V.sathiesh Kumar_Understanding the Discharge Activity Across GFRP Material Due to Salt Deposit Under Transient Voltages by Adopting OES and LIBS Technique_2014

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21, No. 5; October 2014 2283

DOI 10.1109/TDEI.2014.004120

Understanding the Discharge Activity across GFRPMaterial Due to Salt Deposit under Transient Voltages

by Adopting OES and LIBS Technique

V. Sathiesh Kumar, Nilesh J. VasaDepartment of Engineering Design, Indian Institute of Technology Madras

Chennai- 600036, India

R. SarathiDepartment of Electrical Engineering, Indian Institute of Technology Madras

Chennai- 600036, India

Daisuke Nakamura and Tatsuo OkadaGraduate school on Information Science and Electrical Engineering, Kyushu University

Fukuoka 819-0395, Japan

ABSTRACTIn the present study, breakdown characteristics of a glass fiber reinforced plastic

(GFRP) with different salt deposit densities (SDD) under standard lightning impulse

(LI)/ switching impulse (SI) voltages are studied. Optical emission during discharge

process and the discharge current are measured simultaneously. It is observed that

flashover voltage (FOV) reduces with increase in salt deposit density on GFRP material

under LI/SI (irrespective of the polarity of applied voltage) voltages. It is also observed

that irrespective of level of SDD, the FOV is less with SIV compared with LIV. FOV

under negative SIV is less compared with positive SIV. Optical emission corresponding

to Na I emission at 589 nm in discharge plasma follows the discharge current profile

and also the lifetime of Na I emission is high under switching impulse compared with

lightning impulse voltages. Na I emission in the discharge plasma sustains for a longer

period in case of negative switching impulse voltages. Plasma temperature is estimated

using emission lines and it is observed that the local temperature during discharge

process is high under negative switching impulse voltage. With increase in salt deposit

density, the optical emission spectrum of GFRP material in addition of Na I line at 589

nm is observed. Laser induced breakdown spectroscopy (LIBS) technique is proposed

and demonstrated for the detection of a salt deposit on a GFRP material from a stand-

off distance of 1 m. In addition, temporal and spatial profile of laser induced plasma on

the polluted GFRP material is used to quantify the salt deposit. Laser fluence less than

5 J/cm2 is ideally suited to rank the severity of a salt deposit on GFRP material at an

incident laser wavelength of 1064 nm.

Index Terms - Wind blade, pollution measurement, flashover voltage, lightning

impulse, switching impulse, temporal and spatial measurements.

1

1 INTRODUCTIONWIND power is one of important renewable energy

sources. Its use in an off-shore power generation

application is increasing considering the energy crisis and

global warming problem. Wind turbines power handling

capacity varies from 30 kW to 7 MW. A typical 7 MW

wind turbine has a tower height of 160 m and a rotor

diameter of 164 m (i.e., Length of blades= 80 m) [1]. Wind

turbines are open air structures often placed in isolated

areas such as offshore and mountain regions where the

wind condition is expected to be good and they are more

prone to be affected by lightning strikes [2-5]. Particularly

lightning damage to a wind turbine blade are quite serious

since the cost for replacements are remarkably high,

laborious and time consuming. Manuscript received on 20 May 2013, in final form 26 April 2014,

accepted 26 April 2014.



2284 V. S. Kumar et al.: Understanding the Discharge Activity across GFRP Material Due to Salt Deposit under Transient Voltages

Lightning can be classified into upward and downward

initiated with respect to tall structures and also classified

based on its polarity (Positive/Negative), being that of

charge transferred from cloud to ground. Wind turbines are

more prone to be affected by upward initiated lightning, if

its height exceeds above 100 meters [6]. In order to prevent

wind turbine blades from lightning damage, lightning

protection systems, such as point receptors and metallic cap

receptors are placed on blades to minimize the number oflightning strikes [7, 8]. Even though with this type of

preventive measure in place to protect the blades from

lightning damage, numerous lightning strikes to the wind

turbine blades have been reported [9-12]. That to the cost

of wind turbine blade is about 15-20 % of the total wind

turbine system [13].

Glass fiber reinforced plastics (GFRP) are used in the

manufacturing of wind turbine blades due to its high

mechanical strength, stiffness, surface roughness and light

weight. In addition the material is highly hydrophobic in

nature. In an offshore environment, wind turbine blades are

subjected to sustain moisture. Soluble and non-soluble

contaminants such as dust, sand and salt gets deposited on the blade surface and enhances the surface discharge activity on

the blade [14]. Admittedly no actual data is available on level

of salt density deposit on wind turbine blades and for

understanding the fundamental aspect of salt density deposit

on flashover voltage, a methodical experimental studies need

to be carried out. Various research groups have studied

extensively on an influence of pollution on an insulator

model under non-uniform pollution conditions [15]. Based on

the study, it was found that pollution deposit on the surface of

insulator reduces the flashover voltage, irrespective of the

polarity of applied voltage [15]. Since no standards were

formulated for testing the pollution performance of wind

blade structures, in the present work, based on duration oflightning activity, the standard lightning and switching

impulse voltages were adopted (which simulates the summer

and winter lightning activity respectively based on duration

of the activity) to test the pollution performance of the wind

blade structure. Thus it has become important to have an

online condition monitoring technique to understand the level

of pollution deposit in order to prevent the blade from

lightning damage.

Optical emission spectroscopy (OES) technique is a non-

intrusive technique to gather physical information during

discharge process such as elemental analysis, plasma

temperatures and density. OES technique have been used

to study the plasma created during electrical dischargemachining process [16] and also used in measuring the

characteristics of plasma generated during the wire

explosion process [17]. OES technique can be used to study

electrical discharge characteristics on material surface. On

the other hand, Laser induced breakdown spectroscopy

(LIBS) is an analytical tool to find the elemental

composition of samples irrespective of their states (Solid,

liquid or Gas) [18, 19]. It has several advantages like multi-

element detection, in-Situ analysis, minimal sample

preparation, nearly non destructive, high detection

sensitivity and remote detection capability. Forensic

elemental analysis, environmental analysis and real time

radioactive material monitoring are some of the

applications of LIBS. LIBS technique can be utilized for

detecting the contaminant layer on wind turbine blades

using a remote sensing approach.

In the present work, the breakdown characteristics of

GFRP material with different level of pollutant (NaCl) under

lightning impulse voltage of 1.2/50 µs and switching impulsevoltage of 250/2500 µs for both polarities are studied. The

optical emission obtained during surface discharge process is

analyzed using OES technique to understand the level of

degradation of GFRP material for different level of salt

deposit densities (SDD). In addition, laser induced

breakdown spectroscopy (LIBS) technique combined with

temporal and spatial studies is demonstrated for detection and

quantification of a salt deposit on a GFRP material.

2 EXPERIMENTAL SETUP

2.1 OPTICAL EMISSION SPECTROSCOPY (OES)

TECHNIQUECommercially available woven fabric glass fiber

reinforced plastics (GFRP) sample with a surface area of 36

cm2 is selected for carrying out the experimental study.

Surface roughness of the virgin GFRP is measured using a

contact type surface profilometer (Marsurf GD25) and the

average surface roughness (R a) value was found to be about

0.6 μm. GFRP is a polymer composite with major

ingredients of silica sand (SiO2), lime stone (CaCO3) and

soda ash (Na2CO3). Slurry is prepared with kaolin (2100

mg) and NaCl (40, 110, 180, 250, 320 and 400 mg) by

mixing it with demineralised water and sprayed over the

surface of GFRP material to form a thin layer. Equivalent

salt deposit density (ESDD) of different contaminatedGFRP samples are calculated as per IEC 60507 standards

and it is shown in Figure 1 [20, 21]. Kaolin clay is used as a

binding medium and it consists of 40-50% of SiO2, 30-40%

of Al2O3, 0-3.2% of Fe2O3 and 7-14% of H2O.

Figure 1. Determination of ESDD and σ20 as per IEC 60507 standard.

Lightning impulse voltage (1.2/50 µs)/switching impulse

voltage (250/2500 µs) of positive/negative polarity are

generated using a single stage impulse voltage generator by

appropriately varying the values of the source capacitance,

front resistance and the tail resistance. LIV and SIV




correspond to that of summer and winter lightning [9-11,

21-25]. To understand the surface discharge activity in

GFRP material, the quasi-uniform electric field gap consists

of two angular electrode tip cut for 45˚ (with edges smooth)

placed on 3 mm thick GFRP material, as shown in Figure 2.

Figure 2. Experimental Setup: Optical emission spectroscopy (OES)

technique.

The electrode configuration used in the present study

is as per standard IEC-60112 [26, 27]. The electrodes

used are of stainless steel material, and are separated by a

gap distance of 10 mm. The electrode gap is so chosen

such a way that emission lines generated during flashover

be confined and the external factors influence are

minimum in order to use optical emission spectroscopy

technique. One electrode is connected to the high voltage

source and the other electrode connected to ground. The

applied voltage magnitude, wave shape, surface

conductivity of the insulating material decides theflashover voltage (FOV) of the electrode gap. Hence the

FOV was obtained for the range of ESDD (similar to that

of outdoor insulators) under standard LI and SI voltages,

by adopting step stress method [28] as shown in Figure 3.

Figure 3. Flowchart to determine flashover voltage

The test voltage applied at predetermined voltage

magnitude. In the present study, flashover voltages

obtained by carrying out the test 15 times at different

locations on the surface of the GFRP test specimen [25].

Applied voltage and discharge current are measured using a

voltage probe (EP-50k, PEEC.A, Lecroy, 1000:1) and a

current probe (94111-1, ETS Lindgren), respectively.

Integration of power over the time which gives deposited

energy on the sample surface.

Optical emission during discharge on GFRP material

with different SDD is collected using a fiber lens with a

focal length of 4.34 mm and it is coupled to a spectrometer

(190 to 1035 nm, Tec 5) with an NMOS linear image

sensor (S3901, Hamamatsu) using a multimode optical

fiber with a core diameter of 600 µm, 0.39 NA. To perform

emission lifetime studies, neutral sodium atom (Na I, D-

line, 588.99 nm) emission lines are used. Na I emission line

is filtered using a sodium filter (589 nm) and focused on to

a multimode optical fiber and photomultiplier tube (R562,

Hamamatsu).

2.2 LASER INDUCED BREAKDOWNSPECTROSCOPY (LIBS) TECHNIQUE

Figure 4. Experimental Setup- Laser induced breakdown spectroscopy

(LIBS) combined with temporal and spatial measurements.

Laser induced breakdown spectroscopy (LIBS)

technique is proposed and used for the detection of a salt

deposit on a GFRP material from a stand-off distance of 1

m. Nd3+

: YAG laser (LAB-150-10-S2K, Quanta-Ray LABseries, Spectra Physics) beam with pulse duration of ≈ 10

ns is focused on to the sample using a lens of 150 mm focal

length to create plasma on the target surface as shown in

Figure 4. Laser induced plasma contains excited ions,

electrons, and neutral atoms. After a short relaxation time,

plasma with excited particles cools down and de-excite to

their lower energy levels by releasing equivalent energy in

the form of electromagnetic radiation termed as plasma or

plume. Optical radiation emitted from plasma is collected

using lens with a focal length of 200 mm and guided to the




spectrometer (LVC-3011-S1, LVM-200-KS, Lambda

Vision) using a multimode optical fiber with a core

diameter of 400 μm, 0.22 NA. To perform emission

lifetime studies, neutral sodium atom (D-line, 588.99 nm)

emission lines are used. Na I emission line is filtered using

a sodium filter (589 nm) and focused on to a multimode

optical fiber and photomultiplier tube (R562, Hamamatsu).

ICCD camera (PI-MAX 1kB UNIGEN II, Princeton

Instruments) is used for studying the spatial profile of laserinduced plasma on the target surface.

3 RESULTS AND DISCUSSION

3.1 BREAKDOWN CHARACTERISTICS OFPOLLUTED GFRP MATERIAL UNDER LIV/SIV

Figure 5. Flashover voltage for GFRP samples with different SDD: (a)

Lightning impulse voltage, (b) Switching impulse voltage.

Figure 5 shows variation in flashover voltage (FOV) of

GFRP samples with different salt deposit densities under

standard LI/SI voltage of both polarities. It is observed that

FOV reduces with increase in salt density on GFRP

material under LIV/SIV (irrespective of the polarity of

applied voltage). On application of voltage, the discharge

incepts from the high voltage electrode and under SIV, dueto long wave front, time duration of corona streamer phase

is considerable and hence the surface charging occurs and

the electrons and ions formed in the medium with the

applied voltage magnitude are sufficient to cause

breakdown of electrode gap at lower voltages compared

with lightning impulse voltage with short front time. FOV

under negative SIV is less compared with positive SIV.

Similar characteristics were observed by Quisman et al

[29]. In General, from Fig. 5, it is observed that FOV of the

electrode gap under LIV/SIV, the SDD and FOV show

inverse relationship.

On application of transient voltage to the electrode gap,

the discharge incepts at the tip of high voltage electrodeand depending on level of SDD the discharge path alters. If

the level of salt deposit is less the incepted discharge at the

high voltage electrode propagates causing aerial discharge

and gets terminated to the ground electrode. At the same

time, if the SDD is high, discharge incepted at the high

voltage electrode propagates at the interface of the salt

deposit and the GFRP material thereby the level of damage

to the GFRP material is high. This is also confirmed by

optical emission spectroscopy (OES) technique.

Figures 6a and 6b show a variation in energy deposited

during surface discharge process leading to FOV between

the electrodes under the application of LIV and SIV of 18

kV, respectively. It is observed that the amount of energy

deposited is high under SIV (irrespective of level of SDD

and polarity of applied voltage) compared with LIV.

Figure 6. Energy deposit on samples with different SDD: (a) Lightning

impulse voltage, (b) Switching impulse voltage.

3.2 ANALYSIS OF SURFACE DEGRADATIONADOPTING OPTICAL EMISSION SPECTROSCOPY

Figures 7a-7d show typical optical emission spectra of

virgin and polluted GFRP samples obtained during surface

discharge under positive LIV. The optical emission spectraobserved during discharge between the electrode gaps

under LIV/SIV, irrespective of polarity, are the same. This

indicates that the current is not that important in

identification of species produced during breakdown. Once

the breakdown occurs, which is being initiated along the

surface, it survives mostly in air abutting the surface and

hence the duration of the current is not that influential

thereby justifying that testing with LIV/SIV be sufficient

than by testing with standard lightning impulse current of

4/10 or 8/20 µs, (which are not the current wave shape of

natural lightning [30, 31]).

Figure 7. OES spectra (positive LIV): (a) GFRP, (b) Kaolin deposited on

GFRP, (c) SDD= 3 mg/cm2, (d) SDD= 11 mg/cm2.

The characteristic peak of emission lines observed in

OES spectra of GFRP (Figure 7a), kaolin clay deposited on

GFRP material (Figure 7b) relates to its elemental

composition [32]. Figures 7c and 7d show emission spectra

of salt deposited samples with SDD of 3 and 11 mg/cm2,




respectively. As observed in Figures 7c and 7d,

characteristic peak at 588.99 nm relates to a neutral sodium

atom (Na I). With increase in SDD, the characteristic peaks

of GFRP material along with Na I emission line are

observed. This clearly indicates with increase in SDD the

discharge propagates at the interface of GFRP material and

that of the pollutant layer. It is difficult to arrive at a

relationship between the peak of Na I emission line and

SDD for the application of different voltage profiles(LIV/SIV). The intensity depends on density of the salt

deposit and when discharge is of fibrillar in nature

proportionately the emission intensity alters. Also multiple

factors which influences the sensitivity of the measurement

which includes variation in discharge magnitude, the

streamer path formed, discharge plasma emission

magnitude, variation in optical coupling of plasma emission

to optical fiber etc. It is also essential to estimate the local

temperature based on the plasma emission intensity during

discharge propagation to correlate the damage induced on

the GFRP material.

Two prominent emission lines of same atomic species

in the spectra are necessary to determine the approximate plasma temperature during the discharge by using a line-

pair method [16]. Plasma temperature estimation using

emission line intensity is applicable only under a

thermodynamic equilibrium. In practice, thermodynamic

equilibrium is difficult to achieve. Hence an

approximation of Local Thermodynamic Equilibrium

(LTE) is adopted to estimate plasma temperature from the

measured spectroscopic data using Boltzmann-Saha

equations [16]. Discharge plasma temperature on a GFRP

sample under LIV/SIV is estimated using a line pair

method (a special case of Boltzmann's method) in which

the relative intensities of emission lines are used and it is

expressed as,

-2 11.44

1 1 2 2ln

2 2 1 1

E E T

I A g

I A g

(1)

where E1 and E2 are excited energy levels, g1 and g2 are

statistical weight of excited energy levels 1 and 2,

respectively, A1 and A2 are transition probabilities of states,

I1 and I2 are intensity of particular atomic species at λ 1 and

λ 2 wavelength, respectively and T is the plasma temperature

under the condition of LTE. In order to estimate discharge

plasma temperature on a GFRP sample, atomic emissionintensity of Ca I (445.47 nm, 487.81 nm) are used.

Spectroscopic parameters used in estimating plasma

temperature are summarized in Table 1.

Table 2 shows variation in discharge plasma temperature

under LIV and SIV with both polarities for an applied

voltage of ± 23 kV. Based on estimated plasma temperature

using line pair method, deterioration of GFRP sample will

be severe in case of negative SIV and in general the

deviation in estimated plasma temperature is less than 50 K.

Further Na I emission lifetime measurements at 589 nm are

carried out. The Na I emission line profile at 589 nm in

discharge plasma measured along with discharge current

(using resistive short) shows a correlation with discharge

current profile as shown in Figure 8. The lifetime was

estimated at 20 % of the maximum emission intensity.Table 2 shows variation in lifetime of Na I emission at 589

nm under LI/SI voltages. Even though the discharge current

dies down quickly, Na I emission in discharge plasma

sustains for longer period in case of negative switching

impulse voltage and enhances the level of degradation of

GFRP material. Results based on electrical discharge

measurements combined with OES technique clearly shows

that switching impulse voltage induces a significant

damage to GFRP material irrespective of the polarity.

Figure 8. Correlation between discharge current profile and Na I emission

at 589 nm lifetime (Positive Lightning impulse voltage): (a) Discharge

current, (b) Na I-589 nm Intensity.

3.3 DETECTING AND QUANTIFYING THE SALT

DEPOSIT ON A GFRP MATERIAL ADOPTING LIBSLaser induced breakdown spectroscopy (LIBS)

technique is adapted in the present study to identify and

quantify the adhesion of salt deposit on a GFRP material

from a stand-off distance of 1 m. Figure 9 show laser

induced breakdown spectra of GFRP material with an

incident laser wavelength of 1064 nm represents the

characteristic peaks related to its elemental composition

[32]. GFRP is a polymer composite with major ingredients

of silica sand (SiO2), lime stone (CaCO3) and soda ash

(Na2CO3) and the plume formed at the surface of GFRP

Table 1. Plasma Temperature estimation parameters

Line λ (nm) Ek (cm-1) gk Ak (s-1)

Ca I 445.47

487.81

37757.44

42343.59

7

7

8.7 x 107

1.88 x 107

Table 2. Plasma Temperature and Na I emission lifetime variation under

LIV/SIV.

Voltages Temperature (K)Average Na I lifetime

(µs)/Standard deviation (σ)

+ LI 5330 4.12/2.88

- LI 5370 3.32/2.48

+ SI 4400 11/7

- SI 6740 60/36




have composition of Na, Ca, Si etc resulting in multiple

peaks (Figure 9). Figures 10a and 10b show emission

spectra of kaolin clay deposited on GFRP sample and salt

deposited (SDD= 11 mg/cm2) on GFRP sample,

respectively, corresponding to the laser fluence of 3.2 J/cm2

with incident laser wavelength of 1064 nm. As observed in

Figure 10b, the presence of Na I emission at 588.9 and

589.5 nm represents the indication of a salt deposit on the

surface of GFRP. Hence, LIBS can be used to identify the presence of salt deposit on wind blades.

Figure 9. LIBS spectra of GFRP Material (laser wavelength=1064 nm)

with fluence of 3.2 J/cm2. Wavelength Range: (a) 300 to 650 nm, (b) 620

to 950 nm.

Figure 10. LIBS spectra obtained with fluence of 3.2 J/cm2: (a) Kaolin

clay deposited on GFRP Material, (b) SDD- 11 mg/cm2.

Figure 11 shows influence of laser irradiation

wavelength on emission spectra from GFRP material

without salt deposition (i) and with salt deposition

(SDD=11 mg/cm2) (ii). As shown in Figure 11i, with the

laser wavelength of 355 nm (Figures 11i-11a), a broadband

continuum dominates the spectrum due to high absorption

coefficient of sample in that wavelength region. When the

laser irradiation wavelength of 532 nm (Figures 11i-11b) is

selected, the scattering of the laser beam interferes with the

emission spectrum. Particularly, the irradiation laser

wavelength at wavelength of 532 nm interferes with many

elemental emission lines. Nevertheless, when the laser

irradiation wavelength of 1064 nm (Figures 11i-11c) is

used, it is possible to observe emission lines of variouselemental species clearly. Based on Figure 11i it is

observed that the irradiation laser wavelength of 1064 nm is

appropriate for understanding the composition of GFRP

material or any unknown contaminants. Figure 11ii show

influence of incident laser wavelength in detecting Na I

emission line (589 nm) for sample with a salt deposit of 11

mg/cm2. Na I emission at 589 nm could be detected by

using different laser irradiation wavelengths (1064, 532,

and 355 nm). The emission intensity of Na I line was

significantly larger than that of the scattering intensity of

the laser wavelength of 532 nm.

Figure 11. Influence of incident laser wavelength ((a) 355 nm, (b) 532 nm,

(c) 1064 nm) in obtaining the LIBS spectra with fluence of 3.2 J/cm2: (i)

GFRP Material, (ii) SDD= 11 mg/cm2.

Figures 12a and 12b show plots for variation in emission

intensity ratio (Na I at 589 nm / Si I at 570 nm)

corresponding to the laser fluence and salt deposition

density (SDD), respectively. As shown in Figure 12a, with

increase in laser fluence from 2 to 4 J/cm2, a small variation




in intensity ratio is observed for a GFRP sample with SDD

= 9 mg/cm2. For larger laser fluence (> 5 J/cm2), intensity

ratio is found to be randomly varying due to the removal of

salt layer by laser impact on the target surface. Emission

intensity ratio of Na I to Si I emission does not have any

direct correlation with the increase in SDD of samples due

to jitters in laser pulse energy and hence limiting its use to

quantify the concentration of salt deposit on a GFRP

material as shown in Figure 12b.

Figure 12. (a) Influence of laser fluence in detecting the salt deposit, (b)

Variation in relative emission intensity (Na I at 589 nm/Si I at 570 nm)

with salt deposit density.

Figure 13. Temporal studies at 589 nm (Na I) with increase in SDD for a

laser fluence of 3.2 J/cm2.

Figure 13 shows Na I emission lifetime measurements at

589 nm for a laser fluence of 3.2 J/cm2. Based on Figure

13, intensity profile shows a rapid increase and a slow

decay for samples with higher SDD. The lifetime is

estimated at 1/e of the maximum emission intensity. It is

observed that with increase in SDD, Na I emission lifetime

at 589 nm increased. Lifetime increase is due to the

increase in the probability of ionic collisions and re-

excitation of Na I ions in samples with higher salt deposit

density.

Figure 14 shows emission lifetime of Na I emission at

589 nm of salt deposit GFRP material with different laser

fluence (2.6 J/cm2, 5.7 J/cm2) for a laser irradiation

wavelength of 1064 nm. It is observed that a sample with

higher SDD values has a significant increase in emission

lifetime of Na I when the incident laser fluence on the

sample is below 5.5 J/cm2 at a laser repetition rate of 10 Hz.For larger laser fluence (> 5.5 J/cm2), laser pulse impact

force on the sample is very large and hence it removes the

salt deposit. Hence the Na I lifetime measured for large

laser fluence (> 5.5 J/cm2) is the combination of Na I

lifetime of GFRP (beneath layer of salt deposit) and of salt

deposit region. This problem could be minimized by the

reducing the repetition rate of laser thereby successive laser

pulse hits a new distant location on the sample.

Figure 14. Influence of laser fluence on ranking the severity of salt

deposit: (a) Fluence= 2.6 J/cm2, (b) Fluence= 5.7 J/cm2.

3.4 PLUME SPATIAL ANALYSIS USING ICCDSpatial information of laser induced plasma on GFRP

and salt deposit samples is studied using an ICCD camera.

Figures 15a and 15b shows the spatial variation of laser

induced plasma on GFRP and salt deposited GFRP (SDD=11 mg/cm2) with respect to different time scales. After the

initiation of plasma on the target surface, plasma starts

expanding to about approximately 5-8 mm in the direction

perpendicular to the target surface until it gets cools down.

Spatial confinement of plasma generated in a salt deposit

samples is higher when compared to lower SDD samples.

Hence plasma in higher SDD sample sustains for a longer

period as shown in Figure 15. Increase in Na I emission

lifetime is observed with increase in SDD and which is in

agreement with lifetime measurement using the

photomultiplier tube (PMT).




Temporal profile of laser induced plasma obtained using

an ICCD camera for an incident wavelength of 1064 nm is

shown in Figure 16a. Temporal profile of laser induced

plasma shows a rapid increase and a slow decay for

samples with higher SDD and which is in agreement with

the profile obtained using life time measurement by

photomultiplier tube. Na I emission intensity does not have

a direct correlation with increase in SDD. With increase in

laser fluence, plasma intensity increases but not theintensity of Na I at 589 nm as shown in Figure 16b.

Because with increase in laser fluence on the target surface,

partly the pollutant layer gets removed (visual observation)

hence the Na I emission might/might not get increased.

Figure 15. Laser induced plasma- Spatial information: (a) GFRP Material,

(b) SDD= 11 mg/cm2.

Figure 17 shows an influence of incident laser

wavelength on Na I emission lifetime. Na I emission

intensity peak is observed at 25 ns for an incident laser

wavelength of 532 nm and 355 nm due to enhancement in

continuum radiation. For 1064 nm incident laser

wavelength, Na I emission intensity peak is observed at 100

ns as shown in Figure 17. Results based on LIBS studiesclearly show that the spectral measurements can be used to

detect the presence of contaminants through elemental

analysis. LIBS technique is extended for lifetime and

spatial measurements, to rank the severity of contamination

and for determining the pollutant accumulated on a GFRP

material. With increase in laser fluence on the target

surface, partly the pollutant layer gets removed hence the

Na I emission intensity alters. So proper choice of laser

fluence is important in order to quantify the amount of salt

deposit on GFRP.

Figure 16. (a) Temporal information at 589 nm emission wavelength for

samples with different SDD using ICCD camera image, (b) Influence of

laser fluence in plasma emission: (i) Plasma intensity, (ii) Na I-589 nm

intensity.

Figure 17. Influence of incident laser wavelength in determining Na I

emission lifetime.

CONCLUSIONElectrical discharge measurements combined with optical

emission spectroscopy (OES) technique is used to study the

influence of salt deposit on the industrial GFRP material

under lightning impulse (LI)/switching impulse (SI) voltages

of both polarities. Flashover voltage (FOV) of GFRP material

decreases with increase in salt deposit density under

LIV/SIV, irrespective of its polarity. It is observed that theFOV is less under negative SIV compared with positive SIV

and LIV. The characteristic peaks observed using optical

emission spectroscopy (OES) technique is in correlation with

the elemental composition of pollutant deposited on a GFRP

material. Na I emission line at 589 nm could be identified for

samples with different contamination level. With increase in

SDD, the optical emission spectra of GFRP (underneath

substrate) material is observed. Results based on optical

emission spectroscopy studies qualitatively are in agreement

with the electrical discharge measurements.




Laser induced breakdown spectroscopy (LIBS) combined

with temporal and spatial studies is proposed and

demonstrated for detection and quantification of a salt deposit

on a GFRP material from a stand-off distance of 1 m. LIBS

measurements clearly shows that Na I line at 589 nm could

be detected using a spectrometer and its quantification was

possible by temporal and spatial measurements of plasma.

Influence of incident laser wavelength and laser fluence to

induce plasma plays a vital role in quantification of a saltdeposit on a GFRP material. With increase in laser fluence on

the target surface, partly the pollutant layer gets removed

hence the Na I emission intensity alters. Hence an appropriate

choice of laser fluence is important in order to quantify the

amount of salt deposit on GFRP. As a typical value, laser

fluence less than 5 J/cm2 is ideally suited to quantify the salt

deposit on a GFRP material for a laser irradiation wavelength

of 1064 nm.

ACKNOWLEDGMENTSThis work is supported by the grant-in aid from the

Department of Science and Technology (DST), India

(SR/S3/EECE/0115/2010). Sathiesh Kumar is grateful toKyushu University, Japan for providing an opportunity to

do a part of a research work at the Graduate school of

Information Science and Electrical Engineering.

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V. Sathiesh Kumar is currently pursuing

Doctoral Studies in the Department of Engineering

Design, Indian Institute of Technology Madras,

India. He received his Master in engineering

(applied electronics) from Thanthai Periyar

Government Institute of Technology, Vellore in

2007. His area of research includes Laser

Technology and its Applications, Opto-

Mechatronics and in Embedded Systems.

Nilesh J. Vasa received his B.E. degree in

production engineering (1988), Master of

Technology in mechanical engineering (1990) and

Doctor of Engineering in electronic device

engineering (1997) from Mumbai University, IndianInstitute of Technology Madras (IIT Madras), India,

and Kyushu University, Japan, respectively.

Currently he is a faculty member at IIT Madras. His

area of research includes Laser technology, Remote

sensing, Opto-Mechatronics, Optoelectronic Devices, Precision Engineering

and Instrumentation, MEMS Devices. He is a member of Optical Society of

America, SPIE, Laser Society of Japan, Laser Society of India.

Ramanujam Sarathi is currently a Professor and

Head of High Voltage Laboratory, Department of

Electrical Engineering, Indian Institute of

Technology Madras, Chennai, India. He obtained

his Ph.D. degree from the Indian Institute of

Science, Bangalore, India in 1994. His research

area includes Condition monitoring of power

apparatus and nano materials.

Daisuke Nakamura received a Doctor of

Engineering degree in information science and

electrical engineering from Kyushu University,

Japan in March 2009. He has been an Associate

Professor in the Department of Electrical

Engineering, Kyushu University since 2010. His

research interest includes Applications of Laser

Ablation and Laser Spectroscopy. He is a member

of SPIE, the Japanese Society of Applied Physics,

and the Laser society of Japan.

Tatsuo Okada is a Professor at the Department of

Electrical Engineering, Kyushu University, Japan.

He received the Ph.D. degree of Engineering from

Kyushu University, Japan in 1981. His research

interest includes development of tunable lasers,

short wavelength light sources, laser processing,

and spectroscopic imaging.

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