Moisture Adsorption Mechanism of Molecular Sieves in Oil–Paper Insulation: Simulation on Static Adsorption Characteristics

Oil–paper insulation structures are widely utilized inside power transformers. Moisture speeds up the deterioration of the paper insulation and significantly degrades the performance of the transformers’ oil–paper insulation. The molecular sieve is a widely used material for moisture removal from gas and liquid, but its mechanism in moisture removal in transformer insulation is still not clear, especially from a microscopic perspective. In this article, the density functional theory (DFT) and molecular dynamics simulation were adopted to calculate the forces of water molecules in the oil–paper insulation system. Also, the models of different A-type molecular sieves were constructed, and the steady-state adsorption characteristics of water molecules in the sieves were obtained by the Monte Carlo method. The results show that the method of calculating the adsorption energy to characterize the adsorption capacity of moisture by different objects is reasonable. The basic reason why molecular sieves can effectively adsorb moisture in the oil–paper structure is that the adsorption energy between the water molecule and the molecular sieve structure is higher than that between the water molecule and other molecules in the oil–paper insulation. Compared with 3A molecular sieves, 4A molecular sieves have a larger free volume, more adsorption sites, and higher adsorption energy, which indicates that 4A molecular sieves have better moisture adsorption characteristics. This research gives a microcosmic explanation of the moisture adsorption characteristics of the oil–paper insulation structure and the moisture adsorption mechanism of molecular sieve, which may provide a theoretical basis for the application and selection of molecular sieves in the online moisture removal of oil–paper insulation structure.

I. Introduction

The insulation issues of transformers have received continuous attention for the irreplaceable role of transformers in power system [1], [2], [3]. Moisture has been recognized as the “enemy number one” for transformer insulation [4]. The moisture in the oil–paper insulation in the transformer not only comes from the external environment but also is an aging product of cellulose. The increase of moisture content in oil–paper insulation will reduce the dielectric strength [5], reduce the initial field strength of partial discharge [6], and shorten the insulation’s life [7], [8], [9].