3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
Analysis of Very Fast Transient over Voltages (VFTOs) of
Transformer in Gas Insulated Substations (GIS) using Wavelet
Techniques (WT)
Prakasam.K, Surya Kalavathi.M, Prabhavathi.D
*PhD schalor, JNTUCEA, Anantapur India, prabhavathi10@gmail.com, 9966783293,Prof, JNTUHCEH, Hyderabad,
India,munagala12@yahoo.co.in,PhD schalor, JNTUCEAAnantapur, India, gvitgem@gmail.com,9966783293
Keywords: Analysis; Damping; mitigation; surge; transients,
wavelet
Abstract
Switching operating generates very fast transient over
voltages(VFTOs) in gas insulated substations (GIS) and it is
very dangerous to transformers and causes the damage to
insulation because of its short rise time, in some cases its
magnitude may be very close the basic insulation level
(BIL),Mitigation and analysis of very fast transient over
voltages (VFTOs) of power transformer is very important in
gas insulated substations (GIS) as very fast transient over
voltages causes’ damage to the insulation of the power
devices like transformer. In this research work power
transformer is considered as it plays major role in the
electrical substation. Initially a power transformer rating of
500MVA in 765kV/400kV gas insulated substation with the
load of 3 X 500MW capacity has been considered and the
simulation model is developed using Mat Lab platform on the
basis of very fast transient (VFT). The very fast transient over
voltages (VFTOs) generated due to circuit breaker (CB)
operation has been evaluated at transformer, open end and
circuit breaker (CB) and in the next sate of this research work,
mitigation methods like RC filter and Ferrite ring have been
employed and then the effect the proposed damping methods
on peak magnitude of very fast transient over voltages
(VFTOs) [5, 4, 11] has been evaluated. In the further stage,
the effect of terminal components on peak of VFTOs has been
estimated for two time constants 1ns. The outcome of the
proposed techniques again is explored to wavelet transform
(Db4) for extraction of high frequency signals. As wavelet
transform is a powerful tool for analysis very fast transient
signals. The results shows that peak value of VFTOs can be
reduced to considerable level by employing the proposed
damping methods and it has proved that the terminal
components of gas Insulated substation (GIS) also influences
the peak magnitudes of VFTOs.
1 Introduction
The power system is large and interconnected one in which
the transformer plays an important role and thereby it is very
essential to protect it against the internal and external faults as
well as from the over voltages due to any reason particularly
when it is used in gas insulated substation since there is
always possibility continuously the likelihood of reason for
fast transient over voltages. Gas insulation substations (GIS)
have been utilized as a part of force frameworks in the course
of the most recent three decades on account of their high
unwavering quality, simple support, and small ground space
necessity and so on. In India likewise, a couple GIS units are
under different phases of establishment. The basic insulation
level (BIL)[7] required for a gas insulated substation (GIS) is
different from that of the conventional substation because of
certain unique properties of the former. Switching operations
generate very fast transient over voltages (VFTOS) [1, 2, 7, 9]
VFTOS may cause secondary breakdowns inside a GIS and
Transient Enclosure Voltages (TEV) outside the GIS,
Prolonged arcing may produce corrosive / toxic byproducts,
Support spacers can be weak points when arc by products and
metallic particles are present, From the reliability point of
view, partial discharge detection is important. The methods of
detection are of electric systems etc. These methods lack
quality control. For these reasons, VFTOs generated in a GIS
should be considered as an important factor in the insulation
design. The major Problems Associated with the Very Fast
Transient over Voltages (VFTOs) generated in GIS causes ,
Flash over to ground at the disconnecting switch (DS) switch
contacts, Failure of electronic control circuits connected to
GIS due to electromagnetic interference of VFTOs, Dielectric strength is reduced under VFTOs, if non-uniform
electric field is formed by the particles (primarily metallic),
effect on equipments such as bushing, power transformer and
instrument transformers, Transient Enclosure Voltage (TEV)
on external surface of the sheath. This may cause flashovers
to nearby grounded objects
2 Modelling of GIS Components
Many authors have discussed about generation,
computation, mitigation suppression [2, 12], measurement
and analysis of very fast transient over voltages in different
ways and many of the researchers presented their articles
about mitigation and analysis of very fast transient over
voltages (VFTOs) [9, 10, 5] in gas Insulated substations (GIS)
however most of them considered low voltage low rating
transformer and comparison given between the existing and
proposed damping methods. This paper presents a robust
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
wavelet technique based analysis of peak magnitude of very
fast transient over voltages (VFTOs) at transformer, open end
and at the disconnecting switch (DS) along with the terminal
components effects on peak values of VFTOs. An exact peak
value of VFTOs must be measure for proper insulation design
and to protect the transformer or other important electrical
equipments in GIS and this can be achieved by the proposed
technique. The wavelet transforms gives the accurate
measurement [6] since the outcomes of wavelet transform is
based on the time and frequency analysis, unlike individual
time domain and frequency domain. The proposed
765kV/400kV, 500MVA GIS system has been designed on
the basis of very fast transient (VFT) as very fast transients
are travelling wave nature. The components of the proposed
system are designed taking surge impedance, propagation
velocity, formative time length and into account.
2.2 Circuit Breaker
In the proposed work 1.5 model of circuit breaker (CB) [4,
11] fig.2 has been employed for switching transients and it
can be designed as per open and closed conditions. During the
closing operation of circuit breaker (CB) the electric field
increases still sparking occur and the sparking occurs at
power frequency first.
2.1Powet Transformer
The power transform can be designed either by multi
conductor model or by detailed model in general. However in
gas insulated substation the very fast transit over voltages
generated due to switching operations is of high frequency in
Thank you for your cooperation in the range of MHz and non
stationary with very short rise time. The windings transformer
at very high frequency behaves like capacitive network [7,8,
10] there by it should be treated as coactive nature, the series
capacitance and shunt capacitance should be considered to
evaluate an exact value of VFTOs. In the proposed system the
parameters of transformer are calculated on the basis of VFT.
Since the transformer coil at high frequencies behaves as
capacitive network the modelling of transformer[7] has been
designed based on very fast transients(VFT)[7, 8, 9] i.e. The
parameters of transformer are estimated for accurate
simulation results and are given by R1 = 22.8 Ω , R2=22.8
Ω,R3=300 Ω and inductance of the coils L1= 5.µH, L2 = 47.5
µH , L3 = 9.42 µH and the capacitance C1 = 0.84pF, C2 =
1.74pf, C3 = 35.4pF, C4 = 120pF the surge impedance,
velocity of propagation, formative time and length are
considered as well as the series capacitance between the turn
and coil and the shunt capacitance between the turn[6], coil
and grounded core and transformer tank are considered for
accurate results of the peak magnitude of very fast transient
over voltages (VFTOs).The parameters of power transformer
is estimated as per the very fast transient nature for accurate
measurement of VFTOs [1, 5, 8]. Fig1 shows the equivalent
circuit of power transformer in the proposed system.
Fig.2.Equivalent circuit of circuit breaker (CB)
During the closing operation of the circuit breaker
(CB) the charging current flows and charge the load to the
source voltage and voltage collapses and extinguish [5, 7].
The sparking charge depend on the speed of the circuit
breaker (CB)The circuit breaker (CB)4, 11] is represented to
by a PI section contains two travelling wave models [ 7],
which is shown in figure 2, with the parameters as Z1=58 Ω,
L1= 560mm L2=390mm, Z2 16 Ω, L3 = 400mm, C1=20µand
C2=25µF. The sparkle utilized as a part of circuit breaker
(CB) re-strike cases is displayed as an exponentially rotting
resistance Ro e(-t/τ) in arrangement with a little resistance, r
of 0.5 Ω to deal with the lingering flash resistance. Value of
fixed resistance rs has been selected on the basis of the
practical consideration as discussed.
Where, R0 = 1012 Ω, Fixed Resistance =0.5Ω. T (spark time
constant) = 1 ns, Open end section of GIS - The open ended
section of GIS has been presented in the following figure
where a lumped shunt capacitance has been considered.
Assuming the same as a coaxial hemisphere, its capacitance
has been estimated using following equation.
Figure.1.Equivalent circuit of power transformer
The surge impedance, propagation velocity and
formative time can be evaluated from the equations (1), (2)
and (3).
Where, R= internal radius of enclosure, r=external radius of
enclosure
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
3 Methods of Mitigation
4 Simulation Results and Conclusions
In this section, the different possible and efficient methods of
damping like, RC filter; Ferrite ring and also terminal
components like OHTL, XLPE Cable and GIL have been
presented in different sections as follows.
The proposed system has been designed using mat lab plat
form and simulated with different methods as mentioned
earlier for the estimation of Very Fast Transient Over
Voltages at transformer (Vtr), open end (Voc) and at circuit
breaker (Vcb) and the outcomes of the proposed techniques
are shown as follows
3.1 RC – Filter Method
RC filters have been widely used in vacuum circuit
breakers to suppress the over voltages. RC filter is parallel
next to circuit breaker. In the proposed work, the comparable
circuit of 765kV GIS is considered with likeness RC channel.
The main reason of which is that capacitor absorbs the high
frequency component and finally energy is consumed by the
resistor because of this character of capacitor it has been
executed as one of the strategy for mitigation and analysis
The parameters of R and C are picked as per the literature
survey as R can be chosen from 50 to 400 Ω and C can be
from 0.01 to 0.2µF ,R = 100 Ω and C = 0.2µF and recreations
are completed.
Fig. 6.Magnitude of VFTO at Transformer (Vtr)
Fig.7.Magnitude of VFTO at Open end (Voc)
Fig. 4.Equivalent circuit of RC-Filter
3.2
Ferrite ring Method
As per the literature survey it has been observed that, the
VFTOs can damp to a great considerable level of peak using
ferrite rings. The most of the authors tested the ferrite ring
under low voltage and low frequency and conventional
comparison given between the shunt resistor and ferrite ring,
In this research work, we introduced a new method of
analysis of very fast transient over voltages VFTOs which is
known as ferrite ring in association with wavelet transform as
the VFTOs are concern to high frequency transients R = 100
Ω, L = 2H and S at t = 10ms.
Fig. 8.Magnitude of VFTO at circuit breaker (CB)
Fig.9.Magnitude of VFTO at Transformer (Vtr) Db4
Fig.5.Ferrite ring Method
3.3 Terminal Components (OHTL, GIL & XLPE Cable)
In the next stage of this research work, the gas insulated
substation (GIS) has been terminated with over head
transmission line, gas insulated line (GIL) and XLPE cable,
the OHTL and cable parameters are estimated based on surge
impedance (Z0) and propagation velocity (𝝊) with the
equations (1) and (2).The parameters used shown below.
For GIS Bus- Bar, Z0 = 270Ω, 𝝊= 270m/µs, OHTL, Z0 = 350,
𝝊 =300m/µs , XLPE Cable, Z0 = 30Ω, 𝝊= 1.9557X105 km/s
The very fast transient over voltages (VFTOs) at transformer,
open end and at disconnecting switch has been estimated.
Fig. 10.Magnitude of VFTO at open end (Voc) Db4
Fig. 11.Magnitude of VFTO at circuit breaker (CB) Db4
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
Fig. 12.Magnitude of VFTO at Transformer (Vtr)
Fig. 13..Magnitude of VFTO at open end (Voc)
Fig. 14.Magnitude of VFTO at circuit breaker (CB)
Fig. 15.Magnitude of VFTO at Transformer (Vtr) Db4
Fig. 16.Magnitude of VFTO at open end (Voc)
Fig. 19.Magnitude of VFTO at open end (Voc)
Fig. 20.Magnitude of VFTO at circuit breaker (CB)
Fig. 21.Magnitude of VFTO at Transformer (Vtr) Db4
Fig. 22.Magnitude of VFTO at open end (Voc)
Fig. 23.Magnitude of VFTO at circuit breaker (CB)
Fig. 17.Magnitude of VFTO at circuit breaker (CB)
Fig. 24.Magnitude of VFTO at Transformer (Vtr)
Fig. 18.Magnitude of VFTO at transformer (Vtr)
Fig. 25.Magnitude of VFTO at open end (Voc)
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
Fig. 26.Magnitude of VFTO at circuit Breaker (VCB)
Fig. 27.Magnitude of VFTO at Transformer (Vtr) Db4
Fig. 28.Magnitude of VFTO at open end (Voc) Db4
Fig. 29.Magnitude of VFTO at circuit Breaker (VCB)
Fig. 30.Magnitude of VFTO at Transformer (Vtr)
Fig. 31.Magnitude of VFTO at Transformer (Voc)
Fig. 32.Magnitude of VFTO at circuit Breaker (VCB)
Fig. 33.Magnitude of VFTO at Transformer (Vtr) Db4
Fig. 34.Magnitude of VFTO at open end (Voc) Db4
Fig. 35.Magnitude of VFTO at circuit Breaker (VCB)
Fig. 36.Magnitude of VFTO at Transformer (Vtr)
Fig. 37.Magnitude of VFTO at open end (Voc)
Fig. 38.Magnitude of VFTO at circuit Breaker (VCB)
Fig. 39.Magnitude of VFTO at Transformer (Vtr) Db4
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
Fig. 40.Magnitude of VFTO at open end (Voc) Db4
Fig. 41.Magnitude of VFTO at Circuit Breaker (VCB) Db4
Table 1: Peak Magnitudes of VFTOs with different methods
in p.u
PEAK VALUES OF VFTOS IN P.U
No
Damp
OHTL
CABLE
GIL
RC
Filter
Ferrite
ring
Method
Vtr(p.u)
Vds(p.u)
3.28
Voc(p.
u)
2.28
Direct
Db4
2.79
1.88
1.65
Direct
2.43
1.72
2.21
Db4
1.79
1.69
2.18
Direct
1.49
1.48
1.16
Db4
1.52
1.47
1.21
Direct
1.64
1.51
1.86
Db4
1.6
1.51
1.98
Direct
1.18
1.14
1.124
Db4
1.08
1.09
1.047
Direct
1.08
1.12
1.15
Db4
1.08
1.097
1.095
2.12
The magnitude of very fast transient over voltages (VFTOs)
without any damping is found 2.89 p.u at transformer (Vtr),
1.98p.u at open end (Voc) and 1.87p.u at circuit breaker
(VCB).With the application of RC filter the magnitude of
very fast transient over voltages (VFTOs) has been
considerably reduced 1.18p.u, 1.14p.u and 1.1234p.u
respectively and steepness of very fast transient over voltages
(VFTOs) is also considerably reduced and which can be
observed from figure 43 to figure 48, particularly at circuit
breaker it has been considerably reduced to low level.
However the impact of RC filter is very good at transformer
and it has no much effect at open end and the same can be
observed from the same figures. (Very fast transient over
voltages (VFTOs) is 1.01p.u with RC filter at open end)
.Which can be understood that, RC filter can protect the
transformer only, though it reduces the very fast transient
over voltages (VFTOs) to very low level.
To overcome this problem the proposed system is
employed with a new technique named as ferrite ring, with
the ferrite ring method the magnitude of very fast transient
over voltages (VFTOs) has been reduced to very considerable
level 1.08p.u, 1.12 and 1.15p.u at transformer, open end and
circuit breaker respectively. Comparing with the above
methods the ferrite ring has very good impact on very fast
transient over voltages (VFTOs) and can reduce the very fast
transient over voltages (VFTOs) to almost unity and fatherly
the steepness of very fast transient over voltages (VFTOs) is
also considerably reduced. However no of ferrite ring to be
connected is depends on the operating voltage, pressure, surge
impedance, formative time constant and length also.
In the present research work, the influences of terminal
components OHTL, GIL and XLPE cable on magnitude of
very fast transient over voltages (VFTOs) also studied and it
has been observed that the magnitude of very fast transient
over voltages (VFTOs) is high (2.43p.u at transformer,
1.72p.u at open end
and 2.21p.u at circuit breaker
respectively) when GIS is terminated with over head
transmission line (OHTL) and is low when terminated with
cable (1.52p.u at transformer, 1.47p.u at open end and
1.21p.u at circuit breaker respectively) and the magnitude of
very fast transient over voltages (VFTOs) is in between
OHTL and XLPE cable (1.64p.u at transformer, 1.51p.u at
open end and 1.51 at circuit breaker respectively). From the
study of influence of terminal components on magnitude of
VFTOs it has come to conclude that, the magnitude of
VFTOs can be somewhat reduced by terminating with the
XLPE cable.
Finally, by this research work it has come to conclude that by
employing the above mitigation methods the magnitude of
very fast transient over voltages at transformer, open end and
at circuit breaker can be reduced to very considerable level
with RC filter and ferrite ring compared to other methods.
By this study it has been concluded that the terminal
components also influences the magnitude of VFTOs. In the
present research work the formative time constant τ is
considered as 1ns. Last but not the least, by employing
wavelet transform (WT) an exact, crisp and accurate
measurement has been done in this analysis since the wavelet
transform (WT) is a power full tool for extraction of high
frequency transient signals and is tabulated in table 1.
Acknowledgements
Affirmations I might want to express my gratitude to Prof.
Prof M Surya Kalavathi who has taught me to seek after this
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3rd International Conference on Electrical, Electronics, Engineering Trends, Communication, Optimization and Sciences (EEECOS)-2016
work and taught me the "Mitigation methods and Analysis of
very fast transient over voltages of transformer in gas
insulated substation and 'Applications of wavelet transform
(WT) in Electrical Engineering' is my pleasure to recognize
part of my co creator D.Prabhavathi in fulfillment of this
research work.
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