OPGW design and construction and installation matters
The composite overhead ground cable (OPGW for short) is unique to the power system and has a new technology of dual functions of power line ground and fiber communication. As an emerging information transmission channel, OPGW has developed rapidly in China in recent years. It has the characteristics of large communication capacity, anti-interference, safety and reliability, and does not occupy the line corridor. At the same time, it will be the communication cable and high-voltage transmission line ground. Cleverly combined into a whole, its good mechanical properties and electrical conductivity not only meet the lightning protection requirements of ordinary ground wire, but also have good shielding effect of good conductors, which can greatly reduce the electromagnetic hazard of the transmission line to the adjacent weak current line. At present, OPGW does not have a unified production serial number, which integrates various requirements of electromechanical characteristics, thermal stability and communication fiber, and each parameter is mutually restricted. Due to the difference in production process and manufacturing equipment, OPGW manufacturers have different parameters. Therefore, even if the OPGW parameters of each manufacturer meet the requirements respectively, whether the overall characteristics of the product meet the requirements of the line engineering must pass the designer. Carry out verification check.
1 OPGW design conditions
1.1 Requirements for optical fibers
The OPGW is basically used as a communication trunk, which has high requirements on the quality of the core. Currently, a single mode fiber or a dispersion shifted fiber is generally used. When designing the bidding, the core number, working wavelength, attenuation, dispersion, bandwidth, additional bending attenuation, thermal stability, and service life of the optical fiber should be investigated. At the same time, the optical fiber provided by the same OPGW manufacturer must be from the same manufacturer. And belong to the same technical category.
1.2 Requirements for the electromechanical characteristics of OPGW
As an overhead ground wire, OPGW must have sufficient tensile strength to meet mechanical requirements and meet thermal stability requirements in addition to meeting communication requirements. The design shall comply with the provisions of DL/T5092-1999 "Technical Regulations for 110~500kV Overhead Transmission Line Design", and the special requirements of OPGW shall also be considered. The following conditions should be met in the design:
(1) Electrical parameters of the transmission line: voltage, 10 to 20 years vision planning system capacity, single-phase grounding short-circuit current, line relay protection cut-off time, earth conductivity distribution along the line, and allowable load of the line tower.
(2) Meteorological conditions: such as the highest temperature, the lowest temperature, the annual average temperature, the maximum ice thickness, the maximum wind speed, the annual lightning day, and the lightning strike probability of the route. As an overhead ground line, the OPGW should have the same meteorological conditions as the meteorological conditions.
(3) Electromechanical requirements of OPGW: The structure should ensure that the fiber is moisture-proof, anti-squeezing and not damaged; when the OPGW is subjected to 70% UTS, the fiber is not subjected to tensile force; the bearing part should be composed of aluminum alloy wire or aluminum-clad steel wire. It must not contain steel wire; it has good vibration resistance and lightning strike resistance, and its service life is more than 30 years.
2 OPGW design selection
2.1 OPGW structure
The structure of OPGW has great differences due to different manufacturers, and various structures have their own characteristics. As an information transmission tool, the OPGW's force on the fiber has a great influence on the attenuation of the signal. The force of the optical fiber is divided into side pressure and axial tensile force. The method of reducing the side pressure can adopt the structure of the aluminum skeleton and the stainless steel tube light unit, and the method of reducing the axial tensile force is to maintain the proper length of the optical fiber. According to whether the fiber buffer structure is subjected to tensile force during operation, it can be divided into two types: loose sleeve structure and tight sleeve structure. Due to the tight structure of the tight-fitting structure, the fiber optic space is small (almost no excess length). Although the elongation of the OPGW under various meteorological conditions and large-range is not more than a few thousand, the manufacturer of the tight-grid OPGW also promises its products. In the case that the cable tension does not exceed its own UTS, its fiber is not affected, but from the perspective of fiber safety, it is more reasonable to adopt a loose-sleeve structure with appropriate length. From the practical point of view, the widely used loose-sleeve structure mainly has a layered, skeleton, and central bundle tube structure. The excess length of the loose-span structure fiber can be controlled to 5 ‰ to 7 ‰ (one-time excess length) when the OPGW is cabled, which basically ensures that various weather changes cause the OPGW to elongate and the fiber attenuation to be within the allowable value. The length of the stranded loose-sleeve structure fiber can also be controlled to 9‰~10‰ (secondary length) due to engineering needs, but this situation is less, and it is not recommended to implement too much. Among the various structures of the existing OPGW, the structure of the layered steel pipe is the most compact, and the ratio of the effective bearing surface to the total section is the largest. Under the same tension, the total section is the smallest and the wind load is small. Effectively solve the problem of the excess length of the fiber and the anti-side pressure problem of the heavy ice zone.
2.2 OPGW thermal stability
When a short-circuit fault occurs in the transmission line, the short-circuit current through the OPGW is large, and the time is long, which will cause the temperature of the OPGW to rise sharply. In order to prevent the fiber in the OPGW from being damaged by overheating, it is necessary to perform thermal stability calculation on the OPGW, that is, to calculate the ground temperature after the line short circuit according to the system short-circuit current and the protection action cut-off time, which is low. The conditions of the maximum allowable temperature of the OPGW. Therefore, the thermal stability capacity is an important parameter for selecting the OPGW. Since the fault of the transmission line is short and the heat is diffused to a small extent, the heat dissipation process can be considered irrespective of the heat dissipation process, and the heat generated by the current on the ground line is all used for the temperature rise of the conductor.
The general calculation formula is as follows:
Where: IY———allow short-circuit current (A);
TY———the maximum allowable temperature (°C);
T1———Initial temperature (°C), generally 20°C;
A1——-the temperature coefficient of resistance at the initial temperature (1/°C);
R1—the combined resistance at initial temperature (Ω/km);
C0———General heat capacity of metal parts (J/km•°C).
1.02 is the stranding constant; Sa and Ss are the aluminum (including aluminum alloy) and steel cross section (mm2); the combined heat capacity is the sum of the heat capacity of each metal part; t is the fault removal time (s).
At present, the main method for calculating the return current on the ground line is to use the ground impedance (including self-impedance and mutual impedance), the tower grounding resistance, and the grounding resistance of the two ends to establish a mathematical model of the combination of n "Π"-shaped equivalent circuits. The computer programming method is used to solve the mesh matrix equation, and the optimal combination scheme of OPGW and shunt line is selected by calculating multi-segment, multi-combination and multi-scheme. As the closer to the two ends of the incoming and outgoing lines, the single-phase grounding short-circuit current of the transmission line is larger. In the actual engineering design, another ground wire other than the OPGW is usually used as a shunt line for the other ground wire other than the OPGW. The return current flowing through the OPGW is reduced when the power transmission line is short-circuited, and the OPGW is protected.
2.3 OPGW mechanical strength check
In addition to meeting the electrical performance requirements, the OPGW must also perform mechanical strength verification to ensure that its mechanical properties, tensile strength and grounding line stress meet the requirements specified in the regulations. The stress, sag, and load of the OPGW are calculated according to the calculation method of the overhead transmission line. According to the design experience of Hunan Institute in recent years, the mechanical strength of OPGW is usually larger than that of ordinary ground wire such as GJ-50, which can meet the requirements of stress and sag. However, due to the cable unit in the middle of OPGW, the cross-sectional area is generally higher than that of ordinary ground wire. Large, so when selecting the OPGW mechanical parameters, it is important to check the load of the tower grounding bracket under various working conditions.
3 OPGW fittings and anti-vibration
OPGW various fittings are supplied by the manufacturer, including overhanging fittings, tensile fittings, anti-vibration hammers, retaining lines and lowering fixtures. The OPGW needs to take anti-vibration measures, and the number of installations with different spans is provided by the manufacturer.
4 matters needing attention during construction
During the construction, the OPGW Construction Instructions provided by the manufacturer shall be strictly operated, and the number of accessories such as OPGW plate number and fitting model shall be carefully checked and counted. The OPGW must use tension to release the line. During the line-lifting process, the OPGW should keep a certain distance from the ground and other obstacles. Pay attention to limiting the bending radius of the OPGW to avoid damage to the cable core. The OPGW tight line uses the tightener and traction provided by the manufacturer. Net sleeve; OPGW's aluminum-clad steel part and aluminum alloy part can be cut by traditional cutting method, but the stainless steel tube with fiber optic must be cut with stainless steel cut by the manufacturer, and must be careful not to damage the fiber; In addition to the joint position, the OPGW does not allow joints anywhere in the line. Therefore, the OPGW, except for the position of the junction box, can be cut in a small amount according to the construction requirements, and must not be cut anywhere else; the OPGW manufacturer needs to put the OPGW on the line and after the connection. The relevant parameters of the entire OPGW line are responsible. Therefore, during the OPGW release process, the manufacturer will send technical personnel to provide technical guidance and construction supervision for the entire process of OPGW's release and connection.