Lukasz Chmura

Estimation of dielectric loss using damped AC voltages

In this article some fundamental aspects of dielectric loss estimation using DAC voltage are described. Damped ac voltage testing is an alternative method of continuous ac voltage testing. It provides a method of assessing the condition of the insulation of capacitive test objects, e.g., power cables or stator windings of generators. The decay of damped ac voltages used for on-site energizing of power cables, and testing generator stator insulation, can be analyzed to estimate the dielectric loss (tan δ) of the insulation.

Use of dissipation factor for life consumption assessment and future life modeling of oil-filled high-voltage power cables

Tan δ measurements can be used to assess the condition of OF insulation in HV power cables. Data showing the behavior of tan δ as a function of temperature at constant electrical stress, and as a function of electrical stress at constant temperature, at different stages of degradation (or aging) are essential. By combining on-site and laboratory measurements, the remaining service lifetime of a cable can be estimated for various future loading levels using a model incorporating ground temperature, the age of the cable, and its loading history.

Time-to-breakdown and breakdown voltage for oil-impregnated insulation subjected to thermal aging

Oil-impregnated (OI) insulation is widely used in many types of high-voltage components such as transformers and cables. The most important requirement for the insulation is to withstand operational stresses without breakdown for the whole period of operation. That means insulation should be able to operate for a period of 30-40 years. For this reason, assessment of the insulation parameters such as breakdown voltage and time-to-breakdown is always done by rapid and endurance testing. However, such tests are always performed on new samples of the insulation and the electrical stress is the only aging factor. Therefore, for better life-time estimation of the insulation, it is important to know if the life expectancy might change due to thermal aging. In this paper, the results will be presented of an investigation into time-to-breakdown and breakdown voltage for thermally degraded OI insulation. In particular, for samples of insulation characterized by different levels of thermal aging, a comparison will be made of time-to-breakdown and breakdown voltage.

Life curves for new and thermally aged oil-impregnated paper insulation

The life-time of oil-impregnated (OI) insulation is mainly governed by two aging mechanism, i.e. electrical and thermal. The electrical-life law takes the form of inverse power law. In this case the life is inversely proportional to the applied electrical stress. Due to continuous voltage application, the insulation loses the dielectric strength. The process of thermal aging of OI insulation is described by Arrhenius law. The law specifies that the rate of thermal aging is exponentially dependent on applied temperature. Due to thermal aging, the structure of the insulation is changed because of chemical decomposition due to pyrolisys, hydrolisys and oxidation. The changes of physical structure result in change of electrical properties of insulation. In order to determine the life-time of the component, the electrical endurance tests are performed at several voltage levels. By the tests performance the time-to-breakdown at given voltage levels are obtained and the life-line of the insulation together with accompanying parameters can be found. However, the mentioned procedure takes only electric aging into account and the thermal aging is neglected here. Therefore, it is interesting how thermal aging influences changes of electrical life. In this paper, the first results of the experiments investigating influence of thermal aging on the electrical life of the OI insulation and its practical implications will be discussed.

Condition assessment of on load tap changers using dynamic resistance measurements

Resistance measurements on power transformers are a widely accepted method for finding winding defects. Continuity of the tap changer contacts can also be checked when the on-load tap changer (OLTC) is operated during the resistance measurement. Resistance measurements that also record the resistance during OLTC operation are called dynamic resistance measurements. This off-line diagnostic method can be used during regular maintenance to assess the condition of OLTC parts not accessible for inspection. Besides condition assessment of inaccessible parts of the OLTC, this method can also be used to detect maintenance errors that could lead to failure. In particular, these measurements are very helpful in finding contact discontinuity, mechanical defects that influence the operation of the tap changer contacts, problems with the switch time and the transition resistors and can find contact degradation in an early stage.

Life-data analysis for condition assessment of high-voltage assets

Currently, network operators are facing a situation in which their high-voltage assets are reaching or even exceeding their design lifetimes [1]-[3]. The problem of future replacement of assets must thus be considered [4], [5]. Spare parts must be available to ensure replacement of components that fail during operation. In practice, utilities adopt two different approaches to assessing the condition of their assets [6], [7], namely bottom-up and top-down analysis. Bottom-up analysis uses aging characteristics of the materials within a given asset, and diagnostic measurements are performed to assess the physical degradation of the various parts of that asset. In contrast, top-down analysis uses mathematics to analyze the service-lifetime data of the whole population under consideration and to estimate the number of future failures within the population. In practice, both approaches have limitations due to differences in component design, operational conditions, environment, and maintenance programs [1]. An additional difficulty arises from ongoing technological improvements, e.g., in the properties of materials used in high-voltage components over a period of perhaps 40 years. In this paper, parametric statistical methods are used to analyze the time to failure of high-voltage components and to estimate the number of future failures. Attention is drawn to several problems that complicate the statistical analysis of service-lifetime data. Detailed information on the basic theory of statistical analysis of failure data can be found in [5], [6], [8]. Using service-lifetime data provided by a Dutch utility, and Monte Carlo simulations, three case studies of the failure of high-voltage components are presented.

Test procedure and test circuit considerations for on load tap changer dynamic resistance measurement

Resistance measurements on power transformers are offline diagnostic methods that are performed without opening the transformer main tank. In case the power transformer is equipped with an on-load tap changer (OLTC), this resistance measurement can be used to find degradation and defects inside the OLTC. Many OLTC defects as well as contact degradation can be measured by means of dynamic resistance measurements, which is a resistance test during OLTC operation. A low (DC) test current through the on-load tap changer is used during dynamic resistance measurements. This method is in particular sensitive to maintenance errors, contact degradation and contact timing problems. In general, distinction can be made between defects that interrupt the current through the OLTC, defects that affect the timing of the OLTC contacts and contact degradation resulting in abnormal contact resistance.

Life time estimation of serviced aged oil-paper insulated HV power cables based on the dielectric loss measurements (tan δ)

this contribution focuses on the application of dielectric loss measurements for service aged oil-impregnated high voltage power cables. The results of the project focused on condition assessment of service aged oil-impregnated HV power cables will be discussed. The final goal of this project is: to develop by using laboratory and on-site tanδ measurements a tool to support the condition assessment and asset management decisions. By combination laboratory measurements and on-site diagnostics, an investigation have been performed into several complete cable circuits to find the relation between electrical and thermal stresses of paper oil-impregnated insulation, at different aging stages. The result of this investigation, in particular influence of electrical and thermal stressing of samples in different stage of aging, with combination of on-site diagnostics has been applied to develop a tool for supporting asset management decision process.

Reliability estimation for populations with limited and heavily censored failure information

Statistical analysis of the life data, is a useful tool helping to assess the life-time of populations of high-voltage components. More specific, the results of such analysis give overview over the failure behavior of the population under investigation, i.e. number and trend of expected failures. For the analysis, the detailed information about ages and numbers and ages of installed units and failed units has to be collected. Subsequently, the distribution representing the behavior of the population is fitted to the data. The latter allows deriving the time-dependent failure rate function, which in turn, directly indicates the trends of the future failures. However, this method requires homogeneous and independent data of sufficient amount. The latter becomes a problem, particularly that for past periods the failure data is often unavailable. It is important to estimate the population reliability and number of expected failures, for the whole population of components being operated. This is also important in the case when the available failure data comes only from one part of the area where the components are installed. In this paper we will show how to deal with populations where the available failure data is heavily censored, and what will the influence of the data division according to the regions in which the transformers are operated, on the failure expectancy.

On-site diagnosis of XLPE transmission power cables with Damped AC technique

Diagnostics of HV power cables are playing an important role in the meaning of two aspects: 1) on-site diagnostic of new installation, where a commissioning test has to prove the absence of any defects in the cable insulation and accessories, after transportation and installation. 2) On-site diagnostic of a serviced aged power cable where the condition after a period in service or information about workmanship of repairs has to be known. As it is known some installation defects can result in breakdown even after a successful after-laying test. Obtaining more diagnostic parameters like; Withstand Voltage, Partial Discharges (PD) and Dielectric Loss (tan δ), providing a indication of insulation condition and accessory performance. In this paper several examples of on-site tests on new and serviced aged XLPE HV power cables will be discussed. Examples of: after-repair, after-laying, and condition assessment tests will be presented as performed with Damped AC diagnostics.