Bogdan Gorgan

Assessment of transformer mineral oil condition based on dc and ac conductivity

Due to the increase of electrical energy consumption, the power transformers are loaded beyond nameplate rating. The degradation of their insulation systems is intensified and the dielectric properties are worsened. Therefore, the interest of asset managers, regarding the condition of their transformers is a very important topic. Electrical conductivity is strongly influenced by the high operating temperatures and catalysis products (water, gases, acids) that appear as a result of oil and paper degradation reactions during transformer operation. The present paper is focused on an experimental study regarding the influence of water, acids and gases (hydrogen, acetylene, ethylene, methane etc.) resulted from degradation processes on electrical conduction phenomena of mineral oil. For that, the frequency/time variation curves of ac/dc conductivity for different concentrations of water, acids and gases are drawn. Finally, the correlation between ac and dc conductivity and the possibility to assess the ageing state of the mineral oil based on these quantities are analyzed.

Calculation of the remaining lifetime of power transformers paper insulation

This paper presents the remaining lifetime calculation of power transformers paper insulation and consequently of power transformers. The calculations are performed based on two models, which are related to the thermal degradation of the cellulose winding paper insulation: the common IEC loading guide and a paper degradation model. The paper insulation models prediction can be improved by involving data from furfural analysis. The remaining lifetime is extracted from the fault probability (reliability) of the paper insulation. The two models are brought together, to aid the asset manager in the decision making process. A probabilistic approach is used, which can be coupled to analysis in terms of risks, benefits, costs, and availability by the asset manager.

Influence of solar irradiation on power transformer thermal balance

In countries with a high ambient temperature and strong solar irradiation, transformer winding hot-spot temperature may increase over its maximum permissible limit. This can considerably reduce the insulation life of the transformer by enhanced degradation of the paper insulation. According to current loading guides, for each 6 K increase in working temperature, the ageing rate increases with approximately a factor two. Therefore, it is important to take into consideration the impact of the sun on the power transformer thermal behavior. In this paper, a modified hot-spot temperature model is presented to account for the effect of transformer winding temperature rise by solar irradiation. The effects of solar irradiation on transformer winding paper insulation are shown by comparing the degree of polymerisation (DP), the fault probability and the remaining life. Here, the fault probability is defined as the probability that the estimated DP-value at a certain moment in time is below a certain end-of-life criterion (threshold value). An additional winding hot-spot temperature rise of 9 K during the summer and a temperature rise of 6 K during the winter may occur in countries with strong solar irradiation. This may result in a reduction of the remaining lifetime by up to 40%.

Health index calculation of electrical equipments using DiagConsole software

During operational life, electrical equipment from the Electrical Grid is subjected to failure due to external causes (lightning strikes, over voltages) and internal causes (ageing of their insulation systems). Condition assessment of this equipment, in order to avoid premature failure, is a very important topic due to reliability and investments issues. The health index is being used by many utilities due to the fact that it shows the global degradation condition of an equipment so that the beneficiary can choose between the type of maintenance or replacement operations. This paper presents a software (DiagConsole), designed to assess the condition of all the electrical equipment from the Romanian Electrical Grid by calculating their health index. The structure of DiagConsole (offline and online monitoring and diagnosis) is presented, in particular for power transformers. The final part of the paper presents a case study, done on a power transformer for which the health index is calculated using DiagConsole. Based on the health index, the condition of the transformer is evaluated.

PV plant modeling for power system integration using PSCAD software

PV Plants connected to the medium voltage grid do not contribute to the grid stability. In order to prevent grid instability, directives (codes) for connecting PV plants to the medium voltage power grid have been released. The supply and control of the reactive power from the renewable generation plants are becoming important issues to be studied, because they can facilitate the integration of PV in power grids. In this paper, two new models of a 6.09 MW PV plant, used to analyze its grid integration according to the grid code, are presented. The first is a simplified model, without taking into account transformers and cables, while the second one is a more complex model which includes these components. The model was developed using PSCAD-EMTDC software. The final part of the paper presents the active-reactive power (P-Q) charts, calculated at the common coupling point (CCP), for different levels of solar radiation (0% to 100%). Based on these charts, it is determined the maximum output power level which can be generated by the plant, according to the current grid code.

Teilentladungsmessungen an rotierenden elektrischen Maschinen – Erfahrungen und Innovationen

ZusammenfassungDie Teilentladungsmessung (TE-Messung) gilt heute als weltweit akzeptierte Diagnosemethode für die Zustandserfassung des Isoliersystems von rotierenden elektrischen Maschinen. Im Beitrag wird neben ihren Besonderheiten auch auf die unterschiedlichen Ansätze der Messung eingegangen. Die Diskussion wird mit praktischen Beispielen und Versuchen untermauert. Die Vorteile eines digitalen Messsystems mit fortgeschrittenen Möglichkeiten zur Separierung einzelner TE-Quellen oder der Identifikation von externen Störern werden aufgezeigt. Abschließend wird die automatisierte Erkennung phasenaufgelöster Muster als Endstufe der softwarebasierten Auswertehilfe vorgestellt und erklärt.AbstractPartial discharge (PD) measurement is nowadays a worldwide accepted method for the condition-based assessment of stator insulation. The advantage of having a fully digital measuring system, with advanced hardware and software capabilities that allows improved insulation diagnosis by means of PD analysis, is shown. The sensitivity of the PD measurements can be strongly limited by a high noise level. State-of-the-art features based on synchronous multi-channel and multi-frequency techniques for signal separation of noise and PD defects are presented. The examples of data evaluation are described and the use of the automated PD pattern recognition system is discussed.

Photovoltaic power plants contribution to the flicker level in the medium voltage energy network. Case study

In recent years, renewable energy sources, particularly photovoltaic power plants, have become important sources of energy for the National Energy System in Romania. Connecting them to the energy grid may cause frequency variations or fast voltage variations (flicker), significant problems in terms of power quality. The severity of these problems depends directly on the percentage of the photovoltaic system integration, geographic location and grid typology. In Romania, photovoltaic power plants that are connected to the medium voltage networks must comply with legislation (NT. 30, Ord. 74, Ord. 59, NT E 012, EN 50160). Given these aspects, this paper aims to conduct a study on the contribution of photovoltaic power plants regarding the flicker level in the medium voltage network; in this particular case, a 5 MW PV plant is presented.

Monitoring and diagnosis of electrical equipment using a web software. Health index and remaining lifetime estimation

Electrical equipment monitoring and diagnosis are very important for both users and manufacturers. In order to assess equipment conditions, certain periodical tests and measurements must be made and monitoring and diagnosis systems must be installed, together with special software. This paper presents a web software (DiagConsole), which can be accessed remotely using a web browser. The software gathers online and off-line data, estimating the equipments health index, remaining lifetime and presenting the type of maintenance needed to prolong it. A condition assessment report is automatically built and can very well replace the traditional paper database. The web software is installed in two key power plants in Romania. Several electrical equipment are monitored and diagnosed (power transformers, hydrogenerators, etc.) and two power transformers and one hydrogenerator are presented as condition assessment examples.