Electric Field Stress mitigation in a Gas Insulated Substation ...acadpubl.eu/jsi/2017-114-7-ICPCIT-2017/articles/8/13.pdfElectric Field Stress mitigation in a Gas Insulated Substation

Electric Field Stress mitigation in a Gas Insulated

Substation under delamination defect

K.Appala Naidu1,G.V.Nagesh

Kumar2,K.Jaya Ram3 and D.Deepak

Chowdary4

1, 2, 3Vignan‟s Institute of Information

Technology, Visakhapatnam, Andhra

Pradesh, INDIA

4Dr L Bullaya College of Engineering

[email protected]

April 14-15,2017

Abstract

To ensure continuous service and reduce the size of

switchgear in a Gas Insulated substation (GIS), the

insulating property of the supporting spacer plays

an important role; this depends upon the electric

field stress distribution on the surface of the spacer.

The electric field stress should be mitigated at the

Triple Junction (TJ) which is formed by the

conductor, SF6 gas and support spacer. Shape

control techniques yields uniform field stress along

the spacer surface but they may lead to uneven

shapes. To overcome this problem Functional

Graded Material (FGM) is designed for the standard

cone type spacer to obtain uniform field stress along

the surface of spacer to with, Grading to low (GL)

spatial distribution of dielectric permittivity. The

breakdown strength of SF6 gas can be enhanced

with Metal insert electrodes near the triple

junctions. The defects of the GIS spacer like

delamination worsen the electric field distribution

International Journal of Pure and Applied MathematicsVolume 114 No. 8 2017, 131-141ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu

131

and degrade the insulation performance of the GIS

spacer and hamper the safety cum power system

stability. In this paper the electric field distribution

along the spacer surface with and without meal

inserts for various width and heights of

delamination gaps is studied. The use of metal

inserts lowers the field stress and potential

distribution even for longer and narrow gaps.

Key Words : gas insulated substation , functionally

graded material, spacer, delamination, metal inserts

1 Introduction

As the GIS are becoming more compact, the magnitude of electric

field stresses developing within the GIS is gaining more importance

for study [1]. Of all the components within the GIS the electric field

stress developed at the surface of the insulator, which acts as a

support for the inner conductor within the outer enclosure, affects

the insulation integrity of GIS. High field stresses along the surface

of the spacer may result in surface flashover over a period of time.

Junction formed by the electrode, gas insulation and solid insulator

at high voltage and ground electrode ends of the support insulator

called as Triple Junction (TJ). This TJ is another critical area where

high electric field stresses may cause initiation of partial discharges.

This may further result into surface flashover along the surface of

spacer. Spacers are one of the critical components in GIS responsible

for breakdown in dielectric strength and surface flashover, injecting

the need in manufacture of reliable and flashover free spacers for

efficient performance of the gas insulated systems [2-4]. Field

studies along the spacer surface have been considered as one of the

measure in evaluating the performance of the spacer.

Perry, E.R [5] reviewed the performance of various shapes of

insulators like smooth disc, corrugated disc and a cone. Cone type

spacer is seen to have considerable potential. Insulator

contamination leads to weakening of dielectric strength. Misaki, T

[6] et al considered local electric field intensification on a cone-type

spacer which is fitted between flanges in SF6-gas-Insulated

apparatus, a major problem. The improved structure with surface

shape of the spacer and contact position slightly changed proved

fruitful in reducing local field intensification. Till now, various

techniques have been applied to improve the insulation performance

and unwind the electric field intensity in practical gas insulated

switchgears. However, these techniques make the spacer geometry

more complex. In recent years, a novel technique based on

functionally graded materials (FGM) is proposed to improve break

down voltage (BDV) of solid insulators while keeping its structure

International Journal of Pure and Applied Mathematics Special Issue

132

simple [6-8]. Okubo group has proposed the application of FGM in

gas insulated switchgear (GIS) spacer fabricated by centrifugal force

and investigated the effectiveness of E-Field control and BDV

improvement [9]. Some Researchers [10-18] have separately

investigated the design and optimization techniques for FGM

spacers and analysed the E-field distribution on the spacer-SF6 gas

interface. However, there are still some issues about the design,

fabrication and performance assessment of FGM before industrial

usage. One of the problems is effective design methods for the

distribution of material dielectric properties which could be used for

a variety of solid spacer geometries. Besides, since only a few types

of (ascending, descending, U shape, etc.) material distribution can be

achieved by the centrifugal force method, flexible fabrication method

for the FGM spacers is still urgently required.

Efforts like shape control to obtain uniform field stress along the

surface of the spacer and incorporating metal inserts and recessed

electrodes for minimizing the electric field stress at TJs have been

effective. Electron emission is most favoured at triple junction in

GIS. Chakravorti, S [14] designed a method to reduce this electric

stress by inserting metal electrode, which could effectively reduce

the field stress at the triple junction with simultaneous rise in the

stresses elsewhere along the spacer surface. Measurements have

been made on plain cylindrical spacers; spacers with a defect at the

insulator/gas/electrode triple junction results clearly showed that

metal inserts shifted the maximum field to the mid gap region.

Further metal inserts have been found to provide effective shielding

of the triple junction, provided that the mid gap field remains below

the inception level. But these methods increase the complexity of

spacer design and sometimes may also not be economically feasible.

FGM is a technique by which the permittivity of the support

insulator is modulated to obtain uniforms field stresses along the

surface of the spacer for standard shapes. FGM supporting

insulators are practically synthesized by controlling the diffused

fillers in the epoxy by applying centrifuge forces. Efforts have been

made to obtain uniform field stresses along the surface of the spacer

but the minimization of electric field stress at both ends of HV and

ground electrodes TJs needs further study.

In this paper, Conventional cone type spacer geometry is taken for

which field study has been done. Electric field stresses developed by

the cone type spacer on the surface of the spacer and also at the

triple junction is determined. Three types of permittivity graded

support insulators are used to analyse the electric field stress on the

surface of the spacer and also at the triple junction at either ends of

the spacer i.e at inner conductor end or enclosure end. It is found

that the electric field stress at the surface of the spacer is

maintained uniform, the electric field stress at the critical junctions


133

formed by the conductor, solid insulator and SF6 gas is not

maintained at minimum value. Metal inserts are considered as a

means to reduce the stress at the triple point junction. However for

reduction in field stress at triple point junction can be attained with

the proper moulding of metal inserts and stress is determined and

the results are presented and analysed.

2. Electric Field Computation

In a GIS the two main insulating media employed are the SF6 gas

and the solid insulating supports called the spacers. For GIS and its

related applications alumina or silica-filled epoxy matrix are the

commonly used as insulating materials. At steady state the

electrostatic field within anisotropic dielectric material, assuming a

Cartesian coordinate system, and Laplacian field, the electrical

energy W stored within the whole volume U of the region considered

is:

dUgradVWu

2

∫2

1 (1)

(2)

3. Functionally Graded Material Cone type spacer:

Due to low permittivity of gas insulation regioncompared to that of

solid insulator the electric field stress under ac and impulse voltage

application usually gets intensified in the gas region. In order to

minimize this field intensification the FGM spacer is made effective

by modulating the permittivity within the solid insulator. By means

of permittivity graded-FGM electric field stress intensification at the

region of interest can be minimized. The important aspect is to select

the permittivity distribution so as to increase the field utilization

factor of the electric field stress distribution as much as possible.

Conductor supported by FGM Spacer with graded permittivity along

its length is as shown in the Figure 1.

A coaxial cone type supporting insulator with constant permittivity

has uniform electric field stress along its surface but the electric field

stress at TJs should be maintained at minimum to avoid any

initiation of discharge at these locations as major surface flashovers

reported in the literature had attributed high electric field stress at

TJs as the main reason for surface discharges and eventual surface

flashovers. To minimize the electric field stress at the TJs metal

inserts and recessed electrodes are implemented but to the

dxdydzz

V

y

V

x

VW

z

z

y

y

x

x

U

]}∂

∂{}

∂

∂{}

∂

∂{[

2

1 222

∫∫∫


134

disadvantage of complex shape design and fabrication. FGM

supporting insulators have created a new arena for designing

support insulators with standard shapes and with distributed

permittivity. Modulating the permittivity distribution along the

length of the spacer the field stress can be controlled to obtain as per

the requirement. Figure 4 shows FGM type cone type support

insulator. Permittivity-graded materials are processed by means of

applying centrifugal forces after incorporating fillers of different

diameters so as to obtain the permittivity distribution suitable for

attaining uniform field distribution. In constant-ε material the

permittivity remains constant throughout the material as the filler

density is constant all through the material where as in GL-FGM

density of high permittivity filler is increased in the centrifugal

direction by which a high permittivity can be obtained at one end

while low permittivity at the other end as shown in the Figure 2.

Figure 1. Functional Graded Materials for Spacer

4. Results and Discussions

In a GIS the two main insulating media employed are the SF6 gas

and the solid insulating supports called the spacers. For GIS and its

related applications alumina or silica-filled epoxy matrix are the

commonly used insulating materials. In this work a normal cone

type spacer is taken with clearance between the outer electrodes to

that of the inner electrode as 100mm.The inner electrode is applied

with voltages of 72.5 kV while the outer electrode is grounded. The

bus duct is filled with SF6 gas and relative permittivity is taken to be

1.015.The spacer is taken to be of GL-FGM spacer. Electric Field

stress along the surface of spacer is shown without and with metal

inserts in Figures 2 and 3.

From the computed electric field stress along the surface of spacer

which was plotted from enclosure end toward the inner conductor,

Figures 2 and 3 it can be observed that the electric field stresses at


135

the enclosure end of triple junction has is 13.3 KV/Cm in the absence

of metal inserts. For a uniform spacer the electric field stress at the

inner conductor end TJ is low but due to the presence of

delamination defect it is increased to 3.7 KV/Cm as shown in the

Figure 2. In the presence of metal inserts the electric field stress is

reduced at both the TJ i.e. enclosure end and the electrode as shown

in Figures 3. The electric surface potential for GL-FGM type cone

spacer decreases towards the enclosure end. The electric potential

wire frames for GL-FGM type cone spacer is show in the Figures 4

and 5. The electric field stress is maximum at the TJ‟s.

Figure 2. Electric surface Stress along the GL-FGM type cone spacer

with delamination in the absence of metal inserts when applied with

72.5kV


136

Figure 3: Electric surface Potential along the GL-FGM type cone

spacer with delamination in the presence of metal inserts when

applied with 72.5kV

Figure 4. Electric Potential wire frame along the GL-FGM type cone

spacer with delamination in the absence of metal inserts when

applied with 72.5kV


137

Figure 5: Electric Potential wire frame along the GL-FGM

type cone spacer with delamination in the presence of

metal inserts when applied with 72.5kV

6. Conclusion

Spacers are important component in the Gas Insulated systems.

Most of the dielectric strength breakdown and surface flashover

cases were due to spacer failures. Non uniform electric field

distribution along the surface of the spacer and high field stress at

triple junctions are reasons for these failures. Accurate modelling of

the Spacer geometry is required for a better stress spread as it

enhances the life of the component. Seldom stress control by means

of shape modelling results in difficulties like moulding and

fabrication. FGM spacers are viewed as better alternative to shape

moulded spacers as FGM spacers retain their conventional shapes

thereby alleviating the difficulties of moulding and fabrication. The

FGM spacer Gradually Low-FGM considered in this work to analyse

the effectiveness of FGM spacers as supporting insulators in GIS.

Electric Field stresses are computed for FGM spacer keeping more

emphasis on the triple junctions. The standard FGM configurations

fail to answer the problem of TJs a critical area in GIS. In this paper

Electric field stress distributions due to standard FGM

configurations with the delamination defect have been presented

and are compared with metal inserts. The electric field stress at the

TJ is reduced effectively while maintaining uniform field stress

along its surface.


138

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Electric Field Stress mitigation in a Gas Insulated Substation ...acadpubl.eu/jsi/2017-114-7-ICPCIT-2017/articles/8/13.pdfElectric Field Stress mitigation in a Gas Insulated Substation