By Ahmed El-Rasheed
Hot spot detection is one of the most useful condition monitoring measures for electrical systems. It allows early detection of faults and, therefore, helps prevent insulation deterioration and reduce the risk of failures. The temperature of electrical circuits has a dominant influence on insulation life. If a loose joint creates a hot spot, insulation close to that hot spot can suffer serious deterioration due to excessive heating, potentially leading to outages.
Using a thermal camera, which converts invisible infrared radiation into clear images from which temperatures can be read, it is possible to identify components that are overheating and those that are abnormally cool. The images from the camera can be displayed on a monitor in real time or stored for later analysis. The use of a thermal camera makes it easier for electricians and maintenance technicians to hone in on potential failures before they occur, which is an important benefit, particularly in the case of critical equipment where failure will result in a major outage.
Once a hot spot is identified, the right tools must be used to remedy the situation before it progresses. One of the most useful of these tools is a Ductor™, a low-resistance ohmmeter that can accurately measure resistances as low as milliohms or even microohms. Such an instrument allows electricians and technicians to identify any abnormally high resistances within components and joints. The repair or replacement of the high resistance elements will reduce heating and resolve the hot spot problem.
Abnormal heating is, in fact, one of the most common causes of problems in electrical systems and is invariably associated with unusually high resistance or excessive current flow. Infrared imaging allows this abnormal heating to be detected quickly and easily.
Under-sized conductors, loose connections, failing electrical components, or excessive current flow may cause abnormal heating, resulting in dangerously hot electrical circuits. Components can literally become hot enough to melt.
Figure 1: Damage to a motor terminal box
Figure 2: Damage to a conductor in a three-phase connection
Figure 3: Excessive heating of a busbar connection
The photographs in Figures 1 to 3 show insulation failure and major damage resulting from undetected hot spots.
Figure 4: Hot spot on a conductor joint
Figure 5: Excessive heating of a lightning arrester
Figure 6: Hot spot in a three-phase supply terminal
In contrast, Figures 4 to 6 show thermal images of hot spots that have been detected before they led to failures. Many operational situations can lead to the development of hot spots. Examples include:
- Loose joints because of vibrations and shocks
- Loose connections as a result of severe short circuits or aged clamping arrangements
- Mechanical damage to sliding power contacts due to poor handling of equipment
- Increased contact resistance resulting from oxidation or corrosion due to environmental issues such as high humidity and air pollution
- Extended maintenance intervals due to difficulties taking equipment out of service
In the specific case of electrical substations, some of the items whose thermal signatures should be examined for potential precursors to failure include:
- Power transformers (oil levels and pump operation)
- Load tap changers (oil levels, other internal problems)
- Insulator bushings (oil levels and bad connections)
- Standoff insulators (moisture, contamination, degradation)
- Lightning arresters (degradation of metal oxide disks)
- Circuit breakers (oil or SF6 leakage)
- Mechanical disconnects (bad connections, contamination)
- Motor or generator terminals (contamination, bad connections)
- Cable joints and terminations (contamination, poor workmanship)
- Control cabinets (wear and tear on fans, pumps, and other components)
When carrying out thermal imaging inspections on electrical line and substation equipment, some important considerations need to be kept in mind. Among these are:
- Load: the system should be running at 40 % or more of peak load during the inspection – higher, if possible. This loading level will allow enough energy for a hot spot to appear. And once a hot spot is located, one should consider how much more heating will occur when the load increases to 100 %.
- Wind: if the inspection is being performed outdoors, and the wind speed at the component under investigation is 10 mph (16 km/h) or greater, then allowance must be made, particularly considering that the hot spot heat may increase at lower wind speeds.
- Phase comparison: unless there is a load imbalance between the phases, they will typically function at similar temperatures. If a component on one phase is warmer than the equivalent component on the other phases, further investigation is needed.
- Temperature: a hot spot should not be ignored even if it is small and the temperature difference seems insignificant. Even small temperature increases can indicate serious problems. It is advisable to judge an identified hot spot by the potential consequences of failure instead of some predetermined temperature-based prioritisation.
- Accuracy: when carrying out thermal inspections, work well within the imager’s measurement resolution and compensate accurately for both emissivity and background temperature.
For substations, the recommended procedure is to start with the exterior using a thermal imager. Scan the transmission line feeding the station, the circuit from the transmission line, high side insulators (arresters), and then hone in on specific components. For example, on a transformer, look at the bushings, the tap changer tanks, and so on. Recording the results and trending them over time can provide invaluable additional information, as illustrated in Figure 7.
Figure 7: Tracking the temperatures over months for three-phase switchgear in a substation, showing trends and issues detected(Tomas Kozel et al, Medium Voltage Switchgear Temperature Monitoring, ABB, 2016)
When a hot spot is identified, repairs can be carried out during a scheduled shutdown. This avoids an unplanned shutdown and is an effective form of preventative maintenance. When carrying out the repairs, tests should be carried out with a dedicated low-resistance ohmmeter such as a Ductor™. An ordinary digital multimeter (DMM) will not provide sufficiently accurate or reliable results. Let’s see why.
A DMM and a low-resistance ohmmeter both use a similar test voltage of just a few volts, but they use very different test currents. For a DMM, the test current is typically around 5 mA, but the most common test current for a low-resistance ohmmeter is 10 A, and there are types available that use test currents of 100 A, 200 A, or even 600 A. A DMM will read down to a tenth or possibly a hundredth of an ohm, whereas a low-resistance ohmmeter will read in microohms or even tenths of microohms.
So why do these differences matter? The primary function of the low-resistance ranges of a DMM is to perform continuity tests. Electricians use these tests to make certain that there are no wiring mistakes at a junction box and that all connections are correct and tight. In many cases, the actual measurement isn’t needed because the DMM has an audible beeper or buzzer activated at a pre-set resistance value. The handheld DMM is ideal for this type of testing.
In contrast, a low-resistance ohmmeter’s primary purpose is to accurately measure resistances below 1 Ω in applications where merely checking for continuity is not enough. There must be the certainty that the circuit or joint being tested will operate reliably without overheating. Some examples of applications include grounding for lightning protection, fault clearance, the mating of contact surfaces for maximum transfer of energy without heating, and maintenance of bolted connections and solder joints. For demanding applications like these, a change of just a few microohms in resistance can indicate an existing problem, or even a developing problem that needs to be corrected before any damage occurs.
Figure 8: A Megger AVO830 DMM showing 0 Ω when testing a busbar joint
Figure 9: A Megger DLRO10HD low-resistance ohmmeter reading 10 μΩ when testing the first section of the same joint as in Figure 8
Figure 10: The same low-resistance ohmmeter reading 1.131 mΩ when testing the second section of the same joint as in Figure 8, which reveals a substandard connection
Figures 8, 9, and 10 clearly show the difference between low-resistance measurements made with a DMM and a dedicated low-resistance ohmmeter. The DMM measures the resistance of the entire two-section busbar joint as zero and, therefore, does nothing more than confirm continuity. However, the low-resistance ohmmeter gives a reading of 10 μΩ for one section and 1.131 mΩ for the second section. The low-resistance ohmmeter has identified a bad connection on the busbar when the DMM could not find any difference in resistance. In general, low-resistance measurements are compared with the resistance of other similar joints/components to decide whether the value is within the normal range.
Hopefully, this article has demonstrated that the combination of thermal imaging and the use of a dedicated low-resistance ohmmeter is an essential part of electrical preventative maintenance. There is enormous value in carrying out regular thermal imaging inspections because it helps identify issues before they lead to costly damage and disruption. There is also a great deal of value in using the right tools to investigate further and remedy the problems that have been identified by thermal imaging.