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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]

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]

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]

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]

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]

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]

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]

To improve the power density of large current rectifier, this study designs optimally a star‐connected autotransformer which is used in large current rectifier. From the viewpoint of minimising the kVA rating of phase‐shift transformer and conduction loss of rectification devices, it is concluded that the ideal large current rectifier is a combination of dual three‐phase half‐wave rectifiers and a star‐connected autotransformer which is used to be the phase‐shift transformer. To design optimally the star‐connected autotransformer, the winding configurations of the star‐connected autotransformer are analysed, and the corresponding expressions and waveforms of current through windings at different winding configurations are also obtained. Consequently, the kVA rating expressions of the autotransformer under different winding configurations are calculated, which describe the relationship between the winding configuration and load power. From the expressions, the optimal winding configuration, which has the least kVA rating and winding numbers, is obtained. Some simulations and experiments are carried out to verify the theoretical analysis. [ABSTRACT FROM AUTHOR]

A novel non‐isolated resonant quasi‐impedance (quasi‐Z)‐source network DC–DC converter is proposed. The resonant impedance source network is derived from the quasi‐Z‐source network by including the autotransformer‐based resonant cell instead of the second inductor of the quasi‐Z‐network. The leakage inductance of the autotransformer and two resonant capacitors connected in series with the autotransformer windings constitute a high‐frequency resonant tank. At the same time, the resonant capacitors block the DC‐bias current of the autotransformer. The resonant operation with a sinusoidal current of the main switch and diodes enables electromagnetic interference mitigation and improves the efficiency of the converter. Experimental results of a 100 W, 30 V/200 V prototype are presented to verify the analysis results of the proposed converter. [ABSTRACT FROM AUTHOR]

High‐frequency resonances occur frequently in traction power supply systems of high‐speed railways in China. In this study, the autotransformer‐fed traction power supply system for a high‐speed railway was first described, and then the high‐frequency resonance mechanism was analysed. On the basis of this, the input impedance at the train's pantograph was applied to identify the resonant frequencies, and a second‐order high‐pass filter was designed by solving the minimum total input impedance with the steepest descent method. The designed filter can not only suppress the existing resonance, but also avoid a new resonance occurrence. Finally, the resonance mechanism analysis and the designed filter are verified by investigating the harmonic spectrum of the pantograph voltage in a MATLAB/Simulink‐based co‐simulation. [ABSTRACT FROM AUTHOR]

Railway Operators and Infrastructure Owners are required to design the railway to specific national and international, technical and safety performance standards. These standards and codes of practice provide the basis for company 'Codes of Practice', which detail the design methodology, application and system installation. To validate the design and to comply with these standards and codes of practice, Atkins and the University of Birmingham have developed the multi‐train simulator (MTS) to model AC railway electrification infrastructure. The development was carried out under a Knowledge Transfer Partnership between Atkins and the University of Birmingham. The MTS models multiple trains moving on AC traction railway networks following specified timetables. The model of the traction power network covers all types of AC feeding arrangements in the UK, including the rail‐return system, the classic booster transformer system and the autotransformer system. This study addresses the work undertaken by the Knowledge Transfer Partnership and describes the development of AC railway electrification infrastructure modelling based on a multi‐conductor model for MTS. The modelling of multi‐conductors in AC power networks separately, instead of lumping them together, enables more accurate calculations of induced voltage, EMC analysis, return current distribution, positive and negative energy consumptions and loss calculations. [ABSTRACT FROM AUTHOR]

A DC current injection circuit of a trifurcated autotransformer (TAT)‐based 12‐pulse diode bridge rectifier (DBR) is proposed which can help reduction of harmonics in the AC mains. The circuit at the DC side employs a voltage multiplier (VM) circuits namely voltage doubler, voltage tripler and voltage quadrupler and an interphase transformer which provides the required circulating current. This induced circulating current shapes the input AC line current near to a sine wave. The output of VM circuits is connected across the load and thereby reuses the harmonic energy absorbed by the system. Thus, improves the energy conversion efficiency of the proposed system. The TAT‐based 12‐pulse DBR has the implicit ability to prevent the zero sequence components thus expels the necessity of zero sequence blocking transformer. The proposed configuration is analysed, simulated in MATLAB Simulink and the simulation results are presented, which confirms the improvement in power quality parameters in the input AC line current. Further, the viability of the proposed configuration is verified by experimental results which confirm the suitability of the proposed configuration in AC–DC applications. [ABSTRACT FROM AUTHOR]

A modified series combining transformer incorporating an autotransformer for the dual‐mode operation to improve efficiency performance in the low‐output power region is presented. The dual‐mode CMOS power amplifier (PA) with the proposed reconfigurable transformer is fabricated using 40‐nm CMOS technology. Compared with other dual‐mode WLAN CMOS PAs, the proposed PA achieves high‐efficiency performance in both high‐power and low‐power modes; in dual‐mode operation, the PA has average output power levels of 19.2 and 15.2 dBm with power added efficiencies of 17.3 and 14.4%, respectively, while satisfying the error vector magnitude requirements of WLAN 802.11g signals. [ABSTRACT FROM AUTHOR]

ABSTRACT This article presents a reconfigurable monolithic step-up autotransformer. In addition, its reconfigurable operation has been analyzed using a proposed nonideal transformer model. A corresponding complete model for a reconfigurable autotransformer has also been developed on a silicon substrate case. The proposed reconfigurable autotransformer was fabricated using a standard 65-nm CMOS process. The measurement results demonstrate that the fitted curves using the proposed complete model agree well with the measured results. The proposed reconfigurable autotransformer can perform the required impedance changes depending on the power amplifier output power change with low insertion loss. The 50-Ω load impedance is transformed to a low value of 3.28 Ω in high power (HP) mode operation, and it is transformed to a high value of 12.26 Ω in low power (LP) mode operation at 0.9 GHz. The transformer insertion losses in HP and LP mode operations are −1.424 dB and −0.706 dB at 0.9 GHz, respectively. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1662-1666, 2015 [ABSTRACT FROM AUTHOR]

Tapped-inductor provides the property of wider voltage conversion range similar to an auto-transformer and can be integrated to a switched-mode power converter to enhance the power conversion efficiency. Its low component count has attracted applications with extreme voltage conversion ratios. Circuit family is presented and a general tapping concept for diode, transistor or both has been described. Its boost converter version is used as an example for the study. Detailed comparisons between tapped-inductor boost and conventional boost are carried out, which include the comparison of voltage gain, component stress and efficiency with the presence of the parasitic components. Experimental results present solid verification of the analysis of performance of tapped-inductor boost converter under extreme conversion ratios. The proposed study has introduced formulations for both transistor and diode tapping and can be extended to other tapped-inductor DC-DC converters as a family of circuits, and thus provides an understanding of the effect of the parasitic components that affect the efficiency under extreme conversion ratio. [ABSTRACT FROM AUTHOR]

An on-chip implementation of a low loss 1:9 transmission line transformer (TLT) is presented. The TLT was fabricated with broadside coupled microstrip lines in a GaAs HBT process. The wide metal transmission lines allow the high-quality factor and the high-current capability. The measured insertion loss is 0.88 dB at 880 MHz. The equivalent circuit model of the TLT is also presented. The model shows good agreement with the measured data. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1936–1940, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21820 [ABSTRACT FROM AUTHOR]