An HFCT is a current transformer designed for frequencies from hundreds of kilohertz to tens of megahertz, far above the 50 Hz of the network. Its core, usually ferrite and split into two halves, clamps around a conductor without cutting it.
It is fitted where the discharge current returns: the screen of a cable, the earth connection of a bushing or a termination, or the neutral of a machine. The pulse of a partial discharge lasts nanoseconds and therefore has high-frequency content that the HFCT sees clearly, while the power-frequency current hardly affects it.
It is the usual sensor for measurement on cables and rotating machines, both in spot campaigns and in permanent monitoring. Its advantage is that it is fitted without disconnecting anything and gives a signal proportional to the charge, calibratable in picocoulombs; its limitation is that it needs a point where the return path is accessible.