AS 4436:1996 pdf – Guide for the selection of insulators in respect of polluted conditions

AS 4436:1996 pdf – Guide for the selection of insulators in respect of polluted conditions.
In exceptional cases, pollution problems cannot be solved economically by a good choice of the insulator. For instance, in areas having very severe contamination or low annual rainfall. insulator maintenance may be required. This can also occur when the environment of an already built substation (or line) changes due to new polluting industries.
Maintenance normally takes one or more of the following forms:
periodic hand wiping on dc-energized installation or dry cleaning either energized or dc-energized;
— periodic coating with grease compounds:
periodic washing. either energized or dc-encrgizcd.
a) Greasing
Grease compounds used for coating insulators are mainly silicone products or hydrocarbons. The thickness of the applied layer of grease depends on the type of grease and on the degree of pollution:
generally. for the silicone based compounds it is about 1 mm and for hydrocarbon compounds can reach some millimetres.
This type of application requires regular maintenance for removing the grease and recoating and is expensive. It should he pointed out that the greased insulators lose most of their properties of self-cleaning through the rain or through the wind and that under certain conditions of heavy pollution the grease can damage the ceramic or the glass.
The frequency of cleaning and regreasing ranges from some months to some years. depending upon the degree of contamination and the weather conditions. An optimization of these operations shall he reached by a check of the conditions of the grease, taking into account the accumulation rate of the pollution content in the grease and the ageing of the grease itself.
b) Washing
There are two main methods for the washing of insulators to remove pollution:
— by fixed sprays;
— by using a manually controlled portable jet.
The frequency of washing should be such as to avoid significant accumulation of pollution. Thus the objective is to keep the insulators in as clean a condition as possible.
Insulator washing with fixed, automatic sprays is an effective and reliable method of combating pollution. particularly when the deposit rate is high. This technique has high capital cost and low running cost.
Portable jet washing equipment operates under the direct control of suitably trained persons and can be used at more than one site. It has low capital cost and high running cost. Some safety precautions are necessary.
Where the pollution deposit rate is high, a pollution detector is desirable to initiate fixed washing orto call for a manual wash.