During operation, the electrical chains are constantly closed and open. It has long been noticed that at the moment of opening between the contacts, an electric arc is formed. For its appearance, there is quite enough voltage of more than 10 volts and current forces - over 0.1 amps. With higher current and voltage values, the internal temperature of the arc often reaches 3-15 thousand degrees. It becomes the main cause of molten contacts and current-carrying parts.

If the voltage is 110 kilovolts and above, in this case the length of the arc can reach the length of more than one meter. Such an arc represents a serious danger to persons working with powerful power plants, Therefore, it requires its maximum limitation and rapid nutrition in any chains, regardless of the voltage value.

What is electric arc

The most characteristic example is the electric welding arc, manifested in the form of a long electric discharge in plasma. In turn, plasma is mixed ionized gases and pairs of components of the protective atmosphere, the main and additive metal.

Thus, the electrical arc is the combustion of the electrical discharge between the two electrodes located in the horizontal plane. Under the action of heated gases, seeking to top, this discharge bends and becomes visible as an arc or arch.

These properties allowed the use of an arc in practice as a gas conductor, with which electrical energy is converted to thermal, creating a high heating intensity. This process can be relatively easily controlled by changing electrical parameters.

Under normal conditions, the gases do not conduct a current. However, if favorable conditions arise, they can be ionized. Their atoms or molecules become positive or negative ions. Under the action of high temperature and external electric field with high gas strength changes and go to the plasma state with all properties of the conductor.

How the welding arc is formed

• Initially, a contact affecting both surfaces appears between the end of the electrode and the detail.
• Under the action of current with high density, particles of surfaces are quickly melted, forming a layer of liquid metal. It is constantly increasing in the direction of the electrode, after which it comes to the gap.
• At this point, the metal evaporates very quickly and the discharge interval begin to fill ions and electrons. The applied voltage causes them to move to the anode and cathode, as a result, the welding arc is excited.
• The thermal ionization process begins, in which positive ions and free electrons continue to concentrate, the gas arc gap is even more ionized and the arc itself becomes stable.
• Under its influence, metal blanks and electrodes are melted and, being in liquid stateMixed together.
• After cooling, the welding seam is formed in this place.

Electric arc harvesting in switching equipment

Turning off the electrical chain elements should be done very carefully, without damage to the switching equipment. Only the opening of contacts will not be enough, it is necessary to properly repay the arc arising between them.

The combustion and arc gas processes differ significantly among themselves, depending on the use on the network. If there are no special problems with constant current, then in the presence of AC, a number of factors should be taken into account. First of all, the arc current passes the zero mark on each half-period. At this moment, the extraction of energy is stops, as a result of the arc spontaneously goes out, and again lights up. In practice, the current approaches zero before the transition through the zero mark. This is due to a decrease in current and a decrease in energy summarizing to the arc.

Accordingly, its temperature is reduced, which causes cessation of thermal ionization. Intensive deionization occurs in the very gap of the arc. If at this moment there is a quick opening and wiring of contacts, then the breakdown may not happen, the chain will turn off without the appearance of an arc.

In practice, creating such ideal conditions is very difficult. In this regard, special measures were developed on accelerated arc. Various technical solutions allow you to quickly cool the arc gap and reduce the number of charged particles. As a result, it comes a gradual increase in the electrical strength of this gap and the simultaneous growth on it of the reducing voltage.

Both values \u200b\u200bare dependent on themselves and affect the ignition of the arc in the next half-period. If the electrical strength exceeds the regenerating voltage, then the arc will not light up. Otherwise, it will be stable to burn.

Main ways of arc harvesting

Quite often uses the arc elongation method, when in the process of discrepancies of contacts when the chain is turned off, it takes place (Fig. 1). By increasing the surface, the cooling condition is significantly improved, and more voltage is required to support combustion.

1.

In another case, the overall electric arc is divided into separate short arcs (Fig. 2). To do this, a special metal grille can be used. In its plates, an electromagnetic field, a tightening arc for separation, is indispensable. This method is widely used in switching equipment with a voltage of less than 1 kV. A typical example is air circuit breakers.

2.

Quite effective is considered to be quenching in small volumes, that is, inside the extinguishing chambers. These devices have longitudinal slits that coincide on the axes with the direction of the arc. As a result of contact with cold surfaces, the arc begins to be intensively cooled, actively highlighting charged particles in environment.

Using high pressure. In this case, the temperature remains unchanged, the pressure increases, and ionization decreases. In such conditions, the arc is cooled rapidly. To create high pressure, tightly closed cameras are used. The method is especially effective for fuses and other equipment.

The arc harvesting can occur with the help of oil where contacts are placed. When they are broken, arc appears, under the action of which the oil begins to actively evaporate. It turns out to be covered with a gas bubble or a shell consisting of 70-80% of hydrogen and oil vapor. Under the influence of the secreted gases falling directly into the barrel zone, the cold and hot gas inside the bubble is mixed, intensively cooling the arc gap.

Other methods of damage

Electric arc harvesting can be performed due to the growth of its resistance. It gradually increases, and the current decreases to the value insufficient to maintain combustion. The main disadvantage of this method is the long time of quenching, during which a large amount of energy is dissipated in the arc.

An increase in the resistance of the arc is achieved in different ways:

• The elongation of the arc, since its resistance is in direct proportional dependence with the length. To do this, you need to change the gap between the contacts towards the increase.
• Cooling the medium between the contacts where the arc is located. Most often it is used by blowing along the arc.
• Contacts are placed in a gas medium with a low degree of ionization or in a vacuum chamber. This method is used in gas and vacuum switches.
• The cross section of the arc can be reduced, passing it through a narrow hole or reducing the area of \u200b\u200bcontacts.

In the circuits with alternating voltage to clean the arc, the zero current method is used. In this case, the resistance is retained at a low level, until the current value decreases to zero. As a result, the quenching occurs naturally, and the ignition is not repeated again, although the voltage on contacts may increase. A drop to zero mark occurs at the end of each half-period and the arc goes out for a short time. If you increase the dielectric strength of the gap between the contacts, then the arc will remain the extinguished.

The consequences of the electrical arc

The destructive impact of the arc is a serious danger not only for equipment, but also for working people. With an unfavorable coincidence, you can get serious burns. Sometimes the defeat of the arc ends with a fatal outcome.

As a rule, an electric arc occurs at the time of accidental contact with current-time parts or conductors. Under the action of a short circuit current, the wire is melted, air is ionized, other favorable conditions for the formation of a plasma channel are created.

Currently, in the field of electrical engineering, it was possible to achieve significant positive results with the help of modern protective equipment developed against an electric 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 (voltova Arc, arc discharge) - physical phenomenon, one of the types of electric discharge in gas.

Structure of arch

The electric arc consists of cathode and anode areas, an arc column, transitional regions. The thickness of the anode region is 0.001 mm, the cathode region is about 0.0001 mm.

The temperature in the anode region during welding by a melting electrode is about 2500 ... 4000 ° C, the temperature in the arc column is from 7,000 to 18,000 ° C, in the cathode area - 9000 - 12000 ° C.

The arc post is electrically neutral. In any of its cross section there are the same number of charged particles of opposite signs. The voltage drop in the arc column is proportional to its length.

Welding arcs classified by:

• The materials of the electrode - with a melting and uncommunicative electrode;
• The degree of compression of the pillar is a free and compressed arc;
• Used current - arc of constant and arc of alternating current;
• According to the polarity of constant electric current - direct polarity ("-" on the electrode, "+" - on the product) and reverse polarity;
• When using alternating current - one-phase and three-phase arches.

Self-regulation of arc during electric welding

In the event of an external reimbursement, changes in the voltage of the network, the feed rate of the wire, etc. - there is a violation in the steady equilibrium between the feed rate and the melting speed. With an increase in the arc length in the chain, the welding current and the melting speed of the electrode wire are reduced, and the feed rate, remaining constant, becomes greater than the melting speed, which leads to the restoration of the arc length. With a decrease in the arc length, the melting speed of the wire becomes greater than the feed rate, this leads to the restoration of the normal length of the arc.

The effectiveness of the process of self-regulation of the arc is significantly affected by the form of the volt-ampere characteristics of the power supply. The high speed of the oscillation of the length of the arc is performed automatically with rigid chains.

Electric Arc Fighting

In a number of devices, the phenomenon of the electric arc is harmful. This is primarily the contact switching devices used in power supply and electric drive: high-voltage switches, circuit breakers, contactors, sectional insulators on the electrified contact network railways and urban electric transport. When the loads are disconnected by the above apparatus between the opening contacts arise arises.

The mechanism of arc occurrence in this case is as follows:

• Reducing the contact pressure - the number of contact points decreases, resistance increases in the contact node;
• The beginning of the discrepancy of contacts is the formation of "bridges" from the molten metal of contacts (in the places of the last contact points);
• Breaking and evaporation of "bridges" from molten metal;
• The formation of an electrical arc in metal vapors (which contributes to greater ionization of the contact interval and difficulties in the arc quench);
• Sustainable burning of arc with rapid burnout of contacts.

For the minimum damage to the contacts, it is necessary to extinguish the arc at minimal time, making all the efforts to prevent the arc location in one place (when the heat arc moves, released in it will be evenly distributed over the contact body).

To fulfill the above requirements, the following arc control methods are applied:

• cooling of the arc flow of the cooling medium - fluid (oil switch); gas - (air switch, autogas switch, oil switch, elegous switch), and the flow of the cooling medium can pass both along the arc barrel (longitudinal quench) and across (cross-quench); Sometimes longitudinal transverse harvesting is used;
• the use of the extinguishing capacity of the vacuum - it is known that with a decrease in the pressure of gases surrounding the switched contacts to a certain value, leads to an effective arc gas (due to the lack of media for the formation of an arc) a vacuum switch.
• the use of more dug-resistant contact material;
• applying contact material with higher ionization potential;
• application of extinguishing grids (circuit breaker, electromagnetic switch). The principle of applying the exhaustion on the lattices is based on the use of the effect of the near-door drop in the arc (most of the voltage drop in the arc is a drop in the cathode; the extinguishing grid is actually a series of consecutive contacts for the arc.
• using

When operating the electrical circuit, an electrical discharge occurs in the form of electrical 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 3 - 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.

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, increase (current density and temperature - local (in separate areas of contact area) overheating, which further contribute to the thermoelectronic emission, when the speed of movement of the electrons increases under high temperature and they are escaping Surface surfaces.

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, there is high tensions, under the influence of which electrons are broken from the electrode surface. They accelerate B. electric field And when hitting a neutral atom gives him his 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 devices use various methods 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. Arc is usually delayed in a metal grid under the influence of electro magnetic fieldInwiddled in 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 extensive cells with longitudinal slits are widely used (the axis of such a slit coincides towards the axis of the arc barrel). 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.

External arcs are most often provided at the expense of the coil, including 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 placed in oil, the arc arising during their opening leads to intensive 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. Given this, the arc gap is limited to a closed insulating device - splogging 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.

Splogging chambers According to the principle of operation, they are divided into three main groups: with auto-cutting, when high pressure and speed of gas in the arc area are created due to energy released in an arc, with forced oil blows with special pumping hydraulic mechanisms, with a magnetic gas in oil when the arc under The action of the magnetic field is moved to narrow slots.

Most effective and simple dumopy chambers with auto table. Depending on the location of the channels and exhaust holes, the chambers are distinguished, in which the intensive blowing of the gas flow mixture and oil is provided along the arc (longitudinal dirt) or across the arc (transverse dimity). The consideration of arc harvesting methods are widely used in the voltage switches above 1 kV.

Other arc harvesting methods in voltage devices above 1 kV

In addition to the above methods of arc harvesting, it is also used: compressed air, the flow of which along or across the arc is blown up, providing its intensive cooling (instead of air, other gases are used, often obtained from solid gas generating materials - fibers, viniplast, etc. - for The score of their decomposition of the most burning arc), which has a higher electric strength than air and hydrogen, as a result of which the arc burning in this gas, even at atmospheric pressure, is quickly quenched, highly developed gas (vacuum), when opening contacts in which the arc is not It lights up again (goes out) after the first passage of the current through zero.

The electrical arc is a powerful, long-term existing between electrodes under voltage, an electrical discharge in a strongly ionized mixture of gases and vapors. It is characterized by high gases and high current in the discharge zone.

The electrodes are connected to the sources of the variable (welding transformer) or DC (welding generator or rectifier) \u200b\u200bwith direct and reverse polarity.

With a constant current welding, the electrode connected to the positive pole is called an anode, and to the negative - cathode. The gap between the electrodes is called an arc area or arc gap (Figure 3.4). Arc gaps are usually divided into 3 characteristic areas:

1. anodic area adjacent to the anode;
2. cathode region;
3. still arc.

Any ignition of the arc begins with a short circuit, i.e. With the closure of the electrode with the product. In this case, U d \u003d 0, and the current I max \u003d I Cor.zamk. At the site of the closure, a cathode spot appears, which is an indispensable (necessary) condition for the existence of an arc discharge. The resulting liquid metal is stretched with a removal of the electrode, overheats and the temperature reaches, to the boiling point - is excited (ignites) arc.

The ignition of the arc can be performed without contacting the electrodes due to ionization, i.e. The breakdown of dielectric air (gas) gap by increasing the voltage by oscillators (argon-arc welding).

The arc gap is a dielectric medium that needs to be ionized.

For the existence of an arc discharge, u d \u003d 16 ÷ 60 V. The passage of the electric current through the air (arc) gap is possible only if there is electrons (elementary negative particles) and ions in it: positive (+) ions - all molecules and atoms of elements (easier Metals form metals); Negative (-) ions - easier form F, CR, N 2, O 2 and other elements with affinity for electrons e.

Figure 3.4 - Arc burning scheme

The cathode area of \u200b\u200bthe arc is the source of electrons, ionizing gases in the arc interval. Electrons distinguished from the cathode accelerated electric field and remove from the cathode. At the same time, under the influence of this field, + ions are sent to the cathode:

U d \u003d U K + U C + U A;

Anodic area has a significantly larger volume u a< U к.

The arc pillar is the main proportion of the arc gap represents a mixture of electrons, + and - ions and neutral atoms (molecules). Arc post neutral:

Σzar.otr. \u003d Σ pricks post.

Energy to maintain a stationary arc comes from the power source of the IP.

Different temperatures, the size of the anode and cathode zones and a different amount of heat released - causes the existence when welding on a direct current of direct and reverse polarity:

Q A\u003e Q to; U A.< U к.

• upon request large number heat for heating the edges of large metal thicknesses is used direct polarity (for example, when surfacing);
• with thin-walled and non-overwhelming metals overheating, reverse polarity (+ on the electrode).