Investigation Of Various Artificial Polluting Methods Biology Essay

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In artificial pollution tests, there are three different polluting methods used are quantitative Brushing Method, Dipping Method and Spraying Method. Based on measurements of ac flashover performance of Glass Disc and composite Long-rod insulators in the artificial climate chamber, the differences on the flashover voltage of polluted insulators are studied using these three methods. The pollution tests show that there are some differences on the pollution flashover characteristics with different ESDD's using different polluting methods. The mathematical models are developed for the pollution flashover voltage of the insulators. The measured flashover voltages are compared with the calculated flashover voltages using the dynamic mathematical models.

Index terms: Composite insulators, solid layer method, polluting methods, ac flashover voltage, flashover characteristics, dimensional analysis technique.

I. INTRODUCTION

With the rapid development of power grids, flashover of insulators polluted with industrial contaminant, coastal fog, natural dust, and bird feces etc, has become an important problem for the safe operation of transmission lines and the design of the external insulation [1]. Compared with porcelain and glass insulators, composite insulators are of an excellent anti-pollution performance, light weight, maintenance-free and convenience. Therefore, it is widely used in China as well as in other countries.. Pollution flashover is a very complex problem due to several reasons such as modelling difficulties of the insulator complex shape, different pollution density at different regions, non-homogeneous pollution distribution on the surface of insulator and unknown effect of humidity on the pollution. The performance of insulators under polluted environment is one of the guiding factors in the insulation coordination if the high voltage transmission lines. On the other hand, the flashover of the polluted insulators can cause transmission line outage of long duration and over a large area [1-2]. Flashover of polluted insulators is still a serious threat to the safe operation of a power transmission system. It is generally considered that pollution flashover is being ever more important in the design of high voltage transmission lines.

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In the solid layer method of artificial pollution tests, three polluting methods are recommended including spraying, dipping and quantitative brushing method [4-13]. Quantitative Brushing Method (BM) is simple, which has easy-operation in the tests [15-19]. Dipping Method (DM) is more suitable for smaller specimens than larger ones. Spraying method shows some similar results to that of dipping method with small dispersion and it can also be used for larger specimens. The results obtained by different polluting methods (SM, BM or DM) are inconvenient to be compared directly, which will lead to the difficulty of applying the laboratory test results to the practical projects. Therefore, it is significant and valuable to study the differences of various polluting methods.

In order to reduce or avoid losses which are caused by pollution flashover of insulators, a reasonable artificial pollution test can study accurately the external insulation properties in simulating natural conditions and improve test procedures to prevent the occurrence of flashover [1]. In this paper the ac flashover performance of composite long rod insulator and the glass disc insulators are determined experimentally. Based on the testing results, the differences on flashover voltage using different polluting methods are analysed.

The analytical models are to be developed to elucidate the phenomenon of flashover on polluted insulators [18-21]. Although the experiments are indispensible for the study of the insulator behaviour under pollution, they take a long time. For this reason, it would be very useful to predict the performance of insulators under polluted conditions, with a satisfactory accuracy, using analytical expressions according to the polluted insulator model. Dimensional analysis has been developed for calculating the flashover characteristics of polluted insulators and several mathematical approaches have been analyzed. However difficulties still exist and further studies are needed.

This paper has covered minor scope of the work and it should be expanded to a wider extent for a better understanding and broad knowledge.

II. EXPERIMENTAL METHOD

A. Test Equipment

The experimental investigation is carried out in the artificial pollution chamber. The test power is supplied with 120/3 kV test transformer. Both the AC and DC supply can be supplied for the pollution tests. In this project, the pollution tests are carried by using AC supply.

WASHING AND DRYING OF INSULATORS

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PRE-CONDITIONING OF INSULATORS

APPLICATION OF POLLUTION ON INSULATOR SURFACE USING THREE METHODS

FLASHOVER VOLTAGE TESTING

Fig 1: Block Diagram of Pollution Test

B. Test Specimens

The samples are 11 kV Composite insulator (HV)-Type A and 11 kV Glass Disc Insulator (HV)-Type B.

C. Artificial Pollution of Insulators

1) Preconditioning Of Specimens: Before the tests, the specimens are carefully cleaned to ensure removing of all traces of dirt and grease and then dried naturally. The specimen surfaces are coated with a very thin layer of dry kieselguhr to destroy the hydrophobicity. In one hour after the above procedures, the specimens' surfaces are contaminated with the suspension of sodium chloride and kieselguhr which simulate the electric and inert materials. In the investigation, the equivalent salt deposit densities are 0.0103, 0.0319, 0.054 and 0.076 respectively.

2) Artificial Pollution of Insulators: The solid layer method can be employed by using three different methods. Using these three techniques, the test specimens are polluted and the flashover voltage of each specimen obtained from the above said methods are compared.

Brushing Method:

The amount of sodium chloride and Kieselguhr are weighed in an electronic balance for various ESDD's. Then, they are poured into a clean bowl adding appropriate amount of purified water. And the contaminated suspension is brushed on the specimen surfaces by a small brush. The pollutant layer should be applied as uniformly as possible, paying special attention to places which are difficult to reach, such as the transition from the trunk to the sheds. The specimens are contaminated within 1hr using this method.

Dipping Method:

The container that completely accommodating the specimen is taken. The container is filled with purified water. The sodium chloride and kieselguhr is added to the water in the ratio 1:6. To enhance the adhesion of the contamination on the insulator, a small amount of wetting agent is added. The wetting agent used is Yellow Dextrin. It increases the adhesion of the contamination in the surface of the insulator. The specimens are contaminated within 30 minutes.

Spraying Method:

The sodium chloride and kieselguhr are taken in the ratio 1:6 and purified water is added to it. The specimen is mounted horizontally. The suspension is sprayed on to the specimen using a sprayer. The specimens are contaminated within 15 minutes.

D. Test Procedure

1) Wetting: After polluting the insulators by solid layer method, they are allowed for natural drying.

The composite long - rod insulators are dried for 24 hours.

The glass disc insulators are dried for 5 hours.

After natural drying, the insulators are suspended vertically at the centre of the artificial pollution chamber. Then they are completely wetted by using purified water before applying the test voltage.

When the pollution layer on the insulator surfaces are completely wetted, a series of flashover tests are to be carried on. The flashover tests can be carried out on both the AC and DC set up. In this project the entire experiments are done by using AC supply.

2) Flashover Voltage Evaluation: The test method used here is Even-Rising Voltage Method. Using this method the flashover voltage and the standard deviation are calculated as follows:

Increase the voltage at a random rate till the flashover occurs. Record the flashover voltage, Uf. 1-2 minutes later, repeat the above procedures For the polluted specimen, the series of flashover voltage obtained by above procedure expressed as Uf1,Uf2,...Ufn (n is the number of flashovers). The minimum flashover voltage of the polluted insulator is,

Ufm = Min (Uf1, Uf2,...,Ufn) --------------- (1)

About five effective Ufm should be obtained at one pollution degree for each type of specimen. The average flashover voltage, Uav is calculated as:

= (i) --------------------------- (2)

Where, Uav - average flashover voltage, Ufm - minimum flashover voltage, N - total times of effective tests. The standard deviation is calculated as:

- --------------- (3)

Where, (σ%) - standard deviation, Ufm(i) - minimum flashover voltage obtained from the test in the ith time.

3) Measurement of ESDD: After flashover testing, the polluted insulators are dried under bright sunlight. After drying, dry granules of NaCl sticking to the insulator surface are collected by brushing them off with a small paint brush. The collected deposits are then dissolved in 100 ml of distilled water. The conductivity meter is used to measure the conductivity of each collected salt solution. At the same time, the temperature is also recorded. The conductivities at different temperatures are converted to conductivity at 20°C using the formula:

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= [1 - b ( -20°)] --------------- (4)

Where, θ is the solution's temperature in degree Celsius, σ is the volume conductivity at the temperature θ, σ20 is the volume conductivity at temperature 20°C (S/m), b is a temperature depend factor. (b=0.01905). Finally, the Equivalent Salt Deposit Density (ESDD) is determined as:

--------------- (5)

Where, Vol is the volume of the solution in cm3, A is the area of the cleaned surface in cm2, Sa is the salinity of the diluted liquid. It is given as:

= --------------- (6)

Thus the ESDD's at various pollution degrees are calculated.

III. MATHEMATICAL MODELLING

Four fundamental dimensions like length, mass, time and current are used to develop the relationship among the AC flashover voltage and other parameters that affects the flashover voltage in the outdoor environment[18-20]. The calculation of flashover voltage primarily depends on the ESDD, conductivity of the pollution layer, surface area of the insulator and arc constant.

In this paper, two models are developed and three parameters are taken for analysis in each model.

A. Model 1:

In developing this model, ESDD, conductivity of the pollution layer (s) and arc constant (N0) are the fundamental parameters affecting the flashover. The relation among them can be written as:

V=f (ESDD,s, N0)

In order to utilize the algebraic approach to dimensional analysis, it is convenient to display the dimensions of the respective variables by a tabular arrangement. Therefore, the dimensional matrix of the respective variables can be written as:

k1 k2 k3 k4

V ESDD sS N0

L

M

T

A

Where k1, k2, k3 and k4 are the exponents of FOV, ESDD, conductivity of the pollution layer and arc constant. From the above dimensional matrix, homogeneous linear algebraic equations are formed and solved. The values of k2,k3 and k4 in terms of k1 (which represents the FOV) can be expressed as:

k2=k1/2

k3=nk1/n+1

k4=-k1/n+1

By assigning k1=1[21] and Buckingham's π theorem, the final dimensional expression describing AC flashover voltage (FOV) is obtained as:

V= dc ESDD-1/2 sS -n/(n+1) N01/n+1-----------------(7)

Where, V-Flashover voltage

dc - Dimensional constant

ESDD- Equivalent Salt Deposit Density

sS - Conductivity of the pollution layer

N0- Arc constant

B. Model 2:

In developing this model, ESDD, surface area of the insulator and arc constant are the fundamental factors affecting the flashover. The relation among them can be written as:

V=f (ESDD, A, N0)

The dimensional matrix of the respective variables can be written as:

k1 k2 k3 k4

V ESDD A N0

L

M

T

A

Where k1, k2, k3 and k4 are the exponents of FOV, ESDD, surface area of the insulator and arc constant. From the above dimensional matrix, homogeneous linear algebraic equations are formed and solved. The values of k2,k3 and k4 in terms of k1 (which represents the FOV) can be expressed as:

K2 = (-n/n-1) k1

K3 = -(4n-1/2n-2) k1

K4 = (1/n-1)k1

By assigning k1=1[21] and Buckingham's π theorem, the final dimensional expression describing AC flashover voltage (FOV) is obtained as:

V = dc ESDDn/n+1 A (4n-1/2n-2) N0(-1/n-1)-------------------(8)

Where, V-Flashover voltage

ESDD- Equivalent Salt Deposit Density

A - Surface area of insulator

N0- Arc constant

dc - Dimensional constant

The above said models are applied for both the Type-A and Type-B insulators and are compared with the experimental results.

In both the models the value of n is taken as 0.59 and N0 as 360 as per [22]. The dimensional constant(dc) depends on the mass of the insulator(M), the leakage distance of the insulator(L), time for the occurrence of flashover(T) and the current(A), which are the fundamental dimensions taken for analysis.

IV. RESULTS AND ANALYSIS

Experimental Results

After the artificial pollution tests on the insulator samples using different polluing methods, the average flashover voltage Uav and the standard deviation (σ %) is determined and are tabulated below:

Table

Flashover voltages of specimens using various polluting

Methods

type

ESDD

(mg/cm2)

BM

SM

DM

Uav

(kV)

(%)

Uav

(kV)

(%)

Uav

(kV)

(%)

A

0.0103

91.5

1.18

86

0.99

82

0.99

0.0319

74.8

1.14

71

1.15

68

1.2

0.054

61

1.34

55.5

0.74

54

1.5

0.076

45.8

1.85

40.5

1.01

38

2.15

B

0.0103

44.5

3.62

41

1.99

39.5

1.03

0.0319

33.3

2.6

28

1.46

26

1.57

0.054

23.8

3.57

20

2.04

19

2.15

0.076

15.5

1.08

14

2.91

13.5

3.02

B. Influence of ESDD on Uav at three polluting methods

A large number of tests show that the pollution flashover

voltages of the various samples reduce with the increase of salt deposit density. From Table 1, the relationships between Uav and ESDD using three polluting methods are as shown in Figure 2 for samples A and B.

The pollutant layer on the surface of specimen using BM is less uniform than using SM or DM and cannot be simultaneously wetted. The flashover voltages obtained using BM and DM are the largest and smallest under same test conditions respectively..

TYPE A

TYPE B

Fig 2: Average pollution flashover voltages Uav Vs ESDD using three polluting methods

C. Modelled Results:

The parameters of the polluted insulators (Type A and Type B) used in model 1 and model 2 are determined. By using the equations (7) and (8) the flashover voltage is calculated for different ESDD's. The calculated results are compared with the experimental results. From the experiments we observed that the Brushing method shows the relatively larger values. Hence the mathematical outputs are compared with the experimental results of Brushing method.

2. Table

Comparison of modelled results with the experimental results for Type A

S.NO

ESDD

MEASURED FOV

MODEL 1 FOV

MODEL 2 FOV

1

0.0103

91.5

87.34

87.7

2

0.0319

74.8

60.34

73.517

3

0.054

61

48.57

64.43

4

0.076

45.83

39.37

43.98

3. Table

Comparison of modelled results with the experimental results for Type B

S.NO

ESDD

MEASURED FOV

MODEL 1 FOV

MODEL 2 FOV

1

0.0103

44.5

42.07

42.25

2

0.0319

33.33

29.88

30.75

3

0.054

23.83

21.29

22.245

4

0.076

15.5

17.2

14.03

From the modelled results it is found that the calculated values of model 1 is less than that of the model 2 and shows less agreement with the experimental results as shown in the figure below:

TYPE A

TYPE B

Fig 3: Variation of Measured FOV with the Mathematical models

V. CONCLUSION

The pollution tests are conducted for composite and glass insulator and the respective mathematical models are developed. The pollution tests show that the pollution flashover voltages of the samples reduce with the increase of salt deposit density. The standard deviations σ(%) of AC average pollution flashover voltage, which are obtained by BM, SM and DM for glass disc and the composite long rod insulators are 1.08% - 3.62%, 0.74% - 2.91% and 0.99% - 3.02% respectively. We also conclude that the σ (%) of the BM is larger than that by SM and DM, with the non-uniformity pollution layer on the specimens. Among these polluting methods the application of pollution through Dipping method is more suitable for smaller specimens than the larger ones. From the experiments, the spraying method is recommended, because the scatter of the pollutant layer is relatively small to easily meet the requirement of long-rod polymeric insulator and it can save lots of Sodium chloride and Kieselguhr.

The new mathematical relationships have been proposed by Dimensional Analysis Technique. The proposed models utilize the normal parameters to calculate the critical flashover voltage. Hence the new model has been found to be more effective compared to the earlier models. The proposed model 2 is validated with experimental results and is found to be in good agreement. Hence Dimensional Analysis Technique would be very useful to predict the performance of insulators under polluted conditions. Therefore this technique is useful and attractive for studying the performance of polluted insulators.

ACKNOWLEDGEMENT

The authors wish to thank the authorities of National Engineering College, Kovilpatti for their continuous support and encouragement for this work done in the High Voltage Laboratory of the instituition