Development of Testing Method for Static Electrification Within Power Transformer Using Suspended Cellulose Fibers in Insulating Oil

This paper describes a new testing method that has been developed to estimate the quantity of electrification in a power transformer. In addition, the influence that the degradation of both a press board (PB; consists of cellulose fibers) and insulating oil has on static electrification can be evaluated by using the new testing method. In this testing method, cellulose fibers that are suspended in insulating oil in a transformer were used. These fibers might come off from the PB and the surface of insulating paper in a transformer. It was considered these cellulose fibers were useful samples for estimating the electrification level of a PB in an actual power transformer. The insulating oil was taken from a running power transformer, and cellulose fibers in the oil were filtered with a metal filter. This filtering process was isolated from the air. These cellulose fibers and insulating oil (oil with cellulose removed) were testing samples for measuring the quantity of electrification in the static electrification process. Static electrification occurs when insulating oil flows over cellulose fibers on a metal filter. The volume of electrification in the static electrification process can be known by measuring the electric current on a conductor connected between the metal filter and the ground. Experimental results showed that the volume of electrification on a degraded PB was larger than that of a new PB. Besides, there was also a tendency for the volume of electrification on degraded insulation oil (large value for electrostatic charging tendency) to be larger than that of new insulation oil.