Document45

International Journal for Research and Development in Engineering (IJRDE)

www.ijrde.com ISSN: 2279-0500 Special Issue: pp- 261-266

Methods Enriching Power and Energy Development (MEPED) 2014 261 | P a g e

Temperature rise estimation in a 420 kV GIS bus bar

considering skin and proximity effects

1Bavisha T,

1Usa S,

2Ucchintala Ravindra,

3Santosh Kumar A

1Department of Electrical and Electronics Engineering, College of Engineering Guindy, Chennai, India.

2,3Alstom T&D India Ltd., Padappai, Chennai 620006,India

ABSTRACT

Gas Insulated Switchgear should carry high

current permanently. The high current creates

ohmic losses in the current carrying parts. The

heat from the losses increases the temperature of

the current carrying parts due to which their

resistance increases. This will result in a thermal

runaway condition resulting in failure of GIS.

Hence the temperature rise on the current

carrying parts should not exceed a maximum limit

which is defined by standards. Thus the current

carrying capacity is limited by the maximum

operating temperature. Moreover the capital cost

of GIS depends upon the current carrying

capacity. Hence it is necessary to predict the

temperature rise accurately. In this paper, the

temperature rise in a 420 kV GIS bus bar is

estimated using a theoretical model. The

theoretical model is in line with the publication of

CIGRE working group 21.12 which is meant for

the calculation of continuous current rating in

compressed gas insulated cables. However a

different principle and pattern that suits GIS bus

duct is adapted. The temperature rise estimated

using the theoretical model shows good agreement

when compared with the measured temperature

rise.

Keywords: bus bar, power loss, proximity, skin

effect, temperature.

I. INTRODUCTION

Gas Insulated Substation (GIS) is an

electrical substation in which the live parts are

contained in an external metal enclosure with sulfur

hexafluoride (SF6) gas as the insulating medium. It

occupies only 10% of the space required by a

conventional Air Insulated Substation (AIS).This

ratio reduces further as voltage level increases. Other

advantages include improved operator safety and

higher reliability during operation.

In gas insulated switchgear, the current

carrying conductor is enclosed in concentric metallic

enclosure intended to contain the pressurized

insulating gas. The cross sectional view of conductor

and enclosure is shown in Fig 1.An alternating

current flowing through the conductor produces an

alternating magnetic field. The magnetic field

generated by the conductor induces a current in the

external earthed enclosure. These currents produce

joules heating loss which in turn increases the

temperature of both conductor and enclosure. The

skin and proximity effects increase the losses further.

The temperature rise on the current carrying parts

should not exceed a maximum value, beyond which

thermal runaway occurs resulting in the failure of

GIS. Thus the current carrying capacity is limited by

the maximum operating temperature. Hence the

knowledge of temperature profile is important to

understand the performance of GIS and to improve its

design.

Fig 1.Bus bar conductor and enclosure

Due to the advances in computer technology and the

evolution of finite element (FE) model, the FE

technique which solves electromagnetic and thermal




fields has been used for such problems [4],[5],[6].The

difficulty in this technique lies in determining the

convection heat transfer coefficient at the solid-gas

interface. The heat transfer coefficient depends upon

the geometry, temperature and material property. A

coupled finite element and analytical technique has

been used which involves the calculation of heat

transfer coefficient by iterative analytical technique

[2],[3].Uncoupled field analysis where two fields are

solved separately, could not take into account of the

interrelationship between the two field parameters

like temperature dependent resistivity. Therefore

fully coupled field analysis that repeatedly solves the

two fields has been used. But this technique is time

consuming [4].

In this paper a theoretical model to estimate

the temperature rise in a 420 kV GIS bus bar is

developed. The publication of CIGRE working group

[1] is meant for the calculation of continuous current

rating in a single core compressed gas insulated

cables. Since the gas insulated cables and GIS bus

duct are similar in construction, a theoretical model

in line with the CIGRE document is developed to

estimate the temperature rise. However the principle

and pattern that suits GIS bus duct is adapted. The

theoretical model mainly concerns the skin effect,

proximity effect, thermal resistance of gas insulation

and thermal resistance of surrounding ambient. The

accuracy of the developed theoretical model is

verified by comparing with the measured

temperature.

II. ALTERNATING CURRENT RESISTANCE

The alternating current resistance per unit length of

the conductor or outer enclosure is given by

(1)

is the alternating current resistance of the

conductor tube at its maximum operating

temperature, is the direct current resistance of tube

at 20 , y is the coefficient of skin effect, is the

temperature coefficient of electrical resistivity at

20 per 1K temperature difference, is the

maximum operating temperature of the conductor or

the enclosure.

III. SKIN-PROXIMITY EFFECT

Skin and proximity effects are the major source of

losses in transformer and inductor designs, as well as

in AC power distribution systems composed of

separate, round conductors. They cause a non

uniform current distribution with an increase in loss.

A. Skin Effect

The skin effect corresponds to the uneven distribution

of the time-varying current in a conductor. The AC

distribution causes the conductor resistance to exceed

its DC value and produces higher losses. Fig.2 shows

the non uniform current distribution pattern on the

current carrying conductor due to skin effect

Fig.2 Non uniform current distribution due to skin effect

The skin depth equation is given as

(2)

f is the frequency (Hz), is the absolute permeability

(H/m), is the conductivity (S/m).




B. Proximity effect:

The AC current flowing in two round, parallel

conductors is not distributed uniformly around the

conductors. The magnetic fields from each conductor

affect the current flow in the other, resulting in a non-

uniform current distribution, which in turn, increases

the apparent resistance of the conductors. This is

called proximity effect.

Fig 3a.

Conductors carrying

current in

same direction

Fig 3b. Conductors

carrying current in

opposite direction

Fig 3a,3b .Illustrates the non uniform current distribution

when two current carrying conductors are in proximity.

C. Coefficient of skin – proximity effect:

The coefficient of skin and proximity effect for the

inner conductor is given by

(3)

The coefficient of skin and proximity effect for the

outer tube is given by

(4)

is the thickness of inner tube , is the thickness

of outer tube, is the external diameter of inner

tube, is the external diameter of outer tube,

, are parameters that depend

upon the value of z. The parameter z is given by

(5)

is the electrical resistivity at direct current and

maximum operating temperature.

IV. PERMISSIBLE CURRENT

The permissible current is calculated from

the formulae based on the publication of CIGRE

working group 21.12.However the parameters that do

not apply to the GIS are not considered.

(6)

is the current in one conductor(inner tube), is

the alternating current resistance of inner tube at its

maximum operating temperature, is the thermal

resistance of the insulation, is the thermal

resistance of the surrounding ambient, is the

permissible temperature rise of inner tube above

ambient, is the loss factor due to losses in the outer

tube (enclosure).

V. HEAT TRANSFER MECHANISM

Heat transfer takes place by conduction,

convection and radiation. The current carrying

conductor creates power loss in the form of heat. The

generated heat is transferred from the conductor to

the enclosure by convection and radiation. Moreover

the induced current flowing through the enclosure

creates heat. This heat is transferred to the outer

atmosphere via convection and radiation.

VI. THERMAL RESISTANCE

Thermal resistance is a heat property and is

a measurement of a temperature difference by which

an object or material resists a heat flow.

A. Thermal resistance between conductor and

enclosure

Heat losses transmitted from the conductor by

radiation are




=

(7)

Heat losses transmitted from the conductor by

convection are

8)

Thermal resistance between conductor and enclosure

= (9)

is the internal diameter of the outer tube, is

the maximum operating temperature of inner tube,

is the maximum operating temperature of outer tube,

is the temperature of the ambient air, is the

emissivity coefficient of the inner tube, is the

emissivity coefficient of the inner surface of outer

tube, is the gas pressure. is equal to 11.3 if sf6

gas is used as insulating gas whereas it is equal to

5.83 if nitrogen is used as insulating gas.

B. Thermal resistance of enclosure

Heat losses transmitted from the enclosure by

radiation are

(10)

Heat losses transmitted from the enclosure by

convection are

(11)

Thermal resistance of enclosure is

= (12)

is the external diameter, is the temperature

of the outer enclosure, is the emissivity

coefficient of the outer surface, e is the factor

permitting to take into account of proximity

effect of adjacent conductor by means of the

formula

(13)

is the external diameter of enclosure and s is the

axial separation of conductors.

VII. METHODOLOGY

Initial temperature of both conductor and

enclosure was assumed. Maximum allowed

temperature rise is taken as initial temperature. As

per standard IEC 62271-1 the maximum allowed

temperature is 65ºC for the conductor and 40ºC for

the enclosure for 40ºC ambient temperature. Heating

losses, permissible current and ac resistance are

calculated. The losses are substituted in the energy

balance equation in each iteration. The temperature

corresponding to the heating losses that satisfies

energy balance equation is the maximum temperature

rise. This methodology is implemented in the form a

simulation program.

Start

Initialize Tc

Initialize Te

Energy balance equation

Pc

= +

Decrement Te

Calculate alternating

resistance, current and heat

losses




Tc is the temperature of the conductor, Te is the

temperature of the enclosure, Ta is the ambient

temperature.

VIII. ENERGY BALANCING EQUATION

The energy balance equation is given by

Pc = + (14)

Pc + Pe = (15)

Pc is the power loss in the conductor and Pe is the

power loss in the enclosure.

IX. VALIDATION OF METHODOLOGY

The temperature rise in a 420 kV gas insulated bus

bar is estimated using the proposed method. Fig 4

shows the cross-sectional view of 420 kV single core

three phase GIS bus bar model.

Fig.4.cross sectional view 0f 420 kV GIS

The conductor and enclosure are made of Aluminum

alloy. Table I shows the specification of the 420kV

bus bar.

Table I. Specification of 420 kV GIS bus bar

Parameter Conductor Enclosure

Maximum allowed

temperature rise ºC) 65 40

Ambient temperature

(ºC) 30

Frequency (Hz) 50

Rated current(A) 4000

To check correctness of the used methodology the

calculated temperature rise is compared with the

measured temperature rise. The measured

temperature rise is the maximum temperature rise

measured during the temperature rise type test.

(CERDA test report n°7599).

Table II shows the comparison of calculated and

measured temperature rise.

Componen

t

Analytical

Temperatur

e

Measured

Temperatur

e

Percentag

e

Deviation

Conducto

r 84 (ºC) 79.68 (ºC) 5.42%

Enclosure 53 (ºC) 53.3 (ºC) 0.005%

Table II. Comparison of calculated and measured

temperature rise

The table III shows the comparison of temperature

rise calculated temperature rise without considering

skin-proximity effect with measured temperature rise.

Table III. Comparison of temperature rise calculated

temperature rise without considering skin-proximity effect

with measured temperature rise.

Component

Analytical

Temperatu

re

Measured

Temperatur

e

Percentag

e

Deviation

Conductor 86 (ºC) 79.68 (ºC) 7.93%

Enclosure 55 (ºC) 53.3 (ºC) 3.18%

X. CONCLUSION

Temperature rise in a gas insulated bus bar

is a critical design parameter. A theoretical model in

line with the publication of CIGRE working group

Te > Ta

Decrement Tc

Tc > Ta

End

yes

yes




21.12 to estimate the temperature rise in a 420 kV

GIS bus bar is developed. The temperature rise

estimated using the theoretical model shows good

agreement when compared with the measured

temperature rise. The results are found to be more

accurate since the skin and proximity effects are

considered.

REFERENCES

[1] CIGRE Working group 21.12., “Calculation of

the continuous rating, compressed gas insulated

cables in still air with no radiation”, Electra No

100,pp. 65-76, 1985.

[2] S. W. Kim, H. H. Kim, and S. C. Hahn, “Coupled

finite-element analytic technique for prediction of

temperature rise in power apparatus,” IEEE

Trans. on Magnetics., vol.38, no.2, pp. 921-924,

2002.

[3] J.K.Kim, S.C.Hahn, K.Y.Park, H.K.Kim,

Y.H.Oh, “Temperature Rise Prediction of EHV

GIS Bus Bar by Coupled Magneto-thermal Finite

Element Method”, IEEE Trans. Magnetics, Vol.

41, No. 5,pp. 1636-1639, May 2005.588

[4] J.H. Yoon, H.S. Ahn, J. Choi, I.S. Oh, “An

Estimation Technology of Temperature Rise in

GIS Bus Bar using Three-Dimensional Coupled-

Field Multiphysics”, Proceedings of the IEEE

ISEI, 2008, 432-436

[5] Y. Li, S.L. Ho, N. Wang, and J. Guo

,“Calculations of Electromagnetic Field and

Thermal Problem in an Isolated Phase Bus by

Using FE Model” IEEE 2008 .

[6] Kim, H.K.; Oh, Y.H.; Lee, S.H ," Prediction of

temperature rise in gas insulated bus bar using

multi-physics analysis”, IEEE Transmission &

Distribution Conference & Exposition: Asia and

Pacific, 2009.

[7] B. Novák, L. Koller, “Steady-state Heating of

Gas Insulated Busbars” ,IEEE 2008

[8] K. Itaka, T. Araki, and T. Hara, “Heat transfer

characteristics of gas spacer cables,” IEEE Trans.

P.A. , vol. 97, Sept./Oct. 1978

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