CHAPTER-III MODELING OF 245kV GIS SYSTEM FOR...

71

CHAPTER-III

MODELING OF 245kV GIS SYSTEM FOR

ESTIMATION AND SUPPRESSION OF VFTOs

3.1 INTRODUCTION

An accurate representation of each component of a system is

essential for a reliable simulation of its transient performance. This

representation must be done taking into account the frequency range of

the transients to be simulated. Very fast transients (VFT) belong to the

highest frequency range of the power system. The simulations are

suitable for frequencies varying from 100 KHz to 100MHz [13]

Due to the travelling nature of the transients the modeling of GIS

makes use of electrical equivalent circuits composed of lumped elements

and especially by distributed parameter lines, surge impedances and

travelling times. Disconnector switches are used primarily to isolate

operating sections of high voltage installations from each other as a

safety measure. In addition, they must also be able to perform certain

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

busbaror off load connection or disconnection of bus sections, circuit

breakers etc. The connection or disconnection of energized but unloaded

substation sections involve the disconnector having to switch small

capacitive currents, typically few mA[21]. During closing and opening

operations the voltages develop across the switching contacts which

72

subsequently collapse in a series of spark discharges often in extended

sequences. Within just nanoseconds, the channel of such a spark

discharge rapidly establishes a conducting bridge across the contacts.

Lasting few hundreds of micro seconds, it momentarily connects the

potential equalization is accompanied by transient oscillations with very

high frequencies in the adjacent GIS elements, giving to VFTOs. The

frequency and amplitudes depends up on the layout of the GIS network.

The estimation of these transients and rise times are very important in

order to design insulation levels[14]. The fast transients over voltages are

generated during switching operation of disconnectors. VFTO generated

in a GIS should be considered as an important factor in the insulation

design. In EHV class voltages, VFTOs can reach to high amplitudes

and steepness, the insulation failures of 500kV transformers due to

effect of VFTO have happened several times [15]. Hence it is important

to estimate and suppress these over voltages for protection of internal

systems. The simulation depends on the quality of the model of each

individual GIS component. In order to achieve reasonable results in GIS

structures highly accurate models for each internal equipment and also

for components connected to the GIS are necessary.

The disconnector spark itself has to be taken into account by

transient resistance according to the toepler’s equation and subsequent

arc resistance of a few ohms. The wave shape of the over voltage surge

due to disconnector switch is affected by all GIS elements. Accordingly,

73

the simulation of transients in GIS assumes an establishment of the

models for the bus, bushing, elbow, transformers, surge arresters,

breakers, spacers, disconnectors and enclosures and so on. One of the

ways is avoiding dangerous layout of GIS and dangerous operation

procedures of the disconnectors however; this brings a big limitation to

the design and control operation of GIS. Another way is to use high

speed disconnector, but there will be a problem of high trapped charge

on the floating electrode[23]. The exisisting method of suppressing

VFTOs is resistance switching. In this method a resistor of range 400Ω to

500Ω is fitted to disconnector[29,31], in this method in the event of

restriking the resistor is inserted in the circuit, so that ,the over voltages

can be suppressed, but this method is complicated in structure and has

reduced reliability and also the probability of failure of resistors are

great. Therefore this method needs to combine practical considerations.

Another method is using R-C filter circuits, this method has been

widely used in vacuum circuit breakers to suppress over voltages of

arcing, the R-C filters can be applied to suppress the VFT, but in this R-

C filter absorbs high frequency components, consumes the energy of

VFTO, but selection of R and C for different ratings applications is

difficult[18]. Another method suggested by Hongsheng Li is use of metal

oxide arrester but this can inhibit the amplitude of the VFTO, but cannot

inhibit its steepness and high frequency oscillations.

74

In this chapter, first suppression effect of VFTOs has been verified

with switching resistor across the disconnector switch and secondly the

suppression effect of VFTOs verified with new technique of application of

ferrite rings on bus bar. The frequency spectrums are also obtained

using FFT technique. The results obtained from the above methods are

compared.

In this chapter, the single line diagram of 245kV GIS and its

description is given in section 3.2. The representations of important GIS

components are given in section 3.3. The modeling of GIS components is

given in section 3.4. Calculation of various parameters for modeling is

given in the section3.5 A 245kV GIS system considered for modeling to

estimate VFTOs is presented in section 3.6 The EMTP-RV model of the

system given in section 3.7 The Simulation of the EMTP-RV GIS model to

estimate transients due to disconnector switch 1 closing operation with

fixed arc resistance is given in section 3.7.1. The Simulation of the

EMTP-RV GIS model to estimate transients due to disconnector closing

operation with variable arc resistance given in section3.7.2. The

Simulation of the EMTP-RV GIS model to estimate transients due to

disconnector opening operation with fixed arc resistance given in section

3.7.3. The simulation of the EMTP-RV GIS model to estimate transients

due to disconnector opening operation with variable arc resistance given

in section3.7.4. The Simulation of the EMTP-RV GIS model to estimate

transients due to disconnector opening operation with variable arc

75

resistance and trapped charges is given in section 3.7.5. The GIS

simulation model with DS2 opening operation and variable arc resistance

is given in the section 3.7.6. The results of various simulations are given

in the section 3.8.1. The transients on source side and load side of the

disconnector switch with different trapped charges are given in the 3.8.2.

Summary is given in the section 3.9. Various methods for suppression of

VFTOs in GIS systems are discussed in section3.10. The VFTO

suppression using resistance switching is discussed in 3.10.1. The FFT

analysis of reduced VFTOs given in section 3.10.2. Single phase

equivalent circuits of 245kV GIS system with opening and closing

resistance given in 3.10.3. Simulation results with opening and closing

resistance are presented in the section3.10.4. The VFTO suppression

using ferrite rings given in 3.11. The simulation results are given in the

section 3.14. The summary is given in the section 3.15.

76

3.2 A TYPICAL 245kV GAS INSULATED SUBSTATION

Fig 3.1 Single-line diagram of 245kV GIS

3.3 REPRESENTATION OF IMPORTANT GIS COMPONENTS

a) Bus ducts: bus duct can be represented as a loss less transmission

line for a range of frequencies lower than 100MHz. The surge impedance

is calculated from the physical dimensions of the duct (using Eq 3.4).

77

The Experimental results show that the propagation velocity in GIS duct

is close to 95% of the speed of light other devices such as closed

disconnectors can also be modeled as lossless transmission lines.

b) Surge arresters: A surge arrester model is taken into account the

steep front wave effect. The voltage developed across the arrester for a

given discharge current increases, and reaches crest prior to crest of the

discharge current. A detailed model is represent each internal shield and

block individually, and the travel times along shield sections as well as

capacitances between these sections as well as capacitances between

blocks and shields, and the blocks themselves. The detailed model is

shown in the table. Usually the switching operations do not produce

voltages high enough to cause MOAs to conduct its capacitance is taken

into account.

c) Circuit breakers: The representation of circuit breakers is very

complicated in GIS systems because of its internal irregularities. In

addition , circuit breakers with several chambers contain grading

capacitors as these components are not arranged symmetrically, a circuit

breaker has a different transient response depending up on which

terminal is connected to the surge source. A closed circuit breaker can

be represented as a lossless transmission line. The surge impedance is

calculated from the diameters of the conductor and enclosure. The effect

of grading capacitors can be ignored. The representation of a closed

circuit breaker is more complicated because the electrical length is

78

increased and the speed of progression is decreased due to the effects of

the higher dielectric constant of the grading capacitors. If the

intermediate voltages are needed, the breaker is divided into as many

sections as there are interrupters, all connected by the grading

capacitors. A simple model consists of two equal lengths of bus

connected by a capacitor with a value equivalent to the series

combination of sections are calculated from the physical dimensions of

the breaker

d) Disconnector switches: Closed disconnectors are modeled as a

transmission line with distributed parameters. Capacitance of the

switching contacts towards the ground is considered. Disconnectors in

open condition are represented with inter electrode capacitance of the

switching contacts towards the ground is considered.

e) Earth switches: Earth switches can be modeled as lumped

capacitance to ground.

79

3.4 MODELING OF GIS COMPONENTS

ELEMENT MODEL EQUIVALENT

CIRCUIT

CHARACTERISTI

CS

BUS DUCT

Transmission line with

distributed parameters.

Loss in transmission line

because of skin

effect.(Neglected)

Loss free

distributed

parameter

transmission line

SPACER Lumped Capacitance

towards the ground.

C > 20pf

ELBOW


distributed parameters

and capacitance added

in between the line.

Parameters

depending on the

ratio between

conductor and

enclosure radius.

Value of the

capacitance C

depending on the

system topology.

CABLE



Each end of cable is

terminating with a

lumped capacitance.

The values are

Depends up on

voltage rating of

GIS

CURRENT

TRANSFORMER

Lumped capacitance

towards the ground

The values are

Depends up on

voltage rating of

GIS

80

CAPACITIVE

VOLTAGE

TRANSFORMER

Lumped capacitance

towards the ground

BUSHING

(Capacitively

Graded Bushing)

Transmission line of

varying surge

impedances are

connected in series

Zg1, Zg2,.. are

variable surge

impedance in SF6

side. Za1, Za2, …

are variable surge

impedance in air

side.

SURGE

ARRESTER

Arrester capacitance is

considered. Protection

characteristic connected

in parallel with arrester

capacitance

In case of VFT

(0.5µs) the

protection

characteristic is

corrected in

reference to the

characteristic for

the surge 8/20µs.

Inductance of

grounding

connection is

taken into account.

POWER

TRANSFORMER

Lumped capacitance

towards the

ground.Inductive branch

toards ground is

neglected due to a very

high impedance at very

high

frequencies.Nonlinear

behavior of the core is

neglected

Value of

capacitance

depends on the

transformer type,

voltage level,

winding connection

and winding type.

81

DIS-CONNECTOR

CLOSED



Capacitance of the

switching contacts

towards the ground is

considered.

Parameters

depending on the

ratio between

conductor and

enclosure radius.

Value of

capacitance C

depends on the

system topology.

DIS-CONNECTOR

OPENED

Inter electrode

capacitance of the

switching contacts


considered.

C includes spacer

capacitance also.

EARTH

SWITCHING

Lumped capacitance

towards the ground.

SPARK

RESISTANCE (in

case of DS

operation)

It is a non-linear

function of time. It varies

according to the

Toepler’s Spark Law

if t < 1µs, R = 0 Ω

if t > 1µs, R varies

from 0 to 5 Ω

SPARK

(earth fault)

Spark resistance varies

according to Toepler’s

Spark Law. L is the

inductance of the spark

channel.

R is in the range of

1 to 3 Ω

CIRCUIT

BREAKER (C.B)

CLOSED



equivalent capacitance of

switching contacts

The surge

Impedance of C.B

bus duct is less

than 70 Ω because

82


considered.

of additional

capacitance.

CIRCUIT

BREAKER (C.B)

OPENED

The capacitance between

switching contacts is

considered. C.B bus duct

is represented with


on both sides of the

contacts.

The length of bus

duct on both sides

of contacts is

equal. The inter

electrode

capacitance incase

of C.B is high,

because of large

arc of the contacts.

3.5 CALCULATION OF VARIOUS PARAMETERS OF GIS

3.5.1 Calculation of Inductance

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

[16] given below:

Where r1, r2, r3, r4, are the radii of the conductors in the order of decreasing

magnitude and ‘l’ is the length of the section.

−∗

∗+

+

+

××= 1

r

rln

r

r-1

r

r

2 r

rln

r

rln

r

rln 0.001 L

2

1

2

1

2

2

1

2

3

4

1

2

3

1l

3.1

83

Fig 3.2 Cross section of typical GIS System

3.5.2 Calculation of Capacitance in micro farads

The Capacitance is calculated with the assumption that the conductors

are Cylindrical. Capacitance is calculated by using the standard formulae

given below:

0

1 0

2 * * *

2 .3 * ln

r lC

b

a

π ε ε∗=

3.2

Where εo = 8.854 * 10-12, εr = 1

b = Outer Cylinder Radius

a = Inner Cylinder Radius

l = Length of the Section

3.5.3 Calculation of Capacitance due to Spacer

Spacers are used for supporting the inner conductor with reference

to the outer enclosure. They are made with Alumina filled epoxy

material whose relative permittivity (εr) is 4. The thickness of the

spacer is assumed to be the length of the capacitance for calculation.

84

3.5.4 Calculation of Short Circuit Inductance(mH) &

Resistance

Assuming a short circuit fault level of 2000MVA for 245kV system

voltage, Inductance and Resistance are calculated as follows: In the

derivation of short circuit inductance and resistance, the GIS

considered for the study is 200MVA,22.8/220Kv with leakage

reactance of 10%. The symmetrical short circuit MVA will be about 8

to 12 times the rated MVA capacity of the ttransformer.

S V * ph =phI

⇒ phV

S =phI

And V

I * X Z% =

⇒ I

V * %Z X =

But L * f * * 2 X Π=

⇒ f * * 2

X

Π=L

And it is assumed that R = XL

3.5.5 Calculation of Inductance due to Load

For 200MVA, 245KV transformer with 10% impedance and 0.8 power

factor the inductance is calculated as follows:

85

P Cos * I * V * 3 =φ

⇒ φ

=Cos * V * 3

P I

And V

I * X Z% =

⇒ I

V * %Z X =

But L * f * * 2 X Π=

⇒ f * * 2

X L

Π=

3.5.6. Calculation of Variable Arc Resistance

Based on earlier studies in SF6 gas, Toepler’s Spark Law is valid for

calculation of Variable Arc Resistance. The Variable Arc Resistance due to

Toepler’s formulae [5] is given below

= ∗ +

3.3

Where KT = Toepler’s Constant

= 0.005 volt.sec/m for SF 6 under Uniform Field conditions

L = Spark Length in meters

qo = Initial Charge or Charge at the instant of breakdown

t = Spark collapse time in sec.

The value of time varying spark resistance R (t), is calculated until

it reaches a value of 1 to 5 ohms. The integral in the denominator sums

86

up the absolute value of current ‘i’ through the resistance R (t) over the

time beginning at breakdown inception. Thus, it corresponds to the

charge conducted through the spark channel up to time‘t’.

Initial charge qo is an important parameter while considering the non-

uniform fields. But the field between the disconnector contacts is almost

uniform. Therefore qo is very small.

3.5.7 Surge Impedance

A typical 245kV gas insulated bus duct in the substation

considered has an inner conductor of 3.6 inches (8.9cm) and its

grounded outer sheath has a diameter of about 12inches (30.5cm). The

surge impedance of the 245kV SF6 bus can be calculated from the

following formula [6]

= 138√

3.4

Where Z= surge impedance in Ω

= inner radius of outer sheath

= radius of inner conductor

K= permittivity of dielectric (unity for SF6)

Hence, for a typical 245kV Gas insulated bus, the surge impedance is

about 75Ω

Ω== 8.739.8

5.30log

1

138Z

87

3.6 TYPICAL SECTION OF SEGREGATED-PHASE 245kV GIS SYSTEM

Fig.3.3 Single line diagram of typical section of segregated-phase

245kV GIS system

T - Generator transformer E.S - Earthing Switch B1 = Air –to- SF6 Gas Bushing C.B- Circuit Breaker L.A - Lightning arrester C.T- current Transformer P.T - Potential Transformer B2 = SF6 Gas - to – XLPE cable

Table.3.1 Dimensions of 245kV GIS system

Name of the GIS component Distance

in meters

Air-to-SF6bushing(From DS1)

12.6

Lightning arrester(From DS1)

11.3

Potential transformer(From DS1)

9.5

Earth switch(From DS1)

1.5

Current transformer(From DS2) 1.5

Earth switch(From DS2) 2.0

Potential transformer(From DS2) 9.5

SF6-to-XLPE cable termination

13.5

88

3.6.1 Description of the circuit

A typical section of 245kV GIS substation has been considered

for VFTO study. Its single line diagram is shown in Fig. 3.3; a complete

EMTP-RV electrical equivalent models have been developed for the GIS

system.

A substation has 200MVA, 22.8kV/220kV transformer and the

over head line from transformer is connected to 245kV GIS system

through an Air-to-SF6 gas bushing. The length of the over head line is

about 20meters. The typical 245kV GIS system consisting of lightning

arrester, instrument transformers, and high speed earthing switch and

SF6 gas to air bushing and SF6 gas to XLPE cable termination have

shown in the diagram. The surge impedance of the over head line is

considered as 350Ω with wave velocity of 300m/µs.

Due to the travelling wave nature of the VFT the modeling of

GIS makes use of electrical equivalent circuits composed by lumped

elements and especially by distributed parameter lines, defined by surge

impedances and travelling times. The power transformer is assumed as a

source side and load side of the disconnector switch being operated.

It is assumed that, initially the load side circuit breaker is

operated. The fast transient over voltages results at both load side and

source side of the disconnector switch. The switching of capacitive

current is difficult; under these conditions restrikes occurs and cause

large number of transients on the supply and load side. The variations of

89

transient voltages at Air-to-SF6 gas bushing, load side & source side are

calculated during disconnector Switch1 operation and transient voltages

at XLPE cable termination has been estimated during Disconnector

switch2 opening and closing operations using EMTP-RV. The step size

for the analysis taken as 0.1ns and stop time is selected between 5 to

8µsec. In the circuit a great deal of restrikes occurs across the switching

contacts when disconnectors are operated. These restrikes lead to

generation of VFTO; Consequently VFTO appears in the circuit. Due to

random nature of trapped charge at the commencement of disconnector

opening or closing two important parameters variable arc resistance and

trapped charge on the floating bus bar are considered during estimation

of VFTO.

The transients generated due to the operation of a disconnector switch1

have been simulated by the injection of a unit step voltage source. The

patterns of transient voltages at air to gas bushing are estimated during

Disconnector switch 1 closing and opening operations with fixed and

variable arc resistance. The patterns of transient voltages at SF6-to-XLPE

cable termination is estimated during Disconnector switch2 opening

operation the results are presented. The patterns of transient voltage at

source side, load side have been analyzed. The trapped charge is varied

from -0.1p.u to -1p.u. insteps of 0.1p.u. (1 p.u = Vm (ph))

The electrical equivalent models with trapped charge are given

in the Fig. 3.5.9 to Fig 3.5.18. The simulation waveforms are given in

90

section 3.8.1. The patterns of transient voltages at SF6-to-XLPE cable

termination during Disconnector switch 2 opening is presented in the

Fig.3.42. The VFTOs on source side with different trapped charges are

presented in the Table 3.1. The VFTOs on load side with different trapped

charges are presented in the table 3.2. The VFTOs at air to SF6 gas bushing

during opening and closing operation of DS1 are given in the table 3.3.

The disconnector switches DS1, DS2 and circuit breaker arrangement as

shown in the diagram.

The disconnectors are of motor driven, with rated voltage is

245kV and rated short-time current 40/50kA 3sec.

Earth switches are located on either side of the disconnector

switches. The rated voltage is 245kV, rated short time current 40/50kA

3sec, Method of operation is motor driven, Bus bar type is segregated

phase, Rated voltage is 245kV,Rated current is 2500/3150A, Rated short

time current rating is 40/50kA,The rated gas pressure is 0.6Mpa.

The ratings of the circuit breaker are, Rated voltage is 245kV, Rated

current is 1250/2500/3150A rated breaking current 40/50A, rated gas

pressure is about 0.6MPa and method of operation is motor driven with

spring. The instrument transformers are located with in the bay. Their

secondary connections are routed through a gas-tight bushing plate to a

terminal box.

The pressurized SF6 gas in the module serves as the primary

insulation. The high voltage connection to the switchgear is established

91

by means of conductor, which is supported by means of a gas-tight

bushing plate to the terminal box.

The surge arrester consists of metal oxide resistors with a non-

linear current/voltage characteristic. The arrester is flange-joined to the

switchgear via gas tight bushing. In the tank of the arrester module,

there is an inspection hole, through which the internal conductor can be

inspected. At the bottom there are the connections for gas monitoring,

arrester testing, and an operation counter. The SF6gas/air termination is

a combination of an angle type module and an outdoor/ SF6 bushing.

The surge impedance of the 245kV XLPE-600 Cable is taken as 30Ω and

wave velocity of 103.8m/µs.

3.7 EMTP-RV MODEL OF THE SECTION OF 245KV GIS SYSTEM

Fig. 3.4 EMTP-RV model of the section of 245kV GIS system

92

The EMTP-RV equivalent circuit representation of typical

245kV GIS system is shown in Fig.3.4. According to their internal design

all parts of the GIS have been represented thoroughly by line sections

with the corresponding surge impedance and travelling times. The

VFTOs during opening and closing operation of the disconnector switch1

are simulated and estimated for various conditions. The variable arc

resistance with respect to time is given in the software according to

customised equation.

3.7.1 GIS (EMTP-RV) model of DS1 closing operation with fixed arc

resistance (Rarc=0.5Ω)

Fig3.5 The Electrical equivalent network of the GIS system during

disconnector switch 1 closing with fixed arc resistance.

The trapped charge equivalent is considered by assigning different

voltage values to VL i .e from -0.1 to -1 pu. This can be considered as

equivalent magnitude of trapped charge.

VS =VFTO at Source side

VL = VFTO at Load side

Vagb = VFTO at Air-to- SF6 bushing

Z Z Z Z

ZZZZ ZZ

250 75 75 75

75 75 757575 30

+2nF

C1 +

0.003nF

C2 +

0.003nF

C3 +

0.2nF

C4 +

0.1nF

C5 +

0.0045nF

C6 +

0.003nF

C8

+

0.003nF

C10

+

0.003nF

C11 +

0.0045nF

C12 +

0.005nF

C13 +

0.003nF

C14 +

0.003nF

C15 +

0.0045nF

C16 +

0.1nF

C17

+

0.5

r

+

0.003nF

C7

+

1 /_0

+ C9

0.0

05

nF

+0.4nF

C18

VM+

VS

?v

VM+

VL

?v

VM+

?v

VM+

?v

Vagb+

0|2.5ms|0

DS1

93




Z Z Z Z

ZZZZ ZZ

250 75 75 75

75 75 757575 30

+2nF

C1 +

0.003nF

C2 +

0.003nF

C3 +

0.2nF

C4 +

0.1nF

C5 +

0.0045nF

C6 +

0.003nF

C8

+

0.003nF

C10

+

0.003nF

C11 +

0.0045nF

C12 +

0.005nF

C13 +

0.003nF

C14 +

0.003nF

C15 +

0.0045nF

C16 +

0.1nF

C17

+

0.5

r

+

0.003nF

C7

+

1 /_0

+ C9

0.0

05

nF

+

0.4nF

C18

VM+

VS

?v

VM+

VL

?v

VM+

?v

VM+

?v

Vagb+

-1|2.5ms|0

DS1

3.7.2 GIS model of DS1 closing operation with variable arc resistance

Fig.3.6 The Electrical equivalent network of the GIS system during

disconnector switch 1 closing with variable arc resistance

3.7.3 GIS simulation model of DS1 opening operation with fixed arc

resistance

Fig. 3.7 The Electrical equivalent network of the GIS system during

disconnector switch 1 opening with fixed arc resistance.




Z Z Z Z

ZZZZ ZZ

250 75 75 75

75 75 757575 30+2nF

C1 +

0.003nF

C2 +

0.003nF

C3 +

0.2nF

C4 +

0.1nF

C5 +

0.0045nF

C6 +

0.003nF

C8

+

0.003nF

C10

+

0.003nF

C11 +

0.0045nF

C12 +

0.005nF

C13 +

0.003nF

C14 +

0.003nF

C15 +

0.0045nF

C16 +

0.1nF

C17

+

0.5

r

+

0.003nF

C7

+

1 /_0

+ C9

0.0

05

nF

+

0.4nF

C18

VM+

VS

?v

VM+

VL

?v

VM+

?v

VM+

?v

Vagb+

0|2.5ms|0

DS1

R(t)+0

Rt1

94

3.7.4 GIS simulation model of DS1 opening operation with variable

arc resistance

Fig3.8 The Electrical equivalent networks of the GIS system during

disconnector switch 1 opening with variable arc resistance

3.7.5 GIS simulation models of DS1 opening operation with variable

arc resistance and with trapped charge of -0.1p.u to -1p.u


opening operation of disconnector switch 1 with -0.1 p.u. trapped

charge




Z Z Z Z

ZZZZ ZZ

250 75 75 75

75 75 757575 30

+2nF

C1 +0.003nF

C2 +

0.003nF

C3 +

0.2nF

C4 +

0.1nF

C5 +

0.0045nF

C6 +

0.003nF

C8

+

0.003nF

C10

+

0.003nF

C11 +

0.0045nF

C12 +

0.005nF

C13 +

0.003nF

C14 +

0.003nF

C15 +

0.0045nF

C16 +

0.1nF

C17

+

0.5

r

+

0.003nF

C7

+

1 /_0

+ C9

0.0

05

nF

+

0.4nF

C18

VM+

VS

?v

VM+

VL

?v

VM+

?v

VM+

?v

Vagb+

-1|2.5ms|0

DS1

R(t)+0

Rt1



VB = VFTO at SF6-to- XLPE cable termination

Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

95




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

Fig3.10 The Electrical equivalent network of the GIS system during

opening operation of disconnector switch 1 with- 0.2 p.u. trapped

charge

Fig3.11 The Electrical equivalent network of the GIS system during opening

operation of disconnector switch 1 with -0.3 p.u. trapped charge




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

96



charge

Fig.3.13. The Electrical equivalent network of the GIS system during opening

operation of disconnector switch 1 with -0.5 p.u. trapped charge.




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

97



charge



charge




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

98




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

Fig.3.16 The Electrical equivalent network of the GIS system during opening


Fig.3.17 The Electrical equivalent network of the GIS system during opening





Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30

+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF

+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

99


opening operation of disconnector switch 1 with -1 p.u. trapped

charge.

3.7.6 GIS simulation model of DS2 opening operation with variable

arc resistance

Fig.3.19 The EMTP-RV equivalent network of the GIS system during closing

operation of disconnector switch 2 with -1pu trapped charge.




Z Z Z Z

ZZZZ ZZ

250 75 75 75

7575 7575

75 30+

C1

2nF +C2

0.003nF

+C3

0.003nF

+C4

0.2nF

+C5

0.1nF

+C6

0.0045nF

+C7

0.003nF

+ C8

0.003nF

+ C9

0.003nF

+ C10

0.0045nF

+ C11

0.005nF

+ C12

0.003nF

+ C13

0.003nF

+ C14

0.0045nF

+ C15

0.1nF

+R1

0.5

+ C16

0.003nF+

1 /_0

+

0.0

05

nF

C

17

+C18

0.4nF

VM+

?v

VSVM+

?v

VL+

DS1

?v

-1|1E15|0

VM+

m4

?v

R(t)+0

Rt1

250 75 75 75 75

75 75 75 75 30

z z z z z

z z z z z

VS = VFTO at source side

VL = VFTO at load side

Vxlpe = XLPE cable termination

VS VL

+

2nF

C1 +

0.2nF

C4 +

0.1nF

C5 +

0.0045nF

C6 +

0.003nF

C7 +

0.003nF

C8 +0.003nF

C9

+

0.003nF

C10

+

0.003nF

C11

+

0.0045nF

C12 +

0.005nF

C13 +

0.003nF

C14 +

0.003nF

C15 +

0.0045nF

C16 +

0.1nF

C17 +

0.4nF

C18

+

0.0

05

nF

C

19

+

1 /_0

AC1 +

0.003nF

C3+

0.003nF

C2

+1ms|2.5ms|0

DS2

R(t)+0

Rt1VM+

?v

Vxlpe

3.8 RESULTS OF THE SIMULATION

Fig. 3.20 VFTO at Air

DS1 with fixed arc resistance.

Fig. 3.21 VFTO at Air

the DS1 with variable arc resistance.

100

RESULTS OF THE SIMULATIONS

VFTO at Air-to-SF6 bushing during the opening operation of the


VFTO at Air-to-SF6 bushing during the opening operation of

the DS1 with variable arc resistance.

SF6 bushing during the opening operation of the

operation of

101

Fig. 3.22 VFTO at Air-to-SF6 bushing during the closing operation of the


Fig. 3.23 VFTO at Air-to-SF6 bushing during the closing operation of the

DS1 with variable arc resistance.

Fig. 3.24 VFTO at source side of DS1 with variable arc resistance and

trapped charge of

Fig. 3.25 VFTO at load side of DS1 with variable arc resistance and trapped

charge of -0.1p.u.

102

at source side of DS1 with variable arc resistance and

trapped charge of- 0.1p.u.

at load side of DS1 with variable arc resistance and trapped

0.1p.u.



Fig. 3.26 VFTO at sou

charge of -0.2p.u.


charge of -0.2p.u

103

at source side of DS1 with variable arc resistance and trapped

0.2p.u.


0.2p.u.

rce side of DS1 with variable arc resistance and trapped



trapped charge of


charge of -0.3p.u.

104


trapped charge of -0.3p.u.


0.3p.u.



Fig. 3.30 VFTO at source side of DS1 with variable arc resistance

trapped charge of


charge of -0.4p.u.

105

at source side of DS1 with variable arc resistance



0.4p.u.



106






trapped charge of

Fig. 3.35 VFTO at load side of DS1 wi

charge of -0.6p.u.

107




0.6p.u.


th variable arc resistance and trapped


trapped charge of


charge of -0.7p.u.

108




0.7p.u.



109




charge of -0.8p.u.

110


trapped charge of -0.9 p.u.


charge of -0.9p.u.

111


trapped charge of -1 p.u.


charge of -1 p.u.

112

Fig. 3.44 VFTO at SF6 – to – XLPE cable termination during opening of DS2

With variable arc resistance and trapped charge of -1 p.u.

113

3.9 FAST FOURIER TRANSFORM (FFT) ANALYSIS OF VERY FAST

TRANSIENT OVER VOLTAGES

In this section, the frequency spectrums of voltage transients are

obtained using FFT algorithm. The Fourier transform has long been a

principle analytical tool in such diverse fields as linear systems, Optics,

Probability theory, Quantum physics, Antennas and Signal analysis.

However, a similar statement is not true for the Discrete Fourier

transform (DFT). But with the development of the Fast Fourier transform

(an algorithm that efficiently computes the DFT) many facets of scientific

analysis have been completely revolutionized. The Fourier Integral is

defined by the expression:

( ) ∫+∞

∞−

Π−= dtetsfS ftj 2)( (3.5)

Where s (t) is the waveform to be decomposed into the sum of

sinusoids, S (f) is the Fourier Transform of set, and 1−=j . Typically, s

(t) is a function of the variable time and S (f) is a function of the variable

frequency. The Fourier Transform identifies or distinguishes the different

frequency sinusoids and their respective amplitudes, which combine to

form an arbitrary waveform. The Inverse Fourier Transform is defined as

∫+∞

∞−

Π= dtefSts ftj 2)()( (3.6)

Inversion transformation allows the determination of a function of

time from its Fourier transform. The validity of equations (3.5) and (3.6)

114

depend upon certain conditions as if s(t) is integral in the sense

∫+∞

∞−

∞<)(ts (3.7)

Then its Fourier transform S(f) exists and satisfies the inverse

Fourier transform. It is to be noted that the above condition is a

sufficient but not a necessary for the existence of a Fourier transform.

If s(t) = β(t) sin(2πft+α), where f and α rare arbitrary constants. If

β(t+k) < β(t) for | t | > λ > 0, then function s(t)/t is absolutely integrable

in the sense of equation (3.7) then S(f) exists and satisfies the Inverse

Fourier Transform equation (3.6).

3.9.1: Discrete Fourier transforms

1,.....,2,1,0,)()/(1

0

/2 −== ∑−

=

−NnekTgkTnG

N

K

Nnkj π (3.8)

The above expression relates N samples of time and N samples of

frequency by means of the continuous Fourier Transform. “The Discrete

Fourier Transform (DFT) is then a special case for the Continuous

Fourier Transform (CFT)”. If it is assumed that the N samples of the

original function g (t) are one period of a periodic waveform, the Fourier

Transform of this periodic function is given by the N samples computed

by equation (3.8).

115

3.9.2: Inverse Discrete Fourier Transforms

∑−

=

Π −==1

0

/2 1,...,2,1,0,)/(/1)(N

n

NnkjNnekTnGNkTg (3.9)

The above discrete inversion formula exhibits periodicity in the

same manner as the Discrete Transform (DT); the period is defined by N

samples of g(kT). This property results from the periodic nature of

ej2πnk/N.

Properties:

(l) Linearity x (t) + h (t) ↔ X (f) + H (f)

(2) Symmetry H(t) ↔ h(-f)

(3) Time scaling h (kt) ↔ 1/k H (f/k)

(4) Frequency scaling1/k h (t/k) ↔ H(kf)

(5) Time shifting h (t-to) ↔ H (f) ej2πft0

(6) Frequency shifting h (t) e j2πft0 ↔ H (f-fo)

(7) Convolution x(t) h(t) ↔ x(T)h(t- T)dT

The above properties for the Continuous Fourier Transform can be

simply restated with the appropriate notation for the discrete Fourier

transform, as the latter is a special case of the former.

3.9.3: Fast Fourier Transform (FFT):

Consider the discrete Fourier transform

∑ −== − 1,....,2,1,0,)()( /2

0 NnekxnXNnkj π (3.10)

Where we have replaced kT by 'k' and n/NT by 'n' for convenience

of notation. We note that equation (3.10) describes the computation of N

116

equations. For example, if N = 4 and if we let

NjeW

/2π−= (3.11)

Then equation (3.10) can be written as

0

0

0

0

0

0

0

0 )3()2()1()0()0( WxWxWxWxX +=== (3.12)

3

0

2

0

1

0

0

0 )3()2()1()0()1( WxWxWxWxX +===

6

0

4

0

2

0

0

0 )3()2()1()0()2( WxWxWxWxX +===

9

0

6

0

3

0

0

0 )3()2()1()0()3( WxWxWxWxX +===

Equation (3.12) can be more easily represented in matrix form

=

)3(

)2(

)1(

)0(

)3(

)2(

)1(

)0(

0

0

0

0

9630

6420

3210

0000

X

X

X

X

WWWW

WWWW

WWWW

WWWW

X

X

X

X

(3.13)

or more compactly as

X(n) = Wnk Xo(k) (3.14)

Examination of equation (3.13) reveals that since W and possibly

Xo(k) are complex, then N2 complex multiplications and N(n-l) complex

additions are necessary to perform the required matrix computation.

“The FFT owes its success to the fact that the algorithm reduces the

number of multiplications and additions required in the computation of

equation”. To illustrate the FFT algorithm, it is convenient to choose the

number of sample points of Xo (k) according to the relation N= 2r where

'r' is an integer. From the choice of N= 4 = 2r = 22, we can apply the FFT

to the computation of equation (3.13). The first step in developing the

117

FFT algorithm for this example is to rewrite equation (3.13) as

=

)3(

)2(

)1(

)0(

1

1

1

1111

)3(

)2(

)1(

)0(

0

0

0

0

123

202

321

X

X

X

X

WWW

WWW

WWW

X

X

X

X

(3.15)

Matrix equation (3.15) was derived from (3.13) by using the

relationship Wnk = WnkmodN. Recall that [nk mod (N)] is the remainder

upon division of nk by N;

hence if N = 4, n = 2 and k = 3 then

W6 = W2 (3.16)

Since, Wnk+W6 = exp[(-jπ/4)(6)] = exp[-j3π]

exp[-jπ] = exp[(-jπ/4)(6)] = W2 = WnkmodN (3.17)

The second step in the development is to factor the square matrix

in the equation (3.15) as follows:

=

)3(

)2(

)1(

)0(

010

001

010

001

110

100

001

001

)3(

)1(

)2(

)0(

0

0

0

0

2

2

0

0

3

1

2

0

X

X

X

X

W

W

W

W

W

W

W

W

X

X

X

X

(3.18)

For the present, it suffices to show that multiplication of the two

square matrices of equation (3.18) yields the square matrix of equation

(3.15) with the exception that rows 1 and 2 have been interchanged. It is

to be noted that this interchange has been taken into account in

equation (3.18) by rewriting the column vector X(n); let the row

118

interchanged vector be denoted by

=

)3(

)1(

)2(

)0(

)(1

X

X

X

X

nX (3.19)

One should verify that equation (3.18) yields equation (3.15) with

the interchanged rows as noted above. This factorization is the key to the

efficiency of the FFT algorithm. Having accepted the fact that equation

(3.18) is correct, although the results are scrambled, one should then

examine the number of multiplications necessary to compute the

equation. The final equation can be re written as:

=

)0(

)0(

)0(

)0(

010

001

010

001

)3(

)2(

)1(

)0(

0

0

0

0

2

2

0

0

1

1

1

1

X

X

X

X

W

W

W

W

X

X

X

X

(3.20)

i.e. column vector x1(k) is equal to the product of the two matrices

on the right in equation (3.18). Element x1(0) is computed by one

complex multiplication and one complex addition

3.9.4: WEIGHTING FUNCTIONS

A weighting function, w (n), is a sequence of numbers that is

multiplied by input data prior to performing a Discrete Fourier

Transform (DFT) on that data. Weighting (also called window) functions

reduce sidelines of DFT filters and widen main lobes while, fortunately,

not altering the locations of the centers of the filters.

Weighting function selection can be made early in the design process

119

because the choice of FFT algorithm and their functions are independent

of each other. Choice of a weighting function to provide the specified side

lobe level is done without concern for the FFT algorithm that will be used

because they work for any length FFT and they work the same for any

FFT algorithm. They do not alter the FFTs ability to distinguish two

frequencies. The performance measures of weighting functions and

comparison are given in [44].

The different types of weighting functions used are:

(1) Rectangular: for n = 0 to N-l, ω(n) = 1

The rectangular weighting function is just the plain FFT without

modifying the input data samples. The peak of the highest side lobe is

only 13 dB below the main-lobe response, and the side lobe peaks do not

drop off rapidly. This makes it poor for signals with multiple frequency

components that have amplitudes that are more than 6 dB different from

each other.

(2) Triangular: for n = 0 to N/2, w (n) = 2*n/N

n = N/2+1 to N-1, w(n) = 2*(N-n)/N

The triangular window function is used to provide side lobes and

straddle loss lower than the rectangular function. The outstanding

characteristic of this window function is the smaller number of side lobes

than the others.

(3) Sine-lobe: for n = 0 to N-1, w (n) = sin(nπ/N)

The sine-lobe window function provides improved side lobe performance.

120

For power of two FFTs, this window function has a computational

advantage over triangular window function because the coefficients are

the same one as used to compute the FFT.

(4) Hanning: for n = 0 to N-l, w(n) = 0.5*[1-cos(nπ/N)]

The Hanning window function is slightly more complicated to compute

than the sine-lobe. The peaks of its side lobes fall off 50% faster than the

triangular and sine-lobe functions.

(5) Sine-cubed: for n = 0 to N-l, w(n) = sin3(nπ/N)

The sine-cubed function is a natural extension to the side-lobe window

function, but with values that are not used for the complex

multiplications between powers of two building blocks.

(6) Sine to the fourth: for n = 0 to N-l, w(n) = sin4 (nπ/N)

(7) Hamming: for n = 0 to N-l, w(n) = 0.54 – 0.46.cos(2nπ/N)

(8) Blackman:

for n = 0 to N-l , w(n) = 0.42 – 0.5*cos(2nπ/N) = 0.08* 0.08* cos(4nπ/N)

(9) Three-sample Blackman-harries:

(a) For n = 0 to N-l, w (n) = 0.449595 – 0.49364* cos(2nπ/N) = 0.05677*

cos(4nπ/N)

(b) For n = 0 to N-l, w (n) = 0.42323- 0.49775* cos(2nπ/N) = 0.07992*

cos(4nπ/N)

(10) Four-sample Blackman-harries:

(a) For n = 0 to N-l,

w(n)=0.40217 - 0.49703 * cos(2nπ/N) + 0.09892* cos(4nπ/N) - 0.00188*

121

cos(6nπ/N)

(b) For n = 0 to N-l,

w (n) = 0.35875- 0.48829* cos(2nπ/N) + 0.14128* cos(4nπ/N) - 0.01168*

cos(6nπ/N)

The following factors significantly influence the transfer function:

Finite record length:

During the conversion from the analog to digital domain, the

Nyquist criterion must be satisfied. It specifies that all signals should be

sampled by at least twice; preferably ten times the highest frequency

component in the signal. So, this stipulates the minimum limit on the

sampling frequency for a given sampling speed. Higher record lengths

result in an increase in digitizer costs and are also higher record lengths

are accompanied by a decrease in signal-noise ratio.

Quantization:

“The process by which the analog signals are converted to their

digital from is called Quantization". This is an irreversible process, and

can be defined as a nonlinear mapping from the domain of continuous

amplitude inputs onto one of a countable number of the possible output

levels. This results in the so called quantization error, which is the main

cause for the deviation of the signals from the ideal nature.

Windowing:

A very popular signal processing method, namely, windowing is

used to avoid leakage effects due to abrupt truncation of signals. This is

122

a deliberate smoothing of the abrupt truncation of the impulse tail

caused by the finite length of the digital record.

In this chapter the fast Fourier transform (FFT) technique has been

employed to identify the dominant frequencies of the fast transient

voltages. The frequency spectrum has been calculated by considering the

VFTO waveform for the duration of 0.4µs and 0.5µs. The Fourier

transform and Fast Fourier transform, types of Fourier transforms and

weighting functions as well as their application are discussed. The

MATLAB 7.1 is used for this analysis. The time-domine data (APPENDIX-

I) of corresponding signal is transferred to Signal processing tool box in

MATLAB7.1 The corresponding results are presented in the section 3.9.5.

3.9.5 FFT analysis of VFTOs

Fig. 3.45 Frequency spectrum of VFTO at Air-to-SF6 bushing during the

opening operation of the DS1 with fixed arc resistance

123

Fig. 3.46 Frequency spectrum of VFTO at Air-to-SF6 bushing during the opening

operation of the DS1 with variable arc resistance.

Fig.3.47 Frequency spectrum of VFTO at Air-to-SF6 bushing during the

closing operation of the DS1 with fixed arc resistance

Fig.3.48 Frequency spectrum of

closing operation of the DS1 with


arc resistance and trapped charge of

124

Frequency spectrum of VFTO at Air-to-SF6 bushing during the

closing operation of the DS1 with variable arc resistance

Frequency spectrum of VFTO at source side of DS1 with variable

arc resistance and trapped charge of -0.1p.u.

SF6 bushing during the

arc resistance

source side of DS1 with variable


resistance and trapped charge of



125

Frequency spectrum of VFTO at load side of DS1 with variable arc

resistance and trapped charge of -0.1p.u.



load side of DS1 with variable arc






126











127











128





side of DS1 with variable arc






129





side of DS1 with variable






130





DS1 with variable arc






131





with variable arc






132

Frequency spectrum of VFTO at load side of DS1 with vari










133

Frequency spectrum of VFTO at source side of DS1 with

arc resistance and trapped charge of -0.9p.u


arc resistance and trapped charge of -1p.u.



134

Fig.3.68 Frequency spectrum of VFTO at load side of DS1 with variable arc

resistance and trapped charge of -1p.u.

Fig.3.69 Frequency spectrum of VFTO at at SF6 – to – XLPE cable

termination during opening of DS2 with variable arc resistance and

trapped charge of -1 p.u.

135

3.9.6 The transients on source side and load side of the

Disconnector switch with different trapped charges.

Table 3.2 VFTOs on source side with different trapped charges

S.No

Trapped

charge in

p.u

Source side

voltage in p.u.

Rise Time in

(ns)

Highest

frequency

In MHz

1

-0.1 1.61 19.27

11

2 -0.2

1.82 21.27

12.6

3 -0.3 1.93

26.73

14.1

4 -0.4 1.97

26.92

13.7

5 -0.5 2.31

27.12

17.4

6 -0.6 2.37

28.12

21.6

7 -0.7 2.39

28.21

28.1

8 -0.8 2.48

28.78

39.1

9 -0.9 2.71

33.40

39.7

10 -1

2.72

39.20

41.2

136

Table 3.3 VFTOs on load side with different trapped charges

S.No

Trapped

charge in

p.u

Load side

voltage in p.u.

Rise Time in

(ns)

Highest

frequency

In MHz

1

-0.1 1.97 19.27

9

2

-0.2

2.13 21.27

13.1

3 -0.3 2.21

26.73

14.7

4 -0.4 2.27

26.92

17.0

5 -0.5 2.42

27.12

19.1

6 -0.6 2.43

28.12

26.3

7 -0.7 2.69

28.21

29.7

8 -0.8 2.74

28.78

29.1

9 -0.9 2.76

29.34

37

10

-1 2.81 39.20

44

137

Table 3.4 VFTOs at air to SF6gas bushing during opening and closing

operation of DS1

VFTO at air to gas bushing

during DS1 opening operation

VFTO at air to gas bushing

during DS1 closing operation

VFTO

in p.u.

Rise time

in ns

Frequency

In MHz

VFTO

in p.u.

Rise time

in ns

Frequency

In MHz

With fixed arc

resistance

2.31

2.29

11.7

2.19

29.11

24.9

With variable arc

resistance

2.21

2.11

11.1

1.92

25.32

19.7

The Fig 3.20 to Fig.3.23 shows VFTO waveforms at Air-to-SF6 bushing.

Similarly from Fig3.24 to Fig.3.43 shows VFTO waveforms at source side

and load side of DS1 with different values of trapped charges. The

Fig3.45 to Fig3.69 shows the various frequency spectrums obtained at

various locations of the system with different trapped charges. The

results are tabulated in tables 3.2 and 3.4 respectively.

In the case of variable arc resistance the system acts as more oscillatory

circuit there is a superposition of transient state, with some damping

effect results slight decrease in VFTO.

138

3.10 VARIOUS METHODS FOR SUPPRESSION OF VFTOs IN GIS

SYSTEMS

3.10.1 VFTO Suppression Using Opening and Closing Resistor across

Disconnector Switch

During switching operation of disconnector switches and earth

faults in GIS systems very fast transient over voltages occurs and will

stress adjacent equipment and secondary equipment in GIS [39]. With

the increasing of GIS voltage levels, the effect of VFTO should be taken

into consideration. Hence it is advisable to suppress these over voltages

for protection of secondary equipment [40]. One of the exisistng methods

of suppressing these over voltages is by insertion of resistor during

switching. Usually the resistance parameter ranges from 400-500Ω will

be used in this method [41]. In the present application, a resistor of

500Ω is connected in parallel to the disconnector switch and a switch is

connected in series with the resistor. The switch connected in series with

the resistor is closed at the time of maximum voltage is obtained during

second restrike/prestrike at load end. The trapped charge of -0.1p.u,-

0.5p.u&-1p.u are considered for the computation of VFTOs with

resistance switching

139

3.10.2 Fast Fourier Transform (FFT) Analysis of reduced transient

over voltages

In this chapter the fast Fourier transform (FFT) technique has

been employed to identify the dominant frequencies of the transient

voltages. The frequency spectrum has been calculated by considering the

VFTO waveform for the duration of 0.4µs and 0.5µs. The Signal

Processing Tool box in MATLAB 7.1 is used for this analysis. The time

domine data of corresponding signal is transferred into the MATLAB 7.1

as an input file and the corresponding frequency spectrums are

obtained. The results are given in section 3.10.4.

140

3.10.3 Single Phase Equivalent Circuits of 245kV GIS System

with Opening and Closing Resistor

Fig.3.70(a) DS1 opening operation with variable arc resistance and with -

0.1p.u trapped charge and resistance Switching

Fig. 3.70(b) DS1 closing operation with variable arc resistance and with- 0.1

trapped Charge and resistance switching

141

Fig. 3.70(c) DS1 opening Operation with variable arc resistance and trapped

charge of -0.5 p.u. and resistance switching.

Fig. 3.70(d) DS1 closing Operation with variable arc resistance and trapped

charge of -0.5 p.u. and resistance switching

142

Fig. 3.70(e) DS1 opening operation with variable arc resistance and trapped

charge of -1p.u. and resistance switching

Fig. 3.70(f) DS1 closing operation with variable arc resistance and trapped

charge of -1p.u. and resistance switching

143

3.10.4 Simulation Results with opening and closing Resistance

Fig. 3.71(a) VFTO at source side of the DS1 opening operation variable

arc resistance and trapped charge of -0.1p.u and switching

resistance across DS

Fig. 3.71(b) FFT of VFTO waveform during opening of DS1 with variable

arc resistance and trapped charge of- 0.1p.u with

resistance switching

144

Fig. 3.71(c) VFTO at source side of the DS1 closing operation with variable

arc resistance and trapped charge of -0.1p.u and switching

resistance

Fig. 3.71(d) FFT of VFTO waveform during closing of DS1 with variable arc

& trapped charge of -0.1p.u resistance and resistance switching

145

Fig. 3.71(e) VFTO at source side of the DS1 opening operation with

variable arc resistance with trapped charge of- 1p.u and

with resistance switching

Fig.3.71 (f) FFT of VFTO waveform during DS1 opening operation with variable

arc resistance with trapped charge of -1p.u and with resistance

switching

146

Fig.71(g) VFTO at source side of the DS1 closing operation with variable


switching

Fig. 3.71(h) FFT of VFTO waveform during DS1 closing operation with variable


switching

147

Fig. 3.71(i) VFTO at source side of the DS1 opening operation with

variable arc resistance with trapped charge of- 0.5p.u and


Fig. 3.71(j) FFT of VFTO waveform during DS1 opening operation with

variable arc resistance with trapped charge of -0.5p.u and


148

Fig. 3.71(k) VFTO at source side of the DS1 closing operation with variable arc

resistance with trapped charge of- 0.5p.u and with resistance

switching

Fig. 3.71(l)FFT of VFTO waveform during DS1 opening operation with variable

arc resistance with trapped charge of -0.5p.u and with resistance

switching

149

3.11 VFTO SUPPRESSION USING FERRITE RINGS

In this section, a new technique is proposed for the suppressing of

VFTOs by high frequency magnetic rings known as ferrite rings. These

ferrite rings are very effective in inhibiting the high frequency

components of transient over voltages[40] . The ferrite rings are

connected to the bus bars near the disconnector contacts can effectively

resist the steep rise time travelling waves passing through and consumes

energy from the waves [22,40]. The characteristics and design features of

ferrite rings for high voltage applications are discussed in detail. The

mathematical modeling of ferrite rings has been done. The Fig.3.72 (a) to

Fig.3.72(d) shows EMTP-RV equivalent circuits with ferrite ring

equivalents. The simulation results during opening and closing

operations are presented in the Fig. 3.73(a) to Fig. 3.73(l). The proposed

technique is experimentally verified and it is discussed in detail in the

next chapter.

Ferromagnetic rings can be utilized to effectively suppress the

amplitude of VFTO generated with in GIS [23], however selection of

ferromagnetic materials for high voltage applications is of extreme

significance. The ferrite material chosen must have different

characteristics of saturation, magnetic conductivity, and frequency

response and loss characteristics. All these parameters influence the

VFTO suppression effect. The ferrite material is chosen such that the

magnetic flux density is maximum. The magnetic conductivity parameter

150

is complex and nonlinear. The suppressing effect on VFTO is determined

by equivalent inductance of Ferro magnetic ring that relate to the size

and the magnetic conductivity of ferrite ring. The energy loss of the

ferrite ring can know from the equation 3.11.1[22]

! =

√" µ ℎ $%& 3.21

µ is magnetic hysteresis conductivity

H is magnetic field strength

h is magnetic hysteresis coefficient

F is magnetization frequency

V is volume of the ferrite ring

3.11.1 Equivalent characteristics

The suitable ferrite rings can be connected to bus bar of GIS

systems to limit the magnitudes of VFTOs generated due to disconnector

operations. The equivalent circuit of the ferrite ring fixing it on the GIS

conductor bar is equivalent to connecting impedance and inductance

between the disconnector and bus bar. The simulation circuit for VFTO

studies is inductance of the ferrite coil parallel to resistance of the coil.

The effect of reflected waves is neglected.

151

3.11.2 Losing characteristics of ferrite rings

The ferrite ring fixed on GIS conductor should have no influence

on the power frequency electric current and the most loss of the ferrite

ring produces at high frequency, so the energy of the VFTO can be

absorbed. The loss of unit volume of ferrite material is

P = Pe + Ph +Pc 3.22

Total power loss can be expressed as

' = &()* 3.23

K is constant

f is frequency

B is flux density; n and m are the Index parameters, from above equation

the loss of the ferrite ring is in direct proportion to f and B.

3.11.3 Design aspects of ferrite rings

Mn-Zn ferrite is chosen as its high magnetic saturation Bs i.e.

about Bs > 47mt at 250C and the core shape selected is toroid. Ferrite

characteristics as a function of operating conditions. When selecting a

ferrite rings it is necessary to consider some important application

aspects.

The frequency where maximum attenuation is needed will

determined the material requirements. The most suitable ferrite would

offer the highest impedance levels at the very high frequencies, which

usually cover an abroad spectrum core shape, which is usually defined

by bus bar type and size. Installation requirements to decide on an entire

152

or split core type. Attenuation/impedance level of maximum suppression

the Mn-Zn material (3S4) is selected for present application. It can

suppress very high frequencies order of MHz [40]. With MnZn-Ferrite

precise control of material composition has resulted in an increase of its

resistivity to a value of 103Ω m. The additional advantage of 3S4 is that it

does not have nickel which is a heavy metal and therefore a potential

hazard to the environment. Also, its high permeability gives it excellent

high-frequency characteristics.

3.11.4 Specifications of 3S4 ferromagnetic material

S.NO. Symbol Conditions Value Unit

1

µi 25oC;10KHz;0.1mT

1700

mT

2 B 25oC;10KHz;250A/m 300 mT

3 --- 100oC;10KHz;250A/m 140 ---

4 Z 25oC;3MHz ≥25 Ω

5 --- 25oC;30MHz ≥60 ---

6 --- 25oC;100MHz ≥80 ---

7 25oC;300MHz ≥90 ---

8 ρ DC; 25oC =103 ---

9 Tc ---- ≥110 Ωm

10 Density ---- =4800 Kg/m3

153

3.11.5 Impedance behavior of 3S4-Ferrite cores

The inside diameter is fixed by the GIS bus bar dimensions. The

ferrite rings should fit closely around the bus bar to avoid loss of

impedance. Impedance increases mainly with the length of a bus bar or

the number of shields. It depends linearly on length and only

logarithmically on the outer dimensions. The most suitable ferrite core

will be the largest type with an outer diameter matching the bus bar

outer dimensions. If large inner diameter (not fitting the bus bar) and

their shorter length are compensated by using more than one turn. The

Z is proportional to N2, where N is number of turns. It is not

recommended to use more than 2 turns on ferrite core. Although the

higher number of turns results in more impedance. The parasitic inner

winding capacitance, which is also proportional to the number of turns,

will decrease the results in a worse performance at very high frequencies.

Location:

The position of ferrite rings is very important for the best

performance in the application. The ferrite rings are connected near the

disconnector switch in the GIS system.

Material and size

The impedance curve can be divided from a pure material curve

can be derived from a pure material curve, the so called complex

permeability curve. As impedance consists of a reactive and a resistive

part, permeability should also have two parts to represent this. The real

154

partµ,) corresponds to the reactances, and the imaginary part ( µ" the

losses.

= ./0µ, − .µ"23

= wµ"3 + ./µ,3 3.24

Z = R+jx, R = w µ,3 & X= wµ"3

Magnitude of Impedance (Z) = √ + 4 = w3 5 µ, + µ,, Where / = 2п&

L0 = µ 7 89:9

µ = 4п ∗ 10<=

N = Number of turns

>? = Effective area

@? = Effective length

In high power rating applications the ferrite ring can be equivalent

to simply from with uniform magnetic field. It can be similar to a thin

ring ‘dr’. The different magnetic field line length and ferrite ring section

can be expressed with one equivalent length 3? and area >?. The total

magnetic field go through the each section, the equivalent inductance 3A is 3A = µµAB 89

C9

3.25

and equivalent resistance E = ℎ&3AFC9

3.26

Here µA is the relative initial magnetic conductivity h is the magnetic

hysteresis coefficient, “N’s the number of rings. The ferrite ring is

155

composed of a lot of ideal thin rings dr. So lN=2пrH, and the section area

as dA = a.dr then the differential inductance is

3 = 7H@ = 7 H

I. 2пr = 7 ) K I 2пr

From B = µµAI , the initial differential inductance is

3A = µµA7 LпM

by adding all the rings, the inductance of the ferrite ring is

3A = NONA7 K2P B L

A 3.27

Using the similar method the resistance can be obtained as

E = ℎ & 3A7 @ 1A − 1

LB LA

3.28

The initial magnetic conductivity of µA of the Mn-Zn ferrite is

between 2000 and 10,000 and its magnetic hysteresis coefficient scale

ERS" is about 0.1*10<T>/V If the equivalent impedance and inductance of

the ferrite ring are calculated as 70Ω and 0.02mH using above

equations. If the frequency is 100MHz, initial magnetic conductivity is

3000. Magnetic hysteresis coefficient is 0.18 ∗ 10<T > VW , the dimensions

of the ferrite ring are K = 3.33 ∗ 10<% m , L= 9.68*10-3m, ri =19.37*10-3m.

156

VS = VFTO at Source voltage

VL = VFTO at Load voltage

VSF6AGB= VFTO at Air to Gas Bushing

+

2nF

C1

+

0.003nF

C19+

0.003nF

C20

+

0.2nF

C21

+

0.1nF

C22

+

0.0045nF

C23

+

0.003nF

C24

+

0.003nF

C25 +

0.003nF

C26 +

0.0045nF

C27 +

0.005nF

C28 +

0.003nF

C29 +

0.003nF

C30 +

0.0045nF

C31 +

0.1nF

C32

+

0.5

R2

+ RLC

250,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

30,0,0

+

1 /_0

+ C3

4

!v0

.00

5n

F

+ RLC

75,0,0

+

0.4nF

C35

VM+

VL

?v

+

0.003nF

C33

+0.02mH

L2

+

70

R1

VM+

VS?v R(t)+

Rt1

0

+-1|10ms|0

SW1

3.12 EMTP-RV EQUIVALENT CIRCUITS OF 245kV GIS SYSTEM WITH

APPLICATION OF FERRITE RINGS

Fig. 3.72(a) DS1 opening Operation with variable Arc Resistance and trapped

charge of -1p.u. and Ferrite rings.

Fig. 3.72(b) DS1 closing Operation with variable Arc Resistance and trapped

charge of- 1p.u. and Ferrite rings




+

2nF

C1

+

0.003nF

C19

+

0.003nF

C20

+

0.2nF

C21

+

0.1nF

C22

+

0.0045nF

C23

+

0.003nF

C24

+

0.003nF

C25 +

0.003nF

C26 +

0.0045nF

C27 +

0.005nF

C28 +

0.003nF

C29 +

0.003nF

C30 +

0.0045nF

C31 +

0.1nF

C32

+

0.5

R2

+ RLC

250,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

30,0,0

+

1 /_0

+ C3

4

!v0

.00

5n

F

+ RLC

75,0,0+

0.4nF

C35

VM+

VL

?v

+

0.003nF

C33

+0.02mH

L2

+

70

R1

VM+

VS?v

+

1ms/10ms/0

SW1

R(t)+

Rt1

0

157




+

2nF

C1

+0.003nF

C19

+

0.003nF

C20

+

0.2nF

C21

+

0.1nF

C22

+

0.0045nF

C23

+

0.003nF

C24

+

0.003nF

C25 +

0.003nF

C26 +

0.0045nF

C27 +

0.005nF

C28 +

0.003nF

C29 +

0.003nF

C30 +

0.0045nF

C31 +

0.1nF

C32

+

0.5

R2

+ RLC

250,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

30,0,0

+

1 /_0

+ C3

4

!v0

.00

5n

F

+ RLC

75,0,0

+

0.4nF

C35

VM+

VL

?v

+

0.003nF

C33

+0.02mH

L2

+

70

R1

VM+

VS?v R(t)+

Rt1

0

+-1|10ms|0

SW1




+

2nF

C1

+

0.003nF

C19

+

0.003nF

C20

+

0.2nF

C21

+

0.1nF

C22

+

0.0045nF

C23

+

0.003nF

C24

+

0.003nF

C25 +

0.003nF

C26 +

0.0045nF

C27 +

0.005nF

C28 +

0.003nF

C29 +

0.003nF

C30 +

0.0045nF

C31 +

0.1nF

C32

+

0.5

R2

+ RLC

250,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

75,0,0

+ RLC

30,0,0

+

1 /_0

+ C3

4

!v0

.00

5n

F

+ RLC

75,0,0+

0.4nF

C35

VM+

VL

?v

+

0.003nF

C33

+0.02mH

L2

+

70

R1

VM+

VS?v

+

1ms/10ms/0

SW1

R(t)+

Rt1

0

Fig. 3.72(c) DS1 opening Operation with variable Arc Resistance and trapped

charge of - 0.5p.u. and Ferrite rings

Fig. 3.72(d) DS1 closing Operation with variable Arc Resistance and trapped

charge of -0.5p.u. and Ferrite rings

3.13 SIMULATION RESULTS

RINGS.

Fig.3.73(a) VFTO at source side of the DS1 opening operation with variable

arc resistance

Fig. 3.73(b) FFT of VFTO waveform during opening of DS1 with variable

Arc resistance and ferrite rings on bus bar.

158

RESULTS WITH APPLICATION OF FERRITE

VFTO at source side of the DS1 opening operation with variable

resistance, trapped charge of -0.1p.u and ferrite rings

FFT of VFTO waveform during opening of DS1 with variable

resistance and ferrite rings on bus bar.

WITH APPLICATION OF FERRITE

VFTO at source side of the DS1 opening operation with variable

ferrite rings.

FFT of VFTO waveform during opening of DS1 with variable

159

Fig. 3.73(c) VFTO at source side of the DS1 closing operation with variable

arc resistance, trapped charge of -0.1p.u and with ferrite rings.

Fig. 3.73(d) FFT of VFTO waveform during DS1 closing operation with variable

arc resistance with trapped charge of -0.1p.u and with ferrite rings.

160

Fig. 3.73(e) VFTO at source side of the DS1 opening operation with variable arc

resistance with trapped charge of -1p.u and with ferrite rings

Fig. 3.73(f) FFT of VFTO waveform during DS1 closing operation with variable

arc resistance with trapped charge of -1p.u and with ferrite rings.

161

Fig. 3.73(g) VFTO at source side of the DS1 closing operation with

Variable arc resistance with trapped charge of- 1p.u with ferrite

rings Application on bus bar

Fig. 3.73(h) FFT of VFTO waveform during DS1 closing operation with

Variable arc resistance with trapped charge of -1p.u and

With ferrite rings

162

Fig. 3.73(i) VFTO at source side of the DS1 opening operation with variable arc

resistance with trapped charge of -0.5p.u with ferrite rings

Fig. 3.73(j) FFT of VFTO waveform during DS1 opening operation with variable

arc resistance with trapped charge of -0.5p.u and with ferrite rings

163

Fig. 3.73(k) VFTO at source side of the DS1 closing operation with variable

Arc resistance with trapped charge of -0.5p.u with ferrite rings

Fig.3.73(l) FFT of VFTO waveform during DS1 closing operation with variable

Arc resistance with trapped charge of -0.5p.u and with ferrite

rings.

164

3.14 RESULTS

The switching operations in a Gas Insulated Systems leads to very

fast transient Over voltages , these over voltages propagates with in the

GIS chambers with very steep wave front and very high amplitude, and

also stress the equipments in GIS and reduce the reliability of the

switchgear equipment. Such over voltages may cause some faults in GIS

and interrelated components, such as transformers. For knowing the

peak values of VFTO the EMTP-RV software is used, and simulations

have been carried out by designing suitable equivalent circuits and its

models. The parameters like arc resistance and trapped charge on

floating electrode are very important parameters in VFTO estimation.

Different values of trapped charges and variable arc resistance model are

considered for simulations.

There are some deficiencies in the existing suppressing methods.

The exisisting technique for suppression of VFTOs in GIS systems is by

using opening and closing resistance across the disconnector

switch(DS), this method is called resistance switching. There are certain

difficulties are present with this method. usually the Bus ducts are filled

with SF6 Gas at certain pressure in the GIS systems, because of this,

the installation of resistors is difficult in GIS systems as in air blast

circuit breakers. The resistance switching can result material

decompositions and byproducts in the gas, which can increase the

particle contamination and partial discharge problems in GIS systems.

165

VFT have a very short rise time in the range of 4 to 100 ns, and followed

by high frequency oscillations in the range of a few hundreds of

kilohertz to about a few tens of megahertz. The resistance switching

mechanism is not suitable because of large response time during

nanoseconds. In this chapter, a new method is proposed by using high

frequency magnetic rings for suppressing VFTO near by the source has

been researched. In this method ferromagnetic rings are mounted on

the conductors linked to the disconnectors to effectively suppress both

the amplitudes and steepness of VFTOs. The results are compared with

results obtained with resistance switching method. The results are

validated by the experimental results.

In this chapter, first EMTP-RV models of 245kV GIS

system with opening and closing resistance have been developed as

shown in Fig.3.70(a) to 3.70(f) The suppression effect is observed .The

VFTO plots and corresponding frequency spectrums are given in the

Fig.3.71(a) to 3.71(l)

In the next case EMTP-RV models of 245kV GIS system with ferrite

ring equivalent have been developed. They are given in Fig. 3.72(a)

to3.72(d) The VFTO plots and corresponding frequency spectrums are

given in Fig.3.73(a) to 3.73(l) The results are summarized in the Tables

3.5 to 3.7

166

Table 3.5 VFTOs without suppression methods:

S.NO.

Mode of

operation of

Disconnector

switch

Arc

resistance

Type

Trapped

charge

in p.u

Peak

amplitude

of VFTO

in p.u

Rise

time

(ns)

Highest

frequency

componenet

MHz

1 Opening

operation Variable -0.1 1.61 17.16 11.11

2 Closing


3 Opening


4 Closing


5 Opening

operation Variable -1 2.72 39.20 41.22

6 Closing


167

Table 3.6 VFTOs suppression with opening and closing resistance:

S.NO.

Mode of

operation of

Disconnector

switch

Arc

resistance

Type

Trapped

charge

in p.u

Peak

amplitude

of VFTO

in p.u

Rise

time

(ns)

Highest

frequency

componenet

MHz

1 Opening


2 Closing


3 Opening


4 Closing


5 Opening


6 Closing


168

Table 3.7 VFTOs suppression with ferrite rings on bus bar:

S.NO

.

Mode of

operation of

Disconnector

switch

Arc

resistance

Type

Trapped

charge in

p.u

Peak

amplitude

of VFTO

in p.u

Rise

time

(ns)

Highest

frequency

componen

et

MHz

1 Opening


2 Closing


3 Opening


4 Closing


5 Opening


6 Closing


169

3.15 SUMMARY

A simulation models are developed using EMTP-RV for the computation

of the VFTO phenomena. The main advantage of such model is to enable

the transient analysis of GIS systems. A spark collapse time is correctly

simulated by the variable resistance. A GIS system comprising of

spacers, bus bars and disconnectors have been considered for modeling

into electrical network. Cone insulators used for supporting inner

conductor against outer enclosure are assumed to be disk type for

approximate calculation of spacer capacitance. The bus duct capacitance

is calculated using formulae for concentric cylinders. The entire bus

length is modeled as distributed pi-network.

The transients due to switching operations with fixed arc resistance

and for variable arc resistance were calculated. It is observed that the

transients obtained with fixed arc resistance having higher than

magnitudes obtained with variable arc resistance. It is also found that

the magnitudes of the transients at both load side and source side of the

disconnector switch increases with trapped charge on the floating

electrode. It is also found that, the rise times are increased with trapped

charge value. The VFTO magnitudes are estimated at locations air-to-SF6

gas bushing and SF6-to-XLPE cable termination during switching

operations of DS1 and DS2.

170

The peak voltages and rise times are obtained

and presented. The trapped charge magnitude depends up on speed of

operation of the circuit breaker. The trapped charge varied from the

value -0.1p.u. to -1p.u. on floating electrode. It is observed that, for any

length of GIS it was found that the transients due to variable arc

resistance give lower value of peak voltages than that obtained with fixed

arc resistance. It was found that the highest frequency component

increases with increase in the trapped charge on floating electrode.

The switching operations in a GIS systems leads to very fast

transient Over voltages (VFTO), these over voltages propagates with in

the GIS chambers with very steep wave front and very high amplitude,

There are some drawbacks in the existing suppressing methods. The

exisisting techniques for suppression of VFTOs in GIS systems is by

resistance swicthing across the disconnector switch(DS). There are

certain difficulties are present with this method. In this chapter, a new

method that using high frequency magnetic rings suppressing VFTO

near by the source has been researched. In this ferromagnetic rings can

be mounted on the conductors linked to the disconnector to effectively

suppress both the amplitudes and steepness of VFTOs. The results are

compared with results obtained with existing method. The results are

validated by the experimental results.

For knowing the peak values of VFTO the EMTP-RV software is

used, and simulations have been carried out by designing suitable

171

equivalent circuits and its models. The parameters like arc resistance

and trapped charge on floating electrode are very important parameters

in VFTOs estimation. Different values of trapped charges and variable

arc resistance model are considered for simulations.

On the basis of plots, the following conclusions are drawn

1. From Fig.3.45, It is observed that, during opening operation of DS,

VFTO maximum peak and principal frequency component is increased.

2. From Fig.3.67, It is observed that, during closing operation of

Disconnector switch the trapped charge effect is more considerable.

3. From Fig.3.50 to From Fig.3.67, the Peak amplitudes of VFTOs and

rate of rise of first peak have been increased proportionately with the

trapped charge on the floating electrode.

4. From Fig.3.50 to From Fig.3.67, a Wide band of frequencies are

observed, the transient voltages oscillates between two dominant

frequencies around 11MHz and 50MHz

5 From Fig.3.51 to From Fig.3.67 With every decrease in trapped charge

value, damping of VFTO increases

6. From Fig.3.54 to From Fig.3.67With increasing of trapped charge

magnitude, the frequency of transient oscillations has been increased.

7. Comparing results Table3.5 and Table3.6, with the resistance

switching method the VFTO suppressed to 13% to 29% maximum.

8. Comparing results Table3.5 and Table 3.7, with the high frequency

magnetic rings (ferrite rings) method of suppressing VFTOs it is

172

observed that the magnitudes are suppressed by 30% to 69%

maximum.

9. Comparing results from Table3.5 and Table3.7 the high frequency

components are very much reduced with ferrite rings. This can reduce

interference effects to GIS secondary circuits, and can increase the

reliability of the system.

As seen from the above results and discussions, with the proposed

method for suppression of fast transients is advantageous, when ferrite

material is carefully selected for particular voltage and current ratings.

However, this method would not increase the complexity of the structure

of the GIS and it can play a role in the protection of GIS equipment

inside the bus bar. The protection range against high frequency

transients is larger than all exisisting methods.

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