Rudolf Woschitz

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.

Review of requirements and diagnostics of oil/board DC-insulation systems

Since the beginning of the 20th century, the main insulation of power transformers has been built up efficiently with oil-impregnated cellulose, and since the 1960ies as layered oil/board-insulation. The latter represents a hybrid system, consisting of oil filled gaps subdivided by pressboard barriers.

Current challenges and issues of designing HVDC converter transformers

For the insulation of high-power transformers the oil/board insulating system has proven itself for many decades. Owing to constant development in the field of high-performance semiconductor elements, high-voltage direct current (HVDC) power transmission becomes more and more important on account of major advantages involved in these systems. This may become problematic in particular in case of converter transformers as the insulation is not exposed exclusively to a mere AC stress. Depending on the topology of the converter, the transformer insulation is subject to a composed voltage stress (alternating voltage with direct component). This, however, results in serious effects and challenges respecting electric stress of the insulating components of oil and board. In case of mere alternating voltage stress capacitive field distribution takes place according to permittivities. Conductivities in the context of AC electric field are not important, but cause negligible dielectric losses. In case of direct voltage stress they are decisive as field distribution ensues in dependence of their values. Compared to AC stress, this leads to a reduction of stress on the oil but to an increase of stress on the insulating board. During switching processes or polarity reversal a transition from the dielectric displacement field to the steady-state electric DC-field takes place. Polarization mechanisms, in particular interfacial polarization, and considerably differing polarization durations between the materials of oil and board must be taken into consideration. Additionally, the conductivity of insulation materials is dependent on many parameters, such as temperature, humidity and aging. The resulting stress on insulating board in case of steady-state field stress necessitates detailed studies which focus on conductivity with regard to conductivity changes resulting from stresses occurring during operation [1], [2].

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

Gas insulated transmission lines in railway galleries — Part II

The possibility of installing a double-circuit Gas Insulated transmission Line (GIL) in the pilot tunnel of the planned new railway galleries Fortezza-Innsbruck has been throughout described in a previous paper [1]. The importance of this topic has driven the European Community to co-finance a feasibility study in the frame of Trans-European Energy Networks (TEN-Energy Programme). The study will last three years and will be completed at the end of 2005. The Italian (GRTN) and Austrian (TIRAG) TSOs are involved in this study together with the University of Padova. The work is in progress and this paper reports the studies that have been already finished: it is completely focused on the technical feasibility whereas the integration into the EHV Austrian and Italian networks, a comparison with other transmission technologies and the environmental impact will be set out in a next paper.

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.

Mutual inductive interference of 400 kV cable systems

The paper proposes a suitable model for the calculation of the mutual inductive and ohmic interference of 400 kV cable systems particularly of the induced currents and voltages in cable shields, cross bonding joints and earthing systems. Based on practical relevant examples, typical scenarios were defined and the effects of parallel laid 400 kV cable systems are shown and discussed. For the calculation of voltages and currents due to mutual inductive and ohmic coupling of parallel laid underground cable systems, the following parameters have to be considered: electrical symmetry of the cable systems, cable laying arrangement, distances between systems and phases, system and cross-bonding section length, influencing current, cross-section arrangement with detailed sub-section design, earthing configuration of the cable shields and joint cases, parallel earth continuity conductors, specific soil resistivity and laying depth. Considering the isolation materials of cables, the distances between phases and related cable shields, between phases themselves and between three-phase systems are much lower than the distances on overhead lines. Therefore, the mutual coupling, the symmetry of the entire system and the earthing arrangement play an essential role regarding induced voltages and currents. Not only the number of sections, but also the section length varies in many cases. For each of these sections, the impedance matrix is computed by using the self-impedances of all conductors and the coupling-impedances between all conductors using Dubanton’s approximation. These coupled cable sections are combined with the earthing resistances of the joints to form a chain model. The voltage and current distribution across 400 kV cable systems through the inductive interference can be finally calculated in all accessible places. Measurements at 400 kV cable systems of a DSO in Austria were carried out to validate the simulation model and the calculation results, respectively. The presented results show a good correlation with the measurement results.

Partial discharge patterns on cross-linked polyethylene DC power cables

Since electrical energy supply is crucial and HVAC applications play a significant role in that field, work and discussion about PD are mostly related to AC voltage. The situation is different recently because HVDC applications have started to take a part on the electrical energy supply. HVDC cable systems nowadays have been replaced the use of HVAC cable systems for transporting energy over long distances because some advantages it has. Cross-linked polyethylene (XLPE) cables are increasingly taking the place of mass-impregnated (MI) and oil-filled (OF) cables in power transmission because they are easier to maintain and are less likely to cause oil leakage. The number of new installed high voltage XLPE power cable increases because of the increase of the use of XLPE cables. In order to investigate partial discharge (PD) phenomena on a cable, XLPE cables with artificial defects are created and tested under DC voltage. This paper describes the DC voltage partial discharge characteristics for different sources such as surface discharge and void discharge. By comparing new representations of DC PD named “NoDi* Pattern” with conventional “PD magnitude as a function of time”, DC PD patterns can be shown and discussed.

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.