Optimized use of HV composite apparatus insulators

Optimized use of HV composite apparatus insulators needs Test sites and test objects, Measurements and inspections, Ageing characteristics, Pollution performance, Leakage current measurements, etc.

HV composite apparatus insulators

Optimized use of HV composite apparatus insulators:
Test sites and test objects:

Details of the environmental, climatic and electrical stresses at the four test stations are given in Table 1 and details on tested insulators and their exposure time are given in Table 2.
Table 1 Details of the test stations where inspections were carried out.
Table 2 Details on tested apparatus insulators (mostly with alternating profile, except standard profile of arresters at Negev and spiral profile of the breaker at Dungeness) and their exposure time.
According to IEC 60815.
Chamber/support.

Measurements and inspections:
The data parameters which were either continuously monitored or periodically recorded at the test stations included: leakage current and weather parameters (continuously) and visual inspections and hydrophobicity measurements periodically, according to [1]. Inspections of the test sites were carried out on an annual base.

Ageing characteristics:
At the most polluted coastal test sites, i.e. Dungeness and Kelso no deterioration except colour changes due to discharge activity (whitish and dark patterns most likely due to water dripping from the upper sheds), traces of discharge activity and slight erosion has been observed on apparatus insulators during the field tests. The details are shown in Figure 1 and Figure 2. No deterioration at all has been observed at the clean site in Sweden and at the desert site in Israel.

At the same time 15 composite line insulators were installed at the two test sites, i.e. Dungeness and Kelso. These results were partially reported in [2]. They were also complemented by one hollow insulator at each site delivered by the same manufacturer as one of the line insulators. A summary on all tested line composite (and one hollow) insulators and their exposure time is presented in Table 3 together with the description of their deterioration. Examples of deterioration/damages with some comments are shown in Figure 3 and Figure 4.

Figure 1 Dungeness, apparatus insulators. Examples of deterioration: A: breaker-27, whitish rings; B: arrester-20, dark pattern; C: arrester-25, traces of discharge activity; D: bushing-25, slight erosion.

Figure 2 Kelso, apparatus insulators. Examples of deterioration: A: bushing-23, whitish rings; B: arrester-12, dark pattern; C: arrester-16, traces of discharge activity.

Table 3 Details on tested line/hollow insulators and their exposure time.
The shortest insulator flashed over and was removed after 1 year in service.
Line-1, 23 mm/kV, Line-3, 22 mm/kV, Hollow core, 21 mm/kV
Figure 3 Dungeness. Deterioration: comparison of line and hollow insulators.
Line-1, 28 mm/kV, Line-3, 28 mm/kV, Hollow core, 23 mm/kV
Figure 4 Kelso. Deterioration: comparison of line and hollow insulators.

Pollution performance:
Hydrophobicity measurements:

The average Hydrophobicity Class (HC) was measured according to [3] during each inspection for each of the test stations. The HC was averaged for the measurements on top, bottom and sheath. The results presented in Figure 5 show that for both of the coastal test sites the silicone rubber insulators are minimum one HC better than porcelain and the maximum of average HC is between HC 5 and HC 6. In clean conditions of Ludvika, the absolute maximum HC was limited to 4. In desert conditions of Negev, on the top of some of the sheds the HC after 5 years was limited to 5.

Leakage current measurements:
Due to the physics of the pollution event, typically, peak currents can be related to the pollution performance of an insulator. In general, a high leakage current indicates a high probability for flashover, as the partial surface discharges (arcs) over the dry bands have a greater possibility to elongate and to bridge the whole insulator. A pollution event is thus defined as the time when the leakage current pulses exceeds a certain value. Preliminary analysis showed that the Dungeness test station is the worst for the insulators from pollution point of view. Severe storms followed by long periods of high humidity and light rain are worse than more moderate breeze type winds at Kelso followed by powerful subtropical rains, see maximum rain conditions in Table 1. Therefore, typical difference in silicone rubber/porcelain performance during pollution event is presented only for the Dungeness test station. Three test objects with similar creepage distance are chosen for the analysis, i.e. arrester-31 mm/kV; bushing-32 and porcelain hollow-32. In Figure 6 the peak currents (measured at 2 kHz and stored every 10 min.) are presented vs relative humidity and rain.

High currents on porcelain insulator typically started earlier during the storm than on silicone rubber insulators. This behaviour correlates well with worse hydrophobicity (higher HC) of porcelain, see Figure 5. More moisture in the form of light rain is also needed to produce high currents on silicone rubber insulators. In a short run, at some moment during the storm, the peak currents on silicone rubber insulators can exceed those on porcelain. However, in long-term one can expect that the porcelain insulator will have a larger number of high leakage current pulses than silicone rubber insulator. This appeared to be a correct assumption and examples are shown in Figure 7 for a number of significant storms at Dungeness and Kelso (peak currents over 30 mA were calculated). Similar observations have been obtained in the laboratory tests, simulating the coastal environment. These observations are important, because high peak currents are related to the high risk for flashover and therefore, porcelain insulators would more often have a chance to flash over than silicone rubber insulators. Taking into account an average number of high peak currents during the significant storms, it can be shown that a large margin exists in the specific creepage distance of composite apparatus insulators in comparison with porcelain, see Figure 8. To obtain the same number of high current impulses on a composite insulator as on the hollow porcelain, very low specific creepage in the range of 10 mm/kV is indicated.

Thus, for the same risk of flashover during pollution conditions, the creepage distance for composite insulator may be 3 times shorter. Leakage currents measured in clean conditions of Ludvika were 6 mA maximum and measured in desert conditions of Negev were 8 mA maximum, which indicate good hydrophobicity properties and possibility for further reduction of creepage distance.

Accumulated charge:
As it was stated in section 4.2, peak currents are typically related to the pollution performance. The accumulated charge flowing over the insulator surface is used as an indicator of the interaction of the insulator material surface with the deposited pollution and wetting, which is, in turn, related to the ageing characteristics. The differences in accumulated electrical charge are assumed to be mainly due to the material properties and internal E-field grading. Therefore, the porcelain hollow insulators with their stable surface properties and no grading are treated as reference insulators. In this section three silicone rubber arresters installed at each of the test sites are compared with respective reference porcelain hollow insulator. A direct comparison is made during typical pollution event (see Figure 9). A direct comparison of the accumulated charge during the whole period of measurements for all tested apparatus is shown in Figure 10 “A”. In case of arresters the measuring system calculated the charge for the current higher than 1 mA, thus avoiding integration of capacitive current, and in case of bushings the leakage current was collected from the special ring located at the silicone rubber surface close to the grounded flange.

Therefore the results are directly comparable. The highest accumulated charge was obtained on the chamber of the breaker, where the continuous discharge activity produced by internal electrodes has led to the colour changes (see Figure 10 “B”) and even reduced the hydrophobicity from HC 2-3 to HC 5 exactly in the middle of the chamber. For the same type of insulators (e.g. support or hollow insulators without internal structure) a much higher accumulated charge is observed on porcelain insulator than on SIR insulator with a creepage distance that is even lower than on porcelain insulator.

A direct comparison of arresters and hollow porcelain insulator is made using parameter, specific for the arresters according to IEC 60099-4 Annex F, i.e. mean accumulated charge for worst 6-hours during the storms (pollution events), see Figure 10 “C”. According to this parameter the Dungeness station has a pollution level ranging from Heavy to Very Heavy. There is no correlation between the accumulated charge and the specific creepage distance, which previously was also found for porcelain housed arresters [4]. Comparison from year to year reveals no indication of degradation, i.e. additional increase of accumulated charge over time for silicone rubber insulators.

Conclusions:
Based on the experience from the long-term testing of the apparatus silicone rubber insulators the following conclusions can be drawn regarding ageing characteristics and pollution performance.

With regard to ageing characteristics, the results of 2-7 years of field testing show that there is only slight deterioration for the apparatus insulators even with rather short creepage distance and in severe coastal environment. This is a much better performance than that of silicone rubber line insulators tested at the same site. The most important explanation of less ageing is a lower and differently located maximum E-field in the vicinity of HV flanges and lower current density due to a large diameter in comparison with line insulators.

With regard to pollution performance, the short-term and long-term hydrophobicity characteristics of silicone rubber apparatus insulators are better than of the porcelain insulators at the same site. The number of the high pulses of the leakage current provoking a flashover is much lower for silicone rubber apparatus insulators than for the porcelain insulators at the same site.

Considering both ageing as well as pollution performance, it is possible to reduce creepage distance in coastal areas with, as a minimum, one pollution level according to IEC 60815.


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