Modeling of Internal Pressure Dynamics in Mass-Impregnated Nondraining HVDC Cables

Experimental and Simulation Studies on Stable Polarity Reversal in Aged HVDC Mass-Impregnated Cables

“…Porosity (ε p ) is defined as the volume fraction through which a fluid can flow within a solid material. As illustrated in Figure 4a, mass impregnated insulation involves the migration of mass oil through porous paper [16,29]. Consequently, the numerical model for internal pressure in MI cables can be described by integrating the fluid continuity equation and Darcy's law, which represent fluid flux in a porous medium, as shown in Equations ( 16) and (17).…”

“…In this state, if the density of the mass on the conductor side increases due to load-off, a very large negative pressure is required to create an inward flow velocity. However, excessively large negative pressures and pressure gradients inside the insulation are unrealistic based on measurements in the literatures [16][17][18]. Therefore, it is necessary to adjust the porosity below a certain very low pressure level to moderate the rate of density change over time.…”

“…The corresponding porosity change graph is shown in Figure 4b. It is assumed that below 0.02 bar, voids are formed, and at higher pressures, the porosity slightly increases due to the formation of mass films between the papers [16].…”

“…These observations support the widely accepted theory that the cavitation phenomenon, characterized by the formation of shrinkage cavities in the insulation during the cooling cycle after a load decrease or disconnection, substantially undermines cable integrity. Recent studies have reported on the measurement and numerical modeling of internal pressure under various temperature and pressure conditions in HVDC MI cable samples [16][17][18]. The findings demonstrate that rapid load changes increase the internal pressure within the insulation and induce mass flow.…”