In the transformer design, the insulation part generally adopts the traditional design method, that is, the test statistic method is used to determine the life of the transformer insulation. In recent years, modern design methods such as finite element analysis method, reliability design technology and optimization design technology have appeared in the transformer insulation design method. With the development of computer technology, large-scale professional analysis and optimization software has been used for analysis and design, using modern technologies. The design method results in a more economical and reasonable solution than the one designed using traditional design methods. As modern design methods are still under development, the specific content of modern design methods is difficult to determine, but at present there are many more mature methods, summed up are: optimization design, finite element analysis, reliability analysis, modeling design, value engineering , design expert system, computer aided design, etc. In addition, these methods can be used in cross-cutting to form a variety of methods with unique application objects, such as reliability optimization design.
I. Insulation structure of voltage transformer
There are four types of voltage transformers: dry, pouring, oil-immersed and gas-typed according to the insulating medium. The insulation structure of the voltage transformer adopts different methods according to different voltage levels and different operating environments. The low-voltage transformers usually used for measurement are dry type, and the high-voltage or ultra-high-pressure sealed gas-insulated (such as sulfur hexafluoride) transformers are also dry type. The pouring type is applicable to voltage transformers of 35kV and below, and products of 35kV and above are all oil-immersed. Among them, compared with the traditional oil-immersed and inflatable transformers, the epoxy resin casting transformer has the advantages of oil-free, airless, non-enclosure, lifetime maintenance-free, and good insulation performance. It is widely used in the world. A transformer.
1. Dry insulating structure The dry insulating structure is simple to manufacture and low in cost, but its dielectric strength is not high, so the voltage using this structure is generally not more than 380V. The wire uses QZ type enamelled round copper wire. The insulating materials used in the insulation structure are: insulating paper, glass cloth tape, phenolic plastic and so on. Between the coil and the core is used bakelite or plastic frame insulation, insulation between the layers and the primary insulation between the primary and secondary coils, generally yellow wax or polyester film. Instrument voltage transformer insulation between the ring core and the coil, generally the use of insulating cardboard, and then wrapped around a layer of glass ribbon, yellow wax or polyester film tape. The dry structure is simple, but its volume is large. It is only suitable for low voltage indoor devices, or instrument voltage transformers of 10kv and below.
2. Casting Insulation Structure The so-called cast insulation is resin, filler, pigment and curing agent mixed in a certain proportion and then poured into the mold of the primary and secondary windings and other parts of the transformer, and cured. The solid insulation formed by the molding. The solidified hybrid rubber not only fixes the relevant parts, but also is the main insulation of the transformer. Cast insulation has good insulation properties, high mechanical strength, moisture, fire and other advantages. The resin compound adhesive has good fluidity at room temperature or high temperature, can fill small gaps, and is easily poured into more complicated shapes. The resin compound adhesive also has a strong adhesive effect and can firmly fix metals and many insulating materials. Bonded together, it is an ideal transformer insulation molding material, which is widely used in indoor voltage transformers of 35kV and below.
At present, the resins commonly used in transformers in China are epoxy resin and unsaturated resin. Unsaturated resin is inexpensive, can be cured at room temperature, and the casting process is simple, but its electrical and mechanical strength is low, and its heat resistance is poor. In addition, the saturated vapor pressure of unsaturated resin is high, the mixed rubber is not suitable for vacuum degassing, and it is not suitable for pouring. Vacuum is applied, and bubbles are poured in the body. The cure shrinkage of the unsaturated resin is large, and the mixed rubber is easily cracked when it is cured. Therefore, unsaturated resins are only suitable for low voltage products. Epoxy resin has small curing shrinkage and low saturated vapor pressure, and is suitable for pouring under high temperature and high vacuum, so that the blended rubber has better fluidity and can be degassed to the utmost so as to obtain an ideal castable body.
There are two types of casting insulation: semi-casting and casting. The semi-casting method is to cast the coils separately and then install the iron cores. The full casting method is to cast the coils and iron cores together and put them together. The epoxy resin fully-poured voltage transformer has the following characteristics: (1) It achieves oil-free and protects the environment; (2) It achieves maintenance-free, saves a lot of manpower and material resources and power outage time; (3) its manufacturing materials are all For non-flammable or flame-retardant substances, flame retardant, explosion-proof.
The insulation of the core and the coil of the coil is borne by the resin, and no entrapment of air bubbles or conductive impurities is required during pouring. Insulation between coil layers uses cable paper or composite insulation paper. The primary insulation between primary and secondary coils is epoxy resin cartridges, phenolic paper drums, or vacuum-impregnated cable drums. The plastic pouring type has a compact structure and is easy to maintain. It is suitable for indoor devices ranging from 3 to 10 kV.
Epoxy resin is a kind of chemical raw material that has been widely used for a long time. It is not only a flame-retardant and flame-retardant material, but also has excellent electrical properties. It was gradually adopted by the electrician manufacturing industry. Pouring voltage transformers are compact in structure and easy to maintain. With the development of outdoor resins, they will gradually be used in outdoor products of 35kV or more. Pouring voltage transformers are divided into semi-casting and full-casting according to the form of pouring. Among them, the primary winding and each low-voltage winding, as well as the two casings at the outlet end of the primary winding are all cast into a single body. Then the core is assembled with a commonly used semi-casting structure. The advantage is that the pouring body is relatively simple and easy to manufacture. The structure is not compact enough, the iron core will have rust, and it needs regular maintenance; the winding and the core are all poured into one and is called full casting type. Its feature is compact structure, almost no maintenance, but the pouring body is more complex, and the core buffer layer is compared. trouble.
The external insulation of the pouring transformer differs depending on the indoor and outdoor structures. The indoor transformer uses a mixed glue to cast the lead wire of the primary winding into a bushing to ensure the insulation distance between the lead terminal and the base or the exposed iron core. The pouring bushing is generally a cone, a cylinder or a square cylinder, and sometimes an umbrella skirt is also provided as required. . There are many sheds on the outside insulation of outdoor transformers to increase the creepage distance along the surface. It is also sometimes designed as an umbrella skirt to increase its anti-fouling and anti-condensation flashover capabilities. The plastic material of the outdoor pouring transformer is different from the plastic material of the indoor pouring transformer. The epoxy resin, filler, and curing agent used in the outdoor transformer should meet the outdoor operating conditions.
3, oil-immersed insulation structure In China's transformers, the current oil-immersed voltage transformer accounts for a large proportion, its structure is commonly used in 35kV and above all levels of voltage transformers. Lower-voltage indoor products also use this structure. Oil-immersed voltage transformers can be classified into single-stage and cascade-type voltage transformers. Single-stage voltage transformers are used for voltage levels up to and including 220kV. Cascade voltage transformers are used for voltage levels up to 66kV and above.
The insulation of oil-immersed voltage transformers can be divided into: internal insulation in oil and external insulation in air. The main insulation is the insulation of the primary winding and the high voltage lead to the core or the grounding part and to the other windings. Insulation between the core and the core of the tandem type voltage transformer and between the core and the ground is also regarded as the main insulation. Longitudinal insulation is the insulation between windings, layers, and segments.
4. Gas insulated structure SF6 gas is colorless and odorless, has high electrical strength, excellent arc extinguishing performance, good cooling characteristics, non-flammable, and strong arc extinguishing ability, and is an excellent insulating material. Using it for electrical equipment can eliminate the threat of fire, reduce the size of equipment, and increase the reliability of system operation. The emergence of SF6 metal-enclosed combination electric appliance (GIS) has reduced the area of ​​outdoor substations and improved the safety and reliability of operation. The general SF6 gas insulation transformer is used in the GIS equipment supporting equipment. After the transformer is installed on the GIS, it is filled with SF6 gas and has good insulation performance.
The SF6 voltage transformer uses a single-phase two-column core. Its body structure is similar to that of an oil-immersed single-stage voltage transformer. It includes winding end insulation, high-voltage lead insulation, primary windings and iron yoke, and other grounded metal parts such as housings. Insulation. If three single-phase voltage transformers are installed in one enclosure, they also include phase insulation. The inter-layer insulation uses weft and giant adhesive tapes and poly-thin films, and the primary winding section adopts a rectangular or graded pagoda shape. Lead insulation is different depending on whether the transformer is a matched or stand-alone type. At present, domestic manufacturers use the SF6 gap of high-voltage leads and other accessories to ensure the insulation strength.
The error of the transformer using SF6 gas is stable. At present, only the grounding type is used, and the single-item type is used for the phase-separated and fully-closed combinational appliances. The three phases consist of three single-phase transformers and are used for three-phase total-closed combination appliances. There is also a stand-alone single-phase SF6 gas insulated transformer for general open substations.
Second, the factors causing insulation aging
In the long-term operation of electrical equipment insulation, a series of physical changes such as softening or melting of solid media, physical changes of low molecular weight compounds and plasticizers, and chemical changes such as oxidation, electrolysis, ionization, and generation of new substances occur. Changes, resulting in gradual deterioration of electrical, mechanical and other properties such as increased conductance and dielectric loss, brittleness, cracking, etc., these phenomena are collectively referred to as insulation aging. Insulation aging eventually leads to insulation failure and the power equipment cannot continue to operate. The relationship between the life of the insulating material shown and the aging time is shown in Figure 1.
There are many reasons for promoting insulation aging. They mainly include the effects of heat, electricity, and mechanical forces. In addition, there are the effects of moisture (moisture), oxidation, various rays, and microorganisms. The speed of insulation aging is closely related to the insulation structure, material, manufacturing process, operating environment, voltage, and load conditions.
1. Thermal aging Insulation of electrical equipment is caused by high ambient temperature during operation or due to heating of the electrical equipment itself. In 1930, vmmontsinger first proposed the empirical relationship between insulation life and temperature, ie, the 10°C rule. It is believed that the insulation life is approximately halved for every 10°C increase in temperature. In fact, the aging rate of different insulation should be different, so the 10°C rule cannot be applied to all insulation systems simply. In 1948, Dakin proposed a new view that heat aging is actually an oxidation effect of the splitting of polymer chains and is essentially a chemical reaction process. Therefore, the chemical reaction rate equation should be followed:
Lnl=lnα+b/t
Among them, α and b are constants determined by the specific aging reaction, l is the insulation life, and t is the absolute temperature. The proposed equation provides a theoretical basis for the high temperature accelerated aging test and the extrapolation of the test results, making up for the shortcomings of the Montsinger 10°C rule that it is difficult to distinguish aging differences under different conditions.
Under the effect of high temperature, the mechanical strength of the insulation decreases, the structure deforms, the material loses its elasticity due to oxidation and polymerization, or causes the discharge resistance performance to decrease; the insulation breakdown due to the material cracking, the electrical aging life is shortened, because when the temperature increases, The discharge starting voltage is reduced, the discharge intensity is increased, the chemical corrosion generated by the discharge is increased, and the thermal instability can also occur at a lower voltage and frequency. Outdoor electrical equipment can cause the seal to fail due to thermal expansion and contraction, and moisture invades the insulation; or because the coefficient of thermal expansion of the porcelain insulator and the metal part is different, the porcelain insulator breaks when the temperature changes dramatically. However, there are experimental data showing that the test results of the discharge resistance of the material obtained at room temperature cannot be used to predict the performance at high temperatures.
2. Electrical aging Electrical equipment insulation will be affected by the electric field during operation. The electric field strength that insulation withstands has a very big influence on its life span, on the one hand, the field intensity increases, the discharge number increases on the one hand; On the other hand accelerates the process from partial discharge to breakdown. There is no theoretical formula for quantitative description of the aging behavior of insulation under the action of electric field stress. In general, the electrical aging life is not a linear relationship with the field strength, but an inverse power relationship. Local damage may occur in the insulation due to lightning overvoltages and overvoltages. When overvoltage is applied later, the damage gradually expands, eventually leading to complete breakdown.
Electrical aging is an inevitable aging form of all high-voltage electrical equipment. For the insulation of high-voltage electrical equipment, there will be more or less air gap defects on the micro scale or even macro scale. When the external electric field reaches the initial discharge voltage of the air gap, partial discharge occurs, destroying the insulation structure, and gradually reducing its insulation performance. Commonly used single-stress electric aging models have anti-power and exponential models respectively L = Kn
L=αexp(-bE)
In the formula, E is the electric field strength; k, n, α, b are the experimentally determined constants.
The mechanism of electrical aging is very complex, such as the uniformity of the electric field and the frequency of the voltage will affect the speed of the electrical aging. When the solid insulating medium is in a uniform electric field, its breakdown voltage is often higher; but in non-uniform In an electric field, its breakdown voltage tends to be lower. When the same insulation medium is used at different voltage frequencies, the number of discharges increases proportionally with the frequency. Therefore, in addition to thermal breakdown caused by very high frequencies, the electrical aging life of general insulation is inversely proportional to the frequency. In addition, the longevity curves of different materials are staggered.
Many researchers believe that when the applied voltage is lower than the insulative partial discharge starting discharge voltage, the material will not suffer from aging caused by the electric field. Under the conditions of temperature determination, the life curve of the insulating material tends to be an electric field é—½ value type, and when the external electric field to which the insulation is subjected is lower or close to the value of the electric field, its life will tend to be infinite. For the presence of the above-mentioned smell-valued electric field, there are also people with different opinions that the aging of the insulating medium under the applied electric field is a continuous process and there is no electric field evaluation that significantly affects the aging process. Some scholars have theoretically calculated and measured the nonlinear conductivities of air in the transition from subcorona to intense corona in cavitation. The results show that the micro subcorona current at low voltage will cause gas and cavitation surface temperatures in cavitation. The increase. With the increase of the voltage, the sub-corona discharge form is transformed into a strong corona discharge form, and the temperature of the discharge source will continue to increase, indicating that the aging of the insulation medium under the applied electric field is a continuous process without any significant influence on the aging process. Electric field interpretation. If this view is confirmed by more experiments, it will be more persuasive because of its clear physical processes and clear measurement methods.
3. Mechanical force aging Under the action of electromechanical load, weight, vibration, impact and short-circuit current, the insulation will be destroyed and the mechanical strength will be reduced. When there is tensile stress inside the material, its resistance to discharge decreases. However, the compressive stress has little effect on its resistance to discharge. Since the material often has residual tensile stress during its manufacture and application, its effect on the aging life of the material is extremely important.
4. Humidity The relative humidity of the environment has an effect on the resistance of the insulating material to surface discharge. If the insulation is subjected to a surface discharge, the relative humidity of the environment has a significant effect on the discharge resistance of the material. As a result of the discharge at high relative humidity, a semiconducting layer is formed on the surface of the material, causing the discharge to decay. Therefore, in the case of surface discharge, the electrical aging life of the insulating material increases with the increase of relative humidity within a certain range of relative humidity, but at a relatively high relative humidity, the life shortens with the increase of relative humidity. If moisture intrudes into the interior of the insulation, it will cause increased dielectric loss or drop in breakdown voltage. For some insulating materials such as polyethylene, due to the presence of moisture, dendrites can also occur at very low electric field strengths.
5, chemical aging Insulation materials in the role of moisture, acid, ozone, nitrogen oxides, etc., the physical structure and chemical properties will change, resulting in reduced electrical and mechanical properties. For example, transformer oil in the air will produce organic acids due to oxidation, so that the dielectric loss increases: At the same time will also form a solid precipitate, plug the oil channel, affect the convection heat, so that the insulation temperature rise and the insulation performance decreased.
6. Other aging factors Insulating materials used outdoors must be exposed to direct sunlight and aging under UV light. Insulation materials used in nuclear reactors and X-ray devices are exposed to radiation and are subject to aging. In addition, insulation materials in tropical areas are also subject to damage by various microorganisms, the so-called microbial aging.
Insulation materials are often simultaneously subjected to a variety of aging factors in the practical application, and the effect is not a simple superposition of aging effects of various single factors. There is still interaction between them, so the aging mechanism is very complicated.
Third, voltage transformer insulation structure development trend
The principle of voltage transformer is relatively simple, different users, according to the occasion, use of the voltage transformer, the speed of product replacement for the voltage transformer proposed different life requirements, due to different life expectancy, product costs, prices have great differences The insulation design life of the voltage transformer will be designed in accordance with the life expectancy of the user, which will completely change the concept that the old product is used as long as possible.
The stress-strength interference theory in mechanical design will be introduced into the electrical insulation reliability technology. The development of this technology makes the insulation life of the electrical product can be differentiated according to the user's service life of the voltage transformer of different occasions and applications. Become reality.
The weak insulation life cycle of the voltage transformer, that is, the electric field concentration problem caused by the distortion of the potential line caused by the tip electrode of the primary coil at the right angle. This problem can be solved by increasing the insulation of the weak area, but increasing the insulation should be considered in conjunction with the increase in cost; it may also be considered to adopt a shielding method to improve the design of the coil to reduce the stress of the electric field and temperature field to optimize, such as increasing Large coil wire diameter, increased coil diameter to increase heat dissipation, rounding of the right-angle portion of the coil, and the addition of copper pressure equalizing rings to the primary coil solve the problem of electric field distortion. The way of lowering the temperature and increasing the copper pressure equalizing ring all need to increase the cost of copper metal, which is not economically worthwhile. Therefore, it is more economical to round the right angle portion of the primary coil, and the tip electrode and the electrode that causes electric field concentration are selected. To become smoother, the way to improve the electric field distribution is optimized.
I. Insulation structure of voltage transformer
There are four types of voltage transformers: dry, pouring, oil-immersed and gas-typed according to the insulating medium. The insulation structure of the voltage transformer adopts different methods according to different voltage levels and different operating environments. The low-voltage transformers usually used for measurement are dry type, and the high-voltage or ultra-high-pressure sealed gas-insulated (such as sulfur hexafluoride) transformers are also dry type. The pouring type is applicable to voltage transformers of 35kV and below, and products of 35kV and above are all oil-immersed. Among them, compared with the traditional oil-immersed and inflatable transformers, the epoxy resin casting transformer has the advantages of oil-free, airless, non-enclosure, lifetime maintenance-free, and good insulation performance. It is widely used in the world. A transformer.
1. Dry insulating structure The dry insulating structure is simple to manufacture and low in cost, but its dielectric strength is not high, so the voltage using this structure is generally not more than 380V. The wire uses QZ type enamelled round copper wire. The insulating materials used in the insulation structure are: insulating paper, glass cloth tape, phenolic plastic and so on. Between the coil and the core is used bakelite or plastic frame insulation, insulation between the layers and the primary insulation between the primary and secondary coils, generally yellow wax or polyester film. Instrument voltage transformer insulation between the ring core and the coil, generally the use of insulating cardboard, and then wrapped around a layer of glass ribbon, yellow wax or polyester film tape. The dry structure is simple, but its volume is large. It is only suitable for low voltage indoor devices, or instrument voltage transformers of 10kv and below.
2. Casting Insulation Structure The so-called cast insulation is resin, filler, pigment and curing agent mixed in a certain proportion and then poured into the mold of the primary and secondary windings and other parts of the transformer, and cured. The solid insulation formed by the molding. The solidified hybrid rubber not only fixes the relevant parts, but also is the main insulation of the transformer. Cast insulation has good insulation properties, high mechanical strength, moisture, fire and other advantages. The resin compound adhesive has good fluidity at room temperature or high temperature, can fill small gaps, and is easily poured into more complicated shapes. The resin compound adhesive also has a strong adhesive effect and can firmly fix metals and many insulating materials. Bonded together, it is an ideal transformer insulation molding material, which is widely used in indoor voltage transformers of 35kV and below.
At present, the resins commonly used in transformers in China are epoxy resin and unsaturated resin. Unsaturated resin is inexpensive, can be cured at room temperature, and the casting process is simple, but its electrical and mechanical strength is low, and its heat resistance is poor. In addition, the saturated vapor pressure of unsaturated resin is high, the mixed rubber is not suitable for vacuum degassing, and it is not suitable for pouring. Vacuum is applied, and bubbles are poured in the body. The cure shrinkage of the unsaturated resin is large, and the mixed rubber is easily cracked when it is cured. Therefore, unsaturated resins are only suitable for low voltage products. Epoxy resin has small curing shrinkage and low saturated vapor pressure, and is suitable for pouring under high temperature and high vacuum, so that the blended rubber has better fluidity and can be degassed to the utmost so as to obtain an ideal castable body.
There are two types of casting insulation: semi-casting and casting. The semi-casting method is to cast the coils separately and then install the iron cores. The full casting method is to cast the coils and iron cores together and put them together. The epoxy resin fully-poured voltage transformer has the following characteristics: (1) It achieves oil-free and protects the environment; (2) It achieves maintenance-free, saves a lot of manpower and material resources and power outage time; (3) its manufacturing materials are all For non-flammable or flame-retardant substances, flame retardant, explosion-proof.
The insulation of the core and the coil of the coil is borne by the resin, and no entrapment of air bubbles or conductive impurities is required during pouring. Insulation between coil layers uses cable paper or composite insulation paper. The primary insulation between primary and secondary coils is epoxy resin cartridges, phenolic paper drums, or vacuum-impregnated cable drums. The plastic pouring type has a compact structure and is easy to maintain. It is suitable for indoor devices ranging from 3 to 10 kV.
Epoxy resin is a kind of chemical raw material that has been widely used for a long time. It is not only a flame-retardant and flame-retardant material, but also has excellent electrical properties. It was gradually adopted by the electrician manufacturing industry. Pouring voltage transformers are compact in structure and easy to maintain. With the development of outdoor resins, they will gradually be used in outdoor products of 35kV or more. Pouring voltage transformers are divided into semi-casting and full-casting according to the form of pouring. Among them, the primary winding and each low-voltage winding, as well as the two casings at the outlet end of the primary winding are all cast into a single body. Then the core is assembled with a commonly used semi-casting structure. The advantage is that the pouring body is relatively simple and easy to manufacture. The structure is not compact enough, the iron core will have rust, and it needs regular maintenance; the winding and the core are all poured into one and is called full casting type. Its feature is compact structure, almost no maintenance, but the pouring body is more complex, and the core buffer layer is compared. trouble.
The external insulation of the pouring transformer differs depending on the indoor and outdoor structures. The indoor transformer uses a mixed glue to cast the lead wire of the primary winding into a bushing to ensure the insulation distance between the lead terminal and the base or the exposed iron core. The pouring bushing is generally a cone, a cylinder or a square cylinder, and sometimes an umbrella skirt is also provided as required. . There are many sheds on the outside insulation of outdoor transformers to increase the creepage distance along the surface. It is also sometimes designed as an umbrella skirt to increase its anti-fouling and anti-condensation flashover capabilities. The plastic material of the outdoor pouring transformer is different from the plastic material of the indoor pouring transformer. The epoxy resin, filler, and curing agent used in the outdoor transformer should meet the outdoor operating conditions.
3, oil-immersed insulation structure In China's transformers, the current oil-immersed voltage transformer accounts for a large proportion, its structure is commonly used in 35kV and above all levels of voltage transformers. Lower-voltage indoor products also use this structure. Oil-immersed voltage transformers can be classified into single-stage and cascade-type voltage transformers. Single-stage voltage transformers are used for voltage levels up to and including 220kV. Cascade voltage transformers are used for voltage levels up to 66kV and above.
The insulation of oil-immersed voltage transformers can be divided into: internal insulation in oil and external insulation in air. The main insulation is the insulation of the primary winding and the high voltage lead to the core or the grounding part and to the other windings. Insulation between the core and the core of the tandem type voltage transformer and between the core and the ground is also regarded as the main insulation. Longitudinal insulation is the insulation between windings, layers, and segments.
4. Gas insulated structure SF6 gas is colorless and odorless, has high electrical strength, excellent arc extinguishing performance, good cooling characteristics, non-flammable, and strong arc extinguishing ability, and is an excellent insulating material. Using it for electrical equipment can eliminate the threat of fire, reduce the size of equipment, and increase the reliability of system operation. The emergence of SF6 metal-enclosed combination electric appliance (GIS) has reduced the area of ​​outdoor substations and improved the safety and reliability of operation. The general SF6 gas insulation transformer is used in the GIS equipment supporting equipment. After the transformer is installed on the GIS, it is filled with SF6 gas and has good insulation performance.
The SF6 voltage transformer uses a single-phase two-column core. Its body structure is similar to that of an oil-immersed single-stage voltage transformer. It includes winding end insulation, high-voltage lead insulation, primary windings and iron yoke, and other grounded metal parts such as housings. Insulation. If three single-phase voltage transformers are installed in one enclosure, they also include phase insulation. The inter-layer insulation uses weft and giant adhesive tapes and poly-thin films, and the primary winding section adopts a rectangular or graded pagoda shape. Lead insulation is different depending on whether the transformer is a matched or stand-alone type. At present, domestic manufacturers use the SF6 gap of high-voltage leads and other accessories to ensure the insulation strength.
The error of the transformer using SF6 gas is stable. At present, only the grounding type is used, and the single-item type is used for the phase-separated and fully-closed combinational appliances. The three phases consist of three single-phase transformers and are used for three-phase total-closed combination appliances. There is also a stand-alone single-phase SF6 gas insulated transformer for general open substations.
Second, the factors causing insulation aging
In the long-term operation of electrical equipment insulation, a series of physical changes such as softening or melting of solid media, physical changes of low molecular weight compounds and plasticizers, and chemical changes such as oxidation, electrolysis, ionization, and generation of new substances occur. Changes, resulting in gradual deterioration of electrical, mechanical and other properties such as increased conductance and dielectric loss, brittleness, cracking, etc., these phenomena are collectively referred to as insulation aging. Insulation aging eventually leads to insulation failure and the power equipment cannot continue to operate. The relationship between the life of the insulating material shown and the aging time is shown in Figure 1.
There are many reasons for promoting insulation aging. They mainly include the effects of heat, electricity, and mechanical forces. In addition, there are the effects of moisture (moisture), oxidation, various rays, and microorganisms. The speed of insulation aging is closely related to the insulation structure, material, manufacturing process, operating environment, voltage, and load conditions.
1. Thermal aging Insulation of electrical equipment is caused by high ambient temperature during operation or due to heating of the electrical equipment itself. In 1930, vmmontsinger first proposed the empirical relationship between insulation life and temperature, ie, the 10°C rule. It is believed that the insulation life is approximately halved for every 10°C increase in temperature. In fact, the aging rate of different insulation should be different, so the 10°C rule cannot be applied to all insulation systems simply. In 1948, Dakin proposed a new view that heat aging is actually an oxidation effect of the splitting of polymer chains and is essentially a chemical reaction process. Therefore, the chemical reaction rate equation should be followed:
Lnl=lnα+b/t
Among them, α and b are constants determined by the specific aging reaction, l is the insulation life, and t is the absolute temperature. The proposed equation provides a theoretical basis for the high temperature accelerated aging test and the extrapolation of the test results, making up for the shortcomings of the Montsinger 10°C rule that it is difficult to distinguish aging differences under different conditions.
Under the effect of high temperature, the mechanical strength of the insulation decreases, the structure deforms, the material loses its elasticity due to oxidation and polymerization, or causes the discharge resistance performance to decrease; the insulation breakdown due to the material cracking, the electrical aging life is shortened, because when the temperature increases, The discharge starting voltage is reduced, the discharge intensity is increased, the chemical corrosion generated by the discharge is increased, and the thermal instability can also occur at a lower voltage and frequency. Outdoor electrical equipment can cause the seal to fail due to thermal expansion and contraction, and moisture invades the insulation; or because the coefficient of thermal expansion of the porcelain insulator and the metal part is different, the porcelain insulator breaks when the temperature changes dramatically. However, there are experimental data showing that the test results of the discharge resistance of the material obtained at room temperature cannot be used to predict the performance at high temperatures.
2. Electrical aging Electrical equipment insulation will be affected by the electric field during operation. The electric field strength that insulation withstands has a very big influence on its life span, on the one hand, the field intensity increases, the discharge number increases on the one hand; On the other hand accelerates the process from partial discharge to breakdown. There is no theoretical formula for quantitative description of the aging behavior of insulation under the action of electric field stress. In general, the electrical aging life is not a linear relationship with the field strength, but an inverse power relationship. Local damage may occur in the insulation due to lightning overvoltages and overvoltages. When overvoltage is applied later, the damage gradually expands, eventually leading to complete breakdown.
Electrical aging is an inevitable aging form of all high-voltage electrical equipment. For the insulation of high-voltage electrical equipment, there will be more or less air gap defects on the micro scale or even macro scale. When the external electric field reaches the initial discharge voltage of the air gap, partial discharge occurs, destroying the insulation structure, and gradually reducing its insulation performance. Commonly used single-stress electric aging models have anti-power and exponential models respectively L = Kn
L=αexp(-bE)
In the formula, E is the electric field strength; k, n, α, b are the experimentally determined constants.
The mechanism of electrical aging is very complex, such as the uniformity of the electric field and the frequency of the voltage will affect the speed of the electrical aging. When the solid insulating medium is in a uniform electric field, its breakdown voltage is often higher; but in non-uniform In an electric field, its breakdown voltage tends to be lower. When the same insulation medium is used at different voltage frequencies, the number of discharges increases proportionally with the frequency. Therefore, in addition to thermal breakdown caused by very high frequencies, the electrical aging life of general insulation is inversely proportional to the frequency. In addition, the longevity curves of different materials are staggered.
Many researchers believe that when the applied voltage is lower than the insulative partial discharge starting discharge voltage, the material will not suffer from aging caused by the electric field. Under the conditions of temperature determination, the life curve of the insulating material tends to be an electric field é—½ value type, and when the external electric field to which the insulation is subjected is lower or close to the value of the electric field, its life will tend to be infinite. For the presence of the above-mentioned smell-valued electric field, there are also people with different opinions that the aging of the insulating medium under the applied electric field is a continuous process and there is no electric field evaluation that significantly affects the aging process. Some scholars have theoretically calculated and measured the nonlinear conductivities of air in the transition from subcorona to intense corona in cavitation. The results show that the micro subcorona current at low voltage will cause gas and cavitation surface temperatures in cavitation. The increase. With the increase of the voltage, the sub-corona discharge form is transformed into a strong corona discharge form, and the temperature of the discharge source will continue to increase, indicating that the aging of the insulation medium under the applied electric field is a continuous process without any significant influence on the aging process. Electric field interpretation. If this view is confirmed by more experiments, it will be more persuasive because of its clear physical processes and clear measurement methods.
3. Mechanical force aging Under the action of electromechanical load, weight, vibration, impact and short-circuit current, the insulation will be destroyed and the mechanical strength will be reduced. When there is tensile stress inside the material, its resistance to discharge decreases. However, the compressive stress has little effect on its resistance to discharge. Since the material often has residual tensile stress during its manufacture and application, its effect on the aging life of the material is extremely important.
4. Humidity The relative humidity of the environment has an effect on the resistance of the insulating material to surface discharge. If the insulation is subjected to a surface discharge, the relative humidity of the environment has a significant effect on the discharge resistance of the material. As a result of the discharge at high relative humidity, a semiconducting layer is formed on the surface of the material, causing the discharge to decay. Therefore, in the case of surface discharge, the electrical aging life of the insulating material increases with the increase of relative humidity within a certain range of relative humidity, but at a relatively high relative humidity, the life shortens with the increase of relative humidity. If moisture intrudes into the interior of the insulation, it will cause increased dielectric loss or drop in breakdown voltage. For some insulating materials such as polyethylene, due to the presence of moisture, dendrites can also occur at very low electric field strengths.
5, chemical aging Insulation materials in the role of moisture, acid, ozone, nitrogen oxides, etc., the physical structure and chemical properties will change, resulting in reduced electrical and mechanical properties. For example, transformer oil in the air will produce organic acids due to oxidation, so that the dielectric loss increases: At the same time will also form a solid precipitate, plug the oil channel, affect the convection heat, so that the insulation temperature rise and the insulation performance decreased.
6. Other aging factors Insulating materials used outdoors must be exposed to direct sunlight and aging under UV light. Insulation materials used in nuclear reactors and X-ray devices are exposed to radiation and are subject to aging. In addition, insulation materials in tropical areas are also subject to damage by various microorganisms, the so-called microbial aging.
Insulation materials are often simultaneously subjected to a variety of aging factors in the practical application, and the effect is not a simple superposition of aging effects of various single factors. There is still interaction between them, so the aging mechanism is very complicated.
Third, voltage transformer insulation structure development trend
The principle of voltage transformer is relatively simple, different users, according to the occasion, use of the voltage transformer, the speed of product replacement for the voltage transformer proposed different life requirements, due to different life expectancy, product costs, prices have great differences The insulation design life of the voltage transformer will be designed in accordance with the life expectancy of the user, which will completely change the concept that the old product is used as long as possible.
The stress-strength interference theory in mechanical design will be introduced into the electrical insulation reliability technology. The development of this technology makes the insulation life of the electrical product can be differentiated according to the user's service life of the voltage transformer of different occasions and applications. Become reality.
The weak insulation life cycle of the voltage transformer, that is, the electric field concentration problem caused by the distortion of the potential line caused by the tip electrode of the primary coil at the right angle. This problem can be solved by increasing the insulation of the weak area, but increasing the insulation should be considered in conjunction with the increase in cost; it may also be considered to adopt a shielding method to improve the design of the coil to reduce the stress of the electric field and temperature field to optimize, such as increasing Large coil wire diameter, increased coil diameter to increase heat dissipation, rounding of the right-angle portion of the coil, and the addition of copper pressure equalizing rings to the primary coil solve the problem of electric field distortion. The way of lowering the temperature and increasing the copper pressure equalizing ring all need to increase the cost of copper metal, which is not economically worthwhile. Therefore, it is more economical to round the right angle portion of the primary coil, and the tip electrode and the electrode that causes electric field concentration are selected. To become smoother, the way to improve the electric field distribution is optimized.
Plate rolling is a process of continuous three-point bending of sheet metal using a plate rolling machine. The equipment puts the sheet material between the upper and lower work rolls when rolling. The upper roller is raised and lowered vertically, and the two lower rollers rotate and move horizontally relative to the axis of the upper roller. When the upper roll descends, the plate is plastically deformed and bent between the upper and lower work rolls. The continuous rotation of the bottom roller drives the steel plate to advance and retreat through the friction between the plate and the roller to complete the coiling.
1. Pre-bending. When the plate is rolled, there is a length at both ends of the plate that does not bend because it does not touch the upper roller. It is called the remaining straight edge. In the process, the minimum force arm at which the plate starts to bend is called the theoretical remaining straight edge. The bending form (symmetrical bending, asymmetrical bending) is related.
2. Centering. The purpose of centering is to make the generatrix of the workpiece parallel to the roller axis to prevent skew.
3. Roll round. Rolling is the main process of product forming, which is divided into two types: one-time feed and multiple feeds. Multiple feeds are commonly used for rolling thick plates. The number of feeds depends on the technological constraints (such as the maximum allowable deformation during cold rolling) and equipment constraints (such as non-slip conditions and power conditions). When the rebound of the cold coil is significant, a certain amount of overcoiling must be added.
4. Straightening round. The purpose of rounding is to make the curvature of the entire circle as uniform as possible to ensure product quality. The general rounding process is divided into three steps:
(1) Load: According to experience or calculation, adjust the work roll to the position of the required maximum correction curvature.
(2) Rounding: Roll the roller 1 to 2 turns under the corrected curvature to make the entire curling rate uniform.
(3) Unloading: gradually unload the load, so that the workpiece is rolled several times under the gradually reduced correction load.
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