Efficient Plant March 2018 - 28

PdM partial-discharge testing
If PD is found to be coming from an asset during an initial survey, but more information is needed to determine
the cause, advanced techniques and technologies may be required to pinpoint the location of the source.

performed online during normal business
hours without interrupting a facility's electrical service. This is a very effective way to
detect impending failure.
With regard to PD, keep in mind that
signal detection is just the first step in executing an online predictive-testing program.
Understanding the causes and how to best
address them based on survey results is
much more complicated-and may require
expert assistance.

IDENTIFYING PD TYPES
Like developments in IR equipment and
technology, PD testing equipment and technology for field applications has come a long
way in the past 20 years. That said, it's not
just the equipment that makes a PD survey
effective. Partial-discharge-signal detection
is one of the more complex technologies
applied to electrical-distribution assets. It
requires expertise and resources to be able
to interpret information gathered during a
survey. The process can prove challenging
for individuals that don't have experience
diagnosing electrical-equipment problems.
Once a PD signal is detected, knowing the
exact type of PD will help determine steps

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for dealing with the issue. The five PD
types are:
surface discharge
corona discharge
void discharge
floating electrode
particle discharge.
Proper PD classification is essential for
conveying risk and urgency for remediation. PD signals should not be classified by
magnitude alone. Some discharges, such
as a floating electrode, can produce a large
magnitude of PD signal but, because of
its location, may pose little or no threat to
equipment failure. In contrast, a void discharge buried deep within a cable's insulation may generate a small signal but put the
asset at high risk of failure.
Identifying an asset that is emitting a
PD signal may be all the information that
operating and/or maintenance personnel
need. The asset can then be taken offline
for an inspection to identify the cause of
the PD and determine how to deal with it.
Visible evidence of PD can be a deposit of
white powder (nitric acid) or green patina
(verdigris) of copper. Simply wiping away
the powder or cleaning the patina is not a
solution. PD will not simply go away. This
evidence is the result of PD, not the cause.
If PD is found to be coming from an asset
during an initial survey-but more information is needed to determine the cause-
advanced techniques and technologies may
be required to pinpoint the source. These
techniques may include time-of-flight
analysis.
This technology compares the speed of the
radio-frequency signal to the soundwave of
the PD source, or may involve triangulating
the sound signal by examining how long the

sound takes to travel to different sensors.
These advanced technologies can often
narrow the PD location to a very small area.
Having this level of detail makes it easier
to acquire the correct spare parts prior to a
scheduled outage. Personnel, in turn, will
know exactly what needs to be replaced before even opening equipment, thus minimizing downtime.

APPLYING PD SENSORS
When it comes to PD technology, there
are generally two areas of focus: cables and
everything else.
For cables, PD testing helps pinpoint defects in insulation, splices, and terminations.
The most common method is to place a
high-frequency current transformer (HFCT)
sensor around the cable and/or drain shield
before it is earthed. This sensor is designed
to transmit signals in a pre-determined
frequency band to the PD-detection
equipment.
There's some debate in the industry
regarding how far down the cable a sensor
can detect PD. The ability to detect it down
the length of a cable depends on several
variables, including:
size of the PD signal at the source
amount of background noise
distance between the PD signal source
and the sensor
type of cable shield
type of cable insulation
condition of the shield.
For everything else, PD testing is a valuable, predictive tool. In fact, today's technology is very effective at detecting PD in the
vast majority of medium- and high-voltage
electrical-distribution equipment, including:
insulating boards

MARCH 2018

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Table of Contents for the Digital Edition of Efficient Plant March 2018