Using infrared imaging in maintenance
Inspecting the transmitter, looking at every bolt and terminal and even touching components to feel how hot they were was how engineers looked for loose connections and other problems in the past. They were looking for trouble. Bad connections can be hard to spot and are characterized by anything from discolored metal and burned insulation to uncompressed split washers. Occasionally, loose connections could be found and tightened to prevent future problems, but that was back in the day when stations shut down during the night — far from today’s 24/7 operational schedule.
Specifically, transmission lines are an area of concern because it is nearly impossible to detect any problems without offline testing; most transmission line faults result in heat, but they are hard to detect by a touch.
Why would a good electrical connection go bad after working fine for many months or years? There are several ways that good connections can go bad.
When any large electrical system is installed, it’s always recommended to retorque all connections after six months to a year after it’s powered up. There’s such a thing as “cold flow” where metal, including copper, will lose its shape over time and become loose within a compression connector (nut and bolt). (See Figure 1.)
Stress on the conductor will lead to a loosening of the connection. Vibration will also lead to bad connections because it can loosen the bolts. This occurs wherever there are motors or cycling of mechanical components. Generally, the use of split washers and making sure the correct torque is on the bolts will prevent this from happening.
Where there’s high-current flow, the result is heat. When the current cycles on and off, the connections are subject to a heating and cooling cycle that can soften metal, causing it to flow and possibly lose contact. Also, if the conductor and the connector (nut, bolt, bus bar, etc.) are of different metals, they will expand and contract at different rates, which also can lead to contact loss.
Oxidation is another factor in bad connections. All metals will oxidize (except gold), and this creates a coating over the metal that prevents good contact. Oxides will form very quickly on copper exposed to air, but copper oxide can be easily broken with low-contact pressure. Also, the use of joint compounds will prevent air from getting to the metal and prevent oxidation.
In rigid coaxial transmission lines, the inner conductor is the source of all heat-related failures. While the outer conductor has from four to eight (or more) bolts to ensure good contact between sections, the inner relies on compression alone. Bullets are fitted into the pipes that make up the inner conductor, and sometimes springs are used, but over time all of these connections can come loose, resulting in heat buildup. (See Figure 2.)
Wave guides can also develop heating problems because the flat mating surfaces must make a very tight fit to ensure a good electrical/RF connection.
What all these contact problems have in common is heat. Whenever there is resistance in the conduction of electrical current, heat is generated.
Why do electrical connections heat up? This is because a loose connection is not making proper contact between the wire and the terminal. Without a tight connection, the current is carried across a smaller contact area, which results in higher resistance and heat — the larger the contact area, the lower the resistance. The heat leads to a softening of the metal and less contact. Over time, this situation turns into a downward spiral that leads to less and less contact area and higher heat levels. At some point, the insulation can even melt or catch on fire.
In some cases, arcing can occur. This causes pitting of the metal and can lead to more heat buildup; the arc itself can reach temperatures of 11,000 degrees Fahrenheit. Arcing leads to the accumulation of soot and carbon buildup that can lead to even more problems.
This is the case with contacts where the making and breaking of a high-current connection can result in arcing and pitting. Again, this leads to higher resistance and more heat. Sometimes the heat is so high that the contacts fuse together and will not separate.
With rigid coax, once the inner core starts to heat up, the copper will soften and deform. This, in turn, causes even more loss of contact and more heat. Once the temperature reaches a certain level, the Teflon insulators start to melt and give off soot, which coats the inner core and contributes to the heat buildup. (See Figure 3.)
How to prevent problems
You could go through your entire electrical system and try to tighten every screw and bolt, but this would be very time consuming and require a lot of downtime. You would need to shut down and de-energize every system before performing this task. Plus, you could not be sure you got to every connection or if there was still a problem even after tightening. And with pitting, merely tightening a connection will not solve the problem.
Sweeping your transmission line on a regular basis is recommended because it will reveal many problems that could not be detected any other way. But again, you have to take it out of service to perform these tests.
Today, there is a much simpler solution to finding bad connections in electrical or RF systems: inferred thermal imaging. You can buy your own equipment or hire one of many companies that provide inferred thermal imaging for all types of power transfer systems, including electrical and mechanical testing. This is called predictive maintenance technology because it enables engineers to see problems before they occur.
Most equipment heats up before it fails, and with inferred thermal imaging, you can actually see it. The great thing about this type of testing is that it’s performed with power on and without contacting the equipment, making it extremely safe. (See Figure 4.)
Imaging circuit breakers, bus bars and other electrical connections can detect high resistance points, and looking at transformers and capacitors can show internal problems like internal shorts. In addition, observing motors and water pumps can show that bearings are heating up before failure. This type of preventive maintenance can save your company a great deal of money and downtime. You should check with your insurance company to see if they will cover the cost of an inferred thermal imaging inspection because it will save them money in the long run. Correcting faults that are discovered during IR surveys can reduce the risk of an electrical fire, and insurance company statistics show average electrical fire costs on average about $500,000.
What to look for
When looking examining the inferred images, it’s important to look for the differences. Which device stands out as cooler or hotter than the others? The next step is to figure out why it’s different, and that’s where you can head off trouble.
This is where hiring a professional thermographer comes in; they are familiar with the intricacies of thermal imaging and how to interpret the results. Thermographers can be certified as Level 1, 2 or 3, with Level 3 being the highest certification. They will be familiar with how the ambient temperature and the type of material being measured will affect the readings and how to adjust for them.
Typically, an engineer accompanies the thermographer and gives them access to electrical panels and other areas to be surveyed. A report is later sent in showing problem areas with photos and temperature readings. Many times as a result of this report, corrective action can be taken right on the spot to tighten up the problem connection.
If you’re purchasing a thermal imaging camera, there are many to choose from, with prices that range from $4000 to $15,000. The industry standard for these imagers is + 2 percent (or 3.6 degrees Fahrenheit), so you can get very accurate readings. There are several things you will need to learn about different types of surfaces and how they reflect and emit heat energy in order to obtain accurate readings, but in many cases, the object you will be looking for is hotter or cooler than other ones nearby, so precise readings may not be required.
No matter if you hire a certified thermographer for purchase your own imager, you will be able to detect problems before they occur and save your company many thousands of dollars.
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