First, OPGW cable overview

OPGW cable is the optical fiber composite overhead ground wire, which belongs to a composite wire with the communication function of ordinary overhead ground wire and optical fiber. Compared with the traditional power lines, OPGW cable has significant advantages in the main performance: lightning protection function and communication ability, when the OPGW cable is used in the power system, no external cable is needed. The optical fiber is placed in the middle of the OPGW, so the optical fiber is not easy to be damaged, and has high security and reliability. The working performance of OPGW is affected by a variety of factors, such as allowable tension, the spacing of line tower, ice load and wind load, suspension point, etc.

In power systems, there are three main types of OPGW: aluminum frame type, center tube type and laminate type. The main characteristics of the aluminum skeleton structure are that the beam tube unit or optical fiber is built into the aluminum skeleton slot, so as to increase the protection of the optical fiber, so that the optical fiber is less prone to damage. OPGW with center tube structure has the characteristics of light weight and small cable diameter, and it is not easy to cause secondary mechanical fatigue and damage. The fiber of this structure has high mechanical strength and can withstand the impact of large circuit current. Laminar stranded cable structure has high reliability and stability, strong resistance to side pressure and distortion, can withstand large short-circuit current impact, and can obtain secondary residual length.

Two, power system OPGW cable lightning protection grounding technology

1. Lightning strikes OPGW cable

Lightning is a transient process that occurs very quickly and releases a great deal of energy in a short period of time. During a thunderstorm, the charges in the cloud are distributed randomly, so there are multiple concentrations of charges. When the electric field intensity of the charge concentration point reaches a certain range of height, the downward load will be divided into positive and negative two conditions. In the process of the pilot discharge channel gradually moving down, some ion clusters in the air will block it and divide the negative leader into several different numbers. When the negative leader moves down and gradually approaches the overhead ground wire or ground wire, there will be a positive charge accumulation in the ground or ground wire. Along with these scenes, there is lightning and thunder. The duration of this phase is usually short, not exceeding 100s. Because the time is short and the speed is fast, the temperature generated during the lightning strike is extremely high (around 600 degrees Celsius). Such high temperature action on OPGW cable will seriously damage the aluminum alloy on the outer layer of the cable, or even directly cause the cable to be burned.

Although the harm of lightning to OPGW is great, it is not beyond control. Some studies have shown that the energy released by lightning in a short period of time will cause significant fluctuations in the heat of the corresponding area of the transmission line, and after lightning strikes, the current will form a temperature gradient distribution in a certain direction from the transmission line to the routing interface where the tower enters the ground. This shows that lightning strikes can cause changes in the temperature distribution of transmission lines, and OPGW cable can determine the location of lightning strikes according to this law during operation, so as to prevent lightning faults. Lightning strike OPGW cable is a local rapid heating to high temperature and then rapid cooling process. The temperature field generated by lightning strikes changes rapidly with time and space, and the thermal physical properties of OPGW materials also change dramatically with temperature. According to this change the lightning fault can be more accurate judgment and prevention.

2, OPGW lightning protection grounding technology

At present, many experts and scholars have done a lot of experiments and researches on improving the lightning protection capability of OPGW cable, and achieved more results. If some scholars point out that in order to improve the lightning protection performance of OPGW optical cable, you can choose the outer diameter of the optical cable when choosing the conductor with a larger diameter, as the outer diameter of the optical cable. The basis of this conclusion is that the induced current of OPGW cable during operation is mainly concentrated on the surface and outer layer of the cable, so theoretically speaking, the surface or outer layer of the cable should be strong enough. But in fact, the current market many OPGW cable outer layer is relatively weak, cable outer conductor DC resistance is often less than the inner layer. Therefore, the follow-up can start from the outer wire diameter of the optical cable, through the use of larger conductor wire diameter to improve the current resistance of OPGW optical cable, so that OPGW optical cable has strong lightning protection characteristics. In addition to this measure, the conductor wire of OPGW cable can also be optimized, choose a more matching conductor wire so that the shard capacity of OPGW cable can be enhanced, so that the lightning protection characteristics of the cable can be improved. In addition, if the OPGW cable operates in a complex environment, it can be considered to install line lightning arrester, increase insulators, add small tower grounding resistance and so on to improve the lightning protection of the cable, so as to minimize the harm of lightning on the cable.

3, power system OPGW cable lightning protection technology

In power system, OPGW can locate and prevent lightning strikes by a distributed optical fiber temperature measurement system. In the construction of such a temperature measuring system in the power system, the temperature changes of OPGW cable during operation will be timely and comprehensively collected, and then the system conducts in-depth analysis and calculation of various temperature data, compares the calculation results with the preset values in the system, and carries out the inspection and alarm of the monitoring quantity. And according to the environmental conditions of different zones and a variety of ways to judge and analyze the lightning accident, and finally realize the accurate positioning of the lightning accident and pre-processing, the impact of lightning accident on the whole system to the minimum.

The distributed optical fiber temperature measurement system in the power system is mainly composed of the following hardware and software: the hardware parts are electrooptic modulator, pulse drive circuit, acoustooptic modulator, polarization controller, fiber laser, EDFA, fiber ring, etc. The system mainly consists of the following functional modules: background server functional modules (database communication module, data processing module, data acquisition module, etc.) [5]. Each module has different functions and assumes different responsibilities in the system. For example, the data acquisition module mainly collects the length, width, frequency and density of the electrical signal sent by APD. The data acquisition module collects various data based on the parameters and instructions issued by the system server. After the collection of signals, the corresponding electrical signal data of functional modules in the system is restored to the temperature data related to optical fiber length positioning according to the demodulation algorithm of Raman principle, that is, the preliminary distributed original temperature data is obtained. These data are important reference for lightning fault location and lightning fault prevention.

When the distributed optical fiber temperature measurement system is in normal operation, the staff can master the operating temperature data of OPGW optical cable through the system, and can obtain the temperature distribution change track, temperature change record, temperature anomaly location and times. Thus, the OPGW optical cable operation state to make accurate judgment, whether the optical cable is affected by lightning and lightning damage position to make accurate judgment, and then according to the judgment results to make the corresponding operation and maintenance and management decisions, so that OPGW optical cable to achieve normal and stable operation.

4. Conclusion

To sum up, OPGW optical cable is a very important part of the power system, the lightning protection performance of OPGW optical cable directly affects the running state of the system, affecting the safety of users. Therefore in the current background to further increase the OPGW cable lightning protection grounding technology research and optimization, constantly improve the OPGW cable lightning protection technology level. At the same time in the process of system operation, the relevant departments or units should be able to use traditional measures and advanced technology to protect the optical cable according to the actual operating environment of OPGW optical cable and lightning protection needs, so that the OPGW optical cable can run normally and stably.