U=C Iα ,

where U is the voltage across the resistance of the non-linear resistor of the valve arrester, I is the current passing through the non-linear resistor, C is a constant, numerically equal to the resistance at a current of 1 A, α is the valve coefficient.

The smaller the coefficient α, the less the voltage on the non-linear resistor changes when the current passing through it changes, and the smaller the remaining voltage on the valve arrester.

The values ​​of the remaining voltages, given in the passport of the valve arrester, are given for normalized impulse currents. The values ​​of these currents are in the range of 3000-10000 A.

Each current pulse leaves a trace of destruction in the series resistor - a breakdown of the barrier layer of individual carborundum grains occurs. Multiple passage of current pulses leads to a complete breakdown of the resistor and the destruction of the spark gap. The full breakdown of the resistor occurs the sooner, the greater the amplitude and length of the current pulse. Therefore, the capacity of the valve arrester is limited. When evaluating the throughput of valve arresters, the throughput of both series resistors and spark gaps is taken into account.

The resistors must withstand without damage 20 current pulses with a duration of 20/40 µs with an amplitude depending on the type of arrester. For example, for surge arresters of RVP and RVO types with a voltage of 3–35 kV, the current amplitude is 5000 A, for RVS types with a voltage of 16–220 kV–10,000 A, and for RVM and RVMG types with a voltage of 3–500 kV–10,000 A.

To improve the protective properties of the valve arrester, it is necessary to reduce the remaining voltage, which can be achieved by reducing the valve coefficient α of a series non-linear resistor while increasing the arcing properties of the spark gaps.

Increasing the arc-quenching properties of spark gaps makes it possible to increase the accompanying current cut off by them, and consequently, it makes it possible to reduce the resistance of the series resistor. The technical improvement of valve-type arresters is currently proceeding precisely in these ways.

It should be noted that in the valve arrester circuit, the grounding device is important. In the absence of grounding, the arrester cannot work.

The groundings of the valve arrester and the equipment protected by it are combined. In cases where the valve arrester for some reason has a separate from the protected equipment, its value is normalized depending on the level of insulation of the equipment.

Installation of arresters

After a thorough inspection, the arresters are installed on the supporting structures, leveled and plumb with a lining, if necessary, under the base of the sheet steel segments and fixed on the supports with a clamp with bolts.

Surge arresters are protective devices. They are designed to protect the insulation of electrical equipment from surges. Valve arresters are used in switchgears of electrical installations, and tubular arresters are used on power lines.
Valve arresters consist of spark gaps connected in series with a working resistor having a non-linear current-voltage characteristic. In some arresters, shunt resistors are connected in parallel with the spark gaps to evenly distribute the voltage between them.
In the symbols of arresters, the letters mean: R - arrester; V - valve, P - substation (polarized for DC arresters); C - station; M - with magnetic blowing; О - lightweight construction; U - unipolar; K - to limit switching surges. The numbers following the letters in the designation indicate the arrester voltages.
Arresters are characterized by a number of parameters.
The voltage class of the arrester is the nominal value of the mains voltage for which the arrester is intended to operate.
The highest allowable voltage of the arrester is the effective value of the highest voltage guaranteed by the manufacturer, at which the arrester reliably extinguishes the arc.
The breakdown voltage of the arrester is the largest value of the smoothly increasing voltage at the moment of breakdown of the arrester.
The impulse breakdown voltage of the arrester is the highest value of the impulse voltage at the moment of the breakdown of the arrester for a given value of the pre-discharge time. Pre-discharge time - the time from the beginning of the rise of the pulse voltage until the breakdown of the spark gap.
The rated discharge current of the arrester is the amplitude value of the pulsed current that passes through the arrester after its breakdown.
The conduction current of the arrester, the spark gaps of which are shunted by resistors, is the current passing through the arrester when a DC voltage of a given value is applied to it. For arresters that do not have shunt resistors, the current measured in this case is called leakage current.
AC surge arresters serve as the main means of limiting surges and protection against them.
The arrester RVP-6 is shown in fig. 1. It consists of multiple spark gaps 12 and series-connected non-linear vilite resistors b, placed in a porcelain case 7 and compressed by a spiral spring 3. The block of multiple spark gaps includes several single spark gaps connected in series, placed in a paper-baxlite cylinder 4. A single spark the gap consists of two shaped brass electrodes glued to an insulating mecanite or electrocardboard spacer. A non-linear series resistor is made of vilite (vilite-baked mixture of carborundum with liquid glass), which have valve properties, that is, the resistance of carborundum changes depending on the voltage applied to it: the higher the applied voltage, the lower its resistance, and vice versa. The number of spark gaps in the block and vilite disks in the column depends on the value of the rated voltage of the arrester. The planes with which the disks come into contact are metallized with aluminum for better contact, and the side surfaces of the vilite disks are covered with an insulating coating to block the path of leakage currents. Felt or felt pads are placed to prevent displacement of the vilite discs 5. Vilite is not moisture resistant and when damp, its valve properties deteriorate. Therefore, the arrester is sealed with a seal 2 made of ozone-resistant rubber and is closed from above with a metal cap 13. The arrester is connected to the supporting structure with a clamp 11, to the current-carrying wires - with a bolt 1, and to the ground - with a pin 9. Thus, the arrester is switched on between the phase of the electrical installation and the ground loop parallel to the protected isolation.

Rice. 1. Arrester type RVP-6
In normal operation, spark gaps provide isolation between phase and ground. As soon as an overvoltage occurs that is dangerous for the insulation of the electrical installation, a breakdown of the spark gaps occurs, as a result of which the network is connected to the ground through vilite disks. At this moment, the maximum voltage is applied to the vilite disks, so their resistance will be the smallest, and the earth fault current will be the largest. As a result of the discharge to the ground, the voltage in the network decreases, and the resistance of the vilite disks increases. The arc of alternating current goes out when passing through zero, and then is restored again. When the voltage applied to the spark gap is insufficient to maintain the arc at the spark gaps, the first time the current passes through zero, its flow through the spark gap stops.
The modernized arrester RVP with a reduced diameter of spark gaps and vilite disks with reduced dimensions and weight is produced under the name RVO (lightweight valve arrester).

2. Arrester type RVS
Valve arrester RVS (valve station arrester) is produced in the form of five standard elements: RVS-15, RVS-20, RVS-30, RVS-33 and RVS-35. Of these elements, arresters for voltages up to 220 kV are completed. They are installed one on top of the other and connected in series. On fig. 2 shows an RVS element consisting of a porcelain casing 1, inside which there are 2 wilite discs and sets of spark gaps 4, consisting of several single spark gaps 3. Each set is enclosed in a porcelain cylinder 5. All spark gaps and wilite discs are compressed by helical springs 6. The porcelain casing is closed from the end sides with covers, under which sealing rubber 7 is laid. The porcelain casing is reinforced with flanges 8, which serve to fasten the spark gap to the supporting structure, as well as to connect it to buses or wires. Sets of spark gaps are shunted with horseshoe-shaped resistors 9, designed to evenly distribute the voltage between them.
On fig. 3 shows a set of spark gaps consisting of four single spark gaps. Each single spark gap includes two figured brass electrodes 4 separated by a micanite spacer. The spark gaps are placed in a porcelain cylinder 3, closed at the top and bottom with brass covers 1. Horseshoe-shaped shunt resistors 2, made on the basis of carbicide, are attached to the latter.

Rice. 3. Set of spark gaps of the arrester

Rice. 4. Block of spark gaps of the RVM type arrester
For a voltage of 35-500 kV, magnetic valve arresters of the RVM type have been used. They differ from other types of spark gaps by the presence of blocks of magnetic spark gaps (Fig. 4). Such standard blocks of spark gaps, supplemented with disk vilitic resistors, are manufactured for a voltage of 35 kV. The block of magnetic spark gaps consists of a set of single spark gaps 2 separated from each other by ring magnets 3. A single spark gap is made up of two concentrically located copper electrodes 6 and 8, between which an annular gap 7 is formed. The arc arising in the gap rotates under the action of permanent magnets with high speed, which contributes to its rapid quenching A set of permanent magnets and single spark gaps is placed inside a porcelain tire 1, closed with steel covers 5. The magnets and copper electrodes are tightly compressed by a steel spring 4.

Dischargers- are used to limit the resulting overvoltages in order to facilitate the isolation of equipment. The resulting overvoltages are divided into two groups: internal (switching) and atmospheric.

The first ones occur when switching electrical circuits (inductors, capacitors, long lines), arc faults to the ground and other processes. The latter arise when exposed to atmospheric electricity. The dependence of the maximum pulse voltage on the discharge time is called the volt-second characteristic. The main element of the arrester is the spark gap. Volt-second ha-

The characteristic of this gap (curve 1 in Fig.) must lie below the volt-second characteristic of the protected equipment (curve 2). When an overvoltage occurs, the gap must break through before the insulation of the protected equipment. After the breakdown, the line is grounded through the resistance of the arrester. In this case, the voltage on the line is determined by the current I passing through the arrester, the resistance of the arrester and grounding. The lower these resistances, the more effectively the overvoltages are limited, i.e., the greater the difference between the possible (curve 4) and a limited surge arrester (curve 3). The voltage across the arrester during the flow of a current pulse of a given value and shape is called the remaining voltage. The lower this voltage, the better the quality of the arrester.

A tubular spark gap is a spark gap supplemented with a forced arc extinguishing device, in the form of a tube made of a gas-generating material (fiber, vinyl plastic), i.e. the arc of the accompanying short-circuit current is turned off due to the intensive gas evolution by the tube at an increased burning t.

1-tube, 2-rod electrode, 3-ring-shaped electrode, 4-grounded electrode, where there is a buffer volume5, where the potential energy of the compressed gas accumulates. When the current passes through zero, a gas blast is created from the buffer volume, which contributes to the effective extinguishing of the arc. S 1 , S 2 - spark gaps. A specific disadvantage of TR is the presence of an exhaust zone that is dangerous for equipment and maintenance personnel. In TS, the gap is formed by rod electrodes with a steep volt-second characteristic due to the large inhomogeneity of the electric field. In this regard, TR is applicable: for the protection of approaches to the p / st; low-power protection equipment p / st 3-10 kV; protection contact of the alternating current network.

Valve arresters. The main elements are vilite rings, spark gaps and operating resistors. These elements are located inside the porcelain casing , which from the ends has special flanges for fastening and connecting the arrester. The casing of the arrester is sealed at the ends with the help of plates and sealing rubber gaskets. When a flash occurs, successively connected blocks of spark gaps break through. In this case, the current pulse through the working resistors closes to the ground. The resulting follow current is limited by the operating resistors, which create the conditions for extinguishing the follow current arc. R of these resistors is large at Uwork and drops sharply at U. Vilite is used as a material for non-linear resistors with a non-linearity coefficient of 0.1-0.2. Working resistors are made in the form of disks. Single spark gap connections will follow for improved arc quenching conditions. The shape of the electrodes provides a uniform electric field, which makes it possible to obtain a flat volt-second characteristic. The appearance of a charge in the closed volume of the spark gap at a short duration of the current pulse is difficult. To facilitate the ionization of the spark gap, a micanite spacer is placed between the electrodes.

OPN - they use resistors with a large non-linearity (0.04) based on zinc oxide (for 110-500 kV). These resistors allow you to limit the switching of the U at the level of (1.65-1.8) Uf, and the lightning resistors at the level of (2.2-2.4) Uf. The design of the surge arrester is carried out by a series or parallel set of resistance disks, and with Uworking h / z one parallel column of resistors has a current of n * 0.01 mA, i.e. no need for a spark gap. The accompanying current flowing after the device has tripped is small (milliamps), as is the power dissipated in the resistors. This makes it possible to refuse the sequential connection of several spark gaps and makes it possible to connect the surge arrester directly to the protected equipment, which significantly increases the reliability of operation.

28.09.2015

Device, appearance

Regardless of the type, arresters necessarily have spark gaps, as well as resistors: working and shunting. Further, the structure is placed in a porcelain case and closed in all flanges using reinforcing mortars. This is how we see them in substations and switchgears.

Moisture-resistant paint and enamel are used, which are placed on top of the reinforcement. Arresters differ in class voltage, which determines the number of micanite washers (spark gaps are made from them), as well as their ratio with the resistance of the working resistor.

During the operation of the switchgear, when the voltage increases to breakdown, the resistance of the working resistor, on the contrary, drops, which indicates its non-linearity.

Vilite (rarely - tervit) disks are used as the basis for a working resistor. They are distinguished by such a property as hygroscopicity, which explains the need for tightness of the arrester housing and connecting joints.

Main types of arresters

• Arresters RVN, RVO, RVE, RVP and RVS are used exclusively to protect switchgears and other high-voltage equipment from failures during a thunderstorm. The latter have a shorter pulse duration compared to switching ones, which is important for these types of devices, because their capabilities are limited by the ability to extinguish the arc with spark gaps. All conclusions come from the composition of such arresters: the design consists of spark gaps connected one after another and a working resistance.
• RVRD, RVMG and RVM: these arresters on any switchgear are capable of extinguishing the arc. The possibility is achieved due to the magnetic field, which acts from permanent magnets: in the spark gap, the arc stretches and disappears. Devices of these types are able not only to protect switchgear or other high-voltage equipment from the destructive effects of lightning discharges, but also to protect against switching overvoltages of short duration.
• RVMK arresters will be the best protection against switching surges, they have the following modules in their design:
• spark, consisting exclusively of spark gaps,
• valve, which is represented only by resistors,
• the main one, where both working resistors and spark gaps are located.

Arrangement and principle of operation of arresters

1.General information

Tubular arresters

Valve arresters

DC arresters

Surge arresters

Long spark gaps

1.General information

During the operation of electrical installations, voltages arise that can significantly exceed the nominal values ​​\u200b\u200b(overvoltage). These overvoltages can break through the electrical insulation of equipment components and disable the installation. To avoid breakdown of electrical insulation, it must withstand these overvoltages, however, the overall dimensions of the equipment are excessively large, since overvoltages can be 6-8 times higher than the rated voltage. In order to facilitate isolation, the resulting overvoltages are limited by means of arresters and the insulation of the equipment is selected according to this limited overvoltage value. The resulting overvoltages are divided into two groups: internal (switching) and atmospheric. The first ones occur when switching electrical circuits (inductors, capacitors, long lines), arc faults to the ground and other processes. They are characterized by a relatively low frequency of the applied voltage (up to 1000 Hz) and an exposure duration of up to 1 s. The latter arise under the influence of atmospheric electricity, have a pulsed nature of the acting voltages and a short duration (tens of microseconds). The electrical strength of the insulation during pulses depends on the shape of the pulse, its amplitude. The dependence of the maximum pulse voltage on the discharge time is called the volt-second characteristic. For insulation with a non-uniform electric field, a sharply falling volt-second characteristic is characteristic. With a uniform field, the volt-second characteristic is flat and runs almost parallel to the time axis.

Fig.1. Matching the characteristics of the arrester and protected equipment

surge arrester electrical installation

The main element of the arrester is the spark gap. The volt-second characteristic of this gap (curve 1 in Fig. 1) must lie below the volt-second characteristic of the protected equipment (curve 2). When an overvoltage occurs, the gap must break through before the insulation of the protected equipment. After the breakdown, the line is grounded through the resistance of the arrester. In this case, the voltage on the line is determined by the current I passing through the arrester, the resistance of the arrester and grounding Rg. The lower these resistances, the more effectively overvoltages are limited, i.e. the difference between the possible overvoltage (curve 4) and the overvoltage limited by the arrester (curve 3) is greater. During the breakdown, a current pulse flows through the spark gap.

The voltage across the arrester during the flow of a current pulse of a given value and shape is called the remaining voltage. The lower this voltage, the better the quality of the arrester. After the passage of the current pulse, the spark gap turns out to be ionized and is easily broken through by the rated phase voltage. A short circuit occurs to the ground, in which a current of industrial frequency flows through the arrester, which is called accompanying. The follow current can vary over a wide range. In order to avoid turning off the equipment from the relay protection, this current must be switched off by the arrester in the shortest possible time (about a half-cycle of the industrial frequency).

The following requirements are imposed on the arresters.

The volt-second characteristic of the arrester must go below the characteristic of the protected object and must be flat.

The spark gap of the arrester must have a certain guaranteed electrical strength at industrial frequency (50 Hz) and with pulses.

The remaining voltage on the arrester, which characterizes its limiting capacity, should not reach dangerous values ​​for the insulation of the equipment.

The follow current with a frequency of 50 Hz must be switched off within a minimum time.

The arrester must allow a large number of operations without inspection and repair.

Fig.2. Designation of arresters

On electrical circuit diagrams in Russia, arresters are designated according to GOST 2.727-68.

General designation of the arrester

Tubular arrester

Surge arrester valve and magnetic valve

The industry produces valve arresters of the RN, RVN, RNA, RVO, RVS, RVT, RVMG, RVRD, RVM, RVMA, RMVU and tubular series.

The arrester RN - low voltage, is designed to protect against atmospheric overvoltage insulation of electrical equipment with a voltage of 0.5 kV.

The arrester RVN - valve, for protection against atmospheric overvoltage of insulation of electrical equipment.

The RC arrester is designed to protect the devices for monitoring the insulation of high voltage bushings of transformers.

The arrester RVRD is a valve-operated arrester with a stretching arc designed to protect the insulation of electrical machines from atmospheric and short-term internal overvoltages.

The arrester RMVU is valve, magnetic, unipolar, designed to protect against overvoltage insulation of traction electrical equipment in DC installations.

The arrester RA - series A, is designed to protect against overvoltage of the excitation windings of large synchronous machines (turbine generators, hydro generators and compensators) with a rated excitation current of up to 3000 A.

Arrester RVO - valve lightweight design; arrester RVS - valve station; arrester RVT - valve, current-limiting; arrester PC - valve for the protection of electrical installations for agricultural purposes; arresters of the RVM, RVMG, RVMA, RVMK series - valve-type arresters with magnetic arc extinguishing, modifications G and A, combined, designed to protect against atmospheric and short-term internal overvoltages (within the capacity of the arresters) of insulation of equipment of electric power plants and substations of alternating current with a rated voltage of 15 -500 kV.

Tubular arresters RTV and RTF - vinyl plastic or fiber-bakelite, designed to protect against atmospheric overvoltage insulation of power lines and with other means of protection to protect the insulation of electrical equipment of stations and substations with a voltage of 3, 6, 10, 35, 110 kV.

Tubular arresters

Fig.3. Tubular arrester

The tubular spark gap (Fig. 3) during normal operation of the installation is separated from the line by an air gap S2. When an overvoltage occurs, the gaps S1 and S2 break through and the pulsed current is discharged to the ground. After the passage of the pulsed current through the arrester, an accompanying current of industrial frequency flows. In the narrow channel of the holder (tube) 1 of the gas-generating material (vinyl plastic or fiber) in the gap S1 between the electrodes 2 and 3, an arc ignites. Pressure builds up inside the cage. The resulting gases can exit through the hole in the ring electrode 3. When the current passes through zero, the arc is extinguished due to the cooling of the gap S1 by the gases leaving the spark gap. In the grounded electrode 4 there is a buffer volume 5 where the potential energy of the compressed gas is accumulated. When the current passes through zero, a gas blast is created from the buffer volume, which contributes to the effective extinguishing of the arc.

The limiting interrupted current of industrial frequency is determined by the mechanical strength of the holder and is 10 kA for a fiber-bakelite holder and 20 kA for a vinyl plastic, reinforced with fiberglass on epoxy resin. The accompanying current with a frequency of 50 Hz is determined by the location of the arrester and varies in a fairly wide range depending on the operating mode of the power system. Therefore, the minimum and maximum values ​​of the short-circuit current at the place where the arrester is installed must be known.

The minimum current of the arrester is determined by the quenching capacity of the tube. The smaller the diameter of the exhaust channel, the greater its length, the lower the lower limit of the current to be switched off. However, at high currents, high pressure develops in the tube. If the mechanical strength of the tube is insufficient, the arrester may be destroyed. At present, high-strength vinyl plastic arresters are produced with the highest breaking current up to 20 kA.

The operation of the tubular spark gap is accompanied by a strong sound effect and the emission of gases. Thus, the gas emission zone of the PTB-I10 arrester has the form of a cone with a diameter of 3.5 and a height of 2.2 m. When placing arresters, it is necessary that elements under high potential do not fall into this zone.

The protective characteristic of the arrester largely depends on the volt-second characteristic of the spark gap. In a tubular spark gap, the gap is formed by rod electrodes having a steep volt-second characteristic due to the large inhomogeneity of the electric field. At the same time, the electric field in the protected devices and equipment is sought to be made uniform in order to make better use of insulating materials and reduce dimensions and weight. With a uniform field, the volt-second characteristic turns out to be flat, practically little dependent on time. In this regard, tubular arresters with a steep volt-second characteristic are unsuitable for protecting substation equipment. Usually only line insulation (insulation provided by suspension insulators) is protected with them. When choosing a tubular arrester, it is necessary to calculate the possible minimum and maximum short-circuit current at the installation site and, based on these currents, select the appropriate arrester. The rated voltage of the arrester must correspond to the rated voltage of the network. The dimensions of the internal S1 and external S2 gaps are selected according to special tables.

Valve arresters

Rice. Fig. 4. Valve arrester (a) and its spark gaps on an enlarged scale (b)

A surge arrester of type PBC-1O (stationary surge arrester for 10 kV) is shown in Fig. 4a. The main elements are vilite rings 1, spark gaps 2 and operating resistors 3. These elements are located inside the porcelain casing 4, which has special flanges 5 at the ends for mounting and connecting the spark gap. Operating resistors 3 change their characteristics in the presence of moisture. In addition, moisture, settling on the walls and parts inside the arrester, worsens its insulation and creates the possibility of overlapping. To exclude the penetration of moisture, the casing of the arrester is sealed at the ends with the help of plates 6 and sealing rubber gaskets 7.

The work of the arrester occurs in the following order. When an overvoltage occurs, three series-connected blocks of spark gaps 2 break through (Fig. 4b). In this case, the current pulse through the working resistors closes to the ground. The resulting follow current is limited by the operating resistors, which create the conditions for extinguishing the follow current arc.

After the breakdown of the spark gaps, the voltage across the arrester

If the resistance of the arrester Rr determined by the operating resistors is linear, then the voltage across the arrester increases in proportion to the current and may become higher than the permissible value for the protected equipment. To limit the voltage Ur, the resistance Rr is non-linear and decreases with increasing current. The relationship between voltage and current in this case is expressed as

where A is a constant characterizing the voltage across the resistance Rp at a current of 1 A; α - non-linearity index. The case when α=0 is ideal, since the voltage Up does not depend on the current.

The described arresters are called valve arresters, because with pulsed currents their resistance drops sharply, which makes it possible to pass a large current with a relatively small voltage drop.

Fig.5. Volt-ampere characteristic of a vilitic resistor

Vilite is widely used as a material for non-linear resistors. In the region of high currents, its non-linearity index α=0.13-0.2. A typical current-voltage characteristic of a vilite resistor is shown in Fig. 5, a. At low currents, the resistance Rp is large and the voltage increases linearly with increasing current (region A). At high currents, the resistance decreases sharply and the voltage Ur almost does not increase (region B).

Vilite is based on SiC carborundum grains with a resistivity of about 10-2 Ohm m. A film of silicon oxide SiO2 10-7 m thick is formed on the surface of carborundum grains, the resistance of which depends on the voltage applied to it. At low voltages, the resistivity of the film is 104–106 Ohm m. With an increase in the applied voltage, the resistance of the film decreases sharply, the resistance is determined mainly by carborundum grains, and the voltage drop is limited.

Working resistors are made in the form of disks with a diameter of 0.1-0.15 m and a height of (20-60) 10-3 m. With the help of liquid glass, carborundum grains are firmly bonded to each other.

Vilite is very hygroscopic. To protect against moisture, the cylindrical surface of the discs is covered with an insulating coating. End surfaces are contact and metallized.

Usually, several working resistors in the form of disks are connected in series (Fig. 3a shows 10 disks). With n disks, the remaining voltage

To reduce the remaining stress, the number of disks n should be as small as possible.

When current is passed, the temperature of the discs rises. When a current pulse of large amplitude flows, but of short duration (tens of microseconds), the resistors do not have time to heat up to a high temperature. With a long flow of even small currents of industrial frequency (one half-cycle is equal to 10 ms), the temperature may exceed the permissible value, the disks lose their valve properties, and the arrester fails.

The maximum allowable amplitude of the current pulse for a disk with a diameter of 100 mm is 10 kA with a pulse duration of 40 μs. The permissible amplitude of a rectangular pulse with a duration of 2000 μs does not exceed 150 A. The disk passes such currents 20-30 times without damage.

After the passage of the pulsed current through the arrester, an accompanying current begins to flow, which is a power frequency current. As the current approaches zero, the resistance of the wilt increases sharply, which leads to distortion of the sinusoidal form of the current. An increase in the resistance of the circuit leads to a decrease in the current and the phase angle φ between current and voltage (φ-> 0). Figure 5b shows the current curves in the working resistor. Here 1 is the source voltage of 50 Hz; 2 - curve of the circuit current, determined by the inductive resistance X; 3 -curve of the current determined by the working resistor (Rр>>X). Due to the non-linearity of the resistor Rp, the return voltage (power frequency voltage) is reduced. Reducing the rate of current approach to zero reduces the arc power in the area of ​​zero current. All this facilitates the process of extinguishing the arc burning between the electrodes of the discharge gap. Due to the use of brass electrodes in the spark gaps, after the current passes through zero, a gap is formed near each cathode, the electrical strength of which is 1.5 kV. This ensures that the follow current is extinguished when the current passes through zero for the first time and makes it possible to extinguish the arc in spark gaps without the use of special arc extinguishing devices.

The device of the spark gap of the valve arrester is clear from Fig. 4, b. The shape of the electrodes provides a uniform electric field, which makes it possible to obtain a flat volt-second characteristic. The distance between the electrodes is assumed to be (0.5-1) 10-3 m.

The appearance of a charge in the closed volume of the spark gap at a short duration of the current pulse is difficult. To facilitate the ionization of the spark gap, a micanite spacer is placed between the electrodes. Since the dielectric constant of air is much less than that of the mica included in micanite, high electric field gradients arise in the near-electrode volume of air, causing its initial ionization. The resulting electrons lead to the rapid formation of a discharge in the center of the spark gap.

It has been experimentally established that a single spark gap is able to turn off the accompanying current with an amplitude of 80-100 A at an effective voltage value of 1-1.5 kV. The number of single gaps is selected based on this voltage. The number of disks of the working resistor should be such that the maximum current value does not exceed 80-100 A. In this case, the arc is extinguished in one half period.

To ensure a uniform load at an industrial frequency, the gaps are shunted with non-linear resistors 1 (Fig. 4). The thermal stability of the disks is designed for the passage of a follow current during one or two half-cycles.

Internal surges are low-frequency in nature and can last up to 1 s. Due to the low thermal stability, vilite cannot be used to limit internal overvoltages. To limit internal overvoltages, the material tervit, similar to vilit, is used, which has high thermal resistance and an increased non-linearity index α=0.15-0.29.

Fig.6. Combined arrester with tervite resistors

Tervit disks are used in combined arresters (Fig. 6, a), designed to protect against both internal (switching) and external (atmospheric) overvoltages. Both non-linear resistors NR1 and NR2 work with internal overvoltages (curve 1 and a Fig. 6b). During atmospheric overvoltages, due to the high current, the voltage on HP2 breaks through the gap IP2 and the voltage on the protected line decreases (curve 2).

Valve arresters operate silently. The number of operations is recorded by a special recorder, which is connected between the lower output of the arrester and ground. The most reliable are electromagnetic recorders, the armature of which, during the passage of a pulsed current, acts on the ratchet mechanism of the counting device.

With the help of the spark gaps shown in Fig. 4, b, it is impossible to turn off currents of 200-250 A. In this case, magnetic blast chambers with a permanent magnet are used to extinguish the arc. The arc that occurs in the spark gap is driven into a narrow slot with ceramic machines under the influence of a magnetic field. On this principle, spark gaps for voltages up to 500 kV have been created. Increasing the diameter of the discs up to 150 mm allows you to increase their thermal stability. As a result, combined magnetic-valve arresters make it possible to limit both internal and atmospheric overvoltages.

The main characteristics of the valve arrester:

The quenching voltage Ugash is the highest power frequency voltage applied to the arrester, at which the accompanying current is reliably interrupted. This voltage is determined by the properties of the arrester. The power frequency voltage applied to the arrester depends on the parameters of the circuit. If, during a short circuit to the ground of one phase, an overvoltage appears on the free phases, then the quenching voltage applied to the arrester is determined by the equation

where Kz is a coefficient depending on the method of grounding the neutral; Unom - rated line voltage of the network. For installations with a grounded neutral, Kz = 0.8, for an isolated neutral, Kz \u003d l,l.

The quenching current Igash, which refers to the accompanying current corresponding to the quenching voltage Ugash.

The arcing effect of the spark gap is characterized by the coefficient

where Upr is the breakdown voltage with a frequency of 50 Hz of the spark gap.

The protective effect of a non-linear resistor is characterized by a protection factor

where Ures is the voltage on the arrester at a pulsed current of 5-14 kA. This voltage should be 20-25% lower than the discharge voltage of the protected insulation.

4.DC arresters

Fig.7. DC arrester

Valve arresters can be used to protect installations from DC surges. However, extinguishing a direct current arc is much more difficult than alternating current. To use the near-electrode voltage drop, a very large number of spark gaps is required, since the voltage on each pair of electrodes should not exceed 20–30 V.

To extinguish the arc, it is advisable to use magnetic blowing with the help of permanent magnets. The resulting electrodynamic force moves the arc at high speed in a narrow slot of arc-resistant insulating material. As a result of intensive cooling of the arc, its resistance increases and the current stops.

The valve arrester for a network with a voltage of 3 kV DC is shown in Fig. 7. The working resistor 1 consists of two wilite discs connected to two spark gaps 2 with magnetic arc suppression. Reliable contact between gaps and disks is achieved with the help of spring 3, which is also a current-carrying element. The main elements of the arrester are located in a porcelain casing 6, which is closed from below by a cover 7. The sealing of the arrester is carried out by a cover 4 with a rubber seal 5.

Surge arresters

On the basis of zinc oxide, which has a pronounced non-linearity of the current-voltage characteristic, a series of non-linear overvoltage suppressors (SPD) for a rated voltage of 110-500 kV has been developed.

The surge arrester is a non-linear resistor with a high coefficient of non-linearity α=0.04 (against 0.1 -0.2 for vilit). It is connected in parallel to the protected object (between the potential output and ground) without discharge gaps. Due to the high non-linearity at the nominal phase voltage, a negligible current of 1 mA flows through the surge arrester. With increasing voltage, the resistance of the surge arrester decreases sharply, the current flowing through it increases. At a voltage of 2.2Uf, a current of 10 4A. After the passage of the voltage pulse, the current in the arrester circuit is determined by the phase voltage of the network.

Fig.8. Volt-ampere characteristic of OPN-500 limiter

SPDs limit switching overvoltages to the level of 1.8Uf and atmospheric overvoltages to (2-2.4)Uf. From the current-voltage characteristic of the OPN-500 (Fig. 8) it can be seen that when overvoltages decrease from 2Uf to Uf, the current flowing through the resistors decreases by 10 6once. The accompanying current flowing after the device has tripped is small (milliamps), as is the power dissipated in the resistors. This makes it possible to refuse the sequential connection of several spark gaps and makes it possible to connect the surge arrester directly to the protected equipment, which significantly increases the reliability of operation.

High non-linearity of arrester resistors (for high currents α ≈0.04) can significantly reduce overvoltages and reduce the dimensions of equipment, especially at voltages of 750 and 1150 kV.

Long spark gaps

The authors of the RDI idea Georgy Viktorovich Podporkin, Doctor of Technical Sciences, Professor of the Polytechnic University of St. Petersburg, Senior Member of IEEE, and Alexander Dmitrievich Sivaev, Candidate of Technical Sciences, began the first experiments to develop long-spark gaps back in 1989, and in 1992 it was obtained certificate of authorship.

Fig.9. Scheme of a long-spark gap

The principle of operation of the arrester is based on the use of the effect of a sliding discharge, which provides a large length of pulsed overlap over the surface of the arrester, and the prevention, due to this, of the transition of the pulsed overlap into a power arc of industrial frequency current. The discharge element of the RDI, along which the sliding discharge develops, has a length several times greater than the length of the protected line insulator. The design of the arrester ensures its lower impulse electrical strength compared to the protected insulation. The main feature of the long-spark gap is that due to the large length of the pulsed lightning coverage, the probability of establishing a short circuit arc is reduced to zero.

There are various modifications of the RDI, which differ in the purpose and features of the overhead line on which they are used.

The main advantage of RDI is that the discharge develops along the apparatus through the air, and not inside it. This allows you to significantly increase the service life of products and increases their reliability.

Long-spark loop-type arrester (RDIP)

RDIP-10 is designed to protect overhead power lines with a voltage of 6-10 kV three-phase alternating current with protected and bare wires from induced lightning surges and their consequences and is designed for outdoor operation at an ambient temperature of minus 60 °C to plus 50 °C for 30 years.

Modular long-spark arrester (RDIM)

RDIM is designed to protect against direct lightning strikes and induced lightning surges of overhead power lines (VL) and approaches to substations with a voltage of 6, 10 kV three-phase alternating current with bare and protected wires.

RDIM has the best volt-second characteristics, which is why it is advisable to use it to protect line sections subject to direct lightning strikes, as well as to protect approaches to overhead line substations.

RDIM consists of two pieces of cable with a cord made of resistive material. The cable segments are stacked together so that three bit modules 1, 2, 3 are formed.