On-line PD Monitoring Makes Good Business Sense
An essential tool for asset managers to ensure reliable operation, improve maintenance
efficiency and to extend the life of their electrical assets.
Executive Summary
With the increasing age of electrical assets, on-line insulation condition monitoring has become an essential asset
management tool. Compared to routine off-line diagnostic tests, on-line monitoring provides asset managers with
continuous insulation condition status while electrical equipment is in operation. It indicates whenever operational
stress factors are damaging electrical insulation and putting electrical assets at risk of failure.
In this white paper, you will learn how on-line insulation condition monitoring has significant benefits for your organization. Not only can you optimize maintenance schedules based on actual asset condition, the reliable service life
of your aging electrical assets can be maximized to help you get the most out of your investment.
Introduction
Electrical assets, including generators, motors, power transformers, as well as power cables and their accessories,
are all exposed to a variety of environmental and operational stress factors throughout their service life. These stress
factors cause aging and degradation in electrical insulation over time, which can eventually lead to the
asset's early and unexpected failure.
Dielectric diagnostic methods, particularly on-line partial discharge (PD) monitoring, make a crucial contribution to
maintaining the operation safety and reliability of electrical equipment. They provide asset managers with evidence
about changes in the dielectric condition of the insulation. Eventual defects can therefore be detected at an early
stage to allow assets at risk of failure to be removed from service and repaired. This helps asset managers to avoid
costly, unplanned maintenance and outages as well to extend the service life of equipment.
Condition awareness is vital
Not knowing the current condition or performance of
an electrical asset may lead to premature failure, which
leaves the organization with only one option – to
replace the asset, which is the most expensive option.
It is therefore critical that organizations have a clear
knowledge about the insulation condition of their
electrical assets. On-line monitoring can identify
changes and trends in insulation condition, which is
indicative of a developing fault. Maintenance can be
scheduled only when neccessary based on condition
to prevent failure and to avoid its consequences.
That is why many organizations are adopting on-line
monitoring to access insulation condition status. It
enables them to move away from routine, time-based
maintenance to a more condition-based maintenance,
© OMICRON 2015
Insulation degradation is a major cause of asset failure. Shown
here is an exploded bushing resulting from aging insulation.
scheduling downtime and repairs only when required.
The economic benefits include greatly reduced maintenance costs, enhanced efficiency and maximized performance.
Thermal
damage
17%
Indications of insulation degradation
Various statistics show that a significant percentage
of in-service failures in electrical assets are related to
defects in their insulation systems. And many insulation
failures in electrical equipment are often the result of
partial discharges (PD).
Insulation
damage
56%
Mechanical
damage
24%
These are small discharges that occur as the result of
exposure to high electric field strengths and they can
permanently damage the insulation. A certain amount
of PD is normal and tolerable in electrical assets. However when PD exceeds accepted levels, it could be an
indication that insulation degradation is taking place at
a progressive rate.
Bearing
damage
3%
Causes of failure in rotating machines
(Source: CIGRE, 2009 Survey of Hydro Generator Failures)
That is why in addition to insulation resistance,
capacitance and power factor, PD has become a widely
accepted measuring parameter for insulation diagnosis.
PD often identifies voids, cracks or other flaws where
electrical stresses exceed the dielectric strength of the
insulation.
Other
6%
Tap changer
27%
Bushings
17%
Windings
39%
Insulation
11%
Causes of failure in power transformers
(Source: Tenbohlen, S., Diagnosis of Electrical Assets,
ETG Conference 2014)
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Off-line PD testing
During the production and commissioning of electrical
equipment, off-line PD tests are frequently conducted
by the manufacturer to verify the quality and reliability
of the equipment.
Monitoring under real load conditions
By comparison, on-line PD monitoring systems allow
asset managers to collect a vast amount of information
continuously without interrupting normal service. These
monitoring systems record insulation condition data
under real load conditions. Insulation deterioration occurring in operation can then be detected anytime and
early action can then be taken in response to defects
before they lead to failure.
During equipment service lifetime, diagnostic PD measurements are also conducted off-line to check on insulation condition. These are conducted when the equipment is taken out of service during regularly-scheduled
routine maintenance. The frequency of such diagnostic
measurements is often determined by the importance
of the asset and its prior maintenance history.
Periodic vs. permanent on-line monitoring
There are two types of on-line PD monitoring systems
available to choose from – either periodic or permanent
monitoring. Both types of monitoring offer continuous
on-line assessments of PD levels and insulation condition during time intervals specified by the operator.
These off-line PD measurements require not only special
diagnostic equipment to conduct the tests, but since
they are conducted when equipment is not operating,
an external voltage source is also needed. This alone is
often difficult to deploy on-site and adds significantly to
the overall maintenance costs.
Periodic PD monitoring enables operators to observe
changes in PD activity over short, extended periods of
time. Such a monitoring system is typically portable
and can be used for different types of assets. When one
asset has been monitored for a period of time, the system can be easily moved to the next asset to be monitored. With the monitoring software, asset managers
can reliably assess the current insulation condition and
identify which asset is most at risk of failure.
Also important to consider is the significant amount of
time required to prepare electrical equipment for testing, to perform the test and to return the equipment
safely to service.
Most significantly, normal operating conditions, such
as load, operating temperature, vibrations etc., are not
considered when the equipment is taken off-line for
routine testing. Off-line PD testing offers only a snapshot
of the insulation condition state at the time of the test.
It does not indicate how the insulation performs fully
loaded for extended periods under normal operating
conditions.
For assets with increased PD levels, as well as those with
special availability requirements or with unusual loads,
a permanent continuous monitoring system can be
installed to keep a constant eye on the condition state
24/7. Insulation condition status can be viewed anytime
using a web interface to a secured server.
A permanent on-line monitoring system typically consists of PD sensors designed for specific assets, a data
acquisition device as well as monitoring and PD analysis
software running on a central computer. Multiple assets
can be monitored at the same time and the data compared using the same software.
Detailed analysis of defects
Off-line testing does have its advantages, however,
including the ability to test insulation of electrical equipment at voltages above rated values in order to verify
dielectric strength and to detect weaknesses under
specific conditions. It is also beneficial for analyzing the
severity of defects when detected and the likely consequences if no action is taken.
After installation, the monitoring system requires no
human interaction and can be left for several years of
reliable data collection.
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Normal life
Startup
End of life
Decreasing failure rate
Low constant failure rate
Increasing failure rate
Early failures
Chance failures
Wearout failures
Preventing early
failure
Ensuring reliable operation
Extending
lifetime
Diagnostic measurement and analysis when defects are detected
Off-line diagnosis
Periodic on-line
monitoring
Periodic condition assessment to identify assets at risk
Permanent on-line
monitoring
Continuous condition assessment
Failure rate during asset life and implementation of on-line PD monitoring
Periodic and permanent monitoring can be implemented across the complete asset life cycle to increase reliability, prevent
premature failures and extend end of life.
Condition-based maintenance
vs. time-based maintenance
On-line condition monitoring
indicates when action needs to
be taken to prevent premature
failure. It eliminates the "blind"
periods between routine timebased maintenance checks when
unknown stress conditions can
result in failure.
Insulation strength
High
Condition-based maintenance
Condition trigger
Time-based maintenance
Failure
Operating stress
t
Low
Asset service life
4
t = Period between routine
time-based maintenance checks
Alarm triggering when PD levels are too high
Both types of on-line PD monitoring systems record
measurement data over time and visualize trends in PD
activity. An alarm is triggered only in case of service-relevant PD activity that exceeds acceptable limits. When
acquired PD values are below pre-defined threshold
values, they are colored in green in a dashboard style
display. However when measured values are near or
above user-defined threshold levels, they are colored
either in yellow (warning) or red (alarm).
A list of events that triggered the warning or alarm is
provided in a log. The early detection of changes in PD
activity enables asset managers to decide if and when to
schedule downtime and make planned maintenance.
Reducing the frequency of off-line tests
The large amount of real-time data gathered by on-line
monitoring systems can be used for precise condition
assessment.
On-line monitoring software allows operators to view PD
trends. Alarms are triggered if set thresholds are violated.
Ideally on-line monitoring should be used from the beginning when an electrical asset is put into operation to
continuously check for symptoms or evidence of an abnormal insulation condition throughout its service life.
Adding a permanent on-line PD monitoring system
can therefore significantly reduce the requirement for
frequent or routine off-line electrical testing. That is, by
continuously knowing the exact insulation condition
status, off-line tests only have to be conducted when a
detected defect requires additional analysis. This saves
not only time but also the expense associated with offline tests and taking electrical assets off-line.
Off-line PD diagnostics can then be used primarily when
an abnormal situation is detected to assess the severity
of the problem. Taking the necessary equipment off-line
can be planned in advance and based on necessity.
Benefits of on-line PD monitoring
On-line PD monitoring is an essential tool for effective maintenance and asset management with these
benefits:
>> On-line PD monitoring systems enable the
>> On-line PD monitoring answers questions about
>> On-line PD monitoring systems trigger an alarm
effective comparison of historical data and the
visualization of trends.
the present condition of the equipment and its
future performance.
when activity exceeds user-defined thresholds.
>> Continuous knowledge of the insulation state
>> The large amount of real-time data gathered by
saves time and money as maintenance can be
scheduled only when required.
on-line PD monitoring systems can be used for
precise insulation condition assessment.
>> Timely maintenance and repair can help to
extend the service life of assets.
5
Economic benefits of on-line PD monitoring
It is often necessary for asset managers to economically
justify this type of condition monitoring activity to their
organization in terms of the benefits gained.
Savings achieved
Calculating the costs of total asset ownership takes into
account not only the purchase of the electrical asset
itself, but also the costs associated with its installation,
training and overall maintenance. Adding to these costs
is the level of risk based on the probability of a premature failure actually occurring and the negative economic impact of such an event.
A PD monitoring system is an efficiency tool. Rather than
contributing directly to increased revenue, it allows asset managers to detect abnormal conditions and to use
the information to greatly minimize maintenance and
repair costs as well as revenue loss.
>> Risk = Cost of consequences x Probability of a failure
An equipment failure during operation causes a longer outage time and considerable cost expenditure for
repair and replacement. The detection of pending faults
is essential in order to reduce the failure rate and thus
decrease the outage time as far as possible.
>> Benefit = Annual cost of risk without on-line
monitoring vs. Annual cost of risk with on-line
monitoring
The added cost of implementing on-line monitoring
can be easily justified when one considers the costs
incurred without monitoring for not only unplanned
maintenance and outages but also unexpected failure.
The table below shows how the different cost factors
are affected by the application of on-line PD monitoring
systems.
In this respect, the purpose of on-line monitoring is to
prevent major and catastrophic failures and convert
them into issues that can be repaired at a reduced cost
during a planned outage.
Monitoring data also supports strategic decision making to ensure the safe, continued operation of electrical equipment beyond the service life specified by the
manufacturer. This enables asset managers to optimize
their investment in electrical equipment.
Impact of on-line PD monitoring on asset
life cycle costs
>> Investment in an on-line
Implementing an on-line PD monitoring system may
add investment costs associated with an asset. However,
considerably lower overall life cycle costs can be expected as a result of the information and the knowledge
gained from using an on-line monitoring system.
(+)
monitoring system
>> Costs for planned
(–)
maintenance and repair
>> Costs for unplanned
(–)
maintenance and repair
>> Outage costs due to
(–)
maintenance
>> Costs for off-line testing
(–)
>> Insurance fees
(–)
>> Premature equipment
(–)
replacement
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How OMICRON can contribute to your on-line PD monitoring success
Advanced technology
OMICRON periodic and permanent monitoring systems employ modern multi-channel synchronous
and multi-spectral measurement techniques for reliable noise suppression and separation of multiple PD
sources. This enables accurate detection of PD and related defects in electrical insulation.
Customized solutions
Periodic and permanent monitoring systems tailored to a variety of electrical assets and applications.
OMS 605
Portable PD monitoring system
for a variety of electrical assets
www.omicron.at/oms605
OMS 600
Permanent PD monitoring system
for generators and motors
www.omicron.at/oms600
OMS 800
Permanent PD monitoring
system for HV cable systems
www.omicron.at/oms800
MONTRANO
Permanent dielectric condition
monitoring system for power
transformers
www.omicron.at/montrano
Periodic on-line monitoring
Permanent on-line monitoring
Knowledge and support
Expert assistance with system design, installation, training, data analysis and interpretation.
Conclusion
Systems for periodic and permanent on-line PD monitoring identify mechanisms responsible for the deterioration of
the insulation in electrical assets throughout their service life. Insulation defects and degradation can be detected at
an early stage to prevent costly system failures through the early initiation of condition-based maintenance and
repair. Asset condition monitoring information also helps to maximize equipment performance, extend equipment
life, and decrease operating costs. Most importantly, it provides asset managers with greater peace of mind.
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OMICRON is an international company serving the electrical power industry with
innovative testing and diagnostic solutions. The application of OMICRON products
allows users to assess the condition of the primary and secondary equipment on
their systems with complete confidence. Services offered in the area of consulting,
commissioning, testing, diagnosis and training make the product range complete.
Customers in more than 140 countries rely on the company’s ability to supply
leading edge technology of excellent quality. Service centers on all continents
provide a broad base of knowledge and extraordinary customer support. All of this
together with our strong network of sales partners is what has made our company a
market leader in the electrical power industry.
© OMICRON 2015
www.omicron.at • www.omicronusa.com