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In this paper, a reduced computational fluid dynamics (CFD) model is proposed for determining the total heat dissipation and the oil temperature distribution of transformer radiators in oil natural air natural (ONAN) cooling mode. In the reduced radiator CFD model, the air flow simulation is replaced with an optimized air heat transfer coefficient (hair) equation. An experimentally verified full radiator CFD model, which includes the air flow simulation, has been introduced as a benchmark model for evaluating the validity of the reduced model. It was found that one ${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$ can provide sufficient accuracy in simulating the air convection effect of transformer radiators. An ${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$ equation was derived through a parametric study using the full radiator CFD model. The performance of the proposed ${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$ equation was compared with the existing empirical equations, and its validities for different insulating liquids and different ambient temperatures were proven by additional CFD modelling. This ${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$ equation can be transformed into a dimensionless form to help the radiator design and facilitate the complete-cooling-loop (CCL) based transformer CFD modelling. [ABSTRACT FROM AUTHOR]

An accelerated loss of life (LOL) of distribution transformers has been observed in recent years owing to the increasing penetration of electric vehicles (EVs). This paper proposes an evolutionary curriculum learning (ECL)-based multi-agent deep reinforcement learning (MADRL) approach for optimizing transformer LOL while considering various charging demands of different EV owners. Specifically, the problem of charging multiple EVs is cast as a Markov game. It is solved by the proposed MADRL algorithm by modeling each EV controller as an agent with a specific objective. During the centralized training stage, a novel centralized ECL mechanism is adopted to enhance the coordination of multiple EVs. It enables the proposed approach to address the management of large-scale EV charging. When the training procedure is completed, the proposed approach is deployed in a decentralized manner. Herein, all the agents make decisions based solely on local information. The decentralized manner of execution helps preserve the privacy of EV owners, reduce the related communication cost, and avoid single-point failure. Comparative tests utilizing real-world data demonstrate that the proposed approach can achieve coordinated charging of a large number of EVs while satisfying the various charging demands of different EV owners. [ABSTRACT FROM AUTHOR]

Ladder network synthesis is very significant for winding fault diagnosis within transformer. To synthesize the network reversely based on the measurable frequency response analysis (FRA) data is the only applicable way. To date the iron core and non-tested winding, which result in the frequency-dependent network components and multiple ladders in some frequency ranges, are usually not considered in the reverse process and thus the synthesis precision is largely limited. This paper proposes to synthesize multiple ladder networks in different frequency regions to handle this problem. Firstly, the sensitivities and frequency-dependent features of network components are analyzed, based on which the topology and components of the networks in diverse frequency ranges are determined afterwards. Further, the network synthesis strategies in different frequency ranges are elaborately designed. Finally, the synthesis method is applied on a distribution transformer successfully, with the fact that the extracted network parameters meet all the constraints and its FRA curve matches well with the measured one. An intact method of multiple-network synthesis for transformer winding in wide frequency range, considering the iron core and non-tested winding, is offered in this paper. [ABSTRACT FROM AUTHOR]

A pulsed power system for generating intense pulsed electric fields (PEFs) in a very large volume of water was designed, manufactured, and tested as a first step toward the proof-of-principle demonstration of a novel noninvasive prepacked food processing operation. The MV-class system described in this article is based on the Tesla transformer technology and is capable of producing PEFs of 100 kV/cm in water, in a volume approaching 1 L. The electric field distribution in the processing water tank was obtained using an electrostatic solver, benchmarked using a Kerr effect arrangement. This article presents the most important experimental data, followed by a detailed electric field analysis and suggestions for the way ahead. [ABSTRACT FROM AUTHOR]

This article presents a novel health management method of the dry-type transformer to diagnose the early unhealthy behavior and evaluate the transformer's health condition by health score. The health condition diagnosis implemented by a proposed dynamic-weighted-feed-back broad learning system (BLS) (DW-FB-BLS) method, which helps to determine the BLS network structure effectively, and adjusts the weight of features in the online application to avoid reduction of accuracy caused by concept drift. Then, a rational score rule is set to evaluate the health condition of the dry-type transformer by health score, which allows intuitive presentation and preservation of transformer's health condition over a long period. Finally, the effectiveness and validity of the proposed method are verified based on the real field data of dry-type transformer. Satisfactory results for unhealthy behavior diagnosis and health evaluation are obtained, it shows that health management of this article can reflect the real health condition of dry-type transformer appropriately. [ABSTRACT FROM AUTHOR]

Solid-state transformers (SSTs) utilize multiple isolated dual-active-bridge (DAB) converters to deliver power from a medium-voltage ac or dc grid to low-voltage dc or ac loads. The DAB converter is a key component of SSTs. This study proposes a novel SST with loosely coupled resonant DABs (LCR-DAB) utilizing loosely coupled inductive power transfer (IPT) coils instead of high-frequency (HF) transformers. Unlike HF transformers, the large air gap between the primary and secondary coils enables easier packaging and high-voltage insulation of the proposed LCR-SST system. Series–series compensated symmetric resonant tanks are selected for the proposed IPT system. The impact of the phase-shift angle and the circuit parameters on the input impedance, efficiency, and power transfer direction of the proposed LCR-DAB is investigated. By performing theoretical analyses, a circuit parameter design method for LCR-DAB is proposed. In addition, a new design approach for low-loss coils of the LCR-DAB is investigated using finite-element analysis results. The proposed LCR-DAB and SST topologies are evaluated using the experimental results. The coil-to-coil and dc-to-dc efficiencies of a single LCR-DAB were 97.4% and 96.7%, respectively, over an air gap of 3 cm. The dc-to-dc efficiency of the three-level LCR-SST was 95.2% at 2.4 kW. [ABSTRACT FROM AUTHOR]

Turn-to-turn faults between primary windings due to insulation degradation are a major cause of distribution transformer failure, and occur due to high levels of stress such as overloading and overheating. An additional consequence of these faults is an increased voltage on the transformer secondary due to effective change in turns ratio. This paper develops a novel method for early detection of insulation degradation and subsequent inter-turn winding failure by monitoring the transformer secondary voltage. The algorithm is based on a cumulative sum (CUSUM) statistic and compares voltages on neighbouring transformers to flag degrading assets. Results obtained from simulation as well as experimental data show that smart meter measurements can be utilized to achieve very high detection accuracy while keeping costs low. The paper also demonstrates the validity of the algorithm in the presence of measurement noise, residential solar power injection etc. [ABSTRACT FROM AUTHOR]

Transformers are the most expensive element of a power system, and so transformer monitoring systems are becoming more important as loads increase. Under heavy loading conditions, the hot-spot temperature of a transformer is the most critical parameter to determine the limit of loading. Therefore, to make an accurate estimation of hot-spot temperature and maintain the reliability of a power system, dynamic thermal models or methods must be employed. Due to the complexity of thermal phenomena, the methods presented so far needs to be improved to reduce computational errors. This is particularly the case for variable loading conditions and power utilities are still in need of good techniques. This paper describes two methods to determine temperature in single-phase, oil-immersed transformers. The first method calculates average windings temperature (AWT) using the voltage and current from primary and secondary windings. The second method, which uses data from the first method, determines the hot-spot temperature of the transformer. The methods have been investigated by experiments on a 30 kVA single-phase transformer and verified the results using COMSOL ™ software. Our methods have proven more accurate than the IEEE Annex G method. [ABSTRACT FROM AUTHOR]

A novel passive interface circuit with gain for nongenerating (active, i.e., excited/driven) sensors in differential or bridge configuration is presented. The circuit is very simple and consists of only one transformer in the output diagonal of the equivalent bridge configuration comprised of sensor(s), bridge completion impedances, and excitation source. It can be used with resistive, capacitive, inductive, eddy-current, and similar sensors for measurement of temperature, displacement, pressure, torque, etc. The gain achieved depends on the transformer voltage ratio, $n$. Unlike the usual transformer application for voltage raise (gain) where the gain is proportional to $n$ , in this circuit, the high gain is achieved with $n$ close to 1. This is due to positive feedback introduced into the circuit. Transformers with voltage ratios close to 1 exhibit much less leakage and smaller losses which can degrade the (ideal) gain. With current on-chip transformers or miniature transformers available on the market, the practical gain can reach 20 dB. The circuit can be easily integrated into various MEMS sensors. As the circuit is entirely passive, its application extends far beyond the usual semiconductor temperature limits. Depending on the transformer technology, the circuit can be used in a very cold (e.g., cryogenic) or very hot environment (e.g., steam and gas turbines, jet engines, rockets, down-hole drilling, etc.) or exposed to nuclear radiation. [ABSTRACT FROM AUTHOR]

Solid-state klystron modulators are typically based on oil-immersed high-voltage (HV) pulse transformers because of their high performance, robustness, simplicity, and straightforward design. However, pulse transformer size is fundamentally linked to application pulse length, pulse power, and pulse rise time. For high-power applications transformer size quickly becomes very problematic when approaching pulse lengths on the order of one millisecond. This article presents a systematic study of the applicability of HV pulse transformers for such long-pulse high-power applications. Both the single-layer and pancake winding techniques are evaluated, keeping reduction of transformer volume as the main design objective. First, design models and efficient optimization procedures are developed. The proposed models are validated through circuit simulation, 3-D finite element analysis, and comparison with a commercial HV long-pulse transformer. Then, the developed optimization procedure is used in studying the evolution of pulse transformer size when pulse length is varied from 500 $\mu \text{s}$ to 5 ms assuming peak pulse power requirements corresponding to that of the European Spallation Source (ESS) klystron modulators (115 kV, 100 A, 14 Hz). Finally, the developed trends are used to derive general analytical equations expressing maximum attainable pulse length as a function of application parameters and system constraints. [ABSTRACT FROM AUTHOR]

This article deals with a new high efficient and cost effective lattice-Rogowski-coil sensor for partial discharge monitoring of power transformers. The sensor is a thin flat-shape printed circuit board type Rogowski-coil, which can be installed on the internal surface of the transformer tank with minimum disturbance to the normal operation of the transformer. Thanks to the accurate and desirable geometry of the sensor, precise monitoring can be carried out. Also, it was designed to handle the common trade-off between the low resonant frequency and high mutual inductance. An experimental setup for measuring the lumped model parameters of the sensor is made and it is shown that the desired resonance frequency of approximately 10 MHz is obtained. In order to evaluate the performance of the sensor, a specially prepared distribution transformer (20 kV/0.4 kV, 500 kVA) was considered. Partial discharge calibration pulses were injected into different locations of the winding of this transformer, and the ability of the sensor was verified in defect localization. Moreover, the effects of some practical aspects, like the value of the terminating resistor of the proposed special Rogowski-coil, its distance from transformer winding as well as the capability of the extracted features from detected PD signals, on the accuracy of PD localization are studied. [ABSTRACT FROM AUTHOR]

A new method for systematically estimating health index, probability of breakdown, and remaining life for power transformers is presented in this article. The method combines three basic models; a physical winding degradation model, a health index model based on condition monitoring data combined with expert judgement, and a statistics-based end-of-life model. The statistics-based model uses data from a database of scrapped transformers under development in Norway. Combining the first two models with the statistics-based model, an individual and condition-dependent probability of breakdown is obtained. From this, the expected remaining life is calculated. Finally, the stochasticity of the method is utilized for optimization of maintenance and replacement. The method hence provides key decision support for transformer managers, enabling them to identify transformers in poor condition, and to follow-up and prioritize transformers for maintenance and replacement. The proposed method has been implemented in a transformer asset management tool for Norwegian utilities. The usefulness of the method is illustrated by applying it to selected transformers from one of these utilities. Finally, important limitations, uncertainties, and further improvements are discussed. [ABSTRACT FROM AUTHOR]

Fuses are commonly used to provide medium voltage (MV) protection to distribution transformers. Unfortunately, there are certain deficiencies introduced when primary-side fuses are used. In paper mill power systems, given the unique organization of the paper mill loads around individual feeders to facilitate load shedding, it may be advantageous to provide transformer differential relay protection from the primary of the transformer to the load side of the main secondary breaker instead of primary fuse protection. This protection can then trip either a primary-side grounding switch (with fuses) or a primary-side feeder circuit breaker using a fiber or a WiMAX (Worldwide Interoperability for Microwave Access) connection. Protection against overload is provided by a secondary main circuit breaker and by design. [ABSTRACT FROM AUTHOR]

The electric power transformer is a vital apparatus in power systems, and failure prognostics is significant for the protection of this asset. In addition to the asset damage, its unexpected failure would interrupt power delivery and jeopardize the stability of the system. There are several fault diagnosis methods introduced for the detection of this kind of fault; however, their functionality is for the postfault condition when the asset is already damaged, and the operation of the system is interrupted. Electric insulation deteriorations make the transformers susceptive to faults due to thermal and electrical stresses. In this article, the impact of early stages insulation deteriorations on the temperature inside the transformer is studied using a finite-element electromagnetic–thermofluid method and based on the observations an online sensor-based decision-making predictive fault diagnosis approach is proposed. Finally, the results are experimentally verified. [ABSTRACT FROM AUTHOR]

This article proposes a new rotary transformer topology enabling reduction of leakage inductances and significant improvement of power transfer capability compared to traditional topologies thanks to its interleaved windings and resonant compensation circuits. The proposed rotary transformer features a nonferrite rotating part made of lightweight strong nonconductive and nonmagnetic composite material and a large air gap, allowing safe operation at high rotational speeds. Hence, this novel rotary transformer can enable the adoption of wound rotor synchronous machines for electric or hybrid vehicle traction. The performance of the proposed new topology was evaluated and compared to the conventional one. Significant performance improvement was confirmed. A 10-kW proof of concept prototypes was built, and the test has validated the power transfer capability with 95.9% efficiency. [ABSTRACT FROM AUTHOR]

The appealing advantages of high-temperature superconducting (HTS) power transformers over conventional ones have attracted transformer manufacturing companies, power companies, research institutes, and universities worldwide to conduct research and development in this field. Unfortunately, HTS transformers are more vulnerable to mechanical stresses than conventional transformers. The results of the interaction between current carrying windings and leakage magnetic fluxes are the electromagnetic forces, which act on transformer windings. Under short circuit events, these forces are remarkable, and, therefore, catastrophic failure of transformer may arise. Flux-diverter applications have been reported in earlier literatures for increasing critical current or decreasing ac losses in HTS transformers. In this paper, a genetic algorithm based method is employed for the optimal design of flux diverter to minimize the axial short circuit force, in design stage of a sample 132/13.8 kV, 50 MVA three-phase core type HTS transformer. In this paper, the optimal dimensions, placement, and permeability of a flux diverter have been determined. It has been shown that utilizing this optimized flux diverter, the axial short circuit forces have been reduced significantly. Electromagnetic modeling and simulations, by the application of finite element method, have been employed for the verification of the analytical method results. A high degree of consistency has been observed between the analytical results and the simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a novel control strategy to significantly reduce the high-frequency transformer loss in a modular solid-state transformer (SST) based converter system. The modular SST design is based on an ac–dc power-electronic building block comprised of an H-bridge at the ac input and a bidirectional soft-switching isolated dc–dc converter with a medium-voltage insulated high-frequency transformer. The ac terminals of input H-bridges are connected in series and directly coupled to the medium-voltage ac line, while the output dc terminals are all connected in parallel at a low voltage. The key innovation described in this paper, without sacrificing the output performance, is the control scheme that reduces core and winding losses associated with the dc–dc resonant stage by turning off the selected dc–dc converters in synchronism with the line voltage. The prototype for the test described in this paper is based on a modular SST where each module is rated at 50 kVA, with a 200 kHz transformer using 1.7 kV silicon carbide discrete mosfets, but the described control concepts are more broadly applicable to similar topologies. [ABSTRACT FROM AUTHOR]

The electric power transformer is a vital apparatus in power systems, and failure prognostics is significant for the protection of this asset. In addition to the asset damage, its unexpected failure would interrupt power delivery and jeopardize the stability of the system. There are several fault diagnosis methods introduced for the detection of this kind of fault; however, their functionality is for the postfault condition when the asset is already damaged, and the operation of the system is interrupted. Electric insulation deteriorations make the transformers susceptive to faults due to thermal and electrical stresses. In this article, the impact of early stages insulation deteriorations on the temperature inside the transformer is studied using a finite-element electromagnetic–thermofluid method and based on the observations an online sensor-based decision-making predictive fault diagnosis approach is proposed. Finally, the results are experimentally verified. [ABSTRACT FROM AUTHOR]

A traction power-supply system (TPSS) is the only power source for electric locomotives; therefore, its safe and consistent operation is of critical importance to reliable, high-speed operation. Electrical faults can disrupt train scheduling and jeopardize the safety and security of high-speed railways. Thus, it is essential to quantitatively evaluate possible risk factors in TPSSs. To do so, based on 24-h train timetable, this paper defines indices to describe the failure risk to traction power-supply equipment, and a separate set of indices present the risk from various power-quality problems. Combining both the equipment and power-quality risks yields a comprehensive risk-assessment for the TPSS, by means of a weighted evaluation. The proposed risk assessment method can ensure informed personnel by communicating the system's risk state in real-time to identify high-risk operation, thereby avoiding the potential for severe accidents and informing maintenance and operational plans. [ABSTRACT FROM AUTHOR]

Dynamic rating monitoring is a solution to face challenges in both the operation and the planning of power systems. With thermal monitoring, dynamic overloads of transformers are allowed; however, the thermal model used to predict hot-spot must be capable of ensuring the assessment of insulation life expectancy. Hence, this paper presents an online algorithm to estimate and predict transformers’ rating in order to assess the overload capability during short and long times. The overload factors are computed using estimations of top-oil temperature. The main characteristic of this proposal is that the environmental cooling conditions are considered by the fitting of the parameters of a simple equivalent thermal circuit using an extended Kalman filter during the transformer operation. To validate the implementation of the algorithm, the information provided by typical transformer rating monitoring systems is used. Thus, the performance and effectiveness of the algorithm are verified using field measurement data during the normal operation of two power transformers. [ABSTRACT FROM AUTHOR]

In order to dig out more potential of design parameters besides nameplate information, a set of pre-physical modeling methods was proposed and integrated into the dynamic thermal modeling procedure, aiming to optimize the key thermal parameters individually. In this method, a global oil momentum model was created considering the flow resistance, and for the problem of heat transfer between windings, oil flow, and radiator, a whole oil flow thermal model was established based on heat transfer principle and energy conservation. Then, the double logarithmic linear regression was applied to quantify the value of the exponents. According to the overload heat run tests, the temperature calculated by the existing models during multiple overloads is obviously different from the measured temperature, while the improved dynamic thermal modeling procedure with optimized thermal parameters attains estimates more accurately. [ABSTRACT FROM AUTHOR]

Transformer pressure relief valve (PRV) false trips resulting from external short-circuit (SC) faults can have serious consequences and lead to enormous economic losses. In our work, experimental and numerical methods were employed to investigate the cause of these malfunctions. External SC faults were performed on a real full-scale 50 MVA/110 kV transformer, and the oil pressure characteristics inside the tank were measured and analyzed. Moreover, a sequential multi-physics coupling analysis method was proposed. Using the finite-element method, the winding displacements and oil pressure behaviors at the PRV were determined. Experimental and theoretical studies revealed that serious external SC faults can lead to sustained vibrations in the windings and create significant oil pressure fluctuations that may further result in PRV malfunctions due to the abnormally high valve pressure. [ABSTRACT FROM AUTHOR]

Mineral oil is traditionally used in liquid immersed transformers to act as a coolant, an information carrier, and as an electrical insulator. Emerging alternative transformer liquids provide advantages, such as improved fire safety and better biodegradability, which transformer operators would like to utilize. In this paper, an experimental study is conducted to compare the thermal performance of a mineral hydrocarbon transformer oil, a gas-to-liquid hydrocarbon transformer oil, and a synthetic ester transformer liquid as coolants in a zig-zag disc type winding model. Comparisons are made under liquid directed cooling modes and under liquid natural cooling modes. It was found that under both cooling modes, the mineral based transformer oil and the gas-to-liquid based transformer oil behaved almost with comparable liquid flow and temperature distributions. Under liquid directed cooling modes, the synthetic ester gave more uniform flow distribution and delayed the occurrence of liquid reverse flow compared to the other oils. Under liquid natural cooling modes and using the zig-zag disc type winding model, synthetic ester, due to its higher viscosity which caused lower inlet flow rate to develop under the specific tested retrofilling conditions, caused less uniform oil flow distribution within the pass and higher hot spot temperature. [ABSTRACT FROM AUTHOR]

Even though multipulse rectifiers (MRs) are a long established and well-known technology, still their behavior is not fully described in the literature when they are fed by three-phase balanced sinusoidal currents sources. To address the aforementioned gap, this paper presents the operation and properties of current-fed MRs. The undertaken aim is achieved by analyzing the examined topology through circuit analysis and then, the theoretical results are validated through comparisons with the simulated waveforms and experimental results. Furthermore, the expected harmonic content and the duality with traditional voltage-fed MRs (VFMRs) are presented. In the proposed structure, the transformer voltages present a multipulse waveform, instead of its primary currents as in VFMRs. This implies on limiting the voltage steps and its derivative, which might be beneficial to reduce cost and volume of insulation, particularly for MVDC and HVdc applications. Besides that, by actively controlling the primary currents, a possible copper loss reduction is shown in the transformer windings, differentiating the proposed structure from its voltage controlled counterpart. [ABSTRACT FROM AUTHOR]

Residential transformer population is a critical type of asset that many electric utility companies have been attempting to manage proactively and effectively to reduce unexpected transformer failures and life loss that are often caused by overloading. Within the typical power asset portfolio, the residential transformer asset is often large in population, has the lowest reliability design, lacks transformer loading data, and is susceptible to customer loading behaviors, such as adoption of distributed energy resources and electric vehicles. On the bright side, the availability of more residential service operation data along with the advancement of data analytics techniques has provided a new path to further our understanding of residential transformer overloading risk statistically. This paper develops a new data-driven method that combines transformer temperature rise and insulation life loss simulation model with clustering analysis technique. It quantitatively and statistically assesses the overloading risk of residential transformer population in one area and suggests proper risk management measures according to the assessment results. Multiple application examples for a Canadian utility company have been presented and discussed in detail to demonstrate the applicability and usefulness of the proposed method. [ABSTRACT FROM AUTHOR]

A technology scheme that replaces Marx generator with Tesla transformer was put forward to improve the reliability of the pulsed power system. In order to validate the technology scheme, a Tesla transformer was designed and tested. The excitation of the primary consists of two 4- $\mu \text{F}$ polypropylene film capacitors which are separated by a three-electrode field distortion gas switch. A deionized water dielectric pulse forming line (PFL), with 13-nF equivalent capacitance, acts as the secondary capacitor. The actual transformation ratio is 9.7 approximately, and the PFL voltage is as high as 1.2 MV when two capacitors are charged to ±62 kV by the bipolar high-voltage direct current. The prefire probability of switch is described by the Weibull distribution and the working ratio of the switch is elaborately chosen so as to balance the prefire probability and the trigger jitter. For ±62 kV charge voltage and 0.2-MPa pressured air dielectric, the prefire probability of switch is only $1 \times 10^{-7}$ and the jitter of that is 5 ns. When confidence is 0.90, the failure probability of pulsed power machine is $2.8 \times 10^{-4}$ according to the Binomial distribution correspondingly. [ABSTRACT FROM AUTHOR]

A modeling methodology of high-voltage pulse transformers based on high-order generalized equivalent circuits (HOGECs) identified from 3-D finite-element analysis is applied to a prototype, and experimental validation tests are performed. The identified HOGEC of the prototype is used to investigate the influence of the detailed mechanical structure and the local winding configuration on the pulse transformer operation performance. [ABSTRACT FROM AUTHOR]

The purpose of dissolved gas analysis (DGA) in utilities is to help detect the presence of abnormality within transformer. When faults occur in transformers, transformer engineers need to determine the location and risk of the faults. Ultimately, they have to decide, based on DGA, whether to continue operating or not, perform internal inspection, or dispose the transformer. In this study, the fault and failure types in the transformer are suggested to determine the location and risk of the faults. In particular, turn-to-turn insulation faults are classified as degradation and breakdown. These faults are difficult to identify during internal inspection, and have a high possibility of failure. Urgent decision and action are thus required to avoid failure. In degradation of turn-to-turn insulation faults, failures may occur by generating thermal gases in paper during a long period of time. In breakdown of turn-to-turn insulation faults, thermal gases are not generated in paper, and failures are rather due to sudden breakdown of insulation. This study also presents a typical example of a turn-to-turn insulation fault. This example shows the progress of the fault from thermal to discharge, which is common phenomena in winding fault. Based on the findings of this work, transformer engineers can determine by DGA if transformers can be operated with or without internal inspection, or disposed when the fault has not been identified during internal inspection. [ABSTRACT FROM AUTHOR]

A 160-MVA, 220/132/33-kV autotransformer is commonly used as an intermediate connecting transformer (ICT) in an electrical power transmission network. The modified design of such an ICT in order to get a phase angle control along with the voltage control, by intermediate voltage variation, is reported in this paper. Estimation of stray loss and core loss is done both by analytically and realizing 2-D and 3-D models by a finite-element analysis (FEA) method. A 3-D FEA design is further extended with arrangement of shunts to limit the stray loss and the thermal impact. Multiphysics simulation platform is used to link magnetostatic, steady-state thermal and structural FEA models. The composite FEA simulation approach used in this study enables steady-state thermal analysis of a transformer by importing stray loss from the magnetostatic model and the structural deformation in the transformer due to hot-spot temperature. To validate the proposed design, a 1-kVA, 415/230-V, three-phase transformer is designed, built, and tested in the laboratory. Voltage and phase angle at various operating conditions calculated analytically, obtained through FEA and measured experimentally, are in close agreement. [ABSTRACT FROM PUBLISHER]

In this contribution, the oil speed in horizontal channels of an oil-directed cooled winding is investigated by experimental and numerical methods. The presented winding model offers insight into the horizontal cooling channels perpendicular to the main oil-flow direction. To visualize oil flow, tracing particles were added to the cooling oil and illuminated by high-power light-emitting diodes. The particle velocities were then determined by taking photographs with a defined exposure time. The design of the sophisticated winding model is described in the contribution. In addition to the experimental results, this contribution presents a comparison with respective numerical results from 2-D and 3-D computational fluid dynamics calculations. Finally, numerical results from the winding model concerning the oil-flow distribution inside the winding at various operating conditions are presented. The investigation indicated a strong nonuniform oil-flow distribution on the horizontal channels. The presented results give a deep insight into the oil flow and temperature behavior of windings, enabling the designer to optimize the cooling of the power transformer windings. [ABSTRACT FROM AUTHOR]

Frequency response analysis (FRA) has recently been developed as a widely accepted tool for power transformer winding mechanical deformation diagnosis, and has proven to be effective and powerful in many cases. However, there still exist problems regarding the application of FRA. FRA is a comparative method in which the measured FRA signature should be compared with its fingerprint. Small differences of FRA signatures in certain frequency bands might be produced by external disturbance, which hinders fault diagnosis. Additionally, the existing correlation coefficient indicator recommended by power industry standards cannot reflect key information of signatures, namely the extreme points. This paper proposes an improved FRA based on binary morphology and extreme point variation. Binary morphology is first introduced to extract the certain frequency bands of signatures with significant difference. A composite indicator of extreme point variation is adopted to realize the diagnosis of fault level. A ternary diagram is constructed by the area proportions of the binary image to identify winding faults, which has a potential to realize cluster analysis of fault types. [ABSTRACT FROM AUTHOR]

This paper proposes an interleaved-stage ac–dc bidirectional power converter topology feasible to medium-voltage railway systems. It is based on an arrangement that uses input-series-output-parallel configuration with identical power modules. Each module consists of two interleaved legs employing coupled windings so that it is possible to connect a medium-frequency transformer. The topology conception is presented, followed by the adopted control and modulation technique. An estimation of semiconductor losses is also discussed, while a comparative analysis by means of simulation results validates the improved performance of the introduced approach if compared with similar solutions. The topology results are obtainedt through a medium-voltage high-power railway application, demonstrating the converter functionalities and performance under both steady-state and dynamic conditions. It is then possible to state that good performance in terms of efficiency is achieved even though the power stage presents higher component count if compared with the topology that presents the best result, thus making it suitable to high-power ac–dc machine drive applications. [ABSTRACT FROM AUTHOR]

The cost of rare earth (RE) permanent magnet along with the associated supply volatility have intensified the interests for machine topologies, which eliminate or reduce the RE magnets usage. This paper presents one such design solution, the separately excited synchronous motor (SESM) that eliminates RE magnets, but does not sacrifice the peak torque and power of the motor. The major drawback of such motors is the necessity of brushes to supply the field current. This is especially a challenge for hybrid or electric vehicle applications where the machine is actively cooled with oil inside the transmission. Sealing the brushes from the oil is challenging and would limit the application of such a motor inside a transmission. To overcome this problem, a contactless rotary transformer is designed and implemented for the rotor field excitation. The designed motor is built and tested. The test data show that the designed motor outperforms an equivalent interior permanent magnet (IPM) motor, which is optimized for a hybrid application for both peak torque and power. Better drive system efficiency is measured at high speed compared with the IPM machine, while the latter outperforms (for efficiency) the SESM at low- and medium-speed range. [ABSTRACT FROM AUTHOR]

The assessment of two vegetal oils as coolant in Low Voltage Winding of a power transformer with zigzag cooling have been analyzed. These dielectric fluids cooling performance has been compared with a typical mineral oil. To make the study, a 2D-axisymmetrical model of a power transformer has been developed to perform a numerical analysis using a Finite Element Method based software, COMSOL Multiphysics?. Some values are obtained in order to establish the comparison, such as hot-spot temperature or hot-spot factor. Moreover, the influence of the increase of the number of passes of the cooling circuit on the hot-spot temperature has been evaluated for all liquids and compared with the initial design. Results obtained in this work show that the hot-spot temperature is lower for the vegetal oils in the initial design. Furthermore, an increase in the number of passes affect more positively to the mineral oil since similar values of the hot-spot temperature for all liquids are obtained. Values of the hot-spot factor indicates that higher number of passes leads to lower efficient cooling circuits owing to the increase of the pressure drop although the hot-spot temperature decreases. [ABSTRACT FROM PUBLISHER]

In this paper, the design procedure of a 14.4 kV output voltage, 100 kHz transformer with an isolation voltage of 115 kV using Litz wire is presented. All design models, including a generalized magnetic model for the leakage and the loss calculations as well as an electrical model for the parasitic capacitance estimation for the transformer are derived and proven by measurements. For designing the insulation, a comprehensive design method based on an analytical maximum electrical field evaluation and an electrical field conform design is used. The resulting design is verified by long and short term partial discharge measurements on a prototype transformer. [ABSTRACT FROM AUTHOR]

This paper presents the analysis and modeling of the non-minimum phase effects that can occur in the low-frequency response of transformers. This effect is highlighted when the phase response has angles outside the range –90° to +90° and the typical response shape, which consists of two resonance points for the lateral phases and one for the central phase, is not found. Analysis based on physical effects, particularly the grounding capacitances and their association with the response, is presented. A model is proposed for characterizing and reproducing transformer low-frequency response, including the non-minimum phase effect. Actual frequency response measurements were obtained experimentally to assess the modeling. [ABSTRACT FROM AUTHOR]

While frequency response analysis (FRA) technique has been successfully used to assess the mechanical integrity of active parts within power transformers, it still exhibits some drawbacks including its inability to detect incipient and minor winding deformations and the requirement for an expert to analyze the results. Although several papers have investigated the impact of various faults on the transformer FRA signature, no attempt was made to automate and improve the fault detection accuracy of the current technique. The main contribution of this paper is the presentation of a new approach for the FRA technique through incorporating the magnitude and phase angle plots that can be measured using any commercial frequency response analyzer into one polar plot. Contrary to the current industry practice that only relies on the magnitude of the measured FRA signature for fault identification and quantification, the proposed polar plot that comprises more features than the magnitude plot will facilitate the use of digital image processing (DIP) techniques to improve the detection accuracy, standardize, and automate the FRA interpretation process. In this regard, three-dimensional models for two three-phase power transformers of different ratings, sizes, and windings structures are modeled using a finite element analysis technique to simulate various levels of axial displacement (AD) and disk space variation (DSV) at different locations of the transformer windings. Impact of minor fault levels on the proposed polar plot signature is investigated through the application of various DIP techniques. Simulation results are validated through practical measurements on a scaled-down transformer. Results show that the proposed polar plot along with the DIP technique is able to detect minor fault levels of AD and DSV with high accuracy. [ABSTRACT FROM PUBLISHER]

The power transformer heating must be monitored for avoiding fast ageing of insulation of impregnated paper and, consequently, transformer’s premature depreciation. This paper describes the development and the implementation of a thermal model able to calculate the internal temperatures in power transformers using numerical simulations to obtain an online model for estimating hotspot temperatures. In order to validate the proposed procedure, a comparison between numerical results and direct measurement in the power transformer is presented. [ABSTRACT FROM AUTHOR]

Improving the operation of power grid and offering new smart applications, the deployment of a supervisory infrastructure is necessary. One solution to transmit data may be based on the power-line communication (PLC) technology. It consists of the superposition of high-frequency signals with the electrical signal of 50/60 Hz. However, the power grids have not been designed to operate in high frequencies. Therefore, they provide a transmission channel with difficult propagation conditions. This paper deals with the modeling of the medium-voltage/low-voltage (MV/LV) power transformers in PLC frequencies. The proposed models are based on the “lumped model” and “black box model.” They are performed in the frequency band from 1 kHz to 1 MHz on SPICE and MATLAB software, respectively. These models are based on impedance measurements and validated by experimental data. The advantages and drawbacks of each model are detailed from the presentation of the modeling method and simulation results. The comparison of the simulation results show that “black box model” offers good accuracy. The transmission results show that transformers are an important element in PLC studies because they provide significant losses. Moreover, these losses depend mainly on the values of the impedance terminals of the MV/LV transformer. [ABSTRACT FROM PUBLISHER]

This paper investigates the performance of a recently proposed online transformer internal fault detection technique and examines the impact of harmonics through detailed nonlinear simulation of a transformer using 3-D finite-element modelling. The proposed online technique is based on considering the correlation between the instantaneous input and output voltage difference (\Delta \bf V) and the input current of a particular phase as a fingerprint of the transformer that could be measured every cycle to identify any incipient mechanical deformation within power transformers. To precisely emulate real transformer operation under various winding mechanical deformations, a detailed 3-D finite-element model is developed. Detailed simulations with (non)sinusoidal excitation are performed and analyzed to demonstrate the unique impact of each fault on the \Delta \bf V\-\bf I locus. Impacts on harmonic order, magnitude, and phase angle are also investigated. Furthermore, practical measurements have been performed to validate the effect of a winding short-circuit fault on the proposed \Delta \bf V\-\bf I locus without and with the impact of system harmonics. [ABSTRACT FROM PUBLISHER]

The state of a transformer depends on many parameters, which should be taken into account to determine its actual condition. In recent years what is known as health indices of transformers has been defined, which is a practical tool to assess the overall status of these electrical devices. Data from field inspection, laboratory tests, and observations during operation of the transformer are combined. All of these result in an index that provides fairly objective and quantitative information as to the condition of the transformer [1]. [ABSTRACT FROM AUTHOR]

Frequency Response Analysis (FRA) technique is commonly used to assess the mechanical integrity of power transformer core and windings. A few papers investigating the ability of FRA technique to detect bushing faults and transformer oil degradation can be found in the literatures. However, in all of these papers, only magnitude of the FRA signature along with visual analysis was used for fault identification and quantification which may lead to inconsistent conclusions for the same FRA signature when interpreted by various personnel. As such, there is an essential need to standardize and automate the FRA interpretation process to improve its reliability and accuracy. This paper is taking a step forward toward this goal through presenting a new FRA interpretation approach by incorporating the magnitude and phase angle plots of the measured FRA signature into one polar plot that comprises more features than the conventional magnitude plot currently used for fault diagnosis. Moreover, the proposed polar plot facilitates the application of digital image processing techniques (DIP) to automate and standardize the whole process. The new proposed technique is assessed through its application to detect minor transformer bushing faults and insulating oil degradations. In this regard, two transformers of different ratings, winding structure and physical dimensions are simulated using 3D finite element analysis to implement various levels of transformer bushing faults and oil degradation. The obtained FRA polar plots of the two investigated transformers under various bushings and oil health conditions are manipulated using the developed DIP codes to investigate the impact of each fault type / level on the proposed polar plot signature. Also, Practical measurements are performed to validate simulation results and evaluate the feasibility of the proposed technique. Results show the ability of the proposed technique to automate the detection of minor levels of bushing faults and insulating oil degradation with a high degree of accuracy. [ABSTRACT FROM AUTHOR]

HVDC transmission has been adopted widely for its large energy transport capacity, high stability and more economical in long-distance power transmission. Converter transformer is the key equipment in HVDC transmission system. The insulation of converter transformer valve side endures a composite voltage including AC, DC and pulsating component etc. Partial discharge (PD) occurring in a void surrounded by insulating material is believed to be one of the main causes of insulation breakdown and it is the main factor of electrical aging of materials. Therefore, it is important to recognize the prebreakdown phenomena and to evaluate the lifetime of insulating materials. Much research has been carried out to study the partial discharge characteristics under pure AC or DC voltage. Only a few investigations were focused on composite voltage. In order to obtain more understanding of PD characteristics under composite voltage, the PD detection and analysis of oil-pressboard insulation under pulsating DC voltage waveform are studied in this paper. The voltage waveform of converter transformer is analyzed through computer simulation. A 6-pulse DC voltage generator and PD detection system are set up. Influence of the DC component and pulsating component to the PD characteristics is studied in lab test. The experimental results show the DC component and pulsating component affect the PD inception voltage, but the pulsating component has more significant influence to PD characteristics than DC component. The study provides an experimental foundation for the converter transformer insulation breakdown mechanism analysis. [ABSTRACT FROM AUTHOR]

Current solutions for voltage regulation close to the consumers usually require technicians for manually changing the taps of the transformer, which may entail high maintenance costs and uncontrolled voltage levels. In this context, electronic on-load tap changers (OLTCs) are a prospective solution for present-day and future power grids, since they provide automatic voltage regulation. This paper presents the analysis and design guidelines for an electronic OLTC applied to a distribution transformer, encompassing the physical layout design for external access to the transformer tap terminals, the evaluation of current and voltage transients during the commutation process, and the dimensioning of the electronic switches and protection system. The dielectric withstand of the proposed physical layout is also analyzed through the finite-element approach, guaranteeing adequate insulation for the electronic circuits located outside the transformer tank, which facilitates circuitry maintenance and provides high modularity. The electronic switches and the protection circuit of the electronic OLTC are experimentally tested under normal and faulty operational conditions in a 5 kVA, 7.69 kV/220 V, single-phase earth return distribution transformer. [ABSTRACT FROM PUBLISHER]

Increased dc injection from grid-connected power converters and an increased dc-bias sensitivity of recent distribution transformers require unprecedented precision in detecting and suppressing the dc bias in 0.4-kV ac grids. In this paper, an optimum design of the dc-bias sensing reactor is devised, along with a robust, simple-to-use sensing algorithm that does not require calibration or tuning. The sample sensors are used as feedback devices within an experimental setup comprising a three-phase grid-connected inverter and a novel dc-bias suppression algorithm. The steady-state dc bias is suppressed below 65\, \upmuV, the level acceptable even with the most sensitive distribution transformers. [ABSTRACT FROM PUBLISHER]

It is a general belief that mineral oil filled power transformers are designed to operate reliably for up to 40 years. However, there are many transformers installed worldwide operating well beyond such age. Therefore, transformer end-of-life should not be simply indicated by the designed 40 years, but determined by the insulation system end-of-life. [ABSTRACT FROM PUBLISHER]

Although a frequency response analysis (FRA) technique has been extensively used to detect mechanical deformation within power transformers, interpretation of FRA signature still needs a high level of expertise to identify and quantify faults as there is no FRA interpretation code widely accepted yet. All commercial frequency response analyzers can measure the magnitude and phase angle of the impedance, admittance, or transfer function of each phase in a wide frequency range; however, only magnitude is currently used for fault identification and quantification. This paper presents a novel approach for FRA signature interpretation by incorporating the FRA magnitude and phase plots into one polar plot that captures more features of the measured signal than the magnitude plot. Digital image processing-based techniques are employed to automate the fault identification and quantification process. To investigate the impact of transformer rating and size on the proposed technique, two transformers of different ratings and physical geometrical dimensions are simulated using 3-D finite-element analysis to emulate transformer real operation. Short-circuit of various fault levels is simulated at various locations within the high-voltage and low-voltage windings of the two transformer models and the obtained FRA polar plot signature for each case study is analyzed and compared with the healthy signature. Also, practical measurement is conducted to validate the simulation results. Results show that fault level along with fault location can be easily identified using the proposed polar plot signature along with the developed digital image processing technique. [ABSTRACT FROM PUBLISHER]

Transformers are the largest, most expensive, and highly critical components of most utility substations. In order to ensure a long, useful service life, it is critical that a power transformer and its ancillary components are tested regularly for incipient fault modes. [ABSTRACT FROM PUBLISHER]

Smart grid solutions with enabling technologies such as energy management systems (EMSs) and smart meters promote the vision of smart households, which also allows for active demand side in the residential sector. These technologies enable the control of residential consumption, local small-scale generation, and energy storage systems to respond to time-varying prices. However, shifting loads simultaneously to lower price periods is likely to put extra stress on distribution system assets such as distribution transformers. Especially, additional new types of loads/appliances such as electric vehicles (EVs) can introduce even more burden on the operation of these assets, which is an issue that needs special attention. Such extra stress can cause accelerated aging of distribution system assets and significantly affect the reliability of the system. In this paper, the impact of a smart neighborhood load on distribution transformer aging is investigated. The EMS of each household is designed to respond to prices and other signals emitted by the responsive load serving entity within the relevant demand response strategy. An optimization framework based on mixed-integer linear programming is presented in order to define the EMS structure. Then, the equivalent aging of the distribution transformer is examined with a thermal model under different scenarios. The case studies that are presented indicate that the integration of EVs in residential premises may indeed cause accelerated aging of the distribution transformers, while the need to investigate the efficiency of dynamic pricing mechanisms is rendered evident. [ABSTRACT FROM PUBLISHER]