Electric current is changed from one voltage to another with power and distribution transformers. When electricity flows through a coil at one voltage, it induces a current in a second coil. This is when transformers do the job of changing the voltage of the electric current. The number of windings that a coil has decides the amount of change.
In this discussion, we focus on how we can use the infrared inspection to monitor the condition of transformers. Transformers may have connection problems – both internal and external. The coil temperatures in dry transformers are so much higher than ambient, it becomes difficult to detect internal problems before great damage is caused. There are other technologies like built-in temperature and pressure gauges that may be used to assess the condition of transformers, but none is as effective as infrared inspection.
What should you check?
You should use thermal imagers to monitor external connections, cooling tubes, and cooling fans. Thermal imagers are also effective in testing pumps and critical transformer surfaces.
What should you look for?
You may monitor the following external components in oil-filled transformers with thermal imagers:
- High and low-voltage bushing connections – Thermal imaging can easily capture overheating in connections which indicates that the connection is loose or dirty causing high resistance. Overloading can be checked by comparing phases.
- Cooling tubes – Cooling tubes are usually warmer in oil-cooled transformers. Comparatively cooler tubes indicate that oil flow is being restricted. Such differences in temperatures can be easily spotted by thermal imagers so you can get to the root of a problem.
- Cooling fans/pumps – Inspection of fans and pumps should be done when they are running. When the fan is operating, failing bearings will cause it to get hot. This can be detected effectively with a thermal imager.
You can also use thermography to detect if there are any problems in surge protection and if lightning arrestors are leaking. To identify these problems, you’ll have to spot very subtle differences in temperature under conditions that may often be difficult to monitor. Apart from thermal imaging, you should use ultrasound or some more reliable technology as well.
For effective thermal imaging to point out the exact location of an internal problem in the transformer, there should be enough heat generated by the transformer so that it can be detected on the outside. There may be internal problems in the following in oil-filled transformers:
- Internal bushing connections – you should bear in mind that the connections are far hotter than what the surface temperatures read by a thermal imager indicate.
- Tap changers – you can regulate the output voltage in transformers and bring them to the required levels with tap changers. An external tap changer compartment shouldn’t be warmer than the transformer’s body. Please note that the IR inspection results may not be entirely conclusive as all the taps won’t be connected during the inspection.
You would do well to create a regular inspection routine. The transformers on all the essential electrical circuits should be included. Store the thermal images of each of the transformers on the computer and track their temperatures over a certain period using the IR camera’s software. That’ll give you baseline images with which you can compare images taken later. This will give you an idea if the current temperature levels are unusual. You can also gauge if the corrective maintenance action taken has been successful.
Modern thermal imagers come with IR-Fusion which combines a visual light image with an infrared image enabling better identification of issues their better analysis. The dual images can be aligned accurately at any distance making it easier to spot problems.
What should you treat as a “red alert?”
You should give the highest priority for repairs if any equipment is in a condition that poses a safety risk. When the imminent failure of any critical equipment component is noticed, it constitutes a red alert. It is the job of key operations, safety, and maintenance personnel to play critical roles and quantify the “warning” and “alarm” levels for power supply to critical equipment. Personnel involved in monitoring transformers should all along bear in mind that as in an electric motor, there’s a minimum operating temperature in a transformer. This minimum operating temperature is a representation of the maximum temperature rise that can be allowed above ambient where the ambient temperature is specified at 40°C. It’s generally considered that a transformer’s life is reduced by 50 percent if there’s a 10°C rise above the maximum rated operating temperature.
What’s the likely cost of failure?
Transformer failures are very costly for power distribution companies. In Oslo, in the summer of 2005, a transformer failure caused a 50-minute power outage affecting 200,000 customers. It left people trapped in subways and elevators and the power delivery company in charge of the transformer was made to pay as much as 10 million Norwegian kroner (1.6 million USD) as compensation to Norway’s main power supplier, NVE.
You can carry out an analysis of the repair or replacement cost, the lost production opportunity, and lost labor costs caused by a failed transformer at your facility.
The follow-up action needed
After your thermal imager locates a problem, use the imager’s software to prepare a report documenting the findings. The report should include a thermal image and a digital photograph of the concerned equipment. It’ll be an effective way to communicate the findings in your report and to suggest necessary repairs.
Conclusion
Thermal imaging is therefore a very effective technique of locating problems in transformers. All transformers have a standard operating temperature. Any deviations in it indicate problems that can be located by thermal imagers. Once the problem is located, you can prepare a concise report including thermal and digital photographs of the transformer. Timely detection of issues in transformers can help prevent major outages which cause huge losses. Thermal imagers can help create a preventive maintenance routine with timely repairs being a norm, not an exception.
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