Transformer Auxiliary Protection Devices: Part 2: Thermal protection (26Q, 49W), Oil Temperature Indicator (OTI) & Winding Temperature Indicator (WTI)

As I mentioned earlier in the previous articles, power #transformers are critical components in the power grid, and their proper operation and maintenance are essential for ensuring the reliability and availability of electrical power. Transformers are subjected to various stresses and external factors that can cause them to malfunction or fail, resulting in power outages and other problems.

One of the most common causes of transformer failure is thermal overload, which occurs when the internal temperature of the transformer rises above safe levels. This can be caused by various factors, such as overloading, short circuits, insulation failure, or ambient temperature conditions. Thermal overload can lead to insulation breakdown, oil degradation, and other forms of damage, which can ultimately result in transformer failure.

The ANSI 26 function is designed to monitor the temperature of the transformer oil (26Q), such as the oil temperature (top oil or bottom oil). This is important because the oil in a transformer is used to cool the windings and other internal components, and if the oil temperature rises too high, it can lead to damage or failure of the transformer. The ANSI 26 function is commonly used to protect against #overloading, #overvoltage, and other abnormal operating conditions.

The ANSI 49 function, on the other hand, is designed to monitor the temperature of the current-carrying element of the transformer, which is typically the winding hot-spot temperature (49W). The hot-spot temperature is the highest temperature within the transformer winding, and it can be used to estimate the overall temperature of the winding. The ANSI 49 function is commonly used to protect against sustained overloading, #insulation_failure, and other abnormal operating conditions.

What is the reason for the increase in temperature inside the transformer (winding temperature or oil temperature)?

There are several factors that can cause temperature problems in power transformers, which can lead to thermal overload and ultimately #transformer failure. Here are some common causes:

-         High ambient temperature: The temperature of the air surrounding the transformer can affect its internal temperature. When the ambient temperature is high, the transformer may have difficulty dissipating heat, which can cause its internal temperature to rise.

-         Cooling system failure: Transformers are designed to dissipate heat through their cooling systems, which typically involve the circulation of oil or air. If the cooling system fails or is not functioning properly, the transformer may not be able to dissipate heat effectively, leading to temperature problems.

-         Overloading: Overloading occurs when the transformer is subjected to more current than it is designed to handle. This can cause the internal temperature of the transformer to rise rapidly, leading to thermal overload.

-         Slow clearing of faults: When a fault occurs in the power system, such as a short circuit or ground fault, it can cause a surge of current that can overload the transformer. If the fault is not cleared quickly, the transformer may be subjected to excessive current for an extended period, leading to temperature problems.

-         Abnormal system conditions: Abnormal system conditions, such as low frequency, high voltage, and harmonics, can also cause temperature problems in transformers. These conditions can cause the transformer to operate outside of its normal operating range, leading to thermal overload.

-         Aging or degradation of insulation materials: Over time, the insulation materials used in transformers can degrade or break down, which can reduce their ability to withstand high.

In addition to the above, here are some additional internal and external factors that can affect the temperature inside a transformer:

- Airflow restrictions: If the airflow around the transformer is restricted, such as by obstructions or debris

- Poor design: A transformer that is poorly designed may have inadequate cooling systems or insulation materials that are not well-suited for the expected operating conditions

- Manufacturing defects: A transformer that has manufacturing defects, such as poor connections or improper winding placement,

- Voltage fluctuations: Voltage fluctuations, such as those caused by voltage surges or sags,

- Harmonics: Harmonics are frequencies that are multiples of the fundamental frequency of the power system.

What are the effects of temperature increase inside the transformer?

-         Reduced lifespan: Prolonged exposure to high temperatures can cause the internal components of the transformer, such as insulation and windings, to degrade more quickly, shortening the transformer’s overall lifespan. The aging of the insulation at the hottest point in winding typically determines the life span of a power transformer; an oil temperature just 8°C above nominal could reduce the insulation life by up to 50 percent, which can significantly impact the transformer's life expectancy. Thus, even small temperature increases can have a significant impact on the transformer's lifespan.

-         Reduced efficiency: When a transformer operates at high temperatures, it can become less efficient at converting electrical energy from one voltage level to another. This can result in increased energy losses and reduced overall efficiency of the power system.

-         Reduced capacity: When a transformer is subjected to high temperatures, it may be unable to handle its designed capacity, resulting in reduced power output and potential overloading of other components in the power system.

-         Gas formation in transformer oil: High temperatures can cause the oil in the transformer to break down and generate gasses. These gasses can lead to the formation of bubbles or voids in the oil, reducing the effectiveness of the transformer's cooling system and exacerbating the temperature problem.

-         Insulation failure: In extreme cases, such as when the transformer is subjected to a sudden, high-current fault, the internal temperature of the transformer can rise rapidly, causing immediate insulation failure. This can result in catastrophic transformer failure and potential safety hazards.

-         Oil degradation: High temperatures can also cause the oil in the transformer to degrade, reducing its insulating properties and potentially leading to further temperature problems.

-         Increased maintenance costs: When a transformer operates at high temperatures, it may require more frequent maintenance and repair to keep it operating properly. This can result in increased maintenance costs and downtime for the power system.

Multiple Set Point Selection for Thermal Protection in Power transformers:

Both the ANSI 26Q and 49W devices are equipped with multiple set points or stages, which are used to provide a graduated response to temperature increases. The specific set points and stages may vary depending on the manufacturer and the application, but a typical set of stages is:

Stage 1: Cooling initiation - The first stage is typically set to initiate the cooling system of the transformer or equipment to try and bring the temperature back down to a safe level. This stage can be set to activate at a predetermined temperature threshold, such as 10 degrees Celsius above the nominal temperature rating.

Stage 2: Increased cooling - If the temperature continues to rise beyond the first set point, the second stage can be activated, which turns on additional cooling capacity, such as a secondary cooling pump. This stage can be set to activate at a higher temperature threshold, such as 20 degrees Celsius above the nominal temperature rating.

Stage 3: Alarm initiation - If the temperature continues to rise beyond the second set point, an alarm signal is triggered to alert operators to the potential problem. This stage can be set to activate at a higher temperature threshold, such as 30 degrees Celsius above the nominal temperature rating.

Stage 4: Equipment trip (optional) - If the temperature continues to rise beyond the third set point, the final stage can be set to trip the equipment, disconnecting it from the power system. This stage is optional and is typically only used in cases where the temperature has risen to a critical level and poses a safety hazard. This stage can be set to activate at the highest temperature threshold, such as 40 degrees Celsius above the nominal temperature rating.

The use of multiple set points or stages allows for a controlled and gradual response to temperature increases, which can help prevent unnecessary equipment trips and reduce wear and tear on the equipment's cooling systems. This is because initiating cooling or alarm signals at lower temperature thresholds can help prevent temperature problems from becoming severe and causing damage to the equipment or the power system.