detection and control of metalic partical

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On the Detection and Control of Metallic Particle

CO

in Compressed

GI s

Equipment

.

5

M.M. Morcos S.A. Ward

Manhattan, KS USA Shoubra, Egypt

Kansas State University

University of Zagazig

Pwllcla

lnlllalsd

Ereakdwm

AI

W

h r l l c l o

H. Anis

Cairo

University

Giza, Egypt

Abstract

Metallic particles drastically impair the insulation

integrity of compressed gas insulated substation (GIS)

equipment. Such particles present a special hazard when

in close proximity of support insulators. For GIS

equipment to be reliable and economic, the problem of

particle contamination should be overcome. Most GIS

equipment manufacturers employ a variety of techniques

and devices, such as electrostatic particle traps, to

control metallic particle contamination. Conductors in

gas-insulated 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. Free metallic

particles are a major cause of partial discharge (PD) in

GIs. Several methods have been used in checking for

PD activity. Recent diagnostic techniques include VHF

and UHF-band PD detecting systems and ultrasonic

vibration detecting systems. The methods used for the

detection of pre-discharge caused by contaminating

particles in GIS and the means of using this detection as

a diagnostic tool for particle contamination are

presented.

hexafluoride (SFa may be drastically reduced due to the

presence of conducting particles in a gas insulated gap.

Fig.

1

shows the actual breakdown field at the inner

conductor in percent of the theoretical value for SF,, in

the presence of conducting particles

[4].

The effect of

metallic particles

on

the SF, breakdown voltage is more

pronounced at high gas pressures

[SI.

Several attempts

have been made to determine the role of conducting

particles in the breakdown process of compressed gas

insulation [l-71. Cookson et

al

found that elongated

particles greatly reduce the SF, breakdown voltage and

corona onset levels

[3].

In general, the gas breakdown

voltage decreases with the wire particle length, while the

breakdown voltage is not necessarily reduced by

decreasing the particle diameter.

Under the influence of the applied voltage, free

conducting particles become charged and oscillate in the

inter-electrode gap. Particle motion largely depends

upon the

typ of

applied voltage. Under

AC

voltages,

for a wire particle of given radius, the activity increases

with particle length since the particle charge-to-mass

ratio at lifting increases with length. This ratio

Introduction

The presence of particle contaminants in gas insulated

switchgear (GIS) can greatly deteriorate the integrity of

the insulation. Those particles may be insulating or

conducting; insulating particles have little effect on the

insulating behavior of

the

gases. Particles may be free

to move in the electric field, may be fixed on the

electrodes or may be fiied

on

spacers, thus grossly

enhancing electrode surface roughness. Many

experimental results have been published involving

particle contamination in uniform and coaxial fields.

The particles studied are of many different shapes and

sizes such as spheres, filamentary (wires particles), and

fine dust

[l-31.

The withstand voltage of sulphur-

Figure 1 . Degradation in electrical insulation strength

of SF, caused by conducting particles

[4]

0-7803-5035-9/98/ 10.0001998

EEE.



decreases with the spherical particle radius

s

the

particle movement is reduced

in

this case [8].

1000

s

5

2

g 100

9

t 5

d

The effects of fmed conducting particles on AC

breakdown compared to those with particles which are

free to move were examined [9]. Although free particles

initiate breakdown when a particle is at

an

electrode, a

particle fixed on the electrode yields different AC

breakdown voltage/gas pressure characteristics. The

difference in behavior of fmed and free particles is

associated with the particle location at the instant of

breakdown and some statistical effects.

-

-

-

-

-

Particle Detection in G I s

The breakdown of insulation while in service can cause

considerable damage to equipment and to the system to

which it is connected. Failures of this typ often may be

related to the occurrence and severity of partial

discharges (PD) inside the insulation.

Free metallic

particles and metallic particles attached to insulators

greatly influence the insulation performance of the GIS

and are a major cause of PD. Several methods have

been used in checking for PD activity,

the most

commonly used are ultrasonic contact probes and the

electromagnetic coupling devices [10-121.

Metallic Particles as Source of PD

Conducting particles are the most frequently met type of

imperfection in GIS; they enhance the local field stress.

As a result, the intrinsic breakdown field strength of SF,

cannot

be

fully exploited in practical applications. For

SF, pressures of engineering interest and normal levels

of surface roughness, PD inception and breakdown

voltages are almost identical.

However, PD without

breakdown can occur for protrusions above the normal

surface roughness. Classical PD detection schemes are

discussed in [13-151.

Diagnostic Techniaues for Particle Detection

Metallic particles

in

GIS cause PD which can be directly

detected by measuring the voltage signal using capacitive

dividers installed in the GIs. They also can

be

detected

by the measurement of tank potential oscillation,

electromagneticwavesfrom he tank, discharge light and

decomposed gas. Free metallic particles can also be

detected by measuring tank vibration caused by the

bouncing of particles inside the

GIs.

Corona pulses

generated during discharges caused by particle

movement were detected using different techniques

[

11,

12, 161. Diagnostic techniques can be used to detect the

presence of pre-discharge phenomena, evaluate the level

of degradation of the SF, gas, localize possible faults or

flashovers,

and detect the presence

of

anomalous

mechanical vibrations

[

17-21]. Diagnostic methods in

use can be classified s electrical, acoustical, and

optical methods. The main features of these methods are

described below; specific details are reported

in [

171.

Electrical methods

In order to detect the presence of imperfectionddefects,

different parameters are measured; namely, electric,

magnetic and electromagnetic fields (both inside and

outside the GIS), current, and voltage. These

parameters are generated by PD which locally generates

small currents with large frequency spectrum that

propagate throughout the GIS. They can be classified in

three groups according to the frequency range of

detection; conventional PD measurement [18, 19, 221,

high frequency (HF) method, and ultra high frequency

(UHF) method [20 211. Various types of electrical

sensors can be used (coupling devices, field sensors,

antennas, coils, current probes). Fig. 2 shows the

relation between PD and the lengths of free metallic

particles found in a GIs [23]. When metallic particles

attach to insulators, PD are reported to be about 1/10 of

the values in Fig. 2. Considering this phenomenon, the

PD detecting system with a sensitivity level of 10 pC can

detect metallic particles of several millimeters long.

2o

IO

I

I

IO

2

3 0

PARTICLE

LENGTH mm)

Figure 2. Variation of PD with free particle length

[23]

77



This means that the newly developed PD detecting

systems are able to detect harmful metallic particles

more accurately on site [18]. Gas breakdown in GIS

generates fast surges which propagate throughout the

substation, undergo reflections at discontinuities and

excite the chambers into many different resonances at

frequencies that can be as high as several GHz [20].

The UHF technique used frequency detection well above

1 GHz frequency domain where there is no noise but the

imperfections/defects of the GIS [20, 211. The methd

has successfully detected and located several defects

(moving and fixed particles).

Acoustical method

The principle of acoustical methods is the detection of

mechanical vibrations on the external enclosure of the

GIS due to shockwaves from PD, impact of free moving

particles, and vibrations of parts of the GIS [19, 221.

Vibrations are measured by means of sensitive acoustic

transducers of various types, positioned

on

the GIS

enclosure.

An

acoustic-emission sensor working in the

ultrasonic range (resonance around 50

kHz)

is the

optimum sensor type/frequency band choice [191.

Diagnostic information can be obtained from the analysis

and the elaboration of the acoustic signals acquired by

the sensors located at suitable points

of

the GIs.

Analysis in both time and frequency domains are in use.

The method has been found sensitive to defects;

mechanical vibrations due to loose parts and bad erection

are also detected easily [22].

ODtical Method

This method is based

on

the detection of the light

emission produced by faults, PD, etc.

[17,

221.

Dependingon the aim of the diagnostic measurements,

different types of optical sensors can be used, and

different analyses can

be

made on the acquired optical

signals. Optical methods have been used in GIS only for

eventual detection and location of flashovers during on-

site testing and in service.

Mitigation and Control

For GIS and gas insulated transmission line (GITL)

systems to be reliable and economic, the problem of

particle contamination should be overcome. Hence,

improvements in the reliability of GIS/GITL systems

could be achieved. Moreover, this could lead to higher

stress operation of compressed gas apparatus and

consequently to a potential reduction in GIS size and

cost. Various techniques for the elimination of particles

in GIS/GITL systems have been developed. Bundled

conductors

[XI

and corrugated enclosure [25] designs

were investigated in order to determine if these provide

a system more tolerant to the presence of particle

contamination. Although such designs showed some

merit compared to the conventional GIS designs, none of

them resulted in sufficient improvement to allow those

systems to operate reliably at higher stresses. Some

attention has been focused on the techniques of

immobilizing particles; these attempts include dielectric

coatings of the sheath or conductor to increase the

particle lift-off voltages [26], and the use of corrosive

gases to corrode the particles with the aim to reduce

them to fine dust [27]. None of these methods is in use

commercially. Another method of preventing the

contaminating particles from moving in the electric field

has been recently explored using anopen-cell foam/SF,

gas insulation system [28]. However, the dielectric

strength of the f o d g a s system in the presence of

particles was not significantly better than the particle-

initiated breakdown strength of

SF,

alone.

Particle Tram

One philosophy in the design of GIS/GITL systems is to

provide designated low field areas in the system in the

form of particle traps where the particles can be safely

trapped and contained

[4].

One method is to control the

particle interaction with the insulators is to position

particle traps at the insulator. Fig.

3

shows an

electrostatic particle trap mounted around a tri-post

insulator in a coaxial electrode system. Test results

showed that the presence of the particle trap around the

insulator significantly reduced the chance of particle

initiated breakdowns associated with the insulator

[29].

Sheath Insulator

\

Tr

Side View Cross Sect i ona l View

Figure 3. Particle trap mounted around insulator for

protection [29]

78



The electrostatic particle trap is presently being used for

particle control in some commercially available GIS.

The effectiveness of a particle trap is a function of the

slot width and the number of slots per unit length. Also,

particle motion in the electric field will influence

trapping. Experimental observations of the particle

behavior with traps showed that particles were attracted

to the slot edges in a firefly motion; this behavior was

more likely at higher electric fields [25]. With elevated

traps, the applied voltage level had a significant

influence

on

the trapping time. Longer particles require

the conditioning voltage to be applied for a much longer

time before they arrive at the trap surface and fall

through the surface slots.

Particles that were smaller

than the trap elevation had no problems entering the

opening between the trap and the enclosure.

Dielectric Coating of Electrodes

Conductors in GIWGITL systems may be coated with a

dielectric material in order to restore some of the

dielectric strength

of

the compressed gas which is lost

due to surface roughness and contamination by

conducting particles. The improvement in the dielectric

strength of the system, due to coating, can be attributed

to several effects [30, 311. The high resistance of the

coating dielectric impedes the development of pre-

discharges in the gas, thus increasing the breakdown

voltage. The electric field necessary to lift a particle

resting on the bottom of a GIS enclosure is much

increased due to the coating [30]. Once a particle begins

to move in the gas gap under the applied voltage, it may

collide with either conductor. If the conductor is coated,

the particle will acquire a drastically reduced charge, if

any. Thus, the risk of a breakdown initiated by a

discharge is reduced significantly.

Conclusions

Conducting particle contaminants in GIS/GITL systems

play a crucial role which adversely affects the insulation

performance of the system. Efforts are being made to

study different methods of particle detection, control and

elimination for GIS systems to be reliable and economic.

Particle traps are used in some commercially available

GIS/GITL systems. Conductors may

be

coated with a

dielectric material in order to restore some of the

insulation strength of the compressed gas which is lost

due to surface roughness and contamination by

conducting particles.

111

131

[41

151

r61

[71

1111

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