The oil temperature and winding temperature inside a transformer affect the performance and service life of insulating materials, thereby influencing the transformer's lifespan and even directly causing faults. Therefore, it is necessary to monitor the real-time internal temperature of the transformer. According to the temperature measurement principle, there are three types of thermometers: 1. Mercury thermometer; 2. Pressure thermometer; 3. Platinum resistance thermometer. The measured temperature can be used in the following ways: 1. Sent to the main body meter to display in real-time through the pointer; 2. Control the cooler and send alarm signals through the auxiliary contacts of the meter; 3. The analog quantity is sent to the measurement and control device to display the temperature on the monitoring background. Operation and maintenance personnel shall handle abnormal temperature conditions in accordance with the manufacturer's instruction manual and regulations. Due to the high voltage and strong magnetic field inside large transformers, the "pressure thermometer" (principle 2) is generally adopted. There are two types of temperature controllers for transformers: oil surface temperature controller and winding temperature controller, with differences in principle and structure in one aspect. Now, let's directly introduce the winding temperature controller.
First, let's get to know its appearance, as shown in the figure below.
1. The metal pipe fitting at the bottom is the base, and the temperature bulb is placed inside the base. The temperature bulb contains a temperature-sensing medium, which expands when heated. This part is inserted into the oil above the transformer to a depth of about 150mm.
2. The coiled spring hose at the back contains a capillary tube, which transmits the expanding medium to the meter. 
The expanding medium drives the elastic element, and the pointer is driven through the transmission mechanism. The white pointer on the upper dial displays the real-time temperature.
3. There is other information on the dial: a red pointer and four color-coded indicator blocks (red, blue, green, and yellow). a. When the temperature rises, the white pointer turns to the position of the red pointer and then continues to turn right, it will drive the red pointer to turn right together. When the temperature drops, the red pointer will not return with the white one, so the red pointer records the maximum position that the white pointer has reached. b. The four color-coded pointers correspond to four groups of normally open (or normally closed) auxiliary contacts, and the corresponding temperature value is the action value of the auxiliary contact.
The functions of the auxiliary contacts are selected by the user, roughly as follows: K1. Cooler return temperature; K2. Cooler start temperature; K3. High temperature alarm; K4. High temperature trip. Most new meters now have 6 groups of auxiliary contacts, with K5 in gray and K6 in purple, which can be used as needed or kept as spares.
However, the temperature measured above is still the oil temperature, which has nothing to do with the windings. By the way, the position of the windings is inaccessible to the thermometer, so a workaround is adopted, which is where the winding temperature controller differs from the oil surface temperature controller.
The above is just a schematic diagram of the principle. Let's not concern ourselves with how the windings of the transformer on the left are connected. Here, components 1, 4, and 5 are the special features. To put it simply, the principle is to convert the load current in the 1-4-5 circuit into copper temperature rise, and then superimpose it on the oil temperature to obtain the winding temperature.
1 is the secondary coil of the high-voltage side bushing CT. The output current is converted by the current converter (5) into a suitable current using a specific method (since the secondary currents vary among manufacturers due to differences in transformation ratios, etc., current conversion is necessary). The converted current passes through the electric heating element (4), and the heat generated drives an increase in the displacement of the elastic element, resulting in a higher indicated temperature, thus reflecting the winding temperature.
There is also a thermal simulation method here: the converted CT secondary current is used to heat the temperature bulb.
Look at the nameplate: the secondary is used for winding temperature measurement.
Earlier, we talked about the temperature controller. Since there is more to come, we explained the origin of the temperature pointer of the temperature controller at that time. The temperature bulb inserted into the transformer oil senses the oil temperature, and the elastic medium in the temperature bulb transmits the expansion to the pointer, thereby indicating the temperature. Two questions were left unresolved at that time: first, where exactly is heated by the CT current in the winding temperature measurement; second, how the digital temperature displayed on the monitoring background comes from. After carefully studying the instructions and consulting the meter manufacturer, we have obtained the answers. First, the CT current (the current converted by the current transformer) heats the temperature bulb in the transformer. As shown in this figure, the current output from 1 is converted through 5 and then input into 2 to heat the temperature bulb.
The figure below was confusing and hard to understand at first, but now it makes sense. The dashed box on the right should be an enlarged view of the temperature base on the left. The two boxes connected by the red line are actually the same component. This makes it easier to comprehend: inside the temperature base, there are a temperature bulb, an electric heating element (for the winding thermometer), and a PT100 resistor.
Therefore, the description in this paragraph of "Temperature Controller 1" is inaccurate. It cannot be said that there is another thermal simulation method. After researching several models, it turns out that all winding temperature controllers adopt the simulation method.
"1 is the secondary coil of the high-voltage side bushing CT. The output current is converted into a suitable current by the 5 current converter according to a certain method (since the secondary currents vary among different manufacturers due to differences in transformation ratios, etc., current conversion and 折算 are required). The converted current passes through the 4 electric heating element, and the generated heat drives the displacement of the elastic element to increase, so that the indicated temperature also increases, thus reflecting the winding temperature. There is also a thermal simulation method here: the converted CT secondary current is used to heat the temperature bulb."
The second question is: how does the temperature displayed on the background monitoring come about? Let's look at the main transformer temperature measurement circuit diagram.
The thermometer outputs two 4-20mA current signals. Here is a brief introduction to the Pt100 platinum resistor: "Pt100 is a platinum thermal resistor whose resistance changes with temperature. The '100' in Pt100 indicates that its resistance is 100 ohms at 0°C, and approximately 138.5 ohms at 100°C."
The resistance of all conductors changes with temperature, but the change in platinum resistors is stable and significant. The temperature measurement principle works by plotting this corresponding relationship as a curve, then checking the temperature corresponding to the measured resistance against the curve. This converts temperature into resistance, where resistance equals voltage divided by current. A computer processes the electrical signal to obtain the temperature from the Pt100.
Therefore, applying a DC voltage to the platinum resistor in the temperature bulb and outputting the current is how the temperature transmitter works, which is the part in the red frame of the figure below. (This is the basic principle; in practice, temperature transmitters also have circuits for zero adjustment and compensation for accuracy, which don't need to be understood in detail.)
It can also directly output Pt100 (the part in the blue frame). My understanding is that the temperature controller is responsible for leading out the two ends of the resistor (eliminating the need for a transmitter), and the measurement circuit applies voltage by itself for calculation.
As for why the Pt100 has three leads, I checked online and found the following explanation (this part is optional reading): A PT100 platinum resistance sensor has three leads, which can be represented by A, B, C (or black, red, yellow). The rules between the three leads are as follows: the resistance between A and B or A and C is about 110 ohms at room temperature, while the resistance between B and C is 0 ohms because B and C are directly connected internally. In principle, there is no difference between B and C. Generally, display instruments provide a three-wire connection method. One end of the PT100 has one lead, and the other end has two leads, all connected to the instrument. The instrument internally offsets the lead resistance through a bridge circuit.
One final regulation: The temperature displayed by the meter and that on the monitoring background shall not differ by more than 5K. The temperatures indicated by multiple thermometers on site, the temperature display devices in the control room, or the monitoring system should be basically consistent, with an error not exceeding 5K.
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