The
presence of moisture content, in traces
exceeding the permissible limits, can
however drastically reduce the effectiveness
of these properties and there is an
increased possibility of arcing &
internal flashovers due to condensation and
tracking which in turn severely affects
performance reliability of circuit breakers. Additionally, the excessive moisture
in the SF-6 gas & enclosed equipment
also accelerates the formation of compounds
such as SF5, S5F10 and Hydrofluoric acid (HF)
as a result of reactions between SF-6 arc
by-products such as SF2, SF4, S2 F2, SOF4,
SO2, SO2F2, SOF2 & water, and these
compounds being highly corrosive, toxic, and
aggressive, degradation of insulation,
insulating surfaces as well as erosion &
rusting of metal surfaces result due to
aging mechanism and all this in the end can
lead to either SF-6 gas leakages and/or
mechanical & electrical failures.Periodic
Moisture measurement and control therefore
constitute a major part of any effective
maintenance programme for SF-6 gas circuit
breaker & gas insulated equipments. The moisture particularly in liquid
phase is very detrimental to the dielectric
withstand strength of gas insulated
equipments & therefore moisture has to
be maintained at a level such that it does
not condense in to liquid for the entire
range of expected operating temperatures.
Each manufacturer will have their own
specifications for acceptable Dew Point
limits. A typical Dew Point value for
Tropical country like India, of 370C
(180 PPMV) measured at atmospheric pressure
is most satisfactory & favoured for SF-6
breakers up to 400Kv range, since this value
is equivalent to 15.90C at
breaker operating pressure of 7 Kg/cm2,
which is well above lowest operating
Temperatures in India. From this point of view, lowest
acceptable Dew Point of
300C (375 PPMV) can be
fixed as a Danger Monitoring parameter,
since it corresponds to 80C Dew Point at 7
Kg./cm2 breaker operating
pressure. Dew Point readings below 300C
should never be allowed/accepted. Similarly a typical `Dew Point of
400C be made applicable for
new SF-6 Gas Cylinder. In superior quality SF-6 gas cylinder
moisture content is kept less than 5 15
PPMV, thereby assuring Dew Point of above
400C up to 620C.
The necessary spot checks on SF-6 gas system
at site can be easily & quickly made on
sample SF-6 gas from breaker or gas cylinder
by using good quality, reputed Hygrometers
preferably from either Shaw Meters U. K. or
Alpha Moisture Systems U.K.
All care must be taken to follow
instructions given by the manufacturer of
Hygrometer properly towards measurement of
Readings.
Generally Dew Point Meters designed
for site application use Aluminum Oxide
sensors and these are suitable for Dew Point
Measurement at (slightly higher) atmospheric
pressure.
For accurate readings &
repeatability one must take Dew Point
readings at the same gas flow rate (3 to 5
L/min) and pressure (0.5Kg/cm2)
& to facilitate this a suitable pressure
gauge, regulator and flow rate tube assembly
has to be deviced & used while using the
Hygrometers.
In some top end hygrometers all such
arrangements are integral in the instrument
& these hygrometers can be connected
directly to breaker system pressure.
There
are few important points one should know,
while taking Dew Point measurements on
SF-6 gas system at sites and same are
given as under.
1.Conversion
from PPMV to PPMW:
The molecular weight of SF-6 is 146. The molecular weight of water vapour
is 18.Therefore a given volume of SF-6
weights 146/18 i.e. 8.1 times the same
volume of water
vapour. Therefore conversion factor for PPMV to PPMW is 8.1. For example
200 PPMW is equal to 1620 PPMV OR 300 PPMW =
2430 PPMV.
2.The PPMV level is independent of
pressure. That is SF-6 gas having 300 PPMV
at 3 atm pressure would still have 300 PPMV
at 1 atm pressure.
3.When converting/comparing PPMV/PPMW and
dew point readings, the pressure
at which the readings are recorded
is extremely important as Dew Point Temp
increases with increase in pressure for same
PPMV moisture readings. For example
: SF-6 system having 300 PPMV at 22
psig gauge pressure will have dew point of 230C but for same
moisture reading of 300 PPMV, Dew Point
would be 120C at
87 psig gauge pressure.
4. Dew Point measurements vary from
season to season. In summer when Temperatures are high, Dew Point readings will
be showing comparatively higher moisture
content than in winter when Temperature are
low. This is because at higher
temperatures the internal surfaces of
various items enclosed in SF-6 breaker will
tend to de sorb any moisture absorbed in
them previously.
We,
at Ovac Switchgear Services Pvt. Ltd, are
now properly geared up & equipped to
guide & record readings of moisture
content or Dew Points on SF-6 GCBs of any
make & type, at sites for our customers. For this purpose, we have procured one modern & CGL
recommended compact model type SADP mini,
Hand held, Dew Point hygrometer of Alpha
Systems U.K. make, recently, and same is
complete with leather carrying case,
pressure regulator, flow indicator meter
& a pressure gauge, to enable us measure
readings very accurately with consistency.
In
the event Dew Point readings taken show
higher moisture content than permissible we
can
help customers, in undertaking to purify
& dry the SF-6 gas at site with the help
of our imported filter/purifier units, to
the acceptable limits. Our filter/purifier unit has recently
proved its effectiveness, at Chandrapur sub
station of MSEB when in case of three
separate 400Kv GCBs, Dew Point readings of
120C were improved to 390C
even with only ONE pass, to MSEBs full
satisfaction.
The
recommended frequency of monitoring Dew Point in
SF-6 breakers as per M/s.CGL, is three-year after
Installation & once in a year thereafter,
every year. We
request our customers to take full advantage of
our facility, for the purpose of auditing,
monitoring, as well as preventing avoidable
failures in future.
