When most people think of finding the source of power quality problems, they imagine a meter or monitor with half a dozen or more wires connected to the voltage bus or terminals, along with three or four current transformers (CTs) clamped around either the bus bars or the wires. There still is no more thorough and accurate way for determining the source of power quality problems than such equipment, especially for intermittent problems.
However, connecting such equipment usually requires removing a panel or cover to get to the normally unexposed, potentially lethal voltages. Unless the circuit has been verified as de-energized, the proper personal protective equipment (PPE) needs to be worn and safety precautions taken. Suppose a breaker is not operating properly, tripping when the expected current levels from the load haven’t recently changed. Even if you don’t have Superman’s ocular powers, there is another way to look for what evil lurks behind those panels or covers. It involves taking advantage of another region in the electromagnetic spectrum, infrared radiation.
Take the nuisance breaker tripping example described on a home repair Web site: “My electric dryer kept tripping the ganged 30 ampere breakers, yet the dryer was working correctly. There was no evidence of a defective induction motor nor shorted heating elements.”
The person describing the situation probably already had used a meter to determine the dryer was working OK. He removed the panel and tightened the screws on the 10 AWG copper wire to the breaker, and the problem went away. The problem was that the loose connections were causing excessive impedance to the current supplied to the dryer. Higher impedance means more of a voltage drop across the contacts and, therefore, more power or watts being dissipated there. This, in turn, resulted in excessive heating, which conducted into the breaker itself. Being a thermal breaker, this extra heat would bias the trip point to a low current level for its design.
How could the same problem be identified in a less dangerous manner, by extending vision into the thermal spectrum. The more heat being produced, the more thermal radiation and, generally, the shorter the equipment life. Recently introduced infrared cameras make it possible to identify potentially defective parts down to the component level on a PC board with a relatively small amount of training. As shown in the image above, one of the breakers appears to be much “hotter” than the rest. The typical equivalent relative temperature scale from hot-to-cold of the objects is seen and then recreated as visible light on the display by the imager running white–yellow–orange–red–purple–blue–black.
The key is relative temperatures. A white hot object in one image might be 100°F (38°C), and another object could be cold blue at 32°F (0°C). In another picture, those two objects with the same colors might only be 5 degrees apart in temperature at 77° and 72°F.
To see the actual hot connection in the aforementioned example would still require removing the cover (wearing the proper PPE). Even though the connection may have been properly torqued when installed, vibration and thermal cycles can loosen it over time. In most facilities, the current flows during only part of the day, and it often contains heat-generating harmonic currents. The heating/cooling/heating/cooling cycle can result in expansion and contraction of the wires, which can cause the connections to loosen over time. This loosening increases the impedance of the connection, which further increases the heating effects, and the situation just gets worse until addressed.
A thermal imaging camera also can help determine where cooling isn’t taking place properly. If some of a transformer’s fins were significantly different in color than others, a potential problem could safely be identified before it leads to a transformer failure. If it had oil-filled cooling fins and one section was blue while the others were yellow, that may mean the hot cooling oil is not properly circulating there.
The thermal camera’s operating instructions must be reviewed and understood to prevent jumping to the wrong conclusions. Many first-time operators have gotten excited about a component showing up white hot only to discover the range of the image was only a few degrees, making white potentially no worse than blue. However, if the blue equivalent temperature was above the operating temperature of the equipment being scanned, blue could indicate a problem. The general rule of thumb from the InterNational Electrical Testing Association (NETA) states, when the difference in temperature (DT) between similar components under similar loading (O/S) exceeds 27 °F (15°C) or if the DT for a component and ambient air (O/A) exceeds 72°F (40°C), it is a problem that requires immediate attention.
A preventative maintenance program that includes conducting periodic surveys and records the data at specific identified points on equipment can show a trend leading to early detection of thermal-based problems, such as loose connections, blocked cooling mechanisms, damaged bearings and defective components. Thermal imaging cameras let you see what’s going on with those invisible electrons.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.