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4 Prace Naukowe Instytutu Podstaw Elektrotechniki i Elektrotechnologii Politechniki Wrocławskiej. Konferencje

4 2004

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New approach in thermal monitoring of large power transformers applied on a 350 MVA odaf-cooled unit

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Cardillo E. , Feser K.

2004

tom Vol. 40, nr 15

77-81

Large power transformers belong to the most valuable and important assets in electrical power systems. The ageing of the used oil/paper-insulation primarily depends on the hot-spot temperature of the solid insulation system. Hence it is the dominant factor limiting the lifetime of the transformer. Therefore in 1996 a prototype of a stand alone thermal monitoring system was developed and installed on a 350 MVA transformer to observe the hot-spot temperature. The experiences of successfully operation during the last years encouraged us to update the system to the newest stage. The paper describes the state-of-the-art of the thermal transformer monitoring system based on new field bus technology. The advantages of a networked system and the usage of a centrally managed data base shared between multiple monitoring systems are exposed. The intention to implement new features exceeding the conventional monitoring system philosophy is discussed on an intelligent multifunction criteria Fuzzy algorithm for the control of fans of the heat exchanger units. It permits a more cost-effective long-time transformer operation management.

Calculation of the transfer function of a power transformer with online measuring data

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Wimmer R. , Feser K.

2004

tom Vol. 40, nr 15

86-90

Mechanical deformation in transformer windings can be detected with the transfer function (TF). Normally the transformer will be disconnected from the public power supply to record the TF (offline measuring). Another possibility is to calculate the TF from the transients overvoltages generated from switching operations or lightning strikes. With adequate sensors these overvoltages can be recorded during operation of the transformer (online-measuring). On the basis of these online-datas diverse problems like separationof excitation- and response signal, too low signal-to-noise ratio, etc. and their solution will be shown in this paper.

Reliability and influences on dielectric diagnostic methods to evaluate the ageing state of oil-paper insulations

100%

Koch M. , Feser K.

2004

tom Vol. 40, nr 15

95-101

Dielectric measurements and diagnostic methods are established procedures to evaluate moisture as an ageing indicator in commonly used oil-paper insulations of power transformers. Both the measurements themselves as well as the analysis of collected data contain several uncertainties and sources of error. This investigation discusses the sources of error and gives a further insight to the influences of transformer insulation design (geometry), insulation temperature, moisture equilibrium in oil and paper and other aspects regarding practical measurements. The dominating impact on dielectric analysis is related to the insulation temperature rather than insulation geometry. Limits and influences on linear modelling of multilayer insulations by equivalent circuits are described too. The investigation includes all the common dielectric diagnostic methods for oil-paper insulations: Recovery Voltage Method (RVM), Polarisation and Depolarisation Currents (PDC) and Frequency Domain Spectroscopy (FDS).

All-acoustic PD measurements of oil/paper-insulated transformers for PD-localization

100%

Markalous S. M. , Feser K.

2004

tom Vol. 40, nr 15

106-112

Dealing with all-acoustic online PD measurements of oil/paper-insulated transformers one encounters two main tasks. First is detection of a PD and its distinction from impulse-shaped acoustic disturbances in terms of a monitoring decision "PD-yes/no". Second is the localization of the PD which is needed e.g. for risk assessment. There one has to give the acoustic amplitude a relation to the electrical amplitude and get an appropriate Picocoulomb (pC) value for considerations concerning deterioration. In case of doing computations with traveling times an exact all-acoustic geometric localization of the PD origin needs at least four sensor signals. Two variants for the system of non-linear observation equations with four unknowns can be distinguished. Working with time-differences starting from a first-hit reference sensor gives three unknown coordinates in space and an unknown time origin. Another approach presented is stating the system of equations in a GPS (Global Positioning System) form handling pseudo-times. In this case three coordinates and a receiver time offset constitute the unknowns.