Most transformers are cooled by either oil or air while operating at temperatures much higher than ambient. In fact, operating temperatures
of 65 °C for oil-filled units and 150 °C for air-cooled transformers are common. Nevertheless, problems with transformers often manifest themselves in overheating or hot spots, making thermal imaging a good tool for finding problems. Power and distribution transformers change electric current from one voltage to another. They accomplish this process when electricity flowing through a coil at one voltage induces current in a second coil. The amount of change is a function of the number of windings on the coils.
The following section focuses on monitoring external and internal conditions of oil-filled transformers. Dry transformers also can exhibit both external or internal connection problems, and external connection problems can be detected as with oil-filled units. Beyond that, dry transformers have coil temperatures so much higher than ambient, it is difficult to detect internal problems before irreparable damage occurs.
What we check
At a minimum, we use our thermal imager to look at external connections, cooling tubes and cooling fans and pumps as well as the surfaces of critical transformers.
What we look for
In oil-filled transformers we monitor the following external components:
- High- and low-voltage bushing connections. Overheating in a connection indicates high resistance and that the connection is loose or dirty. Also, compare phases, looking for unbalance and overloading.
- Cooling tubes. On oil-cooled transformers, cooling tubes will normally appear warm. If one or more tubes are comparatively cool, oil flow is being restricted and the root cause of the problem needs to be determined.
- Cooling fans/pumps. Inspect fans and pumps while they are running. A normally operating fan or pump will be warm. A fan or pump with failing bearings will be hot. A fan or pump that is not functioning at all will be cold.
For thermography to be effective in pinpointing an internal transformer problem, the malfunction must generate enough heat to be detectable on the outside. Oil-filled transformers may experience internal problems with the following:
- Internal bushing connections. Note: connections will be much hotter than surface temperatures read by an imager indicate.
- Tap changers. Tap changers are devices for regulating transformer output voltage to required levels. An external tap changer compartment should be no warmer than the body of the transformer. Since not all taps will be connected at the time of an inspection, IR inspection results may not be conclusive.
Our thermal imagers now include IR-Fusion®, a technology that fuses a visual, or visible light, image with an infrared image for better identification, analysis and image management.
The dual images are accurately aligned at any distance heightening details so problems are easier to spot.
What represents a “red alert?”
Equipment conditions that pose a safety risk should get the highest priority for repairs. However, the imminent failure of any piece of critical equipment constitutes a red alert. Key operations, maintenance and safety personnel should play roles in quantifying “warning” and “alarm” levels for the power supplies to critical assets. (Note: alarm levels for specific equipment can be set on Fluke handheld thermal imagers.)
Throughout, personnel responsible for monitoring transformers should keep in mind that like an electric motor, a transformer has a minimum operating temperature that represents the maximum allowable rise in temperature above ambient, where the specified ambient is typically 40 °C. It is generally accepted that a 10 °C rise above its maximum rated operating temperature will reduce a transformer’s life by 50 percent.
What’s the potential cost of failure?
For power delivery companies, transformer failures can be very costly. A transformer failure in the summer of 2005 in Oslo, Norway resulted in a 50-minute power outage for 200,000 customers, left people trapped in subways and elevators, and cost the power delivery company responsible for the transformer 10 million Norwegian kroner (£1.2 million, GBP) in compensation to NVE, Norway’s main power supplier. For a failed transformer at your facility, you can do an analysis of the cost of repair or replacement, lost production opportunity and lost labour costs for affected equipment.
Whenever we discover a problem using a thermal imager, we use our associated thermal imaging software to document our findings in a report, including a thermal image and a digital photograph of the equipment.