44098603 Modelling in High Voltage Equipment

7/29/2019 44098603 Modelling in High Voltage Equipment

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MODELLING UHF SIGNAL PROPAGATION IN HIGH VOLTAGE

EQUIPMENT

Fahad: [email protected]

Abstract Detecting partial discharges (PD) inside high voltage equipment is apowerful means of insulation diagnosis that can be used for condition

monitoring. One way of doing this is to use ultra high frequency (UHF) sensors

to detect the electromagnetic waves that radiate from the discharge. The case

study will involve the student becoming familiar with; (i) partial discharge

phenomena and (ii) finite difference time domain (FDTD) method of modelling

transient electromagnetic fields. During the subsequent project, the student will

use the software package XF7 to study a range of cases of practical interest and

provide new insight into the challenges of PD detection and location in power

transformers and gas insulated substation (GIS) equipment.

TABLE OF CONTENTS

1. INTRODUCTION ........................................................................................... 2

2. PARTIAL DISCHARGE (PD). ...................................................................... 2

2.1 Methods of measuring PD .............................................................................................................4

2.1.1 Chemical testing of PD ........................................................................................................ ........5

2.1.2 Acoustic testing of PD ...................................................................................................... ......... ..5

2.1.2.1 Ultrasonic testing of PD ............................................................................................................62.1.3 Electrical testing of PD ........................................................................................................ ........8

2.1.4 UHF testing of PD ........................................................................................................................8

3. FINITE DIFFERENCE TIME DOMAIN (FDTD) ............................................. 9

3.1 Theory ...................................................................................................................................... ........ .10

4. MODELLING OF PD USING FDTD ............................................................ 12

Discussion ................................................................................................................................................14

4.1 Simulation Results ................................................................................................................ ......... ..16

5. CONCLUSIONS AND FUTURE WORK ..................................................... 20

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mailto:[email protected]:[email protected]


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1. IntroductionUHF sensing method of partial discharge (PD) detection in high voltages is fast

attracting vigorous research activities. A theoretical and experimental study of excited

UHF signals in a 132 kV gas insulated substations (GIS) was described by [1]. And a

very sensitive PD detection for GIS equipment in the range of 400 kV has also been

reported in [2, 3]. The major challenge, however, is to try and match the system

frequency response to the high voltage GIS design criteria which might require the

use of an UHF active coupler because most of the PD signal energy is found at higher

frequencies, in order to have the detection cover a wider range of high voltages (132

kV and above). Towards this objective, this project will seek the use XF7 software

package to study various challenging scenarios facing PD detection as well as location

in power transformers and GIS equipment using well known measuring methods such

as the UHF mentioned above and others like the chemical, acoustic and electrical

methods.

2. Partial Discharge (PD).Partial discharges may be defined as small electrical sparks that take place in the

insulation of electrical equipment such as generators, cables, motor windings,

transformers and other switchgear. PD study is particularly of interest to us when

considering sustainable and reliable electric power network operations. Since a

potential PD emerges as a result of an electrical breakdown of a segment within the

insulation, then its analysis method should be measured in a proactive approach by

diagnosing the respective equipment smitten with PD for ensured sustainable

functionality. This is more so the reason PD measurements could be carried out either

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on-line or off-line and periodically or continuously to further predict any need for

future maintenance.

PD measurements are based on breakdown currents caused by erosion of an insulating

materials dielectric strength explained below. Most insulating materials namely

polyethylene, epoxy and polyester do exhibit some kind of dielectric strength

otherwise known as electrical break down strength which is a representation of the

electrical intensity required for current to flow through as well as for discharging the

current. Air when used as an insulating material on the other hand, exhibits a very low

dielectric strength which gives way to a rather extremely brief (fractions of

nanoseconds) passage of current during dielectric breakdown.

Different electrical equipments portray different characteristics when subjected to

different operating conditions. For example, electrical insulations of generators and

motors tend to erode through weakened properties of the epoxy or resins bonding that

coat and insulate the windings when subjected to thermal stresses, chemical attack

and or abrasion to excessive coil movement, resulting in the development of an air

pocket in the windings.

PD are an unwanted phenomena in an electrical power network signalling a wide

range of system malfunction from imminent equipment failure to accelerating the

breakdown process explained above. At instances, ground faults or phase-to-phase

faults could be generated through excessive arcing between ground and live conductor

in the insulation causing its dielectric strength and mechanical texture of the winding

to disintegrate. This may be the starting point of the PD process and could get worse

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considering the global system in which PD tends to spread, inflicting a more serious

damage to a scale of total failure to the entire system or a greater part of it. Although

some PD is harmless, it should be the responsibility of the tester to identify which PD

is harmful and which one is not and to specifically locate the position where the PD

occurs. It has been reported that insulation failure in high voltage levels can cause up

to 90% electrical system failure. This is definitely abominable due to the excessive

costs associated with high voltage equipment and must be avoided through either

conducting on-line or off-line testing. On-line testing may be conducted when in

operation in a normal mode which gives sufficient information as to how the system

has been fairing, whether PD has build up, in which case engineers could take

preventive action and save the system from catastrophic failure by going off-line. This

can save money, time and even lives. With the on-line testing accurate measurements

of the systems performance, accurate assessment of the PD build up as well as the

spread of the PD can easily be ascertained. Off-line testing is often carried out when

the entire system is either down or switched off.

To sum it all, PD could be said to occur when the electric field strength is greater than

the electrical resistive breakdown strength of the insulating material. And should be

tested and identified as early as possible to curb out systems catastrophic failure.

2.1 Methods of measuring PD

On going research has so far identified four ways of measuring partial discharge

named below and are explained in turn:

Chemical

Acoustic

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Electrical

Ultra High Frequency (UHF)

2.1.1 Chemical testing of PD

Activities in PD are evidenced by changes in chemical composition in some particular

phases of the system. Conversely, these changes have been exploited in the detection

of PD activities. For example, liquid insulations often results in developed gaseous

compound usually determined through gas-in-oil analysis. In this process, the

developed fission gases solely depend on the insulation material and its power

density. Precipitation of NOX and ozone concentration is witnessed confirming the

presence of PD activity in an air insulated material due to exposition to chemical

reaction. The main disadvantage of the chemical method lies in its integrative

characteristic which makes it rather difficult to identify the current state of a single

PD in terms of its location, intensity or extent and nature.

2.1.2 Acoustic testing of PD

It is well known fact that most operating equipments produce a broad sound range.

These sounds exhibit high frequency ultrasonic components characterized by short

wave signals which are fairly directional in nature. Therefore, isolating these signals

from background noise and determining their location at a particular instance is a

straightforward matter.

Ultrasonic translators are some kind of airborne ultrasonic instruments capable of

providing a two-way type of information as qualitative or quantitative. Qualitative

type is the ability to hear ultrasounds through noise filtering headphones. Quantitative

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type of information dissemination is carried out through meter readings. This is

accomplished using heterodyning electronic process often adopted by ultrasonic

translators which accurately converts sensed ultrasounds into audible range

recognizable by users through headphones.

It is equally important to be able to hear ultrasounds as it is important to view and

gauge the sonic patterns produced by various equipment. This is the main advantage

of this method as it allows analysts to confirm a diagnosis on the spot by being able to

discriminate among various equipment sounds. The high frequency/short wavelength

nature of ultrasonic signals makes them easy to users to accurately pinpoint their

locations. Humans can sense sounds in the range of 20 Hz ~ 20 kHz with low

frequency sounds in the audible range of about 1.9 cm ~ 17 m long. These tend to be

gross when compared to the sensing pattern of ultrasonic translators operating only

within 0.3 ~ 0.6 cm of length. It can thus be seen that ultrasound wavelengths are far

smaller in magnitude suggesting ultrasonic environment as more conducive for

isolating and locating a source of problems in noisy plat environments.

2.1.2.1 Ultrasonic testing of PD

Ultrasonic testing is especially important in evaluating high voltages exceeding 1000

volts in identifying tracking problems, say, in enclosed switchgear where the tracking

frequency exceeds the fault frequency using infrared thermograph techniques.

Transient current discharges through high voltage lines or across electrical connection

gaps often disturbs the surrounding air molecules and generates ultrasound, normally

perceived as a cracking or frying sound and at times as a buzzing sound. Three basic

electrical problems detectable by ultrasound are:

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Corona effect is the ionization of the surrounding air forming a blue or

purple glow in an electrical conductor, such as an antenna or high voltage

transmission line, when its voltage exceeds some threshold value.

Tracking occurswhen anarc traces the path of damaged insulation, often

referred to as baby arcing.

Arcing is the production of an arc when electricity flows through space such

as occur in lightening process.

Ultrasound testing method is particularly used in detecting for discharges at both

winding ends (phase - phase) and in slot sections (phase - earth). It is also ideal for

high voltage applications such as switchgear, relays, insulators, bus bars, cables,

junction boxes, contactors and substation equipments like transformers and bushings

may as well be tested.

Arcing and corona leakage detection uses similar procedure to that used in detecting

acoustic emissions from mechanical sources. The user listens for cracking or buzzing

sounds rather than listening for rushing or rubbing sound. Generally, an area of

disturbance may be identified and located with a transistor radio or a wide-band

interference locator known as gross detector. A scanning module can then be used to

scan the area globally once a disturbance has been located. The sensitivity of the

module is reduced in the presence of a strong signal to be shown on the meter until

the location of the loudest point.

This method offers a simple means of determining whether a problem exists or not by

comparing sound quality and sound levels among similar equipment which produces

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different tones for envisaged problems. Another advantage is prediction of faults may

be accomplished by an extended period amplitude signals trending.

2.1.3 Electrical testing of PD

In the conventional electrical method, the detection circuit focuses on the capturing of

electrical pulses created by the streamer in the void. It calculates the apparent charge

by measuring and integrating the PD current in the measuring /detecting impedance.

2.1.4 UHF testing of PD

Although it is uncommon to use UHF waves for the detection of PD, however, UHF

detection offers the advantage of localizing the source of discharge as PD occurs

because of sharp geometries created in high voltage insulated equipment. The main

drawback of this method is the extreme sensitivity it offers to external noise.

UHF detection method is particularly applied to large distribution transformers in the

field. In order to avoid sudden failure to neighbouring equipment or cause

unnecessary customer dissatisfaction and economic disruption, it is always important

to determine whether power transformers are suffering from dangerous levels of PD

or from internal arcing.

The use of UHF method was in recent years established as the usual PD measuring

procedure in gas insulated substations (GIS) and increasingly in transformers. The

method is based on the detection of high frequency signals generated in the event of

discharges. PD impulses of momentary duration (< ns) can produce electromagnetic

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waves whose spectrum lies in the GHz range. To this effect, capacitive sensors

resembling antennas have now been developed to detect transient waves.

3. Finite Difference Time Domain (FDTD)Finite difference time domain (FDTD) is a popular computational modelling

technique. It is time domain based and hence can cover a wide range of frequency in a

single simulation run. FDTD methods are of generic class of grid based differential

time domain numerical methods. The time dependent Maxwells equations (in partial

differential form), are descritised using central difference approximations to the space

and time partial derivatives. The resulting finite difference equations are solved in

either software or hardware in a leapfrog manner. The electric field vector

components in a volume of space are solved at a given instant in time, then the

magnetic field vector components in the same spatial volume are solved at the next

instant in time and the process is repeated over and over again until the desired

transient or steady state electromagnetic field behaviour is fully evolved.

Basic FDTD space grid and time stepping algorithm trace back to a seminal paper in

1996 by Kane Yee in IEEE Transaction on Antennas and Propagation [1]. The

descriptor Finite Difference Time Domain and its corresponding FDTD acronym

were originated by Allen Taflove in a 1980 paper in IEEE Transactions on

Electromagnetic Compatibility [2].

Since about 1990, FDTD techniques have emerged as primary means to

computationally model many scientific and engineering problems dealing with

electromagnetic wave interactions with material structures. Current FDTD modelling

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applications range from near DC (ultralow frequency technology involving the entire

earth ionosphere waveguide) through microwaves (radar signature technology,

antennas, wireless communication devices, digital interconnects, biomedical

imaging/treatment) to visible light (photonic crystals, nanoplasmonics, solitons, and

bio photonics) [3]. In 2006, an estimated 2000 FDTD related publications appeared in

the science and engineering literature. In 2008, there were at least 27

commercial/proprietary FDTD software vendors, 8 free software/open source

software FDTD projects and 2 freeware/closed source FDTD projects.

Power transformers are one of the most important and expensive elements of power

system. Severe conditions, such as lightning strikes, switching transients and short

circuits can lead to an immediate failure, especially for aged transformers. Their

insulation strength can degrade to the point that they cannot withstand system events

such as short circuit faults or transient over voltages [1]. Insulation degradation is

frequently linked to PD. The methods of detection of PD are outlined in section 3 with

a report in [2, 3] that one or combination of more than one can be applied.

3.1 Theory

This section presents a brief overview of the FDTD technique for further reading

reference is made to Yee's classic paper [1] and the text by Taflove [2]. In a three-

dimensional FDTD model, magnitudes of the E- and H-fields are stored at points on a

discrete lattice.

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x

zEy

discharge current

Iy(t)

effectivecurrent density

Jy

= Iy

/(x z)

Fig. 1 Discharges can be defined in terms of a current density passing through the

face of a cell in the lattice [9].

TheE- andH- field components are offset in space so that central differencing can be

used to implement Maxwell's equations. An iterative algorithm alternately updates the

EandHarrays with a small time increment t. In rectangular coordinates, distances

between points in the lattice are denoted x, y and z, which are usually made

equal.

Signal propagation through the lattice occurs when a source of electromagnetic

energy is introduced by the appropriate numerical representation. An electrical

discharge current, whose cross-sectional area is smaller than the face of a cell in the

FDTD lattice, can be represented by a current density J, aligned with one of the

electric field components in the lattice. For the example shown in Fig. 1, introducing a

y-directed currentIy(t) requires the following additional operation at each iteration of

the FDTD code:

( )

zx

tItEE

yyy

+

(1)

Where Ey is the y-directed electric field in the cell containing the discharge current

and is the material permittivity at that point. The defined current, flows over one

cell of length y. This approach has been validated for a cylindrical coordinate

system [3].

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A current source in an FDTD model deposits two equal and opposite fixed charges in

the lattice at its end points [4]. If the current source spans a number of adjacent cells,

charge is only deposited at the start and end of the current path, provided that the

currents flowing in each cell are made equal. If one or both ends of the current path

meet the surface of a conducting object, the charge (as represented by electric flux)

that has been created spreads out over the surface of the object in order to satisfy the

boundary conditions. By this means, static fields can be set up within the FDTD

lattice, which might provide the initial conditions for a subsequent fast discharge

event.

4. Modelling of PD using FDTD

The experiment shown in Fig. 2 represents a simple PD detection system. A cell

containing a PD source is mounted inside a hollow cylindrical chamber. The cell

consists of a clear plastic envelope filled with SF6 at a pressure of 100 kPa, within

which a needle (30 mm long) is mounted on the centre pin of a hermetically sealed

50 coaxial connector. An electrode connected to a variable HV supply provides the

potential necessary to electrically stress the needle tip. Current pulses flowing on the

needle are used to trigger the digitizer sweep. Chamber resonances are monitored by

means of an electric field probe, whose output signal is recorded on the same trace by

summing the input channels, A and B. Further details of the PD cell and electric field

probe can be found in [6].

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variable HVAC supply

digitizer UHF amplifier

hollowcylinder

E-field probe

PD source

10 dB

x

z

y

A

B

Fig. 2 Detecting UHF resonances in a cylindrical cavity containing a PD source [9].

For an FDTD simulation of this experiment, the cylindrical cavity was represented on

rectangular lattice with a spacing of 5 mm. Fig.3 shows a cross-section through the

array holding the material permittivity data pertaining to they-component of electric

field. The needle is not represented in this array, but rather is included as a current

source in free space on the axis of the cell. This is achieved by using equation (1) to

incorporate the measured PD current into six cells coincident with the needle position.

Fig. 3 Section through the FDTD lattice atx = 0.3 m in the model of the

experiment shown in Fig. 5. A lattice spacing of 5 mm was used [9].

A typical result for this configuration is shown in Fig. 7, where the amplified output

voltage of the electric field probe is compared with the simulated signal. This requires

a post-processing stage for the FDTD E-field data, using a Fast Fourier Transform

together with a frequency-domain model for the probe transfer function. The resulting

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z

y

PD current is

y-directedover six celllengths in thisregion

HV cable

plastic cell,

r = 2.6


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voltage must also be scaled to allow for the gain of the UHF amplifier. Although the

signals begin to differ after a few ns (probably due to stair casing effects when the

cylinder is approximated on a rectangular grid), the amplitude of the signal was

modelled quite accurately.

-0.10

-0.05

0

0.05

0.10

0.15

(a) Measured

PD current pulse shape shownhere and below for reference.Peak current is 10.3 mA.

0 5 10 15 20time ( ns )

-0.10

-0.05

0

0.05

0.10

0.15

(b) FDTD

Fig. 4 Comparing the measured (a) and simulated (b) UHF signals excited by PD in

the cylindrical chamber of Fig. 2 [9].

Discussion

Condition monitoring remains a research topic of considerable importance to

electrical utilities. In the field of PD detection, the principal interpretation strategies

are based on analyzing phase or time resolved pulse amplitude patterns. Even when

using sensors that detect discharges by means of emissions at VHF or UHF (such as

in GIS), signals are usually converted to simple pulses, devoid of all high-frequency

content. This approach has been adopted for practical reasons, because multiple signal

paths for the original PD pulse can lead to a very complicated time-domain signal, as

shown in Fig. 5. However, the proliferation of mass-market products such as

computers and mobile communications equipment operating in the UHF band will

reduce the cost of electronic hardware for signal processing at these frequencies. We

may therefore benefit by improving our understanding of the high frequency signals

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radiated by electrical discharges, identifying any high-frequency information that

could be used to characterize and locate insulation defects, or to improve

discrimination between PD signals and external sources of interference. FDTD

models could assist with this process by allowing transient fields to be visualized and

by generating signal data that can be used to test new diagnostic techniques.

HVplant

sensor

PDpulse

multiple

signalpaths

Fig. 5 Signals radiated by PD are influenced by surrounding structures, as well as by

the signal source. This can lead to the generation of complicated time-domain signals

[9].

Another field that could benefit from employing FDTD techniques is experimental

research involving high-speed discharges [7]. In certain experiments, quantities of

interest cannot be measured directly. For example, FDTD models could be used to

study interactions between discharge channels and electrode geometry, as may be

illustrated with reference to Fig. 6. Suppose that we wish to determine the shape of a

fast current pulse during the breakdown of a small gap by measuring the electric field

at the surface of the ground plane a short distance from the test object. The radiated

signal will depend on both the forcing function (the current pulse) and the pattern of

current flow over the experimental apparatus. If the experiment was reproduced in an

FDTD model, it should be possible to determine the time-domain transfer function

relating the voltage output from the remote sensor to the current pulse flowing in the

gap. The impulse response predicted using the model could be used to enable de-

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convolution of the experimentally measured sensor output signal so that the temporal

variation of the source current can be recovered.

4.1 Simulation Results

Figure 7 shows two dimensional simulation results in free space as reconstructed from

[10]. The problem space was divided into 60 x 60 cells and the spread for the

Gaussian pulse in equation (9) is taken as 0.5 ns.

Fig. 7 Ez field after 30 times step with Gaussian pulse at centre

Two dimensional simulation results in free space without PMLis depicted in Figure 8

results have been obtained using the sameGaussian pulse simulated at the centre of

the grid after 100time steps in free space without the absorbing boundaryconditions.

Here the pulse is seen to have reached theboundary and reflected. The contour in Fig.

9 is neitherconcentric nor symmetric about the centre.

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Fig. 8 Ez field after 100 times steps without PML

Fig. 9 Contour of Ez field after 100 time steps without PML

Two dimensional simulation results in free space with PML is shown In Fig. 7 and 8

point PML has been used. Comparison of Fig. 8 and Fig. 10 shows how the

reflections get eliminatedwith PML. The outgoing contour of Fig. 11 is circular and

onlywhen the wave gets within eight points (PML) of the problem space, does the

phase front depart from its circular nature.

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Fig. 10 Ez field after 100 time steps with 8 point PML

Two dimensional simulation results in oil with obstructions is presented below. A real

transformer will contain oil as the insulating mediumand also obstructions like the

winding and core. Here such obstructions are simulated by two representative and

scaleddown circular objects of 6 cm radius centred at (15, 30) and (45, 30).

Fig. 11 Ez field after 100 time steps with 8 point PML

This simple geometry is assumed for verification of the algorithm. An oil medium

with relative permeabilityr= 1, relative permittivity r= 2.2 and conductivity = 0

and the 2 circular obstructions with relative permeability r= 1, relative permittivity

r = 1 and conductivity = 5.8x107 (S/m) representing simplified copper winding

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structure are considered. In Fig. 12 Ez field propagation, with pulse originated at

(30, 30), after 100 time steps is shown. Here the PML is also used. Figure 113 is the

contour of the pulse propagation.

Fig. 12 Contour of Ez field propagation after 100 time steps with pulse at (30, 30) in

oil and circular obstructions centred at (15, 30) and (45, 30)

Fig. 13 Ez field propagation with pulse after 100 time steps at (30, 30) in oil and

obstructions centred at (15, 30) and (45,30).

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5. Conclusions and future work

Results presented in this paper have demonstrated that electrostatic fields can be

generated within an FDTD model and shown how such models might be applied to

the study of electrical discharges through the measurement of radiated signals. Using

FDTD models in parallel with experiments may help to improve the accuracy of

breakdown measurements, as well as assisting with the interpretation of the signals

radiated by partial discharges inside electrical plant. One aim of future work will be to

incorporate models of sensor structures into the lattice so that their interaction with

the electromagnetic fields is taken into account.

HV electrode

dischargeE-fieldsensor

insulatingmedium

Fig. 14 Using an FDTD model in parallel with this experiment could allow the

discharge pulse shape to be measured [9].

Interfacing electromagnetic FDTD models with numerical models of the fundamental

processes in the gap, such as those developed by Morrow [8], may yield new insights

into the overall electromagnetic interactions that occur during electrical breakdown.

Thus, an object could be charged to an initial potential by an artificial pulse in the

FDTD lattice, with the subsequent discharge channel parameters being controlled by

the ionization processes of the insulating medium. These in turn would feed back to

the external field, initiating its collapse and radiation of an electromagnetic transient.

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Subsequently, this project would use the software package XF7 to study a range of

cases of practical interest and provide new insight into the challenges of PD detection

and location in power transformers and gas insulated substation (GIS) equipment.

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References

[1] K S Yee, "Numerical solution of initial boundary value problems involving Maxwell's equations in isotropic

media", IEEE Trans. Antennas and Propagation, Vol. AP-14, No. 3, pp. 302-307, May 1966

[2] A Taflove, Computational electrodynamics The finite-difference time-domain method, Artech House, 1995

[3] M D Judd and O Farish, "FDTD simulation of UHF signals in GIS", Proc. 10th Int. Symp. on High Voltage

Engineering (Montreal), Vol. 6, August 1997

[4] C L Wagner and J B Schneider, "Divergent fields, charge, and capacitance in FDTD simulations", IEEETrans. Microwave Theory and Techniques, Vol. 46, No. 12, pp. 2131-2136, December 1998

[5] M D Judd, O Farish and B F Hampton, "Excitation of UHF signals by partial discharges in GIS", IEEE Trans.Dielectrics and Electrical Insulation, Vol. 3, No. 2, pp. 213-228, April 1996

[6] M D Judd, O Farish and B F Hampton, "Modeling partial discharge excitation of UHF signals in waveguidestructures using Green's functions", IEE Proc. Science, Measurement and Technology, Vol. 143, No. 1, pp. 63-70,

1996

[7] A R Dick, S J MacGregor, M T Buttram, R C Pate, L F Rinehart and K R Prestwich, "Breakdown phenomena

in ultra-fast plasma closing switches", Proc. 12th IEEE Int. Pulsed Power Conf., pp. 1149-1152, 1999

[8] R Morrow, "Properties of streamers and streamer channels in SF6", Physical Review A, Vol. 35, No. 4,

pp. 1778 - 1785, February 1987

[9] M.D. Judd, Using Finite Difference Time Domain Techniques to Model Electrical Discharge Phenomena,

Annual report conference on electrical insulations and Dielectric Phenomena, Vol. 2 pp 518 521, October 2000.[10] C. Abraham and S.V. Kulkarni, FDTD Simulated Propagation of Electromagnetic Pulses due to PD forTransformer Diagnostics

44098603 Modelling in High Voltage Equipment

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Transcript of 44098603 Modelling in High Voltage Equipment