TỔNG HỢP CÁC BÁO CÁO KHOA HỌC VỀ CAO ÁP VÀ VẬT LIỆU ĐIỆN CỦA BỘ MÔN HỆ THỐNG ĐIỆN (ĐẠI HỌC BÁCH KHOA HÀ NỘI)

CÁC BÁO CÁO BAO GỒM: 1. NGHIÊN CỨU CÔNG NGHỆ PHÂN TÁCH CÁC PHẦN TỬ CÓ TÍNH ĐIỆN DẪN KHÁC NHAU BẰNG KỸ THUẬT CAO ÁP TĨNH ĐIỆN (Thầy Nguyễn Đình Thắng và Thầy Đinh Quốc Trí). 2. STREAMER INCEPTION IN MINERAL OIL UNDER AC VOLTAGE (Thầy Trần Văn Tớp). 3. ELECTROMAGNETIC CHARACTERISATION OF PANI PU IN MULTILAYERED STRUCTURE, APPLICATION FOR EMI PROTECTION AT MICROWARE FREQUENCY (Thầy Phạm Hồng Thịnh, Thầy Hoàng Ngọc Nhân và Cô Nguyễn Thị Lan Hương).

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1

STUDYING THE TECHNOLOGY FOR SEPARATING ELEMENTS WITH DIFFERENT

ELECTRICAL CONDUCTIVITITES USING HV ELECTROSTATIC TECHNIQUE

Nguyn ình Thng, inh Quc Trí

Trng i hc Bách Khoa Hà Ni

TÓM T T

bày mô hình thit b và kt qu nghiên cu s d ng công ngh phân tách các ht

bng k thut cao áp tnh in Công ngh ng d ng k thut cao áp tnh in ã

ti Vit Nam, vic nghiên cu sâu công ngh và nh h ng ca các yu t khác

áng

ABSTRACT

The separation of elements with different conductivities is widely applied in

industry, agriculture, e-waste processing technology… This paper presents the

developed device and the research results, implementing the technology of

high-voltage electrostatic technique in particles separation Application of the technology

has been studied over the world for many years due to its advantages, such as low

power consumption, high separation performance and environment-friendly However

in Vietnam, the deep research of technology and impact of different factors, such as

the electric field and the environment, has not been invested sufficiently.

T khóa: phân tách ht, x lý cht thi in t, cao áp t nh in

PHN T CN PHÂN TÁCH

1.1 in dn và trng lng riêng

 phc v cho vic nghiên cu

khoáng ly t m Cm hòa (Cm xuyên,

Hà Tnh) C s ly sa khoáng  làm thí nghim da trên s khác bit v tr s in

nh sau (theo [1]):

hình dng rt a dng,  thun tin cho

tính toán và mô phng ng i ta th ng

quy v hình c u, hình elip ho!c bán elip

 ng ca các ht khoáng sn ti m Cm

hòa dao #ng trong khong t 70 n

vic dùng thit b kiu máng nghiêng và

cho hiu sut tách cao [3,5]

2 MÔ HÌNH CA THIT B

Mô hình thit b thí nghim do tác

gi cùng các &ng nghip thit k, ch to

và l'p !t ti phòng thí nghim cao áp và

vt liu ca Tr ng i hc Bách khoa Hà

n#i Vic la chn mô hình thit b kiu này da trên m#t s c s sau:

 Mô hình thit b này hin còn ch a

dng ti Vit Nam: có công sut và hiu sut phân tách cao, d* dàng l'p

!t vn hành hiu ch+nh, tiêu hao ít

tin hành thí nghim theo các ch #

vn hành thc t, nh ng v$i u im cu to n gin nên d* dàng thay <i các thông s k= thut (chiu dài, hình dng, v trí in cc…)

S & mô hình thit b trên hình 1:

tách kiu máng nghiêng

3

nghiêng; 3 - in cc trên; 4 - in cc

d $i (hình tr); 5 - Khay hng sn phm;

 thun tin cho vic phân tích các

kt qu thí nghim  ây tác gi chn s

v trí khác nhau

Các kt qu thí nghim cho phép

c n tách và phân tích nh?ng nh h ng ca

in cao áp tnh in c@ng nh m#t s nh

h ng khác n hiu sut ca mô hình

thit b, trên c s ó có th hiu ch+nh

nh"m hoàn thin mô hình thit b

D $i tác dng ca thit b rung các

ht sX chuyn #ng t ph*u xung máng

nghiêng Lúc này các ht chuyn #ng

hoàn toàn d $i tác dng ca trng lc và

bay vào các khay hng sn phm

Qu= o chuyn #ng ca các ht

ch+ chu tác #ng ca trng lc, lc này

3

4 cos 3

các khay thu h&i ph thu#c vào kích th $c

và t\ trng riêng, nh ng hiu sut phân

Hình 2 Phân b khi lng các ht trong

còn in cc phía d $i – cc tính d ng)

in tr ng sX xut hin gi?a các

in cc v$i máng nghiêng Khác v$i

máng nghiêng sau ó chuyn #ng trong

in tr ng in tr ng tác #ng lên các ht này và làm thay <i qu= o chuyn

4

Thay <i tr s in áp !t lên in cc tc

là thay <i c ng # in tr ng, khi ó có

th thy rt rõ và ghi li s thay <i qu=

- vecto vn tc chuyn #ng ca ht,

t<ng ca các lc tác #ng lên ht, t- thi

- tit din ca ht

!c bit khi ta t;ng in áp  cc

#ng trong in tr ng mnh có xu h $ng bay lên và va p vào in cc trên sau ó

&ng thi m#t s ht có xu h $ng b hút v phía cc d $i (cc tính d ng) iu

in v$i c ng # cao ca các ht So sánh

các khay thu h&i nhiu hn (xem hình 3)

Hình 3 Phân b khi lng các ht trong

3 KT LUN:

Quan sát khay thu h&i sn phm b"ng m't th ng d* dàng nhn thy  phía nh?ng khay n"m  phía xa in cc cha các ht có m u sm hn (các ht Ilmenite)

5

m u sáng hn (Zircon và thch anh) Các

nh"m hoàn thin thit b

hình thit b v$i vic thay <i nhiu thông

u nh c ng # in tr ng, kích th $c,

v trí và góc nghiêng ca in cc…, kt

qu phân tách sa khoáng t m Cm Hòa

Các kt qu nghiên cu trên kh`ng

hình thit b này khi áp dng  tách các loi khoáng sn khác có tính cht g n ging v$i sa khoáng ca m Cm Hòa

TÀI LIU THAM KHO:

6

B# môn H thng in, tr ng i hc Bách Khoa Hà N#i S 1, i C& Vit, Hà N#i

Influence of electrode geometry, liquid conditioning, and consequences for test methods of liquids

O Lesaint Grenoble Electrical Engineering Laboratory (G2E lab)

CNRS, Grenoble-INP and Joseph Fourier University

Grenoble, France olivier.lesaint@grenoble.cnrs.fr

T.V Top now at: Hanoi Polytechnic Institute (IPH)

Hanoi, Vietnam

Abstract—This paper describes an experimental study of

streamer inception in mineral oil under ac voltage, with rod and

point electrodes Positive and negative streamer inception

frequencies versus voltage are investigated in gaps up to 40 cm

with different electrode shapes and different conditioning of the

oil (filtered oil, addition of cellulose particles, water) Streamer

inception probability increases exponentially versus field, and it

is not possible to simply define an “inception voltage” A voltage

(or field) value correlated to an inception probability must be

used to properly compare different experiments (comparison

between liquids, influence of pollution, etc.) Under ac, several

effects are superposed to reduce dielectric strength: “scale

effects”, influence of pollution, long time duration With sharp

points, injected space charges considerably influence

experiments, and the results obtained cannot be extrapolated to

practical applications in which the effect of space charge is

mostly absent

I INTRODUCTION

It has been known for a long time that initiation of

breakdown in liquids under ac voltage is a very complex

process Even in the simplest situation (a liquid between two

metallic electrodes), many parameters are able to influence the

initiation of breakdown:

- the liquid chemical nature;

- “scale” effects Breakdown voltage in quasi-uniform field

decreases when the stressed liquid volume and/or electrode

surface area are increased [1,2] "Volume" or "electrode

surface area" effects are usually interpreted in terms of "weak

points” able to trigger breakdown (solid particles in the liquid

volume, or electrode surface defects) The probability to get

large weak points increases with stressed liquid volume, and

electrode surface area;

- pollution: breakdown voltage in quasi-uniform field under

ac decreases when solid particles (metallic, hydrated cellulose

fibers, …) and water are present [3, 4];

- time: breakdown initiation shows a large statistical

variation, and is strongly affected by the time duration of

voltage application (the longer the time, the lower the initiation

voltage);

- injected space charge may also have an influence: field reduction by homocharges, or increase with heterocharges

It remains very difficult to model and predict the initiation

of breakdown in practical situations This study is devoted to obtain a more comprehensive and quantitative description of these phenomena under ac voltage Since breakdown results from the initiation and propagation of streamers, the study of initiation can be done with two main types of experiments:

- under moderately divergent field, the average field in the gap is very high, and all initiated streamers propagate to breakdown The measured breakdown voltage is equal to the voltage required to initiate a streamer;

- under divergent field (point-plane or rod-plane at large gaps), streamers can appear due to a high local field, but are unable to propagate to breakdown It is thus necessary to detect streamer inception with more sophisticated techniques The main practical advantage of these experiments is the absence of breakdown (no destruction of electrodes, limited degradation of the oil) On the other hand, experiments with large rod electrodes impose us to use high voltage and very large gaps to avoid breakdown

A previous study of streamer initiation was carried out with impulse voltage [5] over a wide range of electrode shapes (from sharp points of μm size to large rounded rods, and with fixed metallic particles on a flat electrode) This study mainly highlighted the effect of “electrode surface” under impulse voltage Under ac, the problem becomes much more complex, since the liquid pollution, time, and injected space charges will have a large influence With short impulses these parameters have a small influence: the time duration is too small to allow motion of particles from the liquid volume up to electrodes, and space charge development is also very limited The experiments carried out here are done with the objective to separate (as far as possible) the effects of electrode shape, time, and pollution by solid particles and water

II EXPERIMENTAL TECHNIQUESThe test cell used was a 150 liter transparent PMMA container (Fig 1) The high voltage electrode is facing a grounded aluminum plane, 50x50 cm in size Steel points (tip radius of curvature rp = 10 to 100 μm), and rods with a

voltage electrode and the distance could be changed without

opening the test cell, and without changing or circulating the

oil, in order to keep exactly the same oil condition The high

voltage supply was a 300 kV test transformer To get very

clean oil, the test cell was included in a closed loop including

an oil processing system (1μm filter, degassing and drying) An

insulating tube allowed us to take oil samples in the test cell to

measure particle and water contents To avoid particle

sedimentation and get an homogeneous and stable particle

content, the oil was continuously stirred with two polyethylene

propellers Cellulose particles, metallic perticles and water can

be added to clean oil Table 1 shows typical results of particle

counting when different amounts of cellulose were added to

filtered oil

Pollution inlet

Drying and filtration unit

PM

Oil sampling

stirrers

HV electrode

Figure 1 Test cell TABLE I Typical particle countings

Particle

size (μm)

Filtered

oil + 0,7 mg/l cellulose

+ 18 mg/l cellulose

+ 70 mg/l cellulose

Streamer detection has to fulfill several main requirements

The size of streamers vary considerably in the experiments:

very small streamers with sharp points at low voltage (charge:

must be very sensitive in order to detect all streamers During propagation of a long streamer, a large number a fast current pulses is detected Conventional PD measurement systems based on pulse detection are unable to record properly streamers: counting of all current pulses leads to considerably overestimate the streamer number actually generated The detection system must also have a very low level of spurious noise, typically less than 1 shot per hour (in some experiments,

a very low number of streamers can appear, typically 1 per hour) The system must be able to count properly such rare events In this study, inception was detected by the streamer light emission using a photomultiplier (PM) This provided a very sensitive detection A “dead time” of 200 μs was fixed after each detection to avoid overcounting streamers The detection threshold was fixed above the background noise of the PM, and the test cell was placed in a totally dark room

III STREAMER INCEPTION FREQUENCYFig 2 shows a typical result obtained with a rod electrode

of medium size (rp = 0.5 mm, distance d = 40 cm), in oil of technical quality without filtration (figures 2 to 4 correspond to the same oil sample, i.e without opening the test cell or circulating the oil)

Figure 2 Inception frequency of positive and negative streamers in oil without filtration, 20ppm water content Rod radius rp = 0.5mm, gap distance

d = 40 cm

The average inception frequency F increases exponentially versus voltage, up to a value ≈ 103

streamers / minute (corresponding to about 1 streamer initiated every half-wave)

The increase of F at higher voltage is then much slower At very low voltage, the exponential variation is still observed down to very low discharge rates (< 10-2 discharge / minute, i.e less than 1 streamer per hour) To obtain significant measurements at very low rates, total durations up to 2 days were used in some experiments In all experiments, no indication of a “threshold minimum voltage” for streamer inception could be obtained The number of positive and

From figure 2, it is quite clear that it is impossible to define

an “inception voltage” In order to make proper comparisons

between experiments (for instance if the liquid nature or

conditioning is changed), only the voltage value corresponding

to an arbitrary discharge frequency (for instance 1

streamer/minute) can be used Most experiments were carried

out in the frequency range 10-1< F< 102 in order to limit both

the duration of experiments, and the degradation of oil at very

high discharge rates

Figure 3: Streamer inception frequency versus voltage with different electrode

radius rp (40 cm gap distance, open dots: negative streamers, full dots: positive

10cm

5cm

-Streamer inception frequency F (minute -1 )

calculated tip field (MV/cm)

10μm 40μm

0.3mm 1mm 2.5mm

8mm

Figure 4: Streamer inception frequency versus calculated maximum field, with

different electrode radius rp and distances d (open dots: negative streamers, full

dots: positive streamers).

Figure 3 obtained with the same oil sample shows the

variation of discharge frequency when the rod (or point) radius

becomes slightly higher than negatives, whereas the opposite is seen with sharp points

Figure 4 shows the same results plotted versus the maximum field calculated at the extremity of rods (or points)

by finite elements method It is very interesting to observe that all measurements carried out with a fixed radius rp at different distances d (5 to 40cm) group together to form a unique plot

This shows that the maximum field is a good parameter to describe streamer initiation in such geometry This figure also shows an exponential increase with the same slope whatever the radius rp, and this tends to prove that the initiation process

is the same in all cases However, the plots corresponding to different radii rp do not group together, and this shows that a single field value does not exist to describe streamer initiation

in all cases Conversely, plots are widely shifted: at a fixed inception frequency, calculated fields are nearly x100 higher with a 10μm point compared to 8mm rod

IV INFLUENCE OF PARTICLES AND WATERThe influence of cellulose particles was studied by adding increasing amounts of a concentrated solution to well filtered oil This concentrated solution was prepared with particles obtained by de-structuring transformer pressboard Fig 5 shows the measured inception frequency measured with either

a sharp point (rp = 40 μm) or large rod (rp = 10 mm), versus cellulose concentration in oil with 35 ppm water

Streamer inception frequency F (minute -1 )

Crest voltage (kV)

Figure 5: Streamer inception frequency versus voltage with two different electrodes (rp = 40 μm and 10 mm), and different cellulose quantity added to filtered oil (40 cm gap distance, 35 ppm water)

This figure shows a quite different behavior with both electrodes With the large rod, a large increase of streamer inception frequency is seen The voltage corresponding to a fixed frequency (for instance 1 streamer/minute) is nearly divided by two between filtered oil, and oil with 25mg/liter

hand, nearly no variation is seen with the sharp point Similar

results are seen on figure 6, were the voltage corresponding to

F = 1 streamer /minute shows a very small variation with rp =

10 μm, and much larger with rp = 8mm Figure 6 also shows

the combined effects of cellulose and water These experiments

show that solid particles (hydrated cellulose) are of primary

importance for the triggering of streamers when the field is low

(large rods), whereas the very high field created by sharp tips is

sufficient to directly induce streamers

Figure 6 Voltage @ 1 streamer/minute versus cellulose and water content,

with two different electrodes: a) rp = 8 mm , b) rp = 10 μm (40 cm gap)

The results are summarized on figure 7, showing the

calculated field at F =10 streamer/minute versus electrode size

On this figure, we have also plotted initiation fields measured

under impulse voltage in the same mineral oil [5] The same

overall tendency is observed with ac and impulses: decrease of

initiation field when the electrode size is increased Two main

zones can be seen on figure 7

In zone I (large rods, rp > 0.5mm), the initiation field under

ac is about half the value measured with impulses This is quite

logical since the duration of voltage application is much longer

with ac Adding particles further decrease the value under ac

When the electrode size is increased, the initiation field under

ac decreases in a similar way as with impulses With impulses,

this effect was mainly attributed to a “surface” effect, since

particles have a negligible influence This shows that in this

zone, all mechanisms able to degrade the liquid properties

superpose under ac: time, pollution, electrode size This is

consistent with observations made for a long time in practical

applications of liquids In zone II (points, rp < 0.5mm), the

slope of the plot changes, and the calculated initiation field

becomes much higher than with impulses This effect is

similar conditions, measurements under impulses are not affected by space charges Since field calculations are carried out without space charges in figure 7, this means that the calculated values with ac are certainly strongly overestimated compared to the field actually present at the electrode extremity

Initiation field @ 10 streamer/minute (MV/cm)

Electrode radius of curvature (mm)

impulse voltage

I II

Figure 7 Calculated tip field @ 10 streamer/minute versus electrode radius, for filtered oil and 25mg/l cellulose (17 ppm water).

V CONCLUSIONSThe experiments presented here show the stochastic character of streamer inception under ac, influenced by the presence of particles and water The inception probability increases exponentially versus voltage, and no “inception threshold” can be observed Injected space charges considerably influence experiments with divergent fields, when the local field exceeds≈ 1MV/cm The results obtained in such conditions are not relevant of practical applications such as transformers, in which the effect of space charge is mostly absent If liquids are compared by measuring partial discharges with sharp points under ac, it is impossible to know which property of the liquid (discharge inception properties or ability

to inject space charges) is revealed by the measurement

REFERENCES [1] W R Wilson, A L Streater and E J Tuohy, "Application of Volume

Theory of Dielectric Strength to Oil Circuit Breakers" AIEE, Trans on

Power App., 1955, pp 677-688

[2] N Giao Trinh, C Vincent and J Régis, “Statistical Dielectric

Degradation of Large-Volume Oil-Insulation”, IEEE Trans PAS,

[5] O Lesaint and T.V Top, "Streamer initiation in mineral oil Part I:

Electrode surface effect under impulse voltage", IEEE Trans on DEI, Vol.9, pp.84-91, 2002 Part II: Influence of a metallic protrusion on a flat electrode", IEEE Trans on DEI, Vol.9, pp.92-96, 2002

Bộ môn Hệ thống điện - Đại học Bách Khoa Hà Nội 12

EMTP Simulation of Induced Overvoltage in Low

Voltage System

Thinh Pham Institute of Material Science

University of Connecticut

Storrs, CT 06269, USA Email: thinh.pham@ims.uconn.edu

Nhung Pham and Top V Tran Department of Power Systems Hanoi University of Technology 1- Dai Co Viet Street, Hanoi, Vietnam Email: tvtop-htd@mail.hut.edu.vn

Abstract—Shielded by high structure surrounding and with short

length of power line, low voltage system is seldom suffered from

direct strokes However, this system is especially threatened by

induced voltage due to nearby strokes The effects of induced

overvoltage may be very harmful to power quality and to low

BIL peculiar to electrical equipment in low voltage system

In this work, the induced overvoltage in a typical low voltage

system in rural areas of Vietnam will be investigated by

EMTP/ATP simulation Rusk model is used to simulate the

current source affecting the low voltage system The influence of

grounding resistance in consumer side and the size of load will be

analyzed The discussion and results may provide useful

information in insulation coordination of low voltage system

Keywords-low voltage system; induced lightning; Rusk’s theory,

ATP/EMTP simulation

I INTRODUCTIONThe limited height of low voltage distribution system

makes it more prone to nearby lightning than direct lightning

Induced lightning causes overvoltage on insulation, which is

usually designed with low BIL, and harms electrical and

electronic devices of such a system In Vietnam, most of

electricity consumers locate in rural areas where distribution

network mainly uses overhead line Furthermore, the distance

between distribution transformer and consumer in those areas

may range from several hundred meters up to kilometers As a

result, low voltage system in the areas is especially threatened

by overvoltage due to induced lightning

Among other theories involved in calculating induced

voltage [1-4], Rusk model [5] is widely used for its easy

handling by analytical formulations [6,7] In this paper,

induced overvoltage in a typical TN low voltage system in rural

areas of Vietnam was simulated in the ATP/EMTP transient

program using Rusk method The effects of grounding

resistance and load size were also analyzed and discussed

II RUSK MODELLightning induced voltage on the transmission line

proposed by Rusk is based on the following assumptions:

• The return stroke current has the shape of step-function

with the maximum value I0, which propagates along

the lightning channel with a constant velocity ν

• This return stroke generates an electric field which is given by:

( ) A(r z t)

t t z r t

z r

where: φ is the scalar potential, ܣԦ is the vector potential, t

is time, r and z are calculated points in cylinder coordinate

• This electric field couples with the transmission line and generates a total induced voltage u(x,t):

dz h

t

t z x z A t x u t x

0

),,()

,(),

where: h is the height of the conductor, uφ (x,t) is the induced voltage in the transmission line due to the scalar potential, Az is the vertical component of the vector potential Those parameters are derived from the transmission line equations:

0),()

,(),(

=

∂

∂++

∂

∂

t t x i L R t x i x t x uφ

(3)

t t h x C t t x u C x t x i

The induced voltage in the line could be considered as the injecting of two current sources, the first one Ie(x,t) is due induced voltage due to scalar potential uφ and the second one Iv(x,t) is due to vector potential A These current sources are defined as [5,7,9]:

x t

t x c vZ t x e

∂

∂

),(

(6)

where Z is the surge impedance of the line, Δx is the line section to be divided for the computation

978-1-4244-6301-5/10/$26.00 @2010 IEEE

return stroke current ν and the charge distribution q0 of the

lightning channel, after some mathematical manipulations

equation 5 and 6 became:

x r z vt r z vt c Z

+

 +

−

2 2

1 2

2

1 0

4

0 )

,

(

γ γ

+

 +

−

−

2 2

1 2

2

1 4

)

,

γ γ

π ν μ

r z vt r z vt

I z

Where r is the distance between the stricken point and the

conductor, 0 and μ0 are dielectric constant and magnetic

constant of the air, ߛ ൌඥଵିሺ௩Ȁ௖ሻଵ మ with c is the speed of light

The injected current sources are connected to the line as shown

in figure 1 [7]

Figure 1 Injected current sources in the EMTP simulation for 1 of

two conductors (phase or neutral) [7]

III SIMULATION

A System configuration

A typical section of 0.4kV distribution line in rural area of

Vietnam as shown in figure 2 was chosen to investigate The

system consists of 3 phase conductors and 1 neutral conductor

A distribution transformer delta-grounded wye 22/0.4kV was used The neutral conductor is commonly grounded with the neutral point of distribution transformer through a resistance of 2 In this TN system, the load is typically grounded through a resistance of 50 (figure 3)

B Modeling method

• Distribution line: A length of 700m of single phase of the line was simulated In order to simulate the maximum induced overvoltage across the load (between phase and neutral), the latter is powered by phase A and N (figure 3) Coupling effect from other phase conductor was neglected for the sake of simplicity The line is assumed to be lossless for the worst case Lighting flashes to a point on the ground in the vicinity of phase A The line was divided into 10 sections of 70m

• Load: An inductance was used to model the induced-lightning response of the load, as recommended in [8] for TN configuration The value of this representative inductance depends on the load size, which varies from 2μH to 10μH

• Distribution transformer: As the neutral of low voltage winding of the transformer is directly grounded, the transformer is represented by a small inductance which is empirically determined

by [8]:

ܮ ൌ ʹͷǤͻߤܪ ൈ ൬ܷܵൈͷͲܸ݇ܣʹ͵ͷܸ ൰ି଴Ǥହସଶwhere S and U are the rated power and rated voltage of the distribution transformer In this case, S=160kVA, U=380V and L=17.89μH

N P

Flashing point x 100m

Figure 2 Configuration of a typical low voltage system

Figure 3 A phase in low voltage system to be simulated and the position of the flashing point (right: distribution transformer, left:

load)

Ie and Iv as described in section II.

C Simulation result

1) Voltage profile along the line

Figure 4 Induced voltage in phase and neutral conductors at

transformer side and load side

A very high value of induced voltage (~50kV) was

observed at the position of the load, on the phase conductor and

neutral conductor Low grounding impedance of the

transformer substantially decreased the induced voltage on

phase and neutral conductor Although the transformer locates

closer to the flashing point than the load, the induced voltage

on phase and neutral conductors at transformer position

(TRANSP and TRANSN) is much lower than that at the load

position

Figure 5 Induced voltage across the load and the transformer

However, the low grounding impedance had a serious

influence on the induced voltage across the transformer With

I0=10kA, the peak induced voltage across the transformer

(v:TRANSP-TRANSN) is about 35kV, nearly three times

higher than that across the load (v:LOADP-LOADN) as shown

in figure 5

2) Influence of grounding resistance of the load

The simulation was performed on the load of small size

(L=10μH) with three values of grounding resistance of the

load: R=20, R=40 and R=60, which correspond to

different value of soil resistivity of rural areas

Figure 6 Dependency of induced voltage across the load on

grounding resistance of the load

Figure 7 Dependency of induced voltage across the transformer on

grounding resistance of the load

It was observed that the more the value of grounding impedance is, the less the induced voltage stresses across the load (figure 6) The voltage behavior is similar to the case of the transformer, as the low grounding impedance decreased the induced voltage on neutral phase but gave rise to the potential difference between phase and neutral conductors However, the grounding resistance of the load did not have any influence on the induced voltage across the transformer (figure 7)

3) Influence of the size of the load

In order to investigate the influence of the load on the induced voltage, the load size was changed from small size (L=10μH) to large size (2μH) according to [8] The computation was performed with grounding resistance of the load R=40  and plotted in figure 8

Figure 8 Dependency of induced voltage across the load on the size

of load

Figure 9 Dependency of induced voltage across the transformer on

the size of load

The same voltage behavior across the transformer was

observed as the previous case in which different grounding

resistances of load were accounted (figure 8) The induced

voltage across the transformer was independent of the load

size, and remained at very high value due to low grounding

impedance of the transformer Larger size of load decreased

induced voltage on it as increasing the grounding resistance of

the load (figure 9) The effect of load size, which is

comparable to that of grounding resistance of the load,

suggested that co-ordination of those two factors may provide

an optimal protection against hazard and induced lightning

IV CONCLUSION

Lighting induced voltage results in harmful effects low

voltage system, on both sides of distribution transformer and of

load The induced voltage on phase conductor is estimated to

be about 40kV for a typical value of lightning current 10kA,

this voltage value is well above the BIL of any equipment in

low voltage system The induced voltage on neutral conductor

greatly depends on the value of grounding resistance

Therefore, induced voltage across the equipment in question

greatly depends on the value of resistance that it is grounded

through As the regulation of electric utilities, the grounding

impedance of distribution transformer is maintained at low

value (typically from 2 to 5) for the purpose of correct

operation, this value increases the harmful effect of induced

resistance Co-ordination between these parameters may fulfill both requirements of safety and of protection against induced lightning

ACKNOWLEDGMENT

This article was funded in part by a grant from the Vietnam Education Foundation (VEF) The opinions, findings, and conclusions stated herein are those of the authors and do not necessarily reflect those of VEF

REFERENCES [1] C Taylor, R Satterwhite and C Jr Harrison, ‘‘The response of a terminated two-wire transmission line excited by a nonuniform electromagnetic field’’, IEEE Trans on Antennas and Propagation, vol.13, no.6, November 1965

[2] A Agrawal, A, H Price and S Gurbaxani, ‘‘Transient response of multiconductor transmission lines excited by a nonuniform electromagnetic field’’, Antennas and Propagation Society International Symposium, vol.18, June 1980

[3] F Rachidi, ‘‘Formulation of the field-to-transmission line coupling equations in terms of magnetic excitation field’’, IEEE Transaction of Electromagnetic Compatibility, Vol 35.,no 3, August 1993

[4] P Chowduri and E.T.B Gross, ‘‘Voltage surges induced on overhead lines by lightning strokes’’, Proc IEE, Vol 114, no.12, December 1967

[5] S Rusk, Induced lightning overvoltages on power transmission lines with special reference to the over-voltage protection of low voltage networks, Royal Institute of Technology, PhD Thesis, Stockhom 1957

[6] H K Hoidalen, ‘‘Calculation of lightning overvoltages using MODELS’’, International Conference on Power Systems Transients (IPST), Budapest, June 20-24, 1999

[7] A E A Araujo, J O S Paulino, J P Silva, H W Dommel,

« Calculation of lightning induced voltages with Rusk’s method in EMTP Part I: Comparison with measurements and Agrawal’s coupling model’’, Electrical Power System Research , vol 60, 2001

[8] H K Hoidalen, ‘‘Lightning induced voltages systems and its dependency on overhead line termination’’, Internationl Conference on Lightning Protection (ICLP), Birmingham, September 14-18, 1998

[9] J G Anderson and T A Short, ‘‘Algorithms for calculation of lightning induced voltages on distribution lines’’, IEEE Trans on Power Delivery, vol 8, no 3, July 1993

Electrical Field Behavior of Transmission Line Insulators in Polluted Area

T Pham Hong * and Tran Van Top Department of Power System, Faculty of Electrical Engineering, Hanoi University of Technology (HUT)

1, Dai Co Viet Street, Hanoi, Vietnam

* E-mail : thinhph-htd@mail.hut.edu.vn

Abstract: Finite element method (FEM) was used to

study the electric field distribution along creepage path

of a cap and pin porcelain insulator string of

transmission line The effect of pollution layer

conductivity and dry band width is considered in order

to investigate electric field behaviors of insulator using

in polluted areas.

INTRODUCTION

Although the use of silicone rubber composite

insulators has been increased significantly in recent

years, porcelain and glass insulators are still

manufactured and remain predominant in distribution

and transmission lines of Vietnam Except for

hydrophilic property, porcelain and glass insulators are

widely used because they offer many advantages: low

cost, flexible maintenance and high strength When

energized in polluted area such as coal industry zone

and coastal area, the insulators are easily contaminated

[1], dry band will be formed and leading to flashover

[2]

Pollution 26%

Other 49%

Figure 1: Service interruption of some 110 kV transmission

lines in Vietnam due to pollution flashover in 2004: in Quang

Ninh province (left side), in 12 coastal provinces of the

central Vietnam (right side)

Exploring 7 million tones of coal per year, Quang Ninh-

a northern province of Vietnam faces not only

environment problem arising from coal dust but also the

outages of power distribution and transmission lines

Field data has recorded about 20% outages in 110kV

transmission line of Quang Ninh province is due to

flashover [3] Power Company 3 which manages

transmission network of 12 coastal provinces in the

central Vietnam has reported that 33% of service

interruption is also due to flashover [4] (Figure 1)

The paper presents the results of finite element (FE)

calculation of the electrical field distribution along a

string of cap and pin porcelain insulator using in current

transmission lines in Vietnam Pollution level and dry

band width are varied in order to investigate their

effects on field distribution along the creepage path

The results will be a good indication for designing insulator, especially for polluted areas

INSULATOR TO BE MODELED

Porcelain insulator type ɉɎ-425 in this study is widely used 35kV distribution networks, 110kV and 220kV transmission lines in Vietnam The number of unit per insulator string depends on rating voltages and operating environment, e.g: the number of units per string of 35kV distribution network is varied from 2 to

4 while 7 or more are usually used in 110kV transmission networks Figure 2 shows the detailed geometry dimension of a porcelain insulator The cap and pin are made of steel and they are embedded in mortar layers in order to fix with porcelain shell The shell is made porcelain with a relative permittivity of 6 and a conductivity of 2.10-13S.m-1 The creepage length

of porcelain shell is 280mm Commercially available FE software ANSYS“ are used for calculation The modeling is carried out with a typical insulator string used in a typical 110kV transmission line, which consists of 7 insulators without corona ring The unit is numbered from the line to ground with unit 1 corresponding to that close to live-line-end and the unit

7 is close to ground-fitting-end Static analysis are performed at steady state condition at f=50Hz An AC voltage of 110kV is applied to the whole string

Figure 2: Geometry dimension of a typical transmission

line insulator used in the calculation The conductivity of pollution layers are selected in accordance with IEC 60815 In this study, the calculation is performed with pollution conductivity varying from ı=8ȝS to ı=20ȝS which correspond to medium and heavy pollution levels Morever, the effect

of mortar layers is neglected in the model

RESULTS AND DISCUSSION Potential and electric field distribution in clean

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insulator string

Figure 3: Equipotential distribution along a clean string

Figure 4: Electric field distribution along a clean string

The calculation is firstly performed with a clean

insulator string Figure 3 shows the equipotential lines

for the clean string Potential is distributed along the

string in accordance with the capacitance of each unit

and the stray capacitance to line and to ground The

potential distribution is found to be maximum in the

unit 1 (near the live-end-fitting) with 18% of total

voltage applied on the string, whereas 13.5% is found in

the unit 7 (near ground-end-fitting) and minimum

potential distribution is found in the unit 5 (12%) These

behaviours are well correlated with the results of

practical field measurements on a test string using a

horizontal sphere gap in the Laboratory for High

Voltage Engineering and Electrotechnical Material at

Department of Power System.

As shown in the figure 4, electric field intensifies as far

from the ground-fitting-end and reaches the highest

Figure 5: Electric field stress along creepage path of unit 1 in

of each unit follows the same trend, high values appear

in the triple junction region (air-cap-porcelain), live-end-fitting, and near the sections with small radius

of curvature In the unit 1, the stress in the triple junction and live-line-fitting take the same value of 2.2kV/mm However, the highest stress of the string is reached in the air-cap-porcelain region of the unit 2 (Figure 6) It is observed that the stress in this region is slitghtly higher that of unit 1 with 2.5kV/mm, but this value is still beyond that needed for corona discharge

−531−

Influence of pollution layer

Figure 7: Modeling of electric field in presence of pollution

layers next to metal cap

A pollution layer with 14.5PS and 25% of upper surface

length (27.5mm) is deposited on each unit (Figure 7) In

this case, the stress distribution is predomenantly

controlled by the conductivity of pollution layer instead

of capacitance distribution like in clean surface case [6]

In this case the electrical stress are “put” from triple

junction region to the pollution layer ends in every unit,

the highest stress is “transferred” from the triple

junction of unit 2 to unit 1 (Figure 8) and remains the

same value as in the clean case (|2.5kV/mm) However

the region in which the highest stress appears is

displayed toward the pollution layer end and this value

is still well beyond that can lead to partial discharge in

the air

Figure 8: Electric field stress along creepage path of unit 1 in

presence of pollution layer of 14.5PS

For other units, the presence of pollution layer reduced

the maximum stress near the cap and tends to linearize

the field distribution along the creepage path As an

exemple, the magnitude of electric field strength along

creepage path in the unit 2 is depicted in the figure 9 It

is observed that the electric field is more evenly

distributed along the creepage path in comparison with

the clean case (Figure 6) The highest stress in the unit 2

is reduced from 2.5kV/mm to 1.2kV/mm and is

transferred from air-cap-porcelain to the pollution layer

end and pin-fitting regions From the point of view of field distribution, the presence of pollution layer plays a positive role in linearizing the field along the insulator string

Figure 9: Electric field strength magnitude along creepage path of unit 2 in presence of pollution layer of 14.5PS

Influence of pollution layer conductivity

Because the presence of pollution layer had negligeble effect on field distribution in others unit, only the unit 1

is analysed to study the influence of pollution layer conductivity Using the same geometry of pollution layer as the previous case (with 25% of upper surface length), the influence of pollution conductivity is studied by performing calculation with different pollution levels: ı=8ȝS, ı=14.5 ȝS and ı=20ȝS The electric field magnitudes at air-cap-porcelain (cap), pollution layer end (PL end) and live-end-fitting (pin) are plotted in the figure 10 It is clear that the stress in cap region decreases with the conductivity, while the electric field in pollution layer ends and live-end-fitting regions slightly increases versus the conductivity

However, the stress in these regions is still inferior to 3kV/mm As a result, the pollution flashover could not occur even in presence of heavy pollution level

0 750 1500 2250 3000

conductivity (ȝS)

cap PL end pin

Figure 10: Influence of pollution layer conductivity on

electric field stress

Influence of dry band width

−532−

As mentionned in introduction paragraph, the flashover

in polluted insulator are initiated by the formation of

one (or more) dry band In order to study the influence

of dry band on electric field distribution, the calculation

has been performed with a dry band inside pollution

layer of 8PS A pollution layer with 50% of upper

surface length is deposited on each unit, a dry band

width ranging from 0 to 1mm is created in the middle of

the pollution layer The stress distribution along

creepage path of unit 1 is depicted in the figure 11 with

a dry band of 0.25mm It is clear that the maximum

strength of 6.5kV/mm appears near the dry band and

increases threefold in comparison with clean case The

highest stress value decreases with the dry band length,

but the stress at dry band are always dominant in

comparison with other regions such as pin-fitting or

pollution layer ends (Figure 12) This indicates that

when a dry band forms inside the pollution layer, partial

discharge could begin from these points and leads to

flashover

Figure 11: Electric field stress along creepage path of unit 1

in presence of a dry band of 0.25mm in the middle of

cap bandgap PL end pin

Figure 12: Electric field stress in different position of

creepage path versus dry band length

Some pictures taken during performing measurement in

the laboratory are shown in the figure 13 Increasing the

applied voltage will lead to flashover which initiates

from the dry band, live-end-fitting and the small

curvature regions These behaviors are well correlated with the results predicted by the simulation in the previous paragraph

Figure 13: Flashover process from live-end-fitting and small curvature of a polluted insulator (from left to right)

CONCLUSION

The presence of a pollution layer on upper surface of insulator strongly modified the field distribution along creepage path With a homogenous pollution layer deposited on each unit, the live-fitting-end region of unit 1 submits the highest stress, but the magnitude of electric field is similar to that in clean case In presence

of a dry band in the middle of pollution layer, the stress reaches maximum value in the dry band and exceeds the breakdown strength of air Flashover can occur from this points and were observed by field measurements

ACKNOWLEDGMENT

Center for Development and Application of Software for Industry (DASI) at HUT is gratefully acknowledged for its help during this study

REFERENCES

[1] J S T Looms, Insulators for high voltages, Peter

Peregrinus Ltd, 1988 [2] David D Jolly, "Contamination Flashover Theory and Insulator Design", Journal of The Franklin Institute,Vol 294, No.6, December 1972

[3] Do Khanh Ninh, “Influence of polluted environment on the performance of glass insulators using in 110kV network of Quang Ninh province”, Master thesis of Hanoi University of Technology, 2006

[4] Le Thanh Giang and Nguyen Quoc Viet, “Modeling of field distribution along a set of insulators”, Conference on student research, Hanoi University of Technology, 2007 [5] Vosloo W L and Holtzhausen J P., “The electric field of polluted insulators”, Africon

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