CN111709136B - Method for calculating the insulation aging degree of power transformers under different ambient temperatures - Google Patents

Abstract

The invention aims to provide a method for calculating insulation aging degree of a power transformer under different external environment temperatures, which is based on an actual structure of an oil immersed transformer, and is characterized in that a temperature field model of the transformer is established by considering heat conduction between a winding and an iron core, convective heat exchange between the winding and the iron core and radiation heat exchange between an oil tank and air, and according to different environment temperatures, the service life of the transformer is predicted on the basis of influence of the environment temperatures on hot spots of the transformer, so that the relative service life loss rate of the transformer under different environments is obtained, and the insulation aging degree of the transformer is obtained. In order to achieve the above purpose, the present invention provides the following technical solutions: s1: on the basis of a three-dimensional physical model of the oil immersed transformer, a temperature rise model of the transformer is established; s2: analyzing the change condition of the hot spot of the transformer according to the different environmental temperatures; s3: a method for predicting the life of a transformer based on the influence of the temperature of a computing environment on the hot spot of the transformer is analyzed; s4: and analyzing the process that the service life of the budget and the relative aging rate follow the temperature change to obtain the relative life loss rate of the transformer at different environment temperatures.

Description

Method for calculating the insulation aging degree of power transformers under different ambient temperatures

technical field

The invention belongs to the research field of service life of electrical equipment, and relates to a method for calculating the insulation aging degree of a power transformer under different external ambient temperatures.

Background technique

The biggest factor affecting the working status of power transformers in real work is heating and insulation problems. Temperature rise is an important reference to measure whether the transformer is safe and stable. "Oil-immersed Transformer Load Guidelines" pointed out that the main factor limiting the load capacity of the transformer is the highest temperature value reached during the operation of the transformer, so this temperature value and the location where this temperature value occurs should be described as accurately as possible. Studies have shown that the expected life of the transformer is inversely proportional to the hot spot temperature of the exponential transformer winding, that is to say, the higher the hot spot temperature, the shorter the expected life of the transformer. The aging of power transformer insulation materials is mainly related to temperature, humidity, degraded substances in transformer oil and oxygen. Considering that transformers will work in different external environments, it is necessary to discuss the aging degree of transformer insulation under different external ambient temperatures. The life of transformers can be predicted, so that people can work more safely and effectively prevent accidents caused by aging.

Contents of the invention

The purpose of the present invention is to provide a method for calculating the insulation aging degree of power transformers under different external ambient temperatures, based on the actual structure of oil-immersed transformers, and considering the heat conduction between windings and iron cores, and the relationship between windings and iron cores and transformer oil. The convective heat transfer and the radiation heat transfer between the oil tank and the air establish the temperature field model of the transformer. According to the difference of the ambient temperature, the life of the transformer is predicted on the basis of the influence of the ambient temperature on the hot spots of the transformer, and the relative life loss rate of the transformer in different environments is obtained.

To achieve the above object, the present invention provides the following technical solutions:

S1: Based on the three-dimensional physical model of the oil-immersed transformer, the temperature rise model of the transformer is established;

S2: According to the difference of ambient temperature, analyze the change of transformer hot spot;

S3: Analyze the method of predicting the life of the transformer based on the calculation of the influence of the ambient temperature on the hot spot of the transformer;

S4: Analyze the process of estimated service life and relative aging rate following temperature changes, and obtain the relative life loss rate of transformers under different ambient temperatures;

Further, step S1 specifically includes adding a solid heat transfer model and a laminar flow model on the three-dimensional physical basis of the oil-immersed transformer, thereby establishing a temperature rise model of the transformer. The solid heat transfer winding includes the heat conduction between solids, and the convective heat transfer should also be taken into account, so the fluid domain control equation needs to be introduced, as shown in the formula:

The heat dissipation inside the natural oil circulation transformer mainly depends on the natural flow caused by thermal buoyancy. According to a large number of experiments, the Reynolds number is related to the fluid density, fluid flow velocity, fluid dynamic viscosity and pipe diameter. The Reynolds number is used to judge the fluid flow state. Dimensionless number, when the Reynolds number of the fluid is less than 2300, the state of laminar flow will be maintained, and when the Reynolds number of the fluid is greater than 2300, the state of turbulent flow will be maintained.

For natural oil circulation oil-immersed power transformers, the flow rate of transformer oil is relatively slow, which belongs to the laminar flow model, and the oil flow can be made into an incompressible fluid in engineering:

In the formula: μ is the dynamic viscosity of transformer oil; ρ is the density of transformer oil; is the principal stress tensor.

Step S2 is specifically to analyze the change of hot spots of the transformer according to the different ambient temperatures: the oil-immersed power transformer operates at different ambient temperatures, and the change of the ambient temperature will change the heat dissipation of the oil tank wall and the radiator, thereby affecting the oil-immersed power transformer. It will affect the temperature rise inside the power transformer.

Taking the 50MVA/110kV oil-immersed power transformer as an example, through the established transformer temperature rise model, under the same structural design and heat source conditions, the ambient temperature under the rated working condition is 263.15K, 268.15K, 273.15K, 278.15K , 283.15K, 288.15K, 293.15K, 298.15K, 303.15K temperature rise of transformer core and low voltage winding.

Under the same heat source and conditions, changing the ambient temperature does not affect the temperature distribution trend but only affects the temperature rise, and the hot spots of the windings appear on the low-voltage windings. List the calculated hot-spot temperatures and average temperatures of the core and low-voltage windings, and compare them The influence of different ambient temperatures on the core and winding hot spots and average temperatures of oil-immersed power transformers was analyzed.

According to Newton's cooling law and Stefan Boltzmann's radiation law, it can be known that the larger the temperature difference, the better the heat dissipation. As the ambient temperature rises, the core, winding hot spot temperature and average temperature will rise accordingly.

Step S3 is specifically analyzing the method of predicting the service life of the transformer on the basis of calculating the influence of the ambient temperature on the hot spots of the transformer. Based on the research on transformer insulation aging at home and abroad, temperature is the biggest factor affecting insulation aging. At present, there is no definite and simple criterion to calculate the life of the transformer, which is usually judged by the expected life. Studies have shown that the life expectancy of the transformer is inversely proportional to the hot spot temperature of the transformer winding, that is to say, the higher the hot spot temperature, the shorter the life expectancy of the transformer. In the range of 80°C to 140°C, the relationship between the expected life of the transformer and the hot spot temperature can be expressed as:

z=Ae ^-Pθ

Among them, z is the expected life of the transformer; A is a constant related to the composition of the material and the moisture and free oxygen in the insulation; P is the temperature coefficient and has nothing to do with the quality of the fiber.

The normal life expectancy of the transformer is calculated according to:

z _N ＝Ae ^-P×98

Relative life expectancy at any temperature expressed in z/z _N :

The relative aging rate is:

v=e ^P(θ-98)

Among them, θ is any temperature.

According to the law of thermal aging, when the winding temperature is lower than 80°C, the loss of the mechanical strength and electrical strength of the transformer insulation is very small and negligible. When the winding temperature is equal to 80°C, the relative aging rate is 0.125. When the winding temperature increases by 6°C, The aging rate increases exponentially, that is, when the winding temperature is 86°C, the relative aging rate is 0.25, and so on for the insulation aging rates at various temperatures.

Step S4 is specifically to analyze the process of estimated service life and relative aging rate following temperature changes, and obtain the relative life loss rate of the transformer under different ambient temperatures; generally, when designing a transformer, the ambient temperature is considered to be 20°C, and the reference value of the hot spot temperature is 98°C. ℃, the expected life of the transformer is 20 to 30 years, and the insulation aging rate at this time is assumed to be 1. When the mechanical strength of the transformer insulation is reduced to 15% to 20% of its rated value, the life of the transformer is considered to be terminated. In engineering, relative life expectancy and relative aging rate are usually used to describe the aging degree of transformers.

The transformer is affected by the ambient temperature and load changes during operation, and the hot spot temperature of the winding changes greatly. When the temperature is higher than 98°C, it will age rapidly, and when the temperature is lower than 98°C, the aging speed will be very slow. If the two parts interact with each other Compensation, that is, the life lost in a period of time is equal to the life lost in the transformer when the winding hot spot temperature is 98°C in the same time interval. It can be considered that the life loss caused by the change of winding temperature in this period of time is the same as the winding temperature is constant at 98°C The loss of life expectancy is equivalent.

The equivalent aging principle can be expressed as:

Among them, T is the time interval; C is a constant.

By quantifying the time in the above formula, the degree of aging at each moment can be obtained, that is, the life loss.

Transformer relative life loss rate V:

Among them, θ _cr is the temperature under the rated load condition, and θ _c is the winding hot spot temperature.

The beneficial effect of the invention is that: the insulation aging degree of the power transformer under different ambient temperatures can be obtained by using the method of the invention.

Description of drawings

Fig. 1 is calculation logic diagram of the present invention

Fig. 2 is the schematic diagram of iron core and winding hot spot temperature under different ambient temperatures of the present invention

Figure 3 is a schematic diagram of the average temperature of the iron core and low-voltage winding under different ambient temperatures

Fig. 4 is the schematic diagram of iron core and low-voltage winding hot spot and average temperature of the present invention changing with ambient temperature

Fig. 5 is a schematic diagram of the variation of insulation aging rate with temperature in the present invention

Fig. 6 is a schematic diagram of the variation of the temperature of the hot spot in the transformer with the ambient temperature of the present invention

Fig. 7 is a schematic diagram of the relative life loss rate of the transformer under different ambient temperatures according to the present invention

Detailed ways:

Below in conjunction with accompanying drawing, the present invention is described in detail

The purpose of the present invention is to provide a method for calculating the insulation aging degree of power transformers under different external ambient temperatures, based on the actual structure of oil-immersed transformers, and considering the heat conduction between windings and iron cores, and the relationship between windings and iron cores and transformer oil. The convective heat transfer and the radiation heat transfer between the oil tank and the air establish the temperature field model of the transformer. According to the difference of the ambient temperature, the life of the transformer is predicted on the basis of the influence of the ambient temperature on the hot spot of the transformer, and the relative life loss rate of the transformer in different environments is obtained. . Specific steps are as follows:

1. On the basis of the three-dimensional physical model of the oil-immersed transformer, the temperature rise model of the transformer is established

Based on the three-dimensional physical basis of the oil-immersed transformer, a solid heat transfer model and a laminar flow model are added to establish a temperature rise model of the transformer. The solid heat transfer winding includes the heat conduction between solids, and the convective heat transfer should also be taken into account, so the fluid domain control equation needs to be introduced, as shown in the formula:

The heat dissipation inside the natural oil circulation transformer mainly depends on the natural flow caused by thermal buoyancy. According to a large number of experiments, the Reynolds number is related to the fluid density, fluid flow velocity, fluid dynamic viscosity and pipe diameter. The Reynolds number is used to judge the fluid flow state. Dimensionless number, when the Reynolds number of the fluid is less than 2300, the state of laminar flow will be maintained, and when the Reynolds number of the fluid is greater than 2300, the state of turbulent flow will be maintained.

For natural oil circulation oil-immersed power transformers, the flow rate of transformer oil is relatively slow, which belongs to the laminar flow model, and the oil flow can be made into an incompressible fluid in engineering:

In the formula: μ is the dynamic viscosity of transformer oil; ρ is the density of transformer oil; is the principal stress tensor.

2. According to the difference of ambient temperature, analyze the change of transformer hot spot

The oil-immersed power transformer works under different ambient temperatures, and the change of the ambient temperature changes the heat dissipation of the oil tank wall and the radiator, which in turn affects the temperature rise inside the oil-immersed power transformer. As shown in Figure 1 and Figure 2.

Taking the 50MVA/110kV oil-immersed power transformer as an example, through the established transformer temperature rise model, under the same structural design and heat source conditions, the ambient temperature under the rated working condition is 263.15K, 268.15K, 273.15K, 278.15K , 283.15K, 288.15K, 293.15K, 298.15K, 303.15K temperature rise of transformer core and low voltage winding.

Under the same heat source and conditions, changing the ambient temperature does not affect the temperature distribution trend but only affects the temperature rise, and the hot spots of the windings appear on the low-voltage windings. List the calculated hot-spot temperatures and average temperatures of the core and low-voltage windings, and compare them The influence of different ambient temperatures on the core and winding hot spots and average temperatures of oil-immersed power transformers was analyzed.

According to Newton's cooling law and Stefan Boltzmann's radiation law, it can be known that the larger the temperature difference, the better the heat dissipation. As the ambient temperature rises, the core, winding hot spot temperature and average temperature all rise accordingly, as shown in Figure 3.

3. Analyze the method of predicting the life of the transformer based on the calculation of the influence of the ambient temperature on the hot spots of the transformer

Based on the research on transformer insulation aging at home and abroad, temperature is the biggest factor affecting insulation aging. At present, there is no definite and simple criterion to calculate the life of the transformer, which is usually judged by the expected life. Studies have shown that the life expectancy of the transformer is inversely proportional to the hot spot temperature of the transformer winding, that is to say, the higher the hot spot temperature, the shorter the life expectancy of the transformer. In the range of 80°C to 140°C, the relationship between the expected life of the transformer and the hot spot temperature can be expressed as:

z=Ae ^-Pθ

Among them, z is the expected life of the transformer; A is a constant related to the composition of the material and the moisture and free oxygen in the insulation; P is the temperature coefficient and has nothing to do with the quality of the fiber.

The normal life expectancy of the transformer is calculated according to:

z _N ＝Ae ^-P×98

Relative life expectancy at any temperature expressed in z/z _N :

The relative aging rate is:

v=e ^P(θ-98)

Among them, θ is any temperature.

According to the law of thermal aging, when the winding temperature is lower than 80°C, the loss of the mechanical strength and electrical strength of the transformer insulation is very small and negligible. When the winding temperature is equal to 80°C, the relative aging rate is 0.125. When the winding temperature increases by 6°C, The aging rate increases exponentially, that is, when the winding temperature is 86°C, the relative aging rate is 0.25, and so on for the insulation aging rates at various temperatures, as shown in Figure 4. It can be clearly seen that the hot spot temperature rises very quickly at 120°C to 140°C, and the insulation aging rate rises very quickly, so the hot spot temperature of the winding is at a temperature exceeding the design value for a long time, which will cause the life of the transformer to be lost rapidly.

4. Analyze the process of estimated service life and relative aging rate following temperature changes, and obtain the relative life loss rate of transformers under different ambient temperatures;

Generally, when designing a transformer, the ambient temperature is considered to be 20°C, the benchmark value of the hot spot temperature is 98°C, and the expected life of the transformer is 20 to 30 years. The insulation aging rate at this time is assumed to be 1. When the mechanical strength of the transformer insulation is reduced to 15% to 20% of its rated value, the life of the transformer is considered to be terminated. In engineering, relative life expectancy and relative aging rate are usually used to describe the aging degree of transformers.

The transformer is affected by the ambient temperature and load changes during operation, and the hot spot temperature of the winding changes greatly. When the temperature is higher than 98°C, it will age rapidly, and when the temperature is lower than 98°C, the aging speed will be very slow. If the two parts interact with each other Compensation, that is, the life lost in a period of time is equal to the life lost in the transformer when the winding hot spot temperature is 98°C in the same time interval. It can be considered that the life loss caused by the change of winding temperature in this period of time is the same as the winding temperature is constant at 98°C The loss of life expectancy is equivalent.

The equivalent aging principle can be expressed as:

Among them, T is the time interval; C is a constant.

By quantifying the time in the above formula, the degree of aging at each moment can be obtained, that is, the life loss.

Transformer relative life loss rate V:

Among them, θ _cr is the temperature under the rated load condition, and θ _c is the winding hot spot temperature.

The change of hot spot temperature with ambient temperature is shown in Figure 5. Calculate the relative life loss rate of the transformer when the ambient temperature increases at 5°C intervals, as shown in Figure 6. According to the method of calculating the loss rate of transformer life due to ambient temperature, the relative life loss rate can be calculated according to the daily average temperature change curve of the transformer in actual operation, and the daily average insulation aging time can be obtained by multiplying the relative life loss rate by time.

The above preferred embodiments are only to illustrate the technical solutions of the present invention, and are not limited thereto. Those skilled in the art can modify, replace or optimize the forms and details without departing from the scope of the claims. Wait for various changes.

Claims (5)

1. A method for calculating the insulation aging degree of power transformers under different external ambient temperatures, characterized in that it comprises: S1: Based on the three-dimensional physical model of the oil-immersed transformer, the temperature rise model of the transformer is established; S2: According to the difference of ambient temperature, analyze the change of hot spots of oil-immersed transformer; S3: Analyze and predict the life of the transformer based on the calculation of the influence of the ambient temperature on the hot spots of the oil-immersed transformer; S4: Analyze and predict the process of transformer life and relative aging rate following temperature changes, and obtain the relative life loss rate of transformers under different ambient temperatures. 2. A method for calculating the insulation aging degree of power transformers under different ambient temperatures according to claim 1, step S1 is specifically: on the basis of the three-dimensional basic physical model of the oil-immersed transformer, consider the winding and iron The heat conduction between the cores, the convective heat transfer between the winding and the iron core and the transformer oil, and the radiation heat transfer between the oil tank and the air establish the temperature field model of the transformer. 3. A method for calculating the insulation aging degree of power transformers under different ambient temperatures according to claim 1, step S2 is specifically, the oil-immersed transformer works under different ambient temperatures, and the change of the ambient temperature makes the oil tank wall And the heat dissipation of the radiator changes, which in turn affects the temperature rise inside the oil-immersed transformer. 4. A method for calculating the degree of insulation aging of power transformers under different ambient temperatures according to claim 1, step S3 is specifically, temperature is the biggest influencing factor of insulation aging, on the basis of calculating the influence of ambient temperature on transformer hot spots A method for predicting transformer life; The life expectancy of variable oil-immersed transformers is inversely proportional to the hot spot temperature of transformer windings, the higher the hot spot temperature, the shorter the life expectancy of oil-immersed transformers; In the range of 80°C~140°C, the relationship between the expected life of the oil-immersed transformer and the hot spot temperature can be expressed as: Among them, z is the expected life of the transformer; A is related to the composition of the material and the moisture and free oxygen in the insulation, and is a constant; P is the temperature coefficient, which has nothing to do with the fiber quality; Calculate the normal life expectancy of an oil-filled transformer: use Expressing the relative life expectancy at any temperature: The relative aging rate is: Among them, θ is any temperature. 5. A method for calculating the insulation aging degree of power transformers under different ambient temperatures according to claim 4, step S4 is specifically, using relative life expectancy and relative aging rate to describe the aging degree of oil-immersed transformers in engineering , analyze and predict the process of oil-immersed transformer life and relative aging rate following temperature changes, and obtain the relative life loss rate of oil-immersed transformers under different ambient temperatures; Oil-immersed transformer relative life loss rate V: in, is the temperature under rated load conditions, /> is the winding hot spot temperature.