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CHAPTER-I

INTRODUCTION

The development and design of compressed Gas Insulated

Substations (GIS) and compressed Gas Insulated Transmission line

(GITL) equipment have progressed drastically for the last three

decades throughout the world because of the excellent insulation

properties of Sulphur hexafluoride (SF6) gas [1]. The Gas Insulated

Substations equipment such as compact substations and

transmission lines is rapidly becoming an important part of high

voltage transmission systems and they are used in key positions in

the power transmission network, where there is insufficient space for

an open air substation, or the land costs are prohibitive. Because GIS

occupies less space compared to conventional Air Insulated

Substations (AIS), in our country, a few GIS units have already been

in operation and a large number of units are under various stages of

installation.

Demand for electric power has become one of the major challenges

for developing countries. In developing countries like India most of the

additional power demand has been met by conventional electric

sources. But, conventional air insulated substations have many

problems such as pollution by salt or dust, meteorological difficulties,

safety etc. Air Insulated power transmission and distribution

substations also suffer variations in the dielectric capability of air to

withstand varying ambient conditions and deterioration of the

exposed components due to oxidization and the corrosive nature of

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the environment. The size of the sub-station is also substantial due to

the poor dielectric strength of air. Hence there is a need to replace the

conventional transmission lines and substations with under ground

cables and Gas Insulated Substations (GIS). Gas Insulated

Substations which are totally enclosed substations are recent

additions to power transmission systems and distribution networks.

Components of the substation are totally enclosed, sheltered from the

elements and if properly designed and built, unaffected by ageing,

thus increasing reliability and requiring little maintenance. Rapid

urbanization and overgrowing population is making the task of

expanding transmission network very difficult due to right way of

problem and limited space availability. Gas Insulated Substations

prove a viable alternative. Due to its many advantages, most of the

utilities and industrial units are opting for Gas Insulated Substations

(GIS) [1]. However these substations suffer from problems that include

generation of over voltages during switching operations like enclosure

faults and the important problem of particle contamination.

Hot line washing and regular maintenance of the substation is

essential, for which it requires spares inventory and man-power in

case of Air Insulated Substations .But, once the GIS has been

installed, it is completely maintenance-free. In Air Insulated

Substations clearance required is more due to poor dielectric strength

of the air; as the dielectric strength of SF6 gas is higher than that of

air, the clearances required are smaller in GIS. Hence the overall size

of each equipment and the complete substation can be reduced to as

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low as 10% of that of conventional air insulated substation.

Conventional air insulated substations have also been suffering from

problems such as pollution by salt or dust, meteorological difficulties,

safety etc., hence GIS has proven to be best alternative in those areas.

Air insulated power transmission and distribution substations

suffer variations in the dielectric capability of air to withstand varying

ambient conditions and deterioration of the exposed components due

to oxidization and the corrosive nature of the environment. The size of

the sub-station is also substantial due to the poor dielectric strength

of air[2]. In order to enhance the life and reliability of a power

transmission and distribution sub-station, it is desirable to protect

the sub-station components from a corrosive and oxidizing

environment. Metal encapsulation of the sub-station elements

provides a simple and effective solution to the problem of durability of

the substations. The use of a bus duct, with pressurized nitrogen gas,

is a good example of devices with metal encapsulation used in power

sub-stations. The size of the container is a direct function of the

dielectric strength of the insulating medium. The container/enclosure

sizes are thus large with a poor insulation like air or nitrogen.

The use of a gaseous medium with higher dielectric strength like

sulphur hexafluoride (SF6) instead of air helps in manifold reduction

in the size of the sub-station components. The grounded metal

encapsulation, on the other hand, makes the equipment safe, as the

live components are no longer within the reach of the operator. The

electric field intensity, at the enclosure surface, is reduced to zero as

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the enclosure is solidly grounded. Using this design philosophy, sub-

station/switchyard equipment, like circuit breakers, disconnectors,

earth switches, bus bars and instrument transformers (both current

and voltage), have been metal-encapsulated or metal-enclosed and

pressurized with SF6 since 1968. The assembly of such equipment at

a sub-station is defined as Gas Insulated and Metal Enclosed System

(GIMES) by the International Electro technical Commission (IEC).The

equipment is popularly known as a Gas Insulated Sub-station (GIS)

system.

The introduction of SF6 gas has revolutionized not only the

technology of circuit breakers but also the layout of substations. The

usefulness of SF6 gas is mainly due to its High dielectric strength,

unique arc quenching ability and Good thermal stability and

conductivity. Sulphur hexafluoride (SF6) is a man-made gas that

became commercially available in 1947[1]. Now a days, it is one of the

most extensively and comprehensively studied molecular gases largely

because of its many commercial and research applications. Besides

the use of SF6 by the electric power industry, other uses include:

semiconductor processing, blanket gas for magnesium refining,

reactive gas in aluminum recycling to minimize porosity, thermal and

sound insulation, spare tires, aero plane tires, “air sole” shoes, voice

communication, atmospheric trace gas studies, wind supersonic

channels and insulation for AWACS radar domes. Its basic physical

and chemical properties, behavior in different types of gas discharges

and uses by the electrical power industry have been investigated

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broadly [3-7]. In its normal state it is, thermally stable, chemically

inert nonflammable, non-explosive and non-toxic. Because of its

relative inertness and nontoxic characteristics, it is generally assumed

to be an environmentally safe and acceptable material in the sense

that it does not interact unfavorably with the biomass.

Sulphur hexafluoride shows several properties that make it

suitable for equipment used in the transmission and distribution of

electric power. Both at room temperature and at temperatures well

above ambient, SF6 act as a strong electronegative (electron attaching)

gas. This principally accounts for its relatively high dielectric strength

and better arc-interruption properties. The breakdown voltage of SF6

is higher i.e., nearly three times higher than that of air at atmospheric

pressure [6]. In addition, SF6 gas has good heat transfer and

insulating properties., when contained has a relatively high pressure

at room temperature. The pressure required to liquify SF6 at 21oC is

about 2,100 kPa [5-9]; its boiling point is reasonably low - 63.8oC, and

allows pressures of 400 kPa to 600 kPa. It is easily liquified at room

temperature pressure, allowing for compact storage in metal cylinders,

there is no handling problems, it is readily available and reasonably

inexpensive. The current price of SF6 for quantity purchases is about

1.2 lakh rupees per cylinder of 50kg. The electrical industry has

become familiar and experienced in using SF6 as electrical equipment.

The usefulness of SF6 gas is mainly due to its,

1. High dielectric strength

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2. Unique arc quenching ability

3. Good thermal stability and conductivity.

In addition to excellent electrical and thermal properties; at normal

temperatures, SF6 gas is also chemically inert, nonflammable, non

corrosive and non toxic.

Generally, there are four major types of electrical equipment that

use SF6 for insulation and/or interruption purposes: Gas Insulated

Substations, Gas Insulated Circuit Breakers, Gas Insulated Bus

duct/Gas Insulated Transmission Lines and Gas Insulated

Transformers. It is estimated [10-12] that for these applications the

electric power industry uses about 80% of the SF6 produced

worldwide, with circuit breaker applications accounting for major

component of power transmission and distribution systems all over

the world, and it employs SF6 almost exclusively. It offers significant

savings in land use, is aesthetically acceptable, has relatively low

radio and audible noise emissions, and enables substations to be

installed in cities very close to the loads.

The increased application of SF6 in Gas Insulated Switchgears/

Substations (GIS), Gas Insulated Cable, electrical accelerators and X-

ray equipment etc. have led to growing concern for investigating the

mechanism of SF6 decomposition and the effects of decomposition

products. In the presence of corona, spark breakdown and electric

power arc, SF6 decomposes into lower oxy-fluorides of Sulphur. These

may react with the electrodes or gas impurities or other solid

dielectrics to form a number or chemically active products. Although

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SF6 is chemically inert and nontoxic, the decomposition products of

SF6 are known to be toxic and corrosive. The accumulation of

decomposition products in the equipment has caused concern

regarding personal safety and material compatibility [1,6,14].

The decomposition of SF6 is greatly influenced by gaseous

impurities. In industrial grade of SF6 the typical impurities are CF4,

N2, O2 (air) and H2O. The gaseous impurities are generally introduced

during filling and partly due to the decomposition of moisture into the

dry SF6 after filling. A survey of major North America utilities revealed

that the average air concentration in SF6 compartments is 500 ppmv

(parts per million volts) and the average moisture content is about 500

ppmv. In practice, GIS environments having the presence of such

impurities are unavoidable.

The Various modules of GIS are factory assembled and are filled

with SF6 gas at a pressure of about 0.3 Mega Pascal's (MPa)) to 0.6

MPa. They are taken to site for final assembly, such substations are

compact and can be installed conveniently on any floor of a multi-

storeyed building or in an underground substation. As the units are

factory assembled, the installation time is substantially reduced. Such

installations are preferred in cosmopolitan cities, industrial

townships, etc., where cost of land is very high and higher cost of SF6

insulated switchgear is justified by saving due to reduction in floor

area requirement. They are also preferred in heavily polluted areas

where dust, chemical fumes and salt layers can cause frequent

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flashovers in conventional out door air insulated substations

[5,15,19].

A Gas Insulated Substation is a compact, multi component

assembly enclosed inside a grounded metallic encapsulation, which

shields all energized parts from the environment. GIS are available

internationally, covering the complete voltage range from 11 kV to

800kV. The thermal current-carrying capacities and the fault-

withstanding capabilities are tailored to meet all the sub-station

requirements[6,8]. More than 100,000 GIS bays have been in service

all over the world since the introduction of such sub-station systems

in the transmission and distribution field.

A Gas Insulated Substation due to its modular construction and

compact nature is a viable alternative to the conventional air

insulated substation. It requires minimum floor space and building

volume. It is suitable for both indoor and outdoor applications. Due to

its enclosed construction, Gas Insulated Substation is operable under

wide range of environmental and operating conditions. Since modules

are pre-fabricated, installation of Gas Insulated Substation at a site is

a quick process involving only an assembly of modules. Maintenance

costs are also reduced. Due to high reliability of the equipment, Gas

Insulated substations can be used for a longer time without any

periodical inspections.

1.1 CLASSIFICATION OF GIS

Gas Insulated Metal-enclosed Substation systems are classified

according to the type of modules or the configuration. The following

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configurations have been evolved over the years and are generally

used:

• Isolated-phase (segregated phase) module

• Three-phase common modules

• Hybrid modules

• Compact modules

• Highly integrated systems

The isolated-phase GIS module consists of an assembly of

individual circuit elements like a pole of a circuit breaker, a single pole

dis-connector, one-phase assembly of a current transformer etc. A

single-phase circuit is formed by using individual components and

pressurizing the elements with gas forming a leak-free gas circuit.

Three such circuits, arranged side by side, form a complete three-

phase GIS bay. The circuits, since assembled individually, require

larger bay width as compared to the other GIS configurations.

In hybrid systems, a suitable combination of isolated-phase and

three-phase common elements is used like three-phase busbar and

single-phase elements, to achieve an optimal techno-commercial

solution. While the three-phase common busbar system simplifies the

connections from the busbar, the isolated-phase equipment prevents

phase-to-phase faults in active modules like the circuit breaker.

Savings in terms of space vary with the design and configuration of

the section.

Hybrid GIS technology has gained popularity, especially in the

medium and high voltage range, where advancements in technology

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have helped to reduce the sub-station size. With hybrid design, it is

possible to construct techno-economical sub-stations providing

additional flexibility for maintaining and expanding sub-stations in

future at lower costs. In compact systems, there is flexibility of

horizontal expansion (same voltage class), while replacement of the

total equipment is necessary for vertical expansion (higher voltage

class).

Compact GIS systems are essentially three-phase common

systems, with more than one functional element in one enclosure. A

single enclosure, housing a three-phase circuit breaker, current

transformer, and earth switches, supports the busbar and the other

feeder elements. The depth of the section is considerably reduced in

this configuration as compared to the three-phase modules. A total

reduction of 47 percent in the equipment area is possible by using

this configuration [16]. Highly Integrated Systems (HIS), introduced in

the year 2000, are single unit metal encapsulated and gas insulated

sub-stations gaining user appreciation as this equipment provides a

total sub-station solution for outdoor/yard sub-stations. The

foundation work is limited to just one equipment, resulting in saving

of substantial installation time.

These units are directly connected to the overhead lines. The

incomer and feeder side connections (bushings) are directly mounted

on the metal enclosure in this system. It is a ready-to-install sub-

station, with pre-defined circuit elements housed, sealed and

pressurized in a single enclosure. A different version of one-unit G1S

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is also available as a Plug and Switch System (PASS) [18]. This variant

can similarly be installed as a single unit, replacing the existing

identical bay of a yard sub-station. A full sub-station can be built by

multiplying these units.

1.2 Components of Gas Insulated Substation

The various circuit components [2] in the main circuit are: Circuit

Breakers, Bus bars, Isolator earthing switches for conductors, Current

Transformers, Disconnecting switches, Voltage Transformers, Cable

ends, Gas supplying and gas monitoring equipment, Earth Switch,

Densimeters and Local control. The various modules are connected in

accordance with the single line diagram shown in Fig. 1.1

Figure 1.1 Single Line Diagram.

1.2.1 Bus bar

The bus bar is one of the most elementary components of the GIS

system. Co-axial bus bars are common in isolated-phase GIS as this

configuration results in an optimal stress distribution. Two sections of

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bus are joined by using plug-in connecting elements. Various sizes of the

bus enclosures [2] are shown in Fig:1.2.

Fig: 1.2 Metal Enclosures for GIS Busbars

1.2.2 Connectors

The high voltage and high current electrical connections from one

module to another in a gas insulated sub-station system are carried

out with the help of spring loaded fingers or bridge contacts and

Multi-lam contacts. The spring-loaded connector and Multi lam

contact are shown in Fig 1.3 and 1.4.

Fig: 1.3 Spring Loaded connector

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Fig: 1.4 Multi- lam contact system

A T-joint (metal casting) used for tapping a high voltage connection

in GIS is shown in Figure 1.5. The joint houses a pair of multi-lam

contacts in the cavity provided for connection to the new element. The

size and diameter of the HT conductor in an isolated-phase system

are governed by the ratio of the conductor and the enclosure [2].

Fig: 1.5 T-joint for a GIS system

1.2.3 Insulating Materials and Insulators

Stable polymers like PTFE (poly tetra fluoroethane) are selectively

used in GIS and associated accessories. Ceramic and high-alumina

ceramics are also used in GIS as solid insulation materials between

the live conductor and the enclosures. However, their poor

mechanical and thermal shock withstanding capabilities and difficult

processing and manufacturing cycles have limited the use of ceramics

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in GIS. Alumina-filled epoxy matrix is a common insulating material

for GIS-related applications.

Disc and post are the preferred shapes and types of support

insulators for GIS. The disc insulators are further sub-divided into

communicating and non-communicating insulators. The two types of

disc insulators [2] are shown in Fig: 1.6 and Fig: 1.7.

Fig: 1.6 Support insulator for GIS (communicating)

Fig: 1.7 Support insulator for GIS (non -communicating)

Fig:1.8 shows the view of a 145 kV, three-phase bus with rib

insulators. The equipment uses two rib insulators per phase for

supporting the individual busbars. A three-phase disc insulator,

Fig:1.8 Three- phase bus

module with rib insulators

Fig: 1.9 Support insulator for

three-phase GIS.

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Fig: 1.9, is used to support a three-phase busbar and to limit the

pressure-rise to the affected enclosure in the conditions of an arc fault [2].

1.2.4 Disconnectors (Isolators)

Isolators are placed in series with the circuit breaker to provide

additional protection and physical isolation. In GIS systems,

motorized isolators are preferred. The fixed contacts are separated by

an isolating gas gap. During the closing operation, this gap is bridged

by the moving contact. A firm contact is established between the two

contacts with the help of spring-loaded fingers or the multi-lam

contacts. The isolation gap is designed for the voltage class of the

isolator and the safe dielectric strength of the gas. Figure 1.10 shows a

cross-section of an isolated-phase GIS isolator. Isolators in high

voltage GIS operate at SF6 pressures of 0.38 MPa to 0.45 MPa. The

operating speed of the isolator moving contact ranges from 0.1 to

0.3m/sec [2].

Fig: 1.10 Cross-section of an isolated- phase GIS isolator

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1.2.5 Circuit Breaker

The circuit breaker is the most critical part of a gas insulated

sub-station system. The circuit breaker in a gas-insulated system is

metal-clad and utilizes SF6 gas, both for insulation and fault

interruption. The SF6 gas pressure in a circuit breaker is around 0.65

MPa. The circuit breaker is directly connected to either current

transformers or the isolators in gas. The circuit breaker enclosure

also serves as the main support element for the individual GIS bay.

The GIS circuit breakers are oriented both in horizontal and vertical

configurations, depending on the system requirements and ease of

installation [2]. It is shown in Fig. 1.11

Fig: 1.11 Cross section of a GIS circuit breaker

1.2.6 Current Transformer

The conventional sub-stations use either live-tank or dead-tank

type current transformers with Oil/SF6 insulation. A porcelain

insulator is used to insulate the low potential section of the current

transformer from the high voltage zone. Ribbon or cut silicon steel

cores are used for the magnetic circuit of the current transformer for

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obtaining the desired ratio and accuracy. Gas insulated current

transformers, with classical coaxial geometry, consist of the following

parts: the tubular primary conductor; an electrostatic shield; ribbon-

wound toroidal core and the gas-tight enclosure. A coaxial

electrostatic shield, at ground potential, is placed between the high

voltage primary and the toroidal magnetic core of the current trans-

former for ensuring zero potential at the secondary of the current

transformer.

1.2.7 Earth Switch

Fast earth switch and maintenance earth switch are the two types

of earth switches used for gas insulated sub-station systems. The

maintenance earth switch is a slow device used to ground the high

voltage conductors during maintenance schedules, in order to ensure

the safety of the maintenance staff. The fast earth switch, on the

other hand, is used to protect the circuit-connected instrument

voltage transformer from core saturation caused by direct current

flowing through its primary as a consequence of remnant charge

(stored online during isolation/switching off of the line). The earth

switch is the smallest module of a gas insulated sub-station system.

1.2.8 Control Panel

Both local and remote control panels are used in GIS. The local

control panel (LCP) provides an access to the various controls and

circuit parameters of an individual GIS bay. The local control panel

facilitates the monitoring of gas pressures, status of the switchgear

element and operating fluid pressures, of oil, SF6 and air. The panel

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has a swing panel and a clear glass door with padlocks. The operator

can verify the status of the circuit through a glass panelled clear door,

containing the mimicked single line diagram, indicators and push

buttons.

The auxiliary gas insulated module or accessories, excluding

control panel, that are required to complete a sub-station are

Terminations, Instrument voltage transformer and Surge and

lightning arrestor.

An instrument voltage /potential transformer, used for metering

and protection, forms a part of the GIS and is gas insulated. This

equipment is directly mounted and connected to GIS, at times with

an isolator /disconnector in series. Both the single-phase and the

three-phase instrument voltage transformers (IVTs) are available for

voltages up to 170 kV. Single-phase IVTs are common for system

voltages higher than 145 kV [2]. A gas insulated surge arrester is a

critical accessory required for a sub-station. This device protects the

system from switching surges. Surge arresters are commonly used for

installation above 170 kV class, where appreciable switching surge

intensity is recorded.

1.3 Advantages of GIS:

1) Very much reduced area and volume requirements resulting

in lower costs.

2) It is suitable for both indoor and outdoor applications.

3) Due to its enclosed construction, GIS is operating under wide

range of environmental and operating costs.

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4) The equipment used in GIS is Pre-fabricated modules. So

Installation of GIS is made very easy at any place.

5) It ensures greatly improved safety and reliability due to

earthed metal housing of all high voltage parts and much

higher intrinsic strength of SF6 gas as insulation.

6) Maintenance costs are also reduced due to high reliability of

the equipment.

7) Elimination of radio interference is achieved with the use of

earthed metal enclosures.

8] It is not necessary that high voltage or extra high voltage

switchgear has to be installed outdoors.

9] They offer saving in land and construction costs.

10 Gas insulated substations can be used for a longer time with

out any periodical inspections.

11] These substations can be located closer to load centers thereby

reducing transmission losses and expenditure in the distri-

bution network.

Compressed gas insulated sub-stations (GIS) consist basically a

conductor supported on insulators inside an enclosure which is filled

with Sulphur-hexafluoride gas (SF6). Gas insulated sub-stations are

high voltage substations, compact in nature, requires less

maintenance when compared to air-insulated sub-stations. The

compactness of GIS is due to use of SF6 gas, which has higher

dielectric strength. Electrical insulation performance of compressed

gas insulated sub-station (GIS) is adversely affected by metallic

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particle contaminants. Free conducting metallic particles, depending

upon their location, shape and size, may lead to serious deterioration

of the dielectric strength of the GIS system and also one of the major

factors which causing breakdown of the system and leading to

unexpected break in power.

The mathematical model has been formulated to simulate the

movement of particle by considering the charge acquired by the

particle, the drag coefficient, the macroscopic field at the particle

location, Reynold’s number and the restitution coefficient.

Typical operating pressures for GIS system are from 240 KPa to

440 KPa. Generally the allowable design levels are used because SF6 is

highly sensitive to field perturbations those are caused by conductor

surface imperfections and also by conducting particle contaminations.

A CIGRE group is suggested that 20% of failures in Gas Insulated

Substation are due to the existence of various metallic particle

contaminations in the form of loose particles. Electrical insulation

performance of gas insulated substation is adversely affected by

metallic particle contamination. The random motion of the particle

due to surface roughness, coefficient restitution, and angle of

incidence when approaching an electrode increases with voltage. At a

sufficiently high voltage the particles cross the gap [20-26]. These

particles may exist on the surface of support insulator, enclosure or

high voltage conductor. Under the influence of high voltage, they can

acquire sufficient charge and randomly move in the gap due to the

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variable electric field. Several authors have reported the movement of

particles with reference to a few parameters.

It is well known that free conducting particles drastically reduce

the dielectric strength of GITL/GIS systems. In practical systems it is

very difficult to avoid metallic particle contamination. The most likely

causes for such contamination are mechanical abrasion, movement of

conductors under load cycling and vibrations during shipment [16].

Such particles are drastically reducing the breakdown voltages as a

result of their movement in the electric field.

When the electric field surrounding a particle is increased, an

uncharged metallic particle resting on the surface of support

insulator, enclosure or a bare electrode will gradually acquire charge

in proportion to the transient voltage. The charge Q accumulated on

the particle is a function of the local electric field E, the shape,

orientation, and size of the particle. When the electrostatic force QE

on the particle exceeds the gravitational force, the particle lifts up. An

additional increase in the applied voltage will make the charged

particle move into the inter-electrode gap. This increases the problem

of flashover. A conducting particle can short-circuit a part of the

insulation distance, and thereby initiate a breakdown; especially its

electrostatic force can cause the particle to bounce into the high field

region near the high voltage conductor.

Some investigations have involved the calculation and

measurement of particle lift off stress, flash over levels and evolution

of the type and pressure of gases, and the motion of the particle under

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the influence of direct, alternating and impulse voltages. All studies

agree that conducting particles can drastically influence the dielectric

strength of SF6 reducing it as low as 10 percent of the

uncontaminated value [16,24]. The breakdown voltage of SF6 gas is

highly reduced in the presence of conducting particles especially at

high pressures.

The presence of particle contamination can therefore be a problem

with gas insulated substations which are operating at high fields. If

the effects of these metallic particles could be eliminated, definitely

this would improve the reliability of gas insulated substations. It

would also offer the possibility of operating at higher fields to affect a

potential reduction in the Gas Insulated Substation size with

subsequent savings in the cost of manufacturing and installation

charges.

At the time of manufacturing of GIS equipment, care should be

taken to ensure that all components are free from metallic particles.

However, metallic contaminants are inevitable in installed systems.

Several methods of conducting particle control and deactivation have

been proposed and some of these in current use are [26]:

a. Electrostatic trapping.

b. Use of adhesive coating to immobilize particles.

c. Discharging of conducting particles through radiation.

d. Coating conducting particles with insulating films.

e. Dielectric coating on the inner surface of the outer enclosure.

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The enclosure in GIS systems may be coated with a dielectric

material to restore some of the dielectric strength of the compressed

gas that is lost due to surface roughness and contamination with

conducting particles. Coating decreases the degree of conductor’s

surface roughness, decreasing the high local electrical fields. Coating

thickness has been varied from a few microns to several millimeters

Also the high resistance of the dielectric coating impedes the

development of predischarges in the gas, thus increasing the

breakdown voltage [27-30]. A variety of coating materials have been

used for example, polymeric films and varnishes, epoxies, paraffin

wax, and anodized aluminum. These materials have a broad range of

physical properties (Volume resistivity, dielectric constant, hardness,

etc.). The volume resistivity of a coating material is considered to be

an important parameter.

A dielectric coating on the inside surface of a GIB enclosure should

inhibit the movement of metallic particles. The electric field required

to lift a particle resting at the bottom of a GIB enclosure is much

increased due to coating [8]. If the enclosure is coated, as the applied

voltage is increased sufficiently, the particle will acquire sufficient

charge to lift against gravity [4]. Once the particle begins to move in

the gas gap under the applied voltage, it may collide with the

conductor. If the conductor is coated, the particle will acquire a

smaller charge, thus the risk of a breakdown initiated by a discharge

is reduced significantly.

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The dielectric coatings improve the insulation performance. Free

conducting particles situated inside the GIS enclosure decrease high

local fields caused by conductor roughness. The coating reduces the

charge on the particle colliding with the coated enclosure, which in

turn reduces the risk of breakdown due to increase of the lift-off field

of metallic particles [4]. An electrostatic force is exerted on the particle

and at an appropriate value of the applied field, this force is sufficient

to overcome the gravitational force thus resulting in levitation and

movement of conducting particle. A dielectric coating on the inner

surface of a GIS enclosure will inhibit the movement of metallic

particle.

In some cases the particle is to use either electrostatic traps or a

combination of electrostatic traps and adhesive coatings [2]. The

charge exchange process, between a free conducting particle and the

electrode system, is significantly modified if the electrodes are covered

with a thin dielectric coating. Electrode coating would improve the

dielectric strength of particle in the GIS systems. The improvement is

due to the following:

i) Decrease in high local fields is caused by conductor

roughness. Experiments show significant improvement in

breakdown strength due to coating [27, 35]

ii) The resistance of the coating impedes the development of

predischarges in the gas [35].

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iii) In the presence of metallic contaminating particles, the field

necessary to lift a particle resisting in a horizontal GIS is

increased due to the coating [27].

iv)The charge acquired by a moving particle upon its contact

with the enclosure is much reduced if the enclosure surface is

coated. This leads to considerable improvement in GIS

insulation strength.

Free conducting particles situated inside the GIB (Gas Insulated

Bus duct) enclosure acquire free charge through a low resistance

contact with the enclosure. An electrostatic force exerted on the

particle, at an appropriate value of the particle, and at an appropriate

value of the applied voltage, is sufficient to overcome the gravitational

force, thus resulting in levitation and movement of conducting

particle. Once the particle is lifted, the attracting force on the particle

due to the image charge decreases, so that the resultant upward force

increases and the particle moves more quickly [5,6,16].

This work presents the movement of a wire like particle inside a

single phase GIB. The movement of a particle has been carried out not

only by its electric field effect on the particle but also considers

Electromagnetic field and Image charge effects. In view of this, the

present work analyzes the Electromagnetic field effect and Image

charge effect on the particle also considered for the calculation of the

movement of the particle .The equations governing the motion of the

particle due to its electric field, Electromagnetic field and Image

charge effects have been derived to obtain the particle trajectories.

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Hence simulation has been carried out to study the effect of fields on

the motion of the particle with and without Image charge effect, with

and without electromagnetic field effect and also with and without

dielectric coating on the enclosure. Trajectories are obtained for

various voltages of aluminum; copper and silver particles have been

presented and analyzed. The results show that there is a considerable

increase in movement of the particle when the electromagnetic field

effect on the particle is considered and a significant increase in

movement of the metallic particle when the image charge effect on the

particle is considered. The results have been presented and analyzed

in the following chapters.

1.4 LITERATURE SURVEY OF THE METALLIC PARTICLE

CONTAMINATION IN GIS

Gas Insulated Substations (GIS) and Compressed Gas Insulated

Transmission Line (CGITL) equipment have been developed and

improved drastically over the last three decades. Several Authors have

studied and surveyed them [1,25]. Considering the growing necessity

of electrical power for developing countries and also in view of the

relatively low per capita power consumption, there is a constant need

for additional power capacity and technological up gradation, even

though non-conventional energy systems have proved to be good

alternative sources for energy. Some of the developing countries,

which are quenching the need of additional power by conventional

electric sources, have shifted the emphasis towards improving the

27

reliability of transmission and distribution systems, ensuring that the

innovations are not harmful to the environment.

It is felt that brief survey of the work done by several Authors and

Researchers in the field is in place here, before going into the details of

the present study.

L.G.Christoporou et al. [1] mentioned that the conventional air

insulated substations have many problems such as pollution by salt

or dust, meteorological difficulties, safety etc. Hence, there is a need to

replace the conventional transmission lines and substations with

underground cable and Gas Insulated Substation (GIS) to overcome

the above problems. Due to its many advantages, most of the utilities

and industrial units are opting for Gas Insulated Substations (GIS). In

our country, a few GIS units have been in operation and a large

number of units are under various stages of installation.

SF6 gas insulated substations are preferred for voltage ratings of

36, 72.5,145,245, 420kV and above. In such a substation, various

equipments like circuit breakers, bus bars, isolators, load break

switches, current transformers, potential transformers and earth

switches are housed in metal enclosed modules filled with SF6 gas.

The SF6 gas provides the phase to ground insulation. As the dielectric

strength of SF6 gas is higher than that of air, the clearances required

are smaller.

L.G.Christoporou et al. [1] discussed that Sulphur hexafluoride

(SF6) is a man-made gas that became commercially available in 1947.

28

It is one of the most extensively and comprehensively studied

molecular gases to date largely because of its many commercial and

research applications. Its basic physical and chemical properties,

behavior in various types of gas discharges, and uses by the electric

power industry have been broadly investigated [2-7]. In its normal

state it is chemically inert, non-toxic, nonflammable, non-explosive

and thermally stable. (It does not decompose in the gas phase at

temperatures T<500C). Because of its relative inertness and nontoxic

characteristics, it is generally assumed to be an environmentally safe

and acceptable material in the sense that it does not interact

unfavourably with the biomass.

Sulphur hexafluoride is the electric power industry’s preferred

gas for electrical insulation and, especially, for arc quenching current

interruption equipment used in the electric power transmission and

distribution systems. Generally, there are four major types of electrical

equipment that use SF6 for interruption and/or insulation purposes:

Gas-Insulated Substations, Gas Insulated Busduct/Gas Insulated

Transmission Lines, Gas Insulated Circuit Breakers and Gas

Insulated Transformers. It is estimated [8-11] that for these

applications the electric power industry uses about 80% of the SF6

produced worldwide, with circuit breaker applications accounting for

major component of electric power transmission and distribution

systems throughout the world, and it is almost exclusively used.

29

SF6- based circuit breakers are superior in their performance to

alternative systems such as high-pressure air blast or vacuum circuit

breakers. For gas insulated transformers, the cooling ability,

compatibility with solid materials, and partial discharge

characteristics of SF6 adding to its beneficial dielectric characteristics

– makes it a desirable medium for use in this type of electrical

equipment. The use of SF6 insulation has distinct advantages over oil

insulation, including the avoidance of breakdown due to charge

accumulation on insulators, no fire safety problems, high reliability,

flexible layout, little maintenance, protected insulation, long service

life, lower noise, better handling and lighter equipment. For gas-

insulated transmission lines, the dielectric strength of the gaseous

medium under industrial conditions is of paramount importance,

especially the behaviour of the gaseous dielectric under metallic

particle contamination, switching and lightning impulses, and fast

transient electrical stresses. The gas must also have a high efficiency

for transfer of heat from the conductor to the enclosure and be stable

for long periods of time (say, 40 years). SF6- insulated transmission

lines offer distinct advantages: cost effectiveness, high-carrying

capacity, low losses, and availability at all voltage ratings, no fire risk,

reliability, a compact alternative to overhead high-voltage

transmission lines in congested areas, and avoidance of public

concerns with high-voltage overhead transmission lines [12-18]

K.Chakrabarti et al. [19] discussed that Sulphur hexafluoride gas

[SF6] is extensively used in compressed gas insulated power

30

apparatus, such as gas insulated transmission line (GITL), gas

insulated sub-stations and switchgear (GIS). SF6, besides being

dielectrically superior to air, is non-toxic, environmentally safe, and

chemically stable. However, in such equipment the SF6 gas, as an

insulating medium, is used well below its ideal dielectric capabilities

(88.4MVmm-1 MPa-1).

Sayed.A.Ward et al. [21] discussed that the use of compressed gas

as insulated media has made possible compact equipment compared

to that air insulation however, the compact construction increases the

operating field intensity. SF6 insulation is extremely sensitive to local

increases in electric field which results from protrusions on electrode.

Triple junction (The region where the electrode, insulator and SF6 gas

meet) in compressed gas, the presence of conducting particles in gas

insulation and the shape of spacers supporting the conductor system

typically results in a reduction in the insulation in the strength of SF6

to about half the theoretical field strength. Metallic particle initiated

breakdown can occur at fields considerably lower than this, reducing

the insulation level by as much as 90% of the surface roughness.

Compressed Gas Insulated Substations (GIS) and Transmission

Lines (CGIT) consist basically of a conductor supported on insulator

inside an enclosure which is filled with SF6 gas. Typical operating

pressures for GIS systems are from 240Kpa to 440 Kpa. Generally the

allowable design levels are used because SF6 is very sensitive to field

perturbations such as those caused by conductor surface

31

imperfections and by conducting particle contaminants. A study of

CIGRE group suggests that 20% of failure in GIS is due to the

existence of various metallic contaminations in the form of loose

particles[22-24]. These particles may exist on the surface of support

insulator, enclosure or high voltage conductor. Under the influence of

high voltage, they can acquire sufficient charge and randomly move in

the gap due to the variable electric field. Several authors have reported

the movement of particles with reference to a few parameters.

K.S.Prakash et al.[25] discussed that electrical insulation

performance of Gas Insulated Substation or Gas Insulated

Transmission line systems is adversely affected by particle

contamination. The accumulated field experience indicated that

sources for such contaminations are mechanical abrasions, movement

of conductor under load cycling and vibration during shipment and

service. Although good designing and quality control would minimize

such contamination, in practice some particles contamination is

unavoidable. These metallic particles may be move freely in the

electric field, or they may be fixed on the electrodes, thus improving

local surface field. In a horizontal coaxial system, if the particles are

resetting on the inside surface of the enclosure, the movement of such

a particle is random in nature, the randomness depend on the

coefficient of restitution and angle of incidence when approaching the

coaxial conductor. The coefficient of restitution being the ratio of

incident and rebound velocities, is dependent on conductor surface

roughness[26-28].

32

J.R.Laghari et al. [16) found that a small amount of fine

conducting particles (<100mg) of less than 30um diameter had no

effect on the alternating and impulse breakdown of a coaxial gap, but

Nitta et al. found that larger amounts of these fine particles (<100mg)

reduced the breakdown voltage. Elongated particles are found to

reduce greatly the breakdown voltage as well as the corona onset

levels in SF6 gas. Generally, the more elongated the wire-particle, the

lower breakdown voltage. The breakdown voltage is not necessarily

reduced by decreasing the wire particles. The breakdown

characteristics of spherical and wire-particles are dissimilar. In the

presence of spherical particles, the alternating breakdown voltage of a

coaxial system increases steadily increasing with pressure. There are,

however, indications of a slight peak beginning to occur in the

breakdown voltage-pressure characteristics of the radius of the

particles increase. In the case of long wire-particles, the breakdown

voltage-pressure characteristic exhibits a pronounced maximum

followed by a gradual reduction[29-32]. No difference is observed

between the results of aluminum and copper wire-particles under

both alternating and direct voltages in a uniform field.

H.Anis et al. [33] stated that the conducting particles can either be

free to move in the GIB or they may stick to energized electrodes or to

an enclosure surface. The presence of contamination can therefore be

a problem with gas insulated substations operating at high fields. If

the effects of these particles could be eliminated, then this would

improve the reliability of compressed gas insulated substation. It

33

would also offer the possibility of operating at higher fields to affect a

potential reduction in the GIS size with subsequent savings in the cost

of manufacture and installation.

The critical dielectric design factor for the present systems is

usually the lightning impulse test where the conductor design field is

of the order of 160 kV/Cm at 440Kpa atmosphere. The present design

levels are of the order of 37% of the theoretical strength of SF6 for

lightning impulse and 19% for the 50Hz factory test. The operating

fields vary from above 6% to 14% of the break down field depending

on the system voltage.

Gas insulated switchgear due to its modular construction and

compact nature is a viable alternative to the conventional air insulated

substation. It requires minimum floor space and building volume. It is

suitable for both indoor and outdoor applications. Due to its enclosed

construction, Gas Insulated Substation is operable under wide range

of environmental and operating conditions. Since modules are pre-

fabricated, installation of Gas Insulated Substation at a site is a quick

process involving only an assembly of modules. Maintenance costs are

also reduced. Due to high reliability of the equipment, Gas Insulated

substations can be used for a longer time without any periodical

inspections.

A.G.Sellar et al. [13] discussed that the attractive features of a Gas

Insulated Substation (GIS), they also suffer from certain drawbacks.

One of them is the outage due to seemingly innocuous conducting

34

particles, which accounts for nearly 50% of the GIS failures.

A.H.Cookson [4] found that the Flash over in a GIS is, in general,

associated with longer outage times and greater costs than in a

conventional air insulated substation. A conducting particle can

short-circuit a part of the insulation distance, and thereby initiate a

breakdown, especially if electrostatic forces cause the particle to

bounce into the high field region near the high voltage conductor.

J.R.Laghari et al. [16] mentioned that the presence of particles in a

compressed gas insulated system can deteriorate its electrical

performance. These particles may result from mechanical abrasions

and damage the system as well as the enclosure during assembly. The

particles may either be insulating or conducting. They may be free to

move under the influence of the applied field or may be fixed on the

electrodes in the form of a protrusion or electrode surface roughness.

Some investigations [2,3] have involved the calculation and

measurement of particle lift off stresses, flash over levels and

evaluation of the type and pressure of gases, and the motion of

particles under the influence of direct, alternating and impulse

voltages. All studies agree that conducting particles can drastically

influence the dielectric strength of SF6, reducing it as low as 10% of

the uncontaminated value [15]. Lahari et al reviewed the electrical

performance of gases under particle contamination. It is very difficult

to predict the magnitude of the reduction in the breakdown voltage as

it depends on the stress level, the type and polarity of voltage applied,

35

the electric field, the type and pressure of gas insulation and above

all, the shape and the size of the particle contaminant.

Several authors conducted experiments on insulating particles.

Insulating particles are found to have little effect on the dielectric

behavior of the gases [4-8]. However the presence of atmospheric dust

containing conducting particles, especially on the cathode, reduces

the breakdown voltage. Bolloud et al. [16] showed that currents of the

order of 10-8A have been observed to flow in uniform fields in SF6 in

the presence of these dust particles. On the other hand, A.H.Cookson

et al. [17] discussed that the breakdown voltage of SF6 is highly

reduced in the presence of conducting particles especially at high

pressures. In addition, depending on the shape of particles as well as

the geometry and voltage level of the system, the particle gets more or

less influenced by the electric field, which in turn makes it hazardous

to the electrical system. The type of gas is found to influence the

particle motion in a gas insulated system containing free particles.

The modes of motion and the necessary stresses for the particle

motion are different in different gases. A.Diessnner et al. [8] first

reported the movement of particles in SF6 and N2 mixtures and

showed that the type of particle movement depends on both the gas

insulation and polarity of the non-uniformity. They described three

different modes of particle motion with negative inner conductor in a

coaxial filled with N2 and SF6 .

36

The free conducting particles become charged and oscillate

between the electrodes under the influence of the applied field. As a

charged particle approaches an electrode, it loses the charge to the

electrode through gas micro discharges. The investigations of

A.H.Cookson [4] revealed that the reduction in the break down voltage

is especially more severe under dc applied voltage than under ac or

impulse voltages. It was also observed that the reduction in the break

down voltage in the presence of the particles under dc voltages is more

pronounced in uniform fields than in non-uniform fields.

S.Zhang et al. [8] stated that a particle-initiated breakdown may

develop only if the particle is at, or near, one of the electrodes. The

breakdown caused by a particle fixed on the high-voltage electrode

occurs at a higher voltage than that when the particle was free. The

conducting particle may be spherical or wire shaped. Wire particles

were chosen for most experiments rather than spheres since they

simulate particles found in practical equipment. The motion of the

particle is different under dc, ac and impulse voltages. The reduction

in breakdown voltage is much greater under dc applied voltages than

under ac or impulse voltages.

H.Kuwahara et al. [9] mentioned that Conducting particles placed

in a uniform ac field lift-off at a certain voltage. As the voltage is

raised, the particles assume a bouncing state reaching a height

determined by the applied voltage. With a further increase in voltage,

the bounce height and the corona current increase until break down

37

occurs. The lift off voltage is independent of the pressure of gas. After

the onset of bouncing, the offset voltage is approximately 30% lower

than the lift-off voltage.

Metallic fillings on the inner conductor of a co-axial line cause a

reduction in the impulse break down voltage. According to studies

made by Kuwahara et al. [9] using copper particles of diameter less

than 30 m decreases the impulse break down voltage by 22% and 8%

under negative and positive polarities respectively in a uniform field.

The negative flash over voltage is lower than the positive flash over

voltage [4].

Utility demands systems that require minor supervision, or

systems that can be easily monitored. Several methods for monitoring

the insulation of GIS systems, including acoustical, chemical and

electrical have been developed by Peterson et al [20]. Recently,

manufacturers have developed systems based on high frequency

capacity couplers [21,22]. Other means of monitoring gas-insulated

systems include Ultra High Frequency (UHF) measuring techniques

[13]. Acoustic measurement and conventional Partial Discharge (PD)

measurements also play an important role in investigation and

supervision of GIS.

K.D.Srivastava et al. [26] mentioned several methods of conducting

particle control and de-activation. Some of them are:

a. Electrostatic trapping

b. Use of adhesive coatings to immobilize the particles

38

c. Discharging of conducting particles through radiation, and

d. Coating conducting particles with insulating films

The dielectric coatings improve the insulation performance. Anis.H

et al. [24] showed the importance of volume resistivity of the coating

material. Free conducting particles situated inside the GIS enclosure

decrease high local fields caused by conductor roughness. The coating

reduces the charge on the particle colliding with the coated enclosure,

which in turn reduces the risk of a breakdown due to increase of the

lift-off field of metallic particles. An electrostatic force is exerted on the

particle and at an appropriate value of the applied field, this force is

sufficient to overcome the gravitational force thus resulting in

levitation and movement of conducting particle. A dielectric coating on

the inner surface of a GIS enclosure will inhibit the movement of

metallic particle.

Coating decreases the conductor surface roughness, is explained

by K.D. Srivastava et al [25,26]. Also the high resistance of the coating

impedes the development of pre-discharges in the gas thus increasing

breakdown voltage of the gas. A very good description of the above

phenomena is given by Parekh et al [27].

A metallic particle resting on a coated enclosure surface may

acquire free charge through several physical processes such as

charging from a charged dielectric surface, conduction through the

coating and through a Partial Discharge (PD), between the particle and

the coating [28]. The electric field required to lift a particle resting on

39

the bottom of a GIB enclosure increases due to the coating. Once a

particle begins to move in the gas gap under the applied voltage, it

may collide with either conductor. The experiments of Feser et al. [29]

revealed that if the conductors were coated, the particle would acquire

a smaller charge, thus reducing the risk of a breakdown initiated by a

particle.

Contamination by metallic particles in Compressed Gas Insulated

Power Transmission Line (GITL) equipment is commonly encountered.

The insulation strength of GITL apparatus deteriorates mainly due to

particle contamination. M.M.Marcos et al. [30], conducted

experiments in the laboratory, which indicate that dielectric coatings

on electrodes not only improve gaseous insulation but also rise the

voltage necessary for initiating a conducting particle movement in

GITL apparatus. The work reported by Srivastava, et al. summarizes

the results of a ten-year investigation into the usefulness of dielectric

coating in GITL apparatus in the presence of metallic contamination.

S.Zhang et al. [40] stated that the effect of dielectric coatings on

the insulation performance of compressed gas and oil insulated

electrodes have been studied by several researchers. Coating

thickness has been varied from a few microns to several millimeters. It

is generally believed that such coatings improve the insulation

performance. A variety of coating materials has been used in these

studies, for example, polymeric films, varnish, epoxies, paraffin wax,

Teflon and anodized aluminum. These materials have a broad range

40

of physical properties like volume resistivity, dielectric constant

hardness, etc. The volume resistivity of the coating material is

considered to be an important parameter, however, experimental

evidence concerning the relative importance of various physical

properties of the coating materials is inconclusive.

Coating thickness has been varied from a few microns to several

millimeters, and the influence of coated electrodes on the insulation

performance has been studied under dc, 60 Hz ac and lightning

impulse voltages. R.F.Gossens et al. [31] found that although a layer

of bakelized paper on the central conductor of a coaxial electrode

arrangement gave a higher lightning impulse breakdown voltage, the

60Hz breakdown voltage was lower. For uniform field, and coaxial

electrode systems, D.J. Skipper et al [32] used nitrogen and sulphur

hexaflouride at pressures of 3.4 Mpa and 1.4 Mpa respectively, and

used a variety of polymeric films, and anodized aluminium with

thickness up to 130m. For nitrogen gas and uniform electric field,

the impulse breakdown strength resulting from coating the electrodes

increased about 50 percent over the uncoated electrodes, and this

value did not differ greatly for pressures between 0.34 Mpa and 3.4

Mpa for all the coating materials used.

A.H.Cookson [4,17,18] extended the scope of the above work, using

40 m and 130 m thickness of high-density polythene coatings in

uniform field geometry with pressures up to 1.5 Mpa for sulphur

hexaflouride and 5 Mpa for nitrogen. Using impulse voltages, they

41

found that the breakdown fields increased about 50 percent over

those for bare electrodes. Dust particles, however, had a deleterious

effect, particularly at higher pressures, and could reduce the

breakdown value by as much as 30 percent.

In additional work, J.M.K.McAlpine et al. [34] observed, in contrast

to the work of Skipper and McNeall [32], that the breakdown strength

is influenced by resistivity of the film material, probably because this

controls the charging of dust particles on the coating and their ability

to act as electron emitting sites. The Vycoat and Neoprene layers,

with a resistivity of about 10 ohm-m, gave the lowest breakdown

fields. The polystyrene varnish and polyolefip layers, with a resistivity

greater than about 10 ohm-m, gave higher breakdown fields.

D.J.Chee-Hing et al [35] conducted an extensive investigation on

the dielectric coated electrodes in compressed SF6 gas for pressures

up to 0.68 Mpa gauge, and under dc and lightning impulse voltages.

Polyurethane, paraffin wax, epoxy and anodised aluminium coatings

were investigated in uniform field geometry. Coating thick nesses up

to 1.5 mm were used. Their results may be summarized thus:

For a given gas gap, there is a substantial improvement

in the dc breakdown voltage, and the breakdown voltage

increases with the thickness of coating. Reducing the volume

resistivity of a coating, by loading it with graphite, lowers the

breakdown voltage; however, the pre-breakdown currents for

graphite-loaded coatings are higher.

42

The lightning impulse breakdown voltage, for a given

gas gap, resistivity, and thickness of coating is generally

higher than the dc breakdown voltage. No “conditioning”

effects appear to occur for either coated or uncoated

electrodes. The coatings punctured after the first breakdown,

and the first breakdown voltage is always higher (5 to 20

percent) than the subsequent breakdown voltages.

The presence of non-conducting debris from repeated

breakdowns substantially lowers the subsequent breakdown

voltage. However, such debris may lead to glow discharges for

polyurethane and epoxy coatings. Glow discharges were not

observed with anodized aluminum coatings.

The coatings acquire a surface charge under dc voltages, and this

charge decays slowly (over several minutes) when the voltage is

removed. Although efforts were made to determine the magnitude and

distribution of the charge, the results were inconclusive. A.E.Vlastos

and S.Rusck [36-38] have investigated the effect of coatings on power

frequency ac breakdown in compressed SF6 and air at pressures up to

1 Mpa, using a coaxial geometry. The relative improvement in the

breakdown voltage is substantially higher for the rough electrode as

compared to highly polished electrodes. They also noted that coatings

on the outer conductor did not matter a great deal, that there was not

a significant “conditioning” effect, and that the first breakdown on a

43

freshly coated electrode was generally the highest in a sequence.

E.Gockenbach [39] and A.Rein et al. [43] reported similar results.

In compressed GIS, the control of metallic particle contamination

by dielectric coating on the inside surface of the enclosure has been

discussed by K.S.Prakash et al [25]. Assuming that the particle

acquires a free charge through the particle discharge, the modeling of

a particle movement is described when the inside surface of the

GIS/GITL enclosure which is coated. Under an applied ac voltage, the

maximum height reached by a particle is much lower for a coated

enclosure than for an uncoated enclosure. When the applied voltage

varies, the maximum height reached by the particle depends on the

magnitude of the applied voltage, phase and velocity of the particle at

which it hits the enclosure. Typical results show that by applying

coating on the enclosure, the maximum height reached by a particle

in a coaxial electrode system can be reduced, thus inhibiting the

movement of particle and also minimizing the possibility of insulation

breakdown [44-46].

The dynamic behavior of free conducting particles of SF6 insulated

systems under switching impulse (SI) superimposed on a dc voltage

has been investigated by R.M.Radwan et al. [47]. This study includes

the influence of the most important design parameters on the particle

motion, such as value of dc switching voltage ratio, particle

parameters, shape of switching voltage and system configuration,

parallel plane and coaxial cylinders. The computations have

44

concluded that the behavior of contaminating conducting particles in

GIS when subjected to a dc bias voltage and SI wave is more onerous

than its behavior under pure switching impulse voltage[48,49].

K.I.Sakai et al. [50] in his paper reported the analysis of spherical

conducting metallic particle motion and the particle-initiated

breakdown in electric fields between diverging conducting plates in

atmospheric air with a dc voltage. Motion of a spherical metallic

particle was estimated by solving the motion equation, and the results

agreed well with the experimental work. It was found that when the

particle is placed on the surface of the horizontal electrode where the

electric field is closely equal to the lifting field, the particle moves

towards the higher electric field region with the effect of Coulomb force

and electric gradient force. This is true for a non-spherical conducting

particle. The results obtained for particle motion suggested that much

attention should be taken in the design of GIS. Moreover, the particle-

initiated breakdown is explained on the basis of the theoretical and

experimental results [51-53]. It was found that one of the reasons for

reduction in breakdown voltage is the effect due to micro discharge

between the particle and charged electrode.

Ultra High Frequency (UHF) technique has been developed to study

the partial discharges produced by free conducting metallic particles

in GIS with a view to predict the probability of particle-triggered

breakdown. It is possible to identify while the particle is capable of

crossing the electrode, which is a needed condition for particle-

45

triggered breakdown. In addition, the technique is used to assess the

size of the metallic particle, which is making use of a qualitative

assessment of the breakdown rise. The implementation of the

continuous discharge monitoring of Gas Insulated Substations are

discussed by A.G.Sellars et al. [54].

In this way, dielectric coating helps in improving the insulation

performance of SF6 and hence is considered to be one of the best

techniques.

Understanding of the dynamics of a metallic particle in a coaxial

electrode system is of vital importance for the presence of metallic

contamination in GIS. If the motion pattern of a metallic particle is

generally known, the probability of a particle crossing a coaxial gap

causing a flash over can be estimated. Moreover, it is of interest to

understand the influence of dielectric coating of electrodes on the

particle charge when the particle is in contact with the dielectric

coating [55,56].

This study does not include particles that are stuck to an

insulating or energized surface, since the problems with fixed

conducting particles are different from those with free conducting

particles. However, in order to make appropriate mathematical models

of particle motion in GIS, it is important to understand the charging

mechanism of metallic particles, which are in contact with coated or

bare electrodes.

46

D.J.Chee-Hing et al. [34] mentioned that a metallic particle is lifted

from a resting position on a grounded electrode, and is kept in motion

by the surrounding electric field and the charge acquired by the

particle. The voltage required to lift the particle is often below the

operating voltage level of the system. Once the particle lifts from the

surface, it changes its direction as well as magnitude, hits the inner

electrode of the system where it gets a new net charge[57-60]. The

magnitude of the net charge depends on the dimension of the particle

and on the value of the surrounding field at the time of impact with

the electrode.

For an ac field, one may expect the motion of a particle to be

random, since the charge on the particle interacts with the

continuously changing electric field. Also, since the charge on it

depends on when it hits the electrodes, the charge varies after each

consecutive impact. However, by statistical analysis of the time

between impacts of the particle with the enclosure, one can find that

the motion of the particle is, in fact, influenced by the power

frequency [27, 61,63]. For an applied, ac field, the maximum height

that the particle can elevate from the electrode is limited compared

with a dc field. If the applied voltage is sufficient, several voltage cycles

might be necessary for the particle to be able to cross the gap. When

isolated conductor GIS and three phase common enclosure GIS were

compared with respect to the effects of criticality of moving particles.

K.Feser et al. [64] found that particle movement was minor in three

phase common enclosures.

47

The primary goal of the simulation was to create a satisfactory

mathematical model of the particle motion in the GIS bus, which

would enable future simulations of the motion of particles with

arbitrary shapes. The simulation considers several parameters like the

macroscopic field at the location of the particle, its weight, viscosity of

the gas, Reynolds number, drag coefficient and coefficient of

restitution on its impact to enclosure. During return flight, a new

charge on the particle is changed based on the instantaneous electric

field. The above simulation yields the particle movement in radial

direction only. However, the configuration at the tip of the particle is

not sufficiently smooth enough to enable the movement

unidirectionally. The randomness of the particle can be adequately

simulated by Monte-Carlo method [67,70,71].

Theoretical and experimental work on particle behavior in electric

fields has shown that the field required to cause “lift-off” of a particle

from an electrode, say, the enclosure of a GIS system, is independent

of gas and pressure, electrode separation and the length of the

particle, but proportional to the mass (or) density of the body [72-75].

The equations governing the particle charge Q and “lift-off” field Elift

from a metallic surface is given in the following chapters. These

equations show that when a wire particle is lifted from the horizontal

to the vertical position (standing on the electrode surface), the particle

acquires more charges and begins to move actively in the electric field.

A vertical wire requires a smaller field to maintain this position than

the field required for lifting it from the horizontal to vertical position.

48

This means that once a particle is lifted it will maintain this position

at a lower field than the lifting field. The reduction in the electric field

strength on the electrode surface is typically 60% of the “lift-off” field.

The particles do not respond to instantaneous changes in the field

because of their inertia. Thus, under ac voltages, particles do not fall

when zero voltage occurs but respond in a time average fashion to the

ac wave form with the motion of a particle depending on the

magnitude and sign of the net particle charge with respect to the ac

field stated by K.D.Srivastava et al. [75].

The elevation above the lower electrode reached by a particle in

the field increases with increasing field above the “lift-off”. For 6.4mm

long aluminum wire [75] in a 76/250mm coaxial system, a doubling of

the field from lift-off is typically required before the particle crosses

the conductor. In the absence of corona discharges from wire

particles, the bouncing activity increases with the increasing particle

length because the particle charge-to-mass ratio increases with the

length. With spherical particles the activity decreases with sphere

radius because the charge-to-mass ratio decreases with radius. Thus,

elongated particles can cross the electrode separation at lower fields

than spherical particles of the same mass can, the crossing field being

inversely proportional to the particle length. Breakdown can occur

when the particle gets close to (or) terminates on the conductor. The

longer the particle, the lower is the resulting breakdown voltage, is

mentioned by K.D.Srivastava et al. [75].

49

J.R.Laghari et al. [16) mentioned that when a conducting particle

is freely located on an electrode, it acquires a charge Q which depends

on the local electric field E, the shape, orientation, and size of the

particle. When the force QE on the particle exceeds the gravitational

force, the particle is elevated. Once the particle is lifted, the attracting

force on the particle due to the image charge decreases, so that the

resultant upward force increases and the particle moves more quickly.

The particle density ρ is assumed to be much greater than the gas

density ρg and any corona taking place at the particle is neglected

[30,73].

Corona discharges (or) avalanche formation occurring from the

particles can greatly affect the motion of the particles by altering the

net charge on particle. One example of such a phenomenon is the

“fire-fly” motion observed with direct voltages [76,77] where a particle

can remain hovering at the negative electrode with intense corona

discharges. With positive polarity dc voltages in a coaxial electrode

system the particles usually move continuously between the

electrodes, but at higher fields they can also remain in a “fire-fly”

position on the outer electrode [76,77]. The discharge activity also

greatly affects the motion, and particle can remain in the mid gap

region for long periods of time and can take several cycles to cross the

gap.

A great contrast between the particle behavior in ac and dc is that

once the particle is lifted in a unidirectional field, it crosses to the

50

other electrode, and as the field is increased, the frequency of the

particle motion between the electrodes increases stated by H.Karner et

al. [76]. Whereas in the ac case as discussed above, a substantial field

increase is required after the particle is lifted, before the particle

crosses the gap, is mentioned by K.D.Srivastava et al. [75].

Particles that are at rest in the bottom of a coaxial system under

normal operating conditions do not represent a direct threat to the

insulation integrity of the system. However, such particles are a

potential hazard. Recent work on superposition voltage studies by

H.Karner et al. [76] demonstrated that particles normally at rest under

the ac voltages may be activated by a switching impulse superimposed

on the 50 Hz ac voltage level above the “fall down” level for the

particles. Similar effects can be obtained by mechanical methods, e.g.,

a hammer blow to the enclosure. Although, under ac voltages at

normal operating levels, the “lifted” particles may not represent a

threat of an insulation failure in the gas gap. These particles are free

to move and thus can be deposited onto the solid insulators of the

system. Once a particle is on the insulator, it will tend to remain there

because of electrostatic adhesion to the solid insulation. In particular,

dc voltages tend to cause particle migration to spacer surface as

reported by Boggs [78].

Particle contamination of solid insulator surfaces can result in

surface flashover well below the clean breakdown voltage for

insulators. Wire type particles on insulators can cause reductions in

51

the system insulation level corresponding to that of free particles in

the gas gap found by K.D.Srivastava et al. [75]. The effects of different

types of voltages on the dielectric strength of insulators for various

types of contaminants are shown in the report published by USA,

Dept. of Energy [80]. The breakdown voltage for contaminated

insulators with an oscillating switching impulse compared to the

standard switching impulses are given by Corner et al [76,77,80].

Fine metal powder has also been found to affect the ac breakdown

level of solid insulation. While the free conducting particles spend only

a fraction of their levitated life-time in a position at the conductor

where they can trigger a break down, a particle adhering to a solid

insulator has similar vulnerable conditions. Thus, flashover along a

spacer surface, once polluted, is more likely to occur than for the gas

gap, particularly, if lightning and switching transient over-voltages are

considered.

The probability of breakdown in a GIS or CGITL system occurring

due to free conducting particles is a function of the accumulated time

and the particles position where the resulting maximum local field

enhancement exceeds the limiting field of the dielectric gas. This has

been demonstrated in tests with different numbers of free conducting

particles before being increased to a higher level found by

K.D.Srivastava et al. [75]. These results also showed that the long-

time voltage-pressure characteristics for breakdown in SF6 were

virtually unaffected by the pressure increasing up to 15 bar.

52

Particle initiated breakdown occurs at considerably lower fields

than those for the surface roughness breakdown and is in general the

lowest for positive polarity voltages. The breakdown voltage is

characterized by the existence of change of gas pressures where the

break down voltage is considerably enhanced over the corona onset

voltage. In addition to the voltage waveform, the state of the particle,

fixed (or) free to move also significantly affects the voltage-pressure

characteristic. A comparison of the voltage-pressure characteristics

was reported by S.J.Dale et al [81]. The enhancement of the break

down voltage was progressively reduced as the particle went from

being fixed to the conductor, fixed but isolated by a small gap, to the

free state. Thus breakdown with free particles occurred well below the

value for fixed particles. The probability of having breakdown with free

particles is a function of the number of particles and the duration of

the voltage; the larger the particle number-time product, the lower is

the observed breakdown voltage. The slight enhanced breakdown

voltage for the particles in the 100 Kpa to 400 Kpa pressure region

cannot be regarded as a “Safety-limit” for operation (or) conditioning of

GIS or CGIT systems but is a statistical event peculiar to the test

results.

For Gas Insulated Systems (GIS) to be reliable and economic, it is

necessary for the particle problem to be overcome. Dust and particles

may unintentionally be introduced into the Gas Insulated

Transmission Systems during assembly (or) field installations[82,83].

The present technique for the elimination of the effect of conducting

53

particles is the use of “Particle Traps” stated by D.H.Peng et al.[84]. In

this system, the particles are caused to move into a low-field region by

the application of a “conditioning” voltage sufficiently high to cause

levitation and migration of the particles into traps but not high

enough to cause flashover. One such design consists of a slotted

metallic cylinder mounted around the spacer and is electrically

connected to the enclosure. Particles migrate along the enclosure, fall

through the slot and are trapped in the low field region at the bottom

of the enclosure under the trap.

The combined effect of moisture, temperature and conducting

particles on the discharge behavior of Sulphur hexafluoride has been

reported by D.H. Peng et al. [84]. Measurements have been made of

the dc corona onset and breakdown voltages for both positive and

negative point/plane electrode systems in compressed SF6 gas.

Presence of moisture lowered the onset voltages and raised the

breakdown voltages significantly[85].

The effect of moisture on flashover voltages in the presence of

metallic particles was also investigated and the results showed that

there was little effect except when moisture was likely to be

condensing on the spacer surface; a little effect was observed, when

the moisture freezes. The reduction in flashover voltage due to

moisture was explained independently in addition to the effect of the

presence of a metallic particle. Complete excursion of conducting

particles and moisture from high-voltage equipment using Sulphur

54

hexafluoride (SF6) insulation, throughout its operation provides

opportunities for contamination by the particles from mechanical

abrasion investigated by G.Berger et al. [86]. Moisture-free (<15 ppm

water) Sulphur hexafluoride is used particularly in tropical countries.

Moist air may leave a thin film in switchgear even after lengthy

evacuation unless the switch gear is kept sealed from a factory to a

site. Similarly, connecting pipes supplying the SF6 to the switchgear

may initially contain moist air.

SF6 switchgear has been found to contain, as much as 800ppm of

water when due precautions were not taken at manufacturer

premises. Moisture is reported to reduce the breakdown strength in

quasi-uniform gaps reported by J.M.K.MacAlpine et al. [87], but no

measurements appear to have been made of the effect of moisture on

corona inception and breakdown voltages or point/plane geometry.

The effect of 100ppm of water vapour on the point/plane corona onset

and breakdown voltages of SF6 gas is reported for various pressures.

Particle contamination has been known to lower breakdown

voltages for many years, as well as spacer flashover voltages [88-92].

Moisture is found to have little effect on flashover voltages in SF6 while

in the gas phase, but condensation has a marked effect [87]. The

combined effect of conducting particles and humidity on the flashover

voltage does not appear to have been investigated to determine

whether there is a cumulative effect.

55

In addition, measurements are reported of flashover voltages with

and without a metallic particle at the triple junction where spacer, gas

and electrode meet, for a range of humidity and temperatures.

Measurements of point/plane [93-97] corona onset and breakdown

voltages have shown that for direct voltages and both polarities, the

onset voltages are significantly reduced, whereas the breakdown

voltages are significantly increased. The onset-voltage effect may be

due to the decrease in ionization due to clustering of water molecules

with the electrons and the consequent formation of space charge. On

the other hand, the increases in the breakdown voltages appear to be

the effect of increased corona shielding due to the lower drift velocities

of the water-clustered ions.

Flashover voltages for spacers in plane-parallel gaps did not appear

to be modified by humidity except where condensation occurred. It is

then presumably controlled by the fluctuations in electric filed near

the electrodes caused by high-permitivity water droplets near, but not

touching, the electrodes. The effect of a metallic particle at the triple

junction was always additional to the condensation effects.

The first generation of gas-insulated switchgear (GIS) that Hitachi

developed was 550 kV. Developed in 1978, this GIS used 4-break

circuit breakers and all of its phases were the separated-phase type.

Later, circuit breakers were changed to the 2-break type. In 1989, the

LIWV (lightning impulse withstand voltage) was lowered by using a

high-performance metal oxide surge arrester, which lowers residual

56

voltage. As a result, a large-radius. 3-phase common spacer was

developed and a 3-phase common enclosure was incorporated into a

more compact main bus. This was the second generation of GIS. The

third generation GIS was for that reduced in size; the tank was

reduced to less than 85% of its original size and the installation area

was reduced to 46% of its original size. Reliability of the GIS was

proven by checking both limit and practical performances. The third

generation GIS, featuring latest technology, is currently in operation

at the Higashi-Gunma Substation of the Tokya electric Power Co.

(TEPCO)

Hitachi has developed smaller and more reliable gas-insulated

switchgear in response to demands for lower costs and stable power

supplies. Reliability of this GIS was improved and enclosure diameter

was reduced in order to minimize the installation space by using new

analysis techniques. The reliability of the GIS was improved by

applying new structural design and manufacturing technologies. For

example, high-precision parts and a GIS-specific assembling device

were used. Large all-weather assembly houses were also used at the

site and the assembly process was improved.

Gas Insulated Substations due to its modular construction and

compact nature is a viable alternative to the conventional air insulated

substations. It requires minimum floor space and building area. It is

suitable for both indoor and outdoor applications. Due to its enclosed

57

construction, GIS is operated under wide range of environmental and

operating conditions because all the parts of GIS or pre-fabricated.

Fig. 1.12 and Fig. 1.13 show the Air insulated substation and Gas

Insulated Substation respectively. Fig. 1.14 and Fig. 1.15 show the

Heavy cluster area and Industrial Pollution respectively. Fig. 1.16 and

Fig. 1.17 show the Yard substation and Gas Insulated Substation

respectively. Fig. 1.18 shows the assembly of Gas Insulated

Substation and Fig. 1.19 shows the Busduct. Fig. 1.20 to Fig. 1.27

show the different operating voltages of the indoor and outdoor Gas

Insulated Substations [2].

The purpose of this thesis is to develop techniques, which will

formulate the basic equations governing the movement of the metallic

particles like aluminum, copper and silver due to its electric field,

electromagnetic field and Image charge effects have been studied to

determine the particle trajectories in Gas Insulated Busduct. Different

types of voltages, generally encountered in power systems have been

considered for the study of particle trajectories.

58

Fig: 1.12 Air Insulated Substation

Fig: 1.13 Gas Insulated Substation

59

Fig: 1.14 Heavy Cluster

Fig: 1.15 Industrial Pollution

60

Fig: 1.16 Yard Substation

Fig: 1.17 Gas Insulated Substation

61

Fig: 1.18 Assembly of Gas Insulated Substation

s

Fig: 1.19 Busduct

62

Fig: 1.20 145kV Indoor Gas Insulated Substation

Fig: 1.21 145kV Underground Gas Insulated Substation

63

Fig: 1.22 230kV Underground Gas Insulated Substation

Fig: 1.23 230kV Outdoor Gas Insulated Substation

64

Fig: 1.24 300kV Gas Insulated Substation

Fig: 1.25 380kV Underground Gas Insulated Substation

65

Fig: 1.26 400kV Gas Insulated Substation

Fig: 1.27 400kV Underground Gas Insulated Substation