network show that the voltage fluctuation and harmonic problems can be minimized with the proposed power electronics interface and control system while the wind energy conversion system captures the maximum power from the wind as wind speed varies. The studies have also demonstrated the ability of the advanced converter to assist the system voltage regulation.
372 citations
TL;DR: In this article, the authors present a graphical approach for modeling and analysis of custom power controllers, a new generation of power electronics-based equipment aimed at enhancing the reliability and quality of power flows in lowvoltage, distribution networks.
Abstract: This paper addresses the timely issue of modeling and analysis of custom power controllers, a new generation of power electronics-based equipment aimed at enhancing the reliability and quality of power flows in low-voltage, distribution networks [2],[3] The modeling approach adopted in this article is graphical in nature, as opposed to mathematical models embedded in code using a high-level computer language The well-developed graphic facilities available in an industry standard power system package, namely, PSCAD/EMTDC, are used to conduct all aspects of model implementation and to carry out extensive simulation studies Graphics-based models suitable for electromagnetic transient studies are presented for the following three custom power controllers: the distribution static compensator (D-STATCOM), the dynamic voltage restorer (DVR), and the solid-state transfer switch (SSTS) Comprehensive results are presented to assess the performance of each device as a potential custom power solution The paper is written in a tutorial style and aimed at the large PSCAD/EMTDC user base
360 citations
TL;DR: In this article, three different definitions of voltage unbalance have been developed by NEMA, IEEE, and the power community, respectively, in order to understand the implications of their use.
Abstract: Author Affiliations: Clarkson University, Potsdam, NY; Department of Electrical Engineering, University of Cape Town, South Africa. Abstract: This letter reviews three definitions of voltage unbalance developed by NEMA, IEEE, and the power community, respectively. The differing definitions of voltage unbalance are analyzed in order to understand the implications of their use. Introduction: In a three-phase system, voltage unbalance takes place when the magnitudes of phase or line voltages are different and the phase angles differ from the balanced conditions, or both. This letter addresses three definitions of voltage unbalance from three different communities and provides a numerical example and analysis to compare them. These definitions have important implications when studying for example, the effects of voltage unbalance on the performance of three-phase induction machines. Definition of Voltage Unbalance: The three definitions of voltage unbalance are stated and analyzed below. NEMA (National Equipment Manufacturer’s Association) Definition: The NEMA definition [1] of voltage unbalance, also known as the line voltage unbalance rate (LVUR), is given by
313 citations
TL;DR: In this paper, the performance of a 4 kW grid-connected residential wind-photovoltaic system (WPS) with battery storage located in Lowell, MA is reported. But, the system was originally designed to meet a typical New England (TNE) load demand with a loss of power supply probability (LPSP) of one day in ten years, as recommended by the Utility Company.
Abstract: This paper reports the performance of a 4 kW grid-connected residential wind-photovoltaic system (WPS) with battery storage located In Lowell, MA. The system was originally designed to meet a typical New England (TNE) load demand with a loss of power supply probability (LPSP) of one day in ten years, as recommended by the Utility Company. The data used in the calculation were wind speed and irradiance of Logan Airport Boston (LAB), obtained from the National Climate Center in North Carolina. The present performance study is based on two-year operation (May 1996 to April 1998) of the WPS. Unlike conventional generation, the wind and the sunrays are available at no cost and generate pollution-free electricity. At around noon the WPS satisfies its load and provides additional energy to the storage or to the grid. On-site energy production is undoubtedly accompanied with minimization of environmental pollution, reduction of losses in power systems transmission and distribution equipment, and supports the utility in demand side management (DSM). This paper includes discussions on system reliability, power quality, loss of supply, and effects of the randomness of the wind and the solar radiation on system design.
310 citations
Abstract: Harmonics – Past to Present Power systems are designed to operate at frequencies of 50 or 60Hz. However, certain types of loads produce currents and voltages with frequencies that are integer multiples of the 50 or 60 Hz fundamental frequency. These higher frequencies are a form of electrical pollution known as power system harmonics. Power system harmonics are not a new phenomenon. Concern over harmonic distortion has ebbed and flowed during the history of electric power systems. Steinmetz published a book in 1916 that devoted considerable attention to the study of harmonics in three-phase power systems. His main concern was third harmonic currents caused by saturated iron in transformers and machines, and he was the first to propose delta connections for blocking third harmonic currents. Later, with the advent of rural electrification and telephone service, power and telephone circuits were often placed on common rights-of-way. Harmonic currents produced by transformer magnetizing currents caused inductive interference with open-wire telephone systems. The interference was so severe at times that voice communication was impossible. This problem was studied and alleviated by filtering and by placing design limits on transformer magnetizing currents. Today, the most common sources of harmonics are power electronic loads such as adjustable-speed drives (ASDs) and switch-mode power supplies. These loads use diodes, silicon-controlled rectifiers (SCRs), power transistors, and other electronic switches to chop waveforms to control power or to convert 50/60Hz AC to DC. In the case of ASDs, the DC is then converted to variable-frequency AC to control motor speed. Example uses of ASDs include chillers and pumps. Due to tremendous advantages in efficiency and controllability, power electronic loads are proliferating and can be found at all power levels – from low voltage appliances to high voltage converters. Hence, power systems harmonics are once again an important problem.
296 citations
TL;DR: In this paper, the authors present a simulation method that provides objective indicators to help system planners decide on appropriate installation sites, operating policies, and selection of energy types, sizes, and mixes in capacity expansion when utilizing PV and wind energy in small isolated systems.
Abstract: The application of renewable energy in electric power systems is growing rapidly due to enhanced public concerns for adverse environmental impacts and escalation in energy costs associated with the use of conventional energy sources. Photovoltaics and wind energy sources are being increasingly recognized as cost-effective generation sources in small isolated power systems primarily supplied by costly diesel fuel. The utilization of these energy sources can significantly reduce the system fuel costs but can also have considerable impact on the system reliability. A realistic cost/reliability analysis requires evaluation models that can recognize the highly erratic nature of these energy sources while maintaining the chronology and interdependence of the random variables inherent in them. This paper presents a simulation method that provides objective indicators to help system planners decide on appropriate installation sites, operating policies, and selection of energy types, sizes, and mixes in capacity expansion when utilizing PV and wind energy in small isolated systems.
263 citations
TL;DR: The literature for current applications of advanced artificial intelligence techniques in power quality, including applications of fuzzy logic, expert systems, neural networks, and genetic algorithms, are surveyed.
Abstract: Increasing interest in power quality has evolved over the past decade. This paper surveys the literature for current applications of advanced artificial intelligence techniques in power quality (PQ). Applications of some advanced mathematical tools in general, and wavelet transform in particular, in power quality are also reviewed. An extensive collection of literature covering applications of fuzzy logic, expert systems, neural networks, and genetic algorithms in power quality is included. Literature exposing the use of wavelets in power quality analysis as well as data compression is also cited.
234 citations
TL;DR: In this article, a novel technique for accurate discrimination between an internal fault and a magnetizing inrush current in the power transformer by combining wavelet transforms with neural networks is presented.
Abstract: The wavelet transform is a powerful tool in the analysis of the power transformer transient phenomena because of its ability to extract information from the transient signals simultaneously in both the time and frequency domain. This paper presents a novel technique for accurate discrimination between an internal fault and a magnetizing inrush current in the power transformer by combining wavelet transforms with neural networks. The wavelet transform is firstly applied to decompose the differential current signals of the power transformer into a series of detailed wavelet components. The spectral energies of the wavelet components are calculated and then employed to train a neural network to discriminate an internal fault from the magnetizing inrush current. The simulated results presented clearly show that the proposed technique can accurately discriminate between an internal fault and a magnetizing inrush current in power transformer protection.
215 citations
TL;DR: In this article, the authors focus on the analysis and mitigation of market power in electricity supply and propose a market power management scheme to identify and mitigate market power abuse by market management measures.
Abstract: Restructuring of the power industry worldwide has brought the issue of market competitiveness to the forefront. While "vertical" market power has been limited by disaggregating generation and transmission, nondiscriminatory access to the transmission system by power suppliers, and the creation of the independent system operator (ISO), "horizontal" and localized market power still survives. In the last few years, the issue of a small number of generators exercising market power has received a great deal of attention, and much has been published on how to identify and mitigate such abuse by market management measures. This paper focuses on the analysis and mitigation of market power in electricity supply.
TL;DR: In this article, an ant colony system (ACS) based optimization approach is proposed for the enhancement of hydroelectric generation scheduling, where the search space of multi-stage scheduling is first determined.
Abstract: In this paper, an ant colony system (ACS) based optimization approach is proposed for the enhancement of hydroelectric generation scheduling. To apply the method to solve this problem, the search space of multi-stage scheduling is first determined. Through a collection of cooperative agents called ants, the near-optimal solution to the scheduling problem can be effectively achieved. In the algorithm, the state transition rule, local pheromone-updating rule, and global pheromone-updating rule are all added to facilitate the computation. Because this method can operate the population of agents simultaneously, the process stagnation can be better prevented. The optimization capability can be thus significantly enhanced. The proposed approach has been tested on Taiwan Power System (Taipower) through the utility data. Test results demonstrated the feasibility and effectiveness of the method for the application considered.
TL;DR: In this article, the authors present three definitions of voltage unbalance developed by NEMA, IEEE, and the power community, respectively, in order to understand the implications of their use.
Abstract: In a three-phase system, voltage unbalance takes place when the magnitudes of phase or line voltages are different and the phase angles differ from the balanced conditions, or both. This letter reviews three definitions of voltage unbalance developed by NEMA, IEEE, and the power community, respectively. The differing definitions of voltage unbalance are analyzed in order to understand the implications of their use.
TL;DR: In this paper, the steady-state performance of an induction motor connected to unbalanced three-phase voltages is analyzed and the effect of the angle of the CVUF on the motor is discussed.
Abstract: Analysis of the steady-state performance of an induction motor connected to unbalanced three-phase voltages is presented. The index of voltage unbalance used in this paper is the complex voltage unbalance factor (CVUF) that consists of the magnitude and the angle. In addition to formulation and discussion of the motor operating characteristics under voltage unbalance, special emphasis is placed on the effect of the angle of the CVUF on the motor. It is found that the angle of the CVUF is also an important factor that merits particular attention in analyzing voltage unbalance issues.
TL;DR: In this article, a comparative study of neural network (NN) efficiency for the detection of incipient faults in power transformers is presented, where the NN was trained according to five diagnosis criteria commonly used for dissolved gas analysis (DGA) in transformer insulating oil.
Abstract: This paper presents a comparative study of neural network (NN) efficiency for the detection of incipient faults in power transformers. The NN was trained according to five diagnosis criteria commonly used for dissolved gas analysis (DGA) in transformer insulating oil. These criteria are Doernenburg, modified Rogers, Rogers, IEC and CSUS. Once trained, the neural network was tested by using a new set of DGA results. Finally, NN diagnosis results were compared with those obtained by inspection and an analysis. The study shows that NN rate of successful diagnosis is dependant on the criterion under consideration, with values in the range of 87-100%.
TL;DR: In this paper, the authors investigated the reliability benefits of adding wind turbine generators (WTG) as an alternative supply in a rural distribution system, and the effects on the wind site selection and the number of wind units were investigated.
Abstract: Wind turbine generators (WTG) used as altenative supply in a distribution system have different impacts on the system reliability performance than conventional altemative supplies due to the variable wind speed. This paper investigates the system reliability benefits of adding WTG as an alternative supply in a rural distribution system. The wind generation interrupted energy benefit (WGIEB), the wind generation interruption cost benefit (WGICB), the equivalent number of conventional generators (ENCG), and the equivalent conventional generator capacity (ECGC) of one MWWTG are introduced. These indices provide direct reliability benefit indicators on the addition of WTG and are important information for system planners to make planning decisions such as the selection of a wind site and the number of WTG. A test rural distribution system is utilized to illustrate the proposed technique. The effects on the system reliability benefits of the wind site selection and the number of wind units are investigated.
TL;DR: In this paper, the authors investigated how mechanical forces within the transformer coils build up under inrush compared to those occurring at short circuit and found that inrush current peaks of 70% of the rated short circuit current cause local forces in the same order of magnitude as those at short circuits.
Abstract: From failure experience on power transformers, it was very often suspected that inrush currents, occurring when energizing unloaded transformers, were reason for damage. In this paper, it was investigated how mechanical forces within the transformer coils build up under inrush compared to those occurring at short circuit. Two-dimensional and three-dimensional computer modeling for a real 268 MVA, 525/17.75 kV three-legged step up transformer was employed. The results show that inrush current peaks of 70% of the rated short circuit current cause local forces in the same order of magnitude as those at short circuit. The resulting force summed up over the high voltage coil is even three times higher. Although inrush currents normally are smaller, the forces can have similar amplitudes as those at short circuit however with longer exposure time. Therefore, care has to be taken to avoid such high inrush currents. Today controlled switching offers an elegant and practical solution.
TL;DR: In this paper, a reduced-order dynamic model for a grid-connected fuel-cell power plant that is suitable for preliminary stability assessment is presented, which is applied to a distributed utility that uses fuel cells and gas turbines to investigate the nature and magnitude of their interaction.
Abstract: This paper describes a reduced-order dynamic model for a grid-connected fuel-cell power plant that is suitable for preliminary stability assessment. Generic voltage and power control loops are included. The model is applied to a distributed utility that uses fuel cells and gas turbines to investigate the nature and magnitude of their interaction. The studies presented in the paper show the effect of the mix between the fuel cell and gas turbine generation on the system stability. The developed model, being simple, could provide a useful tool for the planning of distributed generation.
TL;DR: In this article, a fault location algorithm based on phasor measurement units (PMUs) for series compensated lines is proposed, which does not utilize the series device model or knowledge of the operation mode of the series devices to compute the voltage drop during the fault period.
Abstract: This work presents a new fault location algorithm based on phasor measurement units (PMUs) for series compensated lines. Traditionally, the voltage drop of a series device is computed by the device model in the fault locator of series compensated lines, but by using this approach errors are induced by the inaccuracy of the series device model or the uncertainty operation mode of the series device. The proposed algorithm does not utilize the series device model or knowledge of the operation mode of the series device to compute the voltage drop during the fault period. Instead, the proposed algorithm uses the two-step algorithm, prelocation step and correction step, to calculate the voltage drop and fault location. The proposed technique can be easily applied to any series FACTS compensated line. EMTP generated data using a 300 km, 345 kV transmission line has been used to test the accuracy of the proposed algorithm. The tested cases include various fault types, fault locations, fault resistances, fault inception angles, etc. The study also considers the effect of various operation modes of the compensated device during the fault period. Simulation results indicate that the proposed algorithm can achieve up to 99.95% accuracy for most tested cases.
TL;DR: In this article, a dynamic programming method for solving reactive power/voltage control problems in a distribution system is presented, where the goal is to properly dispatch main transformers under load tap changers, substation capacitors, and feeder capacitors.
Abstract: This paper presents a dynamic programming method for solving reactive power / voltage control problems in a distribution system. The objective of this paper is to properly dispatch main transformers under load tap changers, substation capacitors, and feeder capacitors based on hourly forecast loads of each feeder section and primary bus voltage such that the total feeder loss can be minimized, voltage profile can be improved, and the reactive power flow into the main transformer can be restrained. The constraints that must be considered include the maximum allowable number of switching operations in a day for under load tap changer and each capacitor, and the voltage limit on the feeder and secondary bus voltage is limited. To demonstrate the usefulness of the proposed approach, reactive power /voltage control in a distribution system within the service area of the Yunlin District Office of Taiwan Power Company is performed. It is found that a proper dispatching schedule for each capacitor and under load tap changer can be reached by the presented method.
TL;DR: In this paper, the authors demonstrate the effect of temperature and electric field on the field grading of resistively graded dielectric dc cable for the range of measured material properties and provide an analytical approximation for computing the field of resistive dielectrics.
Abstract: The development of solid dielectric dc transmission class cable is a priority throughout much of the world, to avoid risks associated with placing hydrocarbon fluids in underwater environments. The conductivity of polymeric solid dielectrics tends to be a strong function of temperature and electric field, however. Based on measured material properties, we demonstrate the effect of such dependencies on the field grading of dc cable for the range of measured material properties and provide an analytical approximation for computing the field of resistively graded dielectrics, including the effect of temperature and field-dependent conductivity.
TL;DR: In this article, a control strategy for damping of electromechanical power oscillations using an energy function method is derived, and the effect of the damping effect is robust with respect to loading condition, fault location and network structure.
Abstract: This paper examines the enhancement of power system stability properties by use of thyristor controlled series capacitors (TCSCs) and static VAr systems (SVCs). Models suitable for incorporation in dynamic simulation programs used to study angle stability are analyzed. A control strategy for damping of electromechanical power oscillations using an energy function method is derived. Using this control strategy each device (TCSC and SVC) will contribute to the damping of power swings without deteriorating the effect of the other power oscillation damping (POD) devices. The damping effect is robust with respect to loading condition, fault location and network structure. Furthermore, the control inputs are based on local signals. The effectiveness of the controls are demonstrated for model power systems.
TL;DR: In this paper, a new algorithm for the optimal feeder routing problem using the dynamic programming technique and geographical information systems (GIS) facilities is proposed, where all practical issues, such as cost parameters (investments, line losses, reliability), technical constraints (voltage drop and thermal limits), as well as physical routing constraints (obstacles, high-cost passages, existing line sections) are taken into consideration.
Abstract: Optimal feeder routing is an important part of the general optimal distribution network planning. This paper proposes a new algorithm for the optimal feeder routing problem using the dynamic programming technique and geographical information systems (GIS) facilities. All practical issues, such as cost parameters (investments, line losses, reliability) and technical constraints (voltage drop and thermal limits), as well as physical routing constraints (obstacles, high-cost passages, existing line sections) are taken into consideration. The algorithm developed is validated comparing its results for a simplified study case, with those obtained by an established solver. The effectiveness of the algorithm is further illustrated for a "real-world" study case.
TL;DR: In this article, a pattern recognition technique based on Bayes minimum error classifier is developed to detect broken rotor bar faults in induction motors at the steady state using only stator currents as input without the need for any other variables.
Abstract: A pattern recognition technique based on Bayes minimum error classifier is developed to detect broken rotor bar faults in induction motors at the steady state. The proposed algorithm uses only stator currents as input without the need for any other variables. Initially, rotor speed is estimated from the stator currents, then appropriate features are extracted. The produced feature vector is normalized and fed to the trained classifier to see if the motor is healthy or has broken bar faults. Only the number of poles and rotor slots are needed as pre-knowledge information. A theoretical approach together with experimental results derived from a 3 hp AC induction motor show the strength of the proposed method. In order to cover many different motor load conditions, data are obtained from 10% to 130% of the rated load for both a healthy induction motor and an induction motor with a rotor having 4 broken bars.
TL;DR: In this paper, a real-time digital algorithm based on discrete Fourier transform (DFT) was proposed to estimate the frequency of a sinusoid with harmonics in real time.
Abstract: A precise digital algorithm based on discrete Fourier transforms (DFT) to estimate the frequency of a sinusoid with harmonics in real-time is proposed. This algorithm, that the authors call smart discrete Fourier transforms (SDFT), smartly avoids the errors that arise when frequency deviates from the nominal frequency, and keeps all the advantages of the DFT e.g., immunity to harmonics and the recursive computing can be used in SDFT. These make the SDFT more accurate than conventional DFT based techniques. In addition, this method is recursive and very easy to implement, so it is very suitable for use in real-time. The authors provide the simulation results compared with a conventional DFT method and second-order Prony method to validate the claimed benefits of SDFT.
TL;DR: In this paper, the authors proposed a few optimal locations of FACTS devices and then determined the best optimal location in order to reduce the production cost along with the device cost, and the allocation and requirement are also discussed.
Abstract: Power system restructuring requires expanding unused potentials of transmission systems due to environmental, right-of-way and cost problems, which are major hurdles for power transmission network expansion. FACTS devices can be an alternative to reduce the flows in heavily loaded lines, resulting in an increased loadability, low system loss, improved stability of the network, reduced cost of production, and fulfilled contractual requirements by controlling the power flows in the network. This letter suggests first, the few optimal locations of FACTS devices and then determines the best optimal location in order to reduce the production cost along with the device cost. The allocation and requirement are also discussed. The effectiveness of the proposed methods is demonstrated on an IEEE 14-bus system.
TL;DR: A neural-fuzzy technology-based classifier for the recognition of power quality disturbances that adopts neural networks in the architecture of frequency-sensitive competitive leaning and learning vector quantization is presented.
Abstract: This paper presents a neural-fuzzy technology-based classifier for the recognition of power quality disturbances. The classifier adopts neural networks in the architecture of frequency sensitive competitive learning and learning vector quantization (LVQ). With given size of codewords, the neural networks are trained to determine the optimal decision boundaries separating different categories of disturbances. To cope with the uncertainties in the involved pattern recognition, the neural network outputs, instead of being taken as the final classification, are used to activate the fuzzy-associative-memory (FAM) recalling for identifying the most possible type that the input waveform may belong to. Furthermore, the input waveforms are preprocessed by the wavelet transform for feature extraction so as to improve the classifier with respect to recognition accuracy and scheme simplicity. Each subband of the transform coefficients is then utilized to recognize the associated disturbances.
TL;DR: In this article, a time-sequential simulation technique incorporating the effects of weather conditions and restoration resources in reliability cost/worth evaluation of distribution systems is presented, where the average failure rate is combined with the TVWF to create time-varying failure rates (TVFRs) for each component.
Abstract: This paper presents a time-sequential simulation technique incorporating the effects of weather conditions and restoration resources in reliability cost/worth evaluation of distribution systems. Time-varying weight factors (TVWF) are introduced to represent the effects on component failure rates and restoration times of weather and available restoration resources. The average failure rate is combined with the TVWF to create time-varying failure rates (TVFRs) for each component. The average restoration time is combined with the TVWF to create time-varying restoration times (TVRTs). Studies conducted in a test distribution system show that the TVFR have large impacts on the interruption costs of frequency-sensitive customers and slight effects on others. The TVRT have significant effects on the indices for all the customers. It is therefore important to consider TVRT in evaluating reliability cost/worth of network reinforcement.