Power system inspection with infrared imaging cameras has been a vital part of keeping power generation, transmission and distribution systems up and running for decades. Recent advances in drone imaging technology have elevated this application to a whole new level and increased its adoption. Let's look at some of the things to keep in mind when considering how and why to do electrical inspections with a drone.
Why use thermal imaging cameras
Whether inspecting substations, switchyards, or transmission and distribution lines, there are dozens, if not hundreds of fused connections, insulators, splices, dead ends, and so on that need routine inspection. Checking from a drone has two main advantages: speed and safety. Drone inspections allow teams to quickly scan large areas to find potential problems that need to be further investigated, and maintain power equipment.
Figure 1. Thermal imaging cameras can add speed and safety to electrical inspections with thermal functions.
Qualitative or quantitative: which way to go?
As discussed in previous editions of this DELTA Solutions series, there are two basic types of thermal inspections: qualitative and quantitative. A qualitative inspection is one in which the thermographer looks for thermal anomalies, i.e. differences in thermal energy that should not exist. For example, if three-phase power lines are inspected and the inspector knows that the lines are under a balanced load, they should all look similar. However, if one phase shows an elevated amount of heat at a connection point, that connection should be investigated further to determine the root cause of the elevated heating. This heating could be due to a loose or corroded connection.
Qualitative inspections can also find problems by detecting a lack of heating where it should exist. For example, if the cooling tubes of an oil-filled transformer have a cold zone while the rest is more or less evenly heated, this could mean a low oil level, or something preventing the oil from circulating properly, such as a blockage or a bad pump.
These two examples are considered qualitative because it is not necessary to measure temperature to indicate a problem: the difference in heat is sufficient indication that a thermal anomaly exists that requires further investigation.
Figure 2. Emissivity and T(refl) compensation can be a challenge in electrical inspections.
Quantitative inspections, on the other hand, require precise temperature measurements, and temperature measurements from a drone are very difficult. As a reminder, let's review the elements of quantitative inspections.
First, remember that there are two different types of temperature measurements: apparent temperature and actual temperature. Infrared temperature measurements are values that the camera calculates based on the amount of energy coming from an object, its emissivity and the amount of energy reflected from the object. The inspector must determine the emissivity of the object and the reflected temperature so that the camera can accurately compensate for these variables and produce an accurate temperature measurement. Although there is a widespread opinion among those new to thermography that obtaining accurate temperature measurements is as easy as point and shoot, this is not the case.
On the other hand, an apparent temperature is one that does not compensate for the variable emissivity and reflected temperature. This is the value that most drone thermal cameras display by default. Reporting an apparent temperature may produce a reading that is within the camera's accuracy specification... or it may not, as the operator has no way of knowing. Still, these values can be reported, they just need to be noted as apparent temperatures, not actual temperatures. In many cases, when inspecting energized electrical systems, the only viable option is to report apparent temperatures.
However, some utility companies have developed standard emissivity values for use in thermographic inspections, so check to see if this level of standardization exists in a given program.
Keep in mind that both qualitative and quantitative inspections are valid forms of thermography; neither is more legitimate than the other and both have value. There is a cliché in thermography called the 90/10 rule, which means that inspectors can often get 90% of the information needed for 10% of the effort with a qualitative inspection. For that extra 10% of information needed for a quantitative value, 90% extra operator time and effort will be required. Whether or not obtaining an actual temperature is worth the extra time and effort is a decision left to the individual thermographer and the objectives of the inspection.
Other considerations: size of the measuring point
There is one other thing thermographers must keep in mind when planning and performing inspections of overhead electrical equipment. When it comes to capturing actual temperatures-beyond the emissivity and reflected temperature of the component-it is imperative to consider the size of the camera's measurement spot.
For various reasons, infrared cameras cannot accurately measure the detected energy in a single pixel. This is mainly due to the optical diffusion of the lens. If the energy of a single pixel passes through a lens, it is diffused slightly so that it impinges on some adjacent pixels, which decreases the apparent intensity of that radiation. Therefore, any infrared camera requires, by its very nature, a minimum number of pixels on the lens to achieve an accurate temperature measurement. This number may vary between camera models, but is usually an area of four by four pixels displayed on screen by a circular crosshair.
Figure 3. Accurate Electrical Inspections from a Safe Distance
Most drone thermal cameras display a crosshair on the screen, but this is only a graphic that highlights a single pixel and does not indicate whether the camera is close enough to the object of interest to fill the required number of pixels with that object's energy.
The other side of the measurement spot size coin, which is merely a complex way of indicating whether the camera is close enough to the object of interest to capture a reliable temperature measurement, is the safe approach distance. This varies from one utility or inspection company to another, and is the closest the operator can get to energized lines or equipment. Be sure to find out what the safe approach distance is and adhere to that restriction. Keep these restrictions in mind when selecting a thermal camera, as some thermal payloads, including the FLIR Vue® TZ20-R, offer a 5x optical zoom and are better equipped to provide measurements from a greater distance. Digital zoom magnifies a portion of the image, stretching the pixels, and does not put more pixels on a specific element of the scene.
Figure 4. Thermal zoom provides situational awareness and clarity during electrical inspections
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