Introduction to Distributed Temperature Sensing

Distributed Temperature Sensing Systems (DTS) are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over great distances.

Measuring Principle - Raman Effect
Physical measurement dimensions, such as temperature or pressure and tensile forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the attenuation of the light in the quartz glass fibres through scattering, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor.

Optical fibres are made from doped quartz glass. Quartz glass is a form of silicon dioxide (SiO2) with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (photons) and the electrons of the molecule. Light scattering, also known as Raman scattering, occurs in the optical fibre. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation.

The light scattered back from the fibre optic therefore contains three different spectral shares:

• the Rayleigh scattering with the wavelength of the laser source used,
• the Stokes line components with the higher wavelength in which photons are generated, and
• the Anti-Stokes line components with a lower wavelength than the Rayleigh scattering, in which photons are destroyed.

The intensity of the so-called Anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fibre is derived from the ratio of the Anti-Stokes and Stokes light intensities.

Measuring Principle - OFDR Technology
Latest DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry (OFDR) . The OFDR system provides information on the local characteristic when the backscatter signal detected during the entire measurement time is measured as a function of frequency in a complex fashion, and then subjected to Fourier transformation. The essential benefits of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal, whereby a significantly higher signal to noise ratio is achieved than with conventional pulse technology (OTDR). This technical benefit allows the use of affordable semiconductor laser diodes and electronic assemblies for signal averaging.

The optical frequency domain reflectometry has been developed as a high-resolution measurement process for the characterisation of optical wave guides with length dimensions of just a few millimetres. In contrast, its application for the Raman backscatter measurement was introduced and patented by the company LIOS Technology.

Schematic system set up
The temperature measuring system consists of a controller (frequency generator, laser source, optical module, HF mixer, receiver and micro-processor unit) and a quartz glass fibre (fibre optic) as line-shaped temperature sensor.

The design is three-channel, since an additional reference channel is required besides the two measurement channels (Anti-Stokes and Stokes). Corresponding to the OFDR system, the power output of the laser runs through the sinus-shaped frequency starting from a starting frequency in the kilohertz range through the ending frequency in the high megahertz range within a measurement time interval with the help of the High Frequency (HF) modulator. The resulting frequency shift is a direct measurement of the local resolution of the reflectometer. The frequency-modulated laser light is connected to the fibre optic-sensor via the optical module.

The continuously back-scattered Raman light is spectrally filtered in the optical module and converted into electrical signals by means of photo detectors. Then the measurement signals are amplified and mixed in the Low Frequency spectral range (LF range). The Fourier transformation of the averaged LF signals results in the two Raman backscatter curves. The amplitudes of these backscatter curves are proportional to the intensity of the Raman scattering of the viewed location. The fibre temperature along the sensor cable results from the amplitude ratio of the two measurement channels.


High Reliability and Industrial Strength

The semiconductor laser diode has been thoroughly type tested according the Telcordia GR-468 standard. It fulfils telecom standards with a medium lifetime of more than 25 years. The entire system was comprehensively evaluated by various independent international bodies (e.g. the VdS, the association of German asset insurers) including EMC tests as well as endurance tests at accelerated aging environments. Field data of the huge installed base prove the exceptional high reliability of the controllers.

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Integrated Condition Monitoring for High Voltage Cables: Direct link to SCADA / substation automation

The intrinsic temperature measuring system DTS based on LIOS Technology’s design, with fibre optic sensors either installed within the power cable’s cross section (FIMT - Fibre in metal tube ) or attached to the exterior of the cable, makes it possible to record the temperature profile along an entire cable route continuously, and to pinpoint the exact location of hot spots within a metre.

Most commonly the measured temperature profile data is transmitted via standard interfaces from systems of the DTS as required, and be either displayed or further processed by PC, PLC or SCADA systems. As a result of the increasing demands placed on the ability to network and integrate measuring systems into management systems, LIOS offers network components that summarise DTS data from several DTS units and transport or convert it to the required network standards and protocols like Modbus, DNP3, IEC60870-5 or XML based data interfaces (e.g. POSC).

Equipped with the Embedded Communication Module (ECM) the Distributed Temperature Sensing (DTS) evaluation unit is accessible by Ethernet/ LAN, holds its own IP address and is compatible with DHCP.
The on-board storage facility of measurement data provides automatic buffering of network break downs. It can handle multiple protocols, so the DTS can be directly linked to PLC or SCADA systems and communicates in various industrial protocol standards providing measured data and customised alarm information.
Hereby, the evaluation unit communicates as a TCP /IP server with several clients.

Features of the embedded communication module of the LIOS DTS controller:

• Configuration and operation of the DTS via two independent channels:
  
  
  • RS232
  • Ethernet TCP/IP
  
  
• Onboard storage of measurement data
  
  
  • Buffers up to 72 hours of measurement data and initiates data transfer automatically after communication link was re-established
  • Individual file access to measurement data through CHARON_02 (onboard data storage 1 GB)
  
  
• Allows to operate individual Plug-In-Interfaces for customer specific protocol solutions
  
  
  • The DTS can communicate in parallel with its proprietary protocol and one or more customer specific protocols like DNP3, IEC60870-5-104, IEC61850, POSC WITSML, Modbus (Master/Slave) etc.
  • Individual assignments for protocols and client addresses (IP) possible
  
  
• Ethernet communication to and from clients with high bandwidth
• Module comes with built-in user configurable Firewall and enables reliable operation even in WAN’s
• Secure data transmission and individual authorisation according TLS- Standard (RFC 2246)

The most common industrial protocols used in electrical systems worldwide are introduced in the following section. However, also other protocol standards are available or can be implemented for the ECM. Please contact us in case of questions.

MODBUS
The MODBUS protocol was published by Modicon in 1979. It is a quite simple single master – multiple slave protocol widely used in general purpose SCADA applications. It provides binary (bit) and 16-bit integer data, but no floating point values, polled by the master, i.e. the slave cannot send data spontaneously. The protocol specification is openly published and royalty-free. In the DTS ECM, TCP/IP is used as transport layer.

IEC 60870-5
IEC 60870-5 provides a communication profile for sending basic Telecontrol messages between systems. It was developed by the IEC Technical Committee 57 (Working Group 03) as a protocol standard for Telecontrol, Teleprotection, and associated telecommunications for electric power systems. The result of this work is IEC 60870-5. The protocol provides inter alia bit, integer and floating point data with or without time stamps. Data is transmitted from controlled stations (slaves) either by polling or spontaneously. In the ECM the IEC 60870-5-104 companion standard, defining standard TCP/IP connections as transport layer, is used.

DNP3
DNP3 (Distributed Networking Protocol) is an open standard managed by the DNP User Group. Initially; the DNP3 protocol was developed by Westronic (now GE-Harris) in 1993. It is widely used in electrical systems worldwide, including North and South America, the UK, Southeast Asia, South Africa, Australia, and New Zealand. It has gained wider use beyond electrical utilities, e.g. for water supply as well. The DNP3 protocol is also referenced in IEEE Std. 1379-2000, which recommends a set of best practices for implementing modern SCADA Master-RTU/IED communication links.
The protocol provides bit, integer and floating point data and time stamps. Data is transmitted from the outstation (slave) either by polling or spontaneously. The ECM uses TCP/IP as transport layer, but even in this mode the DNP3 protocol additionally uses its own protocol stack for synchronisation and error checking. This provides extra security.

The ideal sensor embedded in power cables: Fibre In Metal Tube (FIMT)

The optical fibre serves as the distributed temperature sensor for LIOS DTS systems and is typically surrounded by a protective layer. It is preferably encased in a stainless steel tube which significantly increases the mechanical stability of the sensor. Fibre encapsulated in stainless steel tube or short “Fibre In Metal Tube (FIMT)” is a hermetically sealed rugged construction for very long lengths of optical fibres. Furthermore, it is particularly effective in protecting against the hydrostatic pressures, high temperature effects and corrosive environments.
The inside of the FIMT metallic tube may lined with gel to ensure that the sensor cable remains permanently water-proof. This viscous gel protects the fibres from many environmental concerns, prevents damage from microbending conditions and helps to minimize the forces applied during spooling and deployment.
An important parameter to consider with FIMT is the Excess Fibre Length. It is defined by the percentage of excess fibre loaded into the metal tube during processing to relieve induced stresses directly related to the differing thermal coefficient of expansions of each material contained in the construction. FIMT is exposed to drastic and often rapid changes in temperature and pressure, so the Excess Fibre Length must be considered to ensure continuous, robust performance across all operational conditions.
FIMT constructions contain individual or multiple fibres (single mode or multi mode) and are available in sizes ranging from 1 mm to 3 mm outer diameter, variations in effective wall thickness are possible as well. FIMT serves as the core for various fibre optic sensor cable constructions or may be integrated into high voltage cable designs directly.
The FIMT is manufactured from a special stainless steel strip. During the manufacturing process this strip is formed to a tube and welded along its length. The stainless steel tubes must be hermetically sealed for all applications. For this reason the complete weld seam is subjected to a leak proof test by means of eddy current. By means of a drawing process the desired final diameter is attained on the one hand and an increase in strength through cold conversion on the other. During the manufacturing process the optical fibres are inserted into the tube. They are clearly differentiated by means of a specific colour code. At the end of the production process the tubes are subjected to a complete optical attenuation measurement and then documented for back-tracing. Additionally, each length produced is subjected to a weld penetration test, each weld seam undergoes a leak test, and the Excess Fibre Length is checked.
LIOS offers a variety of FIMT constructions perfectly suitable for distributed temperature sensing applications along power cable transmission systems.