Temperature rise test and analysis of transformer with optical fiber temperature controller

1 Introduction

The temperature rise test is an important test to ensure the service life and safe operation of the transformer. The purpose of the temperature rise test is to verify whether the heat generated by the total loss of the main body and the temperature at which the heat dissipation device reaches thermal equilibrium under rated working conditions of the transformer meet the requirements of relevant standards and technical agreements, and to verify the rationality of the product structure and find out the degree of local overheating on the oil tank (or shell) and structural parts.

The hottest point temperature of transformer winding is the determining factor for the safe, economical operation and service life of transformer, because the aging of the hottest point insulation of winding due to overheating may develop into the damage of the whole transformer. Therefore, GB1094.2-1996 "Power Transformer Part 2 Temperature Rise" and IEC354 "Oil-immersed Power Transformer Load Guide" added the calculation of the hot spot temperature rise of winding on the basis of previous relevant standards. However, any theoretical calculation needs to be tested by experiment, and the reliable test is to actually measure the hot spot temperature rise in the experiment.

Zhejiang Electric Power Transformer Co., Ltd. uses optical fiber temperature controllers to measure hot spot temperature rise in transformer temperature rise tests. The test was conducted under the guidance of the National Transformer Quality Supervision and Monitoring Center, and the test process and results fully comply with relevant national standards.

2 Principle of optical fiber temperature measurement

There are three methods for determining the hot spot temperature of the winding: direct measurement, thermal simulation measurement, and indirect calculation. The direct measurement method is to bury sensors in the winding, and use optical fiber to transmit signals under high voltage and high magnetic field conditions to accurately measure the hot spot temperature of the winding online and in real time. It is the preferred method for hot spot measurement in future transformer temperature rise tests.

Take the ThermAsset2 of Lumasense Company in the United States as an example. The hot spot optical fiber temperature controller of the transformer winding is determined by measuring the intrinsic parameters (decay time) of the phosphor alone, which will not change due to physical changes in the optical fiber, so the system is a system that does not require calibration. The phosphor sensor is directly attached to the end of the optical fiber probe. The probe is made of materials that have been verified to be compatible with oil-immersed high-voltage power transformers for a long time and have excellent electrical properties. Since the 1980s, this technology has been used in power transformers (for many years, it has been widely accepted as an industrial standard by major transformer manufacturers around the world). During the temperature rise test, only the optical fiber probe needs to be installed (using the existing tools in the factory), and the data measured by the optical fiber probe can know the actual temperature of the hottest point of the winding, and the operation is simple. The system has 4 to 8 measurement channels with independent output and display. The temperature sensor used is made of a stable high-temperature resistant fluorescent material and is directly placed at one end of the optical fiber. The principle is that when the light pulse emitted by the LED light source is sent to the temperature sensor in contact with the winding through the optical fiber, the pulse excites the fluorescent material of the sensor to produce fluorescence with a longer wavelength. The temperature of the sensor is measured according to the decay time of the returned fluorescence, and then the temperature value and related system parameters are displayed through processing, and the temperature information is transmitted to the control room at the same time. The system can provide accurate winding temperature when the transformer is at peak load and emergency over-nameplate capacity operation, and can enable the transformer to adjust the load in time according to the measured actual winding temperature.

3 Test methods and procedures

GB1094.2-1996 stipulates that the short-circuit method is the standard method for temperature rise test of oil-immersed power transformers.

3.1 Prediction of winding hot spot temperature and location

Before the test, the location and temperature of the hot spot are determined using special calculation software based on the relevant parameters of the winding, including winding method, wire type and parameters, winding size and oil channel size. This article uses transformer temperature field calculation software, which can calculate and analyze the temperature field of pancake windings and cylinder windings. The calculation results of the software have been actually tested on various types of transformers and have high accuracy.

3.2 Installation of fiber optic temperature controller

3.2.1 Sensor Installation

(1) Installation in pancake winding. Insert the optical fiber temperature probe into the pad between adjacent coils. The installation and insulation treatment of the probe in the pad are shown in Figures 1 and 2. After the pad is processed, put the pad into the probe position, as shown in Figure 3.

(2) Installation in cylindrical windings. Since there are no horizontal braces in cylindrical windings, the probe is installed on the vertical braces in order not to damage the insulation structure of the winding. During installation, a slot is made on the vertical brace, and the slot height is the pre-calculated hot spot height. The slot is made at the corresponding position of the brace, and the probe should be able to contact the winding, as shown in Figure 4.

3.2.2 Installation of lead probe (optional)

According to GB1094.3-2003, the temperature rise measurement of the casing is a monitoring measurement, and it is only necessary to prevent overheating. Infrared temperature measurement can be used in the test. However, in order to achieve automatic temperature alarm and obtain continuous temperature data of the casing, optical fiber temperature measurement is used in this test. The installation of the optical fiber probe on the lead is shown in Figure 5.

3.2.3 Installing the tank wall joint plate

The tank wall has four tapered threads for installing the through-hole. After drilling a hole in the tank, the connecting plate can be welded to the tank wall or installed on the tank wall with screws.

3.2.4 Installing the through-hole and internal and external optical fibers

The through-hole is used for optical connection of internal and external optical fibers. The internal and external optical fibers are connected by ST connectors. Connect the SMA connector of the internal optical fiber with a locking nut to the through-hole, as shown in Figure 6. One end of the external optical fiber is connected to the through-hole, and the other end is connected to the optical fiber temperature measurement system body, as shown in Figure 7.

3.3 Experimental process

The short-circuit temperature rise test is carried out in two stages as specified by national standards.

3.3.1 Total loss application stage

The temperature rise stability condition is in accordance with GB1094.2-1996. It should be noted here that the temperature rise of the hot spot cannot be used as the stability standard, because the stability of the top oil temperature rise lags behind the hot spot for a period of time. When the hot spot temperature rise is stable, the top oil temperature rise may not have reached stability.

3.3.2 Rated current application stage

The winding resistance in this stage is measured according to the method required by national standards. The thermal resistance can be measured to obtain the average temperature of the winding. After adding the fiber optic temperature controller, the instrument can automatically record the temperature of the hot spot (the interval time is user-defined). Based on the data, the hot spot temperature at the time of power failure can be found.

3.4 Temperature Measurement

(1) Measurement of ambient temperature: The ambient temperature shall be measured in accordance with the test specifications of national standards.

(2) Measurement of radiator inlet and outlet temperatures. Set up 2 to 3 temperature sensors at the inlet and outlet of the radiator and record the average value of their readings. Record the inlet and outlet temperatures of the radiator oil at regular intervals.

(3) Measurement of top oil temperature rise. The top oil temperature is measured using multiple thermometers inserted into the top of the oil tank and into the connecting pipes of the radiator.

(4) Measurement of the temperature rise of the hot spots in the windings. Use an optical fiber temperature control device to automatically measure the temperature rise of the hot spots in real time, and save a large amount of temperature data through a computer terminal.

(5) Local temperature measurement outside the oil tank. During the transformer temperature rise test, it is necessary to measure whether there is local overheating at the connection between the oil tank wall and the bushing. Usually, an infrared thermometer is used for detection.

4 Project Examples

4.1 Test results

The temperature rise test results with optical fiber temperature controllers meet national standards. The data obtained, especially the hot spot temperature value and position, are of guiding value for the safe and reliable operation of transformers. Table 1 is a comparison of the test results and calculation results of a main transformer.

4.2 Results Analysis

From the installation method of the fiber optic probe, we know that the probe is located outside the winding insulation, and the temperature measured by the probe is the temperature of the insulation layer close to the wire. According to the heat conduction mechanism of heat transfer, there is a temperature gradient between the surface of the copper wire and the outer surface of the insulation paper. Therefore, there is a difference between the measured temperature and the actual hot spot, and the measured value needs to be corrected. According to Fourier's law of thermal conduction:

After correction according to formula (3), the hot spot temperature values ​​measured by the optical fiber are also shown in Table 1.

Relative error:

Substituting the data into formula (4), the test results use the corrected values, and the calculation results of the relative error are shown in Table 2.

As can be seen from Table 2, the relative error of the high-voltage winding is very small, because there are only wire insulation and insulation gaskets between the probe and the copper wire. The relative error of the low-voltage winding is relatively large, because the probe of the low-voltage winding is installed in the support bar, which is not only separated from the hot spot by an insulation layer but also has a certain distance from the hot spot, so the relative error is relatively large.

5 Conclusion

(1) The temperature rise test with fiber optic temperature controller conducted by Zhejiang Electric Power Transformer Co., Ltd. on a 50MVA transformer was carried out. The test process and results were in compliance with relevant national standards.

(2) It is feasible to use the fiber optic probe to measure the hottest temperature of the winding during the temperature rise test. The experience gained will be of great significance for the use of fiber optic temperature measuring probes in future transformer field operation.

Online monitoring of winding hot spot temperature has reference value.

(3) The test results of average temperature rise and hot spot temperature are in good agreement with the predicted results, which verifies the reliability of the temperature field calculation software for predicting hot spots in this test. It also indirectly proves that the error of the hot spot position determined in this test is within a reasonable allowable range.