Transformers are pivotal in our electric power infrastructure, managing voltage levels to ensure electricity is transmitted and distributed efficiently. Their operational stability and performance are critical not only for daily operations but also for avoiding major grid disruptions. One key transformer parameter that is frequently monitored is the hotspot temperature (HST), which represents the maximum temperature within the winding insulation. Accurate transformer hotspot monitoring remains critical for ensuring its reliability and longevity.

Why is this temperature so important? Because it has a direct influence on the aging rate of the transformer’s solid insulation. Studies show that every 6°C rise in HST from 80°C to 140°C doubles the insulation aging rate. That means even small temperature rise can significantly reduce a transformer’s lifespan.

How Elevated Winding Temperatures Lead to Transformer Insulation Breakdown

Transformer windings rely on solid insulation—typically made from cellulose-based paper—to protect them from electrical and thermal stress. Under long-term exposure to high temperatures, this insulation begins to degrade in a process known as depolymerization. In the paper, the breakdown of the molecular chains results in a loss of elasticity and mechanical strength, which eventually causes transformer insulation to fail.

Modern transformers often use thermally upgraded paper, capable of withstanding higher temperatures—up to 110°C. Yet even here, an increase of just 7°C can double the aging rate. For transformers using traditional Kraft paper insulation, the rated HST is lower around 95°C–97°C, as per IEEE/IEC standards, and crossing that limit in emergencies (e.g., reaching 140°C) can spike the aging acceleration factor to nearly 100.

The risks increase when insulation has more than 2% moisture; high temperatures can evaporate this water, creating bubbles that harm the paper and oil insulation. These conditions increase the risk of internal dielectric breakdown—a key driver in unplanned transformer failures due to the thermal aging of transformers.

The Cost of Inadequate Hotspot Temperature Monitoring

Heat is one of the major stress factors in a transformer’s operating environment. Overloads, high ambient temperatures, and internal losses can all cause localized heating, driving up the HST. Without precise transformer hotspot detection, asset managers may not recognize the warning signs until it’s too late.

According to a CIGRE Transformer Reliability Survey, one out of every 200 transformers experience a catastrophic failure each year. Inadequate temperature monitoring is directly responsible for a large number of these failures. The consequences are severe and result in costly repairs, major network outages, and monetary losses. CIGRE estimates the cost of major transformer failures to be roughly $14,000 per MVA for property damage alone.

Real-time transformer hotspot monitoring provides a proactive strategy, providing accurate temperature data directly from the winding structure. This level of alertness aids in the early detection of thermal anomalies, enabling asset managers to address problems before they become costly failures.

Early Warning Signs of a Developing Hotspot

Early indications of thermal stress signs are usually seen in transformers before insulation failure. The warning signs may be minimal, but if identified correctly, they provide great opportunities to act prior to severe damage. Following are typical signs that your transformer might be experiencing thermal stress:

• Abrupt increases in winding temperature indicator (WTI) values, even if top-oil temperature is normal.
• Unusual patterns in real time fiber optic winding sensor data, suggesting the presence of localized heat.
• Variations in the load to temperature curve, which can indicate higher resistance or degradation of insulation.
• Abnormal DGA readings, particularly high levels of carbon monoxide (CO) and carbon dioxide (CO₂)- typical indicators of breakdown of paper insulation.
• Partial discharge activity, usually act as a warning sign before insulation failure and dielectric breakdown.

Even though these warning signs are quite helpful, depending only on traditional indicators might not provide the depth or speed required to know what is occurring within the transformer. Slight variations in thermal behaviour can go unnoticed until they result in insulation failure or unanticipated outages. It highlights the increasing importance of the continuous monitoring of transformer performance. Also, it emphasizes the necessity of developing more advanced techniques for early thermal stress detection, particularly in the winding structure- before it’s too late.

In part 2, we will explore more advanced monitoring techniques that offer deeper insights into transformer hotspot detection. Stay tuned to discover how real-time data from fiber optic winding temperature sensors, predictive analytics, and online monitoring systems can help you detect emerging issues early, reduce thermal stress, and extend transformer lifespan.

Resources:

1. A new method for transformer hot-spot temperature prediction based on dynamic mode decomposition – ScienceDirect
2. CIGRE Transformer Reliability Survey.
3. What is Transformer Monitoring?
4. https://hrcak.srce.hr/file/298079
5. Everything You Need to Know About Transformer Oil and Winding Temperature Indicators | Qualitrol Corp
6. Fiber Optic Hot Spot Temperature Monitoring | Qualitrol Corp