VFTO documentation.doc

5/19/2018 VFTO documentation.doc

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CHAPTER 1

INTRODUCTION

1.1 What is Gas Insulated Switchea!"

A compact, multi component assembly enclosed inside a grounded metallic

encapsulation, which shields all energized parts from the environment. The primary

insulating medium is compressed SF6gas.

It generally consists of

a. !us"bars

b. #ircuit"brea$ers

c. %isconnecting switches

d. &arthing switches

e. #urrent transformers

f. 'oltage transformers

g. #able and bo(es

h. )as supplying and gas monitoring e*uipment

i. %ens meters+. ocal control

)as Insulated Substations -)IS have found a broad range of applications in power

systems over the last three decades because of their high reliability, easy maintenance, small

ground space re*uirement etc.. In our country also, a few )IS units have been in operation

and a large number of units are under various stages of installation.

)IS is based on the principle of operation of complete enclosure of all energized or

live parts in a metallic encapsulation, which shields them from the e(ternal environment.

#ompressed SF6 gas, which has e(cellent electrical insulating properties, is employed as the

insulating medium between the encapsulation and the energized parts. )as Insulated

Substations have a grounded outer sheath enclosing the high voltage inner conductor unli$e

conventional e*uipment whose closest ground is the earth surface.

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The !asic Insulation evel -!I re*uired for a )as Insulated Substation -)IS is

different from that of the conventional substation because of certain uni*ue properties of the

former. )as insulated bus has a surge impedance -01 2hm more than that of the

conventional oil filled cables, but much less than that of a over head line -311 4 511 2hms.

In addition, the )IS is totally enclosed and therefore is free from any atmospheric

contamination. ence, in general the )IS permit lower !I rating than the conventional one.

A )IS re*uires less number of lightning arresters than a conventional one. This is mainly

because of its compactness. The basic consideration for insulation co"ordination is '"t

characteristic. The '"t characteristic of SF6 is considerably flat compared to that of air. Air

can withstand to very high voltages for very short time. 2n the other hand SF6 e(hibits a flat

characteristic. Thus the ratio of basic switching impulse level to basic lightening impulse

level is close to unity for )IS, where as for the conventional substations this ratio varies

between 1.6 and 1.76.

1.# Ad$antaes %& GIS %$e! the C%n$enti%nal' O(en Ai! Su)stati%ns"

/ 'ery much reduced area and volume re*uirements resulting in lower costs.

8 )reatly improved safety and reliability due to earthed metal housing of all high

voltage parts and much higher intrinsic strength of SF6 gas as insulation.3 9ore optimal life cycle costs because of lesser maintenance, down time and

repair costs.

5 &limination of radio interference with the use of earthed metal enclosures.

: It is not necessary that high voltage or e(tra high voltage switchgear has to be

installed outdoors.

6 They offer saving in land and construction costs.

0 These substations can be located closer to load centers thereby reducing

transmission losses and e(penditure in the distribution networ$.

1.* Disad$antaes %& GIS"

Although )IS has been in operation for several years, a lot of problems encountered

in practice need fuller understanding. Some of the problems being studied are

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/. Switching operations generate 'ery Fast Transient 2ver voltages -'FT2.

8. 'FT2 may cause secondary brea$down inside a )IS and Transient &nclosure

'oltages -T&' outside the )IS.

3. Field non"uniformities reduce withstanding levels of a )IS.

5. ;rolonged arcing may produce corrosive < to(ic by"products.

:. Support spacers can be wea$ points when arc by"products and metallic particles are

present.

For these reasons, 'FT2 generated in a )IS should be considered as an important

factor in the insulation design. For designing a substation it is essential to $now the

ma(imum value of 'FT2. 9oreover, this 'FT2 in turn generates Transient &nclosure

'oltages -T&' outside the )IS. ence studies are carried out on estimation of the 'FT2

and T&' levels. For this purpose ;S;I#& can be used.

In )IS, 'ery Fast Transient 2ver voltages -'FT2 are caused by two ways, due to

switching operations, line to enclosure faults and internal insulation flashover.

The internal FT2=s generated have traveling wave behavior of a surge. Since FT2=s

have the characteristics of traveling wave, they can change significantly at different points

within )IS. These FT2=s travel to the e(ternal system through enclosures, gas"air bushings,

cable +oints, current transformers etc. and may cause damage to the outside e*uipments li$ehigh voltage transformers connected to the )IS.

FT2=s can also lead to secondary brea$down in )IS. Further they may give rise to

electro"magnetic interference.

Since the contact speed of the dis"connector switches is low, re"stri$ing occurs many

times before the interruption is completed. &ach re"stri$e generates 'FT2=s with different

levels of magnitude.

%is"connector Switches -%S are used primarily to isolate the operating sections of an

' installation from each other as a safety measure. !eyond this, they must also be able to

perform certain switching duties, such as load transfer from one busbar to another or

disconnection of bus bar, circuit brea$er etc.. Step shaped traveling wave generated between

the dis"connector switch contacts propagates in both directions, reflecting at the components

of )IS, thus resulting in a comple( waveform.

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1.+ The ,ain P!%)le,s Ass%ciated with the -TO a!e as &%ll%ws"

/ Flashover to )round at the dis"connector switch contacts.

8 Failure of electronic control circuits connected to )IS, because of electromagnetic

interference of 'FT2.

3 %ielectric strength is reduced under 'FT2, if non"uniform electric field is formed

by the particles -mainly metallic.

5 &ffect on components such as bushing and transformer.

: Transient &nclosure 'oltage -T&' on e(ternal surface of the sheath. This may

cause flashover to near by grounded ob+ects.

For these reasons, 'FT2 generated in )IS should be considered as an important

factor in the insulation design of not only gas insulated components, but the entire substation.

The 'FT2 generated due to switching operation, the brea$down may occur if a sharp

protrusion e(ists within the )IS. The over voltage pattern and the 'FT2 level changes after

the 'FT2 brea$down. This type of brea$down is $nown as Secondary !rea$down. This type

of brea$down is also possible at the switching contacts during the current interruption. From

the insulation design point of view, this new 'FT2 level and amplitudes of the high

fre*uency components are also important.For designing a substation it is essential to $now the ma(imum value of 'FT2.

ence studies are carried out on estimation of the 'FT2 levels. For this purpose ;S;I#& can

be used. In ;S;I#& simulation a suitable e*uivalent circuit is necessary for each component

of the substation.

From the above it can be seen that the estimation of magnitudes of 'FT2=s are

essential for the design of a )IS. This has been the scope of this pro+ect.

1./ Ai, and Sc%(e %& the P!esent Stud0"

The present wor$ is aimed at calculating magnitude of fast transient over voltages in

)IS due to Switching 2perations and ine"to &nclosure faults by suitably modeling a typical

)IS system. A comparison is made for different lengths of )IS. For better understanding of

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the transients, they are calculated with Fi(ed Arc >esistance and with 'ariable Arc

>esistance. Attempts have been made to compare the transients with oad and without oad.

Therefore in the present study, the following wor$ has been carried out.

/. The ma(imum possible 'FT2 level for 85:?' substation is estimated.

8. The effect of each component of )IS on the 'FT2 level is estimated separately.

3. The length of the cable termination depends on station configuration. From 'FT2

point of view, minimum length of the cable is estimated by considering different

switching operations.

5. A model of the spar$ channel development is proposed for estimating the 'FT2

level.

In #hapter"8, iterature Survey, ;rinciple and )eneration of 'FT2, Secondary

!rea$down, Surges, >e"stri$es and ;re"stri$es, Trapped #harge and #urrent #hopping in

)IS are discussed.

In #hapter"3, 9odelling of )IS #omponents, ;S;I#& models, 9odelling details,

#alculation of ;arameters and &*uivalent circuit of )IS components are presented.

In #hapter"5, The transients due to switching operations and line"to"enclosure faults

with Fi(ed Arc >esistance for different lengths of )IS and also the transients due to fault

along with load and without load are described and analyzed.In #hapter":, The transients due to switching operations and faults with 'ariable Arc

>esistance for different lengths of )IS and also the transients due to fault along with load

and without load are dealt with.

In #hapter"6, Suppression of fast transient over voltages is discussed.

In #hapetr"0, #omparison between the transients due to switching operations with

Fi(ed and 'ariable Arc >esistance for different lengths of )IS. #omparison between the

transients due to fault for different lengths, with fi(ed and variable arc resistance, with and

without oad, and suggestions for the further wor$ are presented.

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CHAPTER #

2ITERATURE SUR-E3

#.1 Int!%ducti%n"

%uring the current operation of dis"connector switch in a )IS, re"stri$es -pre"stri$es

occur because of low speed of the dis"connector switch moving contact, hence 'ery Fast

Transient 2ver voltages are developed. These 'FT2=s are caused by switching operations

and line"to"enclosure faults.

@hen a dis"connector switch is opened on a floating section of switchgear, a trappedcharge may be left on the floating section. In the opening operation of dis"connector switch,

transients are produced and the magnitude of these transients and rise times depends on the

circuit parameters. @hen there is a fault occurs, there is a short circuit in the system.

Transients are also produced due to the faults in the system. %ue to this 'FT2=s are caused

by switching operation can also lead to secondary brea$down with in )IS. >e"stri$ing surges

generated by the dis"connector switches at )IS generally possess e(tremely high fre*uencies

ranging from several hundred ?z to several 9z.

In this chapter, the general layout of 85:?' )IS is given in section 8.8. The literature

survey is presented in section 8.3. )eneration of 'FT2 is discussed in section 8.5. ;rinciple

of FT2 generation is described in section 8.:. Secondary brea$down in )IS is e(plained in

section 8.6. The occurrences of Surges, >e"stri$es and ;re"stri$es in )IS are presented in

sections 8.0, 8.7 respectively. Trapped charge condition in )IS is also discussed in section

8., and necessity of current chopping is described in section 8./1.

#.# Gas Insulated Su)stati%ns"

The general layout of 85:?' )as Insulated Substation comprises the following

components

#ircuit !rea$er

Isolator

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%is"connector Switch

&arthing Switch

#urrent Transformer

'oltage Transformer

!us bar B #onnectors

;ower Transformer

!ushing B #able

@hen designing the )IS, space"associated costs are reduced, resulting in a substantial

reduction in overall station costs, as )IS occupies only roughly /1C of the space re*uired by

a conventional substation. Typical cases for which )IS is undoubtedly the more economicsolution -along with areas of ma+or cost savings are given below

/. Drban and Industrial areas -space, pollution

8. 9ountain areas -site preparation, altitude, snow and ice

3. #oastal areas -salt"associated problems

5. Dnderground substations -site preparation

:. Areas where aesthetics are a ma+or concern -andscaping etc.

#.* 2ite!atu!e Su!$e0"

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S. Ganabu E5, has e(perimentally estimated fast transient over voltages in )IS. The

ma(imum FT2 estimated from observation was 8.0 p.u. This was observed infre*uently and

occurred only at the open end of the bus bars.

S. A. !ogss E:, carried out field tests for measurement of dis"connector switch

operation induced transients and indicated that transients do not e(ceed 8.1p.u. Further it

gives that the trapped charge left during dis"connector switch opening depends on the design

of the switch.

S. 2gawa E6, proved that re"stri$ing surge of dis"connector switches can estimated

by conducting calculations with considerably high accuracy than measured waveforms.

Accuracy of as low as 3C to :C has been achieved for measured and calculated values.

H. aznadar E0, >. @itzmann E7, has developed models for different )IScomponents and conducted e(periments with regard to waveform distortion on various

models consisting of spacers, bushing etc..

Amir 9ansour 9iri E, presented numerical and e(perimental evaluation of the

transient behavior of )IS. @ith the help of electrical e*uivalent circuits of )IS components,

the generation and propagation of transients inside )IS have been evaluated.

obuhiro Shimoda E/1, J. 2zawa E//, describes the method of suppression of

transient over voltages caused by dis"connector switch. This is obtained by insertion of

resistor with appropriate value during switching operation.

T. ). &ngel E/8, determined the resistance of high"current pulsed arc by various

formulae. The results indicate that in the initial stages of discharges -t K 1.:s, e*uation

developed by Toepler and some other authors are identical.

). &c$lin and %. Schlicht E/3, describes the operation and switching procedures with

isolators occurring in )IS and the principle operation of FT2=s generated in )IS.

Tohei itta E/5, describes surge propagation in )IS. Traveling velocity of surges is

e*ual to the velocity of light. Any component, which adds e(tra ground capacitance to the

system should be properly included in the calculation model. Small inductance plays

important in the surge propagation performance of a given system.

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;. 2smo$rovic E/6, describes the formative times and Toepler=s constant approach to

modeling the brea$down event and it depends on the macroscopic parameters of the

insulation.

#.+ Gene!ati%n %& -e!0 ast T!ansient O$e! $%ltaes 4-TO5 in a GIS"

%uring the current operation of dis"connector switch in a )IS, re"stri$es -pre"stri$es

occur because of the low speed of the dis"connector switch moving contact, due to the very

fast voltage collapse within a few nano seconds -ns and the subse*uent traveling waves,

'ery Fast Transient 2ver"voltages are developed. The main oscillation fre*uency of the fast

transients depends on the configuration of )IS. 9oreover, the effect of comple(ity of the

configuration of a )IS on the pea$ value of the transients has been studied in this thesis.

For the development of e*uivalent circuits, low voltage step response measurements

of the main )IS components have been made. Dsing the ;S;I#& the e*uivalent electrical

models are developed. The pea$ value of the fast transients often occurs when circuit

structure is relatively simple, but more fre*uently if the structure is rather complicated. The

propagation velocity of traveling wave generated during dis"connector switch operation is

about 31cm < ns.

The representation of bushing is important for simulating the fast transients.

)enerally, the transit time through a bushing is comparable to or greater than the rise time of)IS generated transients. For this reason, bushings cannot be considered as a lumped element

in estimating the 'FT2 level.

The generation of fast transients can be classified into two types. They are due to the

following

a %is"connector switch operation

b Faults between !us bar and &nclosure

In case of line"to"earth fault, the voltage collapse at the fault location occurs in a

similar way as in dis"connector gap during re"stri$ing. !y this event, step shape traveling

surges are in+ected. For such a surge source inside )IS, two surges traveling in opposite

directions are generated. owever, if voltage collapse occurs at the open end of )IS, only

single surge propagates on the bus.


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Spar$ collapse time is defined as the time to bridge the gap with the spar$ after the

initiation of brea$down. A longer spar$ length causes longer spar$ collapse time. It was also

observed that with a constant SF6gas pressure, a higher inter electrode brea$down voltage

causes longer spar$ collapse time. @ith the same voltage, a lower gas pressure also causes

longer spar$ collapse time.

@hen SF6brea$down occurs it re"combines very *uic$ly, since it has a high electro"

negative property. %ue to this property, re"stri$ing voltages of the order of nanoseconds rise

time are produced. ence FT2=s are mainly because of SF6. As a conse*uence of

characteristics of brea$down in electro"negative gases and short traveling wave times in )IS

resulting from short overall length, transient over"voltages with steeper voltage rise and

higher fre*uencies are produced.

!rea$down in SF6starts initially by avalanche, starting with initiatory electron due to

cosmic radiation, field emission or several other phenomena producing electrons. These

electrons are accelerated by electric field thereby increasing its $inetic energy. As a result,

number of electrons increases because of collisions. According to streamer criteria, first

avalanche occurs followed by chain of avalanches bridging the gap between the electrodes

and thus forming a streamer. Thus, to have brea$down there should be sufficient electric field

to produce se*uence of avalanches and there should be atleast one primary electron to initiate

first avalanche.In the above se*uence of events there e(ists a time lag for initiating electron to be

available in the gap after the voltage is applied. This time lag is termed as the Statistical Time

ag. Similarly the formation of spar$ channel ta$es definite time $nown as Formative Time

ag -Tf and is defined below E/0.

=D

?l5.5T

Tf

@here l L Spar$ ength

?TL Toepler=s #onstant

D L Ignition 'oltage

This time lag is of the order of nanoseconds. Therefore the rise time of FT2=s will be

of the order of nanoseconds. The above phenomenon suggests that the FT2=s are generated

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due to voltage collapse, which occurs when spar$ is produced. This spar$ is produced after a

time lag of Tf.

%is"connector Switches -%S are designed to interrupt small charging current that

flows through the short lines as fast as the circuit brea$er. In this case, since the contact speed

of %S is generally slow, re"stri$ing occurs a number of times before interruption is

completed, resulting in generation of high fre*uency surge voltage each time re"stri$e ta$es

place. %S operation in )IS generates the largest line"to"ground voltage transients imposed on

the switchgear during normal operation.

#./ P!inci(le %& TO Gene!ati%n"

%uring opening operation of %is"connector Switch -%S, transients are produced due

to internal oscillations. The magnitude of these transients and rise times depends on the

circuit parameters li$e Inductance, #apacitance and #onnected oad. Assuming that some

trapped charge is left during opening operation, transients can be calculated during closing

operation of %S.

Fast Transient 2ver voltages generated during %is"connector Switch operation are a

se*uence of voltage steps created by voltage collapse across the gap at re"stri$ing. Specific

over voltage shape is formed by multiple reflections and refractions. 2peration of %is"

connector Switch -%S can be shown by using the below figure

i #.1 Elect!ic Ci!cuit &%! e6(lainin !est!i7es

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@here /L Inductance of Source

#/L #apacitance of Source

#8L #apacitance of )IS 2pen ;art

D/L ;ower Fre*uency 'oltage

D8L 'oltage of )IS Section

The more fre*uent service situation of the isolator is its use to connect or dis"connect

unloaded parts of the installation as is shown in figure 8./. For e(ample, a part of the )IS is

dis"connected by an isolator from a generator or from an overhead supply line, where by the

self"capacitance #8of this part of circuit can be upto several nF, depending on its length.

First re"stri$e across the gap occurs when voltage across the gap e(ceeds the

brea$down voltage. The occurrence of se*uence of re"stri$es is described with the following

figure 8.8.

i #.# -%ltae %& the %(enended GIS side %& the Is%lat%!

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The voltage across the gap is the difference between D/and D8. If it is assumed that

the brea$down voltage D!of the gap increases with increasing separation and therefore with

time as shown in figure 8.8. Then the curve D8can be constructed as follows.

At the instant of mechanical contact separation, D/and D8have the same value, the

voltage D8 continues to retain this value, while D/ changes with power fre*uency. The

voltage -D8" D/ across the gap of the isolator also changes. As soon as, -D 84 D/ e(ceeds

the dielectric strength D!of the gap, a brea$down and thus a first re"stri$e occurs. !oth

electrodes are there by electrically connected by a conducting spar$, whereby )IS section

with initial voltage D8is very rapidly charged to instantaneous value of D /. The transient

current flowing through the spar$ then interrupts as soon as the )IS have been charged to D /

and spar$ e(tinguishes.

The voltage D8now remains constant with time, while the voltage D /, on the side of

supply $eeps changing. This continues until the second re"stri$e occurs with an increased

brea$down voltage D!as a conse*uence of larger separation. ence D 8 follows D/, until

finally at the end of the switching process the gap no longer can be bro$en down. Transients

are also produced due to faults in the system. @hen there is a fault, there will be short circuit

in the system. %ue to this, oscillations occur due to presence of inductance and capacitance

on both sides of the fault section causing transients.

#.8 Sec%nda!0 9!ea7d%wn in a GIS"

'ery Fast Transient 2ver voltages -'FT2 caused by switching operations can lead to

Secondary !rea$downs within )as Insulated Substations.

In the first type, the flashover to ground at the dis"connector switch contacts is due to

the streamer generated during re"stri$e or pre"stri$e between the dis"connector switch

contacts. Secondly, inside the )IS, li$e particles or fi(ed protrusions cause an

inhomogeneous field distribution and insulation can fail. In these two types of earth faults,

'FT2 are developed. The flashover voltages under these two conditions are appreciably

lower than the normal withstand voltages to the ground.

;ractically, it can be observed that,

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/. Streamers are generated from several locations over a contact. Apparently one of

these streamers develops a flashover between the contacts, while the flashover to ground is

caused by the development of the other streamers.

8. The flashover voltage to ground is lower when the spar$ is generated between the dis"

connector switch contacts by an impulse voltage than when the spar$ is simulated with a

piece of wire. This is because of the e(istence of streamers.

;ractically, it can be observed that the 'FT2 induced earth faults are possible at the

dis"connector switch contacts during its operation. This is because of the development of the

enhanced field gradient to earth and later 'FT2 will be generated in the )IS.

The brea$down from the live conductor to the outer conductor is possible under

'FT2 or impulse voltages. Thus it is important to develop a simulation model for the

brea$down and the characteristics of the spar$ channel. The time varying process during

voltage brea$down and the resulting 'FT2 can be measured. The computer simulation

model for this brea$down can be developed. The results obtained with ;S;I#& are compared

with measured values. The time varying process during the building of the spar$ will be

simulated by using the Toepler=s spar$ law.

#. : Su!es in GIS"

The discharge process during each individual re"stri$e begins with a voltage collapse

across the contact gap, which because of the particular brea$down mechanism in

electronegative gases ta$es place within only appro(imately /1 "7 sec. This voltage collapse is

directly related to the formation of the spar$ channel. @ith a typical voltage decrease rate of

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towards the open end of the )IS and is there again reflected. For this reason, the discharge

transient shows a periodicity of double the traveling time of the wave in the )IS.

The amplitudes of the voltage and current surges depend on the re"stri$ing voltage

and on the parameters of the circuit. Therefore very different amplitudes can occur depending

on the comple(ity of the installation.

#. ; ReSt!i7es and P!eSt!i7es in GIS"

%is"connector Switch -%S operation typically involves slow moving contacts which

results in numerous discharges during operation. For e(ample, a floating section of

switchgear between a disconnect switch and an open brea$er -load side may be disconnected

from an energized )as Insulated System -supply side.

For capacitive currents below M / amp, a re"stri$e occurs every time the voltage

between the contacts e(ceeds the dielectric strength of the gaseous medium between them.

&ach re"stri$e generates a spar$, which e*ualizes the potential between the switch

contacts. Following spar$ e(tinction, the supply and load side potentials will deviate

according to the A# supply voltage variation and the discharge characteristics of the load

side respectively. Another spar$ will result when the voltage across the electrode gap

dependent brea$down voltage D!and the potential difference of the load and supply side, D.&ach %is"connector Switch -%S operation generates a large number of ignitions

between the moving contacts. The number of ignitions depends on the speed of the contacts.

The largest and steepest surge voltages are generated only by those brea$downs at the largest

contact gap. Therefore, only a few brea$downs -/1 4 :1 need be considered for dielectric

purpose.

The slow operation and very rapid brea$down give rise to T>A;;&% #A>)& and

traveling wave surges within )as Insulated Substation -)IS.

#. < T!a((ed Cha!e in GIS"

@hen a %isconnect Switch is opened on a floating section of switchgear, a Trapped

#harge may be left on the floating section. The potential caused by this charge will decay

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very slowly as a result of lea$age through spacers. A trapped charge near /.1 p.u -pea$ can

levitate particles.

;article motion under %.# conditions is much more severe than that for A.#

e(citation and may lead to scattering of particles onto insulating surfaces. owever, such

particle motion leads to appreciable -A %.# currents, which will normally discharge the

floating section in a relatively short time.

A trapped charge of / p.u implies that the first brea$down upon closing the disconnect

switch will occur at 8 p.u across the switch contacts and may lead to conductor4to4ground

over voltages of upto 8.: p.u. Thus the magnitude of trapped charge left after operation of a

disconnect switch may be of some conse*uence to switchgear reliability.

%uring recent field tests on a :11 ?' sub station, measurements were made of the

trapped charge left when a %S was opened onto a floating section of switchgear. umerous

measurements led to the conclusion that for this switch, a potential of 1./ 4 1.8p.u is left on

the floating section and that this result is consistent. The reason for this consistent result is

that the negative brea$down occurs at appro(imately /:C greater potential difference than

the positive brea$downs for this switch.

The asymmetry in brea$down voltages leads to the NfallingO pattern near the end of

operation which continues until the potential is low enough that brea$downs can occur

during the rising portion of a power fre*uency cycle as shown in below figure 8. 3.

i #. * 2%ad side $%ltae wa$e&%!, du!in %(enin %& disc%nnect switch

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Two such brea$downs bring the potential bac$ to a large positive value after which

the falling pattern is re"established. The end point of this process is inevitably a transition

from a large negative potential to a slightly positive potential at a gap distance for which the

positive brea$down potential is /./ p. u -pea$ and the negative brea$down potential is

/.8 p. u -pea$. At this point another positive and negative brea$down cannot occur, as a

result 1./ " 1.8 p. u -pea$ is left on the floating switchgear.

The salient features which lead to this small trapped charge are the asymmetry in

brea$down potential and relatively long arcing time. This trapped charge can be controlled

through careful design of contact geometry. For the purpose of calculating transient

magnitudes, a trapped charge of /.1 p. u -pea$ prior to closing of %is"connector Switch -%S

is assumed. 2ne of the methods suggested to suppress these over voltages is by insertion of a

resistor with an appropriate value during switching.

#. 1= Cu!!ent Ch%((in"

@hen a #ircuit !rea$er -#.! is made to interrupt low inductive currents such as

currents due to no load magnetizing current of a transformer, it does so even before the

current actually passes through zero value, especially when the brea$er e(erts the same de"

ionizing force for all currents within its short circuit capacity. This brea$ing of current before

it passes through the natural zero is termed as N#urrent #hoppingO.

The energy contained in the electro"magnetic field cannot become zero

instantaneously. The only possibility is the conversion from electro"magnetic to electro"static

of energy.

i.e. #'8

/,I

8

/ 88 =

I#

,' =

/0


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)enerally in 'acuum or SF6circuit brea$ers the currents chopped are of the order of

: Amps. @hen a constant de"ionizing force is applied by a brea$er for arc interruption, then

force must be high enough to interrupt highest value of short circuit current.

i #. + wa$e&%!, %& %$e! $%ltae with cu!!ent ch%((in

ow, if the brea$er is called upon to brea$ a load current which is less than thehighest short circuit current, then the de"ionizing force would be sufficient enough to force

the arc from its high value straight to zero before the same actually reaches to natural zero.

This results a tremendous amount of over voltage as shown in the above figure 8.5. This

phenomenon is termed as N#urrent #hoppingO.

/7


19/119

#.11 C%nclusi%n"

Switching operations in a )as Insulated Switchgear lead to very fast transient

phenomena. These 'FT=s stress the e*uipment in )IS as well as the secondary e*uipment.

Switchgear reliability can be improved by assuring that dis"connectors minimize the trapped

charge left on the switchgear. >educed trapped charge carries two benefits. Firstly, the

magnitude of dis"connector operation induced transients is reduced and Secondly, the

tendency for free conducting particles to be scattered onto spacers is reduced.

ence it is essential to $now the ma(imum value of 'FT2=s produced in the

switching operation. For this reason ;S;I#& is used. In pspice simulation a suitable

e*uivalent circuits is necessary for each component of the substation. The designed

e*uivalent circuit of each component in the substation using pspice simulation is used in the

5thand :th#hapters.

/


20/119

CHAPTER*

>ODE22ING O GIS CO>PONENT OR

CA2CU2ATION O TRANSIENTS

*.1 Int!%ducti%n"

For accurate analysis of transients, it is essential to find the 'FT2=s and circuit

parameters. %ue to the traveling nature of the transients the modelling of )IS ma$es use of

electrical e*uivalent circuits composed by lumped elements and especially by distributed

parameter lines, surge impedances and traveling times. The simulation depends on the *uality

of the model of each individual )IS component. In order to achieve reasonable results in )IS

structures highly accurate models for each internal e*uipment and also for components

connected to the )IS are necessary.

The dis"connector spar$ itself has to be ta$en into account by transient resistance

according to the Toepler=s e*uation and subse*uent arc resistance of a few ohms. The waveshape of the over voltage surge due to dis"connector switch is affected by all )IS elements.

Accordingly, the simulation of transients in )IS assumes an establishment of the models for

the !us, !ushing, &lbow, Transformers, Surge Arresters, !rea$ers, Spacers, %is"connectors,

and &nclosures and so on.

In this chapter, the modeling concept of )IS is given in section 3.8. The ;S;I#&

models are developed in section 3.3. #alculation of parameters of )IS is described in section

3.5. &(perimental apparatus of )IS is described in section 3.:. Single"line diagram

dimensions of 85:?' )IS are given in sections 3.6 B 3.0 respectively. The e*uivalent circuit

of )IS components is given in section 3.7.

81


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*.# >%dellin C%nce(t"

A )IS system comprising of an Input #able, Spacer, %is"connector Switch, !us bar

of :mts length and load has been considered for modeling into electrical networ$ and

analysis.

The Fast Transient 2ver voltage waveform generated during #losing and 2pening

operation of %is"connector Switch and ine"to"&nclosure faults has been considered for

calculations.

Spacers are simulated by lumped #apacitance. The Inductance of the busduct is

calculated from the diameters of #onductor and &nclosure. #apacitances are calculated on

the basis of actual diameters of inner and outer cylinders of central conductor and outer

enclosure. #one Insulators used for supporting inner conductor against outer enclosure are

assumed to be dis$ type for appro(imate calculation of spacer capacitance.

The busduct can be modeled as a series of ;i"networ$ or as se*uence parameters.

owever in this model, it is considered as distributed ;i"networ$. The Schematic %iagram of

a Typical )as Insulated System -)IS is shown in below figure 3./.

i *.1 Sche,atic dia!a, %& a t0(ical Gas Insulated Su)stati%n

Assuming that some trapped charge is left on the floating section of switchgear during

opening operation of dis"connector switch, a voltage of certain value is considered during

simulation.

8/


22/119

*.* PSPICE >%dels"

To simulate the 'ery Fast Transient 2ver voltages in )IS, ;S;I#& is used. The

e*uivalent circuit of )IS is shown in below figures 3.8 B 3.3.

i *.# E?ui$alent ci!cuit %& GIS

@here,

H/L Surge Impedance of )as Insulated !us duct w.r.to &nclosure Interior surface

H8L Surge Impedance of 2verhead Transmission ine w.r.to &arth Surface

H3L Surge Impedance of &nclosure &(terior Surface w.r.to &arth Surface

#bL #apacitance of the !ushing

# L #apacitance of the #urrent Transformer

i *.* E?ui$alent ci!cuit %& GIS

88


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@here l/, l8be the length of the source side bus bar, length of the load side bus bar, #/

and #8are source side capacitance and the load side capacitance respectively. et H cand lcbe

the surge impedance and length of the cable respectively.

For estimating these voltages, the e*uivalent impedance networ$s for the components

li$e #apacitance, Inductance of the )round @ire, )rounding )rid, Spar$ #hannel, and the

>esistance of )round )rid, Switch -@hich follows Toepler=s Spar$ aw are re*uired.

*.+ Calculati%n %& Pa!a,ete!s"

*.+.1 Calculati%n %& Inductance"

The inductance of the bus duct can be calculated by using the formula E/6

given below

@here r/, r8, r3, r5, are the radii of the conductors in the order of decreasing

magnitude and Pl= is the length of the section.

+

+

+

= /

r

rln

r

r"/

r

r

8r

rln

r

rln

r

rln1.11/,

8

/

8

/

8

8

/

8

3

5

/

8

3

/l

i *.+ C!%ss secti%n %& t0(ical GIS S0ste,

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*.+.# Calculati%n %& Ca(acitance"

The #apacitance is calculated with the assumption that the conductors are

#ylindrical. #apacitance is calculated by using the standard formulae given below

@here oL 7.7:5 Q /1"/8, r L /

b L 2uter #ylinder >adius

a L Inner #ylinder >adius

l L ength of the Section

*.+.* Calculati%n %& Ca(acitance due t% S(ace!"

Spacers are used for supporting the inner conductor with reference to the outer

enclosure. They are made with Allumina filled epo(y material whose relative

permittivity -r is 5. The thic$ness of the spacer is assumed to be the length of the

capacitance for calculation.

*.+.+ Calculati%n %& Sh%!t Ci!cuit Inductance @ Resistance"

Assuming a short circuit fault level of /1119'A for /38?' system voltage,

Inductance and >esistance are calculated as follows

S'Qph

=phI

ph'

S=phI

And'

IQRHC =

85


25/119

I

'QCHR =

!ut ,QfQQ8R =

fQQ8

R,

=

And it is assumed that > L R

*.+./ Calculati%n %& Inductance due t% 2%ad 4T!ans&%!,e!5"

For 6119'A, /38?' transformer with /1C impedance and 1.7 power factor

the inductance is calculated as follows

;#osQIQ'Q3 =

=#osQ'Q3

;I

And'

IQRHC =

I

'

QCHR =

!ut ,QfQQ8R =

fQQ8

R,

=

*.+.8 Calculati%n %& -a!ia)le A!c Resistance"

!ased on earlier studies in SF6 gas, Toepler=s Spar$ aw is valid for

calculation of 'ariable Arc >esistance. The 'ariable Arc >esistance due to Toepler=s

formulae E: is given below

8:


26/119

> L( )+

t

1o

T

ti*

l?

dt

@here ? TL Toepler=s #onstant L 1.11: volt.sec-t, is calculated until it reaches a

value of / to 3 ohms. The integral in the denominator sums up the absolute value of

current Pi= through the resistance >-t over the time beginning at brea$down inception.

Thus, it corresponds to the charge conducted through the spar$ channel upto timePt=.

Initial charge *ois an important parameter while considering the non"uniform

fields. !ut the field between the dis"connector contacts is almost uniform. Therefore

*ois very small.

*./ E6(e!i,ental A((a!atus 4>%dellin details5"

A )IS unit with the following arrangement is assumed for developing the model as

shown in below figure 3.:.

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i *./ S7etch %& E6(e!i,ental A((a!atus

The apparatus has a dis"connector with an earthing switch, four dis$"type spacers, a

load bus bar about /1m long with three post"type spacers and a ::1?' gas bushing

containing stress capacitor.

The / )z surge sensor mentioned in the diagram is located at a distance of /.6m

from the dis"connector. Further, holding the load side bus bar at zero potential, dc voltage

80


28/119

was applied from the high voltage dc power supply to the bushing via a / 9 resistor and

'FT2 waveform of the closing operation was observed.

The dc voltage applied was positive and moving contact of the dis"connector was

located on the load side.

*.8 Sinle2ine Dia!a, %& #+/ - Su)stati%n"

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i *.8 Sinleline dia!a, %& #+/7$ GIS

*.: Di,ensi%ns %& a #+/ - Gas Insulated Su)stati%n"

8


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#omponents of a )IS %istance in meters

/. 2verhead Transmission ine

8. #able

3. #able to ightening Arrester -A

5. #able to 'oltage Transformer -'T

:. 'T to #urrent Transformer -#T

6. #T to &arthing Switch -&S

0. &S to %is"connector Switch -%S/

7. %S/ to !DS"II

. !DS"II

/1. !DS"II to %S3

//. %S3 to &arthing Switch -&S/

/8. &S/ to #ircuit !rea$er -#!3

/3. #!3

/5. #!3 to #!:

/:. %S: to ;ower Transformer -;T

/6. ;T to %S6

/0. %S6 to &arthing Switch -&S8

/7. &S8 to #!5

:111

7111

/.3

8.1:

/.8

1.3:

/.:

1.:

/1

1.:

3.3:

1.5

8.:

1.

//

/:

1.:

1.:

*.; E?ui$alent ci!cuit %& GIS c%,(%nents"

31


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E2E>ENT >ODE2EBUI-A2ENT

CIRCUITCHARACTERISTIC

!DS %D#T

Transmission line with

distributed parameters.

oss in transmission line

because of s$in effect.

S;A#&>umped #apacitance

towards the ground.# 81pf

&!2@


distributed parameters

and capacitance added in

between the line.

;arameters depending on

the ratio between conduct

and enclosure radius. 'alu

of the capacitance #

depending on the system

topology.

#A!&



&ach end of cable is

terminating with a

lumped capacitance.

#D>>&T

T>ASF2>9&>

umped capacitance

towards the ground

#A;A#ITI'&

'2TA)&

T>ASF2>9&>

umped capacitance

towards the ground

!DSI)

-#apacitively

)raded !ushing

Transmission line of

varying surge impedances

are connected in series

Hg/, Hg8, U are variable

surge impedance in SF6

side. Ha/, Ha8, U are

variable surge impedance

air side.

3/


32/119

SD>)&

A>>&ST&>

Arrester capacitance is

considered. ;rotection

characteristic connected

in parallel with arrester

capacitance

In case of 'FT -1.:Vs the

protection characteristic i

corrected in reference to t

characteristic for the surge

7

T>ASF2>9&>

umped capacitance

towards the ground

'alue of capacitance

depends on the transforme

type, voltage level, windin

connection and winding

type.

%IS"#2T2>

#2S&%



#apacitance of the

switching contacts

towards the ground is

considered.

;arameters depending on

the ratio between conduct

and enclosure radius. 'alu

of capacitance # depends

on the system topology.

%IS"#2T2>

2;&&%

Inter electrode

capacitanceof the

switching contacts


considered.

# includes spacer

capacitance also.

&A>T

S@IT#I)

umped capacitance

towards the ground.

S;A>?

>&SISTA#& -in

case of %S operation

It is a non"linear function

of time. It varies

according to the Toepler=s

Spar$ aw

if t K /Vs, > L 1

if t /Vs, > varies

from 1 to :

38


33/119

S;A>?

-earth fault

Spar$ resistance varies

according to Toepler=s

Spar$ aw. is the

inductance of the spar$

channel.

> is in the range of

/ to 3

#I>#DIT

!>&A?&> -#.!

#2S&%


distributed parameters

e*uivalent capacitance of

switching contacts


considered.

The surge impedance of

#.! bus duct is less than

because of additional

capacitance.

#I>#DIT

!>&A?&> -#.!

2;&&%

The capacitance between

switching contacts is

considered. #.! bus duct

is represented with

distributed parameters on

both sides of the contacts.

The length of bus duct o

both sides of contacts is

e*ual. The inter electrod

capacitance incase of #.!

high, because of large arc

the contacts.

T>ASF2>9&>@here r L /V

L /8.7 m

*.< C%nclusi%ns"

A model is developed for the prediction of the 'FT2 phenomena in the circuit of

voltage and current transformers in )IS. The main advantage of such model is to enable the

transient analysis of )IS. A spar$ collapse time was correctly simulated by the variable

33


34/119

resistor. !y this spar$ collapse time, resistance of the 'FT2 is e(tended, and the component

caused by short surge impedance discontinuities such as spacers, dis"connectors and short

bus branches were damped.

A )IS system comprising of spacers, bus bar and dis"connectors has been considered

for modeling into electric networ$. The inductance of the bus bar is calculated from

diameters of conductors and enclosure using standard formulae. #one insulators used for

supporting inner conductor against outer enclosure are assumed to be dis$ type for

appro(imate calculation of spacer capacitance. The busduct capacitance is calculated using

formulae for concentric cylinders. The entire bus length is modeled as distributed pi"networ$.

CHAPTER+

TRANSIENTS DUE TO SWITCHING @ AU2TS WITH

IED ARC RESISTANCE

35


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+.1 Int!%ducti%n"

%uring the switching operation of the circuit, the transients are developed. !y the

calculated values of the circuit parameters in previous chapter, the e*uivalent circuits are

constructed by using ;S;I#& software. !y using the circuits the transients are calculated for

different lengths of )as insulated substation. The transients are also calculated during the

faults with and without load at different distances.

#onsider a circuit with the elements as shown in below figure 5./.

i +.1 Elect!ic ci!cuit &%! e6(lainin Rest!i7es

I/, I8are Isolators and

#! is #ircuit !rea$er

In this chapter, the transients due to switching operations for :mts and /1mts length

)IS are given in section 5.8. The transients due to faults for :mts and /1mts length )IS

without load are presented in sections 5.3./ B 5.3.8 respectively. The results of transients due

to faults for :mts and /1mts length )IS with load are described in sections 5.3.3 B 5.3.5

respectively.

+.# T!ansients due t% switchin %(e!ati%n"

+.#.1 Sinle Phase e?ui$alent ci!cuit &%! /,ts lenth GIS"

The bus duct is divided into three sections of length 8.:mts, /.:mts, and /.1mts

respectively from load side. The )IS bushing is represented by a capacitance of 811pf. A

3:


36/119

Fi(ed >esistance of 8ohms of the spar$ channel is connected in series with the circuit

brea$er. The e*uivalent circuit is shown in figure 5.8.

%ue to trapped charge some voltage remains on the floating section which can create

severe conditions because the first re"stri$e can occur at the pea$ of power fre*uency voltage

giving a voltage of 8. 1 p.u. 2n re"stri$e the voltages on each side will collapse initially zero

and hence creating two /.1 p.u voltage steps of opposite polarities. In this, it is assumed that

re"stri$ing is created at /.1 p.u and "/.1p.u respectively on either side of dis"connector Switch

-%S. The transients due to different switching operations are observed.

i +.# Sinle Phase e?ui$alent ci!cuit &%! /,ts lenth GIS due t% Switchin %(e!ati%n

Dsing the circuit given in Fig 5.8, transients due to closing of the circuit brea$er are

calculated as given in Fig 5.3. 9a(imum voltage obtained is 3.18p.u with a rise time of 31ns.

The graphs are obtained from ;S;I#& simulations and software is given in Appendi("/.

In figure 5.8, the voltages before and after circuit brea$er is ta$en to be /.1 p. u and

" /.1 p.u as the most onerous condition. !ut depending on the time of closing of #.!, the

magnitude of the voltage on the load side changes.

36


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i +.* T!ansient $%ltae wa$e&%!, du!in Cl%sin %(e!ati%n %& C9 &%! /,ts GIS

36


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For different values of voltages on the load side, the magnitudes and rise times of the

voltage waveform are calculated $eeping source side voltages as constant at /.1 p.u. The

values are tabulated as shown in Table 5./.

S. N%

2%ad Side -%ltae

4(.u5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4ns5/ "/.1 8.5:

8 "1. 8.33 /8

3 "1.7 8./0

5 "1.0 8./5 /1

: "1.6 /.6 /8

6 "1.: /.70 /1

0 "1.5 /.01 /8

7 "1.3 /.61 /8

"1.8 /.50 /8

/1 "1./ /.37 /1Ta)le +.1 T!ansients due t% $a!iati%n %& $%ltae %n l%ad side

Similarly by changing the magnitudes of the voltage on the source side, $eeping

voltage on load side constant at "/.1 p.u. Then the transients due to variation of voltage on

source side obtained. The values are tabulated as shown in Table 5.8.

S. N%S%u!ce Side -%ltae

4(.u5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4ns5

/ /.1 8.5:

8 1. 8.37 //

3 1.7 8.86 //5 1.0 8.1 /8

: 1.6 8.18 //

6 1.: /.75 /8

0 1.5 /.03 /1

7 1.3 /.63 //

1.8 /.:1 /1

/1 1./ /.37 /1

Ta)le +.# T!ansients due t% $a!iati%n %& $%ltae %n s%u!ce side

%uring closing operation, the current through the resistance of the circuit brea$er is

shown in Fig 5.5. From the graph, it was found the ma(imum current is 31mA at a rise time

of /8ns.

30


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To introduce current chopping, the circuit brea$er is opened. ence to calculate

transients due to opening operation the #.! is opened at /1ns -say. The transients are

obtained and as shown in Fig 5.:. From the graph, the ma(imum voltage obtained is 8.1/p.u

with rise time of /7ns.

Assuming that there is a second re"stri$e, another switch is connected in parallel to

the circuit brea$er for simulation in ;S;I#& modeling. Transients are calculated by closing

this switch when voltage difference across the contacts of the circuit brea$er reaches

ma(imum value. Transients calculated due to second re"stri$e gives the pea$ voltage of

8.88p.u at a rise time of /6ns as shown in Fig 5.6. The values are tabulated as shown in

below Table 5.3.

>%de %& O(e!ati%n>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec%nds5

%uring #losing

2peration3.18 31

%uring 2pening

2peration8.1/ /7

%uring Second

>e"Stri$e8.88 /6

Ta)le +.* T!ansients due t% switchin %(e!ati%ns &%! /,ts lenth GIS

37


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i +.+ Cu!!ent wa$e&%!, du!in Cl%sin %(e!ati%n %& C9 &%! /,ts GIS

3


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i +./ T!ansient $%ltae wa$e&%!, du!in O(enin %(e!ati%n %& C9 &%! /,ts GIS

51


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i +.8 T!ansient $%ltae wa$e&%!, du!in Sec%nd Rest!i7e &%! /,ts GIS

5/


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+.#.# Sinle Phase e?ui$alent ci!cuit &%! 1=,ts lenth GIS"

The circuit is divided into three sections of /mt, 5mt, and :mts respectively from load

side and by using the below circuit shown figure 5.0. The transients due to closing of the

circuit brea$er are calculated as shown in Fig 5.7. From this graph, the pea$ voltage obtained

is 8.5: p.u at a rise time of 0/ns.

i +.: SinlePhase e?ui$alent ci!cuit &%! 1=,ts lenth GIS due t% switchin %(e!ati%n

To introduce current chopping, the circuit brea$er is opened. The transients are

obtained during opening operation is shown in Fig 5.. From the graph, the ma(imum

voltage obtained is /.85 p.u at a rise time of 6:ns.

Assuming a second re"stri$e transients are calculated by closing another switch at the

time ma(imum voltage difference occurs across the circuit brea$er. The transient obtained

due to second re"stri$e is shown in Fig 5./1. From the graph, the ma(imum voltage obtained

is 8.:/ p.u at a rise time of /80ns.


-%ltae 4(.u5

Rise Ti,e

4nan% sec%nds5

%uring #losing

2peration8.5: 0/

%uring 2pening

2peration/.85 6:

%uring Second

>e"Stri$e8.:/ /80

Ta)le +.+ T!ansients due t% switchin %(e!ati%ns &%! 1=,ts lenth GIS

58


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i +.; T!ansient $%ltae wa$e&%!, du!in Cl%sin %(e!ati%n %& C9 &%! 1=,ts GIS

53


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i +.< T!ansient $%ltae wa$e&%!, du!in O(enin %(e!ati%n %& C9 &%! 1=,ts GIS

55


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i +.1= T!ansient $%ltae wa$e&%!, du!in Sec%nd Rest!i7e &%! 1=,ts GIS

5:


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+.* T!ansients due t% aults"

+.*.1 GIS %& /,ts lenth t% calculate t!ansients due t% &aults"

i +.11 GIS %& /,ts lenth t% calculate t!ansients due t% &aults

The e*uivalent circuit of :mts length )IS is shown in figure 5.//. This circuit is

divided into three sections of /mt, /.:mt and 8.:mts lengths respectively from the load side.

The transients are obtained without fault is shown in Fig 5./8. From this graph, the ma(imum

voltage is obtained at 8.1 p.u at rise time of 5311ns.

Fast transient over voltages are generated not only due to switching operations but

also due to single"line"to"ground faults. A fault at a particular point is e*uivalent to a short"

circuit at that location. This situation can be simulated by connecting a switch at a particular

point and closing it at the pea$ of the voltage.

P!%cedu!e &%! calculati%n %& t!ansients at di&&e!ent distances"

Case 4i5" -%istance of 8.:mts

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i +.1# T!ansient $%ltae wa$e&%!, &%! /,ts GIS with%ut ault' with%ut 2%ad

50


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i +.1* A &ault %ccu!s at a distance %& #./,ts lenth &!%, l%ad side

The circuit is shown in above figure 5./3. From this circuit, the ma(imum voltageacross the circuit brea$er can be found. The transients that are obtained in this case is shown

in Fig 5./5. From this graph, the ma(imum voltage is obtained at 8.18p.u at a rise time of 83

ns.

Case 4ii5" -%istance of 5mts

i +.1/ A &ault %ccu!s at a distance %& +,ts lenth &!%, l%ad side

The circuit is shown in above figure 5./:. From this circuit, the ma(imum voltage

across the circuit brea$er can be found. The transients that are obtained in this case is shown

in Fig 5./6. From this graph, the pea$ voltage is obtained at 8.7p.u at a rise time of :8 ns.

57


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Case 4iii5" -%istance for :mts

i +.1: A &ault %ccu!s at a distance %& /,ts lenth &!%, l%ad side

The circuit is shown in above figure 5./0. From this circuit, the ma(imum voltage


in Fig 5./7. From this graph, the pea$ voltage is obtained at 3./8 p.u at a rise time of /18 ns.

The magnitudes and rise times of :mts length )IS due to faults are tabulated in the

Table 5.:.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 8.: 8.18 83

3 5.1 8.7 :8

5 :.1 3./8 /18

Ta)le +./ T!ansients due t% &aults &%! /,ts lenth GIS with%ut 2%ad

Form the above table, it is clear that as the length of the bus bar between faulted point

and load is increasing, higher degree of oscillations are obtained in the circuit.

5


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i +.1+ T!ansient $%ltae wa$e&%!, at a distance %& #./,ts &!%, l%ad side' &%! /,ts GIS' with%ut 2%ad

:1


52/119

i +.18 T!ansient $%ltae wa$e&%!, at a distance %& +,ts &!%, l%ad side' &%! /,ts GIS' with%ut 2%ad

:/


53/119

i +.1; T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side' &%! /,ts GIS' with%ut 2%ad

:8


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+.*.# GIS %& 1=,ts lenth t% calculate t!ansients due t% &aults"

i +.1< GIS %& 1=,ts lenth t% calculate t!ansients due t% &aults

The e*uivalent circuit for /1mts length )IS is shown in above figure 5./. The above

circuit is divided into three sections of :mt, 5mt and /mts respectively from load side.

The transients are obtained without fault is shown in Fig 5.81. From this graph, the

ma(imum voltage is obtained at /. p.u at a rise time of :701 ns. The transients are

calculated at different distances by short circuiting at their respective distances are given

below.


Case 4i5" -%istance of /mts

i +.#1 A &ault %ccu!s at a distance %& 1,ts lenth &!%, l%ad side

:3


55/119

i +.#= T!ansient $%ltae wa$e&%!, &%! 1=,ts GIS with%ut ault' with%ut 2%ad

:5


56/119

The circuit is shown in above figure 5.8/. From this circuit, the ma(imum voltage


in Fig 5.88. From this graph, the ma(imum voltage is obtained at /.: p.u at a rise time of 78

ns.

Case 4ii5 -%istance of :mts

i +.#* A &ault %ccu!s at a distance %& /,ts lenth &!%, l%ad side

The circuit is shown in above figure 5.83. From this circuit, the ma(imum voltage

across the circuit brea$er can be found. The transients are obtained in this case is shown in

Fig 5.85. From this graph, the ma(imum voltage is obtained at 8.17 p.u at a rise time of 75

ns.

Case 4iii5" -%istance of /1mts

i +.#/ A &ault %ccu!s at a distance %& 1=,ts lenth &!%, l%ad side

The circuit is shown in above figure 5.8:. From this circuit, the ma(imum voltage


in Fig 5.86. From this graph, the ma(imum voltage is obtained at 8.65p.u at a rise time of

/88 ns.

::


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i +.## T!ansient $%ltae wa$e&%!, at a distance %& 1,ts &!%, l%ad side' &%! 1=,ts GIS' with%ut 2%ad

:6


58/119

i +.#+ T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side' &%! 1=,ts GIS' with%ut 2%ad

:0


59/119

i +.#8 T!ansient $%ltae wa$e&%!, at a distance %& 1=,ts &!%, l%ad side' &%! 1=,ts GIS' with%ut 2%ad

:7


60/119

The magnitudes and rise times of /1mts length )IS due to faults are tabulated in the

Table 5.6.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 /.1 /.: 78

3 :.1 8.17 75

5 /1 8.65 /88

Ta)le +.8 T!ansients due t% &aults &%! 1=,ts lenth GIS with%ut 2%ad

+.*.* GIS %& /,ts lenth t% calculate t!ansients due t% &aults with 2%ad"

In this analysis, it has been carried out by connecting a transformer as oad. The load

is represented as a capacitance and short"circuit inductance connected at the end of )IS.

i +.#: GIS %& /,ts lenth t% calculate t!ansients with 2%ad due t% &ault

The e*uivalent circuit for :mts length )IS with load is shown in above figure 5.80.

The transients are obtained without fault is shown in Fig 5.87. From this graph, the ma(imum

voltage is obtained at /.3 p.u at a rise time of :178ns. The transients are calculated at

different distances by short circuiting at their respective distances are given below.

:


61/119

i +.#; T!ansient $%ltae wa$e&%!, with%ut &ault &%! /,ts GIS' with 2%ad

61


62/119



From the above circuit, the ma(imum voltage across the circuit brea$er can be found.

The transients that are obtained in this case is shown in Fig 5.8. From this graph, the

ma(imum voltage is obtained at /.05 p.u at a rise time of 66 ns.




ma(imum voltage is obtained at /.0: p.u at a rise time of 68 ns.

Case 4iii5" -%istance of :mts


The transients that are obtained in this case is shown in Fig 5.3/. From this graph, the

ma(imum voltage is obtained at /.7/ p.u at a rise time of 67 ns.

The magnitudes and rise times of :mts length )IS due to faults with load are

tabulated in the Table 5.0.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 8.: /.05 66

3 5.1 /.0: 68

5 :.1 /.7/ 67

Ta)le +.: T!ansients due t% &aults &%! /,ts lenth GIS with 2%ad

6/


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i +.#< T!ansient $%ltae wa$e&%!, at a distance %& #./,ts &!%, l%ad side' &%! /,ts GIS with 2%ad

68


64/119

i +.*= T!ansient $%ltae wa$e&%!, at a distance %& +,ts &!%, l%ad side' &%! /,ts GIS with 2%ad

63


65/119

i +.*1 T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side' &%! /,ts GIS with 2%ad

65


66/119

+.*.+ GIS %& 1=,ts lenth t% calculate t!ansients due t% &aults with 2%ad"

i +.*# GIS %& 1=,ts lenth t% calculate t!ansients with 2%ad due t% &ault

The e*uivalent circuit for /1mts length )IS with load is shown in above figure 5.38.

The transients are obtained without fault is shown in Fig 5.33. From this graph, the ma(imum

voltage is obtained at /.5 p.u at a rise time of 6188ns. The transients are calculated at

different distances by short circuiting at their respective distances are given below.





ma(imum voltage is obtained at /.58 p.u at a rise time of //8 ns.

Case 4ii5" -%istance of :mts


The transients that are obtained in this case is shown in Fig 5.3:. From this graph, the

ma(imum voltage is obtained at /.8 p.u at a rise time of /85 ns.





6:


67/119

i +.** T!ansient $%ltae wa$e&%!, with%ut &ault &%! 1=,ts GIS' with 2%ad

66


68/119

i +.*+ T!ansient $%ltae wa$e&%!, at a distance %& 1,ts &!%, l%ad side' &%! 1=,ts GIS with 2%ad

60


69/119

i +.*/ T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side' &%! 1=,ts GIS with 2%ad

67


70/119

i +.*8 T!ansient $%ltae wa$e&%!, at a distance %& 1=,ts &!%, l%ad side' &%! 1=,ts GIS with 2%ad

6


71/119

The magnitudes and rise times of /1mts length )IS due to faults with load are

tabulated in the Table 5.7.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 /.1 /.58 //8

3 :.1 /.8 /85

5 /1.1 /.36 /08

Ta)le +.; T!ansients due t% &aults &%! 1=,ts lenth GIS with 2%ad

+.+ C%nclusi%ns"

The transients due to switching operations and line to enclosure faults with fi(ed arc

resistance for different lengths of )IS was made. Transients are calculated along with load

also. It was observed that the transients obtained due to switching operations and faults in

:mts length )IS will affect the system more than that obtained in /1mts length )IS. It was

also found that during fault analysis, as the distance between the fault point and load

increases the magnitudes and rise times of the transients also increase. @hen load is

connected at the open end of )IS, the pea$ voltages and rise times that are obtained due to

short"circuit do not follow a definite pattern.

01


72/119

CHAPTER/

TRANSIENTS DUE TO SWITCHING @ AU2TS WITH

-ARIA92E ARC RESISTANCE 93 USING TOEP2ERS

SPAR 2AW

/.1 Int!%ducti%n"

In previous chapter, transient over voltages calculated on the basis of fi(ed arc

resistance have been presented. It is however, $nown that the resistance of the spar$ channel

varies with current. At the instant of initiation of arc the resistance is very high. As the

current in the arc increases the value of resistance starts decreasing until it saturates at very

low value. In general, the arc resistance appears to be inversely proportional to some function

of current.

Several authors have given arc resistance e*uations which can be divided into two

groups as given below.

/. Inverse integral e*uation reported by Toepler et al. E/8

8. Inverse e(ponential e*uation reported by %emeni$ et al. E/8

These e*uations were numerically evaluated for a given arc current and then

normalized with the e(perimental arc resistance at t L 1.:Vs -appro(imate time of ma(imum

current. 2f all these e*uations, one e*uation has been used for the analysis in this thesis.

!ased on earlier studies in SF6gas, Toepler=s Spar$s aw is valid for calculation of

variable arc resistance. The variable arc resistance due to Toepler=s formulae E/8 is

calculated as given below.

> -t L+

t

1

1

T

-

?

dttiq

l

@here ?TL Toepler=s #onstant

L 1.11: volt.sec


73/119

L Spar$ ength in meters

*1 L Initial #harge

t L Spar$ #ollapse Time in sec.

The value of time varying spar$ resistance > -t is calculated until it reaches a value

of / ohm. Initial charge *1 is an important parameter while considering the non"uniform

fields. !ut the field between the dis"connector contacts is almost uniform. Therefore, initial

charge *1is very small and can be neglected.

@hen a circuit brea$er operates a conducting spar$ channel is established with time

lag of few nanoseconds after the brea$down channel is connected the electrodes. %uring this

time only the spar$ resistance changes from a very large value to very small value. For

homogeneous fields, this time is given by

tzL/3.3 Q1E

KT

@here &1L!rea$down field strength

L 7.6 Q /16volt


74/119

/.# T!ansients due t% switchin %(e!ati%n"

/.#.1 Sinle Phase e?ui$alent ci!cuit &%! /,ts lenth GIS"

i /.1 /,ts lenth GIS with -a!ia)le A!c !esistance due t% switchin %(e!ati%n

Dsing the e*uivalent circuit of :mts length )IS given in Fig :./, transients due to

closing operation of the circuit brea$er are calculated as given in Fig :.8. From this graph,

the ma(imum voltage obtained is 3.37p.u with a rise time of 50ns. The difference between

ma(imum value for Fi(ed and 'ariable Arc >esistance is found to be insignificant.

!y using the above circuit, the transients due to opening operation of the circuit

brea$er is shown in Fig :.3. From this graph, the ma(imum voltage obtained is /.37p.u at a

rise time of 3/ns. The difference between ma(imum value for Fi(ed and 'ariable Arc

>esistance is found to be significant.

Assuming that there is a second re"stri$e, another switch is connected in parallel to

the circuit brea$er for simulation in ;S;I#& modeling. Transients are calculated by closing

this switch when voltage difference across the contacts of the circuit brea$er reaches

ma(imum value. Transients calculated due to second re"stri$e gives the pea$ voltage of

8.::p.u at a rise time of /3ns as shown in Fig :.5.

03


75/119

i /.# T!ansient $%ltae wa$e&%!, du!in Cl%sin %(e!ati%n %& C9 &%! /,ts GIS' with -a!ia)le A!c Resistance

05


76/119

i /.* T!ansient $%ltae wa$e&%!, du!in O(enin %(e!ati%n %& C9 &%! /,ts GIS' with -a!ia)le A!c Resistance

0:


77/119

i /.+ T!ansient $%ltae wa$e&%!, du!in Sec%nd Rest!i7e &%! /,ts GIS' with -a!ia)le A!c Resistance

06


78/119

The magnitudes and rise times of :mts length )IS are tabulated in the Table :.3.

>%de %& O(e!ati%n>anitude %& -%ltae

4(.u5

Rise Ti,e

4nan% sec5

%uring #losing

2peration3.37 50

%uring 2pening

2peration/.37 3/

%uring Second

>e"Stri$e8.:: /3

Ta)le /.1 T!ansients due t% switchin %(e!ati%n &%! /,ts lenth GIS with -a!ia)le

A!c Resistance

/.#.# Sinle Phase e?ui$alent ci!cuit &%! 1=,ts lenth GIS"

i /./ 1=,ts lenth GIS with -a!ia)le A!c !esistance due t% switchin %(e!ati%ns

The e*uivalent circuit of /1mts length )IS is given in Fig :.:, transients due to

closing operation of the circuit brea$er are calculated as given in Fig :.6. From this graph,

the ma(imum voltage obtained is 8.30p.u with a rise time of 06ns.

!y using the above circuit, the transients due to opening operation of the circuit

brea$er is shown in Fig :.0. From this graph, the ma(imum voltage obtained is /.17p.u at a

rise time of 08ns.

00


79/119

i /.8 T!ansient $%ltae wa$e&%!, du!in Cl%sin %(e!ati%n %& C9 &%! 1=,ts GIS' with -a!ia)le A!c Resistance

07


80/119

i /.: T!ansient $%ltae wa$e&%!, du!in O(enin %(e!ati%n %& C9 &%! 1=,ts GIS' with -a!ia)le A!c Resistance

0


81/119

i /.; T!ansient $%ltae wa$e&%!, du!in Sec%nd Rest!i7e &%! 1=,ts GIS' with -a!ia)le A!c Resistance

71


82/119

Assuming that there is a second re"stri$e, another switch is connected in parallel to the

circuit brea$er for simulation in ;S;I#& modeling. Transients calculated due to second re"

stri$e gives the pea$ voltage of /.76p.u at a rise time of :1ns as shown in Fig :.7. The

magnitudes and rise times are tabulated as shown in below Table :.8.


-%ltae 4(.u5

Rise Ti,e

4nan% sec5

%uring #losing

2peration8.30 06

%uring 2pening

2peration/.17 08

%uring Second

>e"Stri$e /.76 :1

Ta)le /.# T!ansients due t% switchin %(e!ati%n &%! 1=,ts lenth GIS with -a!ia)le

A!c Resistance

/.* T!ansients due t% &ault"

/.*.1 GIS %& /,ts lenth t% calculate t!ansients due t% &aults"

i /.< GIS %& /,ts lenth t% calculate t!ansients with -a!ia)le A!c Resistance

due t% &aults


7/


83/119


The e*uivalent circuit is shown in above figure :.. From this circuit, the ma(imum

voltage across the circuit brea$er can be found. The transients that are obtained in this case is

shown in Fig :./1. From this graph, the ma(imum voltage is obtained at /.75 p.u at a rise

time of //8 ns.


From the above circuit shown in fig :., the ma(imum voltage across the circuit

brea$er can be found. The transients that are obtained in this case is shown in Fig :.//. From

this graph, the ma(imum voltage is obtained at 8.:8 p.u at a rise time of 68 ns.

Case 4iii5" -%istance for :mts

From the above circuit shown in fig :., the ma(imum voltage across the circuit

brea$er can be found. The transients that are obtained in this case is shown in Fig :./8. From

this graph, the pea$ voltage is obtained at 8.78 p.u at a rise time of :6 ns.

The magnitudes and rise times of :mts length )IS due to faults with variable arc

resistance are tabulated in the Table :.3.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 8.: /.75 //8

3 5.1 8.:8 68

5 :.1 8.78 :6

Ta)le /.* T!ansients due t% &aults &%! /,ts lenth GIS with $a!ia)le a!c !esistance'

with%ut 2%ad

78


84/119

i /.1= T!ansient $%ltae wa$e&%!, at a distance %& #./,ts &!%, l%ad side &%! /,ts GIS' with%ut 2%ad

73


85/119

i /.11 T!ansient $%ltae wa$e&%!, at a distance %& +,ts &!%, l%ad side &%! /,ts GIS' with%ut 2%ad

75


86/119

i /.1# T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side &%! /,ts GIS' with%ut 2%ad

7:


87/119

/.*.# GIS %& 1=,ts lenth t% calculate t!ansients due t% &aults"

The e*uivalent circuit of /1mts length )IS with variable arc resistance is shown in

below figure :./3.

i /.1* GIS %& 1=,ts lenth t% calculate t!ansients with -a!ia)le A!c Resistance

due t% &ault



From this circuit, the ma(imum voltage across the circuit brea$er can be found. The

transients that are obtained in this case is shown in Fig :./5. From this graph, the ma(imum

voltage is obtained at /.73 p.u at a rise time of / ns.



The transients that are obtained in this case is shown in Fig :./:. From this graph, the

ma(imum voltage is obtained at 8.5: p.u at a rise time of /33 ns.

76


88/119

Case 4iii5" -%istance for /1mts


The transients that are obtained in this case is shown in Fig :./6. From this graph, the pea$

voltage is obtained at 8.73 p.u at a rise time of /38 ns.

70


89/119

i /.1+ T!ansient $%ltae wa$e&%!, at a distance %& 1,t &!%, l%ad side &%! 1=,ts GIS' with%ut 2%ad

77


90/119

i /.1/ T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side &%! 1=,ts GIS' with%ut 2%ad

7


91/119

i /.18 T!ansient $%ltae wa$e&%!, at a distance %& 1=,ts &!%, l%ad side &%! 1=,ts GIS' with%ut 2%ad

1


92/119

The magnitudes and rise times of /1mts length )IS due to faults with variable arc

resistance are tabulated in the Table :.5.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 /.1 /.73 /

3 :.1 8.5: /33

5 /1 8.73 /38

Ta)le /.+ T!ansients due t% &aults &%! 1=,ts lenth GIS with $a!ia)le a!c !esistance'

with%ut 2%ad

/.*.* GIS %& /,ts lenth t% calculate t!ansients due t% &aults with 2%ad"

i /.1: GIS %& /,ts lenth t% calculate t!ansients with -a!ia)le A!c Resistance

due t% &ault

The e*uivalent circuit for :mts length )IS with load is shown in above figure :./0.

The transients are calculated at different distances by short circuiting at their respective

distances are given below.


/


93/119



The transients that are obtained in this case is shown in Fig :./7. From this graph, the




The transients that are obtained in this case is shown in Fig :./. From this graph, the


Case 4iii5" -%istance of :mts


The transients that are obtained in this case is shown in Fig :.81. From this graph, the


The magnitudes and rise times of :mts length )IS due to faults with load are

tabulated in the Table :.:.

S. N%Distance in

4,ts5

>anitude %&

-%ltae 4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 8.: /.51 03

3 5.1 /.83 63

5 :.1 /.35 0

Ta)le /./ T!ansients due t% &aults &%! /,ts lenth GIS with $a!ia)le a!c !esistance'

with 2%ad

8


94/119

i /.1; T!ansient $%ltae wa$e&%!, at a distance %& #./,ts &!%, l%ad side &%! /,ts GIS' with 2%ad

3


95/119

i /.1< T!ansient $%ltae wa$e&%!, at a distance %& +,ts &!%, l%ad side &%! /,ts GIS' with 2%ad

5


96/119

i /.#= T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side &%! /,ts GIS' with 2%ad

:


97/119

/.*.+ GIS %& 1=,ts lenth t% calculate t!ansients due t% &aults with 2%ad"

i /.#1 GIS %& 1=,ts lenth t% calculate t!ansients with -a!ia)le A!c Resistance

due t% &ault

The e*uivalent circuit for /1mts length )IS with load is shown in above figure :.8/.

The transients are calculated at different distances by short circuiting at their respective

distances are given below.





ma(imum voltage is obtained at /.51 p.u at a rise time of //3 ns.









6


98/119

i /.## T!ansient $%ltae wa$e&%!, at a distance %& 1,t &!%, l%ad side &%! 1=,ts GIS' with 2%ad

0


99/119

i /.#* T!ansient $%ltae wa$e&%!, at a distance %& /,ts &!%, l%ad side &%! 1=,ts GIS' with 2%ad

7


100/119

i /.#+ T!ansient $%ltae wa$e&%!, at a distance %& 1=,ts &!%, l%ad side &%! 1=,ts GIS' with 2%ad


101/119

The magnitudes and rise times of /1mts length )IS due to faults with load are

tabulated in the Table :.6.

S. N%

Distance in

4,ts5

>anitude %& -%ltae

4(.u5

Rise Ti,e

4nan% sec5

/ 1.1 1 1

8 /.1 /.51 //3

3 :.1 /.8 /85

5 /1.1 /.58 /01

Ta)le /.8 T!ansients due t% &aults &%! 1=,ts lenth GIS with $a!ia)le a!c !esistance'

with 2%ad

/.+ C%nclusi%ns"

The variable arc resistance is calculated by Toepler=s formulae. Transients are

calculated due to switching operations and faults with variable arc resistance along with load.

For any length of )IS it was found that transients due to variable arc resistance give lower

value of pea$ voltages than that obtained with fi(ed arc resistance. @hen load is connected at

the open end of )IS, the pea$ voltages that are obtained due to faults do not follow a definite

pattern.

/11


102/119

CHAPTER8

SUPPRESSION O O-ER-O2TAGESAND CO>PARISIONS

8.1 Int!%ducti%n"

The fast transient over voltages during switching operation and faults can cause

damage to the system e*uipment. ence it is advisable to suppress these over voltages for

protection of e*uipments. 2ne of the methods of suppressing these over voltages is by

insertion of resistance during switching. )enerally a >esistor of :11 is used for this

purpose E/1.

In this analysis, a resistor of :11 is connected in parallel with the circuit brea$er and

a switch is connected in series with the resistor. The transient over voltages are suppressed

only if the current during contact operation flows through the resistor. The switch connected

in series with the resistor is closed at the time ma(imum

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