In - depth analysis and comparison of transformer temperature rise measurement methods
In the operating systems of various electrical equipment, the transformer is undoubtedly a crucial safety component. It is like the "heart" of the equipment, and its stable operation plays a decisive role in the safety and reliability of the entire system. When the equipment is in normal operation or there is a local fault, the temperature rise of the transformer becomes a key monitoring indicator. Once the temperature rise of the transformer is too high, exceeding the temperature limits that its material parts, such as the skeleton, coil, and paint layer, can withstand, it is very likely to cause the insulation performance of the transformer to fail. The consequences caused by insulation failure are unimaginable. In the mildest cases, it may cause an electric shock hazard, posing a threat to the life safety of operators; in the most severe cases, it may cause a fire hazard, bringing catastrophic losses to the equipment and even the entire site. Therefore, it is particularly necessary to accurately measure the temperature rise of the transformer in the equipment.
Currently, when measuring the temperature rise of a transformer, two main methods are mainly used, namely the thermocouple method and the resistance method. Next, we will elaborate on these two methods in detail.
Thermocouple method
The thermocouple method is a relatively commonly used method for measuring the temperature rise of transformers. Currently, the DR030 digital temperature polling detector can be used to complete this measurement work. During the test, the thermocouple wire needs to be accurately fixed at the measured part of the transformer. There are two specific fixing methods. One is to use adhesive tape for pasting. This method is relatively simple to operate, but there may be a problem of insecure pasting. The other is to use a coating (alumina + solvent) for pasting. This method can make the thermocouple wire fit more closely to the measured part and improve the measurement accuracy. After pasting the thermocouple wire, apply a load to the tested transformer and turn on the power. At this time, you need to wait patiently until the transformer reaches a thermal steady - state or wait for 4 hours before measuring its temperature rise. This is because during this period, the temperature change of the transformer gradually tends to be stable, and the measured data can more accurately reflect its actual temperature rise situation.
Resistance method
The measurement process of the resistance method is relatively more complicated. First, before applying a load to the transformer and turning on the power, a professional measuring instrument needs to be used to accurately measure the cold-state resistance R1 of the transformer. This step is crucial because the cold-state resistance is the basic data for subsequent temperature rise calculations. Then, apply a load to the transformer and turn on the power. Similarly, wait for 4 hours or until the transformer reaches a thermal steady state, and then quickly turn off the power. The word "quickly" here is very critical because once the power is turned off, the temperature of the transformer will gradually decrease. In order to measure the hot-state resistance as accurately as possible, the measurement must be carried out immediately after turning off the power to obtain the hot-state resistance R2 of each winding of the transformer. Finally, the temperature rise of the transformer is calculated using the following formula:
Δt = (R2 - R1) / R1 × (234.5 + t1) - (t2 - t1)
Among them, R1 represents the resistance value at the start of the test (unit: Ω), R2 represents the resistance value at the end of the test (unit: Ω), t1 represents the room temperature at the start of the test (unit: ℃), and t2 represents the room temperature at the end of the test (unit: ℃).
From the above two testing methods, we can clearly find that their measurement focuses are different. The thermocouple method measures the temperature rise of the outer layer of the transformer winding, while the resistance method measures the average temperature rise of the transformer winding. In the relevant national standards, GB4943 stipulates that the thermocouple method is allowed to measure the temperature rise of the transformer winding, but the measured result needs to be increased by 10℃; GB8898 requires using the resistance method to measure the temperature rise of the transformer winding. In order to deeply understand the differences between these two methods and explore whether measuring the primary winding or the secondary winding of the transformer can more accurately reflect the actual situation of the transformer temperature rise when measuring the transformer temperature rise, we conducted temperature rise tests on two power transformers with different structures, namely the transformer with a king-shaped skeleton and the transformer with a drawer-type skeleton, and made a detailed comparison based on the measurement results.
Test results and analysis
Skeleton typeSample numberResistance method (primary)Resistance method (secondary)Thermocouple method (primary)Thermocouple method (secondary)Remarks
King-shaped skeleton1#Before the test: 2.02 kΩ, after the test: 2.17 kΩ, temperature rise: 17.8℃Before the test: 6.40 Ω, after the test: 6.74 Ω, temperature rise: 13.1℃16.5℃14.4℃The temperatures of Sample 1# before and after the test are t1 = 16.8℃ and t2 = 18.1℃ respectively; the test duration is 2h
King-shaped skeletonNo. 22.12 kΩ before the test, 2.27 kΩ after the test, temperature rise of 16.9℃6.51 Ω before the test, 6.88 Ω after the test, temperature rise of 14.1℃17.6℃15.1℃The temperatures of Sample No. 2 before and after the test are t1 = 18.1℃ and t2 = 18.4℃ respectively; the test duration is 2 hours
King-shaped skeleton3#Before the test: 2.07 kΩ, after the test: 2.21 kΩ, temperature rise: 18.0℃Before the test: 6.53 Ω, after the test: 6.88 Ω, temperature rise: 14.0℃16.9℃14.0℃The temperatures of sample 3# before and after the test are t1 = 19.6℃ and t2 = 19.3℃ respectively; the test duration is 2h
Drawer-type skeletonNo. 1Before the test: 0.52 kΩ, after the test: 0.56 kΩ, temperature rise: 17.6℃Before the test: 2.30 Ω, after the test: 2.41 Ω, temperature rise: 12.3℃11.9℃13.0℃The temperatures of Sample No. 1 before and after the test are t1 = 17.6℃ and t2 = 18.4℃ respectively; the test duration is 2 hours
Drawer-type skeletonNo. 2Before the test: 0.52 kΩ, after the test: 0.56 kΩ, temperature rise: 18.7℃Before the test: 2.28 Ω, after the test: 2.40 Ω, temperature rise: 12.9℃12.7℃12.6℃For the No. 2 sample, the temperatures before and after the test are t1 = 18.4℃ and t2 = 19.1℃ respectively; the test time is 2h
Drawer-type skeletonNo. 3Before the test: 0.52 kΩ, after the test: 0.57 kΩ, temperature rise: 20.1℃Before the test: 2.28 Ω, after the test: 2.42 Ω, temperature rise: 14.9℃14.2℃14.1℃The temperatures of the No. 3 sample before and after the test are t1 = 19.5℃ and t2 = 19.2℃ respectively; the test duration is 2 hours
Through in - depth analysis of the measurement results, we can draw the following conclusions:
1. Difference between primary and secondary temperature rise: Whether it is a transformer with a king-shaped skeleton or a transformer with a drawer-shaped skeleton, their primary temperature rise is higher than the secondary temperature rise. Among them, the temperature rise difference between the primary and secondary of the transformer with a king-shaped skeleton is 2 - 4℃, and that of the transformer with a drawer-shaped skeleton is about 5℃. This is because the temperature rise of the transformer coil is mainly caused by the copper loss Pm of the transformer coil. When the transformer is working, part of the heat generated by the copper loss is absorbed by the coil itself, and the other part is dissipated to the surrounding medium through radiation, convection, and conduction on the surface of the transformer, and finally reaches a thermal equilibrium state. In this process, since the copper loss Pm generated by the primary coil of the transformer is relatively large, more heat is generated, which in turn causes its temperature rise to be higher than that of the secondary. Of course, this result is not absolute. There may be differences in the temperature rise generated by transformers with different structures or when transformers operate in different equipment.
2. Comparison of measurement results of transformers with a king-shaped skeleton: For transformers with a king-shaped skeleton, the temperature rises of the transformers measured by the thermocouple method and the resistance method are very close. This may be due to the structural characteristics of the king-shaped skeleton, which make the temperature distribution of the outer layer of the coil and the whole coil relatively uniform. Therefore, the difference between the results obtained by the two measurement methods is small.
3. Comparison of measurement results of transformers with drawer-type skeletons: For transformers with drawer-type skeletons, the primary temperature rise of the transformer measured by the resistance method is nearly 6°C higher than that measured by the thermocouple method, while the secondary temperature rise results of the transformer measured by the two methods are very close. This indicates that in transformers with drawer-type skeletons, there may be a large non-uniformity in the temperature distribution of the primary coil. The resistance method can more accurately reflect the average temperature rise of the primary coil, while the outer-layer temperature rise measured by the thermocouple method is relatively low.
Based on the above analysis, the temperature rise of the transformer winding measured by the resistance method is generally slightly higher than that measured by the thermocouple method. This indicates that the temperature rise value of the transformer winding measured by the resistance method is closer to the actual temperature rise of the transformer. The degree of difference between the measurement results of the two methods is closely related to factors such as the materials used in the transformer, the structure of the transformer, the location of measurement points (for the thermocouple method) when measuring the temperature rise, and the measurement speed.
Measurement suggestions
Based on the above research and analysis, when actually measuring the temperature rise of a transformer, we give the following suggestions:
1. Selection of measurement method: Use the resistance method to measure the temperature rise of the transformer as much as possible, because the resistance method can more accurately reflect the average temperature rise of the transformer winding and is closer to the actual temperature rise of the transformer.
2. Selection of the coil: When measuring the temperature rise of the transformer coil using the resistance method, try to select the primary coil. Since the copper loss of the primary coil is relatively large and the temperature rise is relatively high, measuring the temperature rise of the primary coil can detect the abnormal conditions of the transformer in a more timely manner.
3. Key points for measurement by thermocouple method: When measuring the temperature rise of the transformer winding package by the thermocouple method, the thermocouple tip should be placed between the outer layer of the pressure-sensitive tape and the outer winding of the transformer as much as possible. In this way, the temperature of the outer layer of the winding package can be measured more accurately, and the measurement error can be minimized.
4. Resistance method measurement skills: When measuring the temperature rise of the transformer winding package using the resistance method, it is recommended to add an on - off switch between the lead pins of the transformer winding and the selected measuring instrument. In this way, after the power is cut off, the DC copper resistance value of the transformer winding package can be measured in the shortest possible time, thereby improving the measurement accuracy.
Through in-depth research and comparison of the temperature rise measurement methods for transformers, we can better select appropriate measurement methods and measurement locations, accurately grasp the temperature rise situation of transformers, and provide strong guarantee for the safe operation of the equipment.