High Voltage Power Transformers Research Articles

Hints to lightning breakdown enhancement for synthetic ester insulating oil: Unraveling the impact of molecular chain length on electron structural parameters under electric field stress

The high flash point and biodegradability of synthetic ester insulating oil offer transformers both operational stability and environmental protection. However, its poor lightning breakdown characteristics under long oil gaps and inhomogeneous electric field often lead to rapid streamer discharges, posing challenges and limits to the use of synthetic ester insulating oil and hinders the development of low-carbon, eco-friendly high voltage power transformers. To address this issue, we propose improving the lightning breakdown characteristics through molecular chain length design. Using density functional theory, time-dependent density functional theory, and wave function analysis, we investigate how microscopic factors like molecular polarity, discharge active sites, frontier orbitals, traps, ionization and electron affinity, and excitation processes influence macroscopic discharge characteristics. Our findings show that under an electric field, chain length is positively correlated with electron transfer, enhancing charge polarization. Chain length also correlates positively with dipole moment, facilitating the formation of space charge centers. While chain length minimally affects discharge active sites, the electric field can intensify their formation. Additionally, a longer chain length decreases the energy gap, increasing electron trap levels more than hole trap levels, thus capturing charges longer. Chain length is negatively correlated with ionization energy, promoting electron impact ionization, and has little effect on the excitation energy of S(0) → S(1), making excitation under high electric fields unlikely.

Effect of an electromagnetic field generated by power infrastructure on the spatial orientation of developing sea trout embryos Salmo trutta Linnaeus, 1758

In recent decades, rapid technological advancements have led to a growing influence of anthropogenic electromagnetic fields on aquatic ecosystems. These fields predominantly stem from energy infrastructure facilities such as wind farms, high-voltage lines, underwater transmission cables, and power plants. As a result, there has been increasing interest in understanding the effects of heightened electromagnetic fields on aquatic ecosystems and their inhabitants, including developing fertilized eggs. This study focused on examining the effects of electromagnetic fields generated by high-voltage lines and power transformers, which are integral to transmission nodes and power plants, on the directional axis of symmetry in sea trout Salmo trutta (Linnaeus, 1758) embryos and their survival rates. The research was carried out at “PGE Dolna Odra in Nowy Czarnów”, one of Poland’s largest power plants (53°12′34′′ N, 014°28′16′′ E). Two experimental sites were selected, with electromagnetic field values ranging from 1.36 to 5.28 μT at the first site and 8.97 to 14.18 μT at the second site. A control sample was also established in an area unaffected by anthropogenic electromagnetic fields. The study findings revealed that electromagnetic fields originating from energy infrastructure facilities significantly affect the spatial orientation of sea trout embryos. The embryos’ response was influenced to be influenced by both the strength and direction of the electromagnetic fields. At the first test site, lower field values caused deviations from the natural alignment axis in the geomagnetic field (north-south), resulting in a shift towards the primary field source. In contrast, the second test site, with higher electromagnetic field values, showed no significant spatial orientation in embryo alignment. Additionally, the study highlighted that higher electromagnetic field values accelerated the rate of embryogenesis but also led to an increased mortality rate among sea trout embryos.

Case Study and Comparison of High Voltage Power Transformer Dissolved Gas Analysis Assessed by Different Standards

A power transformer is a vital equipment in a power system. To prevent transformer failure, it is necessary to assess the transformer condition, one of which is through the DGA test. IEC 60599 and IEEE C57.104 become guidance for transformer assessment through the DGA test. DGA is a method of determining transformer conditions and fault diagnosis by assessing transformer oil data samples. H2, CO2, C2H4, C2H2, C2H6, CH4, CO, O2, and N2 are among the dissolved gases measured to monitor transformer conditions. To determine the faults, the IRM, RRM, and DTM are used. The fault detected on 8 transformer case studies showed T1, T2, T3, PD, D1, and D2. Maintenance recommendations for 8 power transformers evaluated were presented.

전력용 변압기의 고장 메커니즘 분석을 통한 실규모 화재폭발 시험 기법 개발

Failures in high voltage power transformers are classified as a kind of disaster accompanied by fires and explosions. It could lead enormous loss of property due to equipment damage and restoration, and black-out of power grid can take place by separating the fault section from the power system. In particular, power facilities including transformers have been intensively installed in Republic of Korea since the 1988 Seoul Olympics. These power equipment is operated for more than 30 years from then. The power transformers in long-term operation are aged and a number of failures tend to increase, therefore, necessity of developing transformer fire prevention and suppression technology is emerging. This paper deals with the development of 154 kV class power transformer real-scale fire test method through its fire and explosion mechanism analysis to advancement fire prevention and suppression technology on power transformer. The test method includes, test items, subjects, facilities, and test site. The established transformer fire mechanism could be applied to develop new investigation method for cause of transformer fire, and real-scale test methodology could be widely utilized for various research such as transformer fire and explosion test, development of its extinguishing technology.

On Improving the Technology of Pulse-Flaw Detection of High-Voltage Power Transformer Windings

On Improving the Technology of Pulse-Flaw Detection of High-Voltage Power Transformer Windings

On improving the technology of pulse defectography of windings of high-voltage power transformers

On improving the technology of pulse defectography of windings of high-voltage power transformers

Mixture of Faults Identification on Power Transformers Using Multi-Method Dissolved Gas Analysis

Utilities rely on the various Dissolved Gas Analysis (DGA) interpretation methods to identify incipient fault within power transformers. However, confusion often arises due to the use of different DGA methods which can have different results or fail to detect mixture of faults (multiple faults). This paper implements the combination of Duval Triangle and Duval Pentagon as one of the most consistent methods to do fault identification based on DGA. Recent historical DGA data of generator step up high voltage power transformers and other several previously published historical data were collected and analyzed. This article aims to assist transformer asset manager in detecting mixture of faults using multi method. The use of combined Duval triangle and Pentagon method is done in accordance to the guideline in IEEE C57.104-2019.

Recognition of overheating in different temperature ranges, which is accompanied by discharges of different intensity, based on the dissolved gas analysis

In wartime and post-war reconstruction, it is the responsibility of a rather ageing fleet of electricity generation equipment to provide a reliable supply of electricity to the households and industry. The challenge is to ensure operational reliability and to extend the service life of this equipment. Non-destructive testing methods are the most practical in terms of operation. For non-destructive testing of high-voltage power transformers, the dissolved-oil gas analysis is the most common and widely used method. This method, using various diagnostic criteria (value of gas ratios, percentage of gases, etc.), allows most of the defects (both electrical and thermal type) occurring in a transformer to be detected. However, during operation, so-called combined defects can occur, i.e. discharges turning into overheating, or overheating accompanied by discharges. In addition, two or more defects of different types can occur in the same transformer. The values of the criteria used for the recognition of the combined defects differ significantly from the values of the criteria typical for thermal or electrical defects, which significantly complicates the recognition process. The purpose of this article is to analyse the values of diagnostic criteria as well as the recognition reliability of overheating in different temperature ranges, accompanied by discharges with different intensity. For a comprehensive analysis, the results of dissolved gas analysis were used for 471 high-voltage power transformers in which overheating in different temperature ranges, accompanied by discharges of different intensity, were detected. The 471 analysed values were split into 38 arrays with similar gas percentages, gas ratios and defect nomograms. Based on an analysis of the recognition reliability of overheating in different temperature ranges, using the norms and criteria regulated by the best known standards and methods, it was found that the highest recognition reliability is provided by the values of gas ratios, which are regulated by the ETRA Square and also by Nomogram method.

Enhancement of the Technique for Calculation and Assessment of the Condition of Major Insulation of Power Transformers

The findings of the analysis of data on the accident rate of power transformers indicate that one of the main causes of their failures is a decrease in the dielectric strength of the insulation. To reduce failures and extend the service life of power transformers in operation, the issue of enhancing the techniques for assessing the condition of their internal insulation becomes relevant. Currently, when selecting the major insulation of transformers, one takes into account the dependency of the dielectric strength of the oil passage on its width. Experts discuss the issues involved in the choice of major insulation while taking into account the effect of the generalized factor being the volume of the oil passage. The solution to that problem largely depends on the study of the statistical characteristics of the dielectric strength of oil passages of different volumes and the effect rated parameters of transformers have on them. The efficiency of the application of such diagnostic characteristics depends on the extent of studies available on them and the establishment of their standardized parameters. The paper proposes a method for estimating the change in the transformer oil volume in stressed oil passages of major insulation of high-voltage power transformers and statistical characteristics of the dielectric strength of these passages while taking into account the effect of the rated values of capacity and voltage of transformers. It is shown that the degree of effect of transformer technical parameters on the statistical characteristics of the dielectric strength of oil passages depends on the quality of transformer oil, which undergoes a change in operating conditions.

Geomagnetically Induced Current Analysis in Malaysian Power Transmission System

For many decades, Geomagnetically Induced Current (GIC) has posed a significant risk over the electrical power grid infrastructures worldwide. The phenomenon occurs due to geomagnetic disturbance (GMD) and related space weather events arising from solar activity. It represents a potential hazard to the secure and safe operation of electrical power grids by causing half-cycle saturation of grounded High Voltage (HV) power transformers, relay misoperation, and increased reactive power demand in the power systems. Previous studies have shown that the occurrence of intense GIC is not limited to high and mid-latitude regions, but powerful space weather events can also result in intense GIC in power systems located in lower geographic latitudes. This study aims to estimate GIC and its impacts on a Malaysian power grid. A network model of the grid was constructed by using the Power System Simulator for Engineering (PSS/E). This represents the first attempt to study GICs in south-eastern Asian power grids since a region is considered to have low GIC risk up to now. During the analysis, firstly, we exposed the entire power network which includes 500 kV, 275 kV, and 132 kV system voltages to different geoelectric field strengths in the 0–180° directions. The applied geoelectric fields were calculated based on benchmark value for 1 in 100 years of GMD events. Then we disconnected the 132 kV low voltage systems from the network model to investigate their influence on the calculated GICs. The results demonstrated that the most vulnerable substations to GMD events and experienced large GICs were those located in the middle of the Malaysian power network. The maximum GIC was recorded at substation 22 with the value of 44.58 A due to the peak electric field of 1.2 V/km at 100° field direction. Also, the results showed that the calculated GICs slightly increased when 132 kV systems were removed from the power network, especially at the substations directly connected to these systems.

Probabilistic hazard assessment: Application to geomagnetic activity

Probabilistic Hazard Assessment (PHA) provides an appropriate methodology for assessing space weather hazards and their impact on technology. PHA is widely used in geosciences to determine the probability of exceedance of critical thresholds, caused by one or more hazard sources. PHA has proved useful with limited historical data to estimate the likelihood of specific impacts. PHA has also driven the development of empirical and physical models, or ensembles of models, to replace measured data. Here we aim to highlight the PHA method to the space weather community and provide an example of how it could be used. In terms of space weather impact, the critical hazard thresholds might include the Geomagnetically Induced Current in a specific high voltage power transformer neutral, or the local pipe-to-soil potential in a particular metal pipe. We illustrate PHA in the space weather context by applying it to a twelve-year dataset of Earth-directed solar Coronal Mass Ejections (CME), which we relate to the probability that the global three-hourly geomagnetic activity index Kp exceeds specific thresholds. We call this a “Probabilistic Geomagnetic Hazard Assessment”, or PGHA. This provides a simple but concrete example of the method. We find that the cumulative probability of Kp > 6−, > 7−, > 8− and Kp = 9o is 0.359, 0.227, 0.090, 0.011, respectively, following observation of an Earth-directed CME, summed over all CME launch speeds and solar source locations. According to the historical Kp distribution, this represents an order of magnitude increase in the a priori probability of exceeding these thresholds. For the lower Kp thresholds, the results are somewhat distorted by our exclusion of coronal hole high-speed stream effects. The PHGA also reveals useful probabilistic associations between solar source location and subsequent maximum Kp for operational forecasters.

Recognition of overheating in different temperature ranges in high-voltage oil-filled equipment by the dissolved gas analysis

One of the significant problems that largely determine the operational reliability of power transformers and, in some cases, the reliability of electricity supply to consumers, is the ageing of this equipment fleet. Therefore, the task of development and improvement of diagnostic methods that allow detecting faults of high-voltage power transformers at an early stage is relevant and practically significant. One of the most widespread methods of non-destructive diagnostics, which allows detecting up to 90 % of all possible defects of high-voltage oil-filled equipment, is the method of dissolved gas analysis. Today, most well-known standards and authors' methods use several diagnostic criteria to detect overheating in different temperature ranges based on the results of dissolved gas analysis. However, these standards and techniques differ considerably in the values of the diagnostic criteria, making it impossible to make an unambiguous diagnosis for the same data using different standards and techniques. Since local overheating of individual parts of insulation and structural elements of high-voltage power transformers is one of the common types of defects and such defects can develop over several years, their development is a serious threat to the insulation of transformers. Therefore, the results of dissolved gases analysis for 1278 high-voltage power transformers in which overheating in different temperature ranges was detected were used for a comprehensive analysis. The 1278 values analysed were split into 31 data sets with similar gas percentages, gas ratios and defect nomograms. Based on an analysis of the recognition reliability of overheating in different temperature ranges, using the norms and criteria regulated by known standards and methods, it was found that the highest recognition reliability is provided by the ratio values regulated by IEC 60599 and the National Standard of Ukraine.

Assessment of the current technical condition of electrical equipment based on an artificial neural network

Assessment of the current technical condition is an important task, so the state of electrical equipment depends on its further operability. Thanks to modern computing devices, it is possible to implement actively artificial intelligence and computer-assisted learning methods that allow achieving high efficiency in data processing. A study was conducted and an algorithm for diagnosing the technical condition based on an artificial neural network was developed. A model based on a multilayer perceptron is proposed, which allows evaluating the technical condition of a high-voltage power transformer. The result of the technical diagnostics of the model is the assignment of the condition to one of the five classes, proposed by the guidelines presented by the International Council on Large Electrical Systems. The methodology is presented on the example of a 250 MVA transformer with a certain defect history, which allowed us to show the reliability and validity of the obtained results. It is shown that the use of the proposed model makes it possible to achieve accuracy in determining the technical condition of 0.95. The introduction of this model into an automated monitoring and diagnostics system will allow assessing the technical condition of electrical equipment in real time with sufficient accuracy.

Data-Driven Anomaly Detection in High-Voltage Transformer Bushings with LSTM Auto-Encoder.

The reliability and health of bushings in high-voltage (HV) power transformers is essential in the power supply industry, as any unexpected failure can cause power outage leading to heavy financial losses. The challenge is to identify the point at which insulation deterioration puts the bushing at an unacceptable risk of failure. By monitoring relevant measurements we can trace any change that occurs and may indicate an anomaly in the equipment’s condition. In this work we propose a machine-learning-based method for real-time anomaly detection in current magnitude and phase angle from three bushing taps. The proposed method is fast, self-supervised and flexible. It consists of a Long Short-Term Memory Auto-Encoder (LSTMAE) network which learns the normal current and phase measurements of the bushing and detects any point when these measurements change based on the Mean Absolute Error (MAE) metric evaluation. This approach was successfully evaluated using real-world data measured from HV transformer bushings where anomalous events have been identified.

Power transformers as excess heat sources – a case study for Denmark

Large-scale heat pumps (HPs), biomass CHPs and excess heat (EH) from industry and data centres are promising district heating (DH) sources. Electricity and thus power transformers (PTs) will be an important part of the future energy system, which opens the possibility to use the thermal losses occurring in PTs for DH. The present paper analyses high voltage PTs in Denmark as DH sources. First, we employ a thermodynamic model of PTs to determine the EH they produce. Subsequently, we analyse thermodynamic properties of heat exchangers and HPs necessary to utilise EH for DH. Finally, we perform a GIS analysis to link the PTs with specific DH networks. From the theoretical amount of excess heat from power transformers (EHPT) available for DH of 0.28 TWh per year, 0.12 TWh or 0.5% of Danish DH demand can reach the consumers. 0.07-0.21 GWh can reach the consumers below the average DH price. The entire EHPT potential can be utilised through HPs, working with an average COP of 4. The sensitivity analysis showed that the EHPT can supply up to 2.26% of the Danish DH demand. Therefore, EHPT is a relatively small DH source on the national scale but could be an important local option.

Potential of Used Cooking Oil as Dielectric Liquid for Oil-Immersed Power Transformers

This paper presents the potential of using treated used cooking oil as a potential dielectric liquid for high-voltage power transformers. The acidity of used cooking oil is higher than the prescribed limit of natural ester for power transformers. Hence, this paper proposes a combination of treatment methods to reduce the acidity of used cooking oil to less than 0.06 mg KOH/g. The treatment methods are: (1) alkaline refining, (2) bleaching and (3) synthetic silicate treatment. The physicochemical and electrical properties (acidity, color, density, flash point, viscosity, water content, breakdown voltage and dissipation factor) of the treated used cooking oil are measured according to standards. The results show that the acidity of the treated used cooking oil is 0.0182 mg KOH/g, which is 99.67% lower than the initial value of 5.4664 mg KOH/g. This acidity is achieved by implementing the following treatment methods: (1) alkaline refining using caustic soda (normality: 2.0N); (2) two cycles of bleaching with Fuller earth's adsorbent (concentration: 10 wt%) in each cycle; and (3) treatment with synthetic silicate absorbent (concentration: 15 wt%). In addition, combined treatment methods affect the color rating of the treated used cooking oil, where the color rating is L0.5. The breakdown voltage and dissipation factor at 90 °C of the treated used cooking oil are found to be 100 kV and 0.00688, respectively. The other physicochemical properties (density, viscosity and water content) of the treated used cooking oil fulfills the BS EN 62770 standard except for the flash point, with a value of 234 °C.

ANALYSIS OF FACTORS INFLUENCING THE INTENSITY OF TRANSFORMER OIL AGEING IN LONG-TERM OPERATION

The results of the analysis of influence of operational factors and peculiarities of transformer design on intensity of oxidative reactions of transformer oils in the tanks of 110 kV high-voltage power transformers are presented. As a result of two-way analysis of variance it was found that with increasing operating time there is a statistically significant increase in the value of the organic acid content in the oil. At the same time the rate of increase in the organic acids content in the transformer oil, operated under different conditions, significantly differs, which indicates a significant influence of operating conditions on the intensity of oxidative reactions. The results of two-way analysis of variance also show that the effects of changes in factor levels are not additive, that is, the effect of a change in the level of influence of one factor leads to a change in the effect of the level of influence of another. In other words, the process of oil oxidation is cumulative and a certain level of organic acids in the oil can be achieved either over a longer period of operation, but with relatively 'light' operating conditions, or over a shorter period of time, but with more 'heavy' operating conditions. In order to determine the factors most affecting the intensity of oxidative reactions, an analysis of the quality of filled oil, operating time, the influence of the region, the influence of the type and nominal characteristics of transformers on 6 data sets with identical rates of oxidative reactions was carried out. The results of the analysis show that the intensity of oxidation reactions is strongly influenced by the operating time, the transformer loading factors, the consumer composition (region of Ukraine) as well as the type and quality of oils. At the same time, factors such as rated capacity, type of transformer, number of windings, and the value of rated voltage on the medium and low voltage windings do not influence the intensity of oxidation of oils. The results obtained allow the correction of the maximum permissible values of oil acidity, taking into account the factors affecting the intensity of oxidation of oils.

Finite difference method for electric field optimization in high voltage power transformer bushings using engineering simulation and 3D design program

The electric field optimization minimizing the field strength on an electrode surface and providing its uniformity is important in designing high voltage power transformer bushings and other apparatus from the viewpoint of efficient utilization of the electric field space. The high voltage power transformer bushing with cylinder electrode system has been designed and tested in this investigation. It was found that the insulation method of the cylinder electrode was the most important factor to lower streamer initiation voltage. The optimized design uses both internal and external elements for electric stress grading at critical parts of the bushing. Applying optimization theory based on charge simulation method, the author developed a computation program for electric field automatic optimization in 3D dielectric axisymmetric field. The results of the computation realized some excellent electrode profiles with uniform electric field distribution. Moreover, the discrepancy from the electric field uniformity on 3D dimensional profile caused by applying it to an axisymmetric electrode was discussed. Then a new electrode with uniform field distribution was obtained by using the computation program for optimization.

Experimental studies on the influence of benzyl benzoate on viscosity of vegetable oil based insulating liquids for power transformer

For insulating liquids in power transformers, the development of low viscous vegetable oil based liquids has been proposed in this work. The viscosity reduction of vegetable oil based liquids has been achieved by processing of the natural esters with different levels of concentration of aromatic solvent of benzyl benzoate. Two categories of vegetable oil based liquids, namely edible such as sunflower oil, palm oil and sesame oil, and non edible such as honge oil, neem oil and punna oil are considered for this investigation. The analysis is also extended to investigate the influence of benzyl benzoate on other critical properties of vegetable oil based liquids such as breakdown voltage, flash point, moisture content, acidity, and density. From the experimental results, it is revealed that inclusion of benzyl benzoate has shown considerable reduction in viscosity of vegetable oil based liquids. Also based on properties of processed natural esters, results have given away the positive encouragement towards the development of low viscous insulating liquids with combined benzyl benzoate and natural esters for application in high voltage power transformers.

Inductance Calculation of HTS Transformers with Multi-segment Windings Considering Insulation Constraints

High-temperature superconducting (HTS) transformers with multi-segment windings have been proposed in earlier literatures for the hysteresis loss reduction and short circuit electromagnetic force mitigation. The optimum distributive ratios for these multi-segment windings have been determined in earlier literatures. Asymmetrical seven segment winding with its optimum distributive ratio (k = 1/12) results in minimization of short circuit electromagnetic forces under normal tap and any adjusted tap. However, insulation design and leakage inductance calculation for the multi-segment winding of an HTS transformer have not been addressed in past literatures. Insulation design is one of the most crucial aspects in the transformer design, especially for high voltage power transformers. The analysis and optimization of the electric field distribution over the winding’s segments insulation in these HTS transformers provide the required dimensions for precise determination of the leakage inductances. In this paper, for the first time, winding insulation design and leakage inductance calculation of a 200 MVA, 230/20 kV HTS transformer with asymmetrical seven segment winding (and k of 1/12), are carried out. The analytical method besides a two-dimensional (2D) finite element method (FEM), through COMSOL Multiphysics software, is utilized for insulation design and calculation of leakage inductances of this transformer.