Effect of Moisture and Load Change on Clamping Pressure in a Transformer Test Rig

During the manufacturing of a power transformer, typically, a fixed clamping pressure is exerted on the winding structure to keep its mechanical integrity during short circuit faults. Various factors, e.g., breathing during load cycles, poor sealing, aging of cellulose materials, etc., introduce moisture in the transformer during its long service life. A typical power transformer operation has a continuous moisture exchange between the cellulose material and the oil. The pressboard samples and controllable heated brass plates are stacked in a test rig. This paper investigates the effect of moisture and load variation on the development of clamping pressure in the test rig. Pressboard samples with two different moisture contents are studied: dry (~0.6%) and wet (~4.7%). Two different heating cycles are investigated: a constant load and a frequent start-stop load. The temperature of the brass plate and oil, relative humidity of oil and pressboard, and clamping force are measured throughout the experiment. It has been observed that the clamping pressure follows the temperature profile of the brass plates. A significant loss of clamping pressure is observed after mounting the wet samples in the rig. The clamping pressure drops further after the first heating cycle as the rig temperature returns to the ambient. However, when the rig returns to room temperature, there is no noticeable decrease in clamping pressure, neither for the dry sample nor for the wet sample after its first heating cycle.