Gas Insulated Substations
Dr. K. D. Srivastava
December 2012
Topics Covered
Section 1. Background
Section 2. Field Experience and
Persistent Design Challenges
Section 3. Operational Experience and
Practices
Section 4. Recent Developments
Bibliography
Section 1
Background
• 1970s-1990s: Gas-filled (SF6) short lengths installed. Many lab models for higher voltages, including three phase designs in a single duct. Also, SF6/N2
• 1990s: 500 kV mass impregnated paper for submarine DC systems in the Baltic Sea
• 1970s-1990s: Low temp. cryogenic/supercon. designs tried. 1990s witnessed the phenomenal growth in HTS technology
Energy and Industrial Culture
• Post World War II, energy (all forms)
usage was growing at the rate of ~3% per
year, in industrial nations
• But in industrial nations electricity usage
was growing by more than 7% by
displacing other forms of energy
• With oil crisis of 1970s and the growing
environmental movement, the energy
picture is very different now!
• In Europe (Western) and North America
the electricity usage is almost constant.
In developing countries, however, the
usage is growing between 7 and 10% per
year.
• Compressed gas cable technology has matured
over the last 30 years, but its potential for bulk
power transport is yet to be exploited and
developed.
• High temperature superconductor technology
is developing rapidly but [is] not yet fully
commercially viable for bulk power transport.
• None of the above three are free from
technological areas of concern!
• However, near urban centres overhead lines
are no longer acceptable to the communities
for environmental and aesthetic reasons.
• What are the alternatives?
• Three choices in technology:
Conventional underground power cables
Compressed gas cables (SF6 - Sulphur
Hexa-fluoride)
Superconducting cables.
• Land costs in urban areas
• Aesthetically “superior” to air insulated
substations
• Not affected by atmospheric pollution
Why GIS? Why GITL?
• Completely sealed (metal-clad) permits
very low maintenance
• Demand for higher energy usage in urban
areas requires increased transmission
voltages; for example, 420 kV
GITL
• In addition to the advantages listed above
for GIS, there is a need for non-aerial
transmission lines near urban areas.
• There are currently only two alternatives:
Underground cables–conventional or
superconducting, or
Gas Insulated Transmission Lines
(GITL)
• GITL, compared to underground cables,
have the additional advantage of reduced
ground surface magnetic fields.
• GIS/GITL installations have the usual
components:
1. Circuit breakers; disconnect,
earthing/grounding switches
2. Current and voltage measuring devices
3. Busduct sections
4. Variety of diagnostic/monitoring devices
Design Features of GIS/GITL
• Installations from distribution voltages
right up to the highest transmission
voltages (765 kV) have been in service for
30 years or more. Both isolated-phase and
three-phase designs are in use.
• SF6 is the insulating medium at a pressure
of 4 to 5 atmospheres. GITL units are
factory-assembled in lengths of 40 to 50
feet.
• The phase conductor is almost always of
aluminium. The outer enclosure is also of
aluminium, although earlier designs used
mild steel. For lower voltages, stainless
steel has also been used.
• Usually busducts are of rigid design
although flexible and semi-flexible designs
have been proposed. None are in use.
Typical Cable Section
Growth of GIS
Growth of GIS Installations Before 1985 January After 1985 January
Voltage GIS CB-Bay-Yrs. GIS CB-Bay-Yrs.
1 230 28669 731 28215
2 227 21252 382 12808
3 123 10362 147 5678
4 45 3870 65 2904
5 26 3252 37 1273
6 - - 2 200
Total 751 67,405 51,078
Voltage Class
1 60 – 100 kV
2 100 – 200 kV
3 200 – 300 kV
4 300 – 500 kV
5 500 – 700 kV
6 >700 kV
5. Current Transformer
6. Potential Transformer
7. Bus Section
8. Cable Termination
Expansion joint
Main Components of GIS
• Busbar and enclosure
• Busduct sections
• Bushing
• Circuit-breakers
• Disconnectors
• Earthing/grounding switches
• Current and voltage transformers and
measuring devices
• Expansion joints
• Diagnostic/monitoring devices
• GIS grounding and control wiring
• Termination modules
Persistent Insulation Challenges
Notwithstanding the high reliability of GIS technology,
both manufacturers and users have to be aware of certain
HV insulation problems inherent in the GIS design.
These are:
1.Reliability of support spacers.
2.Generation of VFTO by disconnect switch operation.
3.Contamination of SF6 gas by metallic particles.
4.Arcing/discharge by-products in SF6.
5.Environmental “green house” effects of SF6.
Applied voltage: 300kV, 0.4 MPa (SF6)
(81kV/div, 20 ns/div
FTO waveform measured by 1-GHz surge sensor
Source: M.M. Rao & M.S. Naidu, III Workshop on EHE Technology,
Bangalore, India, 1995.
• Diagnostic methods for identifying defects in a GIS
installation have been proposed by CIGRE. Many
gross assembly errors and poor quality assurance
procedures can give rise to significant partial
discharges (PD), which in the presence of moisture
may lead to toxic by-products in the SF6 gas.
• Automated insulation condition monitoring
systems, with innovative sensors, are being developed
and installed on GIS and other HV power apparatus.
• New techniques for PD detection/location are
perhaps the most significant developments in GIS
condition monitoring.
n = n0 exp αx
Collisional Ionization in Nitrogen-
Uniform Electric Field
n0 = electrons initially at x = 0
n = electrons at x
α = ionization coefficient for the gas
Effective Ionization Coefficient α′ as a function of Electric
Field Strength and Pressure
Molecular
Formula
BP
°C
Relative Electric
Strength
SF6 -63.8 2.5/760 mm
C4F6 -5 3.9/730 mm
C5F8 25 5.5/600 mm
C5F10 22 4.3/600 mm
CF3CN -63 3.6/753 mm
C2F5CN -30 4.7/735 mm
C3F7CN 1 5.8/550 mm
C8F16O 101 6.3/760 at 180°C
Environmental Impact of SF6
• SF6 is a gas specifically mentioned in Kyoto
protocol. Search is on for a replacement gas or
gas mixture. 80% of SF6 manufactured is used
by the electrical industry. Leakage rates are
<1% per year. References [13-14] are good
papers for an overview and the feasibility of
using SF6/N2 mixture. Equipment with 20%
SF6 is on the market. Table 6 shows some of
the by-products of SF6 breakdown by
arcing/discharges.
Section 2
Field Experience and
Persistent Design Challenges
Transient Ground-Rise in GIS (TGPR)
(Transient Enclosure Voltage) (TEV)
(For earthing practices in GIS installation see:
W G 21.03 Rep. in Electra, No. 151, Dec.,
1993, PP. 31-52)
• TEV or TGPR can be a very serious EMC
and personnel safety problem. Voltage rise
on grounded shields of several kV at
distances up to several km have been
observed in early days.
• Such transient voltages on the “grounded”
enclosure arise from an internal collapse
of voltage in the SF6 gas, internal re-
strikes across circuit breaker or disconnect
switch contacts, or flashover of external
insulation close to GIS, e.g., and air-SF6
bushing.
• Internal voltage collapse produces
travelling waves, in both directions, from
the point of breakdown. Such transients are
often called VFTO (very fast transient
overvoltages).
• At the points of discontinuity (changes in
surge impedance) these VFTO waves get
reflected and refracted. Such transitions
can be modelled as junctions of
transmission lines.
• Being high freq. transients, the currents are
confined to the “skin depth” of the coaxial
conductors.
• Typical impedance junctions are air/SF6
bushing, GIS/cable connections, ground
leads connecting the enclosure to the
earthing grid/mat/plate, or a ZnO arrester.
• Support spacer flanges can also act as sites for reflections.
• For a L-G fault, the step voltage may be: • m
U 100% = (LIWL)GIS x (1.20) x 1.12
1.20 ~ Pos. Polar. Breakdown
Neg. Polar. Breakdown
1.12 is a factor to allow for 100% breakdown under LI, i.e. 2.?.LIWL; ?=6%
• Internal breakdown give a step voltage
rise-time, dependent on gas pressure of
SF6,
Tr(min) ≈ (1……1.5) ns
p
where, p is in mPa.
• For re-strikes, EMTP studies show that
switching a no-load transformer may
produce up to 3 p.u. of overvoltage and
disconnect switch operation may produce
between 1.5-2 p.u. The two, opposite
travelling waves are 50% in voltage
magnitude.
• For a bushing transient the TEV
~ (S1) (Trav. wave)
where S1 = - _2Ze_ Fig. 1
Z1+Z2+Ze
Voltage going out to line is
~ (S2) (Trav. wave)
where S2 = - 2Zg_ Fig. 2a
2Zg+Z3
Zg = surge imp. of ground connection
Ze = surge imp. of enclosure
• Num. example .
Z1 = 60 - 450Ω
Z2 = 350 - 260Ω
Ze = 200 - 90Ω
Zg = 150 - 300Ω
The computed coeff. are:
S1 = 0.54 to 0.78
S2 = 0.54 to 0.75
Note: Significant overvoltages can develop
on the enclosure!
Assessment of surge propagating beyond GIS
Assessment of surge propagating beyond GIS
Propagation of surge down ground connections
Overvoltages on enclosures associated with a cable termination
Effect of surge arrester at point of GIS/Cable sheath interface
VR = discharge from the lightning arrester
Surface Flashover in
Compressed Gases
• Air GASES
• SF6
• Parallel Plane
• Point-to-Plane GEOMETRY
• Coaxial
• Epoxy
• Teflon MATERIALS
• Perspex
• DC
• 60HZ AC VOLTAGE
• Switching & Lightning
• Impulse
• Breakdown & Corona Voltage
• Surface Charge MEAS.
• Pre-Breakdown Current Pulses
• Particle Contamination
Design Principle
The field with the insulator should not
exceed the field at the central conductor
surface without the insulator.
Very difficult to achieve!
Effect of cohesion in case of coaxial electrode
Designs of cast Epoxy insulators
60 Hz breakdown voltage of 102 mm/292 mm coaxial
electrode system with free conducting particles, SF6
pressure 440 kPa, voltage ramp 2 kV/s.
Critical Problems 1. Triple-junction design
2. Tangential vs. normal field at the insulator
3. Surface discharges from partial discharges
4. Presence of metallic particles on the
insulator surface
5. For D.C. applications - the problem of bulk
charging of insulator
6. Poor quality material - voids & other
defects
Reliability of Support Spacers
• Bulk failure is rare - but voids, protrusions,
conducting contaminants may cause sustained
discharges in the bulk and lead to failure.
• Casting is a high temperature process and
differential cooling and contaminants in the
filler (Al2O3) have to be minimized by strict
quality control.
• Very often the PD level generated by these
defects is below the detection sensitivity of
1pc.
• “Intrinsic breakdown of epoxy spacer is
over 1MV/mm - but the material does age.
• Early designs operating AC stress was 10
kV/mm (rms) at maximum locations. Many
of these failed in service in about 5 years.
• Typical stresses now range from 2 kV/mm
(rms) at 145 kV and 4.1 kV/mm (rms) at 800
kV. But some high voltage designs still use
5-6 kV/mm (rms).
• Economic pressure to reduce spacer
dimension since this will affect the enclosure
diam.
• Metallic protrusions and contaminants
exhibit a “silent” initiation phase.
• PD detection requires increasing detection
sensitivity as the spacer size increases with
voltage level of GIS.
• For example, a 500 kV spacer should
perhaps be tested with a detection
sensitivity of about 0.5 pc. Such a level is
difficult to achieve in a factory.
• Improved ultra wideband techniques,
including coupler designs may allow
measurements to 0.1 pc in a factory
environment. With further improvements in
noise filtering, high quality test transformers,
levels of 0.01 pc have been achieved in a
factory setting.
• Another factor is the reduced margin between BIL and operating stress as the voltage class becomes higher.
• When there are voids present, either from the
start or due to slow initiation activity at
protrusions and metallic inclusions, the electron
production rate is too low to start a PD in one
minute of test. (3 electrons/cm3-sec).
• Also, a great deal of detection threshold depends
upon the radial position of cavity.
• Testing spacers in a factory at a higher voltage
would compensate for the lack of initiatory
electrons.
• The question of x-ray irradiation during
spacer testing has now been taken up
seriously by manufacturers. XIPD - X-ray
Induced Partial Discharge - is a new
technique for quality improvement.
• The question of trapped DC charge on a GIS
bus bar and its subsequent impact on spacer
flashover, should not be ignored.
• Even a small protrusion on the central
conductor near a spacer would deposit a “line
charge” on the spacer. The local field at the
“tips” of such a line charge could be high
enough to initiate a local discharge. A trapped
charge of, say, 0.8 pu on a 550kV GIS is
equivalent to a sustained DC voltage of
~340kV in the bus.
• Such a line charge may be particularly
dangerous when the disconnect switch
operates. The combined transient field plus
the line charge filed may be sufficient to
cause spacer flashover.
Typical sequential variations of the breakdown voltage of a coaxial
conductor without and with a composite-profile cone spacer.
• Insulating spacers are widely used in high-
voltage power apparatus. From a withstand
voltage of view spacers are the weakest
components and an improvement in the
understanding of surface flashover
characteristics of such solid insulators is
beneficial for better designs of power
apparatus.
• In the busbar of GIS there could be trapped
charge after disconnect switch operations.
The electrical stress created by these charges
can lower the withstand voltage. Work was
undertaken to determine the changes, if any,
in the early stages of the surface breakdown
under lightning impulse voltage when there is
a prior direct stress.
Test Model and Experiment Set Up
Electrodes and
Spacer
The Test Circuit
Comparison of streak image of surface flashover and gap
breakdown in the air. (a) gap breakdown, (b) surface flashover
• Some papers reported that in busbar of GIS
equipment there could be trapped charge after
disconnect switch operation. From previous
work, it was found the surface charge
accumulated on the spacer surface after applied
impulse voltage.
• The application of DC prestressing will
approximate conditions resulting from disconnect
operation or lightning/switching surge.
DC Prestressing
Flashover voltages for ptfe spacer with SF6 and N2.
Test Model and Experiment Set Up
Electrodes and
Spacer
• The results obtained with the combined dc
and impulse voltages have indicated that a
dc voltage alters the electric field
distribution along the surface of a spacer
• From the experiments, it is clear that the
initiation luminosity of flashover on insulating
spacer is at somewhere between two electrodes.
There would be local field enhancements at
several places. It is not possible or economically
justifiable to employ spacers with perfect or
near perfect surfaces. Hence, improvements in
the withstand voltage can only be obtained by
preventing field enhancements through other
means such as a weakly conductive coating.
• The development of flashover from the
onset of stage one activity when there is a
dc initial voltage is much more rapid than
when there is no dc voltage. The rapid
flashover development can give rise to fast-
fronted transients in the substation.
Predischarge development in SF6.
t=0 is the start of the voltage breakdown at the
gap.
Predischarge development at an insulator surface with
a disturbance near the anode.
The influence of conductive paint near the cathode on the
predischarge development is shown in the next figure.It is evident
that the predischarge formation occurs in the space between
disturbance and anode. As the discharge proceeds in anode direction
the remaining gap between cathode and the disturbance in this case
is bridged very late.
The result is that such a conductive disturbance directly at the
insulator surface causes an advancement of the adjoining
electrode. Thus, it is not surprising that in both cases the
reduction of the electrical strength is comparable.
Predischarge development at an insulator surface with
a disturbance near the cathode.
Predischarge development at an insulator surface
with a protruding disturbance near the anode.
Predischarge development at an insulator surface
with a protruding disturbance near the cathode.
Predischarge development at an insulator surface
with a protruding disturbance near the cathode.
A streak photograph of the surface flashover
before insulator pre-charging.
A streak photograph of the surface flashover
after insulator pre-charging.
Phase resolved accumulated counts, showing the influence of
X-ray intensity. Accumulation time: 20s; text voltage: 40 kV
a No X-rays
b Ion dose rate 5.4 μA/kg (21mR/s)
c Ion dose rate 19 μA/kg (72 mR/s)
d Ion dose rate 36 μA/kg (139 mR/s)
Continuous long time recording of voltage X-rays and
PD activity; SF6 pressure: 600 kPa.
Particle Contamination in
GIS/GITL
Effect of:
• Particle dimensions
• Ambient field non-uniformity
• Gas composition
• Particle deformation
• Number of particles - free
• Duration of voltage application
• Voltage waveform
• Nearness to a spacer
• Electric wind
• Fixed or free particles
Particle Reduction During
Manufacture and Assembly
(GIS/GITL)
• 80-85% of surface area is due to the inside
surface of the enclosure
• Not easy to clean
• Enclosures are normally extruded Al. or Iron
tubes
• Manufactured surface finish is 125 to 65, but,
• Die marks, oxide layers and local damage is always present and these are the main sources of particles.
• Surface conditioning of the enclosure is essential.
Any surface conditioning process must address: oxide layers, Die marks, Burrs and loosely attached machining debris
Sources of Metal Particles in GIS
• Machining debris
• Expansion joints
• Poor mechanical assembly
• Other defects in metal parts
Possible particle locations:
1. Fixed on phase conductor
2. Fixed on enclosure
3. Free to move in elec. field
4. Fixed on spacer
Free particle movement different under DC, AC and Impulse voltages.
Degradation in electrical insulation strength
of SF6 caused by conducting particles.
Loss of dielectric strength of SF6 in the presence of
a 0.45/6.4 mm wire particle in a coaxial system
subject to direct voltages.
Section of a simulated motion of an Al/0.5/10 mm
particle (100 kV, 3 bar, R=0.80).
Section of a simulated motion of a Cu/0.5/10 mm
particle (100 kV, 3 bar, R=0.80).
Hjk
L;l
L;l
L;l
H
L
L
L
Breakdown
voltage profile
of a spherical
particle in an
SF6 parallel-
plane electrode
system.
Comparison of the effect of coefficient of restitution on the
calculated maximum bounce height for 0.45/6.4 mm copper
particles, field strength 2.5 kV/mm peak, 60 Hz.
Metallic Particle Control
• Q-control of machining of components
• Ultra-sonic cleaning of components
• Adhesive tapes/coatings
• Particle traps
• Dielectric coatings
• Conditioning
Conditioning Methods for
Enclosure Surface
1. Chemical etching
2. Sand or glass bead blasting
3. Abrasive finishing using oil oxide paper
4. Mechanical vibration with forced air
flow.
See D.O.E. (US) Report
# DOE/ET/29336-1
August 1983
Particle Control by Dielectric
Coating
• To move in an electric field the particle needs
to be charged
• By coating the inside surface of the enclosure
we may reduce the charge
BUT
A metallic particle on a dielectric coating may
acquire charge by:
• conduction through coating
• by partial discharge between particle and
coating
OR
• by contact charging from and already charged
surface
• Effect on breakdown
• Effect on particle charging
• Effect on maxm. excursion height
• Particle movement “inhibition” pseudo-
resonance
• Breakdown probability
• Experimental results
Why Dielectric Coatings?
Insulator and particle trap for CGIT system.
SF6;
Teflon;
1.5 mm
diameter
steel.
Micro-
discharge
criterion SF6,
2 mm
diameter
spheres,
theoretical
computation.
Effect of applied voltage on maximum height reached by an
aluminum wire particle (0.45 mm dia./6.4 mm long) in a 70/90 mm
GIS/GITL system (_______ uncoated, - - - coated) for a coefficient of
restitution of 0.95.
1.5 mm
diameter steel
spheres,
Polyurethane
coating.
1.5 mm
diameter
steel
sphere,
Epoxy
coating.
Particle movement: Effect of particle length on
time to first gap crossing.
Comparison of calculations and measurements:
Particle motion from calculations and videotape
observation.
Comparison of calculations and measurements:
measured and calculated lift-off fields.
Smoothed curves
of lifting field vs.
pressure for
spherical steel
particles 1.5 mm
diameter.
h j
Migration velocity of particles in 226/89 mm coaxial
electrode system as function of slope at 50 kV rms.
Operational Experience with
GIS/GITL
• Reliability of Support Spacers
• Very Fast Transient Overvoltages (VFTO)
• Transient Ground Rise
• Bushing and Transformer Insulation
• Design of Disconnect Switches
• Metallic particle Contamination
• Discharge By-Products in SF6 Gas
• Environmental Effects of SF6
Emerging Trends in GIS/GITL Technology
• More rigorous factory and on-site
commissioning tests.
• More elaborate/sophisticated monitoring and
diagnostic test equipment.
• Increasing use of GITL, mainly for urban
power feeders. One reason is to minimize
ground level magnetic fields associated with
conventional underground cables.
• Development of DC GIS for incorporating
into expanding national/international
HVDC systems
• Search for replacement gases for SF6. The
most promising is an 80%/20% N2/SF6
mixture. Circuit breakers will continue to
use pure SF6, and least in the near to mid-
term.
• Improved one-break circuit breakers for
compact transmission voltage GIS for
urban centres.
• Replacement of existing AIS by GIS will
accelerate, especially near urban centres.
New Developments
• UHF partial discharge detection
• HVDC GIS
• SF6/N2 mixtures
• Long GITL installations
• Compact substations
GIL/GIS Recent Development
• 70m long prototype by Siemans for 400 kV
system. SF6/N2 mixture
• Simulated 50 year life
• Renewed interest in flexible lines. However,
the biggest challenge is the design of long
100 m sections. How to mechanically
support the conductor!
• Switching impulse tests for SF6/N2 mixture
confirm theoretical models.
• Recycling guidelines for SF6 and
extracting SF6 from SF6/N2 mixtures are
now available.
• Three phase rectangular enclosures for 500
kV class have been tested (~200 cm x 200
cm).
• Long-term field tests for GIL: minimum 1
year on a 100 m section.
• Comparison of aerial lines and GIL must
take into account the total life cycle costs,
over 50 to 70 years.
• Combined voltage and current sensors.
• Highly integrated sub-station layout - a
mixture of metal clad and air-insulated
technology.
• Very thick coatings on conductors.
• For DC GIS a conductive coating on
spacers.
• Using an epoxy enclosure for GIL.
• Japanese ~3 km 275 kV GIL.
Distribution of enclosures on a voltage class basis.
Distribution of short circuit current ranges on
a voltage class basis.
Distribution
of degree of
importance
assigned by
users to the
development
of built-in
technology to
monitor
parameters as
indicated.
Users’ opinion on continuous vs. periodic.
Major
failure
frequency
by voltage
class.
Distribution of major failure causes reported by
users for all voltage classes.
CIGRE Survey 2000: Voltage classes
CIGRE survey
2000
Major failure
frequency (FF) –
2nd GIS survey
total population
and comparison
between the 1st
and the 2nd
survey results.
Identification of main component involved in the failure
from GIS voltage class point of view.
CIGRE
Survey
2000
CIGRE Survey 2000: Identification of main
component involved in the failure from GIS age
point of view (5 most involved components).
The test
cell for oil
and paper
insulation.
The composite electrode system.
A measured fast front step waveform.
A fast front breakdown of oil and paper.
∆ - one layer
paper multiple
FFT data
One layer of
paper multiple
impulse results.
Section 3
Operational Experience
and Practices
In-service fault rate (faults/station-year) vs.
years in service for 25 North American GIS.
Comparison of single-phase enclosed SF6 CGI
bus bars for rated voltages of 230 kV and 550 kV
Calculated
field gradient:
1, 2, 3, & 4: 230
kV bus
conductors
1’, 2’, 3’ & 4’:
550 kV bus
conductors
Cone insulators of
various design
Dimensions and ratings of rigid single phase
GITL underground systems.
Comparison of GITL dimensions for manufacturers.
The dimensions selected reflect the manufacturer’s
design and manufacturing philosophy including
design testing, quality control and manufacturing
tolerances.
Typical design of compressed gas insulated
transmission line. Shipping module is 18 m long
with insulators every 6 m. Other designs may use
only disc or conical insulators. Drawing not to scale.
CGIT system with cast Epoxy tripost and
conical insulators.
Full-scale model GIL and insulating system.
Determination of
the diameters of
conductor and
enclosure.
Example of the
construction of
post-type
particle trap.
Full-scale single-phase model GIL with a length of 168 m.
Fundamental dimensions and material used in GIL
Required specifications.
General view of the 275 kV GIL in operation.
Cross section of corridor for the GIL.
Relative cost of CGIT systems as function of
enclosure diameter.
Cost breakdown of 60 foot CGIT shipping
module (including assembly, labour and testing).
Dimensions and rating of three conductor, buried CGIT systems.
Optimum dimensions for three conductor cable:
Re = 5.56 Rc, R1 – 2.78 Rc.
Designs of three-
conductor CGIT
systems. Post
insulators a-c are
attached to metallic
ring which moves
inside enclosure,
insulators d-f are
attached by welding to
inside of enclosure.
Product Approx. Concentration by
Volume (%)
SOF2 (SF4) 0.5
SOF4 0.085
SiF4 0.085
S2F10 0.026
SO2F2 0.006
SO2 0.002
HF 1.0
Note: SF4 is quickly hydrolyzed to SOF2
Compound TLV by ppmv
SOF2 1.6
SO2 2
HF 3
S2F10 0.01
decomposition
source
main decomposition products toxicity
(weighted)
reactivity
with
atmospheric
humidity formula state abundance
hot
contacts
SOF2
SO2F2
SO2
gas
gas
gas
low
low
low
high
low
medium
medium
low
low
partial
discharges
SOF2
SF4
gas
gas
low
low
high
medium
medium
high
no load
switching
arcs
SOF2
SOF4
SO2F2
gas
gas
gas
low
low
low
high
high
low
medium
medium
low
Rough characterization of decomposition produces resulting from different
sources
decomposition
source
main decomposition products toxicity
(weighted)
reactivity
with
atmospheric
humidity formula state abundance
heavy
switching
arcs
SF4
WF6
SOF2
CF4
HF
CuF2
WO3
gas
gas
gas
gas
gas
solid
solid
medium
medium
medium
medium
low
medium
medium
medium
high
high
non toxic
medium
non toxic
non toxic
high
high
medium
none
low
none
none
internal
arcs
HF
SF4
CF4
Al2F3
Fe2F3
gas
gas
gas
solid
solid
medium
high
medium
high
high
medium
medium
non toxic
medium
non toxic
low
high
none
medium
none
Chemical measurements. Example of chromatographic
measurements. Defect a): PD level of 10-15 pC. SOF2 and
SO2F2 by-products as a function of the time under voltage.
Chemical measurements. SOF2 and SO2F2 by-products measured after various events.
*High sensitivity (0.1 ppmv) chromatography (TCD + FPD/SSD)
**Lower sensitivity (50 ppmv) chromatography (TCD only)
Reactivity and toxicity of gaseous SF6 decomposition products
Threshold limit values (TLV) for different SF6 by-products
Rough characterization of decomposition products resulting from
different sources
Flowchart for the destination of removed SF6
Basic structure of the SF6 reclaiming process
A comprehensive catalogue of guidelines for the handling and
management of SF6 is available from the US EPA.
Diagnostics, Field Testing &
Commissioning
• Joint effort with Customer-Transfer of Know-
how to Users
• Cleanliness of site - Humidity and Dust
Control
• Alignment of Components
• If Factory Assembled Re-check Nuts/Bolts &
Alignment & Level
• Grounding-Transient Voltage Rise
• No Floating Components or Tools Left
Inside!!
• When Filling with Gas - Avoid Condensation
of SF6 - Specially on Spacers
• Very Important to Have a Written Training
Manual for Site Erection and Testing
What Diagnostics to Install
Permanently? • Gas Pressure
• Humidity
• Gas Sampling for SF6 Arcing By-products
• For Switches
• Travel Distance
• Travel Velocity
• Contact Wipe
• Contact Resistance
• Physical Condition - Corrosion
• Capacitive Couplers
• Voltage Feed for AC/Impulse Site Tests
• Other Alarm/Monitoring Devices - Optical
Observations, Acoustic Couplers etc.
• Bus Isolating Links
What Site Equipment?
• Resonant Test-set
• Surge Generator
• PD, Acoustic Measuring Devices
• Vibration Monitors
Site Tests – Insulation
• Conditioning for “Sweeping” Particles
• AC in 15 to 20% Steps, up to 80% of Factory Test Level
• Up to 1.1x The Switching Oscill. Impulse
• No DC Voltage Tests
• One Manuf. Uses X-ray System to Check Alignment and Contact Damage on Site
Oper.
planned
correctly
$
Reliability
Capital
Diagram A
Complexity of Monitoring System
Design!
• CB Related Info.
– Travel
– Position
– Elec. Wear
– Hydraulic System
– Internal Arcing?
PD levels of protrusions according to IEC-270.
PD levels of moving particles (length l = 5 and
7 mm) on the enclosure according to IEC-270.
Signal amplitude for moving particles (length l =
5 and 7 mm) on the enclosure measured by the
UHF-method at CF f = 1.29 GHz.
Transmission coefficient tTEM of the TEM-
mode for a dielectric spacer disc (thickness is 5
cm, εr = 6.5).
Transmission coefficients ITE and ITM for a
dielectric spacer disc (thickness is 5 cm, εr = 6.5).
Disc sensor installed inside a GIS
Equivalent circuit of a disc sensor inside a GIS
Rough representation of a PD pulse in SF6
Four discharges from an impacting A1/0.5/10
particle. Voltage level/phase = 100 kV/086°
(Erms – 6.5 kV/cm at the enclosure).
Multiple discharges from an impacting A1/0.5/10
particle. Voltage level/phase = 120 kV/084°
((Erms – 7.8 kV/cm at the enclosure).
Top trace is for an acoustic sensor.
Defect Detectable in
typical En range
Significant
size
Protrusion on hv-cond. ≥ 4 mm 1 – 2 mm
Protrusion on enclosure ≥ 10 mm 4 – 6 mm
Free particles ≥ 5 mm 3 – 5 mm
Particles on spacer ≥ 12 mm 3 – 5 mm
Gas filled ball yes -
Floating electrode yes -
Defects detected at typical nominal field
strength compared with critical size of defects.
Possibilities, features, items of
advanced GIS Implications, benefits
More parameters measured.
History of events can be stored.
Advanced techniques for
measurement.
Prediction on ageing,
planning of replacement,
retrofit.
Trend analyses possible (gas
density etc.)
The condition of the
installation can be predicted.
Prediction of need for
maintenance (condition-based
instead of preventive).
Reduced maintenance costs.
Increased availability.
Reduced outage costs.
(Continued on next slide)
Possibilities, features, items
of advanced GIS Implications, benefits
Prediction of development
requiring corrective action
(service disturbance and
unforeseen maintenance can
be avoided
Reduced maintenance costs.
Increased availability.
Reduced outage costs.
Reduced redundancy of primary
circuit possible – lower initial
costs.
The conditions can be
checked by distance
(teleservice).
Unmanned operation.
Maintenance can be contracted
to a third party.
Reduced maintenance costs.
(Continued on next slide)
(Continued from previous slide)
Possibilities, features, items
of advanced GIS Implications, benefits
Controlled switching.
Simplified circuit breaker.
Reduced breaker wear.
Reduced system transients.
New current and voltage
transformers.
Reduced space requirement.
More flexibility in location of
measurement devices.
Reduced initial and LCC.
Complete factory assembly and
test of GIS bays.
Shorter delivery lead times.
Enhanced features.
Higher quality.
Optimised system solution.
(Continued from previous slide)
Possibilities, features, of
secondary system Implications, benefits
Reduced number of
connections and interfaces
Reduced spaced requirement.
Shorter lead time.
Lower costs of assembly,
engineering and sitework.
Standardised hardware,
flexible software
Lower initial cost.
Shorter lead time.
Increased flexibility.
Increased opportunities for
self-testing
Higher reliability, higher
availability.
EMC precautions Facilitated, lower cost.
GIS monitoring parameters and sensors.
Function: insulation.
GIS monitoring parameters and sensors. Function: switching.
GIS monitoring parameters and sensors. Function: others.
hh hh
Cost implications of application of advanced technologies to GIS
Summary of the ABB approach to quality assurance of GIS
Quality assurance testing.
1. Combination of tests is
frequent during
development.
2. Movement of contacts,
mechanisms.
3. Some tests out of
batches, pressure
vessels etc.
4. Requalification after
some years of
production.
Methods for insulation diagnostics.
Selected diagnostic methods.
• Conditioning procedures in the field have
to be adapted to move particles to
“harmless” locations. UHF PD detection
techniques help in the process.
• It is now recognized that very small voids
in a spacer may not be detectable through
conventional PD detection techniques, but
may give rise to very low probability
breakdowns under VFTO pulses.
• Also spacers are known to acquire surface
charge if nearby corona exists and under
prolonged exposure to DC stress. Even
AC GIS spacers may be exposed to DC
stress due to trapped charges after
disconnect switch operation. One
manufacturer has proposed the use of
weakly conducting surface coatings.
Monitoring and Diagnostics
• The last decade has seen very significant
advancements in monitoring and diagnostic
technology.
• Increased use of fibre-optics for PC based
control and use of Rogowski coils for current
and capacitance dividers for voltage
measurements will become more common.
Similarly, the use of very sensitive pressure
transducers is being developed for PD detection
in GIS. This method has some advantages over
acoustic detectors.
• Considerable discussions are underway to
develop guidelines for incorporating
advanced technologies for monitoring and
diagnostics. Obviously, some simple
questions have to be answered first!
1. Why do it? What is the added value?
2. Criteria for selection
3. Reliability and estimated life
4. Compatibility with other systems
5. Can it be retro-fitted?
• GIS need less maintenance and it is possible to evolve protocols for condition-based and reliability-centred maintenance. Moreover, environmental impact and investment/risk analyses are becoming necessary.
• There is a more to further reduce the size of GIS especially for EHV/UHV range by incorporating one-break gas circuit breakers. For a 500 kV GIS floor savings of 35% and cost reductions of 20-30% are envisaged.
• Other features are: Pockels cell based VT
and tolerating a higher enclosure
temperature.
• R & D effort is underway to replace AIS
with GIS for 500 kV systems and for re-
furbishing 25 year old GIS.
• Both VHF/UHF PD detection techniques
are used for on-site commissioning. One
manufacturer reports that for 72.5 kV to
245 kV GIS either technique may be
used. Particles on spacers are difficult to
detect. Correlation of signal level to pC in
the PD is not possible.
• VHF (30 – 300 MHz) and UHF (300 –
3000 MHz methods detect the TEM, TE
or TM waves generated by PDs. Below
about 300 MHz only TEM mode can
exist. Using detection above 100 MHz
improves signal/noise ratio.
• Signal conditioning techniques are
adopted, for example, filtering to reject
noisy frequency bands, phase locking and
signal integration, gating of noise sources.
• To identify and differentiate between PD
sources the significant parameters are:
1. Peak signal magnitude / RMS of total
signal.
2. Repetition rate.
3. Periodicity of signal groups and phase
angle of individual pulses with respect to
power frequency.
UHF PD Detection in GIS
• Several sensors/couplers are installed in a GIS. Commonly these are capacitance pick-up devices with appropriate electronics to convert PD signals for transmission over a fibre-optic network to a control room.
• Since particles are the most troublesome course of PD and insulation failure and deterioration, below we examine PD signals from fixed and free particles.
• If it is possible to vary the voltage and
conduct visual examination, we can
determine PD inception/extinction
voltages of free particles.
• Fixed particles give PD levels of up to 25
pC. Periodicity may be either equal to
power frequency or double if voltage is
raised.
• Free particles produce PD when they
strike an electrode or a spacer. Usually
PD levels are low and random. At higher
voltages particles cross the gas gap and
give rise to very high PD levels (100 pC
or more) and may lead to breakdown.
• Current UHF detection works in the range
300 – 3000 MHz and has the advantage of
low noise level.
• The cutoff frequencies for a simple
coaxial waveguide with outer radius “a”
and inner radius “a-b” are expressed as:
• Waveguide modes in a GIS. The resonant
frequencies are given as:
• The cut-off frequency for TEM mode is 0.
Hence all higher modes of TEM exist but
get progressively weaker.
• The resonant frequency for TE and TM
are often only 5-10 MHz apart. Full
consideration has to be given to all
TE/TM modes for proper interpretation of
PD measurements.
• Of course, reflections/attenuations have to
be taken into consideration.
Rough representation of a PD pulse in SF6
Four discharges from an impacting A1/0.5/10
particle. Voltage level/phase = 100 kV/086°
(Erms – 6.5 kV/cm at the enclosure).
Multiple discharges from an impacting A1/0.5/10
particle. Voltage level/phase = 120 kV/084°
((Erms – 7.8 kV/cm at the enclosure).
Top trace is for an acoustic sensor.
Measuring result of the frequency response of
a disc sensor (disc radius r = 5 cm, lD = 7 cm).
Calculated frequency response of a disc
sensor (disc radius r = 5 cm, lD = 7 cm).
• Attenuation of signals occurs due to
several causes: losses in the metal
enclosure and the disc/conical spacers.
Enclosure 2dB/km - very low
Spacers 102 dB/m
• A continuous UHF monitoring system is
needed for GIS which are critical for
power supply system security. In the
U.K., several such systems are in
operation. (National Grid Company and
Scottish Power.)
• UHF signals from a set of 3-phase
couplers is cabled to a node for data
acquisition(DAQ). All nodes of the GIS
are linked by a fibre-optic token ring
network, which can control up to 256
nodes. Network operates at 38.4 kB. PD
signal range may be from 1 pC to 1000
pC and the frequency range is 500-1000
MHz.
• The system software can permit “on-line”,
“event” or “history” modes of operation.
Other on-line condition monitoring data
systems can be integrated with the UHF
PD monitoring system (circuit breakers,
disconnect switches, etc.)
Future Trends in GIS Technology
Particle Control and Management
• It is generally accepted that some metallic
particle contamination will always be
present. Moreover, with the rapid growth
of HVDC systems, managing particle
contamination has become even more
critical.
Possible approaches to mitigate the effects of
such contamination are:
1. More stringent manufacturing quality
control.
2. Larger enclosure diameter to reduce the
operating field at the enclosure.
3. Particle traps.
4. Dielectric coatings on the inside surface
of enclosure and on the central conductor.
• We now know a fair bit about these PD sources, e.g.,
• Moving particles produce PD signals in a random relationship to the 60 Hz wave. The magnitude, however, depends upon size and on the applied voltage.
• A fixed protrusion on either conductor or on a spacer will produce corona signals is a known relationship to the power frequency voltage.
• A floating metallic component will also
produce PD signal in a fixed relationship to
the power frequency waveform, but its
magnitude is the highest.
• So the procedure is to record the full frequency
spectrum, pick some suitable PD signal
frequency and establish its relationship with the
50 Hz / 60 Hz waveform.
• Since the UHF signals are being monitored
the attenuation is high - the enclosure skin
effect contributes a lot to this. As a result
couplers have to be installed at a separation
of not more than 20 m.
• 400 kV GIS in UK (Scottish Power and
National Grid) have perhaps 25 to 30 three
phase sets installed. At this number of
couplers the technique becomes comparable
in number of sensors to the acoustic
methods. Mandatory on all new EHV/UHV
GIS. Loss of a 420 kV GIS may trigger
insurance claims.
• Voids in spacers are unlikely to be
detected by any form of PD test on site.
Quality control during manufacture is the
only answer.
• Floating components may arise due to
corrosive action on nuts/bolts and
intermittent sparking under VFTO.
Partial Discharges in GIS • PDs in GIS arise from several sources:
1. Poor or loose electrical/mechanical
contact between conducting parts.
2. Fixed metallic defects on conducting and
insulating surfaces - protrusions, sharp
edges, deep cuts and metallic particles.
3. Moving metallic particles in the GIS
enclosure.
4. Voids in the spacer bulk material.
• The magnitude and the phase angle of
PDs with respect to 60 Hz varies with the
type of defect. The sources listed above
produce PD signals in the descending
order. That is, floating electrical parts
produce the largest PD. Except for the
moving metallic particles, which give
random signals, the other PD signals have
a definite phase relationship with applied
AC voltage.
• In general PD detection methods may be
grouped into four types:
A. Electrical
B. Acoustic
C. Optical
D. Chemical
Of these, the electrical methods offer the
most sensitivity and versatility for
detection and location. The other three
methods can provide additional
information.
• Some of the PD detection methods are
more suitable for type testing or
development testing, for example,
chemical and optical. For monitoring and
troubleshooting in the field, the electrical
and acoustic sensors are commonly used.
• Conventional PD detection, as per IEC
270, works in the range of 10 KHz to 1
MHz. This is unsuitable on site since the
signal/noise ratio is poor.
• Due to the different media the signal has to
travel, many sensors are required for location
of the PD source.
• An acoustic probe is more useful during
conditioning with AC voltage, since a portable
sensor can be moved around to identify the
location. Also phase relationship with 50/60
Hz is helpful.
• Electrical methods can be further sub-divided
into:
– Conventional PD according to IEC 270
– HF couplers to about 10 MHz
– UHF techniques
• In the field the shielding requirements are
difficult to achieve. UHF techniques in this
respect are simpler since the environment
noise is less likely in these high freq. ranges.
• Since the rise-time of a PD signal is very short
(1ns or less), some of the cavity resonances in
the GIS are excited, and the total capacitance
of a GIS is not a determining factor.
UHF Techniques for PD Diagnostics in GIS
Some fascinating and exciting work is being done in this area.
New data analysis techniques are being explored, e.g.
• Pattern Recognition
• Fractals
• Neural Networks
• Ultra Wide Band Recording of PD Signals
Measurement
Discharge Pattern
Feature Extraction
Data Base ----------> Classification
Decision
A General Procedure for PD Diagnostics in Power Equipment
Clearly, our “Decisions” are as good as our
“Data Base”. Lots of experimentation has been
done and a lot more is needed.
Expertise of disciplines new to power
engineering is being brought to bear on GIS
technology.
So, How Good is our Data Base?
• We know some of the most common sources of PD in
GIS, e.g.,
• Metallic Particles - free moving
• Metallic Particles on spacers
• Protrusions on inner/outer conductors
• Void in a spacer
• Floating metal objects
• --------
• --------
• SF6 Related Info.
– Pressure
– Moisture
– Breakdown By-products
• PD Data
• Sensor Locations
• Data Acquisition
• Data Reduction
• Data Analysis Using Present and Historical Data
INTEGRATED CONTROL, MONITORING AND DIAGNOSTIC SYSTEM
What are Detailed Aspects of UHF PD Detection in GIS?
• The Resonant Frequencies
• What Freq. Range you Select?
• What Type of Coupler?
– Internal
– External
– New GIS
– Existing GIS
• Coupler Location
– Signal/Noise Ratio
– Propagation Through GIS
• Software Design
– Customized
– Signal Analysis
– Data Bank
– Expert or Neural Systems
– Calibration
– Comparisons With Other Data
Partial Discharge Testing of GIS
Purpose: • Developmental tests
• Type tests
• Production tests
• Commissioning tests
• Monitoring/Diagnostic
• PD - very early local breakdown of gas. May lead to failure in time. Corona stabilization makes voltage level for PD much lower than that for breakdown, except for LI and VFTO.
• Quality control is essential for all the components that go into a GIS
• Possible techniques are:
– Electrical
– Acoustic
– Chemical
– Optical
• Optical techniques are best suited for the
developmental and type test stage. However,
an adequate number of windows are [is?]
essential for visual checks during service.
• Chemical methods are best suited for the
developmental, type test and perhaps as a
back-up in the field.
• In practice it is the ratio of downstream stable products SO2F2/SOF2 which offers discrimination as to the source of discharges, for example, tests at CESI show:
Phenomena Time SO2F2 SOF2 Ratio
PD 260 hrs 15 ppml 35 0.43
Disconnector Cap. 200 oper 5 97 0.05
Switching 400 oper 21 146 0.14
Cir. Break. 5 oper @ 31kA <50 3390 <0.01
5 oper @ 18kA <50 1560 <0.03
• Currently, in equipment in service the choice
is between the Electrical and Acoustic
methods. Often both are used, since in some
ways they complement each other.
• Acoustic sensors could be either AE type or
Accelerometers. The PD electrical signal to
the resultant acoustic signal have a very
complex relationship, but it is less susceptible
to environmental noise, and is non-invasive.
Section 4
Recent Developments
SF6 - Global Environmental Impact
• SF6 is non-toxic, very stable chemically.
• It is man-made and its lifetime in upper
atmosphere is very long (800 to 3200 years!)
• Currently, 80% used by elec. power industry.
Other uses are in micro-electronics,
aluminum, magnesium production, tracer gas,
nuclear industry etc.
• 7000 metric tons/yr in 1993 - may reach
10,000 tons/yr by 2010. Allowable
concentration 1000 ppm by vol.
Two areas of Health and Environmental impact:
A. Through its normal use in a work place -
ARCING BY-PRODUCTS
B. Global environmental impact - OZONE
DEPLETION & WARMING
Regarding A. the industry is developing stringent
guidelines to protect workers and to
minimize “leaks” into the global atmosphere.
Use of SF6 in Electrical Power Equipment
Health, Safety and Environmental Effects
• By itself SF6 is non-toxic and the TLV level is
about 1000 ppmv.
• However, many organizations require a much
lower level. Three levels of personnel
protection recommended are:
Level of Protection TLV
Low 1000 ppmv
Intermediate 200 ppmv
High 20 ppmv
Codes of practice define the level recommended.
• Breakdown by-products arise, both under
arcing and under low-energy discharges,
such as corona.
• Above ~500˚C SF6 begins to break up and
at ~3000˚C dissociation is complete.
During the cooling period, at ~1000˚C,
many chemical reactions occur. H2O is a
major factor.
• TLV levels for the by-products are
established; there are national variations.
• IEC 1634 lists many of the relevant data
and controversies!
SF6 As a “Greenhouse” Gas
• ozone depletion, and
• global warming.
CFC + (UV) --> Cl + (CFC) Residue
Cl + O3 --> CI0 + O2
Cl0 + O --> Cl + O2
It is the release of Cl that is responsible for O3 depletion. The following relative role is quoted by IEC1624:
CO2 (60%), CH4 (15%), N2O (5%), CFC (12%), SF6 (10-2%)
There is, however, controversy about these figures! (See IEEE Trans. on Dielec. and Elec. Insul.,
Vol 2, No. 5, 1995, p. 953)
• SF6 concentration in upper atmosphere has
doubled in the past decade. Increasing at
~8.7% / year.
• Elect. industry uses ~80% of world
production of SF6 (~7000 metric tons in
1993), and the production is expected to
grow to ~10,000 metric tons by 2010.
• SF6 is 25000x more effective than CO2 as a
“Greenhouse” gas.
Environmental activists, however, argue that
for estimating a worst case impact we must
assume that ALL SF6 will eventually “leak”
into the global atmosphere.
Estimates show that SF6 concentration in
upper atmosphere is rising at 8.7% per year.
Approx. doubled in a decade. Could reach 10
parts in 1012 by vol. by 2010.
SF6 does not deplete ozone - no chlorine in
its structure.
But SF6 is very effective in absorbing (and
reflecting back to Earth) infra-red radiation.
25000x more effective than CO2!
Present contribution of SF6 to global
warming is <0.01%.
If the present usage trends continue SF6
contribution to the “greenhouse” effect could
reach 0.1% by the end of the 21st century.
No reliable estimates of how much actually
leaks into the Earth’s atmosphere. No
inventory check or validation of used gas
stockpile is maintained.
SF6 can be “destroyed” by incineration at
1100˚C in waste disposal plants.
• The nauseating and tissue irritant effects
often cause the most panic and alarm.
• Several absorbants are quite effective:
Alumina, Soda Lime, Molecular Sieves, and
combinations thereof.
• The most common by-products are: SOF2,
SO2, HF, CF4, SF4, SO2F2, plus the various
metal fluorides.
• S2F10 is formed, most likely, in low energy
discharges. However, at above 200˚C it
decays if H2O is present. Although, it is
difficult to detect, there is reluctant
acceptance of its likely presence.
• The accumulated experience with arcing by-
products suggests that the component to
want/monitor is SOF2.
• HF, of course, is highly reactive and hence
corrosive.
SF6 - N2 Mixtures
• SF6 does not occur naturally in the
environment
• 80% of the world production is used by the
electrical industry
• It contributes about .01% to the
“Greenhouse” effect. But its concentration in
the atmosphere is growing very rapidly.
• It is an efficient absorber of infra-red radiation and its global warming potential is estimated to be ~25,000x greater than that of CO2.
• Its atmospheric life is very long - the half-life, i.e., to be reduced to 37% of its original value, is anywhere between 800 and 3200 years.
• So, there is concern in industry about the long-term prospects for its continued use in switchgear and GIS. Hence, the interest in mixtures.
• No other synthetic gas (fluoro-carbons) is better in its environmental impact.
• Abundant data on the two gases and their
mixtures. Reliable production of breakdown
strength in uniform fields.
• Strong synergism between the two gases.
Small quantities of SF6 in N2 can improve
dielectric strength dramatically.
• All of the dielectric strength of SF6, nearly,
can be achieved by adding less than 20% SF6
into N2.
SF6/N2 Mixtures for GIS?
• SF6/N2 mixtures less susceptible to effects
of field non-uniformity than pure SF6, thus
mitigating the effects of particles and
surface protrusions.
• Less is known about dielectric behaviour
above 1MPa (10 atmos.) PD and corona
have not been as extensively studied in
SF6/N2 mixtures as in either gas alone.
• Also, less is known about chemical stability of mixtures under low energy discharges. Little is known about the production rates of S2F10, S2OF10, S2O2F10.
• SF6/N2 mixtures do not have arc quenching properties of SF6 by itself.
Comparative Limiting (E/P) values for SF6/N2 mixtures
SF6 % (E/P) lim kv/cm.bar
100 88.6
73.1 85.1
50 79.0
20 65.2
10 57.0
5 50.0
• SF6/N2 mixtures are less sensitive to
protrusions and surface roughness than
pure SF6, e.g., for roughness higher than
100.
• The corona stabilization effect is not as
pronounced.
• The arc quenching properties of mixtures
are not as good as pure SF6.
• SF6/N2 mixtures are not particularly better
when it comes to arcing breakdown by-
products (SO2, SOF2, SO2F2, SOF4). Even
a low SF6 content (<10%) still generates
these by-products.
Breakdown
Voltages
DC Breakdown Voltage (kV) of SF6/N2 Mixture in Uniform Field Gap
Measured and calculated 60 Hz ac breakdown voltage values for
SF6/N2 mixtures. Similar behaviour is exhibited under lightning
and switching impulse voltages
Field Test of 1000kV Gas Insulated Switchgear
Basic specifications and ratings
Field Test of 1000kV Gas Insulated Switchgear
Field test items on switchgear
Schematic of a DC GIS Insulation Design
RECENT DEVELOPMENTS -CONT.
• Leakage of SF6 <0.5% / yr
• Combined VT/CT
• Single-break CB for 500 kV
• 1100 kV Prototype GIS
• Refurbishing of old GIS
• Replacement of AIS in urban areas
• Mechanical design to allow for SF6/N2
mixtures
• By 1991 - accumulated experience of 200,000 CBB-YRS. The average age is still only 8 yrs.
• Users expectation is a life between 30 - 40 yrs.
• Asset depreciation period used 20 to 50 yrs.
• GIS is expected to have a longer life that AIS
• RE: Maintenance several categories may be defined and equipment classified, e.g.
• Routine inspection • Preventive maintenance
• Repair maintenance • Corrective/special maintenance and component categories may be:
Active or Passive Primary
Secondary equipment
• Most major utilities have codes of practice for delivering maintenance services for GIS
• Life cycle costs have to be evaluated:
LCC = CI + CP + CR + CO + OC + CD
CI: installation (equip. + land + comm. etc.)
CP: planned corrective
CR: repair
CO: operation
OC: outage
CD: decomm.
The First 1000 kV Underground
Transmission Line Babusci et al - CIGRE paper 21-303,1994
(ENEL & Pirelli Cavi)
• Three 600m lengths in Tuscany, SCOF type
design 1250 mm2
• Water cooled through adjacent pipes.
• Power transfer through 2.4 GVA with water
cooling, up to 7 GVA if internal oil-cooling is
added.
• Project initiated 20 years ago. At that time PPL
was not as well established, so Pirelli decided
to use paper, and the internal oil pressure is 1.3
mPa
• The ends are terminated with one SF6 immersed bushing and one outdoor termination.
• About 50% gain in a.c. dielectric strength if oil pressure raised from 0.3 mPa to 1.3 mPa. Impulse strength up by 10% both at room temp. and at 90C.
• Oil duct 40mm, insulation 35mm thick, outer PE sheath dia. 155mm.
• 2.4 MV LI peak; 1.8 MV SS peak.
• Part of a 1000 kV “Pilot Plant”.
Developmental Testing
Elec. - Mech. - Chemical • PD in spacers
• VFTO Effects on Insulation
• Mech. Vibration
• Combined Elec./Mech. Stress in Spacers
• Chemical Corrosion from SF6 Arcing on Spacers and Contact Surfaces
• Particle Dynamics and Control
• Transient Ground-rise Effects on Control Wiring Insulation
INSULATION HAS TO BE DESIGNED FOR
LOW PROBABILITY BREAKDOWN
SPECIALLY UNDER VFTO
• HF and optical techniques
• Advances in nanotechnology for insulating
materials will have major impact on the design
of GIS
Bibliography
General Bibliography
1. Proceedings of the International Workshop on Gas Insulated Substations, 1985,
Toronto, Canada. Pergamon Press, UK, 1986.
2. IEEE Substations Committee Tutorial on GIS/GIL, 2004. Publication #03TP165,
USA.
3. Electric Power Substations Engineering, Editor John D. McDonald, Second Edition,
CRC Press, New York, USA, 2007.
4. Bibliography on Gas Insulated Substations, IEEE Substations Committee Report,
IEEE Transactions on Pwr. Apparatus & Systems, Vol. PAS-96, No. 4, pp. 1280-
1287, 1977.
5. Addendum I To Bibliography of Gas Insulated Substations, IEEE Substations
Committee Report, IEEE Transactions on Pwr. Delivery, Vol. 4, No. 2, pp. 1003-
1020, 1989.
6. Bibliography of Switchgear Literature, IEEE Committee Report, IEEE Transactions
on Pwr. Delivery, Vol. 5, No. 1, pp. 177-188, 1990.
Specific References
1. R. Kurrer, K. Feser, “The Applications of Ultra High Frequency Partial Discharge
Measurements in Gas Insulated Substations”, IEEE Trans. On Power Delivery, Vol.
PWRD-13, pp.893-905 July 1998.
2. J.S. Pearson, O. Farish et al, “Partial Discharge Diagnostics for Gas Insulated
Substations”, IEEE Trans. On Dielec. And Electrical Insulation, Vo. DEIS-2, pp. 893-
905, October 1995.
3. “Diagnostic Methods for GIS Insulating Systems”, Working Group 15.03, Paper
15/23-01, CIGRE Session 1992.
4. C.J. Jones, O. Beierl et al, “Guidelines for Monitoring Control and Supervision of
GIS Incorporating Advanced Technologies”, Paper 23-203, CIGRE Session 1996.
5. Bo H.E. Wahlstrom, Y. Aoshima et al, “The Future Substation – A Reflective
Approach”, Paper 23-207, CIGRE Session 1996.
6. D. Allan, T. Blackburn et al, “Recent Advances in Automated Insulation Monitoring
Systems, Diagnostic Techniques and Sensor Technology in Australia”, Paper 15-101,
CIGRE Session 1998.
7. A. Kaczkowski, W. Knoth, “Combined Sensors for Current and Voltage are Ready
for Application in GIS”, Paper 12-106, CIGRE Session 1998.
8. M.D. Judd, O. Farish, B. Hampton, “The Excitation of UHF Signals by Partial
Discharges in GIS”, IEEE Trans. On Dielec. And Electrical Insulation, Vol. DEIS-3,
pp.213-228, April 1996.
9. T. Hasegawa, K. Yamaji et al, “Development of Insulation Structures and
Enhancement of Insulation Reliability of 500 kV DC GIS”, IEEE Trans. On Power
Delivery, Vol. PWRD-12, January 1997.
10. K.S. Prakash, K.D. Srivastava, M.M. Morcos, “Movement of Particles in
Compressed SF6 GIS with Dielectric Coated Enclosure”, IEEE Trans. On Dielectric
and Electrical Insul., Vol. DEIS-4, June 1997.
11. J.M. Braun, G.L. Ford et al, “Reliability of GIS Epoxy Insulators: the Need and
Prospects for More Stringent Acceptance Criteria”, IEEE Trans. On Power Delivery,
Vol. PWRD-8, pp. 121-131, January 1993.
12. A. Hjortsberg, G. Homstrom, E. Osterlund, “Current Transmission Systems for
HVDC Including a Solid Insulator Having a Surface Coating of Resin Containing
Chromium Oxide or Iron Oxide”, US Patents #4, 688,142,18, August 1987.
13. M. Meguro, K. Katada et al, “Compact GIS in Harmony with Environment and CAD
Evaluating System for 550 kV Substation Design”, paper 23-202, CIGRE Session
1998.
14. W. Buesch, H.P. Dambach et al, “Application of Partial Discharge Diagnostics in GIS
at On-Site Commissioning Tests”, Paper 15-104, CIGRE Session 1998.
15. L.G. Christophorou, R.J. van Brunt, “SF6/N2 Mixtures: Basic and H.V. Insulation
Properties”, IEEE Trans. On Dielectrics and Electrical Insul., Vol. DEIS-2, October
1995.
16. CIGRE WG 23.10, “SF6 and the Global Atmosphere”, Electra, No. 164, February
1996.
Some Recent Publications of Interest
1. W. Xiomek, M. Reformat, E. Kuffel, “Applications of Genetic Algorithms to Pattern
Recognition of Defects in GIS”, IEEE Transactions on DEIS, Vol. 7, No. 2, pp. 161-
168, 2000.
2. C. Beyer, H. Jenett, D. Clockow, “Influence of Reactive Sf6 Gases on Electrode
Surfaces after Electric Discharges under SF6 Atmosphere”, IEEE Transactions on
DEIS, Vol. 7, No. 2, pp. 234-240, 2000.
3. S. Tenbohlen, G. Schroder, “Influence of Surface Charge on Lightning Impulse
Breakdown of Spacers in SF6”, IEEE Transactions on DEIS, Vol. 7, No. 2, pp. 241-
246, 2000.
4. M.S. Indira, T. S. Ramu, “Motion of Conducting Particles Causing Inadvertent
Outages in GIS”, IEEE Transactions on DEIS, Vol. 7, No. 2, pp. 247-253, 2000.
5. P. Maitly, S. Basu et al, “Degradation of Polymer Dielectrics with Nanometric Metal
Oxide Fillers due to Surface Discharges”, and “Improvement of Surface Degradation
Properies of Polymer Composites due to Pre-processed Nanometric Alumina Filters”,
IEEE Transactions on DEIS, Vol. 15, No. 1, pp. 52-62, and pp. 63-72, 2008.
6. S. Okabe, T. Yamagiwa, H. Okubo, “Detection of Harmful Metallic Particles inside
Gas Insulated Switchgear using UHF Sensor”, IEEE Transactions on DEIS, Vol. 15,
No. 3, pp. 701-709, 2008.
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