Power Transformer Insulation Research Articles

The high-voltage substation configuration influence on the estimated lightning performance

The substation configuration has a great influence on the lightning overvoltages caused by backflashes at the lines connected in front of the substations. In most of the analyses the critical substation configuration is assumed with only one connected line. In the paper the influence of the substation configuration the line performance is analyzed. The mean time between power transformer insulation failures is computed taking into account the monthly distribution of the lightning activities and also the monthly probability distribution of certain substation configurations. The method is illustrated by a 400 kV substation example.

Electrical and chemical diagnostics of transformers insulation. B. Accelerated aged insulation samples

This paper describes the analysis of accelerated aged insulation samples to investigate the degradation processes observed in the insulation from aged power transformers. Short-term accelerated ageing experiments were performed on paper-wrapped insulated conductors and on pressboard samples. The condition of aged insulation samples was investigated by two relatively new diagnostic techniques: (a) measurements of interfacial polarization spectra by a DC method; and (b) measurements of molecular weight and its distribution by gel permeation chromatography (GPC). Several other electrical properties of the paper/pressboard samples were also studied. Possible correlations have been investigated among the different measured properties. The GPC results have been used to predict how power transformer insulation molecular weights change with temperature and time.

Measurements of dielectric stress in EHV power transformer insulation

A relatively high failure rate of the internal insulation of EHV power transformers has been reported by public utilities. This prompted a study of the safety margin of EHV transformers in service and under test. In this project, a number of transient overvoltages were digitally recorded in EHV substations and the transient voltage distribution along the HV winding was measured digitally on many untanked transformers. An evaluation of the safety margin indicates that test impulse waveforms specified in the standards do not always represent system-generated transients, which can jeopardize EHV transformer insulation. The insulation can be designed in such a way to keep a similar safety margin in different parts of the winding, and insulating materials can be used more efficiently.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The kinetics and mechanisms of degradation of cellulosic insulation in power transformers

Cellulose based paper is widely used as electrical insulation in large electrical power transformers and cables. Its deterioration under the combined effects of thermal, oxidative and hydrolytic degradation determines the ultimate life of the insulation system, although other factors may cause it to fail earlier. The use of chemical indicators of degradation, such as 2-furaldehyde, is increasingly being used to assess the condition of the paper and, in time, their cumulative concentrations in the oil will be used to aid estimation of the remaining life of the plant. The activation energy of degradation of cellulose in the temperature range 100–200°C is 111 ± 6 kJ/mole and is independent of the reaction conditions. The degradation rate is significantly influenced by the pre-exponential factor, which increases with increasingly aggressive environments, such as the presence of oxygen and water. Water is a product of degradation and there is some evidence that it auto-accelerates the reaction in oil. The mechanisms of formation of products such as furaldehyde from glucose have been investigated by radiolabelling, but more complex mechanisms may exist for formation from cellulose.

Detection and location of internal defects in the insulation of power transformers

With the development of fast, and low-cost analog-digital converters (ADCs) and new mathematical procedures for waveform description, e.g., time encoded signal processing and recognition (TESPAR), novel approaches to detect and locate internal defects in the insulation of power transformers become feasible if the following requirements are fulfilled: the equivalent circuit of the insulating system, suitable for the fast partial discharge (PD) signals with risetimes of approximately 10 ns, is known; the response of the insulating system to PD signals injected at least at three different sites of each winding (top, center, bottom) is calculated, or measured at the signal tap-off points (i.e., bushings); and the pattern matrix of all calculated or measured signals exists: e.g., encoded and signal processes with TESPAR as a reference. The above mentioned requirements are examined for quasi-simultaneous detection and location of internal defects in power transformers. The method is applied to PD measurements on power transformers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mathematical and experimental analysis of the field drying of power transformer insulation

A mathematical model was developed to predict the adsorption and diffusion of moisture by oil-impregnated transformer insulation during field installation and the diffusion, desorption and evaporation of moisture during vacuum drying practices. A field drying experiment was performed on a new 93 MVA power transformer to verify the accuracy of the drying model. Four paper models were inserted in the transformer during manufacture to monitor, the course of moisture diffusion during the study. Insulation temperatures were also monitored to evaluate a hot oil spray process in uniformly heating the transformer insulation. Good agreement was achieved between mathematical predictions and field moisture measurements. High temperature processes are essential to the rapid removal of moisture during field vacuum drying operations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Estimation for a life model of transformer insulation under combined electrical and thermal stress

A life model for power transformer insulation is proposed. It is the Arrhenius model with a weighting factor for the enhanced reaction under combined thermal/electrical stress. Laboratory tests are used to evaluate the model's parameters. The thermal and thermal/electrical stress aging data on Fish paper are presented and analyzed. The estimates from the life model are close to the experimental results. It is hoped that the life model is extendable to other engineering materials. The model's parameters can be evaluated from two laboratory tests, thermal aging and combined thermal/electrical aging. For the Fish paper, when thermal stress is increased by 30 degrees C, the life is reduced by a factor of 2; at 70 degrees C life was 238 hours and at 100 degrees C life was 120 hours. The material life was further reduced by a factor of nearly 2 when subjected to thermal/electrical stress: at 80 degrees C life was 200 hours, whereas at 80 degrees C and 600 Vac life was 118 hours. Electrical stress is important in accelerating the material degeneration.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Development of low-permittivity pressboard and its evaluation for insulation of oil-immersed EHV power transformers

The development of a low-permittivity pressboard obtained by blending polymethylpentene fiber with cellulose fiber is described. The permittivity is 3.5, which is lower than that of conventional pressboard. With this pressboard, insulating characteristics of intercoil models were investigated under lightning impulse and AC voltage conditions. It is demonstrated that a spacer fabricated from this pressboard material increases the partial discharge inception and breakdown voltages up to 30% compared with conventional pressboard spacers. >

新しい低誘電率プレスボードを用いた超高圧大容量変圧器絶縁の開発

新しい低誘電率プレスボードを用いた超高圧大容量変圧器絶縁の開発

Digital acquisition and processing of partial discharges during acceptance test of HV transformers

A six-channel digital recording system has been developed for monitoring partial discharges (PDs) in the HV insulation of power transformers during acceptance tests. The analog PD signals are converted to digital words describing their apparent charge, polarity and the point on the wave (phase) of the test voltage at which the PD pulse appears. The data are recorded during a 1 s acquisition period, processed, displayed and stored during the time (up to 7 s) the system is preparing for the next acquisition. The complete record of PDs acquired during the test is stored on a hard disk, and specialized programs have been developed for post test evaluation of the transformer performance. Different presentation formats allow easy identification of interference signals induced in the test circuit by external sources, correlation of PD patterns recorded on different transformer windings, and evaluation of changes in PD activity during the test.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Testing Methods for Power Transformer Insulation

Long-term test of alternating current voltage in power transformers greatly contributes ot the improvement of the reliability of insulation. Many kinds of V-t characteristics are collected by experimental research, and typical V-t characteristics of a whole transformer insulation are discussed in the paper.

Diagnostic Methods for Power Transformer Insulation

Concerning diagnostic methods for power transforme. r ins-ulation , gas analysis in oil and partial discharge measurements become useful methods, Typical methods are described in this chapter.

Switching-impulse strength of the external insulation of three-phase power transformers

The switching-impulse strength phase-to-phase and phase-to-earth has been investigated on a power transformer dummy to provide data for the design of its external clearances. The dummy was equipped with 420 kV bushings, but also simulations were used to obtain smaller or larger distances to earth. The clearances were varied between 2 m and 6 m. The influence of rain is included. The report offers design curves for power transformer clearances covering the range of rated switching-impulse withstand voltages up to 1550 kV, phase-to-earth, and up 2400 kV, phase-to-phase.

Vacuum process in drying of power transformer insulation

The drying of transformer insulation to remove absorbed water spans the spectrum of vacuum pumping systems from mechanical rotary piston pumps and mechanical boosters, to liquid ring pumps down to vapor booster and vapor diffusion pumps. Power transformers can weigh 500 000 lb and contain 30 000 lb of insulation. A primary dry of the cellulosic insulation using narrow boiling hydrocarbons as a boiling heat transfer medium occurs in large‐volume tanks of varying capacity from 7000–21 000 ft3. Internal assemblies containing the insulation are first dried to water contents of 0.1% by weight. The pressure and temperature in these tanks range from 25 to 120 °C with pressure levels as low as 0.3 Torr at cycle completion. Secondary transformer drying takes place after internal assemblies are installed within steel transformer tanks. These tanks range in free volume to 3500 ft3. Drying may take place at ambient or elevated temperatures (depending on atmospheric exposure history) at pressures approaching 0.001 Torr.

Drying of power-transformer insulation

To ensure that large high-voltage power transformers will operate reliably for years without requiring unscheduled maintenance, manufacturers thoroughly dry the insulation after manufacture and before commissioning. Using conventional monitoring procedures, it is difficult to establish the residual moisture concentration throughout the insulation.The paper introduces a computational technique which has the potential of calculating accurately the moisture levels in the windings after various periods drying. The method takes into account the evaporative transfer of moisture from the winding surface to the drying medium and may be applied to all drying cycles which incorporate periods of air, vapour-phase or oil-spray heating and vacuum.To demonstrate the accuracy of the method the drying of a model layer winding was predicted and the results were compared with measurements. The excellent correlation of the results is displayed in the paper.

Diffusion of moisture through power-transformer insulation

Moisture in power-transformer insulation has long been recognised as a major cause for the deterioration of the dielectric and for shortening the `service-lives? of units. Engineers have, therefore, striven to find a method that will accurately predict the degradation of the insulation in service conditions. This must involve a knowledge of how the moisture concentration varies in the insulation with time and load. Until recently, there has been no really satisfactory method of doing this, but now, with the advent of fast, large-storage computers, it is practicable to determine these changes in concentration numerically. Accurate computation depends on realistic mathematical modelling of the diffusion processes in both the liquid and solid insulants. These calculations are complex because the rate of moisture transfer depends on the materials, moisture distribution, temperature and oil velocity. The computations involve the simultaneous solution of the fundamental diffusion and heat-transfer equations. A program has been developed to execute these calculations and it should prove to be a powerful tool for both transformer designers and electricity-supply engineers. The program produces sufficient information for engineers to decide whether the insulation of a transformer is in a satisfactory condition for the unit to be used. The reliability of the computations depends on the provision of accurate data, including a value for the diffusion coefficient of moisture through transformer oil. The magnitude of this parameter was unknown and the paper describes the establishment of a technique for measuring the quantity.

High voltage power transformer insulation

High voltage power transformer insulation

Loading of power transformers

For many years, the maximum temperature power-transformer insulation would withstand was regarded as fixed, and was used as the basis of standard kVA ratings and of overload calculations. Later, the life of the insulation was found to be a thermal-aging process; so the maximum permissible temperature now became a variable with time. The derived aging formulas and the parameters in present use, together with the resulting standard overload values, are reviewed. Practical life studies to set bounds to the permissible temperatures have not been successful; so only relative conclusions can be drawn, and the absolute limit is still unknown. It is, however, clear that quite large margins are available over present-day practice, and some part of these have been incorporated in the more recent loading guides. The desirability of increasing present ratings to utilise these margins is considered. Analysis suggests that any extra rating thus derived cannot be economically realised and is better reserved for emergencies. It is concluded that loading capacity is not the only, or even the major, factor in determining the expectation of life of a transformer. There are other, more exigent, limits. Nevertheless, for practical-operation purposes and to permit systematic planning, a standard loading practice, however arbitrary, must be established. The national loading guides fulfil this function in a relatively simple and practical manner. In particular operating conditions where the loading requirements can be more closely prescribed, it is possible to work directly on the hottest-spot temperature and dispense with some of the orthodox restrictions. The ‘integrated system transformer’, to which brief reference is made, is the first practical application of this principle.

A method of analysis of transformer impulse voltage distribution using a digital computer

With the increasing need for economical design of transformer insulation in large present-day power transformers, it is highly desirable to be able to predict at the design stage the distribution of impulse voltages throughout the windings. The paper presents a method of impulse voltage calculation using high-speed digital computer techniques which appears sufficiently accurate and flexible to provide a basis for a routine design procedure.The derivation of the transformer equivalent circuit is discussed, together with the computer programme for the solution of the resulting differential equations. The advantages of the method are indicated, and the results are compared with other recent theoretical work and with direct tests on an experimental transformer.The relative importance of the various winding parameters in the construction of the equivalent circuit is determined experimentally. Finally, possible extensions of the method are discussed and the possibility of complete automation of the procedure is indicated.

A field survey of transformer oil quality

REDUCTION of power transformer insulation by the recently recommended two full basic insulation levels1 has placed renewed emphasis on oil quality. The design of corona-free transformers has eliminated the use of sharpedged electrodes in highly stressed locations, favoring the more uniform dielectric field conditions. Because contamination in these uniform fields has a much greater influence on dielectric strength that in nonuniform fields, the question of oil quality is of further importance.