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• The effect of the aging of paper/pressboard on furfural content in oil was studied. • The effect of oil/paper/pressboard ratio on furfural- DP equation was studied. • The effect of oil replacement on aging evaluation results is corrected. The furfural analysis is an effective method to evaluate the aging state of transformer cellulose insulation. However, the existing research on the furfural analysis mainly focuses on 'oil-paper' system, without considering the oil-paper-pressboard system. In fact, the mass of insulating pressboard occupies a large proportion in the transformer, and there are obvious differences in the mass ratio of oil-paper-pressboard among transformers with different voltage levels, which will affect the effectiveness of furfural analysis. In addition, oil replacement is an oil treatment method to improve the insulation performance of oil, which will lead to the loss of furfural content in oil. In view of this, a series of experiments and studies were carried out to obtain the modified equation between furfural and degree of polymerization (DP) of paper/pressboard insulation with different oil-paper-pressboard mass ratios under oil replacement condition. Firstly, accelerated thermal aging experiment of insulation samples with various oil-paper-pressboard mass ratios was conducted in the lab, and the effect of oil-paper-pressboard mass ratio on furfural was analyzed. Then, the oil replacement experiments were performed, and the oil replacement compensation factor was proposed. Finally, a modified furfural -DP equation was established. This paper attempts to report a method for analyzing the effect of oil-paper-pressboard mass ratio and oil replacement on furfural- DP equation. These findings are expected to improve the application of furfural analysis in field transformers. [ABSTRACT FROM AUTHOR]

Retrofilling a transformer with natural or synthetic esters leads to a reduction of its fire and environmental risk without making a large investment. Nowadays, most of the retrofilled transformers are medium power and voltage units, but their use in higher rated power units is rapidly expanding. Transformer retrofilling presents certain challenges due to the physico-chemical differences between mineral oil and esters. The insulation system of a transformer is designed to work with mineral oil, performing a double function as dielectric and coolant agent; when replacing mineral oil with an ester, whose relative permittivity and viscosity are different, the ability to perform these two functions must be reassessed. In this work, the electric field distribution of a retrofilled transformer has been analyzed using Finite Element Method. The study was conducted on a 280 MVA transformer which was originally designed to operate with its tank filled with mineral oil. The dielectric margins of the different parts of the insulation were calculated, and then compared with the dielectric margins of the transformer insulation before retrofilling. To improve the accuracy of the study, an experimental study was driven to obtain the permittivity values of mixtures of MO and NE in different proportions and the composition of the solid insulation in retrofilled transformers. The study also investigates the influence of the quality of the retrofilling process on the electric field distribution. The analysis shows that some parts of the insulation, such as the oil channel, operate with lower stress when the insulating liquid is changed to an ester, but other regions, such as the solid insulation in the most stressed areas of the insulation, reduce their dielectric margins significantly. • The use of natural ester retrofilled transformers is becoming common. • Diverse permittivity in ester, mineral oils, and impregnated paper alters electric fields. • The solid insulation of a retrofilled transformer remains impregnated in mO • The paper compares the electric field distribution in a real transformer when it uses mineral oil, natural ester or is retrofilled. • Experimental and simulation data are included and analyzed in detail. [ABSTRACT FROM AUTHOR]

This paper deals with a novel hybrid transformer configuration for MV/LV three-phase distribution systems. The configuration involves a standard distribution transformer with an open-end secondary winding, which is connected to the LV grid and a four-legged inverter with a floating DC bus. Unlike previous hybrid transformers, the proposed configuration eliminates the need for a bidirectional converter and auxiliary transformer windings as well. The fourth leg of the inverter is exploited to manage the neutral line current. The main advantage of the proposed configuration is its ability to provide phase voltage control at the distribution grid edge. This enables the compensation of various grid voltage anomalies such as sags, swells and some kinds of interruptions. The effectiveness of the proposed configuration is demonstrated through simulation and experimental results, highlighting its promising performance in real-world scenarios. [ABSTRACT FROM AUTHOR]

This paper aims to estimate the three-phase voltage sensitivity matrix and the network topology of a low-voltage distribution network from smart meter data, which measures voltage magnitude, current magnitude, and power factor with a lead/lag indicator. The targeted networks are three-phase networks with single-phase loads. Understanding network sensitivity and topology is crucial for fault detection, unmetered load identification, and addressing Dynamic Operating Envelope problems. The problem is formulated as a constrained optimization problem to estimate both the three-phase voltage sensitivity and the low-voltage transformer voltage. The estimated voltage sensitivity is used to further identify the network topology, by implementing an enhanced Recursive-Grouping and Backtracking algorithm, as well as a candidate topology selection technique. The proposed method is tested on the 55-node European feeder and several synthetic networks. Compared to the state-of-the-art, the results show a four-fold improvement in the accuracy of voltage sensitivity estimation and substantially fewer mistakes in the topology estimates. The result underscores the efficacy of a three-phase network model and voltage angle approximation in enhancing estimation accuracy. • Estimating voltage sensitivity to current of low-voltage distribution network. • Estimation uses smart meter measurements only. • Transformer voltage is estimated simultaneously. • Approximating unknown phase angle and considering coupling effect on neutral line. • A fast Recursive-Grouping and Backtracking algorithm to find topology. [ABSTRACT FROM AUTHOR]

• New concept of controller load damping factor of power systems using voltage regulators. • Implementation method for a series-type controlled load damping factor controller. • Examination of controlled load damping controller implemented in RTDS. As sustainable natural sources such as wind and solar are increasingly integrated into power grids, conventional fossil fuel-based generating units will be gradually replaced, resulting in enormous pressure on frequency regulation of the novel power systems. This paper proposes a method for implementing a cascaded load-damping factor controller based on the controlled load-damping factor controller. Series-type voltage regulators, namely, solid-state transformers or dynamic voltage restorers, can regulate the voltage of the 10 kV feeder within the allowed range. Therefore real-time control of composite load power is realized, which significantly improves the primary frequency regulation capability of novel power systems with renewable energy generation(REG). The effect of actuator regulation time constant is considered, The impact of dynamic loads on the primary frequency regulation of the controller, designed based on a static load model, is analyzed, The effectiveness of the controller on the primary frequency regulation under different renewable energy penetration rates is discussed, The effectiveness of the controller in smoothing short-term power fluctuations of REG is examined. Finally, the effectiveness of the series-connected controlled load-damping factor controller in primary frequency regulation is verified on the RTDS simulation experimental platform. The proposed control system is characterized by a simple structure, easy implementation, and convenient maintenance, which can realize the coordination and unification of reactive-voltage and active-frequency control of the novel power system in the 110 kV customer substation and is beneficial to practical application in engineering. [ABSTRACT FROM AUTHOR]

• The fault deviation function is constructed to locate the stator ground fault of large hydro-generators and the fault winding potential is calculated accurately which is the basis of the injected current calculation. • The fault point arc suppression is realized by injected current regulation considering the fundamental fault current and the third harmonic fault current. • The voltage variation of the neutral point criterion is proposed to distinguish temporary faults and permanent faults. • All methods proposed in this paper consider the irregular distribution of the grounding capacitance and the grounding transformer impedance. Generator stator ground faults are common, and a reliable ground fault arc suppression method is required to ensure generator safety and avoid unnecessary impact on the power system. A fault arc suppression method is proposed in this paper by regulating the neutral injected current and controlling the fault point voltage to a low level. Considering the required injected current is different for different fault positions on the stator winding, a fault location method and a fault winding potential calculation method are proposed. In addition, a fault type discrimination method based on the neutral fundamental voltage variation is proposed to distinguish temporary and permanent faults. In this paper, the grounding transformer impedance and irregular distribution of the stator grounding capacitance are considered. The PSCAD/EMTDC simulation results verify that the proposed method can effectively realize arc suppression at different fault scenes and the ground fault type can be distinguished effectively. [ABSTRACT FROM AUTHOR]

• The paper presents a new reliable detection approach for interesting faults overcoming the insensitivity problem of conventional protection. • The proposed method detects these faults by monitoring only the magnitude of the measured three-phase voltage at the load side without installing voltage transformers. • A robust threshold value is proposed to guarantee both dependability against all fault conditions and security against the normal unbalance. • The security of the proposed approach against external faults (at the low voltage side) is verified by monitoring the magnitude of the three-phase current. • The faulted feeder isolation and the faulted section isolation are accomplished by a proposed autonomous control method with the aid of the infrastructure of smart grids (available communication system). • All applied test results corroborate the efficacy of the proposed detection method under the considered faults. This paper presents a new and reliable detection approach for different crucial faults in distribution systems. These faults are one-open conductor faults at the far side of the feeder or simultaneous faults that are an open conductor followed by an earth fault at either the source side or load side (downed conductor-return faults). Under downed conductor-return faults, there is no zero-sequence current at the conventional protection point. Furthermore, the negative-sequence component is less than the normal unbalance component of the conventional protection point under one-open conductor faults at the far side of the feeder. Therefore, the presented detection approach is based on the measurements at the low voltage side of the load transformers. The proposed method detects such faults by monitoring only the magnitude of the measured three-phase voltage at the load side without installing voltage transformers. A robust threshold value is proposed to guarantee dependability against all fault conditions and security against the normal unbalance. Also, the security of the proposed approach against external faults (at the low voltage side) is verified by monitoring the magnitude of the three-phase current. Fortunately, the proposed system identifies the faulted section. The faulted feeder isolation and the faulted section isolation are accomplished by a proposed autonomous control method with the aid of the infrastructure of smart grids (available communication system). Extensive investigations are done based on a detailed simulation of the selected distribution feeder using the ATP-EMTP program. For verification purposes, recorded faulted cases are experimentally applied on a real distribution feeder to visualize such fault conditions and confirm the detection impossibility of earth-return faults by conventional protection, significantly if the fault faraway the substation. On the other hand, all applied test results corroborate the efficacy of the proposed detection method under the considered faults. [ABSTRACT FROM AUTHOR]

• A field test and detection method for the winding vibration signal of converter transformer is presented. • The law of vibration intensity distribution of converter transformer box is presented. • The influence of transformer oil and winding heat effect on vibration signal transmission is studied. The converter transformer is important for the high-voltage direct current transmission system. The distribution characteristics of the vibration signal of the tank are significant for the operation status monitoring and the judgment basis for the fault condition of the converter transformer. However, most present research on vibration detection technology focused on ordinary power transformers rather than the HVDC converter transformers. Moreover, the influence of the special structure and the heat accumulation effect of the load current on the vibration signal distribution characteristics is not fully considered. To solve the above problems, this paper analyzes the intensity distribution of vibration signals at different positions on the tank surface and valve side bushings based on the offline load test. At the same time, to study the influence factors of vibration signal distribution, the influence of current heat accumulation on winding characteristics and transformer oil temperature is analyzed. The results show that vibration intensity is related to location and the load current. And the vibration intensity of the tank surface increases with the time of current flowing through the winding, nevertheless, the vibration intensity tends to decrease near the valve-side bushing. The test methods and research on distribution Characteristics in this paper can provide new thoughts for the optimization of the layout of converter transformer vibration detection sensors, and provide a research basis for the application of vibration signal fault diagnosis methods. [ABSTRACT FROM AUTHOR]

• The frequency coupling effect is revisited from the viewpoint of complex circuit analysis. • A grid-tied converter system can be categorized into strongly, weakly coupled, and decoupled specifically. • The frequency coupling effect is caused by the rotation of complex-valued vectors and admittance mismatching. • The phase-shifting transformer-based circuit modeling and analysis framework is proposed. Due to asymmetric control algorithms and topology, the frequency coupling effect (FCE) extensively exists in the grid-tied converter (GTC), increasing the order of traditional impedance models and sacrificing their inherent physical insight. The two problems can be solved using the recently reported order reduction techniques. However, FCE is roughly regarded as a coupling phenomenon in the existing studies, which is not rigorous because a GTC system will present various degrees of FCE in different situations. Therefore, this paper revisits the FCE from the viewpoint of the complex circuit and categorizes the GTC systems into three types for analysis, i.e., strongly, weakly coupled, and decoupled systems. Firstly, the plausibility of using the complex-valued vectors to depict FCE is illustrated, and thus FCE is clarified as resulting from the rotation of complex-valued vectors and the admittance mismatching. Based on this new clarification, the phase-shifting transformer-based circuit modeling framework is proposed. Then GTC system can be intuitively presented by the proposed circuit model, whose mathematical model is still order-reduced. More importantly, coupling paths of strongly, weakly coupled, and decoupled systems can be flexibly modeled by adjusting the ratio of the phase-shifting transformer. Finally, simulations and experiments prove the analysis and proposed modeling framework. [ABSTRACT FROM AUTHOR]

The temperatures of a power transformer have direct influence on the windings insulation deterioration, determining how long the equipment will be able to remain in operation. In order to better predict and study the transformer thermal behavior, a thermal-hydraulic model (TH) for oil-directed (OD) power transformer is proposed. From the constructive dimensions and the operative characteristics of the transformer, the proposed model is able to determine the complete oil flow and temperature distributions over the magnetic core, windings and radiators by iteratively solving an algorithm composed of both hydraulic and thermal resistance networks. A transformer prototype was assembled and equipped with 29 optical fiber sensors as part of a measurement system in order to validate the approach. The details of the active part, tank and radiators modelings are presented in a generalized way, being applicable to different constructive arrangements. The validation results show a maximum relative error of 1.31% for temperature, accurately describing the flow and temperature fields of the prototype, indicating that the model can be satisfactorily used for predicting the hydraulic and thermal behaviors of electric transformers. • Winding convergence through finite volume and thermal-hydraulic coupling. • Thermal radiation modeled for external heat transfer on tank and radiators. • Temperature distribution along winding turns, insulation paper and oil channels. • Prototype equipped with 29 optical fiber sensors for model validation. • TH model validation presenting 1.31% of maximum relative error. [ABSTRACT FROM AUTHOR]

• Traction transformer integrated LCL filtering method with less filter inductors and capacitors is proposed. • Filtering mechanism is revealed by building electromagnetic decoupled equivalent circuit model of innovative transformer. • Prototype of traction transformer with integrated filter windings is designed and manufactured for validation. • Good filtering performance of the proposed method is verified by real-time field-circuit coupling simulation. Converters in AC-DC-AC traction drive system introduce a wide range of high-frequency harmonics. When the high-frequency harmonics contain contents at around the inherent resonance frequency of the traction power supply system, it will cause harmonic resonance in the traction grid. The LCL-type single phase line-side converter is widely adopted to prevent the high-frequency harmonics from entering the traction grid. However, additional four inductors and four capacitors are needed to form the filtering system, which increases the occupied onboard space. In this paper, a traction transformer integrated LCL filtering method is proposed to achieve high-frequency harmonic and resonance suppression and, at the same time, reduce the space occupied by the LCL filter. The equivalent circuit of the traction transformer integrated LCL filtering system is modeled and the mechanism of harmonic and resonance suppression is analyzed. Then, the required equivalent impedances of the traction transformer, the filtering capacitor, and the corresponding controller of line-side converter are designed. Finally, the parameter design of the traction transformer prototype is verified by finite element simulation. And the grid-train system with the traditional L filter, LCL filter, and the proposed traction transformer integrated LCL filter are simulated by real-time field-circuit coupling simulation, which verifies the good filtering effect of the proposed method. [ABSTRACT FROM AUTHOR]

• Probabilistic models based on real data is applied to consider uncertainties. • Battery capacity and charging power modify the aggregated charging demand. • Long-range vehicles using high-power chargers accentuate the overload condition. • High-power chargers reduce undervoltage and unbalance occurrence. The number of electric vehicles with larger battery capacity that charges at higher power levels is increasing in residential areas. This scenario directly impacts users' charging behavior and modifies the demand profile of electric vehicles. This paper evaluates the effects of long-range electric vehicles on total charging demand, transformer loading and voltage quality in a real distribution system. Electric vehicles demand is modeled based on real-world data obtained from a large-scale project developed in the United Kingdom. This study proposes performance indices to perform a qualitative analysis of the system and uses Monte Carlo Simulation to consider uncertainties in residential demand and vehicle charging behavior. The results show that the current trend of adopting long-range vehicles that charge at higher power levels modifies vehicles charging frequency and duration. It increases transformer peak demand, and the number of hours the transformer operates overloaded. Voltage quality violations occur less frequently when three-phase connectors are used to charge vehicles, as they split demand between phases. © 2017 Elsevier Inc. All rights reserved. [ABSTRACT FROM AUTHOR]

• Power quality management for 12-pulse rectifier is established to improve its voltage and current adaptability. • Power quality of distribution systems is managed based on the magnetic potential balance. • An energy-based control scheme is proposed to achieve the optimal power quality. • Lower dc-link voltage and better power quality can be achieved. This paper proposes an energy-based control scheme for optimal power quality management in 12-pulse rectifier integrated distribution systems. In this 12-pulse rectifier integrated distribution system, instead of using a step-down transformer for voltage matching, a back-to-back converter is connected to the taps of star and delta windings on 12-pulse rectifier transformer's secondary side via the coupling LC. Using the magnetic potential balance of harmonics and reactive components between the back-to-back converter and the load side of the 12-pulse rectifier, the power quality of distribution systems can be managed. This topology can improve the voltage and current adaptability of the converter, thereby saving the cost of voltage matching transformer and exploiting the filter converter capacity potential. To ensure global power quality performance, an energy-based control scheme is proposed to improve the power quality management performance by accelerating the energy dissipation of the system. Case studies are performed to verify the effectiveness and superiority of the proposed methodology on low dc-link voltage and power quality. [ABSTRACT FROM AUTHOR]

Power transformers are the first devices used to transfer the electrical energy produced in power plants to the grid to supply the industrial and individual receivers with electricity. The heat generation in windings and core, being an effect of the power losses, is usually dissipated in large units by using mineral oils, which are harmful to the environment. Nowadays, the industry and global society seek environmentally-friendly alternatives. One of the most promising substitute for their high biodegradability, safety in operation, and favourable thermo-physical properties are natural ester oils. For this reason, a numerical study of 8.5 MVA disk-type power transformer cooled using conventional mineral oil and a commercially used rapeseed ester oil is presented in this paper. Moreover, due to different thermal behaviour of the considered oils, the comparison was made for the unit working in different seasons of hot and moderate climate zones (Argentina and Poland). In the numerical approach, electromagnetic (EMAG) and computational fluid dynamics (CFD) models were used for a detailed study of the selected device. In particular, a novel and very efficient EMAG–CFD–CFD coupling procedure was developed to assess the cooling of the large power transformer. Such a coupled computational procedure allowed for the detailed investigation of the power loss, oil flow characteristics, and temperatures with a satisfying computational effort. The results showed that the average windings temperatures are higher by 2–9 K when the ester oil is used, dependent on the ambient conditions. The hotspot temperature in the low voltage windings increased by up to 9 K and up to 18 K in the high voltage windings using ester oil. According to the results, the oil duct construction requires modification in the high voltage region for transformers cooled using mineral oil in cold climate conditions. • Coupled electromagnetic and thermo-fluid model of disk-type transformer was developed. • Advanced coupling approach allowed to perform a detailed CFD simulation efficiently. • Separate detailed CFD sub-models of windings and transformer tank were coupled. • Biodegradable ester oil was compared with mineral oil for the transformer cooling. • Power transformer cooling in Polish and Argentinian climate-zones was compared. [ABSTRACT FROM AUTHOR]

• The entire circulating oil systems considering the radiator model are constructed to study the transformer thermal-fluid coupling. • The multi-scale method combining 3D winding model and 1D radiator model is proposed for ONAN transformer. • Temperature rise test is conducted in a 35 kV ONAN transformer, and winding temperature and radiator temperature are measured by FOSs and thermocouple probes respectively. • Compared with traditional method, the multi-scale model with entire circulating oil systems approximates the experimental results more. The cooling ducts of radiator play an essential role in transformer heat dissipation, while it is always ignored in most researches due to the computation costs, which might result in misleading thermal insulation margin. This paper presents a novel method, the multi-scale thermal-fluid coupling, based on solving models in different dimensions combinedly, which can accurately analyze transformer temperature with the entire circulating oil system. Firstly, finite volume method (FVM) is adopted to investigate the thermal performance of 3D winding model while the radiator cooling duct model is simplified to 1D based on nodal data. Then, a serial coupling method is utilized to map temperature and velocity data between two models, that's to say, 3D winding model and 1D cooling duct model are connected and solved simultaneously. After the theoretical analysis, a 35 kV oil-natural-air-natural (ONAN) transformer is utilized to carry out temperature-rise test. In measurement, dynamic temperature information of winding and radiator is recorded by fiber optic sensors (FOSs) and thermocouple probes respectively. It shows that the multi-scale model on the basis of entire circulating oil systems is in accordance with the experimental results. Finally, a traditional model without the radiator cooling ducts is constructed as a comparison to demonstrate the effect of radiator cooling ducts. The results indicate that the radiator model makes a great contribution to temperature distribution, and it can change the peak value and location of hot spot. Therefore, the multi-scale model, which can combinedly solve models in different dimensions, is a more accurate and efficient method in thermal-fluid coupling, and it can offer guidance to the transformer thermal insulation design. [ABSTRACT FROM AUTHOR]

• Novel approach for relays tuning using detailed mathematical model. • Hybrid Real-Time Power System Simulator is used as a source of necessary data. • Proposed approach allows to adapt protection settings to practical conditions. The correct operation of relay protection (RP) devices largely determines the security of electric power systems (EPS). The key point that in turn determines the behavior of protection in various emergency modes is a determination of their settings. The existing methodologies and tools often do not allow to provide RP settings adequate to the practical conditions of their functioning. This statement is confirmed by the statistics of faults in EPSs. The main reason for this issue is the inability to reliably reproduce transients in EPS via the software programs used in practice to calculate the RP settings. The Hybrid Real-Time Power System Simulator (HRTSim), developed by the authors, allows to reliably reproduce the entire range of normal and emergency modes and processes in EPS of any scale, topology and configuration by using detailed three-phase models of all power system components. Taking into account this possibility and features of the HRTSim, the task of detailed modeling of RP devices, including instrument current and voltage transformers, becomes relevant. The implemented special tools for RP modeling and the HRTSim in general allow to develop a new methodology for RP tuning. At the same time, the typical elements – relays – can be identified for each type of RP that determine their operation algorithm. This paper presents an analysis of the main relays of the most modern microprocessor-based RP devices and proposed principles of their tuning. The developed approach allows to adapt the RP settings to the practical conditions of their functioning in EPSs and to minimize the probability of RP maloperation. [ABSTRACT FROM AUTHOR]

• Phase-shifting transformers cause under-reach for the distance protection. • Improved phase and ground protection elements are proposed. • The proposed method uses local measurements. • Tap position of the PST is the data that needs sharing with remote relays. Phase-shifting transformers (PSTs) provide an economical solution to control the power flow in the electric grids. However, the PSTs vitiate the conventional protection system applied. This paper studies the impact of the delta-hexagonal phase-shifting transformer on the performance of the distance protection relays for the transmission lines. Modified distance protection formulas are derived for both phase and ground faults. The proposed method uses the local measurements at the relay point and does not require voltage or current measurements from a remote substation. Alternatively, the tap position of the phase-shifting transformer is the data needs to be shared between substations which can be accomplished using a simple communication method. The proposed method is evaluated on a simulated transmission line using MATLAB/Simulink packages. The results show the ability of the proposed method to compensate for the PST effect on the distance protection relays. [ABSTRACT FROM AUTHOR]

• Ferroresonance modes after single phase-to-ground faults in isolated neutral network are studied. • The way to topological model of a voltage transformer (VT) is explained. • Parameters of the VT model are found under uncertainties in its design and characteristics. • Possibilities to simplify the VT model by neglecting core losses are considered. It is commonly recognized that correct transformer model (particularly topological one) is a key element for any ferroresonance simulations. This paper represents a line of reasoning by which a reliable model of a voltage transformer (VT) can be constructed given substantial uncertainties in its parameters and characteristics. Starting with catalog data for electric steels and typical leakage inductances of 6-kV VTs, a model of an isolated neutral network with three inductive VTs is initially fitted to reproduce in detail its ferroresonant behavior for a chosen network capacitance. Then, validity of the network model is verified by accurate predictions of ferroresonance processes for all other capacitance values available and thus different ferroresonance modes recorded during full-scale factory tests. Possibilities to simplify the VT model by neglecting dynamic, and then entire core losses and hysteresis are also shown. [ABSTRACT FROM AUTHOR]