1
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
2
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).
3
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
4
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.
5
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, restriking occurs many times before the interruption is completed. Each
re-strike generates VFTO’s with different levels of magnitude [2].
6
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
7
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
8
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
9
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
10
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.
11
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
12
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.
13
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.)
14
1.7 GENERATION OF VERY FAST TRANSIENT OVER VOLTAGES
(VFTOs) IN A GIS:
During the switching operation of dis-connector switch in a GIS, restrikes (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:
15
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 restriking. 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
16
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].
 KT 
Tf = 4.4 ∗ l ∗ 
 U
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.
17
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, restriking 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.
18
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 selfcapacitance 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.
19
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
20
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.
21
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
22
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 restriking 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
23
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).
24
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
25
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.
26
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 electromagnetic to electro-static of energy.
i.e.
⇒
1 2 1
LI = CV 2
2
2
V=
L
∗I
C
27
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 300400Ω 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 disconnector 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 amplitudefrequency 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 timefrequency 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.
transforms
and
She
their
also
discussed
applications.
about
Finally
she
different
wavelet
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, timefrequency 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 timedomain 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 signalprocessing 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.