EXPERIMENTAL AND THEORETICAL STUDY OF BREAKDOWN VOLTAGE INITIATED BY PARTICLE CONTAMINATION IN GIS

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241 EXPERIMENTAL AND THEORETICAL STUDY OF BREAKDOWN VOLTAGE INITIATED BY PARTICLE CONTAMINATION IN GIS Sayed A. Ward 1, Adel A. ElFaraskoury 2, S. Sabry ElSayed 3 1 Head of Elect. Eng. Dept.

1 241 EXPERIMENTAL AND THEORETICAL STUDY OF BREAKDOWN VOLTAGE INITIATED BY PARTICLE CONTAMINATION IN GIS Sayed A. Ward 1, Adel A. ElFaraskoury 2, S. Sabry ElSayed 3 1 Head of Elect. Eng. Dept., Banha University, Cairo, Egypt 2 Extra High Voltage Research Center, Egyptian Electricity Holding Company, Egypt 3 High Voltage Substations, Egyptian Electricity Transmission Company, Cairo, Egypt ABSTRACT The particle contamination inside the Gas-Insulated Systems (GIS), plays a major role to occurrence the Partial Discharge (PD), which causes the insulation failure in the GIS. This paper presents the experimental and theoretical study of breakdown voltage initiated by a metallic particle inside a gap insulated by compressed SF 6 gas. Study the effect of particle contamination on the electric field distribution in the gap. Study the effect of gas pressure, gap spacing, particle shape and the particle size on the breakdown voltage that initiated by the metallic particle. Study the Particle Contamination Factor (PCF) to describe the sensitivity of the dielectric strength at the contaminating gaps. The electric field calculated by the Finite Element Method Magnetics (FEMM) software, which is based on using the Finite Elements Method (FEM). The breakdown voltage initiated by the particle contamination is calculated by the streamer formation criterion. The experimental study carried out in the laboratory with using the pressure vessel and negative high voltage DC source. The measured breakdown voltage for different particle size agreed well with the calculation of breakdown voltage. Keywords - GIS, Breakdown Voltage, particles, SF 6 gas, electric field I. INTRODUCTION Sulfur hexafluoride (SF 6 ), is a man-made gas with excellent dielectric properties. The SF 6 is widely used as an insulating gas in various electric devices, where it used as interrupting medium in the circuit breaker and as insulating medium in GIS, Gas Insulated Transmission Line (GITL), Gas Insulated Bus duct (GIB) and Gas Insulated Transformer (GIT), because the SF 6 has good electrical properties. However, there are some problems effect on the insulation of the SF 6 gas, such as the particles contaminations, the moisture and the decomposition products [1-5]. The Fig.1 shows the statistics of the GIS major failures in the world, it's clearly that, the insulation failure of the SF 6 gas causes the most of problems in the GIS. The partial discharge causes about 80 % of insulation failure in the GIS, where, the particles contaminations are the main player to occur it [6-8], hence, the particles contamination cause above 20% of the failures in the GIS [2]. The particles contaminations inside the GIS may be exist by, the manufacturing process, from mechanical vibrations, from moving parts of the system such as breakers or disconnectors [9], from the negligence during the maintenance inside the GIS or from the corrosion of metallic parts which due to the decomposition products and the moisture as in Fig.2. Anyway, when the particle contamination exist inside the GIS, it is will effect on the electric field and the breakdown voltage. This paper presents the effect of the particle contamination on the electric field and the breakdown voltage. The experimental and theoretical study is as follows, comparison between a clean gap and a gap with particle contamination at the same case. The study will be at different cases of gap spacing, gas pressure, particle shape and the particle size. Mechanism Trouble 18.1% Others 12.3% Gas Leak 12.4% Insulation Failure 57.3% Figure 1 Statistics of the GIS major failures in the world [6]

242 Figure 2 the surface corrosion (one of the sources of particles contaminations in GIS) [10] II. EXPERIMENTAL SET-UP The experimental set-up is schematically illustrated in Fig.

The experimental work was carried out inside the laboratory in dry air at room temperature (22 25 C), and varies SF 6 pressure from 10 KPa to 200 KPa using the pressure vessel with SF 6 cylinder, and

A high-voltage dc source with negative polarity and output voltage up to 80 kv, was used to energize the stressed plate.

2 242 Figure 2 the surface corrosion (one of the sources of particles contaminations in GIS) [10] II. EXPERIMENTAL SET-UP The experimental set-up is schematically illustrated in Fig.3 this set-up has been built up to measure the breakdown voltage of a particle-initiated in the compressed SF 6 -insulated plate to plate gap. The experimental work was carried out inside the laboratory in dry air at room temperature (22 25 C), and varies SF 6 pressure from 10 KPa to 200 KPa using the pressure vessel with SF 6 cylinder, and using plate to plate gap. Stainless steel spherical particles of radii 0.25mm and 0.5mm were used. A high-voltage dc source with negative polarity and output voltage up to 80 kv, was used to energize the stressed plate. The high-voltage source has a voltage metering device, for measuring the output applied voltage, with full scale accuracy of ±2%. The upper metallic part of the vessel was connected to the HV source through a water resistance of 1MΩ as a current-limiting resistor, to prevent any damage of instruments if flashover occurred, and the lower part of vessel was grounded. Figure 3 Schematic diagram of the experimental set-up. The steps of the experimental study as following: 1) Disassembly the vessel and out the part which belong to the gap configuration. 2) Select the type of the gap configuration that we need study on it (plate to plate). 3) Sitting the distance between the two electrodes. 4) Cleaning all inside vessel parts. 5) Putting the metallic particle contamination (in case of contaminating gap). 6) Completely close and assembly by accuracy and carefully. 7) Making a vacuum to the vessel for five hours. 8) Stop the vacuum and wait about two hours to sure that the vacuum and assembly are good. 9) Charging the nitrogen gas N 2 to one bar and wait to an hour to eviction the moisture. 10) Making the evacuation for two hours to out the N 2 gas. 11) Start to charge the SF 6 gas to the vessel until to the test pressure (related to the case of study). 12) Making the preparation of high voltage test. 13) Start to inject the H.V slowly and notice of the breakdown voltage, then record its value at least five measurements were taken for each measuring point to estimate the mean value. 14) Varying the gas pressure from 10 KPa to 200 KPa to test at different gas pressure. 15) All times we need to change the gap distance, the gap configuration or putting the metallic particles on the gap repeat all steps. III. METHODS OF ANALYSIS III.1 Electric field calculation The electric field is calculated with using the FEM throughout this work. The FEMM package, software is used to simulate the problems and to calculate the electric field inside the GIS. FEMM software is a finite element package for solving 2D planar and axisymmetric problems in electrostatics and in low frequency magnetic [11, 12]. The FEMM Version 4.2 is used throughout the work in this paper for computation the voltage and the electric field distributions around the contaminating particles. The steps of the electric field calculation by the FEMM software as following: a) Simulate the two plates and the contaminating particle as in Fig.4a, then define its materials and the type of insulation medium to the FEMM software. b) Applied a unit voltage to high voltage plate and applied zero voltage to the other plate, by insert it to the FEMM software. c) The FEMM software start to distribute the voltage through equipotential lines as in Fig.4c. d) The FEMM software distributes the electric field in the gap between two plates as Fig.4d. e) Take a 3000 value of the electric field distribution alone axis from the surface of particle to the high voltage plate, to use it in the

243 electric field study and the breakdown voltage calculations. Figure 4.a about 10 8 electrons for air and other gases [13]. The avalanche-streamer transition and breakdown take place as in Fig.

The streamer criterion then takes the form: [1,16] zc N C = exp [α (z) - η (z)] dz (1) 0 Where α is the Townsend ionization coefficient, η is the electron attachment coefficient, Z c is the critical

14 (3) Figure 4.b In which α/p and η/p are measured in (m -1. Pa- 1 ) and E/p in (V/m.Pa).

3 243 electric field study and the breakdown voltage calculations. Figure 4.a about 10 8 electrons for air and other gases [13]. The avalanche-streamer transition and breakdown take place as in Fig.5, and the avalanche attains a critical size [1,2,14]. The streamer criterion then takes the form: [1,16] zc N C = exp [α (z) - η (z)] dz (1) 0 Where α is the Townsend ionization coefficient, η is the electron attachment coefficient, Z c is the critical avalanche length when α = η. For SF 6 the dependence of α and η on the electric field E and gas pressure P can be put in the form [14]: α/p = (E/P) (2) η/p = (E/P) (3) Figure 4.b In which α/p and η/p are measured in (m -1. Pa- 1 ) and E/p in (V/m.Pa). After dependence on, the equations (1, 2, 3) and the steamer condition of breakdown, had been designed a software program to calculate that: the breakdown voltage, the ionization coefficient α, the attachment coefficient η, the critical number of electrons N C and the ionization-zone thickness. All previous program calculations at the instantaneous of breakdown voltage and belong to the SF 6 gas only. Figure 4.c Figure 5 Development of an avalanche which initiated by spherical particle in plate to plate gap [1] Figure 4.d Figure 4 (a, b, c and d) the steps of FEMM software to simulate the particle contamination and calculate the electric field distribution around it. III.2 Method of Breakdown analysis The Streamer formation criterion is used to breakdown voltage calculation. The Streamer occurs when, the number of electrons reach to the critical number (N C ) IV. Results and discussions IV.1 Field intensification factor at the particle surface tip In the absence of the contamination particle, the electric field is distributed in the gap related to the type of the gap, at the plate to plate is the same value. But, in the presence of the particle, the electric field is distorted in the vicinity of its surface. The field pattern around the

4 244 particle depends on the particle shape, size and position. The field intensification factor is the electric field strength at the particle tip divided by the field strength at of the same point in the gap at clean gap. The field intensification factor at the particle surface tip is responsible for the development of the corona discharge in the plate to plate gap [2], Fig.6 and Fig.7 show the field intensification factor at the tip of spherical particles, it's about from 3:4, while, Fig.8 and Fig.9 show the field intensification factor at the tip of steel filings particles, it's about from 4:7. small difference with varying radii. (b) The zone of distorted electric field at the bigger spherical particles is larger than the zone of distorted electric field at the smaller spherical particles, so the breakdown initiated by large radius of spherical particles is smaller than the breakdown initiated by small radius of spherical particles. (c) The field intensification factor increases by increasing the length of filings at the same width, while it decreases by increasing the width of filings at the same length. (d) The influence of the gap spacing is small on the field intensification factor, in all previous cases. Figure 6 Field intensification factor of the spherical particles lying at the ground plate at gap spacing 30mm Figure 8 Field intensification factor of the steel filings particles lying at the ground plate at gap spacing 30mm Figure 7. Field intensification factor of the spherical particles lying at the ground plate at gap spacing 50mm Fig.6 is plotted for five spheres of varying radii existing in the plate to plate gap, lying at the ground plate, at gap spacing 30mm, while Fig.7 at gap spacing 50mm. Fig.8 is plotted for four steel filings particles of varying dimension existing in the plate to plate gap, lying at the ground plate, at gap spacing 30mm, while Fig.9 at gap spacing 50mm, we can see that (a) The field intensification factor in the tip of spherical particle is Figure 9 Field intensification factor of the steel filings particles lying at the ground plate at gap spacing 50mm IV.2 Breakdown voltage and ionization-zone thickness initiated by the particle contamination Fig.10 to Fig.13 show the relation between the breakdown voltage and SF 6 pressure at clean gaps and contaminating gaps by different shape and size of particles. From this figures we can see that; (a) by increasing of radii for spherical particles, the breakdown

5 245 initiated by particle is decreasing when the particle is located on the ground plate. (b) By increasing the length of the steel filings particles, the breakdown initiated by particle is decreasing at the same width. (c) By increasing the width of the steel filings particles, the breakdown initiated by particle is increasing at the same length. The Table I and Table 2 show that; (a) the ionization-zone thickness decrease by increasing the SF 6 pressure (b) the ionization -zone thickness increase by increasing the radius of spherical particle when the gas pressure above 100 KPa (c) the ionization -zone thickness increase by increasing the width of steel filings particles, while it small deference by increasing the length. Figure 12 Calculated breakdown voltage initiated by spherical particles in SF 6 gas, with varying radii, lying at ground plate at gap spacing 30mm Figure 10 Measured and calculated breakdown voltage initiated by spherical particles in SF 6 gas, with varying radii, lying at ground plate at gap spacing 5mm Figure 13 Calculated breakdown voltage initiated by steel filings particles in SF 6 gas, with varying dimension, lying at ground plate at gap spacing 30mm TABLE 1 Ionization-zone thickness at breakdown voltage for different radii of spherical particles at varied SF 6 pressure, at gap spacing = 30mm SF 6 pressure (KPa) Ionization-zone thickness (mm), spherical particles r=0.1 mm r=0.25 mm r=0.5 mm TABLE 2 Ionization-zone thickness at breakdown voltage for varying radii of steel filings particles at varied SF 6 pressure, at gap spacing= 30mm SF 6 pressure (KPa) Ionization-zone thickness (mm), steel filings particles L=0.5mm, W=0.25mm L=0.5mm, W=0.5mm L=1mm, W=0.25mm L=1mm, W=0.5mm Figure 11 Measured and calculated breakdown voltage initiated by spherical particles in SF 6 gas, with varying radii, lying at ground plate at gap spacing 10mm IV.3 The particle contamination factor The Particle Contamination Factor (PCF) is used to describe the sensitivity of the dielectric strength to

6 246 local field enhancement in GIS due to the particle contamination. The particle contamination factor is defined as the ratio of the breakdown voltage with particle contamination -to- the breakdown voltage with clean gap [5]. Fig.14 to Fig.17 show the effect of the particles contamination on the breakdown voltage at different shape and size of particles also, at different gap spacing with SF 6 pressure, where we can see that; (a) by increasing the SF 6 pressure, the influence of contaminating particle increase, in any case at low gas pressure the PCF around 0.8 or 0.9, while at high gas pressure the PCF about 0.3: 0.4 at spherical particles and about 0.2: 0.3 at steel filing particle. (b) The influence of the gap spacing on the PCF is small. (c) The influence of bigger spherical particle on the PCF is larger than the influence of smaller spherical particle on the PCF. (c) The dimension of the steel filings particles also, effect on the PCF. (d) The PCF is more influence at the higher length of the steel filings. (e) The PCF is more influence at the lower width of the steel filings. Figure 16 the particle contamination factor for steel filings particles at varying dimension with the SF 6 pressure, lying at ground plate at gap spacing 30mm Figure 14 the particle contamination factor for spherical particles at varying radii, with the SF 6 pressure, lying at ground plate at gap spacing 30mm Figure 17 the particle contamination factor for steel filings particles at varying dimension with the SF 6 pressure, lying at ground plate at gap spacing 50mm Free metallic particles originate mainly from the manufacturing process, from mechanical vibrations, or they may originate from moving parts of the system such as breakers or disconnectors. Depending on The particles may be conducting or insulating in nature, the latter being less harmful. [9]. V. CONCLUSION Figure 15 the particle contamination factor for spherical particles at varying radii, with the SF 6 pressure, lying at ground plate at gap spacing 50mm 1) The presence of contamination can therefore be a problem with gas-insulated substations operating at high fields. If the effects of these particles could be eliminated, then this would improve the reliability of compressed gas insulated substation.

7 247 2) The intensification factor is due to presence of metallic particle in GIS, it is depends on the shape and size of particle, where it about 3:4 in spherical particles, and about 5:7 in the steel filings particles. 3) The presence of contamination particle reduce the breakdown voltage in GIS. The size of particle is effective of breakdown, the steel fling particle is severe than the spherical particle. 4) The ionization-zone thickness decreases by increasing the SF 6 pressure, while it increase by increasing the radius of spherical particle when the gas pressure above 100 KPa and it increase by increasing the width of steel filings particles, while it small deference by increasing the length. 5) The pressure is effective to make the contamination particle is more influence of breakdown in GIS, where the metallic contaminants effects are more pronounced at higher gas pressures. 6) The shape of the particles, as well as the geometry and voltage levels of the system, the particles get more or less influenced by the electric field which, in turn, makes them hazardous to the electrical system. 7) The particle contamination factor (PCF) to describe the sensitivity of the dielectric strength of SF 6 gas, when the particle contamination is exist in GIS. VI. REFERENCES [1] M. Abdel-Salam, A. Ahmed, A. Nasr EL-deen, Inception Voltage of Corona Discharge from Suspended, Grounded and Stressed Particles in Uniform-Field Ga s-insulated- Gaps at Varying Pressures, International Journal of Plasma Environmental Science & Technology, Vol.4, No.1, MARCH 2010 [2] M. M. ElBahy, S. A. Ward, R. Morsi, M. Badawi, Onset voltage of a particle-initiated negative corona in a co-axial cylindrical configuration, IOP publishing Journal of phisics, October 2013 [3] B. Rajesh Kamath, Member, IAENG, J. Sundara Rajan, K. A. Krishnamurthy. Z. Kurian, Partial Discharge Characterization of Nichrome Particle in a Gas Filled Duct, Proceedings of the World Congress on Engineering and Computer Science 2013 Vol I,WCECS 2013, October, 2013, San Francisco, USA [4] M. Siva Sathyanarayana, J. Amaranth, Modelling And Analysis Of Trajectories Of A Wire Like Particle IN A Three Phase Common Enclosure Gas Insulated Bus duct (GIB) With And Without Image Charges, International Journal of Engineering Research and Applications (IJERA),Vol. 2, Issue 3, May-Jun 2012, pp [5] Sayed A. Ward, "Optimum SF 6 -N 2, SF 6 -Air, SF 6 -CO 2 Mixtures Based on Particle Contamination", IEEE International Symposium on Electrical Insulation, Anaheim, CA USA, April [6] CIGRE Task Force (1998) [7] Sahu.K.B.M and Amarnath.J, Effect of various parameters on the movement of metallic particles in a single phase gas insulated bus duct with image charges and dielectric coated electrodes, ARPN J. Eng. Appl. Sci , 2010 [8] Subrahmanya Mkbvsr and Amarnath.J, Image charge effect on metallic particle in single phase gas insulated bus duct (GIB), J. Emerging Trends Eng. Appl. Sci., (JETEAS) , 2011 [9] D. Veena Deepika, Breakdown in GIS due to the presence of metallic particles, NCNTE [10] G.A.S. Gesellschaft für analytische Sensorsysteme mbh,germany, catalog of "SF 6 - Breaker- Analyser -SF 6 - Percentage, Moisture, SO 2 " http/ [11] David Meeker, Finite Element Method Magnetics, Version 4.2, User s Manual, September [12] Sayed A. Ward, M. A. Abd Allah, Amr A. Youssef, Particle Initiated Breakdown Inside Gas Insulated Switchgear for Various Gases Mixtures, International Journal on Electrical Engineering and Informatics. Volume 4, Number 2, July 2012 [13] N. H. Malik, A.A.AL-Arainy, M.I.Qureshi, Electrical Insulation on Power Systems, Book first edition. [14] F.A.M. Rizk, C. Masetti, R.P. Comsa, Particle Initiated Breakdown in SF 6 Insulated Systems Under High Direct Voltage, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-98, No. 3 May/June 1979 [15] Solvay Chemicals, Solvay Fluor, Sulphur Hexafluoride, ts/fluor/sulphur_hexafluoride/sulphur_hexafluoride.asp x. [16] E. Kuffel, W.S. Zaengl, J. Kuffel, High Voltage Engineering Fundamentals, Second edition 2000,

Inception Voltage of Corona Discharge from Suspended, Grounded and Stressed Particles in Uniform-Field Gas-Insulated-Gaps at Varying Pressures

Abdel-Salam et al. 1 Inception Voltage of Corona Discharge from Suspended, Grounded and Stressed Particles in Uniform-Field Gas-Insulated-Gaps at Varying Pressures M. Abdel-Salam 1, A. Ahmed, and A. Nasr