Transformer Temperature Rise Explained

One of the most transformer-critical issues I meet in my work is temperature rise.

We take temperature rise seriously. Once, we replaced a transformer because it was overheated.

In this article, I will answer the most common questions about transformers’ temperature rise. Let’s move forward.

transformer temperature rise

Temperature rise in a transformer refers to the increase in temperature that occurs within the transformer’s components during normal operation.

This rise in temperature is primarily a result of the electrical losses that occur as electric current passes through the transformer windings and the magnetic core.

Understanding and controlling temperature rise is crucial for ensuring the transformer’s reliable and safe operation over its lifespan.

Here’s a breakdown of the key aspects related to temperature rise in transformers:

  1. Operating Temperature:

    • Transformers are designed to operate within a specified temperature range. This range is influenced by factors such as the insulation materials used, the type of cooling (oil or air), and the transformer’s intended application. The operating temperature is the normal temperature at which the transformer functions during its regular load conditions.
  2. Temperature Rise Limits:

    • The temperature rise limit is the maximum allowable increase in temperature above the ambient temperature. It is defined by standards and specifications for transformer design and performance. Temperature rise limits are typically categorized as follows:
      • Top Oil Temperature Rise: The increase in temperature of the insulating oil.
      • Winding Temperature Rise: The increase in temperature of the transformer windings.
  3. Temperature Rise Testing:

    • During the manufacturing and testing of transformers, a temperature rise test is conducted to verify that the transformer can operate within specified temperature limits. This involves applying a load to the transformer and measuring the resulting temperature rise in the oil and windings.
  4. Ambient Temperature:

    • The ambient temperature, or the temperature of the surrounding environment, affects the transformer’s cooling capacity. Transformers are often designed with an ambient temperature specified in their ratings. Higher ambient temperatures can contribute to increased temperature rise and may necessitate adjustments in the transformer’s loading or cooling systems.
  5. Insulation Class:

    • The insulation class of a transformer is a designation that reflects the maximum temperature the insulation can withstand. Common insulation classes include A, B, F, and H, each corresponding to a specific maximum allowable temperature.
  6. Monitoring and Protection:

    • Transformers are equipped with temperature monitoring devices such as Oil Temperature Indicators (OTIs) and Winding Temperature Indicators (WTIs). Protective relays are set to trigger alarms or initiate corrective actions if the temperature rises beyond specified limits.

Controlling temperature rise is crucial for ensuring the longevity and reliability of transformers. Excessive temperature can lead to accelerated aging of insulation materials, reduced dielectric strength, and potential failures.

Therefore, transformers should be operated within their specified temperature limits, and regular monitoring and maintenance practices should be employed to detect and address any issues related to temperature rise promptly.

Why does a transformer get hotter than usual?

Different losses that cause a transformer to get hot, in normal conditions, are losses due to resistance of the windings, Hysteresis, and eddy current losses as I mentioned above.

But, in some cases, the transformer gets hotter than usual. There are other reasons why the transformer gets hotter. I will mention the most common reasons:

  1. When the load exceeds the transformer rating “overloading the transformer”.
  2. Higher ambient temperature.
  3. High moisture in the transformer oil increases the temperature rise because the transformer oil’s ability to dissipate heat decreases.
  4. Blocked ventilation openings of transformers, sometimes by bird nests.
  5. When connecting loads with poor Power Factor.
  6. The misaligned or broken cooling fan of the transformer.
  7. Any issues with the transformer cooling system i.e. air fans or oil pumps?
  8. Any internal fault can increase the temperature of the transformer.

For instance, in my work were did a regular inspection of one of the power transformers, and we noticed a temperature rise by the thermal camera.

We hired a third-party company to make an oil analysis, it found the reason for the transformer temperature rise was higher moisture in the oil.

What are Transformer OTI and WTI?

OTI (Oil Temperature Indicator) and WTI (Winding Temperature Indicator) are devices used to monitor and measure the temperature of different components in a transformer.

These indicators are crucial for ensuring that the transformer operates within its specified temperature limits and for preventing overheating that could lead to damage or failure. Here’s a brief explanation of OTI and WTI:

  1. OTI (Oil Temperature Indicator):

    • The Oil Temperature Indicator monitors the temperature of the insulating oil in an oil-immersed transformer. The insulating oil serves multiple purposes, including providing electrical insulation and acting as a coolant. Monitoring the oil temperature is essential to ensure that the transformer operates within a safe temperature range. The OTI typically consists of a temperature sensor placed in the transformer’s oil, connected to an indicator or monitoring system. If the oil temperature exceeds the specified limits, the monitoring system can trigger alarms or take other protective actions.
  2. WTI (Winding Temperature Indicator):

    • The Winding Temperature Indicator monitors the temperature of the transformer’s windings, which are the coils of wire that carry electrical current. Monitoring the winding temperature is critical because the insulation materials in the windings have temperature limits. Excessive temperature can lead to insulation degradation and reduced transformer life. Similar to the OTI, the WTI includes a temperature sensor installed in the winding, connected to an indicator or monitoring system. If the winding temperature rises beyond the acceptable range, the monitoring system can activate alarms or protective measures to prevent damage.

Both OTI and WTI are part of the transformer’s protective and monitoring systems, and they play a crucial role in maintaining the transformer’s health and preventing failures due to overheating.

Transformers are designed to operate within specific temperature limits, and these indicators help ensure that these limits are not exceeded during normal operation.

In addition to OTI and WTI, other protective devices such as Buchholz relays and thermal overload relays may also be employed to enhance the safety and reliability of transformers.

Read also my other articles:

how is temperature controlled in a transformer?

Power transformer

The temperature in a transformer is controlled through various mechanisms and components designed to monitor and regulate the heat generated during operation. Here are some key aspects of temperature control in a transformer:

  1. Cooling Systems:

    • Transformers are equipped with cooling systems to dissipate heat and maintain temperatures within acceptable limits. There are two main types of cooling methods:
      • Oil Cooling: In oil-immersed transformers, the insulating oil itself plays a crucial role in dissipating heat. Cooling is enhanced by natural convection or forced circulation of the oil through cooling radiators or heat exchangers.
      • Air Cooling: Dry-type transformers use air as the cooling medium. Air circulates around the windings to remove heat. This method is common in smaller transformers or locations where the use of oil is impractical.
  2. Temperature Sensors:

    • Transformers are equipped with temperature sensors, such as thermocouples or resistance temperature detectors (RTDs), strategically placed in the windings and oil. These sensors continuously monitor the temperature rise. The data collected is used for control and protection purposes.
  3. Cooling Fans:

    • Some transformers, especially those with forced oil or air cooling, may have cooling fans that assist in heat dissipation. These fans are activated when the temperature exceeds a certain threshold, helping to enhance the cooling process.
  4. On-Load Tap Changers (OLTC):

    • Transformers may be equipped with on-load tap changers that allow the adjustment of the transformer turns ratio. This feature is used to regulate the voltage and, indirectly, the load on the transformer. By adjusting the tap settings, the current and, consequently, the heat generated can be controlled.
  5. Buchholz Relay:

    • In oil-immersed transformers, a Buchholz relay is often installed. This relay detects internal faults in the transformer by monitoring gas and oil flow within the tank. Severe faults can generate excessive heat, and the Buchholz relay can initiate an alarm or trip the transformer to prevent further damage.
  6. Thermal Overload Protection:

    • Transformers are equipped with thermal overload protection devices that can disconnect the transformer from the power source if the temperature rises above a critical level. This protection is essential for preventing thermal damage to the insulation and other components.
  7. Monitoring and Control Systems:

    • Modern transformers often have sophisticated monitoring and control systems that can provide real-time data on temperature, load, and other operating conditions. Operators can use this information to make informed decisions about the transformer’s operation and maintenance.

These mechanisms work together to ensure that the transformer operates within its designed temperature limits, preventing overheating and prolonging the transformer’s life. Regular maintenance and inspections are also essential to identify and address any issues that may affect temperature control.

I have written a more detailed article about transformer cooling methods, you can check it.

the temperature limits for transformers

The temperature limits for transformers are specified to ensure safe and reliable operation under normal conditions. Different components of a transformer, such as the winding and insulating oil, have their own temperature limits. Here are common temperature limits for transformers:

  1. Top Oil Temperature:

    • The top oil temperature is a key parameter in oil-immersed transformers. The temperature rise of the insulating oil is monitored to prevent excessive heating. The typical temperature limit for the top oil is around 65°C (149°F) to 90°C (194°F) above the ambient temperature.
  2. Winding Temperature:

    • The winding temperature is a critical factor, as the insulation materials in the windings have temperature limits. The winding temperature rise is typically limited to 55°C (131°F) to 65°C (149°F) above the ambient temperature.
  3. Total Temperature Rise:

    • The total temperature rise is the sum of the ambient temperature and the temperature rise due to the transformer’s operation. The total temperature rise limits vary, but it is commonly set to 65°C (149°F) for oil-immersed transformers and 80°C (176°F) for dry-type transformers.

It’s important to note that these values are general guidelines, and specific transformers may have different temperature limits based on their design, insulation materials, and intended applications.

Manufacturers provide detailed technical specifications for each transformer model, including the recommended temperature limits.

Additionally, industry standards, such as those set by organizations like the International Electrotechnical Commission (IEC) or the Institute of Electrical and Electronics Engineers (IEEE), may provide guidelines for acceptable temperature limits.

What happens if a transformer gets too hot?

If a transformer gets too hot and exceeds its designed temperature limits, several detrimental effects can occur, potentially leading to transformer failure and damage. Here are some of the consequences of overheating in a transformer:

  1. Insulation Degradation:

    • The materials used for insulation in transformers, such as paper, pressboard, and other dielectric materials, have temperature limits. Excessive heat can accelerate the aging and degradation of these materials, compromising the transformer’s insulation integrity. This can result in reduced dielectric strength and an increased risk of electrical breakdown.
  2. Reduced Transformer Life:

    • Continuous exposure to high temperatures accelerates the aging process of the transformer’s components. Over time, this can lead to a significant reduction in the transformer’s expected lifespan. Transformers are expensive assets, and premature failure can result in significant financial losses.
  3. Oil Breakdown:

    • In oil-immersed transformers, the insulating oil plays a critical role in dissipating heat. High temperatures can cause the breakdown of the oil, resulting in the formation of sludge and other by-products. Degraded oil can lose its insulating properties and may no longer effectively cool the transformer.
  4. Increased Risk of Faults:

    • Overheating increases the risk of internal faults within the transformer. Insulation breakdown, the generation of gases, and other issues can lead to short circuits and other faults. These faults can cause localized heating, further exacerbating the problem.
  5. Buchholz Relay Activation:

    • The Buchholz relay, which is a protective device installed in oil-immersed transformers, can be triggered by excessive heat. If the temperature rise is due to a fault inside the transformer, the Buchholz relay may initiate an alarm or trip the transformer to prevent catastrophic failure.
  6. Loss of Efficiency:

    • High temperatures can lead to increased losses in the transformer’s core and windings. Loss of efficiency can result in higher energy consumption and reduced overall performance.
  7. Fire Hazard:

    • In extreme cases, overheating can lead to a fire in the transformer. This poses a significant safety risk and can result in damage to surrounding equipment and facilities.

To prevent these issues, transformers are designed with temperature limits and monitoring and protection systems are put in place.

Temperature sensors, cooling systems, and protective relays work together to maintain the transformer within its specified temperature range.

Regular maintenance, inspections, and adherence to recommended loading practices are crucial to ensuring the reliable and safe operation of transformers.

If a transformer shows signs of overheating or experiences a fault, prompt action is necessary to investigate and address the root cause to prevent further damage.

transformer oil temperature range

transformer oil temperature rise

The temperature range for transformer oil is an important factor in ensuring the proper functioning and longevity of a transformer.

Transformer oil, also known as insulating oil or mineral oil, serves several purposes, including providing insulation and dissipating heat generated during the operation of the transformer.

The recommended temperature range for transformer oil typically falls within the following limits:

  1. Normal Operating Temperature:

    • The normal operating temperature for transformer oil is usually around 40°C to 90°C (104°F to 194°F). Transformers are designed to operate efficiently within this temperature range.
  2. Maximum Allowable Temperature:

    • Transformers are usually designed to withstand short-term temperature spikes beyond the normal operating range. The maximum allowable temperature can vary, but it is often around 105°C to 110°C (221°F to 230°F). Beyond this temperature, the insulation and other components may start to degrade.
  3. Alarm and Trip Limits:

    • Transformers are equipped with monitoring systems that include temperature sensors. Alarms are typically set to trigger if the oil temperature exceeds a certain threshold, often around 70°C to 95°C (158°F to 203°F), depending on the specific transformer design. If the temperature continues to rise, a trip mechanism may shut down the transformer to prevent damage.

It’s essential to note that exceeding recommended temperature limits can lead to accelerated aging of the insulation and other components, reducing the transformer’s overall lifespan. Regular monitoring and maintenance are crucial to ensure that transformers operate within the specified temperature range and to address any issues promptly.

Keep in mind that specific transformer designs and manufacturers may have slightly different temperature specifications, so it’s important to consult the transformer’s technical documentation for precise information.

I’ve written a detailed article about How temperature rise affects electrical equipment, Read it for more information.

Temperature rise test of transformer

The temperature rise test is a critical procedure conducted during the manufacturing and testing of transformers. This test is performed to ensure that the transformer operates within specified temperature limits under normal operating conditions. The test involves measuring the temperature rise of various components of the transformer, such as the winding and oil, when the transformer is subjected to its rated load.

Here is an overview of how the temperature rise test for transformers is typically conducted:

  1. Setup:

    • The transformer is placed in a test bay, and all necessary connections are made. This includes connecting the transformer to a power source and loading equipment.
  2. Instrumentation:

    • Temperature sensors, such as thermocouples or resistance temperature detectors (RTDs), are strategically placed at critical locations within the transformer. These locations often include the winding, the top oil, and other key components.
  3. Initial Measurements:

    • The transformer is initially energized at no load, and the baseline temperatures are recorded. This establishes a reference point for later comparisons.
  4. Loading:

    • The transformer is loaded to its rated capacity or a specified load level. The load is typically applied gradually to allow the temperatures to stabilize at each loading step.
  5. Steady-State Measurements:

    • Once the transformer reaches a steady-state condition under the specified load, temperature measurements are taken at the various sensor locations. These measurements provide information on the temperature rise of the transformer components.
  6. Duration of Test:

    • The transformer is maintained at the rated load for a specified duration to ensure that temperature conditions stabilize. This duration can vary but is often several hours.
  7. Temperature Rise Calculation:

    • The temperature rise is calculated by subtracting the initial (no-load) temperatures from the temperatures measured at the steady-state condition. This provides the rise in temperature due to the applied load.
  8. Comparison with Standards:

    • The calculated temperature rise is compared with the specified limits outlined in industry standards and the transformer’s design specifications. These standards often include limits for the top oil temperature rise and the winding temperature rise.
  9. Acceptance Criteria:

    • The transformer must meet or exceed the specified acceptance criteria for temperature rise to pass the test. If the temperature rise exceeds the allowed limits, further investigation and adjustments may be necessary.

The temperature rise test is crucial for ensuring that the transformer can handle its rated load without overheating and that it complies with industry standards and design specifications. It helps verify the transformer’s thermal performance and reliability under real-world operating conditions.

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