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The stable operation of transformers is an important guarantee of power system security. The temperature distribution of main insulation is different in long‐term operation. The temperature difference between the local hottest spot temperature and the average oil temperature in the transformer exceeds 30 K. Therefore, the oil‐paper insulation inside the transformer will exhibit non‐uniform ageing after long‐term operation. At present, frequency domain dielectric response (FDS) technology is widely used in transformer ageing assessment. However, the existing assessment methods are only used for overall insulation ageing assessment. The average moisture content and polymerization degree of the overall insulation between each phase winding are obtained. But it still cannot be used to assess the non‐uniform ageing of the transformer's main insulation. The radial, axial, and two directions non‐uniform ageing models have been made in the laboratory in this study. The FDS and physicochemical properties of non‐uniform ageing models were tested. The variation law of FDS for non‐uniform ageing models was analysed. Meanwhile, the dielectric relaxation model of non‐uniform ageing for main insulation was established. The method of frequency division iterative calculation was proposed. Combined with the Fessler moisture balance formula of ageing oil‐paper insulation, the quantitative assessment method for ageing parameters of the non‐uniform ageing model can be realized. Compared the calculated results with the chemical analysis results, the relative error of the polymerization degree calculation is less than 4%, and the error of the moisture content calculation is less than 0.3%. The calculation results can meet the needs of the engineering apply. Therefore, this method can realize the quantitative assessment of transformer main insulation ageing, which is of great significance to assess the transformer insulation state. [ABSTRACT FROM AUTHOR]

Near Infrared Spectroscopy (NIRS), as one of analysis technologies, has shown promisingly industrial applications for significant properties for recent decades such as fast response, preciseness, non-intrusion etc. Here, the authors employed NIRS coupled with a series of physical and chemical tests to assess the ageing condition of oil-paper insulation, which is responsible for the main insulation type of high-voltage power transformer. Among these procedures, the data analytic algorithms are of utmost importance to determine the evaluation precision. After plenty of trial-and-error, Savitzky-Golay (S-G) convolution was finally utilised to de-noise samples and improve the spectral data quality. The competitive adaptive reweighted sampling (CARS) was used to select the optimal wavelength combination of NIRS, which is found able to fully extract the effectively spectral information and reduce dimensions of spectral data. Based on the above-mentioned techniques, the quantitative analysis model of NIRS was established by partial least squares (PLS), which could synthetically process the spectral data and the degree of polymerisation (DP) of paper samples. The results indicated that compared with the traditional detection methods, the NIRS analysis is a powerful and informative tool to characterise the condition of oil-paper insulation without intrusion or damage to transformers. [ABSTRACT FROM AUTHOR]

The accuracy and feasibility of the traditional XY model of transformer oil–paper insulation system are disputable since it ignored the effect of non‐uniform aging and conductance on dielectric response property. Given this issue, this work attempted to report a modified XY model to overcome these defects. The novelty of this work is in the exploration of the modified XY model as a potential tool for establishing the quantitative correlation among the complex relative permittivity of the liquid insulation, solid insulation, and liquid–solid composite insulation. In that respect, the presented verification experiments proved the feasibility and accuracy of the reported model. The contributions are expected to provide a theoretical basis for the condition evaluation of transformer solid insulation under non‐uniform aging conditions. [ABSTRACT FROM AUTHOR]

Power transformers have been expected to work with safety and reliability over a period of more than 30 years. The life expectancy of transformers depends upon the degree of deterioration of insulating materials, particularly conductor insulating paper. Many attempts have bee made in recent years to estimate the degree of deterioration of transformers but no diagnostic techniques have been put into practical use because of the difficulty of taking into account the service conditions. It is well known that materials change color when they deteriorate, but no colorimetry study has been made on material deteriorations. According to the recent advances in instrumentation and data processing equipment, the color of materials has become easy to measure qualitatively. Therefore, the basic investigation on colorimetry of insulating papers has been made to clarify the applicability to power transformers. Test results showed a strong correlation between the color difference and the deterioration of insulating paper. This report discusses the effect of materials of papers and impregnating oils on colorimetry. [ABSTRACT FROM AUTHOR]

Large power transformers using perfluorocarbon liquid have advantages of high dielectric strength, excellent cooling ability, and nonflammability. To make the transformer more compact, it is very important to improve the insulation material. Recently, the authors succeeded in developing a new type of aramid insulation paper which has no fluff on its surface to avoid field concentration. In this study, an outline of the developed paper and the experimental results of the insulation strength on turn-to-turn and intercoil models are described. It was found that the new aramid paper increases the insulation strength 1.2 (turn-to-case) to 1.7 (intercoil case) times that of the existing insulating paper. [ABSTRACT FROM AUTHOR]

This study reports experimental study of cellulose performance in mixture of mineral and synthetic ester (SE) oils. Solid insulation impregnated with mineral oil (MO) and mixture oil has been subjected to accelerated thermal stresses to understand the deterioration rate of cellulose. Diffusion of oil–moisture mixture into paper and corresponding variation in dielectric strength of paper has been examined. Compatibility of solid insulation with MO and mixture oil is analysed by thermal expansion coefficients, X-ray diffraction analysis and dissolved decay products. Furthermore, chemical functional groups present in aged oils are identified by Fourier transform infrared spectroscopy analysis. It is observed that the performance of cellulose insulation is improved with addition of SE to MO. This is achieved by higher water solubility of esters, which enables higher moisture consumption during hydrolysis and keeps cellulose dry. [ABSTRACT FROM AUTHOR]

A sudden capacity increase in demand during the summer peak, or in contingencies such as malfunctioning transformers, may cause an overload in normal transform-ers. In this paper, on the basis of overloaded transformer operation in distribution substations, thermal aging testing in oil was carried out under various overload patterns, such as short-term overload and long-term overload, but with the winding insulation paper's lifespan loss kept constant. From the results, various characteristics, such as the mean degree of polymerization and production of furfural and (C02+C0), and their effects on the lifespan loss of the insulation paper, were obtained. ©2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(2): 1-8, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). [ABSTRACT FROM AUTHOR]

The bushing of a transformer is one part of the transformer system, but is as vital as the transformer itself because it forms a part of the main circuit. Deterioration and abnormal condition of the bushing possibly cause its destruction, shortage of the main circuit to the Earth, and fire in the systems, so that diagnosis of the bushings is thus very important. At present, mainly visual inspection from outside of the bushing is carried out. Inspection of the inside condition of the bushing by dissolved gas analysis of the inside oil is done only as sampling for selected bushings because it is difficult due to the sealed construction of the bushings. Therefore, a simple diagnostic method for bushings has been expected for a long time. In Eastern Europe, the RVM (Return Voltage Measurement) method based on scanning the polarization spectra was applied to the oil–paper insulation systems inside the transformer as a diagnostic method. In this paper we investigate theoretically the principle of the RVM method applied to bushings with the same oil–paper insulation systems as the transformer by using the equivalent circuit, and the possibility of using this novel diagnostic method for condenser-type bushings is demonstrated by a model experiment. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 21–29, 2000 [ABSTRACT FROM AUTHOR]

DC collection and transmission is one of the major development directions of large‐scale photovoltaic (PV) power system. In order to achieve low‐cost, high‐efficiency and long‐distance transmission of PV power, this paper adopted a DC grid‐connected topology by using multi‐modular cascaded DC‐DC converters, forming an input‐independent and output‐series (IIOS) system. In this system, an LLC type DC‐DC converter based on IGCT and magnetically integrated transformer was proposed in this paper, which can significantly increase the capacity, efficiency and power density of DC‐DC converter. And the equalization unit based on half‐bridge topology was adopted for the case of uneven irradiation of the PV inputs. In addition, this paper proposed an equalization control method based on the Nearest Level Modulation (NLM), which greatly simplifies the design of the acquisition and control system. Furthermore, the parametric design of a 50MW PV system was presented, and the simulation was carried out to verify the feasibility and advantages of the system in this paper. The proposed IIOS system is more suitable for high‐capacity, long‐distance PV power transmission, which can serve as a reference for the development of large‐scale photovoltaic systems. [ABSTRACT FROM AUTHOR]

One of the main reasons for the failure of transformer bushings is moisture penetration. Here, an online monitoring method of moisture in transformer bushings based on an improved humidity indicator card, which belongs to the group of passive physical and chemical monitoring methods, is proposed. First, the operating principles and monitoring system are explained. Next, by means of a moisture simulation device, the moisture migration characteristics between the gas and oil, the influence of the interface radius on the response characteristics of the monitoring device, and the characteristics of the humidity indicator card are studied experimentally. Then, the moisture migration process in micro‐scale is simulated by molecular dynamics simulation. Finally, by changing the mass fraction of the cobalt chloride in the humidity card, the warning values of relative humidity at different temperatures are obtained. The simulation results show that water molecules can migrate from the oil–paper system to the air. Moreover, once the moisture enters the transformer bushing, the proposed humidity indicator card can correctly display the relative humidity at different temperatures. In conclusion, the proposed method provides a new way for online moisture monitoring of transformer bushings. [ABSTRACT FROM AUTHOR]

The phase‐locked loop (PLL) is a critical control module applied in the control system of renewable energy sources (RESs) to synchronize with the grid voltage. When a fault occurs, the dynamic response process of the PLL will generate a phase locking error, resulting in a mass of harmonic components in the fault current of the PV inverter, which further poses challenges to transformer differential protection. To address the challenges posed by the phase locking error in practical engineering applications, this paper proposes an improved scheme that uses a digital computer algorithm to record and store the pre‐fault voltage and introduces the limiting coefficient. Firstly, the memory voltage is used to eliminate the phase locking error of the PLL, thereby mitigating harmonic distortion in the fault current of the RESs and ensuring the reliability of the transformer differential protection. In addition, the limiting coefficient is introduced to eliminate the effect of the increased amplitude of the fault current caused by the memory voltage. The voltage reference value of the inverter is multiplied by an appropriate limiting coefficient k and then output to the physical system. Finally, the effectiveness of the proposed scheme is verified by MATLAB/Simulink simulation. [ABSTRACT FROM AUTHOR]

This study aims to explore the effect of the antioxidant 2,6-di- tert-butyl- p-cresol (DBPC) and acids on the generation and migration of copper sulfide on insulating windings as a result of the reaction of dibenzyl disulfide (DBDS) and copper in oil-immersed transformers. The heating experiments were conducted by adding different amounts of DBPC and different kinds of acids to the naphthenic mineral oil. The result indicates that acids can accelerate the generation of copper sulfide and that DBPC cannot effectively consume DBDS to reduce the corrosion phenomenon, but rather accelerates the deposition of copper sulfide onto the insulating paper. In particular, when the DBPC concentration is approximately 0.9% (w/w) at 130 °C for ten days, the amount of copper sulfide deposited on the insulating paper reaches its maximum. Moreover, excessive use of the antioxidant DBPC accelerates the aging of insulating oil, on the contrary, which adversely affects the mineral oil. Although the addition of DBPC can improve the antioxygenic property of mineral oil, too much of it is inadvisable because the transformers that contain DBPC and DBDS may be at high risk and suffer from breakdown. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

In the actual operation, converter transformer valve side bushings are subjected to high voltages and large current loads with large number of harmonics simultaneously. In our previous papers, using the 3‐D electromagnetic‐fluid‐thermal coupling analysis method, the 3‐D temperature distribution of a converter transformer valve side bushing was researched, and the heating and dissipation characteristics of the bushing were analyzed. However, the electric field distribution of the bushing is affected by the temperature gradient inside the bushing. Related researches about the electro‐thermal coupling field of the bushing are rare and most researches are concentrated on the single factor of voltage or temperature. In this paper, by using the electro‐thermal coupling field simulation method proposed in our previous paper, the effect of the core outline size on the electric field distribution of the bushing under a certain temperature gradient was investigated. In addition, by using the transient electric field simulation method, the electric field distribution of the bushing in one cycle subjected to an actual voltage waveform and the current in temperature rise test simultaneously was analyzed considering the effect of the Joule heating power of the core. The proposed method will be able to provide a computation basis for the structure design and optimization of converter transformer valve side bushings. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

We propose an autoregressive weighted network (ARWNet) time series forecasting model inspired by the idea of ensemble learning. The model adopts the classical autoregressive analysis to optimize weak learners. Meanwhile, the combined weight optimization method is used to construct an efficient, strong learner. With these methodological foundations, the scalability of the framework is greatly enhanced by the possibility of experimenting with other learners to assist in decision‐making. Machine learning can provide great utility and acceptable cost in the prediction process of electricity transformers. Over the years, many research papers on time series prediction have been reported. This work will focus on the analysis using the potential properties in the series: long‐term, continuity, periodicity, and delay. In our experiments, the ETT‐small dataset is used to compare the prediction accuracy of ARWNet and other mainstream models. All results suggest that the proposed ARWNet model demonstrates strong generalization ability and high predicting accuracy with delay characteristics, which outperform current popular time‐series prediction methods. [ABSTRACT FROM AUTHOR]

Cascaded H‐bridge rectifier (CHBR) is widely used in high‐power fields such as STATCOM and power electric traction transformer (PETT) due to its high efficiency and easy expandability. With the high‐reliability operation requirements of power equipment, the fault‐tolerance capability has gradually become a research hotspot nowadays. In order to improve the reliability of the power supply, cascaded H‐bridge rectifier should restore as soon as possible after a fault. However, the traditional fault‐tolerant strategies have a great effect on the grid side and a long recovery time. This paper proposed a quasi‐hot standby fault‐tolerant control strategy. Compared with the traditional strategies, the quasi‐hot standby fault‐tolerant control strategy has a faster recovery speed after the malfunction, and its transient recovery process has less impact on the power grid. Simulation and experiment results show the effectiveness and good recovery performance of the proposed fault‐tolerant control strategy. [ABSTRACT FROM AUTHOR]

The isolated DC/DC converter plays an irreplaceable role in the development of the DC microgrid. However, the multiple discrete high‐frequency transformers occupy considerable volume in power converters due to the connection of multiple isolated converters. In order to reduce the number of discrete transformers and increase the power density as well as the reliability of the power converter, a three‐port isolated DC/DC converter with a decoupling magnetic integrated high‐frequency transformer (DMIHFT) is proposed for the DC microgrid in this paper. The semiconductor count of the proposed converter is reduced by combining one DAB converter and two LLC converters. Three discrete transformers are integrated into a single common core, and magnetic decoupling is achieved among each transformer by rational design at the same time. In addition, the magnetic circuit model derivation process of the proposed DMIHFT and the operation principles of the converter are described in detail. The design guidance of the proposed DMIHFT is present. And then, a 15 kW/20 kHz DMIHFT is designed based on the design guideline, and the simulation is carried out. Finally, a 15 kW/20 kHz DMIHFT prototype is fabricated, and the effectiveness of the proposed DMIHFT is verified by experimental results. [ABSTRACT FROM AUTHOR]

The extra‐high voltage (EHV) transformer winding disks are subjected to dynamic electromagnetic forces caused by short‐circuit currents. These periodically decaying impact forces may endanger the safety of the transformer. The winding constraint conditions around the circumference are different due to the influence of axial pressure structures and magnetic boundaries. Moreover, this difference becomes more evident as the voltage and capacity increase. To investigate the spatial differences and time effects of EHV transformer short‐circuit faults, this paper proposes a reliability evaluation method based on dynamic relative displacement (DRDEM) for the first time. Firstly, the suitable and efficient analysis method is summarized by comparing the deviation among different calculated impedances and the measured value. Then, the space‐time effect of windings is obtained by the static magnetic field and dynamic structural field based on equivalent compression supports. Furthermore, a 50 MVA/110 kV transformer is designed for the destructive short‐circuit test to simulate the short‐circuit ability feature of the EHV transformer. The disassembly phenomena after all the tests indicate that the DRDEM is more adaptable in characterizing the winding ability to withstand short circuit when investigating the spatial differences and time effects. [ABSTRACT FROM AUTHOR]

The valve‐side bushing of the UHV converter transformer is the key equipment in the DC transmission project, and its running state directly affects the security and stability of the power system. This paper analyses the attributes of the physical entities, uses the digital twins to establish the state feature set and proposes a zero‐sample state evaluation algorithm for the valve‐side bushing. First, the geometric, materials and electrical characteristics are analyzed, and the detailed components of carrier current are obtained by empirical mode decomposition. Then, COMSOL is used to establish digital twins, verify the validity of twins with axial heat distribution of bushing, and establish a state feature set with the extreme temperature inside and outside bushing. Finally, the fuzzy clustering algorithm is used to classify the state feature set, and the similarity is used as the index to realize the zero‐sample state evaluation of the valve‐side bushing. Through the demonstration and analysis of examples, the evaluation model solves the problems of difficulty in extracting the internal features, fewer fault samples, and training difficulty, which is conducive to improving the operation and maintenance management level of power transmission equipment. [ABSTRACT FROM AUTHOR]

The descriptions of power equipment defect records are often characterized by colloquial short texts. Standardized classification of a large number of colloquial defect descriptions has laid a solid foundation for building a power equipment knowledge graph and improving the level of intelligence in the field of power inspection. Using deep learning and natural language processing technology, this paper proposes a text classification model for power equipment defect records named Bidirectional Encoder Representations from Transformers with Family Feature Fusion (BERT‐TriF). The model firstly leverages BERT to semantically represent the input text. To extract family history as well as text implicit information, we creatively propose a family feature fusion algorithm for training. An improved multi‐head attention mechanism is developed subsequently to enhance text semantic category features and strengthen the learning ability of the model. By comparing BERT‐TriF and baseline models such as TextCNN, TextRNN, and fastText on the specified and generic text dataset, the experimental results demonstrate that it has better performance, robustness, and universality for short text classification. [ABSTRACT FROM AUTHOR]

When the manufacturers produce capacitor voltage transformers (CVT), the function of harmonic measurement is not considered. Therefore, this paper explores methods to solve the problems of harmonic measurement from the perspective of manufacturers. Using the amplitude‐frequency characteristic curve of CVT under no‐load condition and standard environment as the benchmark, the estimation method of stray capacitance parameters in CVT based on CSO algorithm is proposed. Besides, in order to improve the accuracy of harmonic correction of CVT in different operating environments, this paper deeply studies the influence of oil temperature and industrial frequency on internal parameters and harmonic transfer characteristics of CVT. On this basis, this paper proposes a correction method of CVT harmonic transfer characteristic curves under different operating environments based on BP neural network algorithm. Finally, under the actual operating environment, calculating the amplitude‐frequency characteristic curve of CVT by utilizing the actual harmonic voltages at the secondary side of the TYP3‐0.02HF CVT and RCVT, and comparing it with the amplitude‐frequency characteristic curve of CVT predicted by the BP neural network. The results show that the proposed methods are effective and feasible. [ABSTRACT FROM AUTHOR]

Transformer is a key equipment for constructing power system, and its safe and stable operation is the basis of power system. In order to solve the problems of difficult identification and low detection accuracy of transformer winding at operating state, in this paper, a diagnosis method based on digital twinning technique for transformer winding failure is proposed. Firstly a digital twin transformer model with high simulation degree based on transformer entities was built, and then multiple physical field simulation were used to deduce the information change of oil tank surface vibration information of digital twin transformer under different working conditions, various winding failures. Here, the vibration signal on oil tank is effectively decomposed to extract the feature vector by independent component analysis method and wavelet packet transform, which in turn is based on neural network algorithm to learn and diagnose the failure signal of winding. In order to test the accuracy and feasibility of the method, the diagnostic test of oil tank vibration signal detection of health and fault transformer is carried out on an experimental prototype of transformer with 110 kV, and the diagnostic results show that this method can diagnose the problems occurred in transformer windings with high efficiency and accuracy. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

This paper suggests a new control algorithm for On‐Load Tap Changer (OLTC) of wind farm main transformer that is responsible for voltage regulation of collector network. The conventional control scheme adjusts the voltage of LV side of transformer without considering the wind turbine voltages. The voltage of end‐string wind turbines may reach the upper or lower limits of Over or Under‐Excited operation regions. Voltages close to upper or lower limits bounds the reactive power capability (generation or consumption) of wind farm due to dependency of wind turbine reactive power capability to voltage. Operation of wind turbine at extreme voltages may stress its power electronic devices. Furthermore, minor disturbances at either grid or power plant may trigger the protection system and interrupt generation due to vicinity of operation point to protection thresholds. The suggested technique integrates the OLTC control into the power plant controller and considers both LV side voltage of transformer and wind turbine voltages. By monitoring the wind turbine voltages, the voltage setpoint of OLTC is adjusted to provide voltages closer to middle of range for wind turbines that prevents the wind turbines from reaching the voltage limits. Dynamical simulations conducted in DigSILENT PowerFactory software for both Under‐Excited and Over‐Excited operation regions demonstrate desired performance for the suggested technique. [ABSTRACT FROM AUTHOR]

The increasing penetration of renewable generation and large implementation of the DC loads are challenges that stress the current distribution systems by causing uncertain voltage variations. The AC−DC hybrid distribution network is seen as an innovative solution to address the problem of accommodating highly dispersed sustainable energy. Specifically, the solid-state transformer (SST), which is capable of enhancing power-quality performance and controllability of the power distribution, has attracted increasing focus in this decade as a promising advanced converter. It is of great interest to investigate how to operate the multiple SSTs with different modes in a coordinated manner to optimise the distribution network performance. This paper formulates the coordinated operation model for multiple SSTs to enable fast voltage regulation for the hybrid active distribution network equipped with large-scale penetration of invertor-based solar power and DC load. A hybrid optimal power flow describing the controlling modes of SSTs and operating interaction between AC−DC subsystems is proposed in this paper. The case studies on a modified IEEE 33-AC−DC hybrid power distribution system show that the voltage profile can be optimised by using the flexible controlling resource of SSTs, facilitating the timely performance adjustment in the presence of fast fluctuating distributed generator's output. [ABSTRACT FROM AUTHOR]

Conventional phase‐shifting transformers (PST) have been utilized for power flow control in transmission networks for many years. Compared to the slow reacting PSTs, recently trending unified power flow controllers (UPFC) enable much faster power flow control. This paper introduces a decentralized power flow control scheme for power flow controllers (PFCs) without the need for communication between the PFCs. For each PFC, an influence area is defined. The line loading of the transmission lines in this influence area is monitored in real‐time operation and if there is an overloading of a line, a feedback control scheme is used to mitigate the overloading of the line. As the power flow control of UPFC is dependent of the network topology and the UPFC converter limit, additional control measures are introduced to circumvent potential critical scenarios. Supplementary control measures are also developed to increase the robustness of control scheme for the simultaneous operation of UPFC with slow reacting PST. Using RMS simulation in DIgSILENT PowerFactory, the developed control method is validated in the IEEE 68‐Bus NETS/NYPS test bench network. The method is also validated for UPFCs with overlapping influence areas and the combination of fast acting UPFCs and slow acting PST devices. [ABSTRACT FROM AUTHOR]

Transformer oil is widely used in high‐voltage transformers as a cooling and insulation medium. Electrical and thermal stresses expose transformer oil to degradation, which may produce dissolved decay products (DDPs) that are partially adsorbed on the insulation paper and accelerate the degradation of oil and paper. An evanescent field D‐shaped plastic optical fibre (POF) sensor was fabricated using the side‐polishing technique to detect DDPs in transformer oil. A laser light source and power meter were used in the prototype to measure the output power of the POF sensor. The measurement of DDPs was conducted on seven samples of insulating oil collected from the field. The sensor performance was evaluated based on different sensing areas, and the sensor sensitivity was measured. Results show an excellent correlation between the output power and DDPs. A correlation analysis was conducted amongst sensor output power, transformer oil refractive index and area under the absorbance curve. The performance of the sensor demonstrates improved linearity and sensitivity of 97.49% and 26.6779 µW/RIU, respectively. The POF sensor is easily fabricated, low cost and can be used for real‐time monitoring of high‐voltage transformers. [ABSTRACT FROM AUTHOR]

A reactive matching (RM) power amplifier (PA) can achieve higher efficiency than a distributed amplifier (DA). In this paper, RM PAs that cover 18–40 GHz are proposed by using one‐order and two‐order synthesised transformer networks (STNs) with GaN on Si technology. In this band, the PAs can provide ≥30 dBm and ≥31.9 dBm output power, with power‐added efficiency (PAE) of ≥17% and ≥15% in the continuous mode, respectively. Further, STNs are analysed using the transmission poles method (TPM) instead of the resonant frequency method (RFM). Thus, the limitations of output power and bandwidth are comprehensively derived. In the one‐order STN, the impedance transformation ratio (T1N) has a maximum value (TMAX_1N), limiting the output power. A two‐order STN is proposed, and the impedance transformation ratio maximum (TMAX_2N) can be improved to (TMAX_1N)2. The result of this paper will be helpful for wideband high‐power amplifier applications. [ABSTRACT FROM AUTHOR]

Designing effective camera trajectories in virtual 3D environments is a challenging task even for experienced animators. Despite an elaborate film grammar, forged through years of experience, that enables the specification of camera motions through cinematographic properties (framing, shots sizes, angles, motions), there are endless possibilities in deciding how to place and move cameras with characters. Dealing with these possibilities is part of the complexity of the problem. While numerous techniques have been proposed in the literature (optimization‐based solving, encoding of empirical rules, learning from real examples,...), the results either lack variety or ease of control. In this paper, we propose a cinematographic camera diffusion model using a transformer‐based architecture to handle temporality and exploit the stochasticity of diffusion models to generate diverse and qualitative trajectories conditioned by high‐level textual descriptions. We extend the work by integrating keyframing constraints and the ability to blend naturally between motions using latent interpolation, in a way to augment the degree of control of the designers. We demonstrate the strengths of this text‐to‐camera motion approach through qualitative and quantitative experiments and gather feedback from professional artists. The code and data are available at https://github.com/jianghd1996/Camera-control. [ABSTRACT FROM AUTHOR]

When the lighting conditions are poor and the environmental light is weak, the image captured by the imaging device often has lower brightness and is accompanied by a lot of noise. The paper designs a progressive self‐calibration network model (PSCNet) for recovering high‐quality low‐light‐enhanced images. First, shallow features in low‐light images can be better focused and extracted with the help of attention mechanism. Next, the feature mapping is passed to the encoder and decoder modules, where the transformer and encoder‐decoder jump connection structures can be better combined with the semantic information of the context to learn rich deep feature information. Finally, the self‐calibration module can adaptively cascade the features decoded by the decoder and input them into the residual attention module quickly and accurately. Meanwhile, the LBP features of the image are also fused into the feature information of the residual attention module to enhance the detailed texture information of the image. Qualitative analysis and quantitative comparison of a large number of experimental results show that this method outperforms existing methods. [ABSTRACT FROM AUTHOR]

This paper proposes a DC-DC converter dedicated to high efficiency and high voltage conversion ratio, which is well suited for fuel cell applications. At high voltage conversion ratio the proposed converter, named here mirror boost converter, has a better ratio of active/freewheeling times which, compared with conventional structures, reduces the penalty on efficiency induced by the high conversion ratio. This converter also uses eight interleaved coupled-inductors, which reduces the current and voltage ripple and makes possible to use of smaller passive components. Coupling all phases with an inter-cell transformer (ICT) also improves the power density, volume and losses of the magnetic component compared to separate inductors. A comparison with a classical double boost converter is realized using the same material. The proposed converter reaches a peak efficiency of 96.5% at 17 kW and 95.3% at 42 kW for a voltage conversion ratio around 10. The mirror boost converter has about half the losses of the double boost converter at same fuel cell power and delivers 69% more power at the maximum measured current phase. [ABSTRACT FROM AUTHOR]

Aiming at the problem of calculation speed for distribution transformer on‐line fault diagnosis, and the shortage of on‐line monitoring type and routine test data, an on‐line fault diagnosis model of distribution transformer based on parallel big data stream and transfer learning is established. First, the fault identification feasibility based on the present on‐line monitoring data of distribution transformer is analyzed, and the main indicators of on‐line fault identification are proposed. Second, the on‐line fault identification method based on ARIMA is proposed, on this basis, the on‐line fault identification model of distribution transformer based on big data stream is established, and the distribution transformers that possess hidden fault are selected, which would be diagnosed further. In order to improve the efficiency, the model is completed on Storm platform in parallel form. Then, the distribution transformer fault diagnosis indicator system is construct. In order to achieve fault diagnosis of the above selected distribution transformers, the effective fault information from other distribution transformers is extracted using the transfer learning algorithm TrAdaBoost, which is used as auxiliary data for the fault diagnosis tool training of the distribution transformer to be diagnosed. At the same time, the model is completed on Storm platform to improve the efficiency. Finally, based on the distribution transformer fault data, the distribution transformer fault diagnosis is simulated, and results show that the fault diagnosis is accurate and efficient. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

In this paper, an integrated of buck and half‐bridge high step‐down converter utilizing single‐stage driving design for high‐efficiency energy conversion is proposed. The proposed converter has the ability to instantly and synchronously transfer energy from input to output within one conversion period. The proposed topology is capable of lowering the primary side voltage of the transformer, so the turns ratio could be reduced. The coupling rate is therefore better along with the leakage inductance. Furthermore, the duty cycle operated in an extreme situation is not necessary. As a combination of the advantages of high step‐down conversion, lower voltage stresses, and fewer amount of semiconductor elements, the feasibility of the proposed topology is verified by the implementation of a 300 W prototype. The proposed integrated topology utilizes the single‐stage energy transfer control algorithm to verify that the proposed experimental circuit has a full load efficiency of 92.4%, in the case of input voltage 380 V and output voltage 5 V, and the maximum efficiency, occurring at 100 W, is 94.55%. [ABSTRACT FROM AUTHOR]

The energy exchange between series and shunt converters is the basis for the powerful functioning of the unified distributed power flow controller, which can be realized through the 3rd harmonic current. In this paper, based on the established 3rd harmonic equivalent network of unified distributed power flow controller power system, the influence of the additional loss caused by the 3rd harmonic current on the line transmission capacity is analysed. The mathematical relationship between the neutral point voltage of the transformer and the 3rd harmonic current is constructed, and it is simplified according to the different 3rd harmonic currents. By analysing the power relationship of the 3rd harmonic equivalent network, it is found that there is a contradiction between improving the 3rd harmonic output efficiency of the shunt single‐phase converter and reducing the neutral point insulation voltage of the transformer. The constraint conditions of the 3rd harmonic current are simulated by the advanced digital power system simulator. Finally, the method of obtaining the economical and safe 3rd harmonic current value range is summarized, which provides a theoretical basis and reference for the device parameter design. [ABSTRACT FROM AUTHOR]

This paper proposes a transformer type dc power flow controller (DCPFC) based on stacks composed of sub‐modules (SMs). The converter can perform dc power flow control independently on point‐to‐point as well as multi‐terminal connection, which can be applied to either high voltage or medium voltage dc grids. In order to explain the operation principle, different operating states of the proposed DCPFC are analysed. With phase‐shift modulation and the closed‐loop current control, the proposed DCPFC can achieve bidirectional power flow regulation. Following the theoretical analysis of the proposed DCPFC, simulation results have verified the feasibility of bidirectional power flow control. A downscaled experimental prototype is also built to demonstrate the operation principle and performance of the proposed DCPFC. [ABSTRACT FROM AUTHOR]

This task‐force paper documents and summarizes the models of power transformers that have been proposed, used, and validated in the exercise of digital real‐time simulation. Power transformer is one of the most important equipment in power systems, and its modelling for electromagnetic transient simulation has evolved over time, especially in the area of real‐time simulation. The focus of the paper is to document and archive the models that have been well accepted and used for transient analysis in digital real‐time simulation so that readers can use it as a master document for transformer modelling in real‐time simulation studies. It includes both conventional and specialized models of power transformers that have been broadly acknowledged by the power engineering community. The models provided here come with detailed mathematical representation and their implementation techniques. A comparative study is also performed to illustrate the differences in their performances. In the end, an application guideline has been provided to guide the readers to select the appropriate model for their study. [ABSTRACT FROM AUTHOR]

This study presents a switch-mode assisted linear amplifier (SMALA) designed for use as a frequency-domain spectroscopy (FDS) excitation voltage source for the diagnostic testing of oil/paper insulation systems in large power transformers. The developed voltage source has a low output voltage noise level and low harmonic content and is capable of driving highly capacitive loads with a maximum output voltage of 800 Vp-p in the frequency range of DC to 100 Hz and current limited to 20 mA. Disturbances in the output voltage associated with switching of the main switching amplifier are minimised through the control of the switching time, taking the slew rate of the correction amplifier into account. The switching time of the main switching amplifier can be effectively controlled using a combination of a simple RC filter on the output of the main switching amplifier and controlling the rise-time and fall-time of the MOSFET gate drive voltage. Using the developed SMALA unit, it was possible to perform FDS measurement on an oil/paper-insulated transformer at an elevated excitation voltage. This resulted in a non-linear response current, manifesting as odd harmonics in the measured current. [ABSTRACT FROM AUTHOR]

In field reliability analyses, a data collection period is given to monitor the failure events from the field. Left‐truncation arises due to early failures occurring before the data collection period, and right‐censoring arises for late failures occurring beyond the monitoring period. Naïve analyses of left‐truncated and right‐censored data lead to biased estimation of the population lifetime of interest. A variety of models and methods have been developed to analyze the left‐truncated and right‐censored data for field reliability analyses. The goal of the paper is to review the existing models and methods for fitting left‐truncated and right‐censored data. Our review includes the existing statistical models, such as the exponential, Weibull, lognormal, gamma, Gompertz, Lomax, and spline models. We comprehensively review the statistical issues of maximum likelihood estimation, model selection, residual lifetime prediction, and Bayesian methods. Some of these methods are illustrated through the field reliability analysis of the electric power transformer dataset. [ABSTRACT FROM AUTHOR]

Abstract: A distribution network operator can reconfigure the network topology by operating section switches to improve multiple objective functions, that is, the line losses and the lifespan of transformers. In the context of long term network management, the impact of future electricity deregulation should be taken into account. Because the related literature has not addressed the above problems, this paper presents a multicriteria optimization approach for efficient distribution network management under electricity market deregulation. The main contributions of this paper can be summarized as follows: (a) the time‐variable (time dependent) and nonconvex issues for the distribution network management are formulated such that the time series variations of the line losses and the transformers mechanical operations are modelled, (b) an electricity consumption model under the deregulation, in which consumers flexibly respond to electricity prices, is taken into account, and (c) from the practical viewpoint, the proposed evolutionary many‐objective optimization approach is applied to find approximated Pareto optimal solutions of the nonconvex large‐scale problem within practical operating time. It can be seen from the computational experiment that the proposed method enables the network operator to easily identify a better network topology, which adequately meets the multiple requirements of the operator. [ABSTRACT FROM AUTHOR]

Summary: In the paper, a new compact electrothermal model of the impulse transformer is proposed. This model has a form of a subcircuit dedicated for SPICE and could be used for low power impulse transformers applicable in switch‐mode power supplies used in electronics. It takes into account simultaneously electric, thermal, and magnetic phenomena occurring in the considered device and describe nonidealities of the core and windings. The form of the elaborated model and analytical dependences describing all its components are presented. Correctness of this model is verified experimentally in a wide range of frequency and load resistance for selected constructions of transformers. Satisfactory agreement between the results of computations and measurements is obtained [ABSTRACT FROM AUTHOR]

This paper proposes a novel bidirectional three‐phase triple‐voltage LLC (T2‐LLC) resonant converter, which can be applied as the submodule of DC transformer (DCT) with fixed input and output terminal voltages. A novel series‐connected three‐phase circuit is adopted at medium voltage (MV) side to improve the voltage level of single submodule (SM) and decrease SM number. Therefore, the power density of DCT can be improved. Besides, the proposed converter is open‐loop controlled with fixed switching frequency, and it naturally realizes bidirectional power flow without changing controlling modes according to the power direction. However, the efficiency and voltage conversion ratio of proposed T2‐LLC converter are sensitive to resonant parameters, and consistency of three‐phase resonant parameters should be ensured to achieve three‐phase symmetrical operation. Therefore, design and optimization methods are proposed to realize ZVS turn‐on, reduce power losses and ensure fixed voltage conversion ratio. The operation principle and optimization method are presented and a 1000 V/333.3 V/5 kW platform is used to validate feasibility of the proposed converter. [ABSTRACT FROM AUTHOR]

SUMMARY This paper deals with the magnetic field generated by medium-voltage/low-voltage substations. Particularly, the paper focuses on the computation of the distance of compliance that is defined as the minimum distance from a field source for having field levels lower than a given threshold. In the first part of the paper, some 3D integral models are proposed to compute the magnetic field generated by each field source present in substations (transformers, cables, etc...). In the second part of the paper, we make use of these integral models to develop a simplified analytical approach for the computation of an approximate distance of compliance, that is, a conservative distance that includes the previously mentioned distance of compliance. The advantage of this approach is that it is composed of simple equations that are very easy to be implemented without losing the tridimensionality of the problem. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY This paper proposes a new high-efficiency photovoltaic (PV) converter for grid connection through a high-leg delta transformer, which is composed of a symmetrically connected boost converter and three half-bridge inverters. One of the three half-bridge inverters is connected to the boost converter, and the others are directly connected to the PV terminals. This circuit configuration enables to reduce the power losses in both boost converter and inverter. This paper also proposes a new cooperative control method between the symmetrically connected boost converter and inverter. The control method can reduce the average switching frequency to 75% of that in a conventional one, resulting in a great reduction in switching power loss. Experimental results show that the proposed circuit improves its European efficiency from 91.6% to 94.5%. [ABSTRACT FROM AUTHOR]

SUMMARY Recently, in Japan, large-scale introduction of photovoltaics (PVs) has been under way. This development increases the likelihood of system voltage increase and fluctuations, which can be prevented by employing PV output prediction. In this study, we focus our attention on applying PV output prediction to system operation with the objective of system voltage control. In this paper, 30-min ahead prediction is assumed according to prior research with prediction error consideration. System voltages of 30 min ahead are estimated and are controlled in advance by using transformer taps and static capacitors. In this paper, voltage control is proposed for coordinating control, which is based on one of the games in the game theory, potential game. The validity of the proposed method is confirmed by running simulations using a modified IEEE 30 bus system. By applying the proposed method, both the voltage deviations and control counts improved considerably compared to a conventional method. [ABSTRACT FROM AUTHOR]

SUMMARY This paper presents an experimental discussion on modular multilevel cascade converters based on double-star chopper cells (MMCC-DSCC). Hereinafter, a single MMCC-DSCC is referred to simply as a 'DSCC'. A couple of DSCCs are used to form a front-to-front (FTF) system capable of dc voltage matching and galvanic isolation between two dc grids. The FTF system can be considered as a dc-ac-dc power conversion system including an ac-link high-power transformer. The higher the ac-link frequency, the smaller and lighter are the ac-link transformer and dc capacitors. When the so-called 'phase-shifted-carrier PWM' is applied to the DSCC, theoretical analysis and computer simulation have confirmed that a ratio of the carrier frequency with respect to the ac-link frequency can be reduced to 5/2. This paper designs, constructs, and tests a 400-Vdc 10-kW downscaled FTF system with a carrier frequency of 450 Hz and an ac-link frequency of 180 Hz, where their ratio is 5/2. Experimental waveforms obtained from the downscaled system are compared with simulated ones obtained from a software package, PSCAD/EMTDC, under the same operating and circuit conditions. They agree well each other not only under steady states but also under transient states. [ABSTRACT FROM AUTHOR]

To study the properties of vegetable oil transformer, the mineral and vegetable insulation oil two transformers with the same parameters are studied using the frequency domain spectroscopy (FDS) method. The complex capacitance and the dielectric loss factor are tested by the insulation diagnostic equipment IDAX300, these parameters can reflect the insulation properties of the transformer. The experimental results show that properties of vegetable oil transformer are different to the mineral. The complex capacitance and the dielectric loss factor tanδ of vegetable oil transformer are higher than those of mineral oil transformer, and the moisture content in vegetable oil is higher than that in mineral oil. The test results are also emphatically analysed, which ultimately laid a theoretical foundation for large-scale operation of vegetable oil transformers. [ABSTRACT FROM AUTHOR]

Power electronic transformer (PET) should have a satisfying low‐voltage ride‐through (LVRT) ability to cope with different kinds and severities of voltage sags. To solve the LVRT issue, this paper proposes to coordinate mode switching control (MSC) and superconducting magnetic energy storage (SMES), and it is to realize the targets of promoting voltage recovery, keeping DC‐link stabilization, and alleviating current inrush. First, the theoretical modeling of a PET incorporating with a SMES unit is implemented, and the PET's fault transient characterizes as well as the SMES's potential supportability is analyzed. Then, the coordination philosophy of the MSC and the SMES is presented. For that the voltage sag reaches a certain threshold, the MSC is enabled to remove the PET's normal control, and the SMES collaboratively operates to lower the DC‐link overvoltage, while avoiding the PET damage. In the process of the coordination control handling the voltage transients, a proper reactive current injection is carried out at the PET's output stage to assist the voltage recovery, and the PET can possess a better LVRT margin. Using MATLAB, a 3 MW PET with an 8 H/1.2 MJ SMES unit is modeled, and different voltage sag scenarios and utilization ways of the MSC and the SMES are simulated. The comparative analysis results show that the proposed approach realizes a satisfying LVRT for the PET. [ABSTRACT FROM AUTHOR]

Multiport converters (MPCs) that integrate multiple converters into a single unit have been proposed to reduce the converter count in electric vehicle power systems where multiple batteries are employed. However, conventional MPCs comprising a dual active bridge (DAB) converter and an interleaved PWM converter require feedback control loops and current sensors to balance the inductor currents. Furthermore, the transformer utilization decreases as the duty cycle of the interleaved PWM converter drifts away from 0.5. This paper proposes a novel MPC that integrates a DAB converter and an interleaved PWM converter with an automatic current balancing capability and improved transformer utilization. A 1‐kW prototype with a half‐bridge or full‐bridge circuit on the high‐voltage side was built for the experimental verification, and loss analysis was performed. The results demonstrated the automatic current balancing capability, improved transformer utilization, and flexible power exchange among three ports. [ABSTRACT FROM AUTHOR]

The bidirectional converter device (BCD) can substitute the substation rectifier and the energy feedback system (EFS) by transforming energy between the AC side and DC side. However, the performance of the railway system with BCDs as only converters has not been fully understood. A simulation approach is required to evaluate the system performance and guide the design. In this paper, the traction power supply system considering the capacity constraint of BCDs is modelled first, and the AC/DC power calculating algorithm is studied. A three‐layer double‐loop parameter designing strategy based on the enhanced brute force is proposed, which considers the N −1 principle. In the case study, the power source model for trains and power constraint for BCDs are verified. The boundary configuration set is obtained. The capacity of BCDs is at least 7 MW, while the slope is 0 and the no‐load voltage is between 1730 and 1750 V. Finally, compared with the system with rectifiers and EFSs, the system with BCDs has better performance, which is 46.7 V less of the rail potential, 127.8 V less of network voltage fluctuation, and 735.56 kWh per hour more of the feedback energy. [ABSTRACT FROM AUTHOR]

The general method for identifying the partial discharge type in a power transformer is based on their fingerprints in the form of phase-resolved discharge patterns. In the case of multiple defects, traditional clustering methods can be applied for separation of active sources. However, such an approach is impractical for online real-time monitoring due to the very large data size. In this paper a new method using stream clustering is introduced. The method separates the active sources by processing the signal once it is captured, then only a synopsis of the discharge data is stored. Two stream clustering algorithms: Density Grids and DenStream are employed. Through measurements obtained from laboratory experimental setups (corona, surface discharge, transformer defect model) performance of the proposed algorithms are evaluated. It is shown that stream clustering method is able to separate the constituent components involved in the stream of a multi-source discharge signal without the need to store a large amount of information. The performance of the Density Grids method depends on a limited number of features that it can accommodate. In comparison, the DenStream method can capture more features which enable better separation of active sources at the expense of longer processing time. [ABSTRACT FROM AUTHOR]