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Bolted connections are subject to inspection. Bolted mounting connections without controlled tension. Pressed contact connections

As is known, depending on the design, purpose, method of connecting materials, area of ​​application and other factors, contact connections are distinguished: bolted, welded, soldered and made by crimping (crimped and twisted).
Contact connections include remote wire spacers.

When operating contact connections made by welding, the causes of defects in them may be: deviations from the specified parameters, undercuts, bubbles, cavities, lack of fusion, sagging, cracks, slag and gas inclusions (cavities), unsealed craters, burnt wires of the cores, misalignment of the connected conductors, incorrect choice of lugs, lack of protective coatings on connections, etc.
Thermal welding technology does not ensure reliable operation of welded connectors for large cross-section wires (240 mm2 or more). This is due to the fact that due to insufficient heating during the welding process of the connected wires and the uneven approach of their ends, the outer layers of the wires are burned out, lack of penetration, and shrinkage cavities and slags appear at the welding site. As a result, the mechanical strength of the welded joint decreases. When mechanical loads are less than the design ones, a wire break (burnout) occurs in the anchor support loop, which leads to emergency shutdowns of overhead lines with a short service life. If in welded joint When individual wire conductors break, this leads to an increase in the contact resistance and an increase in its temperature.
The rate of development of the defect in this case will significantly depend on a number of factors: the value of the load current, wire tension, wind and vibration influences, etc.
Based on the experiments conducted, it was found that:

1. a decrease in the active cross-section of the wire by 20 - 25% due to the breakage of individual conductors may not be detected when carrying out IR inspection from a helicopter, which is due to the low emissivity of the wire, the distance of the thermal imager from the route by 50 - 80 m, the influence of wind, solar radiation and other factors;
2. when rejecting defective contact connections made by welding using a thermal imager or pyrometer, it must be borne in mind that the rate of development of a defect in these connections is much higher than that of bolted contact connections with pressure;
3. defects in contact connections made by welding, identified by a thermal imager during inspection of overhead lines from a helicopter, must be classified as dangerous if their excess temperature is 5 °C;
4. steel bushings not removed from the welded section of wires can create a false impression of possible heating due to the high emissivity of the annealed surface.

In contact connections made by crimping, there are wrong selection lugs or sleeves, incomplete insertion of the core into the lug, insufficient degree of crimping, displacement of the steel core in the wire connector, etc. As you know, one of the ways to control crimped connectors is to measure their DC resistance.
The criterion for an ideal contact connection is the equality of its resistance to the resistance of an equivalent section of the whole wire. A crimped connector is considered suitable for use if its resistance is no more than 1.2 times higher than the equivalent section of the whole wire. When the connector is crimped, its resistance drops sharply, but with increasing pressure it stabilizes and changes slightly.
The connector resistance is very sensitive to the condition of the contact surface of the wires being pressed. The appearance of aluminum oxides on contact surfaces leads to a sharp increase in the contact resistance of the connector and increased heat generation.
Minor changes in the contact resistance of the contact connection during their crimping process, as well as the associated low heat generation in the contact connection, indicate insufficient efficiency in detecting defects in them immediately after installation using infrared equipment. During the operation of pressed contact connections, the presence of defects in them will contribute to a more intense formation of oxide films and increase the contact resistance, which can lead to the appearance of local heating. Therefore, we can assume that IR inspection of new crimped contact connections does not allow identifying crimping defects and should be carried out for connectors that have worked for a certain period of time (1 year or more).
The main characteristics of crimped connectors are the degree of crimping and mechanical strength. With increase mechanical strength connector, its contact resistance decreases. The maximum mechanical strength of the connector corresponds to the minimum electrical contact resistance.

Contact connections made using bolts most often have defects due to the lack of washers at the junction of the copper core with a flat terminal made of copper or aluminum alloy, the absence of disc springs, direct connection of the aluminum tip to the copper terminals of equipment in rooms with an aggressive or humid environment , as a result of insufficient tightening of bolts, etc.
Bolted contact connections of aluminum busbars for high currents (3000 A and above) are not stable enough in operation. If contact connections for currents up to 1500 A require tightening the bolts once every 1 - 2 years, then similar connections for currents of 3000 A and above require annual overhaul with the obligatory cleaning of the contact surfaces. The need for such an operation is due to the fact that in high-ampere busbars (busbars of power plants, etc.) made of aluminum, the process of formation of oxide films on the surface of contact joints occurs more intensively.
The process of formation of oxide films on the surface of bolted contact joints is facilitated by different temperature coefficients of linear expansion of steel bolts and aluminum busbars. Therefore, when a short-circuit current passes through the busbar, when it operates with an alternating current load, deformation (compaction) of the contact surface of the aluminum bus occurs in it over a long distance as a result of vibration influences. In this case, the force tightening the two contact surfaces of the busbar weakens, the lubricant layer between them evaporates, etc.
Due to the formation of oxide films, the contact area of ​​the contacts, i.e. the number and size of contact pads (number of points) through which current passes are reduced and, at the same time, the current density increases, which can reach thousands of amperes per square centimeter, as a result of which the heating of these points greatly increases.
The temperature of the last point reaches the melting temperature of the contact material, and a drop of liquid metal forms between the contact surfaces. The temperature of the drop, rising, reaches a boil, the space around the contact connection is ionized, and there is a danger of a multiphase short circuit in the switchgear. Under the influence of magnetic forces, the arc can move along the switchgear busbars with all the ensuing consequences.
Operating experience shows that, along with multi-ampere busbars, single-bolt contact connections also have insufficient reliability. The latter, in accordance with GOST 21242-75, are allowed for use at a rated current of up to 1,000 A, but are damaged already at currents of 400 - 630 A. Increasing the reliability of single-bolt contact connections requires taking a number of technical measures to stabilize their electrical resistance.
The process of development of a defect in a bolted contact connection, as a rule, takes quite a long time and depends on a number of factors: load current, operating mode (stable load or variable), exposure to chemical reagents, wind loads, bolt tightening forces, contact pressure stabilization, etc.
The transient resistance of a bolted contact connection depends on the duration of the current load. The contact resistance of contact connections gradually increases up to a certain point, after which a sharp deterioration of the contact surface of the contact connection occurs with intense heat generation, indicating an emergency condition of the contact connection.
Similar results were obtained by specialists from Inframetrix (USA) during thermal tests of bolted contact joints. The increase in heating temperature during testing was gradual throughout the year, and then there was a period of sharp increase in heat release.

Failures of contact connections made by twisting occur mainly due to installation defects. Incomplete twisting of wires in oval connectors (less than 4.5 turns) leads to the wire being pulled out of the connector and breaking. Uncleaned wires create high contact resistance, resulting in overheating of the wire in the connector with possible burnout. There have been repeated cases of the lightning protection cable AZhS-70/39, twisted at a smaller number of turns, being pulled out from the oval connector brand SOAS-95-3 air lines 220 kV.

Rice. Photo of the place where the remote spacer is attached with a break in the conductors as a result of vibration effects (a) and a diagram of the flow of load currents in the two-wire phase of an outdoor switchgear or overhead line when the conductors are broken at the place where the distance spacers are attached (b)

Distance spacers.

Unsatisfactory design of some designs of spacers, exposure to vibration forces and other factors can lead to chafing of the wire conductors or their break (Fig. 34). In this case, a current will flow through the spacer, the value of which will be determined by the nature and degree of development of the defect.

Analysis of the results of thermal imaging inspection of contact connections

Welded contact connections.

During thermal imaging testing of contact connections, assessment of their condition in accordance with the “Scope and Standards of Testing of Electrical Equipment” can be carried out by the defectiveness coefficient or by the value of excess temperature. Experiments conducted by Yuzhtechenergo revealed the insufficient efficiency of the thermal imaging method for detecting a defect in a welded contact joint at an early stage of development, especially when monitoring contact connections of overhead line wires from a helicopter. For welded contact joints, it is preferable to assess their condition by the value of excess temperature.

Pressed contact connections.

At one time, the values ​​of defectiveness coefficients were used as criteria for assessing the condition of pressed contact connections on outdoor switchgear and overhead lines, i.e. the ratio of the measured resistance or voltage drop across a connector to the resistance of an identical section of a whole wire.
With the advent of CT devices, the condition of pressed contact connections can be assessed by the value of excess temperature or by the defectiveness coefficient.
The question arises about the degree of effectiveness of each of these methods for assessing the condition of pressed contact connections. To solve this problem, Mosenergo carried out load tests on a section of ASU-400 wire with serviceable and defective connectors.
The defectiveness coefficients for direct current (Kx - 9) and voltage drop (K2 = 5) were previously determined. The results of load tests (Table 1) showed that for crimped connectors, the most preferable method for assessing contact connections is based on the excess temperature value.

Thus, at a current of (0.3 - 0.4)/nom, the excess temperature is 7-16 °C, which is quite reliably recorded by the ICT device.
The results of the experiments are in good agreement with the recommendations of the “Scope and standards of testing of electrical equipment.” When assessing the condition of pressed contact connections based on the values ​​of defectiveness coefficients, it is necessary to keep in mind that at the initial stage of manufacture (during installation) contact connections have a defectiveness coefficient of 0.8 - 0.9.

Failure of a crimped contact connection develops gradually and largely depends on compliance with the crimping technology and the pressure developed during this process. The optimal condition is considered to be one in which maximum degree compression corresponds to the minimum value of the contact resistance of the contact connection.

Bolted contact connections.

In both domestic and foreign practice, the most widespread assessment of the condition of a bolted contact joint is based on the value of excess temperature.
The process of defect development in a bolted contact connection was studied by Inframetrix (USA) on an existing connection at a load current of 200 A. The experiment showed that the process of defect development in the absence of external climatic, vibration and other factors and a load stable over time can proceed for a very long time .
Based on the test results, the company proposed the following limit values ​​of excess temperature at rated current:
A)< 10 °С - нормальная периодичность тепловизионного контроля;
b) 10 - 20 °C - frequent thermal imaging control;
c) 20 - 40 °C - thermal imaging control every month;
d) > 40 °C - emergency heating.
The system proposed by the company for assessing the condition of bolted contact connections based on heating temperature, in principle, does not differ from that regulated by the “Scope and standards of testing of electrical equipment.”


Rice. 2. Dependence of the excess temperature of the bolted contact connector on the load current:
1 - with a reduction in the contact area of ​​the contact surfaces by 40%; 2 - the same, 80%

The effect of heating temperature of bolted contact joints on the degree of defect development was studied by Yuzhtekhenergo. For this purpose, load tests were carried out on bolted contact connections by simulating a reduction of 40 and 80% in the area of ​​contact of the contact surfaces (Fig. 35). The possibility of detecting defects of this kind during thermal imaging control was confirmed and it was shown that defects at an early stage of development can be clearly detected at load currents (0.3 - 0.4)/nom.
Cyclic long-term tests of bolted contact connections show that the stability of their contact transient resistance is largely determined by the design of the fastening fittings (the presence of spring washers, etc.). When carrying out thermal imaging monitoring, identifying contact connections with increased heating requires taking certain stabilization measures, for example, shutdown or temporary load reduction. In the latter case, the current /admissible permissible for a given defective contact connection can be determined from the relation

Note. The data given in the table applies if other standards are not established for specific types of equipment.
where /load, ΔTmeas - current and temperature rise of the measured contact connection, respectively; ΔTnorm - excess temperature of a contact connection, regulated by the “Scope and Standards of Electrical Equipment Testing”, depending on the type of coating of the contact surfaces and the environment in which they are located.
Evaluation of the thermal state of electrical equipment and live parts, depending on their operating conditions and design, can be carried out: by standardized heating temperatures (temperature rises), excess temperature, defectiveness coefficient, dynamics of temperature changes over time, with changes in load, by comparing measured temperature values ​​within phases and between phases with temperature values ​​in known good areas.
Limit values ​​of heating temperature for /nom and its excess are given in table. 16.

For contacts and bolted contact connections, the standards given in table. 16 should be used at load currents (0.6 - 1.0)/nom after appropriate recalculation. Recalculation of the excess of the measured temperature value to the normalized value is carried out according to the relation

where ΔTnom - temperature rise at /nom; ΔTrab - the same, at g
slave-
Thermal imaging monitoring of electrical equipment and live parts at load currents of 0.3/nom and below does not help identify defects at an early stage of their development.
For contacts and bolted contact connections at load currents (0.3 - 0.6)/nom, their condition is assessed based on excess temperature. The temperature value recalculated to 0.5/nom is used as a standard.
For recalculation, the ratio is used

where ΔT0.5 is the excess temperature at a load current of 0.5/nom.
When assessing the condition of contacts and bolted contact connections based on excess temperature at a load current of 0.5/nom, the following areas are distinguished according to the degree of malfunction:

1. excess temperature 5-10 °C. Initial degree a malfunction that should be kept under control and measures taken to eliminate it during scheduled repairs;
2. excess temperature 10 - 30 °C. Developed defect. Measures should be taken to eliminate the malfunction at the next time the electrical equipment is taken out of service;
3. excess temperature more than 30 °C. Emergency defect. Requires immediate elimination.

It is recommended to assess the condition of welded and crimped contact connections based on excess temperature or defectiveness coefficient.
When assessing the thermal state of live parts, the following degrees of malfunction are distinguished, based on the given values ​​of the defectiveness coefficient:
No more than 1.2................................................... ... Initial degree of malfunction, Forward

Types of bolts. Metal ones are usually connected with bolts, less often reinforced concrete structures. For connection metal structures apply following types bolts: normal, coarse, high precision and high strength with corresponding nuts and washers.

Rough precision bolts are stamped from round carbon steel with a diameter of no more than 20 mm. They are placed in holes with a gap of 2-3 mm. Such bolts have increased deformability and do not perform well in shear in multi-bolt connections; therefore, their use in connections with alternating forces is not allowed. Rough precision bolts are used, as a rule, in units where one element rests on another, with transmission through a support table, as well as in connections where they do not work or work only in tension.

High-precision bolts are processed by turning lathe with a tolerance of + 0.1 mm. Such bolts are made with a diameter of 10-48 mm and a length of up to 300 mm.

High-strength bolts (otherwise known as friction bolts) are designed to transfer forces acting on a connection through friction. Such bolts are made from high-strength steels and are heat treated to finished form. The bolts are placed in holes 2-3 mm larger than the diameter of the bolt, but the nuts are tightened with a calibration wrench. Such connections are simple, but quite reliable and are used in critical structures.

The diameters for high-precision bolts are assigned equal to the nominal diameters of the bolts. The holes for such bolts have only positive deviations, which ensures installation of the bolt without difficulty. Unlike bolts of normal and coarse precision, the working part of the shaft of a high-precision bolt does not have threading, which ensures fairly complete filling of the hole and Good work for cutting To distinguish high-strength bolts from others, raised markings are applied to their heads.

Assembling connections. The assembly of bolted joints includes the following operations: preparing the joining surfaces, aligning the holes for the bolts, preliminary tightening the joint parts to be joined, drilling the holes (if necessary) to the design size, installing the bolts and final assembly.

Preparation of mating surfaces involves cleaning the mating elements from rust, dirt, oil and dust. In addition, they straighten irregularities, dents, and bends, and also remove burrs on the edges of parts and holes with a file or chisel. These operations are performed especially carefully when connecting parts with high-strength bolts, where the tight junction of all joined elements is one of the main conditions for the reliable operation of the bolted connection.

The surfaces to be joined are cleaned with dry quartz or metal sand using a sandblasting machine; roasting gas burners, steel brushes, chemical treatment.

Sandblasting is more effective than other methods, as it provides a high coefficient of friction for the mating surfaces, but this method is the most labor-intensive.

The most commonly used fire treatment method is using universal burners, which work like natural gas, and on an oxygen-acetylene mixture, and create a temperature of 1600-1800 ° C, which ensures the burning of grease stains and peeling off scale and rust.

One way to clean bolts, nuts and washers is to immerse them in a tank of boiling water and then into a container filled with unleaded gasoline with 10-15% mineral oil. After the gasoline evaporates, a thin continuous film of lubricant remains on the surface of the hardware.

Accurate alignment of the holes of the mounting parts is achieved using pass-through mandrels, which are a rod with cylindrical parts. The diameter of the mandrels should be 0.2-0.5 mm less than the diameter of the hole.

To fix the relative position of mounted elements and prevent their shift 1/10 total number The holes are filled with plugs with a diameter equal to the diameter of the holes. The length of the plugs must exceed the total thickness of the elements being connected. After installing the plugs, the mandrels are knocked out. Packages of connected elements are tightened with permanent or temporary bolts, which are placed through every third hole, but at least every 500 mm.

Holes are drilled using manual pneumatic and electric machines.

Pneumatic machines can be straight, used for working in places where there are no size restrictions, and angular, adapted for working in tight spaces. Pneumatic installations are used to drill holes with a diameter of up to 20 mm.

Electrical machines operate on 220 V AC mains. outdoors Such machines are used complete with a protective switching device, and in closed dry rooms they are grounded, the installer works electric tools wearing gloves and standing on a rubber mat. The safest machines are those with double insulation; they can be used without additional protective measures and when working outdoors.

After drilling holes free of assembly bolts, the bolts are unscrewed and permanent bolts are installed in their place.

The nuts of all bolts (permanent and temporary) are tightened with hand wrenches (regular or ratchet). In this case, one worker holds the bolt head from rotating, and the second one tightens the nut. On bolts of normal and high precision, washers are installed - one under the bolt head and no more than two under the nut. At large number bolts in one connection, electric impact wrenches are used. The bolts are installed from the middle of the joint to the edges. On the nut side there must be at least one thread left with full profile. The quality of tightening is checked by tapping the bolts with a hammer weighing 0.3-0.4 kg. In this case, the bolts should not move or shake.

The nuts are protected from self-unscrewing by locknuts or spring washers. However, under dynamic and vibration loads, these measures are not enough, therefore, during operation, the condition of the installation connections should be systematically monitored and the nuts on loose bolts should be tightened.

Connections with high-strength bolts are shear-resistant and with load-bearing bolts. In shear-resistant connections, bolts are not directly involved in the transmission of forces: all forces applied to the mating elements are perceived only due to the friction forces arising between the shear planes. In connection with load-bearing bolts, along with the friction forces between shear planes, the bolts themselves also participate in the transmission of forces, which makes it possible to increase bearing capacity of one bolt is 1.5-2 times compared to a bolt in shear-resistant connections.

The surfaces of the elements to be connected in these cases are treated as for conventional bolted connections. Before installing bolts, washers and nuts, remove preservative grease. To do this, they are dipped in a lattice container into boiling water, and then into a container with a mixture of 15% mineral oil and 85% unleaded gasoline.

When assembling and installing metal structures, special attention is paid to the tension of the elements being connected. There are several ways to determine bolt tension. On construction site A method is often used to indirectly estimate the tension forces through the torque that must be applied to the nut.

Torque M is determined from the expression: M = KR·a, where P - Bolt tension force, N; d - nominal bolt diameter, mm; K is the bolt torque coefficient.

The tension of the bolts is controlled selectively: when the number of bolts in a connection is up to 5 - all bolts, when 6-20 - at least 5 bolts and when more- at least 25% of bolts in the connection. If during inspection it is discovered that at least one bolt does not meet the established requirements, then all bolts are checked. The heads of the checked bolts are painted, and all connections are puttied along the contour.

Checking the condition of bolted connections

Bolted connections should be inspected by tapping the fastening points with a hammer. All bolted connections must be tightly secured with nuts and locknuts. The corners of the locking plates must be bent and secure the bolt nuts, preventing them from unscrewing. In case of loose fastening, secure them wrenches. Clean from dirt and ice and lubricate fastening units (bolts, hinge joints), working gate, control rulers, bolts, axles and “fingers”. For lubrication, used machine or transformer oil, CIATIM-201 (CIATIM-202), CIATIM-221 or ZhTKZ-65 lubricant are used.

Checking the presence and condition of twists

The presence and condition of the screws is checked by visual inspection and tapping the fastening points with a plumber's hammer. The twists must be installed (according to the approved installation drawings) from galvanized wire with a diameter of 4 mm on the axes of the inter-tip, working, control rods, gate hinge, as well as the attachment points of the external contactor and the electric drive headset strips with an external contactor and 3 mm on the mounting strip of the control rods.

If the twist is broken or does not correspond to the installation drawing, it is replaced with a new one. Operation of fastening units without screws is not allowed.

6.2.16.1 The tightening of bolted connections of the nodal linings of aluminum dome roofs is monitored when dismantling the cards to control beams and support crowns (Table 6.4, lines 12 and 27 and Table 6.5, line 20). Additionally, the tightening of bolted connections in four node linings is monitored according to the diagram shown in Figure 6.18.

Figure 6.18 – Diagram of places for dismantling the hub caps (view dome roof above)

6.2.16.2 Before checking the tightening, the protective caps must be dismantled and a visual inspection of the bolted connection must be carried out. The surface of bolts, nuts and washers should be free of cracks, scale, rust, burrs, dents and nicks on the threads. The bolts must be marked indicating the tensile strength, symbol heat number, the manufacturer's mark is affixed, the marking of bolts of the climatic version HL (according to GOST 15150) must contain the designation “HL”.

6.2.16.3 The tightening of bolted connections is checked by measuring the tightening torque with a torque wrench and a feeler gauge. The number of controlled bolted connections in a unit must be at least:

If the number of bolts in a connection is up to four – all bolts;

From five to nine - at least three bolts;

From 10 or more - 10% of bolts, but not less than three in each connection.

If one bolted connection is detected with non-standard tightening
(subclause 6.2.16.6), double the number of bolted connections is subject to control. If one bolt with abnormal tightening is detected during re-inspection, all bolts in all inspected units must be checked, bringing the tightening torque of each to the required value.

6.2.16.4 To carry out tightening control threaded connections with controlled tightening torque of high-strength bolts of the upper node linings, scale and torque wrenches are used limiting types and probes that meet the requirements given in table 6.10.

Table 6.10 – Requirements for means of monitoring bolted connections

Torque wrenches for monitoring the tightening of high-strength bolts must be calibrated at least once per shift in the absence of mechanical damage, as well as after each replacement of the control measuring instrument or repair of the wrench, in accordance with SNiP 3.03.01-87 (clause 4.27).

6.2.16.5 Before inspecting a bolted connection, it is necessary to set the tightening torque set in project documentation, upon reaching which a click will occur. In the absence of data established in the design documentation, the torque M, Nm, is determined by the formula:

M = K∙P∙d, (6.11)

where K is the average value of the torque coefficient established for each batch of bolts in the manufacturer’s certificate or determined at the installation site using control measuring instruments. For bolts according to GOST R 52644 K = 0.18;

P – design bolt tension specified in the working drawings, N (kgf). In the absence of design data, the calculated bolt tension is determined according to SNiP 2.03.06-85, 8.10 using the formula:

Р = Rbh×Abn, (6.12)

where R bh is the calculated tensile strength of a high-strength bolt, determined by the formula:

R bh = 0.7∙R bun , (6.13)

where R bun is the minimum tensile strength of the bolt, taken according to
SNiP II-23-81* (Table 6.1) and given in Table 6.12.

A bn – cross-sectional area of ​​the bolt, accepted in accordance with GOST 9150, GOST 8724 and
GOST 24705, adopted from the values ​​given in SNiP II-23-81* (see table 6.2) and are shown in table 6.11.

Table 6.11 – Value of the minimum tensile strength of a bolt

Table 6.12 - Sectional areas of bolts

6.2.16.6 The criterion for compliance with the tightening of a bolted connection is the absence of rotation of the nut or bolt.

6.2.16.7 Tie tightness of the top knot lining and aluminum profile, at the joints, should be checked with a probe 0.3 mm thick, which should not pass between the assembled parts to a depth of more than 20 mm according to (SNiP 3.03.01-87). A diagram for checking the junction of the upper assembly lining and the aluminum profile with a probe is shown in Figure 6.19.

1 – junction of the upper nodal lining and the aluminum profile

Figure 6.19 – Scheme for checking with a feeler gauge (this place is indicated by the number 1) at the junction of the upper assembly lining and the aluminum profile