Christof Sumereder

Properties of Polymer Composites Used in High-Voltage Applications

The present review article represents a comprehensive study on polymer micro/nanocomposites that are used in high-voltage applications. Particular focus is on the structure-property relationship of composite materials used in power engineering, by exploiting fundamental theory as well as numerical/analytical models and the influence of material design on electrical, mechanical and thermal properties. In addition to describing the scientific development of micro/nanocomposites electrical features desired in power engineering, the study is mainly focused on the electrical properties of insulating materials, particularly cross-linked polyethylene (XLPE) and epoxy resins, unfilled and filled with different types of filler. Polymer micro/nanocomposites based on XLPE and epoxy resins are usually used as insulating systems for high-voltage applications, such as: cables, generators, motors, cast resin dry-type transformers, etc. Furthermore, this paper includes ample discussions regarding the advantages and disadvantages resulting in the electrical, mechanical and thermal properties by the addition of micro- and nanofillers into the base polymer. The study goals are to determine the impact of filler size, type and distribution of the particles into the polymer matrix on the electrical, mechanical and thermal properties of the polymer micro/nanocomposites compared to the neat polymer and traditionally materials used as insulation systems in high-voltage engineering. Properties such as electrical conductivity, relative permittivity, dielectric losses, partial discharges, erosion resistance, space charge behavior, electric breakdown, tracking and electrical tree resistance, thermal conductivity, tensile strength and modulus, elongation at break of micro- and nanocomposites based on epoxy resin and XLPE are analyzed. Finally, it was concluded that the use of polymer micro/nanocomposites in electrical engineering is very promising and further research work must be accomplished in order to diversify the polymer composites matrices and to improve their properties.

Statistical lifetime of hydro generators and failure analysis

One of the most important questions for manufacturers and users of electric power equipment is the failure probability and the expected lifetime of the produced and installed equipment. This question cannot be answered easily because there are many different parameters that influence lifetime behavior. If a breakdown occurs, it has to be determined whether the machine can made operational again within the technical and economical framework or if it has to be completely or partially replaced. The technical decision concerning repair or replacement often results in electric diagnostic measurements, in which economic decisions determined by corporate policy play a large role. In this paper, the characteristics of typical insulation systems for hydro generators will be discussed and a failure statistics of each system will be calculated. The resulting lifetimes for these systems will then be evaluated with Weibull distributions.

Significance of Defects Inside In-Service Aged Winding Insulations

This study aims to gain insight into the consequences of local defects in stator winding insulation; therefore, optical (microscopic) inspections and voltage-endurance tests have been carried out on in-service aged generator bars. In this context, generator bars with mica-bitumen insulations and generator bars with mica-epoxy insulations were investigated. The mica-bitumen insulations had been in service for 43 years in a 12 kV hydro-generator. The generator bars with mica-epoxy insulation had been dismantled from two identically constructed 10.5-kV generators after 35 and 36 years of service. The time to breakdown values obtained from the voltage-endurance tests were evaluated using a Weibull distribution. Apart from the scale parameter, Weibull exponent of this distribution allowed for the identification of early failures. The voltage-endurance tests carried out on replacement bars for these generators did not yield any early failures; such failures, however, were detected on both types of the in-service aged insulations. In particular, these occurred at high levels of electric stress and at delaminations between the copper conductor and the groundwall insulation. It is concluded that the interpretation of the measured time to breakdown values and their scatter depends on the level of electric stress.

Unconventional diagnostic methods for testing generator stator windings

This paper presented a comparison of different methods of measuring insulation resistance and tan delta (dissipation factor), and of documenting partial discharge activity. The measurements were made on several service-aged generators. The results for one in-service generator, obtained between service inspections, are discussed in detail. Comparing classical insulation resistance measurements and the dielectric response behavior in the frequency domain, good agreement is found. With the insulation time-constant, the dryness can be determined relatively. In the case of resin-impregnated insulation systems, additional investigations are required to determine the humidity absolutely. The determination of tan delta using various test systems and measuring methods was carried out on-site. Because tan delta is frequency and temperature-dependent, the results of series resonance circuit measurements must be modified by correcting for the difference between the measurement frequency and the power frequency. The results obtained using various PD-test systems depend on bandwidth and the circuit configuration. The multi-synchronous system has the significant advantages that the PD activity at several points can be monitored in one measurement, and the three-phase amplitude relation diagram makes it easy to identify the PD source. However, the calibration process is lengthy, so little time is saved overall, and a full set of quadripoles and coupling capacitors is needed for each channel.

Explanation and analysis of oil-paper insulation based on frequency-domain dielectric spectroscopy

In this paper, to effectively distinguish between the aging and moisture of oil-immersed paper (OIP), and to enhance the accuracy of the nondestructive diagnosis of the insulating state for oil-paper insulation, we perform an in-depth study of the frequencydomain dielectric response theory. First, we present a theoretical explanation for frequency-domain spectroscopy (FDS) based on investigations into the mechanism and characteristics of conductivity loss and polarization loss. Secondly, we explore the classical loss characteristics by performing a systematic analysis of the relationship between the complex dielectric permittivity and the frequency based on the measured FDS curves. Finally, we study the influences of the aging condition and moisture content on FDS. We extract some of the characteristic parameters that are related to the insulation condition. The results obtained indicate that at low frequencies, the FDS characteristic was mainly influenced by the conductivity loss, which may be explained by the Dyre model. At high frequencies, the characteristic was mainly influenced by the polarization loss, which may be explained by the H-N model. In addition, the characteristic at mid-range frequencies was due to both the conductivity and polarization loss. The moisture content significantly affects only the conductance process. Therefore, the characteristic parameter, namely the DC conductivity σ 0, which is closely related to the conductance process, can be used to quantitatively describe the moisture content of OIP. Meanwhile, the characteristic parameters α, β, and τ, which are associated with the polarization process, can be used to quantitatively describe the aging status of OIP.

Humidity Absorption of Generator Bars

The absorption of humidity in electric insulation systems is dominant for the behaviour of the insulation resistance and the breakdown strength. For this reason the dryness of the insulation system is tested as a routine test, e.g. the humidity of insulating oil at transformers. The dryness of generators is evaluated by measuring the time constant of the electric insulation. In an Austrian recommendation for the condition evaluation of rotating machines the humidity and insulation condition is evaluated by an empirical established dryness boarder. Since this evaluation method was recognized new insulation materials and production technologies for generator bars developed. This paper should give a view to the behaviour of the physical knowledge base of humidity absorption of solid insulation systems. The surface and volume resistance in dependence to temperature and air humidity for different insulation systems for rotating machines is measured and discussed

Residual voltage endurance of generator insulation systems

The insulation system of a generator is stressed by electrical, mechanical and thermal loads during the whole lifetime. By the means of diagnostic measurements the worsening of the condition within the dielectric system can be evaluated. To ensure the operational safety for the next maintenance period a hipot test with raised level can be applied. In spite of all carefulness it can happen that a breakdown of the insulation system happens. Beside the costs and required time for machine repair contractual damages can result in high costs for the operator. The idea of these investigations was to examine the residual voltage endurance of aged generators. For this reason operational aged generator bars were dismantled carefully from two stators of same type which were set out of operation for renewal. The generator bars were tested by the means of following dielectric methods: Insulation Resistance, Dielectric Dissipation Factor and Partial Discharge as well as accompanying material tests. The test objects were electrically aged with a three time raised nominal voltage with an expected lifetime between 100 and 200 hours. In regular intervals the dielectric tests were repeated periodically. In this paper elected results were presented with the focus on the diagnostic expressiveness of the investigated dielectric parameters. Statistic methods were used to describe and display the aging behavior of the investigated insulation system.

A comparison of partial discharge detection with 50 Hz and 0,1 Hz at XLPE Cables

In this paper the results of different cable test techniques were compared. On the one hand the classical 50 Hz method for partial discharge detection and on the other hand the very low frequency method - VLF - at 0,1 Hz was examined. Additionally the oscillating wave method was applied onsite in an aged medium voltage grid with several joints of a local utility.

Impact of small voids in solid insulating materials

Due to recent developments in insulation technology extensive knowledge of the impact of small voids on the dielectric behaviour and lifetime of the insulating system is required. Our objective is to investigate the PD behaviour of epoxy resin specimens with embedded artificial micro voids in order to determine the degradation mechanisms of insulating materials due to air filled micro voids. Therefore we carried out experiments, where a set of cylindrical test specimens were positioned in an oil vessel between metal electrodes. Whilst applying high electric field strength (up to 15 kV/mm) we determined the electrical partial discharge inception at different temperatures by heating the oil vessel (22 to 85 °C). Besides the partial discharge inception and extinction voltage, the level and rate of PD inception and extinction were also monitored. We found that PD inception took place at a lower voltage level or field strength respectively for specimens with hollow glass microspheres than for specimens of pure resin. Furthermore we recognised different PD patterns for specimen with and without microspheres respectively. The impact of temperature on the PD behaviour of the tested specimens can be described as marginal for temperatures between room temperature and about 50 °C but differs for higher temperatures. The influence of temperature seems to correlate with the properties of the epoxy resin used.

Umfassende Zustandsbewertung von Generatoren

SummaryTechnical diagnostics present a main part in generator maintenance, however, for condition evaluation besides aspects like equipment properties and rate of utilization the results of measurements are to be considered, too. Special emphasis is put on visual inspections which are mostly implemented during revision processes. For a comprehensive condition evaluation different technical and optical investigations have to be applied and the results have to be analyzed. In this way the aim of localization and repair of early failures can be achieved efficiently, so that a reliable operation during the whole life time period can be ensured.ZusammenfassungDie technische Diagnostik stellt einen wesentlichen Teil der Generatorinstandhaltung dar, jedoch sind für die Zustandsbewertung neben den Resultaten dieser messtechnischen Untersuchungen auch Aspekte wie Betriebsmitteleigenschaften und Maschinenauslastung zu beachten. In den Vordergrund ist auch die visuelle Inspektion zu stellen, die zumeist im Rahmen der Revision durchgeführt wird. Für eine umfassende Zustandsbewertung sind daher unterschiedlichste technische und optische Untersuchungen durchzuführen und deren Ergebnisse zu analysieren. Das Ziel zur Lokalisierung und frühzeitigen Behebung von Schwachstellen kann somit effizient erreicht werden, so dass ein zuverlässiger Betrieb während der gesamten Lebensdauer gewährleistet werden kann.