What is the partial discharge ? and how to control it in HV motors and generators ? Part 3/4

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Some plants use PD level tracking as a condition
monitoring tool for their high voltage motors and
generators. Specialized expertise and equipment
are needed to perform the measurements and
analyze the results, and plants should decide
whether they will train their own personnel to do
this work or contract it out to an external service
provider. These decisions should preferably be
taken as early as possible, before the motor or
generator is ordered, because there may be special
hardware requirements, for example, that need to
be accommodated.
Various methods are available to detect the energy
released by PD, typically based on measuring
electrical current pulses, ozone levels, sound, light
or other high frequency radiation. This article
focuses on measurement of electrical current
pulses because this is the most commonly used
method for motors and generators. Capacitive
couplers can be installed in the main terminal box
to allow connection of measuring equipment for
online PD monitoring on-site.
PD pulses are characterized by their magnitude,
polarity, length, and the phase angle at which they
occur. PD is analyzed using special software, which
may be capable of pinpointing the origin of the PD
especially when used by an experienced operator.
The magnitudes of individual PD measurements
do not provide much information. They can be
affected by many factors, including the type of
sensors and measuring equipment used, the
settings (e.g. detection bandwidth) and
calibration, the type of software, and the ambient
humidity, temperature and noise level. Noise
sources that can affect results include welding
equipment, overhead cranes and switchgear, to
name just a few. Furthermore, PD pulses are
—
Detecting and measuring PD
attenuated as they are propagated within the
stator winding itself. Image 2 is a PD pattern with
high background noise, highlighting how noise can
make interpretation of PD measurements difficult.