Gis

Introduction to Gas Insulated

Switchgear and Substations

Dr. M. Mohana Rao

BHEL Corporate R&D

Hyderabad

E-mail: [email protected]

Cable Technology

• 1960s-1980s: Fluid filled systems for HV and EHV

• 1980s-1990s: Low loss PPL systems to match the

paper laminate’s performance for EHV

• 1970s-1990s: Parallel development of XLPE system from

MV up to EHV 275kV XLPE cables in service.

In Japan 500 kV XLPE cables installed.

• 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 mixtures attempted.

• 1970s-1990s: Low temp. cryogenic/supercon. designs tried.

witnessed the phenomenal growth in HTS

technology.

Power Ratings: for conventional cable

technologies

1. Paper fluid-filled

2. PPL fluid-filled

3. XLPE

Insulation Thickness: for conventional

cables from 1990 to 1998

500kV - from ~35mm down to ~ 25mm



Design Stresses: for conventional cables

Paper - from 10kV/mm to ~15kV/mm

PPL - from ~18kV/mm to ~20kV/mm

XLPE - from ~5kV/mm to ~35kV/mm

[Theoretical maxm. stress in 100% SF6 is

~89kV/cm.bar]

Energy and Industrial Culture

• 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.

• The availability of useful forms of energy is not equal

worldwide, and there are major geographical barriers

to the movement of energy in the world.

• World-wide experience in transporting oil, natural gas

and electricity over long distances (thousands of km)

Present Status of “Conventional”

Cable Technology

• Both oil-paper and polymeric cables up to 500 kV

system voltage are in service and commercially

available.

• Experimental designs of oil-paper cable have been

tested for both 750 kV and 1000 kV.

• Cost differentials for such cable when compared to

overhead lines are in excess of 25:1 (some estimates

put this as high as 40:1).

• Technology of making joints is still in an

experimental/development stage.

• At such high operating voltages the margin to the

high voltage “intrinsic” breakdown is lower. very high

quality control is needed.


Cable Technology• Conventional cable technology is very well established

and over the past 100 years there have been many

technological improvements.

• 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.

• This prospect raises the technological and economic

question of:

How does one move large amounts of electrical energy

to major urban centres?

• Over sparsely populated areas, overhead lines are,

perhaps, the only proven and economic option for

long distances.


Cable Technology

• High temperature superconductor technology is

developing rapidly but [is] not yet fully commercially

viable for bulk power transport.

• 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.

Air Insulated Substation (AIS)

1. Bus bar

2. Circuit Breaker

3. Disconnector (line or bus)

4. Earthing switch (line or bus)

5. Current transformer (feeder / bus)

6. Voltage transformer (feeder/ bus)

7. Feeder Disconnector

8. Feeder Earthing switch

9. Lightning / Surge Arrester

10. Cable termination

11. Control Panel.

GIS ASSEMBLY

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Limitations of AIS

• Large dimensions due to statutory clearances and poor

dielectric strength of air

• Insulation deterioration with ambient conditions and

susceptibility to pollutants.

• Wastage of space above

• Life of steel structures

• Seismic instability

• Large planning & execution time

• Grounding-mat is essential for containing touch and

step potentials

• Hot line washing and regular maintenance of the

substation is essential, requires spares inventory and

man-power.

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The need for GIS

• Expansion / up-rating of existing s/s

• Non availability of sufficient space for s/s

• Difficult climatic and seismic conditions at site

• Urban site (high rise bldg.)

• High altitudes

• Limitations of AIS

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.

Clearances

Phase to ground clearance for 132 kV systems is

~1200 mm in air, compared to 80 mm in SF6 gas at

4.0 bar(g).

This gives a direct reduction in dimensions of the

high- voltage equipment- by 15

Consequently the size of SF6 insulated equipment

is around 6% that of air insulated equipment for this

voltage class (132kV)

Equipment size

To 30% for 33kV and below

To 15% for 66kV

To 6% for 132 kV to 170 kV and

To 4-5% for >245kV

GIS

AIS

AIS and GIS in the same number of bay

Old 400kV AIS Substation

New 400kV GIS Substation

Under refurbishment

After refurbishment (in 2005)

New 400kV GIS Substation

Design Features of GIS/GITL

• GIS/GITL installations have the usual components:

• Circuit breakers; disconnect, earthing/grounding

switches

• Current and voltage measuring devices

• Bus duct sections

• 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 more than two decades. 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 of aluminium / copper. The

outer enclosure is also of aluminium, although

earlier designs used mild steel / Stainless steel. For

lower voltages, stainless steel has also been used.

Typical Cable Section

Growth of GIS

Growth of GIS InstallationsBefore 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

300 kV GIS

CB

CT

Bus

400 kV GIS

VT

CB

800 kV GIS

Cone Insulator

Expansion joint

Concept of GIS

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Design of GIS

HT Conductor Enclosure Cone Insulator

Cross section of a Bus Section

Design of GIS

Minimum Isolating Distances

Critical stress (Ecr) at gas pressure, P

= 5.3(1+0.46P)[1+(1/rc0.5

)*(2.82-2.29/P)] kV/mm

Geometric constant (K) for co-axial geometry

=shell dia. / conductor dia.

Field Factor (Ff) = (K-1)/ln K

Safe Working Stress, Es = Ecr/Ef

Minimum isolating distance (x)

= applied voltage / safe working stress mm

Design of GIS

The gas-insulated equipment works under two major

stresses:

1. Electrical stress

2. Mechanical stress

The electrical design results in basic clearances

between HT conductor and the enclosure. Controlling

of electrical stresses on high voltage electrodes is also

an objective of the electrical design.

The mechanical design comprising of equipment

dimensions, support structures and the operating

systems like drives for switching components.

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Design Philosophy

• Safety

• High reliability

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Safety

• Safety is introduced by optimising operating

electrical stresses to safe levels by better inter-

electrode spacing

• Safety is reinforced by increasing the gas

volume and the thermal inertia of the system to

enhance cooling and retain insulation strength

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High Reliability

• Reliability is ensured

– By superior contact systems for CB, and

disconnectors

– Multi-contact and friction free surfaces

are incorporated for long operating

cycles

– Rugged, time proven operating drives are

used

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In-service fault rate (faults/station-year) vs.

years in service for 25 North American GIS.

Calculated field

gradient:

1, 2, 3, & 4:

230 kV bus

conductors

1’, 2’, 3’ & 4’:

550 kV bus

conductors

insulators of various

design

Designs of cast Epoxy insulators

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.

Full-scale model GIL and insulating system.

Determination of

the diameters of

conductor and

enclosure.

Example of the

construction of

post-type

particle trap.

Required specifications.

Fundamental dimensions and material used in GIL

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 3-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.

12/13/2010 29

Medium Voltage

Gas Insulated Substation Systems

12/13/2010 30

Medium Voltage GIS with

Vacuum Interrupters

ISOLATOR cum

EARTH SWITCH

VACUUM

INTERRUPTER

GAS TO AIR

BUSHING

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GVM-36

Metal-clad and Gas Insulated MV Switch Panel

• Vacuum circuit breaker

• Tubular bus bar

• Isolator cum earth switch

• Gas to air termination

• Cast epoxy CT

• Gas insulated VT

• Cable box

• Bay size 650x1800x2540

• Weight 800 kg.

PRESSURE RELIEF

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Bay formation

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Double bus bay

12/13/2010 34CABLE BOX

BUS BAR

INSULATOR

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GVM 36

Mechanism

CB Enclosure

Cable Connection

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36kV GIS with 3-phase double bus

BUS I BUS II

12/13/2010 37

High & Ultra High Voltage

Gas Insulated Equipment

and Systems

12/13/2010 38

GIS Bay

Single Line Diagram

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145 kV GIS

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300 kV GIS

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Single Line Diagram of a GIS

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Three Phase bus bar for 145 kV GIS

Rib Insulator

Enclosure

12/13/2010 46

12/13/2010 47

Circuit Breaker under SC Test at CESI

Mechanism

Line side

Load side

Housing

12/13/2010 48

ISOLATOR / DS

Fixed ContactMoving Contact

HT shield

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Testing of GIS Modules - DS

• Bus Charging current: The current in rms value which

breaker shall make or break when energising or de-

energizing parts of a bus bar system.

• TD1 : Switching of a very short portion of bus duct.

• AC Voltage: 1.1xVph DC Voltage: -1.1*1.414*Vph

• Bus Transfer Current : This is the current disconnector

shall make or breaks when the disconnector transfers

load from one bus system to another.

• The current is 80% of the current rating or maximum

limited to 1600 A.

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12/13/2010 52

EARTH SWITCH (ES)

12/13/2010 53

• EARTH SWITCH (ES)

Fixed Contact

Moving Contact

ES Housing

12/13/2010 54

Design and development of 145 kV,

40 kA Gas Insulated Earth Switch.

The earth switch developed in

the present contribution can

be used to discharge long

lines/cables basically for

removal of DC voltage /

trapped voltage on the line.

The switch can also be used

to discharge the HT line

before opted for maintenance

of the substation.

12/13/2010 55

Testing of GIS modules -

Earth Switch

• 2 No. of Making operations.

• Verification of per-arc characteristic with-

out s/c current.

• Electromagnetic current 50 A @ 1k V.

• Electrostatic Current 0.4 A @ 3 kV.

• 10 No. make/break operating cycles.

12/13/2010 56

12/13/201057

Gas Insulated

Surge Arrester

33 kV

72.5 kV

145 kV

LT Terminal

52 kV

Surge arrester

12/13/2010 58

Testing of GIS modules - Surge

Arrester

• Dielectric Tests : PD, AC 1 Min. , Impulse

• PD test as per IEC - 60270

• AC and Impulse test as per IEC - 60060-1

• Leakage Current Test.

• Residual Voltage test for Lightning

Impulse wave 8/20 s as per IEC-99-4

• V-I characteristics.

12/13/2010 59

Testing of GIS modules - Surge

Arrester

12/13/2010 60

Technological Advancements

• Fast response surge arresters

– A break-through in SA technology

– Patented arrangement of MOV elements help

reducing the inductance of the system.

– Uniform voltage distribution on elements

(~3-6%)

12/13/2010 61

Current Transformer (CT)

12/13/2010 62

Current Transformer

12/13/2010 63

Voltage Transformer (VT)

Secondary Terminal Box

Barrier Insulator

12/13/2010 64

Cable-to-gas termination

Barrier Insulator

Silicon Rubber

BootXLPE

12/13/2010 65

Cable /SF6 Termination

•Next

12/13/2010 66

GAS-TO-AIR BUSHING

12/13/2010 67

Design and Development of Gas-to-Oil

Bushings 72.5 kV for GIS Application

The developed bushing

has been tested

successfully for an

insulation withstand

level suitable to 72.5

and 110 kV GIS. The

equipment has been

withstood at 40 kA for

the time duration of 1

sec with peak current

magnitude of 100 kA.

The development is first

of its kind in India.

12/13/2010 68

Pre-Fabricated Cable Connection for

145 kV Sub-station application

Support Insulator Pre-Fabricated Cable connection

12/13/2010 69

12/13/2010 70

400 kV GIS Test set-up

under HV Test.

12/13/2010 71

400 kV GIS CB Assembly

under HV Test.

12/13/2010 72

400 kV GIS DS/ES

Assembly under HV Test.

12/13/2010 73

• Current Transformer (CT)

400 kV Gas Insulated Transmission Line

(GITL)

Rated voltage = 420 kV (rms)

Rated Current = 2500 A

Frequency = 50 Hz

Impulse withstand Voltage = 1425 kVp

Power Frequency withstand Voltage = 630 kV (rms), 1 Min.

Rated Short time current = 40 kA

Advantages:

The charging current is substantially

reduced.

The dielectric losses are negligible.

The comparatively large diameters of

conductor and enclosure result in low

resistive and heat losses.

The power carrying capability is more than

conventional lines at a particular system

voltage.

The configuration provides highly effective

screening.

12/13/2010 76

TEST FACILITIES

12/13/2010 77

Cascade Transformer : 500 kV(rms), 1 Amp

Impulse Generator

12/13/2010 78

Gas Filled Test Transformer: 325 kV

Test Transformer

Test Object

12/13/2010 79

Computerized Discharge Analyzer (CDA - 3)

PD level (min- max, current & power)

PD Count, Repetition rate,

Phase-angle, Q-rate and Phase plots

12/13/2010 80

Acoustic Insulation Analyzer (AIA)

Assembly of acoustic insulation analyzer on

grounded GIS enclosure.

12/13/2010 82

Proof tests on GIS Enclosure

12/13/2010 83

Proof test on GIS Enclosure

• Gas insulated modules with welded steel

enclosures tested for 2.3 x normal pressure.

• Record of strain.

• No distortion in shape of enclosure.

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Arc Fault tests on GIS Enclosure

12/13/2010 85

Arc Fault test on GIS Enclosure

• Arc fault test at STC rating on GIS enclosure as per

IEC-62271-203.

• Arc fault test at 0.1 sec and 0.3 sec for 40 kA

and above.

• Arc fault test at 0.2 sec and 0.5 sec for 40 kA

below.

• Operation of pressure relief in first stage of

protection.

• No-fragmentation in second stage of protection.

12/13/2010 86

Certification of GIS modules for Internal Arc

Fault at 40 kA.

1. The test has been conducted in two phases, confirming the

main and back-up protection as per IEC 62271-103.

2. For the time duration of main protection (< 0.1 sec), no

external effects of the enclosure except the operation of suitable

pressure relief devices.

Importance:

The main aim of the

present evaluation is to

provide high protection to

the operating personnel at

the substation. This is

possible by limiting the

external effect of the arc to

the appearance to a hole

or a tear in the enclosure

without any fragmentation.

0.1 sec, 40 kA

12/13/2010 87

Certification of GIS modules for Internal

Arc Fault at 40 kA.

1. For the duration of back-up protection (< 0.3 sec), no fragmentation

except burn- through of the enclosure.

2. Certification of GIS modules for the arc fault test enhances the

business potential of the 145 kV GIS equipment. Now, BHEL became

one of a few GIS suppliers, who can offer the equipment with this

certification.

0.3 sec, 40 kA

12/13/2010 88

Site Testing of GIS

12/13/2010 89

Site Testing as per IEC-

60517

• Power Frequency test at 80 % of test voltage level for 1

min or impulse test at 80 % level.

• Power frequency test at 100 % of test voltage level for 1

min across isolator contacts or impulse test at 100 %

level.

• For 245 kV and above PD test as per standards IEC-

60270 : Conventional, VHF/UHF, Acoustic methods.

• AC and Impulse test as per IEC - 60060-1.

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Site Testing as per IEC-

62271-203

• Power Frequency test voltage at 1.1 times phase

voltage or 1.9 times phase voltage for at least 10 min.

• Power Frequency test voltage at phase voltage or line

voltage for at least 30 min.

• System is preferred to test as sections.

• DC voltage test on GIS is not preferable.

Insulation Reliability - Challenges

In view of high reliability requirement of GIS technology,

both manufacturers and users have to be aware of

certain HV insulation problems inherent in the GIS

design. These are:

• Reliability of support spacers.

• Generation of VFTO by disconnect switch operation.

• Contamination of SF6 gas by metallic particles.

• Arcing/discharge by-products in SF6.

• Environmental “green house” effects of SF6.

Insulation Reliability - Challenges

•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.

Design Principle

The electrostatic 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

Surface Flashovers in GIS

• Air GASES

• SF6

• Parallel plane

• Point-to-plane GEOMETRY

• Coaxial

• Epoxy

• Teflon MATERIALS

• DC

• 50 / 60 HZ AC VOLTAGE

• Switching and lightning

• VFTO

• Breakdown and corona voltage

• Surface charge MEAS.

• Pre-breakdown current pulses

• Particle contamination MEAS.

Three phase Bus bar system

12/13/2010 7

Three Phase bus bar for 145 kV GIS

Rib Insulator

Enclosure

12/13/2010 8

Technological Advancements

• Support Insulators specially designed to

avoid external pipe connection for gas

– Cone Insulators

• Non-communicating

• Communicating

– Rib Insulators

• For 3-phase systems

12/13/2010 9

Cone Insulators

245 kV GIS

Rib-Type

(145 kV GIS)

420 kV GIS

145 kV GIS

Critical Problems

1. Triple-junction design

2. Tangenital 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 rare- but the

material does age.

• Economic pressure to reduce spacer dimension since this

will affect the enclosure diameter.


• 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).

• Another factor is the reduced margin between BIL and

operating stress as the voltage class becomes higher.

• PD detection requires increasing detection sensitivity as

the spacer size increases with voltage level of GIS.

• For example, 550 / 800 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.

• 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).

• 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.

• 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.

• The question of trapped DC charge on a GIS bus bar

should not be ignored. 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.

Schematic diagram of electrode geometry with insulator.

Typical sequential variations

of the breakdown voltage of

a coaxial conductor without

and with a 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 bus bar of GIS there could be trapped charge

after disconnect switch operations. The electrical

stress created by these charges can lower the

withstand voltage.

Effect of Trapped charge

• From extensive research work, it was found the surface

charge accumulated on the spacer surface after applied

impulse voltage or DS operation.

• The application of DC pre-stressing will approximate

conditions resulting from disconnect operation or

lightning/switching surge. 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.

Trapped charge – DC prestressing

Test Model and Experiment Set Up

The Test Circuit

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 glow

of flashover on insulator is at somewhere between two

electrodes. There would be local field enhancements at

several places. It is not 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 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.

Trapped charge : DC pre-stressing

Comparison of streak image of surface flashover and gap

breakdown in the air. (a) gap breakdown, (b) surface

flashover

A streak photograph of the surface flashover

before insulator pre-charging.

Pre-discharge development in SF6.

t=0 is the start of the voltage breakdown at the

gap.

Pre-discharge development at an insulator surface

with a disturbance near the anode.

It is evident that the pre-discharge formation occurs in

the space between disturbance and cathode. As the

discharge proceeds in cathode direction the gap between

anode and the disturbance in this case is also bridged

simultaneously.

It is evident that the pre-discharge formation occurs in

the space between disturbance and anode. As the

discharge proceeds in cathode direction the remaining

gap between cathode and the disturbance in this case is

bridged very late.


with a disturbance near the cathode.


with a protruding disturbance near the anode.


with a protruding disturbance near the cathode.

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

• Fixed or free particles

Particle Control During Manufacture and

Assembly (GIS/GITL)

• 80-85% of surface area is due to the inside surface of

the enclosure. This is not easy to clean.

• Enclosures are normally extruded Al. or steel tubes

• Manufactured surface finish is limited.

• 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 is different under DC, AC

and Impulse voltages.

Effect of particle / wire length

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.

Hjk

L;l

L;l

L;l

H

L

L

L

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).

Video shots of

particle motion.

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.

Flexible CGIT system with double corrugated conductor

and injection molded insulators

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 maximum 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 dia. 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.

Effect of coating

on Lifting field of

particles.

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.

12/13/2010 33

Dielectric Coating of HT conductors

and enclosures

12/13/2010 34

Dielectric Coating of HT conductors and

enclosures

Smoothed curves of

lifting field vs.

pressure for

spherical SS particles

1.5 mm diameter.

h j

Effect of PD for different particles

Very Fast Transient Over voltages

(VFTO) and Transient Enclosure

Voltages (TEV) During GIS Operation

Dr. M. Mohana Rao

BHEL Corporate R&D

Hyderabad


What is VFT?

In a GIS, Very Fast Transient Over voltages (VFTO) are generated

mainly due to switching operations.

The voltage collapse across switching contacts takes place in 3 to

20 ns depending on breakdown voltage, electric field non-

uniformity and operating gas pressure.

The short-rise time pulse (i.e., voltage collapse) starts at the

switching contacts that propagate along the gas insulated bus

sections/components and take reflections at different terminations.

Because of superposition of the original pulse with the reflected

pulse, VFTO are developed.

The waveform of these transients depends on the configuration of

the GIS. The VFTO levels are found to be on the higher side for the

following conditions of the switching configurations:

(1) Small length of bus sections on the load side of the switch.

(2). High surge capacitance components on source side of switch

(3.0 P.u.).

DS Operation

GENERATION OF VFTO

Opening Operation

Load

voltage

Source

voltage

Why VFT is a Problem?

12/13/2010 6

Earthing of GIS

GIS Earthing is possible in Two ways:

1. Single Point Earthing

2. Multi-point Earthing.

In single point earthing each enclosure is isolated from

next one and grounded each enclosure at only one

point so that no loop currents.

In multiple point earthing enclosures are electrically

connected and grounded at many locations. In addition

the enclosures of the different phases are connected by

shunts.

12/13/2010 7

Earthing of GIS

single point earthing

multiple point earthing

12/13/2010 8

Earthing of GIS

1. Return

Conductors

2. Earthing

conductors

3. Return and

Earthing

Conductors

4. GIS earthing

mat.

12/13/2010 9

Earthing of GIS

Multipoint Earthing is advantageous than single point

earthing because of the following:

• Reliability of grounding

• Low magnetic field intensity outside the enclosure

• VFT related Flashovers Can be controlled.

12/13/2010 10

Earthing of GIS

In Multipoint Earthing the following aspects are Important:

1. Only small portion of the return current flow through the

earthing conductors.

2. The current induced in the enclosure could be up to 90 to

95 % of the rated current.

3.To avoid excessive currents in grounding grid (earthing

net) the enclosures are connected by inter-phase shunts.

4. Due to multiple eathing connections loops are formed

which carry very high induced currents due to strong

electromagnetic field coupling and low impedances.

VFTO LEVELS?

VFTO – SECONDARY BREAKDOWN

Characteristics of VFT

Frequency Components of VFTOs in GIS

VFTC LEVELS ?

Coupling Phenomena of VFT

Why TEV IS A CONCERN?

Internal voltage collapse produces travelling waves,

in both directions, from the point of breakdown.

Such transients are often called VFTO (very fast

transient over voltages). At the points of

discontinuity (changes in surge impedance) these

VFTO waves get reflected and refracted. Such

transitions can be modeled as junctions of

transmission lines.

Being high freq. transients, the currents are confined

to the “skin depth” of the coaxial conductors.


The very fast transient over voltages and the

associated transient currents generated in gas

insulated section propagates partly to the overhead

transmission line and partly to the exterior surface of

the bus section enclosure.

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. Out of these two, gas-to-air bushing is the

most significant one.

The transient voltages that appear on the exterior

surface of the enclosure during switching operations or

earth faults is known as Transient Enclosure voltages

(TEV) or Transient Ground Potential Rise (TGPR).

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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.

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TRANSIENT ENCLOSURE VOLTAGES (TEV)

GIS enclosure is electrically continuous.

GIS enclosure is grounded at several points.

Induced magnetic field on the metallic structures and the

control circuits are reduced due to the flow of major

portion of return current in the GIS enclosure.

TEV Levels in 800 kV GIS MODEL

Application of ZnO arresters or surge

capacitors at the discontinuities of

electrically small insulated flanges,

GIS enclosures, GIS and connected

equipment enclosures.

Using low impedance copper strips

for shorting of the enclosures.

By using ground strips with large

perimeter (strips of rectangular

bars) to limit high frequency

impedance.

By using multiple ground wires at

the discontinuities like gas-to-air

bushing.

By decreasing the height of the enclosure above the earth surface.

• Support spacer flanges can also act as sites for reflections.

• 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.

Propagation of surge down ground connections

GIS data:.

Z1 = 60 - 450Ω

Z2 = 350 - 260Ω

Ze = 200 - 90Ω

Zg = 150 - 300Ω

Assessment of surge propagating beyond GIS

• For a bushing transient the TEV

~ (S1) (Trav. wave)

where S1 = - _2Ze_

Z1+Z2+Ze

Voltage going out to line is ~ (S2) (Trav. wave)

where S2 = - 2Zg_

2Zg+Ze

Zg = surge imp. of ground connection

Ze = surge imp. of enclosure

S1 = 0.54 to 0.78

S2 = 0.54 to 0.75

Note: Significant over voltages can develop on the

enclosure!

Over voltages on enclosures associated with a cable

termination

Transient Ground-Rises in GIS

(For earthing practices in GIS installation see: W G

21.03 Rep. in Electra, No. 151, Dec., 1993, PP. 31-

52)

Operational Experience with GIS/GITL

• Reliability of support spacers

• Very Fast Transient Over-voltages (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

CIGRE Survey 2000: Voltage classes

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 technology to monitor parameters

Users’ opinion on continuous vs. periodic

Distribution of major failure causes reported by users for

all voltage classes.

A Statistical Study on GIS failures

Major failure

frequency by

voltage class.

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Note

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 composite electrode system

A measured fast front step waveform

A fast front breakdown of oil and paper.

12/13/2010 17

Operation of 145 kV GIS at

APTRANSCo

12/13/2010 19

POWER MODE DEMONSTRATION OF 145 kV

GIS @ RC PURAM SUB-STATION

Pre-fabricated Cable Connection

145 kV GIS

Control Cubicle

12/13/2010 20



145 kV GIS

Load Line

12/13/2010 21



Bus BarT-Isolator

VT

12/13/2010 22

145 kV GIS

12/13/2010 23

0 500 1000 1500 2000 2500

0

100

200

300

TIME, HRS

CURRENT, A

VOLTAGE, kV

POWER, MVA

145 kV GIS @ RC PURAM substation Loading Pattern

Environmental Concerns with SF6

Usage and SF6-N2 Mixtures

Dr. M. Mohana Rao

BHEL Corporate R&D

Hyderabad


Arcing and Discharges in GIS SF6

Insulation and Handling of By-

products

ProductApprox. 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 productstoxicity

(weighted)

reactivity

with

atmospheric

humidityformula 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

humidityformula 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.

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 - reached 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

• 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:

Low 1000 ppmv

Intermediate 200 ppmv

High 20 ppmv

• 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.

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 IEC61624:

CO2 (60%), CH4 (15%), N2O (5%),

CFC (12%), SF6 (10-2%)

SF6 does not deplete ozone - no chlorine in its structure.


• 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.


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.


• 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.

• The nauseating and tissue irritant effects often cause the

most panic and alarm.

• Several absorbents 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.

Effective ionization “co-efficient α” as a function of

electric field strength and pressure.

The table shows the values of relative electric strengths

measured at pressure indicated (mm Hg) relative to air at

the same pressure.

The measurements were made using two polished brass

spheres of diameter 1 inch contained in a glass cell which

could be evacuated. The spark gap was generally 0.015 to

0.020 inch. The apparatus was checked frequently by the

measurement of the relative electric strength of SF6. The

average value of this was 2.5.

Values of Relative Electric Strengths

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

Values of Relative Electric Strengths

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. Equipment with 20% SF6 is on the market.

• So, there is concern in industry about the long-termprospects for its continued use in switchgear andGIS. Hence, the interest in mixtures.

• No other synthetic gas (fluoro-carbons) is better inits 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.

SF6/N2 Mixtures for GIS?

Data Required in the following areas:

• Less is known about dielectric behavior above1MPa (10 atmos.) PD and corona have not been asextensively studied in SF6/N2 mixtures as in eithergas alone.

• Also, less is known about chemical stability ofmixtures under low energy discharges. Little isknown about the production rates of S2F10, S2OF10,S2O2F10. Even a low SF6 content (<10%) stillgenerates these by-products.

• SF6/N2 mixtures do not have arc quenchingproperties 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

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DC Breakdown Voltage of SF6/N2 Mixture in Uniform Field Gap

Measured and calculated 60 Hz ac breakdown voltage

values for SF6/N2 mixtures. Similar behavior is exhibited

under lightning and switching impulse voltages

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.

Emerging Trends in GIS/GITL Technology

• 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 centers.

• Replacement of existing AIS by GIS will accelerate,

especially near urban centers.

New Developments

• UHF partial discharge detection

• HVDC GIS

• SF6/N2 mixtures

• Long GITL installations

• Compact substations

GIL/GIS Recent Development

• 70m long prototype for 400 kV system with 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.

GIL/GIS Recent Development

• 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.

Recent Developments

• Leakage of SF6 <0.5% / yr

• Combined VT/CT

• Single-break CB for 550 kV

• 1100/1200 kV Prototype GIS

• Refurbishing of old GIS

• Replacement of AIS in urban areas

• Mechanical design to allow for SF6/N2 mixtures

• 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 fordelivering maintenance services for GIS

• Life cycle costs have to be evaluated:

LCC = CI + CP + CR + CO + OC

CI: installation (equip. + land + comm. etc.)

CP: planned corrective

CR: repair

CO: operation

OC: outage

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

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

Special Studies In GIS

• Insulation has to be designed for low probabilitybreakdown specially under VFTO.

• Optical techniques for Current / Voltage sensing

• Advances in nanotechnology for insulatingmaterials will have major impact on the design ofGIS

12/13/2010 14

Highly Integrated Gas insulated Substations (HIS)

Hybrid Gas Insulated Substations (H-GIS) is combination of Air

Insulated substation (AIS) and gas insulated substation (GIS). The

technology of H-GIS is based on the concept of combining the

advantages of AIS and GIS.

Technology Type Termination Space Reqt.

AIS Outdoor Porcelain / Composite

clad insulator

High

GIS Indoor/

outdoor

Gas-to-air termination Low

H-GIS Outdoor Gas-to-air bushing Medium

12/13/2010 15


The advantages of H-GIS are as follows:

1. High reliability same as Gas Insulated substations.

2. Extension can be with ease compared to AIS and GIS.

3. Even though H-GIS occupy more space compared to GIS, it requires

less space compared to AIS. The reduction is about 40 % in EHV

level (420 -765 kV).

4. Flexible combination of Unit module enables any lay out. This

facilitates future extension just by addition of unit module in no time.

Cable and termination costs can be cut as they are in outdoor yard

and can be directly connected to transmission lines.

5. Viable as stand-alone modules, no interconnections required from

one bay to another and can be oriented anywhere in substation yard.

Easy retrofits.

6. Utility can choose the limited portions which need to be enclosed in

gas, based on criticality of insulation.

7. Building or basement cost not required.

12/13/2010 16


12/13/2010 17

Highly Integrated Switchgear

12/13/2010 18

12/13/2010 19

Hybrid GIS

12/13/2010 20

12/13/2010 21

Composite insulator- weight comparison

Gis

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