Inspecting electrical systems with infrared imaging cameras is an important part of keeping power generation, transmission, and distribution systems up and running for decades. The latest advances in drone imaging technology have taken this application to a whole new level and increased adoption.
Reasons to use thermal for drones
Whether you’re inspecting substations, switches, transmission lines, or distribution lines, there are hundreds, if not hundreds, of connections that require regular inspection, fused disconnections, insulators, splices, dead ends, and more. .. “Checking these from a drone has two main advantages: speed and safety,” said Greg Nagler, sUAS, Global Business Development Director at Teledyne FLIR. “Drone inspection allows the crew to quickly scan large areas to find potential problems that need further investigation later and safely keep the crew away from the range of energized equipment. increase.”
Qualitative or quantitative
There are two basic types of thermal testing: qualitative testing and quantitative testing. A qualitative test is performed in which the drone thermographer is looking for thermal anomalies, or differences in thermal energy that should not be there. For example, if you are inspecting three-phase power lines and know that they are under a balanced load, they should all look almost the same. However, if the amount of heat at the connection point is increasing in one phase, then the connection should be further investigated to determine the root cause of the increase in heating. This heating may be due to loose or corroded connections.
Qualitative tests can also find problems by detecting lack of heating where some should be. For example, if the cooling tube of an oil-filled transformer has a cooling area where the rest is heated almost uniformly, it may be a low oil level, a blockage or a bad pump.
These two examples are considered qualitative because no temperature measurement is required to indicate the problem. The difference in heat is a good indication that there are thermal anomalies that need further investigation. On the other hand, quantitative inspection requires accurate temperature measurement, and it is difficult to measure temperature from a drone.
Keep in mind that there are two types of temperature measurements: apparent temperature and true temperature. Infrared temperature measurements are values that the camera calculates based on the amount of energy from the object, the emissivity, and the amount of energy reflected from the object. If the emissivity and reflection temperature of the object cannot be determined, the camera will not be able to correct these variables accurately to produce accurate temperature measurements. There is a common view among those who are new to thermography that accurate temperature measurements can be made easily, but that is not the case.
On the other hand, the apparent temperature is the temperature that does not correct the emissivity and reflection temperature variables. “This is the value that most drone thermal cameras display by default,” says Nagler. Reporting the apparent temperature may or may not produce readings that are within the camera’s accuracy specifications. You really have no way of knowing. Still, these values can be reported. It should be annotated as the apparent temperature, not the actual temperature. Often, it is the only viable option when inspecting an energized electrical system that reports sensible temperature.
However, some utilities have developed their own standard emissivity values for thermography inspection, so check to see if this level of standardization exists in your particular program.
Keep in mind that both qualitative and quantitative tests are valid forms of thermography, both are more legitimate than the other, and both are valuable.
Thermography has a truth called the 90/10 rule. In many cases, with 10% effort, you can get 90% of the information you need (qualitative testing). If you need an additional 10% (quantitative value) of information, you may need an additional 90% of time and effort. Whether producing true temperature is worth the extra time and effort is a decision left to the individual thermographer and inspection goal.
Measurement spot size
There are a few other things that drone thermographers need to keep in mind when planning and conducting inspections of overhead electrical equipment. When trying to capture the actual temperature beyond the component’s emissivity and reflected temperature, it is the camera’s measured spot size that needs to be taken into account.
For a variety of reasons, infrared cameras cannot accurately measure the energy detected in a single pixel. This mainly results in light diffusion from the lens. As one pixel of energy passes through the lens, it diffuses slightly and hits several adjacent pixels, effectively reducing the apparent intensity of its radiation.
For this reason, Mr. Nagler said: “Infrared cameras, by their very nature, require a minimum number of pixels on the target to achieve accurate temperature measurements. This number depends on the camera model, but is often a circular reticle screen. Shows in an area of 4×4 pixels. ”
Operators should also keep in mind that most common drone thermal cameras display a reticle on the screen, but this is a graphic that emphasizes one pixel and is targeted to meet the required number. It does not indicate that you are close enough to your target. A pixel with that target energy.
Behind the measurement spot size is a complex way to determine if the camera is close enough to the object of interest to capture a reliable temperature measurement, and a safe approach distance. It depends on the power company and inspection company and is the closest to the energized line or equipment. Check your safe approach and comply with those restrictions.
Thermally equipped drone: an obvious choice for inspection
The speed and efficiency of drone thermal testing is an obvious choice for most utilities and energy companies, but there are many options to consider how to do them. For best results, “consider whether you need qualitative or quantitative data, and whether you can get that data accurately due to equipment and operational limitations,” Nagler said. ..
Be sure to read Part 1 of this series. How to interpret an infrared image captured by a droneAnd part 2, Temperature measurement with drone and infrared camera. And in the coming weeks, check out Commercial UAV News for more articles in this series.