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Precisely interpreting frequency response analysis (FRA) curves is crucial when investigating mechanical malfunctions in power transformer windings (TWs). However, many conventional explanations of FRA features cannot be validated by an equivalent circuit (EC) that represents winding structures using resistance, inductance, and capacitance components in a ladder network. Incorrect interpretation can lead to misdiagnosis and confusion in asset management. Previous ECs are often proposed by analysing winding structures without rigorous verification compared to the corresponding FRA data. The specific EC is acquired using the transfer function (TF) derived from the measured FRA data. This allows for the establishment of the relationship between TW FRA curves, TF equations, and EC topologies. The precise interpretation of FRA curves can be achieved by closely observing the FRA curves created from TFs and ECs. The remarkable similarity between the measured and modelled FRA curves verifies the authenticity of the derived ECs. This significant achievement clarifies many misunderstandings regarding FRA and represents a substantial advancement in FRA technology. [ABSTRACT FROM AUTHOR]

During geomagnetic storms, geomagnetically induced currents (GICs) occur in the power grid. GIC is a quasi‐direct current, which is represented by using a fixed DC current, the authors propose an approach that combines field‐circuit‐coupled and weakly coupled magneto‐mechanical models to investigate the magnetostriction characteristics of single‐phase, four‐column autotransformer cores excited by GICs. A non‐linear magnetostrictive constitutive model that considers the impact of stress and magnetisation intensity on ferromagnetic materials is introduced. The model reveals that the strain in the core initially increases as the magnetisation intensity increases; however, when the core begins to saturate, the magnetostrictive strain weakens as the magnetisation intensity increases. Moreover, the results of multiphysics finite‐element modelling are experimentally verified using a direct current bias platform. The results show that when excited by GICs, there is an enhancement in the vibration of the iron core, and the harmonic content of the vibration acceleration increases considerably, with the odd harmonics most significantly increasing in amplitude. The conclusion drawn is that GICs can exacerbate transformer vibration and significantly increase the content of harmonics, especially odd harmonics. [ABSTRACT FROM AUTHOR]

Abstract Precisely interpreting frequency response analysis (FRA) curves is crucial when investigating mechanical malfunctions in power transformer windings (TWs). However, many conventional explanations of FRA features cannot be validated by an equivalent circuit (EC) that represents winding structures using resistance, inductance, and capacitance components in a ladder network. Incorrect interpretation can lead to misdiagnosis and confusion in asset management. Previous ECs are often proposed by analysing winding structures without rigorous verification compared to the corresponding FRA data. The specific EC is acquired using the transfer function (TF) derived from the measured FRA data. This allows for the establishment of the relationship between TW FRA curves, TF equations, and EC topologies. The precise interpretation of FRA curves can be achieved by closely observing the FRA curves created from TFs and ECs. The remarkable similarity between the measured and modelled FRA curves verifies the authenticity of the derived ECs. This significant achievement clarifies many misunderstandings regarding FRA and represents a substantial advancement in FRA technology.

Abstract During geomagnetic storms, geomagnetically induced currents (GICs) occur in the power grid. GIC is a quasi‐direct current, which is represented by using a fixed DC current, the authors propose an approach that combines field‐circuit‐coupled and weakly coupled magneto‐mechanical models to investigate the magnetostriction characteristics of single‐phase, four‐column autotransformer cores excited by GICs. A non‐linear magnetostrictive constitutive model that considers the impact of stress and magnetisation intensity on ferromagnetic materials is introduced. The model reveals that the strain in the core initially increases as the magnetisation intensity increases; however, when the core begins to saturate, the magnetostrictive strain weakens as the magnetisation intensity increases. Moreover, the results of multiphysics finite‐element modelling are experimentally verified using a direct current bias platform. The results show that when excited by GICs, there is an enhancement in the vibration of the iron core, and the harmonic content of the vibration acceleration increases considerably, with the odd harmonics most significantly increasing in amplitude. The conclusion drawn is that GICs can exacerbate transformer vibration and significantly increase the content of harmonics, especially odd harmonics.

Abstract Once the short circuit damage accident of a large capacity 500 kV transformer occurs suddenly, it will seriously affect the safe operation of the power grid. The article expounds on the calculation principle and evaluation method of the transformer's ability to withstand short circuit, then introduces the design structure of 400 MVA/500 kV short circuit autotransformer and calculates the electromagnetic force and evaluates the ability to withstand short circuit under high voltage ‐ medium voltage (HV‐MV) and medium voltage ‐ low voltage (MV‐LV) conditions. By comparing with typical 110 kV and 220 kV transformers, the short‐circuit characteristics are analysed, the control strategy is proposed, and the largest capacity short circuit prototype with 400 MVA/500 kV autotransformer is developed. Last, the sudden short circuit test is carried out in the large current test centre. The results show that the large capacity autotransformer passed the big current impulse under HV‐MV and MV‐LV short circuit conditions for the first time in the world, and all indicators are good, which confirms the accuracy of the strength check and the effectiveness of the short circuit evaluation method and also proves that the 500 kV autotransformer has strong strength to withstand short circuits.

This paper deals with the voltage regulation strategies implemented in the Italian transmission network to face the increasing problem of high voltages during time periods characterized by low demand. After an introduction in which this very actual problem is discussed, the focus is on tap staggering practices. Although tap staggering is not a new idea, it is gaining practical importance only in the very last few years as a means of enhancing the inductive power drawn from the grid and, therefore, limiting the voltage rise. Accordingly, tap staggering contributes to the mitigation of the problems caused by the increasing penetration of renewable energy sources and thus can allow an increase in the share of renewable energy sources. The paper presents the different tap staggering practices that are being defined by the Italian transmission system operator and reports some tap staggering tests recently performed on large autotransformers as well as phase-shifter transformers. [ABSTRACT FROM AUTHOR]

Once the short circuit damage accident of a large capacity 500 kV transformer occurs suddenly, it will seriously affect the safe operation of the power grid. The article expounds on the calculation principle and evaluation method of the transformer's ability to withstand short circuit, then introduces the design structure of 400 MVA/500 kV short circuit autotransformer and calculates the electromagnetic force and evaluates the ability to withstand short circuit under high voltage ‐ medium voltage (HV‐MV) and medium voltage ‐ low voltage (MV‐LV) conditions. By comparing with typical 110 kV and 220 kV transformers, the short‐circuit characteristics are analysed, the control strategy is proposed, and the largest capacity short circuit prototype with 400 MVA/500 kV autotransformer is developed. Last, the sudden short circuit test is carried out in the large current test centre. The results show that the large capacity autotransformer passed the big current impulse under HV‐MV and MV‐LV short circuit conditions for the first time in the world, and all indicators are good, which confirms the accuracy of the strength check and the effectiveness of the short circuit evaluation method and also proves that the 500 kV autotransformer has strong strength to withstand short circuits. [ABSTRACT FROM AUTHOR]

This paper deals with the voltage regulation strategies implemented in the Italian transmission network to face the increasing problem of high voltages during time periods characterized by low demand. After an introduction in which this very actual problem is discussed, the focus is on tap staggering practices. Although tap staggering is not a new idea, it is gaining practical importance only in the very last few years as a means of enhancing the inductive power drawn from the grid and, therefore, limiting the voltage rise. Accordingly, tap staggering contributes to the mitigation of the problems caused by the increasing penetration of renewable energy sources and thus can allow an increase in the share of renewable energy sources. The paper presents the different tap staggering practices that are being defined by the Italian transmission system operator and reports some tap staggering tests recently performed on large autotransformers as well as phase-shifter transformers.

Actual loads are analyzed, and questions are examined of the justification and choice of the power of autotransformers in 220 – 750 kV power grids, requiring accounting for the modes of operation of power systems with an outlook of 10 or more years. Corresponding methodological provisions are proposed for the justification and choice regarding generalized load ability and load capacity of transformer equipment, as well as a reliability assessment of electrical installations for project planning of the development of electric power systems. [ABSTRACT FROM AUTHOR]

Grid-tied photovoltaic (PV) installations with distributed maximum power point tracking (DMPPT) architectures include a dc–dc module integrated converter (MIC) for managing each PV panel, isolating it from the others, reducing the mismatching effect and maximizing the harvested power. In this article, the autotransformer forward converter with type-Zeta resonant reset (AFZ) is proposed as a DMPPT architecture's MIC candidate. The main characteristics of the AFZ converter are the high versatility due to its voltage step-up and step-down capability; the use of an optimized autotransformer with only two windings, reducing the complexity and power losses of this component; the good dynamic performances, like the forward converter ones; the low number of components and the simplicity and high feasibility associated to the use of just one active switch. Besides, soft switching transitions are achieved thanks to the autotransformer type-Zeta resonant reset. The steady-state theoretical analysis, considering the effect of the autotransformer leakage inductance, is presented. The converter is also studied in the frequency domain, obtaining the small-signal transfer functions. A design procedure based on the requirements of a 100-kW grid-tied photovoltaic installation is described, yielding in a 225-W prototype with efficiencies up to 95.6%. Experimental results validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

Geomagnetically induced currents (GICs) in long high‐voltage transmission lines cause half‐cycle saturation of transformers, malfunction of protective relays, reactive power loss, and voltage instability. This study describes the design for a GIC effects mitigation device (GEMD) that can be installed at a power system autotransformer common node connection to block the flow of GIC DC flux at the cores of the autotransformers. The devices consist of variable resistors, which are controlled by a wide‐area method based on particle swarm optimisation algorithm. The effectiveness of the proposed protective electrical devices is investigated and validated through simulation studies under many operational cases. The test results obtained from 648 cases verify the high performance of GEMD. The proposed method can mitigate the reactive power loss due to GIC in the power system with average improvements by 7.9 and 7.09% over resistive and capacitive blocking devices, respectively. [ABSTRACT FROM AUTHOR]

In the work is given a statistical analysis of power transformer’s testing results obtained by using frequency response analysis method. The research has been done from the amplitude-frequency characteristics of low voltage windings for twelve autotransformers type of “ATDCTN”. The localization method of most informative resonance in amplitude-frequency characteristics spectrum for detecting mechanics faults of winding is proposed.

Despite the good operating results of passive anti‐icing devices installed by the Italian transmission system operator (TSO) on high‐voltage overhead lines (OHLs), the increasing number and consequences of wet snow events have prompted consideration of other anti‐icing measures. In particular, attention focuses on the thermal approach, which is intended to achieve – before, or in the early stage of wet snow events – the anti‐icing current in OHL phase conductors where snow sleeve formation is foreseen. The authors carried out a specific anti‐icing study with reference to a 132 kV critical path in an area of the Apennines, with simulation results illustrated in this study. Anti‐icing measures investigated were (i) load shifting actions and (ii) insertion of shunt reactors used as ballast loads. The results show that shunt reactors are the most effective anti‐icing measure. The main disadvantage of this innovative solution is the degradation of voltage regulation in the area. However, this downside can be limited by combining the insertion of reactors with control of autotransformers under load tap changers. Similar results are obtained from analyses of other critical areas. As a result of these findings, the Italian TSO is now undertaking the installation of the first ballast reactors. [ABSTRACT FROM AUTHOR]

The large current generated by a direct start of the super-large-capacity high-voltage induction motor would have a huge impact on the power grid as well as the motor itself. Traditional soft starters have the shortcomings of discontinuous adjustment, voltage sags, sudden torque mutation, secondary current impact and high cost. To resolve this issue, the authors propose a novel coupled autotransformer and magnetic-control (CATMC) soft-start method. The structure of the new CATMC soft starter combines the functions of the autotransformer and magnetic control reactor via an innovative electric and magnetic circuit design. In this study, the authors analyse the magnetic circuit structure and working principles of the CATMC soft starter. Then, to validate its principle and performance, the authors conduct a simulation study using ANSYS software and design and test an 18 MW/10 kV CATMC soft starter prototype. The simulation results demonstrate that the CATMC soft starter effectively avoids secondary current impact and constrains the motor starting current to

This article investigates the feasibility of interconnecting LCC-HVDC and VSC-HVDC through an un-interrupted DC autotransformer (DC AUTO). The interconnected system aims at taking advantage of two HVDC technologies. To begin with, the system configuration and the potential benefits are illustrated. Then, the corresponding control strategies towards LCC-HVDC, VSC-HVDC, and the un-interrupted DC AUTO are proposed. The installed controllers enable the system stable operation during both normal time and faulted conditions. Finally, a four-terminal test system is built on PSCAD/EMTDC platform to validate the technical feasibility of the interconnected system and the effectiveness of the proposed control strategy.

Conventional unidirectional DC/DC converters for high-voltage direct current application employ DC/AC/DC technology and suffer from high converter cost and power loss. To solve these issues, a unidirectional step-up DC–DC autotransformer (UUDAT) is studied. UUDAT is composed of the series connection of diode bridges and voltage source converters. Topologies of UUDAT is detailed. The harmonic and uncontrollability issues are discussed. Control and the possible application scenario for UUDAT are proposed. The ways of dimensioning the voltage and power rating of the components in UUDAT are studied. Extensive simulations on the power system level verified the technical feasibility of UUDATs.

For large oil-filled transformers, the oil flow and temperature distribution in transformer windings are an essential issue to be addressed. However, the computation of the oil flow and temperature distribution is a complicated engineering problem because of the complex physical structure of the winding area and the coupled effect of electromagnetic, fluid and thermal fields. This study presents a multi-filed coupled approach to compute and analyse the oil flow and temperature distribution of the winding area for an ODFS-334 MVA/500 kV single-phase auto-transformer. The eddy current losses of the transformer winding discs are computed based on the combined finite element and analytical method, and the computational fluid dynamics based on 3D fluid-thermal coupled method is used to calculate the oil flow and temperature properties of the prototype. The computation of the oil flow and temperature distribution reveals accurately the hot spot of the transformer. To validate the feasibility and applicability of the proposed method, the numerical results obtained using the proposed method are compared with those of the experimental ones.

Power transformers are one of the key components of power systems. Therefore, they must be operated efficiently to obtain satisfactory system performance, since any distortion or failure in power transformer structures may cause drastic damages. In order to detect deficiencies in power transformers, various methods are utilised. Frequency response analysis (FRA) is one of the most well‐known methods used for fault detection in power transformers. However, the interpretation of FRA results is subjective, and depends primarily on the opinion of a human expert. In other words, there is no comprehensive, accepted and standardised method to interpret FRA results. Subsequently, fault detection based on FRA is prone to error. In this study, a new mathematical approach is used to interpret the frequency response of power transformers based on evidence theory method. This method is more accurate and less computationally burdensome compared to conventional methods. Simulations are implemented on an operating 125 MVA, 230/132/20 kV autotransformer using finite‐element method and the results are compared to the corresponding FRA results of the same autotransformer in order to evaluate the proposed method. The obtained results indicate the efficiency of the proposed method in detecting probable faults of transformer windings through analysing the FRA results. [ABSTRACT FROM AUTHOR]

When operating power autotransformers that have become widely used in networks of 110 kV and higher, it is often necessary to deal with open-phase modes caused by disconnections of one or two phases in the case of short circuits or in the case of single-phase repairs. This leads to asymmetry of the voltage of autotransformers, which affects the quality of power supply to consumers. For the autotransformer itself, an open-phase operation can be dangerous with respect to overloading of individual windings. The presence of an electrical connection between high and medium voltage causes the specifics in the analysis of their operating modes. In addition, while the theoretical analysis and the general approach to the calculation of asymmetrical modes of two-winding transformers are currently considered in sufficient detail, the task of analyzing the asymmetrical operating modes of three-winding autotransformers remains important. An approach to the calculation of autotransformer and combined asymmetrical operating modes of power autotransformers has been developed on the basis of the method of symmetrical components. The proposed model and obtained analytical expressions allow one to determine the currents and voltages of the phase windings of the autotransformer in a variety of asymmetrical modes of operation on the basis of a unified approach. [ABSTRACT FROM AUTHOR]

This paper focuses on the substation grounding study input parameters that significantly impact the touch or step potential calculations and the ground grid design. The input information includes the soil resistivity measurement, fault current, clearing time, and transmission line information, which are discussed in detail. Many of those input parameters are difficult to be accurately gathered or vary due to environmental or system changes. As a result, a sensitivity analysis of the input parameters is useful to identify the most influential parameters. The result shows soil resistivity, number of overhead ground wires (OHGW), and OHGW materials have the greatest impact on grounding study results. In addition, distinguishing the switching and auto-transformer stations from the true source stations during the fault current distribution analysis is also necessary. A few recommendations for improving the input parameter qualities are also provided. [ABSTRACT FROM AUTHOR]

In general, an autotransformer (AT) is used to improve the voltage sag and inductive interference in power lines and communication lines in the Korean ac electric railway feeding system. In particular, the Korean AT feeding system has a different configuration compared to those of the other countries, such as Japan or France; therefore, it has certain distinctive characteristics. However, a fault locator imported from the other countries has been used in Korea, and the settings of these equipment should be modified and reflect the distinct characteristics of the Korean AT feeding system. Thus, the development of a novel fault-location scheme for the Korean railway system is required. Herein, we analyzed the fault current and developed a specific relationship for use in our new algorithm. We presented the current magnitude of each branch, expressed by the boosting current of the ATs; further, the novel fault-location scheme using this relationship is developed. The novel scheme performance is verified through power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) and the field measured data. [ABSTRACT FROM AUTHOR]

For large oil-filled transformers, the oil flow and temperature distribution in transformer windings are an essential issue to be addressed. However, the computation of the oil flow and temperature distribution is a complicated engineering problem because of the complex physical structure of the winding area and the coupled effect of electromagnetic, fluid and thermal fields. This study presents a multi-filed coupled approach to compute and analyse the oil flow and temperature distribution of the winding area for an ODFS-334 MVA/500 kV single-phase auto-transformer. The eddy current losses of the transformer winding discs are computed based on the combined finite element and analytical method, and the computational fluid dynamics based on 3D fluid-thermal coupled method is used to calculate the oil flow and temperature properties of the prototype. The computation of the oil flow and temperature distribution reveals accurately the hot spot of the transformer. To validate the feasibility and applicability of the proposed method, the numerical results obtained using the proposed method are compared with those of the experimental ones. [ABSTRACT FROM AUTHOR]

Conventional unidirectional DC/DC converters for high-voltage direct current application employ DC/AC/DC technology and suffer from high converter cost and power loss. To solve these issues, a unidirectional step-up DC–DC autotransformer (UUDAT) is studied. UUDAT is composed of the series connection of diode bridges and voltage source converters. Topologies of UUDAT is detailed. The harmonic and uncontrollability issues are discussed. Control and the possible application scenario for UUDAT are proposed. The ways of dimensioning the voltage and power rating of the components in UUDAT are studied. Extensive simulations on the power system level verified the technical feasibility of UUDATs. [ABSTRACT FROM AUTHOR]

Due to its low harmonic, high efficiency, and high power factor (PF), the multipulse autotransformer has been frequently used in the aviation sector. Many studies have been carried out on the theoretical calculation and simulation analysis of the autotransformer. However, the practical application of the multipulse rectifier in the practical engineering field is inevitable. This paper investigates an 18-pulse rectifier power supply of 11 kW being constructed using a multipulse autotransformer to suppress the harmonics of a radar power system, in which the parameters of the various parts are designed in detail, and test results are simulated using MATLAB/Simulink software. The actual test suggests that, at the rated condition, the PF of the rectifier is 0.99, the total harmonic distortion of the input current is below 6.7%, and the efficiency is more than 96%. The power supply is successfully applied to the radar power supply system of a large transport aircraft, and meets the requirements of an environmental test. Furthermore, the results also provide powerful application support for the power supplies and distribution systems in other fields. [ABSTRACT FROM AUTHOR]

This paper presents an improved average model and evaluation of an 18-pulse autotransformer rectifier unit (ATRU) in differential delta configuration. Average models remove the switching behavior of diode rectifiers and high bandwidth transients, which not only facilitates simulation of power systems by reducing computational cost but also enables impedance-based stability analysis for large complex power systems. To experimentally validate the proposed average model, a 2-kW, 18-pulse ATRU has been tested and the results of the derived model are compared with those of the switching model and experimental prototype. Computed transfer function of load impedance from the proposed average model closely resembles those of the switching model and the experimentally measured results validate the modeling procedure. Furthermore, stability analysis of the 18-pulse ATRU may be executed based on the return ratio of source and load impedance. [ABSTRACT FROM AUTHOR]

In order to provide a source for nine phases suitable for 18-pulse ac to dc power, this paper proposes a new structure for a step-up asymmetrical delta-connected transformer for converting three-phase ac power to nine-phase ac power. The design allows for symmetry between the nine output voltages to improve the power quality of the supply current and to minimize the THD. The results show that this new structure proves the equality between the output voltages with 40°- a and 40°+ a phase shifting and produces symmetrical output currents. This result in the elimination of harmonics in the network current and provides a simulated THD that is equal to 5.12%. An experimental prototype of the step-up asymmetrical delta-autotransformer is developed in the laboratory and the obtained results give a network current with a THD that is equal to 5.35%. Furthermore, a finite element analysis with a 3D magnetic field model is made based on the dimensions of the 4kVA, 400 V laboratory prototype three-phase with three-limb delta-autotransformer with a six-stacked-core in each limb. The magnetic distribution flux, field intensity and magnetic energy are carried out under open-circuit operation or load-loss. [ABSTRACT FROM AUTHOR]

In this paper, two large current rectifiers are proposed based on two wye-connected autotransformers. The requirements of the ideal large current rectifier are analyzed, and it is concluded that the large current rectifier has a higher power density and a higher energy conversion efficiency when it is made up of two three-phase half-wave rectifiers and a wye-connected autotransformer. According to theoretical analysis results, the two novel wye-connected autotransformers are designed to feed two three-phase half-wave rectifiers. The two autotransformers can generate two groups of three-phase voltages with a 60o phase shifting, and their kVA ratings account for 95% and 80% of the load power, respectively. These values are less than those of a double star rectifier at 30% and 46%. From the input mains and output side, the power quality of the proposed rectifiers is the same as that of the double star rectifier. Some experiments validate the correctness of the theoretical analysis. [ABSTRACT FROM AUTHOR]

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The existing 24-pulse autotransformer rectifier unit (ATRU) needs interphase reactors for parallel work of the rectifier bridges, and its output voltage cannot be regulated. Aiming at these problems, a step-up and step-down asymmetrical 24-pulse ATRU is proposed in this paper. The connections and turns ratios among transformer windings are well designed. In addition, a 15-degree phase difference is formed between two of the 24 voltage vectors produced by the transformer, which makes the four rectifier bridge groups produce a 24-pulse DC voltage without interphase reactors. Meanwhile, by adding extended winding to each phase of the transformer, wide-range regulation of the ATRU output voltage can be realized, and the reasonable voltage regulation range is between 0.2 and 1.6. The superposition of the voltage vectors and the principle of the voltage regulation are analyzed in detail. Furthermore, the turns ratio of the windings, winding current, output voltage, and kilovolt-ampere rating are all derived. Finally, the simulations and experiments are carried out, and the correctness of the principle and theoretical analysis of the new 24-pulse ATRU are verified. [ABSTRACT FROM AUTHOR]

To improve the accuracy of fault location system, several short-circuit tests need to be conducted before being brought into service in autotransformer (AT) feeding systems for high-speed railways in China. However, no systematic algorithm yet exists to evaluate the consistency of the current distribution of short-circuit tests. A methodology is proposed in this paper to address this problem. Based on Kirchhoff’s current law and the generalized method of symmetrical components, the current deviations of the AT feeding systems are analysed and then normalized with the short-circuit current as they vary greatly with systems and short-circuit sites. It is also found that the short-circuit current varies with the calculation methods, and its unbiased standard deviation also reflects the consistency of the short-circuit test. The mean and maximum of the current deviations, as well as the unbiased standard deviation of the short-circuit current, show the consistency of the short-circuit test from different aspects, although the last two items are highly relevant. Therefore, a unified evaluation index is defined as the sum of the three items, and then applied in two case studies to test its performance. The results show that, the proposed index can clearly distinguish the consistency of the short-circuit tests and may be used to sort the short-circuit tests for fault location systems. Besides, some short-circuit tests may have very poor consistency indices, and thus are not applicable to the tuning of fault location systems. In the authors’ opinion, the determination of the threshold of the proposed index needs further investigation. [ABSTRACT FROM AUTHOR]

Frequency response analysis (FRA) is an effective approach for detecting mechanical damage in the windings of a transformer. However, the application of FRA to estimate the winding condition of autotransformers (ATs) employed in the China High-Speed Railway is problematic. Because the split windings of the ATs are connected inside the tank, it is impossible to measure the FRA curves for each split winding outside the tank directly. Therefore, it is difficult to identify fault winding and effectively diagnose the overall condition of the windings. This paper explores the features of FRA curves under different fault conditions to help distinguish the fault winding. An equivalent circuit model is built for the AT with split windings, and the electrical parameters of the windings are determined using the finite-element method. The model is then employed to simulate typical axial displacement faults. The results show that the first antiresonance and resonance of the frequency response for connections involving series windings or series and common windings may be visible features for diagnosing winding faults and the fault level. Specifically, the first and second resonances of the frequency response for the common winding connection may be regarded as the primary feature for identifying fault windings. [ABSTRACT FROM PUBLISHER]

SUMMARY: In this paper, a new method for synthesizing a higher‐order passive complex filter using autotransformers is proposed. The proposed filter can be synthesized approximately by transforming the conventional one. Since the proposed transformation replaces all transformers with autotransformers, the proposed filter has no floating windings. As examples, both first‐order and third‐order complex filters are synthesized. The validity of the proposed method was confirmed through computer simulation and experiment. Due to approximate synthesis, the transfer function of the real output is different from that of the imaginary output. However, both of them have roughly the same response near the passband. Experimental results show good agreement with the theoretical value. It means that the proposed method is valid for the practical application. [ABSTRACT FROM AUTHOR]

In this paper, a switched reluctance motor (SRM) with phase windings similar to an autotransformer is presented. The new winding structure allows for fast demagnetization. This enhances the machine performance at high speeds. During demagnetization, the dc bus voltage is applied to a portion of the phase winding, and accordingly a higher negative demagnetization voltage/turn is applied. This results in the faster discharge of the phase excitation currents. The power electronic converter configuration is modified to facilitate the new winding topology. However, the number of switches is the same as a conventional half-bridge converter used in SRMs. The proposed SRM drive with the modified winding configuration is verified through finite element analysis based circuit simulations and experimental results. [ABSTRACT FROM AUTHOR]

Fractional horsepower motors are used to power exhaust fans such as those found in bathrooms and oven hoods. There was an increase in fires attributed to these motors shortly after the year 2000. Still, many argue that these motors cannot cause fires because they contain a thermal cutoff (TCO) unit that shuts off electrical current when the TCO reaches a specific temperature. In this paper we describe an unsafe failure mode that can occur after extended use. As fans age, the bearings wear out, resulting in heating of the coil wires. Excessive heating results in break down of the wire insulation, which can lead to a short between wires and create an autotransformer configuration. An autotransformer can support high current in only part of the wiring, resulting in resistive heating sufficient for ignition. With the low thermal conductivity of the coil (0.0162W/(cm∘C)) and the poor performance of the TCO (no cutoff up to 260∘C), a large temperature gradient is possible. The gradient allows for ignition in one part of the motor without activating the TCO. We experimentally validated the proposed mechanism in an isolated motor and a complete fan assembly resulting in ignition after 18 s and 23 s. The assembled fan burned for more than 6 minutes before we extinguished the flames for safety reasons. Our results are consistent with field results from actual fires. [ABSTRACT FROM AUTHOR]

This paper proposes a current-based Newton-Raphson power flow method for an autotransformer (AT)-fed railway power supply system. The power flow formulation based on the NR method for such a multi-conductor system is revised. The train’s position variation and load variation cases were studied on the impacts of a dynamically moving train to the performance of the proposed power flow method. The Nakhon Ratchasima – Nong Khai railway line was used to study the convergence characteristics of the proposed method when implementing in a practical situation. The results of the proposed NR method were compared with those of the Gauss-Seidel (GS) and sequential linear power flow (SLPF) method for the same power system. The analysis and comparison of convergence behaviour of all three methods have been done in details. By varying train’s position, the GS method showed the great drop in the number of iterations and execution time at the AT’s positions but it appeared to be the maximum execution time for the SLPF method, while the proposed NR did not show substantial changes. For the case of load variation, GS and SLPF method showed an increase in the number of iterations and execution time with the load while the proposed NR was almost unchanging with the load. Regardless of memory usage, the proposed NR showed better convergence for both train position and load variation cases. [ABSTRACT FROM AUTHOR]

This paper proposes a newly formed model for time-domain analysis of single-phase autotransformer based on superposition of collinear vectors. Parameters of the model are determined with parameter identification procedure presented as a step-by-step algorithm using two no-load states. Identification is completed by solving minimization of least squares using genetic algorithm. This procedure also verifies the model and returns the total relative error referred to effective current value under 10%. Saturation is taken into account with nonlinear transcendent functions that are differentiable in whole domain, while the hysteresis is neglected. [ABSTRACT FROM AUTHOR]

Low-frequency oscillation (LFO), harmonic resonance, and harmonic instability phenomena in high-speed railways (HSRs) result from the interactions between multiple electric trains and the traction network. A train–network interaction system and a unified impedance-based model are therefore presented to reveal the mechanism of these three phenomena in Part I of the two-part paper. Part II of the paper presents and compares the detailed simulation and experiment results to investigate the available possible influential factors, verifications, and discussions of the three phenomena. The influential factors include the following: 1) four-quadrant converter (4QC) parameters involving the control parameters of the current controller (CC), the voltage controller (VC), and the phase-locked loop (PLL); 2) network parameters involving the utility power system, traction transformer, primary/secondary supply lines, catenary lines, and autotransformers (ATs); and 3) different numbers and positions of trains and railway lines will also be considered and discussed. In order to validate the theoretical results, the time-domain simulation and experiment system have been conducted. Finally, the differences and the relations among the three phenomena are discussed. [ABSTRACT FROM AUTHOR]

Complete and accurate spatial magnetic field description is the premise of effectively assessing the power supply capability of a high-speed railway (HSR). Its evaluation indicators are the current distributions and the integrated impedance of traction network. This paper proposes a spatial magnetic-field description method for the auto-transformer (AT) power supply system. Due to the limitations of previous approaches, all the real loop circuits of the AT system are considered for structuring a loop circuit matrix. At first, different description processes are divided, respectively, into those for the right side and the left side of the load. Then, considering that two types of return conductors exist in an AT system, a certain current ratio deduced in existing studies is introduced. As the introduced current ratio of the left side of the load is approximate, an iterative scheme is adopted. By constantly adjusting current ratio to satisfy a constraint condition of circuit voltages, accurate traction network impedance is obtained. Last, in order to verify the effectiveness of the proposed description method, two real-life experiments in a Chinese HSR line are performed, which indicate that the proposed method can not only directly reflect complete and accurate current distribution, but also deduce the exact traction network integrated impedance. [ABSTRACT FROM AUTHOR]

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In a small-to-medium-scale wind energy conversion system (WECS), diode rectifiers rather than pulse-width modulated (PWM) rectifiers are widely adopted due to the features of high reliability and low cost. However, large current harmonics are induced in the generator phase current by commutations of diode rectifier, eventually causing large torque ripple on the drive-train of the WECS and making it more likely to be fatigue breakdown. In this paper, a 12-pulse autotransformer rectifier unit (ATRU) with reduced volume and weight is proposed for WECS application. By analyzing the characteristics of the input current and output voltage of the ATRU within the entire operation range of WECS, the method to properly design its parameters is proposed. A 1.2 kW direct-drive WECS demonstration platform using ATRU is built in the lab. A modified sensorless overall power control strategy is adopted to control the WECS. Experimental tests are carried out and the results not only validate the feasibility of implementing ATRU in WECS applications, but also prove its high torque ripple reduction ability. [ABSTRACT FROM AUTHOR]

The operation of a three-phase induction machine on single-phase supply has been an approach used for electromechanical energy conversion in rural communities with limited or no access to the three-phase grid. Such single-phase to three-phase conversion can be achieved by passive and active means. In passive methods, fixed capacitors are used for starting and running the motor. Single-phase to three-phase conversion with reduced switch count is desirable, as this leads to a lower cost. However, it is a challenge to start an induction motor using such a power converter and to maintain balanced three-phase voltage under all loading conditions. It is shown in this work that balanced excitation cannot be achieved under all conditions with variable capacitor emulation method. An active phase-converter configuration and its control that ensures balanced three-phase power supply at the motor terminals under all operating conditions is proposed. Also, this approach has the ability to soft-start the motor. It is shown that this can be done without drawing excessive currents from the power converter or from the single-phase grid. A design methodology is presented for the selection of the autotransformer taps to limit startup transient currents to the desired level, based on the analytical results. Simulation and experimental results are presented that validate the proposed topology and control. [ABSTRACT FROM PUBLISHER]

This paper presents the design and analysis of a polygon-connected autotransformer based 20-pulse AC-DC converter which supplies direct torque-controlled induction motor drives (DTCIMDs) in order to have better power quality conditions at the point of common coupling. The proposed converter output voltage is accomplished via two paralleled 10-pulse AC-DC converters each of them consisting of five-phase diode bridge rectifier. An autotransformer is designed to supply the rectifiers. The design procedure of magnetics is in such a way that makes it suitable for retrofit applications where a six-pulse diode bridge rectifier is being utilized. The proposed structure improves power quality criteria at AC mains and makes them consistent with the IEEE-519 Standard requirements for . Furthermore, near-unity power factor is obtained for a wide range of DTCIMD operations. A comparison is made between six-pulse and proposed converters from the viewpoint of power quality indices. Results show that input current total harmonic distortion (THD) is less than 8% for the proposed topology at variable loads. A laboratory prototype of the proposed polygon-connected autotransformer-based 20-pulse AC-DC converter is developed and test results are presented to validate the developed design procedure and the simulation models of this AC-DC converter under varying loads. [ABSTRACT FROM AUTHOR]

In this paper, a novel large current rectifier using the fork-connected autotransformer and dc-side current injection method is proposed. The fork-connected autotransformer outputs two sets of three-phase voltages with 60° phase difference, and feeds two three-phase half-wave rectifiers. Compared with the isolated line-frequency transformer, the proposed autotransformer has lower kilovoltampere (kVA) rating under the same load power. An optimal designed single-phase full-bridge inverter is used to inject the compensation currents into the dc side of the two three-phase half-wave rectifier, which can reduce the harmonics in input line current to be an acceptable level. The kVA rating of the single-phase full-bridge inverter is very lower, and the proposed large current rectifier draws nearly sinusoidal currents from the ac main after using the dc-side injection. A 4 kW prototype is set up to validate the theoretical analysis and evaluate the performance. [ABSTRACT FROM PUBLISHER]

Owing to the installation of autotransformers at regular intervals along the line, distance protection relays cannot be used with the aim of locating ground faults in 2 × 25 kV railway power supply systems. The reason is that the ratio between impedance and distance to the fault point is not linear in these electrification systems, unlike in 1 × 25 kV power systems. Therefore, the location of ground faults represents a complicated task in 2 × 25 kV railway power supply systems. Various methods have been used to localize the ground fault position in 2 × 25 kV systems. The method described here allows the location of a ground fault to be economically found in an accurate way in real time, using the modules of the circulating currents in different autotransformers when the ground fault occurs. This method first needs to know the subsection and the conductor (catenary or feeder) with the defect, then localizes the ground fault's position. [ABSTRACT FROM AUTHOR]