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CLEAN-FOG AND SALT-FOG
Mojtaba Rostaghi-Chalaki, A Shayegani-Akmal, H Mohseni
To cite this version:
Mojtaba Rostaghi-Chalaki, A Shayegani-Akmal, H Mohseni. HARMONIC ANALYSIS OF LEAK- AGE CURRENT OF SILICON RUBBER INSULATORS IN CLEAN-FOG AND SALT-FOG. 18th International Symposium on High Voltage Engineering, CIGRE, Aug 2013, Séoul, South Korea.
pp.1684-1688. �hal-01514188�
HARMONIC ANALYSIS OF LEAKAGE CURRENT OF SILICON RUBBER INSULATORS IN CLEAN-FOG AND SALT-FOG
M. Rostaghi-Chalaki 1*, A. A. Shayegani-Akmal1 and H. Mohseni1
1 The School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran.
*Email: <[email protected]>
Abstract: Environmental and electrical stresses have deterioration effects on Silicon Rubber (SIR) insulators. In coastal areas due to high amount of salt, humidity and dust suspended in the air, the deterioration process is intensive. In this paper, based on IEC 60507 the leakage current (LC) of 20kV SIR insulators has been investigated in clean-fog and salt-fog with and without artificial pollution. Tests have been executed with six similar SIR insulators. A pair of polluted and clean SIR insulators is subjected to clean-fog; other two pairs are subjected to salt-fog with 5 and 10 kg/m3 salinity respectively. Results show that combination of pollution and salt-fog is more destructive than clean-fog, where the main reason may be the increase of conductivity. Dry band arcing makes insulator perform nonlinearly and some LC waveform harmonic growth. The Fast Fourier Transform (FFT) method, used to find the harmonic spectrum of the LC. Results, also illustrate the coordination between the 3rd and 5th harmonic components of the LC and insulators surface condition.
1 INTRODUCTION
In recent years, due to high advantages of SIR insulators such as low cost, light weight, high mechanical strength, low surface tension energy and good electrical strength compared to porcelain insulators, they have been widely used over the world for power transmission and distribution systems [1]. According to polymeric SIR insulators material type, the most effective problem is environment conditions such as salt, humidity, dust, rain, acids and ultra violet which can lead to chemical changes. These material modifications affect the reliability and long-term performance of the SIR insulator while being in service [2-4]. The hydrophobicity is one of the SIR insulator abilities to deposit pollutants on the insulator surface which can restrain or limit LC. By hydrophobicity decreasing due to the effects of environmental conditions, the LC on the surface increases and results in a chemical process on the surface which can finally erode it. Erosion is an irreversible and non-conducting degradation of the surface of the insulator that occurs by loss of material and it is one of the most important proofs of low hydrophobicity [5-7]. The LC waveform is a good indicator of the insulator surface condition. It consists of ohmic and capacitive components. The LC condition is more dependent to insulator surface hydrophobicity. When it is hydrophobic, the LC is usually capacitive and the waveform is sinusoidal, otherwise the hydrophilic surface makes the LC more and more resistive. Generally, capacitive current depends on the geometry and permittivity of the insulators. While, the resistive component is strongly dependent on layers of contamination and erosion on the SIR insulators surface [8-10]. With increasing of the LC amplitude due to losses hydrophobicity, since the distribution of the surface current density is not uniform, a
specific path is heated and dried. This path is the main reason for forming and expanding the electrical arc which is called dry band arcing. Dry band arcing is the main reason of the appearance of distortions on the LC waveform and can led to an increase in the level of its harmonic components. This is one of the main causes of aging in polymer insulators [9, 11-13]. With high hydrophobicity, in the first steps of aging, the LC is almost low. However, at the critical aging point which comes after a while, electric forces help water droplets to be joined and make a conducting path with lower resistant for LC [14].
In this paper a comparison between LC of SIR insulators in clean-fog and salt-fog conditions, with and without presence of pollution has been proposed. Based on experimental results it is seen that with presence of pollution, the 3rd and 5th harmonic components of the LC is modified significantly, and this can be assumed as a criterion for polymeric insulators aging.
2 EXPERIMENT 2.1 Specimen
Six similar 20kV High Temperature Vulcanized (HTV) SIR insulator are used for this test.
Figure 1: 20kV HTV SIR insulator used in the test.
This kind of insulator has alternative sheds. The specimens were tested based on IEC 60507 [15].
Configuration and dimensions of this specimen are shown in Figure 1 and Table 1 respectively.
Table 1: Characteristics of the 20kV HTV SIR insulator used in the test.
Parameter Dimension
Voltage in kV 20
Mechanical strength in kN 70
Creepage distance in mm 750
Arcing distance in mm 260
Shed diameter in mm 110/80
Core diameter in mm 25
2.2 Test setup
Tests have been performed in the fog chamber with dimensions of 2m×2m×2m. The fog generation system consists of two nozzles and an air conditioner. The fog is injected to the chamber by a fog generation system through these nozzles.
The high voltage (HV) transformer with the ratio of 380 V/ 100 kV (10 kVA) is employed to energize the insulators to the required voltage stress. Test setup schematic is shown in Figure 2. The data acquisition system consists of a digital oscilloscope which monitors the voltage across the shunt resistor with an aim to measure the LC value. A protection system is prepared to protect the oscilloscope against unexpected overvoltages, as well.
Figure 2: Test Setup Schematic.
2.3 Test Method
The six SIR insulators are divided to three equal groups. One insulator from each group is selected and then based on IEC 60507, these three specimens were polluted with 100g Kieselguhr, 10g NaCl and 1000g tap water composition while the other three specimens remained clean. In first step, a pair of polluted and clean insulators stands
vertically in the fog chamber containing clean-fog.
In second step, a pair of polluted and clean insulators was mounted vertically in the fog chamber containing salt-fog with 5 Kg/m3 salinity.
In last step, a pair of polluted and clean insulators was mounted vertically in the fog chamber containing salt-fog with 10 Kg/m3 salinity. An 11.54 kV voltage (a single phase to ground voltage) is applied to a pair of polluted and clean insulators in the fog chamber in each step. Moreover, the LC data is recorded continuously in all steps with data acquisition system.
3 LEAKAGE CURRENT ANALYSIS 3.1 Leakage Current in Clean-Fog Test
Figure 3 shows the LC waveforms for polluted and clean insulators in the clean-fog test.
(a)
(b)
Figure 3: Time domain and FFT spectrum of leakage current waveforms of SIR insulators subjected to clean-fog. (a), clean insulator; (b), polluted insulator.
(a)
(b)
Figure 4: Time domain and FFT spectrum of leakage current waveforms of SIR insulators subjected to salt-fog with 5 kg/m3 salinity. (a), clean insulator; (b), polluted insulator.
(a)
(b)
Figure 5: Time domain and FFT spectrum of leakage current waveforms of SIR insulators subjected to salt-fog with 10 kg/m3 salinity. (a), clean insulator; (b), polluted insulator.
In Figure 3a, the amplitude of the LC is insignificant because of none pollution, high resistance and good hydrophobicity on the surface of clean SIR insulator. In the presence of clean-fog the LC has some harmonics where the percentage of the 3rd harmonic component is lower than the 5th harmonic component. The FFT spectrum of the LC waveform in figure 3a confirms this claim. Figure 3b shows that in the presence of contamination, the amplitude of the LC increases significantly.
This occurrence is the result of hydrophobicity decreasing which helps forming of conductive paths and reducing resistance on the surface of insulator. Also FFT spectrum analysis of the LC illustrates that the 3rd Harmonic component is greater than the 5th harmonic component because increasing of the LC dries some paths on the surface and dry band arcing happens through this
paths. Dry band arcing makes the LC waveform become nonlinear partly.
3.2 Leakage Current in Salt-Fog Test with 5 kg/m3 salinity
In the salt-fog with specified salinity the amplitude of the 5th harmonic component decreases and it increases for the 3rd harmonic component as shown in Figure 4a. The main point is the more conductivity of the salt-fog toward clean-fog. Due to FFT spectrum waveform 5 kg/m3 salinity is a critical point where the 3rd and 5th harmonic components are relatively equal and an acceleration aging is expected by increasing the salinity. Figure 4b shows that by an increase in the conductivity, the aging process is accelerated. As it can be seen in the figure, the growth in the 3rd harmonic component is more than the 5th harmonic component.
3.3 Leakage Current in Salt-Fog Test with 10 kg/m3 salinity
With higher salinity the time domain LC waveform becomes more nonlinear and the 3rd harmonic component grows significantly as shown in figure 5b. Corona and dry band arcing are observed in this condition. According to figure 5a the LC waveform is partly nonlinear and the 3rd harmonic component grows smoothly with respect to the 5th harmonic component.
3.4 Relation between LC Harmonic components and Insulators Surface Condition
In first step, the amplitude of the 3rd harmonic component of clean insulator is lower than the 5th harmonic component whereas in polluted insulator the amplitude of the 3rd harmonic component is more than the 5th harmonic component and the amplitude of other harmonic components are very small so they could not be used as a criterion (Figure 3). In second step, the amplitude of the 3rd harmonic component of clean insulator is relatively equal to the 5th harmonic component while in polluted insulator the amplitude of the 3rd harmonic component has more growth than the 5th harmonic component (figure 4).
Table 2: The amplitudes of harmonic components of leakage current for clean insulators.
Fog salinity
Harmonic components
amplitude (uA) %3rd %5th 1st 3rd 5th
Clean-fog 12.26 0.87 2.72 7.09 22.18 5 kg/m3 11.20 1.78 1.81 15.89 16.16 10 kg/m3 25.13 6.12 0.97 24.35 3.85
Table 3: The amplitudes of harmonic components of leakage current for polluted insulators.
Fog salinity
Harmonic components
amplitude (uA) %3rd %5th 1st 3rd 5th
Clean-fog 378.3 62.75 48.76 16.59 12.89 5 kg/m3 1078.7 251.55 20.67 23.32 1.91 10 kg/m3 1201.4 451.00 17.41 37.54 1.45
In last step, in both conditions of clean and polluted insulator the amplitude of the 3rd harmonic component is more than the 5th harmonic component like the previous case where in polluted insulator the amplitude of the 3rd harmonic component has more growth than the 5th harmonic component (Figure 5). Tables 2 and 3 show the
amplitudes of LC low harmonic components. The percentage of each harmonic component was calculated by equation (1):
%nth harmonic = (nth/1st) × 100 (1) Where: nth = Amplitude of the nth harmonic
component
1st = Amplitude of the fundamental harmonic component
Combination of pollution and salt-fog is more disastrous than that of pollution with clean-fog, and the main reason is increase of conductivity. The contamination makes insulator perform nonlinearly and some Leakage current waveform harmonic growth. Tables 2 and 3 illustrate the coordination between the 3rd and 5th harmonic components of LC and insulators surface condition. With increase of pollution the ratio of 3rd / 5th increases so it can be considered as a good criterion for determining severity of pollution on the surface of insulators.
4 HYDROPHOBOCITY
The Hydrophobic characteristic of SIR insulators mostly leads to form water drops than a continuous layer. In this regard, conducting paths are prevented to be formed on the surface of SIR insulator, so the LC will be minimized and the flashover voltage will be maximized. Figure 6 illustrates that in a polluted insulator conducting paths are formed, and the LC increases consequently, while water drops are formed on the surface of clean insulator and thus it leads to low LC.
(a) (b)
Figure 6: The hydrophobicity of specimens. (a), clean insulator; (b), polluted insulator.
5 CONCLUSION
This study proves that harmonic analysis can indicate a relation between LC and pollution severity on the surface of SIR insulator and this can be declared as a good criterion for determination of insulators condition. A high proportion of the 3rd harmonic component with respect to the 5th harmonic component represents the situation has become worse for insulator.
When clean insulator is subjected to clean-fog, the proportion of the 3rd harmonic component with respect to the 5th becomes low, while fog is one of the deterioration factors of insulators. But when subjecting the insulator to a combination of pollution and fog (especially salt-fog) the 3rd harmonic component directly grows by increases in pollution severity. Based on experimental results the main reasons for this behaviour can be named as hydrophobicity decreases in pollution condition, withering of some areas on the surface due to LC heat which leads to dry band arcing, and corona discharge.
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