Contents I. Introduction
With the pursuit of high efficiency in electric power transmission and renewable energy, there comes a rapid development in high voltage DC transmission systems [1] . The converter transformer is an essential part of the DC transmission system. It is well known that the oil-paper insulation system is the “heart” of the transformer. Transformer oil is usually taken as the blood of the transformer, whose security decides the safety of the whole power system. The insulating oil inside a transformer normally contacts metal, iron core and pressboard insulation. Metal filings or cellulosic residual can be formed in the oil [1 – 4] , almost 94% of them are cellulose particles [5] . There is a particles shedding phenomenon for transformer in operation, especially for aged transformers with old oil-paper insulation. If the impurities of converter transformers in presence of particles during operation form a bridge in a period of time, it will be a well conductive path in the converter transformer, which may cause partial discharge, even the whole insulation damage of the medium [5 – 9] . The bridging phenomenon can be a deadly risk to transformers. Moreover, because of normal aging of the insulation, the moisture and impurities increase gradually [10] . Moisture plays a detrimental role in the oil-paper insulation lifetime by reducing the thermal resistance and electrical breakdown strength and is regarded as “the first enemy” after temperature [11] . The production of moisture is inevitable in the service of a transformer. Field experience shows that the moisture content of a transformer is usually <0.5% in the initial stage of its operation, and it may increase to 4%–8% at the last stage of its life [12] .
