[Show abstract][Hide abstract] ABSTRACT: The paper, prepared by CIGRE WG D1.03 (TF 09), presents the guidelines for risk assessment procedure on defects in GIS based on PD diagnostics. The procedure, described in detail in CIGRE Technical Brochure 525, starts with sensitive PD measurement to detect the critical defects and follows with identification of the type of the defect and its location inside the GIS. This information taken together with other essential data from laboratory measurements, manufacturer's experience, design aspects and trend analysis of the PD activity, are the base for the estimation of the criticality of the defects. Finally, the risk assessment is performed based on the estimated dielectric failure probability and failure consequences that can be different in case of on-site testing or in service activity.
Full-text · Article · Dec 2013 · IEEE Transactions on Dielectrics and Electrical Insulation
[Show abstract][Hide abstract] ABSTRACT: The article presents the results of extensive investigations aimed at determining the moisture content in a large population of power transformers (161 units). The project was carried out during the period of 2003-2011 in Poland, Germany, and Sweden. All results were obtained by means of the Frequency Response Spectroscopy (FDS) method. Among the transformers tested were grid transformers, generator step-up (GSU) transformers, and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed moisture variation with age was found to depend on transformer design and operating conditions. It is suggested that, based on the presented statistical results and knowledge concerning transformer type and age, one may discriminate between probably dry transformers and transformers that require actual moisture analysis and therefore prioritize further condition assessment.
No preview · Article · Jun 2013 · IEEE Transactions on Dielectrics and Electrical Insulation
[Show abstract][Hide abstract] ABSTRACT: The measurement of partial discharges (PD) is a non-destructive and sensitive diagnostic tool for the condition assessment of insulating systems. Two major tasks of PD measurements may be distinguished, (i) PD detection, hence providing evidence and the type of the PD and (ii) the location of the PD. The question "Where is the PD source?" is amongst others tremendously important for scheduling and starting maintenance/repair actions cost and time efficiently or to perform a risk analysis. Here the possibility to geometrically localize the flaw, by means of arrival times of acoustic PD signals, gets an extremely interesting option. Precise acoustic arrival times are consequently essential to accurately locate PD in a power transformer. The averaging of acoustic PD signals helps to enhance the acoustic sensitivity. The acoustic detection limit is lowered significantly and the determination of the arrival times is made possible for weaker PD. Supplementary steps, like automatic objective arrival time determination or additional wavelet-based de-noising further improve the overall location accuracy. A new location approach works with pseudo-times and allows for the use of robust direct solvers instead of the previously used iterative algorithms.
Preview · Article · Jan 2009 · IEEE Transactions on Dielectrics and Electrical Insulation
[Show abstract][Hide abstract] ABSTRACT: Since regulating transformers have proved to be efficient in controlling the power flow and regulating the voltage, they are more and more widely used in today's environment of energy production, transmission and distribution. This changing environment challenges protection engineers as well to improve the sensitivity of protection, so that low-current faults could be detected (like turn-to-turn short circuits in transformer windings) and a warning message could be given. Moreover, the idea of an adaptive protection that adjusts the operating characteristics of the relay system in response to changing system conditions has became much more promising. It improves the protection sensitivity and simplifies its conception. This paper presents an adaptive adjustment concept in relation to the position change of the on load tap changer for universal differential protection of regulating transformers; such a concept provides a sensitive and cost-efficient protection for regulating transformers. Various simulations are carried out with the Electro-Magnetic Transients Program/Alternative Transients Program. The simulation results indicate the functional efficiency of the proposed concept under different fault conditions; the protection is sensitive to low level intern faults. The paper concludes by describing the software implementation of the algorithm on a test system based on a digital signal processor.
Preview · Article · May 2008 · IEEE Transactions on Power Delivery
[Show abstract][Hide abstract] ABSTRACT: According to the applicable standards, electromagnetic compatibility emission measurements are carried out in the frequency domain using a measuring receiver. In this paper, the theory, setup, and algorithms, as well as the practical aspects including the examples of a precompliance measuring system in the time domain, are presented. The advantage of this system is that the measurements can be carried out approximately 10-100 times faster with sufficient accuracy and signal-to-noise ratio in the detector modes peak, quasi-peak, and average.
Preview · Article · Dec 2007 · IEEE Transactions on Electromagnetic Compatibility
[Show abstract][Hide abstract] ABSTRACT: The rising number of electronic and electric systems in a modern vehicle makes automotive EMC more and more important. Due to steadily decreasing development cycles, EMCMeasurements of the whole vehicle are only possible in a relative late state of the development time. By means of measuring transfer functions in a vehicle, one can categorise different places of installation in face of their possible disturbance behaviour. The comparison of different transfer functions of vehicles can help to estimate the potential disturbance risk e.g. to the build-in radio-, TV- or mobile phone antenna.
[Show abstract][Hide abstract] ABSTRACT: The transfer function (TF), also called frequency response, can be used as a diagnostic method to detect mechanical and electrical defects. The assessment of FRA (Frequency Response Analysis) results is done by a comparison between a recent and a previous measurement, which is called the reference measurement. An acknowledged problem with FRA is that up to now there is no international standardized procedure for comparison. Analyses are done by experts but run the risk of being judged with subjectivity. Thus, the need for an algorithm enabling quantitative determination of the identicalness of two FRA measurements within a certain frequency range is obvious. The article will present different algorithms to assess the FRA: expectation, standard deviation, correlation factor and analysis of resonance frequencies. The performance of these algorithms is verified using data sets obtained during measurements for the investigation of sensitivity of the FRA method to mechanical winding displacements. The sensitivity of the algorithms increases by splitting the frequency range into several parts. Finally the algorithms are compared to Chinas standard for FRA assessment.
[Show abstract][Hide abstract] ABSTRACT: The frequency response analysis (FRA) is a well-established method used for assessment of the mechanical and electrical conditions of power transformer windings. The evaluation of the winding condition is done by comparison between an actual FRA and a reference FRA, which are carried out previously. A deviation between both FRA curves can indicate a change of the winding condition of the tested transformer. According to the standard of knowledge, the connections between measuring device and transformer terminals, as well as the grounding technique are of main importance for a good repeatability. Additionally, a power transformer is a large construction that needs long cables and grounding lines for the signal transmission. Against this background, it is clear, that the geometric arrangement of the measuring setup may not be the same as it was at the reference measurement and this change can have an influence on FRA results. Therefore, an excellent repeatability of the measurements is needed in order to prevent any problematic to distinguish between measurement failures and real damage within the transformer.
[Show abstract][Hide abstract] ABSTRACT: The paper presents the improvement in design of the conductor connecting the windings and bushings in oil power transformers. This is a sensitive part of large transformers and there is a need to optimise its manufacturing time and costs. The thermal problem of the heating of this conductor with increased insulation thickness on a part of it is treated using a non-linear two-dimensional thermal model. The experiments performed made determining of some problematic parameters (heat resistance of oil-paper insulation and the paper to oil convection heat transfer coefficient) of the thermal model feasible. The results obtained are of practical interest in the design practice of interconnections, but also affect the important parameters of heat transfer by devices with oil immersed paper insulation.
Full-text · Article · Feb 2006 · Electrical Engineering
[Show abstract][Hide abstract] ABSTRACT: 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.
Preview · Article · Feb 2005 · IEEE Transactions on Power Delivery
[Show abstract][Hide abstract] ABSTRACT: The diagnostic examination of partial discharges (PD) for the condition assessment of insulating systems of transformers according to the IEC60270 standard is normally carried out in test bays to control new products after a manufacturing process. An increasing demand for assessing the insulating quality of aged equipment which is met by means of onsite or online PD measurements is noticed. Online/onsite acoustic PD measurements of oil-paper insulated transformers could be grouped into two major tasks. First is to provide evidence of PD (detection) as sensitive as possible. Second is the in many respects important determination of the failure location (localization of the PD). Depending on whether mixed-acoustic (i.e. electric or electromagnetic triggering) or all-acoustic (acoustic triggering) measurements are used three different approaches for the system of nonlinear observation equations with three (space coordinates (x, y, z) of the PD) or four unknowns (an additionally unknown temporal origin) are presented and discussed. The new approach within the acoustic signal processing which works with pseudo-times allows for utilization of robust direct GPS (Global Positioning System) solvers instead of the previously used iterative algorithms. Laboratory experiments as well as onsite/online measurements given in the contribution show the advantages of the new positioning algorithms.
[Show abstract][Hide abstract] ABSTRACT: In course of cost reduction efforts at electric utilities the budgeting for physical assets appears to be the most crucial planning instrument. The operational practice shows yet that the existing cost prediction tools are not sensitive enough for reflecting fast condition developments in the periodic maintenance budget plan. As a result, decision makers take financial consequences of assets degradation into consideration only with substantial time delay. This leads to sub-optimal ma-nagement of interacting business processes and con-sequently to considerable losses in the entire organi-zation. The present contribution will demonstrate an ite-rative technique for determination of asset degradation required for budget calculation. The approach combines financial and engineering information and it allows the decision makers to follow the technical and financial risks as a function of time. The article will also illustrate, that the assets amortization and the required maintenance budget might be calculated by deploying the Fuzzy-Logic in real time.
[Show abstract][Hide abstract] ABSTRACT: Partial discharge (PD) diagnosis for large power transformers as one of the most powerful tools to evaluate insulating quality and characteristics should be carried out as permanent monitoring or onsite measurements with adequate sensitivity. This allows conditioned based maintenance strategies, serves asset appraisements and permits continuous risk assessment. Online/onsite electric PD measuring procedures according to the IEC60270 standard so far show a limited reachable sensitivity e.g. caused by corona. Since PD also appear as source of mechanical waves in the ultra-sonic range and electromagnetic waves up to the UHF (ultra high frequencies) range additional acoustic or electromagnetic measurements are possible and helpful in online/onsite measurements. Comparative investigations with the acoustic and the UHF method revealed a very moderate damping of the UHF signals in oil and in the solid insulations in contrast to resulting attenuation of the acoustic signals. Thus a significantly higher sensitivity especially for hidden PD defects could be reached with the UHF method. Sensitive electromagnetic UHF PD measurements help to identify and localize faults which are not detectable with acoustic single impulses by simple averaging of the acoustic signals. Furthermore broad-band investigations of UHF spectra revealed cavity resonances which possibly make up the basis for advantageous narrow-band measurements.
[Show abstract][Hide abstract] ABSTRACT: Impulse tests are performed on power transformers to assess their insulation integrity. During these tests, impulse voltages are applied, and the resulting voltage and current waveforms are recorded. After postprocessing of these data, the presence of faults in the winding should be detected. Different methods and models have been developed for pattern recognition of transformer winding current and classification of impulse faults, . The drawback of these methods is the representation of a fault as a short circuit with constant conductance. In this paper, the breakdown and arc were modeled to represent a more realistic fault during impulse tests. Using this model, it is possible to generate more realistic data, which are necessary to develop more reliable fault-detecting algorithms and, as a result, better pattern recognition and classification of impulse test waveforms. The model has its roots in the classical Mayr equation, and it is simulated with TACS and MODELS modules in EMTP. The simulation results verify the ability of the suggested method to represent disk-to-disk breakdown and arc fault during impulse testing.
No preview · Article · Aug 2004 · Electrical Engineering
[Show abstract][Hide abstract] ABSTRACT: In this paper, a novel and accurate method is presented for evaluating the series and parallel resistances for detailed modeling of power transformers. In the proposed method, the black box model of power transformers as well as the natural frequencies of its winding are estimated by the impulse test. Then the series and parallel resistances can be evaluated for most upper and lower natural frequencies with reasonable accuracy to be implemented in the detailed model. The exact estimation of these resistances is important for the damping determination of transformer winding oscillations. The proposed model enables the designers to correct the design data sheets before mass production by performing the impulse test on the prototype. In the present study, a comparison between the simulation and test results verifies the validity of the proposed method.
No preview · Article · Aug 2004 · Electrical Engineering
[Show abstract][Hide abstract] ABSTRACT: This paper surveys the work on the improving of the thermal modelling of oil power transformers. The original algorithm for the hot-spot temperature calculation is exposed. The algorithm describes the physical phenomena better and provides higher calculation accuracy then the algorithms in the valid IEC standards and in the new draft. The algorithm is convenient to be practically realised on the microprocessor thermal relay. The research of the important practical problems of unknown starting hot-spot temperature and of thermal characteristics change during a long-term operation of a transformer is also overviewed. The prospect for future work is a further research of other configuration of the thermal observer. Also, the natural step after the research and extensive experimental verification of the algorithms is the application in commercial microprocessor thermal protection and monitoring systems.