Electrical systems are among the
most valuable assets in your
plant and can have the biggest
impact on your bottom line.
Their production and management
cost is high, and failures
almost always lead to
catastrophic losses. Electrical
systems are being operated at
higher levels, even while
systems are aging—which affects
both the life and the
reliability of the assets.
Today’s asset managers are
facing the increased challenge
of maximizing their aging
electrical infrastructure with
fewer qualified technical
in-house resources, stricter
regulatory requirements for
worker safety, and shrinking
maintenance budgets. Advances in
technology, including the use of
Partial Discharge Testing, are
giving asset managers new
approaches to achieve improved
reliability and performance of
critical electrical assets.
What is Partial Discharge
(PD)?
Partial discharges are small
electrical sparks that occur
within the insulation of medium
and high voltage electrical
assets. Each discrete partial
discharge is the result of an
electrical breakdown of an air
pocket within the insulation.
These discharges erode
insulation and eventually result
in insulation failure.
According to the National Fire
Protection Association (NFPA
70B), the leading cause of
electrical failures is
insulation breakdown. The
National Electrical Code (NEC)
states that these partial
discharges are the first
indication of insulation
deterioration. Research from the
IEEE Gold Book, Table 36,
indicates that cables,
switchgear, and transformers
suffer the greatest losses from
insulation failure.
For more than 50 years,
companies have performed Partial
Discharge Testing on electrical
assets as part of ongoing
predictive maintenance programs.
Data obtained through Partial
Discharge Testing and Monitoring
can provide critical information
on the quality of insulation and
its impact on overall equipment
health. Because partial
discharge activity is often
present well in advance of
insulation failure, asset
managers can monitor it over
time and make informed strategic
decisions regarding the repair
or replacement of the equipment.
These predictive diagnostics
help companies to prioritize
capital and MRO investments
before an unexpected outage
occurs. Partial discharge
testing results can help predict
future performance and
reliability of critical assets,
including:
-
Cables, splices, and
terminations
-
Power transformers and
bushings
-
Switchgear
-
Motors and generators
Failures are not limited to
service aged equipment.
Acceptance testing on
newly-installed equipment builds
in reliability right from
startup. Acceptance Testing can:
-
Verify original
manufacturers test data and
identify damaged insulation
that occurred from improper
installation, poor design,
and/or poor workmanship
during or after
installation.
-
Identify premature failures
and capture baseline data to
trend asset health over the
asset’s life cycle to ensure
maximum return on
investment.
Integrated Partial Discharge
Solutions
Emerson’s integrated Partial
Discharge Testing and Monitoring
solutions include both online
and offline testing as well as
periodic and continuous
monitoring of your electrical
assets. Depending on your
specific operating requirements
and application, Emerson can
customize a program to best fit
your needs. Choose from the
following solutions:
-
Online Partial Discharge
Testing
-
Handheld Survey
-
Periodic Partial Discharge
Testing
-
Continuous Online Monitoring
-
Ultrasonics
-
Offline Partial Discharge
Testing
-
Tan-Delta
-
Very Low Frequency Testing (VLF)
Online Partial Discharge
Testing
Online testing is performed
while the equipment is energized
at normal operating voltages.
The testing is conducted during
real operating conditions, under
typical temperature, voltage
stresses, and vibration levels.
It is a nondestructive test and
does not use over voltages that
could adversely affect the
equipment. Online Partial
Discharge Testing is relatively
inexpensive compared to offline
testing that requires
interruption of service and
production. For critical
facilities that operate 24x7,
this is the best solution for
identifying insulation
condition.
Emerson conducts online periodic
testing using a PD Surveyor and
a Portable PD test unit with
noninvasive sensors. The PD
Surveyor is the first
multi-purpose handheld device
able to conduct the initial
pre-screen to identify the
critical equipment for testing.
Typically only 5 to 10% of
medium and high voltage assets
will have significant levels of
PD activity. This prescreening
will provide the required data
to develop an overall test plan
to ensure focus on the right
assets, minimizing unnecessary
spending. The pre-screen also
provides a safety check prior to
opening panels or performing
work.
Once the assets are prioritized,
Emerson’s test engineers conduct
the periodic PD measurements
using non-invasive, calibrated
PD sensors including High
Frequency Current Transformer (HFCT)
sensors, Transient Earth Voltage
(TEV) sensors, or Airborne
Acoustic sensors. These sensors
are inductively, capacitively,
or acoustically coupled to the
equipment to provide online PD
testing while the equipment
remains energized. Each point of
attachment is tested for just a
few minutes, enabling a large
number of assets to be tested
quickly and easily. The
technology can also provide
conventional offline partial
discharge measurements in
accordance with International
Electrotechnical Commission (IEC)
requirements. Emerson’s advanced
test application captures the
partial discharge signals
synchronously across the 60Hz
power cycle, allowing the test
engineer to observe
phase-related patterns of
discharge—online and in
real-time. Emerson’s analysis
application provides an
automatic PD “criticality” level
based on magnitude and number of
PD pulses per power cycle. The
PD reader uses
easy-to-understand,
knowledge-based methods to
provide PD pulse discrimination
regarding source of partial
discharge (cable, switchgear,
etc.) while automatically
presorting electrical “noise”
pulses. The application also
includes an automatic RF noise
reduction function.
Permanently Mounted Sensors
For “hard to access areas” or
areas that pose a safety
concern, you can use permanently
mounted sensors for periodic
online PD detection. These HFCT
sensors are installed by Emerson
engineers and remain on the
equipment for online access and
diagnostic information. Often,
these sensors must be installed
during an outage or plant
downtime. Once installed, no
further outage or disruptions
are required for online testing.
Periodic Partial Discharge
Testing
Sometimes, you may need to
conduct continuous monitoring
over a short period. A portable
PD Monitor can be used to
perform continuous monitoring
from one hour up to three
months. Through continuous
monitoring, Emerson will trend
your assets over a longer period
of time to provide a more
accurate assessment and
recommendation. This solution is
less expensive than installing a
permanently-mounted PD
monitoring system.
Continuous Online Monitoring
For your most critical assets
that have demonstrated high
levels of partial discharge,
including those that are nearing
the end of their life,
continuous online PD monitoring
may be the answer. Measure and
analyze data on your electrical
assets and provide remote access
via LAN or modem to deliver
continuous and accurate data on
the condition and capability of
the equipment. Continuous
monitoring will also provide
timely alerts to maintenance and
will identify impending failures
before an unplanned shutdown
occurs.
Ultrasonics
When problems occur in medium
and high voltage electric
equipment, such as partial
discharge and corona, it will
produce sound waves that can be
detected by Ultrasonic testing.
Performed without disrupting
plant or facility operations,
Ultrasonic testing is a
non-destructive, noninvasive
predictive maintenance tool.
Because it maintains insulation
integrity, it is commonly
employed in applications such as
cable terminations, switchgear,
busbars, and transformers.
Ultrasonic measurement is most
powerful on a comparative basis
and can significantly increase
the reliability of partial
discharge detection when used
with other Emerson partial
discharge testing technologies.
Offline Partial Discharge
Testingg
Offline Partial Discharge
Testing offers a significant
advantage over other
technologies because of its
ability to measure the cable
system’s response to a specific
stress level and predict its
future performance without
causing a fault. Offline testing
is also known for its ability to
pinpoint the exact defect
location on field-aged
equipment, enabling the asset
manager to accurately plan for
maintenance and repair. The
challenge with offline testing
is that the equipment must be
taken offline and out of
service. Offline testing is also
commonly used in acceptance
testing on newly-installed
cables. When deenergization is
preferred, Emerson can conduct
offline testing as part of your
predictive maintenance program.
Tan-Delta Testing
Dissipation Factor (Tan-Delta)
is one of the most powerful
offline nondestructive
diagnostic tools used to monitor
the condition of extruded cable
insulation. Capacitance and
Tan-Delta values from new
insulation are used as benchmark
readings. By comparing periodic
readings of the capacitance and
Tan-Delta of your insulating
material with the benchmark
readings, you can measure the
deterioration of the insulation,
predict the life expectancy, and
plan maintenance and repairs
before an unexpected outage
occurs. A significant advantage
of Tan-Delta testing is the
ability to detect water trees—a
major source of insulation
failures in aged, extruded
cables. Tan-Delta testing can be
performed during the
manufacturing process. The
testing frequency depends on the
rate of change of the insulation
quality, history of past
failures, environmental
conditions, etc. High levels of
environmental moisture, elevated
temperature, and chemicals or
pollution may require more
frequent testing.
Emerson’s test engineers can
help you evaluate your
requirements and establish a
program that best meets your
needs.
Very Low Frequency Testing (VLF)
VLF AC Hipot testing was
developed in the early 1980s as
a substitute for DC Hipot
testing. DC Hipot testing is no
longer an acceptable test method
for field aged extruded cables
because of its destructive
nature and inability to
determine insulation quality.
Although VLF testing is
primarily used to perform hipot
or withstand testing in cables,
it is also suitable for testing
transformers, switchgear,
rotating machines, and other
electrical assets, and is
frequently used as a voltage
source for offline testing. A
VLF hipot test is a go/no-go
test and is not a diagnostic
test, but is one of the best
ways to check the AC integrity
of an electrical asset to
determine a pass or fail result.
Similar to offline partial
discharge testing, the challenge
of this test is that the
equipment must be taken out of
service.
Maximize Uptime
Partial discharge testing plays
a critical role in determining
the health of your electrical
assets and ensuring maximum
uptime. It also provides asset
managers with the critical
information to target
maintenance resources to the
areas that require the most
attention. Depending on your
specific operating requirements
and application, Emerson can
help you develop and implement
the right combination of
technology, online or offline,
to meet your maintenance
requirements. Improve
operational efficiency and
worker safety by working with
the experts at Emerson.
