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This paper proposes a systematic method for the identification of the load circuit parameters (say the $\boldsymbol{R}$ , $\boldsymbol{L}$ , and $\boldsymbol{C}$ elements) based only on the information of the instantaneous voltage and current measured at the point of common coupling (pcc). Geometric Algebra (GA) and concepts of differential geometry are used to produce a rigorous mathematical framework. The identification is formulated as a multidimensional geometrical problem that is solved conveniently by means of GA. Once the passive elements of the load have been identified, the active and reactive powers can be computed from first electromagnetic principles (Maxwell Equations). The theory is general and is verified with linear and nonlinear circuits. The paper shows single-phase circuits but the theory can be extended to three-phase circuits. The method is easy to program and has shown to be very robust for all tested cases. Because of its generality, the method presented will find applications beyond electric circuits. [ABSTRACT FROM AUTHOR]

Permanent emergency lighting is a safety device whose lamp is always on and continues to operate when a mains failure occurs. The circuit has to charge the battery and drive the lamp while the grid is present and keep the lamp running from the battery in case of mains failure. This might require up to three different converters: front-end converter, electronic ballast, and battery charger. Since an emergency lamp must be on for as long as the local regulations require during a grid failure, efficiency is key. This article presents a permanent emergency lamp based on a series resonant current regulator that operates both as a ballast and as a battery charger. In this circuit, the power converter is placed in series with the LED array, operating as a controllable nondissipative impedance. A methodology based on the study of energy balances during resonant operation intervals is presented, aiming to design and build a laboratory prototype with low component count and high efficiency. [ABSTRACT FROM AUTHOR]

Forced ventilated underground installations are challenging to design due to the complex mathematics of turbulent fluid flow. Previous methods have tackled this problem, but only recently, the effect of confinement on a single cable has been considered. In this paper, the effect of cable confinement on the practical case of ventilated trefoil installations is investigated. The complete Computational Fluid Dynamics (CFD) problem is solved using Finite Element Method (FEM) simulations and then validated by laboratory experiments using pipes. FEM simulations are then used to vary the current, the wind velocity, and the distance of the trefoil from the tunnel wall to get a comprehensive picture of the phenomena. This data is used to create a new empirical equation for the Nusselt number for a wide range of operating conditions. The principle of similitude is validated for a scaled-down model of a large tunnel and then used to analyze the thermal rating of tunnels. The new equation improves the IEC standard equations by accounting for the effect of cable confinement on trefoil installations. An important finding of this paper is that a lower maximum permissible current rating is obtained, which if ignored can present a serious risk for thermal damage. It was also found that the cables must be installed three cable diameters away from the wall to prevent the derating effect of confinement. [ABSTRACT FROM AUTHOR]

This paper presents a distributed voltage regulation method based on multi-agent system control and network self-organization for a large distribution network. The network autonomously organizes itself into small subnetworks through the epsilon decomposition of the sensitivity matrix, and agents group themselves into these subnetworks with the communication links being autonomously determined. Each subnetwork controls its voltage by locating the closest local distributed generation and optimizing their outputs. This simplifies and reduces the size of the optimization problem and the interaction requirements. This approach also facilitates adaptive grouping of the network by self-reorganizing to maintain a stable state in response to time-varying network requirements and changes. The effectiveness of the proposed approach is validated through simulations on a model of a real heavily-meshed secondary distribution network. Simulation results and comparisons with other methods demonstrate the ability of the subnetworks to autonomously and independently regulate the voltage and to adapt to unpredictable network conditions over time, thereby enabling autonomous and flexible distribution networks. [ABSTRACT FROM AUTHOR]

Dynamic thermal rating for underground cables with balanced phase currents has been extensively studied in the literature. Paradoxically, distribution feeders operate inherently unbalanced and no publications are available for unbalanced thermal rating. Because of the lack of technology, utilities operate with very conservative margins using the maximum of the three-phase currents to determine the line loading. This practice leads to the significant underutilization of underground cables. To fill the gap this paper presents a physically sound and accurate transient electro-thermal model of unbalanced three-phase distribution cables considering variations in the load and environment. Moreover, the model proposed in this paper allows harvesting the large hidden capacity of distribution systems trapped in the thermal inertia of underground cables. A convenient visual tool is developed to uncover hot spots and underutilized energy routes. This facilitates real-time network reconfiguration based on predicted load graphs and dynamic thermal conditions. The proposed model is also useful to study load growth, perhaps eliminating or deferring capital investments needed to upgrade the capacity of the existing underground feeders. The IEEE 37-bus distribution network is used as a case study. The results show that the proposed methodology has a high potential to defer investments, for instance, one could save $5M for a small system like the IEEE 37-bus distribution network which would have been necessary under current conservative practice. [ABSTRACT FROM AUTHOR]

In this letter, a new method is proposed to measure the magnetizing inductances of dual reversible models of single-phase two-winding transformers. Different from all available methods, a dc source is used to excite the transformer to produce deep-saturation through an air-core inductor. A small ac source is applied to the other winding to measure the two magnetizing inductances with only one test, significantly reducing the cost. The cost is further reduced because the necessary equipment is of smaller size than that for the available methods. The proposed method is validated experimentally and with finite elements simulations for two single-phase transformers. [ABSTRACT FROM AUTHOR]

This paper presents a new method for detecting the early stages of unstable conditions in large-scale power systems. A reduced order dynamic model is introduced for each control area that includes an aggregated generator and associated controllers. An innovative nonlinear observer proposed that can simultaneously estimate the parameters and states of the model benefiting from available online measurements. A sensitivity analysis is used to improve the efficacy and performance of the proposed estimator. The estimated states of the proposed equivalent generator model are then used to define a Real-Time Transient Stability (RTSA) index. The recommended RTSA index demonstrates to be a clear measure of the distance to instability and prevents economic losses due to blackouts and cascading system trips. The PSS/E simulation results on two case studies using show the excellent performance of the proposed index. [ABSTRACT FROM AUTHOR]

Closed-form determination of fault current limiter (FCL) impedance for symmetrical faults was reported previously using a mathematically rigorous method. In this paper, extensions of the method to asymmetrical faults are presented. Single line-to-ground faults and line-to-line faults are studied. As shown in this paper, although the material corresponding to asymmetric faults use the same basic concepts described before, the different nature of asymmetrical faults and application of symmetrical components theory lead to a different formulation. Also, the graphical representation of asymmetrical faults is as contour plots instead of the circle diagrams for three-phase faults. A three-bus and the IEEE 39-bus systems are used to illustrate the virtues of the method for the proper selection of FCLs impedance. Results demonstrate that the FCL designed, as shown in this paper, works properly for asymmetrical faults. The new formulation and graphical representation for asymmetrical faults are compatible with traditional power system short-circuit analysis software. An example with both symmetrical and asymmetrical faults is presented to illustrate how a full FCL impedance design can be achieved with the method. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the magnetic effects of steel tanks on different three-phase transformers. Removable steel tanks are designed and built for transformers with distinct sizes and core topologies. Corresponding tanks of nonmagnetic materials are also built to study the effects of tank material. Zero-sequence tests, open-circuit (no-load) tests, ferroresonance, geomagnetically induced current (GIC), and energization (inrush current) tests are performed on the transformers with and without tanks. The results indicate that steel tanks have a significant effect on the zero-sequence impedance, ferroresonance performance, GIC, and no-load losses in steady state, while the tank has no appreciable effect on inrush currents. [ABSTRACT FROM AUTHOR]

This paper exploits smart inverters of distributed energy resources (DERs) for the mitigation of fault-induced delayed voltage recovery (FIDVR). According to the IEEE Standard 1547-2018, smart inverters with voltage ride-through and var injection capabilities are promising technologies for mitigating FIDVR. This paper quantitatively investigates the effectiveness of smart inverters for FIDVR from a system-wide perspective. It shows that the penetration of smart inverters and induction motors are important factors affecting mitigation success. In addition, the over-voltages at the end of FIDVR due to load disconnections can be prevented with coordinated control of DERs and automated switching capacitors. The control scheme of smart inverters used for FIDVR is extended from the distributed volt/var droop control to include low- and high-voltage ride-through capabilities. The IEEE 13-bus feeder, the 8500-node test feeder, and the IEEE 390-node low-voltage network test system are used to illustrate the impact of DER penetration. For example, one can determine the residential air conditioner penetration level (for a given system at certain DER penetration level) that makes a system susceptible to FIDVR. The effects of solar intermittency are also studied. [ABSTRACT FROM AUTHOR]

This two-part paper is intended to inform power system engineers, electrical engineering academicians, and suppliers of electrical apparatus of the threat of wildfires initiated from mal-operation of electrical grids and the unexploited opportunity to develop proper solutions and preventive means to such lethal events. This part (Part I) reviews and categorizes research in different fields of science and industrial projects that attempt to address wildfire issues. The topics include prediction and prevention means, detection methods, monitoring and surveillance techniques, suppression methods, allocation and mapping algorithms, and a summary of research and educational efforts. Subsequently, this paper highlights the damages and negative effects that a wildfire can cause to the electric grid and the interruptions to its continuous operation. Finally, this paper analyzes and categorizes the various scenarios of faulty electrical networks that may lead to wildfires. Part I of this paper provides the ground work and information for the solutions and discussions presented in Part II. [ABSTRACT FROM AUTHOR]

In this part of the paper (Part II), available technical solutions to minimize or prevent wildfires caused by power networks are reviewed in more detail. These methods include fundamental changes to power system equipment, periodic checkups and maintenance, identification of faults via voltage/current waveforms, changes to protection and relaying settings and coordination, and installation of new power system apparatus. The strengths and drawbacks of each method are discussed. The discussion section brainstorms some future possible solutions that smart-grid technologies can enable. The paper concludes that there has neither been enough research in universities nor enough investment by industry to develop viable preventive solutions for wildfires that initiate from the daily operation of electrical power grids. This, in the authors’ opinion, is the crucial time for investment in research and development to tackle the lethal wildfire problem while electric power networks offer “the lowest hanging fruits to pick.” [ABSTRACT FROM AUTHOR]

This two-part paper presents a method to mitigate half-cycle saturation of transformers caused by monopolar operation of neighboring HVDC transmission lines while keeping the ability to detect the zero-sequence fault currents (ZSFC) when ground faults occur. Part I of this paper has presented the mitigation principles and device design of the proposed neutral current blocking switch. In Part II, the performance of the proposed method to permit the circulation of zero-sequence current is investigated. An operation strategy is proposed that simultaneously allows the mitigation of half-cycle saturation and the detection of ZSFC. Simulations on a widely-used 500 kV system show that the proposed mitigation technique, using a sub-synchronous switching frequency (no higher than 30 Hz), can effectively mitigate the half-cycle saturation while allowing the circulation of ZSFC. The novel mitigation method exploits the characteristic differences between half-cycle saturation (dc) and asymmetric faults (ac). The method provides an implementable solution to the dc-bias phenomenon because it delivers concurrently dc-bias mitigation, minimal impact on ground fault detection, and no switching stresses on the power electronic switches. The proposed technique can also be applied to the mitigation of geomagnetically induced currents. [ABSTRACT FROM AUTHOR]

When HVDC transmission systems operate in the ground return mode, transformers in adjacent ac systems frequently suffer half-cycle saturation. In this two-part paper, an effective mitigation method using power electronic switches is proposed that does not compromise the ability of the protection system to detect fault currents. In Part I, a study is conducted to find the physical performance of the half-cycle saturation and give a technical solution to the inrush-like half-cycle saturated currents. Power electronic switches, opening the neutral connection to ground, are utilized to mitigate the saturation without compromising the reference ground of the transformer. The determination of the switch parameters, switching frequency, and duty cycle is performed. Details of the mitigation transient are studied by comparing simulations and laboratory experiments to validate the efficacy of the proposed power electronic switch for the mitigation of half-cycle saturated currents. In Part II, the influence of the proposed switch on the detection of ground faults is investigated. Proper parameter and operation strategies are selected to effectively detect the ground faults and ensure the fast response of protective relays. The techniques proposed are applicable to mitigate geomagnetically induced current. [ABSTRACT FROM AUTHOR]

This paper presents a novel circuit model for eddy-current effects for multi-winding transformers. It presents the theoretical framework to model eddy currents applicable to layer or disk windings with n-layers for two-dimensional (2-D) transformer arrangements. The new white-box model is physical, dual, and leakage reversible. Additionally, it does not have mutually coupled elements or negative inductors. The paper presents a complete circuit for transformer windings (including capacitances) and core. The model accurately predicts the variation of resistance (dc component, skin, and proximity effects) and leakage inductance of windings from dc to hundreds of kilohertz. Being derived from the principle of duality, the circuit elements can be physically related one-to-one to the distribution of flux and current in the winding geometry. A practical winding discretization, based on field penetration depth for the given frequency, allows estimating the parameters using very simple formulae requiring only geometrical information and properties of materials. Model verification is done using finite element simulations. It is shown that the circuit model matches very well with 2-D FEM simulations. The circuit can be easily implemented in any circuit simulation software like EMTP-RV, PSCAD, ATP, etc., by simply dragging and dropping elements. [ABSTRACT FROM AUTHOR]

Accurate modeling of three-phase asymmetric (three- and five-limb) transformers is still a challenge. In this paper, a method for the calculation of the major hysteresis loop for three-phase asymmetric transformers is proposed. All necessary information can be obtained from measurements taken at the transformer terminals without breaking the connections and using only standard tests. For illustration and validation, the method is applied to Wye–wye and Wye–delta connections for three-limb and five-limb transformers, but the method is applicable to all transformer connections. The obtained hysteresis loops are implemented in three-phase dual-reversible models. Simulation results are validated with laboratory experiments. The excellent agreement between simulations and measurements gives great confidence that the method is practical for engineering work. [ABSTRACT FROM AUTHOR]

An optimized two-zone model based on the critical degree of saturation is proposed in this paper for ampacity calculations of underground power cables. Moisture migration is fully considered based on geological soil models. Comprehensive finite-element method (FEM) simulations are performed to study the effect of the moisture content and its effects on soil thermal resistivity and critical temperature rise. The critical degree of saturation (Scr) is used in the improved two-zone model instead of the critical temperature rise (Δθx) used in the IEC Standard 60287. The comparative analysis presented between the new method and the IEC Standard shows that IEC is more susceptible to changes in the ambient conditions. The use of a fixed critical temperature rise may yield significant errors. Shortcomings of numerical solutions are discussed highlighting the advantages of the new (optimized) two-zone model. The optimized method is useful for the design of new installations as it accounts for changes in soil thermal resistivity and critical temperature rise. The new method has been validated with available test results. [ABSTRACT FROM AUTHOR]

In this paper, a mathematically rigorous method is presented to calculate the impedance of FCLs based on a desirable change of short circuit currents. The method is found on the principle of superposition and can be easily incorporated into standard short circuit current calculations of power systems. The changes of the short circuit currents for each FCL installed are easily calculated without having to build a new impedance matrix for each new case. In addition, the method gives a clear graphical representation of the impact of the FCL on the power system taking advantage of the FCL impedance locus in the complex plane. The resultant plots provide valuable insight and information to utilities, system planners, and FCL manufacturers. FCL manufacturers can use these plots to identify the FCL impedance value and angle subject to the manufacturing and system constraints. The plots also allow utilities and system planners to compute the changes of fault current distribution caused by the FCL quickly and accurately. Results show that FCLs can increase short circuit currents in other branches. High-limiting FCLs may cause larger increase in short circuit currents of other branches and therefore a compromise may have to be made between fault current limitation and the maximum additional currents that are acceptable in other branches. Examples on a three-bus system and an eleven-bus system are presented for illustration and validation. [ABSTRACT FROM AUTHOR]

A comprehensive time series power flow (TSPF) framework is proposed for the analysis of fault-induced delayed voltage recovery (FIDVR). TSPF bridges the gap between static power flow simulations and time-domain simulations for FIDVR analysis. FIDVR events can be simulated faster with TSPF, while transient simulations normally require much longer time. In the TSPF framework, a random model for the disconnection of the induction motors is proposed to determine the load for different “snapshots” during FIDVR events. Regression analysis is used to predict the parameters needed in the simulations. There is no need to simplify the network topology or aggregate loads into clusters as in measurement-based load modeling approaches. The techniques presented in this paper successfully reproduced two FIDVR events recorded in heavily meshed distribution networks in New York City in 2010 and 2015. The paper uncovers that the proper modeling of motor protections (thermal and under-voltage) is key to properly predict FIDVR events. [ABSTRACT FROM AUTHOR]

One of the most widely used installation types of power cables is inside nonmetallic ducts. Ampacity calculations for this type of installations were summarized back in 1957 by Neher and McGrath. The method has been standardized in the IEC 60287 family of standards. It is shown in this paper that there is a noticeable mismatch between the IEC standard calculations for steady-state conditions with experimental tests and finite-element method simulations. After an in-depth investigation, it was found that the source of the discrepancy between reality and the standard is an inaccuracy in the calculation of the heat transfer of the air between the cable surface and the internal duct surface. The standard calculations simplify the problem too much, resulting in relative large errors. A more precise formulation, yet simple enough for computer implementation, is proposed in this paper. The accurate calculation of the steady-state temperature is obtained from first heat transfer principles. Numerous laboratory experiments and finite-element simulations are used to evaluate the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, an extension to the multiwinding BCTRAN model is proposed for the study of low-frequency saturating transients in single-phase transformers. The conventional experimentally obtained BCTRAN (winding leakage) susceptance matrix is retrofitted with a previously missing experimentally obtained nonlinear (core magnetizing) model. The assembled model gives accurate results for transients in all terminals. The model parameters can be acquired from information available in the nameplate plus terminal tests without the need of detailed transformer design or construction information. Only needed are standard no-load and impedance tests plus a newly introduced saturation test. Very simple formulas are proposed to compute the parameters. Illustrative examples on how the parameters are computed are given for three transformers rated 1.8, 5, and 75 kVA. For model validation, laboratory inrush currents tests are carried out and compared with simulations using the EMTP-RV. The results show that the simulations and experimental results match very closely giving great confidence in the model correctness and parameter estimation method. [ABSTRACT FROM AUTHOR]

This paper discusses the feasibility of using customer coupon demand response in meshed secondary networks. Customers are rewarded by coupons to achieve the objective of optimal operation cost during peak periods. The interdependence of the locational marginal price and the demand is modeled by an artificial neural network. The effect of multiple load aggregators participating in customer coupon demand response is also investigated. Because load aggregators satisfy different proportions of the objective, a fairness function is defined that guarantees that aggregators are rewarded in correspondence with their participation toward the objective. Energy loss is also considered in the objective as it is an essential part of the distribution system. A dynamic coupon mechanism is designed to cope with the changing nature of the demand. To validate the effectiveness of the method, simulations of the proposed method have been performed on a real heavily-meshed distribution network in this paper. The results show that customer coupon demand response significantly contributes to shaving the peak, therefore, bringing considerable economic savings and reduction of loss. [ABSTRACT FROM AUTHOR]

This paper proposes three new operation strategies such that a set of dc microgrids in distribution systems can cooperate pursuing particular objectives: 1) loss reduction; 2) full unbalance compensation; and 3) partial unbalance compensation. Loss reduction is a regular strategy for distribution utilities operating their systems efficiently by optimal dispatch of three-phase in/out powers of dc microgrids. Full and partial unbalance compensation strategies can solve single- or two-phase overloading problems of substation transformers temporarily and slow down their aging process. To validate the performance of the proposed operation strategies, several steady-state studies are performed in the IEEE 123-bus test system. According to the simulation results, the operation losses can be reduced by up to 34% with 15.2% DG penetration. The maximum single-phase loading of the substation transformer can be reduced by up to 30%. [ABSTRACT FROM AUTHOR]

In this paper, a new network structure is proposed to improve reliability and reduce losses using superconducting fault current limiters (SFCLs) for primary distribution systems. SFCLs are used to tie two (or more) radial primary feeders to form loops. SFCLs diminish the short-circuit current and the looped operation significantly reduces the active power loss while simultaneously increasing reliability. With this technique, there is no need to upgrade the upstream circuit breakers and settings of protection devices when operating in a loop. It is demonstrated that there is a tradeoff between benefits (reliability improvement and loss reduction) and acquisition cost of SFCLs. The technical feasibility and benefits are discussed. A comprehensive economic analysis based on optimization methods is conducted to determine the number and location of SFCLs in a typical system considering costs and benefits. Two case studies are selected: The reliability standard system known as “Bus 4 of RBTS” and the 84-bus distribution network from Taiwan Power Company. The results are compared with conventional distribution system reconfiguration. The comparison makes evident the superiority of the new method. [ABSTRACT FROM PUBLISHER]

This paper presents a novel online diagnostics method capable of detecting winding deformations in two-winding single-phase transformers. The main idea is to identify changes in the short-circuit impedance. The combination of 3-D Lissajous curve methods with a Butterworth low-pass filter allows for the accurate determination of winding deformation of large power transformers in real time. The method is very robust and capable of detecting deformations at the early stage even when the measurements are noisy. Only information already available to the differential protection relay is needed. The proposed diagnostics method has been validated with circuit and finite-element simulations plus a lab experiment. The results show that the proposed online diagnostics technique has the ability to identify winding deformation problems under severe conditions, such as nonsinusoidal input, nonlinear loading, and measurement noise. Under ideal conditions (no signal noise), the inductive identification error of the proposed online diagnostics method identifies the parameters with less than 0.09% error. When accepting a measurement noise of 1%, the error on the identification of inductance is less than 0.13%. [ABSTRACT FROM PUBLISHER]

Conservation voltage reduction (CVR) plays an important role in energy savings and shaving peak demand. This paper quantifies energy savings for refrigeration loads (RLs) with experimentally validated air conditioner and refrigerator models. It is demonstrated that peak demand reduction and energy savings exist simultaneously during the implementation of CVR. A 1666-bus real meshed network with residential, small, and large commercial customers is simulated for the calculation of energy savings. Different working patterns: weekday and weekend; weather conditions: hottest and average summer days during peak load of the year; and load types: mixed load (ZIP and RLs) and ZIP load are studied. In addition, the IEEE 8500-node radial system with 100% residential customers is also used to show the energy savings of CVR. It is concluded that the economic benefits of CVR are larger for meshed networks than for radial systems. It is also shown experimentally and by simulation that CVR produces significant savings for both utilities and customers. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel control strategy such that downstream non-synchronous microgrids can perform load balancing functions. The proposed method can eliminate (or reduce) the unbalance of currents at the substation transformer of radial distribution systems. The load balancing ability of microgrids has been studied for two different scenarios: 1) using communications between the microgrid and the substation; and 2) using only local measurements at the microgrid. The limitations posed by the availability of measurements and system topology requirements for the success of the process are discussed. Two numerical examples are provided to validate the proposed control scheme using time domain and time-sequence power-flow simulations. [ABSTRACT FROM PUBLISHER]

In this paper, a novel method is proposed for the estimation of the design parameters of single-phase distribution transformers using data acquired from terminal measurements along with nameplate data and external tank dimensions. Transformer parameters that can be measured from terminals, for example, leakage inductance, saturation inductance, and winding resistance, are considered first. These parameters are analytically correlated with the geometrical information of the transformer structure. A system of nonlinear equations is derived accordingly. Winding dimensions and the number of turns are computed with acceptable engineering accuracy. The core dimensions (including its cross-sectional area) are calculated using winding information. The obtained data can be utilized to develop white- or gray-box models to be used in the investigation of the thermal and/or electromagnetic behavior of power transformers as presented in several research studies published to date. Examples on laboratory and utility-grade transformers are shown for illustration and validation of the proposed design parameter estimation method. [ABSTRACT FROM AUTHOR]

Cables in ventilated and ladder-type trays have been extensively studied and are rated according to ANSI/NEMA standards. The National Electric Code (NEC) provides guidelines on ampacity for cables installed in ventilated and ladder-type trays. However, for solid bottom trays, there is very little published material; there are neither standards nor guidelines. This paper proposes a methodological approach for the thermal rating of power cables installed in solid bottom trays with and without cover. An analog thermal–electrical circuit is derived from first thermodynamic principles. The circuit parameters are easy to compute. The method is completely general and is applicable to trays with any number of cables (not only to integer fill depths). The validity of the method is corroborated with numerous finite-element method simulations and laboratory experiments. A hand-worked example is given for illustration. [ABSTRACT FROM AUTHOR]

This paper shows that there is great potential for saving money when reconfiguring distribution systems at an optimal frequency. Studies are conducted on hourly, daily, weekly, monthly, and seasonal reconfiguration plans, based on the 8760 hourly loads of a year. Switches that actively participate in the reconfiguration process are first identified. These sectionalizers are substituted by smart (remotely controlled) switches. According to the results, two economic indices: total savings and return on investment are used to determine the optimal switching frequency. Numerical simulations are conducted on three distribution systems including one with 119 buses that is commonly used as a benchmark for reconfiguration of large-scale distribution systems. The results reveal that there exist large potential gains on frequent switching reconfiguration. Therefore, detailed studies must be carried out to determine the benefits of the dynamic reconfiguration for particular distribution systems. [ABSTRACT FROM AUTHOR]

This paper introduces a three-phase reversible transformer model for the study of low-frequency transients. The model topology is obtained from the direct application of the principle of duality by drawing basic circuit elements on top of the transformer frame. The model parameters are obtained from terminal and physical size measurements as well as the characteristics of the iron core. The implementation of the model is based on standard drag-and-drop circuit elements available in electric circuit simulation programs. The model is validated with inrush current and normal open circuit laboratory measurements. The main advantage of this model is that the parameters remain the same for the simulation of different operating conditions from all terminals. This is true for transients involving deep saturation, normal loading operation, or open circuit. [ABSTRACT FROM AUTHOR]

A method is proposed to prioritize the repair or replacement of out-of-service transformers that feed a heavily meshed secondary grid. Priority assigned to the restoration of a specific transformer is based on the risk reduction that results from this replacement. Risk is defined as the reduction in the probable number of customers out of service should the transformer return to service. This measure of risk addresses the possibility of network collapse following feeder failures (occasioned by load-induced failure of transformers or feeders) and local customer impact on the secondary network. The prediction of risk makes extensive use of load predictions for feeder sections, network transformers, and secondary mains. A software tool has been developed to implement the equations proposed in this paper. This software gives system planners and operators the ability to quickly and economically select the next transformer to be repaired or replaced. [ABSTRACT FROM PUBLISHER]

The use of a horizontal arrangement of wall shunts is proposed in this paper as a cost-effective way to reduce the stray losses in power transformers. This paper compares the performance of horizontal wall shunts with the available alternative (vertical shunts). A 3-D finite-element analysis (FEA) is used for the calculation of stray losses in tank walls, and other structural parts. A novel hybrid numerical/analytical method is proposed for the calculation of stray losses inside the magnetic shunts. The proposed method is based on the double Fourier series expansions of the magnetic field distribution at the surface of the shunts, which is determined using 3-D FEA. A 200 MVA power transformer is investigated as a case study where the stray losses are calculated with and without the vertical and horizontal shunts. A parametric FEA is carried out to find the optimal placement of the horizontal shunts on the tank walls. Results show that the proposed horizontal magnetic shunts arrangement are as effective as the conventional vertical shunts in reducing the stray losses while reducing the weight of the shields, therefore providing a cost-effective method for magnetic shielding of the transformer tank walls. [ABSTRACT FROM AUTHOR]

This paper presents a laboratory test procedure to determine the parameters of the inverse hysteresis model (IHM) applicable to single-phase toroidal transformer low-frequency modeling. The model parameters are obtained through a fitting procedure of the history-independent IHM of a series of open-circuit experiments and waveform recordings. These experiments can be easily executed during the transformer acceptance testing at the factory laboratory. The performance of the model is verified under various operational conditions, such as sinusoidal excitation, varied frequencies and voltages, and non-sinusoidal excitations. The results show that the model performs very well under all tested conditions giving great confidence that the proposed method can be used for parameter determination. [ABSTRACT FROM PUBLISHER]

This paper applies a known stochastic approximation method to solve a two-phase optimal power dispatch problem under load uncertainty using a novel realistic model for random loads. The stochastic power dispatch problem is solved with the Robbins–Monro method applied with the Kiefer–Wolfowitz procedure with random directions. The constraints of this optimization problem have been investigated and considered by truncated algorithms. Two numerical examples are presented for illustration of the significant improvement obtained with the proposed method. In the first example, the optimizable cost of the system can be reduced by 1.6% under 2.68% load variation. In the second example, the results show that the possibility of voltage violations is reduced from 49.5% to 0.01%. All the improvements are compared with deterministic optimal solutions validated with scenario-based Monte Carlo simulations. The method is 60 times faster than scenario-based Monte Carlo for similar accuracy for the IEEE 30-bus test system. [ABSTRACT FROM PUBLISHER]

In this letter, the principles for controlling the saturation inductance of toroidal transformers by leaving unwound sectors are presented. It is shown that inrush currents can be reduced substantially with this technique. This concept is supported by finite-element simulations, transient analyses, and laboratory experiments. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of utilizing dc links to merge heavily meshed urban distribution networks in dense-load areas to increase reliability and expand operational flexibility. It provides a cost-benefit evaluation of utilizing dc-link technology to interconnect three segments of New York City electric distribution networks with complex grid configurations. The outcome of this work highlights the advantages provided by dc links, such as increased reliability and power quality, improved voltage support, and demand relief for feeders at or above capacity limitations. Furthermore, the study shows that dc links may provide a better alternative to transformer installations, feeder upgrades, and/or capacitor additions, and offer the opportunity to postpone large capital investments for system upgrades (such as building a new substation) due to demand increase. The study was carried out with power-flow simulations using field-validated power-flow data. [ABSTRACT FROM AUTHOR]

This paper describes the advantages of using nonsynchronous microgrids in networked systems containing densely concentrated loads. The nonsynchronous bus arrangement, in addition to allowing for the integration of substantially larger distributed generation, completely isolates transient disturbances from and to the network and the microgrid. Significant is the fact that distributed generators installed in the microgrid do not contribute to the short-circuit current that needs to be interrupted by the substation breakers. The behavior of the grid and the microgrid is investigated by comparing: the occurrence of faults, voltage reduction, and losses, in the presence and absence of the microgrid. The benefits of the dc microgrid are made evident with steady-state and transient studies performed on a real distribution network in New York City. [ABSTRACT FROM PUBLISHER]

A comprehensive literature review shows that transformer-based solutions are superior for the mitigation of inrush currents than external (to the transformer) solutions. The use of air gaps and low-permeability (iron) materials are known techniques for this purpose. This paper investigates the effectiveness of these approaches for reducing inrush and phase-hop currents. Studies are carried out on toroidal transformers, due to their broad application in power electronics devices. Contrary to common belief, this paper demonstrates that air gaps do not reduce the inrush currents when a transformer is fully demagnetized. However, inrush currents can be mitigated by the use of low-permeability iron materials. It is also demonstrated that air-gaps significantly reduce inrush currents when transformers have residual flux, e.g., for phase-hop conditions. Analytical expressions are derived to compute the mitigation factor for a specific gap length. The results and formulae presented in this paper are verified with laboratory experiments, transient simulations with validated circuit models, and 2-D finite element simulations. [ABSTRACT FROM AUTHOR]