Cable temperature under load GOST Permissible cable heating temperature

Power cable line is a line for transmitting electrical energy, consisting of one or more parallel cables with connecting cables. locking and end couplings (seals) and fasteners. In power cable lines, paper and plastic insulated cables are most widely used. The type of insulation of power cables and their design affect not only the installation technology, but also the operating conditions of power cable lines. This is especially true for cables with plastic insulation. So, as a result of loads changing during operation and additional heating caused by overloads and short-circuit currents, pressure arises in the cable insulation from polyethylene (polyvinyl chloride), which increases with heating, which can stretch the screens and sheaths of the cables, causing their residual deformation. During subsequent cooling, due to shrinkage, gas or vacuum inclusions are formed in the insulation, which are centers of ionization. In this regard, the ionization characteristics of the cables will change. Comparative data on the temperature coefficient of volumetric expansion of various materials used in power cable designs are given in Table 1.

Table 1. Temperature coefficients of volumetric expansion of materials used in the construction of power cables

It should be noted that the highest value of the temperature coefficient of volumetric expansion occurs at temperatures of 75-125°C. corresponding to insulation heating during short-term overloads and short-circuit currents.

Impregnated paper insulation of cable cores has high electrical characteristics. long service life and relatively high temperature heating Cables with paper insulation better retain their electrical characteristics during operation in the event of frequent overloads and associated additional heating.

To ensure long-term and trouble-free operation of cable lines, it is necessary that the temperature of the cable cores and insulation during operation does not exceed permissible limits.

The long-term permissible temperature of the conductors and their permissible heating at short-circuit currents are determined by the cable insulation material. The maximum permissible temperatures of power cable cores for various core insulation materials are given in Table. 2.

Table 2. Maximum permissible temperatures of power cable cores

Note: The permissible heating of cable cores made of polyvinyl chloride plastic compound and polyethylene in emergency mode should be no more than 80°C, of ​​vulcanizing polyethylene - 130°C.

The duration of cable operation in emergency mode should not exceed 8 hours per day and 1000 hours. for the service life. Cable lines with a voltage of 6-10 kV, carrying loads less than rated, can be overloaded for a short time under the conditions given in table. 3.

Table 3. Permissible overloads in relation to the rated current of cable lines with voltage 6-10 kV

Note: For cable lines that have been in operation for more than 15 years, overloads should be reduced by 10%. Overloading cable lines with a voltage of 20 ÷ 35 kV is not allowed.

Any power cable line, in addition to its main element - the cable, contains connecting and end joints (terminals), which have a significant impact on the reliability of the entire cable line.

Currently, during the installation of both end couplings (seals) and connecting couplings, heat-shrinkable products made of radiation-modified polyethylene are widely used. Radiation irradiation of polyethylene leads to the production of a qualitatively new electrical insulating material with unique sets of properties. Thus, its heat resistance increases from 80 °C to 300 °C during short-term operation and up to 150 °C during long-term operation. This material is distinguished by high physical and mechanical properties: thermal stability, cold resistance, resistance to aggressive chemical environments, solvents, gasoline, and oils. Along with significant elasticity, it has high dielectric properties that are maintained at very high temperatures. low temperatures. Heat-shrinkable sleeves and terminations are mounted both on cables with plastic and cables with impregnated paper insulation.

The laid cable is exposed to aggressive environmental components, which are usually chemical connectors diluted to varying degrees. The materials from which the cable sheath and armor are made have different corrosion resistance.

Lead is stable in solutions containing sulfuric, sulfurous, phosphoric, chromic and hydrofluoric acids. Lead is stable in hydrochloric acid at concentrations up to 10%.

The presence of chloride and sulfate salts in water or soil causes a sharp inhibition of lead corrosion. Therefore, lead is stable in saline soils and seawater.

Nitric acid salts (nitrates) cause severe corrosion of lead. This is very significant, since nitrates are formed in the soil during the process of microbiological decay and are introduced into it in the form of fertilizers. Soils according to the degree of increasing their aggressiveness towards lead shells can be distributed as follows:

• saline;
• limestone;
• sandy;
• chernozem;
• clayey;
• peat.

Carbon dioxide and phenol significantly enhance lead corrosion. Lead is stable in alkalis.

Aluminum is stable in organic acids and unstable in hydrochloric, phosphoric, and formic acids. and also in alkalis. Salts have a very aggressive effect on aluminum, the hydrolysis of which produces acids or alkalis. Of the neutral salts (pH = 7), salts containing chlorine are the most active, since the resulting chlorides destroy the protective film of aluminum, therefore saline soils are the most aggressive for aluminum shells. Sea water, mainly due to the presence of chlorine ions in it, is also a highly aggressive environment for aluminum. Aluminum is quite stable in solutions of sulfates, nitrates and chromium. Corrosion of aluminum increases significantly when in contact with a more electropositive metal, such as lead, which occurs when installing couplings unless special measures are taken.

When installing a lead coupling on a cable with an aluminum sheath, a contact lead-aluminum galvanic couple is formed, in which aluminum is the anode, which can cause destruction of the aluminum sheath several months after installation of the coupling. In this case, damage to the shell occurs at a distance of 10-15 cm from the coupling neck, i.e. at the place where protective covers are removed from the shell during installation. To eliminate harmful effects Using similar galvanic couples, the coupling and bare areas of the aluminum sheath are covered with cable composition MB-70(60), heated to 130 °C, and adhesive polyvinyl chloride tape is applied on top in two layers with 50% overlap. A layer of tarred tape is applied on top of the adhesive tape, followed by coating it with bitumen topcoat BT-577.

Polyvinyl chloride plastic is non-flammable and highly resistant to most acids, alkalis and organic solvents. However, it is destroyed by concentrated sulfuric and nitric acids, acetone and some others. organic compounds. Under the influence elevated temperature And solar radiation polyvinyl chloride plastic compound loses its ductility and frost resistance.

Polyethylene is chemically resistant to acids, alkalis, salt solutions and organic solvents. However, polyethylene under the influence ultraviolet rays becomes brittle and loses its strength.

The rubber used for cable sheaths is highly resistant to oils, hydraulic and brake fluids, ultraviolet rays, and microorganisms. Solutions of acids and alkalis at elevated temperatures have a destructive effect on rubber.

Armor made from low carbon steel usually fails much before the shell begins to corrode. Armor is highly corrosive in acids and very resistant to alkalis. Sulfate-reducing bacteria, which produce hydrogen sulfide and sulfides, have a destructive effect on it.

Covers made of cable yarn and bitumen practically do not protect the sheath from contact with the external environment and are destroyed quite quickly in soil conditions.

Electrochemical protection of cables from corrosion is carried out by cathodic polarization of their metal sheaths, and in some cases, armor, i.e. imposing a negative potential on the latter. Depending on the method of electrical protection, cathodic polarization is achieved by attaching a cathode station, drainage and tread protection to the cable sheaths. When choosing a protection method, the main factor causing corrosion in given specific conditions is taken into account.

The brand of power cable characterizes the main structural elements and the scope of application of cable products.

The letter designations of the cable structural elements are given in table. 4.

Table 4. Letter designations of cable structural elements

Note:

1. The letters in the cable designation are arranged in accordance with the cable design, i.e. starting from the core material and ending with the outer cable covering.
2. If at the end of the letter part of the cable brand there is a letter “P” written through a dash, this means that the cable has a flat cross-section and not a round one.
3. The designation of the control cable differs from the designation of the power cable only in that the letter “K” is placed after the material of the cable core.

After the letters there are numbers indicating the number of main insulated conductors and their cross-section (through the multiplication sign), as well as the rated voltage (through a dash). The number and cross-section of conductors for cables with a zero conductor or grounding conductor is indicated by the sum of numbers.

The most widely used cables are the following standard core sections: 1.2; 1.5; 2.0;2.5; 3; 4; 5; 6; 8; 10; 16; 25; 35; 50; 70; 95; 120; 150; 185; 240 mm.

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Measurement of the temperature of the cable sheaths must be carried out in places where the cable operates in the most difficult conditions (places where the cable intersects with heat and steam pipelines, in bundles of existing cable lines, in sections of the route with dry or high thermal resistance soil), during the period of maximum load cable.
To determine the temperature difference D£cab, t0b should be taken as the maximum temperature value, and the current value I should be taken as the maximum line load.
Measuring heating temperatures of cable sheaths or environment can be done using thermocouples, resistance thermometers or thermometers.
When monitoring cable heating, you should keep in mind the following temperature ranges, which are most often encountered: cable sheath temperature up to +60°C; soil temperature from -5 to + 25°C; air temperature from -40 to +45°C.
From the given data it follows that the temperature ranges are only a few tens of degrees, and often the temperature difference between the cable sheath and the environment is more than 10-20 "C. This requires the use of very sensitive temperature indicators.

a) Thermocouple method

When controlling cable heating with thermocouples, it is necessary that they create e.g. in the operating temperature range. d.s. about 0.5-1 mV, which will allow the use of millivoltmeters and galvanometers available in laboratories.
The most sensitive are thermocouples made from chromel-kopel alloys, which develop thermo-e. d.s. at 6.9 mV at 100° C.
Copper-constantan thermocouples (4 mV per 100°C) can also be used.
Thermocouples must have two junctions, one located on the cable and the other at the point at which the temperature is constantly recorded by a sensitive and accurate thermometer (the cold junction temperature).
To create good contact For thermocouples with a cable sheath, it is advisable to caulk the working junction into a lead petal (a disk with a diameter of 3-4 cm, a thickness of 2-3 mm) and use, as they are called in practice, “petal” thermocouples. Such a petal is securely fixed to the cable with taffeta or keeper tape.
If there are no leaf thermocouples, you should first place soft staniol under the working junction and only then press the thermocouple tightly to the cable sheath by wrapping it with thick fabric tape.
When monitoring cable heating, at least two thermocouples should be placed in one place for mutual monitoring of readings and reserve in case of breakage of the working junction.
Usually, in practice, it is necessary to control in any area the temperature of several adjacent cables, on which a group of thermocouples (up to 10-20 pieces) is installed.
All cold junctions of these thermocouples are usually brought to one place, where their temperature is recorded by a thermometer. In this case, to the obtained temperature reading on the instrument scale, it is necessary to add the ambient temperature (at the location of the ends of the “cold” junction) if it is positive, and subtract it if it is negative.

It is good to place the cold junctions in a vessel with melting ice or snow. This gives a stable cold junction temperature of 0°C until all the ice or snow has melted, and the millivoltmeter reading (usually calibrated in degrees) immediately gives the temperature of the cable sheaths in degrees Celsius without correction for ambient temperature, since it is equal to zero.
The ends of the thermocouples are connected to a contactor with a switch, to which a portable millivoltmeter (galvanometer) is connected during measurements.
Potentiometers with a sensitivity of at least 0.05 mV per division can also be used for measurements.

b) Thermal resistance method

A more sensitive method is to control the heating of cables using thermal resistances.
Thermal resistances are made of thin insulated wire with a diameter of 0.05-0.07 mm, which has a large temperature coefficient (change in resistance when heated)
The thermal resistance value should be at least 5-10 Ohms (usually 20-30 Ohms).
Several meters of thin wire are fixed on a piece of thick sheet electrical cardboard so that the wire strands are located on one side of the sheet (Fig. 45). For greater mechanical strength, the output ends of the resistances are made of thicker insulated wire.
To prevent the wire threads from spreading out and getting tangled, it is necessary to secure them to the plate with bakelite varnish.

Rice. 45. Winding thermal resistance tapes for measuring temperatures on cable sheaths.
1 - ends for connecting the thermoelement to the bridge; 2 - transition to a large cross-section wire.
To protect the wire threads from breaking, place a piece of thin cable paper on top of them, also lubricating it with bakelite varnish.
After making the thermal resistance, the sheet on which it is attached should be given a cylindrical shape by winding it around a rod with a diameter of 40-50 mm.
The value of the ohmic resistance of thermoelements after a one-hour exposure at a constant temperature is accurately measured on the bridge.
So, for example, if the thermal resistance is made of copper wire with a diameter of 0.05 mm and has a resistance of 20 Ohms at room temperature (+20° C), then when the temperature of the cable changes by 1° C, the change in resistance will be about 0.1 Ohm, which can be established with sufficient accuracy for practice using conventional measuring bridges.
Sometimes, based on local conditions, the thermal resistance must have very small dimensions, for example, for laying cables on the lead sheath in the gaps of the lower armor tape (the upper armor tape is cut). In these cases, very thin wire with high resistivity should be used.
Recently, semiconductor thermal resistances have been used to measure cable temperatures.

c) Thermometer method

In cases where the cables are located in a tunnel, duct or rooms, their temperature can be monitored directly with thermometers. The thermometer scale should be no more than 50-100° C.
For ease of connection to the cable, the thermometer should have an end with a mercury head bent at a right angle. Soft staniol is placed under the mercury head of the thermometer, after which the thermometer is pressed tightly to the cable by winding and tightening with fabric tape.
If continuous or periodic automatic recording of cable heating temperatures is desired, then thermocouples or thermal resistances must be connected to electronic potentiometers such as EPD-07, EPD-12, EPP 09 specially installed for this purpose.
When installing thermocouples, resistance thermometers or thermometers, it is important to maintain the cable cooling conditions unchanged.
In tunnels or ducts this concerns the ventilation of cables. It is not allowed to install any partitions, fill the spaces between individual shelves with anything, etc.
When laying cables in trenches, after thermocouples or thermal resistances have been laid, the hole is filled and compacted with the same soil.
Temperature measurements can begin no earlier than 24 hours after the pit is closed and the covers over the cables are restored. This is dictated by the need to warm up the soil and create a normal thermal field around the cable.
The ends from thermocouples or resistances are brought out to the wall of a nearby room or placed and secured in a control well specially equipped for this purpose.
Depending on the monitoring results, the load on the cable line increases or decreases, or measures are taken to improve cable cooling.

The heating temperature of the cable cores on which the KVV type termination is mounted should not exceed 65 C under prolonged load. Seals of this type have high chemical resistance, with the exception of concentrated hydrochloric acid, chlorocarbons and other materials that have a destructive effect on polyvinyl chloride.
The heating temperature of the cable cores, and therefore the current, is limited by the permissible temperature for the cable insulation and depends on the cable insulation material. The cable cross-section is selected according to the PUE tables, which take into account the temperature of the cable core.
The heating temperature of the cable cores is controlled by a thermometer (thermocouple) installed on the cable sheath.
Thermal drop D for cables 16 - 240 mm2 depending on the load current. Checking the heating temperature of cable cores is carried out by measuring the temperatures of their metal sheaths.
Thermal difference D (for cables 16 - 240 mm2 depending on the load current. The heating temperature of the cable cores is checked by measuring the temperatures of their metal sheaths. For measurements it is recommended to use thermistors or thermocouples and only in as a last resort thermometers.
Symbols of underground structures. It is very difficult to directly measure the heating temperature of the cable cores; therefore, the heating of cables during their operation is monitored by measuring the heating temperature of the cable sheath.
In table 1 - 65 show the permissible excess heating temperature of cable cores during a short circuit. It is accepted that before the short circuit the temperature of the cable cores did not exceed the permissible temperature for heating in a long-term mode.
To increase the durability of cables of this type, it is necessary to set the heating temperature of the cable cores to no more than 90 C.
Such cables after exposure to short circuit current must be inspected, the terminations must be repaired if necessary, and high voltage tests must be carried out. If the heating temperature of the cable cores is higher than the specified values, the cables are considered unsuitable for further use and must be replaced immediately.
Permissible long-term current loads on cables with voltages up to 35 kW inclusive with insulation made of impregnated cable paper in a lead, aluminum or laminated polychlorinated-nyl sheath are accepted in accordance with the permissible heating temperatures of the cable cores according to GOST.
The laying of cables inside the boxes must be carried out in accordance with the requirements of the PUE for laying cables in cable channels. In this case, the distance from the structures to the front wall of the box is not standardized. The heating temperature of the cable cores should be no more than specified in § 1 - 3 - 9 PUE.
Losses in a cable consist of losses in the cores, insulation and sheath. Losses in insulation and sheathing can be negligible or significant. The heat flow caused by losses in all cable elements flows radially from the center of the cable outward through the thermal resistance of the various elements and causes overall overheating of the cable. This overheating, taking into account the basic soil temperature, determines the temperature on the cable core. The heating temperature of the cable core should not exceed the limit established for this insulation.

A correctly calculated and properly executed electrical network does not guarantee the elimination of the possibility of emergency situations leading to unacceptable electrical overheating when a short circuit occurs.

For example, a similar situation, as noted in the work, occurs when a load is connected to an electrical outlet through an extension cord. Starting from a certain length of the extension wire added to the group line, the resistance of the phase-zero circuit increases to a value at which the short circuit current will be less than the operating threshold of the electromagnetic release of the circuit breaker. Therefore, when installing electrical installations, it is advisable to take into account the possibility of abnormal operating conditions of electrical wiring.

In accordance with the “Temperature Limits for Electric Cables for a Rated Voltage of 1 kV under Short Circuit Conditions,” the temperature of the cable cores (up to 300 mm 2 inclusive) with PVC insulation during a short circuit should not exceed 160 degrees. Reaching this temperature is allowed for a short circuit duration of up to 5 seconds. With such a short circuit duration, the cable insulation does not have time to heat up to the same temperature. For longer short circuits, the maximum heating temperature of the cores must be reduced.

Let us consider the occurrence of a similar situation using the example of using a group “C” circuit breaker. The time - current characteristic of the switch is shown in Fig. 1. In the given characteristics, zone “a” - thermal release and zone “b” - electromagnetic release are highlighted. The graph shows two curves 1 and 2 of the dependence of the switch operation time on the current, which show the limits of the technological spread of the switch parameters during its manufacture. For automatic circuit breakers of group “C”, within the technological spread, the multiple of the operation current of the electromagnetic release to the rated operation current of the thermal release is in the range from 5 to 10. We are only interested in curve 2 for alternating current (AC), which shows the maximum operation time of the circuit breaker. As can be seen from the graph in Fig. 1, with a slight decrease in the short-circuit current below the operating threshold of the electromagnetic release, the response time of the circuit breaker is determined by the thermal release and reaches a value of the order of 6 seconds.

Rice. 1 Time is the current characteristic of group C machines.

Let's try to find out what happens to the cables during the period of time during which the thermal release operates. To do this, it is necessary to calculate the dependence of the temperature of the cable cores on the time of passage of currents through them, close to the operating threshold of the electromagnetic release.

Table 1 gives the calculated temperatures of the cable cores depending on the duration of the short circuit (at different currents) for a cable with copper cores with a cross-section of 1.5 square meters. mm. Cable of this cross-section is widely used in lighting residential and public buildings.

To calculate the temperatures of the cable cores, the calculation method from “Calculation of thermally permissible short-circuit currents taking into account non-adiabatic heating” was used.

The temperature of the cable cores is determined by the formula:

Θ f = (Θ i +β)∙exp(I AD 2 ∙t/K 2 ∙S 2) - β (1)

where, Θ f is the final temperature of the cable cores o C;

Θ i - initial temperature of the cable cores o C;

β is the reciprocal of the temperature coefficient of resistance at 0 °C, K, for copper β=234.5;

K is a constant depending on the material of the conductive element, A s 1/2 /mm 2, for copper K = 226;

t - short circuit duration, s;

S - cross-sectional area of ​​the conductor, mm 2;

I SC - known maximum short circuit current (rms value), A;

I AD =I SC /ε - short circuit current determined on the basis of adiabatic heating (rms value), A;

ε - coefficient taking into account heat removal to neighboring elements;

X, Y - constants used in the simplified formula for cores and wire screens, (mm 2 / s) 1/2; mm 2 /s, for cables with copper conductors and PVC insulation X=0.29 and Y=0.06;

Calculations were made for a cable temperature before a short circuit of 55 degrees. This temperature corresponds to the operating current passing through the cable before a short circuit occurs on the order of 0.5 - 0.7 of the maximum permissible continuous current at an ambient temperature of 30 - 35 degrees. Depending on the expected operating conditions of the electrical installation, the temperature of the cable cores before a short circuit can be changed when designing the electrical network.

Table 1

From Table 1 it can be seen that the maximum short-circuit current (if the electromagnetic release fails), which does not cause heating of the cores above 160 degrees in 6 seconds, is approximately 100 A. That is, a cable with a cross-section of 1.5 mm 2 can be protected by an automatic circuit breaker of the "group" C" with a rated current of no more than 10A.

When manufacturing cables, the cross-section of the cores is often underestimated. An underestimation of the cross-section by 10% is common. It is not difficult to find cables with a large cross-section in the markets.

Table 2 gives the calculated temperatures of the cable cores when the cross-section is reduced by 10%. As can be seen from the table, the C10 circuit breaker does not protect such a cable with 100 percent reliability.

For the most critical objects, especially those with building structures made of combustible materials, it is advisable to select a circuit breaker when designing an electrical installation according to Table 3, in which the core sections are given with a 20% understatement. Protection of such cables will be provided by automatic circuit breaker C6 or B10, in which the multiple of the operating current of the electromagnetic release to the rated operating current of the thermal release is in the range from 3 to 5. This will significantly increase the reliability of electrical wiring.

Table 2

Table 3

When choosing a cable, a lot of different parameters are taken into account, from the cross-section of the cores to the insulation material. Why is it important to know details such as shell material? After all, its main function is to protect against electric shock. If the insulation copes with this task, then more attention needs to be paid to the more important characteristics of the cable. Unfortunately, many people make this mistake; in fact, the permissible heating temperature of the cable and the insulation material are extremely related to each other. Each type of protective shell is designed for a certain temperature; if it exceeds certain values, the aging process of the insulation is accelerated. This seriously affects the life of the cable, and often the equipment connected with it. The permissible heating temperature of the cable is the parameter on which not only the load capacity of the cable depends, but also the reliability of its operation. Permissible heating temperature of a cable with different types of insulation. All types of materials used as insulation of current-carrying conductors have their own physical characteristics. They have different densities, heat capacities, and thermal conductivities. As a result, this affects their ability to withstand heat, so vulcanizing polyethylene can maintain its performance characteristics up to 90ºC. On the other hand, rubber insulation can withstand a significantly lower temperature load - only 65ºС. The permissible heating temperature of a PVC cable is 70 degrees and this is one of the most optimal indicators. One of the most important indicators is the permissible cable heating temperature c. This type of cable is used extremely widely and is designed to work with different voltages. That is why you should be careful in this characteristic; it changes as follows:

• for a voltage of 1-2 kV, the maximum permissible temperature for cables with depleted and viscous impregnation is 80ºС;
• for a voltage of 6 kV, insulation with viscous impregnation can withstand 65ºС, with depleted impregnation 75ºС;
• for a voltage of 10 kV, the permissible temperature is 60ºС;
• for a voltage of 20 kV, the permissible temperature is 55ºС;
• for a voltage of 35 kV, the permissible temperature is 50ºС.

All this requires increased attention to the long-term maximum load of the cable and operating conditions. Another insulation material in demand today in the electrical industry is cross-linked polyethylene. It has a complex structure that provides unique performance characteristics. The permissible heating temperature of the cable and cross-linked polyethylene insulation is 70ºС. One of the leaders in this parameter is silicone rubber, which can withstand 180ºC. What can overheating of a cable lead to? Exceeding the permissible heating temperature of the cable leads to the fact that the insulation properties change dramatically. It begins to crack and crumble, resulting in a risk of short circuit. The service life of the cable is seriously reduced with each degree exceeded. This requires more frequent repairs and costs, so it is better to initially use the cable that is designed to solve specific problems. But this is not enough; it is necessary to regularly monitor the temperature of the shell, especially in places where overheating can be assumed. These may be places near heat pipes or create unfavorable conditions for cooling.