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CHAPTER I

1. INTRODUCTION:

The increase in demand for electricity and the growing energy

demand in metropolitan cities have made it necessary to extend the

existing high voltage network right up to the consumer. For reliable power

supply and economic advantages, Gas Insulated Substations (GIS) have

been installed in increasing number over the last 20years and several

units are under erection. GIS of up to 800kV have been developed and are

being widely used. Initially, GIS were installed only where land costs are

and requirements of environmental compatibility were the main

considerations over a period of time as a result of rapid progress of GIS

technology, GIS have become economical and popular.

GIS is “compact, multi component assembly enclosed inside a

grounded metallic encapsulation, which shields all energized parts from

the environment the primary insulating medium is SF6 gas. It generally

consists of

a. Bus bars

b. Circuit breakers

c. Disconnecting switches

d. Earthing switches

e. Current transformers

f. Voltage transformers

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g. Cables and boxes

h. Gas supplying and Gas monitor equipment

i. Density meters

j. Local control

Gas Insulated Substations (GIS) have found a broad range of

applications in power systems over the last two decades because of their

high reliability, easy maintenance, small ground space requirement etc.

In our country also, a good number of GIS units have been in operation

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

GIS 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 external environment. Compressed SF6 gas, which has

excellent electrical insulating properties, is employed as the insulating

medium between the encapsulation and the energized parts. Gas

Insulated Substations have a grounded outer sheath enclosing the high

voltage inner conductor unlike conventional equipment whose closest

ground is the earth surface. The Basic Insulation Level (BIL) required for

a Gas Insulated Substation (GIS) is different from that of the

conventional substation because of certain unique properties of the

former. Gas insulated bus has a surge impedance (70 Ohm) more than

that of the conventional oil filled cables, but much less than that of a

over head line (300 – 400 Ohms).

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In addition, the GIS is totally enclosed and therefore is free from

any atmospheric contamination. Hence, in general the GIS permit lower

BIL rating than the conventional one. A GIS requires less number of

lightning arresters than a conventional one. This is mainly because of its

compactness. The basic consideration for insulation co-ordination is

Volt-time characteristic. The Volt-time characteristic of SF6 is

considerably flat compared to that of air. The air can withstand to very

high voltages for very short time. On the other hand SF6 exhibits a flat

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

lightening impulse level is close to unity for GIS, where as for the

conventional substations this ratio varies between 0.6 and 0.86[1].

1.1 ADVANTAGES OF GIS OVER THE CONVENTIONAL, OPEN AIR

SUBSTATIONS:

1) GIS station occupies only about 10% of the space required by a

conventional air insulated substation

2) GIS can be installed either under ground or indoors and in

heavily populated areas

3) GIS are also conveniently used in coastal areas (salt pollution)

and industrial and urban locations where space and pollution

are the main considerations

4) These substations are generally located closer to the load

centers there by reducing the losses in transmission and

distribution networks

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5) GIS Systems are immune to atmospheric conditions and

pollution, the outages get reduced and coupled with their

increased reliability, the overall maintenance costs are

minimized

6) Estimation of radio interference with use of earthed metallic

enclosures

1.2 DISADVANTAGES OF GIS:

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

problems encountered in practice need fuller understanding. Some of the

problems being studied are:

1. Switching operations generate Very Fast Transient Over voltages

(VFTO).

2. VFTO may cause secondary breakdown inside a GIS and Transient

Enclosure Voltages (TEV) outside the GIS.

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

4. Partial discharges with in the enclosures can cause break downs

5. Metallic particle contamination

6. Transient electric field and transient magnetic fields

7. Field non-uniformities reduce withstanding levels of a GIS.

8. Support spacers can be weak points when arc by-products and

metallic particles are present.

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For these reasons, VFTO generated in a GIS should be considered

as an important factor in the insulation design. For designing a

substation it is essential to know the maximum value of VFTO. Moreover,

this VFTO in turn generates Transient Enclosure Voltages (TEV) outside

the GIS. For designing a GIS systems it is essential to know the

maximum value of VFTO moreover, this VFTO in turn generates TEV

outside the GIS, hence studies are carried out on estimation of the

VFTO.

In GIS, Very Fast Transient Over voltages (VFTOs) are caused by

two ways, due to switching operations, line to enclosure faults and

internal insulation flashover.

The internal FTO’s generated have traveling wave behavior of a

surge. Since FTO’s have the characteristics of traveling wave, they can

change significantly at different points within GIS. These FTO’s travel to

the external system through enclosures, gas-air bushings, cable joints,

current transformers etc. and may cause damage to the outside

equipments like high voltage transformers connected to the GIS.

FTO’s can also lead to secondary breakdown in GIS. Further they

may give rise to electro-magnetic interference.

Since the contact speed of the dis-connector switches is low, re-

striking occurs many times before the interruption is completed. Each

re-strike generates VFTO’s with different levels of magnitude [2].

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Dis-connector Switches (DS) are used primarily to isolate the

operating sections of an HV installation from each other as a safety

measure. Beyond this, they must also be able to perform certain

switching duties, such as load transfer from one bus bar to another or

disconnection of bus bar, circuit breaker etc. Step shaped traveling wave

generated between the dis-connector switch contacts propagates in both

directions, reflecting at the components of GIS, thus resulting in a

complex waveform.

1.3 THE MAIN PROBLEMS ASSOCIATED WITH THE VFTO ARE AS

FOLLOWS:

1. Flashover to Ground at the dis-connector switches contacts.

2. Failure of electronic control circuits connected to GIS, because of

electromagnetic interference of VFTO.

3. Dielectric strength is reduced under VFTO, if non-uniform electric

field is formed by the particles (mainly metallic).

4. Effect on components such as bushing and transformer.

5. Transient Enclosure Voltage (TEV) on external surface of the

sheath. This may cause flashover to near by grounded objects.

6. Transient electromagnetic field causes malfunctioning of secondary

equipment.

1.4 SUPPRESSION OF VERY FAST TRANSIENT OVER VOLTAGES

Switching an unloaded bus bar in GIS with a disconnectror can

cause multiple restriking of the disconnector gap and produce steep

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travelling waves propagating on the bus bar. Reflection and

superimposition of the travelling waves forms very fast transients in GIS

systems.

These VFTOs can reach to high amplitude and steepness. The

suppression of VFT in GIS has been a long standing problem. One way is

to avoid dangerous layout of GIS and dangerous operation procedures of

the disconnectors. However, this brings a big limitation to design and

control operation of GIS, another way is design and control operation of

GIS and other way is to use high speed disconnector. This can reduce

the occurrence probability of VFT, but cannot avoid VFT completely. The

new VFT suppression method is that putting high frequency magnetic

rings on GIS bus bar can increase the local inductance of the bus bar,

resist the travelling waves passing through, consume the energy of the

waves and therefore make suppression effect of VFT. Modeling of ferrite

rings has been proposed and simulations have been performed to

investigate how the ferrite rings influence the travelling waves.

1.5 TRANSIENT ELECTROMAGNETIC PHENOMENA IN GIS SYSTEMS:

In gas insulated substations (GIS) very fast transient overvoltages

are generated during switching operation of disconnector switch (DS) and

circuit breakers (CB). These transient currents (VFTC) have rise times of

about 3-20ns The magnitude of transient currents could be a few kA

depending on the location of the switch operated in the GIS these

transient voltages and currents radiate electromagnetic fields during its

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propagation along the gas insulated HV bus as the associated

frequencies are in the range of few MHz to about 300MHz[3].The

transient electromagnetic fields thus generated in turn may leak out

into external environment through discontinuities like non-metallic

flanges, SF6 to air bushing ,SF6 to cable termination, non-metallic

viewing points etc. These transient fields may couple with the control

wiring and data cables and produce transient current and voltage on

them leading to malfunctioning of control equipment during switching

operations is a severe problem in some cases. The estimation of transient

EM fields is gaining importance in recent times for characterizing the

transient EM fields or transient electromagnetic interference EMI in a

GIS, it is essential to predict the transient EM field emission from a gas

to air bushing due to the very fast transient currents generated during

switching events. The characterization of transient EMI in terms of time

domain and their frequency spectra for the highest expected levels is very

important for temporary upset or permanent damage of the control

equipment

In practice, in GIS stations the wiring between the instrumentation

is poorly shielded due to lack of light weight and flexible cables and

simple low cost shielding terminations [4]. The control circuit shielding is

very important, because VFTO produces voltages and currents in the

control circuits, which get added up at the terminals of the equipment

resulting in complex transient waveforms that are further complicated by

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the presence of resonance in the cable itself. Experimental

measurements have shown that open circuit voltages on control wiring

within the GIS building can reach up to 1000V during disconnector

switch operations. Similar voltages on the control wiring that is directly

exposed to the fields from the gas/air bushing can reach values up to

10kV [5].Due to improper shielding of the equipment housings, cable

termination boxes and similar devices, certain portions will be exposed to

transient electromagnetic fields. Estimation of magnitudes of these fields

are essential to decide the quality of shielding to be provided to the

control equipment mounted directly on the switchgear or on the bus bar

enclosure [6].

Complete and effective shielding design against VFTOs is extremely

difficult in the frequency range of 1MHz to 30MHz [7]. The substation

configuration switchgear design and control equipment wiring process as

well as the life of the electronic equipment influence the design of the

shielding. Improvements in these areas will help to improve the

effectiveness of shielding thus reducing the effect of electromagnetic

transients on the equipment and personel.GIS stations have to meet high

standards of reliability in operation and therefore the risk of failure due

to over voltages must be kept to a minimum. The main parameters that

gives rise to over voltages are the substation layout, the connections to

the external transmission lines, arrangements of earth wires, the tower

grounding and location of lightning arresters. When connecting a GIS to

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an over head lines, the lightning performance investigations have to be

carried out at very high voltage levels, more than 245kV class, it must be

ensured the specified insulation levels. The insulation levels for GIS

chosen from IEC 71 given in IEC 60517.

The switching over voltages are higher than voltages between the

lines for three phase encapsulation. The dielectric strength between

phases and phase to earth is to be designed carefully. To minimize the

risk of failures by over voltages and the maximum over voltages has to be

safety factor. If the over voltages are obtained on experience, then it is

called conventional safety factor in the case of lightning over voltages and

it depends on the system voltage. The insulation coordination of GIS is

mainly depends on very fast transient over voltages, the greater attention

has to be paid on these to adopt standard insulation strategy. From the

above it can be seen that the estimation of magnitude of very fast

transient voltages and very fast transient currents and its suppression

methods, estimation of transient magnetic fields and transient electric

fields are also very important for design of a GIS.

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1.6 VARIOUS COMPONENTS OF GIS

The general lay out of the 245kV GIS comprising of the following

components.

1.6.1 Circuit breakers for GIS systems:

The synchronized axial blast method is used which significantly

boosts circuit breaking performance and also hand holes are provided to

access to the interrupter contacts for inspection and replacement. The

overall size of the circuit breaker in a GIS is considerably reduced due to

the absence of porcelain insulators and the use of short terminal

connections. Since the breaker chamber is at earth potential the

clearances between adjacent bays or bus bar systems is reduced. Also

the energy requirement of the operating mechanism is considerably low.

The breaker is normally provided with a hydraulic spring opetrating

mechanism for each phase to facilitate single-phase auto-reclosing.

1.6.2 Disconnector switches:

In GIS, the disconnecting switches are used for electrical isolation

of circuit parts. Components of the disconnector switches are mounted

in a enclosure with the active parts supported by insulating spacers

these are two types 1) Axial Disconnectors 2) T-type Disconnectors

1.6.3 Earthing switch:

In GIS, earthing switches are used to facilitate grounding of

conducting parts during maintenance. They are generally slow acting

devices that are operated during the off state of the GIS equipment

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opening of these switches can be done either by an electric motor or

normally. Fast acting switches suitable for GIS equipment and these are

driven by motor operated drives.

1.6.4 Current transformers:

Current transformers are required for measurement of the current

flowing through the equipment and for providing protection. The current

conducting bar forms the single turn primary while the secondary coils

are located on a toroidal iron core which is placed concentrically around

the primary. The output of secondary winding is brought out through

high gas trough.

1.6.5 Voltage transformers:

Voltage transformers are required for measurement of the voltages

in the network for operating the protection system. The transformers are

of resin cast type for lower voltages and SF6 gas insulated type for higher

voltages. In case of a gas insulated voltage transformer, the primary and

secondary windings are concentrically on the core. The spacer insulator

is used for high voltage connections and for isolating the gas space from

the GIS equipment. The low voltage leads are connected to the terminal

box mounted on the enclosure through gas tight bushing insulators.

The voltage transformer is fitted with gas valves for evacuation and filling

up of the SF6 gas. The ratings are up to 200VA. They shall be foil-gas

insulated.

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In brief a typical 245kV Gas Insulated Substation comprises the

following components:

Circuit Breaker

Isolator

Dis-connector Switch

Earthing Switch

Current Transformer

Voltage Transformer

Bus bar & Connectors

Power Transformer

Bushing & Cable

When designing the GIS, space-associated costs are reduced,

resulting in a substantial reduction in overall station costs, as GIS

occupies only roughly 10% of the space required by a conventional

substation. Typical cases for which GIS is undoubtedly the more

economic solution (along with areas of major cost savings) are given

below:

1. Urban and Industrial areas (space, pollution)

2. Mountain areas (site preparation, altitude, snow and ice)

3. Coastal areas (salt-associated problems)

4. Underground substations (site preparation)

5. Areas where aesthetics are a major concern (Landscaping etc.)

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1.7 GENERATION OF VERY FAST TRANSIENT OVER VOLTAGES

(VFTOs) IN A GIS:

During the switching operation of dis-connector switch in a GIS, re-

strikes (pre-strikes) 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 subsequent traveling waves, Very Fast

Transient Over-voltages are developed. The main oscillation frequency of

the fast transients depends on the configuration of GIS [8]. Moreover, the

effect of complexity of the configuration of a GIS on the peak value of the

transients has been studied in this thesis.

For the development of equivalent circuits, low voltage step

response measurements of the main GIS components have been made.

Using the EMTP-RV the equivalent electrical models are developed. The

peak value of the fast transients often occurs when circuit structure is

relatively simple, but more frequently if the structure is rather

complicated. The propagation velocity of traveling wave generated during

dis-connector switch operation is about 30cm/ns. The representation of

bushing is important for simulating the fast transients. Generally, the

transit time through a bushing is comparable to or greater than the rise

time of GIS generated transients. For this reason, bushings cannot be

considered as a lumped element in estimating the VFTO level. The

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

to the following:

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a) Dis-connector switch operation

b) Faults between Bus bar and Enclosure

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-

striking. By this event, step shape traveling surges are injected. For such

a surge source inside GIS, two surges traveling in opposite directions are

generated. However, if voltage collapse occurs at the open end of GIS,

only single surge propagates on the bus.

Spark collapse time is defined as the time to bridge the gap with the

spark after the initiation of breakdown. A longer spark length causes

longer spark collapse time. It was also observed that with a constant SF6

gas pressure, a higher inter electrode breakdown voltage causes longer

spark collapse time. With the same voltage, a lower gas pressure also

causes longer spark collapse time.

When SF6 breakdown occurs it re-combines very quickly, since it

has a high electro-negative property. Due to this property, re-striking

voltages of the order of nanoseconds rise time are produced. Hence FTO’s

are mainly due to properties of SF6. As a consequence of characteristics

of breakdown in electro-negative gases and short traveling wave times in

GIS resulting from short overall length, transient over-voltages with

steeper voltage rise and higher frequencies are produced.

Breakdown in SF6 starts initially by avalanche, starting with

initiatory electron due to cosmic radiation, field emission or several other

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phenomena producing electrons. These electrons are accelerated by

electric field thereby increasing its kinetic 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

breakdown there should be sufficient electric field to produce sequence of

avalanches and there should be at least one primary electron to initiate

first avalanche.

In the above sequence of events there exists 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 Lag. Similarly the formation of spark

channel takes definite time known as Formative Time Lag (Tf) and is

defined below [17].

∗∗=

U

K l 4.4 T

T f

Where l = Spark Length

KT = Toepler’s Constant

U = Ignition Voltage

This time lag is of the order of nanoseconds. Therefore the rise time

of FTO’s will be of the order of nanoseconds. The above phenomenon

suggests that the FTO’s are generated due to voltage collapse, which

occurs when spark is produced. This spark is produced after a time lag of

Tf.

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Dis-connector Switches (DS) are designed to interrupt small

charging current that flows through the short lines as fast as the circuit

breaker. In this case, since the contact speed of DS is generally slow, re-

striking occurs a number of times before interruption is completed,

resulting in generation of high frequency surge voltage each time re-strike

takes place. DS operation in GIS generates the largest line-to-ground

voltage transients imposed on the switchgear during normal operation.

1.7.1 Principle of FTO Generation:

During opening operation of Dis-connector Switch (DS), transients

are produced due to large number of re strikes between the contacts. The

magnitude of these transients and rise times depends on the circuit

parameters like Inductance, Capacitance and Connected Load. Assuming

that some trapped charge is left during opening operation, transients can

be calculated during closing operation of DS.

Fast Transient Over voltages generated during Dis-connector

Switch operation are a sequence of voltage steps created by voltage

collapse across the gap at re-striking. Specific over voltage shape is

formed by multiple reflections and refractions. Operation of Dis-connector

Switch (DS) is shown in the Fig. 1.1.

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Fig 1.1 Electric Circuit for explaining re-strikes

Where L1 = Inductance of Source

C1 = Capacitance of Source

C2 = Capacitance of GIS Open Part

U1 = Power Frequency Voltage

U2 = Voltage of GIS Section

The more frequent service situation of the isolator is its use to

connect or dis-connect unloaded parts of the installation as is shown in

Fig.1.1. For example, a part of the GIS is dis-connected by an isolator

from a generator or from an overhead supply line, where by the self-

capacitance C2 of this part of circuit can be up to several nF, depending

on its length.

First re-strike across the gap occurs when voltage across the gap

exceeds the breakdown voltage. The occurrence of sequence of re-strikes

is described with the following Fig. 1.2.

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Fig 1.2 Voltage of the open-ended GIS side of the Isolator

The voltage across the gap is the difference between U1 and U2. If it

is assumed that the breakdown voltage UB of the gap increases with

increasing separation and therefore with time as shown in Fig 1.2. Then

the curve U2 can be constructed as follows.

At the instant of mechanical contact separation, U1 and U2 have the

same value, the voltage U2 continues to retain this value, while U1

changes with power frequency. The voltage (U2 - U1) across the gap of the

isolator also changes. As soon as, (U2–U1) exceeds the dielectric strength

UB of the gap, a breakdown and thus a first re-strike occur. Both

electrodes are there by electrically connected by a conducting spark,

whereby GIS section with initial voltage U2 is very rapidly charged to

instantaneous value of U1. The transient current flowing through the

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spark then interrupts as soon as the GIS have been charged to U1 and

spark extinguishes.

The voltage U2 now remains constant with time, while the voltage

U1, on the side of supply keeps changing. This continues until the second

re-strike occurs with an increased breakdown voltage UB as a

consequence of larger separation. Hence U2 follows U1, until finally at the

end of the switching process the gap no longer can be broken down.

Transients are also produced due to faults in the system. When there is a

fault, there will be short circuit in the system. Due to this, oscillations

occur due to presence of inductance and capacitance on both sides of the

fault section causing transients.

1.8 SECONDARY BREAKDOWN IN A GIS:

Very Fast Transient Over voltages (VFTO) caused by switching

operations can lead to Secondary Breakdowns within Gas Insulated

Substations.

In the first type, the flashover to ground at the dis-connector switch

contacts is due to the streamer generated during re-strike or pre-strike

between the dis-connector switch contacts. Secondly, inside the GIS, like

particles or fixed protrusions cause an inhomogeneous field distribution

and insulation can fail. In these two types of earth faults, VFTO are

developed. The flashover voltages under these two conditions are

appreciably lower than the normal withstanding voltages to the ground.

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1. 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.

2. The flashover voltage to ground is lower when the spark is

generated between the disconnector switch contacts by an impulse

voltage than when the spark is simulated with a piece of wire. This

is because of the existence of streamers.

Practically, it can be observed that the VFTO induced earth faults

are possible at the disconnector switch contacts during its operation. This

is because of the development of the enhanced field gradient to earth and

later VFTO will be generated in the GIS.

The breakdown from the live conductor to the outer conductor is

possible under VFTO or impulse voltages. Thus it is important to develop

a simulation model for the breakdown and the characteristics of the

spark channel. The time varying process during voltage breakdown and

the resulting VFTO can be measured. The computer simulation model for

this breakdown can be developed. The results obtained with EMTP-RV are

compared with measured values. The time varying process during the

building of the spark will be simulated by using the Toepler’s spark law.

1.9 SURGES IN GIS:

The discharge process during each individual re-strike begins with

a voltage collapse across the contact gap, which because of the particular

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breakdown mechanism in electronegative gases takes place within only

approximately 10-8 sec [9]. This voltage collapse is directly related to the

formation of the spark channel. With a typical voltage decrease rate of

1013 v/s (100 kV in 10 ns), it is the stimulus for a traveling wave, which

propagates away from the gap into the installation.

After a certain travel time the wave front reaches the open end of

the GIS section, is then reflected and travels back again crossing the gap

that is still short-circuited by the spark, until it reaches the next

discontinuity in the surge impedance, such as for example the connection

of the GIS to the overhead line.

Here it is now partly reflected. On this partial reflection the wave

splits itself into a Reflected and a Transmitted component. The Reflected

component travels a second time towards the open end of the GIS and is

there again reflected. For this reason, the discharge transient shows a

periodicity of double the traveling time of the wave in the GIS.

The amplitudes of the voltage and current surges depend on the re-

striking voltage and on the parameters of the circuit. Therefore very

different amplitudes can occur depending on the complexity of the

installation.

1.10 RE-STRIKES AND PRE-STRIKES IN GIS:

Disconnector Switch (DS) operation typically involves slow moving

contacts which results in numerous discharges during operation. For

example, a floating section of switchgear between a disconnect switch and

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an open breaker (load side) may be disconnected from an energized Gas

Insulated System (supply side).

For capacitive currents below 1Amp, a re-strike occurs every time

the voltage between the contacts exceeds the dielectric strength of the

gaseous medium between them.

Each re-strike generates a spark, which equalizes the potential

between the switch contacts. Following spark extinction, the supply and

load side potentials will deviate according to the AC supply voltage

variation and the discharge characteristics of the load side respectively.

Another spark will result when the voltage across the electrode gap

dependent breakdown voltage UB and the potential difference of the load

and supply side, U.

Each Dis-connector Switch (DS) 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 breakdowns at the largest

contact gap. Therefore, only a few breakdowns (10–50) need be considered

for dielectric purpose.

The slow operation and very rapid breakdown give rise to ‘TRAPPED

CHARGE’ and traveling wave surges within Gas Insulated Substation

(GIS).

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1.11 TRAPPED CHARGE IN GIS:

When a Disconnect Switch is opened on a floating section of

switchgear, a Trapped Charge may be left on the floating section. The

potential caused by this charge will decay very slowly as a result of

leakage through spacers. A trapped charge near 1.0p.u (peak) can levitate

particles [10]

Particle motion under D.C conditions is much more severe than

that for A.C excitation and may lead to scattering of particles onto

insulating surfaces. However, such particle motion leads to appreciable

D.C currents of few µA, which will normally discharge the floating section

in a relatively short time.

A trapped charge of 1 p.u implies that the first breakdown upon

closing the disconnect switch will occur at 2 p.u across the switch

contacts and may lead to conductor–to–ground over voltages of up to 2.5

p.u. Thus the magnitude of trapped charge left after operation of a

disconnect switch may be of some consequence to switchgear reliability.

During recent field tests on a 500kV sub station, measurements

were made of the trapped charge left when a DS was opened onto a

floating section of switchgear. Numerous measurements led to the

conclusion that for this switch, a potential of 0.1 – 0.2p.u is left on the

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

consistent result is that the negative breakdown occurs at approximately

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15% greater potential difference than the positive breakdowns for this

switch.

The asymmetry in breakdown voltages leads to the “falling” pattern

near the end of operation which continues until the potential is low

enough that breakdowns can occur during the rising portion of a power

frequency cycle as shown in below Fig.1.3.

Fig. 1.3 Load side voltage waveform during opening of disconnect switch

Two such breakdowns bring the potential back 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

breakdown potential is ≈ 1.1 p. u (peak) and the negative breakdown

potential is ≈ 1.2 p. u (peak). At this point another positive and negative

breakdown cannot occur, as a result 0.1 - 0.2 p. u (peak) is left on the

floating switchgear.

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

asymmetry in breakdown 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 1.0p.u (peak) prior to closing of Dis-connector Switch (DS) is

assumed. One of the methods suggested to suppress these over voltages

is by insertion of a resistor with an appropriate value during switching.

1.12 CURRENT CHOPPING:

When a Circuit Breaker (C.B) 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 breaker exerts the same de-ionizing force

for all currents within its short circuit capacity. This breaking of current

before it passes through the natural zero is termed as “Current

Chopping”.

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. CV 2

1 LI

2

1 22 =

⇒ I C

L V ∗=

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Generally in Vacuum or SF6 circuit breakers the currents

chopped are of the order of 5 Amps. When a constant de-ionizing force is

applied by a breaker for arc interruption, then force must be high enough

to interrupt highest value of short circuit current.

Fig 1.4 waveform of over voltage with current chopping

Now, if the breaker is called upon to break a load current which is

less than the highest short circuit current, then the de-ionizing force

would be sufficient enough to force the arc from its high value straight to

28

zero before the same actually reaches to natural zero. This results a

tremendous amount of over voltage as shown in the above Fig 1.4. This

phenomenon is termed as “Current Chopping”.

1.13 FREQUENCY CHARACTERISTICS OF VFTOS/VFTCS IN GIS

SYSTEMS AND ITS IMPORTANCE:

Very fast transient over voltages generated due to switching

operations of disconnectors in GIS system. These VFTOs and associated

VFTCs radiate electromagnetic fields (EM) fields during its propagation

through the coaxial GIS bus section as the associated frequencies are in

the range of few MHz to about a hundreds of MHz. The transient

electromagnetic fields in turn, leak out into the external environment

through discontinuities such as SF6 gas to air bushing, gas to cable

termination, non-metallic viewing ports, insulated flanges etc. in addition

to the radiated EM field coupling, conducted mechanisms are also

responsible for the coupling of very fast transient currents to the control

wiring.

The size of the electrical components in GIS are much smaller than

AIS(Air Insulated Substation), so the frequencies of multiple reflection of

travelling waves on the bus bars of GIS at least ten time higher than the

AIS. The characteristic impedance of the high voltage bus bars in GIS is

about five times smaller than that of AIS. i.e 60Ω in GIS instead of 300-

400Ω in AIS. Therefore the capacitive currents of off-loaded bus bars in

GIS are larger than such capacitive currents in AIS. Because of the lower

29

characteristic impedance as well as large gradient of electric field between

the prestrike and restrike arcs in SF6 gas under pressure in reference to

grounded enclosure. This leads to an important difference of impedance

at the junction of the GIS and high voltage over head line, bushings, open

circuit breaker or disconnetor contacts as well as open air bus bars.

These differences are, during switching operations, origin of high standing

voltage and current waves. The amplitudes of voltage waves depends on

the voltage level and it is comparable of the current waves are inversely

proportional to characteristic impedance.

The metal enclosure of the GIS presents a shielding

discontinuity, i.e. at the junction with high voltage over head lines or

cables; it becomes an important source of radiation. The consequences

of the features, during switching operations are very high potentials

induced in the grading system of the substation and in the secondary

circuits. These problems are more dangerous in usually small distances

in GIS systems.

As said above, the transient response of the control circuits is a function

of the frequency content of the VFTC. Hence it is essential to segregate

VFTC waveform into time and frequency scales simultaneously. For this

purpose a GOBOR wavelet function is used to obtain time-frequency

spectrum of the VFTC waveform at important locations of the 245kV GIS.

30

1.14 TRANSIENT ELECTRIC FIELD AND MAGNETIC FIELDS

GENERATION AT GIS STATIONS

The estimation of transient magnetic and electric fields are very

important in the GIS systems because usually the high frequency EM

fields are leak out at the bus bar terminations i.e. air-gas bushing, cable

termination during disconnector switch (DS) operation. The transient

voltages and currents radiate electromagnetic fields during its

propagation along the gas insulated high voltage bus as the associated

frequencies are in the range of few MHz. to about 300MHz [11]. The

transient electromagnetic fields thus generated in turn may leak out into

external environment through discontinuities like non-metallic viewing

ports etc. these transient fields may couple with the control wiring and

data acquisition systems etc. Malfunctioning of the primary /secondary

equipment have been reported by many authors during switching

operations of the GIS due to induce and /conducted voltages on control

circuits. It is essential to predict the transient EM field emission from a

gas to air bushing due to the very fast transient currents generated

during switching events. The characterization of these transient EM fields

are essential, because these are very sensitive to VLSI circuits, SCADA

systems etc.

The estimation of emission levels from the gas insulated equipment

during switching events is found to be important to the EMC design of

control devices operating in such EM environment and hence to ensure

31

reliable operation of the system. So, the estimation of Transient EM fields

are important in GIS systems to avoid temporary upset or permanent

damage of control equipment.

1.15 LITERATURE SURVEY:

1.15.1 Literature survey on VFTOs

Working group of 33/13-09, CIGRE [1] deals with the given

qualitative description of origin of VFTOs associated with gas insulated

substation. They also described the effect of these VFTOs on different

equipments like transformers, circuit breakers and bushings etc.

B.P. Singh [2] et al deal with the modeling concept of a typical GIS

and various over voltages generated due to switching operations and line

fault for fixed and variable arc resistances and in the presence or

absence of load. J.Meppelink [3] et al described the origin of very fast

transients in GIS. The different types of very fast transients are

classified. Switching operations in GIS lead to VFTOs. The effects due to

these VFT’S are radiate transient magnetic fields and transient electric

fields at bushing terminations and cable terminations.

Ivo Uglesic[4], et al described the problems associated with VFT’S

in Gas insulated sub stations. He describes the electromagnetic

compatibility (EMC) of secondary equipment in the 123kV GIS. The

cause for malfunctioning of bus bar protection scheme due to

disconnector switch operation was discussed. Continuous monitoring of

32

GIS equipment against VFT’S and influence on secondary equipment and

some precautionary measures also recommended.

K. Diederich[5] et al, described the Origin of Very Fast Transients

in GIS. Switching operations in a GIS lead to very fast transient

phenomena, which can be subdivided into internal and external very fast

transients. These VFT’s stress the equipment in GIS as well as the

secondary equipment. S. Yanabu [6] et al has experimentally estimated

fast transient over voltages in GIS. The maximum FTO estimated from

observation was 2.7p.u. This was observed infrequently and occurred

only at the open end of the bus bars.

S. A. Bogss [7] et al carried out field tests for measurement of dis-

connector switch operation induced transients and indicated that

transients do not exceed 2.0p.u. Further it gives that the trapped charge

left during dis-connector switch opening depends on the design of the

switch. S. Ogawa [8] et al, proved that re-striking surge of dis-connector

switches can be estimated by conducting calculations with considerably

high accuracy than measured waveforms. Accuracy of as low as 3% to 5%

has been achieved for measured and calculated values. Z. Haznadar [9] et

al & R. Witzmann [8] et al, have developed models for different GIS

components and conducted experiments with regard to waveform

distortion on various models consisting of spacers, bushing etc.

Amir Mansour Miri[10] et al, presented numerical and experimental

evaluation of the transient behavior of GIS. With the help of electrical

33

equivalent circuits of GIS components, the generation and propagation of

transients inside GIS have been evaluated. Nobuhiro Shimoda [11] & J.

Ozawa [11], 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. G. Engel

[12] et al, determined the resistance of high-current pulsed arc by various

formulae. The results indicate that in the initial stages of discharges (t <

0.5µs), equation developed by Toepler and some other authors are

identical.

G. Ecklin and D & Schlicht [13], describes the operation and

switching procedures with isolators occurring in GIS and the principle

operation of FTO’s generated in GIS. Tohei Nitta [16] et al, describes surge

propagation in GIS. Traveling velocity of surges is equal to the velocity of

light. Any component, which adds extra 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.

P. Osmokrovic [17] et al, describes the formative times and

Toepler’s constant approach to modeling the breakdown event and it

depends on the macroscopic parameters of the insulation.

V.Vinod kumar[18] et al describes the VFTO computations on

420kV GIS system. The variation of VFTO peak along the nodes for

disconnector and circuit breaker operations as well as the variation of

34

VFTO with different trapped charges have been studied. The results

indicate a distinct pattern of variation of VFTO Peak along the nodes of

the GIS in the case of disconnector switch operation as compared to that

of circuit breaker operation. D.Povh,[19] et al reported the phenomena of

very fast transients in a gas insulated substations. The corresponding

GIS equivalent circuits are useful to simulate VFT’s in the range of 100

KHz to 50MHz. validation of simulations is proved by comparing the

results with measurement results.

Jinsong tao[20] et al describes disconnect switches operation is

most frequent process in switch yard and substation and it is also a

source of electromagnetic interference to control circuits in substation or

environment for its radiated and conducted characterstics. V.Vinod

kumar[21] et al estimated the magnitudes of very fast transient over

voltages in a 420kV GIS. The variation of peak along with the GIS bus

nodes for disconnector and circuit breaker switching operations.

Xiang Zutao[22] et al describes suppression of very fast transient

over voltages in a GIS systems. Suppressing with MOA and resistance

switching are discussed. The calculated results are compared with the

measured results. The results shows that VFTO’S are suppressed up to

some extent. L.I.Qing–min [23] et al describes the use of ferromagnetic

rings suppress the amplitude and steepness of VFTO generated with in

GIS. Based on equivalent description of the complex magnetic spectrum

of ferromagnetic materials as well as the application of transmission line

35

theory, a simulation model is established in frequency domain to analyze

the suppressing characteristics of very fast transients in GIS.

Z. Haznadar [24] et al presented modeling techniques of GIS

components to estimate very fast transient electromagnetic transients.

Very fast electromagnetic transients caused by switching operation in

gas insulated substations are estimated using most suitable models. The

results are compared with the results obtained from field tests. Mastake

Kawada [25] et al describes non contact method for detecting insulation

fault. To detect the wide-band electromagnetic wave emitted during

switching operation of disconnector switch using wavelet transform. The

wavelet transform provides a direct quantitative measure of spectral

content, dynamic spectrum in the time-frequency domine.

Lu lu & Lin Xin [26] et al describes the three dimensional electric

field of 800kV SF6 Air chamber is analyzed by using the finite element

based software. For the complex analysis the extra-high voltage

disconnector chamber is considered to estimate electric filed intensity

distribution. Theoretical basis for construction design of disconnector is

supplied according to analyzed results of electric fields.

Toshihiro Hoshino [27] et al describes detection of electromagnetic

by antennas to diagnose the insulation performance of GIS to establish

high sensitive diagnosis technique of GIS insulation, frequency

charaterstics of electromagnetic wave are established. Based on these

results how the aperture condition contributed to the radiation

36

characteristics of electromagnetic wave. Finally they concluded that the

electromagnetic wave radiated from GIS aperture was based on different

radiation mechanism in frequency. C.M.Wiggins[28] et al has measured

transient electric field and magnetic fields and he described the

characterization of the these fields .the dominant frequency components

are also presented. The dominant frequencies of switching transients are

observed between 0.5MHz to 120MHz. J.A.Martinez [29] et al describes

the guide lines for digital simulation of very fast transients in gas

insulated substations and also he discussed about the origin of VFT over

voltages, their propagation and effects on GIS equipment is included.

Meng Tao [30] et al described calculation of VFTO’S in GIS

systems. He also discussed the suppression effect of cable and SF6 bus

bar. He pointed out that VFTO peak values may be restricted by

increasing the length of the cable and by varying the capacity of shunt

capacitor.

Zhang Bo [31] et al describes VFT effect on generator transformer

insulation in 500kV GIS. He simulated Tian Huangping pumped storage

power station in east china power system. The magnitudes are analyzed

for various conditions.

C.Y.Lui[32] et al describes the effect of VFT on GIS spacer

insulation, and he discussed the critical transient fields occured along

the surfaces of cone type insulators, and he estimated various fields

levels at various voltage conditions. OS.Singa [33] et al describes the

37

study of the very fast transient over voltages with SF6-N2 gas mixtures as

the insulating medium. With different percentages of SF6 he recorded the

VFTO levels during disconnector switch operation. In mixtures no

significant difference in the VFTO levels is seen between pressure 2-4

bar.

Shigeto Fjita[34] et al describes the effect of VFTO on shell type

transformer. In his work inorder to simulate waveforms of VFT invading

a transformer a simple equivalent circuit for shell type transformer in a

high frequency range is derived using this model, the over voltages are

analyzed for various configurations of GIS. Akihiro Ametani [35] et al

describes the effect of VFTO particularly on low voltage control circuit

cable through a measuring voltage transformer. He found that the

oscillating frequency of the switching surge exceeds certain limit. He

investigated switching surge characterstic in the low voltage control

circuits. Laboratory tests are performed accordingly to IEC 61000-4-12.

K.Feser[36] et al describes very fast transients occur in a gas

insulated substations stresses the equipment in GIS , adjacent

equipment. The different types of very transients are classified and their

characterstic parameters are summarized based on the measurements.

They concluded transient over voltages are of two types internal and

external, but the frequency spectrums have not clearly discussed.

Jinsong Tao [37] et al describes the electromagnetic interferences

is most common during disconnector switch operation. Peak values of

38

high frequency interferences are different according to the time of

operation of disconnector switch. The value at zero crossing of power

frequency voltage wave shape is far less than at peak point time.

Joy Thomas .M [38] et al carried out EMTP simulations on 420kV Gas

Insulated sub station. The variation of VFTO peak along the nodes for

disconnector and circuit breaker operations as well as the VFTO with

various operations as well as the VFTO with various trapped charges

have been studied. They concluded that the VFTO peak levels are not

proportional to the increase in the trapped charge.

Xulianyuan [39] et al describes the effect of GIS apparatus

parameter on a very fast transient over voltages. The suppression effect

of cable and SF6 bus bar is discussed and influence of the shunted

capacitor is analysed in this paper. They concluded that the VFTO peak

values may be restricted by increasing the length of the cable and SF6

bus abr.

Liu weidong [40] et al describes the suppressing methods of VFTO

wee discussd. Simulation tests were conducted on GIS model, with time

variant resistor as a arc in switch. Different magnetic materials were

used to suppress the VFTOs. They concluded that the VFTOs distinctly

suppressed by amorphous material (R2KB). Lu Tiecheng [41] et al

describes the simulation of 500kV GIS substation to estimate VFTOs

using EMTP, they also discussed about various factors influences the

over voltages. The VFTO effect on transformer insulation is discussed.

39

Thomas H. Dodds[42] et al presents the development of basic insulation

level. The unique properties of the ga insulated substation are analysed

and insulation coordination design techniques are utilized to determine

BIL ratings for the substation eequipment. Tavakoli.A [43] et al discussed

regarding effective factors which effect on wave shape and level of the

VFTO and VFTC. The effect of trapped charge compensation resistor,

hybrid compensation filters L and T types and variation of arrangement

of switchgear is discussed. And they concluded the considerable when

compared to hybrid compensation filters L and T.

J.A.Martinez [44] et al describes the modeling guide for digital

simulation of VFT in Gas insulated substations. They discussed

regarding origin of VFTs, propagation and effects. Several examples

corresponding to actual cases with detailed data input and validated

simulation results are presented. In the digital simulation, they

considered the arc resistance is a variable resistance which varies

experimentally decreasing resistance and the resultant time for break

down considered as 10ns. The investigation clearly shows that detailed

information of the internal design of GIS and also external equipment

like bushings, transformers etc.

J.C.Mendes [45] et al describes the EMTP simulations of a

standard GIS disconnector when interrupting small capacitive currents.

They mainly discussed high frequency transformer model to with stand

VFT, the evaluation of the VFT voltage stress on the HV terminal of the

40

transformer directly connected to the GIS. They estimated voltage stress

to design transformer winding insulation.

Amit kumar[46] et al investigated phenomena of VFT in Gas

Insulated S ubstations, the typed EHV-GIS substation is modeled and

simulated using PSCAD4.2 version. They considered closed disconntor is

fixed impedance and open disconnector as a capacitance of few pF. They

concluded that lengths of disconnector and bushing do not affect the

VFT magnitude and transient frequency

M.Mhohan Rao [47] et al estimated transient electromagnetic (EM)

fields generated during switching operations in a Gas Insulated

Substation depend on the wave shape of very fast transient over voltages

and very fast transient over currents. The peak magnitudes of VFTC and

their dominant frequency content at various locations have been

computed in 245kV GIS for different switching operations as well as

substation configuration. S.Carsimamovic [48] et al estimated very fast

transients due to disconnector operation in a 400Kv gas insulated

substation. They mainly discussed breaking of small capacitive currents

by disconnector. Transient over voltages are computed and discussed for

different switching conditions and points of buses as well as for different

points at grounding enclosure.

Selim Trabulus [49] et al investigates integration of digital

measuring technology in gas insulated substations. They discussed

conventional technology and modern technology and intelligent GIS

41

systems. They discussed sensors and actuators used in the GIS systems.

They also discussed difficulties with conventional instrument

transformers used in the GIS systems. M.Mohan Rao[50] et al estimated

transients included on control cables and secondary circuit of

instrument transformers in a GIS during switching operations. The effect

of the length of control circuit, type of grounding of the cable sheath

characteristics of the transient fields, type of secondary circuits and LC

loading of the control circuitry on the induced voltages have been

analyzed and reported. They concluded the transient voltages appearing

at the secondary circuit of the instrument transformers are in the order

of instrument transformers are in the order of a few kilovolts,

characterized by type of control circuit and are more for the potential

transformers than for the C.Ts. the reduction in the induced voltage

levels due to the attenuating transient fields is more for the PT secondary

circuit than for the CT secondary circuit.

Marcin stosur[51] et al estimated very fast transient over voltages

at the transformer terminals, resulting from a disconnectro switching

operations in a typical GIS substation. They proposed disconnector

model as a single spark model, with including many re or pre-strikes has

incorporated. N.H.Malik [52] et al carried out experiments to evaluate

performance of SF6 Gas under various pressures and with various gas

mixtures. The SF6+N2 and SF6+CO2 gas mixtures are subjected under

positive 150/1500µs switching impulse applications in non-uniform field

42

electrode systems. The gas mixtures pressure is varies from 1-5bar

during the experimentation. Tian Chi [53] et al proposed mathematical

model based on 500Kv GIS and 800kv GIS to study the amplitude-

frequency of VFTO and its effects on power transformers. The protective

effect of shunt resistor of disconnector switch has been verified. They

concluded that compared with 550kv GIS, the rated voltage of 800kv is

increased by 0.5 times, but relative value of VFTO appears almost same.

The insulation can be distorted by the resonant over voltages.

Li-Ming Zhou [54] et al estimated fast transients and high

frequency in a 15kv distribution cable. They conducted that the

attenuation (or) loss in power cables is generally seen as bad attribute,

they also concluded that high frequency loss of power cables protects

power system components from the detrimental effects of fast surges.

A.R.Memari [55] et al discussed mitigation of magnetic field near

load carrying conductors of an existing power line. They proposed a new

method of estimating magnetic field near high voltage conductor the

computed results are compared with the experimental results.

C.M.Wiggins [56] et al has experimentally estimated bus current

transients, electric and magnetic field transients. The current transients

coupled onto control and CT cables produced by switching operations in

a115kv ring bus have been measured and characterized. The transients

during opening and closing operations of disconnector switches have

been observed under normal conditions. Ali F. Imece [57] et al has given

43

modeling guide lines for the digital simulations involving fast front

waveforms. They mainly focused on lightning surges on the line

conductors near the GIS system. The selection of surge arrester location

and rating for protection of transmission lines in the Gas insulated

substations were discussed.

Smajic [58]et al presented a detailed analysis of very fast transient

electromagnetic (VFT) initiated by disconnetcor switching operations in

gas insulated substations. They also performed electromagnetic

simulations performed on real life GIS geometries along with modeling

details are presented. Igor Ivankovic [59] et al describes problems with

electromagnetic compatibility (EMC) of the secondary equipment in the

132kv gas insulated substation through measurements and computer

simulations were conducted in order to determine magnitudes and

frequencies of the transient voltages and currents in the secondary

circuits.

Ing. M artin Mach [60] et al investigates the magnetic field

emission caused by two different types of medium voltage switchgears;

because of complexity of the substation geometry they suggested

simplifications of the real arrangement. The 3D solution models are

obtained. They also discussed some aspects associated with processing

models characterized by pressure of incommensurable sub domains and

mathematical aspects of solution. W.K.Dialy [61] et al estimated

magnetic fields generated by typical distribution substation based on

44

currents in the grounding systems, distribution feeder neutrals, over

head ground wires and ground grid loops. They concluded that mainly

magnetic fields are significantly influenced by the ground wires their

study revealed that circulating equipment caused noticeable

distributions in the magnitudes of magnetic fields.

Robert G.osen [62] et al described a technique to calculate the

coefficients of spherical and finite length cylindrical multiple sources

whose positions have been defined by the user. The differences between

actual fields and equivalent source fields can be used to determine

additional source locations. S.E.Wright [63] et al estimated electric and

magnetic fields of switching transients in a 500kv gas insulated

substation. Peak vertical electric fields of 16kV/m and horizontal

magnetic fields of 212 A/m have been measured near SF6 to air bushing

of a Gas Insulated substation. The dominant frequencies of switching

transients are observed between 0.5MHz and 120MHz.

Zijad Bajramovic [64] et al investigated very fast transient

electromagnetic transients (VFT) in air insulted and gas insulated

substations caused by switching operation of disconnectros are

investigated. Digital simulations of VFT for different models of power net

work are performed using EMTP-ATP. The peak values estimated.

I.O.Habibalah [65] et al estimated magnetic fields on a typical

230kV substation in Saudi Arabia. The simulation of magnetic fields was

done by using SBCALC magnetic field modeling program developed by

45

EPRI environment division running the resulting magnetic field

environment in a variety of graphical formats including contour and

three dimensional maps. The measured and modeling results were

coordinated and showed almost full agreement. The results were

reasonably lower than the limits presented by international guidelines.

Bojan Trkulja [66] et al calculated electric field based on integral

equations approach and suited to solving large problems is presented.

The integral equations are solved by both the methods BEM and

Galerkin. They compared the numerical results with measured fields, the

comparison shows good agreement. They mainly concentrated on

computation of low-frequency quasi-static electric fields. Antonio F.

Otero [67] et al describes the simulation of the magnetic field generated

by electrical lines through a simple model accurately predicts the

measured values. FFT analysis of the magnetic frequency and amplitude

of the possible induced currents.

B.Vancia [68] et al describes the electric field calculations at the

line end of 275kV substation. They observed that the fields can be high

enough to cause damage to insulators due to corona discharge; they

suggested grading devices need to be used to reduce the electric field to

acceptable levels. Charles L. Wagner [69] et al investigated insulation

coordination for gas insulated substations. They describe an analysis

made to determine the BIL ratings and protective distances for the gas

insulated equipment in a insulated substation at various system levels

46

and lightning arrester ratings. They developed analysis to select required.

BIL ratings for either conventional (or) gas insulated lighting arresters

located at the terminals of the gas insulated substations.

Pinches D.S [70] et al describes modeling of GIS systems for

computation of VFTOs. Simulations have been performed with the use of

the model of a very typical GIS system, based on state of the art GIS

modeling. Song fan [71] et al describes three dimensional finite element

analysis of GIS disconnector. The distribution of magnetic field intensity

and the values of eddy currents are calculated. The FEM models of

100kv GIS disconnector has developed. The distribution of magnetic field

intensity of the disconnector and distribution of current density of the

disconnector are given.

LiLinling [72] et al describes the construction of 800kv gas

insulated transmission line (GITL) they also discussed the filling

pressure of SF6 gas is 10psi (gauge reading) at 1000meters altitude. The

same amount of SF6 at 2500m will result in 12.09 PSI on the gauge

reading. They suggested 12.09 PSI on the gauge reading. They suggested

that the filling pressure should not exceed 0.48kpa to 0.58Mpa on gauge

reading. Miroslav Krepela [73] et al describes the transient phenomena

originating from switching of 400kv SF6 circuit breaker. They analyzed

the unsymmetrical phase inrush currents during a circuit breaker

switching and over voltages provoked by current chopping and reigniting

of electrical arc are analyzed.

47

M.Gamlin [74] et al presents recent experience regarding reliability

of GIS/GIL. They described the high voltage test systems with their

advantages and limitations and the applicable partial discharge

measuring methods. The onsite testing of a GIS terminated by a SF6 air

bushing. S.Huigang [75] et al desrcribes a method and device for

suppressing vacuum switch re-striking voltage. They proposed a

magnetic ring string provided around the conductor and a resistor is

coupled to the conductor which can limit the over voltage both in

steepness and amplitude, and it is simple in structure and reliable in its

own insulation.

Anastasia S. Safigianni [76] et al investigates the electric and

magnetic fields values in the area of indoor gas insulated substation

100MVA, 150/20kV in xanthi, Greece. They have used isotropic

magnetic-field probe and the measurement ranges 5nT to 10nT and

0.1V/m to 100kV/m. the measured magnetic and electric field values are

very small in the supervision room, they are slightly higher values at SF6

to air bushing termination. Marjan Popov [77] et al estimated very fast

transient over voltages by both measurements and simulations in layer

type distribution transformer. The modeling of the transformer and

computations are verified by measurements. They suggested observing

transients with a longer period of time, the influence of frequency

dependent core losses, the influence of frequency dependent core losses

must be taken into account.

48

P.S.Nickel [78] et al described a time domain model for predicting

transient bus currents, electromagnetic fields, and cables coupling

effects caused by switching actions in high voltage substations. They

estimated EM field data at both normal and abnormal conditions. Hu

Qin [79] et al described optimized charge simulation method to calculate

condition surface electric field of 800kV. Surface electric field is

calculated when the charge is in the optimal position, the distribution of

the surface electric field and its maximal value calculated accurately. The

charge simulation method advantages are discussed.

Garry H.Rodrigue [80] et al describes quantitatively how the

wavelet transform can be effective mathematical tool for the analysis of

transient signals. They have used various types of wavelet functions to

analyze electro cardiogram signals. J.Ozawa [81] et al describes transient

phenomena in power systems due to switching operation faults. The

transient voltage waveforms are analyzed using fast transient voltage

waveforms and critical frequencies are identified. Suppression of fast

transient over voltages during DS switching is discussed. Soo-Hwan Cho

[82] et al describes a time-frequency analysis of power quality

disturbances using winder wavelet transform. The various PQ

disturbances including voltage swells, voltage sag, harmonics, inter

harmonics, transients with multiple high frequencies and voltage

fluctuations will be thoroughly investigated by using this new time-

frequency analysis method.

49

J.Amarnath [83] et al estimated VFTO levels using PSPICE models.

Equivalent circuits were developed for 245kV GIS system, various

lengths of GIS sections. The maximum level of VFTO observed is

2.72p.u. The VFTO levels are estimated using fixed arc resistance and

variable arc resistance. Transient’s suppression is verified with parallel

resistance across the Disconnectors.

Gayan Wije weera [84] et al describes a new type of electric field

sensor that has been fabricated using micro machine technology. The

sensor requires minimal operating power with the shutter being driven

by a 75mV drive signal while consuming only 70µW. The sensor has a

linear response to the electric field amplitude. Ying Wang [85] et al

describes operation of GIS disconnectors can leave a DC voltage on the

switchgear which can cause charging of spacer surfaces. They explained

about probability of surface flashover when disconnector is closed. They

concluded that because of trapped charge left on electrode causes

surface charge distribution across the spacer. The failures in some

550kv disconnector switches were discussed. RUAN quanrong [86] et al

describes the opening and closing resistor method to suppress VFT

generated during disconnector operation. They have data analysis to 28

different connection modes using EMTP program. Data shows that open

/close resistance is larger than 200Ω, the amplitude of VFT is reduced by

15%.

50

A. Sabot [87] et al describes the insulation coordination of GIS.

They conducted that the insulation coordination practice mainly dealing

with fast over voltages. They presented on site test procedures related to

the insulation coordination. The dielectric diagnostic techniques are also

discussed.

J.M.Garablenkow [88] et al conducted tests on the subject of

capacitive switching currents by disconnector switches in gas insulated

substations with various test objects and arrangements. They also

described the influence of various test parameters such as voltage, gas

pressure and distributed capacitances etc. Vinodh kumar .V [89] et al

estimated very fast transient over voltages in a 520kV gas insulated

substations using EMTP software. They observed variation of VFTO

peaks at various nodes of the GIS bus in the case of disconnector

operation rather than circuit breaker operation. It is also noticed that the

variation of VFTO peak levels are not in direct proportion to the trapped

charge present on the high voltage bus. Sakai.K [90] et al discussed

extremely low frequency (ELF) magnetic field environment in a

500kV/275kV Gas Insulated Substation. They also investigated magnetic

field distribution around the 3-phase GIS. From the results they

characterize the distinct features of magnetic field distribution in

500kV/275kV Gas insulated substation by comparing with another type

of substation.

51

M.MohanRao [91] et al described a numerical characterization of

the transient electromagnetic field generated in a gas insulated

substation during operation of a disconnector switch. The frequency

spectrums of various EM field emissions are characterized. Finally, the

transient EM field emission levels from gas to air bushing due to VFTC

are established.

V.S.Rashkes [92] et al estimated VFTO levels during disconnector

operations in a open air substations .they observed that highest spark

over voltage observed across the contacts of the breaker. The

calculations have showed that 787 and 525kv substations with branched

buses very high frequency over voltages (VHFOV) could result in

flashovers on a bus open end with un favorable ratio of bus lengths. For

transformer insulation protected with an arrester VHFOV are not

dangerous .some protective measures are recommended.

Hiroshi Koyama [93] et al discussed a correlation between the

ultra-high-frequency (UHF) signal (mV) and apparent charge (9pc) for

onsite calibration regarding partial discharge (PD). The sensitivity line of

3-phase gas insulated switchgear (GIS) is compared to that of single

phase GIS and differences in defects are also discussed. Eduardo

ZABALA [94] et al explained an over vie of wavelet transforms

applications in power systems. Mainly they have analyzed transient

waveforms other than transform approach. They also discussed multi

52

wavelet and second generation wavelets to improve the actual and future

applications.

Hongsheng li [95] et al estimated very fast transient over voltages

caused by disconnecting switch operation using ATP-EMTP program. The

various protection measures including metal oxide arrester, R-C

absorbers, opening and closing resistors and applications of ferrite rings

have been discussed Shun-Li Lu .[96] et al describes a new method of

measuring power frequency magnetic fields due to distorted sinusoidal

currents in power system. In the field actual test results are

accomplished by a standard area-type measurement set up to identity

and characterize magnetic fields in a 69kV SF6 GIS substation.

M.R.Iravani [97] et al submitted a report on modeling guide lines

for the investigation of fast transients, and also proposed new method of

digital-computer time domain simulation methods. They proposed

sample test systems and typical time-domain simulation results.

Hussain H [98] et al presented measurement results of power frequency

magnetic fields on 275kV GIS substation in Malaysia. The measurements

cover the magnetic fields at important locations in a substation. The

results are presented in various formats. Statistical representation of

measured data also presented. The results from the measurements will

be used for future simulations and for comparison purpose.

C.M.Wiggins[99] et al estimated electromagnetic interference levels

on sensitive electronic equipment are quantified experimentally. They

53

also reviewed measuring techniques for interference voltages, currents

and electric and magnetic fields. The nominal maximum field and control

wire interference levels expected in the switchyard are estimated using

high frequency transient coupling models. Farag A.S [100] et al proposed

methodology to evaluate the magnetic field in a gas insulated substation.

Proposals for magnetic field measurements wee given relations between

the fields and various design factors are studied in order to optimize

substation parameters to reduce the magnetic fields.

Qingmin Li [101] et al describe high frequency magnetic coils used

in a Gas Insulated substations based on magnetic spectrum of

ferromagnetic materials different magnetic materials are used to

suppress the VFTO levels at gas insulated substations.

A.Ardito [102] et al carried out digital simulations of the overall

substation behavior during electromagnetic transients. A design and

modeling of GIS bushings against the very fast transient over voltages

have been made in this work. The initial partial discharges in the

bushing have been discussed. LU Tiebing [103] et al estimated radiated

magnetic field from the junction between GIS enclosure and high voltage

cable. The estimated results are compared with the results obtained from

the software TEMFGIS. They concluded that the frequency of over

voltages generated by closing operation of 125kV disconnector switch is

250MHz. The generated fields are strongly depending up on the location

of switches.

54

Huijuan Zhang [104] et al have analyzed the electromagnetic

transients when switching the no load bus in the 500kV substation by

the ATP software the results are compared with actual results. They

concluded that simulation model has a better accuracy and it is proved

that electromagnetic transient simulation is reliable and effective.

D.E.Thomas [105] et al describes switching transients EMI

response measurements on several types of substation cables and

internal cable wires. The measures and predictable voltage transients in

a gas insulated substation and air insulated substations are presented.

Model predictions are compared to and validated against measured wire

transients.

Amara Graps [106] is introduced wavelets in the digital signal

processing field. She describes the basics of wavelets beginning with

Fourier, compares wavelet transforms with Fourier Transforms,

properties and other important aspects of wavelets. Also, the applications

of wavelets like image processing, musical tones, and de-noising of noisy

data have been used. The basic concept behind wavelets is to analyze the

signal according to scale. Wavelets are functions that satisfy certain

mathematical requirements and are used in representing data or other

functions. The author described wavelet overview, historical perspective,

basis of functioning of wavelet, different Fourier transforms like Fast

Fourier transforms, and Discrete Fourier transforms. Windowed Fourier

transformers. Further, she also compared the Fourier transforms with

55

Wavelet transforms. She also discussed about different wavelet

transforms and their applications. Finally she concluded with

advantages of wavelet over Fourier transforms. In recent years,

researchers in applied mathematics and signal processing have

developed powerful wavelet techniques for the multi-scale representation

and analysis of signals. The main wavelets in use are Daubechies, Graps,

Morlet, Kaiser, Lee and Yamamoto, Mallet, Resinkoff and Burrus, Rioni

and Vetterli, Mexican, Gabor, etc. These new methods differ from the

traditional Fourier techniques. Wavelets localize the information in the

time-frequency plane; in particular, they are capable of trading one type

of resolution for another, which makes them especially suitable for the

analysis of non stationary signals. Chien-Hsing Lee et al. [107] provides

lot of literature survey of recent wavelet developments in power

engineering applications. This survey includes detection, localization,

identification, classification, compression, and storage and network

system analysis of power disturbance signals. In each case, they proved

them with some general information and brief explanation. The authors

explained about wavelet properties in the context of power engineering

applications like decomposition and reconstruction of a signal, time-

frequency localization of signals using different wavelets. The Morlet or

Daubechies wavelet has the best time-frequency localization and is

confined with the Heisenberg uncertainty principle.

56

Rosa M de Castro Fernandez et al. [108] presented a descriptive

overview of wavelet transform application in power systems. The main

publications carried out in this field have been analyzed and classified

by choosing those that are more representative of certain feature, related

to their contribution or continuity of a line of investigation. J.P.Antoine

[109] reviewed the general properties of the wavelet transform both in its

continuous and discrete versions in one or two dimensions and some of

the application in signal and image processing. It is a fact that most real

life signals are non-stationary. They often contain transient components,

sometimes very significant, physically, and mostly cover a wide range of

frequencies.

Karen L.Butler-Purry et al. [110] and. The analyzed data was

obtained from simulations and experiments. Time frequency analysis

was performed using Discrete Wavelet Transform (DWT). From the time-

domain results, it is observed that distinct spikes are conveyed in the

high frequency bands during incipient activity.

El Sayed M. and Tag Eldin [111] used a new approach for

classifying transient phenomena in power transformer. Discrimination

between internal faults, external faults with current transformer

saturation and magnetizing inrush current is achieved by wavelet

transforms. The wavelet transform is applied for the analysis of the

power transformer transient phenomena because of its ability to extract

information from the transient signals in both time and frequency

57

domain. The authors used a new algorithm for Wavelet transforms for

extracting different features of the transient voltages. Fast

electromagnetic transients are typically non-periodic signals, which

contain both high-frequency oscillations and localized impulses

superimposed on the power frequency and its harmonics.

A.P.S. Meliopoulos et al. [112] used the transient analyzers via

wavelet method of dynamical systems. This method consists of the

traditional frequency domain analysis to capture the steady state

operation of the system and a wavelet based transient analysis, which

captures the disturbances. The authors have named this method

Wavelet-based Transient Analysis (WBTA). The authors implemented

Daubechies wavelets. The results obtained using this method are

compared and verified with a numerical time-domain analysis method. In

the signal and image processing community there is chance of having

noise, or more generally, an unknown error in the actual signal.

Sylvain Durand and Jacques Froment [113] proposed a model to

reconstruct wavelet coefficients using total variation minimization

algorithm. The authors focused on two promising approaches, which are

Wavelet thresholding and total variation minimization. The approach is

motivated by wavelet signal de-noising methods where thresh holding

small wavelet coefficients are available. Wavelet de-noising consists of

decomposing the noisy data into an orthogonal wavelet, superimposing

the wavelet coefficients smaller than given amplitude, using a so-called

58

soft or broad thresh holding, and transforming the data back into the

original domain.

Tertulien Ndjountche et al. [114] used the technique for noise

reduction and is based on the Hidden Markov Tree (HMT) structure,

which can efficiently model the statistical characteristics of practical

signals. Wavelet transform can localize the signal information in the

time-frequency plane with different resolutions. This property has been

found to be useful in noise reduction.

D.T.Nguyen and J.A.Hoang [115] used wavelet transform to detect

the disturbances on electricity supply. The work carried out by these

authors is a demonstration of diversity of types of disturbances on

electricity supply and consequently the complexity of the digital signal-

processing task required in order to analyze and to identify the

disturbances. The authors propose use of FFT for the separation of the

power supply frequency and its harmonics before resorting to the wavelet

of the analysis for the transient disturbances. Transient disturbance is

carefully isolated from the rest of the signal and is detected using wavelet

transform. Features such as onset time, duration and dominant

frequencies of a disturbance can be accurately and efficiently extracted

using this wavelet transform technique for disturbance identification.

D.C. Robertson et al. [116] employed wavelet transform method for

analyzing electromagnetic transients associated with power system faults

and switching. This method is very effective for the non-periodic, wide-

59

band signals associated with electromagnetic transients for analyzing the

source of transients.

Summary:

The detailed literature survey has carried out on VFTOs and its effects.

Many authors described that very fast transient over voltages depends

up on arc resistance, effect of trapped charge and circuit configuration

and voltage ratings etc. few authors have considered it as a fixed arc

resistance most of the authors considered as a variable arc resistance.

Ivo uglesic et al described the problems of electromagnetic compatibility

of secondary equipment in GIS systems . they also recommended

precautionary methods . Yanabu S et al has experimentally estimated

fast transient over voltages in GIS, but they obtained VFTO of 2.7pu at

the ope end of the bus bars. Amir Mansour et al presented VFTO

propagation inside the GIS. Ozawai et al describes the method of

suppress of transient over voltages by inserting resistance during

switching operation. Toheri nitta et al describes that ground capacitance

has significant effect on propagation of trvelling waves. Vinodh kumarr et

al describes the the VFTO computations on 420kV GIS system. In the

earlier literature suppression of VFTO at origin is not discussed and also

suppression of high frequency components in VFTO and VFTC have not

discussed. The use of time frequency analysis is also not discussed in

detail.

60

ORGANISATION OF THESIS

CHAPTER-I

Introduction to Gas Insulated Substations, advantages of GIS over the

conventional open air substations, disadvantages of GIS have been

presented. The principle of VFTO generation, secondary breakdown

phenomena and prestrike, trapped charge effect and current chopping in

GIS are discussed. The very fast transient phenomena in GIS,

suppression techniques, transient electromagnetic phenomena in GIS

have been discussed. The Literature survey on Very fast transient over

voltages and measurements, Transient electric and magnetic fields in

GIS systems and Wavelet applications for transient analysis have been

discussed.

CHAPTER-II

The statement of the problem, main contribution of the thesis and brief

organization of the thesis have been discussed.

CHAPTER-III

Modeling of GIS components, the EMTP modeling concepts have

been presented. The calculation of parameters and equivalent circuits of

GIS components, modeling of arc are presented. VFTOs are computed for

fixed arc resistance and variable arc resistance conditions. The VFTO

computations have been made with and without trapped charge and the

results are presented. The Fast Fourier Transform (FFT) technique using

MATLAB 7.1(Signal Processing Tool Box) has been used in order to

61

identify the dominant frequencies of the transient voltages/currents. The

frequency spectrums have been calculated for the finite time duration of

5µsec.

The various VFTO suppression techniques have been discussed.

First case the suppression effect of VFTOs is verified with resistance

switching. Next case, the suppression effect of VFTOs is verified by

proposed technique of application of high frequency ferrite rings.

Modeling concepts of ferrite rings has been discussed. The VFTOs are

computed with and without ferrite rings. The frequency spectrums of all

the cases were obtained. The results of both the techniques have been

compared and discussed in this chapter.

CHAPTER-IV

A series of experiments have been conducted on 1-Phase, SF6,

3.3kV GIS system with disconnector arrangement. The VFTOs are

captured by specially designed high frequency capacitive surge sensor.

To know the effect of trapped charge on VFTO, a voltage of 4.7kV (D.C) is

applied to floating electrode in steps of 10%. The VFTOs are captured

recorded through Digital Storage Oscilloscope of 250 MHz, 1GS/s with

FFT. The data is recorded through Digital storage oscilloscope (DSO) of

250MHz; 1GS/s with FFT function then it is transferred to computer

system through RS-232 serial communication port provided. The VFTOs

are captured during both opening and closing operations of the

disconnector switch. Proposed method of suppressing VFTOs is verified

experimentally. The corresponding frequency spectrums are also

obtained for all the cases. The suppression effect of VFTOs by both

resistance switching and ferrite application methods are compared and

results are analyzed.

62

CHAPTER-V

The fast transient electromagnetic fields (FTEMF) in GIS systems

are computed. The FTEM fields at four important locations were

computed by using ELECNET & OPERA field computational soft ware’s.

The EM field measurements are carried out at 245kV GIS substation

along with ABB, India as a part of GIS (SCADA) automation project, and

the corresponding results are presented. The results obtained from EM

field computations are compared with the measured data. The final

conclusions are presented in this chapter.

CHAPTER -VI

The VFTCs generated during Disconnector Switching in a 245kV

are analyzed using wavelet transforms. The time-frequency spectrums of

VFTCs at two important locations are obtained. The variations of

current magnitude with time for different frequencies associated

with the VFTC are calculated. The results are discussed.

CHAPTER-VII

In this chapter conclusions and scope for future work have been

presented.

SUMMARY:

In this chapter the statement of the problem and main objectives of

the thesis have been discussed. The brief outline on the thesis

organization has been explained.