Distributed Temperature Sensing: Optimum Utilization of Energy Transmission Backbone in Eastern Europe

LIOS Technology successfully provided an integrated thermal monitoring solution for a strategically important interconnection aerial line in Eastern Europe.

The LIOS DTS system was supplied to provide distributed temperature measurement by means of a fibre integrated into a replaced optical phase conductor (OPPC). The picture above shows a straight joint of the OPPC.
The DTS covers the entire 17 km range of the three-core bundle 400kV transmission line. The applied Optical Frequency Domain Reflectometry (OFDR) principle ensures a temperature survey even over long distances at the decisive spatial resolution of 1 m.
It provides an invariant spatial resolution along the entire sensor length and a satisfactory temperature resolution of less than 1°C, which ensures early identification and clearly measurement of atypical hotspots (e.g. tower shadows). In addition to the phase conductor temperature, the electrical and metrological data are integral parameters for the established dynamic rating system (RTTR). All data are made online available through the SCADA system of the utility.
The integrated monitoring solution generates:

• Safety - Avoid and respond appropriately to critical load situations
• Optimization  - Utilize the existing grid capacity to an optimum
• Dynamic and Flexibility - With dynamic rating to a flexible energy backbone

Sistema Óptico de Medición de Temperatura Distribuida (DTS)

Distributed Temperature Sensing

Principios básicos de Sistema Óptico de Medición de Temperatura Distribuida (DTS)
Los sistemas de fibra óptica son adecuados no sólo para la transmisión de información, sino también como sensores distribuidos localmente. Las magnitudes físicas de medida como la temperatura o las fuerzas de compresión y de tracción pueden influir en las fibra de vidrio y modificar localmente las propiedades de los conductores de luz en la fibra. Como resultado de la atenuación de la luz en las fibras de vidrio de cuarzo producida por la dispersión, se puede determinar el lugar de una influencia física externa, de manera que la guía de ondas de luz se puede utilizar como un sensor lineal.
El llamado efecto Raman es especialmente apropiado para la medición de la temperatura con guíaondas de luz de vidrio de cuarzo. En el interior de la fibra de vidrio, la luz se dispersa en fluctuaciones de densidad microscópicas, que son menores que la longitud de onda. En la retrodispersión, junto al coeficiente de dispersión elástica (Dispersión de Rayleigh) en la misma longitud de onda que la luz incidente, también se encuentran componentes adicionales en otras longitudes de onda, que están acoplados a la oscilación molecular y, por tanto, a la temperatura local (Dispersión de Raman).

Procedimiento de medición
El sistema de medición de temperatura por fibra óptica (DTS) se basa en un procedimiento de retrodispersión Raman por fibra óptica. El detector de calor (sensor de temperatura) propiamente dicho, es un cable guíaondas fibroóptico sensible al calor y a la radiación. Por medio de un aparato de evaluación (reflectómetro óptico Raman) se pueden determinar con resolución espacial los valores de temperatura en la fibra de vidrio del cable guíaondas. Las guíaondas de luz tienen atenuaciones mínimas. La atenuación mínima posible de las fibras de vidrio está limitada por la dispersión Rayleigh de la luz, causada por la estructura amorfa de la fibra de vidrio. Además de la dispersión de Rayleigh, si se producen influencias térmicas en el material de fibra de vidrio se origina otra dispersión de la luz, la llamada dispersión de Raman. Los cambios de temperatura inducen vibraciones reticulares en la estructura molecular del vidrio de cuarzo. Si la luz incide en estas oscilaciones moleculares estimuladas térmicamente, se produce una interacción entre las partículas de luz (fotones) y los electrones de la molécula. En el guíaondas de luz tiene lugar la dispersión de luz dependiente de la temperatura (dispersión de Raman) que, a diferencia de la luz incidente, se desplaza espectralmente en una cantidad equivalente a la frecuencia de resonancia de la vibración reticular.
En comparación con la dispersión de Rayleigh, la dispersión de Raman posee un coeficiente de dispersión muy pequeño, prácticamente irrelevante, y no puede medirse con la técnica clásica OTDR. La intensidad de la banda Anti-Stokes depende de la temperatura, mientras que la banda Stokes es prácticamente independiente de ésta. La medición de la temperatura local en un punto cualquiera de la guía de ondas,  resulta de la “proporción de las intensidades de luz Anti-Stokes y Stokes”. Una particularidad de esta técnica de Raman, es la medición directa de la temperatura con una escala Kelvin. Utilizando un procedimiento de retrodispersión óptico Raman, se puede medir la temperatura a lo largo de la fibra de vidrio como una función de lugar y de tiempo. El procedimiento de retrodispersión más conocido es el sistema OTDR (OTDR: Optical Time Domain Reflectometry). Éste trabaja según un procedimiento de eco de impulsos, mediante el que el nivel y el lugar de dispersión se determinan a partir de la diferencia de tiempo de propagación entre la emisión y la detección de los impulsos de luz. En comparación con la luz dispersa de Rayleigh, en la medición de luz dispersa de Raman existe una señal de retrodispersión más pequeña con un factor 1000. Un sensor de temperatura Raman distribuido localmente con la técnica OTDR, por lo tanto, sólo es factible con fuentes de láser pulsado (en láser de estado sólido general) de alta potencia (caras) y con rápidas técnicas de promediación de señal igualmente caras.
El sensor térmico Raman OFDR (OFDR, Optical Frequency Domain Reflectometry) desarrollado por la empresa LIOS Technology GmbH, no trabaja como la técnica OTDR en un margen temporal, sino en una gama de frecuencias. El procedimiento OFDR proporciona información sobre el desarrollo térmico local, cuando la señal de dispersión detectada durante el tiempo de medición completo se mide como una función de frecuencia de forma compleja (función compleja de transmisión) y, a continuación, se somete a la transformada de Fourier. Las ventajas fundamentales de la técnica OFDR son el modo prácticamente continuo del láser  y la detección de banda estrecha de la señal óptica de retrodispersión, por lo que se alcanza una relación señal-ruido notablemente más alta que con la técnica por impulsos. Esta ventaja técnica posibilita la aplicación de diodos de láser semiconductorizados económicos y la utilización de módulos electrónicos más económicos para la promediación de señales. Frente a ésta, se encuentra la medición, técnicamente difícil, de la luz dispersa de Raman (medición compleja según cantidad y fase) y un costoso procesado de señal mediante el cálculo de la Transformada rápida de Fourier (FFT), con exigencias más elevadas de linealidad de los módulos electrónicos.

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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Containment Integrity Monitoring Solutions for LNG Carriers

LNG Carrier

LIOS Technology distributed temperature measurement system (DTS-System) is suited ideally to detect reliably and to locate precisely any temperature anomaly caused by an accidental release of LNG. Our innovative and patented OFDR evaluation method enables the determination of a continuous temperature profile along the entire length of an optical fibre offering thousand of data points at a single measurement. Based on our expert knowledge and our impressive record of safety installations we have developed suitable alarm processing algorithm to detect temperature events fast, reliably and precisely. Also multiple temperature events are identified and pinpointed simultaneously allowing the operator to initiate proper counter measures rapidly.
With the capability to use multiple optical channels and a measurement range of  several kilometres per channel the LIOS Technology DTS system is a cost - sensitive solution for an entire cryogenic supervision including containments, foundations, pumps and piping.  The system supplies precise temperature information which facilitates the control of cool-down operations like load- and unload procedures. Additionally an optional loop set-up of the sensor cable enables a redundant operation of the system. Consequently the entire measurement length keeps maintained in case of single fibre break.  Summarized the operator takes the following benefit from using LIOS Technology DTS system:

• Full information of containment integrity – everywhere at any time
• Significantly reduced off-times due to exact pinpointing of leaks
• Enabling effective and automatic counter measures
• Fast detection of slow leaks as well as sudden leaks
• Better control of load and unload procedures (cool-down operations)
• Maximum safety of people, assets and environment
• Redundant operation thanks to loop set-up of the sensor cable
• Reduced installation efforts – additional wiring is not required
• Low maintenance efforts and cost
• Long lifetime

  Read more:

	LNG and LPG Containment Integrity Monitoring Solutions
	Information centre for LNG operations

Superior Technology and Industrial Strength – State of the Art DTS Production

The LIOS' DTS (Distributed Temperature Sensing) technology has been successfully proven in critical applications like fire detection in road and rail tunnels,  power cable and transmission line monitoring, in oil & gas exploration for permanent downhole monitoring and for industrial induction furnaces surveillance, where these systems have been equipped in worldwide projects with more than 2000 permanent installations since 1997.

Key advantages of Raman OFDR distributed temperature sensing (DTS) systems by LIOS Technology:

• Reliable system design with approved key components from the telecom industry
• The OFDR technology enables to provide sophisticated temperature surveillance at commodity prices
• Invariant spatial resolution along the entire sensor length of 1 m or 50 cm even at most remote distances.
• Direct link to SCADA systems and flexible data handling
• International Approvals and Certificates – type tested and accredited
• Impressive track record of more than 2000 installations in permanent operation

At LIOS’ new facilities at Seiler Höfe, Cologne, Germany we experience ideal surrounding conditions as an industrial manufacturer of fiber optic DTS sensor systems. In conscious of the strong requirements being made in the safety market, we provide a highly proven product based on the compliance with international quality standards recognised by impartial and competent partners for type testing our products like the VdS (Association of German Property Insurers), Deloitte or EXAM and is furthermore reflected in our implemented quality management system, certified according to DIN ISO 9001 / Edition 12/2008 and our environmental management system according to ISO 14001.

• Industrial DTS production with impressive track record
• Modern and efficient production procedures
• Comprehensively type tested
• 100 % applied Factory Acceptance Test (FAT)
• Customised Site Acceptance Test (SAT) through experienced staff on site
• High Tech made in Germany

Related information on LIOS DTS technology:

• The technology behind LIOS OFDR distributed temperature sensing systems