What is the partial discharge ? and how to control it in HV motors and generators ? Part 1/4

what causes partial discharge?
Partial discharge (PD) occurs in high voltage (HV)
electrical equipment like cables, transformers,
motors and generators. It is a kind of very small
spark that occurs due to a high electrical field. In
industrial high voltage motors and generators,
electrical stress can reach several kilovolts per
millimeter and this can give rise to PD either
inside the insulation (internal PD) or on the
surface (external PD).
Internal PD generally occurs in microscopic airfilled
voids that remain inside the insulation, with
the number and size of the voids depending on
the procedures followed during manufacturing.
Charges build up around the voids and when the
local electric field stress across a void exceeds
the breakdown field stress, a discharge occurs
that short-circuits the void. Special equipment is
required to detect and measure internal PD.
External PD occurs on the surface of the
insulation, when the local electric field stress
exceeds the threshold to ionize the air. Factors
behind external PD can include poor air
clearances, contamination on the end-windings,
and variations or defects in the corona protection
in the slots or on the end-windings. External PD is
generally easy to detect by visual inspection as it
usually results in a deposit of white powder on
the surface of the insulation.
PD generates a short-duration plasma burst and
emits UV light, both of which stress the
insulation. It causes rapid localized temperature
spikes and creates chemically aggressive
substances such as nitrogen oxides, ozone and
nitric acid. These effects impact the surrounding
insulation, which should therefore be designed to
withstand PD.