MITIGATION AND ANALYSIS OF VERY FAST TRANSIENT OVER VOLTAGES(VFTOS) OF TRANSFORMER IN 765KV GAS INSULATED SUBSTATION (GIS) USING WAVELET TRANSFORM

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MITIGATION AND ANALYSIS OF VERY FAST

TRANSIENT

OVER

VOLTAGES(VFTOS)

OF

TRANSFORMER

IN

765KV

GAS

INSULATED

SUBSTATION (GIS) USING WAVELET TRANSFORM

K.Prakasam

, Prof. M. Surya Kalavathi

, D.Prabhavathi

1

Department of EEE, JNTUCEA, JNTUA, (India)

2

Department of EEE, JNTUHCEH, (India)

3

Department of EEE, JNTUCEA, JNTUA, (India)

ABSTRACT

Mitigation and analysis of very fast transient over voltages (VFTO) is very important in gas insulated substations

(GIS) .In this paper a power transformer rating of 400MVA in 765kV/400kV gas insulated substation has been

considered and different techniques for mitigation and analysis of very fast transient over voltages (VFTOs) at the

transformer are applied and the same results are analyzed by the application of discrete wavelet transform (DWT) as

wavelet transform gives the accurate results. The proposed system has been designed with Mat Lab software

platform and the system is simulated to evaluate the peak values of VFTOs generated at the both at transformer and

open end with and without damping methods (Ferrite ring and nono crystalline). The results show that the peak

value of VFTOs can be considerably reduced by introducing the above damping methods. The outcomes are

explored to wavelet transform for transient information. By the application of wavelet transform it has concluded

that an exact measurement VFTOs can be obtained

Keywords: Damping, Ferrite, Peak, Transient, Surges, Wavelet.

I. INTRODUCTION

During switching operation of disconnecting switches in gas insulated substations number of re strikes and pre

strikes occurs and due to which voltage collapses with in very short duration (nanoseconds) at switching gap. The

travelling surges generated by the switching operations travel either direction of the system. The super imposition of

these reflected waves cusses rise of voltage which is known as very fast transient over voltages(VFTOs).The VFTO

in GIS are greater concern if GIs is operating at higher voltage level. The major problems associated with the very

fast transient over voltages (VFTOs) generated in GIS causes , Flash over to ground at the disconnecting switch

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(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. Which necessitates suppressing the magnitude of VFTOs?

II. MITIGATION AND ANALYSIS OF VFTOS

The new approaches are presented in this paper. A matched ferrite ring, nanocrystalline and RF resonator formed by

shielding inside the GIS. The very fast transient over voltages (VFTOs) in the proposed system has been studied

well and the most suitable methods (ferrite ring and nanocrystalline) have been chosen for mitigation of VFTOs and

are presented in the following section. The parameters of proposed system have been estimated based on very fast

transient nature. The surge impedance (Z0), velocity of propagation (υ) and formative time (τ) can be evaluated from

the following equations.

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, this paper introduced a new method of

analysis of very fast transient over voltages VFTOs which is known as wavelet transform as the VFTOs are concern

to high frequency transients. The comparable circuit of ferrite ring is shown in the following figure3.

Fig.1.Equivalent circuit of ferrite ring

Most of the authors tested the ferrite ring under low voltage and low frequency and conventional comparison given

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KHz to 3MHz) and have high Dielectric strength. The nanocrystalline is modeled as R, L connected in series and

connected in parallel with capacitor. The parameters of R= 150Ω, L=0.002mH and C = 0.001nF chosen in the

design of nanocrystalline. The equivalent circuit of nanocrystalline is shown in the following figye4.

Fig.2.Equivalent circuit nanocrystalline

The RF resonator is a high frequency radio frequency device tuned to a dominant harmonic frequency. The RF

resonator can be electrically treated as LC circuit tuned to high frequency and low quality factor. The estimated peak

magnitudes of VFTOs with RF resonator have been produced in the next section. From the outcomes of the

proposed system it has been cleared that the peak magnitude of VFTOs can be considerably reduced.

2.1 Modelling of Proposed system

The simulation model of proposed system has been done using mat lab platform. The parameters of the system are

estimated based on very fast transient nature (VFT).

58

58

OHT L

80pF 765kV/400kV GIS Si mul ati on Model

Discre te , Ts = 5e -005 s.

powe rgui v + -Voltage Measurement2 v + -Voltage Measurement1 v + -Voltage Measurement Vcb T o Workspace2 Vtr

T o Workspace1

vds T o Workspace

Scope2 Scope1 Scope -K-Gai n1 -K-Gai n Conn1 Conn2 DS

DC Voltage Source

80pF1

80pF 51

4nF1 4nF

242 120nf8 120nf9

120nf7 120nf6 120nf52 120nf50 120nf49 120nf48 120nf47 120nf46 120nf45 120nf44 120nf43 120nf42 120nf41 120nf40 120nf4 120nf39 120nf38 120nf37 120nf36 120nf35 120nf34 120nf33 120nf32 120nf31 120nf30 120nf3 120nf29 120nf28 120nf27 120nf26 120nf25 120nf24 120nf23 120nf22 120nf21 120nf20 120nf19 120nf18 120nf17 120nf16 120nf14 120nf13 120nf12 120nf11 120nf10 120nf1

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III. SIMULATION TEST RESULTS

The simulation test has been carried at transformer and at open end to estimate the magnitude of very fast transient

over voltages (VFTOs) and rise time. The simulation outcomes of the proposed system have been presented in this

section.

Fig.4.VFTO at Transformer under no damping

Fig 4 represents the VFTO at the transformer under no damping conditions. From the figure 4 it can be observed that

the peak magnitude of VFTO is 2.09p.u and the rise time is 48ns

Fig.5.VFTO at Transformer with wavelet transforms (Db4)

Fig 5 represents the VFTO at the transformer under no damping conditions estimated with wavelet transform (Db4).

From the figure 5 it can be observed that the peak magnitude of VFTO is 2.02p.u.

0 50 100 150 200 250

-2 -1.5 -1 -0.5 0 0.5 1 1.5

Time in (ns)

M

ag

nit

ud

e

of

V

olt

ag

e

in

p.

u

VFTO at Transformer (Vtr) with Ferrite ring

Peak

Fig.6.VFTO at Transformer with ferrite ring

Fig 6 represents the VFTO at the transformer with ferrite ring. From the figure 6 it can be observed that the peak

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Fig.7.VFTO at Transformer with ferrite ring

Fig 7 represents the VFTO at the transformer with ferrite ring with wavelet transform (Db4). From the figure7 it can

be observed that the peak magnitude of VFTO is 1.38p.u.

0 50 100 150 200 250

-1.5 -1 -0.5 0 0.5 1 1.5

Time in (ns)

M

ag

ni

tu

de

o

f V

ol

ta

ge

in

p

.u

VFTO at Transformer (Vtr) with Nanocrystalline

Peak

Fig.8.VFTO at Transformer under no damping

Fig 8 represents the VFTO at the transformer under with ferrite ring. From the figure 8 it can be observed that the

peak magnitude of VFTO is 1.18p.u and the rise time is 32ns

Fig.9.VFTO at Transformer under no damping

Fig 9 represents the VFTO at the transformer with ferrite ring with wavelet transform (Db4). From the figure it can

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0 50 100 150 200 250

-2 -1 0 1 2

Peak

Fig.10.VFTO at open end under no damping

Fig 10 represents the VFTO at the transformer under no damping. From the figure it can be observed that the peak

magnitude of VFTO is 1.81p.u and the rise time is very more.

Fig.10.VFTO at open end under no damping

Fig 10 represents the VFTO at the transformer under with ferrite ring. From the figure 10 it can be observed that the

peak magnitude of VFTO is 1.81p.u and the rise time is very more.

0 50 100 150 200 250

-1.5 -1 -0.5 0 0.5 1 1.5

Time in (ns)

M

ag

ni

tu

de

o

f V

ol

ta

ge

in

p

.u

VFTO at Openend (Voc) with Ferrite ring

Peak

Fig.11.VFTO at open end with ferrite ring

Fig 11 represents the VFTO at open end with ferrite ring. From the figure 11 it can be observed that the peak

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Fig.12VFTO at open end with ferrite ring

Fig 12 represents the VFTO at open end with ferrite ring with wavelet transform (Db4). From the figure12 it can be

observed that the peak magnitude of VFTO is 1.25p.u.

0 50 100 150 200 250

-2 -1.5 -1 -0.5 0 0.5 1 1.5

Time in (ns)

M

ag

ni

tu

de

o

f V

ol

ta

ge

in

p

.u

VFTO at Open end (Voc) with Nanocrystalline

Peak

Fig.13.VFTO at open end with ferrite ring

Fig 13 represents the VFTO at open end with nanocrystalline. From the figure 13 it can be observed that the peak

magnitude of VFTO is 1.27p.u and the rise time is >200ns.

Fig.14.VFTO at open end with ferrite ring

Fig 14 represents the VFTO at open end with nanocrystalline. From the figure 14 it can be observed that the peak

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0 50 100 150 200 250

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2

Time in (ns)

M ag ni tu de o f V ol ta ge in p .u

VFTO at Transformer with RF Resonator

Peak

Fig.15.VFTO at transformer with RF resonator

Fig 15 represents the VFTO at transformer with RF resonator. From the figure 15 it can be observed that the peak

magnitude of VFTO is 1.02p.uand rise time is 18ns.

Fig.16.VFTO at transformer with RF resonator

Fig 16 represents the VFTO at transformer with RF resonator with wavelet transform (Db4). From the figure 16 it

can be observed that the peak magnitude of VFTO is 1.02p.uand rise time is 16.7ns.

0 50 100 150 200 250

-3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

Time in (ns)

M ag ni tu de o f V ol ta ge in p .u

VFTO at Open end (Voc) with RF Resonator

Peak

Fig.17.VFTO at open end with RF resonator

Fig 17 represents the VFTO at open end with RF resonator. From the figure 17 it can be observed that the peak

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Fig.18.VFTO at open end with RF resonator

Fig 18 represents the VFTO at open end with RF resonator. From the figure 18 it can be observed that the peak

magnitude of VFTO is 1.02p.uand rise time is 16.7ns.

TABLE 1 PEAK MAGNITUDE OF VFTO IN P.U

Test Results

Method

V

V

No Damp

Direct

2.09

1.82

Wavelet

2.02

1.67

Ferrite ring

Direct

1.45

1.24

Wavelet

1.48

1.22

Nanocrystalline

Direct

1.17

1.12

Wavelet

1.15

1.15

RF Resonator

Direct

1.02

1.1

Direct

1.02

1.07

IV. CONCLUSIONS

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By employing the ferrite ring, the peak magnitude of VFTO at transformer has been reduced by 30.62%

and with nanocrystalline it has been reduced by 46.55% as well as with RF resonator the peak magnitude

reduced by 51.12%. In the second stage of research work the peak magnitude of VFTO at open end has

been reduced by 32.97% and with nanocrystalline it has been reduced by 39.45% similarly with RF

resonator it has been reduced by 42.16%.

In the proposed system the percentage of reduction has been analyzed with reference to no damping

conditions. However the accurate estimation of peak magnitudes of VFTOs has been achieved by the

application of wavelet transform (Db4).

ACKNOWLEDGMENT

Acknowledgments I would like to express my gratitude to Dr.M Surya Kalavathi, madam garu who has

encouraged me to pursue this work and taught me the “art of „Analysis and Mitigation methods Very Fast

Transients over Voltages (VFTOs) in gas insulated substations‟ and „Applications of Wavelet Transform

in Electrical Engineering‟ is my pleasure to acknowledge role of my co author D.Prabhavathi in

completion of this work.

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