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Most power transformers operating in a power system possess oil-paper insulation. A serious defect of this type of insulation, which is associated with long operation time, is an increase in the moisture content. Moisture introduces a number of threats to proper operation of the transformer, e.g., ignition of partial discharges (PDs). Due to the varying temperature of the insulation system during the unit’s normal operation, a dynamic change (migration of water) takes place, precipitating the oil-paper system from a state of hydrodynamic equilibrium. This causes the PDs to be variable in time, and they may intensify or extinguish. Studies on model objects have been conducted to determine the conditions (temperature, humidity, time) that will have an impact on the ignition and intensity of the observed phenomenon of PDs. The conclusions of this study will have a practical application in the evaluation of measurements conducted in the field, especially in relation to the registration of an online PD monitoring system.

Kraft paper in combination with dielectric oil is the most common as insulation system used in power transformers. The most used oil in power transformers is mineral oil. However, dielectric oils based on natural esters possess some advantages in comparison with mineral oil such as higher biodegradability, fire safety and availability. Therefore, they might be the most ideal substitute for mineral oil. The introduction of a new material requires the evaluation of its degradation rate because this determines transformers’ life span. In order to assess the performance of new insulating systems based on vegetable oils, simulations to obtain the hot-spot temperatures in power transformers using vegetable oils and controlled laboratory experiments of ageing were carried out. The reason is that hot spot temperature inside windings of the oil-immersed power transformers is one of the main manifestations of the thermal stress which leads to aging of insulation systems. The research leading to these results has received funding from multiple sources during years but we would specifically like to acknowledge the support received in the later stages from the Spanish Plan Estatal de I+D under the grant agreement DPI2013- 43897-P.

This paper deals with a low-cost method for the assembly of flexible substrate antennas and UHF RF identification silicon (Si) chips. Such a method exploits a magnetic coupling mechanism, thus not requiring for galvanic contacts between the Si chip and antenna itself. The magnetic coupling is established by a planar transformer, the primary and secondary windings of which are implemented on flexible substrate and Si chip, respectively. As a result, the Si chip can be assembled on the antenna with a mere placing and gluing process. First, the idea has been validated by theory. Electromagnetic simulations of a square heterogeneous transformer (1.0-mm side) show a maximum available power gain (MAG) of -0.4 dB at 868 MHz. In addition, the heterogeneous transformer is also quite tolerant with respect to misalignment between primary and secondary. An offset error of 150 μm reduces the MAG to - 0.5 dB. A sub-optimal matching strategy, exploiting a simple on-chip capacitor, is then developed for antennas with 50- Ω input impedances. Finally, the idea has been experimentally validated exploiting printed circuit board (PCB) prototypes. A PCB transformer (1.5-mm side) and a transformer rectifier (two-diode Dickson multiplier) have been fabricated and tested. Measurements indicates a MAG of -0.3 dB at 868 MHz for the transformer and the capability of the developed rectifier to supply a 220-kΩ load at 1.5 V with a - 2-dBm input power.

The adoption of prognostics for critical assets has the potential to advance asset management in the power industry significantly. Whereas diagnostic techniques can identify the presence of incipient faults, prognostics aims to predict the future state of a given asset [1], [2]. Prognostics can therefore be used to estimate the remaining useful life (RUL) of the asset, and help plan maintenance while minimizing the risk of failure in service. Prognostics requires a good model of the process of deterioration, from inception through to failure [1]–[3]. Deterioration may be due to aging, as in the case of paper insulation, or it may be due to a fault. Regardless of the cause of deterioration, prognostics is useful only if the deterioration is slow enough that maintenance (repair or replacement) can be scheduled during the predicted RUL. Thus, prognostics is not superior to diagnostics if the deterioration is so rapid that failure cannot be prevented.

Careful monitoring of high voltage equipments and diagnosing the critical conditions before they lead to a disaster are prerequisites of condition-based maintenance. Moisture content in a transformer is regarded as one of the major factors in diagnosing its conditions. It causes many problems for a power transformer including electrical breakdown between either its windings or one winding with neutral, increase in the amount of partial discharge and sundry minor problems. Since paper insulation of a power transformer carries large portion of water content, determining moisture content in this part of the transformer is essential. However, the problem is that the direct measurement of moisture in paper is impossible. Therefore, various methods have been proposed to measure the moisture content in a transformer but each one has its limitations. In this study, an approach is introduced to measure water content in a transformer by analysing the moisture dynamics in oil, tracking its variations and analysis of parameters such as temperature, without necessity of disconnecting the transformer from the power grid.

La estimación de la pérdida de vida es útil para la gestión de transformadores de potencia. Un método, no invasivo, es estimar la edad funcional del papel aislante del transformador, mediante las guías de carga. Para esto, el perfil de temperatura del punto caliente es calculado a partir de características técnicas del transformador, los perfiles carga y temperatura ambiente y un conjunto de ecuaciones diferenciales. En la práctica, la información disponible para este análisis es incompleta. En este artículo se presenta un método para estimar la carga histórica y los perfiles de temperatura ambiente experimentados por el transformador, cuando existe falta de datos. Para este fin, el método emplea una red neuronal artificial y simulaciones de Monte Carlo. El método es aplicado a un transformador de potencia de 30 MVA. Los resultados obtenidos son analizados en una sección de validación para finalmente dar las conclusiones del trabajo. A non-invasive method useful for asset management is to estimate the functional age of the insulating paper of the transformer that is caused by thermal aging. For this purpose, the hot-spot temperature profile must be assessed by means of some transformer characteristics, the historical load, ambient temperature profiles and a set of equations. In many in-service unit cases, the available data is incomplete. This paper proposes a method to deal with the lack of data. The method is based on the estimation of the historical load and ambient temperature profiles by using an artificial neural network and Monte Carlo simulations. The probable loss of total life percentage of a 30 MVA power transformer is obtained through the proposed method. Finally, the corresponding results for the assessed transformer, a model validation section and conclusions are presented.

Power transformers are one of the most critical components of power systems, and failures often result in significant economic loss from the interruption of power and damage of assets. The establishment of maintenance programs that signal abnormal conditions in transformers is essential for the reliable operation of these equipments. However, in a competitive market, it is important to maximize investments by depleting the remaining life of the equipments before replacing them. The end of life of a transformer comes about by aging of its solid insulation, which is mainly Kraft paper, pressboard, and wood, and these materials age mainly by hydrolysis reactions. Although transformer lifetimes are considered to be about 30 years, experience shows that some transformers remain in service for more than 50 years [1]. Aging of cellulosic insulation depends mainly on operating temperature, moisture, oxygen, and acids. The value of these factors depends on the operating conditions and maintenance practices, which are not always accurately quantified. Having a method to estimate the remaining life of a transformer is essential for owners to assess the risk of keeping a transformer in service or the necessity of replacing it. Unfortunately, estimating the remaining life of transformer insulation is not an easy task because it is not possible to extract paper samples from the active part of the transformer to make direct determinations, and estimations are always based on indirect estimations. On the other hand, the transformer temperature is not homogeneous, so the condition of the paper varies within the transformer, which makes the analysis even more complex. The determination of the furanic compounds content in the transformer oil is one of the more valuable methods to diagnose the aging condition of transformer solid insulation [2]. Unlike other markers, for example CO or CO2, which are produced during the degradation of cellulosic insulation but also mineral oil, furanic compounds are exclusively generated as a by-product of the aging reactions of only cellulose insulation, and that makes the diagnosis more reliable.

On load tap changing (OLTC) transformer has become a vital link in modern power systems. It acts to maintain the load bus voltage within its permissible limits despite any load changes. This paper discusses the effect of different static loads namely; constant power (CP), constant current (CI) and constant impedance (CZ) on the maximum power transfer limit from the generation to the load centre through the OLTC branch and in turn on the static stability margin of power systems. Then the paper introduces a novel approach for the on-line determination of the OLTC settings using artificial neural network (ANN) technique in order to improve the power transfer capability of transmission systems. The proposed approach is tested on a sixbus IEEE system. Numerical results show that the setting of OLTC transformer in terms of the load model has a major effect on the maximum power transfer in power systems and the proposed ANN technique is very accurate and reliable. The adaptive settings of OLTC improve the power transfer capability according to the system operating condition. Keywords: OLTC, Static Load, Voltage Stability, ANN.

The work presented in this paper deals with the investigation of circuit breaker prestrike effect that occurs during energizing a distribution transformer. An experimental test setup that consists of a supply transformer, a vacuum circuit breaker (VCB), a cable and a test transformer is built, and the prestrikes in the VCB are recorded. The test transformer is a prototype distribution transformer, with installed measuring points along transformer windings in each phase. Voltage oscillations are measured along the windings and transformer terminals. The transformer is modeled by lumped parameters extracted from telegrapher's equations in discrete form. Voltage oscillations during switching-in operations are recorded and calculated with and without a cable installed between the VCB and the transformer. Computed voltages show good agreement with the measured voltages. Described method can be used by transformer manufacturers to estimate voltage wave forms during switching or lightning, to provide useful information for insulation coordination studies, and to investigate resonance effects in transformer windings.

The extraction of winding resistance from impedance measurements needs a compensation of undesirable effects, e.g. core loss and distributed winding capacitance. This paper rigorously shows that the core loss (or core-loss resistance) measured with the two-winding method always includes the effect of the winding mutual resistance. At high frequencies, this effect becomes more prominent and can cause an overestimation of the measured core-loss resistance. As a result, the compensated winding resistances can be significantly underestimated. To mitigate this effect, this paper proposes to measure the core-loss resistance on an auxiliary 1:1 transformer with single-turn windings. Consequently, it is scaled to obtain the actual core loss. The proposed analysis and method is applicable to multiwinding systems. For validation, this paper considers a gapped transformer with litz-wire winding for high-frequency operations. The experimental results are validated against the results from its three-dimensional finite-element (FE) model. The litz-wire winding is considered in the FE model by means of a homogenization approach. With the method proposed in this paper, the experimentally extracted winding resistances become more accurate and are in good agreement with the FE results., SCOPUS: ar.j, info:eu-repo/semantics/published

This paper proposes a new high-efficiency PV converter for grid connection through a high-leg delta transformer. The converter 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. As a result, this circuit configuration enables to reduce the power losses in both boost converter and inverters. 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 the switching power loss. Experimental results confirm that the proposed circuit configuration makes it possible to improve its European efficiency from 91.6% to 94.5%.

Transformers are one of the most important components of the power system. It is important to maintain and assess the condition. Transformer lifetime depends on the life of its insulation and insulation life is also strongly influenced by moisture in the insulation. Due to importance of this issue, in this paper a new method is introduced for determining the moisture content of the transformer insulation system using dielectric response analysis in the frequency domain based on artificial bee colony algorithm. First, the master curve of dielectric response is modeled. Then, using proposed method the master curve and the measured dielectric response curves are compared. By analyzing the results of the comparison, the moisture content of paper insulation, electrical conductivity of the insulating oil and dielectric model dimensions are determined. Finally, the proposed method is applied to several practical samples to demonstrate its capabilities compared with the well-known conventional method.

The Ironless Inductive Position Sensor (I2PS) has been introduced as a valid alternative to Linear Variable Differential Transformers (LVDTs) when external magnetic fields are present. Potential applications of this linear position sensor can be found in critical systems such as nuclear plants, tokamaks, satellites and particle accelerators. This paper analyzes the performance of the I2PS in the harsh environment of the collimators of the Large Hadron Collider (LHC), where position uncertainties of less than 20 μm are demanded in the presence of nuclear radiation and external magnetic fields. The I2PS has been targeted for installation for LHC Run 2, in order to solve the magnetic interference problem which standard LVDTs are experiencing. The paper describes in detail the chain of systems which belong to the new I2PS measurement task, their impact on the sensor performance and their possible further optimization. The I2PS performance is analyzed evaluating the position uncertainty (on 30 s), the magnetic immunity and the long-term stability (on 7 days). These three indicators are assessed from data acquired during the LHC operation in 2015 and compared with those of LVDTs.

This paper presents dynamic control and a dead-time compensation method of an isolated dual-active-bridge (DAB) dc-dc converter. Conventional phase-shift control methods for the DAB converter may cause dc offsets in both inductor current and transformer magnetic flux density in transient states. The dynamic control method in this paper independently controls the diagonal switches in each H-bridge converter to modify the duty ratios in transient states. This method enables both inductor current and transformer magnetic flux density to be settled within a half switching period without any dc offsets. Moreover, the phase-shift error caused by the dead time is analyzed in terms of the switching angle. This analysis reveals that the dead time causes the switching-angle error in only one of the two H-bridge converters and may cause dc offsets. A new dead-time compensation method based on this analysis is proposed to eliminate the dc offsets and to achieve a good dynamic response. A 5-kW experimental system verifies the validity of the proposed control method.

WOS: 000432411800062 No-load losses are caused by the magnetizing current that needed to energize the transformers core. These losses are constant and occur 24 h a day, regardless of the load. So, the customers should pay the fixed monthly cost for these losses. In recent years, amorphous alloy has been used to improve distribution transformers performance with the decrease in No-load losses and the increase in the efficiency of transformers. This paper outlines the 2605SA1 amorphous core as a new development of the amorphous alloy with a higher saturation induction. This paper presents a comparison between traditional distribution transformers and amorphous distribution transformers. Distribution of flux density in two different transformers with amorphous core and M5-type silicon steel core has been simulated using 3-D finite element modelling. The simulation and modelling results have been compared with the tests results of 630 kVA, 34.5/0.4 kV prototypes manufactured in the TEK TRANSFORMER factory in Turkey. A good agreement has been obtained between the simulation and experimental results. The simulation and test results show that amorphous core considerably (about 63%) reduces the no-load power losses as well as efficiency of transformers.

Medium Voltage DC (MVDC) distribution Power Systems for all-electric ships (AES) can be regarded as functionally composed of three subsystems, namely the power sources, the load centers and the distribution network. Extensive use of power electronics is required for connecting power sources and load centers to the MVDC bus and for protecting the MVDC power system through properly placed DC circuit breakers. In this paper, an overview is given of the power electronics equipment found in the literature and on the market that could be suitable for use in future AES MVDC power systems. Some industrial experiences regarding DC generator systems, energy storage apparatus and solid-state DC circuit breaker prototypes are reported in the paper as examples of state-of-the-art realizations. Different DC/DC converters, which can be employed as solid-state transformers, are also discussed and a structure obtained by combining them is proposed.

The subject of this paper falls under the development of power electronics for more electric aircraft systems. The study concerns the design of a converter whose function is to generate a 300-V dc bus from a standard 28-V dc avionic network. Various systems, such as motor drives, need to be connected to the 28-V dc network. Their design could be simplified significantly, and standardized, by introducing this intermediate 28–300-V stage. The step-up converter in the proposed configuration is integrated as a part of the system. The power specification is 10 kW, which leads to very high-current values on the 28-V side. The choice of the converter topology, therefore, needs to match this central constraint. The proposed converter uses six boost cells, associated to constitute a series–parallel architecture. This arrangement leads to significantly more optimal sizing for high step-up ratios than conventional boost-cell configurations. Cell interconnection is implemented by means of a monolithic InterCell transformer to reduce the weight of the magnetic part. This paper describes the topology, design, and construction of the lab prototype, as well as experimental results obtained during testing.

The stability of the grid-connected inverters adopted in distributed energy resources (DERs) highly depends on the characteristics of the grid impedance. Hence, various active damping methods and adaptive control algorithms have been proposed for the control of the grid inverters. Differently from those existing solutions, this paper proposes to use a smart transformer (ST) to shape the grid impedance to interact with the controllers of local inverters, aiming at improving the stability of the local controllers as well as the overall grid. A main advantage of this solution is that it offers a pervasive service to all the available DERs, reducing the cost and design effort of the local controllers. Three active damping methods are presented in this paper and the implementation issues in the ST LV-side voltage controller are given. Moreover, the design criteria of the active damping methods considering the control performance of ST and the stability requirements of local DERs are presented. Analysis and experiments are carried out to verify the effectiveness of the proposed solution.

Conventional dielectric response measurement techniques, for instance, recovery voltage measurement (RVM), frequency domain spectroscopy (FDS) and polarization–depolarization current (PDC) are effective nondestructive insulation monitoring techniques for oil-impregnated power transformers. Previous studies have focused mainly on some single type of dielectric measurement method. However, the condition of oil paper insulation in transformer is affected by many factors, so it is difficult to predict the insulation status by means of a single method. In this paper, the insulation condition assessment is performed by grey relational analysis (GRA) technique after carefully investigating different dielectric response measurement data. The insulation condition sensitive parameters of samples with unknown insulation status are extracted from different dielectric response measurement data and then these are used to contrast with the standard insulation state vector models established in controlled laboratory conditions by using GRA technique for predicting insulation condition. The performance of the proposed approach is tested using both the laboratory samples and a power transformer to demonstrate that it can provide reliable and effective insulation diagnosis.

This paper proposes a novel multiphase transformer connection scheme which converts three phase balanced AC input to seven phase balanced AC output. Generalized theory to convert a three phase utility supply into any number of phases is presented. Based on the proposed generalized principle, a three phase to seven phase power converting transformer design is presented with connection scheme, analysis and simulation and experimental results of the proposed three phase to seven phase conversion transformer. The proposed transformer in this paper is analyzed and compared with the connection scheme for seven phase available in the literature. The connection scheme is found to have higher power density, lower core area and lower core requirement as compared to the available connection scheme of the same rating. Impedance mismatching between different phases of the transformer is observed in the three phase to seven phase transformer available in the literature. As this mismatching introduces error in study of per phase equivalent circuit diagrams as well as imbalance in voltage and currents. The present design also addresses the impedance mismatching issue and reduces mismatching in the proposed transformer design. A prototype of the proposed system is developed and waveforms are presented. The proposed design is verified using simulation and validated using experimental approach.

A single phase transformer-less inverter is introduced in this paper. The negative polarities of the input voltage and output terminal have common ground. Therefore, the leakage current problem that is common in PV systems is eliminated naturally. In addition, the proposed inverter has fewer components compared with its counterparts and only one switch conducts during the active states which enhance the inverter efficiency. The proposed inverter is analyzed in details and compared with some existing topologies. The performance of the proposed inverter is validated using the simulation results. A single phase transformer-less inverter is introduced in this paper. The negative polarities of the input voltage and output terminal have common ground. Therefore, the leakage current problem that is common in PV systems is eliminated naturally. In addition, the proposed inverter has fewer components compared with its counterparts and only one switch conducts during the active states which enhance the inverter efficiency. The proposed inverter is analyzed in details and compared with some existing topologies. The performance of the proposed inverter is validated using the simulation results.

The aging of electrical insulation material or a system is a main issue for designers of high-voltage (HV) machines. Precise determination of the life cycle of electrical insulation is one way of improving the efficiency of electrical machines involved in the production and transmission of electrical energy. Much effort has been devoted to preparing statistical or physical methods of Electrical Insulating System (EIS) life time estimation in the real operation of electrical machinery. The main aim of this paper is to introduce a new physical-statistical model of thermal aging respecting the threshold value. This model is based on thermal aging model and the main difference between this model and previously published models is taking into account the threshold value of degradation factor. The complete design of this model is presented in this paper, including functions defining the threshold value of the effect of the degradation factor depending on the temperature. Proposed model was verified by accelerated thermal aging test at selected temperatures (160, 170, 180 °C) and time intervals (0, 120, 240 h) on a commonly used transformer board. The breakdown voltage was set as an indicating parameter of the level of thermal aging and was measured according to standard IEC 60243-1. Collected data from these measurements were used for threshold value determination (431.23 K) and verification of proposed physical-statistical model of thermal aging respecting the threshold value.

In the present world, modern power system networks, being a complicated combination of generators, transmission lines, transformers, circuit breakers and other devices, are more vulnerable to various types of faults causing stability problems. Among these faults, transient fault is believed to be a major disturbance as it causes large damage to a sound system within a certain period of time. Therefore, the protection against transient faults, better known as transient stability control is one of the major considerations for the power system engineers. This paper presents the control approach in the transmission line during transient faults by means of Static Synchronous Series Compensator (SSSC) in order to stabilize Single Machine Infinite Bus (SMIB) system. In this paper, SSSC is represented by variable voltage injection associated with the transformer leakage reactance and the voltage source. The comparative results depict that the swing curve of a system increases monotonically after the occurrence of transient faults However, SSSC is effective enough to make it stable after a while.

This paper presents the concept of the generalized power source in an active distribution network. After the energy storage system (ESS), distributed generators (DG), and demand-side controllable load are connected to the active distribution network, part of the system load can be undertaken by these ESS, DG, and controllable load instead of relying on the capacity of the substation, which greatly improves the reliability of the system. The above-mentioned distributed energy resources in the active distribution network are collectively referred to as the generalized power source (GPS). Substation credible capacity refers to the capacity of the GPS to supply the distribution network that is equivalent at the high voltage distribution network level when considering the faults of the main transformer, 110 kV lines, and circuit breakers. Considering the uncertainties and control strategies of GPS, this paper takes a specific distribution area as the background to study its substituted substation capacity. Based on a sequential Monte Carlo evaluation framework and the principle of constant reliability, an evaluation method of the GPS credible capacity is proposed. In order to verify the effectiveness of the method proposed in this paper, the credible capacity of various typical connection modes of a high voltage distribution network is quantitatively analyzed in the case study, which can provide a reference for the capacity planning of an active distribution network substation and grid structure selection.

This paper takes a typical 220 kV three-phase three-limb oil-immersed transformer as an example, this paper building transient field-circuit coupled model and 3D coupled magneto -fluid-thermal model. Considering a nonlinear B–H curve, the magneto model uses the field-circuit coupled finite element method (FEM) to calculate the magnetic flux distribution of the core and the current distribution of the windings when the transformer is at a rated current and under direct current (DC) bias. Taking the electric power losses of the core and windings as a heat source, the temperature inside the transformer and the velocity of the transformer oil are analyzed by the finite volume method (FVM) in a fluid-thermal field. In order to improve the accuracy of the calculation results, the influence of temperature on the electrical resistivity of the windings and the physical parameter of the transformer oil are taken into account in the paper. Meanwhile, the convective heat transfer coefficient of the FVM model boundary is determined by its temperature. By iterative computations, the model is updated according to the thermal field calculation result until the maximum difference in hot spot temperature between the two adjacent steps is less than 0.01 K. The result calculated by the coupling method agrees well with the empirical equation result according to IEC 60076-7.

This paper presents a model of a bidirectional MV/LV solid-state transformer (SST) for distribution system studies. A modular multilevel converter configuration is used in the MV side of the STT. The LV side uses a three-phase four-wire configuration that can be connected to both load and generation. The model developed for this work has been implemented in MATLAB/Simulink, and its behavior has been tested by carrying out several case studies under different operating conditions. The simulation results support the feasibility of the SST and its advantages in comparison to the conventional transformer. The paper also includes a discussion of the main model limitations and the future work.

In the paper results of simultaneously conducted measurements achieved using capacitive and inductive sensors are presented according to different PD model sources immersed in a mineral transformer insulation oil. All measurements are preceded under laboratory conditions using typical measurement set up commonly applied for on-site PD detection: measuring impedance and capacitor and high frequency current transformer (HFCT) are used respectively. Measuring frequency and voltage level influence as well as phase resolved PD patterns analysis are investigated in the research. Various fundamental PD signal descriptors assigned for selected frequencies are also proposed and compared for chosen sensors. The main purpose of the presented research is to compare PD measurement results achieved using selected type of sensors during laboratory measurements and to point the best application areas in fields of PD detection in high voltage apparatus under normal operating conditions. Furthermore a proper measurement results interpretation coming from different sensors as well as measurement conducting problems and achieved patterns disparities are also discussed in the paper.

The impact of the increasing connection of distributed generation to medium voltage (MV) feeders, with particular reference to photovoltaic (PV) units, justifies the investigation on secondary voltage/VAR control (VVC) schemes able to improve the utilization of available control resources and to reduce reactive power flows. The paper deals with a secondary VVC scheme based on a distributed multi-agent approach that requires only the estimation of the reactive power flows between the buses where the PV units with reactive power control capability are connected. Phasor Measurement Units (PMUs) are used to get the relevant information. In general, distributed control approaches are expected to work adequately even by using communication infrastructures with lower performances than those required by centralized approaches. The paper addresses such an issue by the analysis of the distributed VVC performance when a shared cellular network is used for the cooperative adjustment of PV inverters reactive power outputs and of tap positions of transformers equipped with on-load tap changers. The analysis is carried out by using a specifically developed ICT (Information and Communications Technology)- power co-simulation platform. It is shown that the VVC scheme has adequate performances also in the presence of significant levels of background traffic and data loss.

State-of-the-art PWM AC/DC converters often suffer from high switching losses due to hard-switching and thus are limited in their efficiency and power density. In this paper, the concept of the integrated Triangular Current Mode (iTCM) operation is introduced, which enables soft-switching over the entire AC mains period and can overcome the limitation of PWM, resulting in a higher system performance. Thereby, an LC-circuit is added to the well-known full-bridge-based PWM AC/DC converter, which internally superimposes a high triangular current to the AC mains current in order to reverse the current direction in the semiconductors in each switching cycle and by this enabling soft-switching operation of all devices. In this paper, the iTCM concept is presented in detail and its performance is compared to the standard PWM and TCM approaches. Furthermore, the proposed iTCM concept is applied to a 10 kV SiC-MOSFET-based bidirectional 25kW single-phase AC/DC converter operated from the 6:6 kV medium-voltage AC grid. In this case, compared to PWM, the iTCM concept allows to increase the switching frequency by almost a factor of five while the total semiconductor losses are reduced by more than 40%.

—For Smart Transformers (ST), reliability is one of the major problems when compared to the traditional low frequency transformers. Modular ST with advanced control algorithms could increase the reliability compared to the nonmodular solutions. This could be achieved by distributing differently the power among the cells depending on their aging. This unequal power transfer is a challenge for Cascaded HBridge converters (CHB). ST based on a CHB rectifier with different capacitor voltage balancing schemes are analyzed in the paper. The goal is to evaluate the best scheme for unequal power transfer while maintaining the dc-link voltages constant. The paper focuses on validating the operating power unbalance limits of each cell of the modular system for unbalanced power transfer while maintaining the voltage balance in the medium voltage dc-link stage. The power unbalance limits are established with analytical calculations and verified through simulations and experiments

Transformer monitoring systems are increasing in number and having a clear overview of the whole fleet of monitored transformers is becoming crucial in an effort to provide the end user with an easy way to monitor its transformer assets and react accordingly to the monitored situations. This paper will introduce a Fleet monitoring solution that provides the architecture and tools to accomplish the set tasks of connecting and accessing information of all the monitoring systems from their dedicated locations. Standard transformer monitoring system configuration is made of a local unit on the transformer for collecting and processing raw data from the sensors and transferring the data to another unit, usually a server located in the substation which stores it in the database. The server unit is also responsible for the presentation of data to the end user and communication with supervisory control systems. This configuration is well capable of achieving its purpose of monitoring the transformer state and operation. The challenge is in accessing the monitored data. Traditional approach for accessing the data was to access the servers locally in the substations. Responding to the monitored conditions was done by connecting critical alarm signals from the monitoring system to the substation control system and in case of an alarm indication the operator from the substation was required to access the monitoring system for further details and inform personnel responsible for the maintenance of transformer units. The transformer maintenance personnel were then required to check the monitored data either by connecting remotely to the monitoring system, which in most cases is not possible due to security and technical restrictions, or to travel to the remote location to investigate the issue. This approach was both very time consuming and costly. There is also a consideration that the modern substations are becoming unmanned or have just a basic crew which may consist of personnel not qualified or trained for the required tasks. The challenge was to enable a centralized access to the distributed monitoring systems across multiple substations. An architecture, both hardware and software, was developed, that accomplishes this task. The architecture is highly modular so that the components of the system can be distributed and integrated into an overall system which is fully customizable to the end user requirements. During development one of the main considerations was to make the integration of already existing systems as seamless as possible. The result is a Fleet monitoring solution that centralizes access to the information from all connected monitoring systems while at the same time does not interfere with the work of dedicated monitoring systems at remote locations. This way the data can be presented directly to the right personnel. This paper will also discuss further goals and directions of development for transformer fleet monitoring.

This paper presents an ensemble-boosting algorithm (EBA) for classifying partial discharge (PD) patterns in the condition monitoring of insulation diagnosis applied for electrical assets. This approach presents an optimization technique for creating a sequence of artificial neural network (ANNs), where the training data for each constituent of the sequence is selected based on the performance of previous ANNs. Four different PD faults scenarios were manufactured in the high-voltage (HV) laboratory to simulate the PD faults of cylindrical voids in methacrylate, point-air-plane configuration, ceramic bushing with contaminated surface and a transformer affected by the internal PD. A PD dataset was collected, pre-processed and prepared for its use in the improved boosting algorithm using statistical techniques. In this paper, the EBA is extensively compared with the widely used single artificial neural network (SNN). Results show that the proposed approach can effectively improve the generalization capability of the PD patterns. The application of the proposed technique for both online and offline practical PD recognition is examined.

In this paper authors present the result of a study oriented to assess the metrological characteristics of measuring systems used for verifying the variation of the ratio errors and phase displacements of inductive voltage transformers vs. temperature. Experimental results show that the designed equipment as well as the set up realized can be considered as a benchmark for the verification of the variations of ratio errors and phase displacements of up to 0.1-accuracy class voltage transformers vs. temperature. In addition, the sources that more contribute to the uncertainties on the ratio and phase errors have been investigated and discussed in the paper along with suggestions on where to act to further improve the accuracy of the presented set up.

Modern Supervisory Control and Data Acquisition (SCADA) systems collect any available data from the systems they monitor and control. Some of the data available in modern SCADA systems include types of the data that can be used for centralized monitoring of transformers. Paper presents possible advanced analysis and reports for monitored transformer units using available SCADA data to be used for monitoring of the life of the transformer and help to schedule regular, predictive and corrective types of maintenance. Paper shows which type of data is available, and what percentage of transformers has the kind of data used for monitoring. Examples of measurement data collected are shown for representative transformers. There is an overview on tap changer position moves and its impact on transformer lifetime and servicing interval. For transformers with available temperature of winding an annualized relative ageing rate is calculated.

A useful tool to support the decision-making process in power transformer management is risk assessment. There are few practical methodologies to assess a Transformer Risk Index (TRI). In addition, such proposals do not consider the latest advances in techniques for transformer health valuation, and they also have other drawbacks. This paper proposes a practical method to undertake risk analysis of power transformer fleets, which deal with the stated problem. The proposal appropriately considers the best attributes of the methods reported in literature in order to compute the two components of TRI, i.e., the failure probability factor and the consequence factor. Moreover, this paper contributes to the risk analysis issue by including risk matrices and clustering techniques to support the decision-making process. The presented method was tested on a fleet of fourteen transformers. This approach serves as a practical and reliable tool for asset management in power utilities. La evaluación de riesgo es una herramienta útil para apoyar el proceso de toma de decisiones para la gestión de transformadores de potencia. Existen pocas metodologías prácticas para evaluar un índice de riesgo del transformador (TRI). Además, tales propuestas no tienen en cuenta los últimos avances en técnicas para la valoración de la salud del transformador, entre otros inconvenientes. En este artículo se propone un método práctico para el análisis de riesgo en flotas de transformadores de potencia, el cual plantea soluciones a los problemas mencionados. La propuesta incluye los mejores atributos de los métodos reportados en la literatura, con el fin de calcular los dos componentes del TRI, es decir, el factor de probabilidad de falla y el factor consecuencias de la falla. Por otra parte, este trabajo contribuye con el análisis de riesgos mediante la inclusión de matrices de riesgo y técnicas de agrupamiento que permiten apoyar de manera robusta el proceso de toma de decisiones. El método presentado es probado en una flota de catorce transformadores. Este enfoque sirve como una herramienta práctica y fiable para la gestión de los activos de las empresas eléctricas. Fil: Romero Quete, Andrés Arturo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Energía Eléctrica. Universidad Nacional de San Juan. Facultad de Ingeniería. Instituto de Energía Eléctrica; Argentina Fil: Gómez, Héctor David. Conocimiento y Servicios de Ingeniería; Colombia Fil: Molina, Juan David. Conocimiento y Servicios de Ingeniería; Colombia Fil: Moreno, Germán. Conocimiento y Servicios de Ingeniería; Colombia

In this paper, a dual-transformer-based asymmetrical triple-port active bridge converter (DT-ATAB) is proposed to interface two different dc-sources and a load. DT-ATAB consists of three active power electronic converters and two high-frequency transformers. All switches of these converters can be turned on with zero-voltage switching to reduce the switching losses. The bidirectional power flow operation is possible between the ports. The DT-ATAB also reduces the circulating powers between the ports for well-matched transformer turns ratios as compared to those in the other existing triple-port active bridge converters (TAB). Furthermore, the magnetic short-circuit conditions arising in the three-winding transformer of the TAB are mitigated in DT-ATAB. The principle of operation, steady-state analysis, various modes of operation (three-port and two-port modes), and a closed loop controller of DT-ATAB are presented. The theoretical analysis of this paper is verified using both simulation and experimental studies. The illustrated results show that DT-ATAB can be used as a promising multiport converter to interface the multiple sources and load to achieve wide-ranging outputs with the minimal losses.

When subject to earthquakes, some objects and structures, such as statues, obelisks, storage systems, and transformers, show a dynamic behavior that can be modeled considering the object/structure as a rigid block. Several papers have studied the dynamic behavior of both stand-alone rigid blocks and systems where rigid blocks have been paired with safety devices to prevent or delay the overturning of the blocks. Although the safety devices have generally been proven to be effective, their effectiveness changes substantially varying the parameters that characterize the system and the seismic input. This paper compares the seismic responses of stand along rigid blocks with those of blocks coupled with two candidate safety devices: an isolating base and a pendulum mass damper. To account for the relevant uncertainties, probabilistic seismic demand models are developed using a Bayesian approach. The probabilistic models are then used along with the overturning capacities of the blocks to construct fragility curves that give a prediction of the probability of overturning occurrence as a function of some characteristics of the blocks, of the safety devices, as well as of the seismic excitation, i.e. the slenderness of the body and the peak ground acceleration. The data needed to develop the probabilistic model are obtained integrating the nonlinear equations of motion of the two systems subject to selected ground motions. In the end, some numerical examples are proposed.

Port facilities have to be organized as microgrids to pursue an efficient electrical utilization based on the four “l”-pillars in the sustainable energy use: levelled, less, local, load (lele-lolo). This paper deals with the special case of STS cranes characterized by high power peaks and a low load factor that require an over-sized electrical network and cause high costs because electrical utilities charge greatly the power peaks. For these reasons, this paper discusses a comprehensive peak shaving solution that implements the coordination of cranes' duty cycles with a power optimization tool and, as energy storage systems, both ultracapacitors and flywheels, allowing the port operator to optimize their power demand, as well as their network design. The suggested “hybrid” solution combines, on the board of each crane, an adequate number of ultracapacitors and in the shore transformer substation, flywheel's modules operating on the simultaneous behavior of the cranes group.

In this paper, a proposed pulse width modulation (PWM) control method for the isolated current-mode resonant DC-DC converter with MHz level switching frequency is presented. The circuit topology is same as a conventional resonant converter with synchronous rectification and without any additional components. The control technique for the output voltage regulation is proposed with the unique PWM control for synchronously-rectifying switches. By using the transformer's leakage inductance and the PWM control, the boost conversion can be realized. In addition, to achieve the zero-voltage switching (ZVS) operation, phase-shift between primary and secondary-side switches is adapted. The ZVS operation can maintain for primary-side switches. In this paper, proposed technique for achieving stable ZVS operation has been discussed. Some experiments have been done with 5MHz isolated DC-DC converter which has Gallium Nitride field effect transistor (GaN-FET), and the total volume of the circuit is 16.14cm3. The data show that the maximum power efficiency is 89.4%., 29th Annual IEEE Applied Power Electronics Conference and Exposition, APEC 2014; Fort Worth, TX; United States; 16 March 2014 through 20 March 2014, Applied Power Electronics Conference and Exposition (APEC), 2014 Twenty-Ninth Annual IEEE, 6803494; 2014

Impedance-source networks are an increasingly popular solution in power converter applications, especially in single-stage buck–boost power conversion to avoid additional front-end dc–dc power converters. In the survey papers published, no analytical comparisons of different topologies have been described, which makes it difficult to choose the best option. Thus, the aim of this paper is to present a comprehensive analytical comparison of the impedance-source-based buck–boost inverters in terms of passive component count and semiconductor stress. Based on the waveform of the input current, i.e., with or without a transformer, and with or without inductor coupling, the impedance-source converters are classified. The main criterion in our comprehensive comparison is the energy stored in the passive elements, which is considered both under constant and predefined high frequency current ripple in the inductors and the voltage ripple across the capacitors. Two-level and multilevel solutions are described. The conclusions provide a “one-stop” information source and a selection guide of impedance-source-based buck–boost inverters for different applications.

International audience; Power transformers play a critical role in electric power networks. Such transformers can suffer failures due to multiple stresses and aging. Thus, assessment of condition and diagnostic techniques are of great importance for improving power network reliability and service continuity. Several techniques are available to diagnose the faults within the power transformer. Frequency response analysis (FRA) method is a powerful technique for diagnosing transformer winding deformation and several other types of problems that are caused during manufacture, transportation, installation and/or service life. This paper provides a comprehensive review on FRA methods and their applications in diagnostics and fault identification for power transformers. The paper discusses theory and applications of FRA methods as well as various issues and challenges faced in the application of this method.

In this paper proved the possibility of developing passive electronic inductive elements based replace metal wire that is wound inductor, the wire is made of electret. The relative permeability of the electret S  10 000, several orders of magnitude greater than the permeability of conventional insulation materials, i < 10, resulting current in the wire acquires properties bias current. The essence of innovation is to replace the source of of magnetic induction flow that pervades the core of the coil. According to the theory of electrodynamics, current bias, in contrast to conduction current, generated no movement of charge along the wire, but the change of the charge in the local volume.Equivalence bias current and conduction current is manifested in the possibility of forming a magnetic field. The flow through magnetic induction coil core regardless of the current it generates, creates voltage at its ends.The paper also shows the numeric characteristics that determine the effective frequency range, specified the reason why electric a wire with і < 10 can not generate magnetic flux through the core and serve as a passive reactive component.