Analysis of partial discharges measurements and generator technology evolution

The use of partial discharge measurements a means of monitoring generators achieves two goals: (1) detecting as soon as possible any mechanical flaw occurring in a generator and, after technological study and visual inspection, working out a suitably adapted maintenance; and (2) monitoring internal electrical aging of insulation systems so as to plan stator refurbishments. It is noted that, with insulation systems using shellac or asphalt type binder, the main flaw is due to internal electric stress on the ground wall. This usually leads to generator refurbishment. With insulation systems using polyester or epoxy resin, internal aging of the ground wall is much slower, whereas mechanical flaws due to loose slot wedge, vibrations, and loose magnetic core clamping are found more often. When they occur, maintenance work should be carried out to avoid any fast erosion of the ground wall. Asphalt insulated generators can either be monitored using the dissipation factor test or partial discharge measurements; but this is no longer true with modern insulation systems, where external mechanical flaws- as well as internal ones- can only be detected using partial discharge measurements