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External damage monitoring of underground cables in distributed fiber optic vibration monitoring system

External damage monitoring of underground cables in distributed fiber optic vibration monitoring system

  • Classification:Optical fiber temperature measurement solution
  • Number of views:A
  • Release date:2022-04-24 00:00:00
  • Description
  • Performance
  • technical

Monitoring significance

In recent years, in order to meet the requirements of urban construction and environmental beautification, more and more power lines are constructed by cable, and the proportion of underground cables in the transmission and distribution system is increasing, and the scale is growing rapidly. However, in the process of service, the cable is often damaged by external forces, and the hidden trouble is frequent and the type, location and time are difficult to predict, and the advantages of safe and reliable cable power supply are seriously affected. Therefore, to ensure that the cable power supply is not damaged by external forces has become an urgent problem for the power operation department. However, the traditional operation and maintenance technology can not effectively match the growth rate of power infrastructure, resulting in cable lines and cable channel operation and maintenance work is facing great pressure.

The system relies on distributed fiber optic vibration sensing technology to achieve cable breakage monitoring, real-time measurement of vibration around the transmission line, location of anomalies, and identification of potential threats to the vertical distance of the cable, so as to effectively assess the threat level of external breakage. The system can provide reliable transmission line anti-external damage early warning function, effectively ensure the safe operation of power system.

DAS measurement principle

This product uses DF-DAS40 series distributed fiber optic vibration sensor to build a monitoring platform. In principle, phase sensitive optical time domain reflectometer (Φ-OTDR) technology is used to detect vibration and realize the monitoring of external broken events. Φ-OTDR usually uses a narrow linewidth laser of kHz level as the light source, and senses the external disturbance of the fiber by injecting the detection light pulse into the sensing fiber and detecting the resulting Rayleigh backscattering (RBS) signal in the sensing fiber. The technology has high sensitivity and fast measurement response, and can realize long-distance fully distributed sensing without blind area, which is very suitable for monitoring all kinds of micro-disturbance events around the buried cable.

In terms of measurement principle, when external disturbance occurs, the length and refractive index of the optical fiber will change with the external disturbance event, which will eventually lead to the change of the phase of the RBS optical signal generated at the corresponding position of the optical fiber. Φ-OTDR can quantitatively measure the external disturbance events by analyzing the RBS phase change before and after the external disturbance occurs. Its principle is shown below:

Through signal processing, the phase change information of the disturbance position can be extracted, and the fiber length change caused by the external disturbance on the fiber to be measured can be obtained, and the external sound field can be reconstructed finally. Combined with signal analysis, it can accurately locate and quickly identify construction phenomena that occur near the cable tube corridor, achieve early warning, rapid inspection and investigation, reduce the probability of failure, and ensure the safe operation of the cable.

The system carries out statistical analysis on the time domain characteristics of vibration signals, studies the law of signal shape change with time, extracts typical characteristic variables such as short-time energy, average zero crossing rate, resonant frequency, sub-band energy and Shannon entropy as the basis for signal judgment and recognition, and combines noise suppression methods such as spectrum reduction and adaptive filtering to effectively reduce the false alarm rate. On the one hand, this system can realize the monitoring and positioning of the hidden dangers of a single line or multiple series/parallel cables, forming a full-line cable monitoring system, and on the other hand, it can realize the whole-process control of all the hidden dangers of the cables, so as to combine the lines and surfaces.

Relying on this system, the construction vibration around the buried cable can be effectively monitored in real time, the abnormal signal can be understood in time, and the probability of buried cable failure can be effectively reduced. Once there is abnormal construction and damage around the cable, the system can issue timely warning. After the fault occurs, it can achieve accurate positioning, quickly determine the location of the fault point, reduce the workload of manual inspection, and improve the efficiency of fault repair. The system equipment is installed in the equipment room, and the one-core optical fiber of the communication cable is used as the sensing device and data communication, without the need to install additional sensors and power supply. The section of photoelectric composite cable laying can be monitored in the whole line, and there is no blind area in the whole line. The system software provides the access interface of detection and warning data for the user's secondary development and integration. The monitoring and early warning technology architecture of "line and surface combination and mobile Internet" built by the system can greatly reduce the fault incidence rate and the work intensity of inspection, and improve the work quality.

The hardware device of the system adopts the standard 2U structure as shown in the figure above. Its internal structure is compact and stable, and it can be installed in the cabinet of the communication room of the substation. The installation of the device is convenient, only a single end of the sensing fiber can be connected to complete the installation of the device, and remote monitoring and data transmission can be realized by using the network interface.

Description of system parameters:

System type


Technical parameter

Vibration sensing cable

Ordinary single-mode optical fiber (SM 9/125, attenuation <0.2dB/km@1550nm) allows long-term tensile strength of 300N, short-term 800N; Allow long-term lateral compressive strength 200N/10cm, short-term 800N/10cm, working life ≥20 years; At the same time, it is required to have anti-corrosion and anti-aging outer sheath.

Monitoring distance

Single channel ≤ 50km (40km for typical applications)

Positioning accuracy

Positioning accuracy < 5 meters

Division of the area

The range of each defense area is 5 meters to 5 kilometers, and the software is adjustable

System alarm response time

< 5s

System interface


Monitoring mode

Real-time distributed monitoring alarm, with positioning function

Frequency response


Controller power parameters

220VAC, 50-60Hz

Controller power



Provide electronic map interface, automatic pop-up alarm information

Alarm output

The system can distinguish between intrusion alarms and system failures, and alarms can be output to third-party systems over a LAN network.

Processor operating temperature


Processor operating humidity

0-90%HR,No condensation

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