Optical current sensors or so called optical current transformers (OCT) based on the Faraday effect have an intrinsic temperature dependency of the current sensitivity caused by the natural drift of the Verdet constant as well as birefringence in the sensing medium. In order to reach high class accuracy of up to 0.2%, comparable to inductive current transformers, these effects have to be compensated.
Two application examples of RFBG sensor arrays that are intended for high temperature profile measurements in chemical reaction vessels and in a gas turbine exhaust duct are described. The array for the chemical reaction vessel includes four sub-arrays with six measurement points each, distributed over a length of 2.3 m. By employing a largescale test facility, temperature profiles extending over 2.3 m with temperatures in the range of 200 °C to 500 °C were acquired with this RFBG array and an excellent agreement with thermocouple data was demonstrated. The second application reported is the measurement of the radial temperature distribution within the exhaust duct of a 7 MW gas turbine. Therefore, a three-point RFBG array was mounted radially in the exhaust gas stream and significant changes in the measured temperature gradient were observed when the engine was in idle or in full load operation mode. In addition to these field deployments of RFBG sensors, long-term annealing experiments at 450 °C in the research lab showed only a shift of the nonlinear temperature calibration curve without changing its shape, allowing recalibrations of RFBG sensor arrays at single temperature points. These examples demonstrate the suitability of RFBG based multipoint high temperature sensing for industrial applications.
This paper presents a new method of identifying an optical sensor like Fiber Bragg Grating1,2 (FBG) temperature sensors. The identifier is a pattern inscribed into the core of an optical fiber by a femtosecond laser. A Rayleigh backscattering interrogator is used to readout the identifier.
A temperature measurement of a high voltage bushing (HV-Bushing) is presented. An optical fiber with several inscribed Fiber Bragg Gratings (FBGs) was used to measure the temperature at given positions and a Rayleigh-based measurement technique was used to measure the temperature (profile) between the FBGs. The used optical frequencies for the FBG and Rayleigh measurement had to be separated to achieve good results. Voltages of up to 1 MV were applied to the HV-Bushing during the measurement.
Low temperature Pyrometry at temperatures beyond 150°C is limited in the measurement speed due to slow pyroelectric detectors. To detect the circumferential temperature distribution of fast rotating machines a novel Fiber Optical Pyrometer Type is presented here.
Commercial distance sensors basing on the triangulation principle are highly accurate and reliable. However due to their contained electronics and optoelectronics they cannot be used in harsh environments such as high temperatures and strong electromagnetic fields. An all fiber optical triangulation sensor principle is presented here which can be used for tip clearance measurements of rotors of large engines such as power generators and turbines.
A common way to explore oil out of tar sand is to use a technique called Steam Assisted Gravity Drainage SAGD. This method can be enhanced by using an inductive heater (EM-SAGD). To monitor the heat dissipation of the inductor a measurement system for this harsh electromagnetic environment is needed. In this paper different optical temperature measurement systems are compared to find the most suitable system for this kind of application. A field test with great results was performed, where the performance of the inductor and the FBG measurement system were demonstrated.
Several fiber optical temperature sensor systems are commercially available. However for many industrial and electrical power applications very simple and cost effective solutions are required which easily can be integrated into existing condition monitoring facilities. A simple two wavelength interrogation system has been designed and combined with the most appropriate fiber optical temperature probes to serve applications in electrical power generation.
Inside of large electrical engines such as power generators and large drives, extreme electric and magnetic fields can
occur which cannot be measured electrically. Novel fiber optical magnetic field sensors are being used to characterize
the fields and recognize inner faults of large power generators.
The increasing quantity of renewable energy in electric power generation leads to a higher flexibility in the operation of
conventional power plants. The turbo generator has to face the influence of frequent start-stop-operation on thermal
movement and vibration of the stator end windings. Large indirect cooled turbo generators have been equipped with
FBG strain and temperature sensors to monitor the influence of peak load operation. Fiber optic accelerometers measure
the vibration of the end windings at several turbine generators since many years of operation. The long term reliability of
fiber optic vibration, temperature and strain sensors has been successfully proved during years of online operation. The
analysis of these data in correlation to significant operation parameter lead to important diagnostic information.
The raising demand for increase of efficiency and reduction of costs in power generation causes a mind change and
promotes the commercial use of fiber optical sensors for health monitoring and control purposes.
The increasing need of energy and the increasing share of renewables in electric power generation demands higher
flexibility in the operation of conventional power plants. Turbo generators have to face higher stress during operation
without consuming additional life time. For the first time in a shop test a new generator design was extensively evaluated
by using about 250 fibre optic sensors - mostly new developed - to control temperature, strain, movement and vibration.
Various spectral-encoded fiber optic sensor concepts and advanced system solutions for application in energy facility
monitoring have been investigated. The technological maturity, high performance and reliability of multiplexed fiber
Bragg grating (FBG) sensor arrays and networks for the measurement of temperature, dynamic strain, air flow, and
magnetic field distributions in electric power generators increasing their efficiency will be demonstrated by selected
examples of field testing under harsh environmental conditions. For high-temperature combustion monitoring in gas
turbines, beside silica FBGs with enhanced temperature stability also sapphire FBGs and Fabry-Perot sensors have been
tested and evaluated as well as fiber-based black-body thermal radiation sensors. Finally, the potential of FBG sensors
for application in cryo-energetic facilities such as super-conductive high-power motors and experimental nuclear fusion
reactors will be discussed.
To verify optimization measures of power generators to improve the energy efficiency and to monitor critical parameters,
fiber optical sensors have been developed and investigated. A fiber optical hot wire anemometer based on the thermooptic
effect of Fiber Bragg Gratings was investigated to measure the flow distribution along the stator core. Fiber optical
magnetic field sensors, based on the strain-optic effect of FBGs, were used to measure the magnetic field distribution on
the end windings of a power generator. A novel fiber-optical accelerometer was used to measure the end winding
vibrations. In this paper the functionality of each sensor is described and results of field test under real conditions are
shown and discussed.
Electric energy will become the backbone of future industrial development. Fossil fuel resources are still available, but
the exploration will become increasingly costly. The share of renewable sources in power generation will increase
significantly. Focusing on energy efficiency, condition monitoring systems will be implemented along the complete
power generation and distribution chain. Smart sensors will function as the eyes for controlling the smart grid.
Multiple fiber optic sensors have been developed for strain, vibration, temperature, magnetic field and air flow
measurement in electric power generators. This paper describes the recent state of development and reports about
todays field experience.
Their small size, the multiplexing capability, and the high elasticity of glass fiber can provide reasons to use optical fiber
temperature sensors instead of approved conventional thermo couples for temperature monitoring of hot gas turbine
components.
Large power engines are permanently increasing efficiency. Knowledge of thermal and mechanical loads is
getting important. A novel coupling technique allows to interrogate FBG temperature and strain sensors on
rotating parts which are normally not accessible.
It has been demonstrated that Fiber Bragg Grating Sensors can measure at very low temperatures in Helium
cryostats. The intrinsic temperature sensitivity is very low but can be significantly enhance by the thermal
expansion of polymer carriers. Temperature and mechanical strain changes can be measured quite accurately.
For a long time electric power was taken as a natural unlimited resource. With globalization the demand for energy has
risen. This has brought rising prices for fossil fuels, as well as a diversification of power generation. Besides
conventional fossil, nuclear plants are coming up again. Renewable energy sources are gaining importance resulting in
recent boom of wind energy plants. In the past reliability and availability and an extremely long lifetime were of
paramount importance. Today this has been added by cost, due to the global competition and the high fuel costs. New
designs of power components have increased efficiency using lesser material. Higher efficiency causes inevitably higher
stress on the materials, of which the machines are built. As a reduction of lifetime is not acceptable and maintenance
costs are expected to be at a minimum, condition monitoring systems are going to being used now. This offers potentials
for fiber optic sensor applications.
For a long time electric power was taken as a natural unlimited resource. With globalisation the demand for energy has
risen. This has brought rising prices for fossil fuels, as well as a diversification of power generation. Besides
conventional fossil, nuclear plants are coming up again. Renewable energy sources are gaining importance resulting in
recent boom of wind energy plants. In the past reliability and availability and an extremely long lifetime were of
paramount importance. Today this has been added by cost, due to the global competition and the high fuel costs. New
designs of power components have increased efficiency using lesser material. Higher efficiency causes inevitably higher
stress on the materials, of which the machines are built. As a reduction of lifetime is not acceptable and maintenance
costs are expected to be at a minimum, condition monitoring systems are going to being used now. This offers potentials
for fibre optic sensor application.
Knowledge of the gas flow distributions, their mass velocity and turbulences, in chemical reactors, thermodynamic
engines, pipes, and other industrial facilities may help to achieve a more efficient system performance.
In our novel approach, optical fibre Bragg grating (FBG) sensors have been used for measuring the temperature of a
heated element, adapting the principles of conventional hot-wire-anemometers. Because of the multiplexing capability
of FBG sensors, the gas mass flow distribution can be measured along the sensor array. The length of the heated and
sensor-equipped element can be easily adapted to the cross section of the gas flow, from <10 cm up to several metres.
The number and distances of FBGs distributed over this length defines the spatial resolution and is basically limited by
the sensor signal processing. According to FBG sensor lengths < 5 mm, spatial resolutions of gas flow measurements of
less than 1 cm can be achieved.
Conventional heated wire anemometry in combination with distributed fiber optical temperature sensors based on fiber Bragg gratings offer the possibility of distributed flow measurements.
Fibre-optic sensors can show significant advantages against other sensor technologies when installed in the right application. We demonstrated successfully the use of fibre-optic current, temperature and strain sensors in power generators, power combustion turbines, and electric infrastructure and preventive maintenance for transportation systems. All solutions offer enhanced efficiency and availability of the units they were installed in.
A sensor network consisting of unified fiber grating based temperature and strain sensor pads has been developed for integration in carbon/aluminum composite current collector strips, and has been tested in electrical trains on commercial railways. The fiber optic sensor network measures value and position of both con-tact forces and impacts under real-time conditions, immediately at the high voltage location of the interface between overhead contact line and current collector.
KEYWORDS: Sensors, Fiber Bragg gratings, Temperature metrology, Carbon, Temperature sensors, Data acquisition, Modeling, Power supplies, Silver, Time metrology
For the first time online temperature measurements with FBG temperature sensors on railway overhead lines could be demonstrated. All sensors measured successfully under outdoor conditions over a two year period. As a first experimental result the excess temperatures due to current load are small compared to ambient sources of temperature change. In the future this first result will be analyzed under different seasonal and current load conditions. Furthermore the modeling and first field test have demonstrated the possibility to acquire information about the interaction between OCL and current collector via optical strain gauges integrated into conventional current collector strips.
This paper reports on the first realization of embedding fiber optical Bragg grating temperature sensors inside the stator bars of a 120 MVA air cooled power generator. Furthermore first measurements of the conductor temperature inside the bushings have been carried out during a generator prototype test run.
Optical fibre current and voltage sensors for power industry have needed about 20 years of devleopment time to gradually emerge at the market. The advantageous potential separation is confronted with an expectation of high reliability and low costs.
Electric and magnetic fields can be measured optically, either directly via Pockels and Faraday effect or indirectly via Piezo effect. In high voltage applications fiber optic sensors are especially attractive for their ability of easy potential separation. For electric fields the Pockels effect in BSO, BGO crystals is mostly read out polarimetrically. The current is mostly measured via the Faraday effect in fibers or bulk optics polarimetrically, but also by means of a Sagnac interferometer. Electrooptic voltage and magnetooptic current sensors have quite a matured state of development and shown their benefits in a lot of field tests and installation in high voltage switch gears.
A cost-effectively designed polychromator using holographic grating as spectral dispersive element and CCD line array as detector is applied to the multiplexing of a many-element fiber grating sensor network for measuring temperature and stationary strain. The influence of intensity and polarization fluctuations in the superluminescent diode as a broadband light source, and in the fiber transmission lines as well have been minimized by reducing parasitic reflections and introducing depolarizing elements. Other error sources of the sensor read-out stability have been reduced by appropriate peak fitting procedures of the CCD pixel intensity distribution and by the consumption of mechanically stable sensor heads for temperature and strain sensing with low cross sensitivity. A special mounting technique allowing compensation of thermally induced Bragg wavelength shifts is used for obtaining a wavelength reference in the polychromator. Thus, stability of wavelength readout is only noise-limited with a rms value of about 0.2 pm and yields a resolution and stability for measurement of temperature <EQ 0.1 K and for stationary strain <EQ 0.l5 (mu) (epsilon) .
Optical current sensors using the Faraday effect promise significant advantages over conventional current transformers. One of the remaining problems is the temperature dependence of the optical current measurement mainly caused by intrinsic or stress induced linear birefringence. By an appropriate annealing process most of the linear birefringence in a fiber coil can be removed. But still a significant temperature drift remains. For AC measurements we have developed a novel compensation method, which uses the DC part of the output signal to compensate the temperature drift. We have achieved a reduction in temperature sensitivity by a factor of 8 to about 0.5% over a temperature range from room temperature to 80 degree(s)C. The overall accuracy of the current sensor is now limited only by the system's noise and stability.
Magnetooptic current sensors are gaining interest due to the following advantages: simple potential separation, immunity against electromagnetic interference, high dynamic range and bandwidth and compact design. Magnetooptic current transformers - MOCT - offer a complete separation of energy and signal distribution in power industry.
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