System for measurement of relative permittivity and dielectric dissipation factor of insulating liquids

U ovom diplomskom radu razvijena je laboratorijska metoda za određivanje relativne permitivnosti i faktora dielektričkih gubitaka izolacijskih tekućina. Mjerenja su obavljena na uzorku transformatorskog ulja u normiranim opsezima jakosti električnog polja i temperature, koje zahtjeva norma IEC 60247. Metoda se temelji na posrednom određivanju tih veličina putem mjerenja kapaciteta i kuta gubitaka ispitnog modela. Metoda je realizirana na principu ''mosnih'' metoda za određivanje kapaciteta i kuta gubitaka, gdje je u jednoj grani spojen etalonski kondenzator zanemarivog kuta gubitaka, dok je u drugoj ispitni model, izveden kao cilindrični kondenzator. Za obradu mjernih rezultata korištena je akvizicijska kartica NI 6356 koja je preko USB-a spojena na računalo i upravljana preko sučelja napravljenog u programu LabVIEW. Kako bi prilagodili ulazni signal u karticu na maksimalnih 10V, izlazi iz kondenzatora su spojeni na ulaze (invertirajuće) dualnog operacijskog pojačala LT1113. Vrijednosti parametara povratne veze oba pojačala su odabrane tako da se na njihovim izlazima dobije napon pogodan za karticu (max. 10V). Također, njihove vrijednosti uzrokuju gotovo jednak fazni pomak, čime je olakšan konačan proračun faktora dielektričkih gubitaka.In this thesis a laboratory method has been developed in order to determine relative permittivity and dielectric dissipation factor of insulating liquides. The measurements have been performed on a sample of transformer oil in a standardized range of electric field intensity and temperature, which is required by IEC 60247 norm. This method is based on an indirect determination of these parameters by measuring of capacity and loss angle of a test model. This method is realized on a principle of ''bridge circuit'' method in order to determine capacity and loss angle, where, in the ''known leg'' there is a etalon capacitor of the negligible loss angle, and in the ''unknown leg'' there is a test model – cylindrical capacitor. In order to process the results of measurements a Data Acquistion NI 6356 was used, which was connected to the computer via USB, and controlled via an interface made in LabVIEW program. In order to adjust the input signal to the maximum of 10V, output ports from the capacitor were connected to the input ports (invert) od a dual operational amplifier LT1113. The parametar values of both amplifiers feedback were chosen in such a way, that on their outputs a voltage suitable for the NI 6356 (max. 10V) was carried away. Also, their values cause almost an equel phase shift, witch facilitates the final calculation of dielectric dissipation factor