What Is an Overhead Power Line?

Overhead lines mainly refer to overhead open lines, which are erected above the ground, and are transmission lines that use insulators to fix power transmission lines on pole towers standing on the ground to transmit electrical energy. Installation and maintenance are relatively convenient and the cost is low, but they are easily affected by weather and environment (such as wind, lightning, pollution, snow, etc.) and cause failures. At the same time, the entire transmission corridor occupies a large area of land, which is likely to cause electromagnetic interference to the surrounding environment. .

Overhead lines should widely use steel core aluminum stranded wire or aluminum stranded wire. The cross-section of the aluminum stranded wire of the high-voltage overhead line shall not be less than 50 square millimeters, the cross-section of the core aluminum stranded wire shall not be less than 35 square millimeters, and the cross-section of the empty wire shall not be less than 16 square millimeters.

The cross section of the conductor should meet the needs at the maximum load.

The selection of the cross section should also meet the voltage loss not more than 5% of the rated voltage (high-voltage overhead line), or 2% to 3% (lighting lines with high visual requirements), and should meet certain mechanical strength. ^[1]

Relevant regulations must be followed during construction and completion acceptance to ensure construction quality and line safety.

Reasonably choose the path, it is required that the path is short, the corners are small, and the transportation is convenient, and a certain safe distance from the building should be maintained.

According to the requirements of relevant regulations, the overhead lines must be within a safe distance from the ground and other facilities. ^[2]

Overhead line ground fault

1.Ground fault phenomenon

The grounding of the line can be divided into: single-phase grounding, two-phase grounding and three-phase grounding. There are two types of ground faults: permanent ground and transient ground. The former is usually the breakdown of the insulation wire, and the latter is usually the lightning flashover and the foreign matter on the wire. The most common of these is the single-phase grounding of overhead lines.

2. Judgment of ground fault

By detecting the voltage of the line and identifying the ground fault.

3.Finding the ground line

The method of determining the ground line is generally to try to pull each line. Follow these steps to handle single-phase ground faults:

(1) Determine whether a single-phase grounding has actually occurred;

(2) Identify which phase is grounded;

(3) Find which line is grounded;

During operation, determine the order of opening the line according to the actual conditions of the load, the length of the line, the length of the line, or the failure rate of the line. If a line is opened, the ground signal disappears, indicating that the ground is on the line.

4.Find the ground point

For the short-term overhead transmission line to find the grounding point, personnel can be arranged to conduct a comprehensive inspection along the line, but for the long-term overhead transmission line to find the grounding point, the preferred method should be used. First, segment the tension rods at 1/2 of the length of the line, disassemble the three-phase drain wires of the line to divide the entire line into two sections, and then use a 2500V megohmmeter to measure the insulation resistance of the three-phase wires. According to the measurement results, it can be determined that one section of the line is grounded or both sections are grounded. Secondly, the search is continued based on the judgment results, and the search scope is gradually narrowed. When the grounding range is reduced to a certain extent, personnel can be arranged to conduct a comprehensive inspection along the line. This saves time, reduces labor, and improves work efficiency.

For high-voltage transmission lines, in addition to line protection devices, substations will also be equipped with fault recorders, traveling wave range finders, and some are also equipped with wavelet range finders. The wavelet ranging device can accurately determine the location of the ground point, and the accuracy can reach 5km, and the accuracy can generally reach 1 ~ 2km. Very useful for troubleshooting.

5, matters needing attention

Finally, it is worth mentioning: Why must the three-phase drain wire of the line be disconnected when measuring the insulation resistance of the line in sections, and then measure the insulation resistance of the three-phase wires separately?

The reasons are:

(1) Some lines are long, and the wires are transposed on the way. When the phases A, B, and C are not marked, the insulation resistance of the faulty phase can be prevented from being missed and misjudgment caused.

(2) It is considered that when the single phase is not completely grounded, the phase with the lowest voltage to ground must be the grounded phase, so only the insulation resistance of one phase is measured. In fact, the faulted phase may be missed and prone to errors;

(3) The line may be grounded at multiple points. Therefore, when an overhead line is grounded, it must be carefully tested and judged accurately, and must not be sloppy during work.

Overhead line overhead wire failure

(1) In the distribution line, because the lines are horizontally arranged and the distance between the lines is small, if the conductors in the same range have different arc sags, the conductors will sway differently during high winds, causing the conductors to collide with each other and cause phases Short circuit, so we must strictly check during construction, pay attention to the tension of the conductors, make the three-phase conductors have equal slack, and be within the specified standard range. When the line is inspected, the above problems should be found and arrangements should be made in time.

(2) The wind blows off the branches and falls on the line, or throws metal objects on the guide line, which will also cause short-circuits between the wires and even break the wires. In addition, ultra-high cars may cause short circuit or disconnection when working under the line or under the line with cranes. Therefore, there should be a certain separation distance on the crossing and crossing lines.

(3) The wires are damaged due to long-term erosion and oxidation by moisture, atmosphere and harmful gases. Steel wires and lightning conductors are most susceptible to corrosion. When the wires are found to be severely corroded during inspections, they should be replaced in a timely manner. ^[3]

Overhead line insulator failure

(1) Porcelain insulators on the line are contaminated by harmful components in the air, and in humid weather, the contaminated layer absorbs moisture and enhances the conductivity, which not only increases power loss, but also easily causes flashover. accident.

(2) An unqualified porcelain insulator is mistakenly installed on the line or due to aging of the insulator, a flashover breakdown occurs under the power frequency voltage. In this regard, the porcelain insulators with flashover traces found during inspections should be replaced in a timely manner, and the new porcelain insulators that have been replaced must undergo a withstand voltage test.

(3) The porcelain insulation part is damaged by external force, and cracks or breakage occur. When a large porcelain skirt is broken or there are cracks from the edge to the top, it should be replaced, otherwise the insulation will be lowered and a flashover accident will occur. ^[3]

Malfunction of overhead poles and fittings

(1) Due to the influence of soil and moisture, the wood poles will decay and often cause down pole accidents. Therefore, when wood poles are used, the roots of the wood poles should have anti-corrosion measures, such as asphalt coating or tied piles.

(2) An accident caused by an external force collision of a cement rod, such as a car or tractor collision.

(3) The wire is not uniformly stressed, which makes the tower tilt. At this time, the cable of the electric pole should be tightened or the line should be adjusted.

(4) In the place where the wire is vibrating, the metal fitting screws are liable to fall off by accident due to vibration. Therefore, during the inspection and cleaning, carefully check whether the parts of the metal fitting are in good contact. ^[3]