Drying of power cables. Drying the insulation of the windings of electrical machines. Drying paper insulation. types of moisture. Kinetics of the drying process

* SZ twist: from 2-100 strands, blanks of small section

Types: 1- rotation of the attachment is always in one direction - a periodic change in the direction of twisting. 2- alternating rotation change

Minus: change of direction of twisting - it is necessary to brake and accelerate - the pitch of twisting changes at the moment of stop and stop, it is impossible to create the required degree of compression during twisting.

Plus: very high performance.

Used as a prefix.

* Single strand frame machines

* Combined machines twisting pairs into bundles. A small number of payoff drums, the practical part is installed as a prefix on the torsional part (up to 10 pairs)

1
- platter

2- paratorsion attachment

3- distribution socket

5- traction device (caterpillar)

6-receiver

*
With a large number of pairs, units with a rotating receiving device are used (twisting 30-50 pairs)

1- paratorsion prefix

2- distribution socket

4- rotating receiving device

5.1 Drying paper insulation. types of moisture. Kinetics of the drying process.

Purpose of the operation: To remove moisture from the paper in order to increase the durability of the cable and the initial parameters.

Humidity to which it is necessary to dry: 0.5 - 0.2%, up to 35 kV inclusive.

Less than 0.1%, 110 kV and above.

Paper is a colloidal, fibrous material (95% cellulose)

Moisture reduces the electrical characteristics of the paper,  V decreases, tg increases, practically does not change. Moisture causes crystallization of rosin in the impregnating composition (the volume changes and voids appear in which polarization and aging of the insulation can occur)

Requirements:

Remove moisture to the extent necessary.

Drying should be carried out in such a way that there is no thermal destruction.

Drying time

Types of moisture:

chemically bound moisture - the OH group that is part of the cellulose, this moisture cannot be removed.

absorption - a monomolecular layer of water accumulating on the surface of paper and capillaries. Removed by drying, but requires a lot of energy.

capillary moisture - directly located in the capillaries. Remove dry. The easiest.

Kinetics of the drying process : drying - evaporation of moisture from the surface of the paper into the environment.

It is necessary to provide:

pr. moisture transfer (from thickness to surface)

evaporation from the surface

Evaporation from the surface is determined i = B (PS- Po)* S, i is the amount of evaporation, V is the evaporation coefficient, Ps is the vapor pressure at the insulation surface, Po is the ambient pressure.

Moisture transfer can be carried out:

Moisture conductivity.

AT
all types of transfer in the direction of decreasing humidity under the influence of external factors. K - coefficient of moisture conductivity,  0 - specific gravity of water. U is the humidity gradient.

The best option when they matchi And andi T and directed towards the surfacei = i And + i T

Reliable and uninterrupted operation of the cable largely depends on the quality of the insulation. It must have such dielectric strength that the possibility of electrical breakdown at the voltage for which this cable is designed is excluded.
Impregnated paper insulation of cable cores has high electrical characteristics, long service life, relatively high allowable temperature. All this and low cost have provided impregnated cable paper with a leading position in cable insulation.
Paper for insulation of cable cores for voltages up to 35 kV inclusive is produced with a thickness of 0.125 mm grade K-12 and 0.175 mm grade K-17 from unbleached, sulphate pulp, predominantly natural color (GOST 645-59). To color the phases in multi-core cables, the upper tape is used from colored paper.
Cable paper is applied by winding the core with unimpregnated paper tapes. There are the following ways of winding multilayer paper insulation: end-to-end, with positive overlap and with negative overlap.
End-to-end winding is characterized by the fact that when the tape is applied, the edge of one turn comes into contact with the edge of the adjacent one. This winding method is rarely used, since it has a serious drawback: when the insulated core is bent, the inner part of the tapes bulges in the compression zone, and the outer part diverges in the tension zone.
In positive overlap winding, one edge of the tape overlaps the edge of the tape of the previous turn. This winding method reduces the flexibility of the strand and often causes wrinkles and even cracks in the paper at the overlap when the strand is bent. This method is used in cables only for winding the lowest layers of insulation located directly at the core, since this excludes the possibility of coincidence in the first layers of paper tapes, which is very important for ensuring the dielectric strength of the insulation. The use of a positive overlap for the outer mites gives greater smoothness to the outer layer of insulation.
The most common way is the negative overlap winding, i.e. with a gap. The presence of a gap between the tapes allows the cable to be bent within certain limits without the danger of damaging the paper insulation. The gap between two adjacent turns in this case is in the range of 0.5-2 mm. The gaps between the turns of adjacent tapes located on top (vertically) should not coincide in order to avoid deterioration of the electrical characteristics of the insulation. However, when applying a large number of tapes, it is not possible to avoid gap coincidences, therefore, the number of coincidences of insulation tapes according to GOST 340-59 on power cables with impregnated paper insulation should not exceed that specified in the standards.
According to the requirements of GOST 340-59, in the insulation of cables of 6 kV and above, more than three tapes located one above the other and two tapes directly adjacent to the core are not allowed to coincide.
In the process of insulating the cores, in addition to the coincidence of the gaps between the tapes, tears of the tapes may appear.
The coincidence of longitudinal cracks or cuts over a length of more than 50 mm in two tapes located one above the other is considered as one.

It should be noted that the development of sliding discharges will be most difficult in cases where the gaps are located under the middle of the tape of the next layer, while the gaps adjacent vertically will be covered with only one layer of paper, and this place will naturally be electrically weakened. For this reason, the isolation technology provides for gap bridging by the next layer by about one third of the width of the tapes.
Of great importance is the width of the paper tapes used for winding. A wide tape hinders the development of sliding discharges between the tapes, allows you to increase the winding step, and hence productivity. However, an excessive increase in the width of the tapes does not ensure a tight winding of the cores, leads to the appearance of wrinkles, cracks and breaks in the paper tapes when the cable is bent. The width of the tapes is usually set depending on the diameter of the wrapped core, while the larger the diameter of the core or cable, the greater the allowable width of the paper tapes.
The width limits of paper tapes for conductors, depending on their diameter, established by the factories of the domestic cable industry, are given in Table. 2-4.
In the case of a sector core, the width of paper tapes is selected according to the equivalent diameter, which is equal to the perimeter of the core divided by π.
The application of paper insulation should be tight, without folds or wrinkles. The presence of folds, wrinkles, leaks in the insulation leads to the formation of voids, air inclusions, which reduce the reliability of the insulation under operating conditions.
The sharp edges of the sectors of the cores cause uneven winding density of the paper insulation, as well as an increase in the electric field strength. An increase in the radius of curvature of the edges of the sector conductors leads to a more uniform distribution of the electric field and an increase in the electrical strength of the insulation.
The thickness of the insulating layer, normalized by GOST 340-59, is given in Table. 2-5 and 2-6.
The deviation of the insulation thickness between the conductors or | M1chzhdu the residential and shell is allowed no more than: for cables 1 kV - minus 0.18 mm, for cables above 1 kV minus 0.24 mm.
Paper tape insulating layer is usually | are superimposed in different directions, and the layer of insulation adjacent to the core is superimposed in the direction of the twisting of the wires of the upper layer of the core. Reversing the direction of the applied tapes of the insulating layer makes it possible to obtain cables without excessive rigidity and a tendency to twist. Paper insulation is applied on a twisting and insulating machine, which simultaneously twists the stranded core and seals it.
The insulated cores of cables, in which each core is leaded separately, come from the twisting and insulating machines directly to the dryer. Insulated cores for multi-core cables from twisting and insulating machines are wound onto drums and sent to machines for general twisting of cores into a cable. The twisting of insulated cores into a cable differs from the twisting of uninsulated cores only in a smaller number of twisted cores and a large twisting pitch. With a general twisting of insulated conductors into a cable, they are given two movements - one rotational around the cable axis and the other rectilinear
The general twist is characterized by two main parameters: the pitch and the direction of the twist, which are of great importance, as will be seen later, when connecting cables to each other.

The pitch of the total twisting of the cores is the length of the manufactured cable per revolution of the twisting device. The step length is determined by the factory standard depending on the diameter of the cable under the sheath.

Each core of its own color during one step makes a complete revolution around the cable axis, successively occupying any position in the cross-sectional area of ​​the circle from 0 to 360 ° (like a clock hand). Each next step of the twisting device is a repetition of the previous one both in the length of the step and in the sequence of placement of the cores in the cross-sectional area of ​​the circle.
Thus, the construction length of the cable manufactured by the factory:
where ι is the pitch length of the total twist; n is the number of steps. When twisting insulated cores, the gaps between the cores are simultaneously filled with paper bundles or sulphate paper with a thickness of not more than 0.08 mm and belt insulation is applied over the twisted cores. The paper bundle, filling the free space between the cores to a round shape, makes it difficult to move the impregnating composition along the cable and thereby increases the electrical strength of the cable. The twisting of insulated cores at all cable industry plants in the Soviet Union is carried out in one direction - the right. This is determined by the conditions for laying and connecting individual construction lengths to each other during the construction of cable lines.
Since 2 tons of cable paper with a moisture content of 7-9% (about 140-180 kg of water) are consumed for the manufacture of insulation of 1 km of a 35 kV cable with a cross section of 3X95 mm2, the cable from general twisting machines enters special vacuum boilers for drying and removing moisture from the paper insulation and air, the presence of which reduces the electrical and physical characteristics of paper insulation.
Drying is carried out at a temperature above 100 ° C, and after 2-3 hours air and water vapor begin to be pumped out of the boiler. The drying time depends on the design of the cable and equipment. To speed up and improve the quality of drying, the process is carried out with simultaneous heating of the cores of the inner part of the cable with electric current.
After the drying process is completed, the paper insulation of the cable is impregnated with an impregnating composition.
After the end of the process of impregnation with a heated composition in a vacuum boiler, the baskets with the cable are installed for cooling in the open air in the drying and impregnation department. In this case, the volume of the impregnating composition in the insulation (as a result of cooling) decreases and, as a result, additional feeding of the insulation occurs with the composition in the basket.
Impregnation with oil-rosin composition significantly increases the electrical strength of the paper insulation of cables.
The impregnating composition is made from mineral oils and rosin. For the impregnation of cables up to 35 kV inclusive, a very viscous mineral oil of the P-28 brand (GOST 6480-53) is used, obtained from the residues of oil distillation, called brystok, which is characterized by high resistance to oxidation and low gas emission during ionization.

The most important characteristic of the impregnating composition is the viscosity. The composition must, on the one hand, be less viscous in order to ensure complete impregnation of the paper, as well as cable laying without preheating at a temperature of at least 0 ° C, otherwise, when the cable is bent, the individual tapes of cable paper will not be able to slide relative to each other, which will break the paper tapes and damage the insulation in these places. On the other hand, when laying on steeply inclined and vertical sections of the route, the impregnating composition, which is not viscous enough, will gradually drain from the upper sections to the lower part of the cable. As a result, the upper section of the cable is devoid of part of the impregnating composition, which degrades the quality of the insulation of this section. At the same time, an increased pressure of the impregnating composition is created in the lower section of the cable, which can lead to rupture of the cable sheath.
For the impregnation of cables, the composition MP-1 is used, which has a viscosity of 6-7.5 according to Engler1 at 70 ° C, and MP-2, which has the same viscosity at 80 ° C. The main electrical characteristics of impregnating (oil-rosin) compositions and cable paper are given in Table. 2-8.
Comparison of the data in Table. 2-8 shows that the dielectric strength of the impregnated cable paper is 1.3-2.2 times that of the impregnating compound and 13-16 times that of the unimpregnated cable paper.
Impregnation of cables with depleted insulation, intended for vertical laying, is carried out with a less viscous composition MP-2. Insulation depletion is carried out in the same boilers after the impregnating composition has been removed from them.
In cables with individually lead-coated conductors with depleted insulation, the impregnating composition should not leak out at a temperature of 85 ° C and in cables with a common lead sheath - at a temperature of 75 ° C.
The depletion of paper with an impregnating composition leads to a decrease in the electrical strength of the insulating layer, so the paper insulation of cables with depleted impregnation thickens.
The thickness of the insulation of cables 1-3 kV with depleted impregnation is the same in thickness with the insulation of cables of the same voltage with normal impregnation. This is explained by the fact that the thickness of the insulation for cables for these voltages is determined by the requirement of mechanical strength, under which the resulting thickness of the paper insulation has a sufficient margin for electrical strength.
At present, cables with depleted insulation are rarely used for vertical and steep sections of the route, since the use of belt-insulated cables and cables with separately lead-coated cores for a cable line at a line operating voltage of 10 kV requires the use of special couplings.
In this regard, at present, much attention is paid to impregnating compositions containing synthetic ceresin as one of the components.
In accordance with GOST 340-59, in 20-35 kV cables over the core, in 6-10 kV cables with separately leaded cores over the insulation and in cables with a common lead sheath over the belt insulation, shielding must be performed by applying a layer of semi-conductive paper. Shielding, the arrangement of conductive surfaces in relation to the insulating material of the cable, is one of the best ways to regulate, limit and reduce the strength of the electric moth.

In cables with viscous impregnation, with a difference in levels along the laying route and under the action of heating, the impregnating composition moves in the radial and longitudinal directions. This leads to the formation of gas inclusions and the occurrence of ionization processes in them, which can lead to damage to the cable insulation.
The use of semiconductor screens along the core and under the lead sheath, where the increase in volume due to the pressure of the impregnating composition and the low elasticity of the tit under operating conditions reaches from 0.5 to 20% of the insulation volume, significantly improves the ionization characteristic of the cable and increases the reliability of its operation.
Under the lead sheath of cables according to GOST 340-59, every 300 mm, the designations of the manufacturer and the year of manufacture of the cable must be clearly marked on the surface of the insulation or on a special tape. In cables under a lead sheath with a diameter of less than 20 mm, instead of a special tape, a tape or thread of the color assigned to the manufacturer is allowed.
In multi-core cables, the upper insulation tape of one core must be made of natural-colored paper, the second core - red or natural-colored paper with a red stripe, the third core - any other color or natural-colored paper with a stripe of any other color. In four-core cables, the top tape of the neutral core must be made of natural-colored paper.
The distinctive coloring of the cores was introduced to determine the direction of the phase sequence of a three-phase system, to ensure the correct connection of the same phases to each other according to their colors during the installation of individual building lengths of the cable, as well as to connect the same-name phases of the busbars of the equipment of the switchgear of electrical installations with cable lines.
Plastic core insulation is used for cables up to 3 kV, manufactured in accordance with GOST 16442-70*. Polyvinyl chloride and polyethylene are used as plastics.
Polyvinyl chloride is a solid polymerization product of vinyl chloride, it is flame retardant and highly resistant to heat aging, water, alkalis, dilute acids and other active chemicals, oils and gasoline. In its pure form, polyvinyl chloride is not used due to its rigidity and brittleness at low temperatures.
To increase the elasticity and frost resistance of polyvinyl chloride, hard-to-evaporate organic filler liquids (plasticizers) are added to it; to improve the electrical insulation characteristics and reduce the cost, kaolin, talc, calcium carbonate, etc. are added to it; to increase resistance at high temperatures - stabilizers; to increase its light fastness - special dyes.
Cables with PVC insulation are most widely used for voltages up to 1,000 V. The disadvantage of PVC insulation is its thermoplasticity. Heating of the core by load currents can cause some softening of the insulation and displacement of the core from the central position during operation. The dielectric strength of insulation made of polyvinyl chloride plasticate, in addition, depends on the time spent under AC voltage.
In order to avoid an increase in dielectric losses in the insulation, these cables can be manufactured for voltages not exceeding 10 kV.
Cables with PVC insulation are made in a sheath only from PVC. The thickness of the sheaths, depending on the diameter of the cable under the sheath, is 1.8-2.6 mm.

Cables laid in the ground are provided with the usual protective covers and armor.
Polyethylene is one of the synthetic polymers that has the greatest application and promising wide use as cable insulation, especially cables for steep and vertical sections of the route. Polyethylene has good mechanical properties and a wide temperature range resistance to acids, alkalis, moisture and has high 9lwhtroieolation characteristics.
Depending on the density, polyethylene is classified into low, medium and high density.
Compared to low density polyethylene, high density polyethylene has a higher melting point and greater mechanical strength. With the introduction of carbon black or graphite into it, semi-conductive polyethylene can be obtained for shielding purposes.
Cables with polyethylene insulation are mass-produced by the domestic industry for voltages up to 10 kV and experimentally 20, 35 kV.

Unlike cables with impregnated paper insulation, the electrical calculation of cables with plastic insulation is carried out not according to the maximum, but according to the average electric field strength, since the field strength in cables with plastic insulation depends markedly on the core radius.
The operating field strength of the developed designs of plastic-insulated cables produced by the cable industry has magnitudes.
The thicknesses of the insulating layer applied by hot pressing for cables up to 3 kV with plastic insulation are given in Table.
For cables of 10 kV and higher with polyethylene insulation, the choice of screens is the most important issue in the reliability of the cable. The shield must be well bonded to the polyethylene insulation and have the same thermal expansion coefficient as the insulation, so that no voids form between the semi-conductive layers and the cable insulation when the cable load changes. These cables are shielded both on the core side and on the sheath side. In this case, the core is pressed with a thin layer of semi-conductive polyethylene, on which the main polyethylene insulation is applied, shielded from above with colloidal graphite or semi-conductive polyethylene.
Plastic insulation for a voltage of 6 kV is shielded from the side of the shell, for which purpose semi-conductive and metal (copper or aluminum) screens are applied over the core insulation.
On 6 and 10 kV cables with plastic insulation and a sheath, the conductivity of the screen tapes must ensure the magnitude of the earth fault current that occurs under operating conditions

Electrical machines are subjected to drying when wetting the insulation of windings and other current-carrying parts. e.g. during transport, storage, installation and repair, as well as when the unit is stopped for a long time.

Drying the insulation of the windings of electrical machines without special need causes additional unjustified costs, and if the drying mode is not maintained correctly, the winding will also be damaged.

The purpose of drying is to remove moisture from the insulation of the windings and increase the resistance to a value at which the electric machine can be energized. The absolute resistance, MΩ, of insulation for electrical machines that have undergone major repairs must be at least 0.5 MΩ at a temperature of 10 - 30 ° C.

For newly installed electrical machines, this value must not be lower than the values ​​​​given in Table. 2, and for electric motors with voltages above 2 kV or more than 1000 kW, in addition, it is necessary to determine the ratio R60 / R15 with a megohmmeter ka6c.

If the obtained data indicate an unsatisfactory condition of the insulation, the electrical machines are dried.

The removal of moisture from the insulation of the winding of an electrical machine occurs due to diffusion, which causes moisture to move in the direction of the heat flow from the hotter part of the winding to the colder one.

The movement of moisture occurs due to the difference in humidity in different layers of insulation, from layers with higher humidity, moisture moves to layers with lower humidity. The difference in humidity, in turn, is created by the difference in temperature. The greater the temperature difference, the more intense the drying of the insulation. For example, by heating the internal parts of the winding with current, it is possible to create a temperature difference between the internal and external layers of insulation and thereby speed up the drying process.

To accelerate the drying of windings heated to the limiting temperature, it is advisable to periodically cool to ambient temperature. Therefore, the efficiency of thermal diffusion is the greater, the faster the surface layers of the insulation are cooled.

Tab. 1. Approximate drying time for electrical machines

During the drying process, it is necessary to heat the windings and steel gradually, since with rapid heating the temperature of the internal parts of the machine can reach a dangerous value, while the heating of the external parts will still be insignificant.

The winding temperature rise rate during drying should not exceed 4 - 5°C per hour. According to the PTE of electrical installations of consumers, the measurement of insulation resistance relative to the machine body and between the windings is carried out for windings of electrical machines with a voltage of up to 660 V inclusive at 1000 V, and for electrical machines with a voltage above 660 V - with a megaohmmeter at 2500 V.

However, according to GOST 11828 - 75, the resistance of the windings of electrical machines for a rated voltage up to 500 V inclusive is measured with a megohmmeter designed for 500 V, windings of electrical machines for a rated voltage above 500 V - with a megohmmeter for 1000 V. Therefore, PTE to some extent tighten the requirements for testing insulation with a megaohmmeter.

Produced at a winding temperature of 75°C. If the winding insulation resistance was measured at a different temperature, but not lower than 10 °C, it can be recalculated to a temperature of 75 °C.

Before drying the insulation of the windings of electrical machines, the room must be cleaned of debris, dust and dirt. Electrical machines must be carefully inspected and purged with compressed air. During drying, the insulation resistance of each winding of the electrical machine is measured with respect to the grounded machine housing and between the windings (Fig. 1).

Each time before the measurement, it is necessary to eliminate residual charges in the insulation; for this, the winding is grounded to the housing for 3-4 minutes. In addition, when drying the windings of electrical machines, it is necessary to measure the temperature of the windings, the ambient air, and the drying current. In practice, as a result of drying the windings of electrical machines, the insulation resistance at a temperature of 750 ° C should not be lower than the data in Table. 2.

Tab. 2. The smallest permissible insulation resistance of the windings of electrical machines after drying

Drying of the windings of electrical machines by the method of inductive losses in steel

In recent years, rational methods have been introduced for drying electric motors by inductive losses in the stator steel when the machines are stationary, not related to the passage of current directly in the windings. With this drying method, there are two varieties: losses in the active steel of the stator and losses in the stator housing.

Heating of electric motors is carried out by magnetization reversal losses in the active steel of the stator of the AC motor or the inductor of the DC machine from the alternating magnetic flux created in the machines in the stator core and the machine housing.

It is created by a special magnetizing winding wound on the machine body along its outer surface with the conductors pulled under the frame (Fig. 1, a) or onto the housing and end shields (Fig. 1, b), an alternating magnetic flux can also be created by inductive losses in the active steel of the stator and the body of the electric machine (Fig. 1, c).

The rotor of an asynchronous or synchronous machine must be removed in order to be able to wind magnetizing turns on the stator.

Rice. 1. Drying of electrical machines due to inductive losses in steel: o-in the machine body, b - in the body and end shields, c - in the body and stator active steel

The magnetizing winding is made with an insulated wire, the cross section and the number of turns are determined by the appropriate calculation.

During the drying process, the insulation resistance of the windings of electrical machines decreases in the first period of drying, then increases and, having reached a certain value, becomes constant. At the beginning of drying, the insulation resistance is measured every 30 minutes, and when a steady temperature is reached, every hour.

The results are recorded in the drying log and at the same time curves (Fig. 2) are plotted for the dependence of the insulation resistance and winding temperature on the duration of drying. Measurements of insulation resistance, winding and ambient temperature continue until the electrical machine is completely cooled down.

Drying of the windings of the electrical machine is stopped after the insulation resistance is practically unchanged at a constant temperature for 3-5 hours and ka6c is not lower than 1.3.

Rice. Fig. 2. Dependence curves of insulation resistance 2, absorption coefficient 3 and winding temperature 1 of the electric machine on the duration of drying

Drying the insulation of the windings of an electric motor in a drying oven

Page 44 of 45

While the drying and impregnating operation is extremely important to obtain proper cable quality, drying and impregnating methods vary widely from factory to factory. Prof. Whitehead, who published in 1928 his research on drying and impregnation of cables, which he began on behalf of the American Institute of Electrical Engineers, says that he found the widest variations in this respect in American factories, namely from six days of drying at high vacuum and with preliminary drying in air until complete absence of drying at 20 hours. welding in hot impregnating mass and under reduced pressure. The same diversity is observed in Europe, and here the Heaver'a method used in the English Glover'a plant, as already mentioned above, stands apart. All this points to a lack of uniformity in understanding the significance of the process and its course, and to a relatively small experimental study of it.
It is known that the quality of a dielectric depends very much on the presence of moisture in it, so its complete removal is very important. The cable insulation before drying contains a lot of moisture, which takes a very long time to remove without special measures. H. Mailer gives the following simple calculation in this regard:
Cable for 35 kV, 395 m.n. with a length of 1,000 tons, has a paper weight of 2,000 kg, which at 7% humidity gives a water content in the cable of 140 kg. If such a cable is placed in a vacuum apparatus with a volume of 8 m3 and dried with a current of dry air at 20 ° C, then the volume of the vacuum apparatus must be changed 1000 times, provided that the air is removed each time completely saturated with moisture. The need for such a large volume of dry air during natural drying indicates the need for artificial drying measures: heating and vacuum. However, both have their drawbacks: high vacuum makes heat transfer from the boiler walls to the cable very difficult; the amount of steam contained in a given volume of the vacuum apparatus is less at reduced pressure than at high pressure; rapid evaporation causes a rapid drop in cable temperature, making drying difficult. Therefore, the usual, or, as the British say, "routine" drying method basically consists in the fact that the cable immersed in a vacuum apparatus is first heated at atmospheric pressure and with the boiler lid open with the help of steam passed into the coil or boiler jacket. This heating lasts for from several hours to 2-3 days at a temperature of 110-120 C, and the time is set according to production experience or laboratory testing. After such heating, the boiler is closed with a lid and a vacuum is created in it, at which drying continues at the same temperature of 110 - 120 ° C. For the most part, a vacuum of the order of 90-95% is given, but new modern installations reach pressures of up to 5 mm and even up to 2 mm rt. Art., and for especially high-voltage cables with the help of laboratory-type mercury pumps, a higher vacuum is also achieved. At such high vacuums, it is necessary to use a vacuum-welded impregnating mass, otherwise it foams strongly when entering the boiler.
Both during the heating process and during the drying process, not all cable elements increase their temperature equally. As measurements show, the copper core of the cable reaches a temperature of 100-110 ° C only after a very long time of continuous drying, about a day or more; after 5-6 hours. this temperature reaches a value of the order of only 60-80 ° C. Sometimes drying under vacuum is interrupted by the inlet of dry gas (air or preferably carbon dioxide), thereby increasing the temperature of the core, and then again a vacuum is applied: this is the so-called shock drying. It must be borne in mind that when the vacuum is interrupted, the temperature of water evaporation rises, and therefore the drying of the cable also stops. Currently, instead of drying by pushes, electric current heating is often used, which greatly speeds up the drying process. Such heating is always carried out with direct current, because with alternating current a very high voltage of the current source is required due to the high inductive resistance of the dried cable. Generally speaking, speeding up the drying process is advantageous not only in terms of better utilization of equipment and savings in the steam used to heat the vacuum dryer, but also in terms of improving the quality of the insulation, since paper can be damaged during prolonged heating. Electric drying is usually not economically advantageous, since it absorbs a large amount of energy, but there are still reasons to use it if there are not enough vacuum devices or if you want to shorten the process.
For low-voltage cables with voltages up to 3 kV, and sometimes up to 6 kV, the drying process is often completely omitted and replaced by boiling in a hot mass of a cable usually preheated by current. The moisture in this "cooking method" is removed during the cooking process. This method has some economic advantages, but it does not give any technical advantages in terms of improving the quality of the cable. In the brewing method, it is recommended to preheat the cable with electric current or in another way, since otherwise the cold cable lowers the temperature of the impregnating mass too much and thereby complicates the brewing process.
In the manufacture of a cable for very high voltage, before the end of drying, the vacuum apparatus is sometimes filled with carbon dioxide, which is then evacuated. The purpose of this operation is to replace, on the one hand, the reactive oxygen of the residual air with neutral carbon dioxide, and on the other hand, to reduce internal voids in the cable, since carbon dioxide dissolves much more in the impregnating mass than air, which leads to a reduction in the original voids.
The process of drying and impregnation of the cable is usually carried out in the same boiler in order to avoid contact of the cable with air, because the dry cable is very hygroscopic. The hot impregnating mass is sucked in thanks to the vacuum prevailing in the boiler. The temperature of the suction mass is usually on the order of 115-135 ° C, and according to N. Mflller'y even 140 ° C. Such a high temperature of the impregnating mass is necessary, since at the end of drying the temperature of the copper core does not reach 100 ° C, and since penetration mass through the paper stops at about 80 ° C, then at a lower temperature of the inlet mass, there can easily be a danger of under-impregnation of the cable, since the mass should cool especially strongly near the relatively cold copper core and adjacent layers of insulation. The second circumstance necessitating a high temperature of the impregnating mass is that in order for the mass to penetrate into all the pores of the paper, a hot mass is needed when its viscosity is sufficiently low.
In order to obtain a good and deep impregnation, the process of suction of the mass into the boiler must be quite slow and last at least 1-2 hours. If the suction goes quickly, then there will be a lot of air in the cable, because it is impossible to achieve an absolute vacuum in the boiler. In addition, the impregnating mass entering the vacuum apparatus foams strongly, since, at reduced pressure, gases dissolved in it begin to leave it, while during slow impregnation, some of these gases are removed by suction with pumps. In well-designed installations for the impregnation of high-voltage cables, the impregnating mass is degassed and, to prevent the reverse dissolution of gases in it and to prevent oxidation, is kept under vacuum; such a mass during impregnation no longer foams. Sometimes the mass is stored under nitrogen, which has a low solubility coefficient.
In order to improve impregnation, it is sometimes carried out in shocks, changing vacuum to pressure, further details of this impregnation method will be given later in the description of drying and impregnation control. Sometimes, when impregnating, a pressure increased by 3-4 at is applied in order to drive the impregnating mass into the cable. In order to allow this impregnation, Krupp boilers are designed for this increased pressure. Practice, however, has not fully justified this method, as will be seen from what follows, and it has now been almost universally abandoned.
The impregnation of the cable should be as complete as possible to ensure good dielectric and thermal properties of the cable. Since the impregnating mass has a very high coefficient of thermal expansion, the cable must be cooled before applying the lead sheath. It is good practice for high voltage cables to cool the cable so that the temperature of the cooled cable is 4-5°C higher than the ambient temperature, and cooling below ambient temperature is not allowed to avoid deposition of moisture from the environment on the cable.
The description of the drying and impregnation process and equipment will begin with a description of the manufacture of oil-rosin impregnating mass. Cooking of this mass is carried out either in the same vacuum apparatus in which the cable is impregnated, or, more conveniently, in special boilers. In FIG. 207 shows one of these Rot boilers, this boiler has a diameter of 4.2 m, is heated by a coil and is equipped with an agitator that makes 30 rpm. In such boilers, rosin is usually loaded first, and then oil is poured. Cooking is carried out with steam heating for several hours at a temperature of about 120 ° C until all the rosin is dissolved in the oil and its foaming, which depends on the release of vapors and moisture, stops. The impregnating mass for high-voltage cables is boiled under vacuum in order to eliminate the dissolution of gases in it and prevent oxidation. The freshly cooked mass should usually stand for several days in order to allow the hydroxy acids contained in the rosin to fall out of solution, otherwise they may eventually fall out in the cable insulation. Sometimes at cable factories contact cleaning of oil with the help of bleaching clays is put. Often, oil is also filtered through conventional filters to eliminate mechanical impurities.

Both of these types of drying are approximately the same, only drying on drums in the vast majority of cases is carried out in vertical, and not in horizontal boilers, as shown in Fig. 210. The relative advantages and disadvantages of drying on drums and in baskets are as follows:

Fig. 207. Boiler for cooking impregnating mass firm Rot.
The cables enter the drying and impregnation either wound on iron drums, onto which they are taken from three-phase machines, or in the so-called iron baskets, into which they are rewound from the drums. Drying of cables on drums is shown in Fig. 208, which shows three cable drums prepared for drying in a horizontal kettle and connected to each other and to special terminals for drying with electric current. The basket view is shown in Fig. 209, which shows a perforated basket converted into a deaf one.

Fig. 208. Cable drying on drums in horizontal boilers.

When drying in the basket, the cable must be rewound at least once into the basket from the take-up drum, and in this case the cable goes into the lead press "against feathers", i.e. with the top layer of paper applied with a positive overlap, the paper can be pulled up into the press.


Fig. 209. Basket for drying and impregnating cables.
The advantages of drying in baskets are that the basket can be made deaf, i.e. without holes, open only from above, which allows the cable to be cooled not in a vacuum apparatus, but in a special room, which greatly increases the use of vacuum apparatuses, with on the one hand, and allows the cable manufacturing process to be carried out without contact of the uncooled cable with air, on the other hand.

Fig. 210. Scheme of drying in a vertical boiler.

When drying on a drum, unnecessary rewinding of the cable is eliminated, but it becomes almost inevitable that the cable is transferred by air after impregnation to special cooling tanks, otherwise the use of equipment for drying and impregnation will be negligible. In addition, it is very difficult to crimp thin cables from drums, since a lot of effort is required to turn the drum in a thick cold mass. Then, with the commonly used drying and impregnation equipment, the cables on the drums must be turned over on edge before drying.
Vacuum dryers can be divided into the following three types: vertical kettles, horizontal kettles and ovens. The scheme of the vertical boiler is shown in Fig. 210, here, inside the boiler, a drum with a cable immersed in the boiler is shown by a dotted line. The layout of the horizontal boiler is shown in Fig. 211, such a boiler is opened by moving the carriage with the boiler cover fixed on it; this boiler is completely unsuitable for receiving baskets. In FIG. 212 shows a view of a Krupp oven; this cabinet is equipped with swivel plates on which baskets with cable are placed. Such cabinets are suitable only for cable drying, and the cable must be rewound into baskets.
For the impregnation of power cables, the most accepted type of boiler is the vertical boiler. Modern boilers for very high voltage cables are built very large, namely for receiving baskets up to 3 and 4 liters in diameter, for ordinary needs they are limited to boilers for baskets with a diameter of 2-2.5 m. Usually, one boiler includes from two to three baskets . In these boilers, drying can also be carried out on drums. The great convenience of this type of boilers is that during the impregnation, with the lid open, you can observe the state of the mass mirror and, by its state, judge whether the impregnation has ended or not, since after impregnation, gas bubbles and moisture should not be released from the mass. These boilers are heated either by a steam coil or a steam jacket. Steam jacketed boilers are more expensive than serpentine boilers, but better as serpentines are often out of tune. In addition, with a jacket it is easier to clean the boiler, superheated steam can be used, which is beneficial. A further advantage of the jacket is that it more easily tolerates the cooling of the boiler by running cold water into it.

Fig. 211. Scheme of drying in a horizontal boiler.
In America, it is customary to use oil instead of steam to heat boilers. Against the use of oil, however, the objection is made that oil is flammable; the distillation products developing from it require a special device for removal; when the oil is cooled, very high pressure must be applied at the beginning of the process, which greatly increases the cost of the installation.
Horizontal boilers for the production of power cables are used very rarely, and in fact they are not suitable for this purpose, because they have the following main disadvantages:

Fig. 212. Drying cabinet firm Fr. Krupp, Grusonwerk.

1. During the impregnation, the mass is greedily absorbed by the cable, and the mirror of the impregnating mass quickly decreases, due to which the upper part of the drum with the cable may not be impregnated if the mass is not collected during the process itself, which is very inconvenient.

2. Since the boiler filled with mass cannot be opened, it is necessary to lower the mass from the boiler in a hot state, which adversely affects the quality of the cable.
The first of these shortcomings, however, is quite easily eliminated by the device on top of the boiler of special reservoirs with an impregnating mass, from where its consumption is replenished. The disadvantage of horizontal boilers is that they are more difficult to keep clean than vertical boilers. The generally accepted opinion is that vertical boilers are more suitable for the production of power cables, horizontal - for the production of telephone cables, and cabinets - for drying telephone cables of small diameter, which should also be dried in baskets.
The typical scheme of the drying-impregnating device is shown in Fig. 213. Here A is an iron drum with a cable; B - vacuum apparatus; C - vacuum pump; D - tank with impregnating mass; E - surface condenser for water vapor sucked from the cable.
Under production conditions, the control of cable drying consists in monitoring the viewing window of the condenser, which shows whether the exhausted steam is condensing or not.


Fig. 213. Scheme of a drying-impregnating device for cables impregnated with a viscous mass.
The drain cock at the condenser also makes it possible to monitor the condensate water drain and roughly judge the stage of the process, however, both of these methods are very primitive and do not provide an accurate definition of the process. Currently, to establish a typical drying and impregnation regime, there are several methods based on measuring the electrical characteristics of the cable during drying and impregnation. The first report on the use of such a method was made by W. A. ​​Del Mag in 1924. According to this report, American cable factories used measurement during drying and impregnation of the electric capacitance of the cable using alternating current. Direct current was not used, since with it the measurement results fluctuate very strongly due to inevitable temperature fluctuations and due to significant electrical absorption.


Fig. 214. Change in cable capacitance during drying and impregnation according to W. A. ​​Del Mag
The nature of the change in capacitance over time according to W. A. ​​Del Mag'u is shown in Fig. 214. As can be seen from this figure, at the beginning of the process, the capacitance grows very strongly, apparently partly due to an increase in the temperature of the cable, and partly due to cable sweating. Then the capacitance starts to fall, and starting from some time, becomes constant. The moment when the capacitance has become constant corresponds to
obviously the end of the drying process. When the mass is admitted into the boiler, i.e., at the beginning of impregnation, the cable capacitance first increases very quickly, then the increase slows down, and finally, the capacitance becomes constant, which corresponds to the end of impregnation. It should be noted that in Fig. 214 the scale for the value of the capacity during impregnation is taken several times less than for drying.

Fig. 215. Change in cable capacitance during impregnation according to P. Junius.
Of several subsequent reports on the development of methods for controlling drying and impregnation by electrical changes, the work of P. Junius'a, produced at the German cable factory Hackethal Draht u. Kabelwerke. Junius took curves of capacitance versus time with a K. W. Wagner bridge using an alternating voice frequency current. Most curious are his observations on the impregnation process. He especially clearly showed the effect of pressure shocks on the degree of impregnation. In FIG. 215 shows, according to Junius, the dependence of the electric capacitance on the impregnation time, and it can be seen that during impregnation under vacuum, the capacitance increases relatively slowly, which indicates a gradual increase in the degree of impregnation. When pressure is applied to the vacuum apparatus by the inlet of atmospheric air, the container immediately jumps upwards, which indicates the compression of air bubbles in the cable.
When vacuum is applied again, the capacitance value drops again, but not to the previous value. Repeated shocks of pressure give again an increase in capacitance to a certain constant limit value. The degree of gap between capacitance limit and vacuum capacitance indicates the degree of cable evacuation.
However, it should be pointed out that the ionization curve given by P. Junius for the cable for which the curve of Fig. 215, had no inflection point.
This way of examining drying and impregnation provides a criterion by which P. Junius evaluates some of the artificial methods used in the cable impregnation process. Some factories try to raise the ends of the impregnated cable so high that they come out of the impregnating mass during the impregnation. By this they try to prevent the penetration of the mass from the ends of the cable, because then the degree of impregnation of the cable can be judged from the cut end. P. Junius considers such a conclusion of the ends to be harmful, because when the boiler is opened, the impregnating mass is pressed into the cable under the action of external pressure, and at the ends of the cable coming out of the mass, at the same pressure, air will be pressed into the cable through the ends.
Another artificial method is that during impregnation, at certain intervals, pressure is applied to the boiler so that the mass penetrates more perfectly into the paper layers. P. Junius does not consider this method to have great advantages, since the mass is expelled from the paper layer when the pressure is stopped by the pressure of the air bubbles compressed in the cable insulation. P. Junius offers the following rational impregnation method:
A cable with a tight fit is put on one end of the cable in the impregnating boiler (without a lead sheath) to create a vacuum inside the cable; this clutch is connected to a special powerful vacuum unit. When the boiler is closed, the cable is evacuated both through the sleeve and through the boiler.

Fig. 216. Scheme of oil-filled cable impregnation according to E. F. Nuezel'io.
Electrical testing is a very lengthy procedure that can only be applied to type tests. Currently, there are ways to control the degree of drying of the cable, by passing the air and steam sucked from the boiler through indicators that chemically indicate the presence or absence of water vapor.


Fig. 217. Scheme of oil impregnation of a filled cable at the Sevkabel plant.

Let's stop with me on the features of drying and impregnating oil-filled cables. As mentioned above, these cables are dried (or rather dried) after the lead sheath is applied, so the equipment for drying these cables is significantly different from the usual one. In FIG. 216 is a diagram of the connection of devices for impregnating with oil filled cable, given by E. F. Nuezel'eM)