LITERATURE SURVEY

LITERATURE SURVEY

CHAPTER 2

LITERATURE SURVEY

2.1. LEAKAGE CURRENT:

Any current flowing from hot conductor to ground over the outside surface of a device is called leakage current. In case of insulators on a transmission line, it is the current that flows over the surface of insulator, and, if no ground exists, the current flowing from a conductive portion of a device to a portion that is intended to be non conductive under normal conditions.

Fig.2.1 Low resistance path over the surface of an insulator

2.2 IMPORTANCE OF LEAKAGE CURRENT FOR INSULATORS:

In case of insulators, the leakage current may not always pose a public safety hazard. Still it becomes a very important factor to consider in design, selection, and installation of a transmission line. The reason for this is the insulator itself whose performance is drastically affected with increased leakage current. The current cannot pass inside the insulator, but a path of relatively low resistance exists over the surface as shown in Fig. 1. This is actually the interface between insulator surface and the air. This path has low resistance than the air around insulator. This is more accurately called surface leakage current path. A small amount of leakage current flows over this path and can never be eliminated at all. However, very low levels of leakage currents have been achieved in modern insulator designs. Before invention of polymeric insulators, the ceramic insulators with reasonably low leakage current were common. Such insulators are still available in form of discs of different sizes and diameters these insulators can be attached together to make an insulator for any voltage level. The lower side of has the grooves. These are meant to increase the surface leakage path and, hence, leakage resistance. The leakage resistance is directly proportional to length of surface of insulator from energized end to dead end. The increase in leakage resistance decreases the leakage current.

Fig. 2.2 Ceramic insulator shapes with increased leakage resistance.

If we don’t use such a grooved structure then we have to increase the overall length of insulator string by adding more discs; this approach is not cost effective. But the more severe problem is the increase of the swing of conductors attached to it. It may result is sever conductor vibration and even breaking of conductor. Another question arises is why not to use these grooves on upper side of discs also. This has been tried in past a long ago and resulted in severe pollution accumulation on upper side creating bowel like structure upward that easily fills with dust and salt pollutants and never be washed with rain. Hence, no ceramic insulator today has grooves on top side.

2.3 FLAT PLATE MODEL AND EXPERIMENTAL SETUP

2.3.1 Equivalent Insulator Model

The simplified geometrical models equivalent to actual insulator are being widely used for the purpose of flashover analysis. Among these models, the basic flat trough model has merited extensive attention in the context of pollution flashover. So the proposed model, equivalent to standard disc insulator made of an insulating glass material with two copper terminals, one on cap and another at the pin. A simplified plan of the insulator model is shown in Fig. 1

Fig. 2.3 Flat Trough Model

Fig. 2.3 Flate Trough Model

2.3.2 Experimental Setup

A proposed equivalent insulator trough model [1] of dimension18.5x 0.6x 0.2cm is used for the contamination flashover experiments. The principal application of this equivalent model would be to help simulate as much as possible the practical conditions of high voltage insulators in the application of low voltage itself. In artificial testing, a contaminant is usually substituted by a dissolved mixture of an inert binder-Kaolin and NaCl salt. The inert binder is supposedly non-conducting and the quantity of salt represents the level of contamination. Contamination salt solution was prepared for various NaCl values of 15g, 20g, 25g, and 30g. The mixture, usually dissolved in distilled water is known as slurry which is thoroughly mixed as per IEC standard before coating the trough is initially washed and wiped clean and dry.

Fig. 2.4 Experimental Setup

The slurry is poured so that it rolls off uniformly in the trough. A test voltage of 230V, 50 Hz was applied across the terminals and the leakage current is monitored through the suitable measuring meter from the instant of application of voltage till the formation of dry band. The dry band was precisely located on the model. Its shape, contour of growth and locations were physically measured. The test results either in a flashover or a withstand.The conductivity and ESDD has also been calculated from the deposited contaminations. The contamination can thus be classified as light, medium or heavy according to the IEC standard.

2.3.3 ESDD Calculation

In any insulator severity of pollution is characterized by the Equivalent Salt Deposit Density (ESDD). The procedure for calculating the ESDD [8] is as follows: After the test has been completed, the deposits were collected by a small brush from the contaminated plate and mixed with 1litre of distilled water to get the solution for specific area of the glass plate. This process is repeated for the other samples of salt solutions. The conductivity of each collected salt solution is measured using a conductivity meter which is initially calibrated using 0.1N KCl solution. At the same time temperature is also recorded. The conductivities at different temperature are converted to 20° temperatures by the expression as,

σ ₂₀ = σ _Ө [ 1-b (Ө – 20)]

Where,

θ is the solution temperature °C

σθ is the volume conductivity at a temperature θ°C (S/m)

σ20 is the volume conductivity at a temperature 20°C (S/m)

b is the factor depending on the temperature θ as given in Table I.

The salinity S_a of the solution is determined by the following expression as,

S_a = [ 5.7 σ ₂₀ ]^ ^1.03

Finally, the equivalent salt deposit density can be determined by the following expression

ESDD =[ S_a * V / A ]

Where,

V is the volume of the solution, cm3

A is the area of the cleaned surface, cm2

According to IEC 60815, pollution site severity classifications are shown in Table II.

Table I. values of b at different temperatures

Table II. Pollution Site severity (IEEE definitions)

2.4 ARTIFICIAL CONTAMINATION TEST

2.4.1 Light Contamination

Insulators are mostly affected by flashover due to the deposition of NaCl salt particles. Therefore the equivalent model was uniformly sprayed with the slurry solution consisting of 15g NaCl, 40g Kaolin and 1 litre of distilled water. Leakage current started to flow on the surface of insulator due to the pollution. It is observed that there is an increase in leakage current magnitude when compared with clean surface condition, which is mainly because of increase in surface conductivity. Dry bands have started to form on the polluted surface after reaching the maximum leakage current of 42mA. No flashover could be seen and the test results in a withstand. Similarly for 20g NaCl, the leakage current increased to 90mA and the time taken for the formation of dry band is reduced compared to 15g.The test results in a Withstand.

2.4.2 Medium Contamination

Experiments were repeated for 25g NaCl. It is noticed that the magnitude of maximum leakage current increased to 130mA. It is because that the current magnitude depends on the level of contamination and the amount of moisture on the insulator surface. The test results in withstand but the field exceeds the withstand capability and it initiates the arc discharge.

2.4.3 Heavy Contamination

Finally the insulator model is contaminated by 30g NaCl solution and experiments were repeated in a similar way. Due to the high contamination of NaCl the magnitude of leakage current goes upto 220mA. The high magnitude of leakage current caused the heating effect which leads to rapid dry band formation and partial discharges across these dry bands. Due to the higher resistance at pin and cap end the heat dissipated in that location may be greater and therefore moisture dried rapidly. After 10 minutes dry bands could not sustain the applied voltage cause the scintillations to occur which ultimately leads to flashover.

CHAPTER 4

RESULTS AND DISCUSSIONS

4.1 ESDD

The correction conductivity, salinity and ESDD have been calculated for various tests as per IEC-507. The measured Salinity, ESDD and conductivities for 15g, 20g, 25g and 30g of NaCl salt are shown in Table III.

Table III. Values of conductivity, Salinity & ESDD using salt solutions

4.2 LEAKAGE CURRENT

Table IV Experimental test result

Table IV Experimental test result

Table V. Test results for various pollution levels

The following points were observed from the results shown above in Table IV and V.

i. For light contamination of 15g NaCl, the maximum leakage current measured is 42 mA. The calculated ESDD from the contamination deposit is 0.0396 which indicates the light degree of pollution.

ii. For 20g NaCl, maximum leakage current is 96mA which results in withstand and the corresponding ESDD of 0.0579 also indicates the low level of pollution.

iii. For 25g the leakage current reaches a peak of 130mA the field exceeds the withstand capability, initiates an arc discharge and extends several arcing which is actually preceded before the flashover. ESDD value is 0.0725 shows the medium level of pollution.

iv. For 30g the leakage current measured is 106mA and reached a peak of 220mA. It results in flash over after 10 minutes of wetting. ESDD shows the heavy pollution level for 0.1128.

v. The leakage current shows that the pollution severity can be correlated with the surface conductivity and ESDD. When these quantities reach the critical value, the flashover is imminent.

vi. Using the proposed model, the leakage current were measured and found similar [3] [4] from experiment. These revealed the equivalent model with 230V supply could be used for flashover prediction and analysis in the place of the standard high voltage insulator.

4.3 VARIATION OF ESDD AND CONDUCTIVITY WITH SALT

CONCENTRATION:

Fig. 4.1

4.4 VARIATION OF ESDD WITH CONDUCTIVITY:

Figure 4.2

4.5 LEAKAGE CURRENT AND VOLTAGE OF INSULATORS TWO

ENDS:

Figure 4.3

4.6 Leakage current VARIATIONS IN terms of

voltage of insulator’s Two ends:

Figure 4.4

CHAPTER 5

CONCLUSION

Leakage current in case of high voltage insulators are important to consider even they may not always pose safety hazard for public. The reason is that insulators are also affected a lot with the amount of leakage current flowing on their surface. Increase in leakage current causes a net generation of heat on surface. This heat damages the insulator surface and also causes a slightly burned path with low resistance over the surface. Such a path gives rise to dry band arcing. This can lead to failure or flash over of insulator. The development of a complete optical sensor system for the detection of partial discharges on insulator strings of overhead lines has been described.

5.1 FUTRURE WORK:

The performances of the sensor systems are being monitored, and possible operational problems are under analysis. The microcontroller software is now under modification to implement a universal serial bus communication between the PC and the processing module.