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NEW APPARATUS SET UP AT ENEL TO MONITOR POLLUTION DEPOSIT AND PILOT CLEANING OPERATIONS ON OUTDOOR INSULATORS

, and . (1996)

Abstract

Most utilities have demanded in these last years monito-ring systems able to survey the severity of the pollution deposit on insulators and alert when this parameter rea- ches a preset level. Cleaning operations are to be carried out at the alarm time on insulators so that surface withstand is recovered and flashovers are prevented. A part of the devices built so far monitor the polluted condition of a sample insulator, exposed in the area of the electric plant whose insulators are to be watched, in a position chosen as representative of the site. In some devices the sample insulator is periodically lowered in a tank where its surface deposit is washed off and dissolved in a given water volume, whose conductivity is then measured. The conductivity will increase in time owing to the cumulative effect of the consecutive immersions of the insulator in the same washing water. In apparatus more recently devised the deposit, instead of being washed off from the insulator surface is artificially wetted on it, having the insulator periodically put into an apposite humidification chamber. The deposit severity is then evaluated by a surface conductance measurement, whose result by means of the forced wetting is no longer dependent on the natural humidity conditions. A new monitoring apparatus designed and set up at ENEL employs as a probe insulator a special glass unit, with a post type shape and a height of 30 cm, whose shank and sheds are hollow. By the inside circulation of a refrigerating fluid the surface temperature of the insulator lowers to the dew point of the surrounding air, condensation occurs and the pollution deposit is moistened. An electronic control leads the process, checking the surface conductance current and stopping the insulator cooling when no increase of this current is any more noticed. It is therefore prevented any washing effect which could alter the deposit conditions. The surface conductance value determined at the stop of the cooling is recorded and compared with the preset alarm level. The subject apparatus offers positive advantages in compariSon with the other solutions. It does not employ any washing or humidification chamber with the moving parts needed to put inside the sample insulator every time the pollution measurement is to be performed. Moreover cooling makes the pollution deposit wet in a very gradual and uniform way, just like dew naturally does on insulators in service. The process is apt to be strictly controlled so that any deposit washing is prevented and 'the representativity of the probe insulator is maintained in time. In addition to these favourable characteristics the apparatus has a cost lower than the other devices' even considering the. more sophisticated among them. By providing a few electric plants properly chosen in a region with the apparatus, a sentinel net will be organized able to pilot the maintenance of the insulators in operation in all that region. The subject paper describes the characteristics of this innovative monitoring apparatus and refers the positive results both of the laboratory tests carried out on it and of a field experience in progress.

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