P
artial Discharge (PD) is a localised electrical discharge in an insu-
lation system that does not completely bridge the electrodes. It
is a phenomenon that occurs only at higher ac voltages (above
2 000 Vac rms (root mean square) line voltages). PD was originally
believed to be a problem only above 4 000 V (see section on low
voltage (LV) inverter-fed motors).
The higher the voltage, the more destructive the activity; the
higher the altitude, the lower the voltage for the onset of destructive
PD. PD is a leading cause, and indicator, of insulation breakdown. PD
arises in a motor winding when electrical stresses in the air rise above
withstand levels. It is usually caused as a result of low energy dis-
charge pulses that occur continuously during operation which cause
the insulation to slowly break down and eventually fail. PDs cause
premature failure of the insulation – shortening the life of the machine.
PD in MV machines [1, 2]
Types of machine insulation
Type II
• Machine with rated voltage >700 V rms
• Form wound
• Expected to withstand PD activity during its life
Generally, MV machines are form wound with rectangular conduc-
tors, the insulated coils filling the slot perfectly, without voids. The
insulation system is designed to eliminate voids where possible, and
would consist of conductor insulation, turn insulation with, possibly,
a corona shield over the turn insulation, a main wall insulation with
a corona shield to ensure good void-free contact with the core, and
stress grading at each end of the coils where the coils leave the slot
section in the core.
Mica is extensively used inMVmachine insulation systems owing
to its excellent PD-resistant properties.
Common problem areas are voids:
o In the insulation between the conductors and core in the slot
o Between the conductors and the inner corona shield
o Between the outer surface of the coil and core
o Between the inner corona shield and the main insulation
o Within the main insulation due to de-lamination
o Within the main insulation due to an incomplete resin fill
o At the coil surface where the coil exits from the slots
o In the air gaps between the coils of different potential
o In the air gaps between the winding connectors and the terminal
leads
Figure 1: Potential area of PD in slot section of coils. (Typical cross section
of a 3 turn 11 kV form wound coil.)
Figure 2: Example of PD in an 11 kV machine in the overhang.
Figure 3: Example of PD damage in the slots of an 11 kV machine after
many years of service.
Is Partial Discharge an
issue?
By H du Preez, Consultant
nt
We know that Partial Discharges take place in all medium and high voltage machines and equipment, but low voltage motors, operating with
electronic adjustable speed systems, may also be subjected to partial discharge conditions.
DRIVES, MOTORS + SWITCHGEAR
The higher the voltage, the more destructive the
activity; the higher the altitude, the lower the voltage for
the onset of destructive PD which is a leading cause, and
indicator, of insulation breakdown.
PD in voids between conductors and
insulation
PD in voids in main wall insulation
PD in voids in main wall and corona
shield
PD between phases
in overhang
Corona shield 'eaten'
away by PD in the slot
section of line and
near line coils
Coils connected to
the star point are
unaffected and show
no signs of PD
PD in de-lamination of main wall
insulation
Electricity+Control
July ‘14
16
