An arc extinguishing lightning protection device with a multi-arc extinguishing chamber structure has been gradually applied to the overhead line lightning protection, but the process and key factors of the arc building of the internal multi-phase flow suppression arc still need to be studied. In this paper, based on the theory of arc magnetohydrodynamics and Euler’s high velocity flow field model, a simplified model of arc coupled polyphase longitudinal air flow in a multi-pipe structure is established, the approximate solution of the model is made, and the development process of polyphase air flow and the extinction mechanism of the arc are obtained. At the same time, the arc quenching process of polyphase longitudinal air flow is also studied experimentally. The experimental results show that the continuous arc of power frequency can be stopped quickly and will not be reignited under the condition of polyphase air flow.
A multi-arc extinguishing chamber structure can improve the uniformity of the electric field distribution of the gap, ensure the breakdown stability of the gap, and turn the long arc into many series short arc, which is conducive to the arc extinguishing, and its application in the field of circuit breakers and switches has been relatively mature, so some researchers began to apply it in the lightning protection field of transmission and distribution network. In order to solve the problem of power frequency arc building caused by lightning,1–4 the line lightning protection device based on the structure of the multiple arc extinguishing chamber is generally installed in parallel on the insulator bypass or spiral installed on the outside side of the insulator. Under the lightning overvoltage, it will take precedence over the insulator to flashover and breakdown and conduct on the whole to release lightning energy to the ground. After the lightning impact, in the impulse stage, the multi-chamber arc extinguishing device can produce the impact airflow and the power frequency arc, which is still in the embryonic stage, can effectively extinguish the power frequency continuous current avoidance face lightning trip.5–13
In this paper, the multi-pipe arc extinguishing device has multiple arc compression pipes, and the impact arc will be sharply compressed after entering the pipeline. Under the action of compression suction and transient thermal conduction of the arc, the axial high speed compression burst air flow can be formed. The arc in the pipeline is driven by the compression-explosion airflow and ejects from the nozzle between the adjacent pipes. Compared with the traditional multi-chamber arc extinguishing structure, the arc extinguishing structure in this paper lengthens the distance between each arc initiating electrode, weakens the near-cathode effect of zero crossing arc extinguishing, strengthens the effect of heating air extinguishing in the impact stage, and makes the arc extinguishing time shorter. This kind of arc extinguishing mode can begin to accumulate the pressure burst air as the arc extinguishing energy in the impact stage, and after the impact stage, the pressure burst air has formed a certain strength and speed, but the arc that is still in the early stage of arc construction is relatively fragile, and it is difficult to develop into a stable power frequency arc under the action of high speed air.
However, at present, the arc extinguishing process of longitudinal blowing in the internal arc extinguishing chamber and the characteristics of arc extinguishing under the combined action of multi-phase longitudinal blowing are still to be further studied. In this paper, the development characteristics of the arc in the unit pipe are studied. By analyzing the characteristics of ac arc extinguishing, the most favorable conditions are found. The arc compression and heat transfer process are modeled and analyzed to grasp the triggering mechanism of longitudinal airflow.
II. ANALYSIS, MODELING, AND SOLUTION OF GAS-ARC COUPLING DEVELOPMENT PROCESS
A. Analysis on the mechanism of multi-point truncation arc in longitudinal airflow
In the multi-pipe structure, the arc is extremely compressed in the pipe, and the energy of the arc column accumulates instantaneously, leading to a sharp rise in the central temperature of the arc column and a huge temperature difference between the arc column and the inner wall of the pipe. Therefore, there will be strong heat conduction in the pipe. At this time, a large amount of gas has been inhaled between the arc column and the inner wall of the pipe. Under the strong convective heat transfer, the temperature of these gases will rise rapidly and the volume will increase rapidly.14,15 Due to the pressure from the inner wall of the pipe, the radial expansion flow develops into the axial longitudinal airflow.
The overall arcing pipeline space structure and the longitudinal direction of air flow are shown in Fig. 1; units arranged in a certain space structure between the compression pipeline, not only can spin the arc, and each pipe axial since inflation will flow in the adjacent pipe arc trajectory turning points to instability in the form of electric arc and the arc more truncation, block the subsequent energy supply, and extinguish arcs quickly and efficiently.
The velocity of the center of the arc column and the velocity of the maximum arc temperature in the laboratory coordinates are both Va. When the axial compressional burst flow moves in the arc column with velocity V in the same coordinate system, the experienced temperature can be expressed as follows:
The voltage balance equation of the circuit after arc establishing is
If the temperature gradient experienced by the arc column coupled with the axial compressional burst flow is ∇T, then the temperature gradient change experienced by the compressional burst flow is
Under the action of the pressure explosion flow, the velocity of the maximum temperature in the center of the arc column quickly reaches the maximum, that is, it moves synchronically with the pressure explosion flow. Therefore, what the pressure burst air flow experiences is the temperature gradient of the maximum temperature ∇mT of the arc column, and the change rate of this temperature gradient relative to time is zero, and it can be obtained as follows:
This formula can be converted to
Here, we can see that the inflation pressure air blasting speed will greatly affect the mutation rate of movement of the turning point arc and make the next arcing pipe movement distance increase, and the energy dissipation rate of the arc and its movement speed are proportional to the movement distance, so in each mutation turning point arc energy dissipates drastically and finally forms the energy the breakpoint. There are energy breakpoints at the adjacent arc-extinguishing elements, and the multi-point truncation of arc is realized.
B. Mathematical model of coupling arc with longitudinally airflow
Each single pipe in the multi-pipe structure can produce a compressional burst flow, and the movement of the air flow and the arc in the pipe basically conforms to the conditions of longitudinal arc blowing. Based on the above analysis, the longitudinal blown air coupled arc model is used to model the arc extinguishing process in the pipeline. In the modeling process, the axial development of air flow in the pipeline is mainly considered, and other velocity components can be ignored. By establishing the conservation equations of mass, momentum, and energy as well as the equations of electromagnetic characteristics and adding other closed equations, the next solution needs can be met. The simplified equations are as follows:
Mass conservation equation:
Here, ρ, t, and u are the fluid density, time, and velocity vector, respectively.
Momentum conservation equation:
Here, p, I, and μ are the absolute pressure, second order unit tensor, and viscosity coefficient, respectively.
Energy conservation equation:
The above simplified partial differential equations are actually equations of the arc column boundary layer in which the momentum conservation equation is divided into axial and radial directions. The radial momentum conservation reflects the conservation relationship between the radial pressure gradient and Lorentz force, while other equations such as Ohm’s law and the gas state equation remain unchanged. Compared with the initial strongly coupled nonlinear partial differential equations, the whole system is greatly simplified, which provides a condition for solving and analyzing the equations.
III. LONGITUDINAL AIRFLOW COUPLED ARC SOLUTION
Due to the above model for arc and parameters of airflow in thermodynamics, transport and other some of the features of both the hypothesis and simplification, so after for the simplified longitudinal arc blow air coupling model to solve related parameters, precision may have certain deviation, but this does not affect this section general rules of the process of blowing arc physics research. The 10/200 μs impact lightning wave with the amplitude of 18 kA and the power frequency arc of 5 kA were selected as the solving objects of the coupling analysis.
The analytical results of the arc current and the average temperature in the pipeline are shown in Fig. 2. It can be seen that after the lightning shock wave breaks down the pipeline, the current in the pipeline rises rapidly, reaching a peak of 13.8 kA around 30 μs. Compared with the original lightning wave shape, the peak amplitude of the current decreased by nearly 22% and the peak time was delayed by about 20 μs, which indicated that after the construction of the arc of the pipeline, the compression flow was triggered quickly and used for the early arc, and the arc development was inhibited to a certain extent in the stage of impact arc. After the current peak, the arc current drops sharply to about 2.5 kA within 70 μs due to the natural attenuation of the impact arc component and the gradual enhancement of the compression flow. Next, under the coupling effect of power frequency current, the arc current rises to 3.9 kA, but it is still not enough to change the trend of arc extinction. The formation of arc fractures causes the arc to finally completely extinguish and not reignite at about 330 μs.
It can be seen from Fig. 2 that the arc temperature change curve is obviously slightly behind the current change curve. When the arc current rises rapidly, the temperature obviously does not change so fast. It can be seen that this is mainly because the arc temperature change has thermal inertia, which leads to the arc current falling to the trough. The average temperature inside the pipe peaked at about 4000 K. At this time, in the high temperature environment, the movement speed of various ions in the air in the pipeline is accelerated, the energy is rapidly accumulated, and the dissociation effect is significantly enhanced, so the rise of arc current appears. After that, the effect of strong airflow blowing belt blows a large section of high temperature arc out of the pipeline and accelerates the convective heat dissipation of the air inside the pipeline, so it can be seen that the average temperature inside the pipeline falls. The rate of change in the period of temperature decrease is much smaller than that in the initial stage of arc construction, and the temperature decrease is not beneficial to the generation of pressure burst airflow, and the thermal inertia of arc is added, so the average temperature inside the pipeline rises to about 3400 K after a short decline. As analyzed in the previous paragraph, after the second current peak, the arc fracture has been formed and the input channel of power frequency energy has been cut off, so the temperature does not show signs of rising after the second peak. After 330 μs, due to the extinction of the arc, the intensity of the pressure burst flow also weakened to a lesser extent, and the natural heat dissipation process began inside the pipeline. It can be seen that the decreasing rate of temperature gradually slows down because the heat dissipation coefficient is proportional to the size of the temperature, and the lower the temperature is, the slower the heat dissipation is. Eventually, the average pipe temperature dropped to less than 1000 K at 500 μs.
In order to study the change of pressure burst flow in the pipeline, it is necessary to analyze the pressure and velocity at different positions in the pipeline. Figure 3 shows the position diagram of the pipe and the definition of positive and negative directions during the analysis. Figure 4 shows the distribution of pressure and velocity in the pipeline with the position and time of the pipeline. It can be seen that when the 10 μs arc is just established, a large pressure has been generated in the middle of the pipeline, and the airflow velocity is more than 100 m/s. However, the pressure at both ends of the pipeline does not change much, and only a small airflow velocity flux is present. This shows that the pressure burst flow is first triggered in the middle part of the pipeline and then develops to both ends of the pipeline. As the arc current gradually rises, the pressure in the pipeline and the speed of the pressure burst air flow are improved in an all-round way at 50 μs. Since the middle part of the pipeline is the starting point of the air flow trigger, the pressure and the speed of the air flow near the middle part rise significantly faster than other positions near the end of the pipeline. At 100 μs, the air velocity in the middle of the pipeline reached a peak of about 840 m/s, and the pressure was nearly 9 atm. On both sides of the pipeline, the air velocity was about 560 m/s and the pressure was nearly 7 atm. Combined with the current and temperature curves, it can be seen that the energy accumulation in the middle of the pipeline has reached its peak. As the air in the middle becomes a strong airflow moving toward the fracture, the pressure and airflow velocity distribution inside the pipeline will change significantly. In the following 100 μs, the overall air flow velocity and pressure in the pipe drop sharply, among which the air flow velocity in the middle drops to only 140 m/s, and the pressure also decreases by about 3 atm compared with that at 100 μs. This is mainly because there is not enough air inside the pipe to maintain the continuous production of the air flow due to the ejector of the pressure burst air to the outside of the pipe. Since both ends of the pipeline are the only outlet for the compression-burst air to be ejected, the air velocity near the port is still about 280 m/s. It can be observed that the pressure and velocity in the pipeline are no longer concentrated in the middle of the pipeline but gradually develop to both sides of the pipeline. As the air flow drives the arc out of the outlet of the pipeline, the arc fracture is basically formed. At 300 μs, the air velocity inside the pipeline further drops, and the air velocity and pressure at both ends of the pipeline are still greater than those in the middle of the pipeline. At 500 μs, the arc has gone out for a period of time; at this time, there is negative air velocity on both ends of the pipeline; this is mainly because after the explosion pressure air, pipeline internal medium has a substantial reduction in a short period of time, which leads to the pressure inside the pipeline; especially, the central pipe external pressure is far less than atmospheric pressure, so they will have a suction effect of outside air flow to the pipe flow phenomenon.
The IEC standard initially stipulated that the arrester with series gap outside the band needs to carry out the power frequency continuous current stop test. Later, the power frequency continuous current stop test was added as an important type test to detect the performance of the arrester in the new national and industrial standards of the arrester in China.
Figure 5 shows the test circuit diagram of the power frequency continuous flow stopper test, where IG is the impact circuit, which can produce 1.2/50 standard lightning waveform; TT is a power frequency loop that can output ten complete rated voltage waveforms. The circuit can trigger the impulse voltage breakdown test sample at any phase angle of power frequency voltage to meet the test requirement that the lightning strike time should be random. At the same time, the circuit can ensure that the voltage applied at both ends of the sample after breakdown does not exceed 10% of the rated voltage to avoid abnormal reignition caused by test conditions.
The test steps are as follows:
The test items are installed at both ends of the 10 kV insulator in parallel, and the series clearance distance is adjusted to the maximum allowable distance.
Start the power frequency system, apply the rated voltage of 13.2 kV at both ends of the test, and maintain it for 3 min.
Start the shock system to release the standard lightning wave whose amplitude is greater than 50% of the lightning shock discharge voltage of the test product to cause the breakdown discharge of the test product. Five times of lightning impact (positive polarity) should be applied to the positive and negative polarity of power frequency voltage half wave, respectively, and the applied time should be kept 30°–0° in front of the peak value of power frequency half wave.
Record the voltage waveform measured by the measuring system, and analyze the waveform.
According to the requirements, five lightning impulse voltages were applied at 90° and 270°, respectively, of the power frequency voltage, and the multi-pipe arc extinguishing device all cuts off the continuous current in the first half wave cycle of the power frequency voltage.
Here, we intercept some typical test waveforms to analyze the power frequency continuous flow cutting effect of the multi-pipe arc extinguishing device. Figures 6 and 7 show the voltage measured by the system when the lightning impact occurs at 90° and 270° power frequency voltage phase, respectively. Among them, the voltage waveform measured by CH2 is the residual voltage waveform of the test sample, and CH3 and CH4 are the voltage borne by both ends of the test sample and the output voltage of the power frequency system, respectively.
It can be analyzed from the waveform in the figure that after the lightning impact overvoltage occurs, the voltage at both ends of the test product quickly falls to near the zero point, and the test product is broken down. At the same time, due to the rapid action of the device, the pressure burst air is generated instantly and used as a flasticity arc, and the voltage at both ends of the test product is restored to the output voltage of the power frequency system after a short oscillation. Within half a cycle after the breakdown of the test sample, under the impact disturbance, the output voltage of the power-frequency system was slightly higher than the rated voltage of the test sample, but still within the allowable range of 10%, so no signs of reignition of the test sample were found. In addition, it can be seen from CH2 that, due to the large number of series air gaps in the sample, the residual pressure after the lightning impact is basically equal to the arc path voltage of the arc, and the value is very small, only about tens of volts, which is also an advantage of the multi-pipe arc extinguishing device compared with the traditional lightning arrester.
On the whole, the multi-pipe arc extinguishing device can act quickly after breakdown, and the time of cutting off power frequency continuous flow is far less than the standard time required by half cycle. Its insulation characteristics can be quickly recovered and not easily reignited under the effect of the elongating, compound, and dissociating of the arc caused by the compression explosion airflow.
As can be seen from Fig. 8, in another arc extinguishing test, under the action of multi-phase longitudinal air flow, the arc is extinguished in a very short time and no reburning phenomenon is observed.
The coupling arc model of longitudinal blown air flow was approximated by taking the impact thunder wave of 10/200 μs with the amplitude of 18 kA and the power frequency arc of 5 kA as the analytical objects. The results show that the arc current, the average temperature of the pipeline, the air velocity, and the pressure of the pipeline are similar. When the arc current reaches the peak value of 13.8 Ka at 30 μs, the average temperature of the pipeline is still in the rising stage. When the current drops to about 3 kA, the temperature rises to the peak value of 4000 K. Under the action of the pressure explosion airflow, the arc is extinguished near 330 μs, and the pipeline also begins to dissipate heat naturally, and the average temperature drops to 1000 K at 500 μs.
In the power frequency continuous current blocking test, the rated working voltage is applied at both ends of the device, and then the lightning shock is triggered at different phase angles to cause the device to break down. The results show that the devices can cut off the continuous current in the first half wave cycle after the breakdown, and the devices have good residual voltage characteristics.
This work was supported by the National Natural Science Foundation of China (Grant No. 51467002). The authors would like to thank all the staff of high voltage laboratory at Guangxi University and Hubei Electric Power Research Institute.
Conflict of Interest
The authors have no conflicts to disclose.
The data that support the findings of this study are available from the corresponding author upon reasonable request.