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.
Why GIS?
Why GITL?
•
Land costs in urban areas
•
Aesthetically “superior” to air insulated substations
•
Not affected by atmospheric pollution
•
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.
Design Features of GIS/GITL •
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
•
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
751
67,405
Total
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
51,078
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 NitrogenUniform 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 restrikes 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_ Z1+Z2+Ze Voltage going out to line is ~ (S2) (Trav. wave)
Fig. 1
where S2 = - 2Zg_ 2Zg+Z3
Fig. 2a
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
VR = discharge from the lightning arrester
Effect of surge arrester at point of GIS/Cable sheath interface
Surface Flashover in Compressed Gases • Air • SF6 • Parallel Plane • Point-to-Plane • Coaxial
GASES
GEOMETRY
• Epoxy • Teflon • Perspex
• • • •
DC 60HZ AC Switching & Lightning Impulse
MATERIALS
VOLTAGE
• Breakdown & Corona Voltage
• Surface Charge • Pre-Breakdown Current Pulses • Particle Contamination
MEAS.
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. 2. 3. 4.
Triple-junction design Tangential vs. normal field at the insulator Surface discharges from partial discharges 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 highvoltage 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
DC Prestressing • 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.
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 fastfronted 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.
Predischarge development at an insulator surface with a disturbance near the cathode. 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 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. 2. 3. 4.
Fixed on phase conductor Fixed on enclosure Free to move in elec. field 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 parallelplane 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. 2. 3. 4.
Chemical etching Sand or glass bead blasting Abrasive finishing using oil oxide paper 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
Why Dielectric Coatings? • • • •
Effect on breakdown Effect on particle charging Effect on maxm. excursion height Particle movement “inhibition” pseudoresonance • Breakdown probability • Experimental results
Insulator and particle trap for CGIT system.
SF6; Teflon; 1.5 mm diameter steel.
Microdischarge 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.
hj
Smoothed curves of lifting field vs. pressure for spherical steel particles 1.5 mm diameter.
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 midterm.
• 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.
CIGRE Survey 2000
Identification of main component involved in the failure from GIS voltage class point of view.
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 of paper multiple impulse results.
∆ - one layer paper multiple FFT data
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 threeconductor 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
Rough characterization of decomposition produces resulting from different sources
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
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 Knowhow 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
Diagram A
planned corre ctly Capital O per.
$
Reliability
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 TEMmode 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
Protrusion on hv-cond. Protrusion on enclosure Free particles Particles on spacer Gas filled ball
Floating electrode
Detectable in typical En range
Significant size
≥ 4 mm ≥ 10 mm ≥ 5 mm ≥ 12 mm yes yes
1 – 2 mm 4 – 6 mm 3 – 5 mm 3 – 5 mm -
Defects detected at typical nominal field strength compared with critical size of defects.
Possibilities, features, items of advanced GIS More parameters measured. History of events can be stored. Advanced techniques for measurement. Trend analyses possible (gas density etc.) Prediction of need for maintenance (condition-based instead of preventive).
Implications, benefits Prediction on ageing, planning of replacement, retrofit. The condition of the installation can be predicted. Reduced maintenance costs. Increased availability. Reduced outage costs. (Continued on next slide)
(Continued from previous slide)
Possibilities, features, items of advanced GIS
Implications, benefits
Reduced maintenance costs. Prediction of development Increased availability. requiring corrective action Reduced outage costs. (service disturbance and Reduced redundancy of primary unforeseen maintenance can circuit possible – lower initial be avoided costs. Unmanned operation. The conditions can be Maintenance can be contracted checked by distance to a third party. (teleservice). Reduced maintenance costs. (Continued on next slide)
(Continued from previous slide)
Possibilities, features, items of advanced GIS
Implications, benefits
Simplified circuit breaker. Controlled switching. Reduced breaker wear. Reduced system transients. Reduced space requirement. New current and voltage More flexibility in location of transformers. measurement devices. Reduced initial and LCC. Shorter delivery lead times. Complete factory assembly and Enhanced features. test of GIS bays. Higher quality. Optimised system solution.
Possibilities, features, of secondary system Reduced number of connections and interfaces Standardised hardware, flexible software Increased opportunities for self-testing EMC precautions
Implications, benefits Reduced spaced requirement. Shorter lead time. Lower costs of assembly, engineering and sitework. Lower initial cost. Shorter lead time. Increased flexibility. Higher reliability, higher availability. Facilitated, lower cost.
GIS monitoring parameters and sensors. Function: insulation.
GIS monitoring parameters and sensors. Function: switching.
hh
hh
GIS monitoring parameters and sensors. Function: others.
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. 2. 3. 4. 5.
Why do it? What is the added value? Criteria for selection Reliability and estimated life Compatibility with other systems Can it be retro-fitted?
• GIS need less maintenance and it is possible to evolve protocols for conditionbased 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 refurbishing 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
Spacers
102 dB/m
- very low
• 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
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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 Low Intermediate High
TLV 1000 ppmv 200 ppmv 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 byproducts 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 longterm 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.
SF6/N2 Mixtures for GIS? • 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 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 % 100 73.1 50 20 10 5
(E/P) lim 88.6 85.1 79.0 65.2 57.0 50.0
kv/cm.bar
•
•
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 byproducts (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. 12801287, 1977. 5. Addendum I To Bibliography of Gas Insulated Substations, IEEE Substations Committee Report, IEEE Transactions on Pwr. Delivery, Vol. 4, No. 2, pp. 10031020, 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. 893905, 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. 161168, 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. 241246, 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.
