Low Voltage Winding Research Articles

Research on temperature distribution characteristics of oil‐immersed power transformers based on fluid network decoupling

AbstractDue to the complex structure and large size of large‐capacity oil‐immersed power transformers, it is difficult to predict the winding temperature distribution directly by numerical analysis. A 180 MVA, 220 kV oil‐immersed self‐cooling power transformer is used as the research object. The authors decouple the internal fluid domain of the power transformer into four regions: high voltage windings, medium voltage windings, low voltage windings, and radiators through fluid networks and establish the 3D fluid‐temperature field numerical analysis model of the four regions, respectively. The results of the fluid network model are used as the inlet boundary conditions for the 3D fluid‐temperature numerical analysis model. In turn, the fluid resistance of the fluid network model is corrected according to the results of the 3D fluid‐temperature field numerical analysis model. The prediction of the temperature distribution of windings is realised by the coupling calculation between the fluid network model and the 3D fluid‐temperature field numerical analysis model. Based on this, the effect of the loading method of the heat source is also investigated using the proposed method. The hotspot temperatures of the high‐voltage, medium‐voltage, and low‐voltage windings are 89.43, 86.33, and 80.96°C, respectively. Finally, an experimental platform is built to verify the results. The maximum relative error between calculated and measured values is 4.42%, which meets the engineering accuracy requirement.

Simulation and experimental analysis of dynamic thermal rise relaxation characteristics for dry-type distribution transformer

Abstract The temperature distribution characteristics of dry-type distribution transformers reflect their overall performance and quality. Distribution transformer daily load changes dramatically, its temperature fluctuates within a certain range. However, most of the research results on transformer temperature rise focus on the steady-state temperature characteristics of transformers, and lack of research related to dynamic temperature rise characteristics. This paper carries out a multi-physics coupling simulation and experimental verification of the dynamic temperature rise relaxation characteristics. The simulation uses the “electromagnetic-flow-thermal” multi-physics coupling analysis method to simulate the steady-state temperature rise characteristics and dynamic temperature rise relaxation characteristics of windings under different load rates. Then, this paper also explores the correlation between the dynamic temperature rise relaxation characteristics and load rate at the typical positions of high-voltage windings and low-voltage windings. The results show that the temperature rise of the winding shows the law of increasing the saturation index function with the increase of the load application time at any load rate. A quantitative characterization model of dynamic temperature rise relaxation behavior at different positions of transformer windings is constructed. It is found that the steady-state temperature rise value of different parts of the winding increases regularly as a power function with the increase of load rate, while the constant value of dynamic temperature rise relaxation time in different parts of the winding decreases as a power function with the increase of load rate. The verification results of the quantitative characterization model of dynamic temperature rise relaxation behavior show that the maximum error of the measured and theoretical steady-state temperature rise value is only 1.56 K, and the maximum error of the constant value of dynamic temperature rise relaxation time is only 0.082 h. These findings provide valuable insights into the performance and quality of dry-type distribution transformers, and can help improve their design and operation for better efficiency and safety.

Study of the thermal state of the transformer depending on the operating mode

Accurate analysis and prediction of the transformer's thermal condition depending on the operating mode, for example, in cold winters with a shortage of electricity, allows for effective planning of regular maintenance. In the course of the work, mathematical models were created to analyze the thermal state of the transformer, in particular, models for finding the temperature of the upper layers of oil and the highest temperature on the transformer winding. The data from these mathematical models were verified by comparing them with an already identified analog model. It was determined that the difference between the results is no more than 7 %. It has been established that the thermal state of the transformer is influenced by the ambient temperature much more than by the load. This is due to the fact that without cases of overload and emergencies, the load on the transformer, depending on the season, does not change significantly. It has been determined that the highest time utilization and the highest temperature on the high and low voltage windings are observed in August, which coincides with the peak ambient temperature. The lowest temperature on the windings and the lowest life utilization of the transformer are observed in January, which also correlates with the lowest ambient temperature. It is determined that under such operating conditions, given that the nominal service life of the transformer is 20 years, the actual service life will be approximately 90 years. It was also found that the reduction in service life in winter is 5 times less than in summer. This allows us to predict a reduction in maintenance needs during the cold months and more intensive maintenance in the summer. In addition, such models can predict potential problems and emergencies, which can significantly reduce the risk of unforeseen outages and increase the reliability of power supply. Regular monitoring and analysis of the thermal condition of the transformer makes it possible to respond quickly to changes in operating conditions and make timely maintenance decisions, which helps to optimize costs and increase the efficiency of power grids.

Simulation of temperature rise in low voltage winding of natural oil circulation transformer and analysis of oil flow heat dissipation

Abstract The heat dissipation performance of an oil-immersed transformer is the key to its stable operation. Winding loss is a crucial part of transformer heat loss, and winding heat dissipation greatly influences the rise in transformer temperature. In this paper, depending on the finite element method, the magnetic field distribution of a 66 MVA (225/26.4 kV) natural oil circulation transformer is analyzed, and the DC and eddy current losses of the low voltage winding are calculated. Based on the finite volume method, the temperature field distribution of the low-voltage winding is calculated, and the heat flux distribution of the contact surface between the disc insulation paper and the oil flow is obtained. It is found that there are transition points and extreme points between the upper and lower surface of the disc and the inner and outer surface of the disc, and the heat flux on the upper and lower surface of the disc conductor has obvious alternating oscillation. This study provides direction for further analysis and understanding of the heat dissipation mechanism and structural design of natural oil circulation transformers.

Analysis of interturn short circuit in regulating winding of power transformer based on field-circuit coupling

The interturn short circuit fault is one of the common faults in power transformers. At present, research on interturn short circuit faults focuses on high, medium, and low voltage windings, while there is relatively little research on interturn short circuit in regulating windings. Specifically, there is a lack of reported studies on the transient electromagnetic processes, magnetic field distribution, and electromagnetic force characteristics of interturn short circuits in regulating windings unconnected to the circuit. This study presents an actual fault scenario involving interturn short circuits occurring in the untapped portion of the regulating windings of a specific power transformer. A field-circuit coupled model was established to analyze the transient electromagnetic processes during the fault, and the model’s effectiveness was validated by comparing its results with actual fault recording data. Additionally, the magnetic field distribution and electromagnetic force characteristics during the fault were analyzed, and discussions were carried out regarding various ratios of short-circuit turns in the regulating windings. The results indicate that even when an interturn short circuit occurs in the portion of the regulating winding that is not connected to the circuit, the current in the short-circuited turns can reach several tens of times the rated value. Additionally, the leakage magnetic field and the electromagnetic force experienced by the short-circuited ring also increase significantly. The short-circuit ratio has a significant impact on the current of the short-circuited ring, leakage magnetic field intensity, and electromagnetic force. This study contributes to a better understanding of the impact of interturn short-circuit faults in the untapped portion of the regulating windings, offering crucial technical support for fault diagnosis and prevention of power transformers.

Investigating temperature rise dynamics at hot-spots within dry-Type transformer windings: A comparative analysis across varied loading rates and an extrapolative computational model

The dynamic temperature data of a transformer can be utilized to accurately assess its load capability and operational longevity. This paper presents a study on the temperature rise characteristics of hot-spots in dry-type transformer windings and develops a extrapolative computational model based on various load rates, utilizing the results of transformer temperature rise tests. Firstly, the dynamic temperature rise characteristics of B-phase low-voltage (LV) winding and high-voltage (HV) winding hot-spot, along with its correlation calculation model with varying load rates, were determined based on the results of temperature rise tests. Subsequently, an extrapolative computational model for the temperature rise of the winding hot-spot, utilizing a three-section continuous load application approach, was proposed employing the lumped parameter method. The validity of this model was confirmed through verification. Lastly, a temperature rise test was conducted on another dry-type transformer to validate the universality of the model. Finally, a different dry-type transformer is chosen, and a temperature rise test is conducted to confirm the general applicability of the model. The results indicate that the calculation model for the hot-spot temperature rise of the winding under three consecutive loads is associated with the dynamic temperature rise pattern of the hot-spot of the winding between the temperature rise test with a small load rate and the temperature rise test with a large load rate. The maximum deviation between the hot-spot temperature rise test of the winding of SCB14-800/10-NX2 transformer and the results of the steady-state temperature rise value calculation is only 1.3 K, with a maximum error of the time constant value of the temperature rise being only 0.05 h. Similarly, the maximum deviation of the steady-state temperature rise value obtained from the hot-spot temperature rise test and the extrapolative computation of SCB-1250kVA/10 kV transformer winding is only 2.03 K, with a maximum error of the temperature rise time constant value being only 0.09 h. The extrapolative computational model proposed in this study conducts the temperature rise test under a small load rate instead of a large load rate, addressing the industry challenge of the challenging implementation of transformer on-site temperature rise tests and high power loss. This approach enhances inspection efficiency.

Localization of Dual Partial Discharge in Transformer Windings Using Fabry–Pérot Optical Fiber Sensor Array

The power transformer is one of the most critical core devices for energy exchange in power systems, and its safe and stable operation is directly related to the reliability of the power grid. Partial discharge is the main cause of insulation degradation and failure of high-voltage electrical equipment. Online monitoring and accurate localization of partial discharge can provide information on the aging of power equipment, which is of great value for improving the safe operation and maintenance of the power grid. Internal dual partial discharge in transformer windings is a more complex type of fault. Since it is located inside the windings, the signal is attenuated and distorted, making it difficult for traditional monitoring methods to capture such partial discharge signal The Fabry–Perot optical fiber sensor is an ultrasonic detection method that can be built into the transformer interior. This sensor has high sensitivity and a small size, enabling flexible placement at different locations inside the transformer for precise partial discharge detection. Especially for the narrow space inside the high and low voltage windings, F–P sensors can form an array, utilizing the array’s directivity to locate the fault points. In this study, an ultrasonic detection system based on the F–P optical fiber sensor array was developed. The system utilizes a directional cross-localization algorithm based on the multiple signal classification (MUSIC) algorithm to accurately locate dual partial discharge sources. This partial discharge detection system was applied to a 35 kV single-phase transformer, enabling the localization of dual partial discharge sources within the high and low voltage windings. Combined with experimental results, this method exhibits high localization accuracy and is particularly suitable for detecting partial discharge phenomena that occur within or between transformer windings.

Structure design of 500 kVA HTS transformer and Joule loss in superconducting windings

HTS transformers can provide lower loss and higher efficiency with smaller size and lighter weight compared with traditional transformers. In this paper, we designed a 500 kVA HTS transformer and developed its simulation using finite element software based on the H-formulation, and the simulation incorporates a heat model and E-J power law. The simulation shows that there is a large radial flux leakage at the end of the low-voltage (LV) windings (superconducting windings), leading to a reduction in critical current density and an associated increase in joule loss in the LV winding. In order to reduce joule loss, various LV winding structures were designed. The results demonstrate that either increasing the axial distance between the coils at the LV winding end or installing a flux diverter outside the LV windings can effectively diminish the radial flux leakage at the LV winding end, consequently reducing joule loss in the LV winding. The impact of the size, position and relative permeability of the flux diverter on radial flux leakage and joule loss are also studied. These structure optimizations and the corresponding effects have important significance to the design of HTS transformers.

Diagnosis of Radial Deformation and Axial Displacement Faults in Power Transformer Windings using X-ray and Compton Backscatter Imaging

Continuous and uninterrupted transmission and distribution of electrical energy is highly dependent on the flawless operation of power transformers. Failure of these critical components can lead to network power interruptions and consequently have severe detrimental consequences. Therefore, timely detection and diagnosis of power transformer faults is crucial not only to prevent the escalation of faults but also to minimize repair and maintenance costs. Conventional methods for inspecting windings faults have limitations such as the need to take the transformer out of service, unreliable results, and the addition of auxiliary devices inside the transformer, such as dielectric windows. In contrast, the proposed method in this paper utilizes a Compton Backscatter Imaging (CBI) or X-Ray Transmission (XRT) device located outside the transformer, eliminating the need for any additional equipment. This enables online scanning of any number of transformers to assess their health status and identify the type, extent, and progression of potential internal faults. Experimental results demonstrate the feasibility of the proposed method for detecting radial deformation and axial displacement and disk space variations faults of varying magnitudes and locations in both high-voltage and low-voltage windings of power transformers.

Quasi-one-dimensional approach to heat transfer: General formulation and its application to transformer windings

A quasi-one-dimensional approach to conduction heat transfer with convective boundary conditions is developed for bodies of variable geometry. The novelty of the method is demonstrated on the windings in oil-filled distribution transformers with partial cooling ducts. We obtain analytical solution for the windings average temperature rise and tangential temperature distribution and find them in an excellent agreement with the FEM simulation, numerically confirming the validity of the reduction from 2D to 1D. When the quasi-one-dimensional approach is applied to calculation of temperature rise for the low and high voltage windings in industrial setting we obtain 1% and 5% discrepancy with the industrial tests respectively. Hence, the method offers reliable estimates of the temperature rise, and temperature distribution in a chosen direction, in bodies of variable geometry.

Research on the Technical and Economic Development of Large Megawatt Wind Turbines Based on Medium-Voltage Electrical System

INTRODUCTION: With the advent of the "bidding era" and "parity era" in the wind power market, the competition of the whole machine factory is becoming more and more fierce, and the capacity of the single fan is getting larger and larger, which becomes the key to the design of the fan electrical system of the large-capacity unit (5.XMW or more). At present, the low-voltage wind power system (690V,1140V) is the common solution for wind turbines. However, due to the limitation of the cable section of low-voltage electrical system and the increase of the rated current of the generator, the increase of the capacity of a single machine makes more cables from the generator side to the grid, and the cost also increases.
 OBJECTIVES: Aiming at the future large megawatt wind power market, the medium voltage Doubly-Fed electrical system solution is proposed to increase the higher generation and electricity income of wind farms and reduce the manufacturing cost of wind farms.
 METHODS: The technology and economy of medium voltage and low voltage electrical system are compared.
 RESULTS: With the gradual increase of single capacity, the economy of medium pressure wind power generation system is getting better and better, and the higher the height of the tower, the better the economy. At the same time, the reduction of the rated current of the generator brings about the reduction of line loss and the increase of power generation. The number of cables is greatly reduced, and the construction cost and difficulty of cable laying will be greatly reduced.
 CONCLUSION: In response to the technical trend of large-capacity wind turbines in the future, the medium-voltage wind power generation system has a good application prospect, both from the economic and technical point of view.

Optimized Design of New 24-pulse Rectifier

Compared with the conventional 24-pulse rectifier, the low-voltage winding of the rectifier transformer must be connected in the y and d modes, so that the voltage phase of the low-voltage side of the transformer is separated by 30° in sequence. This paper uses the phasor method to obtain a set of phase voltages that move towards 15° through the equivalent of three-phase voltages, so that the two sets of voltages produce a phase difference. Then, through decoupling and voltage reduction, the electrical quantity is converted into a signal quantity, the voltage level is reduced, and then the electrical quantity output is obtained to obtain the required voltage value. Finally, the required DC power is obtained through the rectification system. The effectiveness of the reverse model of this paper is verified by comparing the results and efficiency with the conventional 24-pulse rectifier.

Calculation of electromagnetic force and temperature distribution of amorphous transformers in different operating modes by finite element method

The study used the Finite element method (FEM) by ANSYS 3D software to calculate the distribution of electromagnetic force (EMF) and temperature on the high voltage and low voltage windings (HLVWs) of the amorphous steel core 3 phase transformer (ASCT) which has a power of 1000kVA –22/0.4kV under different load cases: No load, full load and short circuit (SC). The obtained results show the exact temperature distribution and location where the highest temperature is found on the HLVWs of ASCT. From the analysis of the temperature distribution on the HLVWs, it is shown that when the transformer falls into an SC fault, it causes the greatest EMF and thermodynamic force, causing extremely heavy consequences for the transformers. The obtained results help designers, manufacturers, and operators have the most suitable options to improve strength of SC and increase the life of the transformer.

Double B-type magnetic integrated transformer based input-parallel output-parallel LLC resonant converter modules

The LLC resonant converter used in high-power situations suffers from the problems of high conduction loss and current stress, which can be solved using input-parallel output-parallel (IPOP)-connected converter modules. However, this leads to a multiple increase in the number of magnetic components, which reduces power density. Magnetic integration technology is an effective way to reduce the volume of converters. Currently, the magnetic integrated transformer based on EE-type cores is widely used to realize miniaturization, and it uses leakage inductance instead of resonant inductance to improve power density. However, leakage inductance is difficult to control, and the external radiated magnetic field will produce serious eddy current loss and electromagnetic interference. This article proposes a novel double B-type magnetic integrated transformer, which can integrate the magnetic components of two LLC resonant converters simultaneously and where the resonant inductances are wound independently. The structure contains four low reluctance branches, which are used as the cores of the transformer and the resonant inductance. The decoupling integration method, which integrates the four components into a single core, has been used to increase core utilization and improve power density. On this basis, the transformer’s high- and low-voltage windings are cross-arranged to reduce the magnetic field intensity in space, further decreasing the loss and electromagnetic interference. Compared with the EE-type magnetic integrated transformer, the volume of the proposed structure is reduced by 5.9%. A 400W experimental prototype is built, and the results verify the validity of the design.

Numerical study of the cooling capacity of several alternative liquids in zig-zag cooling system of a power transformer

Mineral oil is the usual coolant of the power Transformers. Nonetheless, the high risk of fire of this liquid due to its low ignition point and its scarce biodegradability has promoted the development of dielectrics alternative fluids with better physical-chemical properties. However, these new liquids have to be as good coolants as mineral oil in order to maintain the power transformer performance. This numerical study analyzes the coolant capacity of some of these new commercial liquids (a natural ester, a synthetic ester, and a silicone oil) by using the zig-zag cooling system of the low voltage winding of a real power transformer. Two types of thermal-fluid studies (parametric and time-dependent studies) have been made by using a 3D model of this system. The comparison of temperatures obtained from the parametric study (different inlet flow rates) allow us to say that the worst coolant is the silicone oil. Also, it can be say that a significant increase in the inlet flow rate produces mild drops of hot-spots temperatures and no modification in the initial temperature distribution. For the time dependent study (25% of overload during 1 hour), the hot-spot temperature exceed the limits, which negatively affects the aging speed of the insulating paper and therefore the transformer lifetime.

Simulation calculation of fluid field- thermal field of oil-immersed transformer and optimization of winding structure parameters

The optimization method of winding structure parameters of oil-immersed transformer is proposed to reduce the hot spot temperature of transformer and the metal conductor consumption based on the coupling calculation method of electromagnetic-fluid-thermal. Firstly, according to the electrical and structural parameters of the oil-immersed transformer, a 3D simulation model is established by using finite element software, and the distribution of the thermal field and the fluid field around the transformer as well as the fluid field and thermal field of the low-voltage winding are obtained. In order to accurately reflect the temperature rise distribution characteristics of the low-voltage winding, a 2D model of the low-voltage winding of the transformer was established considering the calculation efficiency and accuracy, and the thermal field and fluid field distribution of the two low-voltage winding models were compared and analyzed. The temperature error was less than 5.98?, which verified the accuracy of the equivalent model. On this basis, the hot spot temperature of low voltage winding under different structural parameters is obtained by combining the Latin hypercube experiment design and thermal field simulation method, and the response surface model of winding hot spot temperature and structural parameters is established. Taking the hot spot temperature of low voltage winding and the amount of metal conductor as optimization objectives, combined with the response surface model, the optimal solution set of Pareto is obtained by using NSGS-? optimization algorithm, and two kinds of optimal design results are obtained on the Pareto front surface. The results show that the hot spot temperature is reduced by 4.16% and the metal conductor consumption is reduced by 13.79% in scheme 1; the hot spot temperature is reduced by 0.51% and the metal conductor consumption is reduced by 28.46% in scheme 2. The research results have important guiding significance for the optimization of oil-immersed transformer.

Analysis and Discussion on the Current State and Demand of Energy Supply for Highway Transportation Facilities

Energy supply is a crucial factor in realizing the future transformation of highway transportation. Determining a comprehensive and sustainable energy supply network that aligns with the demands of future highway transportation facilities - ensuring they are "safe, reliable, environmentally friendly, efficient, and sustainable" - poses a key challenge for current highway developers and operational managers. This study examines the types and energy consumption of significant energy-consuming facilities along expressways. It delves into the current state of power supply and distribution technologies for highway transportation facilities, including low-voltage, medium-voltage, and complementary wind and solar power systems. Additionally, the research analyzes existing issues within power supply and distribution systems for highway transportation facilities. The study investigates the load characteristics and spatial distribution of electricity-consuming facilities along expressways, concluding with an analysis of the future trends in energy demand for highway transportation facilities. This comprehensive exploration encompasses aspects such as the scale of energy demand, the diversification of energy supply, and the need for a secure, efficient, and intelligent energy supply. The findings aim to provide valuable insights for developing innovative, diverse energy supply, and control technologies for safe, efficient, and interconnected highway transportation facilities.

Detection and localization of LV winding radial deformation in transformers using electromagnetic waves - a feasibility study

Recently, online methods based on electromagnetic waves have been proposed to detect the mechanical defects of high voltage (HV) windings in power transformers. In this article, for the first time, the possibility of online detecting and locating the radial deformation (RD) of low voltage (LV) windings using electromagnetic waves is presented. In the proposed method, a high frequency and wideband signal is sent to a simplified model of transformer winding via small antennas. The antennas are connected to the inner side of the oil tank cover through Radio Frequency (RF) cables, so that online monitoring can be realized with minimal changes in the transformer structure. The data of the reflected signals in different states is recorded in a database and sorted as primary data bank. By comparing the results of the sound state with measured signals, the possible faults can be detected. In this paper, the detection and location of radial deformation is estimated by using regression tree based on three features. Several cases are simulated by Computer Simulation Technology (CST) software, and their verification are conducted by a setup in laboratory. Based on comparison results, it can be claimed that the proposed method has LV windings radial deformation online detecting and locating ability with an acceptable accuracy.

Research on Magnetic Characteristics and Short-Circuit Force of 3D Wound Core Transformer

The problem of eddy current loss in power transformers directly affects the technical economy of the transformer, which is a problem that cannot be ignored in the design and calculation of power transformers. Studying how to reduce the eddy current losses caused by leakage magnetic fields is a difficult point in transformer design. The load loss of low-voltage foil copper windings in a 10 kV/400 kVA 3D wound core transformer is calculated, and the three-dimensional finite element models of the transformer are established by Using Magnet software and COMSOL Multiphysics software, and the calculation results are compared. At the same time, the short-circuit force of foil windings is shown in the simulation. The results show that the three-dimensional finite element models are correct.

Design, development, and testing of a 6.6 MVA HTS traction transformer for high-speed train applications

High-temperature superconducting traction transformers (HTSTTs) have the merits of small size and lightweight in comparison with their conventional counterparts. This article reports the development progress of a 6.6 MVA HTSTT operating at 65 K, including the design, testing, and system cooling. The introduction of flux diverters and an optimized winding design realized a short-circuit impedance higher than 43% and AC loss less than 3 kW. The insulation structure was designed to pass insulation tests specified in standard in China GB/T 25120-2010. An open cooling system with reduced pressure was developed, which realized the efficiency of the 6.6 MVA HTSTT above 99%. Before assembling the prototype transformer, we conducted tests for critical current and dielectric performance of the HTS double pancake coils (DPCs) used in high-voltage (HV) and low-voltage (LV) windings to verify the current-carrying and insulation performances of each DPC. Finally, we measured the critical current and no-load loss of the HTSTT prototype at 77 K. Test results showed that the mass of the transformer is 33% less than conventional transformers. At 77 K, the critical current of the LV winding and HV winding is higher than 700 A and 50 A, respectively. Moreover, the HTSTT on a no-load test reached the test voltage of 25 000 V and loss of 6 kW. In the next step, we will continue to conduct experimental research, and verify the feasibility of the HTSTT on the train, and develop a circulating cooling system, all meeting the commercial requirements of the HTSTT.