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