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How to monitor the settlement of substation foundation using distributed fiber optic sensing technology

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In order to solve the foundation settlement problem of some substations built in low-lying areas, a substation foundation settlement monitoring scheme is proposed using distributed fiber optic sensing technology. On the basis of introducing distributed fiber optic sensing technology, stress optical cables with strong anti-interference ability were studied as sensing elements. The deployment methods of surface deformation monitoring optical cables, deep deformation monitoring optical cables, foundation pile settlement monitoring optical cables, and connecting optical cables were introduced separately. Finally, two types of substations were selected as pilot application objects. Analysis of the monitoring results showed that the peak and valley positions in the monitoring positioning map can determine the degree of fiber optic cable relaxation, and then determine the direction and amplitude of strain. The proposed monitoring scheme can well meet the needs of substation foundation settlement monitoring. This can provide reference and assistance for the advancement of foundation settlement prevention and control technology in substations.

Substation is an important hub of the power network. With the rapid development of the economy and society, land resources are becoming increasingly scarce. In order to ensure normal power supply in high concentration neutral power load areas, substations are sometimes forced to be built in certain special geological areas. In the the Pearl River Delta region of Guangdong Province, the geological foundation has high water content and deep soft soil layer. Due to urban development and changes, some substations built on soft soil layer and river alluvial soil layer have foundation settlement problems. The southern subtropical rainy climate has strengthened the geological erosion and infiltration of substations, and is also prone to secondary disasters such as cracking and tilting of ground buildings, posing a potential threat to the operation of substation equipment.

To prevent and treat the settlement problem of substation foundation, while implementing the site selection, construction, and supervision of substations in various regions, it is also necessary to pay attention to the monitoring, early warning, and treatment of substation settlement problems. At present, the main methods for monitoring geological subsidence in substations include manual inspection, video monitoring, geological displacement monitoring, etc., which have poor real-time performance, inability to detect and eliminate hidden dangers in a timely manner, or insufficient accuracy, and difficulty in judging when the characteristics of the phenomenon are not obvious. In recent years, distributed fiber optic sensing technology has been widely promoted and applied due to its advantages of good technical economy, long monitoring distance, and the ability to measure signals at wide spatial positions. Distributed fiber optic sensor technology is based on effects such as Rayleigh scattering, Raman scattering, and Brillouin scattering in optical fibers. The sensing distance and accuracy of optical time-domain reflection measurement based on Rayleigh scattering are limited, and the return signal of Raman scattering technology is weak. Therefore, in recent years, there has been more research on Brillouin scattering based fiber optic sensing technology in China. In view of the urgent need for monitoring substation foundation settlement, distributed optical sensing technology is used to develop a device system for monitoring substation foundation settlement. This device system can reduce the difficulty of preventing settlement disasters in substations, grasp the impact of geological foundation settlement on substation equipment, and provide auxiliary decision-making and effectiveness evaluation methods for substation prevention and control of foundation settlement.

Distributed optical sensing technology, due to the non-uniformity of the fiber material itself, when light propagates in the fiber, it will propagate in directions other than the original direction, which is the scattering phenomenon of light propagation in the fiber. Among various scattering phenomena, there exists a type of Brillouin scattering, which is the result of the coupling effect between the light waves propagating into the fiber and the sound waves existing inside the fiber, ultimately leading to a change in the frequency of the scattered light compared to the initial incident light. The factors affecting the difference between the two include the scattering angle of the scattered light and the characteristics of the sound waves.

Research both domestically and internationally has found that the frequency change (frequency shift) of Brillouin scattering light in optical fibers exhibits a linear relationship with the axial strain of the fiber and the ambient temperature. Under constant temperature conditions, the tensile strain experienced by the fiber can be directly reflected by the Brillouin frequency shift.

By eliminating the influence of temperature while setting a temperature reference, a single linear relationship between the Brillouin frequency shift value and the axial strain in the fiber can be obtained. By measuring the frequency shift values at various positions in the entire fiber using an induction element, the corresponding change in strain at each position can be calculated, which can then be applied in related stress measurement fields. This is Brillouin fiber optic sensing technology. The working process can be simply described as: using a narrowband laser to generate an initial light source, dividing it into two paths. One path of light is modulated into optical pulses, amplified, and transmitted along the sensing fiber to generate a reverse Brillouin scattering light signal for detection; The other path of light generated by narrowband lasers is made into frequency shifted light and coherent with Brillouin scattering light. The coherent processed signal is input into a computer for analysis to obtain temperature or strain measurement results. The BOTDA system is a dual input system, and the sensing fiber mainly conducts energy carried by the Brillouin frequency shift between the pump light and the detection light. If the frequency shift values of the pump light and the detection light are closer to the Brillouin frequency shift values, the energy value transmitted by the sensing fiber is greater. In actual measurement, it is necessary to gradually adjust the frequency difference between the pump light and the detection light according to a specific set value. Generally, frequency scanning is used to obtain the discrete points below each frequency value in the spectrum. After fitting, the complete Brillouin scattering spectrum reflecting the frequency shift value at each position can be obtained. Finally, the temperature or strain values can be calculated and converted based on linear relationships.

Monitoring optical cable

Considering that the monitoring of foundation settlement in substations requires high accuracy in monitoring methods, and the monitoring units installed in the ground must have strong anti-interference ability, traditional optical fibers are more sensitive and fragile, and cannot meet the requirements. For the convenience of construction and monitoring, this article studies and designs a stress optical cable with fixed-point function. This optical cable has a segmented identification function. In actual installation, personnel only need to use special fixtures to continuously arrange the optical cable and the main nodes of the monitoring object based on the cracking situation of the on-site house for a fixed length, in order to achieve full coupling between the optical cable and the monitoring object. By fixing the optical cable in segments, effective measurement of the monitoring section can be achieved, providing convenience for strain point positioning and data analysis, especially for deformation conversion. At the same time, this type of optical cable can be reinforced with reinforcement bars according to the engineering situation, ensuring the toughness of the optical fiber. Therefore, it has good mechanical properties and tensile and compressive properties, which is convenient for construction under special conditions and can withstand various harsh working conditions.

Fiber optic cable deployment plan

As a sensing unit, the stress optical cable has the advantages of passivity, corrosion resistance, aging resistance, radiation resistance, etc. It has strong plasticity and is suitable for deployment of complex terrain in the field. At the same time, the optical cable used in this layout scheme is both a sensing optical cable and a transmission optical cable, facilitating the connection of the monitoring host in the monitoring area and the substation machine room. According to the on-site installation and debugging situation, the BOTDA monitoring instrument adopts a spatial sampling interval of 0.5 meters. In order to effectively identify the small deformation results obtained from surface deformation monitoring, deep deformation monitoring, and foundation pile settlement monitoring, at least 2 meters of optical cables are reserved when the measurement method changes during construction to complete the identification of spatial resolution and temperature calibration. The specific fiber optic cable layout plan includes surface deformation monitoring fiber optic cable layout, deep deformation monitoring fiber optic cable layout, foundation pile settlement monitoring fiber optic cable layout, and connection fiber optic cable layout.

Surface deformation monitoring

Fiber optic cable deployment

The surface deformation monitoring optical cable can monitor the horizontal deformation of landslides, and the monitoring optical cable is laid using a 2m fixed point stress optical cable.

Layout method of surface deformation monitoring optical cable

When laying optical cables, first dig a trench with a width of 17 cm and a depth of 10 cm along the design direction of the optical cable, then lay the stress optical cable in the trench, lay the armored optical cable in the trench, and keep it in a straight state. Use angle iron and metal clamps to couple the optical cable with the ground layer at the fixed point of the optical cable, and pass through PVC pipes for protection between the fixed points; Backfill and compact the optical cable with undisturbed soil, and measure the strain of the optical cable using a BOTDA monitor during backfilling. It is recommended that the optical cable generate less than 500 microstrain (microstrain: one millionth of the change in mechanical size relative to the original size); Record the actual direction and markings of the optical cable, and after the cable is laid, backfill the trench.

Layout of deep deformation monitoring optical cables

In order to provide early warning for gradual and sudden foundation settlement, the method of on-site sampling and deep hole settlement monitoring in this layout plan is used to measure the deformation situation in the deformation zone in advance.

Layout method of deep deformation monitoring optical cable

When deploying deep deformation monitoring optical cables, drill a bare hole with a diameter of 200mm at a selected location using a drilling rig; Using a heavy hammer and steel pipe pressurization method, place the optical fiber into the bottom of a 15 meter hole; In order to increase the measurement range, one 2m fixed point optical cable and one 10m fixed point optical cable were selected for deployment, and the strain of the optical cable was monitored using a BOTDA monitoring instrument; Afterwards, when backfilling the borehole, it is necessary to calculate that only 20cm of clay balls should be filled at the fiber optic cable node position, and the remaining positions should be backfilled with undisturbed soil to ensure good coupling between the fiber optic cable node and the geological layer. At the same time, the tightness of the fiber optic cable should be continuously adjusted to ensure that the strain generated by the cable does not exceed 500 microstrain.

Monitoring of pile settlement

The basic principle of monitoring the settlement of foundation piles for fiber optic cable installation is to first drill a hole that reaches the bedrock using a drilling rig, then make a benchmark installation, and place the monitoring fiber optic cable between the benchmark installation and the foundation pile to be monitored. Since the benchmark pile does not produce any settlement changes, the strain changes of the fiber optic cable can be monitored using BOTDA monitoring instruments to determine the settlement changes of the foundation pile. The production method of benchmark piles is to first drill a hole into the bedrock with a drilling rig at a safe distance of 6 meters from the high-pressure equipment, with a depth of about 19 meters. Then, weld a steel pipe with a diameter of 160 mm and place it here. Pour concrete into the steel pipe, and the height of the steel pipe from the ground surface is about 3 meters. The installation method of the foundation pile settlement monitoring optical cable is to weld the angle iron with the steel pipe of the reference pile during installation, drill holes on the angle iron, and fix the stainless steel pulley with screws; Lift a 0.5 meter long cement pile with one end of stainless steel wire, connect the other end to a steel plate, and connect the steel plate to the monitoring pile; Fix the nodes of the monitoring optical cable and the steel plate of the monitoring pile with metal clamps; Fix the other node of the monitoring optical cable to the angle iron of the reference pile through a metal fixture; The optical cable between the benchmark installation and the monitoring pile is protected with PVC pipes, which are fixed to the steel wire; It is recommended to adjust the tensioning device under the monitoring of the BOTDA monitor to achieve a strain of 1/20 of the full range generated by the optical fiber; Finally, fix the monitoring optical cables between the remaining 4 monitoring piles and the benchmark piles in sequence.

Layout of connecting optical cables

Due to the placement of BOTDA monitoring instruments in the computer room, there is a certain distance between the settlement hazard monitoring area and the computer room. Therefore, it is necessary to install and lay a connecting optical cable between the monitoring optical cable and the monitoring instrument, as shown in Figure 6. The stress optical cable is laid horizontally in the key monitoring area of the substation. Some optical fibers are not suitable for burial underground, and it is necessary to fuse jumper wires on the surface of the optical fiber and add certain protective measures. Generally, a layer of metal hose or armored metal corrugated pipe can be nested outside.

Data analysis of pilot application of fiber optic cable laying method for connection

Monitoring of foundation settlement in 110 kV substation

The 110 kV substation is located around the industrial area. Due to settlement and other reasons, the 110 kV substation has obvious cracks and cracks on the walls. In order to monitor the deformation of the building walls, monitoring optical cables are fixed on the surface of the building walls using fixtures; In order to monitor the settlement and deformation of the tower foundation outside the substation, a foundation pile settlement monitoring optical cable is installed. Through BOTDA data collection, a total of 1541 sampling points were identified. In addition to monitoring the starting and ending ends of the optical cable, the monitoring positioning map was divided into three parts: tower foundation deformation monitoring section, station ground deformation monitoring section, and wall deformation monitoring section.

There are four peaks in the positioning map of the tower base deformation monitoring section, which correspond to the four sections of optical cables laid out. The three valley positions are reserved spare cables and can be used as temperature reference optical cables.

The ground deformation monitoring sections inside the station are all in varying degrees of tension. Surface deformation can cause changes in the tension of these two optical cables, and the Brillouin frequency shift value will correspondingly change. The direction and magnitude of surface deformation can be determined by its linear relationship with strain.

The wall deformation monitoring section consists of a tensioned optical cable section and a relaxed optical cable section. The tensioned optical cable is a fixed optical cable at both ends for monitoring wall deformation, and the data is reflected in the local peak position in the monitoring positioning map. The relaxed optical cable is the connecting optical cable between the two fixed optical cables, which can be used as a temperature reference optical cable. After cracks appear on the wall, the tightness of the optical cable will change, leading to changes in the Brillouin frequency shift value and inferring the degree of strain, which can determine whether cracks appear on the wall.

Monitoring of foundation settlement of 220 kV substation on the embankment

The 220 kV embankment station of the power supply bureau is located on the southeast side of the aluminum plant. The topography of the station area is mountainous and leveled land. Except for a small amount of hilly terrain in the northeast corner, the station site is located in other areas with relatively flat terrain. The Quaternary covering layer of the station site is mostly caused by alluvial and siltation, mainly consisting of cohesive soil, silty soil, and sand. The underlying bedrock is Cretaceous sandstone. The southwestern part of the station area was originally a fish pond, which was backfilled and leveled during the construction of the station. Currently, the settlement of the 220 kV busbar pillars in this area is relatively severe, with a drop of about 10 cm between the two pillars. The ground settlement is significant at 20-30 meters, and the edge wall is damaged due to settlement, presenting a wave like pattern on the horizontal line of the wall edges. The height of the outer slope of the substation is 7-9 meters. Currently, due to the unstable foundation of the slope, no drainage ditch has been constructed, and the embedded PVC drainage pipes have shown significant deformation and damage. In order to use distributed passive optical sensing technology to monitor geological foundation settlement disasters in substations and achieve online monitoring of geological foundation settlement disasters in substations, data was collected through BOTDA, with a total of 2031 sampling points. In addition to monitoring the beginning and end of the optical cable, the monitoring positioning map is divided into three parts: foundation pile deformation monitoring section, deep deformation monitoring section, and surface deformation monitoring section. There are a total of 5 foundation piles set up in the settlement monitoring section, and their monitoring data features are the same. There is a trough between two peaks, and the trough position is reserved for the top of the monitoring pile foundation, which can be used as a temperature reference optical cable.

The peak position of the deep deformation monitoring section is the suspension point of the optical cable above the ground. This suspension point is relaxed after the natural settlement of the backfill soil is completed. The optical cables in this section are in different degrees of tension, and deep settlement will gradually reduce the degree of tension.

The valley position of the surface deformation monitoring section is the relaxation section near the wall, with two monitoring optical cables in grooves on both sides of the relaxation section. The optical cables in these two sections are in varying degrees of tension, and surface deformation can cause changes in the tension degree of these two optical cables, thereby determining the direction and magnitude of surface deformation.

A distributed fiber optic sensing technology is proposed to monitor the foundation settlement of substations by utilizing the linear relationship between the frequency value of Brillouin scattering light and stress changes. In order to improve the anti-interference ability of optical fibers and meet the accuracy requirements, a stress optical cable with segmented identification function was designed as a sensing element. This article introduces the deployment methods of four types of optical cables: surface deformation monitoring optical cables, deep deformation monitoring optical cables, foundation pile settlement monitoring optical cables, and connecting optical cables. Through pilot application results in two different substation environments, it verifies that distributed fiber optic sensing technology has good effects in substation foundation settlement monitoring, providing a new solution for improving the monitoring ability of substation foundation settlement faults.




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