When opening the contacts of the switch, the current is not interrupted. According to the law of Lenz in the chain, EDC E L \u003d -LDI / DT occurs, which prevents the change in current. The latter finds for itself a path through the gas gap between the diverging contacts of the switch, which overlaps the electric arc. To interrupt the current, the arc must be repaid. In alternating current circuits, favorable conditions for arc harvesting occurs every time the current comes to zero, i.e. 2 times during each period. The diameter of the arc pillar, the temperature and ionization of the gas decrease sharply. At some point in time, the current comes to zero and the arc discharge stops. However, the chain is not yet interrupted.

After zero current in the gas gap, to some extent ionized, the deionization process continues, i.e. The process of turning it from the conductor in the dielectric, and in the electrical circuit, the process of recovery of voltage on the contacts of the switch from a relatively small voltage on the arc to the network voltage begins. These processes are interrelated. The outcome of the interaction of the arc gap with an electrical circuit depends on the ratio between the energy supplied to the gap, and the loss of energy in it, depending on the extinguishing device of the switch.

If during the entire transition process the loss of energy is dominated, the arc will not appear again and the chain will be interrupted. Otherwise, the arc appears again and the current will take place for another half of the period, after which the interaction process will repeat. The function of the switch lies not so much to "extinguish" an arc, but rather to eliminate the possibility of its new ignition by efficiently deionizing the gap with various artificial means. At the same time, the exceptional property of the gas is used - quickly, for several microseconds, turn from the conductor into a dielectric, capable of resisting the restoring network voltage.

To understand the device and switches, it is necessary to familiarize themselves with physical processes in the arc interval during the shutdown process. This article discusses the methods of arc harvesting in air and oil switches.

Physical processes in the arc spacing of the switch at high pressure

Electric arc, or rather arc discharge, is called an independent discharge in gas, i.e. The discharge flowing without an external ionizer characterized by a high current density and a relatively small drop of voltage at the cathode. The high pressure arc is discussed below, i.e. Arc discharge at atmospheric and higher pressure.

The following areas of arc discharge are distinguished:

• region of cathode drop in voltage;
• anode area;
• still arc.

The region of the cathode drop in the voltage is the thinnest layer of gas at the surface of the cathode. The voltage drop in this layer is 20-50 V, and the electric field strength reaches 10 5 10 6 V / cm. Energy, summing up from the network to this area, is used on the selection of electrons from the surface of the cathode.

Electron release mechanism can be double:

• thermoelectronic emission with refractory and refractory electrodes (tungsten, coal), the temperature of which can reach 6000 K and above
• auto-electronic emission, i.e. Emissioning electrons from a cathode by the action of a strong electric field with a "cold" cathode.

The current density on the cathode reaches 3000-10000 a / cm 5. The current is concentrated on a small brightly illuminated platform, called the cathode spot. Released electrons move through an arc pillar to the anode.

At the Anode, positive ions acquire acceleration towards the cathode. Electrons go to the anode and form a negative charge in a thin layer. The voltage drop at the anode is 10-20 V.

The processes in the arc pillar are of the greatest interest in studying the switches, since various types of exposure to arc are used to harvest an arc. The latter is a plasma, i.e. Ionized gas with a very high temperature and the same content of electrons and positive ions per unit volume.

The high temperature in the arc pillar create and maintain electrons and ions involved in the thermal chaotic movement of neutral molecules and atoms, but also have a directional movement in the electric field along the axis of the arc, determined by the particle charge sign. This movement prevents neutral gas. The frequent collisions of electrons and ions with neutral particles occur. Since the length of the free mileage of the electrons at high pressure is small, the loss of energy during elastic collisions with molecules and atoms per collision, small and insufficient for the ionization of particles. However, the number of collisions undergoes electrons is very large. As a result, the electron energy is transmitted to the neutral gas in the form of heat.

The average energy of "electronic gas" cannot significantly exceed the average energy of the neutral gas, since the additional energy acquired by electrons and ions in its directional movement along the axis of the meadow pillar is small compared to the thermal energy of the gas. Consequently, ions, electrons, as well as neutral atoms and molecules are in thermal equilibrium. In this case, the specific ionization of the arc pillar is fully determined by the temperature and when one of these values \u200b\u200bchanges, the other is inevitably changed.

Since at high gas pressure atoms and molecules overwhelmingly prevail over electrons and have almost the same high temperature, most of the excited and ionized atoms and molecules are obtained by collisions between neutral particles, and not when collisions with electrons. Thus, electrons are ionized not directly during collisions with neutral particles (as it occurs in vacuo), but indirectly, increasing the temperature of the gas in the arc column. Such an ionization mechanism is called thermal ionization. The source of the energy required for thermal ionization is the electric field.

In an arc pillar there are loss of energy, which in the steady state are equalized by the energy obtained from the network. The bulk of energy is carried out from a arc pillar excited and ionized atoms and molecules. Due to the difference in the concentrations of charged particles in an arc pillar and the surrounding space, as well as the difference in temperatures of the ions diffuse to the surface of the arc pillar, where they are neutralized. These losses must be assessed by the formation of new ions and electrons associated with the cost of energy. In the steady state, the voltage gradient in the arc column is always that the ionization compensates for the loss of electrons through recombination. The voltage gradient depends on the properties of the gas, the state in which it is (calm, turbulent), as well as on pressure and current. With increasing gas pressure, the voltage gradient increases due to the reduction of the free mileage of electrons. With increasing current, the voltage gradient decreases, which is explained by an increase in the cross-sectional area and the temperature of the arc pillar. The arc pillar seeks to take such a section so that in the underlying conditions of energy loss were minimal.

The dependence of the voltage gradient E \u003d DU / DL in the arc column from the current with a very slow change in the latter is the static characteristic of the arc (Fig. 1, a), depending on the pressure and properties of the gas.

Fig.1. Volt-ampere characteristics of the arc:
a - static characteristic;
b - dynamic characteristics

In the steady state of each point, the characteristic corresponds to some section and the temperature of the arc pillar. When the current changes, the arc pillar must change its section and the temperature in relation to new conditions. These processes require time, and therefore the newly established state occurs not immediately, but with some delay. This phenomenon is called hysteresis.

Suppose that the current suddenly changed from the value i 1 (point 1) to the value i 2 (point 2). At the first moment, the arc will retain its sections and the temperature, and the gradient will decrease (point 2 "). The resulting power will be less necessary for current I 2. Therefore, the section and temperature will begin to decrease, and the gradient will increase until a new steady state appears at point 2. On a static characteristic. With a sudden increase in current from the value i 1 to the value I 3, the voltage gradient will increase (point 3 "). The power supply to the arc will be greater required for current I 3. Therefore, the cross section and the column temperature will begin to increase, and the voltage gradient will decrease until a new steady state occurs at point 3 on the static characteristic.

With a smooth change in current at some speed, the voltage gradient does not have time to follow the change in current in accordance with the static characteristic. With increasing current, the voltage gradient exceeds the values \u200b\u200bdetermined by the static characteristic, and when the current decreases the voltage gradient less than these values. Curves E \u003d F (i) When the current changes at a certain speed are the dynamic characteristics of the arc (solid lines in Fig. 1, b).

The position of these characteristics relative to the static characteristic (see a dotted curve) depends on the speed of change of current. The slower the current changes, the closer is the dynamic characteristic to static. In specified conditions of arc discharge, there can be only one static characteristic. The number of dynamic characteristics is not limited.

When analyzing electrical chains, it is customary to operate with the concept of resistance. Therefore, they are also talking about the resistance of the arc, understanding the ratio of the tension in the electrodes to the current. The arc resistance is impermanent. It depends on the current and many other factors. As the current increases, the arc resistance decreases.

Fig.2. Stress on an arc with alternating current:
a - the arc voltage as a current function;
6 - arc voltage as a function of time

The volt-amps characteristic of the alternating current arc is shown in Fig. 2, and. For a quarter of the period, when the current increases, the voltage curve lies above the static characteristic. The next quarter of the period when the current decreases, the voltage curve lies below the static characteristic.

The arc is ignited at points 1 and 3 and fuses at points 2 and 4. In Fig. 2, B shows the characteristic of the arc as a function of time. Intervals 2-3 and 4-1 correspond to an unstable state at which there is an intensive interaction of the arc with a permanent chain R, L and C. These short time intervals, the duration of which is several microseconds, are used for intensive deionization of the gap between the switch contacts to prevent the new Ignition of arc. Depending on the conditions, the interaction process may end two: or the arc will go out and the chain will be interrupted, or the arc appears again and the interaction process will repeat after half the period under more favorable conditions.

Arc harvesting in air switches

In air switches, the arc is quenched in the high pressure air flow. The switching device of the switch (Fig. 3, a) is a chamber in which two nozzles are placed that serve simultaneously with contacts. The exhaust sides of the nozzles are connected to the low pressure area. In the dilution of contacts, due to the pressure difference, the flow of air appears, directed to the nozzle symmetrically in both directions.

Fig.3. OUGOGAXING DEVICE OF THE AIR CONTROL WITH DUT BONSTANDING:
a - scheme;
b - Pressure distribution along the axis

In Fig. 3, it is shown the distribution of pressure along the axis. In the middle of the interval between the nozzles there is a stream inhibition point, the pressure in which is indicated by P O.

In both directions, the pressure decreases and reaches the nozzles in the necks of approximately half of the P o. Through the necks, the pressure continues to fall to the pressure of the exhaust.

The process of arc harvesting flows as follows. A arc arises between the discontinuous contacts, which under the action of the air flow is quickly transferred along the axis. In this case, the reference stains of the arc move inside the nozzles in a stream, as shown in Fig.3. The arc in the interval between the nozzles has a cylindrical shape.

Fig.4. The temperature distribution in the transverse direction on the plot between the nozzles:
a - arc;
b - thermal border layer

The temperature distribution in the transverse direction is shown in Fig.4. In the arc area, it is approximately 20,000 k and sharply falls to the heat border layer in the resulting arc. Here the temperature varies in the range of 2000 to the temperature of the cold air. As current approaches, the diameter of the cylindrical part of the arc is quickly reduced. With a current equal to zero, it is less than 1 mm. However, the temperature in this part of the arc is still very high (15,000 K).

The most important factor contributing to the arc is the turbulence in the border layer between the arc and its surrounding relatively cold air. Due to the high temperature of the arc, the gas density in the post is approximately 20 times less than in the environment. Therefore, the gas velocity inside the arc pillar is significantly higher than the speed in the adjacent layers (the speed is inversely proportional to the root of the square from the density). Due to the diffusion of particles from the region at high speed to the region at low speed and in the boundary layer there are significant cutting forces, vortices are formed and the entire volume acquires high turbulence. A relatively cold non-ionized gas is made in the arc pillar, as a result of which the pillar loses its homogeneity. It is split into thousands of finest conductive threads that continuously change their shape and position (Fig. 5).

Fig.5. Influence of turbulence on an arc post near zero current (scheme)

They have a high temperature and high specific ionization and are surrounded by cold weakly ionized gas. It is known that the diffusion rate from the cylindrical volume is inversely proportional to the square of the diameter. The thinner of the ionized threads, the faster there is a particle exchange with an ambient and less ionized medium. Turbulence increases diffusion many times. It manifests itself particularly sharply in the necks of the nozzles, where the plasma speed is maximum - 6000 m / s. After zero, for a short period of time calculated by microseconds, the decay of the conductive channel occurs and the further decrease in temperature is determined by the thermal border layer, the cooling of which is much slower.

Fig.6. Scheme of substitution explaining the effect of arc and container resistance

Fig.7. Electric circuit arc interaction

The arc resistance and the container turned on in parallel to the arc gap (Fig.6) has a significant effect on the shutdown process. If you neglect the arc resistance, the current i 0 \u003d i M sinɷt is suitable for zero almost linearly (Fig. 7). However, the arc resistance is not zero. Therefore, the current I B in the arc spacing of the switch decreases:

(1)

where t 0 is the moment of opening the contacts.

As can be seen from the figure, the stress on the arc varies in accordance with the volt-ampere characteristic. The current reduction rate is significantly reduced over the last 5 ... 10 μs before coming to zero. This time is not enough, but it is several times more than the arc time constant and therefore significantly affects the state of the arc at zero of the current (point 1). The arc is easily fading. The arc resistance varies and the crooked PVN. The process of recovery of the voltage begins at point 1; The voltage reaches the maximum at point 2, when i l \u003d i c \u003d 0.

Stage of a possible thermal breakdown

If the gas temperature in the gap does not decrease to a certain critical value determined by the gas property and pressure, the gap will retain its conductivity after zero (point 1) and the residual conductivity current (Fig. 8) appears under the action of PVR.

Fig.8. Razing arc with a delay,
caused by the appearance of current residual conductivity

Under favorable conditions, it is small and quickly fades (point 2). However, if the cooling process is not intense enough, the residual conductivity current increases; Repeated plasma warming occurs, the process of ionization and arc occurs again. This phenomenon got the name of the heat breakdown, since the electric breakdown is impossible, since the gap ionized and did not purchase even electrical strength.

There will be such a breakdown or not, depends on the outcome of two interrelated processes occurring in the interval, of which one is determined by the integral in the time of the input of the power (the current and voltage of the interval), and the second - the integral of the loss time caused by thermal conductivity and convection. This means that the interaction process will continue until the current disappears or the arc will not appear again. The thermal breakdown phenomenon is characteristic of the first 20 μs after zero current under conditions when the speed regenerating the voltage is large, for example, with unsuccessful KZ.

Stage of a possible electric breakdown

If the heat breakdown did not occur, the intertek interval continues to be exposed to PVN. The arc channel has an even increased temperature and reduced density. After a few hundred microseconds after zero, when the PVN reaches the maximum value, the stage of a possible electrical breakdown occurs. It is based on no energy balance, but the electron formation process in the electric field. If an increase in the concentration of electrons exceeds some critical value, the formation of a spark will occur, which will go into an arc discharge.

Arc harvesting in oil switches

In oil switches, contacts are blocked in oil, but due to the high temperature of the arc resulting between contacts, oil decomposes and arc discharge occurs in the gas environment. Approximately half of this gas (by volume) make up the oil pairs. The rest consists of hydrogen (70%) and hydrocarbons of various composition. Gas These are combustible, but in oil the combustion is impossible due to the lack of oxygen. The amount of oil decomposed by the arc is small, but the volume of the formed gases is great. One gram of oil gives approximately 1500 cm 3 of gas shown to room temperature and atmospheric pressure.

The arc harvesting in oil switches is most efficient when the use of quench chambers, which limit the arc zone, contribute to the pressure in this zone and the formation of the gas blast. Through an arc pole. Figure 9 shows the scheme of the simplest quenching chamber.

Fig.9. Scheme of the simplest oily camera switch

In the process of shutdown, the contact rod 1 moves down. There is an arc between contacts 1 and 2. Intensive gas formation occurs and the pressure in the chamber is rapidly increasing. Relatively cold gas formed on the surface of the oil is mixed with the plasma of the arc. The border layer comes into a turbulent state promoting deionization. However, the arc cannot go out until the distance between the contacts reaches some minimum value determined by the reducing voltage. This minimum interval is formed when mobile contact is still in the chamber. When the rod leaves the camera limits, the gases with force are thrown out. There is a gas blow, directed along the axis, which contributes to the arc gash.

After popping the arc, the contact rod continues its movement to provide the necessary insulating distance in the disconnected position.

The voltage on the oil switch arge is at least 3 times more than that of the air switch. The electric strength of the gap is restored faster (at a speed of about 2 kV / μs). Therefore, with the same current of the shortcut, the exhausting device of the oil switch can be calculated for twice the voltage and twice the larger wave resistance than the air blasting device.

Characteristic properties of air and oil switches

In the air switches, the blowing in the arc gap is created from the external source of energy and does not depend on the disconnected current. After zero, the regenerating voltage is applied to a short gap filled with hot ionized gas. The rate of recovery of the electric strength of the gap is determined by the cooling of the gas and removing it from the gap flow of fresh air. This requires time and therefore the process of restoring the electric strength of the gap is larenched.

Fig.10. Characteristics of regenerating electrical strength
arc interval of the air circuit breaker

Figure 10 shows the typical curves of the reducing electrical strength of the arc interval of the air circuit breaker. They have an S-shaped form. In this case, the basic stage of the process of restoration of the electric strength of the gap flows at a rate not exceeding 1-2 kV / μs, and starts after 10-15 μs after the zero current value. With an increase in the disconnected current, the delay increases, and the speed of the reduction of electrical strength decreases. The lower dotted curve corresponds to the case of unsatisfactory operation of the switch, since the process of recovery of electrical strength of the gap flows too slowly. The nominal shutdown current of the air switch is limited to the recoverable electrical strength of the gap.

In oil switches to form a gas blast, the energy of the arc itself is used. The pressure in the jacket chamber and the strength of the blast in the first approximation is proportional to the disconnected current. The more the latter, the more efficient the deionization of the gap and its electric strength is restored faster. However, as the current increases, the mechanical stresses increase in the parts of the quivering chamber. Therefore, the rated shutdown current is limited by the mechanical strength of the jacuing chamber.

The characteristic properties of air and oil switches are manifested when the asymmetric current of the KZ is disconnected. As is known, high-speed switches in the presence of appropriate relay protection are blocked by its contacts when the aperiodic component of the disconnected current does not have time to plunge. Therefore, these switches must be able to turn off both symmetrical and asymmetric current, i.e. Current, not displaced or displaced relative to the time axis, depending on the conditions. The asymmetry of the current β (the relative content of the aperiodic component in the CW current) is defined as the ratio of the aperiodic component to the amplitude of the periodic component of the CW current by the time τ opening the contacts of the switch

(2)

The asymmetry of the disconnected current depends on the time constant of the chain T a \u003d x / (ɷr), as well as from τ - the opening time of the switch contacts, taking into account the timing of relay protection time. The more time constant and the faster the switching of the switch, the greater the asymmetry of the disconnected current. Generators, transformers and reactors have the greatest time constant. Therefore, the greatest asymmetry should be expected at shorts near the generators and busbars of stations. Calculations show that the current asymmetry, disconnected by high-speed switches installed in the main power steering stations, can reach 80%. Less high-speed switches under the same conditions may occur with asymmetry of about 40-50%. Switches installed in distribution networks are found with asymmetry not exceeding 20%.

In the presence of an aperiodic component in a shut-off current:

• increases current current;
• the time intervals between the moments when the current reaches zero, becomes unequal: they alternately more or less semi-period;
• the rate of change of DI / DT current is reduced when it approaches to zero value;
• returning voltage on the switch of the switch decreases.

Increasing the current current value and change the time intervals between zero current values \u200b\u200bmay under adverse conditions to significantly increase the energy released compared with the energy allocated in the absence of an aperiodic component of the current. The energy released in the arc determines the gas ionization in the gap, and in the oil circuit breakers, also the amount of gases formed and pressure in the chamber, therefore, mechanical voltages in the elements of the switch, the degree of melting contacts, etc.

Reducing the speed of change of current when approaching it to zero reduces the ionization of the gap to the time of the arc pops, which facilitates the process of disconnection.

Reducing the return voltage also facilitates the disconnection process.

Fig.11. Returning voltage at asymmetry of the disconnected current

As can be seen from Fig.11, the periodic component of the current of the CZ I n is shifted relative to the voltage of the network by an angle φ close to π / 2. If the closure phase is α \u003d φ, then the aperiodic component is absent, the moment of the arrival of the current to zero value and the arc deposit is close to the moment the voltage maximum. Returning voltage is determined by the ordinate AB. When you closed at any other time in the composition of the disconnected current, the aperiodic component and the moment of arrival of the current is shifted to zero. In the case under consideration, with α \u003d 27 °, the return voltage after a large half-wave current is determined by the ordinate A "B", and after a small half-wave - ordinate A "B" (when constructing curves, periodic and the aperiodic components are taken conditionally unlucky).

From the above analysis, it follows that in the presence of an aperiodic component in the disconnected current, a number of new factors affect the disconnection process, the part of which will take this process, the other part facilitates it.

The outcome effect of the aperiodic component depends on the properties of the switch.

Oil switches, which turns off the ability of which is limited by the mechanical strength of the intake chamber, have a significant stock in the reducing electrical strength of the arc interval. Increasing the active value of the turned off current, due to the presence of the aperiodic component, increases the severity of the shutdown, since the energy released in the arc increases, and the facilitating factors introduced by the aperiodic component of the CW current (reducing the speed of the current approach to zero and reduce the return voltage), are not used oil switches. These switches say that they are sensitive to the current, since the energy released in the arc is determined mainly to the current.

Air switches that turns off the ability of which is limited by the electrical strength of the gap, use facilitating factors introduced by the aperiodic component of the current (decrease in the speed of reducing current and returning voltage). Increasing the active value of the incisable current caused by the aperiodic component does not increase the severity of shutdown, since the accusable weighting and facilitating factors are compensated. About such switches it is customary to say that they are sensitive to tension.

When the disconnecting power switch is selected, the asymmetry of the shut-off current CW should be taken into account. However, normalized (nominal) asymmetry values \u200b\u200bβ nom are set as the same for oil and air switches.

January 17, 2012 at 10:00

When operating the electrical circuit, an electrical discharge occurs in the form of an electric arc. For the appearance of an electric arc, it is enough that the voltage on the contacts is above 10 V at a current in the circuit of about 0.1a and more. With significant stresses and currents, the temperature inside the arc can reach 10 ... 15 thousand ° C, as a result of which contacts and current-carrying parts are mounted.

At 110 kV voltages and above the length of the arc can reach several meters. Therefore, the electrical arc, especially in powerful power circuits, on voltage above 1 kV is a greater danger, although serious consequences can be in the voltage settings below 1 kV. As a result, the electric arc must be limited as much as possible and quickly repayment in the voltage circuits both above and below 1 kV.

Causes of electric arc

The process of the formation of an electrical arc can be simplified as follows. When contacting contacts, the contact pressure and a corresponding contact surface decreases, the transition resistance (current density and temperature increases - local (in separate areas of contact area) overheating, which further contribute to the thermoelectronic emission, when the electron motion speed is increased under the influence of high temperatures. Spit from the surface of the electrode.

At the time of contacts, there is a chain break, voltage is rapidly restored at the contact gap. Since there is little distance between the contacts, an electric field of high tension occurs, under the influence of which electrons are broken from the surface of the electrode. They accelerate in the electric field and when they hit a neutral atom give it their kinetic energy. If this energy is enough to tear at least one electron from the shell of a neutral atom, then the ionization process occurs.

The formed free electrons and ions constitute the plasma of the arc barrel, that is, an ionized channel in which the arc burns and the continuous movement of particles is ensured. At the same time, negatively charged particles, primarily electrons, move in one direction (to the anode), and the atoms and molecules of gases, devoid of one or more electrons, are positively charged particles in the opposite direction (to the cathode). Plasma conductivity is close to metals conductivity.

In the barrel, the arc passes a high current and a high temperature is created. This temperature of the arc barrel leads to thermoionization - the process of formation of ions due to the impact of molecules and atoms with high kinetic energy at high speeds of their movement (molecules and atoms of the medium, where the arc is burning, disintegrate on electrons and positively charged ions). Intensive thermoionization supports high plasma conductivity. Therefore, the voltage drop in the length of the arc is small.

In the electrical arc, two processes continuously flow: in addition to ionization, the deionization of atoms and molecules. The latter occurs mainly by diffusion, that is, the transfer of charged particles into the environment, and the recombination of electrons and positively charged ions, which are reunited in neutral particles with an impact of energy spent on their decay. In this case, the heat sink occurs in the environment.

Thus, three stages of the process under consideration can be distinguished: the ignition of the arc, when due to the impact ionization and emission of electrons from the cathode, the arc discharge begins and the ionization intensity is higher than the deionization, the sustainable burning of the arc, supported by thermoionization in the arc barrel when the intensity of ionization and deionization is the same, The population of the arc when the deionization intensity is higher than the ionization.

Methods of arc harvesting in switching electrical apparatus

In order to disable the elements of the electrical circuit and exclude damage to the switching machine, it is necessary not only to open its contacts, but also to pay off the arc appearing between them. The processes of arc harvesting, as well as burning, with variable and constant current are different. This is determined by the fact that in the first case, the current in the arc each half-period passes through zero. During these moments, the release of energy in the arc stops and the arc each time spontaneously goes out, and then lights up again.

Almost current in the arc becomes close to zero slightly earlier than the transition through zero, since when the current decreases, the energy caused to the arc decreases, respectively, the temperature of the arc is reduced and thermoionization ceases. At the same time, the deionization process is intensively underway in the arc gap. If at the moment to break up and quickly breed contacts, then the subsequent electrical breakdown may not happen and the chain will be disabled without arc. However, it is extremely difficult to do it extremely difficult, and therefore adopt special measures of accelerated arc harvesting, providing cooling arc space and reduce the number of charged particles.

As a result of deionization, the electrical strength of the gap gradually increases and at the same time the regenerating voltage is growing on it. From the ratio of these values \u200b\u200band depends, whether the arc period will turn around on the next half or not. If the electric strength of the gap increases faster and turns out to be greater than the regenerating voltage, the arc will no longer light up, otherwise the sustainable burning of the arc will be ensured. The first condition and determines the task of arc harvesting.

In the switching apparatuses use various ways of arc harvesting.

Extension of the Arc

When contacting contacts in the process of turning off the electrical circuit, the arc appeared stretched. At the same time, the conditions for cooling the arc are improved, since its surface increases and more voltage is required for burning.

Dividing long arc on a number of short arcs

If the arc formed during the opening of the contacts is divided into short arcs, for example, tightening it into a metal grille, it will go out. The arc is usually tightened into the metal grille under the influence of the electromagnetic field, carried in the lattice plates with vortex currents. This arc harvesting method is widely used in switching devices to voltage below 1 kV, in particular in automatic air switches.

Cooling arc in narrow slots

Arc harvesting in small volume is facilitated. Therefore, the switching chambers with longitudinal slots are widely used in switching devices (the axis of such a slit coincides towards the arc axis). Such a gap is usually formed in cameras from insulating arc-resistant materials. Due to the contact of the arc with cold surfaces, its intensive cooling occurs, the diffusion of charged particles into the environment and, accordingly, fast deionization.

In addition to the slots with flat-parallel walls, cracks are also used with ribs, protrusions, extensions (pockets). All this leads to deformation of the arc barrel and contributes to an increase in the area of \u200b\u200bcontact with the cold walls of the chamber.

The drawing of the arc into narrow slits usually occurs under the action of a magnetic field that interacts with the arc, which can be considered as a conductor with a current.

The external magnetic field for the movement of the arc is most often provided by the coil, included consistently with the contacts, between which arc occurs. Arc harvesting in narrow slots are used in devices for all voltages.

High pressure arc

At a constant temperature, the degree of gas ionization drops with increasing pressure, while the thermal conductivity of the gas increases. All other things being equal conditions, this leads to enhanced arc cooling. The arc harvesting with the help of high pressure generated by the arc itself in tightly closed cameras, is widely used in fuses and a number of other devices.

Arc harvesting in oil

If the circuit breaker contacts are placed in oil, the arc arising during their opening leads to intense evaporation of oil. As a result, a gas bubble (shell) is formed around the arc, consisting mainly of hydrogen (70 ... 80%), as well as water vapor. The released gases at high speed penetrate directly into the zone of the arc barrel, cause mixing of cold and hot gas in the bubble, provide intensive cooling and, accordingly, deionizing the arc gap. In addition, the deionizing ability of gases increases the pressure generated during the rapid decomposition of the oil.

The intensity of the exhausting arc process in oil is higher, the closer arc with oil and oil is moving faster in relation to the arc. Considering this, the arc gap is limited to a closed insulating device - an extinguishing chamber. In these cameras, there is a closer touch of oil with an arc, and with the help of insulating plates and exhaust holes, working channels are formed, according to which oil and gases are moving, providing intensive blowing (dirt) arc.

Physical foundations of burning arc. When operating the contacts of the electrical apparatus due to the ionization of the space between them, an electric arc occurs. The gap between the contacts does not remain conductive and the passage of the circuit current does not stop.

It is necessary to ionization and the formation of the arc that the voltage between the contacts was about 15-30 V and the circuit current of 80-100 mA.

When the space ionization between the contacts fill its gas (air) atoms are disintegrated into charged particles - electrons and positive ions. The flow of electrons emitted from the surface of the contact, which is under the negative potential (cathode), moves towards a positively charged contact (anode); The flow of positive ions is moving to the cathode (Fig. 303, a).

The main carriers of the current in the arc are electrons, since positive ions, having a large mass, are moving significantly slower than electrons and is transferred therefore a unit of time is much less electric charges. However, positive ions play a big role in the process of burning arc. Approaching the cathode, they create a strong electric field near it, which acts on the electrons available in the metal cathode, and take them off from its surface. This phenomenon is called auto-electronic emissions (Fig. 303, b). In addition, positive ions continuously bombard the cathode and give it their energy that goes into heat; In this case, the temperature of the cathode reaches 3000-5000 ° C.

With increasing temperature, the movement of electrons in the metal of the cathode is accelerated, they acquire greater energy and begin to leave the cathode departing into the environment. This phenomenon is called thermoelectronic emission. Thus, under the action of auto and thermoelectronic emissions, all new and new electrons come from the cathode.

With its movement from the cathode to the anode, electrons, facing its path with neutral gas atoms, split them into electrons and positive ions (Fig. 303, B). This process is called shock ionization. New, so-called secondary electrons begin to move to the anode and, with their movement, split all new gas atoms. The considered gas ionization process is avalanche-like character, just as one stone, abandoned from the mountain, captures all new and new stones on its path, generating avalanche. As a result, the gap between two contacts is filled with a large number of electrons and positive ions. This mixture of electrons and positive ion is called plasma. In plasma formation, thermal ionization plays a significant role, which occurs as a result of an increase in temperature causing an increase in the speed of movement of charged particles of gas.

Electrons, ions and neutral atoms forming plasma are continuously faced with each other and exchange energy; At the same time, some atoms under electrons are punched into the excited state and emit excess energy as light radiation. However, the electric field acting between contacts causes the bulk of positive ions to move to the cathode, and the main mass of electrons is to the anode.

In the DC electrical arc in the steady mode, the thermal ionization is determined. In an alternating current arc, shock ionization is played with a significant role in the transition of a current, and over the rest of the arc burning time is thermal ionization.

When burning arcs simultaneously with the ionization of the gap between the contacts, the return process takes place. Positive ions and electrons, interacting with each other in the intercontaznaya space or when the chamber is hit on the walls, in which the arc is burning, form neutral atoms. This process is called recombination; With the termination of ionization recombination It leads to the disappearance of electronics and ions from the interelectrode space - it is deionization. If recombination is carried out on the chamber wall, it is accompanied by the release of energy in the form of heat; During recombination in the interelectrode space, the energy is released as radiation.

When contacting the walls of the camera in which contacts are located, the arc is cooled that. leads to the strengthening of deionization. Deonization occurs as a result of the movement of charged particles from the central areas of arcs with a higher concentration into the peripheral areas with low concentration. This process is called electron and positive ion diffusion.

The burning area of \u200b\u200bthe arc is conditionally divided into three sections: a cathode zone, an arc barrel and anodic zone. In the cathode zone there is an intensive emission of electrons from a negative contact, the voltage drop in this zone is about 10 V.

In the barrel of the arc, plasma is formed with approximately the same concentration of electrons and positive ions. Therefore, at each moment of time, the total charge of positive plasma ions compensates for the total negative charge of its electrons. The large concentration of charged particles in the plasma and the absence of an electrical charge in it determine the high electrical conductivity of the arc barrel, which is close to the electrical conductivity of the metals. The voltage drop in the arc bar is approximately proportional to its length. The anode zone is filled, mainly by electrons suitable from the arc barrel to a positive contact. The voltage drop in this zone depends on the current in the arc and the size of the positive contact. The total voltage drop in the arc is 15-30 V.

The dependence of the voltage drop U dg acting between the contacts from the current I passing through the electrical arc is called the volt-ampere arc characteristic (Fig. 304, a). Voltage U s, in which the arc ignition is possible at a current i \u003d 0, called ignition tension. The value of the ignition voltage is determined by the contact material, the distance between them, temperature and the environment. After occurrence

the electrical arc of its current increases to the value close to the load current, which proceeded through the contacts before the disconnection. At the same time, the resistance of the cross-stage gap falls faster than the current increases, which leads to a decrease in the drop in the voltage U dg. The combustion mode of the arc corresponding to the curve A is called static.

When the current decreases to zero, the process corresponds to the curve B and the arc stops with a smaller stress drop than the ignition voltage. The voltage u r, in which the arc goes out, is called voltage of quenching. It is always less than the ignition tension due to an increase in the contact temperature and an increase in the conductivity of the intertek intertension. The larger the current reduction rate, the less the tension of the arc valve at the time of the cessation of the current. Volt-ampere characteristics B and C correspond to a reduction in current at different speeds (for a curve with more than for curve B), and straight D corresponds to a practically instantaneous reduction in current. This nature of the volt-ampere characteristics is explained by the fact that with a rapid change in current, the ionization status of the intertek intersion does not have time to follow the change in current. It requires a certain time to deionize the gap, and therefore, despite the fact that the current in the arc fell, the conductivity of the gap remained the same corresponding to the large current.

Volt-ampere characteristics B - D, obtained by rapid change in current to zero, are called dynamic. For each intercontract, the material of the electrodes and medium, there are one static arc characteristics and a plurality of dynamic, prisoners between curves A and D.

When burning alternating current arc during each half-period, the same physical processes occur as in the DC arc. At the beginning of a half-period, the tension on the arc increases according to the sinusoidal law to the value of the ignition voltage U z - section 0-A (Fig. 304, b), and then after the arc occurs as the current increases - the section A - B. In the second part of the half-period, when the current begins to decline, the voltage on the arc increases again to the value of the valve of the zing U g when the current is downtown to zero - section B - s.

During the next half period, the voltage changes the sign and according to the sinusoidal law to the value of the ignition voltage corresponding to the point A 'volt-amps characteristic. As current increases, the voltage decreases, and then rises again when the current is reduced. Arc voltage curve, as can be seen from fig. 304, B, has the shape of a cut sinusoid. The process of deionization of charged particles in the interval between the contacts continues only a small fraction of the period (plots 0 - a and s -a ') and, as a rule, it does not end during that time, as a result of which the arc occurs again. The final arc harvesting will take place only after a series of repeated ignitions during one of the subsequent current transitions through zero.

The resumption of the arc after the transition of the current through zero is due to the fact that after the current decline to the zero value, the ionization that exists in the arc bar will not immediately disappear, as it depends on the plasma temperature in the residual branch of the arc. As the temperature decreases, the electrical strength of the intertek intertension increases. However, if at some point in time, the instantaneous value of the applied voltage will be larger than the breakdown of the gap, then it will be sampled, an arc will occur and the current of another polarity will flow.

Termination conditions arc. The conditions for quenching DC arc depend not only on its volt-amps characteristics, but also on the parameters of the electrical circuit (voltage, current, resistance and inductance), which includes and disable the contacts of the device. In fig. 305, and show the volt-ampere characteristic of the arc

(Curve 1) and the dependence of the voltage drop on the R resistor included in this chain (direct 2). In the steady mode, the voltage U and the current source is equal to the sum of the voltage drops in the DG and IR arc on the R. resistor when the current changes in the circuit to them is added e. d. s. self-induction ± e L (depicted by shaded ordines). Long burning of the arc is possible only in modes corresponding to points A and B, when the voltage U and - IR, applied to the gap between the contacts, is equal to the voltage drop U dg. In this case, in the mode corresponding to the point A, the burning of the arc is unstable. If during the burning of the arc at this point the characteristics of the current for some reason increased, then the voltage U DG will be less than the applied voltage U and - IR. The excess of the applied voltage will cause an increase in the current, which will grow until it reaches the value of i c.

If in the corresponding point A mode, the current will decrease, the applied voltage U and - IR will become less u dv and the current will continue to decrease until the arc goes out. In mode, the corresponding point in, the arc is stable. With an increase in the current Over I in the voltage drop in the DUG U DG, there will be more applied voltage U and - IR and the current will begin to decrease. When the current in the chain becomes less i in, the applied voltage U and - IR will become more u DG and the current will begin to increase.

Obviously, to ensure arc harvesting in the entire predetermined range of current i from the highest value to zero when the chain is turned off, it is necessary that the volt-ampere characteristic 1 is located above direct 2 for the disconnected chain (Fig. 305, b). At the same time, the condition of the voltage drop in the U DG arc will always be more applied to it the voltage U and - IR and the current in the chain will decrease.

The main means of increasing the incidence drop in the arc is an increase in the length of the arc. When the low voltage circuits are blurred with relatively small currents, quenching is ensured by the appropriate choice of the contact solution, between which arc occurs. In this case, the arc goes out without any additional devices.

For contacts that tear down the power chains needed to quench the length of the arc are so high that it is no longer possible to carry out such a solution of contacts. In such electrical devices, special extinguishing devices are installed.

Splogging devices. The arc harvesting methods can be different, but they are all based on the following principles: Forced arc elongation; cooling of the intertekny gap by air, vapor or gases; Separation of arcs per number of separate short arcs.

When lengthening arc and removing it from contacts, an increase in the voltage drop in the arc column and the voltage attached to the contacts becomes insufficient to maintain the arc.

Cooling of the interteklifting gap causes increased heat transfer of arc post into the surrounding space, as a result of which charged particles, moving from the inside of the arc to its surface, accelerate the deionization process.

The separation of arcs into a number of individual short arcs leads to an increase in the total drop in the voltage in them and the voltage applied to the contacts becomes insufficient for the sustainable support of the arc, therefore it is quenched.

The principle of quenching by elongation of the arc is used in devices with protective horns and in the switches. The electrical arc arising between contacts 1 and 2 (Fig. 306, a) during their opening, rises up under the action of the power F B, created by the flow of air heated, stretched and lengthened on diverging stationary, horns, which leads to its gas. An electrodynamic force created as a result of an arc current interaction with the magnetic field arising around it also contributes to the elongation and arc. In this case, the arc behaves like a conductor with a current in a magnetic field (Fig. 307, a), which, as shown in Chapter III, tends to push it out of the field limits.

To increase the electrodynamic force F e, acting on the arc, into a chain of one of the contacts 1 in some cases includes a special extinguishing coil 2 (Fig. 307, b), creating a strong magnetic field in the arc formation zone

the thread of which f, interacting with the current I arc, provides intensive blowing and arc harvesting. The rapid movement of arcs along the horns 3, 4 causes its intensive cooling, which also contributes to its deionization in the chamber 5 and the gas.

Some devices use methods forced cooling and stretching arc compressed air or other gas.

When opening contacts 1 and 2 (see Fig. 306, b) the arc appearance is cooled and blew out from the contact zone of a jet of compressed air or gas with FB power.

An effective means of cooling the electrical arc followed by its junction are the extinguishing chambers of various designs (Fig. 308). The electrical arc under the action of the magnetic field, the air flow or other means is driven into a narrow slit or a chamber's labyrinth (Fig. 308, a and b), where it is closely in contact with its walls 1, partitions 2, gives them warm and goes out. Wide use in electrical devices e. p. p. Find the labyrinth and slot chambers, where the arc is extended not only by stretching between contacts, but also by its zigzag curvature between the chamber partitions (Fig. 308, B). Narrow slit 3 between the walls of the chamber contributes to the cooling and deionization of the arc.

It is based on the separation of arc to a series of short arcs based on the separation of arcs on a series of short arcs (Fig. 309, a) embedded inside the exhaust chamber.

Deion lattice is a set of a number of separate steel plates 3, isolated relative to each other. The electrical arc resulting between the opening contacts 1 and 2 is separated by a grid on a number of shorter arcs connected in series. To maintain the combustion of the arc without its separation, the voltage U is required, equal to the sum of the ceremonic (anode and cathode) drop in the voltage U e and the voltage drop in the arc column U Art.

When separating one arc on a short arc, the total voltage drop in the column of all short arcs will continue to be Nu E as in one common arc, but the total stress drop in all arcs will be NU e. Therefore, to maintain the burning of the arc in this case, voltage will need

U \u003d Nu E + U Art.

The number of arcs n is equal to the number of lattice plates and can be selected so that the ability to resistant burning of the arc with this voltage U was completely excluded. The effect of such a principle of quenching is effective both at a constant and variable current. When the AC transitions through the zero value to maintain the arc requires a voltage of 150-250 V. in connection with this, the number of plates can be selected significantly less than at a constant current.

In fuse fuses with an aggregate in melting insert and the occurrence of an electrical arc due to increased gas pressure in the cartridge ionized particles move in the transverse direction. At the same time, they fall between the grains of the aggregate, cool and deionize. The grains of the aggregate, moving under the action of overpressure, split the arc to a large number of microdes than and ensures their quenching.

In fuses without aggregate, the case is often made from a material from a rude gas that is heated. Such materials refers, for example, Fiber. In contact with the arc, the housing is heated and gas that contributes to the arc gas. An arc in oil switches of AC (Fig. 309, b) with the only difference is that instead of dry filler, non-combustible oil is used here. In the occurrence of arc at the time of opening the movable 1, 3 and fixed 2 contacts, its quenching occurs under the action of two factors: allocations of a large amount of hydrogen that does not support the combustion (in the oil used for this purpose, hydrogen content 70-75%), and intensive cooling of the oil arc Due to its high heat capacity. The arc goes out at the moment when the current is zero. Oil not only contributes to the accelerated arc jam, but also serves as insulation of current and grounded structural parts. To clean the arc in the DC circuit, the oil does not apply, since under the action of an arc, it quickly decomposes and loses its insulation qualities.

In modern electrical devices, the arc harvesting is often carried out by a combination of two or more considered

above the methods (for example, with the help of an extinguishing coil, protective horns and deion lattice).

Electrical arc valve conditions determine the disconnecting ability of protective devices. It is characterized by the greatest current, which can turn off the device with a certain arc harvesting time.

With a short circuit of the electrical circuit connected to the source of electrical energy, the current in the chain increases by curve 1 (Fig. 310). At the time of T 1, when it reaches the value that the protective device is adjusted (setting current I y), the device is triggered and turns off the protected chain, as a result of which the current decreases by curve 2.

The time countable from the moment of signaling to shut down (or inclusion) of the device until the start of the opening (or closure) of the contacts is called your own T C apparatus. When the moment is turned off, the opening of the contacts corresponds to the arc occurrence between the diverging contacts. In the circuit breaker, this time is measured from the moment the value of the setpoint T 1 until the arc appears between the contacts T 2. Time of burning Dougie T DG is called from the moment of the appearance of the arc T 2 until the current termination of the current T 3. The total time of shutdown T P is the amount of their own time and the burning time of the arc.

The electrical arc is a type of discharge characterized by a high current density, high temperature, high gas pressure and a small voltage drop on the arc gap. In this case, there is an intensive heating of electrodes (contacts), on which the so-called cathode and anode spots are formed. The cathode glow concentrates in a small bright spot, the hotspot of the opposite electrode forms an anode spot.

In the arc you can note three areas, very different processes in them. Directly to the negative electrode (cathode) arc adjacent the region of the cathode drop in the voltage. Next is the plasma barrel of the arc. Directly to the positive electrode (anode), an area of \u200b\u200banodic drop in voltage is adjacent. These areas are schematically shown in Fig. one.

Fig. 1. The structure of the electric arc

The size of the areas of the cathode and anodic drop in the voltage in the figure is greatly exaggerated. In fact, their length is very small for example, the length of the cathode drop in voltage has the value of the order of the path of the free movement of the electron (less than 1 MK). The length of the area of \u200b\u200bthe anodic voltage drop is usually somewhat larger than this value.

Under normal conditions, air is a good insulator. Thus, a 1 cm voltage required for a breakdown of the air gap of 1 cm. In order for the air gap to become a conductor, it is necessary to create a certain concentration of charged particles (electrons and ions) in it.

How does electric arc occur

The electrical arc, which is a flow of charged particles, in the initial moment of discrepancies of contacts arises as a result of the presence of free electrons of the gas gap and electrons emitted from the surface of the cathode. Free electrons that are in the gap between the contacts are moved at high speed in the direction of the cathode to the anode under the action of the electric field forces.

The field strength at the beginning of the discrepancy of contacts can reach several thousand kilovolts per centimeter. Under the action of the forces of this field, electrons are broken from the surface of the cathode and moved to the anode from it the electrons that form an electronic cloud. The initial flow of electrons created in this way is formed in the future intensive ionization of the arc gap.

Along with the ionization processes, the arionization processes are in parallel and continuously. The processes of deionization are that, with a convergence of two ions of different signs or a positive ion and electron, they are attracted and, facing, neutralized, in addition, the dressed particles are moved from the combustion area of \u200b\u200bshower with a greater concentration of charges into the environment with a smaller concentration of charges. All this factors lead to a decrease in the arc temperature, to its cooling and extinction.

Fig. 2. Electric arc

Arc after ignition

In the established combustion mode, the ionization and deionization processes in it are in equilibrium. The arc bar with an equal number of free positive and negative charges is characterized by a high degree of gas ionization.

The substance, the degree of ionization of which is close to one, i.e. In which there are no neutral atoms and molecules, called plasma.

Electrical arc is characterized by the following features:

1. A clear outlined boundary between the arc bar and the environment.

2. High temperatures inside the arc barrel reaching 6000 - 25000K.

3. High current density and arc barrel (100 - 1000 A / mm 2).

4. Small values \u200b\u200bof the anode and cathode drop in the voltage and is practically independent of the current (10 - 20 V).

Volt-ampere characteristic of an electric arc

The main characteristic of the DC arc is the dependence of the stress of the arc from the current, which is called volt-ampere characteristic (WA).

The arc arises between the contacts at a certain voltage (Fig. 3), called the ignition voltage of the UV and the distance dependent between the contacts, on the temperature and pressure of the medium and on the speed of contacts. The arc argument voltage is always less than the voltage U h.

Fig. 3. Volt-ampere characteristics of DC arc (a) and its substitution scheme (b)

Curve 1 is a static characteristic of the arc, i.e. obtained with slow change. The characteristic has a falling character. With increasing current, the stress on the arc decreases. This means that the resistance of the arc gap decreases faster, whose current increases.

If one or another speed reduce the current in the arc from i1 to zero and at the same time fix the voltage drop on the arc, then the curves 2 and 3. These curves are called dynamic characteristics.

The faster to reduce the current, the lower the dynamic flocks will be. This is explained by the fact that when the current decreases such parameters such as the cross section of the trunk, the temperature, do not have time to quickly change and acquire values \u200b\u200bcorresponding to a smaller current value when steady.

The voltage drop on the arc gap:

Ud \u003d u s + edid,

where U s \u003d U K + U A is a near-reflux voltage drop, ED - a longitudinal voltage gradient in the arc, ID - Dina arc.

From the formula it follows that with an increase in the length of the arc, the voltage drop on the arc will increase, and the Wah will be placed above.

Electric arc struggle when designing switching electrical apparatuses. Electric arc properties are used in and in.

• Electric arc (Voltov arc, arc discharge) - physical phenomenon, one of the types of electric discharge in gas.

  It was first described in 1802 by Russian scientist V. Petrov in the book "News of Galvani-Voltovsky experiments through a huge battery, which sometimes from 4,200 copper and zinc circles" (St. Petersburg, 1803). The electrical arc is a special case of the fourth form of a substance - plasma - and consists of an ionized, electrically quasi-larger gas. The presence of free electrical charges provides the conductivity of an electric arc.

  The electrical arc between the two electrodes in the air at atmospheric pressure is formed as follows:

  With an increase in the voltage between the two electrodes to a certain level in the air between the electrodes, an electric breakdown occurs. The voltage of the electrical breakdown depends on the distance between the electrodes and other factors. The ionization potential of the first electron of metals atoms is approximately 4.5 - 5 V, and the storage voltage is twice as much (9 - 10 V). It is necessary to spend energy to the output of the electron from the metal atom of one electrode and the ionization of the atom of the second electrode. The process leads to the formation of a plasma between the electrodes and the burning of the arc (for comparison: the minimum voltage for the formation of a spark discharge, a little exceeds the electron output potential - up to 6 V).

  To initiate a breakdown with the existing voltage, the electrodes approach each other. During the breakdown between the electrodes, a spark discharge usually occurs, a pulsed closure electrical circuit.

  Electrons in spark discharges ionize molecules in the air gap between the electrodes. With sufficient power of the voltage source in the air gap, a sufficient amount of plasma is formed for a significant drop in the breakdown voltage or air interval resistance. In this case, spark discharges turn into an arc discharge - a plasma cord between the electrodes, which is a plasma tunnel. The arc arise is, in fact, the conductor and closes the electrical circuit between the electrodes. As a result, the average current increases even more, the heating arc is up to 5000-50000 K. It is believed that the alert of the arc is completed. After the ignition, the stable burning of the arc is ensured by thermoelectronic emissions from the cathode heated by the current and ion bombardment.

  The interaction of electrodes with an arc plasma leads to their heating, partial melting, evaporation, oxidation and other types of corrosion.

  After the ignition, the arc may remain stable when the electrical contacts are diluted to a certain distance.

  When operating high-voltage electrical installations, in which the emergence of an electric arc is inevitable, the fight against it is carried out using electromagnetic coils combined with extinguishable chambers. Other ways are the use of vacuum, air, emelegase and oil switches, as well as methods for taking current to temporary load, self-tearing electrical circuit.