Periodic Testing or Continuous Monitoring?
Last year US industry spent over $600 billion on plant and equipment maintenance. According to industry experts, at least $200 billion was wasted. Other industry statistics suggest that 80% of all failures of plant and equipment occur on a random basis and only 20% of the failures are age related. This means that 80% of failures have not been detected with common test practice and therefore has not been prevented. Can the situation be improved?
Traditionally, partial discharge (PD) measurements have been performed on equipment on a periodic basis, one to 4 times per year. The periodicity is based on the analysis of the most current reading, trend analysis, availability of resources, etc. Typical equipment monitored includes motors, generators, switchgear, cables, bus duct and transformers.
In rotating equipment development of partial discharge (PD) is usually a very slow process, but this is not always the case. We have seen many cases of very fast development of partial discharge (PD) on a wide variety of equipment.
Issues with Periodic Testing Programs
An advanced periodic partial discharge (PD) analyzer is expensive. In some cases you may need more than one analyzer for different types of equipment like one for Hydro generators and another for Steam Turbine units. Our online continuous partial discharge (PD) monitoring device is universal and is not application specific.
Keeping personnel up to date on the use of the partial discharge (PD) equipment and associated software; if the analyzer is used infrequently, staying up to date is difficult. Things are forgotten and historical data is lost for comparison purposes. We have seen many cases where the analyzer is sitting in a storage room gathering dust and has not been used in several years. This is mainly due to the trained person has left the company or has been reassigned to another location or job function and no one has picked up the responsibilities to continue with the testing program.
One of the key diagnostic tools in interpreting partial discharge (PD) measurements is trending. In order to obtain a decent trend, on a two times per year basis, it will take two years to just start a trend.
Times between tests are typically 6 - 12 months (if on-line) and frequently even longer for off-line tests.
Many Partial Discharge – insulation failure problems will manifest themselves in a lot shorter period of time. One wants to know quickly if a problem is starting. If a problem is identified at an early stage, remedial action can usually be taken to mitigate the cause and prevent a failure and the need of a major repair later.
During a periodic test and all of a sudden a problem is detected, one starts to ask the following questions:
- When did the problem start?
- How fast is it degrading? Not only the velocity of change, but also the acceleration of change.
- How fast will it continue to degrade?
The answer is - you really do not know! In periodic mode one will need, at least, two consequent tests to establish a rate of defect development which means another year of testing. Therefore, one does not have enough qualitative and quantitative data to make a proper judgment at the time of problem manifestation, so arbitrary decisions are made.
In many cases partial discharge (PD) activity is unstable. It may not be active today, but active tomorrow. Through continuous monitoring, a wide variation of PD activity is frequently observed. Many factors can affect PD activity. A few include:
- Voltage
- Load
- Temperature
- Humidity
- Vibration
- Pressure
As more and more data is collected in conjunction with other dynamic factors, much is being learned. Correlation of the certain dynamics with partial discharge activity provides a tremendous amount of insight as to the diagnostics capabilities that exist.
Most accepted partial discharge (PD) standards avoid the stating of specific levels or quantities to determine acceptability limits. Instead, they stress trending. In order to perform proper trend analysis, one should make sure that all dynamics are nearly identical at each testing interval. This is very difficult (some time is impossible) to accomplish and time consuming.
At this time, no correction factors are known to account for the change in dynamics. Another reason for the avoidance of establishing levels is that even a very low level of partial discharge (PD) activity that is trending upward will be signaling a problem. Conversely, a high level of partial discharge (PD) activity that is stable should be of concern, but at least one knows that the problem or defect is not getting worse, which should provide some level of comfort.
Taking periodic readings is labor intensive, time consuming and frequently requires an expert in taking and analyzing the data to extract valid information. Maintenance organizations that are being driven to do more with less are looking for ways to increase reliability with fewer resources. The only true way to reduce maintenance costs is to take the “work” out of it. That means taking the labor out of it. This is the essence of condition based maintenance.
Advances in technology now allow the continuous monitoring of partial discharge in most equipment found in industrial and power generation facilitates rather than rely on periodic measurements. There are many deficiencies in making periodic on-line as well as off-line tests. These include:
A continuous monitoring system will overcome all of the deficiencies listed above and will be an effective condition based maintenance tool and will save your company money by:
Finding a problem is not left to chance. The monitor will identify a problem in its earliest stages of defect development. This will provide one with sufficient information as to the growth rate and the severity of the defect.
Provide information as to what phase the defect is on and generally what type of defect such as:
- Corona - Surface Discharge
- Void type of defect
- Insulation Delamination
- Ground wall deterioration (slot Discharge)
- Loose high voltage connection
It is even possible to localize even further the location of the defect.
- No labor is required to conduct the tests. The equipment does that by itself. You can then use your limited resources for other important tasks as well as your time can be better spent finding solutions to problems instead of finding problems.
- Unnecessary maintenance is reduced because you will be constantly testing and will have accurate data on which to base decisions.
- More accurate data as tests are conducted under real operating conditions No outage is required to perform the test, therefore no loss of productivity of the monitored asset.
- No infant mortality failure patterns are introduced via more invasive testing procedures.
- No surprises, no forced outages, increased safety of personnel. You will always be aware of conditions and/or problems.
- Other dynamics such as temperature, humidity and load current can be easily correlated to partial discharge (PD) activity, which provides addition insight to diagnostics. No need to go to several sources and gather the information.
- Provides the opportunity for remote diagnostics. The expert does not need to come out to the field for basic diagnostics. A site visit by an expert will be the exception and not the rule. This can be done by emailing data to an expert or perhaps have a modem connected to the monitor so the expert can dial-in and download the information for analysis.
- One can make an evaluation of a piece of equipment based on its own history and not be comparison to other equipment. This will make the detection of subtle problems easier
- One can easily monitor worsening conditions so can either defer repairs or allow time to plan an outage. We all know by planning an outage by having the right people and the right materials will significantly reduce the cost of repairs.
Summary
This Application Note reviewed the features and advantages of a continuous monitoring system
for partial discharge activity on a wide variety of equipment. A continuous monitoring system can save the user a considerable amount of money by having the ability to have both quantitative and qualitative data in order make a more informed decision. Some of the ways the use of the continuous monitoring system can save company money include:
- No labor required to conduct tests.
- More efficient deployment of personnel. Labor is freed up to fix problems instead of look for them and to take care of other important matters.
- More accurate data as tests are conducted under real operating conditions.
- Testing methodology / sensors of our Systems more directly measure insulation problems than alternative methods.
- Finding a problem is not left to chance (like interval testing). One will know when a problem started and how rapidly it is progressing, thereby allowing a better outage / service decision.
- Unnecessary maintenance is reduced because you’re constantly testing and have more accurate data on which to base decisions. In effect, you achieve predictive based maintenance vs. interval / chance maintenance.
- You can prioritize which equipment receives service first.
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