Impact on theFlashover Process

In the field,because ceramic insulators are hydrophilic, surface pollution can be consideredfully wetted, forming a continuous conductive film of water. In the case of SRinsulators, however, their hydrophobic surface prevents formation of such afilm and thus provides superior electrical strength. But understanding thisonly looks at surface hydrophobicity and not at the entire pollution layer.Research has shown that, under UV and temperature effects, LMW species fromwithin the silicone bulk will migrate to the surface with the help of silicaand other non-soluble ingredients. After this, a hydrophobic cover willessentially form around the salt, such that it becomes difficult for moistureto get inside the pollution layer. Therefore, the total amount and rate of saltdissolution will decrease and the conductivity of water on the surface will belower. Even if the surface becomes fully wetted by rain or fog, the hydrophobiccover will slow down the dissolution rate.

Another key issuein this regard concerns the different impact on SR insulators of wetting fromweather. When precipitation intensity is low, the surface of these insulatorsbecomes wet only gradually meaning surface salts cannot be completely dissolvedout. When precipitation rate is high, pollution will drain quickly away fromthe insulator and salt in the hydrophobic pollution layer will not have time todissolve completely in the water. Therefore, when the SR insulator experiencesflashover, only a portion of the salt in the pollution is actually dissolved inthe water, i.e. not all the salt is “effective” in the flashover process.

Impact onPollution Measurement Results

The present methodfor ESDD measurement on SR insulators follows the same process used for ceramicinsulators. According to this methodology, the pollution is washed out anddissolved in 300 ml water in order to get all the pollution without salt lossor residue.

However, becauseof hydrophobicity transfer into the pollution layer, the salt dissolutionprocess during the above pollution measurement method is different from thatwhich actually takes place during the flashover process. For one, the methodattempts to dissolve all the salt from the pollution while, for SR insulators,only a part can be dissolved out in the wet condition and be effective duringflashover. Second, the method removes all the pollution by completelydestroying hydrophobicity and then dissolving the pollution in water. But, asdiscussed, in the case of SR insulators, salt is dissolved out only graduallyfrom the surface over a period of time. Therefore, the ESDD obtained under thepresent methodology does not accurately characterize the pollution level thatis effective for flashover of such insulators. Research should ideally beconducted to find new test methods to obtain the effective salt deposit densityfor flashover of SR insulators. In short, hydrophobicity transfer into thepollution layer of SR insulators has a significant impact on salt dissolutionsuch that only part of the salt in the pollution layer plays a role in theflashover process. ESDD obtained under the present measurement method simplydoes not reflect this fact.

Effective Equivalent Salt DepositDensity for SR Insulators

Based on operatingexperience as well as laboratory tests, a different concept was proposed tocharacterize the actual and effective amount of salt involved in the process ofpollution flashover of SR insulators.

Definition of EESDD

Effective equivalentsalt deposit density (EESDD) could be defined as the equivalent NaCl weight ofdissolved (i.e. the effective) salt in the wet pollution layer per unit area ofSR insulator. It could also be referred to as effective ESDD, ECDD (equivalentcontamination deposit density) or EDSDD (equivalent dissolved salt deposit density).

According to thedefinition of ESDD in IEC 60815, the effective equivalent salt deposit densitywould be calculated as follows:

EESDD = S / A0

where S is theequivalent NaCl weight of measured dissolved salt (in mg) and A0 is the wholetest area.

Artificial PollutionTest Results & Analysis of EESDD. According to this definition of EESDD, aseries of artificial pollution test were conducted with differenthydrophobicity transfer times and pollution severities. Fig. 1 shows theresulting EESDD/SDD curves versus hydrophobicity transfer time, i.e. the periodfrom the time a sample is polluted to when it is measured.

From Fig. 1, one can seethat the ratio of EESDD/SDD decreases significantly after one or two days ofhydrophobicity transfer and reaches a steady state value after about 4 days.Moreover, once hydrophobicity has sufficiently transferred into the pollutionlayer, measured EESDD values are only 20 to 30% of the original SDD values.This means that only a portion of the salts in the pollution layer havedissolved out while the remainder are protected within the hydrophobicpollution layer. The dissolved part of salt is therefore the effective saltwithin the pollution and, in this regard, the better the hydrophobicity, theless salt will dissolve out.

MeasurementMethod for EESDD

According to itsdefinition, a new measurement method is proposed to obtain EESDD. This methodreflects the actual dissolution process and is easily conducted in the field orin the laboratory.

Measurement Procedure

The procedure to obtainboth the wettability class (WC value) and the EESDD involves a series ofspecific steps:

1. Prepare the testequipment and samples, i.e. obtain A0,
2. Spray the sample to obtain the WC value and then collect all the dropletsfrom the test sample into a beaker,
3. Spray the sample an additional 25 times and collect all the droplets fromthe test sample into the same beaker,
4. Dilute the water in the beaker to 100 ml and measure the conductivity. Thencalculate the salt weight, S (in mg).
5. Calculate EESDD = S / A0
6. (If required to measure NSDD), follow the IEC standard method to obtainNSDD1 of the 100 ml and NSDDR of residue sample pollution, i.e. NSDD = NSDD1 +NSDDR.
7. (If required to obtain ESDD) obtain the ESDDR of the residue samplepollution (following the IEC standard method). Then, ESDD = EESDD + ESDDR.

2. Measurement Results ofSR Insulators with Natural Pollution

3. A site pollution surveywas conducted in China from 1999 to 2000 involving a total of 50 SR insulatorsthat were selected and tested. These insulators were from differentmanufacturers and operated in different types of service environments includingurban, marine, power plants, cement plants, chemical plants, brick factories,farms, etc. The EESDD of SR insulators was tested using the method describedabove and both EESDD and ESDD of the different shed surfaces were obtained.Fig. 2 is the measurement result of one particular SR insulator that operatedin a rural farm environment for 1½ years under 110 kV AC. There were 14 shedson the insulator having diameters of 120/80 mm (shed numbers from 1 through 14were counted from the energized end). Wettability class (WC) test resultsshowed that hydrophobicity generally became better from the high voltage end tothe ground end, with values from WC5 to WC2 for upper surfaces of sheds and WC7to WC5 for the lower surfaces.

4. EESDD measurementresults showed that:

5. 1. More than 20% of thesalts were not dissolved out in the wet condition.
2. EESDD and ESDD have considerable consistency. If the ESDD is higher, theEESDD of the same surface is similarly higher than on other surfaces.
3. For shed upper surfaces, the WC value is lower than that of lower surfacesand the EESDD/ESDD ratio is also lower. For shed lower surfaces, the WC ishigher and the EESDD/ESDD is also higher.

Comparing Figs. 1 and 2,the test results demonstrate considerable consistency. EESDD actually existsand is measurable, i.e. both the concept and the test method are valid andproven.

Conclusions

1. This article analyzesthe impact of hydrophobicity transfer on the salt dissolution process, whichmakes the flashover process of ceramic insulators and SR insulators quitedifferent. Present site mapping based on ceramic pollution data is thereforeclearly not optimized for utilizing SR insulator configurations.
2. The dissolution of salt from the pollution layer is slowed by theinsulator’s hydrophobicity transfer property such that only a portion of thesalt in the pollution layer actually dissolves out. This results in SRinsulators having lower wet pollution conductivity than ceramic insulators
3. A new concept, referred to as EESDD, has been proposed to characterize theeffective ESDD of SR insulators. It is an actual and measureable value asverified by test results.
4. A methodology for measuring EESDD is proposed and was applied to a test ofSR insulators in service. This method requires carrying out the WC test firstand then spraying the surface several times more. All the droplets on samplesurface during these steps are collected in a beaker before being diluted in100 ml of water. The equivalent salt, S, could then be calculated according tothe conductivity of the 100 ml solution. The corresponding EESDD is obtained bysimple division, S/A0.

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