The Future of Predictive Technologies is Here Now
In today’s competitive environment, iti mips erative to capitalize on advancens in
technology that allow new approaches to the maintenance of a facility’s physical assets.
These include reliability-centered mateinnance, predictive diagnostics, condition
monitoring and expert systems. These apporaches have had a significant factor in
reducing unscheduled downtime and realizing gains in lost profit opportunities.
Studies have shown that 80% of equipment failures occur on a random basis and only 20% are age related. This indicates that whatever our current maintenance practices are, they are not overly effective. Reasons for this phenomenon include:
- Many defects develop quickly (weeks, days or hours) that are not caught during
normal periodic maintenance. Time inet rvals between outages have increased
over time, therefore less maintenance is being performed.
- Some components of equipment are rarely, if ever, inspected or tested such as the main bus structure of medium voltage switchgear and bus duct.
- Outages are shorter and more compact, therefore less effective maintenance is
provided due to budget and time constraints.
- Equipment has been “Valued Engineered”, which means all of the tolerances in
clearances, etc, are at the absolute minimum to reduce cost and yet pass
standards. The old adage of a built in “safety margin” is now a misnomer.
- Many new insulating systems are being utilized and are not time tested and are
being thermally, mechanically and electrically stressed to their limits.
- Many past periodic online maintenance activities have been severely limited due
to new Arc Flash and Safety requirements.
Many times money is spent when not necessary. Many repair/replace decisions are
made due to limited qualitative and quantitative information. Also, many times “Ultra
Safe” decisions are made due to not having enough valid and reliable information or
made due to internal corporate politics (what the boss wants to hear).
RELIABILTY AND FAILURE STATISTICS
The IEEE Standard 493-1997, IEEE Recommended Practice for Design of Reliable
Industrial and Commercial Power Systems – Gold Book completed end-user surveys
over two time periods. This data is relatively old, but at the same time it does represent
the majority of existing medium voltage electrical equipment in service today. Table 1
and Figure 1 show the comparable hours of downtime per year of a variety of medium
and high voltage industrial equipment.
Note: Average downtime hours per year is calculated by multiplying the Failure Rate per
Year times the Average Hours per Outage.
The cost of an unplanned outage can be calculated by multiplying the Average
Downtime Hours per year by your hourly plant cost of an outage. Even with a low
hourly outage cost, a rapid ROI can be realized. It must be noted that switchgear and
large power transformers are identified as two of the highest critical assets, mainly due
to the high number of hours per outage. We know from experience that we already
perform extensive maintenance on transformers such as visual inspections, power
factor testing and dissolve gas analysis, yet the failure rate is in the top four. We also
know we perform limited maintenance on the main bus compartments of switchgear and
bus ducts, mainly due to outage constraints.
Figure 1 – Percentage of Downtime Hours per Year by Equipment Type
On-Line Monitoring
Use of On-line monitoring and predictive technologies can alleviate the shortcomings
inherent in many of our current maintenance practices. If you think these capabilities
are beyond your budget constraints, it is time to rethink. The same advances that allow
us put a powerful computer on our desk for a relatively low price has put these new
monitors and technologies in the reach of most budgets. Many of these technologies
have become more intelligent and thus require less expertise in interpreting the results.
This is not always the case and it is difficult for companies to employ enough qualified
personnel at each site to understand the large variety in data from various types of
equipment. Many monitors provide more information than the typical end user can
understand but are available to the expert for additional diagnostics and prognostics. A
new acronym has emerged for these Intelligent Electronic Devices (IED).
Unfortunately there is no one technology that is the “Holy Grail” for electrical equipment
condition assessment. In many cases, multiple technologies need to be employed in
order to perform a complete diagnosis. Most monitoring systems are designed to alert
the user of a problem or unusual condition and provide additional diagnostic data.
Most of these technologies have built in communication capabilities that allow for
remote Predictive Monitoring via Ethernet, Serial communications such as ModBus,
DNP 3.0 or customized data streams and wireless and analog modems. Remote
predictive monitoring allows companies to streamline expertise at centralized locations
or outsource to the appropriate experts. This can be done on a continuous or periodic
basis or be event driven. Event driven systems send out an alert via a phone, pager,
email or fax to the appropriate person, who then communicates back to the monitor for
further analysis and recommendations.
What New On-Line Predictive Technologies are Available Today?
Infrared Monitoring
Performance of infrared surveys is probably the most common predictive technology used today. It is utilized on all types of equipment found in the electrical power
distribution systems. It is somewhat limited on medium voltage equipment such as
switchgear since it is difficult to “scan” certain areas due to limited access. Also, more
problems are found in low voltage equipment due to the higher current levels. Today,
there are wireless infrared sensors available that can be placed at connections such as
bolted connections and finger clusters that will communicate back to a central source.
The US Navy has also utilized specialized infrared sensors in low voltage switchboards
for years that connect back to a central location.
Dissolved Gas Analysis (DGA)
DGA on transformer oil use been utilized longer than any other predictive technology for
electrical equipment. Improvements over time on the analysis of the results have led to
better diagnostics. DGA used to be limited to just the oil in the main tank but have now
been validated for us in load tap changers and Oil Circuit Breakers. There are several
on-line continuous gas monitors available in the market today. These include Key Gas
and Seven Gas monitors. Also, portable instruments are available to provide on-site
DGA rather than sending the samples to a lab and having to wait for results.
Motor Current Signature Analysis
By analyzing the signatures of currents on three phase motors, one can diagnosis many
problems, such as broken rotor bars, air gap problems, misalignment, improper
mounting, eccentricity and problems with gear boxes and loads. Most of this technology
is based on making periodic on-line measurements, but strides are currently being
made to build this intelligence in to the motor starter protective devices.
Electrical Partial Discharge
A partial discharge (PD) is a small spark that occurs in an insulation system that does not go completely from phase to phase or phase to ground. Therefore, measurement
and analysis of this discharge can provide an early warning of insulation failures in
equipment rated 3,000 volts and higher. Measurement of PD can be done on a periodic
or a continuous basis. In either case, specialized sensors must be installed to detect
these pulses created by the small spark. Proven applications of this technology include
cables, bus duct, switchgear, motors, generators and transformers. Use of PD
monitoring on MV switchgear is an excellent use of this technology and can
compensate for the lack of traditional maintenance on main bus structures.
Large Power Transformer Monitoring Systems
Most industrial facilities do very little monitoring of their Large Power Transformers.
Modern day systems now control and monitor all aspects of a transformer including
temperatures, loads, cooling systems, pressures, bushings and windings. Four great
examples include:
- The monitoring of the loads on the cooling fans and pump circuits to indicate abnormal conditions such as locked rotor or loss of cooling capacity.
- Monitoring the temperature differential across the connection board between the
main tank and the load tap changer compartment
- Continuous monitoring of the power factor and capacitance of High Voltage Bushing
- Central data concentrator and communication RTU for all third party monitors such as PD and DGA.
SUMMARY
Experience has taught us that half the battle in ensuring electrical equipment reliability is to keep it Clean, Dry, Cool and Exercised. If these four items are accomplished, then
currently available on-line predictive technologies can take care of the rest. With
depleting technical expertise in electrical equipment and the pressures of running
facilities 24 x 7 x 365, these new technologies offer the ability to fill the maintenance
gap while reducing costs and increasing power system reliability. Due to the decreasing
cost of these IED’s and the high cost of unplanned outages, a rapid ROI is achieved.
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