Calculation Model and Accuracy Study of Transformer Standard Oil Preparation

This article introduces a treatment method, component concentration calculation model, and device for preparing standard oil using transformer oil. The base oil is prepared by deeply purifying the transformer oil and measuring its background value. Based on the target component concentration, the appropriate volume of standard gas is determined, and the background value of the combined oil is used in the component concentration calculation model to determine the theoretical value of the standard oil. The standard oil value that has been prepared is in good agreement with the anticipated target value, thereby simplifying the purification process and cutting down on the processing cost of used transformer oil. Simultaneously, a fully automatic integrated device has been developed for the advanced purification of transformer oil and the production of standard oil. This device comprises a vacuum pump, an oil storage tank, an electromagnetic valve, an oil cylinder, and various sensors, all managed by sophisticated software. The standard oil for transformers can be prepared and stored stably for an extended period. The standard oil output is processed under sealed and unchanging pressure conditions to ensure a consistent concentration of components in the standard oil.


Introduction
Transformer oil is a mineral insulating oil consisting of numerous hydrocarbon molecules with varying molecular weights.When faults such as discharge or overheating occur within oil-filled electrical equipment, they can break certain C-H and C-C bonds, which can accompany the production of a small number of active hydrogen atoms and unstable carbon-hydrogen compound radicals.These hydrogen atoms or radicals rapidly recombine through complex chemical reactions to form hydrogen gas and low molecular weight hydrocarbon gases, including methane, ethane, ethylene, acetylene, and others, while potentially producing solid carbon particles.The oxidation of oil and insulating paper produces a minor amount of CO 2 and CO [1,2] .These characteristic gases get dissolved in the transformer oil, and their concentration is closely linked to the type and extent of the equipment's defects.By accurately detecting the gas content dissolved in these oil types, it becomes feasible to identify defects in oil-filled electrical equipment.Transformer standard oil is a standard sample that contains accurate characteristic gas content, which is used to simulate transformer oil after various types of faults occur during the operation of transformers.A substantial quantity of standard transformer oil is employed in calibrating online chromatographs and training professional technical skills [3] .The used transformer oil cannot be directly utilized in preparing standard transformer oil because it contains impurities such as hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide, and oxygen.This article presents a rapid and efficient purification technique capable of highly purifying used transformer oil and accurately preparing transformer oil concentrations, facilitating transformer oil recycling [4] .

The principle of gas dissolution in oil
Based on the principle of headspace chromatography, a closed system under constant temperature and pressure conditions causes equilibrium between the gas and liquid phases in the oil sample, resulting in the dissolution of gases.The concentration of dissolved gases in the oil sample can be calculated by measuring the concentration of each component gas in the gas phase [5] .The calculation is performed based on the following Formula (1).
In the formula: X i -The concentration of component i in the oil sample, µL/L.C ig -Balance condition, the concentration of dissolved gas component i in the gas, µL/L.K i -At the test temperature, the distribution coefficient of dissolved gas component i after gasliquid equilibrium.
V g -Gas volume under equilibrium condition, mL.V l -Liquid volume under equilibrium condition, mL.

Preparation method of standard oil
To prepare the blank oil, the collected waste transformer oil is first filtered, purged with nitrogen, and adjusted to the saturation level of nitrogen dissolution in the transformer oil.Furthermore, the chromatograph identifies the remaining quantity of dissolved components in the refined blank oil and incorporates this information into the computation of theoretical values.The theoretical values of the components that are prepared for subsequent standard oil have been corrected.After considering the amount of oil in the container and the theoretical values required for preparation, the volume of component gases to be added is precisely calculated [6] .These gases are precisely quantified and evenly dissolved in the oil during circulation.The theoretical concentration of a single component in the standard oil is calculated as the following Formula (2).
In the formula: cs-Concentration of standard gas, µL/L.C t -Target concentration of standard oil, µL/L.C b -Background concentration of this component in purified oil, µL/L.K i -Dissolution distribution coefficient of oil gas in this component.V o -Oil volume, mL.V g -Volume of added standard gas, mL.Finally, we use a chromatograph to detect the concentration of the prepared standard oil components.Standard oil is stored and used in a sealed state.

Structure of the device
The impurities in used transformer oil mainly include solid particles and dissolved gases.Solid particles are removed using small aperture metal filters.Dissolved gases are removed by nitrogen blowing and vacuum pumping [7] .The structure of the device is shown in Figure 1.The standard oil preparation process is shown in

Device workflow
3.2.1 Oil filtering process.We turn on the vacuum pump and solenoid valve to thoroughly evacuate the system.We suction the oil from the storage tank and introduce it into the oil cylinder, followed by the addition of sufficient nitrogen to achieve saturation.To begin the process, we activate the circulating pump, which causes the oil to flow from the oil cylinder, pass through the atomizer and degassing chamber, and repeat this cycle until the designated duration is reached.Any excess nitrogen and impurities are removed during this time, resulting in a highly purified transformer oil.To detect the background value of this oil, we utilize a gas chromatograph.

Preparation process of standard oil.
To heat the oil to 50℃, we activate the solenoid valve and circulating pump.We automatically calculate the volume of standard gas required to be added to the oil tank based on the target temperature of the standard oil.The oil continues to circulate until it is evenly distributed.We utilize a gas chromatograph to identify the standard amount of this oil.To remove the standard oil, we open the solenoid valve, and the oil cylinder piston will ascend.

Experimental research
According to GB/T17623-2017, "Gas Chromatographic Determination of Dissolved Gas Components in Insulating Oil" [8] and QGDW 10536-2017, "Technical Specification for Online Monitoring Device for Dissolved Gas in Transformer Oil" [9] , we use gas chromatography to determine the concentrations of various gas components in recovered oil, filtered purified oil, prepared standard oil, and stored oil after being stored for some time.

Degassing efficiency test 4.1.1 Relationship curve between degassing efficiency and oil filtration time.
We select the total hydrocarbon content in the oil as the detection object and examine the relationship curve between the degassing efficiency of total hydrocarbons and time.During the degassing process, the total hydrocarbon content is detected every 5 minutes to obtain the relationship curve between the degassing efficiency of the total hydrocarbon components and time, as shown in Figure 3.
It can be seen that more than 90% degassing effect can be achieved in 30 minutes.After 30 minutes, the degassing efficiency enters the slowly decreasing zone, so the degassing time of the device is set to 30 minutes.

Oil degassing effect test of transformers with different background concentrations.
A comparative test was conducted on the degassing effect of transformers with low and high concentration component contents.The test conditions were oil temperature of 50 ℃ , top space degassing, room temperature of 20℃, atmospheric pressure of 101 kPa, and cyclic degassing time of 30 minutes.The detection items include hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, and carbon dioxide.The test results are shown in Table 1 It can be seen that the components in transformer oil have achieved good degassing effects under high and low concentration conditions and have been circulating for 30 minutes under degassing conditions.After oil filtration, the residual concentration of the components is lower than the required value of the standard, meeting the configuration requirements of the subsequent standard oil.

Accuracy test for standard oil preparation
The inner diameter size of the main cylinder is Φ250*530 mm, with an oil content of 30 L and a temperature control range of 50±0.3℃.The oil distribution pressure is negative, ranging from-(10-20) kPa.The detection items include H 2 , CO, CO 2 , CH 4 , C 2 H 6 , C 2 H 4 , and C 2 H 2 .Simulate types of faults such as overheating faults, discharge faults, and arc faults, prepare transformer oil standard oils with different concentrations of components, overlay the background concentration of the purified oil, detect the actual component concentration of the prepared transformer standard oil and the preset theoretical value, calculate the absolute deviation, compare with the standard requirements, and inspect the accuracy of the component concentration of the prepared transformer standard oil.The test results are shown in Table 2.

Standard oil storage stability test
The prepared standard oil sample is stored in the device, and after long-term storage, its component content will change.When the change exceeds 20% of its original component content, the standard oil sample is deemed invalid.After preparing standard oil, the storage period is 48 hours, at most 168 hours.
The relative deviation and allowable deviation of the detection results of component concentration changes are shown in Figure 4.The loss rate at 168 hours is 5.62%, which is better than the standard requirement of 10%.

Conclusion
This device and method can deeply purify the used transformer oil step by step through nitrogen replacement and vacuum pumping and prepare it into various typical fault transformer standard oil with any component concentration value for recycling.The background value of the purified oil can be included in the theoretical value of the standard oil calculation, which not only makes the actual prepared standard oil more in line with the expected target value but also greatly reduces the purification difficulty and processing cost of used transformer oil.The overall sealing of the device is good, and the prepared transformer standard oil can be stably stored for a long time.The extraction process of standard oil can be automatically controlled through operating software, which is convenient to operate.While ensuring safety in use, the process is carried out under sealed and constant pressure conditions, maintaining a constant component value in the standard oil.

Figure 2 .Figure 1 .
Figure 1.Appearance and Internal Structure of the Device.

Figure 3 .
Figure 3. C1+C2 Degassing efficiency curve.Figure 4. Detection of standard oil storage stability.The detection results show that the highest value of component concentration change in transformer standard oil storage for 48 and 168 hours is H 2 .The loss rate at 168 hours is 5.62%, which is better than the standard requirement of 10%.

Figure 4 .
Figure 3. C1+C2 Degassing efficiency curve.Figure 4. Detection of standard oil storage stability.The detection results show that the highest value of component concentration change in transformer standard oil storage for 48 and 168 hours is H 2 .The loss rate at 168 hours is 5.62%, which is better than the standard requirement of 10%.

Table 1 .
. Test results of oil degassing effect of transformers with different background concentrations.

Table 2 .
Accuracy test results of preparation of standard oils with different concentrations.demonstrate that all relative deviations fall below the permissible standards for relevant deviations.The actual range of component concentration is 2 µL/L to 13294 µL/L, and the results of the accuracy tests on the prepared standard oils with varying concentrations meet the standard requirements.