The influence of polution on the electrical insulators Текст научной статьи по специальности «Электротехника, электронная техника, информационные технологии»

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10. Турцевич А.С., Гранько В.И., Красницкий В.Я. Специальные технологические среды в производстве СБИС и УБИС // Зарубежная электронная техника. 1991. № 4. С. 3 - 38.

11. Достанко А.П., Баранов В.В., Ануфриев Л.П. и др. Повышение стабильности твердотельных структур при ионной бомбардировке // Электронная обработка материалов. 2001. № 2. С. 43 - 45.

Поступила 28.08.03

Summary

The study of the basic electrophysical parameters of thin Mo films obtained by magnetron sputtering depending on the conditions of deposition was made. It is shown that the films obtained at deposition rate about 4 nm/s and at less than 1 Pa Argon pressure to the substrates heated up to temperature over 450°C have the best reproducibility.

S. Ababei, I. Gheorghiu

THE INFLUENCE OF POLUTION ON THE ELECTRICAL INSULATORS

University of Bacau, 157 CaleaMarasesti str., R0-5500, Bacau, Romania

Introduction

The pollutant agents that are present in the environmental air have a direct influence on the electrical properties of insulators. The surface properties of the insulators are the first ones to be affected.

Test conditions

A study on the influence of some pollutant agents on the electrical insulator was done. Electrical insulator types 2025, 2026, PSG6, PSG12 and VLKS were tested individually or in groups of two or more elements, in gradually increasing pollution conditions.

Nitrogen oxide, chlorine and sodium chloride were used as pollutant agents, because, these gases usually are emitted by chemical plants. Electrical insulators with a clean surface as well as covered with sili-cone vaseline were used. These insulators were exposed during a six month period to polluted medium with nitrogen oxides and sulfuric anhydride.

The tests were performed in the following conditions:

- The insulators were tested individually or in groups, in conditions of constant humidity and constant applied voltage while the concentration of pollutant agent in the environment air was gradually increasing.

- The insulators were tested individually or in groups in conditions of constant pollutant agent concentration in environmental air and constant applied voltage while relative air humidity was gradually increasing.

- The insulators were tested individually or in groups in conditions of constant pollutant agent concentration in environmental air and constant humidity while applied voltage was gradually increasing.

Theoretical considerations

Applied voltage and humidity effects on the polluted surface of insulator results in the appearance of leakage currents. The values of these currents are determined by the conductivity of superficial film on the surface. This leads to the conclusion that the dependence of leakage current on conductivity and applied voltage is as follows:

© Абабей С., Георгиу И., Электронная обработка материалов, 2004, № 3, С. 74 - 80.

74

I = f (y,U) (1)

As it can be seen from the above expression, if the continuity conditions of the film are realized the voltage gradient remains constant, the film voltage influences like a linear element. In these conditions the current keeps a sine waveform shape on the insulator surface and depends on conductivity variation, pollution and humidity degree.

If the continuity and uniformity aren't assured on the length of the leakage pass, the current depends on even more factors such as: conductivity, shock ionization, voltage and the film uniformity degree.

To clarify this function lets consider time period t1 in which the current keeps a sine waveform shape on the insulator's surface. In this case the expression will be as follows:

i (t )i = f (R) or i (t )i = f (y,U ) (2)

Due to physical and chemical conditions, an increase in the pollution phenomenon will lead to the forming of shock ionization current in addition to the one leaking through the film.

In the subsequent time period t2 an equivalent scheme of insulator can be considered as two elements, linear and nonlinear ones, connected in parallel. During this period, the expression for current is presented as equation (3).

: i (t )2 = G (a,U) + F (j,U) (3)

where a is the shock ionization coefficient.

It is clearly seen that the resistance increases with the increase of leak current i(t) so:

if R then y and F ( y,U

In the period t2 , the current is, in fact, the ionization current according to the following expression:

I (t )2 = G (a, u) (4)

This current influences the deformed character of the i(t) function. This happens because of the fact that the discharges take the form of electric arc discharges, which extend on the insulator's surface and the following expressions are valid:

Y = Co Ck sin(k&t-9) (5)

i

n n

i(t) = C0 Ak sinkat + ^Bk coskat (6)

i i

where Ak and Bk are Fourier coefficients, which can be determined by decomposing of the function into simple wave forms. Because the leakage current depends both on the humidity and pollution degree it is important to know its variation according to these factors.

Experimental results

Statistical analysis of the measurements

This relation shows the interdependence between the leakage currents from the insulator surface and the pollution degree and the humidity determined by conductivity modifications. The analysis of experimental data dispersion presented in table 1 shows a linear relation between the leakage current (y) and the humidity degree (named x) as the following function:

yx = bo + b • x (7)

By statistical processing of the measurement data, which are shown in table 1, the following solution for b0 and b1 in the matrix form was obtained:

B = u = (8)

The theoretical regression function yx, which has the nearest values to the effective values y is:

bo 0.03

bi 0.0054

yx = 0.03 + 0.0054 • x

(9)

In this formula b0 ^ 0, though it is very small. This can be accounted for the fact that a small leakage current exists even in the absence of pollution when the insulator is subjected to a voltage.

In figure 1 the dependence between the leakage current and the pollution degree is presented. In table 2 the results of the statistical processing for the determinations of the leakage current are presented, the humidity being variable while voltage is constant.

The dispersion diagram for these data reveals a relation between the leakage current and humidity,

as:

yx = b0 - b1 x2 (10)

To calculate parameters b0 and b1 the data from table 2 were used. The solution in matrix form is:

B =

b0 0.12

bi 0.00074

The final expression for regression function is:

(11)

yx = 0.12 - 0.00074x2

Table 1. Measurement results

(12)

X 0 550 900 1250 1460 fy yfy y2fy xfx x2fx xyfxy

0.01 5 5 0.05 0 0 0 0

3 4 4 12 36 2200 1210000 6600

5 2 2 10 50 1800 1620000 9000

7 2 2 14 98 2500 3125000 17500

7.5 1 1 7.5 57.25 1460 2131600 10950

fx 5 4 2 2 1 14 43.55 7960 8086600 44050

xfx 0 2200 1800 2500 1460 7960 S xfx =( x + x2 + x3 + x4 + x5 )• fx S yfy =( + y 2 + y3 + y4 + y5 )• fy S x2 fx = (x12 + x22 + x32 + x42 + x52 ) • fx S f =(x1 y + x2 y 2 + ... + x5 y5 )• fxy

X fx 0 1210000 1620000 3125000 21316000 8086600

Vfy 0.05 12 10 14 7.5 43.55

V fV 0 36 50 98 7.5

xyfxy 0 6600 9000 17500 10950 44050

Information matrix |x* x| = 14 7960 7960 8086600

Inverse information matrix l * |-1 x x = 8086600 -7960

113212400 - 63361600 113212400 - 63361600 -7960 8086600

113212400 - 63361600 113212400 - 63361600

Factor matrix 1 „1 43.55 x y = 1 1 44050

System solution |x* y x* y = 11 I ?0 0.03 jj 0.0054

Regression function yx = b0 + b1 x = 0.03 + 0.0054x

This function can be made linear by taking logarithm, because the coefficient b1 is very small, that is why the regression line can be approximated by a straight line. The approximation will simplify the calculations, which are necessary for the determination of the regression function in the case when both pollution and humidity influences are taken into consideration.

□ 500 1000 N20

------experimental regression line

-theoretical regression line

Fig. 1. The leakage current variation vs N2O5 quantity

Using the experimental data from table 3 and by denoting the leakage current by y and the humidity and pollution with x1 and x2, respectively, the relation function can be approximated with a straight line that has the expression:

y ( x2 )= b0 + b1 xi + b2x2 (13)

Table 2. The result of the statistical processing for the determination of the leakage current

x Y x2 x3 x4 xy x2y

1 0 0.1 0 0 0 0 0

2 68 3 4624 314432 21381376 204 13872

3 74 4 5476 405224 29986576 296 21904

4 81 4.5 6561 531441 43046721 364.5 29524.5

5 92 6.5 8464 778688 71639296 598 55016

6 100 7.5 10000 1000000 100000000 750 75000

E 415 25.5 35125 3029785 2.66-108 2212.5 195316.5

Information matrix |x* x| = 6 415 35125 3029785

3029785 -35125

Inverse information matrix 1 * 1-1 x x = 18178710 -14576875 - 415 18178710 -6 14576875

18178710 -14576875 18178710 - 14576875

Variable matrix lx* H = 25.5 2212.5

Normal system solution |x* y\ 1 x* y 0.12 0.00074

Regression function yx = b0 -b1 x2 = 0.12 - 0.00074x2

^^ x — x1 ^b x2 ^b x3 ^b x^ + x 5

Sum Z y = + H2 + H3 + H4 + H5

Z xy = x1 y1 + x2H2 + x3 H3 + x4y 4 + x5 Hs

Using the experimental data from table 3 the matrix solution can be obtained as:

b0 0.1

B = \ = 0.0082

b2 -0.017

The theoretical regression function is:

yx = 0.1 + 0.0082x1 - 0.017x2 (15)

mA 4

U = ct 'A

P = ct 467 mg/m3

yx= 0.12 - □.□□□74

x, %

□ 50 100 ------experimental regression line

-theoretical regression line

Fig. 2. The leakage current variation vs. humidity Table 3. Leakage current vs. humidity x1 and pollution degree x2

y x1 x2 x12 x12 x1y x2 y x^2

1 0.1 0 0 0 0 0 0 0

2 0.75 170 44 28900 1938 127.3 33 7480

3 1 440 68 193600 4624 440 68 29920

4 2.4 650 83 637000 6889 1560 1560 53950

5 5.5 980 100 360400 10000 5390 5390 98000

E 9.65 2240 295 1246600 23449 7517.5 7051 189350

Relation between leakage current and both pollution and humidity is presented in figure 3.

Fig. 3. The leakage current vs. humidity and pollution degree

This variation shows that on an OA part of the leakage current curve is directly proportional with the pollution and humidity. Experimental data show that after the point A on the leakage current curve is no longer directly proportional to pollution degree, instead it has disorderly values. Superficial discharges appear on the insulator surface in this area.

Using the electrical insulator subjected to working voltage is dangerous at this level of pollution as soon as electric breakdown can occur at any time.

A small increase of working voltage at this level of pollution or humidity will result in acceleration of partial discharges and electric breakdown through insulation may start unless preventive actions are taken.

In the most unfavorable cases (extremely high humidity or intense pollution) the electric breakdown through the insulation takes place much earlier and leakage current increases up to hundreds of mA.

The minimum value of the leakage current, at which the electric breakdown begins, determines the security level at which the insulator can be used.

Table 4. Leakage current values for insulator type 2025

Formula and results

y X Relative humidity

0.1 0 44

0.75 170

1.25 570

4.6 2150

6.5 3354

1.5 460 74

2.6 810

3.7 1420

7.1 1960

0.1 0 83

1.5 460

3.2 950

3.8 1250

6.8 1750

0.1 0 100

2 320

4 799

5.5 980

8 1464

0.1 0 100

0.8 450

1.2 1160

6.5 3354

7.4 3459

0.1 0 100

1.5 592

2 851

4 1586

6.5 2150

(x*x)-1

(x*y) (x*x)-1 (x*y)

13.1 0.217

32531 0.0018

14.9 0.15

21958 0.0033

yx

16225616 -6244

43340544 -6244 43340544 5

43340544 43340544

6725700 -4650

12006000 -4650 12006000 5

12006000 12006000

0.217 + 0.0018x

0.15 + 0.0033x

5721000 -4390

9333400 9333400

-4390 5

9333400 9333400

3244797 -3563

-9450172 9450172

-3563 3

15.4 20394

0.15 0.00358

0.15 + 0.00368x

9450172 -9450172

19.5 20938

0.14 0.005

0.14 + 0.005x

25392814

-8531

-119716916 -8531

119716916 5

119716916 -119716916

16.9 50975.5

0.04 0.0009

0.04 + 0.0009x

8212561

38380601 -5179

-5179 38380601 5

38380601 -38380601

14 0.09

22909 0.001

0.09 + 0.001x

(X ' X ) =

-S-

det |x' x det x' x|

-Sx Sf

det x' x\ det x' x

(x' y ) =

S y

S xy

(X ' X )(X 'Y ) =

yx = b0 + bxx

Sx 2 S y-S xS xy S x2 S f-(S x )2

S f S xy-S xS y

S x2 S f-(S x )2

The variation curves of the effective values of the leakage current for insulator type 2025 in the different test conditions are shown in figure 4 using data from table 4. It is seen from figure 4 that the behavior of the insulators during the pollution process and its effect on them can be characterized by the curves gradient. Worth to note that the values of the leakage current are close one to another at the beginning of the dangerous areas even though the pollution values that determine these areas are very different. This way, the difference in leakage current for points A1, A3, exposed for 6 months to pollution, and points A2, A4 points, unexposed to pollution, is only 1 mA.

The oscilloscope visualization is another way to show the existence of two areas, safe and unsafe, in the process of insulation pollution. We observed that in the first part (OA area) leakage current waveform is sine, while after point A it is deformed. This area is characterized by the appearance of some harmonics. Figure 5 shows the curves shape obtained by oscilloscope visualization in two areas.

Fig. 4. The leakage current vs. pollution degree: ¡.Insulator with vaseline, after 6 month exposure; 2. Clean insulator; 3.Greased insulator, after 6 month exposure; 4. Recently greased insulator

Fig. 5. Leakage current on the surface of insulator

Conclusions

The presence of pollutant agents, the relative humidity and the applied voltage have a different influence on the leakage currents.

In the most unfavorable cases (extreme humidity or intense pollution) the electric breakdown through the insulation takes place much earlier. The same can be said about appearance of partial electric breakdowns. Leakage current during electric breakdown increases very fast and can achieve values as high as hundreds of mA.

The dependence of leakage current intensity on the content of pollutant agents, the relative humidity and the applied voltage for different insulator types can be expressed by some analytical expressions.

REFERENCES

1. Pop Eugen, Vasile Stoica. Masurari in energetica. Editura Facla, Timisoara, 1981.

2. Suciu Iacob. Bazele echipamentelor electrice. Editura Facla Timisoare, 1996.

3. Peicov Alexandru, Tusaliu Petre. Aparate electrice. Scrisul Romaneasc, Craiova, 1998.

Received 12.07.03

Summary

The different pollution agents, which are present in the environmental air, influence the electrical insulators properties. In this essay, the way in which pollution agents influence the electrical insulators properties at working voltage and different humidity was studied. Analytical relations were obtained. Using these relations, the evolution of electrical insulator properties in the presence of pollution agents can be determined.