Transformer Insulation System Research Articles

Adaptive-Optimization-Based Digital Simulation Database for Nonuniform Aging Diagnosis of Transformer Insulation System

Adaptive-Optimization-Based Digital Simulation Database for Nonuniform Aging Diagnosis of Transformer Insulation System

Expert system for improving and controlling insulation system of service transformers using fuzzy logic controller

Expert system for improving and controlling insulation system of service transformers using fuzzy logic controller

Calculation of Charge Transport Phenomena and Mobility Analysis in the Insulating Oil Using First Principle and Marcus Theory.

Oil-paper insulation is widely used as a reliable composite insulation system in power transformers. The dielectric property of oil insulation plays an important role in the reliable operation of power equipment. To recognize the charge transfer process in composite insulation, the mobility of the charge in aged insulating oil is studied. However, few studies have been conducted on the microscopic mechanism of charge transport phenomena at the molecular level. In this research, we have studied the molecular electronic structure and the distribution of holes and electrons in the insulating oil by first-principles calculation. By combining with Marcus theory, the corresponding electron coupling energy, reorganization energy, and free energy are obtained. The corresponding charge hopping model is chosen by the parameter relation, and the hopping rate is calculated. At last, the mobility of holes and electrons in insulating oil within the insulation is simulated by the Monte Carlo method. Other possible charge migration methods are also studied and discussed for the comparison. It is observed that the transfer integral of electrons is 2 orders of magnitude larger than that of holes, which is mostly due to the localization of lowest unoccupied molecular orbitals (LUMO). The hole and charge transfers accord with Marcus hopping, the adiabatic charge transfer model, and the charge hopping rate is obtained. The actual free energy action under an external electric field is obtained by calculating polarizability and permittivity. Monte Carlo simulation is used to obtain the charge transfer image and mobility under an actual electric field. Possible types of traps and mobility of ions and clusters in the insulating oil are also studied.

Transformer hot spot temperature estimation through adaptive neuro fuzzy inference system approach

Transformer performance and efficiency can be enhanced by effectively address the properties of its insulation system. The power transformer insulation system weakens as a result of operational thermal stresses brought on by dynamic loading and shifting environmental patterns. Winding hot spot temperature is a crucial metric that must be maintained below the prescribed limit while power transformers are operating so as to maintained power system reliability. This is due to the fact that, among other variables, the time-dependent aging effect of insulation depends on transitions in hot spot temperatures. Due to the non‐linear nature of the conventional mathematical models used to determine these temperatures, and complexity of thermal phenomena, investigations still need to be exercised to fully understand the variables that associate with hot spot temperature computation with minimum error. This paper explores the possibilities of enhancing top oil and hot spot temperature estimation accuracy through the use of an adaptive neuro-fuzzy inference (ANFIS) technique. The paper presents an adaptive neuro fuzzy model to approximate the hot spot temperature of a mineral oil-filled power transformer based on loading, and established top oil temperature. Initially, a sub-ANFIS top oil temperature estimation model based on loading and ambient temperature as inputs is established. Using a hybrid optimization technique, the ANFIS membership functions were fine-tuned throughout the training process to reduce the difference between the actual and anticipated outcomes. The correctness and reliability of the created adaptive neural fuzzy model have been verified using real-world field data from a 60/90MVA, 132 kV power transformers under dynamic operating regimes. The ANFIS model results were validated against field measured values and literature-based electrical-thermal analogous models, establishing a precise input-output correlation. The developed ANFIS model achieves the highest coefficient of determination for both TOT and HST (0.98 and 0.96) and the lowest mean square error (7.8 and 10.3) among the compared thermal models. Correct determination of HST can help asset managers in thermal analysis trending of the in-service transformers, helping them to make proper loading recommendations for safeguarding the asset.

Natural esters as sustainable alternating dielectric liquids for transformer insulation system: analyzing the state of the art

Natural esters as sustainable alternating dielectric liquids for transformer insulation system: analyzing the state of the art

Failure Analysis of Transformers under Impulse Test

Transformers are the most critical and costly components in AC power systems. Insulation is the weakest link in transformers, and the design and quality of a transformer's insulation system shall be confirmed by performingdielectric tests according to National or International standards. The impulse test is more critical among the dielectric tests, as it verifies the integrity of the whole insulation system of the transformer. However, concluding the impulse test result is difficult when minor discrepancies exist in the earth's current waveforms. This paper presents two such case studieswith distribution class transformers.

A Comparison Between Bubble Formation on Kraft and Thermally Upgraded Papers in Synthetic Ester

Bubble formation endangers the integrity of the oil-paper insulation system of power transformers. In this research, a test setup is created to detect bubbling inception temperature (BIT) for Kraft and thermally upgraded papers (TUPs). The paper samples are dried, prepared at six different moisture levels, and immersed in synthetic ester for the experiments. The paper strips are wound around a cartridge heater, and a controller unit raises its temperature. Results of the experiments revealed that BITs for Kraft paper are comparable to those reported in the literature. Usage of synthetic ester is shown to cause insignificant changes in the BITs. The causes of discrepancies among the BITs obtained via different test setups are discussed in detail, and an equation is presented to estimate BIT in Kraft paper based on four experimental data sets. The tested TUP is shown to have significantly lower BITs than Kraft paper, which is in contrast to the previously reported data. Scrutinizing TUP at temperatures beyond BITs revealed that the behavior of bubble formation differs from that in the Kraft paper. In the test setup with Kraft paper, bubbles are formed progressively at temperatures higher than BIT. In contrast, occasional sporadic bubbles emerge from TUP beyond the BIT until a stream of bubbles, similar to the case of Kraft paper, is observed at several degrees higher than BIT in the TUP.

Application of a Rogowski Coil Sensor for Separating Internal and External Partial Discharge Pulses in Power Transformers

Partial discharge (PD) measurement is an essential tool for evaluating the health condition of a transformer insulation system. The Conventional PD measurement systems have a major drawback since they cannot discriminate the internal PD pulses and external pulse-shaped interferences, especially in online and on-site conditions. Various de-noising techniques (e.g. wavelets) have been also used in the literature, which is effective for extracting the PD signal from the noisy signals or classifying the PD types. However, the PD pulses and some of the randomly external pulse-shaped interferences have similar time-frequency characteristics, which makes them difficult to separate. In this paper, a new cost-effective and simple logic method is proposed for the separation of these interferences. This uses the polarity comparison of the output voltages of the Rogowski Coil (RC) sensor, mounted on the bushing, and measurement impedance, connected to the test tap. The same polarity represents the internal pulse, while the opposite polarity indicates the external pulses. A distribution transformer ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$20/0.4\ {\bm{kV}},\ 500\ {\bm{kVA}})$</tex-math></inline-formula> and a condenser bushing ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$72.5\ {\bm{kV}})$</tex-math></inline-formula> were considered to evaluate the performance of the proposed system. The results confirm the effectiveness of the proposed PD measurement system for separating the pulses.

Bubble Effect Phenomenon in Modern Transformer Insulation Systems Using Aramid-Based Materials and Alternative Insulating Liquids

One of the possible causes of transformer failures is high moisture in the winding insulation system. In an extreme case, when the critical temperature is exceeded, a sudden release of water vapor from the transformer insulation, called the bubble effect, can occur. This article analyzes the initiation temperature of the bubble phenomenon in various solid insulation materials (Kraft cellulose paper and aramid-based high-temperature papers such as Nomex® 910 and Nomex® 926) immersed in two electro-insulating liquids (mineral oil and Midel 7131 synthetic ester). The initiation temperature of the bubble effect depends mainly on the moisture content of the solid insulation, but it was found to be slightly lower for high-temperature materials than for cellulose. However, after taking into account the differences related to uneven water absorption of the tested materials, the differences in the initiation temperature of individual solid materials are very small. Synthetic ester, compared to mineral oil, slightly increases the bubble initiation temperature, regardless of the solid material used.

Bubbling Phenomena in Liquid-Filled Transformers: Background and Assessment

The degradation of the insulation system in liquid-filled power transformers is a serious concern for electric power utilities. The insulation system’s ageing is accelerated by moisture, acids, oxidation products, and other decay particles (soluble and colloidal). The presence of these ageing by-products is detrimental to the insulation system and may further lead to premature ageing and serious consequences. The ageing mechanisms of oil-paper insulation are complex, highly interrelated, and strongly temperature-dependent. The operating temperature of the transformer insulating system has a direct relationship with the loading profile. The major aspect that is witnessed with the fluctuating temperatures is moisture migration and subsequent bubble evolution. In other words, gas bubbles evolve from the release of water vapor from the cellulosic insulation wrapped around the transformer windings. The models presented in the existing standards, such as the IEC Std. 60076-7:2018 and the IEEE Std. C57.91:2011, are mainly based on the insulation temperature, which acts as a key parameter. Several studies have investigated the moisture dynamics and bubbling phenomenon as a function of the water content in the paper and the state of the insulation system. Some studies have reported different prototypes for the estimation of the bubble inception temperatures under selected conditions. However, there are various attributes of the insulation system that are to be considered, especially when expanding the models for the alternative liquids. This paper reviews various evaluation models reported in the literature that help understand the bubbling phenomenon in transformer insulation. The discussions also keep us in the loop on the estimation of bubbling behavior in alternative dielectric liquids and key attributable factors for use in transformers. In addition, useful tutorial elements focusing on the bubbling issue in transformers as well as some critical analyses are addressed for future research on this topic.

Aging Analysis of Transformer Insulation at Weakest Region: Dielectric Parameters Extraction via Immune Optimization

The conventional method is insufficient for determining the worst aging state of traction transformer oil-paper insulation due to the nonuniform aging. To address this issue, an extended relaxation distribution model based on multipopulation immune algorithm (ERD-MPIA) is proposed. This work collects frequency-domain spectroscopy (FDS) on the oil-paper system in a variety of nonuniform aging phases. Then, the MPIA is used to optimize the ERD model to establish the appropriate relaxation parameters for characterizing the worst aging stages. The verification results demonstrate that ERD-MPIA is capable of accurately decoupling the FDS of oil-paper systems subjected to nonuniform aging, allowing for the determination of the ideal relaxation parameters. Additionally, a distinct association is discussed between the relaxation parameters and the worst aging states. In that regard, the findings are expected to evaluate and detect the worst aging states of the traction transformer insulation system.

Evaluasi Kinerja Sistem Isolasi Transformator Kertas Kraft Terendam Minyak Mineral Pada Sel Uji Penuaan Termal Dipercepat

The transformer insulation system in the form of oil-immersed paper, is the most important part of a transformer. This isolation system needs to be ensured that it has the appropriate characteristics, and is resistant to stress due to transformer operation. The most commonly used method for evaluating insulation life is the accelerated thermal aging test. Insulating oil-immersed paper is sealed in a vessel with a ratio in the Accelerated Thermal Aging Chamber with a volume capacity of 3600 ml. It takes about 3000 ml of oil to be poured into a tube with a ratio of insulating oil to insulating paper rolled on copper which is 10:1 and then heated in a thermal oven to accelerate aging. This study aims to develop an Accelerated Aging Chamber for evaluating the performance of transformer isolation systems. The developed test cell is able to reach a set-point of 150°C in 16 minutes with on/off control while PID control in 37 minutes with a Human Machine Interface for monitoring and collecting temperature data in real time, and maintaining that temperature until the end of the experimental period. Theresults of the sample will later be tested for the characteristics of the transformer insulation, namely tensile strength, color scale, andbreakdown voltage due to thermal stress.

Water Content in Transformer Insulation System: A Review on the Detection and Quantification Methods

Water can be an irritant to a power transformer, as it is recognized as a major hazard to the operation of transformers. The water content of a transformer insulation system comprises the water in the transformer insulation oil and in the cellulose paper. The increase in the water content in the insulation system leads to reduced breakdown voltage, accelerated aging of the oil–paper insulation system, and the possibility of producing bubbles at high temperatures. Therefore, various techniques have been applied to measure the water content in both oil and paper insulation. This article comprehensively reviews and analyzes the methods (technically or nontechnically) that have been used to monitor the water content in transformer insulation systems. Apart from discussing the advantages and major drawbacks of these methods, the accuracy, measurement time, and cost of each technique are also elucidated in this review. This review can be extremely useful to the utility in monitoring and maintaining the good condition of transformers. Based on the reviewed methods and their challenges, a few future research directions and prospects for determining the water content in transformer insulation systems are outlined, such as utilizing artificial intelligence and enhancing current techniques.

Influence of Gelling in Natural Esters Under Open Beaker Accelerated Thermal Aging

The present work is focused on the oxidation aspects (focused on gelling) of natural esters and its influence on the performance of transformer insulation systems. Even though natural esters are not suitable for breathing units, the scenario considered simulates a defect in the sealing system and thus bringing the liquid in direct contact with atmospheric air. Commercially available natural ester liquids are thermally degraded under open beaker conditions in the presence of cellulose with a controlled aging history. The accelerated aging conditions are maintained until the formation of sol and gel is evident in the bulk of the liquid. Later, the insulating liquids and insulating papers are analyzed to understand the behavioral impact of sol and gel on the insulation. The discussions in this article are supported by the physicochemical and electrical characterizations of the degraded liquids and papers. The characterizations include Fourier transform infrared spectroscopy (FTIR), ac breakdown voltage (BDV) failure probability rate, X-ray diffraction (XRD), degree of polymerization (DP), and viscosity. It is found that before the formation of gelling, sol is witnessed in the bulk of the liquid. The presence of the sol highly influences the viscosity of the liquid and aids in rapid acceleration of the oxidation process. It is understood that that the gelling phenomena in natural esters do not have a noticeable impact on the liquid BDV failure rate and paper degradation.

Frequency Range of UHF PD Measurements in Power Transformers

Although partial discharge (PD) measurement is a well-accepted technology to assess the quality of the insulation system of power transformers, there are still uncertainties about which frequency range PDs radiate and which frequency range should be evaluated in a measurement. This paper discusses both a UHF PD frequency range obtained from studies investigating laboratory experiments and a frequency range from numerous practical use cases with online and on-site measurements. The literature review reveals a frequency spectrum of ultrahigh-frequency (UHF) PD measurements in the range of 200 MHz to 1 GHz for most publications. Newer publications extend this range from 3 to 6 GHz. The use cases present UHF PD measurements at transformers with power ratings up to 1000 MVA to determine frequency ranges which are considered effective for practical applications. The “common” frequency range, where measurements from all use cases provide signal power, is from approximately 400 MHz to 900 MHz, but it is noted that the individual frequency range, as well as the peak UHF signal power, strongly varies from case to case. We conclude from the discussed laboratory experiments and practical observations that UHF PD measurements in power transformers using either valve or window antennas, according to Cigré, are feasible methods to detect PD.

Partial Discharge Localization in Power Transformers Using Invariant Extended Kalman Filter

This paper proposes a novel invariant extended Kalman filter (IEKF), a recently modified version of the extended Kalman filter (EKF), to estimate partial discharge (PD) location in a transformer insulation system model. An acoustic signal measurement is utilized to localize the PD location. Unlike conventional EKF methods, where the correction term used to update the state is linearly proportional to the output error, the correction term of the proposed algorithm is independent of the output error, resulting in a fast response with a significant reduction in the estimation error. In contrast to the EKF, the proposed method can successfully mimic the nonlinear dynamics and mitigate measurement noise stochasticity. Moreover, even if the measurement model fails to fully capture the PD’s dynamics, the IEKF will still sustain a reasonable performance. In contrast, conventional EKFs can easily diverge if a mismatch between the measurement model and the true measurement occurs. Experimental results are shown to verify the proposed method’s performance compared to a recently published variant of the EKF.

Effect of Electrical Accelerated Aging on DC Resistivity of Mineral Oil Used in Power Transformers

Power transformers are still equipment of primary importance within power grids. The recent paradigm changes registered in the field of energy require the interconnection of electrical grids of much larger areas, even up to the level of an entire continent. In this context, knowing the state of the electrical equipment and that of the power transformers is an essential aspect to ensure safe operation conditions. The present work is related to the vulnerability of the oil–paper insulation systems of power transformers to different aging factors and their quick degradation under certain conditions. In this regard, the study proposed in this paper considers the measurement of DC resistivity on mineral oil samples contaminated with small amounts of accelerated aging oil under the action of high-strength electric field and partial discharges. In the absence of standardized procedures for accelerated electrical aging of mineral oil, an original method presented and discussed in previous articles is used. The obtained results clearly indicate that under the action of electrical stress, the mineral oil resistivity decreases significantly, showing that the oil’s electric properties worsen.

Frequency-Dependent Dielectric Spectroscopy of Insulating Nanofluids Based on GTL Oil during Accelerated Thermal Aging

Improving the dielectric properties of liquid-insulating materials is a current problem in research into the insulation system of a power transformer. Modern optimization of insulating liquids involves the potential use of unique synthetic esters enriched with nanoparticles. This study presents the results of the dielectric response of liquefied gas-based (GTL) insulating liquids during accelerated thermal aging. The dielectric relaxation spectroscopy method was used in the frequency domain to point out power losses as an imaginary part of a complex electric modulus. The relaxation spectra express the validity of applying this complex dielectric parameter. The polarization processes of the base oil alternately change position in the low-frequency band during thermal aging. Fullerene nanofluid undergoes three phases of dielectric loss changes during thermal aging. In the case of magnetic nanofluid, the effect of electric double-layer polarization disappeared after 500 h of thermal aging. It was found that with the gradual increase in the thermal aging time, there is no gradual increase in the dielectric losses investigated in the measured frequency spectrum. This study shows that the concentration of the two types of nanoparticles independently causes a different dielectric response to an applied AC electric field in the GTL base fluid.

Experimental Study of Sealed Tube Accelerated Aging Test Parameters and Determination of the Thermal Class of Transformerboard

The latest 2011 edition of the IEEE C57.100 standard introduced a new sealed tube accelerated aging test (STAAT) procedure and methodology for evaluating the thermal index (TI) of liquid-immersed transformer insulation systems. The application of this standard becomes increasingly important, as new insulating materials (both solid and liquid) have been developed, and their thermal performance shall be qualified. The experimental work presented in this article investigated the effects of different test parameters on the aging test results and data interpretation. The new findings on the dependence of the aging process on the ratio and type of materials in the system, as well as on the selected aging criteria provide a better understanding of the accelerated aging and interpretation of the data along with practical recommendations for the improvement of the test procedure. In parallel, for the first time, the TI of the Transformerboard, as a component of the so-called industry-proven insulation system, was investigated and defined based on the standard industry test. The results confirmed that the Transformerboard’s thermal class is, at least, 120 °C; this validates an empirically based application of this material as a part of the 120 °C class insulation system in both power and distribution transformers.

The detection and classification of partial discharges in power transformer using the acoustic method

Partial discharges are the main causes of degradation of the insulation system in power transformers. Their permanent monitoring is therefore of fundamental importance, in the way that it ensures a better assessment of the level of electrical insulation. Several types of partial discharges are present in power transformers depending on the components of the electric field to which the insulation is subjected. Many on-line methods are used to the continuous measurement of partial discharges, including the acoustic method which doing both measurement of electrical parameters and localization of the sources of partial discharges. This article deals with both recognition and classification of partial discharges by analysis of acoustic signals. The goal is to reproduce various types of partial discharges present in the transformers (surface, internal, and corona discharges) by several structures of electrodes in a prototype transformer filled with insulating oil. The generation of partial discharges is done using an experimental setup consisting of a controlled 220V/50kV HV transformer connected to a prototype of transformer containing an electrode system. The experimental procedure are carried out according to the protocol of the IEC 61270 standard. The measurement of the acoustic signals on the tank is carried out using a conventional broadband sensor from the company ACOUSTIC EMISSION (GI150). Both signal analysis obtained in the frequency domain using the time-frequency response and high-resolution frequency analysis (MUSIC) makes it possible to clearly distinguish the type of discharge produced.