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.