Download Environmental Problems with SF6 Gas and Potential Use of Diluted

The Current Use of SF6 Gas by Electric Power Industry in Saudi Arabia and its
Possible Replacement by Diluted SF6 Gas Mixtures
By
N.H. Malik, M.I. Qureshi and A.A. Al-Arainy
College of Engineering, King Saud University,
P.O. Box 800, Riyadh 11421, Saudi Arabia.
ABSTRACT
SF6 gas insulated equipment plays an important
role in electric power networks all over the world
including GCC countries, due to its merits as compared
to traditional air and oil insulated equipment. As a
result, its production as well as its amount that is
released in atmosphere by utilities worldwide due to
equipment leakages and maintenance problems has
increased. Recently, it has been declared as a potent
greenhouse gas. Therefore, worldwide efforts are being
made to develop its universal substitute. However, in
the absence of its alternate and to reduce its used
content, its mixtures with other insulated gases are
being investigated. In this investigation, initially a
survey was carried out about the types and number of
SF6 filled equipment that are in use in the Kingdom of
Saudi Arabia by the local power utilities, and the
amount of SF6 gas released due to leakages and
maintenance, etc. In addition, systematic experiments
were also carried out to evaluate the performance of 8%
SF6+N2 and 8% SF6+CO2 gas mixtures subjected under
positive 150/1500 s switching impulse applications in
non-uniform field electrode system. The gas mixture's
pressure was varied in a range of 1 – 5 bar which is the
pressure in which most of SF6 insulated equipment
operates in the local electric power utilities. The results
are discussed in the light of physical phenomena that
control the breakdown process in such gas mixtures.
Keywords: SF6 gas, gas insulated equipment, SF6-CO2
mixtures, SF6-N2 mixtures, switching
impulse breakdown.
1. INTRODUCTION
Sulfur hexafluoride (SF6) is a man-made gas that
became commercially available in 1947 [1]. It exhibits
many properties that make it suitable for equipment
utilized in electrical transmission and distribution
systems. SF6 is a strong electronegative or electron
attaching gas both at room temperature and at
temperatures well above the ambient, which principally
accounts for its relatively high dielectric strength and
good arc-interruption properties. The breakdown
voltage of SF6 is nearly three times higher than air at
atmospheric pressure [2]. Furthermore, it has good heat
transfer properties and it readily reforms itself when
dissociated under high gas pressure conditions in an
electrical discharge or an arc so that it has a fast
recovery and it is self-healing. Most of its stable
decomposition byproducts do not significantly degrade
its dielectric strength and are removable by filtering. It
produces no polymerization, carbon, or other conductive
deposits during arcing, and it is chemically compatible
with most solid insulating and conducting materials
used in electrical power equipment at temperatures up to
200 C. It offers significant savings in land use, is
aesthetically acceptable, has relatively low radio and
audible noise emissions, and enables substations to be
installed in cities very close to the load centers [3,4].
However, SF6 has some undesirable properties as
well. For example, it forms highly toxic and corrosive
compounds when subjected to electrical discharges.
Moreover, nonpolar contaminants, e.g., air. are not
easily removed from it. Its breakdown voltage is
sensitive to water vapor, conducting particles, and
conductor surface roughness. Decomposition of SF6
due to electrical discharges may proceed as per the
following reactions:
SF6 + e

SF4 + 2F + e
SF6 + e

SF2 + 4F + e
SF6 + e

SF5 + F + e
SF5 + SF5

S2F10
SF4 + H2O

SOF2 + 2HF
SF4 + O

SOF4
SOF2 + H2O

SO2 + 2HF
Some of the byproducts as shown above are formed
under high power arc discharges while others are
produced under lower power corona discharges in
presence of oxygen and moisture. SF6 itself is nontoxic
and a human can survive indefinitely in a mixture of
20% O2 and 80% SF6. However, SF6 will not support
life and can cause suffocation. Being extremely dense,
it will accumulate in low-laying areas, requiring care if
being exhausted to atmosphere. Moreover, some of the
discharge byproducts of SF6, such as S2F10, SOF4, and
HF, can be very toxic having working day exposure
limits in the range of a few ppm. Therefore, it is
prudent to assume that any SF6 equipment which has
been in service for some time may contain some toxic
species, thereby requiring extreme care in its handling.
It is generally believed that if moisture is absorbed by
molecular sieves and oxygen can be prevented from
being present in SF6 equipment, the metal fluorides
which constitute the main discharge byproducts are
relatively harmless. Thus chemical adsorbents such as
soda-lime and activated alumina are used in some SF6
equipment to keep the harmful byproduct concentration
within tolerable limits [4].
SF6 gas insulated equipment forms a major
component of electrical power networks all over the
world including GCC countries, due to its magnificent
merits as compared to traditional air and oil insulated
equipment.
On global basis, its production has
exponentially increased in the last decade and so does
its amount that is being released into atmosphere by
utilities due to equipment leakages and maintenance
problems. Recently, it has been declared as a potent
greenhouse gas, since it is an efficient absorber of
infrared radiation particularly at wavelengths near to
10.5 m. It possesses a high global warming potential,
which for 100 years horizon is estimated to be about
24000 times greater than that of CO2 [4]. Worldwide
efforts made to develop its universal substitute have not
met a success, yet. However, to reduce its used content,
its mixtures with other gases are being investigated in
several research centers around the world. Recently,
efforts have been made to study diluted mixtures of SF6N2 and SF6-CO2 gases and reports show promising
results.
In Saudi Arabia and other GCC countries, electric
power utilities use large number of SF6 gas insulated
equipment. Present research work was executed in two
phases. In the first phase, a Kingdom-wise survey was
carried out to determine the types and number of SF6
filled equipments that are in use in the local power
utilities and industry. In addition, the total amount of
SF6 gas used and its percentage released to the
atmosphere during maintenance, refilling etc. was
estimated.
In the second phase, a systematic
experimental study was undertaken to study the
switching impulse breakdown characteristics of diluted
mixtures of SF6 gas with N2 and CO2 having 8% of SF6.
The paper presents the results and analysis and
implications of this survey and the experimentally
observed characteristics.
2. USE OF SF6 IN INDUSTRY AND ITS
RELEASE TO ATMOSPHERE
Generally, there are four major types of electrical
equipment that use SF6 for insulation and/or interruption
purposes: gas-insulated circuit breakers, gas-insulated
transmission lines, gas-insulated transformers, and gasinsulated substations. It is reported in reference [5] that
for these applications, the electric power industry uses
about 80% of the SF6 produced worldwide, with circuit
breaker applications accounting for most of this amount.
Gas-insulated equipment is now a major component of
almost all power transmission and distribution systems
allover the world and it employs SF6 almost exclusively.
Depending on the particular function of the gasinsulated equipment, the gas properties that are the most
significant vary. For circuit breakers, the excellent
thermal conductivity and high dielectric strength of SF6
along with its fast thermal and dielectric recovery are
the main reasons for its high interruption capability.
SF6-based circuit breakers are superior in their
performance to alternative systems such as highpressure air blast or vacuum circuit breakers. For gasinsulated transmission lines, the dielectric strength of
the gaseous medium under industrial conditions is of
paramount importance, especially the behavior of the
gaseous dielectric under metallic particle contamination,
switching and lightning impulses, and fast transient
electrical surges. The gas must also have a high
efficiency for transfer of heat from the conductor to the
enclosure and be stable for long periods of time of  40
years. SF6-insulated transmission lines offer distinct
advantages due to their cost effectiveness, high-carrying
capacity, low losses, availability at all voltage ratings,
no fire risk, higher reliability, a compact alternative to
the overhead high-voltage transmission lines in
congested areas, and avoidance of public concerns about
high-voltage overhead transmission lines.
Since, the use of SF6 has increased in recent
decades, more quantities of this gas are being released
to the atmosphere. In turn, this has resulted in increased
concentration of SF6 in the atmosphere [6]. Measurements have shown that the amount of SF6 in the atmosphere has been increasing at a rate of ~8.7%/yr,
from barely measurable quantities in 1980 to levels of
~3.2 pptv (~3.2 parts in 1012 parts) by volume in 1992.
The atmospheric concentration of SF6 could reach 10
pptv by the year 2010 depending upon the assumptions
of release rates and economic growth forecasts [4].
In many industrial applications SF6 is not
recoverable, and its release into the environment by the
electric power industry comes from normal equipment
leakage, maintenance, reclaiming, handling, testing, etc.
At present efforts are being made so that SF6 leakage
rate from the electric power-system equipment is
limited to 0.5% to 1% per year.
Nevertheless, decreasing the rate of SF6 leakage
and increasing the level of SF6 recycling are high
priorities since they will both curtail production needs
of SF6 and thus will reduce the quantities of SF6 that are
eventually released into the environment. Indeed, efforts
are being undertaken by the electric power industry to
better monitor the gas pressure in SF6-insulated
equipment and the amount of SF6 released into the
environment [4]. These efforts include improved
methods to quantify and stop leakages, better pumping
and storage procedures, setting of standards for recycling, manufacturing tighter and more compact
equipment, development of sealed-for-life electrical
apparatus, gradual replacement of older equipment
which normally leaks at higher rates, and
implementation of a sound overall policy of using,
handling, and tracing SF6.
3. POTENTIAL OF SF6 AS A
GREENHOUSE GAS
Greenhouse gases are atmospheric gases that
absorb a portion of the infrared radiation emitted by the
earth and return it to earth by emitting it back. Potent
greenhouse gases have strong infrared absorption in the
wavelength range from ~7 m to 13 m and occur
naturally in the environment. Examples of such gases
are H2O, CO2, CH4 and N2O, etc. Other examples of
such gases are man-made gases that are released into the
environment, such as fully fluorinated compounds
(FFC); combustion products such as CO2, nitrogen and
sulfur oxides and SF6. The effective trapping of infrared
radiation by the greenhouse gases and its re-radiation
back to earth results in an increase in the average
temperature of the earth's atmosphere. The effect is
known as the "greenhouse effect". The man-produced
contribution to the greenhouse effect shifts the balance
between incoming and outgoing radiation at the top of
the earth's troposphere toward the former, causing
"global warming".
Sulphur hexafluoride is an efficient absorber of
infrared radiation, particularly at wavelengths near 10.5
m. Additionally, unlike most other naturally occurring
greenhouse gases (e.g., CO2, CH4), SF6 is largely
immune to chemical and photolytic degradation and
therefore its contribution to global warming is expected
to be cumulative and virtually permanent. Although the
determination of the lifetime of SF6 in the environment
(the time taken for a given quantity of SF6 released into
the atmosphere to be reduced via natural processes to
~37% of the original quantity) is highly uncertain
because of the lack of knowledge concerning the predominant mechanisms of its destruction, it is very long;
estimates range from 800 years to 3,200 years [7]. The
strong infrared absorption of SF6 and its long life time
in the environment are the reasons for its extremely high
global warming potential, which for a 100-year horizon
is estimated to be ~24,000 times greater than that of
CO2, the predominant contributor to the greenhouse
effect. Therefore the concern about the presence of SF6
in the environment derives exclusively from this very
high value of its potency as a greenhouse gas. While
the potency of SF6 as a greenhouse gas is extremely
high, the amount of SF6 in the atmosphere is too small
to have significant environmental consequences.
Estimates of the relative contribution of SF6 to nonnatural global warming, using estimated SF6
concentration levels range from 0.01% to 0.07% and in
100 years this value could become as high as 0.2% [5].
However, government and environmental protection
agencies, electrical, chemical, and other industries using
or interested in the use of SF6 have expressed concerns
over the possible long-term environmental impact of
SF6 and the electric power industry is responding in a
multiplicity of ways to better control SF6 than in the
past and to reduce its releases into the environment.
The best response to the concerns over the possible
impact of SF6 on global warming is to prevent the
release of SF6 into the environment. Clearly the most
effective way to do this is not to use SF6 at all. Nonelectronegative gases that are benign and environmentally ideal, such as N2, normally have low dielectric
strengths. For example, N2 has a dielectric strength
about three times lower than SF6 and lacks the
fundamental requirements for use by itself in circuit
breakers. Nonetheless, such environmentally friendly
gases might be used by themselves at higher or lower
pressures, as the main component in mixtures with
electronegative gases, including SF6 at partial
concentrations of a some percent. Suggestions have
been made repeatedly over the last two decades to use
high-pressure N2 and mixtures of N2 with SF6 for
insulation, arc quenching, and current interruption.
Mixtures of N2/ SF6 have been and are being used in
circuit breakers under severe weather conditions.
Besides SF6/N2, other mixtures in use include SF6/CF4
and SF6/He. Dielectric properties of SF6 gas mixtures
with other gases are also being investigated since 1970s
and considerable literature exists on this subject.
In GCC countries, as have been done by other
utilities worldwide, industrial users, manufacturers and
installation contractors of SF6 filled equipment, should
be encouraged to minimize the intentional release of SF6
gas to the atmosphere. Moreover, the issue can be
discussed at various administrative levels in order to
highlight the importance of the problem. Such efforts
will result in a reduction of the total amount of this gas
released to the atmosphere, thereby, decreasing its
adverse environmental effects.
4. SF6 EQUIPMENT IN SAUDI ARABIA
Uniform field AC breakdown data was retrieved
from the literature [8] and was used to derive an
equation that can be used to estimate breakdown values
in a large range of 'pd' values.
A survey carried out by the authors show that SF6
filled equipment is extensively being used in the
Kingdom of Saudi Arabia. The equipment used by
electric utilities includes:
(a) SF6 circuit breakers with voltage rating from 13.8
kV to 380 kV. At present, the total number of such
breakers is more than 3000. The gas used in these
breakers has a pressure which varies in the range of
0.5 bar to 7.5 bars (gauge). Total amount of SF6
gas used in these breakers is estimated to be over
200 tons.
(b) SF6 insulated GIS have voltage ratings from 13.8
kV to 380 kV. At present, the total number of GIS
sections in use are over 15000. The gas used in
these GIS sections has pressure in the range of 0.2
bar to 7 bars (gauge). Total gas used in this
equipment is over 500 tons.
(c) Gas insulated lines (GIL) operating at 380 kV have
a total length of over 30 km. SF6 gas pressure used
is in range of 3.5 bars to 6 bars, while the total
amount of SF6 gas used in GIL is about 20 tons.
It is clear from these results that large quantities of
SF6, is being used in a variety of SF6 filled equipment
while some amount is certainly being released annually
to atmosphere due to gas leakages, malfunction or other
problems.
The survey further showed that a variety of
methods are being used to monitor SF6 equipment
which are aimed at detection of the gas leakage which is
accompanied by changes in the gas pressure and
density. Usually either pressure or density is monitored
with two levels for alarm and tripping. Various types of
gas detection sensors are available. Different utilities
use different methods for detection of gas leakage, as
well. Moreover, for storage of SF6 and its quality
check, proper methods exist and are being used by local
utilities. Usually gas is stored in cylinders of different
size ranges. Before using the gas, its purity and
moisture content is checked by suitable tests such as
dew point test or gas purity tester, etc.
5. EXPERIMENTAL SYSTEM AND
PROCEDURES
To simulate non-uniform field stress distribution, a
point-plane electrode set up was employed. In this case,
point electrode used was in the shape of a 110 mm long
rod with a tip radius of 1.0 mm. The plane electrode
was made of aluminium with polished surface.
The pair of electrodes assembly was inserted inside
a pressure test vessel. It was 66 cm long and 15 cm in
diameter and constructed from transparent epoxy and
designed to operate in the gas pressure range of 1 bar to
6 bars. It could be evacuated with the help of a vacuum
pump down to 100 mbar to evacuate air prior to
uploading it with gas mixture or gas. Three types of
gases i.e. SF6, N2 and CO2 were investigated to evaluate
their insulating behavior under the application impulse
voltages. All the gases were of technical grade and had
a purity of  99%. The mixture of gases was formed
based on gas partial pressures and was obtained by
initially filling the pressure vessel to a predetermined
pressure of SF6 after its prior evacuation, and then it
was further filled with the other gas up to a total
pressure level of 6.0 bar absolute. Based on the partial
pressure values, a mixture of required ratio of 8% SF6N2 or 8% SF6-CO2 gas mixture was used in these
studies.
High voltage lightning and switching impulses
were generated using a multi-stage impulse generator
that is capable of delivering output in the voltage range
of (0 – 1000) kVp for both polarities. It is a ten stage
generator, in which each stage can be charged up to 100
kV to deliver an impulse energy of 4 kJ per stage.
Fig. (1) shows the complete experimental system
that was used to generate and measure the impulse
voltages as applied to the non-uniform field gaps using
point-plane electrodes. GC-223 is the control unit of the
impulse generator.
Fig. (1): Experimental set-up for non-uniform field breakdown study under impulse voltages.
To acquire output voltage wave shape and measure
its parameters, beside a peak impulse voltmeter (not
shown here), a digital impulse measuring system
(DIMS) was also used. It is equipped with a 10 bit, 100
MS/s PC-based digitizer, which can reproduce applied
output impulses of both polarities with great accuracy.
These impulses can be stored and retrieved at will when
these are required for comparison and evaluation.
RESULTS AND ANALYSIS
6.1 Uniform Field AC Breakdown Voltage of SF6
The uniform field AC breakdown voltage data of
SF6, the following regression equation was derived for
Vb as a function of pd values obtained from the
literature [8]. In this equation Vb is in kV and pd is in
bar cm.
Vb = 90.2(pd) – 2.164(pd)2 + 0.021(pd)3
(1)
In reference [9], the following equation has been
proposed to estimate Vb for uniform field gap as a
function of pd where Vb is in kV and pd is in kPa cm.
Vb = 132 (pd)0.915
(2)
A comparison of equations (1) and (2) as shown in
Fig. (2) illustrates that equation (1) provides a more
accurate fit to the experimental data over wider range of
pd values in uniform field gaps for pure SF6.
Vb (kVpeak)
For lightning and switching impulse studies, the
5% (V5), 50% (V50) and 90% (V90) breakdown voltage
values were found using statistical method, where the
breakdown probabilities (Pr) were measured at several
fixed voltage levels by using ten impulse applications at
each set voltage level.
Then, from the plotted
breakdown probability breakdown voltage curves, the
values for V5, V50, and V90 were obtained. On these
normal probability plots, the 0.01% of probability has
been plotted as = 0%, whereas 100% breakdown voltage
is placed at 99.99% value of probability.
6.
Fig. (2):
Comparison plots of Vb and 'pd' values calculated using equation (2) (dotted line from reference [9]), and
using equation (1) (solid line). Data points derived from reference [8].
6.2
SF6 and its Mixtures under Surge Voltages
Power networks are commonly subjected to
lightning and switching surge stresses during their life
time and so is the fate of gas filled insulated
components that form its important integral elements.
Study of the impact of such voltage waveforms on the
insulating characteristics of gas insulated gaps is very
important. In case of lightning impulse voltage, it was
shown by Yoshitake et al. [10] that merely an addition
of 2% of SF6 to N2 gas increases the breakdown strength
of the mixture by 240% than that of pure N2 gas and
exhibits strong synergistic effect. Similarly, Hayakawa
et al. [11] have shown promising performance of SF6
mixtures with CO2 gas. Moreover, it was shown that
though the V50 breakdown voltage in pure SF6 gas
exhibits higher values under positive lightning impulse
(LI) voltages in coaxial electrode system than under
negative polarity LI, yet the behavior is opposite under
SF6-N2 gas mixtures. Although breakdown in highly
non-uniform field gaps for SF6 gas and its mixtures with
other gases have been studied extensively, the switching
impulse voltage behavior in dilute mixtures (with SF6
content 10%) has not been studied properly. The
experimental results presented here address this aspect
of the investigation.
Fig. (3) shows the breakdown voltage (Vb) versus
breakdown probability distribution (Pr) of SF6 gas
maintained at 4.0 bar under lightning impulse 0.9/45 s
waveform of both polarities. It is clear from the 'P r-Vb'
curves that breakdown probability of gas varies very
sharply between 10% and 100% with the applied
voltage with the exception of scatter observed between
20~40% Pr values under both polarities. The scatter is
slightly more under negative polarity than under
positive polarity. However, the breakdown voltages
under positive polarity are much smaller than under
negative polarity and hence show the important role of
positive polarity, both in the design of gas insulated
equipment as well as in research of phenomena that lead
toward breakdown of gas insulation.
Fig. (4) illustrates the breakdown probability
distributions as a function of voltage in SF6 gas under
the applied switching impulse (SI) voltage of 150/1500
s waveform and having a positive polarity for pressure
of 4 bar, respectively. It is clear that the probability
distributions are non-Gaussian and are accompanied
with large scatter that typically lies in a bigger range
(20-80%) of Pr values. Moreover, as the pressure is
increased, the range of voltage (V20-80 = V80 – V20) in
which this scatter is observed, increases systematically
Fig. (3): Effect of polarity on the 'Pr-Vb' curves for 100% SF6 gas maintained at 4.0 bar, under standard lightning
impulse voltages of positive and negative polarities.
to maximum at around 4 bar and then declines sharply
at 6 bar as shown in Fig. (5). This characteristic is
similar to the typical Vb-p variation of a rod-plane gap
under AC stress, and illustrates that this scatter is related
to the corona manifestations in the electrode gap. Anis
and Srivastava [12] have reported four different types of
pre-breakdown discharges in rod-plane gaps when
subjected to positive switching impulse voltages. These
were termed as single pulse discharge, multiple-pulse
discharge, incomplete breakdown pulses and corona
free breakdowns. The relative proportions amongst
these forms depend on the degree of field nonuniformity and the applied pulse magnitude. The single
discharge pulses prevail at pressures below the critical
pressure, while multiple-pulse discharges are associated
with leader-like form of discharge which occur at lower
pressures and more likely under over-voltages. The
incomplete breakdown pulses occur at intermediate
pressures just below the critical pressure. The final one
is the direct breakdown that occurs in stresses above
critical pressure level. In addition, the maximum scatter
in discharge onset voltage is seen to occur at gas
Fig. (4):
pressure corresponding to corona stabilization region.
Corona stabilization has the indirect effect of increasing
the scatter in the onset voltages. However, under
positive polarity LI, it was shown more recently by
Yoshitake et al. [10] that at intermediate gas pressures
where the Vb is higher, the partial discharges are
composed of three successive pulses. The first one is
associated with streamer discharge while the other two
occur with some time delay and are shown using streak
camera to be associated with the leader discharge. The
impulse breakdown was verified to propagate stepwise.
It is most likely that the multiple corona pulses observed
by Anis and Srivastava [10] are a similar manifestation
like the triple pulse phenomena observed by Yoshitaka,
under lightning impulses. The only difference is that in
case of switching impulse (SI) the field gets
comparatively more time to play a role in establishing
an avalanche or number of avalanches resulting in the
formation of several pulses. However, in this case too,
it is anticipated that the last two or more pulses in the
bunch occur with time delay and are associated with the
formation of a leader.
Pr-Vb curve for SF6 gas maintained at 4.0 bar, under positive switching impulse of 150/1500 s waveform. d
= 29 mm.
Fig. (5): Comparison of the variation of V(20-80) as a function of gas pressure due to SF6 gas, SF6-N2 and SF6-CO2
mixtures having 8% SF6.
Figs. (6) displays an example of the breakdown
probability distribution profiles as function of SI applied
voltage for SF6-N2 gas mixture. It is clear that in this
gas mixture too, like in pure SF6 gas, a large scatter
occurs in Vb values in the probability range of 20-80%.
It was found that (20-80) increases with the increase in
pressure up to a level of 5 bar, where after it exhibits a
sharp decline, as shown in Fig. (5).
Fig. (6): Pr-Vb curve for 8% SF6-N2 gas mixture maintained at 5.0 bar, under positive switching impulse of 150/1500
s waveform. d = 29 mm.
Figs. (7) illustrates a selected example of
breakdown probability distribution as a function of
applied voltage for SF6-CO2 mixture. A similar trend in
scatter is observed in this gas mixture, too. However, in
this case the scatter (20-80) increase almost
monotonically up to a gas mixture presence of 5.0 bar
before observing a sharp decline as in other two cases.
From the (Pr–Vb) plots for SF6 gas, SF6-N2 and
SF6-CO2 mixtures and for each gas pressure, the
breakdown values corresponding to 5%, 50% and 95%
probabilities i.e. V5, V50 and V95 were determined. Figs.
(8) through (10) illustrate the variation of these
parameters with the applied gas pressure. The V5 values
represent the near maximum dielectric withstand
capability of SF6 and its gas mixtures and are used in
the design of gas insulated systems and show how such
equipment will perform under non-uniform field and
positive switching impulse voltages. From the results, it
becomes clear that in point-plane gap spaced at 29 mm,
the SF6 gas pressure of ~2 bar gives the optimum value
of withstand voltage level. However, if the applied
stress level is enhanced then 2.0 bar pressure is still the
best choice for SF6 gas insulated equipment, since both
Fig. (7):
the V50 and V95 values exhibit the maximum at this
pressure.
In case of 8% SF6-N2 gas mixture, the optimum
pressure level where all the three voltage parameters
investigated exhibit maximum value also stands at 2.0
bar. Interestingly, both the SF6 and SF6-N2 mixtures
exhibit a voltage withstand level of ~130 kVp at this
pressure level.
In case of 8% SF6-CO2 mixtures almost all the
three voltage parameters increase monotonically up to a
pressure of 4.0 bar, while beyond that pressure level, a
decline can be noticed in V5, V50, and V95 values. At
2.0 bars the V5 value of SF6-CO2 mixtures exhibits
relatively 76% of withstand voltage level as compared
to that of pure SF6 and SF6-N2 gas mixture. However,
in 8% SF6+CO2 mixture this voltage withstand level is
achieved at an elevated gas pressure of 4.0 bar. This
shows that SF6-CO2 gas mixtures could also be
promising substitutes for SF6 but it will need the GIS to
be designed to operate at much higher gas pressure.
Pr-Vb curve for 8% SF6- CO2 gas mixture maintained at 6.0 bar, under positive switching impulse of
150/1500 s waveform. d = 29 mm.
Fig. (8): Breakdown voltage versus gas pressure curves for SF6.
Fig. (9): Breakdown voltage versus gas pressure curves for SF6-N2 mixture with 8% SF6.
Fig. (10): Breakdown voltage versus gas pressure curves for SF6-CO2 mixture with 8% SF6.
7. CONCLUSIONS
[5]
G. Mauthe, L. Niemeyer, B.M. Pryor, R. Probst,
H. Brautigam, P.A. O'Connell, K. Pettersson,
H.D. Morrison, J. Poblotzki, and D. Koenig, "SF6
and the Global Atmosphere", Electra, No. 164,
pp. 121-131, February, 1996.
[6]
L.G. Christophorou and R.J. Van Brunt, "SF6
Insulation: Possible Greenhouse Problems and
Solutions", NIST-IR 5685, July 1995.
[7]
U.S.
Environmental
Protection
Agency,
"Electrical Transmission and Distribution
Systems, Sulfur Hexafluoride, and Atmospheric
Effects
of
Greenhouse
Gas
Emissions
Conference", EPA Conference Proceedings
Report, August, 1995.
[8]
N.H. Malik, and A.H. Qureshi, "Calculation of
Discharge Inception Voltages in SF6-N2
Mixtures", IEEE Trans. on Electrical Insulation,
Vol. EI-14, No. 2, pp. 70-76, 1979.
[9]
R.S. Nema, S.V. Kulkarni and E. Hasain,
"Calculation of Sparking Potentials of SF6 and
SF6-gas mixtures in Uniform and Non-Uniform
Electric Fields", IEEE Trans. on Elect. Insul.,
Vol. 17, No. 2, pp. 70-75, 1982.
The following pertinent conclusions can be derived
from this study:
1.
2.
3.
SF6 equipment used in Kingdom of Saudi Arabia
includes circuit breakers (CBs), GIS, and GIL. The
CBs and GIS operate in the voltage range of 13.8
kV to 380 kV in which gas pressure varies from 0.2
bar to 7.0 bar gauge. The number of CBs that are
currently in use, well exceeds 3000, whereas GIS
sections installed stand more than 15,000.
Accumulative length of GIL which operates mainly
in 380 kV networks exceeds 30 km. The total
amount of SF6 gas contained in these units at
present exceeds 700 tons. Beside that, large
quantity of this gas is kept stored as spare for future
use or maintenance purpose at different sites.
Certain amount of SF6 gas is also released annually
to the atmosphere due to malfunction or faults etc.
Dielectric behavior of SF6 gas and its mixtures with
other insulated gases is reasonably well understood
for uniform field configurations. An empirical
relation is proposed which fits the experimental
data over large range of 'pd' values.
Under surge voltages, addition of small quantities
( 8% of SF6) to N2 or CO2 can significantly
improve their impulse withstand characteristics. In
such electrode configurations the pre-breakdown
discharges play a significant role in shaping the
overall profile of the breakdown phenomena.
Under switching surges, such discharges lead to
large scatter in the breakdown probability
distribution, which was found to depend on gas
pressure, mixture type as well as impulse
waveform parameters. In this regard both the
wavefront as well as wavetail values play
important role.
REFERENCES
[1]
G. Gamilli, "Gas-insulated Power Transformers",
Proc. IEE, Vol. 107 A, pp. 375-382, 1960.
[2]
L.G. Christophorou and R.J. Van Brunt, "SF6/N2
Mixtures, Basic and HV Insulation Properties",
IEEE Trans. on Dielectrics and Electrical
Insulation, Vol. 2, pp. 952-1003, 1995.
[3]
L.G. Christophorou, J.K. Olthoff and R.J. Van
Brunt, "Sulphur Hexafluoride and the Electric
Power Industry", IEEE Electrical Insulation
Magazine, Vol. 13, No. 5, pp. 20-24, 1997.
[4]
N.H. Malik, A.A. Al-Arainy, and M.I. Qureshi,
"Electrical Insulation in Power Systems", Marcel
Dekker Inc., New York, USA, 1997.
[10] Y. Yoshitake et al., "Impulse Partial Discharge
Propagation Mechanism Under Non-Uniform
Electric Field in N2/SF6 Gas Mixtures",
Proceedings of 13th Int'l Symposium on High
Voltage Engineering, 2003.
[11] N. Hayakawa et al., "Impulse Partial Discharge
and Breakdown Characteristics Under NonUniform Field in CO2 and N2/CO2 Gas Mixtures",
i.b.i.d., 2003.
[12] H. Anis and K.D. Srivastava, "Pre-breakdown
Discharges in Rod-Plane Gaps in SF6 Under
Positive Switching Impulses", IEEE Trans. on
Electrical Insulation, Vol. EI-16, No. 6, pp. 552563, 1981.