【Summary description】 The robustness and optimization of power communication system is one of the essential conditions for realizing smart grid. As the foundation of power communication system, optical fiber transmission network has achieved rapid development. In order to meet the special needs of power system communication, special power optical cable emerges at the historic moment. After nearly 30 years of development, special power optical cable is not only increasingly mature but also promising.

The robustness and optimization of power communication system is one of the essential conditions for realizing smart grid. As the foundation of power communication system, optical fiber transmission network has achieved rapid development. In order to meet the special needs of power system communication, special power optical cable emerges at the historic moment. After nearly 30 years of development, special power optical cable is not only increasingly mature but also promising.
1. Characteristics of various power special optical cables
Special power optical cable is set up through the unique line tower resources of the power system, attached to the power line and the photoelectric composite optical cable attached to the power tower, make full use of the overhead power line corridor. Special power cable is less likely to be damaged by external force, high reliability, although its own cost is relatively high, but the construction cost is low. Relying on the power system's own line resources, special optical cable avoids conflicts and entanglements with the outside world in terms of frequency resources, routing coordination, electromagnetic compatibility, etc., and has great initiative and flexibility.
According to the way of laying and defending, special power optical cables can be divided into optical fiber composite overhead ground wire OPGW, optical fiber composite overhead phase wire OPPC, all-metal self-supporting optical cable MASS, all-medium self-supporting optical cable ADSS, ground cable binding optical cable ADL and ground cable winding optical cable GWWOP, etc.
2. Comparison between OPGW and ADSS
Here, OPGW optical cable and ADSS optical cable are analyzed and compared in terms of characteristics, applicable characteristics, common faults and solutions.
A. Feature difference
The outstanding feature of OPGW is to combine the communication optical cable and the overhead ground wire on the high voltage transmission line into a whole, the optical cable technology and the transmission line technology integration, become a multifunctional overhead ground wire, not only lightning rod, overhead optical cable, but also shielding line, in the completion of the construction of the high voltage transmission line, but also completed the construction of the communication line. Very suitable for new transmission lines. There are three main types of common OPGW structure, which are aluminum tube type, aluminum skeleton type and (stainless) steel tube type.
The all-medium self-supporting optical cable ADSS adopts the high-strength aramid yarn with high elastic modulus as the tensile component in its manufacturing. At the same time, the geometric size of the optical cable is small, and the cable weight is only one third of that of the ordinary optical cable. It can be directly hung on the appropriate position of the power pole and tower, and the extra load on the pole and tower is small. Its outer sheath through neutral ionization impregnation treatment, so that the cable has a strong resistance to electric corrosion, can ensure the cable in the strong electric field life; Optical cable is made of non-metallic materials, good insulation performance, can avoid lightning strikes, power line failure, will not affect the normal operation of the optical cable; Using the existing power tower, can not power off construction, and the power line with the pole erection, can reduce the cost of the project. A variety of ADSS fiber optic cable structures can be summarized into the most important center tube type and layer stranded type 2.
B. Applicable characteristics
OPGW optical cable is mainly used in 500kV, 220kV, 110kV voltage grade lines, affected by power outage, safety and other factors, mostly in the application of new lines. The application characteristics of OPGW are as follows [4] : (1) the high voltage over 110kV line, the file distance is large (generally above 250M); (2) Easy to maintain, easy to solve the problem of line crossing, and its mechanical characteristics can meet the requirements of large line crossing; (3) The outer layer of OPGW is metal-armored, which has no effect on high voltage corrosion and degradation; (4)OPGW must be cut off during construction, and the loss of power outage is large. Therefore, OPGW should be used in the newly built high-voltage lines above 110kV; (5)OPGW performance indicators, the greater the short-circuit current, the more need to use good conductor armor, the corresponding reduction of tensile strength, and in the case of a certain tensile strength, to improve the capacity of short-circuit current, only to increase the metal cross-sectional area, resulting in the cable diameter and cable weight increase, so the line tower strength raised safety issues.
ADSS optical cable is widely used in 220kV, 110kV, 35kV voltage class transmission lines, especially in the built lines used more. It can meet the requirements of large span and sag of power transmission lines. The standard ADSS design is up to 144 cores. Its characteristics are as follows [4] : (1) the theoretical value of fiber tension in ADSS is zero; (2)ADSS cable is fully insulated structure, installation and line maintenance can be live operation, which can greatly reduce the loss of power outage; (3) The expansion rate of ADSS can remain unchanged in the range of large temperature differences, and it has stable optical characteristics at the limit temperature; (4) Electric corrosion-resistant ADSS cable can reduce the electric corrosion of high voltage induction field on the cable; (5) The ADSS cable is small in diameter and light in weight, which can reduce the influence of ice and wind on the cable, and its influence on the strength of the tower is also small; (6) The ADSS uses new materials and smooth shape design, giving it superior aerodynamic characteristics.
C. Common faults and solutions
OPGW is affected by short circuit fault, when the line fault short circuit current impact OPGW cable, stainless steel unit instantaneous high temperature, must improve the short circuit current capacity of the cable, in order to reduce the impact of short circuit fault on the cable. According to Q=I2t, the heat resistance of OPGW can also be improved by limiting the size and duration of the actual short circuit current.
In addition to short circuit fault, lightning is another factor that causes instantaneous high temperature of OPGW cable. Compared with the short-circuit fault, the instantaneous current intensity of lightning strike is larger, but the duration is very short, so the heat capacity of the temperature rise caused by lightning strike is smaller than that generated by short circuit. However, the short-circuit current acts on the entire metal section of OPGW cable, while the lightning current is only limited to a small section of one or several metal monofilms. The concentration of energy causes this small piece of wire to become hot enough to partially or completely melt it. This is the main reason why lightning strikes OPGW fiber optic cables are broken.
The main fault in the application of OPGW optical cable is the broken strand caused by lightning strike. The current solutions are as follows:
1) Develop the outer strand material with lightning resistance. Developed in Brazil in 2000, the outer strand is made of a high-grade galvanized steel wire and aluminum tubes that protect the fiber. High-grade galvanized steel requires more energy to melt under lightning strikes.
2) The outer strand wire should be aluminum clad steel wire as far as possible, and the thickness of aluminum clad steel wire should be thickened.
3) Try to increase the design air gap between the outer strand and the inner strand to avoid heat transfer.
4) With the same material, a larger diameter of the outer strand is adopted. After the material and structure of OPGW cable is determined, its lightning resistance characteristics are determined.
As ADSS cable is running near high voltage wires, there is a strong electric field around, it is very likely to occur electrical corrosion phenomenon caused by cable damage.
5) In different voltage levels of the line, the priority is to choose the electric mark coat, 110kV and above the line must choose the electric mark material.
6) For 110kV and above lines, anti-corona coils can be considered to effectively reduce the electric field on the surface of the fittings and optical cables and reduce leakage current.
7)220kV and above lines and strong tower hanging point field, shockproof hammer on the cable surface creepage corrosion is much safer than shockproof whip.
8) Control the space potential of cable suspension point, 110kV does not exceed 15kV, 220kV does not exceed 20kV.
Distributed optical fiber temperature sensors can be used to monitor electrical corrosion failures.
3. Special power cable with similar characteristics
Different special power cable has its own characteristics, meet different requirements of use, applied in different occasions. The following is a comparison of special optical cables with similar characteristics in practical application.
3.1 OPGW optical fiber composite overhead ground wire and OPPC optical fiber composite overhead phase line
In the power grid, some lines do not have overhead ground wire, but phase line is essential. In order to meet the requirements of optical fiber networking, similar to OPGW technology, adding optical fiber in the traditional phase structure with appropriate methods, it becomes optical fiber composite phase line (OPPC).
At present, OPGW and ADSS are widely used in 110kV to 500kV line backbone network for communication. For some difficult to choose OPGW, ADSS optical cable lines, OPPC is the supplement products of OPGW, ADSS optical cable.
3.2 All-media self-supporting optical cable ADSS and metal self-supporting optical cable MASS
It is considered that the MASS optical cable and the existing tower should be erected on the same pole as the ADSS optical cable. In order to reduce the extra load on the tower, the MASS optical cable should be small in structure and light in weight. Therefore, the MASS optical cable structure adopts the center tube type, that is, the stainless steel optical fiber unit is stranded outside a layer of galvanized steel wire or aluminum clad steel wire, usually from the cost consideration, mainly galvanized steel wire. Because MASS optical cable is of metal structure, the problem of electrical corrosion can be easily solved through good grounding treatment and selection of weak electric field installation points.
3.3 Non-metal Lash overhead Optical Cable (AD-Lash) and non-metal winding optical cable GWWOP
They use automatic bundling machine and winding machine will cable bundle and wind in the ground or phase line, the common advantages are: optical cable light weight, low cost, installation quickly. In the ground or 10kV/35kV phase line can be installed without power failure; The common disadvantages are: Due to the use of organic synthetic materials to do the outer sheathing, so can not withstand the line short circuit phase line or ground wire generated high temperature, there are external sheathing material aging problem, construction need special machinery, in the construction of operational, safety and other problems more, and it is vulnerable to external damage, such as bird damage, shooting, etc., Therefore, it has not been widely used in power system. However, this kind of technology has not been eliminated or abandoned in the world, and is still used to a considerable extent.
4. Conclusion
Special power cable has high reliability, long life, easy installation, low overall cost, good security and other outstanding advantages, in the power system communication has been more and more applications, technology is also in continuous progress, materials are increasingly updated. At the same time, the demand for special fiber optic cables also tends to be diversified and high standards, how to improve the characteristics of special power fiber optic cables from the structural design, material selection and other aspects has profound significance, the development of special power fiber optic cables will usher in a better tomorrow.