Monitoring system for bridges

Fiber optics products can be used to monitor the condition of different bridges; it evaluates in real-time depreciation during the exploitation, provides public safety, and cuts the expenses for maintenance.

Data collected by these fiber optics products allow decreasing regular maintenance expenses of the constructions. All the information is stored in an orderly manner to enable sorting through long and short-term tracking periods. Using a monitoring system helps to use the facility in a more efficient way. Monitoring fiber optics products help to build the construction and to optimize the load in different situations, and it will make the utilization time.

The following parameters are measured with fiber optic product:

  • Relative linear beam deformation
  • Temperature next to the deformation place
  • The inclination of the supporting structure
  • Other physical dimensions

Beam sensors are located on the right and left sides of the bridge and the fiber optic product controls the pressure on the entire length of the construction, bridge conditions during the peak load and at relaxation time, relative changes in different structure elements during the exploitation period, climate changes effect.

Fiber optic product for inclination tracking controls the potential shift of the bridge over time. And the fiber optic product located at the central part of the bridge tracks the vibration and the danger of resonating frequencies.

The amount of sensors per bridge is always determined case by case. In general, it depends on the central part between the supporting structures. The part that takes the heaviest load is the central part of the bridge and it has to be measured precisely.

These sensors help to find weak parts of the structure and prevent damages and catastrophes.

Monitoring loadbearing structures of large scale public facilities with fiber optic sensors

There are plenty of different fiber optic applications of FBG sensors; it can monitor a variety of structures, buildings, and other facilities. This technology is often used in construction work and to track different parameters in industrial production and other areas.

Fiber optic applications include effective monitoring of loadbearing structures in different public facilities. Most of the modern construction sites are technically sophisticated and uniquely designed. In the process of project development, certain safety measures are planned, however, it may change in time after the building utilization has started. Regular monitoring becomes crucial in this context and it may prevent different accidents. The automated monitoring fiber optic system allows tracking the technical condition of the object in real-time with minimal human involvement.

Fiber optic Bragg gratings (FBGs) are used as a sensing element in this monitoring fiber optic system. This kind of sensors doesn’t have electronic components and it makes them resistant to electromagnetic waves and fire safe. Fiber optic Bragg gratings are inscribed in the fiber with a UV laser. FBG in each sensor reflects the specific fiber optic wavelength with a spectrum width of approximately 1 nm. The grating is affected by mechanical and temperature impact and its physical change cause the shift in the fiber optic wavelength of the reflected light. The fiber optic wavelength shift is measured, which allows measuring the deformation and temperature change. Two gratings are used simultaneously to separate these parameters, where one of them is isolated from the mechanical impact. This allows tracking the temperature impact on the second grating and measuring the deformation. One optic fiber may have multiple gratings, each one of them provides a response at its own length, and the distance between gratings vary from 10 mm to several kilometers.

One of the most common methods to investigate sensors is to scan the FBG aggregate with a tunable source. The tunability range is usually around 100 nm (1500 – 1600 nm), the investigation frequency is from 1 to 500 Hz, and the number of parallel channels can be up to 8 channels.

Usually, it is enough to interrogate sensors once per 6 hours, when they are installed on a large public facility. Each sensor has 2 critical measurement levels (yellow and red), when this level is reached the optic fiber system sends out an alarm signal. The frequency of interrogation increases to 1 time per 30 minutes. Moreover, the notifications are sent out in case of malfunctioning. The software allows viewing the data collected from all the sensors as the graphs of relative deformation and temperature.

This fiber optic application of FBG sensors works perfectly well for large constructions, such as stadiums.

What is DTS?

A distributed temperature sensing system (DTS) is a device made to monitor power cable with fiber optic temperature sensors. DTS measures temperature on the large distances, greater than 30 km. It records the temperature along the cable as a continuous profile. DTS spatial resolution is 1 m with accuracy within ±1°C at a resolution of 0.01°C.

Adding RTTR to distributed temperature sensors allows achieving more precision when measuring high-temperature wire. DTS has some level of uncertainty when it detects the temperature, and its level increases when the conductor operates in an emergency situation. This happens because the conductor heating is detected with the time lag. Also, the temperature differs depending on the loading level. The higher the loading level is, the less precise are the measurements.

What is DTS measuring principle?

Certain factors, such as temperature or pressure, affect the optic cable and change the light transmission characteristics in the fiber. Optical fibers operate as linear temperature sensors when the light is projected in it through scattering to determine the location of the physical effect on the outside.

Optical fiber is made of doped quartz glass, silicon dioxide and has an amorphous solid structure. Its molecules interact with the photons when light falls on the thermally excited molecular oscillations. This process leads to light scattering, also called Raman scattering. The difference between the incident light and scattered light is that the later experiences a spectral shift that is equivalent to the lattice oscillation resonance frequency. The scattered light has three spectral shares, namely the Rayleigh (laser source wavelength scattering), the Stokes line components (lower frequency), and the anti-Stokes line components (higher frequency). Anti-stokes depend on the temperature, hence it is possible to calculate the fiber temperature as a ratio between anti-Stokes and Stokes line components.

What is DTS used for?

Distributed temperature sensing has multiple applications in different industrial segments to monitor power cables, tunnels, and industrial buildings, to detect leakages, and to control high-temperature wires. Recently DTS has been applied in monitoring ecological factors, such as groundwater, rainforests, river streams, etc.

Dynamic Cable Rating

In recent years a lot of research has been done in order to increase the power flow of underground cables and to develop the equipment to effectively monitor the weather and thermal state of the cable by creating accurate thermal models. As a result of applying dynamic thermal rating technologies, the capacity is usually increased by 5 to 15%. Few factors determine the thermal rating of the underground, among them the soil temperature and thermal resistivity of the earth, and they change very slowly, don’t get affected much by weather and current loading.

The main challenge with underground installations is to accurately measure the maximum current value that can flow through the circuit breakers.

The current capacity carried in specific cable circuit breakers depends on certain aspects, such as cable construction, the soil, the temperature, and the sheath-bonding method. Only the soil properties are variable, and the others are constant.

The soil is affected by the weather change in different seasons and the cable heating, hence the current carrying capacity changes drastically. Dynamic current rating of a cable circuit is a crucial factor in order to utilize it in the full capacity all year round; because it is always the biggest challenge for power operators to choose the right power load for the underground cable.

That is why monitoring thermal conditions of the buried cable circuits and installed distributed temperature sensing (DTS) systems is crucial.

A real-time operating system is created to capture different load current parameters, cable surface, and soil temperatures to provide input to the real-time operating system. The data about current and temperature is passed to a computer through the fiber optic connection. The computer gathers load and temperature data and provides an updated ampacity rating.

Once the dynamic current rating data is received, it has to be analyzed within a set period of time. The results are usually used to develop risk management strategies. There are certain challenges when calculating line ampacity, such as conductor properties and atmospheric conditions, which have to be considered. Each of these factors increases the level of uncertainty when determining ampacity.

RTTR – Real Time Thermal Rating System

A real-time thermal rating is a monitoring system. It helps to effectively use current-carrying capacity. Basically, it allows avoiding making assumptions about the current load, and instead to ensure that it is used in the most efficient way and the probability to exceed the acceptable temperature is low.

Smart grid technology, a real-time thermal rating system, has been created to rate the electrical conductors affected by the local weather conditions. It provides accurate real-time temperature measurements and current reading along the entire high-temperature wire. The RTTR is embedded in the cable and calculates the capacity of the current under specific conditions. It is a perfect solution to monitor power cable performing under abnormal conditions such as different emergencies, energy outages, etc.

RTTR is often used with the DTS system of temperature sensors because it gives more accurate data and allows monitoring operations in the real-time mode. For cables that have temperature sensors (DTS) embedded or touching it, the temperature is monitored continuously and the rating can be indicated accordingly. The cables without DTS have their operational temperature are calculated based on the real-time installation condition and loading. There are two types of RTTR to monitor power cable: self-contained and environmentally based.

Self-contained real-time temperature collects the data along with the entire circuits; the embedded fiber optic cable measures the internal temperature, and the attached one measures the sheath temperature.

Environmentally based RTTR measures soil temperature and its direct effect on the cable. It also measures soil thermal resistivity, which affects the heat exchange rate between the cable and the external environment.

RTTR usually uses the following parameters for the calculations:

  • The ground type (soil, clay, sand, gravel, thermal backfill)
  • Burial Depth
  • Cable Type
  • Cable Structure
  • Other cables laid in close proximity

Rating calculations of the high-temperature wire are based on the data derived from monitoring the underground cable. Standard static ratings are usually conservative and understate the real feeder capacity; hence the feeders are not loaded fully most of the time. The real-time thermal rating allows determining the times when the cable is not loaded fully and when certain actions need to be taken.