# High Efficiency Compound Vacuum Oil Gas Separation Technology Based on Integrated Oil Filter of Ultra-High Voltage Transformer

Authors: Jin Guan; Xuechao Hu; Huan Liu; Yaoguo Yang
Journal: International Transactions on Electrical Energy Systems
 (2022)

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## Abstract

In order to improve the filtering effect of vacuum oil purifier, the author proposes the research and application of high-efficiency compound vacuum oil-gas separation technology based on electric ultra-high voltage transformer integrated oil purifier. The author studies under the condition of constant oil temperature and flow rate, the effect of vacuum degree on the filtering effect of transformer oil vacuum oil filter. Using the research results, a frequency conversion automatic vacuum oil filter is designed, which makes the processing process have the characteristics of simple operation, high degree of automation, and high work efficiency. The test results show that if the transformer oil processed by the variable frequency automatic vacuum oil purifier is new oil, its water content is less than 45 mg/kg and the gas content is less than 12%; when the breakdown voltage is greater than 25 kV, the transformer oil index can meet the following requirements after the equipment is filtered for one cycle: breakdown voltage ≥75 kV; water content ≤5 mg/kg; and gas content ≤0.1% (volume ratio). After one cycle (after 100 min), the variable frequency automatic (three-stage) vacuum oil filter can greatly shorten the time of transformer oil circulating and filtering.Conclusion. The designed frequency conversion automatic vacuum oil purifier is obviously better than other transformer oil vacuum oil purifiers.


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## Body

## 1. Introduction

Transformer is important equipment in the power system, mainly used to change the voltage of alternating current and reduce losses during power transmission. According to different cooling methods, transformers can be divided into dry-type transformers and oil-immersed transformers. Dry-type transformers are small in size and clean in technology, and they are often used in indoor environments with high requirements for fire and explosion protection; oil-immersed transformers have large capacity, good heat dissipation, low price, and are more widely used, especially in occasions with high requirements on capacity and load [1]. Transformer oil is used as a medium for insulation and heat dissipation, and its quality will directly affect the safety and reliability of the transformer. The impurity content is usually required to be kept at a low level to avoid corrosion of fuel tanks, wires, and insulating materials. However, during the long-term operation of the transformer, it is affected by electrical, chemical, and environmental factors, and some performance indicators of the pressure converter oil may not meet the requirements for safe operation. The traditional practice is regular manual inspections and preventive testing (Figure 1). In addition to making judgments and discovering abnormalities based on the appearance, sound, temperature, and instrument values of the equipment, the inspectors also need to conduct preventive offline (power failure) inspections on the transformers on a regular basis and filter the transformer oil that does not meet the requirements of safe operation to make the technical indicators of the transformer oil return to normal [2]. Over the years, a certain proportion of transformer failures have been caused by excessive impurities in the oil. At the same time, the transformer is damp due to the aging of the seal, that is, the moisture in the transformer oil increases, which has a great impact on the electrical performance of the transformer oil, thus threatening the safe operation of ultra-high voltage transformers.Figure 1
Technical route.


## 2. Literature Review

Cai et al. proposed a new filtering method, according to the mechanism of bionic glomerular filtration, the bioelectric field in the kidney-type filtration process is replaced by the “low-energy heterogeneous isoelectric field,” and the rubber-plastic composite biomaterial with variable pore size is used, it replaces glomerular filtration [3]. A high-efficiency HEP100S three-stage transformer oil vacuum oil filter was proposed by Sato et al. The oil purifier adopts three vacuum pumps, and its biggest advantages are high vacuum rate, high filtration precision, and high purification efficiency, and the transformer body can be evacuated while the transformer oil of the ultra-high voltage equipment is vacuum filtered [4]. Sharma and Chaurasia used gas chromatography to analyze the components of dissolved gases in transformer insulating oil and designed a computer fault diagnosis program: the program simplifies the calculation procedures of the chromatographic result processing and improves the reliability and accuracy of data processing and diagnosis [5]. However, due to the analysis of a single gas, and as the three-ratio method is used in the analysis process, the diagnostic error is large. Selva et al. used the neural network method to establish a prediction model for the parameters of transformer oil, such as flash point, water-soluble acid, acid value, breakdown voltage, and moisture and dissolved gas of oil, as it provides a strong basis for fault diagnosis and elimination, and the diagnostic error is further reduced, and the accuracy is high, which can ensure the safe and economical operation of the transformer [6]. Huang et al., based on neural network theory and fuzzy theory, expounded the blackboard expert system for transformer oil and gas diagnosis, and this type of object-oriented technology reflects the characteristics of an expert system, that is, it is flexible and has outstanding performance; through the application of fuzzy theory, it can not only solve the problems existing in the process of diagnosing faults such as fuzziness, but also at the same time, based on neural network functions, modes, etc., the system as a whole has higher fault diagnosis accuracy. The problems faced in the single-method diagnosis process are thus effectively overcome, making the diagnosis of transformer insulation more accurate [7].The author purifies the transformer oil and effectively filters out the solid particles, water, gas, etc., in it, so as to ensure the safe operation of the power grid. The degree of vacuum and the temperature of the oil are key factors in the effectiveness of oil treatment. A large number of secondary vacuum oil purifiers in operation have poor vacuum lifting capacity, and there are likely to be traces of acetylene and other impurities in the treated transformer oil, resulting in low efficiency. A three-stage vacuum oil filter with large flow rate and frequency conversion control is developed, which can adapt to UHV equipment [8].


## 3. Research Methods

### 3.1. Overall Structure Design of Vacuum Oil Filter

The vacuum oil filter is mainly composed of heating, filtering, and vacuum systems. Its working principle is as follows: under the vacuum state, due to different saturated vapor pressures of oil and water, the water in the transformer oil vaporizes and evaporates and is pumped into the atmosphere by the vacuum pump, and the water can be separated from the oil and the composition is stable. The research design strives to achieve the best matching state of vacuum and temperature.The frequency conversion automatic vacuum oil purifier adopts a three-stage vacuum unit, which is composed of a frame, an imported pump, a heater, a filter, a vacuum system, an electrical control system, and an outlet pump.


#### 3.1.1. Three-Stage Vacuum System Structure

The principle of frequency conversion automatic vacuum oil purifier is the following: the vacuum pump group is composed of 2 Roots pumps (008, 009) and 1 mechanical vacuum pump (010) in series, 2 frequency conversion motors control oil intake and oil discharge, and multiple switching commands of the oil level in the vacuum separation chamber control the speed of the motor [9]. The working process is as follows: after the transformer oil is filtered by the coarse filter (001), it is sent to the oil filter system under the drive of the oil inlet pump (002), then it enters the vacuum separation chamber (005) to separate the gas contained in the transformer oil, and the water contained in it is vaporized and evaporated by the Roots pump (008, 009) and the mechanical vacuum pump (010). The degassed and dehydrated transformer oil is sent to the fine filter (013) with a filtration accuracy of 0.5 μm by the oil outlet pump (012) and then enters the power transformer. When the isolation valve (006) is closed, the mechanical vacuum pump (010) and the Roots pump (008, 009) are started. In addition, as an auxiliary function, the independent vacuum circuit (007) can be used to directly vacuum the transformer body and the airtight container.


#### 3.1.2. Vacuum System Design

The speed of evaporation and dissipation of the moisture in the transformer oil in the vacuum separation chamber determines the dehydration efficiency of the oil filter, the heated transformer oil circulates from the top of the vacuum separation chamber to the bottom by gravity, in this free fall state, and the moisture and gas in the oil escape and are quickly pumped into the atmosphere by the vacuum pump [10]. The main feature of the vacuum system of variable frequency automatic vacuum oil purifier is that in addition to the professional functions of water, gas, and particles, it can also vacuum the transformer body and airtight container and dry the windings of the transformer.In addition to the pumping capacity of the vacuum pump itself, the vacuum degree of the vacuum separation chamber is also affected by the resistance of the pipeline to the gas.(1) Three-Stage Vacuum System Design. Evacuate the 500 kV transformer body with a volume of 40 m3, from atmospheric pressure 1.013×105Pa to working pressure (133 Pa), and the ultimate pressure of the transformer body is kept less than 80 Pa, while the general vacuum system takes about 75 minutes to evacuate. In order to improve the efficiency, the three-stage vacuum system is designed to take less than 40 minutes to evacuate.In order to improve the vacuuming efficiency of the vacuum unit, the high-efficiency vacuum system scheme is designed as follows: 2-stage Roots pump + 1-stage mechanical vacuum pump are connected in series, and the three-stage vacuum unit formed is used as the pumping system.The calculation of vacuum pump selection includes the following steps: (1) according to the analogy method, choose Roots pump 008 as the main pump, and the pumping rateSz = 0.278 m3/s; (2) calculate the effective pumping rate Se=1/4∼1/2Sz of Roots pump 009, pumping rate Sz = 0.139 m3/s; and (3) select mechanical vacuum pump 010 as the backing pump, and the effective pumping rate needs to be selected with reference to the multiple of Roots pump 008. Its effective pumping rate Sq=1/8∼1/4Sz and effective pumping rate Sq = 0.056 m3/s. The pumping capacity of the 500 kV transformer body vacuum pumping test is verified, and the test results are shown in Table 1.Table 1
Performance index of vacuum pump (cooled by air cooling).
ProjectNominal pumping rate (m3·s−1)Maximum differential pressure (Pa)Ultimate vacuum (Pa)Motor power (kW)Roots pump 0080.55652005 × 10−15.5Roots pump 0090.27852005 × 10−14Vacuum pump 0100.084Straight to the atmosphere104(2) Calculate the Pumping Time of the Three-Stage Vacuum System. The pumping time is calculated as formula (1) by ignoring other influencing factors such as air leakage, pipeline, and pipeline accessories:(1)t=2.3Kq⋅VS⋅lgPiP,where t is the pumping time, s; Kqis the correction coefficient, which is related to the pressure P when the vacuum pump is exhausted (as shown in Table 2); S is the nominal pumping rate of the vacuum pump, m3/s; V is the volume of vacuum separation chamber, m3; P is the pressure after pumping at time t, Pa; and Pi is the pressure at the beginning of the vacuum pump pumping, Pa.Table 2
Correction factors.
Correction factor1.0∼0.1101∼1.01.0 × 102∼1011.0 × 103∼1.0 × 1021.0 × 104∼1.0 × 1031.0 × 105∼1.0 × 104Kq642.251.751.51.25The startup conditions of the vacuum pumps at all levels are different during prepumping, and the startup sequence is as follows: first start the mechanical vacuum pump (010) at normal pressure, that is, at standard atmospheric pressure; when the vacuum sensor detects that the vacuum degree in the vacuum separation chamber (005) reaches a certain value, the Roots pump (009) is started in sequence; and when the vacuum sensor detects that the vacuum degree in the vacuum separation chamber (005) increases to a certain value, start the Roots pump (008) until the vacuum degree in the vacuum separation chamber (005) reaches the set operating value [11, 12]. In the above three pumping stages, the effective pumping rates of the vacuum units are different, and the prepumping time needs to be calculated in segments, the total pumping time is composed of the sum of the three pumping times, and the time is shown in Table 3.Table 3
Air extraction schedule for each stage (cooling by air cooling).
ProjectVacuum separation chamber volume (m3)Pumping rate (m3·s−1)Pressure at the start of vacuum pump evacuation (Pa)Pressure after pumping (Pa)Correction factor (kW)Pumping time (s)Mechanical vacuum pump 010400.0841 × 1057 × 1031.51 590.4Roots pump 008400.5564 × 1031 × 1021.75289.9Roots pump 009400.2787 × 1034 × 103475.4Total pumping time1955.7It can be seen from Table3 that the total pumping time is 1 955.7 s, about 33 min. Calculations show that the pumping capacity of the three-stage vacuum unit exceeds that of the two-stage vacuum unit commonly used in the market, can meet and exceed the technical requirements, and greatly improve the working efficiency of the vacuum oil filter [13].(3) Design Heating Power. In order to ensure the efficiency of the oil purifier, the power calculation of the heater is very important, according to the characteristics of the oil purifier and the operating environment, the empirical temperature rise can be 6°C/h in the initial design, and 100 min is a working cycle. Set the temperature rise Δt of the transformer oil to 10°C.Transformer oil is incompressible during repurification. Considering the constant volume thermal system during treatment, the heatq absorbed by 1 kg of transformer oil processed by an automatic vacuum oil purifier is expressed as the following formula:(2)q=η⋅PρQ=CvΔt,where η is the thermal efficiency which is taken as 0.45 in the calculation (considering that the heat loss during the oil filtering process is relatively large); P is the heating power, kW; Q is the work flow, m3/s, and the value is 0.002 498; ρ is the transformer oil density, kg/m3, which is 895; Cv is the specific heat capacity of the transformer oil constant volume, J/kg; K, the value is 2.15; and Δt is the temperature rise of the transformer oil treated by the oil filter in one cycle which is taken as 10°C.According to formula (2), we can get the following: P=CvΔtρQ/η=2.15×10×895×0.002498/0.45=120.15kW, so the power of the heater is 120 kW.(4) Operation Control of Frequency Conversion Automatic Vacuum Oil Purifier. The use of frequency converter speed regulation technology is a key factor for high-quality vacuum oil purifiers, which can avoid damage to the oil inlet pump (002) or the oil supply solenoid valve (004) due to frequent opening and stopping [14]. The use of inverter power supply can weaken the starting and impact of the three-stage vacuum oil purifier, which can gradually increase the output voltage and frequency of the motor, so the starting current and impact of the motor are relatively small; thus, ensuring the performance and stability of the equipment.The heaters are divided into three groups, temperature adjustment and control sensors are installed on the three groups of independent heaters, and make sure that the temperature of the transformer oil being processed is kept constant at around 60°C. The heater adopts fuzzy control theory, uses temperature sensor, and realizes dual control of vacuum oil purifier through fuzzy mathematical operation [15].The circuit is equipped with the functions of interruption, overcurrent, and overtemperature alarm and protection and integrates precision filtration with high-efficiency dehydration and degassing as much as possible.


## 3.1. Overall Structure Design of Vacuum Oil Filter

The vacuum oil filter is mainly composed of heating, filtering, and vacuum systems. Its working principle is as follows: under the vacuum state, due to different saturated vapor pressures of oil and water, the water in the transformer oil vaporizes and evaporates and is pumped into the atmosphere by the vacuum pump, and the water can be separated from the oil and the composition is stable. The research design strives to achieve the best matching state of vacuum and temperature.The frequency conversion automatic vacuum oil purifier adopts a three-stage vacuum unit, which is composed of a frame, an imported pump, a heater, a filter, a vacuum system, an electrical control system, and an outlet pump.


### 3.1.1. Three-Stage Vacuum System Structure

The principle of frequency conversion automatic vacuum oil purifier is the following: the vacuum pump group is composed of 2 Roots pumps (008, 009) and 1 mechanical vacuum pump (010) in series, 2 frequency conversion motors control oil intake and oil discharge, and multiple switching commands of the oil level in the vacuum separation chamber control the speed of the motor [9]. The working process is as follows: after the transformer oil is filtered by the coarse filter (001), it is sent to the oil filter system under the drive of the oil inlet pump (002), then it enters the vacuum separation chamber (005) to separate the gas contained in the transformer oil, and the water contained in it is vaporized and evaporated by the Roots pump (008, 009) and the mechanical vacuum pump (010). The degassed and dehydrated transformer oil is sent to the fine filter (013) with a filtration accuracy of 0.5 μm by the oil outlet pump (012) and then enters the power transformer. When the isolation valve (006) is closed, the mechanical vacuum pump (010) and the Roots pump (008, 009) are started. In addition, as an auxiliary function, the independent vacuum circuit (007) can be used to directly vacuum the transformer body and the airtight container.


### 3.1.2. Vacuum System Design

The speed of evaporation and dissipation of the moisture in the transformer oil in the vacuum separation chamber determines the dehydration efficiency of the oil filter, the heated transformer oil circulates from the top of the vacuum separation chamber to the bottom by gravity, in this free fall state, and the moisture and gas in the oil escape and are quickly pumped into the atmosphere by the vacuum pump [10]. The main feature of the vacuum system of variable frequency automatic vacuum oil purifier is that in addition to the professional functions of water, gas, and particles, it can also vacuum the transformer body and airtight container and dry the windings of the transformer.In addition to the pumping capacity of the vacuum pump itself, the vacuum degree of the vacuum separation chamber is also affected by the resistance of the pipeline to the gas.(1) Three-Stage Vacuum System Design. Evacuate the 500 kV transformer body with a volume of 40 m3, from atmospheric pressure 1.013×105Pa to working pressure (133 Pa), and the ultimate pressure of the transformer body is kept less than 80 Pa, while the general vacuum system takes about 75 minutes to evacuate. In order to improve the efficiency, the three-stage vacuum system is designed to take less than 40 minutes to evacuate.In order to improve the vacuuming efficiency of the vacuum unit, the high-efficiency vacuum system scheme is designed as follows: 2-stage Roots pump + 1-stage mechanical vacuum pump are connected in series, and the three-stage vacuum unit formed is used as the pumping system.The calculation of vacuum pump selection includes the following steps: (1) according to the analogy method, choose Roots pump 008 as the main pump, and the pumping rateSz = 0.278 m3/s; (2) calculate the effective pumping rate Se=1/4∼1/2Sz of Roots pump 009, pumping rate Sz = 0.139 m3/s; and (3) select mechanical vacuum pump 010 as the backing pump, and the effective pumping rate needs to be selected with reference to the multiple of Roots pump 008. Its effective pumping rate Sq=1/8∼1/4Sz and effective pumping rate Sq = 0.056 m3/s. The pumping capacity of the 500 kV transformer body vacuum pumping test is verified, and the test results are shown in Table 1.Table 1
Performance index of vacuum pump (cooled by air cooling).
ProjectNominal pumping rate (m3·s−1)Maximum differential pressure (Pa)Ultimate vacuum (Pa)Motor power (kW)Roots pump 0080.55652005 × 10−15.5Roots pump 0090.27852005 × 10−14Vacuum pump 0100.084Straight to the atmosphere104(2) Calculate the Pumping Time of the Three-Stage Vacuum System. The pumping time is calculated as formula (1) by ignoring other influencing factors such as air leakage, pipeline, and pipeline accessories:(1)t=2.3Kq⋅VS⋅lgPiP,where t is the pumping time, s; Kqis the correction coefficient, which is related to the pressure P when the vacuum pump is exhausted (as shown in Table 2); S is the nominal pumping rate of the vacuum pump, m3/s; V is the volume of vacuum separation chamber, m3; P is the pressure after pumping at time t, Pa; and Pi is the pressure at the beginning of the vacuum pump pumping, Pa.Table 2
Correction factors.
Correction factor1.0∼0.1101∼1.01.0 × 102∼1011.0 × 103∼1.0 × 1021.0 × 104∼1.0 × 1031.0 × 105∼1.0 × 104Kq642.251.751.51.25The startup conditions of the vacuum pumps at all levels are different during prepumping, and the startup sequence is as follows: first start the mechanical vacuum pump (010) at normal pressure, that is, at standard atmospheric pressure; when the vacuum sensor detects that the vacuum degree in the vacuum separation chamber (005) reaches a certain value, the Roots pump (009) is started in sequence; and when the vacuum sensor detects that the vacuum degree in the vacuum separation chamber (005) increases to a certain value, start the Roots pump (008) until the vacuum degree in the vacuum separation chamber (005) reaches the set operating value [11, 12]. In the above three pumping stages, the effective pumping rates of the vacuum units are different, and the prepumping time needs to be calculated in segments, the total pumping time is composed of the sum of the three pumping times, and the time is shown in Table 3.Table 3
Air extraction schedule for each stage (cooling by air cooling).
ProjectVacuum separation chamber volume (m3)Pumping rate (m3·s−1)Pressure at the start of vacuum pump evacuation (Pa)Pressure after pumping (Pa)Correction factor (kW)Pumping time (s)Mechanical vacuum pump 010400.0841 × 1057 × 1031.51 590.4Roots pump 008400.5564 × 1031 × 1021.75289.9Roots pump 009400.2787 × 1034 × 103475.4Total pumping time1955.7It can be seen from Table3 that the total pumping time is 1 955.7 s, about 33 min. Calculations show that the pumping capacity of the three-stage vacuum unit exceeds that of the two-stage vacuum unit commonly used in the market, can meet and exceed the technical requirements, and greatly improve the working efficiency of the vacuum oil filter [13].(3) Design Heating Power. In order to ensure the efficiency of the oil purifier, the power calculation of the heater is very important, according to the characteristics of the oil purifier and the operating environment, the empirical temperature rise can be 6°C/h in the initial design, and 100 min is a working cycle. Set the temperature rise Δt of the transformer oil to 10°C.Transformer oil is incompressible during repurification. Considering the constant volume thermal system during treatment, the heatq absorbed by 1 kg of transformer oil processed by an automatic vacuum oil purifier is expressed as the following formula:(2)q=η⋅PρQ=CvΔt,where η is the thermal efficiency which is taken as 0.45 in the calculation (considering that the heat loss during the oil filtering process is relatively large); P is the heating power, kW; Q is the work flow, m3/s, and the value is 0.002 498; ρ is the transformer oil density, kg/m3, which is 895; Cv is the specific heat capacity of the transformer oil constant volume, J/kg; K, the value is 2.15; and Δt is the temperature rise of the transformer oil treated by the oil filter in one cycle which is taken as 10°C.According to formula (2), we can get the following: P=CvΔtρQ/η=2.15×10×895×0.002498/0.45=120.15kW, so the power of the heater is 120 kW.(4) Operation Control of Frequency Conversion Automatic Vacuum Oil Purifier. The use of frequency converter speed regulation technology is a key factor for high-quality vacuum oil purifiers, which can avoid damage to the oil inlet pump (002) or the oil supply solenoid valve (004) due to frequent opening and stopping [14]. The use of inverter power supply can weaken the starting and impact of the three-stage vacuum oil purifier, which can gradually increase the output voltage and frequency of the motor, so the starting current and impact of the motor are relatively small; thus, ensuring the performance and stability of the equipment.The heaters are divided into three groups, temperature adjustment and control sensors are installed on the three groups of independent heaters, and make sure that the temperature of the transformer oil being processed is kept constant at around 60°C. The heater adopts fuzzy control theory, uses temperature sensor, and realizes dual control of vacuum oil purifier through fuzzy mathematical operation [15].The circuit is equipped with the functions of interruption, overcurrent, and overtemperature alarm and protection and integrates precision filtration with high-efficiency dehydration and degassing as much as possible.


## 3.1.1. Three-Stage Vacuum System Structure

The principle of frequency conversion automatic vacuum oil purifier is the following: the vacuum pump group is composed of 2 Roots pumps (008, 009) and 1 mechanical vacuum pump (010) in series, 2 frequency conversion motors control oil intake and oil discharge, and multiple switching commands of the oil level in the vacuum separation chamber control the speed of the motor [9]. The working process is as follows: after the transformer oil is filtered by the coarse filter (001), it is sent to the oil filter system under the drive of the oil inlet pump (002), then it enters the vacuum separation chamber (005) to separate the gas contained in the transformer oil, and the water contained in it is vaporized and evaporated by the Roots pump (008, 009) and the mechanical vacuum pump (010). The degassed and dehydrated transformer oil is sent to the fine filter (013) with a filtration accuracy of 0.5 μm by the oil outlet pump (012) and then enters the power transformer. When the isolation valve (006) is closed, the mechanical vacuum pump (010) and the Roots pump (008, 009) are started. In addition, as an auxiliary function, the independent vacuum circuit (007) can be used to directly vacuum the transformer body and the airtight container.


## 3.1.2. Vacuum System Design

The speed of evaporation and dissipation of the moisture in the transformer oil in the vacuum separation chamber determines the dehydration efficiency of the oil filter, the heated transformer oil circulates from the top of the vacuum separation chamber to the bottom by gravity, in this free fall state, and the moisture and gas in the oil escape and are quickly pumped into the atmosphere by the vacuum pump [10]. The main feature of the vacuum system of variable frequency automatic vacuum oil purifier is that in addition to the professional functions of water, gas, and particles, it can also vacuum the transformer body and airtight container and dry the windings of the transformer.In addition to the pumping capacity of the vacuum pump itself, the vacuum degree of the vacuum separation chamber is also affected by the resistance of the pipeline to the gas.(1) Three-Stage Vacuum System Design. Evacuate the 500 kV transformer body with a volume of 40 m3, from atmospheric pressure 1.013×105Pa to working pressure (133 Pa), and the ultimate pressure of the transformer body is kept less than 80 Pa, while the general vacuum system takes about 75 minutes to evacuate. In order to improve the efficiency, the three-stage vacuum system is designed to take less than 40 minutes to evacuate.In order to improve the vacuuming efficiency of the vacuum unit, the high-efficiency vacuum system scheme is designed as follows: 2-stage Roots pump + 1-stage mechanical vacuum pump are connected in series, and the three-stage vacuum unit formed is used as the pumping system.The calculation of vacuum pump selection includes the following steps: (1) according to the analogy method, choose Roots pump 008 as the main pump, and the pumping rateSz = 0.278 m3/s; (2) calculate the effective pumping rate Se=1/4∼1/2Sz of Roots pump 009, pumping rate Sz = 0.139 m3/s; and (3) select mechanical vacuum pump 010 as the backing pump, and the effective pumping rate needs to be selected with reference to the multiple of Roots pump 008. Its effective pumping rate Sq=1/8∼1/4Sz and effective pumping rate Sq = 0.056 m3/s. The pumping capacity of the 500 kV transformer body vacuum pumping test is verified, and the test results are shown in Table 1.Table 1
Performance index of vacuum pump (cooled by air cooling).
ProjectNominal pumping rate (m3·s−1)Maximum differential pressure (Pa)Ultimate vacuum (Pa)Motor power (kW)Roots pump 0080.55652005 × 10−15.5Roots pump 0090.27852005 × 10−14Vacuum pump 0100.084Straight to the atmosphere104(2) Calculate the Pumping Time of the Three-Stage Vacuum System. The pumping time is calculated as formula (1) by ignoring other influencing factors such as air leakage, pipeline, and pipeline accessories:(1)t=2.3Kq⋅VS⋅lgPiP,where t is the pumping time, s; Kqis the correction coefficient, which is related to the pressure P when the vacuum pump is exhausted (as shown in Table 2); S is the nominal pumping rate of the vacuum pump, m3/s; V is the volume of vacuum separation chamber, m3; P is the pressure after pumping at time t, Pa; and Pi is the pressure at the beginning of the vacuum pump pumping, Pa.Table 2
Correction factors.
Correction factor1.0∼0.1101∼1.01.0 × 102∼1011.0 × 103∼1.0 × 1021.0 × 104∼1.0 × 1031.0 × 105∼1.0 × 104Kq642.251.751.51.25The startup conditions of the vacuum pumps at all levels are different during prepumping, and the startup sequence is as follows: first start the mechanical vacuum pump (010) at normal pressure, that is, at standard atmospheric pressure; when the vacuum sensor detects that the vacuum degree in the vacuum separation chamber (005) reaches a certain value, the Roots pump (009) is started in sequence; and when the vacuum sensor detects that the vacuum degree in the vacuum separation chamber (005) increases to a certain value, start the Roots pump (008) until the vacuum degree in the vacuum separation chamber (005) reaches the set operating value [11, 12]. In the above three pumping stages, the effective pumping rates of the vacuum units are different, and the prepumping time needs to be calculated in segments, the total pumping time is composed of the sum of the three pumping times, and the time is shown in Table 3.Table 3
Air extraction schedule for each stage (cooling by air cooling).
ProjectVacuum separation chamber volume (m3)Pumping rate (m3·s−1)Pressure at the start of vacuum pump evacuation (Pa)Pressure after pumping (Pa)Correction factor (kW)Pumping time (s)Mechanical vacuum pump 010400.0841 × 1057 × 1031.51 590.4Roots pump 008400.5564 × 1031 × 1021.75289.9Roots pump 009400.2787 × 1034 × 103475.4Total pumping time1955.7It can be seen from Table3 that the total pumping time is 1 955.7 s, about 33 min. Calculations show that the pumping capacity of the three-stage vacuum unit exceeds that of the two-stage vacuum unit commonly used in the market, can meet and exceed the technical requirements, and greatly improve the working efficiency of the vacuum oil filter [13].(3) Design Heating Power. In order to ensure the efficiency of the oil purifier, the power calculation of the heater is very important, according to the characteristics of the oil purifier and the operating environment, the empirical temperature rise can be 6°C/h in the initial design, and 100 min is a working cycle. Set the temperature rise Δt of the transformer oil to 10°C.Transformer oil is incompressible during repurification. Considering the constant volume thermal system during treatment, the heatq absorbed by 1 kg of transformer oil processed by an automatic vacuum oil purifier is expressed as the following formula:(2)q=η⋅PρQ=CvΔt,where η is the thermal efficiency which is taken as 0.45 in the calculation (considering that the heat loss during the oil filtering process is relatively large); P is the heating power, kW; Q is the work flow, m3/s, and the value is 0.002 498; ρ is the transformer oil density, kg/m3, which is 895; Cv is the specific heat capacity of the transformer oil constant volume, J/kg; K, the value is 2.15; and Δt is the temperature rise of the transformer oil treated by the oil filter in one cycle which is taken as 10°C.According to formula (2), we can get the following: P=CvΔtρQ/η=2.15×10×895×0.002498/0.45=120.15kW, so the power of the heater is 120 kW.(4) Operation Control of Frequency Conversion Automatic Vacuum Oil Purifier. The use of frequency converter speed regulation technology is a key factor for high-quality vacuum oil purifiers, which can avoid damage to the oil inlet pump (002) or the oil supply solenoid valve (004) due to frequent opening and stopping [14]. The use of inverter power supply can weaken the starting and impact of the three-stage vacuum oil purifier, which can gradually increase the output voltage and frequency of the motor, so the starting current and impact of the motor are relatively small; thus, ensuring the performance and stability of the equipment.The heaters are divided into three groups, temperature adjustment and control sensors are installed on the three groups of independent heaters, and make sure that the temperature of the transformer oil being processed is kept constant at around 60°C. The heater adopts fuzzy control theory, uses temperature sensor, and realizes dual control of vacuum oil purifier through fuzzy mathematical operation [15].The circuit is equipped with the functions of interruption, overcurrent, and overtemperature alarm and protection and integrates precision filtration with high-efficiency dehydration and degassing as much as possible.


## 4. Analysis of Results

### 4.1. Test Method

Use the transformation project to carry out the necessary tests, and the transformer oil to be treated is 15 t, which is filtered by a three-stage transformer oil vacuum oil filter with a flow rate of 150 L/min, before vacuum filtering the oil, the method of adding evenly stirred pure water to the oil in the oil tank [16]. In the actual operation process, the detection instruments and detection methods are limited; in this test, the water content in the transformer oil was tracked and detected for 2 hours continuously, and the time interval was 10 minutes, and then sampling was carried out. The technical parameters of the transformer oil before treatment are shown in Table 4.Table 4
Technical parameters of transformer oil before treatment.
Technical parameters before filteringFlow/m3·h−1Water content/mg·kg−1Gas content/%Initial oil temperature/°CDielectric strength/kVAmbient temperature/°CTransformer oil9 0004812.0538.42924.7


### 4.2. Test Items

Breakdown voltage: it is a very important indicator to test the ultimate electrical stress resistance of the transformer.Gas content: it is mainly used to analyze the causes of transformer failures, it can detect specific gases such as CO and CO2 in the oil, and the dissolved gas content in the transformer oil is strictly monitored and controlled.Water content: China’s power industry stipulates that the water content in transformer oil should be less than 10 mg/kg.If the transformer oil processed by the variable frequency automatic vacuum oil purifier is new oil, its water content is less than 45 mg/kg, the gas content is less than 12%, and the breakdown voltage is greater than 25 kV, after using the equipment for one cycle of filtration, and the transformer oil index can meet the following index requirements: breakdown voltage ≥75 kV; water content ≤5 mg/kg; and gas content ≤0.1% (volume ratio) [17, 18].In order to test and check the operation effect of the variable frequency automatic vacuum oil filter, firstly turn on the heater, ensure the temperature of the oil entering the vacuum separation tower, record the test data, and draw the vacuum degree and saturation curve of the three-stage vacuum oil filter (as shown in Figure2).Figure 2
Vacuum degree and saturation curve of the variable frequency automatic vacuum oil filter.It can be seen from Figure2 that with the increase of the vacuum degree of the vacuum oil filter, the saturation of water in the oil is significantly reduced, after 100 min (1 cycle) treatment, and the test results of the oil filter are shown in Table 5 [19, 20].Table 5
Test results of frequency conversion automatic (three-stage) vacuum oil filter.
ProjectWorking vacuum/PaBreakdown voltage/kVAir content (volume)/%Water content/mg·kg−1Ambient temperature/°CAutomatic (three-stage) vacuum oil filter9.7760.0452.826


## 4.1. Test Method

Use the transformation project to carry out the necessary tests, and the transformer oil to be treated is 15 t, which is filtered by a three-stage transformer oil vacuum oil filter with a flow rate of 150 L/min, before vacuum filtering the oil, the method of adding evenly stirred pure water to the oil in the oil tank [16]. In the actual operation process, the detection instruments and detection methods are limited; in this test, the water content in the transformer oil was tracked and detected for 2 hours continuously, and the time interval was 10 minutes, and then sampling was carried out. The technical parameters of the transformer oil before treatment are shown in Table 4.Table 4
Technical parameters of transformer oil before treatment.
Technical parameters before filteringFlow/m3·h−1Water content/mg·kg−1Gas content/%Initial oil temperature/°CDielectric strength/kVAmbient temperature/°CTransformer oil9 0004812.0538.42924.7


## 4.2. Test Items

Breakdown voltage: it is a very important indicator to test the ultimate electrical stress resistance of the transformer.Gas content: it is mainly used to analyze the causes of transformer failures, it can detect specific gases such as CO and CO2 in the oil, and the dissolved gas content in the transformer oil is strictly monitored and controlled.Water content: China’s power industry stipulates that the water content in transformer oil should be less than 10 mg/kg.If the transformer oil processed by the variable frequency automatic vacuum oil purifier is new oil, its water content is less than 45 mg/kg, the gas content is less than 12%, and the breakdown voltage is greater than 25 kV, after using the equipment for one cycle of filtration, and the transformer oil index can meet the following index requirements: breakdown voltage ≥75 kV; water content ≤5 mg/kg; and gas content ≤0.1% (volume ratio) [17, 18].In order to test and check the operation effect of the variable frequency automatic vacuum oil filter, firstly turn on the heater, ensure the temperature of the oil entering the vacuum separation tower, record the test data, and draw the vacuum degree and saturation curve of the three-stage vacuum oil filter (as shown in Figure2).Figure 2
Vacuum degree and saturation curve of the variable frequency automatic vacuum oil filter.It can be seen from Figure2 that with the increase of the vacuum degree of the vacuum oil filter, the saturation of water in the oil is significantly reduced, after 100 min (1 cycle) treatment, and the test results of the oil filter are shown in Table 5 [19, 20].Table 5
Test results of frequency conversion automatic (three-stage) vacuum oil filter.
ProjectWorking vacuum/PaBreakdown voltage/kVAir content (volume)/%Water content/mg·kg−1Ambient temperature/°CAutomatic (three-stage) vacuum oil filter9.7760.0452.826


## 5. Conclusion

The test results show that, in one cycle (after 100 min), the variable frequency automatic (three-stage) vacuum oil purifier can greatly shorten the time of circulating and filtering transformer oil; the work efficiency is improved; the cycle of the whole project is shortened; it can meet the requirements of ultra-high voltage transformer oil treatment in China; and the treatment efficiency is improved, the equipment has high practicability and promotion value, and the market prospect is optimistic.



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*Source: 9989686-2022-09-07.xml*