C. Sumereder

Moisture determination and degradation of solid insulation system of power transformers

The reliability and operational risk of power transformers is strongly dependant on the condition of the electrical insulation system. Different kind of stress can be distinguished, normally a combination of electrical and thermal stress is present as well as the influence of humidity in the insulation system has to be considered. The so called multi-stress load results in an accelerated ageing process and the degradation of the insulation system. The condition of the mineral oil can be determined by sampling and analyzing the oil by the means of DGA and chemical-physical analysis. The solid insulation system mainly consists of transformerboard. The condition determination of the cellulose can be done by sampling, with is quite expensive and difficult. New dielectric methods which operate on the effect of response function have established in the last years. In this paper the focus should be directed on the mechanism of cellulose ageing and dielectric measurement.

Polarization-depolarization measurements at insulation systems for rotating machines

For the condition evaluation of the electric insulation systems, dielectric measurements are a very important tool for the diagnosis. Beside partial discharge and dissipation factor measurements the dielectric response of the electrical insulation system gives further information about the aging process and the condition of the insulation medium. The measurement and analysis of the polarization and depolarization current (PDC) is done according to the classical equivalent circuit of parallel and serial resistances and capacitances. This paper should give a view to PDC measurements at insulation systems for rotating machines.

Condition Evaluation of Hydro Generators

Taking a look at the structure of the hydro power plants of the Austrian utilities many of the electric machines are reaching a critical age. The risk of a failure or a break down can cause high financial consequences. For this reason a concept for the condition evaluation of hydro generators was developed. The focus of this procedure was to collect operational data; machine data and protocols of technical diagnosis measurements in order to form an objective analysis by weighing each factor. The aim was to make a kind of benchmark of all investigated machines. Several comparison charts were generated. This should result in making the right decisions about further operations, maintenance activities or replacement measures

Lifetime management of power transformers

Power transformers belong to the most important and most cost-intensive equipment of electrical power transmission and distribution. If an error in a transformer occurs, this causes not only an interruption of the electrical power supply of far areas but also large economic losses beyond that. Thus, the operation of transformers must guarantee a continuous and error free power supply over decades. Occurring errors and thus self-announcing possible failures of transformers should be detected in time, in order to initiate suitable measures for error correction. A continuous entry of the isolation status can prevent large errors, extend service life of the system and optimize maintenance. Thus, apart from higher system availability an avoidance of unplanned disconnections and expensive repairs are achieved. The emphasis of this contribution is to present methods which permit a substantially more exact and more reliable predicate about the current status of the isolation system of liquid-isolated power transformers.ZusammenfassungLeistungstransformatoren zählen zu den wichtigsten und kostenintensivsten Betriebsmitteln der elektrischen Energieübertragung und -verteilung. Ein Fehler in einem Transformator hat nicht nur den Stromausfall der angekoppelten Netzbereiche zur Folge, sondern bringt auch wirtschaftliche Verluste mit sich. Der Einsatz von Transformatoren erfordert daher einen möglichst sicheren und unterbrechungsfreien Betrieb über mehrere Jahrzehnte. Eintretende und sich ankündigende Fehler an Transformatoren sollten rechtzeitig detektiert werden, um geeignete Maßnahmen zur Verhinderung oder die Behebung dergleichen einleiten zu können. Eine kontinuierliche Erfassung der Isolationseigenschaften ermöglicht eine Vorbeugung langer fehlerbedingter Unterbrechungen sowie eine Verlängerung der Wartungsintervalle und damit eine Optimierung der Instandhaltung. Abgesehen von der Erzielung einer höheren Verfügbarkeit werden ungeplante Abschaltungen und teure Reparaturen verhindert. Der Schwerpunkt dieser Veröffentlichung liegt in der Präsentation einer Methode, die auf dem Vergleich bekannter Maßnahmen der Zustandsbeurteilung basiert, welche eine exaktere und wirtschaftlichere Aussage über den aktuellen Status des Isolationssystems von flüssiggekühlten Leistungstransformatoren ermöglicht.

Extension of generator lifetime by winding renewal

Two hydro generators were chosen to renew the stator winding. The motivation of this investment was an expertise about the condition of the generators. Beside technical diagnosis the operational dates and maintenance history was taken into account as well as the age of these generators. The stator windings were replaced with an insulation system that had the benefit of a 10 % power increase. The aged generator bars were tested in laboratory with the focus on the residual lifetime. Voltage endurance tests and optical investigations were done and statistically analysed. The result of these investigations confirmed the decision of the renewal of the stator winding

Aging behaviour of cross-linked polyethylene (XLPE) as an insulating material for high (HV)- and extra-high voltage cables (EHV)

The objective of this paper is the scientific investigation of the electrical long-time response of XLPE-insulation. The electrical XLPE-insulation system is susceptible to aging when subjected to the thermal, electrical, environmental and mechanical stresses experienced under service conditions. Based on extensive experimental research work concerning the breakdown behaviour of XLPE, it is shown in this paper that beyond the statistical history, space charge processes contribute to the AC dielectric strength decrease of thick-walled XLPE-insulation. Values for the AC breakdown strength from long time tests on a cable model and 2 real cables are given and an example for calculation with approximation methods, to compare the values, is carried out. Weibull statistical analysis of area and volume effects on the breakdown strength in XLPE, for a coaxial cylindrical electrode configuration, is examined.

Dielectric measurements as quality criterion for liquid nitrogen

For the operation of high temperature superconducting (HTS) devices it is necessary to cool the HTS conductor down to the operating temperature of about 77 K. Depending on the application warm or cold dielectric insulation systems are used in electrical power systems and high voltage apparatus. Liquid nitrogen (LN/sub 2/) is an excellent dielectric liquid, but there is less knowledge about the quality criterions and their measuring methods. For the measurement of dielectric parameters (e.g. breakdown voltage, partial discharge, relative permittivity) the classical measuring methods have to be adapted. This paper gives a view to the results of dielectric measuring methods with LN/sub 2/. The measurements were done with different degrees of LN/sub 2/ pureness.

Transformer condition determination methods

Beside the classical oil analysis there were new methods to determine the condition of the electric insulation system of transformers. The aging of the insulation system is influenced dominant by the water content of mineral oil. For this reason the moisture determination with chemical analysis and dielectric measurements were essential.

Quality control of XLPE cables by means of impulse voltage test

High voltage cable lines are rated among the most expensive investments in the electric energy supply system. Due to economic and-operational requirements these lines are to live up to the highest expectations as far as reliability and adequate life time are concerned. Therefore it is recommended to undertake impulse voltage tests with increased level apart from tests as provided in the relevant standards. Thanks to this examinations impurities in the insulation and in the interface between insulation and semi conductive layers can be recognized fast and easily for they cause lower levels of the impulse strength. Particularly so-called protrusions in the interface between insulation and semi conductive layers or inadequate adhesion of the semi conductive layers to the insulation, both defects that can occur during the production of the cable, are subjected to the test. During operation these impurities can cause increased local electrical field strength in the insulation as well as water trees and electrical trees. Additionally they speed up the aging process in the insulation of the cable and this can lead to early faults respectively a shorter life time. These production related defects are not detectable by standard tests as partial discharge measurement or voltage tests with power frequency. Due to our investigations and good experience in the field of quality control of XLPE cables we carry out this test for all Austrian and some German utilities.

Condition evaluation of generator bars

The condition evaluation of electric power equipment gained very much importance in the last years. Generally it forms the basis of maintenance strategy, risk evaluation and the decision over renewal or reinvestment. Dielectric measurements are a very important tool for the technical diagnosis of electric insulation systems like generator bars of rotating machines. Beside partial discharge and dissipation factor measurements the dielectric response of the electrical insulation system gives further information about the aging process and the condition of the insulation medium. The measurements and analysis of the polarization and depolarization current (PDC) are done in time domain and for the interpretation of the test results in the frequency domain the discrete Fourier transformation is applied. This paper should give a view to conventional dielectric measurements and the PDC analysis at insulation systems for rotating machines. Generator bars in resin rich and VPI technology were tested in different stadiums of age, mode of operation and load. For the results of the practical tests a mathematical model was found to show the correspondence to the existing models of the classical equivalent circuit of parallel and serial resistances and capacitances.