RU2778138C1 - Method for protective grounding of an overhead power line under voltage - Google Patents

Abstract

FIELD: electric power industry.

SUBSTANCE: invention relates to the field of electric power industry. According to the method on an overhead line that is energized, only the supports of the overhead line are subject to grounding. The value of the maximum touch voltage on the overhead line supports and the potential equalization zone during the work are determined. Within the potential equalization zone, metal sheets, grids are placed on the ground, portable grounding devices are installed, and they are electrically connected to each other and connected to artificial and natural grounding conductors.

EFFECT: ensuring reliable protection of repair personnel from electric shock when working on overhead power lines (OL) under voltage.

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Description

The invention relates to the electric power industry and can be used to ensure safety measures during the production of work on overhead power lines (VL), which are under voltage.

Rules for labor protection during the operation of electrical installations [Rules for labor protection during the operation of electrical installations. Registered with the Ministry of Justice of Russia on December 30, 2020, No. 61957 and published on January 1, 2021] defines security measures during work on overhead power lines.

A known method of compensating the touch voltage at the place of work on the overhead power line brought out for repair [RF Patent No. 2752875 IPC H02H 05/10, publ. 08/11/2021, Bull. No. 23], in which the overhead power line is grounded at the ends in the switchgears of substations, at the work site the phase wires of the overhead power line brought out for repair are connected to the ground loop of the support with the main ground, additional grounding is installed, separated from the ground loop of the support. According to the method, a potential equalization zone is determined during repair work, covering workplaces where it is necessary to ensure the protection of repair personnel, as well as places where equipment is concentrated, within the potential equalization zone, safe repair work on an overhead power line is ensured by equalizing the potential of the equalization zone to the potential of grounding and ground loop of the support, use potential equalization means for this, for example, metal meshes or sheets, connect the potential equalization means to each other and connect to the additional grounding and ground loop of the support.

The disadvantage of this method is the impossibility of its use when performing repair work on an overhead line that is energized.

The closest technical solution is a method of protective grounding of an overhead power line [RF Patent No. 2747778 IPC H02G 01/02, publ. 05/14/2021 Bull. No. 14], including the grounding of an overhead power line electrically connected by portable groundings to its own grounding loop of the support or an artificial grounding conductor mechanically immersed in the ground to a depth of at least one meter. According to the method, the overhead transmission line is additionally grounded by electrically connecting it with the help of grounding wires to existing natural grounding conductors.

The disadvantage of this method is the impossibility of its use when performing repair work on an overhead line that is energized.

During operation on the overhead line, work is provided that can be performed without removing the voltage. For example, on metal supports of overhead lines, the following types of work are performed:

conducting a riding inspection, checking and restoring welded and bolted joints;

straightening deformed corners of lattice supports;

painting elements of metal supports;

checking and restoring the fastening of the support, coating the threaded joints with anti-corrosion grease, measuring the transient resistance at the place of the bolted connection of the grounding conductor with the body of the support;

cleaning the surface of the foundation from earth and grass;

excavation of the foundation for inspection of foundations and grounding device, assessment of mechanical damage to foundations, cracks, violations of the waterproofing layer, corrosion and weathering of the surface layer of concrete;

repair of damaged foundations;

restoration (strengthening) of the grounding device of supports;

restoration of the deboning of the foundations of the supports;

other types of work.

When carrying out these works, no technical measures are provided, the repair personnel can touch the metal support and be exposed to high touch voltage. The reality of such a danger is associated with the possibility of single-phase (asymmetric) short circuits in 110-220 kV networks during repair work on overhead lines. Single-phase short circuits have a rather high probability of occurrence, and in the presence of an equipotential connection of workplaces on overhead lines with local and extended elements of electrical complexes, these elements may be under high voltage. According to statistics, about 70% of the total number of short circuits are single-phase.

The time of existence of such a danger is very short (determined by the duration of the existence of a short circuit), and the place and the root cause of the short circuit may not be recognizable due to the remoteness from the place of work on the overhead line and not show any signs of existence, except for the creation of an invisible, but dangerous high touch voltage .

Despite the high probability of these events, the above cases were not officially recorded and were not considered by commissions investigating the causes of accidents on overhead lines. They also did not appear in accident reviews, but they may appear in them under other definitions, for example, as "heart failure". This is possible, since a member of the team who was next to the person who received an electrical injury cannot accurately determine whether he was subjected to short-term exposure to high touch voltage or not, since the high voltage source was far from the workplace on the overhead line.

When a worker loses consciousness, the primary resuscitation procedures undertaken by the team members may be ineffective, and the arriving medical worker can only ascertain death from heart failure. The reason for this is that short-term exposure to high voltage can cause the heart to fibrillate and stop, which outwardly manifests itself in the same way as heart failure.

It should be noted that the regulatory documents do not consider the possibility of removing a hazardous potential to the workplace on overhead lines in the event of an accident.

In essence, the remote potential is a change in the potential of the wire or other electrically conductive parts of the elements of the overhead line, leading to an unacceptable value of the contact voltage at the workplace on the overhead line. In case of single-phase short circuits on a 110 kV overhead line, the source of the remote potential is due to the presence of an equipotential connection between each overhead line support along an uninsulated lightning protection cable. This connection contributes to the distribution of the potential of the emergency tower, where the short circuit current and the voltage drop across the resistance of the "lightning wire-support-grounding device" system, to all other towers.

As an example, consider a single-phase short circuit on an energized 110 kV overhead line in case of damage or a decrease in the insulation level (surge) of a suspension string of insulators (figure 1). At present, the task of calculating the current of a single-phase short circuit for overhead lines of 110 kV and above has been practically solved in the framework of ensuring the functioning of relay protection and automation.

The magnitude of the short-circuit current depends primarily on the concentrated power in the considered section of the network, is different for different points on the same overhead line, and changes when the considered point moves along the overhead line. For example, for a double-circuit overhead line 110 kV "CHPP GA3-2 - Substation Zarechnaya" (Nizhny Novgorod energy system), 15.61 km long, calculated using the program "TK3-3000" ("ESP Novosibirsk"), the distribution of single-phase short circuit currents along the overhead line is given in table 1.

Table 1. Distribution of the current value of a single-phase short circuit when moving a point along an overhead line with two-way power supply.

To analyze the mechanism of occurrence and determine the magnitude of the remote potential on the overhead line, you can use the calculated values of the short-circuit current at an arbitrarily chosen point of the overhead line and a more simplified design scheme. It is advisable to focus on the analysis of the short circuit current spreading paths along the lightning protection cable and the grounding devices of the towers. The calculation scheme in this case will correspond to Fig.2.

The equivalent resistance of the system of lightning protection cable and supports (Z _equiv ) at the point of short circuit is determined by the following relation (1).

where R ₀ is the resistance of the grounding device of the supports (Ohm); Z _T - impedance modulus of the ground wire in span (Ohm), which is determined from the expression:

where l - span length (km); ρ _T - specific active resistance of the ground wire (Ohm/km); _Dz - equivalent depth of zero sequence current return through the earth, ~ 1000 m; r - radius of the ground wire (m); d _i - distance from the i-th wire of the overhead line to the ground wire (m); Z ₁ , Z ₂ - input resistances of a homogeneous chain, consisting of n links, formed by the resistances of the grounding devices of the supports and the ground wire, determined by the expression:

where Z ₀ is the resistance to which the homogeneous chain is closed (Ohm); n is the number of spans of a homogeneous section of overhead lines; th(nk) - hyperbolic tangent; Z _c , k - parameters of the overhead line, the values of which are determined as:

The equivalent resistance of such a chain depends on the number of links forming it and on the location of the short circuit on the overhead line, that is, on the x-coordinate of the considered point on the overhead line.

Change in the values of the equivalent resistance of the lightning protection cable system and supports for the previously mentioned double-circuit overhead line with the following given parameters: C-50 type lightning protection cable, number of spans n=78, span length l=0.2 km, obtained by calculations using formula (1), taking into account the values (2) and (3), are reflected in table 2. and in Fig.3.

Table 2. Calculated values of the remote potential on the supports of overhead lines.

As shown in figure 3, when moving along the 110 kV overhead line from the GAZ CHPP to the Zarechnaya substation, from support No. 1 to support No. 78, the resulting resistance of the "Lightning protection cable-support-grounding device" system first increases to a certain value, depending on the resistance grounding of each support and the length of the span (lightning wire), then stabilizes at some level. For example, if the resistance of the grounding devices of the supports is 5-6 ohms (as shown by the results of the measurements) and the length of each span is 200 m, the resulting resistance value is in the range from 0.42 to 1.5 ohms. At the same time, the currents in the lightning protection cable from both sides, when moving from the GAZ CHPP to the Zarechnaya substation, first decrease from 22 kA to 15 kA, then, as they approach the Zarechnaya substation, they increase to 42 kA. The value of the removal of the potential U _vp while changing, as shown in the graph (figure 3): in the area of CHP GAS increases to 30 kV, then decreases to 22 kV, then, as you approach the substation Zarechnaya, increases again to 45 kV. This is the source of the dangerous removal of the potential, which creates an unacceptable contact voltage at the work site on the overhead line.

The voltage class of 110 kV overhead lines has a number of differences in comparison with higher voltage overhead lines. Firstly, the level of insulation and the length of the string of insulators, and, consequently, the direct distance from the wire to the grounded structure of the support through the air on a 110 kV overhead line is less than on a higher voltage overhead line. Secondly, the security zone and the width of the glades on the 110 kV overhead line are smaller. Thirdly, 110 kV overhead lines, as a rule, have a denser configuration and are located within or near settlements. For these reasons, natural, man-made and other extraneous influences lead to the failure of 110 kV overhead lines much more often and the isolation distance is more often blocked by all kinds of conductive surges (especially in spring), by various mechanisms during the construction of facilities in the protected zone, as well as by birds during the nesting period. For example, a surge about 1 meter long can lead to overlapping of the garland on a 110 kV overhead line, but practically cannot on an overhead line of 220 kV and above.

Thus, the non-insulated fastening of a lightning protection cable on a 110 kV overhead line and the potential carry-over values that are not provided for are unacceptable from the point of view of ensuring the electrical safety of maintenance personnel during the operation of energized overhead lines.

Grounding means the intentional connection of conductive parts of electrical installations to a grounding device with a special wire or device. The main function of grounding, from the point of view of electrical safety, is to reduce the time of action of the emerging dangerous voltage by initiating the operation of relay protection to turn off the emergency section of the electrical installation with a switch or other switching device.

However, this is only effective when the dangerous voltage is applied through the switching device associated with the high-speed protection relay. If the switching device does not have or does not work with high-speed relay protection, or a dangerous voltage at the workplace arises directly (for example, remote potential, atmospheric overvoltage, etc.), then this grounding function is not feasible. In this case, to ensure the electrical safety of the maintenance personnel, grounding should reduce the value of the contact voltage to the maximum allowable value.

The concept of "grounding" in regulatory documents [for example, Rules for labor protection during the operation of electrical installations. Series 17. Issue 53. - M .: Closed Joint Stock Company "Scientific and Technical Center for Research on Industrial Safety Problems", 2013. - 192 p.; Intersectoral rules on labor protection (Safety rules) for the operation of electrical installations, POT RM -016-2001, RD 153-34.0-03.150-00. - M.: "Izdatelstvo NTs ENAS", 2003. - P.98.] is given not only as a universal, but also a comprehensive technical measure to ensure electrical safety, performed during the installation of equipment and in the preparation of a workplace in any existing electrical installation. The regulations also consider the procedure and places for installing these groundings in electrical installations, the requirements for the personnel who install them, as well as the limits of the workplace to which these groundings apply. In fact, this event consists in connecting de-energized conductive parts of an electrical installation to grounding devices using special grounding conductors.

On the overhead line there are grounding devices of individual supports, in which the potential equalization zone is limited by the area limited by the ground loop of a particular support. The possible boundaries of the workplace almost always go beyond the potential equalization zone of the grounding device of the support.

Since the grounding device of the support in this case cannot provide simultaneous equalization and equalization of potentials, it cannot provide the system of labor safety standards (SSBT) established in [GOST 12.1.038-82]. Electrical safety. Maximum permissible values of contact voltages and currents (with Change No. 1). The system of labor safety standards: Sat. GOSTs. - M.: IPK Standards Publishing House, 2001. - 9 s] the value of the maximum allowable contact voltage.

This is due to the fact that the resistance of the grounding device of the overhead line support is normalized according to the criteria of lightning protection and the operation of relay protection and automation. Therefore, in emergency modes, when a large short-circuit current flows through the grounding device of the support, the potential of this support can significantly exceed the value of the maximum allowable contact voltage.

The objective of the invention is to create a method of protective grounding of overhead power lines, which allows you to reliably protect workers from electric shock when working on energized lines.

The task is achieved by the method of protective grounding of an overhead power line (VL) under voltage, including grounding of the overhead line, portable ground electrodes are electrically connected to artificial ground electrodes mechanically immersed in the ground, grounding of the overhead line is additionally electrically connected using ground wires to existing natural ground electrodes. According to the method, on an overhead line that is energized, only the overhead line supports are subject to grounding, based on the possible short-circuit currents and parameters of the overhead line, the value of the maximum touch voltage on the overhead line supports and the potential equalization zone during work are determined, taking into account the maximum touch voltage within the potential equalization zone metal sheets, grids are placed on the ground, portable groundings are installed, and they are electrically connected to each other and connected to artificial and natural grounding conductors.

Fig. 1 illustrates the flow of currents of a single-phase short circuit with the removal of the potential on the grounded structures of the overhead line support.

In Fig.2. the design scheme for determining the equivalent resistance of the system is shown: a lightning protection cable and supports in case of a single-phase short circuit.

In Fig.3. distribution graphs along the line of values are presented: 1 - short-circuit current (kA); 2 - the resulting resistance of the "lightning wire-support-grounding device" system (Ohm); 3 - potential levels on the grounded elements of the supports of the 110 kV overhead line "CHPP GAZ - Substation Zarechnaya" with a single-phase short circuit at each considered point (support).

In Fig.4. the support and grounding elements of the overhead line are shown, designed to ensure the safety of work on an overhead line that is under voltage.

In Fig.4. the following designations have been introduced: 1 - support of an overhead line under voltage; 2 - interconnected metal meshes, sheets; 3 - portable grounding; 4 - stationary ground loop (artificial ground electrode); 5 - natural ground electrode; 6 - potential equalization zone, in which safe work is ensured.

The method of protective grounding of an overhead power line that is energized is implemented as follows.

Previously, before performing work on an overhead line that is under voltage, the possible short-circuit currents are calculated and, taking into account the parameters of the overhead line, the value of the maximum touch voltage on the overhead line supports is determined. Based on the short-circuit currents, the maximum contact voltage, the necessary measures to ensure the safety of personnel during work, as well as the potential equalization zone, are determined.

Before starting work on the overhead line, the potential equalization zone 6 around the overhead line support 1 is fenced with signal tapes, and the equipotentiality of the surface of the potential equalization zone 6 is ensured by placing metal grids 2, sheets 2 and other potential equalization means 2 on the ground. If necessary, install additional portable grounding 3. Metal grids 2, sheets 2, portable grounding 3, stationary grounding loops (artificial grounding conductors) 4, natural grounding conductors 5 and the ground loop of the support 1 VL are interconnected.

Thus, within the fenced-off potential equalization zone 6, the safety of maintenance personnel is ensured when performing work on an energized overhead line.

It should be noted that the prototype method is designed to ensure the safety of work on an overhead line that is being repaired, and cannot ensure the safety of a person on an overhead line that is energized.

Claims (1)

The method of protective grounding of an overhead power line (VL) under voltage, including grounding of the overhead line, portable ground electrodes are electrically connected to artificial ground electrodes mechanically immersed in the ground, grounding of the overhead line is additionally electrically connected using ground wires to existing natural ground electrodes, characterized in that on an overhead line that is energized, only the overhead line supports are subject to grounding, based on the possible short circuit currents and the parameters of the overhead line, the maximum touch voltage on the overhead line supports and the potential equalization zone during work are determined, taking into account the maximum touch voltage within the potential equalization zone, they are placed on ground, metal sheets, grids, install portable grounding, as well as electrically connect them to each other and connect to artificial and natural grounding conductors.

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