On transmission lines, ready-made towers can be used to install communication lines. ADSS optical cables can provide fast and economical transmission in power systems due to their unique structure, good insulation, high-temperature resistance, and high tensile strength. Transmission channels are favored by power communications. In recent years, there have been many accidents in China in which ADSS optical cables were burned out due to electrical corrosion after being used for several years, which directly affected the normal operation of communication lines and even caused line tripping. At present, there are many reasons for the electrical corrosion of ADSS optical cables, such as breakdown, tracking, corrosion, etc. This article briefly analyzes the causes of electrical corrosion of ADSS optical cables and proposes solutions, hoping to help prevent such accidents and solve existing problems.
1. Principle of electric corrosion
The direct causes of electrical corrosion of ADSS optical cables are mainly dry strip arc and corona discharge. Since the ADSS optical cable is located near high-voltage conductors or towers, the spatial electric field around the conductors places the ADSS optical cable at a spatial potential. When the ADSS optical cable is in a high electric field and dirty environment, a resistance layer is formed on the surface of the ADSS optical cable. Under the action of the space potential, a current (called ground leakage current) is generated between the surface of the ADSS optical cable and the grounding hardware, and the heat generated by the current causes The water evaporates, and the water evaporates to a certain extent, forming a small dry zone on the surface of the ADSS optical cable, blocking the current. As a result, potential accumulation occurs at both ends of the dry zone. When the induced voltage is high enough to penetrate the air, a discharge occurs to form an arc. This is often referred to as a dry zone arc. The arc discharges and the current is generated again. Repeatedly, the dry-band arc forms dendritic carbonization channels, that is, electric traces, on the ADSS optical cable sheath, which can cause damage to the polymer. The heat generated by the dry-band arc can cause the cross-linked polymer to The bonding force is gradually lost and corrosion occurs. If the heat of the arc is sufficient, the sheath will melt into holes, a process called galvanic corrosion or tracking. Electrical corrosion first damages the optical cable sheath, then corrodes the aramid yarn, and finally causes the optical cable to break.
2. Factors affecting electrical corrosion
2.1 Electric corrosion caused by dry strip arc
In the 110-220kv high-voltage power grid, optical cables run in high electric fields and polluted environments for a long time. The surface of the optical cable has a high potential, and its surface is affected by the polluted environment to form a semi-conductive layer. In this way, the ADSS optical cable and the grounded hardware generate a certain leakage current, and the heat generated by the current flows from the middle of each optical cable to both ends, causing the water on the surface to evaporate to form a local dry area. When this potential accumulates to a high enough level, the ADSS optical cable discharges and forms Arc, as the number of discharges increases, the outer sheath of the optical cable will crack or even break. It can be seen that the main culprit of electrical corrosion in ADSS optical cables is this dry band arc.
2.2 Electric corrosion caused by corona discharge
Because ADSS optical cables are made of non-metallic materials and have small wire diameters, although anti-shock whips are added 15-20cm outside the pre-twisted wires during construction, the breeze vibration frequency and vibration amplitude are higher than those of metal wires with larger diameters. a lot of. Therefore, the contact surface between the end of the pre-twisted wire and the end of the anti-vibration whip or anti-vibration hammer is small, resulting in uneven distribution of the electric field; secondly, the induced voltage changes much faster along the direction of the optical cable than elsewhere; when the field strength reaches a certain height, the pre-twisted wire The end and the end of the anti-shock whip become discharge electrodes to generate corona discharge. Electrical corrosion caused by corona discharge will cause the outer sheath of the optical cable to crack, the internal aramid yarn to carbonize and deteriorate, and the tensile strength of the optical cable to decrease. Eventually causing cable breakage.
2.3 Design and construction of optical cables
The design location of hanging points
By calculating the field strength, choose a place with a smaller field strength to set up the hanging point. At present, field strength calculation software is basically used, which is a static mathematical model. Calculate the field strength under several extreme conditions and select appropriate optical cable hanging points. These softwares do not consider the tower for calculation. In fact, the tower, as a ground electrode, will have a great impact on the calculation results, so it should also be combined with the on-site conditions. Actual situations should be handled with caution.
Construction
Compared with other factors, the impact of electrical corrosion caused by the construction of optical cables is smaller, but it still exists. During the construction process, there are three main reasons that may cause damage or wear to the outer sheath of the optical cable:
- Failure to use tension to pay off the cable in accordance with the construction specifications for power optical fiber communication projects, causing the optical cable to be scratched by stones or branches on the ground;
- When the cable is laid out , The pulley does not match the outer diameter of the optical cable;
- The hardware scratches the outer sheath of the optical cable. Once the outer sheath of the optical cable is scratched, electrical channels will most likely appear over time and electrical corrosion will occur.
3. Preventive measures for electrical corrosion of ADSS optical fiber cables
There are many reasons for the formation of electrical corrosion. At present, no optical cable manufacturer or designer can completely eliminate the occurrence of electrical corrosion. Through years of practice and operation experience of manufacturers, designers, construction personnel and users, and some domestic optical cables due to electrical Analysis of accidents caused by corrosion and cable breakage are summarized as follows for reference.
(1) Selection of outer sheath material of ADSS optical fiber cable
When the space potential of the ADSS optical cable is less than or equal to 12kV, high-density polyethylene material (HDPE) can be used; when the space potential of the ADSS optical cable is greater than 12kV, special anti-tracking sheath materials should be used. The type of outer sheath material used for the specific ADSS optical cable should be determined according to the specific conditions of the site.
(2) Selection of ADSS optical fiber cable suspension point location
ADSS optical cables should be suspended in spaces with zero space potential or equal field strength. Due to the uneven distribution of space potential and field strength in the 110-220kv high-voltage power grid, reliability requirements for the ground and across objects must also be met. At the same time, Affected by environment and temperature changes, etc. The hanging position of the specific hanging point on site should be determined according to the specific conditions of the site to meet the above needs.
(3) Prevent wear and tear on the surface of the optical cable, the installation position of the anti-vibration whip, and the protection of hardware during optical cable construction
During the construction process of optical cables, it is easy to cause damage or wear to the outer sheath of the optical cable. After the outer sheath of the optical cable is worn, scratched, and cracked, dry-strip arcs are generated, and the electrical corrosion of the optical cable is aggravated, which in turn shortens the service life of the optical cable. Therefore, although the damage or wear of the outer sheath of the optical cable caused by the ADSS optical fiber cable construction is relatively minor, it is necessary to take remedial measures and have the construction party carry out professional protective treatment on the scratched parts. For example: after wrapping special insulating tape inside and outside the pre-twisted wire port, apply anti-electrical corrosion paint on the outside of the insulating tape.
Studies have found that if the distance between the anti-vibration whip and the end of the hardware is far enough to a certain distance, the arc phenomenon can be appropriately reduced or even eliminated. However, during the construction process and users did not pay enough attention to the distance between the anti-vibration whip and the hardware, which was quite random and should be paid attention to. It is recommended that the distance between the anti-vibration whip and the optical cable hardware should be greater than or equal to 50cm. If conditions permit, you may wish to use an anti-vibration hammer.
Spray "water-increasing agent", mica mist powder base, or ceramic powder base on ADSS optical fiber cables and hardware to prevent the generation of "dry strip arc".
(4) Install a corona suppression ring
Because the two ends of the hardware and the spiral square vibrator are not smooth, the electric field intensity between them is uneven, which in turn causes corona discharge. Installing a corona suppression ring at the end of the hardware can basically eliminate end discharge. This not only protects the outer sheath of the ADSS fiber cable, but also extends its operating life on overhead lines.
Summary of optical cable manufacturers:
The phenomenon of cable breakage caused by ADSS cables is common, and many factors are caused by the electrical corrosion of ADSS optical cables. The factors analyzed above are summarized as follows: Which type of optical cable outer sheath should be considered according to the space potential of the optical cable; the position of the optical cable hanging point has an impact on electrical corrosion. Whether ADSS Cable can be erected on this line, in addition to scientific calculations, actual on-site inspection and survey are also required; during the construction process, tension is used by the power optical fiber communication engineering construction specifications to avoid damage and scratches to the outer sheath of the optical cable as much as possible, and anti-vibration whips are required. Leave a suitable distance between it and the hardware.