Operation & Maintenance Manual
05
STRUCTURAL DETAILS & GENERAL ARRANGEMENT
3.3 - SF6 GAS SYSTEM
The gas-tight compartments and the gas seals ensure long
service life.
A gas pressure monitor which is temperature compensated
provides accurate readings on the internal gas condition of the
switchgear.
3.3.1 - SF6 Gas Pressure Monitoring Device
The Breaker and Busbar compartment is sealed with SF6 gas at
1.35 kg/cm2 @ 20 deg. C, which is continuously monitored by a
density monitor.
SF6 Gas density monitor shall be provided for each separate
compartment consisting of SF6 gas.
WIKA make Gas density monitor with model number
233.52.063 is as shown in gure 3.3.1 below:
The safe permissible Gas pressure range is between 1.2 to
1.35 bars @ 20 deg. C
Some special features of the gas density monitor are as
follows:
- Hermetically leak tight, therefore no negative impact by
atmospheric pressure
uctuations and differences in
altitude.
- Local readout with alarm contacts.
- Temperature compensated.
The Gas Density Monitor is available with maximum two
magnetic snap action switching contacts. These contacts can
be normally close (NC) or normally open (NO).
These 2 contacts can be used for alarm and tripping purpose.
Let us consider switching contacts are normally closed
(NC). The terminal number details are as shown in
gure 3.3.2 :
When gas pressure inside tank is more than 1.25 Kg/cm² then
these two contacts shall be open.
1. When gas pressure decreases to less than 1.25 Kg/cm²,
contact with terminal no. 3-4 becomes Close. This contact
can be used to give an alarm, so that gas can be re lled
before gas pressure falls to minimum permissible pressure.
When the gas is re lled and the internal pressure exceeds
1.25 Kg/cm² then the switch is automatically reset (becomes
open).
2. When gas pressure decreases to less than 1.20 Kg/cm²,
contact with terminal no. 1-2 becomes Close. This contact
can be used to trip VCB. When the gas is re lled and the
internal pressure exceeds 1.20 Kg/cm² then the switch is
automatically reset (becomes open).
Note:
The switching point is not adjustable
3.4 - PRESSURE RELIEF DISC
The bursting disc is located on the rear of the breaker chamber
of every panel. For the bus compartment, it is located at the top
rear with an arcing chute for the main busbar direct arcing
produced to the rear. When the gas pressure in the
compartment exceeds 3.5kg/cm² during an arc fault, this disc
will burst.
MOLECULAR SIEVES
For proper working condition of the SF6 insulated switchgear,
the moisture content of the gas must be kept below 150ppm.
Condensation within the gas compartment is taken care of by
the molecular sieve, which absorbs moisture.
3.4.1 - GAS FILLING
During operation or lifetime of the equipment it may happen
that there is leakage in the tank due to some unavoidable
circumstances and the pressure on the SF6 manometer on the
Figure 3.3.1 - Gas pressure monitor (model 233.52.063)
Figure 3.3.2 - Normally closed switching contacts
1
2
3
4
SF6 GAS
LIMIT S/W
