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1768 S. Zhang et al.: Inner Insulation Structure Optimization of UHV RIP Oil-SF6Bushing Using Electro-thermal Simulation

DOI 10.1109/TDEI.2014.004211

Inner Insulation Structure Optimization of UHV RIPOil-SF6Bushing Using Electro-thermal Simulation

and Advanced Equal Margin Design Method

Shiling Zhang, Zongren Peng andPeng LiuState Key Lab. of Electrical Insulation and Power Equipment

Xian Jiaotong University

Xian, 710049, China

ABSTRACTThe iso-margin method based on the classical analytical equations is widely used in the

development of extra high voltage (EHV) bushing. Therefore, its effectiveness and

feasibility have been fully validated. However, with the theoretical formula, it is

difficult to evaluate the influence of stray capacitance and temperature on the internal

electric stress of the ultra high voltage (UHV) bushing. Its necessary to further

improve the traditional iso-margin method. Firstly, the basic principle of iso-margin

method has been briefly described. After that, the E-field distribution of condenser

bushing was investigated on the Finite Element Method (FEM) computing platform.

Then, the mathematical model of the advanced equal margin design method was

established. Furthermore, this paper presents the program flow of optimization which

combines the simulation of electro-thermal coupling process and the particle swarm

optimization (PSO) algorithm. The proposed methodology was applied to the design of

the prototype of the UHV resin impregnated paper (RIP) oil-SF6 bushing, which

realizes the uniform axial E-field distribution of the bushing condenser. Meanwhile, the

partial discharge margin between adjacent foils can be equal. Moreover, the hot-spot

temperature is lower than the operation limiting temperature of the RIP material. The

bushing condenser was fabricated according to the optimal structure design. The

prototype of bushing has passed through all the type tests. In this paper, the FEM

electro-thermal coupling simulation and the advanced equal margin design method

were applied to the inner insulation structure optimization of the UHV RIP oil-SF6bushing. Meanwhile, the proposed methodology provides some theoretical guidelines

for the future research and development of other types of bushing on the UHV level.

Index Terms - UHV RIP oil-SF6bushing, equal margin design method, finite element

method (FEM), electro-thermal coupling simulation.

1INTRODUCTION

HIGHvoltage condenser bushing has been widely used in

the UHV power transmission system. In this kind of power

equipment, a capacitor core is installed between the center

conductor and the outer flange as its main insulation.

Meanwhile, the multi-layer metal foils are embedded in the

crepe paper layers to make the distribution of electric stressmuch more even. Therefore, the reasonable design of the

bushing core could lead to much longer lifespan and higherreliability. In general, the traditional method, for instance the

equal step and equal capacitance design method, is

fundamental. Yet the overall electrical performance needs tobe improved due to the uneven axial E-filed at the foil edges.

In the 1980s, the equal thickness but unequal step design

method was proposed to minimize the size of the condenser.

In the 1990s, the iso-margin method was used. It effectively

enhances the partial discharge inception voltage (PDIV)compared with the traditional methods. In addition, it has

been proved theoretically and verified by experiments [1-4].

This method has been developed into a software package

which has significantly improved the bushing products.In recent years, a new type of insulating material called the

resin impregnated paper (RIP) with the superior thermal and

electrical performance is prevalent. In the RIP structure, the

oil phase is completely eliminated. The RIP technology

preserves all the essential advantages of OIP and is free fromthe fire hazard and unsatisfactory mechanical property. In

addition, the AC UHV transmission system is developing

rapidly in China, which raises the higher requirements for thedesign and manufacture of the UHV RIP bushings.

There are some literatures concerning the design method of

bushing, specifically about the computation of E-field andManuscript received on 22 July 2013, in final form 23 March 2014,

accepted 18 April 2014.

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21, No. 4; August 2014 1769

temperature distribution in the condenser of bushing. The

paper of Jyothi [5] proposed a mathematic method to solvethe nonlinear differential equation with the thermal and

electric boundary conditions. Paper of Monga [6] applied the

FEM electric field computation to optimize the internal fieldshaper of gas fileld high voltage composite bushing. Paper of

Mohammad Hesamzadeh [7] used an advanced optimal

approach based on the improved genetic algorithm, which

obtains the well-distributed electric stress.In the AC UHV case, the physical dimension of bushing

condenser is quite large. On the other hand, the thermal flow

in the condenser is intimately linked to two electro-thermal

heat sources, namely, eddy heat due to the conductor currentand dielectric loss due to the applied voltage respectively. The

latter one is quite significant on the UHV level. It is therefore

imperative to improve the design method of bushing in thefollowing aspects: 1) to calculate the internal radial and axial

E-field stress as accurately as possible, 2) to incorporate the

nonlinearity of resistivity with the temperature into the

thermal and electric field distribution [8]. Concretely, the

radial and axial electric stress is affected by the metal foils

lengths and their relative positions. But the stray capacitancedue to the existence of metal parts shall also affect the internal

E-field distribution to some extent. Additionally, under the

rated operating condition, the temperature gradient isestablished across the bushing core, which, more or less,

modifies the E-field distribution. However, the traditional

design method cannot take these factors into consideration [9-12].

In engineering practice, to get an accurate analytical

solution of a complex problem is difficult. Fortunately,

numerical methods such as the FEM can be utilized to the

design of bushing condenser. Firstly, the classic analyticalformulas were given to calculate the electric stress and

complete the rough design of the foils lengths and their

relative positions. Secondly, the FEM was used to simulatethe distribution of thermal and electrical field with a full-scale

model. The interactive program flow of optimization process

which combines the electro-thermal coupling simulation andthe particle swarm optimization (PSO) algorithm had been

dealt with in detail. Finally, the above advanced iso-margin

design method was used in the optimization design of the

UHV RIP oil-SF6 bushings inner main insulation. The

prototype had passed through all the type tests.

2 THE TRADITIONAL ISO-MARGIN DESIGNMETHOD

The photo of a bushing condenser and internal parallelfoils arrangement with the relevant parameters are shown inFigure 1. n isthe number of foils inside the bushing condenser.

Considering the thin solid insulation in the parallel-plane

geometry of bushing condenser, the partial discharge

inception voltage (PDIV) of the k th insulating layer kU (k=1n) is shown

0.5k kU Wd (1)

where Wis the harmful partial discharge coefficient, and dkis

the thickness of k th solid insulating layer between adjacentfoils. Equation (1) is an empirical formula obtained by fitting

the experimental result. It has been used in the optimal design

of HV transformer bushing [13].With the iso-capacitance design method, PDIV of the entire

bushing core Uis expressed by0.5

minU nWd (2)

where dmin is the minimum value of the insulations

thickness. Theoretically, ifkU denotes the actual voltage

drop of the k th insulating layer, it is feasible to equate the

partial discharge margin (PDM) of the insulating layer. The

Figure 1. Thephoto of a bushing condenser and the foils arrangement. r0rn and l0ln are the radius and length of the innermost and outermost foils. dkis the thickness of kth solid insulating layer between adjacent foils.

1k and

2k are the upper and lower step lengths of the kth foil.

PDM is expressed by /k kU U . Then the overall PDIV of

the bushing core'

Ucan be rewritten as follows:'

0.5 0.5

1 1

( )n n

k k

k k

U U W d Wn d

(3)

'

Uis the PDIV obtained with the iso-margin design method.

The average insulation thickness d which is described by

equation (4) is larger than dmin.

1

1 n

k

k

d dn

(4)

The careful comparison between equations (2) and (3)

indicates that'

U U . Therefore, the iso-margin method can

effectively improve the PDIV. According to the design

conception of iso-margin method, the ratio of partialdischarge inception voltage to actual voltage drop on each

insulating layer can be equated to a constant A (A>1). In

equation (5), kE is the partial discharge inception electric

stress of the kth insulating layer. It is expressed by /k kU d .

rkE is the actual radial electric stress. With the reference to

equation (1), then0.5/ ( )k kk k k

k rk rk rk

U d W d U EA

U E E E

(5)

The thickness of the kth insulation layer can be obtained

2( )rkkAE

dW

(6)

With the reference to Figure 1, equation (7) is reduced to

2

0

1 1

( )n n

rkn k

k k

AEr r d

W

(7)

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Solving equation (7), the constantA now becomes

2 0.5

1

0.5

0

( )

( )

n

rk

k

n

W E

Ar r

(8)

Equations (5)-(8) can form the iterative format, which isgiven by equation (9). The superscript represents the

calculated values in the first iteration.(1) (1)

2 (1) 2 2 2 0.5

0 0

0 0

(1) (1) (1) (1) 01 (1) (1)

0

( ) ( (ln / ln )( ))

( ) 1

2 ln( )

rk k n

k k n

nk k k rk

n k k

A E r r

d l l l l W r r

l lUr r d E

r r r l

(9)

If the insulation layer between adjacent foils is positionedaccording to the bushings geometry, the insulation layer

voltagekU is obtained

(1)(1) (1) (1)

(1)(1) 1

(1) (1)

1

2 ln krk k k k

k

k k

rE l r

rU

l l

(10)

The upper and lower step lengths are determined by

equation (11).1a kE and 2a kE are the upper and lower axial

electric stress of the kth foil.(1)

(1)

1

1

kk

a k

U

E

(1)(1)

2

2

kk

a k

U

E

(11)

After a few steps of iterations, the equal PDM of each

insulating layer between adjacent foils can be obtained

numerically. Meanwhile, the radius and length of each foil

can also be worked out. Based on the iso-margin method, the

large and small foil design method is used, which inserts thesmall foils into the large ones to shield the high electric stress

zone near the foil edge, as shown in Figure 2.

Figure 2. Thestructure of the large and small foils arrangement. InsertXJ1small foils between DJ1large foils near the inner conductor, insert XJ3smallfoils between DJ3 large foils near the flange, and insert XJ2 small foils

betweenDJ2large foils in the middle of the bushing core.

3 THE ELECTRO-THERMAL SIMULATIONMETHOD

With regard to the RIP core, the axial heat flow is

negligible due to the large ratio of its length to radius.

Therefore, in the steady-state condition, the heat-flow can be

treated as a radial-outward flow, as shown in Figure 3.

Figure 3. The steady-state heat-flow pattern in the bushing core. Tmax is themaximum temperature at the interface between the conductor and the

insulation, Ta is the ambient temperature.

The heat flow continuity formula in the steady state can be

written as2

1 ( ) ( )0

d dT r d r r

r dr dr dr

(12)

where is the thermal conductivity of the insulation. ( )T r ,

( )r is the axial temperature and the potential value at the radius

position rin the insulation respectively. is the AC conductivity

of the insulation. The E-field stressE(r) may be written as:

( )( )

d rE r

dr

(13)

can be expressed by the relative permittivity r and the

loss factortan .

02 tanrf (14)

where f is the operating frequency,0 is the permittivity of

vacuum. For the bushing condenser, the axial electric stress

distributionE(r)is expressed by

0

2 22 00

0 0

0

( )( )

2ln( ) ln( )

ln( )

n

n n

n

U l lE r

r l l r r l

rr r

r

(15)

where U is the applied voltage. The boundary conditions of

the above model have been assumed as

1

0 1

( )

2

( )

r r

ar r

PdT r

dr r

T r T

(16)

where P1 is the heat generating in the conductor per unitlength. r is a large distance from the external boundary. By

solving equations (12)-(16), the temperature distribution T(r)

is written as the sum of ( )T r and ( )T r . The detailed

expressions of ( )T r and ( )T r are given in Appendix A.

( ) ( ) ( )T r T r T r (17)

( )T r is the contribution of the eddy heat due to the current,

and ( )T r is the contribution of the dielectric loss due to the

applied voltage. The closed-form solution only exists on

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condition that and are independent on the temperature.

However, take the RIP material used in the UHV oil-SF6

bushing condenser as an example, and , as functions of

the temperature, have been obtained by the experimental

measurements. The AC conductivity varies like U shape

in the temperature range of 0-110 , and rises markedly in

the range of 110-140shown in Figure 4.

Figure 4. The experimental and fitting curves of AC conductivity which is afunction of the temperature for the RIP material used in the UHV oil-SF6

bushing. And subplot (a) can be obtained when subplot (b) is enlarged in thetemperature range of 0-110.

The dual-exponential function was used to fit the nonlinear

relationship between the conductivity and the temperature, then1 2

1 2

T Te e (18)

and1 =6.624e-13 S/cm, 1 =-0.01015, 2 =3.3e-16 S/cm, 2 =0.06816

for the RIP material of the UHV oil-SF6bushing.

In Figure 5, the experimental results indicate that both thethermal conductivity and the heat capacity of the RIP

materials used in the UHV oil-SF6bushing are functions of

the temperature. Therefore, a kind of new electro-thermal

coupling method based on the FEM is proposed to calculate

the temperature distribution of bushing. The method takes thenonlinearity of the RIP material into account.

Figure 5. The experimental results show that the thermal conductivity andheat capacity of the RIP materials used in the UHV oil-SF6 bushing are

functions of the temperature.

Two problems in the computing technology need to be

addressed: 1) one is that how to combine the two separateprocesses, the electric stress and temperature calculation; 2) the

other is that how to incorporate the nonlinearity of the RIP

material into the calculation. For the first problem, the physical

settings used for the electric stress and temperature calculation

should be respectively defined. In each physical setting, the

separate solid model should be established. After that, the

model should be meshed into small elements with the material

attribution of ( )T and ( )T . Afterwards, the boundary

conditions, including the voltage in the setting of electric

analysis and the temperature in the setting of thermal analysis,

are applied to the meshing model, as shown in Figure 6.

Figure 6. The principle of the electro-thermal coupling method based on theFEM. The temperature and dielectric loss can interact in the settings of

electric and thermal analysis.

For the second problem, the new solution has been proposed

using ANSYS APDL codes. In the traditional finite element

analysis, the study object shall be the volume with its material

properties considered consistent. From the above analysis, it can

be concluded that the temperature gradient could be established

across the bushing condenser which leads to the gradient

distribution of the material properties. In order to show the

nonlinearity, the study object should be modified to the tiny

elements. The reason can be interpreted as follows: the volume of

a tiny element is much smaller than that of the overall bushing

condenser. Therefore, the material properties within the local

volume can be treated identically. The high calculation accuracy

can be achieved if a large number of elements are generated

inside the bushing condenser. The solution of the E-field and

temperature of every element can be obtained by applying the

iterative calculation, as shown in Figure 7.

Figure 7. The iterative calculation of the electro-thermal coupling methodbased on the FEM.

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Initially, the temperature distribution T(r) across the

bushing condenser is roughly determined by (A1)-(A3).

Then, the material properties (e.g., ( )T and ( )T ) of each

element could be modified according to the experimental

results. Firstly, the electric stress E(r) and the dielectric

loss of every element can be obtained by the harmonic

field analysis in the setting of electric calculation.

Secondly, the thermal boundary conditions shown in

equation (16) and the dielectric loss are used in the setting

of thermal calculation to recalculate the temperature

distribution. Then, the material attributions of each element

are modified again. If the inequalities in equation (19) are

satisfied, the iterative process is assumed to have

converged, and then output the E-field and temperature

distribution.

1

1

u uT T

1

2

u uE E (19)

where u is the iteration number, and 1 , 2 are specifiedsmall numbers, Tand Eare temperature and electric stress

of every element. If the inequalities in equation (19) are

not satisfied, set the new interaction as u=u+1 and repeat

the calculation.

4 THE ADVANCED EQUAL MARGINDESIGN METHOD

The traditional iso-margin method should be

transformed into an optimum problem with the relevantrestriction. It has been validated that the temperature has

apparent influences on the radial stress Er, the axial stress

Eaand the partial discharge margin PDM. Up till now, thetemperature distribution has been obtained by the electro-thermal coupling method. Furthermore, this methodology

should be combined with the traditional equal margin

method. In the first place, the fitness function is given inAppendix B. Its effectiveness has been validated in [7].

The optimization parameters of the bushing include the

foil lengths lk (k=1 to n-1), the foil radiuses rk(k=1 to n-1)

and the foil thicknesses dk (k=1-n). The full-scale FEMmodel of the bushing can be established with the

combination of randomly selected structure parameters.

And then, based on the electro-thermal coupling method,

the electric and temperature field calculation can be carried

out. Ultimately, the values of Er, Ea and PDM of thebushing condenser are used to calculate the value of the

objective function (B1). If the value reaches the minimum

point, it means that the current combination of structureparameters is the optimal solution, which ensures that Er,

Ea and PDM meet the pre-select control value, and

distribute uniformly. Moreover, the PSO intelligent

optimization algorithm is used to solve the optimumproblem [14]. In this paper, the PSO algorithm is

implemented in the ANSYS computing environment. The

flow chart of the advanced equal margin method

combining the FEM and PSO is shown in Figure 8.

Figure 8. Theflow chart of the advanced equal margin design method.

In the first place, input the bushing condensers material

parameters, testing voltage, rated operating current and the

size of the condenser. The initial values of the foil length,the foil radius and the insulation thickness of adjacent foils

are obtained with the traditional iso-margin method.

Secondly, in the ANSYS computing environment, the full-

scale FEM model should be created to calculate the electric

stress and temperature distribution of the bushing

condenser with the electro-thermal coupling method.

Thirdly, extract the radial stress Er, the axial stress Eaand

the partial discharge margin PDMto calculate the value of

the fitness function. The PSO operation program will

continue until the best solution (the minimum of the fitness

function) is found. Finally, the flow chart of the advanced

equal margin method returns the optimum combination of

parameters.

5 APPLICATION OF THE ADVANCEDEQUAL MARGIN DESIGN METHOD

Implement the proposed advanced equal margin method tothe optimization of the inner insulation structure in the UHV

RIP oil-SF6bushing. Its basic technical specifications chosen

for the case design are as follows:

The highest voltage for the equipment (Um) 635 kV

The rated power frequency withstand voltage (Un) 1200 kV

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The rated frequency 50 Hz

The rated current 3150 A

The designed outer contour of the condenser is shown in

Figure 9, and the initial parameters of the outer contour are

listed in Table 1.

Figure 9. The outlet of the UHV RIP oil-SF6bushing.

Table 1.Theparameters of the UHV RIP oil-SF6bushing condenser.

Parameters l0 (mm) ln (mm) r0 (mm) rn (mm) n

Value 4405 1045 78 320 95

The actual testing and operating environment of the UHV

oil-SF6bushing is shown in Figure 10, which includes the GIS

bus-bar, the transformer oil tank, the gas-side shield electrode

and the intermediate flange.

Figure 10. The testing environment of the UHV oil-SF6bushing.

The previous analysis indicates these metal attachmentshave certain impact on the inner E-field distribution of the

bushing condenser. Therefore, the full-scale model has been

established for the E-field and temperature field calculation,

shown in Figure 11, which is consistent with the actual testing

and operating environment.

In the calculation of the temperature field, the joule heating

of the insulation medium and the eddy heating of the inner

conductor should be considered together. The electric stress

and temperature distribution of the UHV oil-SF6 bushing

under the full-scale model is shown in Figure 12. This figure

Figure 11. The full-scale calculation model of the UHV oil-SF6bushing.

indicates that the region with the highest electric stress is

mainly inside the bushing condenser. And the hottest-spot

temperature is about 95 , which locates on the interface of

the inner conductor and the bushing condenser. Moreover,

the hottest-spot temperature is limited below the control

value under the full load. However, the allowed limiting

temperature of the RIP material will be determined by thelong-term ageing of the actual RIP core in the future work.

Based on the test results, the optimization of the bushing

condenser and the inner conductor structure should befurther carried out. At present, the inner insulation structure

of the UHV oil-SF6 bushing has been optimized with the

proposed advanced iso-margin design method. The results of

foil length lk, foil width dk and foil step2k

(k=1-n) are

shown as Table 2.

Figure 12. The E-field and temperature distribution under the full-scalemodel of the UHV oil-SF6bushing.

Table 2.Theoptimum design of the inner insulation structure.

k2k

(mm) lk(mm) dk(mm) rk(mm) Erk(kV/mm) Eak(kV/mm) PDM

1 20 4365 1.6 79.6 4.32 0.73 1.40

10 20 4023 2.0 95.6 3.81 0.73 1.41

20 22 3601 2.5 118.1 3.34 0.72 1.42

30 24 3135 3.0 146.0 3.00 0.71 1.43

40 25 2637 3.4 178.7 2.81 0.72 1.42

50 25 2133 3.4 212.7 2.83 0.73 1.41

60 23 1651 2.8 243.5 3.15 0.72 1.42

70 19 1229 1.9 266.6 3.85 0.72 1.43

75 18 1045 1.6 275.1 4.33 0.73 1.40

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a) The radial stress

b) The axial stress

c) The partial discharge margin

Figure 13. The radial stress, axial stress and partial discharge margin oneach capacitive layer for conventional and optimum design.

Figure 13 shows that the proposed method is promising

with consideration to the essential technological limitations.

The radius of the bushing condenser has decreased from 320

to 275 mm, and the total number of large and small foils also

has decreased from 95 to 75, so the more compact condenserhas been obtained. The U shape curve of the radial stress

cannot be changed since it is determined by the intrinsic

structure of the condenser bushing. In the conventional design,the axial stress of Band Bis not equal. But in the optimum

design, the axial stress of AandAis equal. The radial stress

is slightly bigger after the optimization due to the more

compact design of bushing condenser. However, the radialstress at any site of foil is still smaller than the control value

which is set to be 4.5 kV/mm. The axial stress Ea and the

partial discharge margin PDM of the optimally designed

bushing are more uniformly distributed. The maximum value

of the axial stress has a drop from 0.85 to 0.75 kV/mm, andthe minimum value of the partial discharge margin has a rise

from 1.2 to 1.4. As a result, the partial discharge inception

voltage (PDIV) of the entire bushing condenser is upgradedfrom 762 to 889 kV. Based on the optimized structure of the

inner insulation in the UHV RIP oil-SF6 bushing, the

prototype has been manufactured in Figure 14.

Figure 14. Theprototype of the UHV RIP oil-SF6bushing.

6 THE ELECTRICAL AND THERMALEXPERIMENTAL VERIFICATION

The new design method and the prototype of a new

production should be verified by the experiment [15-17].

With the experimental verification, the effectiveness of theelectro-thermal simulation and the advanced iso-margin

design method can be evaluated. Therefore, the electrical and

thermal experiments have been carried out.

6.1 THE ELECTRICAL EXPERIMENT

The electrical performance test is conducted according tothe standard GB/T4109-2008 AC voltage above 1000V

insulated bushing or IEC60137-2008. It is the first time to

manufacture the prototype of the UHV RIP oil-SF6bushing in

the world. Therefore, the type tests reported in the paper are

slightly different from the IEC 60137-2008. Some of the typetests can be considered as the researching tests. The

experimental set up is shown in Figure 15.

a) The outdoor setup b) The indoor setup

Figure 15. Theonsite type tests of the UHV RIP oil-SF6bushing.

The testing items and the results are showed in Table 3.The measured dielectric loss value tan under the power

frequency voltage of 120, 300 and 667 kV should not exceed0.5 %. And the capacitance of the condenser should remain a

constant. Under the dry withstand voltage test at the power

frequency, there should be no external flashovers and internal

breakdowns under 1200 kV for 5 min. Under the appliedvoltage of 953 kV, the amount of partial discharge should be

smaller than 10 pC. When the voltage drops to 667 kV, thepartial discharge level should be smaller than 5 pC. Under the

lightning impulse dry withstand voltage 2400 kV and the

switching impulse dry withstand voltage 1950 kV, the

external flashovers and internal breakdowns cannot occur.

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Table 3.The electrical type test results.

The type tests The test results

The dielectric loss factor and

capacitance measurements

The ambient temperature: 15

The relative humidity: 58 %

120 kV tan: 0.45 %

Cx: 384.1 pF

300 kV tan: 0.46 %

Cx: 384.1 pF

667 kV tan: 0.47 %

Cx: 384.1 pF

The dry withstand voltage test at

power frequency



The applied voltage: 1200 kVThe duration time: 5 min

no flashover and breakdown occur

After the test, the repeated

measurement data of dielectric loss

factor and capacitance is as follows:

120 kV tan: 0.47 %

Cx: 384.3 pF

300 kV tan: 0.47 %

Cx: 384.3 pF

667 kV tan: 0.46 %

Cx: 384.3 pF

The partial discharge

measurement

The pre-applied voltage: 1100 kV

The testing voltage: 953 kV

The partial discharge quality: 5 pC

The testing voltage: 667 kV

The partial discharge quality: 2 pC

The system background noise: 1 pC

The lightning impulse withstand

voltage test



The Pos. full-wave: (2377-2401) kV.

The Neg. full-wave: (2381-2397) kV.

The Neg. chopped-wave: (2649-2741)

kV


After test, the repeated measurement

data of dielectric loss factor and

capacitance is as follows:

120 kV tan: 0.43 %

Cx: 384.9 pF

300 kV tan: 0.43 %

Cx: 384.9 pF667 kV tan: 0.43 %

Cx: 385.0 pF

The switching impulse dry

withstand voltage test



The Pos. full-wave: (1925~1946) kV

The Neg. full-wave: (1930~1958) kV


After test, the repeated measurement

data of dielectric loss factor and

capacitance is as follows:

120 kV tan: 0.47 %

Cx: 384.1 pF

300 kV tan: 0.47 %

Cx: 384.1 pF

667 kV tan: 0.47 %

Cx: 384.2 pF

Table 3 indicates that the results of the electrical type tests

done for the UHV RIP oil-SF6bushings prototype can readily

satisfy the requirements of IEC60137-2008 or GB/T4109-

2008 AC voltage above 1000V insulated bushing. This proves

that the improved iso-margin design method is effective on

the UHV levels bushings.

6.2 THE THERMAL EXPERIMENTThe temperature-rising experiment of the UHV RIP oil-SF6

bushing has also been carried out. The temperature-rising

process at key points around the bushing can be recorded by

the sensors. The arrangement of the temperature sensors is

shown in Figure 16. The basic principle for arranging the

temperature measurement points is to focus on the metal parts

on both ends of the bushing condenser. Furthermore, the

effective contact of the sensors with the measurement points

must be ensured, shown in Figure 17. During the temperature-

rising test, the ambient temperature of the testing hall was

about 22 . And the transformer oil was heated to 80 . The

rated value of the current flowing through the bushings inner

conductor is set to be 3150 A. The current which is applied to

the bushing condenser lasted nearly 10 hours. And the

temperature was recorded every one hour. Moreover, the

temperatures changing processes of all the key measuring

points are shown in Figure 18.

Figure 16. The temperature measuring points on the contour of the bushing

condenser. 1, 16, 19, 20the current carrying inner conductor. 2, 3, 4, 5, 15,

17, 18, 21the aluminum accessories. 10, 11the middle flange. 22, 23

the transformer oil. 9SF6. 6, 7, 8, 9, 12, 13, 14 the RIP bushing

condenser.

Figure 17. The installation of temperature sensor with the silver plating

Figure 18. The temperature-rising curves of the measurement locations.

It is concluded that the temperature of each measuring

point increases dramatically in the range of the starting 5

hours, and reaches the steady state in the range of the

remaining 5 hours. The steady-state temperature in the oil side

of the bushing is higher than that in the SF6 side. It also can be

found that the hottest-spot temperature with the value of 95

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locates at the inner conductor of the bushing which fits well

with the electro-thermal simulation result.

7 CONLUSION

The paper has combined the electro-thermal coupling

simulation with the PSO algorithm to improve the traditional iso-

margin design method. Then, the optimal structure of the inner

insulation is used to develop the prototype of the UHV RIP oil-SF6bushing. The following conclusions can be obtained:

(1) The stray capacitance and temperature have significant

influences on the internal E-field distribution of the UHV

bushing after evaluating quantitatively by the FEM. Based on

this, it is important to improve the traditional design method,

and obtain the uniform radial stress, axial stress and partial

discharge margin.

(2) The electric and thermal properties of the RIP material

used in the bushing condenser are functions of the operating

temperature which determines the electric and thermal

distribution inside the bushing condenser. Therefore, it is

reasonable to take the nonlinear characteristic into

consideration during the design of the actual bushing with the

proposed electro-thermal coupling method.

(3) The improved iso-margin design method has been used in

the optimization design of the UHV RIP oil-SF6bushing. The

prototype has passed through all the type tests, which proves

that the proposed iso-margin design method is effective and

reasonable.

(4) The long-term aging performance evaluation of the RIP

materials based on the accelerated thermal and electric stress

will be carried out in the follow-up work. The aim is to

evaluate the allowed operation temperature of the UHV RIP

oil-SF6 bushing. Based on the test results, the structure

optimization of the bushing condenser and the innerconductor should be further carried out until the hottest-spot

temperature is lower than the allowed operation temperature

of the RIP material.

APPENDIXA. THE CLOSED-FORM SOLUTION OF THE HEAT

FLOW CONTINUITY EQUATION

The closed-form solution of the heat flow continuity

equation is described as:

1( ) ln( )2

a

P rT r T

r

(A1)

0

ln( ) ( ( ln ln( )(ln( ln )

ln

1) ln( ln ) ln ln( ln ) ln ))

b N r r M bT r m b N r

N N b N r r

b N r r b N r r

(A2)

Where2 2

20 0

0 0

2 2

0 0

( )

2 ln( ) ln

ln

n nl l l l M Nr r

r r

b l N r m U

(A3)

B. THE FITNESS FUNCTION OF THE ADVANCED

EQUAL MARGIN DESIGN METHOD

The fitness function used in the advanced equal margin

design method is available in [7]. However, for the quick

reference, the expression and descriptions are described as:

1

[(1 ) (( ) 1) ( )]

[(1 ) (( ) 1) ( )]

[(1 ) (( ) 1) ( )]

r

r

a

a

k

Ek

E

kn

EkE

k

kk PDMPDM

Max ERCMinX X

Max ERC

Max EACMinMin X XMax EAC

Max PDCMinX X

Max PDC

(B1)

subject to

1 (B2)

where , and are the weight values of different design

parameters, which satisfy the equation (B2). X is an integer

variable which may be either 0 or 1.ERC is the control value of

the radial stress,EAC is the control value of the axial stress, and

PDC is the control value of the partial discharge margin. Then

take the radial stress as an example. If the maximum value of the

radial stress is greater thanERC, the value ofX is set to 1. Theminimization of ( ) /kErax ERC ERC makes Er as close as

possible toERC. Otherwise, if the maximum value of Eris less

thanERC, X is set to 0. The minimization of (( / ) 1)r

k

EMin Max

guarantees the maximum and minimum ofErwill be as close aspossible. The uniform distribution of Er can be obtained. The

optimization strategy ofEa andPDM is identical to that ofEr.

REFERENCES

[1] Z. Guoqiang, Z. Yuanlu and C. Xiang, Optimal Design of high voltagebushing electrode in transformer with evolution strategy, IEEE Trans.

Magnetics, Vol.15, pp.1690-1693, 1999.

[2] J.V.Champion and S.J.Dodd, Inter-foil Electrical Breakdown in High Voltage

ERIP Condenser Bushings, IEEE 7th Intl. Conf. Solid Dielectr., pp. 329-332, 2001.

[3] R. Platek, Robert Sekula and Bogusz Lewandowski, Fluid Influence on

Dynamic Characteristics of Transformer-bushing System Using Fluid

Structure Interaction (FSI) Approach, IEEE Trans. Dielectr. Electr. Insul,

Vol.17, pp. 408-416, 2010.[4] C.Ko, S.Zhang, Improved Transient Hot-Spot Temperature Calculation

Method of High Voltage Bushings, IEEE Trans. Power Delivery, Vol.24, pp.

1295-1301, 2009.

[5] N.S.J yothi, T.S. Ramu and M. Mandlik, Temperature Distribution in Resin

Impregnated Paper Insulation for Transformer Bushings, IEEE Trans.Dielectr. Electr. Insul, Vol.7, pp. 931-938, 2010.

[6] S. Monga, R.S. Gorur, P. Hansen and W. Massey, Design Optimization of

High Voltage Bushing Using Electric Field Computations, IEEE Trans.

Dielectr. Electr. Insul, Vol.13, pp. 1217-1224, 2006.[7]M. R. Hesamzadeh, N. Hosseinzadeh and P. Wolfs, An Advanced Optimal

Approach for High Voltage AC Bushing Design, IEEE Trans. Dielectr.

Electr. Insul, Vol.15, pp. 461-466, 2008.

[8] M.K. Pradhan and T.S. Ramu, Estimation of the Hottest Spot Temperature(HST) in Power Transformers Considering Thermal Inhomogeniety of the

Windings, IEEE Trans. Power Delivery, Vol.19, pp. 1704-1711, 2004.

[9] W. Cao, W. Shen, B. He and K. Wu, Structural Design for DC Bushing Core

Based on the Material Life, IEEE Trans. Dielectr. Electr. Insul, Vol.20,

pp.281-288, 2013.[10] C. C. Reddy and T.S. Ramu, On the Computation of Electric Field and

Temperature Distribution in HVDC Cable Insulation, IEEE Trans. Dielectr.

Electr. Insul, Vol.13, pp.1236-1244, 2006.

[11] C. C. Reddy, Theoretical Maximum Limits on Power-Handling Capacity ofHVDC Cables, IEEE Trans. Power Delivery, Vol. 24, pp. 980-987, 2009.

[12] J.D. Chen and Z.Y. Liu, Dielectric Physics, Beijing China, Mechanical

Industry Press, pp. 240-280, 1982 (in Chinese).

8/10/2019 06878005.pdf

10/10


[13] Z. Fang, J. Jicun and Z. Ziyu, Optimal Design of HV Transformer Bushing,

IEEE 3rd Intl. Conf. Properties and Applications of Dielectric Materials,

Tokyo, Japan, pp. 434-437, 1991.[14] A. A. Eajal, M. E. El-Hawary, Optimal Capacitor Placement and Sizing in

Unbalanced Distribution Systems with Harmonics Consideration Using

Particle Swarm Optimization, IEEE Trans. Power Delivery, Vol. 25,

pp.1734-1741, 2010.[15] J.Rocks, N. Koch, R Platek and T. Nowak, Seismic Response of

Transformer-bushing Systems, IEEE Trans. Power Delivery, Vol.19,pp.131-137, 2004.

[16] W.Lampe, D.Wikstrom and B.Jacobson, Field Distribution on an HVDCWall Bushing During LaboratoryRain Tests, IEEE Trans. Power Delivery,Vol. 6, pp. 1531-1540, 1991.

[17] P.J. Lambeth, Laboratory Tests to Evaluate HVDC Wall Bushing

Performance in Wet Weather, IEEE Trans. Power Delivery, Vol.5, pp.

1782-1793, 1990.

Shiling Zhangwas born in Chongqing, China, in 1986.He received the B.Sc. degree in electrical engineering

from Xian Jiaotong University, Xian, China, in 2008.

Now he is working for the Ph.D. degree in high voltageand insulation technology at Xian Jiaotong University.

His research fields of interest are in structure design and

optimization of extra-high voltage and ultra-high voltage

insulation system, especially the internal and external

insulation system of extra-high voltage and ultra-highvoltage bushings.

Zongren Peng was born in Shaanxi province,China, in 1953. He graduated from Major of

Insulation, Dept. of Electrical Engineering, XianJiaotong University, Xian, China, in 1977. He is

currently a professor at State Key Laboratory of

Electrical Insulation and Power Equipment, XianJiaotong University. His research fields are the high

voltage and electrical insulation technology,including the insulation structure optimization and

the calculation of complex fields; the forming

mechanism of space charges in dielectrics and itsmeasurement methods; and the materials, structure and electrical propertiesof UHV AC/DC bushings.

Peng Liu was born in Ningxia, China, in 1979. Hereceived the B.Sc., M.Sc.. and Ph.D. degrees inelectrical engineering from Xian Jiaotong University,

China in 2001, 2004, and 2010, respectively. He is a

researcher at the State Key Laboratory of Electrical

Insulation and Power Equipment, in Xian JiaotongUniversity, since 2011. His research topics focus on

the dielectric properties of epoxy resin and its

composites, including the space charge behavior of

polymeric insulation materials.

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