AGL-CS-021 — AGL Transformer Overheating Investigation
The AGL Knowledge Hub | Engineering Case Study Series. A thermal-diagnostic investigation where one isolating transformer operated at a significantly higher temperature than adjacent units — from thermal survey through connection resistance and insulation testing to replacement.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During a scheduled thermal inspection of a runway edge lighting circuit, one isolating transformer was found operating at a significantly higher temperature than adjacent transformers.
The associated runway edge light remained operational, and the Constant Current Regulator (CCR) was maintaining the selected current.
No insulation alarm was active, and the transformer had not yet failed electrically.
The investigation ultimately identified excessive heating caused by a high-resistance primary connection and deteriorated transformer insulation, resulting in abnormal power dissipation within the transformer assembly.
This case demonstrates that thermal abnormalities often provide early warning of transformer failure before electrical failure becomes apparent.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand normal operating temperatures of AGL isolating transformers.
- Recognize abnormal transformer overheating.
- Differentiate load-related heating from fault-related heating.
- Perform thermal and electrical correlation.
- Investigate high-resistance connections.
- Evaluate insulation deterioration under thermal stress.
- Apply condition-based transformer replacement criteria.
3System Background
An AGL isolating transformer normally generates heat due to:
- Copper losses.
- Core losses.
- Magnetic losses.
- Load current.
Normal heating is expected.
Abnormal overheating may result from:
- High-resistance primary connections.
- High-resistance secondary connections.
- Internal winding deterioration.
- Core damage.
- Insulation degradation.
- Moisture contamination.
- Overloading.
- Poor heat dissipation.
Temperature is an important predictive maintenance parameter.
4Problem Statement
The reported condition was:
- One transformer significantly hotter than adjacent units.
- Associated light operational.
- CCR healthy.
- No circuit outage.
- Transformer age: 10 years.
- Dry weather conditions.
Is the overheating caused by loading, connection resistance, insulation deterioration, or an internal transformer defect?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Light operation | Normal |
| Transformer surface temperature | 82°C |
| Adjacent transformers | 48–52°C |
| Weather | Dry |
The temperature difference was large enough to indicate abnormal localized heating.
6Initial Engineering Hypotheses
The maintenance team considered:
Normal load variation.
High-resistance primary connection.
High-resistance secondary connection.
Internal winding deterioration.
Core-loss increase caused by insulation aging.
7Thermal Diagnostic Principle
Excessive heating may occur due to:
This creates a progressive thermal degradation cycle.
8Test Equipment
The investigation used:
- Thermal imaging camera.
- True-RMS multimeter.
- Micro-ohmmeter.
- 5 kV insulation resistance tester.
- Transformer test equipment.
- Approved AGL circuit test equipment.
- Circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Thermal Survey
Thermal imaging showed:
| T21 | 50°C |
| T22 | 49°C |
| T23 | 82°C |
| T24 | 51°C |
| T25 | 50°C |
Transformer T23 was clearly abnormal.
Step 2: Load Verification
Measurements confirmed:
- Circuit current normal.
- No overload condition.
- Same loading as adjacent transformers.
Overloading was eliminated as the primary cause.
Step 3: Primary Connection Inspection
Inspection found:
- Oxidized primary termination.
- Loose compression connection.
- Minor discoloration.
- Elevated contact resistance.
Micro-resistance measurement showed significantly higher resistance than adjacent transformers.
Step 4: Transformer Insulation Testing
Results:
| Primary continuity | Normal |
| Secondary continuity | Normal |
| Insulation resistance | 18 MΩ |
| Adjacent transformers | Greater than 1000 MΩ |
The transformer insulation had deteriorated.
Step 5: Internal Examination
After replacement, inspection revealed:
- Insulation discoloration.
- Resin cracking.
- Localized thermal damage.
- Oxidized winding terminations.
- Evidence of prolonged overheating.
The overheating had been occurring for an extended period.
10Root Cause
The transformer overheating was caused by:
- A high-resistance primary connection.
- Progressive insulation deterioration.
The high-resistance connection produced excessive I²R heating, while the deteriorated insulation reduced the transformer's ability to dissipate heat safely.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the overheated transformer.
- Repaired the primary connection.
- Installed a new approved transformer.
- Verified connection resistance.
- Performed insulation resistance testing.
- Conducted thermal verification.
- Restored the circuit.
- Compared transformer temperatures.
- Updated transformer condition records.
12Post-Repair Verification
The transformer temperature returned to the same range as adjacent units.
13Root-Cause Analysis
Transformer overheating detected.
Abnormal temperature confirmed.
Normal.
High resistance found.
Insulation deterioration confirmed.
Thermal damage identified.
High-resistance connection and insulation deterioration.
Replace transformer and repair connection.
14Why This Case Was Important
A transformer may:
- Continue operating.
- Pass continuity tests.
- Maintain light output.
while already experiencing dangerous thermal deterioration.
Thermal monitoring can identify developing failures before:
- Insulation breakdown.
- Winding failure.
- Secondary open circuit.
- Earth leakage.
- Complete transformer failure.
15Engineering Lessons
Thermal inspection is a valuable predictive tool.
Thermal inspection is a valuable predictive-maintenance tool.
High-resistance connections generate significant heating.
High-resistance connections can generate significant heating.
Temperature comparison is highly effective.
Temperature comparison between similar transformers is highly effective.
Overheating accelerates insulation aging.
Overheating accelerates insulation aging.
Condition-based replacement prevents failures.
Condition-based replacement can prevent operational failures.
16Engineer's Decision Point
You observe: One transformer at 84°C. Adjacent transformers at 50°C. CCR operating normally. Light functioning correctly. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about transformer overheating, based on this case study.
Q1What is AGL-CS-021?
AGL-CS-021 is an engineering training case study on an isolating transformer found overheating during a thermal inspection of a runway edge lighting circuit. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2How hot was the transformer compared with its neighbours?
The transformer surface temperature was 82°C while adjacent transformers ran at 48–52°C. That difference was large enough to indicate abnormal localized heating, even though the light was operating and the CCR was healthy.
Q3Can a transformer be overheating while everything still works?
Yes. The light stayed operational, the CCR maintained 6.6 A, no insulation alarm was active and the transformer had not yet failed electrically. Thermal inspection revealed a developing problem that normal operation did not.
Q4What was the root cause?
A high-resistance primary connection combined with progressive insulation deterioration. The connection produced excessive I²R heating, while the deteriorated insulation reduced the transformer's ability to dissipate heat safely.
Q5Which tests were used to correlate the heating?
The team used a thermal imaging camera, true-RMS multimeter, micro-ohmmeter and a 5 kV insulation resistance tester, along with transformer test equipment and circuit drawings. The recommended step is to correlate the thermal result with connection resistance and insulation testing.
Q6How was the transformer repaired?
The team isolated the circuit, removed the overheated transformer, repaired the primary connection and installed a new approved transformer. They verified connection resistance, tested insulation resistance, conducted thermal verification, restored the circuit and updated condition records.
Q7How was the repair verified?
The transformer temperature fell to 51°C, in line with adjacent units. Connection resistance was normal, insulation resistance was greater than 1000 MΩ and the light operated normally.
Q8What should you investigate first if one transformer is at 84°C while its neighbours are at 50°C?
Perform thermal correlation with connection resistance and insulation testing. Ignoring the temperature difference, replacing the light fixture or increasing CCR current is not the right first step.
Engineering & Expert Notes
Compare Against Neighbouring Units
ThermographyRelative temperature against similar transformers under similar load highlights the abnormal one.
Inspect the Primary Connection First
TerminationsA hot primary lead often indicates a resistive joint, which can be repaired without changing the transformer.
Heat Shortens Insulation Life
AgeingSustained overheating weakens insulation. Plan replacement for units that have been running hot.
Re-Check Temperature After Repair
VerificationConfirm the temperature has dropped under comparable conditions before closing the job.
20Case Study Conclusion
Transformer overheating is often an early warning indicator of connection deterioration, insulation degradation, or internal transformer defects that may not yet be visible through routine operational testing.
Systematic thermal imaging, resistance measurement, insulation testing, and comparative temperature analysis can identify developing transformer failures before they result in insulation breakdown, winding failure, or operational outages.
In AGL engineering, temperature is often the first visible symptom of an electrical problem that has been developing inside the transformer for months or years.
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