Measurements on pressboard to understand the effect of solid insulation condition on monitoring of power transformer winding clamping pressure

Abstract

Pressboard is the main building material of a power transformer winding structure. Through-thickness compression behaviour of pressboard has a direct influence on the transformer winding clamping pressure. Being a cellulose based polymeric material with a porous structure, the through-thickness compression behaviour of pressboard is highly non-linear. Also, the mechanical properties of pressboard are moisture, temperature, and ageing status dependent. However, information on the exact behaviour of the through-thickness compressibility of pressboard under typical operating conditions of a power transformer has not been well documented in the literature. This has posed difficulties in developing reliable techniques to detect loose clamping conditions in power transformer windings. This study presents the results of laboratory measurements on the effect of moisture, temperature and ageing on the through-thickness compression behaviour of oil-impregnated pressboard. From the measured results, it was found that the compressibility of pressboard could increase with increasing temperature, ageing and moisture ingress. It was also found that moisture and ageing tend to exaggerate the thermal softening of pressboard. Subsequent finite element analysis confirmed that transformer clamping pressure has significantly altered with the pressboard condition.

Investigation on Insulation Performance of Thermally Aged Natural Ester Oil Impregnated Pressboard

“…Liao et al [28] have observed that with ageing hydrogen bonds between OIP fibres brake reducing the crystallinity of structure non-linearly, making it brittle. The surface of OIP contains highly hydrophilic hydroxyl groups, which aids the breakage of inter-polymer hydrogen bonds, resulting in loss of mechanical strength [29].…”

The vibration response of transformer windings under harmonic excitations and its applications

“…Dissolved gas analysis (DGA) can perform real-time monitoring of the gas composition in the insulating oil [4], and identify whether discharge or high temperature phenomena occur inside the tank, but it is not sensitive to the winding deformation, pressboard misalignment and other faults that do not produce gas. The current studies show that the clamping force reduction will degrade the short-circuit strength of the winding mechanical structure, which is one of the main causes of transformer damages [5,6]. Therefore, a fault diagnosis approach for transformer windings that moves from offline to online is urgently needed.…”

Classification of Cellulosic Insulation State Based on Smart Life Prediction Approach (SLPA)

“…Earlier, the oil-impregnated CSKP failure was attributed only to electrical and thermal stresses [4]. However, CIGRE identified moisture as a most deleterious agent in the rapid ageing of the CSKP insulation [5,6]. Consequently, moisture, along with thermal and electrical stresses, produces a rapid degradation of the CSKP insulation.…”