Asset-Management of Transformers Based on Condition Monitoring and Standard Diagnosis [Feature Article]

In this paper, a methodology is developed to use data acquisition derived from condition monitoring and standard diagnosis for rehabilitation purposes of transformers. The interpretation and understanding of the test data are obtained from international test standards to determine the current condition of transformers. In an attempt to ascertain monitoring priorities, the effective test methods are selected for transformer diagnosis. In particular, the standardization of diagnostic and analytical techniques are being improved that will enable field personnel to more easily use the test results and will reduce the need for interpretation by experts. In addition, the advanced method has the potential to reduce the time greatly and increase the accuracy of diagnostics. The important aim of the standardization is to develop the multiple diagnostic models that combine results from the different tests and give an overall assessment of reliability and maintenance for transformers.     

The selection of the frequency range for high-voltage on-sitetesting of extruded insulation cable systems

High-voltage on-site tests of extruded insulation cables are necessary to check the quality of the installation of the system. The test voltage should simulate the stress under service conditions, and generate the same failure mechanism. With respect to an optimum design of test systems, a frequency range of 20 to 300 Hz, proposed in the latest IEC Draft for on-site testing of extruded insulation cables, seems to be reasonable. The test results on model cables can be used for real cables concerning the influence of the waveform on the breakdown voltage. The withstand voltage decreases with increasing frequency. The withstand voltage and the electrical breakdown field strength are very close together for frequencies between 20 Hz to 300 Hz. Mechanical defects, as well as water trees, reduce the breakdown voltage at 0.1 Hz more than at 20 to 300 Hz, but the absolute test voltage values are higher, and the breakdown mechanism is different compared with voltages of power frequency or adjacent frequencies. For on-site tests, frequency-tuned resonant test systems (ACRF) can be recommended because they have a very good weight-to-test power ratio, and very low power demands     

Dielectric spectroscopy techniques as quality control tool: a feasibility study - [feature article]

In this article, the feasibility of using time- and frequency-domain dielectric spectroscopy measurements to monitor the condition of oil-impregnated paper (OIP) condenser bushings is discussed.     

Life mangement of SF6 circuit breakers based on monitoring and diagnosis - [Feature Article]

In order to provide an accurate estimate of a circuit breaker's remaining lifetime, reliable measurements of ageing and failure characteristics and condition monitoring, are necessary. In order to obtain this documentation, it is necessary to monitor the performance parameters which are most sensitive to aging. Following comprehensive data acquisition and data analysis, it should be possible to develop a comprehensive electro-thermo- mechanical life model which accurately describes the aging behaviour of circuit breakers.     

Specific investigations to quantify heavy damage causes on loading resistor modules

In this paper, premature damage to some static inverter loading resistor modules is analyzed. Furthermore, investigations were carried out to determine the origins and causes of this serious damage occurring on the resistor modules in service. Investigations were preformed on the insulating liquid samples as well as new resistor modules. From the results obtained, it seems that the origin of the damage is from a long-standing, free-burning arc between two resistor windings, following mechanical breakdown.     

Determining the test voltage factor function for the evaluation of lightning impulses with oscillations and/or an overshoot

In order to remove the present IEC 60060-1 and IEEE Standard 4 ambiguities in the evaluation of the test voltage when oscillations and/or an overshoot appear superimposed in a lightning impulse, at the end of the 1990s, a European Project within the Standard, Measurement and Testing (SMT) program was developed by KEMA (Netherlands), Schering Institute (Germany), LCOE (Spain), and NGC (United Kingdom) in coordination with the CIGRE Working Group 33.03. The conclusion of this European Project is a new evaluation method called test voltage function K(f) derived on the breakdown behavior of different dielectric media, that removes the standard ambiguity to determine the test voltage of an impulse with oscillations and/or an overshoot. This paper presents the experimental results of the project and deals in depth with the definition of K(f).     

Challenge of mixed insulating liquids for use in high-voltage transformers. II. Investigations of mixed liquid impregnated paper insulation

As society depends more and more on electrical power, the demands for highly reliable power supplies are increasing. This paper examines the compatibility of oil mixtures with cellulose and aramid papers that are used in transformer insulation.     

Assessment of the actual condition of the electrical components in medium-voltage networks

Due to the large amount of electrical equipment and the costs of an individual diagnosis in medium-voltage networks, a general consideration of some representative electrical components is necessary to assess the actual condition of the electrical equipment. For a life model of electrical components, the individual aging phenomena of representative electrical components, as well as the general aging mechanisms of insulating materials, are taken into account in this paper. The aging of insulating materials can be estimated by an electrical breakdown occurring in electrical components so that the relationship between lifetime and failure probability of electrical components may be studied using the life model, the probabilistic failure model, and the enlargement law which are reasonably able to investigate the cause & consequence of failure in medium-voltage networks. Thereby, a new approach is developed for the assessment of the actual condition of electrical components in terms of failure time, failure probability, hazard rate, and other related variables of electrical components in medium-voltage networks. These reliability issues will support the decision-making in future deregulation of the electric energy market. Furthermore the model is verified through some electrical components like conductors, cables, transformers, and circuit-breakers with their specific failure statistics. It is demonstrated that the method is able to assess the actual condition of electrical components in medium-voltage networks with reasonable, accurate data.     

Distortion-free measurement of high impulse voltages

It is shown that every high voltage divider has a limited bandwidth of some MHz, resulting in a limitation for the measurement of front oscillations or front-chopped lightning impulse voltages. It is pointed out that a voltage divider can be easily calibrated and can be constructed to prevent EMC-problems during impulse voltage measurement. Using a spherical electric field sensor with optoelectronic data transmission, a voltage measurement system with an upper bandwidth of 25 MHz has been constructed. The advantage of this system is the distortion-free measurement of all impulse test voltages in any circuit. After a short review of the principle and characteristics of the sensor, some practical applications are highlighted, showing that it can improve high-voltage measurement