1

CHAPTER-I

INTRODUCTION

1.1 INTRODUCTION

The increasing demand for electricity and the growing energy

demand in metropolitan cities have made it necessary to extend the

existing high voltage network right up to the consumer. During the

last two decades for reliable power supply and economic advantages

[1], Gas Insulated Substation (GIS) have found a broad range of

application in power systems, because of their high reliability, easy

maintenance, small space requirement etc. In our country, a good

number of GIS units have been in operation and a large number of

units are under various stages of installations. GIS is based on the

principal of operation of complete enclosure of all energized or live

parts in a metallic encapsulation, which shields them from the

external environment. Compressed Sulphur Hexafluoride (SF6) gas,

which has excellent electrical properties, is employed on the insulating

medium between the encapsulation and the energized part. GIS have

a grounded outer sheath enclosing the high voltage conductor.

Gas Insulated Substation comprises the following components-

circuit breakers, isolator, disconnector switch, earthing switch,

current transformers, voltage transformers, Busbars and connectors,

power transformers, surge arrestors, cable termination, Gas supplying

and Gas monitor equipment, Density meters and Local meters.

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It can be observed from the literature that GIS sub-station posses the

following advantageous features [2-5]

� GIS station occupies only about 10% of the space required by

conventional air insulated substation.

� GIS can be installed either underground or indoors and in

heavily populated areas.

� GIS are also conveniently used in coastal areas and industrial

and urban locations where space and pollution are the main

considerations.

� These substations are generally located closer to the load

centres there by reducing the losses in transmission and

distribution network.

� GIS systems are immune at atmospheric condition and

pollution, the outages get reduced and coupled with their

measured reliability, and the overall maintenance costs are

minimized.

� High service reliability due to non-exposure of high voltage parts

to atmospheric influences.

Disadvantages of GIS

Although GIS has been in operation for several years, a lot of

problems encountered in practice need full understanding. Some of

the problems being studied are:

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� Switching operation generate Very Fast Transient over Voltages

(VFTO).

� Prolonged arcing may produce corrective/toxic by-products.

� Partial discharges within the enclosures can cause break

downs.

� Metallic particle contamination.

� Transient electric field and transient magnetic fields.

� Field non-uniformities reduce withstanding levels of GIS.

In general, spacers are used to isolate different sections.

Spacers are either cone (or) disc shaped of large majority of spacers

are using alumina filled epoxy. Epoxy support spacers in GIS have

been highly reliable. Spacer insulation is the single most critical

component for the dielectric performance of GIS units. In modern GIS

design, internal stresses below 4kV/mm (rms) are used. The

requirements on GIS insulators are many and they must be able to

[6-8].

� Withstand the high internal and surface electric fields, typically

up to 4.0kV/mm (rms) for continuous operation.

� Withstand forces during transportation.

� Withstand short circuit forces.

� They must be made of a non-tracking material, so that no

conducting tracks occur during testing.

� Must be relatively insensitive to surface contamination.

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If it is a gas tight insulator, it must withstand a test pressure of

3.25 times the maximum working pressure and should be leak tight

so that not more than 0.5% of gas is lost per year. In a GIS, the

insulating media employed are the SF6 gas and the solid insulating

supports. The behavior of the insulating system depends on the basic

properties of the gas and the surface and volume properties of the

solid insulators. Spacers acquire charge from corona sources,

ionization in the gas and discharges from metallic particles.

Discharges from metallic particles and spacers together give the

lowest breakdown voltage. In recent years, there has been an

emphasis on the long term reliability of the epoxy spacers.

Moving particles give rise to both partial discharges and acoustic

signals. Particles can attach to spacer surface and can cause flashover

of the spacer. Forced services outages caused by spacer problems are

mainly due to excessive operating stresses.

The main types of defects that occur in GIS are free particles

and protrusions on electrode surfaces in the gas medium, surface

contamination by gas impurities and defects within the bulk of the

solid material. The above defects when present, give rise to local field

enhancements, which cause flashovers. If the field enhancements are

small, then there will only be streamer corona without any leader

development leading to breakdown. These corona discharges

chemically react with solid insulating spacers and modify their

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properties. When the applied voltage is further increased, the steamer

corona transforms into a leader discharge and the leader growth will

then be influenced by the space charges due to corona stabilization

under the application of slowly rising a.c or switching impulse

voltages. Breakdown occurs after the time that is necessary for the

leader propagation and voltage collapse. The time will be longer under

the application of steeply rising impulses (rise times lower than a few

microseconds) leading to higher breakdown voltages. Therefore, there

is a minimum voltage at which breakdown occurs under the

application of such fast rising impulses.

The defects in the SF6 gas used in the GIS include free floating

particles [9, 10] and metallic protrusions on electrode surfaces.

These particles can give rise to partial discharges in the vicinity of the

high voltage conductors. Free metallic particles on the electrodes get

charged and can traverse the complete gap in the case of a.c voltage

and the extent of this motion depends on the size of the particles,

their weight and the magnitude of the a.c field. During the motion of

the particles, charge exchange occurs by sparking before they hit the

surface.

The main defects include particles and flashover tracks on the

insulation surfaces as well as surface contamination while cracks and

voids within the bulk of the insulation can also cause problems.

Metallic and other particles can be sticking to the insulator surface,

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and they behave like protrusions on the electrode surfaces. The

breakdown voltage when particles are present will depend on the

tangential field on insulator surface, the field enhancement due to the

particles themselves, and the surface charges caused by earlier partial

discharges. These particles are considered to adhere to the insulator

surface at critical areas where they get charged by partial discharges

and move into low stress areas until the partial discharges extinguish.

These partial discharges give rise to surface tracks and reduce the

insulator surface resistance. Due to partial discharges or flashovers

during testing, the solid insulating material will be carbonized due to

the high temperature of the spark resulting in conducting track. A

track can be formed by the energy of single flashover or by successive

flashovers at same location.

These tracks from leakage paths eroding the insulator surface

over a period of time. Gaseous impurities and SF6 products also

contaminate insulator surface. Air, N2 and lower compounds of SF6 up

to 10% were found to have only a minimum effect on the dielectric

properties of SF6 gas. During the operation of GIS, SF6 decomposition

products are formed and the spacer should have adequate resistance

to these decomposition products. All GIS units in service use cast

epoxy spacers. In order to avoid the interface problem, where voids or

small gas gaps can initiate volume puncture or surface flashover, the

spacers are either cast directly on to the conductor or on to metal

inserts[11,12], or have the interface well shielded. Decomposition

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products of SF6 gas due to the partial discharges [13] and

disconnector switching are also observed to have no significant effect

but the main contaminant that effects the dielectric integrity is the

moisture in the gas. Moisture can be condensed on the insulator

surface and at the ambient temperature it can cause dew formation

on the insulator surface. A dew point of -5˚C is considered safe to

ensure that no harmful condensation of moisture in liquid form gets

deposited on the insulator surface.

Within the GIS, non-uniform fields are always present due to

the presence of dust, floating metallic particles, fixed particles in the

form of electrode surface roughness, condensation of moisture on the

insulator surfaces, etc.

The insulation strength of compressed SF6 is greatly decreased

by contamination in the form of conducting particles. Electrical

insulation performance of GIS systems is adversely affected by

metallic particle contamination. The accumulated field experience

indicates that sources for such contamination are mechanical

abrasions, movement of conductors under load cycling and vibrations

during shipment and service [14]. These particles may be free to move

in the electrical field or may be fixed to the conductors thus

enhancing local surface fields. In a horizontal co-axial system with the

particles resting on the inside surface of the enclosure the motion of

such particle is random in nature. The dynamics of wire particles in a

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horizontal co-axial system are studied because they approximately

correspond to the type of particles encountered in practice.

Another important source of field non-uniformity within the GIS

is sharp points or mechanical edges. These defects are often of minor

importance under normal power frequency voltages. However, steep

fronted impulse voltages such as lightning, impulse or very fast

transients can significantly decrease the di-electric strength of GIS

assembly in the presence of these particles. Good design and the

adoption of quality assurance methods at all levels will enable the GIS

manufacture to limit the quantity and size of any residual particles in

a modern GIS to insignificant levels. Although the likelihood of particle

effects in GIS is very small, it still does exists, which is why research

is in progress to develop diagnostic and analytical methods for

detecting and localizing them.

When the shape of the spacer is changed from an annular disc

to a conical disc keeping the thickness at the base constant, the field

around the insulator also changes. The field on the spacer surface

increases when the angle of inclination reduces while the maximum

stress occurs on the surface of the inner conductor covered by the

insulator. A shielding electrode having a suitable shape is used to

avoid this field concentration on the conductor. However, the field

concentration was observed to be zero when an annular disc insulator

is used.

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In recent years, from the view points of the environment friendly

and efficient power transmission, electric power equipment tends to be

compact and be operated under higher voltage. In a gaseous

insulation system, a solid insulator plays an important role for

mechanical support for holding insulation clearance between high

voltage and low voltage electrodes. In the insulation design of a gas-

solid composite insulation system which is typically included in GIS

and a Gas Insulated Transmission Line (GIL), etc., the insulation

technique in the gas-solid interface heavily becomes important as well

as the insulation both in gas gap and inside the spacer. In the

insulation of a gas-solid interface, various factors of significance are

contamination particles, voids, cracks, E-field intensification at triple

junction and charging on the spacer surface, as well as the electric

field distribution on the spacer surface with a perfectly pure condition.

For these reasons, the spacer insulation in the practical gas insulated

switchgears, are made improved by various techniques, for examples,

controlling the spacer shape, additional shielding electrodes for

relaxation of E-field, and the introduction of thin layer made of a low

conductivity material on the spacer surface. In addition, a lower

permittivity is being applied to the spacer. However, these techniques

lead to the complicated structure of the spacer which limits the

flexibility of the spacer design and increases the manufacturing cost.

In order to overcome the limitations, it is necessary to propose a new

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concept on the spacers by keeping their simple structure and

configurations.

With a new concept for spacer insulation an application of a

Functionally Graded Material (FGM) which has been developed

originally for the structural material under thermal or mechanical

severe stress in special environment. In electrical applications for us,

the FGM spacer has spatial distribution of dielectric permittivity and

can make the E-field distribution in and around the spacer more

suitable, thus achieving the efficient E-field control by keeping the

simple configuration of the spacer.

1.2 LITERATURE SURVEY

Ibrahim A. Metwally [15] in his research reported that, from the

last two decades, the evolutionary development of GIS has resulted in

higher integration of a number of new technologies to enhance

performance and reliability by reducing defects, having more compact

designs, and reducing maintenance intervals and costs. Incremental

improvements are continuing in interrupter technology, such as self-

extinguishing features at Medium Voltage (MV) and resistance

interruption at Extra and Ultra-High Voltages (EHV and UHV). In

addition, SF6 gas technology for circuit breakers, zinc oxide (ZnO) for

arresters, radio communication for condition monitoring, and a choice

of porcelain or polymer composite for the full range of equipment are

also some of the technologies integrated or innovated by GIS

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manufacturers in recent years. Recently, ac GIS ratings have reached

up to 1,100kV rated voltage and 50kA (rms) rated short-circuit

breaking current. In addition, 1,200kV ac GIS are going to be visible

very soon. Moreover, 500 kV dc GIS for dc transmission systems have

become available [16].

Epoxy or cast resin solid insulators are used as spacers in GIS.

They represent the weakest points in GIS systems as the electric field

on their surfaces is higher than that in the gas space [17]. Generally,

the higher the operating voltage of GIS, the higher is the failure rate

due to the higher electric field strength. In particular by means of

monitoring and diagnostic systems as about 61% of the Failures could

have been detected PD in compressed SF6 GIS arise from protrusions,

free conducting particles[18], floating components, and bulk

insulation defects (voids). These defects represent about 53% of the

total main failure causes in GIS. Some techniques are used for the

mitigation and control of particle contaminations in GIS are particle

traps, dielectric coating of the electrodes, the use of SF6 gas mixtures,

and the use of FGM as solid spacer with optimizing its profile. The

ultra-high frequency and acoustic emission techniques can be used

for GIS PD monitoring system, dramatic reduction in failure rates can

be achieved when using such systems.

G. Schoffner et al [19] discussed that Gas Insulated

Transmission Lines (GIL) area means of bulk electric power

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transmission at extra high voltage consists of tubular aluminum

conductors encased in a metallic tube that is filled with a mixture of

SF6 and Nitrogen gases for electrical insulation. Since the first

installation of GIL in 1975, second generation GIL has been developed

that is more economically viable and its design optimized both for

installation and operation. Where GIL is installed in combination with

Gas Insulated Switchgear (GIS), compact solutions can be delivered in

order to supply large amounts of electric power to meet the high

demand of large cities and industry [20]. These new possibilities can

mitigate power flow problems, reduce the risk of failure of electrical

transmission systems and enable the installation of optimum

solutions regarding technical, economical and environmental aspects.

The requirements to installations for high voltage power transmission

and distribution have changed. HIS and GIL as innovative products

offer new possibilities to cope with these new requirements.

Depending on each situation, by a coordinated application of the

different techniques of GIS, HIS, AIS, OHL and GIL the optimum

solution will be provided regarding technical, economical and

environmental aspects [21].

D.I.Yang et al explains that the insulator [22] made in epoxy

resin was widely used in SF6 GCB (Gas Circuit Breaker) and Gas

Insulated Switchgear because it’s electrical and mechanical property

are efficiency. Especially, spacer that was used for supporting the

conductor and gas division in GCB and GIS is regarded as of the

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important component affected on the lifetime of the power apparatus.

The authors report that two different results obtained in the

development of several type spacers for GIS [23]. Firstly, for the three-

phase spacer of 362kV GIS, they presented the optimal design that

was obtained by electric field analysis and mechanical stress analysis

using commercial program.

N. Giao Trinh et al [24] showed that in Electrostatic-field

optimization of the profile of the gas dielectric interface was studied as

a means of improving the dielectric performance of epoxy spacers. An

optimum disc shaped spacer is defined with a dielectric cone angle of

750, assuming a dielectric constant of the epoxy resin of 5 or higher.

The dielectric performance of the optimum disc shaped spacer is

found to be limited, however, to about 85% of that of the conductor

system without spacer. A new composite-shaped spacer was developed

which combines the advantage of the long leakage distance of a cone

shaped profile with that of the quasi-uniform field distribution of a

disc shaped profile. Tests indicate that a dielectric performance

comparable to that of a conductor system without spacer [25] is

possible with the new composite-shaped spacer. From this profile

optimization study on epoxy spacers for use in compressed

SF6insulated cables, the following conclusions are made [26].

� For simple cone-shaped interfaces, a range of optimal angles

could be defined as a function of the relative dielectric constant

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εr of epoxy. For practical values of εr, 5 and higher, the optimal

di-electric cone angle ranges from 650 to 800.

� Metal inserts embedded in the epoxy can have a beneficial effect

when located near the metal-epoxy-SF6 junction, since they

artificially reduce the local field intensity at these junctions.

� The best ac performance obtained with the optimum disc-

shaped spacer is about 85% of the intrinsic disc dielectric

performance of the test conductor without spacer at the

nominal operating gas pressure of 0.4 MPa.

J.M. Braun [27] stated that the Bulk failure by electrical treeing

of the solid dielectric in Gas Insulated Switchgear is relatively, the

general deterioration process in a void can be described as follows, “A

high field at the void location and the low dielectric strength of the

contents of voids result in partial discharges in the cavity”. This leads

over time to erosion and enlargement of the cavity and generation of

electrical "tree" channels which eventually bridge the insulation and

cause failure. Partial discharges in epoxy insulation occur when a

combination of a sufficiently high electric field stress and a discharge-

initiating free electron is present in a void. The process depends,

among other parameters, on the gas content within the void [28]. In

modeling partial discharge characteristics within spacers and

decomposition and pressure of gases are found inside the voids are of

prime concern. The gases that could be found are obviously residues

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of the epoxy curing process and include entrapped air, curing

byproducts, as well as thermal decomposition byproducts.

R. M. Radwan [29] et al describes that the effect of the spacer's

dimensions and its relative permittivity on the total electric field

distribution. These effects will be also outlined for a practical spacer's

shape. The field behavior near the triple Junction has been explained

[30].

� For the spacer's shape, the spacer's thickness has a

considerable effect on the maximum field value on its surface.

The optimum value of this thickness is 0.5p.u.

� The relative permittivity of the spacer's material has a

considerable effect on the field distribution especially around

and near the high voltage and low vo1tage electrodes.

� For a practical spacer's shape, the maximum electric field on

the convex side is about 20% higher than that on the concave

side and they occur at Rx=1.5 and 1.65 p.u. respectively.

� The electric field distribution near the triple junction has a

peculiar behavior. Theoretically, it becomes infinity or zero

depending on the spacer's relative permittivity and the spacer's

inclination angle "�" or in other words the spacer's thickness

"zs.".

J. Jia [31] et al resulted that In GIS, particles near spacer in

GIS tend to cause apparatus faults by leading flashover breakdown

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along spacer surface. Metal inserted spacer as a method is designed in

purpose to prevent particles from lifting and adhering to spacer. The

author’s showed that, the influence of metal inserted spacer on

particle motion in non-uniform electrical field under DC voltage is

calculated for three type spacers. The results show that metal inserted

spacer has good performance in preventing particle from lifting and

adhering to spacer for disk and ribbed spacer [32]. Also, metal

inserted electrode shows a different influence range on particle motion

for different spacers. The results can be used to analysis AC condition

[33-34] considering the root-mean-square value of voltage.

K. Itaka [35] et al discussed that the Problems concerning local

electric field intensification on a cone-type spacer which is fitted

between flanges in SF6-gas-insulated apparatuses were investigated.

Conventional structures, in which flat surfaces of the spacer come in

contact with rounded corners of the flange, sometimes cause

flashovers at considerably low voltages because of local field

intensification [36]. In the improved structure proposed by the

authors, surface shape of the spacer and contact position are slightly

changed in order to avoid local field intensification. Field calculations

and experiments verified that the improved structure is effective for

actual use. Problems concerning local field intensification on a cone-

type spacer fitted between flanges in SF6-gas insulated apparatus

were investigated. The results are summarized as follows:

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� The conventional structure sometimes caused flashover at quite

low voltages because of local field intensification.

� It was made clear quantitatively that the above characteristic is

caused since the spacer makes contact with the flange at the

interface between rounded surface and flat surface which has

increased electric stress, and since the small gas gap near the

spacer-flange interface becomes like a wedge.

� An improved structure was designed avoiding these problems.

� The effectiveness of the proposed structure is made clear by

field calculations and experiments. Since these results can be

applied for the design of not only cone-type spacers but also

disc-type spacers, they are significant for the insulation design

of practical gas-insulated apparatuses.

N. Giao Trinhn [37] et al studied that the spacer and a

composite-profile cone, were evaluated in a coaxial conductor 2.5 X 7

cm in diameter under the influence on the V-t characteristics [38] of

the conductor when subjected to repeated applications of impulse

voltages of constant wave shape and increasing magnitude. The

results show that an insulating spacer can reduce the critical

withstand voltage and yield smaller dispersion in the breakdown

voltages. These effects can be minimized by adopting a design that

favors breakdown in the gas rather than along the spacer interface

[39]. The following conclusions are made.

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� The presence of a spacer results in a reduced withstand voltage

of the conductor, a shorter delay time to breakdown and less

dispersion of the breakdown data.

� The influence is more pronounced under negative polarity and

at higher gas pressures.

� Proper design of the spacer, aimed at preventing breakdowns

developing along the interface, can minimize the effects on the

V-t characteristic of the conductor.

� The influence of the spacer is also more pronounced in a 50%

SF6 - 50% N2 mixture than in pure SF6.

� Insulating spacers were observed to cause a temporary

reduction in the withstand capability of the cable, associated

with electrostatic charging of the insulators.

V.V-Akimov [40] et al stated that DC electric strength of pure

SF6 gaps is almost the same as that for AC ones. However, DC

electrical strength of real insulation systems including support epoxy

spacers is apparently lower than that with AC. One of the major

causes of such phenomena relates with the difference between AC and

DC spacer electric field formation mechanisms. There are free electric

charge accumulation processes on the spacer surface during long-

term DC voltage application [41]. This may lead to substantial

distortion of an initial (capacitive) field distribution near the spacer

surface and as a result to decrease in flashover voltage. In this

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connection, the design criteria developed for AC spacers are not

enough valid for their using with DC insulation.

0. Farish [42] et al discussed that in compressed-Gas-insulated

equipment, the weakest point in the system is often at the interface

between the gas and the solid spacers used to support the conductors.

The low dielectric strength is attributed to the effects of surface

charges, or to ionization at high-field sites such as the gas electrode-

spacer "triple junction", there have been some studies of surface

charge development under dc stress relatively little is known about

the way charge builds up on a surface and how the surface charge

influences the breakdown process under impulse conditions [43]. In

the present work a study was made of impulse flashover of model

spacers under conditions where:

� Surface charge was allowed to accumulate as a result of

repeated impulse stressing.

� A fault was simulated by introducing an annular gas gap at the

triple junction

� The spacer surface was recharged with a line charge or uniform

charge distribution

� Metal inserts were used to shield the triple junction and move

the high-field site to the mid-gap region.

An important feature of the study was that, in all cases, the

surface charge density was measured before and after each impulse-

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voltage application, the equipment was designed so that the complete

surface of the spacer, including both triple-junction regions, could be

scanned by a charge probe. For a plain spacer, the response to

impulse stress is determined by conditions at the impulse junction.

For defects of a few tens of microns, discharge activity begins at about

70% of the limiting field strength but the breakdown level is only

slightly affected, with large defects at the cathode triple junction the

onset level is considerably reduced and charge can be deposited over

most of the spacer surface [44]. If charge is allowed to accumulate the

impulse strength can be reduced by as much as 30%. When controlled

charging methods are used to create regions of high charge density

the strength can be as low as 50% of the gas-only value, even for

pressures of 1 bar, with the greatest reduction occurring for

deposition of hetero charge. Inserts can provide effective shielding of

the triple junction. However, they introduce a normal field component

which can attract surface charges. This may be detrimental in a

system in which charges are produced as a result of micro discharge

activity in the gas.

K. Tekletsadik [45] et al discussed about the Breakdown or

flashover arcs in Gas Insulated Systems (GIS) that produce a magnetic

force which influences the path of the arc and has a decisive effect on

the spacer-surface damage experienced during flashovers. A mild-steel

flange and aluminum disc piece were made to hold the test epoxy-

resin spacer and to have an access to take arc photographs in an

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open-to-air configuration at the open end of the GIS [46-48], with

special spacer-electrode arrangements to study the arc lift off and

push onto the surface during flashover on both sides of the spacer.

The principle of arc dynamics is discussed, along with, experimental

arrangements, results obtained from arc photographs and the effects

of the arc path on spacer-surface damage.

Breakdown and flashover arcs in GIS produce a magnetic force

which influences the motion of the arc. The arc dynamics are found to

have a decisive effect on the spacer-surface damage experienced

during flashovers. A flashover arc can be pushed onto the surface of

the spacer or lifted off the surface depending on which side of the

spacer surface failed. An arc path which is pushed onto the surface

causes more severe surface damage. It should be noted by designers

that there is, in many cases, a strong probability that the current flow

will come from one side of the spacer, and in these cases the spacer

geometry can be modified to allow the magnetic-field effect to lift the

arc away from the surface of the spacer.

Shigemitsu Okabe[49] et al stated that The insulation strength

decreased the most when the lighting impulse voltage was applied to

internal insulation of the spacer, In the experiment in which

alternating current voltage is applied for a long period of time, it was

found that there is no decline in the insulation properties even after

the voltage is applied for the equivalent of 30 years when the electric

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field intensity is 12kVrms/mm or less although the combination with

the multiple lightning impulse application may bring about damages

to the spacer insulation. The degradation mechanism caused by

generation of micro-pits was also understood through simultaneous

microscopic observation of the surface and of the interface between

the electrode and epoxy.

In order to contribute to high reliability and rational Insulation

design of Gas Insulated Switchgear, V-N characteristics [50] (the

dielectric breakdown voltage vs. number of repetitions of voltage

application characteristics) regarding the internal insulation and

creeping insulation of the epoxy spacer, the main insulating element

of GIS, were obtained against the lightning impulse voltage and the

switching impulse voltage. Further, effects of long-time ac voltage

application on spacer degradation were examined and the following

results were obtained.

� Regarding “V-N characteristics (internal insulation, creeping

insulation)”The gradient “n” of V-N characteristics of the

epoxy spacer internal insulation. The proportional decrease

of dielectric strength was largest when the lightning impulse

was applied to the epoxy spacer internal insulation.

� Micro discharge traces were observed both on an embedded

electrode surface and resin surface on the interface in a

spacer to which impulse voltages were applied repeatedly or

ac voltage was applied for a long time. The generation of the

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micro-discharge-traces was influenced by the roughness of

the electrode surface and flaking on the electrode interface.

Spacer insulation degradation caused by impulse and ac

voltage application is assumed to lead to erosion due to the

spread of discharge traces when the electric field is

intensified at the micro protrusions and the small areas of

flaking on their tips.

T. Nitta et al [51] observed that various factors controlling the

flashover of solid insulators in pressurized SF6, are reviewed and their

influences in gas insulated systems are discussed from a practical

point of view. Flashover voltage of clean insulator surface is under the

influence of the insulator-metal contact as well as the macroscopic

electric field distortion due to the high dielectric permittivity of solid

insulator. Conducting particles or even fine metal powder can reduce

the flashover voltage. Their effects are strongly dependent on the

position they are located, the size of the insulator and gas pressure.

Humidity of SF6, gas should be strictly governed in SF6, gas insulated

apparatuses, and since the condensation of water can decrease

flashover voltage considerably. Decomposition products of SF6, due to

the arcing in switchgears are deleterious to epoxy insulators [52]

particularly when silica is used as their filler. The decomposition

products decrease the leakage resistance on the insulation surface.

The field strength near positive electrode is enhanced by the

electrolytic effect in the surface conduction layer. In some extreme

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condition, it initiates tracking on the insulator surface. Some of the

important factors influencing the flash over characteristics on the

surface of solid insulators in compressed SF6 [53].

R. M. Radwan [54] et al stated that the Solid insulating spacers

are one of the critical components affecting reliable performance of

Gas Insulated Systems. The breakdown strength of GIS is strongly

influenced by the roughness of the spacer's surface and defects

produced from improper manufacturing. Also, GIS are likely to be

contaminated with non-conducting and conducting particles,

produced during mechanical abrasion or arcing occurring during

operation of the isolating switches and circuit breakers. The presence

of a conducting particle in a GIS can strongly influence the dielectric

performance of the system. This depends on the type, location and

density of the particles. Studies reported on scaled models and on an

actual spacer with a particle fixed on its surface have revealed drastic

reduction in the system breakdown voltages. Therefore the knowledge

of the electric field intensity around the spacer's defects and

conducting particles on its surface contributes towards better

understanding of its surface flashover phenomenon [55]. The Finite

Element Method has been employed to compute the electric field at

the dielectric interface. It is an efficient technique for solving field

problems. The following conclusions are drawn:

25

� The electric field on the spacer's surfaces is strongly affected by

the presence of surface defects. It may increase to almost 150 %

or 185%from its value without protrusion or depression

respectively.

� There is no noticeable effect of changing the defect’s position on

the electric field intensification; the Defect Field Factor (DFF) is

almost constant at 1.5 and 1.85 for protrusion and depression

respectively.

� The electric field on the spacer's surfaces is also strongly

affected by the presence of conducting particles. It increases at

the particle location on the spacer's surface. It reaches almost

1.3 its normal value for a particle of 2 mm.

� The Particle Field Factor (PFF) is almost constant at 1.15,

wherever the particle is located, for a particle of 1 mm.

� The electric field at the spacer's surface decreases, at the

particle location, with the increase of the vertical elevated

distance of the flying particle, and it is almost negligible when

the particle elevated distance "hp" is 5 times the particle's

diameter.

� For an adhered particle, the electric field reaches almost 3.6 its

normal value for a particle of 2 mm. Also the PFF is almost

constant at 3.6, wherever the particle is located for a particle of

2mm.

26

Hideo Fujinami [56] et al discussed that the Mechanism and

effect of the dc charge accumulation on the surface of solid insulating

support (spacer) have been studied in compressed SF6 gas, using

various cylindrical model spacers [57-59]. The distribution of surface

charge has a close relation with the normal component (gas side) En of

electric field on the spacer surface. The maximum charge density can

be estimated from the condition of En = 0. When voltage is applied in

a polarity opposite to pre-stressed dc, surface charge increases the

maximum field strength in the arrangement, thus resulting in the

reduction of the insulating ability. It is possible to estimate the lowest

flashover voltage due to surface charge only from numerical field

calculations [60]. An anti-charging spacer shaped along electric lines

of force has been proposed and studied. Mechanism and effect of the

dc charge accumulation have been studied in compressed SF6 gas,

using various cylindrical model spacers.

The main conclusions are as follows.

� The surface charge distribution on a spacer has a close relation

with the normal component (gas side) En of the electrical field on

the surface, and is also influenced by the surface roughness.

� Possible causes of surface charge are (a) micro discharge or field

emission from surface projections, (b) motion of dust particles,

and (c) natural ionization of SF6 gas in a prolonged time range.

Charge carriers drift through the gas along electric lines of force

27

up to the maximum charge density on the spacer surface given

by the condition of En = 0.

� Numerically calculated results of maximum charge density

showed good agreement with the experimental values.

� The flashover voltage of a spacer with surface charge can be

estimated by composing the two fields due to the surface charge

and due to the applied voltage without charge. It is possible to

estimate the lowest flashover value that is in the safest side,

only from numerical field calculations.

� An anti-charging spacer profile which has no normal field

component on the surface was proposed and verified

experimentally.

T. Nitta [61] et al stated that a technique to design, fabricate

and test dc gas-insulated switchgear has been developed to apply the

advantages of compressed gas insulation to metal-enclosed HVDC

equipment [62]. Charge accumulation on solid insulators is one of the

fundamental problems which have to be solved in establishing the

design stress for HVDC equipment. The present theory is a review of

the studies which have been performed in the development of a +125

kV HVDC gas-insulated converter station and +500 kV HVDC-GIS.

The properties and mechanisms of surface charging, the optimum

design of the spacer and its breakdown characteristics are

summarized. Problems associated with capacitive probe measurement

of surface charge and a practical solution to obtain the charge

28

distribution on the spacer, are presented. Charge accumulation on

spacers in HVDC gas insulation has been studied as one of two

critical factors in designing the insulators applicable to 550 kV HVDC-

GIS. From the results, the following conclusions have been drawn [63-

65].

� The analytical computation method to evaluate charge

distribution on a conical spacer from the capacitive Outputs has

been developed. At present the method is the only way to

transform the probe measurements into charge densities on the

spacer surface.

� In an industrially clean system, the charge carriers are

transported from the surface of the conductor and the sheath to

the surface of the spacer through the gas phase.

� Negative charges due to field emission from micro protrusions

and/or micro dust attached to the highly stressed parts of the

conductor and the sheath is the source of charged carriers.

� We should design the physical configuration of the spacer and

the electrodes for DC-GIS in such a way that the surface of the

spacer intersects the electric field lines in acute angle as

possible. We should take care to avoid the local enhancement of

the electric stress on the conductor and sheath. This practice is

different from avoiding the sharp edges which have influenced

the insulation design of AC-GIS.

29

� The conical and post spacers which are selected as the optimum

design for HVDC-GIS exhibit satisfactory results even at dc

polarity reversal.

M.M. Morcos [66] et al observed that the use of compressed gas

as the insulating medium has made it possible to use compact

equipment compared to that with air insulation. However, the

compact construction increases the operating field intensity. Sulphur

hexafluoride (SF6,) gas insulation is extremely sensitive to local

increases in electric field, which results from protrusion 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 inside

its grounded casing. The influence of a metallic particle attached to

the spacer is particularly significant in the decrease of the dielectric

strength of the SF6, [67] insulated system. Therefore, for development

of highly reliable compact gas-insulated systems, it is vital to reduce

the effect of metallic particles. The flashover withstand of a gas spacer

interface is a limiting factor in the design and operation of a SF6, gas

insulated system. The surface flashover shows a strong sensitivity to

the metallic particle contamination [68] of the spacer surface. The

particle may cause a flashover at a small fraction of the clean gas gap

breakdown voltage. The particles initiate spacer flashover at low

voltage values, not only for AC and DC voltages, but also for impulse

and oscillating impulse voltages. Therefore, it is reasonable when

30

commissioning a SF6 gas-insulated system to carry out tests with a

voltage wave form for which the particle-contaminated insulation is

more sensitive and/or to use diagnostic measurements in order to

detect the presence of particles.

H.Maekawa [69] al presented that the detecting of Partial

Discharge (PD) in gas insulated switchgear is one of important

monitoring terms. Authors studied the behavior of surge due to PD in

gas insulated switchgear. PD can be detected by catching the

electromagnetic wave radiating from the insulated spacer between

enclosures, because coaxial between central conductor and enclosure

is not completely in insulating spacer part. Antenna can be caught the

electromagnetic wave by catching the several waves from different

propagating paths. In the same time, PD location [70] in GIS can be

estimated. In this system, they have developed and applied to actual

500kV GIS and well performed. In the study, obtained results are

summarized as follows.

� PD in GIS can be detecting by catching the electromagnetic

waves with detecting antenna placed by insulated spacer.

� By finding the arrival timing of signals obtained by each

antenna and calculating the time domain differences among

them, the location of PD can be estimated easily.

D.A. Mansour [71] et al showed that the high reliability, less

maintenance and compact size of Gas Insulated Switchgears have

31

made them the primary choice for many utilities. However, sometimes

insulation defects inside GIS can be a serious threat to safe operation

of GIS and can lead to costly disruption of supply. As insulation

failure usually starts with partial discharge (PD) activity, author’s

investigates the differences in PD characteristics [72] in SF6 gas

among different types of defects. The defect types considered in this

study are particles in a gas gap; particles adhered on a spacer surface

and spacer/electrode detachment. Different experiments were made

for sequential PD measurements [73] using the system of PD-Current

Pulse Waveform Analysis [74] (PD-CPWA). The PD phase

characteristics, PD pulse number and PD current were analyzed for

the different defect types. Also the ratio of voltage increment to phase

increment at the next PD pulse appearance (∆u/∆φ pattern) was

obtained and compared for each defect type. Experimental results

shows that correct identification of defects can be achieved based on

considered PD characteristics. Partial discharge characteristics were

measured and analyzed with a wideband (4 GHz, 20 GS/s) measuring

instrument to identify the type of different defect types inside GIS.

Different electrode setups were built for simulating possible defect

types in GIS. Three types of defects were examined for

spacer/electrode detachment, particles in a gas gap and particles

adhered on a spacer surface.

Naoki Hayakawa [75] et al stated that A metallic particle

appeared in a gas insulated switchgear sometimes adheres on a solid

32

spacer surface. If the adhered metallic particle is exposed to a surge

high voltage, a breakdown (BD) may be induced. Therefore, it is

eagerly demanded to diagnose its risk correctly under the service

voltage by partial discharge (PD) measurement. In his research,

particle-initiated surface PD characteristics were systematically

studied in 0.4 MPa SF6 gas by changing the sizes of particles. PD

inception voltage [76], temporal change of PD current and the PD

pulse number were analyzed in detail. Furthermore, comparing with

PD characteristics of particles in a gas gap, the influence of the solid

insulator on the PD characteristics was clarified. It was found out that

PD characteristics greatly changed with time owing to electric charges

deposited on a spacer surface. PD characteristics of various metallic

particles on the epoxy plate were measured and analyzed using the

ultra-high speed measurement system. Temporal change of PD

characteristics and dependence of PD characteristics on the particle

size is analyzed. The following results are obtained.

� The electric field strength near the metallic particle tip was

intensified extremely when a metallic particle was fixed on the

epoxy plate. And PDIV decreased by about 20 ~ 30%.

� PD didn’t appear in several cycles after the voltage application

even if the applied voltage was much higher than PDIV.

Temporal change of PD characteristics was extremely large

immediately after the voltage application.

33

� PD current increased with the particle diameter. PD pulse

number depended on the particle diameter and time.

Immediately after the voltage application, the PD pulse number

decreased with metallic particle diameter. However, after several

minutes, the PD pulse number started to increase with the

particle diameter.

� Complex PD characteristics of different sizes of particles were

qualitatively explained with the surface charges accumulated on

the epoxy plate near the particle tip.

Hirotaka Muto [77] et al studied that as a mean of diagnosing

partial discharge (PD) signals propagate inside a Gas Insulated

Switchgear, a study for the leakage of electromagnetic waves [78-79]

(EM-waves) emitted from the insulating spacer was implemented. The

EM-waves leaking out from the solid insulator have the resonance

frequencies [80] depend on the spacing between adjacent bolts in the

direction of the flange circumference, because the leakage portion is

the equivalent of a slot antenna. In the present work using an

electromagnetic analysis model which has a simulated spacer on a

concentrically-shaped GIS tank, the output characteristics of the EM-

waves that leaked out from the slit ere analyzed under various

conditions such as the spacing between adjacent bolts the width of

the spacer, the dielectric constant of the spacer and the form of the

flange. Also the actual measurement by the experimental equipment

used to simulate the model was implemented for comparison with the

34

analytical results. Consequently, the optimal specifications of the

sensor and the measurement method used to achieve highly-sensitive

detection for practical use are summarized and proposed as well as

evaluating the effectiveness of the electromagnetic analysis model are

adopted.

Katsumi Kato [81] et al describes that the applicability of the

FGM spacer to gas insulated power equipment. In the FGM spacer,

they gave the spatial distribution of dielectric permittivity to control

the E- field distribution inside and outside the spacer. Firstly, E-field

simulation results when applying the FGM by a finite element method

are presented, in which they show that effective reduction of the

maximum field strength by applying the FGM. Next, a fabrication

technique of the FGM spacer sample with not only step-by-step but

also continuous changes of permittivity [82] is presented by use of

centrifugal force.

Finally, authors proposed the application of FGM as a spacer

material for gas insulated switchgears. The application effectiveness

was verified by numerical simulation and experimental results and

they made continuously changed distribution of permittivity and

controlled it by applying the centrifugal force [83]. They optimized the

fabrication condition for the permittivity distribution and these

fabrication techniques are expected to be extended to future electric

power equipment.

35

Masahiro Hanai [84], et al describes that for the size reduction

and the enhancing reliability of electric power equipment, the electric

field stress around insulators should be considered enough. For the

relaxation of field stress, the application of FGM with spatial

distribution of dielectric permittivity can be an effective solution.

Investigating the applicability of FGM for reducing the electric field

stress on the electrode [85] surface with contact to solid dielectrics,

this was one of the important factors dominating a long-term

reliability of the insulating spacer.

At last they implemented the application of FGM for reducing

the electric field stress on the electrode surface in contact with solid

insulators, which was one of the important factors dominating a long

term insulating property [86] of the solid spacer. The FGM application

effect was verified by numerical simulation of electric field and life

time estimation. They made U-shape permittivity distribution [87] and

controlled it by applying the centrifugal force, their application

duration, author’s made various types of the Y-shape permittivity

distribution. These fabrication techniques [88] are expected to be

extended to the actual application of FGM to the electric power

equipment and estimated a long-term insulation performance for the

fabricated FGM sample and found the significant effect for life time

extension by the application of FGM. Finally, authors verified that

high performance of electrical insulation of solid spacer could be

obtained by a permittivity graded FGM application.

36

Heung-Jin Ju,[89], et al investigated for use as an insulator in

high voltage applications. The FGM was able to relax the electric field

concentration around a high voltage electrode and along gas-insulator

surface. The FGM, spacer the permittivity of which was gradually

changed, exhibited a considerable reduction in the maximum electric

field when compared to a conventional spacer with a uniform

permittivity. It is difficult to apply a gradual permittivity variation in

the FGM spacer to real product processing due to its complicated

shape. Thus, in this work, the electrode shape in gas insulated

switchgear was changed in order to increase the possibility of real

FGM insulator. Consequently, the insulation capability of the

switchgear with the optimally designed FGM spacer [90] can be

efficiently improved.

They implemented to improve the insulation capability

capability of switchgears, an elliptical FGM spacer was proposed. The

electrode shape was modified and optimization of the FGM spacer

configuration was performed. By modifying the electrode shape, the

maximum electric field intensity was slightly reduced [91] and the

electrode structure was simplified. The maximum electric field was

also efficiently reduced through the application of the FGM spacer.

Consequently, the optimal designed elliptical FGM spacer model was

more efficient than the uniform permittivity spacer model in improving

the insulation performance.

37

Hitoshi Okubo [92], et al proposed a new concept for solid

insulation, an application of FGM. By controlling the distribution of

dielectric permittivity inside solid insulators [93], one can achieve the

efficient field control with keeping simple configuration of solid

insulators. In this paper, authors described a computer-aided

optimization technique for the FGM solid insulators. By controlling

the filler particle concentration in the matrix, an optimized field

distribution is obtained.

Authors verified the compact design of gas insulated

equipment, they proposed the application of FGM to the solid spacer.

Firstly, they proposed a computer-aided optimization technique for the

FGM solid insulators. In the optimization process, permittivity

distribution of the FGM solid insulator is sequentially modified for

minimizing the electric field stress in and around FGM solid

insulators. Consequently, they successfully developed the

optimization techniques on permittivity distribution in FGM solid

insulators. Next, in order to verify the optimization efficiency, they

carried out the optimized distributions of permittivity in the cone

spacer calculation model. Finally, they could confirm the significant

effect of FGM application for gas/solid composite insulation system

[94].

Hitoshi Okubo [95], et al describes that the electrical

insulation designs of GIS spacer, they need to control electric field

38

distribution around solid spacer, especially around the triple junction.

For this purpose, they proposed the application of FGM [96], which

has spatial distribution of dielectric permittivity, to the spacer of SF6

GIS. In this, they discussed applicability of FGM with numerical

simulation and fabrication conditions. They firstly investigate the field

control effect of the FGM spacer with a conical shape, by FEM.

Secondly; they fabricated FGM spacer with continuously graded

distribution of permittivity by applying the centrifugal force. As for the

filler material to control permittivity, they selected TiO2 rutile crystal

particle. In order to obtain the optimum permittivity distribution, the

centrifugal forces, their application duration, the diameter distribution

filler particles, volume ratio of filler versus resins and so on were

controlled.

They implemented the compact design of electric power

equipment, author’s proposed the application of FGM to the solid

spacer. As one of the estimations of the applicability to the GIS

spacer, they introduced conical spacer models. Furthermore, in order

to fabricate effective FGM spacer, they made continuously graded

distribution of permittivity and controlled it by applying the

centrifugal force. Finally, they optimized the fabrication condition and

obtained the permittivity gradient from 4.0 to 9.0 by using TiO2

particle filler. These techniques of permittivity distributions are

expected to be applied to future electric power equipment.

39

Summary: The following aspects have been observed from the

Literature cited above,

� GIS have been used as an improvement for several problems,

such as space saving of substations, reliability and safety

installation, maintenance costs and environmental

disturbances.

� The SF6 having many advantages like non-toxic, superior

cooling characteristics, greater dielectric strength and arc-

quenching properties.

� The conventional spacer sometimes caused flashover at quite

low voltages because of local field intensification.

� Conventional spacer having the weakest point in GIS system as

the electric field on their surface is higher than that in the gas

space.

� The failure of the conventional spacer have been detected PD in

compressed SF6 GIS arise from protrusions, free conducting

particles, floating particles and bulk insulation defects(voids).

� GIS systems used cast filled epoxy resins spacers that are cast

in metallic moulds under vaccum and cured under carefully

temperature conditions and this is to ensure that the casting is

free from voids and cracks, and to ensure good adhesion to the

conductor or metal sleeve on which the spacers are cast.

40

� The conductors used in GIS are made of alluminium alloy or

high conducting copper on the other hand, materials used for

enclosure are mild steel, stainless steel or alluminium alloys.

� GIS has to withstand different types of voltage stresses that

occurs in the network such as continous power frequency

voltages, temporary over voltages and transient over voltages.

� In a GIS, the insulating media employed are the SF6 Gas and

the solid insulating support. The behavior of the insulating

system depends on the basic properties of the gas and surface

and the volume properties of the solid insulators.

� The insulation of Gas-solid interface have to consider various

factors like contamination particles, voids, cracks, E-Field

Intensification at triple junction and charging on the spacer

surface.

� The spacer insulation in Gas insulated switchgear are made

improved by various techniques. For example, controlling the

spacer shape additional shielding electrodes for relaxation of E-

Field and the introduction of the thin layer made of a low

conductivity material on the spacer surface. In addition a lower

permittivity material is being applied to the spacer.

� This techniques leads to be complicated structure of the spacer

which limits the flexibility of the spacer design and increases

the manufacturing cost.

41

� A new concept of spacer insulation, an application of a

Functionally Graded Material (FGM) which is developed for the

structural material under thermally or mechanically severe

stress in special environment.

� The application of FGM spacer has spatial distribution of

Dieletric permittivity inside. By the control of the distribution of

dielectric permittivity, it can make the E-Field distribution in

and around the spacer more suitable.

1.3 STATEMENT OF THE PROBLEM

The present and future trend in electric power equipment tends

to be compact and be operated under high voltage. The modular

design of GIS offers a high degree of flexibility to meet layout

requirements of both substation as well as power station switchgear,

making efficient use of available space. GIS technology has reached a

stage of application where wide ranges of GIS equipment up to highest

voltage of 800kV are available with many unique features. In a

gaseous insulation system, a solid insulator (spacers) plays an

important role for mechanical support for holding insulation clearance

between High Voltage (HV) and Low Voltage Electrodes (LV). In the

insulation design of a gas-solid composite insulation system which

typically include in Gas Insulated Switchgears (GIS) and a Gas

Insulated Transmission Line (GIL), etc., the insulation technique in

the gas-solid interface becomes important as well as the insulation

42

both in gas gap and inside the spacer. In the insulation of gas-solid

interface, various factors like, contamination particles, voids, cracks,

E-field intensification at triple junction and charging on the spacer

surface have to be considered. For these reasons, the spacer

insulation in the practical gas insulated switchgears are made

improved by various techniques, for examples, controlling the spacer

shape, additional shielding electrodes for relaxation of E-field, and the

introduction of thin layer made of a low conductivity material on the

spacer surface. In addition, a lower permittivity is being applied to the

spacer. However, these techniques lead to a complicated structure of

the spacer which limits the flexibility of the spacer design and

increases the manufacturing cost. In order to overcome the

limitations, it is necessary to propose a new concept for spacer’s

design which maintains simple structure and configurations, with the

use of FGM application to the solid spacer, the properties of the

insulator can be changed to get the required relative permittivity at

specific location. FGM insulator is proposed for which the permittivity

of the spacer is sequentially modified for minimizing the electric field

stress in and around the FGM insulators.

Mainly, the various types of insulator shapes have been

applied for single phase and three phase systems and the electric

fields contours along the surface of insulator spacers are

obtained for conventional and FGM. The results obtained from the

numerical simulation of various insulator shapes of conventional and

43

FGM have been compared and analyzed. From those results it is

understood that the applicability of the FGM to the solid dielectrics is

for improvement of the electric field stresses.

1.4 MODELLING TECHNIQUE

In the proposed work, in order to determine (estimate) the

critical electric field values, first numerical simulations have been

carried out for various insulator shapes for both single and three

phase GIS systems. The E-field around the surface of insulator along

X-axis and Y-axis has been obtained for various shapes of insulating

spacers. Next, to reduce the critical filed on surface of conductor in

both single phase and three phase GIS systems by using FGM. The

modelling of each material has been discussed in detail. In order to

estimate these stresses for single phase GIS, the dimentions are

considered like outer diameter of the enclosure is 241mm and

thickness is 6.4mm. The outer Diameter of the conductor is 89mm

and thickness is 0.7mm and the materials used for conductor and

enclosure is Aluminum Alloy and for Insulator is Epoxy resin and Gas

is SF6.

For three phase GIS models, the dimentions are considered like

the each phase conductor diameter of the conductor is 89mm, the

diameter of the enclosure is 508mm, the thickness of the enclosure is

6.4mm and the thickness of the conductor is 12.7mm. The materials

used for different geometries are, for conductor and enclosure material

44

it is Aluminum Alloy, for insulator material it is Epoxy resin, and the

Gas used is SF6

The results obtained from the numerical simulation of various

insulator shapes of conventional and functionally graded material

spacers have been compared and analyzed with Finite Element

Method (FEM). From those results the applicability of the FGM to the

solid dielectrics for improvement of the electric stress and the long

term insulation performance in electric power equipment is verified.

1.4.1 Functionally Graded Material (FGM)

The term Functionally Graded Material (FGM) refers to solid

objects or parts that usually consist of multiple materials or

embedded components, that is, they are materially heterogeneous.

The term “heterogeneous object” is defined for those objects with

and/or multiple material objects with clear material domains.

A FGM consists of a material whose properties change from one

surface to another according to a smooth continuous function based

on the position throughout the thickness of the material. Electric

power equipments tend to be compact and then operated under higher

electric field stress. Especially, in gas insulated power equipments

such as GIS, solid insulators play a critical role for electrical

insulation. To improve the insulation performance of solid insulators,

it is needed to control the electric field distribution around the solid

insulators. With a new concept on solid insulators with keeping their

45

simple structure and configuration, the application of FGM with

permittivity distribution to solid spacers for GIS, and made the

fundamental investigation of FGM.

In GIS/GIL designs often use gas tight spacers to separate

different bus compartments. It is preferable to limit the bulk field to

below 4kV/mm (rms). Most spacers are cast, filled epoxy resin

systems. Resins are usually bisphenol A, cycloaliphatic or hydantoin.

Fillers like silica and quartz have been used and are necessary for

good thermal and tracking properties and minimal shrinkage during

casting. The epoxy formulations are, however, proprietary.

Silica/quartz fillers are subject to corrosion damage when SF6 arcing

by-products are present. In the FGM solid insulators, spatial

distributions of relative permittivity are given for the control of the

electric field distribution in and around the solid insulators.

Conventional materials have constant relative permittivity distribution

on the contrary, FGM materials have continuously graded relative

permittivity distribution by the arrangement of filler particles. As a

filler, Al2O3, SiO2 or TiO2 particles are applied with several 10 µm ~

sub µm diameter. In order to relax the stress concentration, the

application of FGM is expected to be effective by giving the suitable

relative permittivity (εr) distribution inside the insulators.

46

1.5 ORGANISATION OF THESIS:

CHAPTER I Presents introduction to Gas Insulated Substations and

the various factors like contamination particles, voids, cracks, E-field

intensification at triple junction and charging on the spacer surface in

GIS are discussed. Since, the spacer insulation in the particle gas

insulated switchgears, are made improved by various techniques for

examples, controlling the spacer shape, additional shielding electrodes

for relaxation of E-field, and the introduction of thin layer made of low

conductivity material on the spacer surface. This chapter verifies the

effects of FGM on the improvement performance by numerical

simulation.

CHAPTER II Presents the statement of problem, introduces the

concept of FGM, main contribution of the thesis and Organization of

the thesis.

CHAPTER III Presents the application FGM and also different

Geometry’s applied for single phase bus duct, the obtained electric

field stress for different insulators along the surface of spacers are

compared and analysed for conventional and FGM spacer is

presented.

CHAPTER IV Presents different Geometry’s applied for three phase

bus duct the obtained electric field stress for different insulators along

the surface of spacers are compared and analysed for conventional

and FGM spacer is presented.

47

.CHAPTER V Presents the conclusions of the proposed research work.

The future scope of this work is also discussed.

1.6 SUMMARY

The statement of the problem formulation has been done and

the contributions of the proposed research work have been given in

detailed. Moreover, the organization of the thesis has been given in

this chapter.