A smart way to minimize test time for transformer dielectric measurements

Dielectric response measurements for the moisture and oil conductivity assessment of transformers are well understood, internationally accepted and growing in use. Of the two principal dielectric response methods, an AC method called Dielectric Frequency Response (DFR, also Frequency-Domain Spectroscopy, FDS) is preferred due to its robustness against noise. The time requirement of a DFR measurement is lengthy compared to that of other electrical test methods. An earlier approach to accelerate test time combined DC (time domain) and AC (frequency domain) test methods. This article discusses the limitations inherent to that approach and presents today’s multifrequency test solution that minimizes test time for DFR measurements without compromising accuracy

Dielectric and mechanical assessment of cellulosic insulation during transformer manufacturing

Due to the impact of cellulose of paper insulation on transformer life, it is imperatire to remove moisture from the oil and the solid insulation. Several techniques have been implemented during manufacturing of power transformers to reduce water content in transformers. These drying processes can involve different costs and time, and they can damage the insulation paper. In this work, a drying process has been implemented in the laboratory trying to simulate the most aggressive conditions that can be suffered by the paper in transformer manufacturing in a real industry. Once the moisture content of papers was lower than 0.5%, the effect of the drying process on paper degradation was evaluated using the analysis of mechanical and dielectric properties and the degree of polymerization. Different commercial papers were studied to quantify the possible degradation induced by the drying process. The results of the mechanical strength study showed a reduction on the degree of polymerization from 1100 to 850 after 4 days of drying. The dielectric analysis of the samples showed different behavior in one of the solids evaluated and it was also found a decreased hygroscopic capacity of degraded samples in comparison with new samples.This research is under the National Research Project “Gestión del Ciclo de Vida de Transformadores Aislados con Fluidos Biodegradables” (PID2019-107126RB-C22/ AEI /10.13039/501100011033). The authors also wish to thank “Fundación Iberdrola” for its financial support for the research project: “Análisis de las Propiedades Dieléctricas de Aislamientos Sólidos Impregnados con Líquidos Dieléctricos”. C. Méndez would like to acknowledge the Spanish Ministry of Science, Innovation and Universities for the financial support for the FPU grant (FPU19/01849)

Assessing Ageing Condition of Mineral Oil-Paper Insulation by Polarization/Depolarization Current

Accurately assessing the ageing status of oil-paper insulation in transformer is essential and important. Polarization and Depolarization Current (PDC) technique is effective in assessing the condition of oil-paper insulation system. Though the PDC behaviour of mineral oil-paper insulation has been widely investigated, there is no report about how to make the quantitative analysis of mineral oil-paper insulation ageing condition by PDC. The PDC characteristics of mineral oil-paper insulation samples were investigated over the ageing period at 110°C. A new method for assessing the ageing condition of mineral oil-paper insulation by calculating the depolarization charge quantity was proposed. Results show that the depolarization charge quantity of mineral oil-paper insulation sample is very sensitive to its ageing condition. The stable depolarization charge quantity could be used to predict the ageing condition of mineral oil-paper insulation

Dielectric and mechanical assessment of Kraft and Diamond Dotted paper aged with commercial vegetable oil

The use of vegetable oil (natural ester) in electrical devices like power transformers is increasing due to their high biodegradability and better safety. The lifespan of power trans- formers is mainly defined by cellulose insulation condition, which usually works together with dielectric oil as electrical insulation and also as mechanical winding protector and compactor. That is why the aim and results of this research shows us not only the dielectric parameters evolution, but also the relationship between the mechanical factors and the moisture content of thermal accelerated ageing processes, with commercial vegetable oil, of Kraft paper and Diamond Dotted Paper (DDP). These are two of the most common insulating materials in electric power transformers. In addition, the new tests have been done by a different method of paper ageing analysis.This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement Nº 823969 - BIOTRAFO

CHECKING OF THE CONDITION OF TRANSFORMERS AND THE EFFICENCY OF OIL REGENERATION WITH RVM (RETURN VOLTAGE MEASUREMENT)

Up to now the insulation in HV power transformers has been made from oil/paper therefore the diagnosis of this kind of insulation will be also important in the next forty-fifty years. The lifetime of this equipment strongly depends on the condition of the insulation system. The ageing process of oil/paper insulating systems is a very complex and complicated phenomenon. In order to get a well-supported decision on the further operation of aged transformers, relevant information would be necessary on the condition of the oil-paper insulation. We also know that the oil-paper insulation has almost always inhomogeneous condition considering the temperature, moistening and ageing processes. The classical methods (insulation resistance, loss tangent, etc.) characterise the insulation by single measured value. This single value is not sufficient for relevant characterisation and diagnosis of such a complex system with lots of tons of insulation and with almost always inhomogeneous distribution of temperature, moisture and ageing product. If we measure the polarization spectra in three states (new insulation in equilibrium and uniform polarization spectrum, new insulation with not uniform distribution of polarization spectrum, later reaching again the uniform distributions) the shape of three polarization spectra will be different. Comparing the polarization and classical methods of this three case we can realize that sometimes the “classical single values” are almost the same but the polarization spectra are different. The three response methods provide very practical information about the insulation system (e.g. moisture, ageing). The moistening and accumulation of ageing by-products change the distribution of interfacial polarisation in the range of long time-constants. These promising response methods measure the polarisation distribution in the range of long time-constant (with other words, in the low frequency range). The three test methods were: Return Voltage measurement (RVM), the C and tgδ measurement in range of some tens of mHz to 50 Hz (FDS=Frequency Domain Spectroscopy), and the measurement of DC charging and discharging currents (PDC=polarisation and depolarization currents) up to some thousands of sec. These equivalent methods (RVM, FDS and PDC) are able to follow the changing of condition of insulation contrary to classical methods. Therefore a little bit surprising that until now the convenient standards is missing considering the polarization methods. In an earlier Hungarian research work (Budapest University of Technology) almost all the necessary measurements have been realized, consequently we are in possession of fundamental data considering the polarization methods. This paper would like to show a short review about the RVM technique, the correct interpretation of RVM data and a case study for the checking of the efficiency of the oil reclamation with RVM technique

Review Of Modern Diagnostic Techniques For Assessing Insulation Condition In Aged Transformers

Cellulosic paper and oil insulation in a transformer degrade at higher operating temperatures. Degradation is accelerated in the presence of oxygen and moisture. Power transformers being expensive items need to be carefully monitored throughout its operation. Well established time-based maintenance and conservative replacement planning is not feasible in a current market driven electricity industry. Condition based maintenance and online monitoring are now gaining importance. Currently there are varieties of chemical and electrical diagnostic techniques available for insulation condition monitoring of power transformers. This paper presents a description of commonly used chemical diagnostics techniques along with their interpretation schemes. A number of new chemical techniques are also described in this paper. In recent times a number of electrical diagnostic techniques have gained exceptional importance to the utility professionals. Among these techniques, polarisation/depolarisation current measurement, return voltage measurement and frequency domain dielectric spectroscopy at low frequencies are the most widely used. This paper describes analyses and interpretation of these techniques for transformer insulation condition assessment

Life span estimation of oil-impregnated paper high voltage current transformers based on long duration tests under simultaneous thermal and electrical stresses

A large number of power system apparatus around the world are over 30 years old in operation. Replacement of power system assets involves high investments. On the other hand, the failure of such equipment causes a high economic impact. The lifespan estimation of high voltage apparatus is an important information to determine the best moment for the replacement. Although the current transformer is an essential component in the electrical power system and its failure may result in serious consequences, there are no accelerated aging tests for this equipment prescribed by technical standards. Life estimation for this equipment is generally made based on field experience, something that cannot be done for new models and/or new manufacturers. In this work, experimental results of life tests performed on reduced models, representing high voltage current transformers with oil-impregnated paper insulation, are presented and discussed. The analysis is based on the application of a multi-stress model, which combines the Montsinger rule and Inverse Power Law. The proposed approach considers the existence of threshold values for electrical and thermal stresses, below them, electrical and thermal degradation are negligible. An order of magnitude for the electrical stress threshold value (E0) and for the electrical aging exponent (n) is obtained from the application of the test results to the model used in this investigation. The scope of the investigation includes several diagnostic tests which are performed after interruptions of the long-duration test. The diagnostic tests include some classical high voltage power frequency measurements, tests on oil samples and finally, dielectric frequency response (DFR), one of the most recent insulation diagnostic techniques. The application of the thermoelectrical multi-stress model to the results indicates that the estimated lifespan for the test samples is about 50 years, that the exponent related to electrical stress n is in the range of 1.3-3 and that the electrical stress threshold E0 is in the range of 1.17 to 1.21 p.u. The results of diagnostic tests carried out as part of this investigation revealed the relevant variation of several parameters along the aging test and, combined with the findings obtained in the autopsy of the failed test samples indicated a dielectric failure caused by the combined action of electrical and thermal stresses. The variations of these parameters are of great value for evaluating the degradation conditions of the insulation over its service life. The cross-analysis of several different diagnostic tests is recommended for decision-making regarding the replacement of equipment in service

Moisture diagnosis of transformer oil-immersed insulation with intelligent technique and frequency-domain spectroscopy

Universities of Guangxi; Brunel Research Initiative and Enterprise Fund; Education Department of Guangdong Province; 10.13039/501100001809-National Natural Science Foundation of China; 10.13039/501100004607-Natural Science Foundation of Guangxi Province

Investigation Of Polarization And Depolarization: Current Measurements For The Assessment Of Oil-Paper Insulation Of Aged Transformers

Moisture and ageing strongly influence the dielectric properties of oilrpaper insulation system of power transformer. Moisture measurement in oil sample generally gives inconclusive information since oilrpaper moisture equilibrium is temperature dependent and takes a long time to be in equilibrium. Direct moisture measurement of paper sample is not practicable for in-service transformers. The measurement and evaluation of the 'dielectric response'and conductivity is one possible way of diagnosing a transformer insulation condition. In a recent research project, polarization and depolarization current measurement has been used for assessing the condition of oilrpaper insulation. The polarization and depolarization current PDC analysis is a non-destructive dielectric testing method for determining the conductivity and moisture content of insulation materials in a transformer. On the basis of this analysis it is possible to take further actions like oilrefurbishment, drying or replacement of the winding of the transformer. This paper presents a description of the PDC technique with the physical and mathematical background and some results of PDC measurements on several transformers. Analyses and interpretation of the field test data are also presented in this paper