Identifying Problems in Their Early Stage
"..The key to our staying ahead of any developing problems will be to identify them at as early a stage as possible, followed by investigating the cause and deciding on the correct remedial action…" says Gary Williams, manager of Network Services at ForticBC . Identifying problems at early stages, as we know, can save direct and indirect incurred costs. But, how do you identify problems at early stages? What are early stages? What equipment do you use? How do you know that you are looking at problems?
Early stage can be referred to as during the manufacturing phase. End users totally rely on specifications, warranties and manufacturers' guarantee. When human error occurs in the manufacturing process the end user gets a device with hidden failure, which can develop anytime into an unexpected serious disaster regardless of known/expected aging processes. Commissioning is certainly a solution.
A couple of utilities from North America and Asia reported that spacers that were purchased and installed on OH transmission lines had to be replaced after noticing, during commissioning, corona discharge emanating from a certain batch of spacers.
FortisBC experienced failures of fuse cutouts made with porcelain housing. Wide temperature wings caused some fuse cutouts to encounter problems, most often due to moisture penetration and subsequent cracking of the porcelain. The conclusion of a study performed was that these failures were related to poor design and quality control problems during manufacture.
Early stage can also mean human errors during renovation of old lines or construction of new. We might expect, in this case, additional problems due to poor design, poor equipment selection and improper installations. Inverted corona rings, absence of grading rings, using insulators' types that do not match geographical and climatic conditions, wrong torque, inadequate clearance etc. Corona cameras, are good indicators of such errors. Absence of corona rings is immediately recorded with a corona camera. Loose bolts, loose wires, loose connections and air gaps are being pinpointed with corona cameras. Early detection of such occurences stops disastrous heat causing wires to melt wires or conductors from getting nicked.
Early stage, on the other hand, means paying attention to degrading processes that develop when conditions change. New variables such as temperature variations, contaminants mixture, change in load, precipitation, additional nearby installations can trigger equilibrium shift and develop into degrading accelerating aging process. In July 2012 the US National Oceanic & Atmospheric Administration (NOAA) announced that USA has experienced its hottest 12-month period since record keeping began some 115 years ago. The earth is rapidly becoming a place of weather extremes. Most insulation on existing lines and at substations has been dimensioned according to standards based on historical experience with service parameters such as precipitation, wind, humidity, temperature, etc. With new environmental conditions and new pollution flashover problems there is a need for on-line monitoring of insulators as well as new assessments.
In her thesis "Improved Condition Monitoring of Composite Insulator" Prof. Da Silva explored the early stages of degradation of polymer insulators in service under different environmental conditions. According to Prof. Da Silva "Corona discharge activity is reflected as leakage current pulses and they constitute the main ageing phenomena in the early ageing of outdoor insulators. Corona discharges on polymer surfaces have low intensity and reduced heat …but they are in closer contact with the surface and can last for longer periods and result in changes to the hydrophobicity, roughness profile and oxidation rate. Strong discharges on polymeric surfaces in the presence of moisture are mobile therefore, it will not last long enough to cause ageing " Dr. Da Silva claimed that at the beginning of the ageing process of a polymer surface, corona activity on water droplets is the main factor to produce the hydrophobicity loss. Material deterioration depends on a combination of discharges intensity and duration. The effect of small discharges, which appear on a polymeric surface in the presence of water drops over a longer time, is worse than of strong mobile discharges. Having this in mind, knowledge of which are the properties or characteristics of the first leakage current pulses caused by water droplet presence and electric stress give information on the worst conditions for the insulation surface.
Equipment vs. knowledge
To be able to identify existing degrading processes lineman use elaborated NDT imagers such and Infrared and Ultraviolet technology. These cameras are in fact screening tools that assist operators observe abnormalities and outstanding signals, while the judgment whether the observed condition is a problem and how serious it is must be based on previous knowledge of the performance of the inspected component and its characteristics.
Infrared cameras are basically temperature evaluators taking infrared emissions that cannot be seen with naked eyes and converting them into visible images. The output of an IR camera depends on pre-entered input of: emissivity, reflective temperature and atmospheric conditions, as well as on focus, distance and temperature scale. Furthermore, a thermographer is expected to be able read the outcome image and distinguish between an abnormal to normal condition. Corona cameras, on the other hand, take UVc radiation that cannot be seen with bare eyes, which is emitted during partial discharge, arcing and flashover, and convert them into visible signals overlaid on the original emitting sources. Neither previous settings nor the electrical load of a line are needed to get the correct data out of the camera. But, knowledge of the grid components, the material of the inspected component and the processes that lead to partial discharge are necessary when analyzing findings.
It is impossible to declare on an existing problem without understanding the operating characteristics of the components under survey. Proper background in UVigraphy as well as in theromography is a pre requisite to using an IR and a corona camera. Both cameras are easy to operate. In fact they are fully automatic. But reading the real meaning of the image and being able to diagnose correctly necessitates more than pulling a trigger. Take for example the case where insulators are glazed with a resistive glaze that causes their surface to heat a few degrees. The purpose in using such a glaze is to discourage moisture accumulation. If such an insulator is being examined with an IR camera the thermal image might indicate degradation due to the detected high temperature. Such a fallacious could be done by an uneducated thermographer who does not know the system under survey. On the other hand, UVigraphers that observes high corona activity on insulators with water drips might perceive a disaster, when in fact the drops are creating temporary high e-field that will soon be evaporated.
Ofil offers engineers, asset managers and maitenance teams to enhance their understanding of corona and of the processes that lead to corona emission by attending seminars on corona testing. Ofil ecourages its customers to take these seminars and benefit from the strength that knowledge provides. More information in CITI's website: www.citi-training.com