Can transformer winding hot spot monitoring be used in high - voltage transformers?

When I first started working as a technical sales lead at a transformer monitoring solutions provider, I’d often get asked the same question by substation engineers and maintenance managers: “Why do I need winding hot spot monitoring when I already track dissolved gas levels in my high-voltage transformers?” It’s a fair question—for years, traditional transformer health monitoring relied on offline testing and a handful of key metrics, but the shift to real-time, proactive asset management has made technologies like our Transformer Winding Hot Spot Monitoring system non-negotiable for high-voltage (HV) units. Today, I want to break down exactly how this technology works, why it’s critical for HV transformers, and how it integrates with the other tools you likely already have in your monitoring stack.

First, let’s ground this in what a high-voltage transformer actually does and why heat is such a big deal. HV transformers—think the ones that step power up for long-distance transmission or down for industrial and residential use—operate at voltages 100kV and above, which means they handle massive electrical loads. Inside, copper windings carry current, and as resistance creates heat, that heat has to dissipate through the insulating oil and tank walls to the atmosphere. The “hot spot” isn’t just the average temperature of the windings; it’s the single highest temperature point along the winding, usually near the top of the HV or LV coils where oil flow is restricted and current density is highest. For context, every 10°C increase above the designed hot spot temperature cuts a transformer’s lifespan roughly in half. That’s a simple rule of thumb from transformer physics that engineers have relied on for decades, but until recently, we couldn’t measure that hot spot directly—we just estimated it from oil top-temperature or average winding temperature, which are blunt tools at best.

So how does Transformer Winding Hot Spot Monitoring work in practice? Our system uses fiber optic sensors that are embedded directly into the winding during manufacturing (or retrofitted into existing transformers with minimal downtime) to measure temperature at multiple points along the coil. The sensors are small, robust enough to withstand the 100+ kV electric fields inside the transformer, and they transmit data via light signals, so there’s no risk of electrical interference that can skew readings from traditional thermocouples. Once we have real-time hot spot data, we pair it with load data, oil flow rates, and ambient temperature to feed into predictive algorithms that not only show current temperature but also forecast how the hot spot will behave as loads fluctuate—something that’s especially important as grids add renewable energy, which creates variable, unplanned load swings that older transformers weren’t designed to handle.

Now, let’s address the core question: Can this technology be used in high-voltage transformers? The short answer is a resounding yes, but let’s dive into the specific use cases that make it indispensable for HV assets. First, HV transformers are a huge capital investment—often costing millions of dollars, and a single unplanned failure can take a substation offline for days, leading to millions in lost revenue for utilities and industrial operators. For example, a chemical plant I worked with last year had a 230kV transformer powering its entire production facility. They were using offline oil testing and a Transformer Dissolved Gas Analyzer to track faults, but when a partial winding short caused a localized hot spot, the dissolved gas readings didn’t spike until weeks later—by then, the winding insulation was already compromised. Installing our hot spot monitoring system let them catch the 12°C over-temperature before insulation damage occurred, allowing them to schedule a repair during a planned outage instead of a catastrophic failure. That’s the value of real-time hot spot data that you can’t get from any other monitoring tool.

Another key use case is for transformers under dynamic load, which is becoming increasingly common as grids integrate solar, wind, and battery storage. Traditional load tap changers and temperature models rely on steady-state load assumptions, but when a transformer is cycling between low overnight loads and peak midday loads (especially with renewable energy’s variable output), the hot spot can spike rapidly. I had a conversation with a utility engineer in the Midwest a few months ago who was dealing with a 115kV transformer that was overheating during midday solar peaks. Their existing thermal model estimated a safe hot spot, but actual measurements from our hot spot system showed a 20°C over-temperature that only showed up at the winding’s top turn. That transformer was operating at the edge of its designed capacity, and without direct hot spot monitoring, they would have risked failing it during a period of high demand. Adjusting their load schedule slightly cut the hot spot back to safe levels, extending the transformer’s life by an estimated 8 years, according to their calculations.

But here’s the thing: hot spot monitoring isn’t meant to work in a vacuum. It’s a critical piece of the transformer health monitoring puzzle, and it integrates seamlessly with other tools utilities already use. Let’s talk about how it pairs with the other systems in our portfolio. For example, our Transformer Dissolved Gas Analyzer tracks gases like methane, ethylene, and acetylene that indicate incipient faults, but it can take days or weeks for these gases to accumulate enough to be detected. Hot spot monitoring catches the thermal stress that’s often the root cause of those gas-producing faults early, so maintenance teams can investigate before gases start spiking. Then there’s our Transformer Core Grounding Current Monitor, which tracks stray currents that can cause localized heating in the core and structural parts of the transformer—this works in tandem with hot spot data to confirm if overheating is coming from windings or core components. And if you’re monitoring partial discharges, our Online Partial Discharge Monitoring System for Transformer flags electrical insulation damage, while hot spot data tells you if that damage is being driven by excessive temperature. Together, these tools create a 360-degree view of transformer health that’s far more reliable than any single metric.

Now, let’s clear up a common misconception: some engineers think hot spot monitoring is only for new transformers, but retrofitting is absolutely possible. Our team has installed the Transformer Winding Hot Spot Monitoring system on transformers that are 30+ years old, with minimal downtime. The process works by carefully inserting fiber optic sensors through the oil valve on the transformer tank, routing them along the existing winding without disrupting the insulation, and connecting them to a monitoring unit mounted on the outside of the tank. Most retrofits can be done during a routine maintenance window, so there’s no need for a full unit replacement, which saves utilities tens of thousands of dollars compared to buying a new HV transformer.

Another point to consider is regulatory and industry standards. Organizations like the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC) have updated their guidelines in recent years to explicitly recommend real-time hot spot monitoring for HV transformers, especially those in critical applications like grid transmission or industrial power supply. For example, IEEE C57.12.90 now includes hot spot temperature as a key parameter for transformer load management, meaning utilities that rely on this standard need accurate hot spot data to optimize their transformer usage and meet regulatory requirements.

I’ve also seen hot spot monitoring make a huge difference for fleet management. Large utilities with dozens or hundreds of HV transformers can use the data from our Transformer Winding Hot Spot Monitoring system to prioritize maintenance. Instead of doing time-based maintenance (where every transformer gets serviced every 5 years, regardless of actual condition), they can shift to condition-based maintenance, focusing on units that show early signs of overheating. This not only reduces maintenance costs but also extends the life of transformers that are operating well within their design limits, leading to significant long-term savings.

Of course, no technology is perfect, so it’s important to address the limitations of hot spot monitoring to be transparent. Fiber optic sensors need to be installed correctly—bad routing can cause signal loss or incorrect readings, which is why working with an experienced provider is key. Also, hot spot data is most valuable when paired with load, oil temperature, and ambient data, so a standalone hot spot system without that context won’t give you the full picture. But when implemented right, with integration to other monitoring tools and expert support, these limitations are minimal.

Let’s circle back to that engineer’s original question from the start: “Why do I need winding hot spot monitoring when I have dissolved gas analysis?” The answer is simple: different tools catch different types of faults, and hot spot monitoring fills a critical gap that other tools can’t. Dissolved gas analysis catches chemical faults that have already progressed, core grounding current monitors catch stray current heating, partial discharge monitors catch electrical insulation flaws—but hot spot monitoring is the only tool that gives you real-time, direct data on the most damaging stress point in your HV transformer. For assets that cost millions, that kind of visibility isn’t just a nice-to-have—it’s a necessity to avoid costly failures, optimize asset performance, and extend lifespan.

If you’re responsible for maintaining high-voltage transformers, whether you’re a utility, industrial plant, or substation operator, I’d encourage you to think about how hot spot monitoring fits into your overall strategy. If you’re already using other transformer monitoring tools like Transformer Dissolved Gas Analyzer or Online Partial Discharge Monitoring System, adding our Transformer Winding Hot Spot Monitoring system will create a more complete picture of your assets’ health. If you’re just starting to move from offline to real-time monitoring, our team can help you design a solution tailored to your specific fleet, whether you have a handful of HV transformers or a large transmission network.

Ready to discuss how Transformer Winding Hot Spot Monitoring can protect your high-voltage assets? Our team is here to walk through your needs, answer questions, and help you build a monitoring strategy that fits your budget and operational goals. Don’t wait until a small overheating issue becomes a catastrophic failure—reach out today to start the conversation about securing your transformer assets for years to come.

REFERENCES

1. IEEE Std C57.12.90-2015, “Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers,” Institute of Electrical and Electronics Engineers, 2015.
2. IEC 60076-7:2018, “Power Transformers - Part 7: Loading Guide for Oil-Immersed Power Transformers,” International Electrotechnical Commission, 2018.
3. Tenbohlen, S., et al., “Advanced Transformer Monitoring: A Review of Technologies and Applications,” IEEE Transactions on Power Delivery, vol. 33, no. 4, 2018, pp. 1853-1862.
4. “Proactive Maintenance of High-Voltage Transformers,” Electric Power Research Institute (EPRI) Report 3002001245, 2021.