Although the topic of biofilm formation on ceramic and polymeric outdoor insulation has been widely addressed in the literature, up to now there has never been a direct comparison between different materials under identical conditions. This edited contribution to INMR by Lars Jonsson of Hitachi Energy reports on such a comparison involving 20 samples and carried out in line with the requirements of DIN EN 60068-2-10:2006-03.

The surface of all specimens was inoculated with a mixed suspension of different fungi and incubated under optimal conditions. After the test period, hydrophobicity was determined as well as hydrophobicity recovery after cleaning. An electrical withstand test was also conducted under wet conditions on a 145 kV bushing covered with a biological growth.

It has long been a concern among asset owners that polymeric materials for outdoor insulation may be more prone to biological growths than are ceramic materials. Many publications in this area were therefore reviewed, and it was discovered that there has never been a direct comparison between different materials under identical conditions. This then became the goal of the present research.

Most published reports and studies of biological growths on ceramic insulators refer to experience from tropical or sub-tropical environments.

There are several different ways micro-organisms can influence the structure and function of synthetic polymers. The main mechanisms are biofouling (contamination), degradation of leaching components, corrosion, hydration, penetration, and discoloration. The surface does not need to support growth to be affected; the mere presence of a biofilm could interfere with material function.

Results from past studies have confirmed that silicone rubber is highly resistant to corrosion induced by microorganisms. However, some organisms that produce pigments can cause serious discoloration, not removable through cleaning.

This study aimed to understand the potential effects of biofouling on polymeric and ceramic insulators used as external insulating housings on bushings. The susceptibility of different types of outdoor insulators to biological growth was evaluated by determining the surface hydrophobicity class of the materials.

The reason behind choice of this evaluation method is that the most common effect of biofilm formation on insulators is reduction in hydrophobicity, which could reduce wet flashover voltage. As such, surface hydrophobicity measurement by water contact angle measurement would be the most suitable approach.

The polymeric insulators studied included 4 different formulations of HTV (High Temperature Vulcanization) silicone rubber, one type of LSR (Liquid Silicone Rubber) and one ceramic material (see Table 1).

Experiment

A total of 20 specimens with a surface area of approximately 25 cm2 were cut from each sample. The surface of the specimens was cleaned with isopropanol and rinsed with de-ionized water. After drying, the specimens were placed in a dust-free container and kept under standard laboratory conditions (23°C, 50% RH) for at least 24 hours before further treatment. Half of the specimens were then exposed to accelerated ageing; the other half were not. Fig. 1 illustrates this approach.

Accelerated Ageing: UV Exposure

The UV exposure test of the 6 different materials was carried according to ISO 4892-2:2013 Method A Cycle No. 1 with a dark period of 8 hours. The total requested exposure time was 1000 hours.

Biological Test: Mold Growth, DIN EN 60068-2-10

Testing was performed according to DIN EN 60068-2-10:2006-03. The surface of the specimens was inoculated with a mixed suspension of different mold fungi (9 types; concentration of the suspension: 1.0 x 106 spores/ml) and incubated under optimal conditions to allow development of the fungi. The test climate was 28 ± 2°C and relative humidity was 95 ± 4%.

Two variations of the test were carried out: for example, in the case of UV aged test objects, 5 pcs were tested with addition of saccharose while 5 pcs were tested without. After 2 weeks and 4 weeks, infestation on the surface was evaluated by visual examination.

Hydrophobicity Classification Test

Once biological testing was completed, hydrophobicity class was determined. Sample surfaces were inclined 25° with respect to the plane surface and continuously sprayed for 25 seconds using de-ionized water. Immediately after the end of spraying, the hydrophobicity class, HC, was determined according to:

1. IEC 62073-2003 (Guidance on the measurement of wettability of insulator surfaces); and
2. DL/T 810-2002 (Electric Power Industry Standard of the People’s Republic of China – Technical Specification for ±500 kV D.C. Long Rod Composite Insulators), where HC 1-2 corresponds to a hydrophobic surface and HC 6-7 to a hydrophilic surface.

After determining hydrophobicity class, the fungi covering the insulator surface (the so-called biofilm) were removed using a tissue damped with isopropanol and then rinsed with de-ionized water. Specimens were placed in a dust-free container and kept under standard laboratory conditions (23°C, 50% RH) 48 consecutive hours before the hydrophobicity class was determined once again, using the same method as described above.

Results are presented in Table 2 and Fig. 2.

Second Biological Test on UV-Aged Insulation Materials

To understand if and how soon fungi will again form a biofilm on those insulators that already underwent fungal infestation, a second biological test was performed. To encourage fungal growth, only the UV-aged materials were considered in this run, and the test was performed under one variation with the addition of saccharose.

Compared with the first test, the HC levels of all HTV silicone materials were similar or somewhat improved, while the hydrophobicity of the LSR insulator and the porcelain insulator was remarkably superior to the previous level (i.e. from HC 4-5 to HC 1-2, and from HC 5-6 to HC 1-2, respectively.) The reason for such improved hydrophobicity in these materials is not clear and further investigation may be needed.

Case Study

A 145 kV class service bushing fitted with an HTV polymeric insulator, returned by a concerned asset owner from an installation in tropical environment, was tested for wet performance. The objective was to find out whether the electrical performance under wet conditions had been reduced because of biofilm formation. The tests were selected to reflect as many operating conditions as possible and performed in the following sequence:

• Hydrophobicity classification, IEC TS 62073;
• Power frequency voltage withstand test with partial discharge, capacitance and tanδ, level measurement. Level 10-310 kV, 50 Hz;
• Wet power frequency voltage withstand test according to IEC 60137, level 275 kV, 50 Hz, 1 minute;
• Wet power frequency voltage withstand test according to IEEE std 4, previous practice in the USA, level 275 kV, 50 Hz, 10 seconds;
• Power frequency voltage withstand test with partial discharge, capacitance and tanδ, level measurement. Level 10-310 kV, 50 Hz;
• Hydrophobicity classification after cleaning the insulator with isopropyl alcohol and a resting period of 48 hours, IEC TS 62073;
• Wet power frequency voltage withstand test according to IEC 60137, level 275 kV, 50 Hz, 1 minute;
• Wet power frequency voltage withstand test according to IEEE std 4, previous practice in the USA, level 275 kV, 50 Hz, 10 seconds;
• Power frequency voltage withstand test with partial discharge, capacitance and tanδ, level measurement. Level 10-310 kV, 50 Hz.

Fig. 3 shows the test circuit for the wet power frequency withstand test.

Hydrophobicity Classification

The classification was done in line with IEC TS 62073. Table 3 and Fig. 4 summarize findings.

Electrical Testing Under Dry Conditions

The study began with an electrical routine test in line with IEC 60137 ed. 7 to verify that the bushing’s internal insulation was intact. The bushing passed the test and had a partial discharge level of less than 5 pC at system voltage and withstood 310 kV during the dry 1-minute AC voltage withstand test.

Electrical Testing Under Wet Conditions

Wet power frequency voltage withstand test, according to IEC 60137 ed. 7.

Precipitation rate during test:
• Vertical component: 2 mm/min
• Horizontal component: 1.5 mm/min
• Conductivity: 113 µS/cm

Wet power frequency voltage withstand test (previous practice in U.S.), according to IEEE Std. 4, 2013.

Precipitation rate during test:
• Vertical component: 5 mm/min
• Horizontal component: 3.7 mm/min
• Conductivity: 151 µS/cm

Cleaning & Re-Testing

The insulator was cleaned with isopropyl alcohol with some of the black biological material still visible after cleaning. The bushing then rested for 48 hours before hydrophobicity classification was conducted again. Hydrophobicity class was HC 1-2.

The wet power frequency voltage withstand tests, as described, and finally the electrical routine test were then repeated. All tests fulfilled required criteria.

It was concluded that cleaning was not necessary to maintain wet power frequency voltage withstand according to IEC 60137.

Summary & Conclusions

Publications have addressed the topic of biofilms that can readily form on HV outdoor insulation, both ceramic and polymeric. Most cases have been reported from tropical environments.

Biofilm formation on the insulators occurs predominantly in areas shaded from direct sunlight, suggesting that partial shading is necessary to prevent desiccation. Insulator geometry thus also influences extent of biofilm formation.

The most reported effect of biofilm formation on insulators is reduction in hydrophobicity, which reduces their wet flashover voltage. Such reduction in wet flashover voltage has typically been higher for ceramic insulators than for polymeric insulators.

In general, several publications state that silicone rubber is highly resistant to corrosion induced by microorganisms. Any biofilms can be effectively removed with a combination of washing and subsequent wiping. Biofilms can also contain organisms that produce pigments causing discoloration that is not removable through cleaning. These problems are aesthetic and do not affect insulator performance.

Looking specifically at experience with silicone rubber, some publications have reported that extent of biofilm formation varies when comparing different types of silicone rubber insulators, i.e. growth appears dependent on type of rubber formulation. However, since the insulators were also of different designs, effect of shed geometry could not be excluded.

When comparing different types of silicone rubber test plates in a controlled laboratory environment, it has been reported that amount of biofilm formation differs between formulations. It was then speculated that the different degrees of biofilm formation were due to differences in the added fillers, other unknown additives or simply the result of a difference in surface roughness.

Controlled laboratory testing has shown that all virgin materials support biological growth to various degrees. Addition of an external nutrition source (e.g. saccharose) increased amount of biofilm, as expected. It was noted that extrusion grade silicones exhibit lower biofilm coverage and higher hydrophobicity (HC 2-3), compared to injection molded silicones (mostly LSR) and porcelain, which were slightly less hydrophobic (HC: 3-5). UV-aged materials exhibited higher coverage of biofilm compared to corresponding virgin samples.

Re-colonization was slower after cleaning the silicone and porcelain surfaces.

A study was conducted on a 145 kV service bushing covered by biological growth, returned by a concerned asset owner. This bushing fulfilled the wet AC voltage withstand performance, as per IEC 60137, without cleaning.