The 5th International Conference on Electrical Engineering – Boumerdes (ICEE-B) October 29-31, 2017, Boumerdes, Algeria.
((0 0 ©2017
A Study of the Behavior of Water Droplets Under The Influence of Non-Uniform Electric Field in Silicon
Rubber
S.Benharat
Research center in industrial technologies CRTI BP 32 El ALIA 16111,
P.O. Box 64, Cheraga 16014, Algiers, Algeria sbenharat@hotmail.fr
S.Bouazabia
Electrical and Industrial Systems laboratory (LSEI) University of sciences and technologies Houari Boumediene
(USTHB)
BP 32 El ALIA 16111, Algiers, Algeria sbouazabia@yahoo.fr
A. Haddad
Advanced High Voltage Engineering Research Centre School of Engineering, Cardiff University
Cardiff CF24 3AA, UK
Abstract— Water droplets on the surface of silicon rubber were investigated under the influence of non-uniform electric fields. Several parameters of water droplets were investigated, such as the positioning of the droplets regarding the electrodes.
The most striking conclusion is that the flashover voltage and electric field distribution depends on the positioning of the droplets from the electrodes.
Keywords— water droplets; non uniform Electric field;
COMSOL; finite element; insulating barrier; silicon rubber I. INTRODUCTION
The insulating materials used for equipment of high voltages during operation for many years, are stressed by various factors, such as moisture, particles, and pressure [1, 2, 3, 4]. The objective of this study is to investigate the dependency of the electric field on moisture and water droplets using a silicon rubber.
Water drops or water films in high voltage equipment, why and where did that occur? The answer to this question allows us to classify the high voltage equipment in indoor and outdoor equipment [5].
In indoor high voltage equipment, for example high voltage switchgear and bus bars, synthetic materials are frequently used and the surfaces of this class of material can accumulate water drops. These accumulated water drops can also later become water films.
For outdoor high voltage equipment’s, rainwater drops are usually deposited on the surfaces of exposed equipment parts during or after precipitation. So by, conductors of overhead lines and sheds of outdoor insulators are those of most equipment parts, which can considerably accumulate multiple water drops on the surfaces [6-7].
In this work we examine the influence of the position of the insulating barrier and the water droplets on the electric field
distribution in non-Uniform electric field system using silicon rubber.
The numerical study of the electric field distribution in rod - plane systems avoids generally expensive destructive testing.
Indeed, we make a digital study to determine the values of the electric field by using the computer software Comsol [8]
II. SYSTEM MODELING a. System presentation
The study system (Fig .1) consists of an arrangement of rod- plane electrode gap of d, between the two electrodes is inserted an insulating barrier of width w, thickness e and relative permittivity εrsolid, placed at the distance x from the rod electrode. The rod electrode of radius rp, is connected to the high voltage HV and the plane electrode of width Lp, is grounded. The system studied containing air of relative permittivity εr.
Fig. 1. Study System X
w ɛr
HV
rp
d
Lp
Insulating barrier e
Plane
((0 0
To evaluate the effect of the barrier on the behavior of the system, it assumes that a change in the electric field is induced by the presence of the insulating barrier; its distribution depends of dimensions and position of the barrier.
In this paper the gap distance, barrier width and thickness and plane width are constants and they are 5cm, 4cm, 2mm and 9 cm respectively.
b. Electric Filed computation
The field values are obtained by solving the Maxwell equations governing the static state.
1 With :
(2) ε : Dielectric permittivity, : Charge Density, E : Electrical
field.
Taking into account the local form of the Ampere law:
0 (3) Where:
(4) V: Electrical voltage.
Combining equations 1 and 4 gives the Poisson equation in the case of a homogeneous material:
∆ 5 Neglecting the space charges and considering the
calculation on the x-y plane, this equation is written as:
0 6 The resolution of this equation in the study system can be done only by a numerical calculation. To do this, the COMSOL software [5] is used, with considering a Neumann boundary condition on a rectangular border around the system:
0 7 III. RSULTS AND DESCUSION
From Fig. 2, 3, 4, 7, 8 and 9 it is evident that the droplet water plays a significant role in determining the flashover voltage in silicon rubber. Moreover, from these figures is indicated that the positioning of silicon rubber and droplets influences the flashover voltage and electric field distribution (if we compare the electric field contour when the barrier with droplets is placed at 0% from the rod electrode and the electric field contour when the barrier with droplets is placed at 20%, 60% and 100% from the rod electrode, unless if we compare between the electric field contour when the barrier is placed 0% from the rod electrode but the droplets are in different horizontal position from the rod electrode (0cm, 0.25cm and 0.75 cm).
From the results it is evident that the droplet leads to a reduction of the flashover and electric field [9]. This is clearly shown in Fig. 5 and 10. The positioning of the droplets from
the electrodes play a vital role. This can be explained by the fact that near the electrodes ionizing phenomena are easier to obtain, so with easier ionization the flashover voltage becomes lower. The latter is reminiscent of the relative easiness with which partial discharges take place in solid dielectric cavity, when one side of the enclosed cavity touches one of the electrodes [10].
Fig. 2. Electric field contour for 4 water droplets (x=0%, first droplets near the rod electrode).
Fig. 3. Electric field contour for 4 water droplets (x=0%, the droplets moved horizontally from the rod electrode by 0.25 cm)
Fig. 4. Electric field contour for 4 water droplets (x=0%, the droplets moved horizontally from the rod electrode by 0.75 cm)
((0 0 Fig. 5.Electric field distribution with and without droplets in different
horizontal position
Fig. 6. Comparison between electric field without droplets and with droplets in different horizontal position (Logarithmic scale)
Fig. 7. Electric field contour with water droplets (x=20%).
Fig. 8. Electric field contour with water droplets (x=60%)
Fig. 9. Electric field contour with water droplets (x=100%).
In Fig. 10 to 13, a direct comparison between the electric field distrebution in dry silicon rubber and wet silicon rubber, is made. From the results it is clear that the dry silicon rubber tends to give higher electric field results than the wet silicon rubber. This is a strong indication that dray silicon rubber improves the flashover voltage.
The position of the insulating barrier and water droplets effect the electric field values, it takes a maximum values when the barrier is near the rod electrode and decreases if the barrier moves to the plane elctrode. The electric field on the barrier surface without water droplets is higer than the electric field with water droplets , it decreases from 450 kV/cm to 290kV/cm respectively at x=0% fig.10, but the values is almost the same if we compare between the the barriers used with and without droplets at 20%,60% and 100% from the rod electrode fig.11 to 13.
1 101 201 301 401
0 0.5 1 1.5 2
Electric field ,E (kV/cm)
On the barrier surface (cm)
Without water droplets droplets closed to the rod droplets 0.25 cm from the rod droplets 0.75 cm from the rod
1 4 16 64 256
0 0.5 1 1.5 2
Electric field ,E (kV/cm)
On the barrier surface (cm) Without water droplets
droplets at 0 cm from the rod electrode droplets 0.25 cm from the rod droplets 0.75 cm from the rod
((0 0 Fig. 10. Electric field distrebution on the barrier surface (x=0%)
Fig. 11. Electric field distrebution on the barrier surface (x=20%)
Fig. 12. Electric field distribution on the barrier surface (x=60%)
Fig. 13. Electric field distribution on the barrier surface (x=100%) IV. CONCLUSION
In this paper, water droplets positioned on Silicon rubber were investigated under the influence of non uniform electric fields. Droplet positioning horizontaly on the barrier surface plays a dominant role in determining the flashover voltage.
According to the results, it is clear that by using water droplets , the value of the flashover voltageand electric field decreases. Other obtained results enable us also to affirm, that the electric field applied to the water surface is controlled by the electrode geometry and spacing between water surface and the electrode.
REFERENCES
[1] CIGRE 349, “Moisture equilibrium and moisture migration within transformer insulation systems,” CIGRE working group A2.30, 2008 [2] M. Koch, M. Fischer, S. Tenbohlen, “ The breakdown voltage of
insulation oil under the influences of humidity, acidity, particles and pressure,” International Conference APTADM, 2007, Wroclaw, Poland [3] P. J. Griffin, Water in transformers – so what!, National Grid Conference
on Condition Monitoring in High Voltage Substations, Dorling, 1995 [4] E. Gockenbach, H. Borsi, “Performance and new application of ester
liquids,” Proceedings of 2002 IEEE 14th International Conference on Dielectric Liquids, 2002, Graz, Austria
[5] Adolphe, A. Beroual “Study of the Behavior of AC Discharges of Water Drops on both Conducting and Dielectric Solid Surfaces,” IEEE Transactions on Dielectrics and Electrical Insulation Vol. 17, No. 5;
October 2010
[6] C. Roero and T. H. Teich, “Water drops on high voltage transmission lines,” Electrostatics Society of America (ESA) Annual Meeting University of Alberta, Edmonton, Canada, 2005.
[7] M. Amin, M. Akbar and S. Amin, “Hydrophobicity of Silicone Rubber used for Outdoor Insulation (an overview)”, Rev. Adv. Mater. Sci. Vol.
16, pp. 10-26, 2007.
[8] COMSOL , "Introduction to comsol multiphysics,” 1998-2015.
[9] KIND, D.—KAERNER, H. : High-Voltage Insulation Technology, Vieweg & Sohn, Braunschweig Germany, 985.
[10] MASON, J. H. : Discharges, IEEE Trans. Electr. Insul. 13 (1978), 211–
238.
0 100 200 300 400 500
0 0.5 1 1.5 2
Electric field, E (kV/cm)
On the barrier surface (cm)
With water droplet Without water droplet
0 50 100 150 200
0 0.5 1 1.5 2
Electric field, E (kV/cm)
On thebarrier surface (cm)
Without water droplet With water droplet
0 50 100 150 200
0 0.5 1 1.5 2
Electric field , E (kV/cm)
On the barrier surface (cm)
With water droplets Without water droplets
0 50 100 150 200
0 0.5 1 1.5 2
Electric field , E (kV/cm)
On the barrier surface (cm)
With water droplets Without Water droplets
