Title: font: times; size: 18 point; style: plain; justified: center; capitalization: first word and Names only

Studies on Insulation Oil Contaminants in Electrical Power Transformers

G.Latha

Department of Physics,AaruPadai Veedu Institute of Technology,VMRF,Chennai,India

latha.physics@avit.ac.in

Abstract: Contaminants of insulating silicone oil in power transformers has been analysed through Microwave absorption and FTIR studies. Holy fiber has been proposed to quantify contaminants by optical studies. FEM analysis is implemented to study mode pattern changes in oil infiltrated fiber due to the changes in the refractive indices of oil samples and the same is verified experimentally. Studies have been performed to sense even early , a very mild degradation using image processing.

Key words: Silicone oil,Contaminants, Solid core Photonic Crystal Fiber.

1. Introduction

The life of the transformer being connected with that of its insulation, the evaluation of the transformer insulation is essential to assess the condition of the unit when new and after several yearsof in-use service. The breakdown voltage at power frequency, is one of the most controlled parameters such as Temperature, Amount of contamination describing the liquid’s function as an insulant. The influence of these parameters and the quantification of the charging tendency of insulating oil have been carried out by employing different experimental methodologies, developed in laboratories. One of the most commonly applied measurements is the mini static tester (MST) developed by Westinghouse Electric Corporation[1]. Online condition monitoring of transformers is an essential element that helps to ensure the continuity and reliability of their operation. This therefore allows one to reduce costs and economic losses associated with their unavailability.This is achieved by collecting data provided by a sensors.. A multitude of different measurable variables can be collected for on-line monitoring. Online monitoring systems show the capability to detect oncoming failures within active parts, including dissolved gases in oil, oil temperature and oil level, temperature of the ambient air and of the cooling medium etc.

The periodic monitoring of transformer oil is a preventive measure to avoid high repaircost, long power downtimes and safety risks associated with transformer breakdown and ensure the reliability ofelectric power transmission and distribution networks. As transformer oils are intended to offer electricalinsulation under high electrical fields, any significant reduction in the dielectric strength indicates thecapability of oil performce. Characteristic contaminants that degrade the performance oftransformer oils are dissolved gases (increases with temperature), moisture and solid contaminants whichare mainly metal containing colloidal oxides [3,4].Oil contamination is tested quantitatively using densitymeasurements, titration techniques and gas chromatography apart from qualitative observation of optical colour.Optical methods involving tracking of change in resonant wavelengths using LPFBG’s have been proposed [5]. These sensors offer all the benefits of small size, light weight and immunity fromelectromagnetic interference associated with optical fibers, but involve higher costs due to requirement offrequency tracking. If simple conventional evanescent field sensing is used to track contamination using etchedsingle mode fibers, the drawback is that it can track only changes until the RI of contaminated oilapproaches to that of fiber RI and therefore cannot be used if RI of oil is high.Therefore we propose a comparatively simple technique which involves tracking the RI change introduced by the contaminants through low cost CCD based imaging of the modal pattern changes in a oil infiltrated solid core photonic crystal fiber (SCPCF). This technique should be able to track changes even if the RI of the oil is higher than that of the solid core.

In SCPCF guidance occurs because of the index difference in the cladding.But if the RI of the fluid filling the holes increases unlike the conventional fibers the guidance in the solidcore PCF will decrease and the each of the fluid filled holes will resemble a waveguide by itself leading tothe PCF now behaving like a multiwaveguide system. Eventually if the RI of the infiltrate increases again,the effective index of the micro structured layer increases and the core index will be less than that of the fluidinfiltrated cladding and now the PCF will again guide light through the core region due to the photonic bandgap guidance. The index contrast between the solid core and that of the fluid filling the holes in a solid coresilica PCFs sweeps from positive to negative, if the RI of the fluid increases and this will result in a change inits guided mode profile. The mode profile change monitored using a CCD camera can be used as a simple sensor to measure the changes in fluid RI’s and hence the degradation of the silicone oil intransformers. It will be possible to identifylow to high RI contaminants.Estimated the contaminats in insulating oil by the diectric ,FTIR studies.

2.Estimation of RI of Silicone oil samples

Silicone oil used for insulation in transformers will get contaminanted due to ageing and other environmental conditions which reduces the efficiency of a transformer.Silicone fluids are chemically inert and have good oxidation resistance. There are two modes of degradation namely thermal breakdown and oxidation in used silicone oil . Oxidation of silicone fluid will take place very slowly at temperatures above 175°C . When it oxidizes, silicone fluid polymerizes, gradually increasing in viscosity until gelation occurs. This progression takes place without the formation of disagreeable acids or sludges. In addition, the dielectric properties of the longer-chain silicone molecules are similar to the dielectric properties of fresh silicone fluid. Thermal breakdown of silicone fluid begins at temperatures above 230°C. At these temperatures the longer polymer chains will slowly begin to degrade to form more volatile cyclic silicone material[2].As silicone oil gets contaminated with carbon, hydroxyl ions, the refractive index of the oil increases and this isestimated.The RI of silicone oil samples collected from the local TNEB office has been measured in the laboratory using the minimum deviation technique with equilateral hollow prisms. Table 1 gives the measured RIs of the four samples used for testing and their optical appearance.

Table.1 Measured RI’s of Silicone oil samples

3.Microwave absorption studies to estmate dielectric constant of oil samples

The insulation structure of power transformers consists of oil and cellulose, whose dielectric properties are strongly influenced by moisture, temperature and aging. Their condition can therefore, be evaluated using dielectric response measurements. Indeed, the dielectric response, which is a unique characteristic of the particular insulation system, can provide indication into aging and moisture content of the transformer insulation . There are three methods referred to dielectric response analysis (DRA) Accumulated solid contaminants have their own unique resonant frequencies at which absorption occurs. Contaminants in silicone oil can be identified by measuring its dielectric constant[6].The contaminants present in silicone oil can be found out by observing the variation of the dielectric constant with frequency. The loss tangent tan and’ were evaluated.The complex permittivities of the samples were tested using vector network analyser and Agilent Technologies 85070E dielectric probe kit.

Figure 1: Dielectric studies of silicone oil samples

Three-term calibration was performed using known standards of air, short-circuit and deionised water before testing the silicone oil samples. Fig 1 gives the plot of loss tangent for the pure and contaminated oil samples. It can be seen that contaminant introduces an absorption peaks at 13.32GHz, 17.3GHz and 17.5GHz. For the sample S2, the peak at 13.32GHz was not observable. 17GHz and 13.5GHz may be attributed to soluble colloidal metallorganicnanocomposites of Ni/AlN, Al2O3 or Fe2O3 due to aging oil and solid insulations.

4.FTIR analysis

Complex molecular structures and mixtures lead to more absorption bands and morecomplex spectra, usually with overlapping bands. The analysis of infrared spectrainvolves the correlation of absorption bands in the spectrum of an unknown compound withthe notorious absorption frequencies for types of bonds. Analysis of these absorptioncharacteristics reveals details about the molecular structure of materials in the sample.Significant for the identification of the source of an absorption band are intensity (strong ,medium or weak), shape (broad / sharp), and position (cm-1) in the spectrum. Intensity isinfluenced by the magnitude of change in the dipole moment that takes place in that mode,and the associated energy involved in dissociation of the bond. Analysis of these absorption characteristics gives the details of the molecular structure of materials in the sample, about the peaks of intensity, shape whether it is broad or sharp, and position in terms of cm-1 are significant for the identification of the source of an absorption band in the spectrum. The magnitude of change in the dipole moment in a particular mode influences the intensity and the associated energy involved in dissociation of the bond.FTIR analysis is performed for silicone oil samples using Fourier Transform Infrared spectrophotometer(JASCO6300) with a resolution of 0.07 cm-1 and frequency 20Hz (SNR is 50,000:1) at Chemistry Laboratory, B.S Abdul Rahman University, Chennai. Table 2 gives the characteristic functional groups and their IR absorption lines seen in silicone oils.

Table .2 Characteristic IR Absorptions in Silicone oil samples

(m=medium, w=weak, s=strong)

Frequency in

cm-1

Bond Type

Functional group

3000–2850(m)

C–H stretch

Alkanes

2260–2210(w)

C≡N stretch

Nitriles

1470-1450(m)

C–H bend

Alkanes

1370–1350(m)

C–H rock

Alkanes

1335–1250(s)

C–N stretch

Aromatic amines

1250–1020(m)

C–N stretch

Aliphatic amines

O–H bend

Carboxylic acids

Figure 2 : FTIR Studies of oil samples

FTIR spectrum of pure oil in Fig.2 indicates the presence of strong C-H stretch, C-H rock and C-H bend absorption peaks and the weaker CN and C-N lines all of them being the typical signature of silicone oil. The FTIR results of contaminated (sample3) and severely contaminated (sample4) oil samples are compared to that of pure oil to facilitate easy interpretation of the results.A positive T indicates absorption bands and negative values indicate an increase in transmission when compared to pure oil. Of particular interest are the weak negative regions marked out with black dashed circles which indicate increase in transmission due to the breaking of the CN and C-N bonds in contaminated silicone oil samples which in turn implies release of nitrogen into the oil. This could aid the formation of the (Ni,Al)/ AlN metal nano particle colloid.

5. Optical studies of Contaminated Oil samples

5.1 FEM modeling of oil infilt LMA-20SCPCF

FEM simulation of the fluid infiltrated LMA-20 has been carried out using COMSOL software. In the Cross section of a PCF, the inner cladding, consisting of six rings of triangularly arranged holes in a matrix material, surrounds a core consisting of a missing hole at the center of the fiber. The matrix material extends beyond the cladding, forming a region we call outer cladding. The natural external boundary of the fiber separates the outer cladding from the outside air medium. The core and outer cladding have same RI, whereas the inner cladding’s effective index is lowered by the presence of holes.LMA-20 SCPCF procured from NKT photonicswith a core diameter about 20 μm in which the air holes with diameter 6.4μmin a hexagonal pattern with a pitch of 13.2μmhas been theoretically modeled using finite element method to determine the mode profile in infiltrated SCPCFas depicted in Fig.3.

Figure3: (a) Micrgraph,(b) Recorded image(c) Field distribution (d) mode pattern by FEM in LMA-20SCPCF

As RI of infiltrant increases, the contrast decreases and the mode becomes lessconfined leading to gradual increase in the effective index of the Gaussian mode sustained in the core. As noilapproaches that of the silica core and slightly exceeds it, the trend in effective mode index changes and fluctuates depending on where the power confinement occurs, in the holes or in the surrounding silica region of the microstructured cladding.One should be aware of arising problems when working with PCFs for sensing applications. The flow of fluids is governed by the laws of micro fluidics and the surface tension is predominant in such micro channels. The air holes of a photonic crystal fiber allow the positioning of liquid and gaseous samples in close proximity to the fiber core without removing the cladding of the fiber.In fully oil filled configuration oil flows into all air channels of PCF by capillary forces.Propagation properties of fiber were investigated with the use of laser source (diode laser@650nm).

5.2 Experimaental studies

LMA PCF is cleaned, cleaved and dipped into the sample of silicone oil such that infiltration of its microholes takes place through capillary action. The infiltrated sample is then placed horizontally and laterally scanned along the length of the fiber using laser diode (LD) source at 650 nm to examine the extent of oil filling in PCF by capillary action. The diffraction pattern obtained when the laser light passes across the fiber, transverse to its length is observed on the screen kept on the opposite side. The pattern observed when the laser beam passes through the PCF regions with microholes filled with air, and when filled with oil is different. Fig.5 shows a mesh plot of the brightness distribution of the diffraction patterns observed when laser light passes perpendicular to the PCF length with micro holes filled with 4(a) air and 4(c) oil. The periodicity and the fringe patterns observed especially those for the prominent lines extent fully along the dimension of the laser spot.When the laser beam covers the PCF wherein one section the micro holes are filled with oil and the other is filled with air, the diffraction pattern consists of two distinct halves with the fringe pattern distinctly different in each half and this serves as a clear indication of the length up to which oil has infiltrated into the PCF. This is illustrated in Figure 4(b)where it can be seen that unlike in Figure 4(a) or 4(c), the interference fringes are not continuous along the dimension of the laser beam as the periodicity and pattern of the fringes in each half are different. Typical infiltration lengths in all the studies reported here is around few centimeters.

Figure 4: Brightness plots of experimentally recorded diffraction pattern when laser beam passes through PCF regions with microholes

filled with (a) air (b) partially air and oil (boundary region) (c) oil

Experimental verification of the mode profiles need infiltration of oil in to the all holes of the PCF , which can be done by placing distal end of PCF immersed in oil for particular period of time or by injection technique such that the fluid probed into the holes and one can record the mode profiles at the distal end using CCD camera. Figure 5 depicts schematic of the experimental set-up used for observing the mode profile of the LMA 20. The light output from a 650nm laser diode was coupled into the PCF using a 10X microscope objective (MO) and the output end was imaged onto a CCD camera using a combination of 20X MO and an eyepiece with additional white light LED illumnation from the CCD end adopting the method as cited in [6]. In order to avoid bleaching of the CCD camera, a beam splitter attanuator was used before the 10X MO to decrease the input power.

Figure 5: Schematic diagram of Experimental setup for imaging mode patterns in LMA-20 fiber

Table.3 Comparison of “neff” values with fluid RI changes to distinguish mode patterns in LMA-20PCF

Mode patterns in Infilt LMA-20

With Silicone oil samples

Refractive Index

Of oil samples

Effective mode index

Values by FEM(neff )

Recorded imagesfrom

Experiment

LMA-20Fiber with airholes

1.0

1.456399

Infilt LMA-20 With pure oil (S1)

1.367

1.456390

With contaminated oil

(S3)

1.4674

1.456388

with- Severely contaminated oil

(S4)

1.4738

1.456407

6.Conclusions

Contaminants responsible for oil degradation has been analysed by FTIR and dielectric studies. An original approach of silicon oil contamination studies based on tracking RI induced mode profile changes of the oil infiltrated LMA fibers is demonistrated. The sensing ability of the LMA fiber probe originates from the mode profile response of the oil infiltrated fiber. The study reveals that detection of contamination of 20% or less can be done effectively with this technique is also a low cost alternative to RI induced spectral change tracking using optical spectrum analyzers. Mode pattern change easily identifiable for change from pure to contaminated oil.Spread in the evanescent field can be used to indicate the onset of contamination .The intensity distribution of the images are also tracked using image processing technique for more sensitive detection system.This simple image processing techniques is a low cost alternative to RI induced spectral change tracking.

References

1. Oommen, T.V. Static electrification properties of transformer oil. IEEE Trans. Electr. Insul. 23, 1988, 123–128.

2. Rusell Martin “Experiences in Service with New Insulating Liquids” working groupA2.35 October,2010.

3. Yaw-Dong Wu, Jian-Jang Lee, and Tien-Tsorng Shih “Proposal for Large Mode Area Photonic Crystal Fibers” Progress In Electromagnetic Research Symposium Proceedings, Moscow, Russia, and August, 2012, 19–23.

4. Ron Spittel*, Klaus Mörl, Hartmut Lehmann, Julia Mörbitz, HartmutBartelt“Optical properties of liquid filled photonic crystal fibers”DGaO Proceedings 2009.

5. V.Mishra, Subash. C Jain, ‘Fuel Adulteration Detection Using long period fiber grating sensortechnology’ Indian Journal of Pure &Applied Physics, Vol 46(02), February 2008,106-110.

6. Fofana, I.; Hemmatjou, H.; Farzaneh, M. Low Temperature and Moisture Effects on Polarization and Depolarization Currents of Oil-Paper Insulation. Electr. Power Syst. Res. , 2010, 80x, 91–97.

7. Sheu F.W, J.Y Chen,“Observing cross-sectional images and average optical patterns of photonic crystal fibers using partially incoherent laser light”, J. Europe. Opt. Soc. Rap. Public. 8, 2013,13003.

8. Beatriz Larri ´ on Miguel Hern´aez, Francisco J. Arregui, Javier Goicoechea,Javier Bravo, and Ignacio R.Mat´ıas“Photonic Crystal Fiber Temperature Sensor Based on Quantum Dot Nano coatings”Hindawi Publishing Corporation, Journal of Sensors ,2009.

9. Yongqin Yu, 1, 2 Xuejin Li, 1, 2,* XuemingHong, 2Yuanlong Deng, 2 Kuiyan Song, 2YoufuGeng, 1, 2 Huifeng Wei, 3 andWeijun Tong3 “Some features of the photonic crystal fiber temperature sensor with liquid ethanol filling”©2010 Optical Society of America.