Detecting damaged or defective line components before they fail and cause serious problems is necessary to ensure the safe operation of overhead networks. It is therefore not surprising that finding reliable methods to inspect lines has always been a priority among grid operators worldwide. Responding to this need, industry has introduced products and methodologies intended to help make inspection procedures faster, more accurate and more economical. This INMR article from 2013, contributed by Boris Horn, a U.S. based aerial inspection specialist, offered a practical overview on how aerial inspections compare to inspection from the ground and can contribute to better information about the condition of overhead line assets.
Periodic aerial inspection of power lines is now commonplace across many developed countries but still focuses on visual inspection of conductors, structures and surrounding vegetation. It is usually regarded as a measure to supplement ground-based patrols and climbing inspections, which are regularly implemented in easily accessible areas. In mountainous terrain or other difficult to access regions, line inspection is often possible only by helicopter.
Such inspection is typically done by utility maintenance personnel in a helicopter, most often equipped only with binoculars or a hand held camera with zoom lens. Specialized infrared and ultraviolet cameras are still not widely installed in helicopters in spite of the fact that these devices are now frequently part of ground-based inspections of lines and substations.
Infrared Thermography vs. Corona Inspection
The question of which technology is more suitable for inspection of overhead lines is often asked but there is no standard reply. Customers ideally want one solution for their inspection routine that is easy to use, fast, inexpensive and that offers a reliable and clear indication of faulty components. Unfortunately, such a universal concept is not yet available. As a result, companies carrying out such inspection tend to follow somewhat different procedures and data analysis is also often done differently due to lack of industry standards.
From an aerial inspection standpoint, the three alternative inspection methodologies are: infrared thermography, corona measurement and standard visual patrol, with or without a daylight camera. A first step toward evaluating which of these technologies will gives the best results involves reviewing the types of problems that have historically affected the line to be inspected. The most suitable technology can then be recommended for identifying every category of fault. For example, if the majority of past problems have involved conductor connections, an infrared thermographic survey will probably be the best choice due to its unique benefit in detecting hotspots.
IR cameras work mostly in the spectral range from 7.5 μm to 13 μm and detect overheated elements by ‘visualizing’ temperature differences. However, care must be taken to always use a calibrated IR camera so that any temperature difference between similar components can not only be visualized but also objectively evaluated in Celsius/Fahrenheit or Kelvin.
IR camera systems are sometimes used for power line surveys. They have excellent sensitivity but do not always have the temperature measurement capability needed. It is difficult, for example, to measure the absolute temperatures of metal parts from a distance because of their emissivity and reflected temperature needs to be set as a parameter in the camera.
Emissivity depends on, among other things, weathering of the component and reflected temperature and it is hard to specify this on round-shaped parts. If these parameters are not known in advance, inspection results may not be accurate. Therefore it is preferable to only compare temperatures of components of the same type and on the same circuit, using only the delta value as the basis for evaluation.
Another variable in choosing the most suitable inspection technology involves load. If the grid is operated at a level less than 30% of nominal load, IR technology will probably not be effective since the minimum load to conduct an IR survey should be about 40%. Every resistor causes heat but, according to Ohm’s law, current is required. Theoretical temperature increases can be calculated and show that these will be marginal at small loads (e.g. less than two Kelvin). This may not be a problem to measure in a laboratory but, with environmental factors such as wind, reflection, etc., a temperature difference of less than five K will be difficult to detect in the field and require exceptional data and analysis.
By contrast, a UV camera is effective as long as there is high voltage on the line without impact from load level. However, environmental factors such as humidity can influence performance. A UV camera is sensitive in the spectral range of around 240 nm to 280 nm and able to detect partial discharges and arcing. While major discharges can be heard and visualized at night, relying on this alone is not practical for field inspection since small discharges cannot easily be seen. Incipient problems such as broken conductor strands or damaged insulators are usually easier to detect using a UV camera.
Discharges are indicated by small flashes in the image and can be quantified by counting (i.e. events/second).
The latest development from one manufacturer can even determine the wattage of a discharge.
There is still debate about the correct interpretation of corona measurements. Nevertheless, it can generally be assumed that existence and distribution of corona and partial discharge activity should be similar on the same components. This is based on the assumption that the components are of equal age and operate in the same service environment and under the same current and voltage levels.
IR and UV technology can be used at every voltage level but the more suitable technology needs to be determined individually for every inspection project.
Fortunately, these requirements are usually met since components and parts installed along most power lines tend to be identical. If increased corona activity is detected on only one specific component, it is likely that it is defective, without exact benchmarks or limits on the extent of the problem. Such an approach has proven itself many times in the field and is sufficient for most utilities.
Both IR and UV have their preferred applications when it comes to inspection and neither provides a universally better methodology for inspecting power lines. Camera manufacturers sometimes battle on technology, but application of one or the other should ideally be defined individually for each project. At the same time, there are plenty of examples where neither technology alone was able to deliver the desired results. When the budget allows, it is therefore best practice to inspect lines using both IR and UV.
Bringing Inspection Technology ‘Into the Air’
There are growing requests these days to combine data collected during ground-based inspection with visual flight patrols. Typically, this leads to a situation where an operator is required to sit in the helicopter with a handheld IR or UV camera.
Naturally, after a few hours the operator becomes tired and not able to focus effectively on the inspection. Installing external displays helps but the problem of a ‘shaking camera’ still exists. It is physically and mentally demanding to sit in a vibrating helicopter, close to power lines, and focus constantly on rapidly changing images. A potential fault may be visible for only seconds, so if the operator is not attentive, a defect can be missed.
Aerial inspection with a camera that is integrated into a gimbal is therefore not only more convenient but also likely to be far more reliable. The gimbal is a gyrostabilized platform that allows the rotation of the camera around a differential axis to eliminate vibrations from the helicopter. Using this platform, any number of sensors can be integrated and all will have the same direction of view. The gimbal operator has a display for each sensor and can comfortably monitor all channels.
Software features such as auto-tracking of a certain area help take photos and measurements of any line. The video streams and pictures are normally correlated with a GPS and time stamp so that findings can later be linked to a specific line section and the structures being inspected. The camera also helps the pilot keep an exact track. For high quality video, it is essential that the helicopter maintain the same distance to the line being inspected. Moreover, IR cameras need to be focused exactly and each change in distance requires the operator to adjust the camera.
Once the helicopter is airborne, as much data as possible should be captured. Unfortunately, alternative inspection technologies sometimes need different aerial maneuvers. For example, service providers are frequently asked to perform a 3D laser scan together with the line inspection. The resulting three-dimensional model of the line and terrain allows determination of the coordinates of structures, while measuring sag of conductors and inclination of towers. Theoretically, both systems could be installed simultaneously, but then the helicopter would be required to fly over the line at the same altitude and exactly in its center point.
For both IR and UV inspections, this is not ideal and risks that faults will not be detectable. Rather, experience indicates that best results, i.e. finding the most detectable faults, will be delivered by flying next to the conductors or even lower. Moreover, a cold, homogeneous sky is the preferred background, especially for an IR camera. Therefore, while combining cameras with different wavelengths during a single inspection flight makes sense, trying to link such inspection with other aerial tasks may prove problematic.
Operator skill and experience are crucial to the success of any line inspection – whether land-based or aerial. While most discussions about inspection focus only on technology, the man-machine interface is crucial. The operator also needs to be focused since faults, especially at an early stage, are difficult to recognize and often only visible to the trained eye. Verification is also needed to confirm that any particular finding is not a false indication or reflection.
Processing and analysis of data from the aerial inspection generally take as long as the flight itself and should be completed by experts in nondestructive testing (NDT) who also have knowledge of the electrical field. A wrong indication could lead to unnecessary repairs. Suitable training is available for infrared technology (e.g. certification according EN 473 in Europe or ANSI/ASNT CP-105 in the U.S.). Manufacturers also offer training on proper use of UV cameras. Still, the combination of these technologies and the skills that are key to their effective aerial application can be mastered only through experience.
Helicopter Types & Safety
There are many possibilities when it comes to choosing a helicopter on which to install inspection equipment and the pre-qualified provider or existing fleet are obvious choices. Still, decision makers should be aware that the type of helicopter selected is also a key safety factor. Flight altitude during inspection is relatively low, speed is only between 30 knots and hovering, and the helicopter is close to compulsory current-carrying elements. Moreover, three people plus heavy inspection equipment and a full tank together exhaust the weight limit. These factors are all critical in case of an incident. If the power line goes through a high-lying area, a powerful helicopter with enough reserve capacity should be used despite the impact on costs.
A camera system can be mounted on almost every common type of helicopter and allows a greater distance to the line being inspected, thereby increasing safety compared to simple visual patrol. In some countries, a twin-engine helicopter is mandatory for inspection over populated areas.
Frequency of Line Inspection
Every grid operator has their own answer as to what constitutes the optimal period between successive aerial inspections. A standard interval is 12 months, sometime a bit more or a little less to allow for seasonal variations. The age of the line is one factor governing inspection frequency, but newly built lines are also being inspected. Other variables to consider might include the line’s general condition, service environment (especially if there is heavy influence of maritime pollution or high dust) and the findings of the last inspection. Relative importance of the line is also a key issue.
Cost of aerial inspection depends a great deal on helicopter model selected and also can vary by country due to local fuel costs. For example, a single-piston engine helicopter such as the Robinson R44 is not comparable in operating cost to a twin-engine turbine Eurocopter AS355. Both are frequently used but the per minute expense of the AS355 is probably four times higher.
The costs for the inspection equipment are generally constant and often represent but a fraction of the total expense. It should be noted that inspection speed may need to be adjusted for thermographic inspection since IR sensors have limits on frequency and this can marginally impact helicopter expenditures.
In general, the incremental costs associated with carrying out an aerial IR/UV inspection compared to visual inspection alone are negligible and should not be a consideration.
Aerial inspection utilizing cameras having different wavelengths is currently available and already performs reasonably well. Still, new sensors with higher resolution as well as superior methods of data analysis continue to improve the methodology and manufacturers are working toward these goals.
A more interesting topic for the future is therefore how best to transport inspection cameras and sensors. Quads, drones, unmanned aerial vehicles (UAVs), crawlers, robots, gyrocopters and even slow flying planes are all being developed and tested in this regard. Helicopters are presently used in the vast majority of aerial IR/UV inspections and may yet prove the best option. Still, tests with professional UAVs are being carried out by different suppliers and appear promising. While the purchasing and operating costs of such models are high and the effort to prepare for such an inspection is more than with a manned helicopter, this technology may one day prove superior.
In places such as the United States and in many other countries, power grids are increasingly old while renewal budgets are limited. This places more emphasis on monitoring the condition of line assets even as new projects to replace ageing infrastructure are on the drawing boards.
Aerial IR/UV inspections have demonstrated their value in meeting this need and will continue to be an important part of maintaining reliability of power supply. Given this, more and more utilities are starting to make use of advanced inspection methods and recognize that only the combination of different inspection technologies will deliver optimal results.
Proper application of IR and UV cameras increase safety of the power network by reducing unplanned outages due to defective or damaged line components. Moreover, they do this without significantly increasing overall operating costs. At the same time, different technical requirements, service conditions and aviation rules may demand customized solutions for their successful application from the air.