A method for simulating instantaneous pressure in the elevated area of ​​a converter transformer under arc fault

Abstract

The present invention discloses a method for simulating the instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault, which relates to the technical field of converter transformers and comprises the following steps: importing a grid model into a simulation software, respectively constructing a mass transfer mechanism and an energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states in the simulation software, wherein the reaction mechanism of transformer oil in different physical states includes a phase change reaction of transformer insulating oil vaporization, a pyrolysis reaction of oil vapor, and a cracking gas production reaction induced by an arc; in the constructed simulation software, arc channels at different positions are set for each time step, and a heat source is set in the arc channel to simulate an arc fault, and an instantaneous pressure cloud map of the elevated seat area corresponding to the time step is obtained; a key position in the instantaneous pressure cloud map is selected, and the corresponding instantaneous pressure values ​​are connected to obtain an instantaneous pressure change curve. The present invention restores the pressure change process of the elevated seat area during actual arc discharge.

Description

Technical Field

[0001] The invention relates to the technical field of converter transformers, and in particular to a method for simulating instantaneous pressure in a converter transformer elevated seat area under an arc fault. Background Art

[0002] With the continuous construction of UHV DC projects, the number of converter transformers in operation continues to rise, and their reliability is directly related to the safe and stable operation of the power grid. Converter transformers have large capacity and usually use mineral oil as the main insulation and heat dissipation medium. They have the characteristics of large oil filling volume, high voltage level and complex insulation structure. Once a penetrating high-energy arc discharge fault occurs, a large amount of gas will be generated locally and rapidly and violently, which can easily cause the steel structure of the oil tank shell to fail and explode and catch fire. The transformer riser is a cylindrical additional device installed on the transformer oil tank. It is used to install components such as transformer bushings. It plays a role in raising the height of the bushing electrical connection point, enhancing the insulation performance between the bushing and the oil tank, and providing a stable support for the bushing. In recent years, there have been many explosions and combustion accidents caused by penetrating arc discharges in the oil in the converter transformer riser and other areas. The fire started quickly, the burning time was long, and the fire area was large, causing huge economic losses and serious social impacts.

[0003] Since arc fault tests of real-size power transformers are dangerous and expensive, simulation is usually used in the prior art to simulate the instantaneous pressure in the riser area of ​​the converter transformer, and then to carry out explosion-proof verification of the transformer. With the improvement of computer technology and the advancement of theoretical foundations, the simulation research on the propagation characteristics of arc pressure in oil has gradually developed from the initial pressure estimation method to two-dimensional and three-dimensional simulation models.

[0004] The existing simulation of the converter transformer riser generally adopts a fluid mechanics simulation model that is closest to the actual physical process. This model generally simulates arc faults by directly injecting artificially set gases. However, the simulation process of this model does not involve chemical reactions and lacks energy conversion processes. The physical and chemical mechanism is too simple and cannot restore the pressure change process in the riser area during actual arc discharge. Summary of the invention

[0005] Based on the defects of the above-mentioned prior art, the present invention provides a method for simulating the instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault, which solves the problem that the simulation process of the existing fluid mechanics simulation model does not involve chemical reactions, lacks energy conversion processes, and the physical and chemical mechanisms are too simple, and cannot restore the actual pressure change process in the elevated seat area during arc discharge.

[0006] The present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for simulating instantaneous pressure in a converter transformer elevated seat area under an arc fault, comprising the following steps:

[0008] Construct a simulation model of the converter transformer riser and perform mesh generation to obtain a mesh model;

[0009] Importing the grid model into the simulation software, setting the boundary conditions, material properties, time steps and step intervals of the grid model, and building the mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states in the simulation software, wherein the reaction mechanism of transformer oil in different physical states includes the phase change reaction of transformer insulating oil vaporization, the pyrolysis reaction of oil vapor and the cracking gas production reaction induced by electric arc;

[0010] In the constructed simulation software, arc channels at different positions are set for each time step, and heat sources are set in the arc channels to perform arc fault simulation, so as to obtain multiple instantaneous pressure cloud maps of the elevated seat area; each of the instantaneous pressure cloud maps includes instantaneous pressure values ​​at different positions in the elevated seat area;

[0011] The same key position in multiple instantaneous pressure cloud maps is selected, and the instantaneous pressure values ​​corresponding to the same key position at different time steps are connected to obtain the instantaneous pressure change curve.

[0012] Preferably, the mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states are respectively constructed in the simulation software, wherein the mass transfer mechanism of the phase change reaction of the vaporization of insulating oil is as follows:

[0013] When the liquid phase temperature is higher than the saturation temperature, the liquid phase evaporates, and the mass transferred from the liquid phase to the gas phase is:

[0014] ;

[0015] When the gas phase temperature is lower than the saturation temperature, the gas phase condenses, and the mass transferred from the gas phase to the liquid phase is:

[0016] ;

[0017] In the formula, and are the mass transfer terms from gas phase to liquid phase and from liquid phase to gas phase during phase change, r L is the liquid density, α L is the volume fraction of the liquid phase, r V is the gas phase density, α V is the volume fraction of the gas phase, Tsat is the saturation temperature, coefficient is the coefficient related to the reaction, T L is the liquidus temperature, T V is the gas phase temperature;

[0018] The energy transfer mechanism of the phase change reaction of insulating oil vaporization is as follows:

[0019] ;

[0020] In the formula, L h is the latent heat of vaporization, S h Energy transfer caused by phase change reaction.

[0021] Preferably, the mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states are respectively established in the simulation software, wherein the mass transfer mechanism of the thermal decomposition reaction of oil vapor is as follows:

[0022] 1mol C 20 H 38 The pyrolysis produces 4.632 mol of pyrolysis molecules smaller than their molecular weight, including 1.052 mol of C2H4, 0.805 mol of C3H6, 0.255 mol of CH4, 0.227 mol of H2, 0.191 mol of C4H8, 0.182 mol of C4H6, and 0.144 mol of C2H6; the reaction rate constant is as follows:

[0023] ;

[0024] In the formula, k is the reaction rate constant, A is the pre-exponential factor, E a is the activation energy of the pyrolysis reaction, R is the gas constant, T is the reaction temperature;

[0025] The energy transfer mechanism of the pyrolysis reaction of oil vapor is as follows:

[0026] ;

[0027] In the formula, H is the enthalpy change of the pyrolysis reaction, is the operator symbol, p For the pyrolysis reaction, is the standard molar formation enthalpy of the substance, f is formation, m is mole, It is the standard state.

[0028] Preferably, the mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states are respectively constructed in the simulation software, wherein the mass transfer mechanism of the cracking gas production reaction induced by the electric arc is as follows:

[0029] The gas production rate caused by each W arc power is 47.3 mL / s, of which 73.5% is H2, 18% is C2H2, 5.2% is CH4, 3.1% is C2H4, and 0.2% is C2H6;

[0030] The energy transfer mechanism of the arc-induced cracking gas production reaction is as follows:

[0031] ;

[0032] In the formula, H is the enthalpy change of the pyrolysis reaction, e For the cracking reaction.

[0033] Preferably, the simulation model is meshed based on the watertight geometry workflow to obtain a mesh model, wherein the mesh model includes a plurality of computing units.

[0034] Preferably, the material properties include material properties of transformer oil, oil vapor and pyrolysis molecular gas mixture in each computing unit in the grid model.

[0035] Preferably, the arc channel is located at the center of the highest temperature area of ​​the simulation model at each time step, and the arc channel radius is the same.

[0036] Compared with the prior art, at least one of the above technical solutions adopted by the present invention can achieve the following beneficial effects:

[0037] When the simulation model is imported into the simulation software, the present invention introduces the mass transfer mechanism and energy transfer mechanism of the phase change reaction of the liquid insulating oil vaporization of the transformer, the pyrolysis reaction of the vaporized oil vapor at local high temperature, and the cracking gas production reaction caused by high-energy electrons in the arc core channel area, forming a more complete gas production mechanism, mass transfer and energy transfer process. In the specific simulation, arc channels at different positions are set for each time step for simulation, and the pressure generated by the arc channels at different positions is updated with the time step, thereby obtaining the instantaneous pressure cloud map of the elevated seat area corresponding to the time step, and connecting multiple instantaneous pressure cloud maps to obtain the pressure change curve of the elevated seat area, thereby restoring the pressure change process of the elevated seat area during the actual arc discharge. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0039] Figure 1 The present invention is a flow chart of a method for simulating instantaneous pressure in a converter transformer elevated seat area under an arc fault. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] Table 1 compares the advantages and disadvantages of existing three-dimensional simulation models. Referring to Table 1, the existing models do not involve reactions, so the type of gas has no effect and is generally represented directly by air. It can be seen that the amount of injected gas is very critical. The existing scheme gives this gas amount according to the proportional coefficient summarized in the arc gas production experiment. The disadvantages are: (1) The arc faults involved in the existing arc gas production experiments are difficult to compare with actual faults in terms of scale and energy, and the proportional coefficient has limitations; (2) The proportional coefficients given by different literatures are also quite different.

[0042] The present invention believes that the gas production of transformer oil arc discharge is mainly divided into two categories, one is cracking under the action of electric field, and the other is vaporization and pyrolysis from high temperature. The electric arc triggers the electric cracking of the oil in the center of the arc to produce a part of the gas, while the local high temperature vaporizes the surrounding oil and even further pyrolysis reaction. The process involves energy conversion, physical state change and multiple reactions, and the mechanism is more in line with the actual situation. At the same time, the proportional coefficients of the amount of gas produced by arcs of different scales and energies are different, which can also echo the situation in the literature. The gas production process of cracking under the action of electric field.

[0043] Table 1 Advantages and disadvantages of 3D simulation models

[0044]

[0045] In order to simulate the instantaneous pressure change in the elevated seat area of ​​the converter transformer under arc fault more realistically and effectively, the present invention proposes a method for simulating the instantaneous pressure in the elevated seat area of ​​the converter transformer under arc fault, which is specifically a two-phase flow fluid mechanics simulation method including chemical reaction, including the study of reaction mechanism, the establishment of coupling mechanism, the construction of simulation model and reasonable iteration scheme in the simulation process. Specifically, the following steps are included:

[0046] S1: Construct a simulation model of the converter transformer riser.

[0047] According to the actual converter transformer riser drawing, a 1:1 modeling was performed to obtain a simulated geometric model, analyze the operating conditions of the converter transformer and the flow and heat transfer state of the transformer oil, find suitable boundary conditions such as symmetry surfaces and walls, and reasonably simplify the calculation domain. Based on the watertight geometry workflow, it was meshed to obtain multiple calculation units. According to the actual arc fault case, the arc position and initial shape were analyzed, and the mesh was locally encrypted.

[0048] S2: Import the mesh model into the simulation software, set the boundary conditions, material properties, time steps and step intervals of the mesh model, and build the mass transfer mechanism and energy transfer mechanism of the phase change reaction of transformer insulating oil vaporization, the pyrolysis reaction of oil vapor and the cracking gas production reaction induced by the arc in the simulation software.

[0049] The research reaction mechanism involved in the simulation of the converter transformer riser in the present invention includes the phase change reaction of the vaporization of the insulating oil, the further pyrolysis reaction of the oil vapor and the cracking and gas production reaction caused by the high-energy electrons in the core channel area.

[0050] Among them, the reaction mechanism of the phase change reaction of insulating oil vaporization is to measure the saturation temperature of insulating oil through experiments. The reaction mechanism of the further pyrolysis reaction of oil vapor is to apply ReaxFF (Reactive Force-Field) reaction field molecular simulation technology, select the representative structure of transformer oil molecules, and establish a multi-molecule simulation system. Under the NVT ensemble, the reaction path, products and reaction rates of different reaction temperatures are simulated, and the chemical reaction equation and reaction rate expression are summarized. The NVT ensemble refers to an ensemble that keeps the number of particles (N), volume (V) and temperature (T) of the system unchanged during the simulation process. The reaction mechanism of the cracking gas production reaction caused by high-energy electrons in the core channel area is to build a low-energy arc test platform to reduce the impact of pyrolysis gas production, and the gas production and gas composition of arcs with different powers are obtained experimentally.

[0051] In the Fluent software, the two-phase flow mixture component transport model is enabled, and the action mechanisms of the three reactions are established. The action mechanisms include mass transfer process, energy transfer process and definition of material properties.

[0052] The mass transfer process includes the mass transfer process of phase change reaction, pyrolysis reaction and electrolysis reaction.

[0053] Mass transfer process of phase change reaction: When the liquid phase temperature is higher than the saturation temperature, the liquid phase evaporates, and the mass transferred from the liquid phase to the gas phase is:

[0054] ;

[0055] When the gas phase temperature is lower than the saturation temperature, the gas phase condenses, and the mass transferred from the gas phase to the liquid phase is:

[0056] ;

[0057] In the formula, and are the mass transfer terms from gas phase to liquid phase and from liquid phase to gas phase during phase change / kg·m -3 ·s -1 , r L is the liquid phase density / kg·m -3 , α L is the volume fraction of the liquid phase, r V is the gas phase density / kg·m -3 , α V is the volume fraction of the gas phase, T sat is the saturation temperature / K, coefficient is the coefficient related to the reaction / s -1 , the physical meaning is the inverse of the reaction relaxation time, T L is the liquidus temperature, T V is the gas phase temperature.

[0058] Mass transfer process of pyrolysis reaction: Transformer oil is a mixture. The present invention uses C 20 H 38 Molecular representative, 1 mol C 20 H 38 Pyrolysis produces about 4.632 mol of pyrolysis molecules, and the molecular weight of the pyrolysis molecules is less than that of C 20 H 38 , of which C2H4 is about 1.052 mol, C3H6 is about 0.805 mol, CH4 is about 0.255 mol, H2 is about 0.227 mol, C4H8 is about 0.191 mol, C4H6 is about 0.182 mol, C2H6 is about 0.144 mol, etc. The activation energy of the reactionE a =2.586×10 5 J.mol -1 , pre-exponential factor A =1.628×10 16 s -1 , set the reaction rate constant accordingly:

[0059] ;

[0060] In the formula, k is the reaction rate constant, R is the gas constant, with a value of 8.314 J / (mol•K), T is the reaction temperature.

[0061] Mass transfer process of cracking reaction: Under the action of electric arc, transformer oil produces about 47.3 mL of free gas per kJ of arc energy, that is, the gas production rate caused by each W of arc power is 47.3 mL / s, of which about 73.5% is H2, 18% is C2H2, 5.2% is CH4, 3.1% is C2H4, and 0.2% is C2H6.

[0062] The energy transfer process includes the energy transfer process of heat source input, phase change reaction, pyrolysis reaction and electrolysis reaction.

[0063] Heat source input: The heat source input is set at the core position of the arc channel, and the heat source power is calculated through the arc voltage and current waveforms.

[0064] Energy transfer process of phase change reaction: Energy transfer caused by phase change reaction S h =( ṁ VL - ṁ LV ) L h ,in L h is the latent heat of vaporization / J·kg -1 .

[0065] Energy transfer process of pyrolysis reaction: The enthalpy change of pyrolysis reaction is shown as follows:

[0066] ;

[0067] In the formula, H is the enthalpy change of the pyrolysis reaction, is the operator symbol, p For the pyrolysis reaction, is the standard molar formation enthalpy of the substance. The data can be obtained by consulting the manual. f is formation, m is mole, It is the standard state.

[0068] Energy transfer process of electrolytic cracking reaction: The enthalpy change of electrolytic cracking reaction is as follows:

[0069] ;

[0070] In the formula, H is the enthalpy change of the pyrolysis reaction, e For the cracking reaction.

[0071] Define material properties: Material properties are determined by the components in each calculation unit and are presented as volume fraction average or mass fraction average. The components in each calculation unit may be different, including liquid content, gas content, and the ratio of different types of pyrolysis molecules in the gas phase.

[0072] S4: In the constructed simulation software, arc channels at different positions are set for each time step, and a heat source is set in the arc channel to perform arc fault simulation to obtain the corresponding instantaneous pressure cloud map of the riser area.

[0073] It was observed in the experiment that the arc channel is always in the gas phase and is easily carried away from the initial position by the gas mass, with a tendency to move upward. Therefore, in the simulation, the position of the arc channel, that is, the position of the heat source input, is updated every certain time step (for example, 5ms), and the final calculation result is obtained by continuous iteration. The calculation results include the instantaneous pressure cloud map of the riser area at multiple time steps.

[0074] Select the key position in the instantaneous pressure cloud map. The key position is the position where the explosion has occurred and the position where the maximum instantaneous pressure is located. Connect the instantaneous pressure values ​​corresponding to the key position at different time steps to obtain the instantaneous pressure change curve. Selecting different key positions can obtain different instantaneous pressure change curves.

[0075] The calculation results of the present invention also include graphs showing changes in the shape, position and quantity of gas generated by the arc and graphs showing changes in temperature distribution.

[0076] Arc channel update principle: the arc channel is considered to be located at the center of the highest temperature area, but the arc channel radius remains unchanged by default.

[0077] Although the preferred embodiments of the present invention have been described, those skilled in the art may make other changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0078] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault, characterized in that: The following steps are involved: Construct a simulation model of the converter transformer riser and perform mesh generation to obtain a mesh model; Importing the grid model into the simulation software, setting the boundary conditions, material properties, time steps and step intervals of the grid model, and building the mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states in the simulation software, wherein the reaction mechanism of transformer oil in different physical states includes the phase change reaction of transformer insulating oil vaporization, the pyrolysis reaction of oil vapor and the cracking gas production reaction induced by electric arc; In the constructed simulation software, arc channels at different positions are set for each time step, and heat sources are set in the arc channels to perform arc fault simulation, so as to obtain multiple instantaneous pressure cloud maps of the elevated seat area; each of the instantaneous pressure cloud maps includes instantaneous pressure values ​​at different positions in the elevated seat area; The same key position in multiple instantaneous pressure cloud maps is selected, and the instantaneous pressure values ​​corresponding to the same key position at different time steps are connected to obtain the instantaneous pressure change curve.

2. A method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault as claimed in claim 1, characterized in that: The mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states are respectively established in the simulation software, wherein the mass transfer mechanism of the phase change reaction of insulating oil vaporization is as follows: When the liquid phase temperature is higher than the saturation temperature, the liquid phase evaporates, and the mass transferred from the liquid phase to the gas phase is: ； When the gas phase temperature is lower than the saturation temperature, the gas phase condenses, and the mass transferred from the gas phase to the liquid phase is: ； In the formula, and are the mass transfer terms from gas phase to liquid phase and from liquid phase to gas phase during phase change, ρ L is the liquid density, α L is the volume fraction of the liquid phase, ρ V is the gas phase density, α V is the volume fraction of the gas phase, T sat is the saturation temperature, coeff is the coefficient related to the reaction, T L is the liquidus temperature, T V is the gas phase temperature; The energy transfer mechanism of the phase change reaction of insulating oil vaporization is as follows: ； In the formula, L h is the latent heat of vaporization, S h Energy transfer caused by phase change reaction.

3. A method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault as claimed in claim 2, characterized in that: The mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states are respectively established in the simulation software, wherein the mass transfer mechanism of the pyrolysis reaction of oil vapor is as follows: 1mol C 20 H 38 The pyrolysis produces 4.632 mol of pyrolysis molecules smaller than their molecular weight, including 1.052 mol of C2H4, 0.805 mol of C3H6, 0.255 mol of CH4, 0.227 mol of H2, 0.191 mol of C4H8, 0.182 mol of C4H6, and 0.144 mol of C2H6; the reaction rate constant is as follows: ； In the formula, k is the reaction rate constant, A is the pre-exponential factor, E a is the activation energy of the pyrolysis reaction, R is the gas constant, T is the reaction temperature; The energy transfer mechanism of the pyrolysis reaction of oil vapor is as follows: ； In the formula, H is the enthalpy change of the pyrolysis reaction, is the operator symbol, p For the pyrolysis reaction, is the standard molar formation enthalpy of the substance, f is formation, m is mole, It is the standard state.

4. A method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault as claimed in claim 3, characterized in that: The mass transfer mechanism and energy transfer mechanism corresponding to the reaction mechanism of transformer oil in different physical states are respectively established in the simulation software, wherein the mass transfer mechanism of the cracking gas production reaction induced by the arc is as follows: The gas production rate caused by each W arc power is 47.3 mL / s, of which 73.5% is H2, 18% is C2H2, 5.2% is CH4, 3.1% is C2H4, and 0.2% is C2H6; The energy transfer mechanism of the arc-induced cracking gas production reaction is as follows: ； In the formula, H is the enthalpy change of the pyrolysis reaction, e For the cracking reaction.

5. The method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault according to claim 1, characterized in that: The simulation model is meshed based on the watertight geometry workflow to obtain a mesh model, wherein the mesh model includes a plurality of computing units.

6. A method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault as claimed in claim 5, characterized in that: The material properties include material properties of transformer oil, oil vapor and pyrolysis molecular gas mixture in each calculation unit in the grid model.

7. The method for simulating instantaneous pressure in the elevated seat area of ​​a converter transformer under an arc fault according to claim 1, characterized in that: The arc channel is located at the center of the highest temperature area of ​​the simulation model at each time step, and the arc channel radius is the same.

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