**2. Optical fiber sensors**

Optical fiber sensors (OFS) came just after the invention of the optical fiber in the 70's. At the beginning of this era, optical devices such as laser, photodetectors and the optical fibers were very expensive, adequate only to the already saturated telephone network in which companies would pay any price to transmit more information and more telephone calls. With the great diffusion of the optical fiber technology in the 80´and on, optoelectronic devices became less expensive, what favored their use in OFS.

OFS can be applied in many branches of the industry but we will concentrate here their applications through our experience in the electric power industry.

In this area, the operators need to measure and monitor some important physical parameters that include:


Some of these parameters, depending on where they are located, are very difficult or even impossible to be conventionally monitored because of a well-known paradigm of the electrical power industry: An electric sensor cannot be close enough to a high potential in order to break the electric rigidity of the air, which is about 1 kV/cm. This would cause a short circuit when the current would flow from high voltage to ground potential by the sensor's connecting wires. The best option to avoid this catastrophic effect is the OFS, because the fiber is made of dielectric materials and therefore it is possible to be placed very close or even touch a high potential conductor and they do not necessary need electrical power at the sensor location.

OFS can be built using several physical principles and materials. They have specific characteristics that are well exploited when applied to the electric power industry and in this case OFS offer a large number of advantages over conventional sensors. The most important are:


In conclusion, when it comes to telemetry, optical fibers perform telemetric measurements at distances much longer than conventional telemetry protocols and media. Additionally, due to its virtually infinite capacity to multiplex, one can mix different kinds of signals in one single fiber therefore saving many kilometers of copper wires, which is also welcome by the

In this article we will concentrate on applications of telemetry over optical fiber and on

Optical fiber sensors (OFS) came just after the invention of the optical fiber in the 70's. At the beginning of this era, optical devices such as laser, photodetectors and the optical fibers were very expensive, adequate only to the already saturated telephone network in which companies would pay any price to transmit more information and more telephone calls. With the great diffusion of the optical fiber technology in the 80´and on, optoelectronic

OFS can be applied in many branches of the industry but we will concentrate here their

In this area, the operators need to measure and monitor some important physical

Some of these parameters, depending on where they are located, are very difficult or even impossible to be conventionally monitored because of a well-known paradigm of the electrical power industry: An electric sensor cannot be close enough to a high potential in order to break the electric rigidity of the air, which is about 1 kV/cm. This would cause a short circuit when the current would flow from high voltage to ground potential by the sensor's connecting wires. The best option to avoid this catastrophic effect is the OFS, because the fiber is made of dielectric materials and therefore it is possible to be placed very close or even touch a high potential conductor and they do not necessary need electrical power at the sensor location. OFS can be built using several physical principles and materials. They have specific characteristics that are well exploited when applied to the electric power industry and in this case OFS offer a large number of advantages over conventional sensors. The most

optical fiber sensors which encompass telemetry and sensor in one single media.

devices became less expensive, what favored their use in OFS.

Distance between stationary and rotating or moving parts

applications through our experience in the electric power industry.

maintenance personnel.

**2. Optical fiber sensors** 

parameters that include:

Impedancy (µΩ)

Gas concentration

 Temperature Pressure

important are:

 High immunity to EMI Electrical insulation Absence of metallic parts

 Local electrical power not required Lightweight and compactness

 Vibration of structures and machines Electric current (from A to kA) Voltage (from mV to MV)

Leakage current of insulators (µA to mA)

Strain (µє)


The high immunity to electromagnetic interference (EMI) is a strong requirement for sensing in electromagnetic contaminated environments, e.g. RF-field and high electric and magnetic fields present in power lines.

The insulation is another special requirement, because as these sensors are inherently electrically insulated (dielectric) and do not require external power, this means that there is no electric path from the power line to ground, which means high personnel security. Therefore the optical fiber sensors can work at high electrical potentials and in potentially explosive environments.

Optical fibers can be used as sensors by modifying a fiber so that the measurand interferes on the guided light and modulate light parameters such as intensity, phase, polarization, wavelength, or transit time of light over the fiber. Sensors that vary the intensity of light are the simplest, since only a simple source and detector are required.

We can divide OFS in three basic categories: intrinsic, extrinsic and evanescent field based.

Extrinsic fiber optic sensors use an optical fiber, normally multi-mode, to transmit modulated light from either a non-fiber optical sensor or an electronic sensor connected to an optical transmitter. In this case the optical fiber is used only to transmit light to and from the sensor. This kind of sensor sometimes is called hybrid sensor for it enclosures different technologies such as optics and electronics.

In intrinsic sensors the light does not leave the fiber and the light modulation takes place inside the fiber. This kind of sensor presents the major benefit to have the ability to reach otherwise inaccessible places and without the need of electrical energy at the sensing location.

The third category is the evanescent field based sensor. Due to the total internal reflection phenomenon that occurs in the core-cladding interface of the fiber, the light propagating in the fiber has two components - an oscillatory field in the core and an exponentially decaying field in the cladding. The latter field, referred to as the evanescent field, is the key to sensing and is based on the modulation of the light amplitude in the core of the fiber by the optical properties of the surrounding medium.

When developing an OFS we can use the fiber for: a) conducting light; b) to be the sensor itself; and c) for both applications, that is, sensing and conducting light to and from the sensing area.

An optical fiber is a thin, flexible, transparent glassy filament that acts as a waveguide, or "light pipe", to transmit light from the light source to the photodetector located at the two ends of the fiber. They are mainly used for telecom and sensing but find many uses in the industry, research sciences, medicine, entertainment etc.

By the 70's all telephone cables and microwave links in the planet were already saturated. The solution came when Charles Kao and George Hockham of the British company Standard Telephones and Cables (STC) promoted the idea that the attenuation in the existing optical fibers could be reduced below 20 decibels per kilometer (dB/km), making fibers a practical communication medium. They proposed that the attenuation in fibers available at the time was caused by impurities that could be removed by chemical processes. They correctly and systematically theorized the light-loss properties for optical fiber, and pointed out the right material to use for such fibers — silica glass with high purity. This discovery earned Kao the Nobel Prize in Physics in 2009.

Optical Fiber Sensors 7

Due to their larger diameter, it is simpler to work with open optics and easy handling. POFs are cheaper than their counterpart as well as the peripheral components and devices, such as connectors, LEDs and photodetectors. They also present more resistance to strain (larger modulus of elasticity) which means more reliable networks. Finally, many interfaces can be built in laboratory what makes the maintenance cost much lower than when dealing with

Of course POFs have disadvantages too. POF only transmits visible and near infrared light, so we cannot use the available technology of telecommunications such as 1300 nm and 1500 nm telecom windows. Additionally, POF has a very high attenuation in the visible

The other issue is the temperature because plastic materials cannot withstand high temperatures as much as glasses. POFs can operate only up to 70 to 85oC. However, some specials POFs have been developed mainly for harsh environment such as in car networks. In these applications POFs have to withstand temperatures as much as 150oC. Table 1 shows

The attenuation of silica fibers is negligible for sensing distances (around 1 km), but when using a POF for transmitting light, the first thing to have in mind is the high attenuation the

This section will present real applications of OFS and telemetry in the electrical power industry. The techniques presented here have been tested in the field mainly in high voltage

 Maintenance costs More resistance to strain

Easy handling

silica fibers.

spectrum (see Fig. 2.3).

some examples.

Cheaper peripherical components

No need of special skill for splicing and connectorization

Fig. 2.3. Optical attenuation of silica fibers and POFs.

POF impinges to the light.

**3. Case studies** 

The crucial attenuation limit of 20 dB/km was first achieved in 1970 by researchers at the American glass maker Corning Glass Works, now Corning Incorporated. They demonstrated a fiber with 17 dB/km attenuation by doping silica glass with titanium. A few years later they produced a fiber with only 4 dB/km attenuation using germanium dioxide as the core dopant. Such low attenuation allowed optical fiber to be used in telecom from the 80's until today when the telecom fiber presents an attenuation of only0.25 dB/km.

Although polymeric optical fibers (POF) are around us much longer than silica fibers, only in the last decade they start to attract attention for LANs and small industrial networks and their use for sensors has just emerged few years ago. Figure 2.1 shows the different diameters as comparing POFs with silica fibers.

Fig. 2.1. Relative comparison of diameters in different kinds of fibers. SI-POF=step-index polymeric optical fiber; PCS=plastic cladding silica fiber; MM Silica=multimode silica fiber; SM silica=single mode silica fiber; PF-GI-POF=perflorinated graded-index POF. The light color represents the cladding and dark color the core.

The first report of poly-metil-meta-acrylate (PMMA) POF dates from 1968 when Du Pont presented a POF with an attenuation of 500 dB/km. From then on several laboratories are keeping trying to decrease the attenuation in order to apply POF in telecom. Figure 2.2 shows the results of those efforts.

Fig. 2.2. History of the attenuation improvement of PF-GI-POF.

Comparing POF and silica fibers by the attenuation, silica fibers are much better. However, when constructing a fiber sensor using POF instead of silica, we have some additional advantages:

The crucial attenuation limit of 20 dB/km was first achieved in 1970 by researchers at the American glass maker Corning Glass Works, now Corning Incorporated. They demonstrated a fiber with 17 dB/km attenuation by doping silica glass with titanium. A few years later they produced a fiber with only 4 dB/km attenuation using germanium dioxide as the core dopant. Such low attenuation allowed optical fiber to be used in telecom from the 80's until today when the telecom fiber presents an attenuation of only0.25 dB/km. Although polymeric optical fibers (POF) are around us much longer than silica fibers, only in the last decade they start to attract attention for LANs and small industrial networks and their use for sensors has just emerged few years ago. Figure 2.1 shows the different

Fig. 2.1. Relative comparison of diameters in different kinds of fibers. SI-POF=step-index polymeric optical fiber; PCS=plastic cladding silica fiber; MM Silica=multimode silica fiber; SM silica=single mode silica fiber; PF-GI-POF=perflorinated graded-index POF. The light

The first report of poly-metil-meta-acrylate (PMMA) POF dates from 1968 when Du Pont presented a POF with an attenuation of 500 dB/km. From then on several laboratories are keeping trying to decrease the attenuation in order to apply POF in telecom. Figure 2.2

Comparing POF and silica fibers by the attenuation, silica fibers are much better. However, when constructing a fiber sensor using POF instead of silica, we have some additional

diameters as comparing POFs with silica fibers.

color represents the cladding and dark color the core.

Fig. 2.2. History of the attenuation improvement of PF-GI-POF.

shows the results of those efforts.

advantages:


Due to their larger diameter, it is simpler to work with open optics and easy handling. POFs are cheaper than their counterpart as well as the peripheral components and devices, such as connectors, LEDs and photodetectors. They also present more resistance to strain (larger modulus of elasticity) which means more reliable networks. Finally, many interfaces can be built in laboratory what makes the maintenance cost much lower than when dealing with silica fibers.

Of course POFs have disadvantages too. POF only transmits visible and near infrared light, so we cannot use the available technology of telecommunications such as 1300 nm and 1500 nm telecom windows. Additionally, POF has a very high attenuation in the visible spectrum (see Fig. 2.3).

The other issue is the temperature because plastic materials cannot withstand high temperatures as much as glasses. POFs can operate only up to 70 to 85oC. However, some specials POFs have been developed mainly for harsh environment such as in car networks. In these applications POFs have to withstand temperatures as much as 150oC. Table 1 shows some examples.

Fig. 2.3. Optical attenuation of silica fibers and POFs.

The attenuation of silica fibers is negligible for sensing distances (around 1 km), but when using a POF for transmitting light, the first thing to have in mind is the high attenuation the POF impinges to the light.

#### **3. Case studies**

This section will present real applications of OFS and telemetry in the electrical power industry. The techniques presented here have been tested in the field mainly in high voltage

Optical Fiber Sensors 9

geometrically compatible with standard POFs and even after cutting and polishing it

Fig. 3.1.1 shows a top view picture of the temperature sensor prototype (conditioning equipment) where the key components are assigned. The LED package was polished almost reaching the semiconductor chip thus maximizing the light caption. Light launching was made through butt-coupling the polished LED and a carefully terminated POF. Optical pulses of 32 ms time-width from the LED were generated to pump the ruby crystal at 15.6 Hz. A miniature 1x2 POF-coupler is used to send pump pulses toward the ruby crystal glued at the end of the

Fig. 3.1.1. Top view picture of the temperature sensor prototype (conditioning equipment). Fig. 3.1.2 shows the picture of the one-POF-probe with 4 mm in diameter. The POF was terminated at the other end with a standard HP plastic connector. With this configuration, it can be detached from the conditioning equipment box. The fluorescence response from the crystal, passing through the same POF, was conveyed into the other port of the coupler. Due to the back reflections at the many optical interfaces, the fluorescent signal could have been buried under the intense excitation signal. Thus, in order to avoid a saturation of the detection stage and to enable the fluorescence response to be detected exclusively, a red long-pass filter was placed before the Si photodiode. The electrical signal generated from the

Fig. 3.1.2. Picture of the miniaturised POF-probe with hemispherical ruby crystal.

The field prototype probe (Fig. 3.1.3) was designed to work in the field under 25 kV. The POF with the ruby crystal is inserted inside the probe up to its tip where it touches the

POF-probe and to collect the fluorescence toward the Si-photodetector.

remains at low cost.

**3.1.2 The prototype** 

photodiode is amplified and processed.

copper conductor of the coil.

transmission lines, in substation equipments and in hydroelectric generators, all in a connected-to-the-grid basis.

#### **3.1 Application of POF and ruby for temperature measurement in an electric power substation**

#### **3.1.1 Introduction**

Temperature is a very important parameter for the electric power industry because insulators, copper conductors, iron core of transformers, insulating oil and every equipment are very sensitive to the temperature which has to be kept under strict control during all times. Nevertheless, when dealing with high voltage, sometimes one cannot use conventional electric sensors particularly when working near high voltage areas. This case reports the development of a temperature sensor system using the fluorescence technique.

The fluorescence effect can be used as an indicator and generate a signal proportional to a specific parameter need to be monitored. In the same way, fluorescent materials can be used as sensors. It is well known that the fluorescence decay time of some crystals is proportional to the temperature. Therefore, one way to build a temperature sensor is by the measurement of the time constant of the exponential decay that produces a linear relationship with the temperature.

Optical fibre sensors offer a large number of advantages over conventional sensors such as high immunity to electromagnetic interference, electrical isolation and the absence of metallic parts, a strong requirement for sensing in electromagnetic contaminated environments, e.g. RF/microwave. The sensor probes are inherently electrically insulated system and external power is not required for their operation, they can work at high electrical potentials and in potentially explosive environments. It can be made as lightweight, compact, disposable of low cost and is highly chemically inert even against corrosion.

The fluorescence based sensors offer the advantage of a near-zero background, because the wavelength of the emitted light is always larger than that of the excitation light, which makes then in principle much more sensitive and error immune than those that change only the absorption when the temperature varies [Asada and Yuki, 1994, Grattan and Zhang, 1995]. Previously, experiments with commercial polystyrene fluorescent fibres as temperature sensor were done [Ribeiro et al., 2003]. Although it features some advantages as compatibility with standards POFs, a weak fluorescence signal with time-decay < 100 ns was measured, thus requiring a much complex electronics. Furthermore, the polystyrene can withstand only up to ~70oC thus limiting its usefulness for the electrical energy industry. Ruby has been used for fluorescence thermometry because it is of low cost, easily available, POF compatible, requires low cost source (blue or green ultra-bright LEDs), Sibased photodetection and simple electronics. Additionally it presents strong intensity and long lifetime of fluorescence signal. The fluorescence peaking at 694 nm wavelength features a long-decay time of 2-4 ms. Persegol and co-workers [Persegol et al., 1999] described a POFbased temperature sensor in the range –20oC to +120oC with an accuracy of 2oC for early detection of faults in medium-voltage (36 kV) substations. They used heavily-doped ruby powder packaged at the POF end as fluorescent material pumped with a green LED. Two POF-probe were used, one for pumping the ruby and the other for bringing the fluorescence back to the photodetector.

In this case study we describe the temperature sensor prototype development based on the ruby crystal and a one-probe-POF for "low" and "high" temperatures. Low cost passive and active components as couplers, connectors, adapters, LEDs etc were used. Ruby crystals are

transmission lines, in substation equipments and in hydroelectric generators, all in a

**3.1 Application of POF and ruby for temperature measurement in an electric power** 

Temperature is a very important parameter for the electric power industry because insulators, copper conductors, iron core of transformers, insulating oil and every equipment are very sensitive to the temperature which has to be kept under strict control during all times. Nevertheless, when dealing with high voltage, sometimes one cannot use conventional electric sensors particularly when working near high voltage areas. This case reports the development of a temperature sensor system using the fluorescence technique. The fluorescence effect can be used as an indicator and generate a signal proportional to a specific parameter need to be monitored. In the same way, fluorescent materials can be used as sensors. It is well known that the fluorescence decay time of some crystals is proportional to the temperature. Therefore, one way to build a temperature sensor is by the measurement of the time constant of the exponential decay that produces a linear relationship with the

Optical fibre sensors offer a large number of advantages over conventional sensors such as high immunity to electromagnetic interference, electrical isolation and the absence of metallic parts, a strong requirement for sensing in electromagnetic contaminated environments, e.g. RF/microwave. The sensor probes are inherently electrically insulated system and external power is not required for their operation, they can work at high electrical potentials and in potentially explosive environments. It can be made as lightweight, compact, disposable of low

The fluorescence based sensors offer the advantage of a near-zero background, because the wavelength of the emitted light is always larger than that of the excitation light, which makes then in principle much more sensitive and error immune than those that change only the absorption when the temperature varies [Asada and Yuki, 1994, Grattan and Zhang, 1995]. Previously, experiments with commercial polystyrene fluorescent fibres as temperature sensor were done [Ribeiro et al., 2003]. Although it features some advantages as compatibility with standards POFs, a weak fluorescence signal with time-decay < 100 ns was measured, thus requiring a much complex electronics. Furthermore, the polystyrene can withstand only up to ~70oC thus limiting its usefulness for the electrical energy industry. Ruby has been used for fluorescence thermometry because it is of low cost, easily available, POF compatible, requires low cost source (blue or green ultra-bright LEDs), Sibased photodetection and simple electronics. Additionally it presents strong intensity and long lifetime of fluorescence signal. The fluorescence peaking at 694 nm wavelength features a long-decay time of 2-4 ms. Persegol and co-workers [Persegol et al., 1999] described a POFbased temperature sensor in the range –20oC to +120oC with an accuracy of 2oC for early detection of faults in medium-voltage (36 kV) substations. They used heavily-doped ruby powder packaged at the POF end as fluorescent material pumped with a green LED. Two POF-probe were used, one for pumping the ruby and the other for bringing the fluorescence

In this case study we describe the temperature sensor prototype development based on the ruby crystal and a one-probe-POF for "low" and "high" temperatures. Low cost passive and active components as couplers, connectors, adapters, LEDs etc were used. Ruby crystals are

cost and is highly chemically inert even against corrosion.

connected-to-the-grid basis.

**substation 3.1.1 Introduction** 

temperature.

back to the photodetector.

geometrically compatible with standard POFs and even after cutting and polishing it remains at low cost.

#### **3.1.2 The prototype**

Fig. 3.1.1 shows a top view picture of the temperature sensor prototype (conditioning equipment) where the key components are assigned. The LED package was polished almost reaching the semiconductor chip thus maximizing the light caption. Light launching was made through butt-coupling the polished LED and a carefully terminated POF. Optical pulses of 32 ms time-width from the LED were generated to pump the ruby crystal at 15.6 Hz. A miniature 1x2 POF-coupler is used to send pump pulses toward the ruby crystal glued at the end of the POF-probe and to collect the fluorescence toward the Si-photodetector.

Fig. 3.1.1. Top view picture of the temperature sensor prototype (conditioning equipment).

Fig. 3.1.2 shows the picture of the one-POF-probe with 4 mm in diameter. The POF was terminated at the other end with a standard HP plastic connector. With this configuration, it can be detached from the conditioning equipment box. The fluorescence response from the crystal, passing through the same POF, was conveyed into the other port of the coupler. Due to the back reflections at the many optical interfaces, the fluorescent signal could have been buried under the intense excitation signal. Thus, in order to avoid a saturation of the detection stage and to enable the fluorescence response to be detected exclusively, a red long-pass filter was placed before the Si photodiode. The electrical signal generated from the photodiode is amplified and processed.

Fig. 3.1.2. Picture of the miniaturised POF-probe with hemispherical ruby crystal.

The field prototype probe (Fig. 3.1.3) was designed to work in the field under 25 kV. The POF with the ruby crystal is inserted inside the probe up to its tip where it touches the copper conductor of the coil.

Optical Fiber Sensors 11

From the straight line slope shown in Figure 3.1.5 the sensitivity is calculated to be 22.5 μs/oC corresponding to an estimated temperature resolution of ~1oC. A sensitivity of 9 μs/oC has been reported in the literature. However, our sample presented a larger

Plastic materials cannot withstand high temperatures as much as glasses. Standard POFs usually can operate up to 70 to 85oC. However, some special POFs have been developed mainly for harsh environment as in car networks applications. Some of those "hightemperature" POFs had been disclosed in the literature but still impose severe limitations

Table 1 shows the attenuation given by the manufacturer for three POFs (Mitsubishi Rayon Company) corresponding to centre wavelengths of blue/green LEDs and ruby R-line

From Table 1 one can see that the EH4001 POF-probe attenuation is the same despite the use of blue or green LED regarding a maximum temperature of 85oC for which this POF can

Table 2 shows comparatively the attenuations for 10m of POF-probe length when a

Fibers type DH4001 and FH4001 can withstand up to 115oC and 125oC, respectively. However, our choice as "high temperature" POF-probe was the heat resistant-grade DH4001 (1 mm core with black XPE jacket) because it features total attenuation of 8.8 dB that is much smaller than 40.0 dB presented by FH4001 regarding the green LED as the

The system has been installed at the harmonic filter of Furnas Substation in the city of Ibiúna, State of São Paulo, Brasil to allow the technicians to monitor the temperature in four points of the coil of this reactor in real time. If the temperature reaches 80oC an alarm is issued in order to shut down the transmission line. Fig 3.1.6 shows one of the four installed sensors, Fig. 3.1.7 shows the coil with two of the sensors and Fig. 3.1.8 shows the control software which screen shows the graph of the four temperatures as well as the ambient

Fiber Type 470 nm 525 nm 694 nm EH4001 (datacom-grade) ~ 0.10 dB/m ~ 0.10 dB/m > 0.40 dB/m DH4001 (heat-resistant, 115oC) 0.95 dB/m 0.48 dB/m 0.40 dB/m FH4001 (heat-resistant, PC core, 125oC) 4.00 dB/m 2.70 dB/m 1.30 dB/m

> 525 nm (pump) + 694nm (fluorescence)

sensitivity probably due to the re-absorption phenomena [Persegol et al., 1999].

Table 1. Attenuation of three POFs at some key wavelengths. PC = polycarbonate.

EH 4001 1.0 + 4.0 = 5.0 dB 1.0 + 4.0 = 5.0 dB DH4001 9.5 + 4.0 = 13.5 dB 4.8 + 4.0 = 8.8 dB FH4001 40.0 + 13.0 = 53.0 dB 27.0 + 13.0 = 40.0 dB

Table 2. Attenuation for 10 m of POF-probe for different pump wavelengths.

for temperature sensing [Ribeiro et al., 2003].

maximum temperature of 110oC is allowed to be reached.

470 nm (pump) + 694nm (fluorescence)

(694 nm).

withstand.

excitation light source.

**3.1.4 Field installation** 

temperature.

Fig. 3.1.3. High voltage probe.

#### **3.1.3 Prototype tests**

Fig. 3.1.4 shows in the top the oscilloscope trace of the square shape pump pulses. Bottom trace shows the fluorescence signal at room temperature exhibiting a clear exponential time-decay.

Fig. 3.1.4. Oscilloscope traces of pump (top) and fluorescent (bottom) light signals at room temperature (23oC).

The exponential decay shown in Fig. 3.1.4 can be expressed as:

$$\mathbf{P(t) = P\_0 \exp[-t/\tau(T)]}$$

where *P(t)* is the output light power at a time *t*, *P0* is the light power at t=0 and *τ(T)* it the time-decay constant at temperature *T*. Fig 3.1.5 shows the measurements of fluorescence time-decay *τ* against the temperature *T* with a typical relaxation time of about 5.0 ms.

Fig. 3.1.5. Fluorescence time-decay against the temperature.

Fig. 3.1.4 shows in the top the oscilloscope trace of the square shape pump pulses. Bottom trace shows the fluorescence signal at room temperature exhibiting a clear exponential time-decay.

Fig. 3.1.4. Oscilloscope traces of pump (top) and fluorescent (bottom) light signals at room

P(t) =P0exp[-t/τ(T)] where *P(t)* is the output light power at a time *t*, *P0* is the light power at t=0 and *τ(T)* it the time-decay constant at temperature *T*. Fig 3.1.5 shows the measurements of fluorescence

time-decay *τ* against the temperature *T* with a typical relaxation time of about 5.0 ms.

The exponential decay shown in Fig. 3.1.4 can be expressed as:

Fig. 3.1.5. Fluorescence time-decay against the temperature.

Fig. 3.1.3. High voltage probe.

**3.1.3 Prototype tests** 

temperature (23oC).

From the straight line slope shown in Figure 3.1.5 the sensitivity is calculated to be 22.5 μs/oC corresponding to an estimated temperature resolution of ~1oC. A sensitivity of 9 μs/oC has been reported in the literature. However, our sample presented a larger sensitivity probably due to the re-absorption phenomena [Persegol et al., 1999].

Plastic materials cannot withstand high temperatures as much as glasses. Standard POFs usually can operate up to 70 to 85oC. However, some special POFs have been developed mainly for harsh environment as in car networks applications. Some of those "hightemperature" POFs had been disclosed in the literature but still impose severe limitations for temperature sensing [Ribeiro et al., 2003].

Table 1 shows the attenuation given by the manufacturer for three POFs (Mitsubishi Rayon Company) corresponding to centre wavelengths of blue/green LEDs and ruby R-line (694 nm).



From Table 1 one can see that the EH4001 POF-probe attenuation is the same despite the use of blue or green LED regarding a maximum temperature of 85oC for which this POF can withstand.

Table 2 shows comparatively the attenuations for 10m of POF-probe length when a maximum temperature of 110oC is allowed to be reached.


Table 2. Attenuation for 10 m of POF-probe for different pump wavelengths.

Fibers type DH4001 and FH4001 can withstand up to 115oC and 125oC, respectively. However, our choice as "high temperature" POF-probe was the heat resistant-grade DH4001 (1 mm core with black XPE jacket) because it features total attenuation of 8.8 dB that is much smaller than 40.0 dB presented by FH4001 regarding the green LED as the excitation light source.

#### **3.1.4 Field installation**

The system has been installed at the harmonic filter of Furnas Substation in the city of Ibiúna, State of São Paulo, Brasil to allow the technicians to monitor the temperature in four points of the coil of this reactor in real time. If the temperature reaches 80oC an alarm is issued in order to shut down the transmission line. Fig 3.1.6 shows one of the four installed sensors, Fig. 3.1.7 shows the coil with two of the sensors and Fig. 3.1.8 shows the control software which screen shows the graph of the four temperatures as well as the ambient temperature.

Optical Fiber Sensors 13

Fig. 3.1.9. Temperature monitored by the four transducers.

Fig. 3.1.10. Temperature monitored during an electric power shortage.

Experimental results of a simple and low cost four-point temperature POF sensor prototype based on time-decay of the ruby fluorescence pumped with blue (or green) ultra-bright LED are presented. The major drawbacks of polymer optical fibres are their restricted temperature range and relatively high losses. However many applications do not exceed a temperature of more than 100oC and requires a sensing distance smaller than 10 m. The best choice for temperatures up to 115oC was the DH4001 as the POF-probe pumped with green LED. The developed prototype is quite compatible with a 1-mm-core silica fibre or a hybrid POF + silica

fibre-probe where the later may be put in contact with the hot surface to be sensed.

**3.2 Application of a POF-based current sensor for measuring leakage current in** 

The leakage current of insulators in a high voltage transmission line is due to the increasing conductive deposited yielded by environment pollution. The more common pollution is the salt-spray produced by winds in areas close to sea shore. The salt deposit on the insulator surface offers to the electrical current alternative paths to the ground, thus connecting the high voltage to ground potential. Although this current is only a few tens of milliamperes, when multiplied by the total number of insulators located in that particular transmission line, the total leakage current can reach so high values that can trigger over current protection devices leading to electrical power line interruption. When an electric arc occurs,

**3.1.6 Concluding remarks** 

**500 kV transmission line 3.2.1 Introduction** 

Fig. 3.1.6. One of the four sensors installed on the top of the coil.

Fig. 3.1.7. The reactor coil and the sensors installed.

Fig. 3.1.8. Control software screen showing the graph of the four temperatures as well as the ambient temperature.

#### **3.1.5 Results**

Figures 3.1.9 and 3.1.10 show the graphs of the measurements taken at two different dates.

Fig. 3.1.8. Control software screen showing the graph of the four temperatures as well as the

Figures 3.1.9 and 3.1.10 show the graphs of the measurements taken at two different dates.

Fig. 3.1.6. One of the four sensors installed on the top of the coil.

Fig. 3.1.7. The reactor coil and the sensors installed.

ambient temperature.

**3.1.5 Results** 

Fig. 3.1.9. Temperature monitored by the four transducers.

Fig. 3.1.10. Temperature monitored during an electric power shortage.

### **3.1.6 Concluding remarks**

Experimental results of a simple and low cost four-point temperature POF sensor prototype based on time-decay of the ruby fluorescence pumped with blue (or green) ultra-bright LED are presented. The major drawbacks of polymer optical fibres are their restricted temperature range and relatively high losses. However many applications do not exceed a temperature of more than 100oC and requires a sensing distance smaller than 10 m. The best choice for temperatures up to 115oC was the DH4001 as the POF-probe pumped with green LED. The developed prototype is quite compatible with a 1-mm-core silica fibre or a hybrid POF + silica fibre-probe where the later may be put in contact with the hot surface to be sensed.
