The principle of self-energizing arc extinguishing is to maximize the energy of the arc itself, heat the sf6 gas in the expansion chamber, increase the pressure of the gas in the expansion chamber, form a high-speed airflow in the nozzle, and have a strong heat exchange with the arc. Arc purpose. Therefore, the self-energy arc extinguishing principle is a breakthrough and development of the electrical arc theory research.
Since the arc energy can be used to establish the necessary pressure for the arc extinguishing chamber, the operating mechanism is not required to provide a large compression work; since the arc extinguishing chamber air pressure is established by the arc, high gas pressure opening speed is not required, The operation power of the mechanism is reduced; under the premise of maintaining the same air blowing pressure, the diameter of the cylinder of the arc extinguishing chamber is reduced, and the mass of the moving parts of the transmission system is reduced, so that the operating power of the mechanism is significantly reduced. Therefore, according to theoretical calculations, the operating power of the self-powered SF6 circuit breaker can be reduced to 20% of the single-pressure type, although in practice the operating power is reduced to a desired level due to insufficient utilization of arc energy and structural design and manufacturing constraints. 25% to 30% is possible and has been achieved.
Development and development, based on the first law of thermodynamics and the basic principles of fluid dynamics, make appropriate assumptions for the designed arc extinguishing chamber, and establish a mathematical model with functions of pressure, temperature and M quantity. In this paper, the fourth-order Runge-Kutta method is used to solve the differential equation of temperature versus time in the operation, and the program is written in Fortran% language. The programmed program is debugged on Powerstation4.0, and the obtained data is plotted on METLAB software. The curve of change.
The establishment of a mathematical model of the pressure characteristics of the arc extinguishing chamber gives a schematic diagram of the structure of the self-powered SF6 circuit breaker. The main parameters of the calculation and analysis in this paper are: geometric overtravel 40mm, average opening speed 6770m/s, and the initial state parameters are absolute in the arc extinguishing chamber. The inflation pressure is 5 MPa, the temperature is 293 K, and the time is taken to zero. The volume of the thermal expansion cylinder of the circuit breaker is calculated to be 0.1 L, 0.2 L and 0.3 L. The circuit breaker is in the open state.
The moving contact from the closed state to the open state is a continuous and changing process. The pressure, temperature and density of the gas in the arc extinguishing chamber are different at each moment. In this paper, the following assumptions are made in the calculation process: The gas is distributed to the gas; since the breaking process is very rapid and there is no heat exchange with the cylinder wall, the process can be regarded as an adiabatic process; in order to better illustrate the problem, the gas volume in the arc extinguishing chamber is divided into three parts, as shown . In the figure, 1 is a thermal expansion cylinder (upstream zone), 2 is an external gas (downstream zone), and 3 is an internal gas of a moving arc contact (downstream zone). Part 1, 2, and 3 gases are considered as ideal gases. Ignore the friction of each part, and the distribution of each part of the gas state parameters can be treated as a centralized parameter.
The constant volume specific heat of SF6 gas is calculated by the following formula: Here, the arc extinguishing chamber and the contact system movement are divided into three processes, and each process differs greatly due to the position of the moving and static contacts and the opening or not of the nozzle.
In the first process, the distance from the contact in the closed position to the moving contact is 40mm. This process is the overtravel phase of the moving contact. At this time, the moving contact is not completely separated from the static contact, and the nozzle is not opened yet. The 1, 2, and 3 parts are still three isolated systems, and there is no exchange of mass and energy between them. Therefore, the first part is a closed thermodynamic system. The following sections 1, 2, and 3 are discussed separately. .
For the first part of the gas, the mathematical equation can be written as the process to find the temperature r value at each moment.
The gas pressure of the arc extinguishing chamber can be derived from the ideal gas state equation: 3 parts of gas, assuming a large capacity gas, because the first part of the gas does not leak to it, the change of the first part of the gas parameter has little effect on it, and may not consider. The gas state in parts 2 and 3 is still the initial parameter.
In the second process, the second part leaves the arcing contact from the static arc contact, that is, the contact stroke is from 40mm to 81mm, and the moving contact is located at the smallest section of the nozzle. Since the contacts generate an arc separately, the arc energy is continuously input into the pressure chamber, the pressure rises, and the temperature of the arc extinguishing chamber is heated. At this time, the arc energy and the arc radius change continuously with the change of the phase angle of the short-circuit current. One part of the gas exchanges energy and mass with both the inside and the downstream of the static arc contact, and the gas leakage area is It varies with the radius of the arc. When the current is at 90., the arc area is the largest and the blockage is the most serious. When the arc current is zero, the arc area is the smallest and the gas discharge area is the largest. At this time, the arc extinguishing chamber 1 and the downstream zone 2 have both arc energy exchange and mass exchange, and also have gas exchange with the three parts. The energy balance equation can be written as 3 parts of 1 gas and 3 parts of gas. Mass exchange; a is the gas enthalpy; Mi is the coefficient of the 1 part arc energy input; g is the arc energy.
The arc chamber pressure is the mass loss rate among them, which is the gas leakage area. d includes A and heart. 2 is the leakage area of ​​the nozzle, which is the area of ​​the ring surrounded by the diameter of the nozzle and the arc; the name 3 is the leakage area of ​​the static arc contact; 0 is the gas leakage parameter.
In the case of subcritical flow rate, then 〃 is the constant entropy index.
In the case of supercritical flow rate, the inside is the pressure on the downstream side of the nozzle, which is approximately 5MI, and Per is the critical pressure. In the third process, the arcing contact leaves the nozzle to the completely open state. The arc is pulled longer, but the geometrical length of the energy exchange between the arc and the arc extinguishing chamber 1 is relatively stable, and the share of gas energy exchange with the downstream zone is getting larger and larger, and the arc energy is mostly lost to the outside. At this time, the pressure in the pressure chamber continues to rise. Under the action of the pressure difference, the air flow forms a high-speed flow in the nozzle, and a strong energy exchange with the arc occurs, and the arc is finally extinguished when the arc current crosses zero. At this stage, the mass of one part of the gas is exchanged with the inner part 2 of the static contact and the outer part 3 of the spout. The energy balance equation can be written as shown in equations (4) to (6).
Among them, 2 is the energy coefficient of the third part of the arc unit time delivery, and its value is smaller.
The relationship between the arc voltage gradient and the working pressure and the diameter of the nozzle was obtained by studying the SF6 gas test: the pressure difference was made.
In the mathematical model of the self-powered SF6 circuit breaker, the input of the arc energy is greatly influenced by the changes of parameters such as pressure and temperature, and for the self-energizing arc extinguishing chamber, the structure of the arc extinguishing chamber is reasonably designed and determined. The size of the arc extinguishing chamber and the effective heating of the arc extinguishing chamber gas are crucial for designing a circuit breaker with excellent breaking performance. In the calculation of the arc chamber parameters, it is critical to determine the arc heating coefficient well.
After reading a large amount of data, the arc coefficient initially used in the research of this project is: 0.20.3 in the second process and 0.10.15 in the third process. 2 Computer-aided design and method selection In this study, the main The solution is to solve the one-dimension differential equation. After systematic analysis, the fourth-order Runge-Kutta method is used to solve the differential equation of temperature versus time in the operation, and the program is written in fortran90 language.
Using the fourth-order Runge-Kutta method, the problem of solving the differential equation can be transformed into a computational problem, in which each step needs to calculate the function value /fc > 04 times, and the step size A can be taken as equal.
Set the step size.
3 Data analysis and conclusions In order to better illustrate the problem, this paper calculated the volume of three different arc chambers, which are 0.1L, 0.2L and 0.3L. From the above, it can be clearly seen that the three pressures and temperature change with time. The curve of the fluctuations. When the contact stroke is less than 40mm, the pressure value changes little, and the temperature does not change significantly. At this time, there is no mass loss due to the nozzle clogging. However, when the moving contact is pulled away from the static arc contact, the pressure rises rapidly due to the intervention of the arc energy, and the temperature also rises rapidly. Compared with the curve 2 and the curve 3, the pressure of the curve 1 rises the fastest and reaches the maximum value. This is because its volume is relatively small, and the heat generated by the arc can easily bring the arc chamber pressure to a very low value in the shortest time, which is disadvantageous for long-arc arc extinction. Curve 2 and curve 3 are more moderate than curve 1. With the end of the first half of the current, the pressures of curves 1, 2, and 3 all have a significant downward trend. At this time, the arc vent opens, the gas quality begins to drain, and the temperature rises very slowly. After zero crossing, the pressure rises rapidly and the temperature continues to increase. At this time, it can be clearly seen from the figure that the pressure rise of the curve 1 has not been the first high, because in the first half wave, the gas discharge is accelerated due to the high air pressure, and the second half wave is obtained. There is not enough gas to blow the arc and the pressure rise is reduced. For curve 2 and curve 3, the gas enthalpy is sufficient, which is beneficial to the effective arc blowing when the arc crosses zero. At this time, due to the large pressure difference, the arc unblocking period is lengthened, and the gas loss rate is increased. The gas quality is degraded. By the zero crossing of the first week, for curve 2, the pressure is maintained at a level of 23.4 MPa, and the temperature reaches a maximum of 3080 K. For curve 3, the pressure is maintained at a high level of 21.9 MPa, and the temperature is also reached. The maximum value of 1935K, from 5, can also be seen under the same inflation pressure conditions, curve 1 due to small volume, the volume is not very good, when the current crosses zero, the gas loss rate is also at a very low level, and the curve 2. Curve 3 has enough gas to blow the arc, and there is a high gas loss rate at this time, but the pressure of curve 2 at zero crossing is better than that of curve 3. After zero crossing, the gas quality is opened due to the nozzle. A large amount of gas leakage, within a few milliseconds after arc extinction, the pressure suddenly drops, the gas mass loss rate decreases, and the gas quality also drops to a few grams.
4 Conclusions The research on the breaking capacity of self-energized SF6 circuit breakers with several different structural parameters was carried out. The results show that the volume of the arc chamber has a great influence on the success of the breaking, and plays an important role in the breaking process. Under the three different volumes, the optimal volume of the arc-extinguishing chamber cylinder is calculated to be around 0.2L. The pressure characteristics of the self-powered SF6 circuit breaker are well represented.
(Finish)
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