The Effects of Salt Contamination Deposition on HV ... - IEEE Xplore

ISBN: 978-1-4799-8903-4 2015 IEEE 11th International Conference on the Properties and Applications of Dielectric Materials (ICPADM)

The Effects of Salt Contamination Deposition on HV Insulators Under Environmental Stresses M Majid Hussain, S Farokhi, S G McMeekin

M Farzaneh

School of Engineering & Built Environment Glasgow Caledonian University Glasgow, United Kingdom

Chair holder, NSERC/Hydro-Quebec/UQAC (CIGELE) University of Quebec at Chicoutimi Chicoutimi (Quebec), Canada

high voltage stations and line-insulators near coastal areas is a major concern for power failure for utilities [1].

Abstract—This paper examines the issue of salt contamination accumulation, flow of leakage current and surface flashover on high voltage insulators. Artificial contaminants were prepared using NaCl and distilled water. With long-term accumulation of salt particles on high voltage insulators in the presence of wind, ambient temperature, humidity, fog and moisture, conducting layers is formed on insulators’ surface. This conductive layer provides an ideal path for leakage current to flow from the high voltage conductor to the grounded side of the insulators. Insulators’ layer moistened and heated due to environmental stresses which cause an increase in leakage current and dry bands formation. Partial arcs can occur across the dry bands. Under favourable conditions a complete flashover might occur. The performance of the polluted insulators mainly depends on the conductivity of the polluted surface layer or on the equivalent salt deposited density of the polluted surface layer, which are affected by environmental conditions. Insulator is tested with different conductivities in a series of experiments including ESDD and NSDD measurement. COMSOL Multiphysics software is used to simulate and determine the electric field and potential distribution as well as the resulting leakage current flow on its polluted surface. The measurement results have been compared with the COMSOL results and found to be in good agreement. Outcomes will lead to the modelling of insulators and electric field response of insulators under different conductivities and environmental stresses

Near coastal areas most of the insulator contamination is due to airborne sea salt particles. Small water bubbles are released from the tips of sea waves due to sea wave’s actions in stormy weather. Wind is the main carrier for these water bubbles, which evaporates the water of small bubbles to form even smaller bubbles of brine [2]. If the relative humidity is low, these bubbles will leave dry, crystalline, salt particles on the surface. These salt particles are deposited on the insulators’ surface. Insulators’ in coastal areas that are extremely close to the sea can be exposed directly to sea salt spray during periods of strong winds. Due to high temperatures and ultraviolet radiation, the water will be dried while leaving a white salt layer on the surface of insulator. The deposition of sea salt on insulator’s surface mainly depends upon the wind velocity and distance from sea [3]. In coastal areas saline pollution is more prevalent than inland areas. As a result probability of surface flashover is high due to high pollution conductivity which allows flow of higher leakage currents. This can cause insulator accelerated aging and frequent and intense discharge activity. Sea salt is not the only source of pollution on high voltage insulators: road salt, industrial pollution and desert pollution also contribute to this process [4]. Nitric acid is another kind of pollution which produces contamination. However, in this paper only the sodium chloride (NaCl) is considered as contamination. Holte et al [5] showed that sodium chloride has a significant effect on minimum ac flashover voltage.

Keywords— Salt Contamination; Environmental Stresses; Leakage Current; Surface Flashover; COMSOL Multiphysics;

I.

INTRODUCTION

The reliability of high voltage insulators is vital for the safe operation of the power system’s networks. The insulators are scattered throughout the networks, even a single insulator fault may lead to premature failure of power system’s networks. The majority of the high voltage insulators are exposed to variable environmental conditions. Several factors made the modeling and design of high voltage insulators more important and complicated: non-uniform distribution of pollution on insulator surface, different pollution level in different regions along with the development of new plans of marine renewable energy and growing investment in oil and gas industries where the insulators are located in harsh climate conditions. The performance of outdoor insulators is an important issue for power system utilities and correlated to local operating climate conditions. Salt deposition processes and surface flashover on

The utilization of a polymer insulator has been increased day by day; composite insulators are also very reactive to the electric and potential field strength. The electric and potential field distributions were based on applied voltage, insulator design and phase spacing. The composite insulators have linearity when compared with ceramic insulators [6]. In this paper we made a comparison of electric field and potential field distribution and leakage current properties of silicon rubber insulator under salt contaminant conditions. The relationship between them and their properties are discussed.

978-1-4799-8903-4/15/$31.00 ©2015 Crown

616


II.

IV.

TEST EQUIPMENT AND PROCEDURE

The composition of contaminating suspension consisted of 30g of kaolin in one liter of de-mineralized water with 0.25g, 0.5g, 0.75g and 1.0g of salts (NaCl), the salinity of 10kg/m3, 20kg/m3, 30kg/m3 and 40kg/m3 was adopted. The polymer insulator is contaminated by 0.3gm salt solution and dried for 24 hours. The dry particles of salt were recovered by a brush from the pollutant porcelain insulator and mixed with 300 ml water to make a solution for polymer insulator. The conductivity is measured with CM 180 conductivity meter with conductance range 20µS to 200mS of collected samples of solution. Temperature is also recorded. The all conductivities at different temperature are converted to 200C using [7]. (1) σ20 = σθ [1-b (θ-20)] Where: θ = is the solution temperature (0C). σθ = is the volume conductivity at temperature of θ 0C (S/m). σ20 = is the volume conductivity at temperature of 20 0C (S/m). b = is the factor depending on temperature of θ (see table II).

A. Test Equipment The experiments were carried out in an environmental chamber. A 33-kV AC voltage was supplied to the sample through wall bushing. The other technical parameters are as follows: rated current 1 A, rated capacity 200kV and short circuit impedance less than 1 percent. B. Test Sample TABLE I.

DETAILS OF INSULATOR USESD FOR ANALYSIS

Insulator type

Suspension

Applied voltage Creepage distance (mm) Number of sheds Leakage Distance (mm) Shed Diameter (mm)

33kV 180 06 200 20

The insulator was energized at a nominal phase to ground voltage of 33kV, specific creepage distance of 5 mm/kV (with 10% overvoltage). III.

MEASUREMENT PROCEDURE OF ESDD

TABLE II.

MEASUREMENT

Leakage current of the polymer insulator was measured continuously using shunt resistance and HIPOT leakage current testers. Maximum leakage current was recorded during 3 hours voltage application. Environmental stress at the test insulators i.e. humidity level and temperature were recorded.

VALUE OF ‘B’ AT DIFFERENT TEMPERATURES

0

θ, C

b

4 8 12 16 20

0.03101 0.02612 0.02439 0.02191 0.01903

The salinity Sa of the solution is determined by the following expression, kg/m3 (2) Sa= (5.7σ20)1.03 The equivalent salt deposit density has been determined by the following expression as, (3) ESDD = Sa •V/A mg/cm2 Where: σ20 = the volume conductivity at temperature of 20 0C (S/m). ESDD = is equivalent salt deposit density (mg/cm²) V = the volume of distilled water (cm³) ESDD of the insulator is measured in lab during experiment. The values are measured when it is removed during the experiment. The pollutants have deposited in lab. It is observed that the ESDD of the insulator increases with pollutant accumulation period.

Fig.1. shows the relation of leakage current and relative humidity of polymer polluted insulator. The NaCl and kaolin are taken in the ratio 1:4 and distilled water is added to it. The insulator is mounted horizontally in an environmental chamber. The suspension is sprayed on to the insulator. The insulator is polluted for 20 minutes. The leakage current is measured under different relative humidity levels as a function of the ambient temperature. It can be seen that leakage current increases with temperature and also the magnitude of leakage current increase with the relative humidity. It can be seen from the graph that leakage current and relative humidity has a direct relation at constant pressure and applied voltage.

Fig.2. shows that for salt pollution from 0.25g to 1.0g the variation of conductivity and ESDD is almost linear. Other researchers have a different argument about the relation of conductivity and ESDD, according to one researcher the value of conductivity is equal to the square root of the ESDD [8]. TABLE III.

CONDUTIVITYAND ESDD OF CONTAMINATED INSULATOR

Solution (gm/ml)

0.25 0.5 0.75 1.0 Fig. 1. Leakage current with temperature at different relative humidity

617

σ20 (S/cm)

0.021127 0.028453 0.045652 0.048241

ESDD (mg/cm3)

0.059432 0.07182 0.12783 0.14012


A. Physics The physics used for simulation 1- Charge conservation for all domains of model 2- Zero charge (for all boundaries of model except for the electrode 3- Initial values for all domains of model as electric potential V = 0 4- Electric potential for two electrodes boundaries B. Boundary Conditions Top end of the high voltage insulator terminal was energized with power frequency AC voltage at 33 kV, while the bottom end of the insulator was connected to ground, 0 V. The energisation voltage corresponds to the rms phase to earth potential when considering the insulator subjected to very heavy pollution conditions, according to BS EN 60815 standard [7]. The outer edges of the air background region are assigned with a boundary that assumes zero external current and electromagnetic sources. The symmetry line of the insulator model was set to be the axial-symmetric axis on the x-y plane.

Fig. 2. Variation of conductivity and ESDD

V.

ELECTRIC FIELD AND POTENTIAL DISTRIBUTION

COMSOL Multiphysics is one of the most effective modules to perform a design and field simulation analysis on high voltage insulators. In this research it is used to simulate a polymer insulator. The Finite Element Analysis is done by using electrostatics (es) physics. The 33kV voltage applied to insulators with 6 weather sheds under uniform polluted layer with thickness of 0.07mm and conductivity 0.02µS. Configuration modeled by 2D symmetric module is shown in fig. 3. As 2D representation in COMSOL is always the first option and preferred due to its simplicity and fast processing time [9].

C. Meshing After specifying material properties and boundary conditions, for accurate computed simulation results meshing analysis was performed on insulator surface (see fig. 4)

Fig. 4. Meshing of Insulator Model

D. Simulation Results Fig. 5 and 6 show the simulation results of electric field and potential distribution on the surface of polluted polymer insulator. In electric field distribution, the E-field magnitude is high near the copper metal electrodes at high voltage end, as more amount of heat is generated at the live end, while relatively smaller in other regions. The electric field strength on the insulator surface has been increased due to the impact of pollution layers.

Fig. 3. Insulator Model

Material properties that used for insulator modeling is given in table IV below. TABLE IV. Materials

FRP Core Pollution Layer Copper Silicon Rubber Air Breakdown

VALUES OF SIMULATED INSULATOR

Relative Permittivity (εr)

6.5 7.5 1.0 3.9 1.0

Conductivity (S/m)

1.0 * 0-14 5.0 * 10-7 5.5 *106 1.0 *10-14 1.0 *10-15

618


Fig.8. shows the voltage distribution along a single unit of polymer insulator obtained from a 2D COMSOL. It is cleared from the voltage distribution curve that the maximum value appears on the high potential side and decreases on the ground side. From the results, it can be predicted that dry bands and partial discharges have occurred on the insulator sheds near the terminal where tangential field are likely high. Leakage current along the pollution sheds is largely driven by the tangential electric field. The flow of current causes surface heating, leading to the formation of dry bands. VI.

CONCLUSIONS

Performance of polymer insulators used in high voltage power transmission lines is very critical. The measured values of leakage current with variation of temperature and humidity are found to have a linear relationship. The electric field along the insulator has increased and is non-uniform due to the pollution layer. The E-field magnitude is high at the energized end and low at un-energized end.

Fig. 5. Electric field lines of polluted insulator

REFERENCES [1]

[2]

[3]

[4] Fig. 6. Electric potential lines with polluted insulator [5]

[6]

[7] Fig.7. Leakage current on top-side of shed

[8]

[9]

Fig.8. Voltage distribution along a single unit of insulator

619

N. Ravelomanantsoa, M. Farzaneh, and W. A. Chisholm, “A simulation method for winter pollution contamination of HV insulators,” Electrical insulation conference, pp. 373-376, June 2011. Y. Taniguchi, N. Arai, and Y. Imano, “natural contamination test of insulators at Noto testing station near Japan sea,” IEEE Transaction on power Appratus and Systems, Vol. PAS-98, No. 1, pp. 239-244, Jan/Feb 1979. M. A. Salam, H. Goswami, and Z. Nadir, “Determination of equivalent salt deposit density using wind velocity for a contaminated insulator,” Journal of Electrostatics, Vol. 63, pp. 37-44 , 2005. R. Hernanz, A. Jose, C. Martin, J. Jose, M. Gogeascoechea, Joseba, Z. Belver, and Inmaculada, “Insulator pollution in transmission lines,” Internatiuonal conference on renewable energies and power quality. K. C. Holte, J. H. Kim, T. C. Cheng, Y. B. Kim, and Y. Nitta, “Dependence of flashover voltage on the chemical composition of multicomponent insulator surface contaminants,” IEEE Transaction Power Apparatus and Systems, Vol. 95, pp. 603-609, 1976. Y. Tu, H. Zhang, Z. Xu, J. Chen, and C. Chen, “ Influences of electric field distribution along the string on the ageing of composite insulators,” IEEE Transaction on Power Delivery, Vol. 28, No. 3, pp. 1865 – 1871, 2013. BS EN 60507, “Artificial pollution tests on high-voltage ceramic and glass insulators to be used on a.c. systems,” 2014. L. An, X. Jian, and Z. Han, “Measurement of equavalent salt deposit density (ESDD) on a suspension insulator,” IEEE Transaction on Dielectric and Electrical Insulation, Vol. 9 No. 4, pp. 562-568, August 2002. I. Sebestyen, “Electric-field calculation for HV insulators using domain decomposition method,” IEEE Transactions on Magnetics, Vol. 38, No. 2, pp. 1213-1216, March 2002.