Experience with compact epoxy-mica capacitors for rotating machine partial discharge detection

Partial discharge pulses in operating motors and generators are often detected by either RF current transformers or high-voltage capacitors. Since the mid-1970s, an 80 pF capacitor has become popular for detecting partial discharges in hydrogenerators and, more recently, in motors. The 80 pF capacitor is made from XLPE power cable, resulting in an inexpensive, yet bulky, capacitor. A reliable 80 pF capacitor has now been developed using a mica and epoxy dielectric. The epoxy-mica capacitor is much more compact making it significantly easier to install. In fact, the installation time for the epoxy-mica capacitor is typically half the time needed to install the cable-type capacitor. More than 600 capacitors have now been installed on hydrogenerators, motors, and small turbine generators. This article discusses the design of the new capacitor, presents its electrical characteristics, and provides some examples of its installation.