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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.

In this paper, a black-box vacuum circuit-breaker model is developed and its parameters are calculated. The developed model is deterministic and does not include the stochastic behavior of arc interruption. The phenomena, such as the current chopping, high-frequency current quenching capability, and rate of rise of dielectric strength are treated as deterministic and calculated using mean values. The vacuum breaker model is verified using a laboratory test setup, and simulation results show good agreement with the obtained measurements. The withstand voltage of the breaker during appearance of the voltage restrikes shows good agreement with the measurements. Moreover, the magnitude and repetitiveness of the high-frequency transients obtained in simulations are within a 10% margin when compared with measurements.

In the above paper, a truncated and, therefore, incorrect biography of Stanley Zocholl was published. We publish the correct biography for Mr. Zocholl here. Stanley Zocholl (LF’95) received the B.S. and M.S. degrees in electrical engineering from Drexel University, Philadelphia, PA, in 1958 and 1973, respectively. He joined Schweitzer Enginering Laboratories in 1991 in the position of Distinguished Engineer. He was with ABB Power T&D Company Allentown (formerly ITE, Gould, BBC) since 1947, where he held various engineering positions, including Director of Protection Technology. He holds over a dozen patents associated with power system protection using solid state and microprocessor technology and is the author of numerous IEEE and protective relay conference papers. Mr. Zocholl is an IEEE Life Fellow and a member of the IEEE Power Engineering and Industry Applications Societies. He is also a member of the Power System Relaying Committee and past chair of the Relay Input Sources Subcommittee. His biography appears in Who’s Who in America. He received the Best Paper Award of the 1988 Petroleum and Chemical Industry Conference and the Power System Relay Committee’s Distinguished Service Award in 1991.

In the original paper (see ibid., vol.17, p.983-90, 2002), it was shown that three-phase transformers with three-limb cores are commonly used in power systems. Therefore, it is important to analyze the (/spl lambda/-i) characteristics for the three-limb and the zero-sequence (/spl lambda//sub 0/-i/sub 0/) characteristics characteristics at no-load, symmetric, and asymmetric loads. The original authors did address this important topic, and the author of this paper would like to commend them for their extensive work.

The author argues that the main contribution of the original paper (see ibid., vol.17, p.983-90, 2002) is the definition of symmetric /spl lambda/-i characteristics for asymmetric three-phase transformers with three limbs. The analysis shows that such transformers have magnetizing currents with more than one maximum and minimum. Such three-limb transformers are relatively insensitive to the generation of reactive power Q-and the resulting voltage drop V-due to balanced DC currents caused by imperfect gating, geomagnetically induced currents, and nuclear explosions producing a low-frequency pulse. In this paper , the author asks various questions about the original authors' results.

In order to make the best use of the existing assets, monitoring of power transformers provides the ability to continuously assess the overload capability of transformers. This can be more economically advantageous than the extension of the network by installing a new transformer. During the overloading periods, the cooling system of a power transformer plays a decisive role to transfer the heat generated in the windings to the surrounding ambient. The faulty operation of the cooling system should be detected by an online monitoring system before the transformer is overloaded. In this paper, an online algorithm is presented for malfunction detection of the cooling system based on the calculation of the standardized error in the calculated top-oil temperatures. Moreover, for calculation of the top-oil temperature, a new thermal model is proposed which uses a few design-dependent variables and considers different heat-transfer modes inside a transformer. For validation of the algorithm, the measured data during the normal operation of three transformers are used—two of them experienced failures in a part of their cooling system. The proposed model and algorithm can be easily implemented and integrated into monitoring systems due to their simplicity and good accuracy.

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs.

Understanding mechanisms of switching transient overvoltages in modern electrical power systems is a necessity to ensure a proper design of power plants and switchgear and the required level of reliable and secure system operation. High fidelity plant modelling and accurate transient analysis is a prerequisite for understanding the mechanisms of how overvoltages are created and whether or not the voltage withstand capabilities of system components will be exceeded. This research is focused on switching overvoltages typical for an offshore wind farm power collection grid configuration that comprises vacuum circuit breakers (VCBs), cables and transformers. An in-depth understanding of the prestrike effects in VCBs is a prerequisite for studying switching transient overvoltages. In this paper, the impact of VCB parameters (e.g., stray capacitance and withstand voltage ability)and cable length on the transformer terminal voltage during closing operation was studied. A wind farm power collection system was modelled in ATP-EMTP environment. To validate the results obtained through computer simulation, field measurements from an actual system were used.

The differential protection of the transformer, generator, or transmission line under normal operating conditions may misoperate when the adjacent transformer is switched-on. To research the cause of the misoperations, impacting factors, such as magnetizing inrush, sympathetic inrush, and current-transformer saturation are analyzed based on the field-recorded fault data and the simulation results in this paper. It is disclosed that the CT transient or CT local transient saturation, which are caused by the long-lasting dc component from the magnetizing inrush current, will lead to misoperation. To solve this problem, a new CT saturation detection algorithm based on the dc component in secondary current is proposed to prevent this sort of misoperation, and the effectiveness of the proposed algorithm is proved by the field-recorded fault data and simulation results obtained under different conditions.

This paper presents verification of frequency-response analysis (FRA) and vibroacoustic-method (VM) test methods complementarity for the assessment of the mechanical condition of a transformer's active part. Experiments performed on real power transformers under field conditions are discussed. The presented results cover three cases of faults which would be difficult to detect or correctly interpret with the application of a single method only. The first case shows the interfering influence of radiators on VM results. In the second case, a loosened winding with unchanged geometry, which cannot be detected by the FRA method, is discussed. The third presented case shows the results of an experiment based on introducing faults into the active part followed by FRA and VM tests. These faults were loosening of the core and loosening of winding with its slight stretching. The first fault could only be detected, as expected by VM results analysis, while the second one was confirmed by both methods; however, FRA assessment quality depends on the method of test data comparison.

A methodology based on computer simulations and aiming to estimate the lifespan of transformer windings after the occurrence of short circuits is presented in this paper. The Von Mises and fatigue criteria are employed to determine the failure proximity of the windings. The obtained results contribute to the evaluation of the influence of electromechanical effects in transformer windings and to the determination of the number of short circuits that a transformer winding can support. The simulation results may also be employed in the design of transformers and failure diagnosis, to increase the security margin against mechanical failures.

Currently, the differential function is the most used for the power transformer protection, leading to a reliable discrimination between internal faults and other events. However, the conventional phasor-based differential protection function presents difficulties in the detection of some internal faults and its performance depends on the harmonic restraint and blocking functions in order to avoid relay misoperation during inrush currents. However, internal faults and other disturbances present transient, which can be properly detected by using the wavelet transform. This paper recreates the phase current and the negative-sequence current differential elements by means of the boundary wavelet coefficient energy. The proposed method was evaluated by using representative simulations of internal faults, external faults, and transformer energizations in two different power systems. By using the boundary wavelet coefficient energy instead of phasor estimation, the proposed method was quite fast, accurate, was not affected by inrush currents in transformer energizations and fault clearance, and could be used in a real-time application with low computational burden. In addition, the proposed method presented no failure in fault with overdamped transients, was scarcely affected by the choice of the mother wavelet, presented no time delay associated with the wavelet filtering, and was not affected by typical noise.

With the application of ultra-high-voltage (UHV) technology, how to reduce the construction cost of the UHV substations is becoming an important and emergent issue. In this paper, two kinds of one-column-varistor arresters are used to replace conventional four-column-varistor arresters for busbars or transformers or both of them. The switching overvoltage and lightning intruding overvoltage are calculated to evaluate the feasibility of using one-column-varistor arresters in UHV substations. At the same time, the energy absorbed by and current flowing through the arresters, and the thermal stability of these one-column-varistor arresters are analyzed to ensure their safe operations. It is shown that conventional one-column-varistor arresters can be used for busbars and transformers in UHV substations to replace conventional four-column-varistor arresters and could highly reduce the construction cost of the UHV substation, but the safety of the substation will remain the same.

Energy policies worldwide are mandating large-scale integration of solar panel (SP) generators with inverters on distribution systems. This causes several SPs to be connected to a distribution transformer. The SP and its interfacing inverter alter the performance characteristics of the transformer. In addition, when new sources and loads are connected to a distribution system, from an asset-management perspective, it is imperative to understand and quantify their effect on distribution system components. This paper presents a two-step study on the effects of SP on distribution transformers via simulation and experiments. In step one, the simulation work quantifies the amount of harmonic distortion caused by SP and associated inverters in distribution transformers considering solar farms and rooftop residential installations. The simulation work uses network topology, load, and generation data of a Canadian utility. Various inverter technologies, output powers, carrier signal frequencies, filtering techniques, numbers of active inverters, and transformer configurations are studied. In step two, level of harmonic distortion observed in simulation is created in a laboratory environment using a commercial inverter for SP applications. A three-phase dry-type transformer is tested to observe the effect of higher harmonic distortion on core and winding temperatures of the transformer. Experimental results conclude that under the worst case loading scenario (i.e., full load with active power flow reversed), the transformer lifetime expectancy is anticipated to decrease by 8.3%.

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.

Interturn short-circuit faults are the leading cause of power transformer's failure. If not detected at the incipient stage, these faults usually develop into more severe faults that would result in irreversible damage to the transformer, unexpected outages, and consequential losses. This paper presents a transformer interturn fault detection system (TIFDS) for the detection of an interturn fault at the inception stage in power transformers. TIFDS is immune to supply voltage imbalance, constructional asymmetries, and instrumental errors. It also takes care of unequal load distribution. The TIFDS algorithm makes the transformer symmetrical in its entire three phases and sets primary-side neutral current to zero value. This is achieved by computing and applying various correction coefficients. However, under the interturn fault condition, TIFDS characterizes the behavior of neutral current. Simulation and experimentation studies are carried out on the 400/110-V, 3 $\times$ 2.5 kVA power transformer. Presented test results demonstrate the effectiveness of the proposed system.

The proliferation of the voltage-source converter-based high-speed trains has resulted in significant distortions in voltage and current waveforms in both traction power supply system and the utility system. The dynamic behaviors of the high-speed trains (HSTs) make the assessment of such power-quality (PQ) problems quite difficult. There is an urgent need for techniques that can quantify the collective PQ impacts of modern trains during a 24-h period. Dynamic behavior modeling of the modern trains during the operating duty period between two station stops is studied here for PQ assessment. The 24-h profiles of the train timetable and rail infrastructure are entered to compute the information, including train positions, speeds, power consumptions, etc. Moreover, six sets of the measurement-based Norton-equivalent model under different operations are implemented to represent the dynamic harmonic behaviors of the train. In addition, the systemic modeling of the utility system, traction lines, and Scott-connection transformer is also described. After comparing the results of calculations and measurements, the proposed model is ideally effective for analyzing the consequences of HST's dynamic behavior and system topology that are involved in fundamental power flow and harmonic power flow in order to evaluate the comprehensive PQ impacts in a companion paper.

In this paper, the design of a 1/30 kV, 10 MW modular isolated dc–dc converter is presented. The design and optimization of a high-frequency transformer as the key part of such a converter is addressed. Efficiency curves for different semiconductors and frequencies are presented in order to find an optimum frequency-enabling adequate transformer volume reduction with a suitable compromise with the converter efficiency. It was found that for this voltage level and size, it is no point to go above 6 kHz from the transformer perspective since the isolation requirement that leads to that the size is not reduced much more for even higher frequencies. At 5 kHz, the efficiency of the transformer having a nanocristalline core reached 99.7% while the power density was about 22 kW/l. For the entire dc–dc converter, an efficiency of 98.5% was reached at 5 kHz switching frequency using SiC metal–oxide semiconductor field-effect transistors as switching elements while the efficiency when using insulated-gate bipolar transistors reached 97.2% at the same frequency.

Secondary or low-voltage (LV) networks are applied in many large cities throughout the U.S., and in some other countries. Various grounding schemes have been employed for the primary systems supplying these secondary networks. In this paper, characteristics of the different grounding schemes for the substation and primary feeders that supply the LV networks are reviewed. The impacts of primary system grounding on the ground fault currents, the unfaulted line-to-ground voltages, as well as the voltage sags in the LV network have been considered for the single line-to-ground fault on the primary. The differences between systems with network transformers having the delta- or grounded-wye-connected primary windings are discussed. The relationships between grounding characteristics and protection settings, including actual examples, have been documented.

Power system transient analysis relies on the deployment of models with ever-increasing accuracy that represents the system's frequency-dependent components with respect to the phenomena being scrutinized. A key issue in this context is the construction of a model yielding passive properties for each system component. Such properties are commonly compromised by corrupting noise in measurements that serve as input to the computation of blackbox models. In addition, model passivity could be further compromised during parameter estimation. This paper undertakes the task of developing a proposal to build an optimization methodology based on linear matrix inequalities (LMI-based) which is intended for obtaining an optimal, passive, blackbox model for a $n$ -terminal power transformer. The resulting model can be further realized as a circuit representation in order to simulate the transformer dynamics in an Eletromagnetic Transient program (EMT type). The results derived from two operating power transformers are later provided to confirm how effective the procedure herein proposed is.

In distribution power systems, feeder voltages can be very sensitive to changes in load and/or distributed generation. This paper introduces a solid-state-transformer-based local voltage-control strategy to reduce variability distribution system bus voltages. An online dynamic volt-var control (VVC) algorithm is proposed to regulate bus voltages by injecting or absorbing reactive power through a solid-state transformer (SST). The proposed algorithm does not require any communication between the SST and the substation and makes control decisions locally. The main goal of the voltage-control algorithm is to enforce strict voltage constraints on the system voltages. The proposed control algorithm is validated in a radial and meshed distribution system using PSCAD.

Ground return currents (GRCs) in ac systems generated by HVDC monopolar operations can result in half-cycle saturation of transformers. Considering the ground potential rises of substations, which is represented by mutual resistances in this paper, an improved model of GRC flow in aca systems is presented. This model is verified by the IEEE benchmark test case for geomagnetically induced current (GIC) and the measured neutral direct current and voltage on neutral blocking devices (NBDs). Based on this model, how the dc bias current in ac systems is affected by mutual resistance is analyzed using measured earth resistivities and representative ac systems. Furthermore, the impact of mutual resistances on GICs is discussed. In the end, an enhanced model for optimal configuration of GRC mitigation is also proposed. The actual application of the optimal NBD configuration has been proven to be cost-effective in mitigating the GRC flow.

Double 12-pulse ultra HVDC transmission systems of $\pm $ 800 kV and $\pm $ 660 kV have larger transmission capacities, more complicated configurations, more complex operation modes, and more components, in comparison to $\pm $ 500 kV or less HVDC transmission systems. Hence, the reliability evaluation of the ultra HVDC transmission system is more complicated than that of the HVDC system. To thoroughly describe different operating modes, this paper presents two new reliability measures for ultra HVDC transmission systems, that is, partial monopole forced outage times and partial bipole-forced outage times. Reliability evaluation models for individual subsystems, including converter transformers (CT), ac filters, converter valve groups, and smoothing reactors have been built considering operation modes and configuration characteristics. In particular, the reliability model of the CT subsystem was built by considering the type and the number of standby CTs, and that of the ac filter subsystem was built by considering the capacity state table of filters. A reliability evaluation model for ultra HVDC transmission systems was proposed by combining reliability logic relationships among subsystems and using the state enumeration method. Case studies of practical ultra HVDC transmission systems illustrate the correctness of indices and the proposed models. In addition, sensitivity analysis and standby analysis of major components were also conducted.

Uncalibrated instrument transformers present at the inputs of phasor measurement units (PMUs) can significantly degrade their outputs. This also causes problems in downstream applications that use PMU data. This paper presents a method for calibrating voltage transformers online using synchrophasor measurements. The proposed approach aims to find the optimal locations where good quality measurements must be added in order to bring the calibration error of all the measurements below a predefined threshold. The IEEE 118-bus system, the IEEE 300-bus system, and a 2383-bus Polish system have been used as the test systems for this analysis. The advantage of the proposed approach is its effectiveness and robustness.

In this paper, an accurate analytical method is proposed to compute the maximum inrush currents for single-phase transformers. The worst case scenarios for inrush currents caused by switching at the voltage zero crossing with the highest possible residual flux are investigated. Hysteresis is modeled with piece-wise linear approximations. Hence, it is assumed that the transformer is always operating on linear sections of the magnetizing curve. The maximum inrush current is computed with simple steps from the solution of the linear governing differential equations. The method is simple and, thus, suitable to be included in the transformer design program (or even using a calculator). Comprehensive studies are carried out to evaluate the proposed method versus laboratory measurements and Electromagnetic Transients Program simulations. The results indicate a significant improvement from published formulae in the literature. The principal advantages of the technique are its accuracy and simplicity. A step-by-step numerical example illustrates how easy it is to estimate the maximum inrush currents.

This paper presents a general method for building equivalent electric circuits of power transformers, including eddy current effects in windings and core. A high-frequency equivalent dual model for single- and three-phase transformers with two multilayer windings is derived from the application of the principle of duality. The model is built from elements available in circuit simulation programs, such as Electromagnetic Transients Program (EMTP)–Alternative Transients Program, EMTP-RV, PSCAD, and PSpice. The parameters of the frequency-dependent leakage inductance and winding resistance are computed with analytical formulae obtained from the solution of Maxwell’s equations that are based on the geometrical dimensions and material information. Ideal transformers are utilized to isolate the electric components (winding resistors and capacitors) from the magnetic components (inductors). The physically correct connection points for electric and magnetic components are clearly identified. The proposed methodology is successfully validated versus finite- element simulations and laboratory measurements.

Current-transformer (CT) saturation threatens accurate discrimination between internal fault current and magnetizing inrush current of a transformer and endangers the proper operation of a differential relay. Thus, compensation of CT saturation could substantially improve the performance and security of transformer differential relays. So far, almost all of the technical literature in this field has focused exclusively on fault current compensation. However, this paper proposes an efficient compensation algorithm to reconstruct the fault and inrush currents which are distorted by CT saturation. Some of the attractive features of the proposed method are its accuracy, fast response time, desired sample-by-sample output, no dependency on power system topology, immunity against noise and harmonics, and its simplicity. These features are extracted through the application of numerous test cases reflecting a wide range of variations in fault and inrush currents. The proposed algorithm proves to be fast, accurate, and reliable.

This paper describes the concept and development of dry-type transformers for underground distribution networks, capable of operating in the open air, partially submerged or completely submerged without enclosure and in contact with water. The rating is in the range of 500 kVA to about 2000 kVA and 15- or 25-kV voltage. Immersion depth is limited to 3 m, due to the limits of the plug-in connectors on the high-voltage (HV) side of the transformer. A tap panel is provided on HV coils for $\pm $ 5.0%. Dimensions and losses are comparable to those of liquid-immersed transformers. Electrical insulation is fully solid. The HV coils are shielded and the shield is grounded.

In this paper, a winding condition assessment model using vibration signals is presented, which can be used to diagnose power transformers online. The basic principle of this model is based on the correlation analysis of winding vibrations. In the model, the fundamental frequency vibration analysis is used to separate the winding vibration from the mixed signal. Then, a health parameter is proposed via principal component analysis. Another parameter is also proposed to detect the fault locations for suspected faulty transformers. In laboratory tests, the model is validated on a specifically designed 110-kV transformer. During the tests, man-made winding deformations are simulated to compare the vibrations under different conditions. The model has also been tested on several in-service power transformers. The preliminary study shows that the proposed model is feasible to assess the power transformer winding condition.

The purpose of core demagnetization is twofold: 1) to reduce the inrush currents when transformers are energized; and 2) to make sure that the frequency-response analysis (FRA) tests are consistent to avoid false diagnoses of damage during transportation. The significance of demagnetizing is presented on field measurements of an 80 MVA unit with FRA measurements. A new demagnetizer device with an alternating polarity dc voltage source is prototyped. Experimental verification of this prototype is presented for the demagnetization of transformers. A nearly complete demagnetization was observed in the laboratory for a small single-phase isolation transformer. The method proposed in this paper is applied to three-phase transformers with different core configurations and connections. Topologically correct modeling and numerical simulations confirm the full demagnetization of all branches of three-phase (three- and five-limb) transformer cores. Inrush current measurements and FRA plots before and after demagnetization confirm the effectiveness of the process.

A novel optimization-enabled electromagnetic transient simulation approach is proposed for calculating the resistor setting of a multistage impulse generator for insulation impulse testing of high-voltage transformers. This approach enables test engineers to overcome most of the major challenges attributed to the use of conventional trial-and-error methods for determining resistor settings, including the excessive time consumption and the potential damage to the test transformer due to a greater number of trial tests. The proposed method uses the frequency response of the test transformer to synthesize a circuit model for it. The test setup, including the test transformer and the impulse generator, is simulated using an electromagnetic-transients-type simulator. A genetic-algorithm-based approach is used to optimize the setting of the impulse generator. Optimized resistor values are then used in impulse testing of a three-phase power transformer. Different cases of impulse generator resistor arrangements are studied in this paper and simulated waveforms are compared with those obtained from measurements.

In this paper, a new equivalent circuit for the transformer winding is presented by which the eddy current losses in the transformer can be represented accurately at high frequencies. The new model enables representing the frequency-dependent impedances of a coil system using the idea of mutually coupled series Foster circuits. An effective method for the determination of model parameters is also provided which involves using the vector-fitting (VF) method for the rational approximation of the system impedance matrix using a certain number of common stable poles. A set of nonlinear equations is then established using the VF results that can be solved with the method of fixed-point iteration to obtain the parameters of the equivalent circuit in a fast and accurate way. The new model can be used for the calculation of resonance transients within the transformer, the situation wherein the maximal value of overvoltages is highly dependent to the damping effects or the loss phenomena in the transformer. Since the model only contains constant lumped parameters, it can be used for transient calculations in the time domain directly using the electromagnetic transients programs, such as the Alternative Transients Program or Electromagnetic Transients Program.

This paper proposes a novel dc power-flow controller for a multiterminal HVDC system, which can be inserted in series on transmission lines to manage the dc power flow. Its key features include: 1) simpler circuit topology; 2) fewer components; and 3) the elimination of an external power supply and low-frequency high-voltage-insulation isolated transformer. A three-terminal HVDC system, including the dc power-flow controller, is studied and its operation principle and characteristics are analyzed. A simulation mode is built to test the feasibility and effectiveness of the proposed dc power-flow controller with cases of power injection step down, power loss of one terminal, and power-flow reversal. A three-terminal dc system is built in the lab to validate the dc power-flow controller. The results show that the proposed dc power-flow controller can enhance the performance and controllability of a multiterminal HVDC system.

Power transformers have important roles as voltage converters in substations, which directly affect the safe operation of power grids. Online monitoring of winding deformation, which is one of the most common transformer faults, is crucial in protecting transformers from damage, particularly those induced by short-circuit current failures, and ensuring their safe operation. This study proposes an improved online monitoring method for winding deformations based on the Lissajous graphical analysis of voltage and current. A practical measurement system is also presented and discussed in this paper. A load normalization method is proposed to solve the problem of Lissajous diagrams changing as load varies. Finally, transformer and winding experiments are performed to verify the validity of the proposed load normalization method and the feasibility of the proposed monitoring method in distinguishing winding deformations with different degrees and locations.

This paper presents a novel mitigation technique to reduce the effects of geomagnetically induced currents (GICs) on high-voltage power systems. The method consists of connecting switching devices at the neutral grounding connection point of transformer banks. Only one transformer bank needs to be grounded through a switch to reduce GIC in a two-terminal system. For multiterminal systems, $n-1$ switches are necessary and the operation is independent from each other. The switching frequency and the duty cycle are selected from a tradeoff between the effectiveness of the method and the detection of fault currents. Transient simulations on a two-bus 230/500 kV system show that the proposed switching at the neutral successfully mitigates the GIC. This proposed technique opens the door to a new family of GIC mitigation methods.

The main purpose of this paper is to provide a methodology to monitor harmonics produced by power transformers during geomagnetic disturbances. The advocated scheme leverages the existing infrastructure and is based on the embedded operating quantities of microprocessor-based differential protection relays. The characteristics of single-phase transformer magnetizing current are discussed. The second harmonic signature is a good indicator of half-cycle saturation and can be mapped into an effective geomagnetically induced current. The proposed scheme was tested in closed-loop using a real-time digital simulator and validated with field measurements.

In this paper, a new concept for analyzing the dielectric severity supported along transformer windings when a transformer is submitted to a nonstandardized transient voltage waveform induced by switching operation of a vacuum circuit breaker (VCB) is examined. There are two new factors considered for evaluating the severity supported by the insulation windings in factory and in service. One factor is based on the methodology suggested by the authors called time-domain severity factor and another one is the frequency-domain severity factor. Both methodologies are here applied to two case studies in order to show and compare their capabilities and sensibilities to detect the risk of dielectric failures. The analyzed case studies, performed as practical applications, consist of: 1) a power transformer submitted to damped oscillatory waves at the resonant frequency and 2) a power system consisting of a power transformer connected to a VCB.

Black-box transformer high-frequency models can be determined via frequency sweep measurements and curve fitting based on rational functions. This procedure often leads to unacceptable model behaviors around 50/60 Hz, making it hard to properly initialize the model for high-frequency transient events. In this paper, we overcome the accuracy problem by merging the frequency sweep data with a data set calculated from a standard 50/60-Hz model. The merging is performed using low-order filters, ensuring a smooth transition between the data sets. The resulting data set is subjected to model extraction via curve fitting and passivity enforcement. The modeling procedure is demonstrated for a 300-kVA distribution transformer. The model is applied in an example of circuit-breaker transient recovery voltage (TRV) simulation, requiring good accuracy at both 50 Hz and at high frequencies.

High penetration of distributed generation (DG) has led to frequent voltage fluctuations in the distribution network. This paper describes the design of a partially rated, power-electronic-assisted onload tap-changing (OLTC) autotransformer. Positive and negative compensation of the grid voltage can be achieved on feeders that have high distributed generation and/or loading. A novel design of taps comprised of several no-load switches and a single semiconductor-mechanical hybrid switch have been proposed, that requires reduced voltage rating and a number of switches. In steady state, the mechanical switch in the hybrid switch conducts the load current resulting in low steady-state losses. During the tap change process, the OLTC uses semiconductor switches, namely insulated-gate bipolar transistor /metal-oxide semiconductor field-effect transistor, thus achieving arc-free tap change and long lifetime of switches. The OLTC system has been customized for both low-voltage and medium-voltage three-phase distribution networks. An open-delta configuration for the medium-voltage application has been proposed that requires only two OLTC units to control all three line voltages. Simulations are carried out to verify the steady-state and transient operation of the proposed OLTC.

Power-line communications is a promising technology to help automate buildings, since it utilizes the in situ power cabling as a communications channel. However, couplers are required to inject and extract the communication signal from the power grid. Most power-line communications couplers make use of a small transformer to adapt impedance levels while also providing galvanic isolation. The cost and size of these transformers have been hindrances in the quest for compact, economic couplers. Previous attempts to eliminate this coupling transformer, while maintaining impedance adaptation, have not been successful in reducing component cost or physical size. In this paper, a novel approach is followed: 1) a suitable dual-function bandpass matching circuit is designed for ordinary electronics, whereafter 2) the specifications of this bandpass matching circuit are upgraded to function safely in the power-grid environment as a coupler. Therefore, a matching circuit is transformed into a compact power-line coupler, which further exhibits bandpass filtering and excellent impedance-adapting performance. Simulations as well as laboratory measurements are shown which confirm the accuracy of the design. Finally, practical 220-V measurements in an office block are presented, which prove the versatility of this novel coupler when power-grid conditions fluctuate.

Fault detection, fault prognosis, and life expectancy estimation of transformers are important issues in improving the reliability of smart grids. Regular maintenance checks can detect the transformer's faulty conditions; however, such checks can only be performed limited times annually due to high cost and disruption of service. Therefore, faults that occur between such checks take a long time to be detected. This paper proposes a simple online monitoring algorithm that uses a minimum set of sensor feedback to estimate oil-immersed forced-air cooling transformer's life expectancy parameters. Abrupt changes or sufficient deviations of these estimations from their nominal values can be used as an indicator of transformer fault. The algorithm can also estimate the transformer-life expectancy during normal operation. A transformer-monitoring prototype has been developed based on the proposed algorithm. The transformer-monitoring prototype that uses wireless communication capability to transmit transformer life expectancy parameters to the substation has been tested, verified with lab experiments, and deployed to a utility substation.

The predictive power of transient ferroresonance simulations depends mainly on the accuracy and fidelity of the power transformer model used for simulating complete systems involving all relevant network components. This paper considers three ways of transformer iron-core modeling relevant for ferroresonance simulations that are presented in detail. The presented transformer models are suitable for the state-space form of differential equation systems that describe the ferroresonance effect. Before solving the obtained differential equation systems, their numerical eigenvalues were analyzed in detail. This eigenvalue analysis has revealed a very stiff nature of the obtained equation systems. The obtained equation systems are solved by using a A- and L-stable numerical differentiation formulas (NDF) numerical technique, with the aim of suppressing undesired numerical oscillations. The obtained numerical results are verified by comparison against the available experimental results. The presented analysis shows that the suggested transformer model based on a hysteretic core inductor provides the most accurate results in terms of voltage and current waveforms and their peak values during the steady-state and transient ferroresonance. The analyzed transformer core models can be implemented in the existing Electromagnetic Transients Program-type simulation tools by using a combination of the trapezoidal and proposed NDF2 method.

In this paper, the prestrike and the restrike effects during switching a vacuum circuit breaker (VCB) connected to a dry-type foil-winding transformer are theoretically and experimentally analyzed. The analysis is conducted by performing a wide range of experimental three-phase switching on/off tests for a 3.75-MVA prototype test transformer connected to a VCB by a cable with different lengths and for different loading conditions. A refined transformer model is successfully applied for the computation of the voltage distribution along the transformer windings. In particular, the winding properties are separately investigated, for which the core influence for frequencies above some tens of kilohertz is studied, and the terminal impedance variation for a broad frequency range is accurately calculated. The simulated results are excellent. Based on these results, an analysis was performed to evaluate the severity of the switching operations by applying spectral analysis of the recorded signals. The statistical analysis of the overvoltages with respect to the cable length and the load value shows that long cables do not guarantee lower overvoltages because of the load and its interaction with the system that makes each case unique.

In this paper, a new high-frequency black-box model of the power transformer is presented for the calculation of transferred lightning overvoltages. The model is based on frequency-response analysis measurements and the application of four-terminal network theory. The computational verification of the presented model was successfully performed on a white-box transformer model in Electromagnetic Transients Program-Restructured Version software. The experimental verification was carried out on 130-kVA and 36-MVA three-phase power transformers. The presented calculation and measurement results confirm the validity of the proposed model for full and chopped lightning impulse voltages.

This paper analyzes the temporary overvoltages (TOVs) that may appear in open-phase states in distribution networks with resonant grounding. The high overvoltages recorded during the open-phase states exceed the overvoltages corresponding to the existing theory. The detailed analytical study of the temporary overvoltages is performed. The closed-form expression of the neutral voltage is derived. The conditions for maximum phase-to-earth overvoltages are obtained. The case study for open-phase faults in the 33-kV resonantly grounded distribution network is described. The dependence of the TOVs on the number of distribution feeders, on the feeder loading, on detuning factor, on the additional damping factor, as well as on the saturation of the voltage transformers are analyzed. Remedies for the reduction of TOVs are considered. Detuning of the Petersen coil is compared with the permanent connection of the wattmetric resistance in parallel to the Petersen coil.

Due to load growth, aging infrastructure, and a competitive environment, innovative solutions are required by electrical utilities to enhance the utilization of transformers which are cost intensive. This paper proposes a demand response optimization model based on transformer hottest-spot temperature. The optimization model quantifies the improvement of transformer utilization through DR. The proposed model is applied to a typical Finnish residential primary and secondary distribution transformers for case studies of load with and without DR. The results show that the loading on the transformers can be significantly increased without sacrificing the life of transformers. The gain in utilization depends on the DR capability of the load. Significant monetary benefits can be achieved with the deployment of the proposed model in a real system.

Power transformers are important equipment of a substation from the generation, transmission, and distribution of electricity to end users. The costs associated with purchasing a power transformer in the voltage class of 500 kV (100 MVA) are a few million. The fines imposed by regulatory agencies are significant when there is unavailability of equipment for any defect or failure. Therefore, energy companies have been struggling in preventive and predictive maintenance in order to maintain this equipment in an effective maintenance program, avoiding the occurrence of failures. There are various techniques that are utilized for diagnosis and analysis of transformer failure, but little has been discussed about mechanisms that assist in decision making when it is necessary to overload the transformer, especially in emergency situations. In this paper, we present a framework that unifies the step of fault diagnosis of power transformers with the process of decision making, considering the current operating conditions as well as the life of the equipment. The assistance to the decision-making methodology is based on risk analysis with indicators derived from the failure rate and Arrhenius theory. The validation of the method is performed with a case study using data from a utility.

In this paper, a previously published model for the representation of the leakage inductance of multiwinding transformers is enhanced to support accurate calculations of low-frequency transients, including inrush currents, series ferroresonance, and geomagnetic-induced currents. The new circuit is obtained from the principle of duality and, therefore, is physically consistent. The unique characteristic of the improved model is that the very deep saturation behavior of the iron core is properly represented for each winding simultaneously (reversible model) without changing parameters. The hysteresis cycle and iron-core losses are also included. In addition to its reversible terminal behavior coupled with physical consistency, the proposed model can be built with circuit elements available in Electromagnetic Transients Program-type programs, and all of the parameters can be computed from terminal tests. The model is validated by comparing computer simulations versus laboratory measurements for three- and four-winding transformers.

Directional relaying is one of the most important features of protective relays in situations where the fault current direction is not fixed. Most transmission lines, meshed distribution networks, and modern smart grids, including dispersed generation (DG) units, are examples of these situations. Conventional directional relaying schemes have some shortcomings and cannot be used in all practical cases. The application of voltage as a reference quantity for the detection of fault direction is a common practice in transmission lines. Nevertheless, this approach is not applicable in distribution networks due to the absence of potential transformers. A recent solution is the application of prefault current, instead of voltage, as a reference quantity. However, this method requires the voltage for the detection of power-flow direction. In this paper, a novel directional relaying scheme is introduced which is able to detect the fault direction using only the postfault current. The proposed scheme overcomes all of the mentioned problems in conventional methods. The efficiency of the proposed scheme has been approved over extensive simulation studies in different operating conditions.

This paper develops computational models for undeformed and deformed transformers, using the finite-element method (FEM) to calculate frequency dependent parameters accounting for diamagnetic properties. In this manner, properly estimated inductances and capacitances can be derived and applied into a winding model for frequency response analysis (FRA). This research uses a hybrid winding model, so that frequency responses in the high frequency range ( >1 MHz) can be explored for the investigation of radial winding deformation. Meanwhile, computational models with respect to winding radial deformation are constructed, so that corresponding inductances and capacitances in specific radial deformed cases can be obtained by FEM. Therefore, the influence of the capacitances as well as the inductances can be taken into account for FRA of radial deformation in high frequencies. The frequency response in the undeformed case is compared with the experimental data to verify the accuracy of the frequency dependent parameters and mathematical winding models. The analyzed results in radial deformed cases are compared with the fault features derived from experimental studies reported in relevant literatures.