Search

The effects of furfural generation and partitioning on the aging assessment of oil-paper insulation of power equipment were investigated. Oil-paper samples with various furfural contents were prepared using an accelerated thermal aging experiment. The mass of the furfural generated during the aging of insulation paper was calculated and used to establish the quantitative equation between the accumulated furfural and the degree of polymerization (DP) of the insulation paper. Results showed that the reciprocal of the DP value was linearly related to the amount of furfural generated by the insulation paper. Effects of temperature, moisture, and temperature on the furfural partitioning ratio between oil and paper were analyzed using a furfural partitioning experiment, and a furfural partitioning model was established. Results showed that the furfural partitioning ratio in oil was positively correlated with temperature and moisture but negatively correlated with the DP of the insulation paper. A method for assessing the age of an insulation paper by integrating the furfural generation and partitioning models was proposed and applied on a 500-kV reactor that was returned for factory maintenance. The application result indicated that the method could accurately assess the overall aging condition of the oil-paper insulation of the power equipment. [ABSTRACT FROM AUTHOR]

Traction transformers are one of the most important equipment in the traction power supply system. Thermal aging of transformer insulation system determines its reliable life. To achieve an estimation of the insulation life of traction power transformers operated under real load condition, the effects of traction loads on the thermal aging of oil-paper insulation in traction transformers were investigated. First, the design principles for the accelerated thermal aging experiment were studied and proposed by considering magnitude (i.e., load rate) and frequency (i.e. spare time and lasting time) of the traction load. Then, an experimental system was established in the laboratory to conduct extensive accelerated thermal aging experiments, in which the traction loads were mimicked by using positive temperature coefficient (PTC) heating plates and a programmable logic controller (PLC). Finally, the experimental results were analyzed and discussed to reveal the effects of traction load on the thermal aging of oil-paper insulation in traction transformers. [ABSTRACT FROM AUTHOR]

Oil-immersed paper is a weak link in internal insulation of oil immersed power transformer, whose aging state distributes inhomogeneously. However, most of the current assessment methods fail to consider the spatial distribution of aging areas inside transformer. Aiming at this problem, this paper proposes the inversion detection method to obtain the resistivity of oil-immersed paper in various regions nondestructively, and uses the resistivity of oil-immersed paper to assess directly the insulation state of oil immersed power transformers. Finite element method is applied to establish the mapping relationship between the resistivity of oil-immersed paper in different regions, and the dielectric loss factor obtained from the port of transformer. Back propagation neural network is used to learn inversely this mapping relationship so as to calculate the resistivity. Applying the proposed method on a 10-kV transformer, the calculated results are close to the measured values. The study shows that the inversion method can effectively calculate the oil-immersed resistivity in various regions, which represent the aging state in different regions of the transformer. [ABSTRACT FROM AUTHOR]

Reliability and power quality are two of the most discussed topics in the utility industry today. Many groups have performed power quality studies with varying results. One large industrial customer in northern New York State which experienced numerous process interruptions due to power system disturbances was the focus of this study. Monitoring methods, results, inadequacies of standards, and unique mitigation techniques used to conquer their problems are discussed in this paper.

The paper presents results of a laboratory study on the effect of temperature and moisture level of paper-oil insulation on the parameters of the Cole-Cole model. It also discusses how to determine the values of coefficient \alpha and time constant \tau (parameters of the Cole-Cole model) of the characteristics resulting from the experimental frequency spectroscopy method (FDS) of paper-oil insulation. [ABSTRACT FROM PUBLISHER]

Oil-paper insulation ageing of transformer has characteristics of spatial locality due to uneven temperature distribution. The transformer is sealed, it is difficult to obtain the local ageing state of oil-paper insulation by sampling directly. Currently, there are no effective detection method to detect the insulation state of local regions. The oil-immersed paper resistivity inversion method can nondestructively assess the local state of oil-paper insulation. However, the current inversion methods still have shortcomings and need to be improved. In this paper, combining the back propagation neural network (BPNN) and Newton downhill method (NDM), an improved inversion method is proposed to optimize the current inversion method, so as to reduce the effect of initial value on it and make calculated results converge better. Finally, a 3D 10 kV transformer model is established to verify the effectiveness of the proposed method. The resistivity calculated by the improved method show that the method can provide initial resistivity values for technicians and improve the convergence and accuracy of current inversion method, which is more suitable for extending to the engineering practice. [ABSTRACT FROM AUTHOR]

Dielectric testing techniques, in both time and frequency domains, are currently widely used by power utilities for assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that results of these tests are highly influenced by the operating temperature during measurements. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system is highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with an understanding of the physical behavior of the insulation system in response to temperature. A circuit model, which describes the dielectric behavior of the transformers main insulation system, has been investigated in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been observed between the operating temperature and the equivalent model parameters that can be used as additional information for better interpretation of the dielectric test results. This paper thus reports a detailed study on the effects of temperature on dielectric measurements of a transformer under controlled laboratory conditions. Some results of practical on-site testing are also presented to demonstrate the possibility of errors that may be introduced in dielectric test results analysis unless temperature effects are taken into consideration. Index Terms--Conductivity, depolarization current, dielectric response, dissipation factor, equivalent model, polarization current, recovery voltage, temperature, transformer insulation.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This first part of the paper describes the environmental and electrical stresses encountered in service that influence the risk of flashover on line insulators. Part II of the paper describes the strength of typical line insulation in these conditions and deals with selection and mitigation methods. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulation coordination, line insulators, snow.

Knowledge about moisture content in oil impregnated paper insulation is essential when estimating remaining lifetime of power transformers. Direct evaluation of moisture content is rarely possible due to inaccessibility of the internal insulation system in transformers. Therefore, various indirect estimation techniques are utilized. Frequency domain spectroscopy (FDS) measurements of transformer insulation belong to this group. To perform high quality interpretation of results of FDS measurements a good knowledge on dielectric responses in oil impregnated pressboard and paper is required, especially as it refers to their variation with water content and temperature. The aim of this paper is to provide an open access to the frequency domain spectra of oil impregnated paper and pressboard samples, which can then be used in modeling of the results of diagnostic measurements in power transformers. Index Terms--Conductivity, dielectric frequency response, modeling, moisture content, oil-paper insulation.

Activation energy is popularly used for the estimation of remaining life of transformer insulation. It is defined as the average rate of all reactions that happen with cellulose. Existing literature shows that the activation energy of oil–paper insulation can be obtained from polarization depolarization current (PDC) and return voltage measurement (RVM) data that are measured at a specific temperature. It is practically difficult to ensure the same measurement temperature for both PDC and RVM data. On the other hand, PDC data and its analysis get influenced by de-trapping current. This de-trapping current is generated by ionic charge carriers that get freed from trap sites during PDC measurement process. Formation of these trap sites is related to physical, chemical reactions that happen at oil–paper interface. This paper proposes a methodology which uses de-trapped charge, dislodged from deep and shallow traps, to assess insulation condition and for the prediction of activation energy. Thus, eliminating the need of RVM data. The proposed method is tested using data collected from various real-life in-service transformers. [ABSTRACT FROM AUTHOR]

The effects of moisture, oxygen, and acidity upon the aging of Kraft and thermally upgraded ('Insuldur[R]') papers have been studied in detail. The results for Kraft are consistent with the kinetic model and parameters proposed earlier by Emsley et al., and with the principle that moisture promotes acid-hydrolysis by causing carboxylic acids to dissociate. Moisture is released during the aging of Kraft and, hence, its aging is auto-acceleratory. In contrast, Insuldur consumed moisture under the same conditions, aged more slowly, and its aging was less accelerated by added moisture. 2-Furfuraldehyde (2FAL), which is a dehydration product of pentosans and, hence, an index of moisture production, is also released from Kraft during aging, but not from Insuldur. Acids are, however, produced from both types of paper. Theoretical explanations for these findings are proposed, and their practical implications for transformer maintenance are discussed. Index Terms--Acids, aging, hydrolysis, maintenance, oil, oxidation, paper, transformer, 2-Furfuraldehyde, water.

Dynamics of water penetration in mass impregnated cable insulation has been studied. For experimental purposes, artificial damage has been inflicted to a 40-cm-long cable sample and water ingress has been continuously monitored by frequency response measurements. A similar experiment has been conducted on 2.8-m-long cable sample, where both frequency response and time-domain reflectometry (TDR) measurements have been performed. After termination of both experiments, actual moisture content has been measured radially and axially. Based on dielectric measurements, a model of water ingress has been developed and diffusion coefficients have been estimated for mass impregnated cable paper. Index Terms--Dielectric response, Karl Fischer titration, mass impregnated cable, moisture content, oil paper insulation.

Dielectric spectroscopy (dielectric response measurements) has been applied for nondestructive estimation of humidity in oil-paper cable insulation. The experiments have been based upon two field-aged cables, 20 and 50 years old. Paper samples from these cables have been characterized and subjected to environments with different relative humidity. Dielectric loss and capacitance have been measured in a frequency range 1 MHz to 1 kHz and related to the moisture content determined by Karl Fisher titration. A method has been verified where the moisture content is correlated to the minimum value of loss tangent (tan [delta]). A number of field measurements have been performed where the method has been applied to estimate the moisture content in the distribution cables. Index Terms--Dielectric response, Karl Fischer titration, mass impregnated cable, moisture content, oil paper insulation.

Special measures may be needed to select line insulators for transmission and distribution lines in locations exposed to freezing conditions. This second part of the paper describes the strength of typical line insulation in these conditions and then deals with selection and mitigation methods based on the in-service stresses described in Part I. Index Terms--Climate, contamination, flashover, fog, high voltage, ice, insulation coordination, line insulators, snow.

An extensive revision of the IEEE C37.011 'Application Guide for TRV for AC High-Voltage circuit Breakers' has been in process for the last four years. The main aim was to introduce the two- and the four- parameter description of the transient recovery voltage (TRV) as used in IEC Switchgear standards that have been introduced in the revisions of Standards C37.04 and C37.06. These two standards are being concurrently revised, but IEEE C37.011 provides the necessary explanations on TRV characteristic in advance. Non-specialist engineer users of the guide will better understand these standards. Index Terms--Circuit breaker, standard, transient recovery voltage (TRV).

Test methods for evaluating flashover voltage of ceramic and nonceramic insulators under ice, snow, and cold-fog conditions are recommended. The paper describes the procedure to be followed at various stages of an evaluation test including preparation of the test object, insulator precontamination, ice, snow, and cold-fog deposits as well as voltage applications and flashover voltage evaluation. Some relevant parameters for ice accretion, flashover voltage evaluation, artificial pollution, and power source requirements are taken tram IEC 60129, IEC 60507, and IEEE Std.-4-1995 on standard techniques for high-voltage testing. Index Terms--Contamination, flashover, fog, high voltage transmission, ice, insulator, snow, statistical interpretation, test methods.

Recently developed monitoring-systems show the need as also the possibilities for online- and offline-onsite-diagnostics. The limit in sensitivity for electrical pd-measurements according to IEC 60270 which is reached using filters for sinusoidal disturbances and compensation for corona impulses is still not satisfactory. The acoustic partial discharge measurement (pd-measurement) is also a well known and reliable method often used by transformer manufacturers in testing. In laboratory setups a gain in sensitivity of the acoustic in comparison to the electrical pd measurements could be established. This is reached by adapting the sensors to the propagation path, a modern but moderately priced acquisition hardware and the introduction of new signal processing algorithms. The developed sophisticated measuring systems show remarkable results in online acoustic pd-measurements onsite. Index Terms--Acoustic measurements, partial discharges, signal processing, transformers.

Many utilities still have paper-insulated pipe type cables in service. In the near future they may have to replace some of these systems due to local increases in electric power demand or aged cable insulation. In many cases the steel pipes are well protected against corrosion and thus suitable for further use. For Retrofitting in such installations, XLPE-insulated cables offer several advantages. Felten & Guilleaume Energietechnik AG has developed a new three-core XLPE-insulated 64/110 kV cable with reduced insulation thickness (10 mm), that can be pulled into a pipe. In co-operation with VEW ENERGIE AG, the cable performance is being investigated in a field test, that started in October 1996. This long-term loading cycle test will last for three years. The test voltage, applied between conductor and earth, amounts to 127 kV. The temperature is measured with the help of an optical-fiber measurement system. This device is computer controlled and works as a remote monitoring system. Keywords: Retrofitting, power cables, cross linked polyethylene insulation, paper insulation, pipe cables, optical-fiber measurements, temperature measurement, cable ratings, power cable installation, trenchless technology

Power capacitors are widely used for regulating voltage and improving the power factor, and the type in most frequent use today are capacitors made of oil-impregnated paper film, a growing number of which will soon have been in operation in the field for 20 years, which has prompted us to make study of techniques for diagnosing deterioration in oil-impregnated paper film capacitors. It was found that thermal stress causes deterioration of the insulating paper, which increases the amount of dissolved CO and C[O.sub.2] in the oil, and lowers the partial discharge inception voltage, determining the useful lifetime of the capacitor. Based on this idea, techniques have been proposed for diagnosing deterioration, consisting of caution level and danger level, from actual stress in the field. Further, a trial calculation was made for the number of years and months it takes for a capacitor to reach the caution level, from an investigation of capacitors in service for years.

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and B.C. Hydro. [V.sub.50] (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 [[micro]second]), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The [V.sub.50] results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The Volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns.

The electric characteristics and insulation design strength of a liquid-nitrogen-impregnated synthetic insulation was considered. It found to detect the impregnation of liquid nitrogen by measuring the electrostatic capacitance of the cable. [Epsilon] tan [Delta], an index of the dielectric loss, was 0.31% for cellulose paper and 0.18% for semisynthetic paper, PPLP and OPPL. It is found that the decline of the thickness dependence of the breakdown strength of the liquidnitrogen-impregnated insulating cable is steeper than that of the OF cables. It is possible to design the insulation strength of the 66 kV cable to be 10 kV/mm.

This paper exploits smart inverters of distributed energy resources (DERs) for the mitigation of fault-induced delayed voltage recovery (FIDVR). According to the IEEE Standard 1547-2018, smart inverters with voltage ride-through and var injection capabilities are promising technologies for mitigating FIDVR. This paper quantitatively investigates the effectiveness of smart inverters for FIDVR from a system-wide perspective. It shows that the penetration of smart inverters and induction motors are important factors affecting mitigation success. In addition, the over-voltages at the end of FIDVR due to load disconnections can be prevented with coordinated control of DERs and automated switching capacitors. The control scheme of smart inverters used for FIDVR is extended from the distributed volt/var droop control to include low- and high-voltage ride-through capabilities. The IEEE 13-bus feeder, the 8500-node test feeder, and the IEEE 390-node low-voltage network test system are used to illustrate the impact of DER penetration. For example, one can determine the residential air conditioner penetration level (for a given system at certain DER penetration level) that makes a system susceptible to FIDVR. The effects of solar intermittency are also studied. [ABSTRACT FROM AUTHOR]

This paper briefly describes the special publication, 'Fault and Disturbance Data Requirements for Automated Computer Analysis', prepared by Working Group I 11 of the Relaying Practices and Consumer Interface Protection Subcommittee of the IEEE Power System Relaying Committee. The publication describes the analysis objectives and the data necessary to facilitate automated analysis. The address for ordering a copy from the IEEE is in the reference section.

Compared to conventional ac cables, the current rating of HVDC cable circuits is more complicated because the permissible current is not only limited by the operating temperature, but also by the electrical stress within the insulation. Therefore the IEC60287 current rating standard is considered insufficient for some HVDC applications because it fails to include the stress limitation. This paper presents an analytical methodology to calculate an electrical stress-limited rating for HVDC cables, which considers the maximum dielectric stress as the current limiting factor. This paper represents the first part of a project which has comprehensively reviewed cable rating calculations for mass impregnated HVDC cable systems. [ABSTRACT FROM PUBLISHER]

In this paper, we discuss a better method to measure the paper water content of transformers using cellulose isotherms. The advantage of the new method is that the transformer oil and cellulosic insulation do not need to be in thermodynamic equilibrium. Instead, the temperature needs to be cycling in order to determine when the direction of water migration, whether out of or into the oil changes. The reversal in water migration can be used to determine the vapor pressure of the water adsorbed by the cellulose. This method was tested using 12 different conditions similar to those within a normally operating transformer. The results showed close agreement when this method is used. The significance is that isotherm equations can now be used even when insulation media are not in equilibrium. [ABSTRACT FROM PUBLISHER]

Voltage rise caused by high levels of distributed generation is manifesting as voltage regulation challenges for many electricity network service providers. In this environment it would be ideal to reduce supply voltage magnitudes, however, many network operators are hesitant to do so due to concerns related to consumer appliance performance at reduced supply voltage magnitudes. Voltage regulation requirements are defined by network standards and network service providers must ensure voltages remain within specified limits. Through an evaluation of domestic appliance performance when supplied at various voltage magnitudes, this paper examines the impact of varying voltage levels on residential appliances. Equipment energy demand, operation and actuation were monitored for each applied voltage magnitude. While no equipment failures were recorded, appliance behaviour varied significantly with applied voltage magnitude. Individual appliance conservation voltage reduction (CVR) factors have also been established. The results highlight the importance of good voltage regulation and provide substantiated appliance performance figures for future studies. The outcomes of this paper allow electricity network service providers to understand the implications of supply voltage magnitude on domestic appliance performance, whether it be understating of the impact of higher voltage magnitudes caused by distributed generation or implications of reducing voltage magnitudes to provide headroom for distributed generation integration. [ABSTRACT FROM AUTHOR]

Utilities prefer to perform a condition-based assessment of power transformer insulation in the least possible shutdown time. It is essential to estimate the values of various insulation condition sensitive performance parameters with higher accuracy. In the present work, a technique for the estimation of performance parameters, moisture content of paper, dissipation factor of the entire oil-paper insulation, paper conductivity, and activation energy is proposed. The evaluation of these parameters using reported techniques requires complete measurement of polarization and depolarization current profile, which requires around 20000 s of measurement time. The present method uses a forecasted polarization current profile obtained from 600 s of measured polarization current data. Detrended Fluctuation Analysis is applied to the forecasted polarization current data obtained from various in-service power transformers to obtain a suitable parameter. It is shown that this parameter maintains well-defined relationships with parameters; moisture content of paper, dissipation factor of oil-paper insulation, paper conductivity, and activation energy. The proposed technique is beneficial for utilities as it eliminates the requirement of measuring depolarization current and return voltage measurement for estimating activation energy. [ABSTRACT FROM AUTHOR]

Traditionally, water mobility inside the cellulosic insulation, as those used in power transformers, has been modeled using the so-called Fick's diffusion theory, considering that the water movement in this porous material only depends on local conditions, such as temperature and moisture concentration. However, recent experimental works have demonstrated that the moisture movement inside the cellulose insulation could be also influenced by a global condition, that is, the insulation's thickness. In this paper, the influence of the insulation thickness in the mobility of moisture inside the cellulosic materials, such as Kraft paper and pressboard, is analyzed. In addition, the improvement in moisture dynamics estimation obtained when this variable is included into the diffusion models has been proved. [ABSTRACT FROM PUBLISHER]

In this paper, we present an update to the models used to calculate the remaining life of transformer paper insulation. A drawback to the current IEEE method is that it does not take into account the availability of water and oxygen on the life expectancy of Kraft paper insulation. We therefore set out to test an algorithm which does take these into account. For our investigation, we loaded three test transformers and ran them to near their end of life. Kinetic equations were applied to model the fall in the degree of polymerization of paper. Our model showed better agreement with the test results than that given from using the IEEE standard. The IEEE standard gives life expectancy for Kraft paper being aged in minimal oxygen under dry conditions, which is not necessarily representative of an old transformer. Wet paper and using high levels of oxygen can age nearly 40 times faster. The IEEE method needs to take the synergistic effect of water and oxygen on increasing the rate of paper aging into account. [ABSTRACT FROM PUBLISHER]