Bálint Németh

Condition monitoring of power transformers using DGA and Fuzzy logic

Dissolved Gas Analysis (DGA) is a widely used technique to estimate the condition of oil-immersed transformers. The measurement of the level and the change of combustible gases in the insulating oil is a trustworthy diagnostic tool which can be used as indicator of undesirable events occurring inside the transformer, such as hot spots, electrical arcing or partial discharge.

Transformer condition analyzing expert system using fuzzy neural system

Power transformers being the major apparatus in a power system, thus the assessment of transformer operating condition and lifespan have obtained crucial significance in latest years. Dissolved gas analysis (DGA) is a sensitive and reliable technique for the detection of incipient fault condition within oil-immersed transformers, which provides the basis of diagnostic evaluation of equipment health.

Extra low frequency electric and magnetic fields during live-line maintenance

Nowadays different live-line maintenance methods are widely applied because of their economic and technical benefits. The different valid regulations are not always strict enough to guarantee the proper safety of the workers. In these regulations the criteria of the investigation is the shielding of the electric field. The proper way to shield electric field is wellknown: all the conductive clothing applied on high voltage live-line maintenance follow the principle of a Faraday cage. Although this principle seems theoretically easy, in practice many of conductive clothing cannot guarantee the proper electric shielding during the work. It is necessary to compare different conductive clothing to determine which construction is the most effective to protect the worker against electric field [1-4]. Another important thing is the proper shielding of extra low frequency magnetic field. Besides the electric field, this field is present during different live-line works as well. The long-term health effects of this magnetic field are still unexplained and there is not any effective practical way yet to shield it. Laboratory measurements and simulations are needed to investigate the magnetic field inside the different types of conductive clothing as a function of current flowing through the conductors near the live-line workers. The topic is practical and up-to-date: it is essential to minimize the risk on the workers during any type of work, especially during live-line maintenance. It is not acceptable to use any live-line methods without any effective shielding, if the loading current generates higher magnetic field than the limits of the valid regulations.

Shielding efficiency of conductive clothing during live-line maintenance

Nowadays live-line maintenance (LLM) is one of the most common methods to repair and maintain energized elements of the power grid. The main advantage of this technique that it represents a balance between technical and economic factors: no consumer disturbance is needed, there is not any service outage caused and losses can be kept as low as possible. Because of these reasons different live-line methods are becoming more and more popular on all voltage levels. Special techniques are available for low, middle and high voltage grids. On high voltage levels “barehand method” is widely applied worldwide. The main principle of this method is an enclosed metal surface around the worker acting as a Faraday cage. Inside an ideal Faraday-cage electric field is zero from outer source. The efficiency of a clothing mainly depends on the size of holes (so called “Faraday-holes”) on the surface. A proper face mesh can guarantee the safety of the live-line worker. Calculations, simulations and measurements in the High Voltage Laboratory of Budapest University of Technology and Economics have proved that poor, coarse or missing face shield decreases the efficiency by the increase of body current.

Diagnostic evaluation of power transformers by an expert system

Power transformers are the most important and expensive equipment in the electrical transmission systems. The transformers average age in the power system in Hungary is approximately 25 years. It is not economical to replace these equipment only because of their age. We have to know and manage more information about the insulation, because the status of the insulation is determines the expected lifetime. A transformer can only be operated efficiently, reliably and economically, if we know its actual state expanding on all essential information. The operational reliability is influenced the most by the quality of the insulation but even other state markers/features play an important role. The quality of the insulation depends on the mechanical (temperature, vibration etc.) and electrical (voltage level, loading) influences. Besides these parameters there are a lot of things what affect the status of the insulation. These are the weather and the location of the equipment in the grid (rural or an industrial location). This paper is a review of an expert system which considers the parameters listed above. This expert system is developed in the High Voltage Laboratory of the Budapest Technical University and Economics. This evaluating system makes a technical and economical evaluation and comparison by marking the status of the transformers based on only a few parameters (status and the reliability of the status). This status contains the environmental influences, the operating values and the diagnostic results (Recovery Voltage Measurement (RVM), Dissolved Gas Analyzing (DGA) Oil test results etc.). The status can be calculated in the following way. Each physical effect (e.g.: oil test, RVM method etc.) has a value, and these physical effects are divided to physical values. These values are classified (e.g.: H2 content in the oil is 5 μl/l) to a classification value with their weight factor. Then the status code is calculated for the status attributes, with the classification value and a weighting. The global status factor is calculated with the status attributes (DGA, RVM, Oil, etc.) and their weight factor. The status diagram contains more status value and drawing these points in the function of the time, this diagram shows the changing of the status of the transformer. With the status diagram and with technical and statistic support, the failure level and some important information can be enumerated in the status system to manage the transformers lifecycle. In the electricity systems it is very important for the operating these equipments to have an expert system with suitable efficiency so the past record is also taken into consideration during the diagnoses.

Adaptation of the application of portable protective air gap to the Hungarian live-line maintenance technology

The purpose of this paper is to examine the usability of portable protective air gaps in case of high voltage live-line maintenance in the Hungarian transmission network. After the examination of the characteristics of the network where the PPAG would be used, the operating range of the device has to be determined, then the gap distance has to be adjusted based on measurements with different voltage waveforms, different arrangements and regarding some other parameters affecting the sparkover characteristics. For these reasons measurements were performed in the High Voltage Laboratory (HVL) of Budapest University of Technology and Economics (BUTE); the results of the measurements were evaluated and are introduced in this paper.

Applicability of the dielectric response methods of diagnostics of power transformers: Hungarian experiences

The actual life of transformers strongly depends on the real operating conditions and the residual life of these units can spread in a fairly broad scale. The key factor is the presence of moisture and ageing products because they strongly influence the condition of oil-paper insulation. Therefore recent attention has been directed to methods of determining moisture content and ageing products in the transformer. There are two fundamental dielectric processes in transformer insulation: the conduction which is characterized by conductivity, and the polarization characterized by the polarization spectrum. Both processes are in very strict relation to the structure of the material. Any changing of the structure caused by the deterioration of insulation brings a typical alteration of the dielectric processes and the dielectric parameters of insulation as well. Thus the shape of polarization spectrum is characteristic not only to the insulation material, but to its deteriorating processes to its condition as well. Especially the long-time constant range of the polarization spectrum gives a lot of information about the deteriorating processes of insulation. The Hungarian results — one detail has been only shown in this paper — could improve the interpretation of results of spectrum methods; it is useful in evaluating the condition of the oil-paper insulation and allows observation of the progress of the aging and moistening process in oil-paper insulation.

Complex Examination of a Cable Terminal Failure

The medium voltage cable networks act important role in electric energy distribution. The most important part of cable lines are the cable joints and terminals from the point of view of failures. In the present case study breakdown of a cable terminal appeared repeatedly in a medium voltage distribution board. The possible reasons were: penetration of atmospheric overvoltage from the connected medium voltage distribution line, assembling inaccuracy, appearance of partial discharges by not properly building-up the distribution board or lack of fitting of the termination and the cable. All of the reasons were examined: data from relays and lightning detection system, fitting of termination in laboratory and the arrangement of distribution board were analysed. Stresses in service were simulated in the laboratory. The results and the methods of examination are discussed.

Practical results and opportunities of evaluation of transformer diagnostic by an expert system

In the High Voltage Laboratory at Budapest University of Technology and Economics developed expert system, called Transformer Status Indicator (TSI) provide a good solution for technical and economical evaluate and for compare multiple transformer diagnostically results. The power transformer is the most important equipment in the transmission and distribution system, so the operators have to know the condition of their equipment because of the maintenance and operating too. This expert system is the principle of condition based maintenance strategy. The system consider discrete diagnostically results (e.g.: oil test, dissolved gas analyzing (DGA), recovery voltage measurement (RVM), furan content, insulation resistance, dielectric loss factor (tgd)). For the comparison we need to consider some other parameters what do not indicate the status of the insulation but it has influence for that. These parameters are: voltage level, nominal power, weather, location in the grid, and the location: industrial or rural environment. The expert system using the data listed above and can characterize the condition of the insulation of the transformer only a few parameters (status of the insulation, reliability and theoretically minimum and maximum values of it) independently from the power, voltage level etc. to compare each transformer condition to make a best decision for change or refurbish the equipment. In the research work, after the developing process, the expert system tested by real data from the Hungarian distribution system. The source of the testing data is 13 HV/MV distribution transformers in Hungary. In this paper we are focusing the results and their evaluating possibilities. Because of the number of test transformers we can use statistical methods to get a good result. Either possible evaluating way is to calculate the average decreasing speed of the status of the insulation. If the operators know each work effect (maintenance and/or reconstruction) in the status of the insulation, the necessary time and volume of this work can calculate using the TSI expert system.

Effect of Voltage Stress on Diagnostic Parameters of Low Voltage Cables

The most important property of the insulation is the dielectric strength, because the main function of the insulation is isolating of the electrodes on different potential. Due to the stresses the dielectric strength is decreasing till it reaches the limit of safety operation. Aging tests are performed on insulations to predict the expected lifetime of the insulation in service. The enhanced voltage stress is widely used as the means of accelerated aging test. In the accelerated aging test the controlled parameter is the voltage stress and the determined variable is the time to breakdown. The aim of the present investigation was to determine the effect of voltage stress on the parameters measured by diagnostics methods. The samples for the examination were taken from polymeric insulated low voltage cables. The samples were periodically aged by voltage stress and insulation diagnostics tests were performed on the samples after every aging period. The results are discussed in this paper.