Thermal behavior evaluation of a new solid insulating paper in natural ester and mineral insulating oils

Abstract

The conventional transformer oil and paper insulation system consists of mineral oil and cellulose-based kraft insulating paper. Kraft paper can be "thermostabilized" by cyanoethylation or cellulose acetylation reactions, or by the addition of stabilizing agents such as urea, melanin and dicyanamide. When high operating temperatures are expected, thermally upgraded kraft (TUK) paper is often used (thermal class: 120, although in IEC, the average winding temperature rise remains 65 K). An alternative to cellulose-based paper is the synthetic dielectric material Nomex®, composed of aramid polymer. Aramid paper has a considerably higher thermal rating of 180+ vs. 120 for TUK. Introduced into the market a few years ago is a new solid dielectric material named Nomex® 910. This Hybrid insulating paper is an engineered cellulose paper. The composite solid dielectric paper consists of cellulose and web-like binders made from the same high-temperature polymer as other Nomex® brand papers and according to literature has a thermal class of 130 in mineral oil, filling the blank space in thermal class between 120 (TUK) and 180 (aramid).IEEE Std C57.100-2011 uses a reference technique to determine a thermal class for a candidate system using the "industry proven system" as a reference. This system for IEEE applications is inhibited mineral oil and thermally upgraded kraft paper. IEEE C57.12.00 defines this insulation system life with an Arrhenius curve defined as the minimum acceptable life. The basis of the curve is 180,000 hours at the defined hot spot temperature of 110 °C. When determining the life curve and therefore also the thermal class of a new candidate system, the end-of-life criteria for the reference "industry proven system" must first be characterized. The procedure calls for ageing three test cells each for three different test temperatures. The test cells are placed in an oven at the prescribed temperatures for a length of time determined by the defined Arrhenius curve. The samples are then tested for tensile strength or degree of polymerization (DP). This value of the remaining tensile strength or DP then becomes the end-of-life criteria for the new candidate system. This paper documents the combination results of insulation systems composed of natural ester (corn-based oil – Biovolt) and mineral insulting oils (MIO) with both thermally upgraded and Hybrid paper. The expectation is that the Arrhenius life curves, equations and thermal index/thermal class will be established for these four insulation system combinations. These will in turn then be available for verification of other insulation systems by comparison to the appropriate combination, in accordance with IEEE Std C57.100-2011, appropriately modified for the specific reference curve selected.