Research on the multi-parameter-based detection scheme for current transformers

Oil-immersed current transformers are commonly used as a type of electrical equipment, but they may experience some faults and defects during actual operation. When detecting these faults and defects, the usual detection object is temperature, which is a single detection quantity and easily leads to misjudgment. This article introduces typical defects in oi-limmersed current transformers, conducts simulation analysis, studies the temperature field changes of current transformers when three typical defects occur, and studies the relationship between temperature and pressure to determine the feasibility of using temperature and pressure as the detection object, to improve the accuracy of detection. The results show that by using temperature and pressure as measurement values of a new detection method, the problem of single measurement parameters that cannot determine the internal condition of the equipment can be solved. By monitoring changes in temperature and pressure in real-time, misjudgment due to external factors can be avoided based on the relationship between temperature and pressure.


Introduction
Oil-immersed current transformers are a type of electrical equipment that uses transformer oil as a heat dissipation medium.Currently, oil-immersed current transformers are widely used in high-voltage and ultra-high-voltage power systems.However, in the actual operation of the power grid, oil-immersed current transformers often experience faults [1] that can affect the normal operation of the grid.These faults can often lead to equipment damage and even cause explosions or fires in severe cases, posing a serious threat to personal safety [2][3].
To address the faults that frequently occur in current transformers, various fault detection methods and case studies have been developed domestically and internationally, with the majority focusing on temperature or other related quantities that change with temperature.However, these detection methods have a single detection quantity and are susceptible to external factors when judging the operating status of the transformer, which can lead to misjudgment of internal defects in oil-immersed current transformers.
This article focuses on studying the temperature field conditions of different defects in oil-medium transformers, as well as the relationship between three different types of defects with different severity levels and internal temperature and pressure.The feasibility of simultaneously using temperature and pressure as detection targets is determined, which can lead to a new online monitoring scheme.

Methods to solve a typical problem
Some common faults in oil-immersed current transformers include abnormal dielectric loss, damage to the main insulation, and excessive dissolved gases in the oil.In response to the current problem of inaccurate parameter estimation of the internal condition of oil-immersed current transformers, this article considers establishing a multi-parameter online monitoring method.This article uses a simulation model to simulate the temperature fields of the three different types of defects under different defect levels, determine the relationship between the defect levels and internal temperatures for different defect types, and then calculate the pressure changes by using the temperature-pressure calculation formula in a closed space.The calculated values are compared with the field values to determine the accuracy of the proposed method.

Fault simulation and temperature field analysis
At an ambient temperature of 25℃, the three typical defects of oil-immersed current transformers were simulated.The distribution of the maximum temperature in the temperature field was obtained through finite element simulation under different defect levels [4].

Turn-to-turn short circuit.
We simulate the temperature field changes of the transformer during a secondary winding interturn short circuit, and the results are shown in Figure 1.The relationship between the applied power consumption and the maximum temperature that the body can reach is shown in Figure 2.

Moisture inside the transformer.
When the device is affected by moisture, the simulation results of its temperature field are shown in Figure 3.The relationship between the applied power consumption and the maximum temperature that the device can achieve is shown in Figure 4.

Relationship between different defect levels and temperature
When the temperature of electrical equipment exceeds 50℃, it is considered to have safety hazards.When the temperature is between 60℃ and 75℃, it is considered a general defect.When the temperature is between 75℃ and 90℃, it is considered a serious defect.If the temperature exceeds 90℃, it is considered a major defect.According to the simulation results, the relationship between the different levels of the three typical defects and temperature at the same time is shown in Table 1.

Pressure-temperature calculation for typical defects in oil-immersed current transformers
When a fault occurs inside a transformer, gas may be generated, which leads to an increase in pressure.During a fault, the speed of temperature diffusion is slow, and by measuring pressure changes, abnormalities can be detected more quickly.There is a certain functional relationship between the internal temperature and pressure of the transformer.Under normal conditions, the internal pressure and temperature of the equipment can reflect the status of the oil with the height variable ∆h of the expansion vessel.The relationship between the oil volume variable ∆ and the temperature variable ∆ is as follows: where G denotes the amount of oil, γ denotes the density of the oil, and α denotes the expansion coefficient of the oil.By knowing the height of the expansion vessel h, we can obtain real-time temperature inside the equipment, as well as the relationship between ∆, ∆ℎ, and P. Therefore, by observing the position of the pointer of the expansion vessel, we can obtain the temperature and pressure inside the equipment.
In this paper, we investigate the changes in various parameters of the equipment within a temperature range of -40℃ to +120℃.By using the calculation method for pressure and temperature in a sealed space, we calculate the pressure inside the equipment at different temperatures and compare them with actual values [6].The comparison chart is shown in Figure 7.As can be seen from the above comparison diagram, the percentage error between the calculated and actual values of pressure is within 0.25%, and the overall numerical deviation is within 1 Kpa, which meets the requirements.

Conclusion
This article simulates different typical defects and analyzes their temperature fields, determines the relationship between defects and temperature, and obtains the temperature variation rules of the equipment under different defect states.This provides data support for the setting of temperature sensors in new online detection schemes.The research content calculates the corresponding pressure based on the measured temperature value and compares the pressure value with the actual value to eliminate the interference of external factors.The new detection method designed by taking pressure and temperature as measurement data can solve the problems caused by insufficient data volume.

Figure 1 .
Figure 1.Simulation of the change of the temperature field of the short circuit between turns.

Figure 2 .
Figure 2. The maximum temperature change of the transformer when the turn-to-turn short circuit occurs.

Figure 3 .
Figure 3.The simulation of the change of the temperature field in the dampness inside the transformer.

Figure 4 .
Figure 4.The maximum temperature change of the transformer when the interior of the transformer is damp.3.1.3Partial discharge at the bottom of the transformer.When partial discharge occurs at the bottom of the transformer [5], the internal temperature field is simulated.The simulation results are shown in Figure The relationship between the applied power consumption and the maximum temperature that the transformer body can achieve is shown in Figure 6.

Figure 5 .Figure 6 .
Figure 5. Simulation of the change of the temperature field of the discharge at the bottom of the transformer.

Figure 7 .
Figure 7.Comparison chart of on-site measured pressure value and calculated value.

Table 1 .
Relationship between three typical defects of different degrees and temperature.