A Fundamental Study on Electrical Insulation of N2/SF6 Gas-Insulated Electric

Power Apparatus

HITOSHI SATO,1 KEIICHI MORITA,

1 TAKASHI KIKKAWA,

1 NAOKI HAYAKAWA,

2

and HITOSHI OKUBO2

1Meidensha Corporation, Japan

2Nagoya University, Japan

SUMMARY

In this paper, we carried out a case study on funda-

mental insulation design for the application of N2/SF6 gas

mixtures to a 275-kV gas-insulated busbar. First, we inves-

tigated the impulse ratio in N2/SF6 gas mixtures and point

out that the insulation design might be based on the negative

switching impulse voltage in the case of lower SF6 content.

Second, we estimated the tank diameter and SF6 gas amount

with consideration of the electric field strength on conduc-

tor surface and temperature rise in gas mixtures. We clari-

fied that the tank diameter at the lower SF6 content is mainly

determined by the electric field strength on the conductor

surface and quantified the optimum gas pressure and SF6

content for a target reduction of SF6 gas amount. © 2002

Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 9�16,

2002; Published online in Wiley InterScience (www.

interscience.wiley.com). DOI 10.1002/eej.1163

Key words: N2/SF6 gas mixture; GIS; insulation

design; impulse ratio; tank diameter.

1. Introduction

Because of its high insulation characteristics, SF6 gas

is widely used as a primary insulation medium in high-volt-

age electric power apparatus, including gas-insulated sys-

tems (GIS) and gas-insulated transmission lines (GIL).

However, since the Kyoto Conference on global warming

(COP3), SF6 has been recognized as a gas with significant

impact on global warming, and the total amount used as

well as amount output are to be restricted in the future, as

is the case for CO2 [1].

Given this situation, N2/SF6 gas mixtures have started

to be studied for use as an insulating gas for high-voltage

electric power apparatus [2, 3]. This is because N2 gas has

a very low negative load on the global environment and is

cheap, and when mixed with SF6 gas, it has a positive

synergism for insulation characteristics [4�7]. However,

current gas insulation technology has developed toward the

use of pure SF6 gas, and so when considering the practical

development of N2/SF6 gas mixtures for insulated electric

power apparatus, the insulation characteristics of the gas

mixtures must be understood under varying conditions, and

the insulation design standards for apparatus must be

reevaluated.

In this paper the authors perform a fundamental study

on the insulation design standards for the use of N2/SF6 gas

mixtures for coaxial cylinder devices like a GIS busbar and

GIL. First, based on existing data the authors consider the

impulse ratio, the allowed electric field on the conductor

surface, the conductor diameter, the tank diameter, and the

temperature rise in the conductor and tank, using the SF6

gas mixture ratio and the gas pressure as parameters. Based

on these results, the authors consider the possibility of

reducing the amount of SF6 gas used and the use of N2/SF6

gas mixtures for GIS.

In this study, a 275-kV GIS was conceived for use

with the N2/SF6 gas mixture. The structure of the GIS is

determined by the cross-sectional dimensions of the co-

axial cylinder electrode, the basic structure. A coaxial

cylinder electrode has a simple shape, its electric field

distribution can be found analytically, and the gas vol-

ume can be readily calculated. Consequently, it was used

for quantitative analysis. The SF6 gas mixture ratio, the

gas pressure, and the tank diameter were used as parame-

ters for this analysis.

© 2002 Wiley Periodicals, Inc.

Electrical Engineering in Japan, Vol. 139, No. 4, 2002Translated from Denki Gakkai Ronbunshi, Vol. 121-B, No. 4, April 2001, pp. 449�454

Contract grant sponsor: Public Project for the Proposal of New Industries

Development through NEDO (New Energy and Industrial R&D Organi -

zation).

9

2. Impulse Ratio

Figure 1 shows the insulated breakdown voltage (gas

pressure P = 0.45 MPa, absolute pressure) when (a) a

negative lightning impulse (LI�), (b) a negative switching

impulse (SI�), or (c) a 60-Hz alternating current (ac) volt-

age is applied in a N2/SF6 gas mixture in a coaxial cylinder

electrode (φ 89 mm / φ 226 mm) as per Cookson and

Pedersen [8]. Note that the positive lightning impulse (LI+)

and the positive switching impulse (SI+) are given by the

following equation in this reference.

Positive lightning impulse insulation breakdown

voltage:

Positive switching impulse breakdown voltage

Vt: Average insulation breakdown voltage

R: SF6 gas mixture ratio

Pt: Gas pressure

Pa: Atmospheric pressure (0.1 MPa)

Based on Fig. 1 and the equations above, the insula-

tion breakdown voltages for LI+, LI�, SI+, and SI� were

divided by the respective ac insulation breakdown voltages

(peak values), and these values, representing the depend-

ence on the mixture ratio for the impulse ratio, are shown

in Fig. 2. This figure makes clear that for each SF6 gas

mixture ratio, the impulse ratio for LI+ was low by com-

parison, and that the SI� impulse ratio was low compared

to the SI+ ratio. In addition, it shows that as the SF6 gas

mixture ratio rises, the LI and SI impulse ratios also tend to

rise, albeit slowly.

In general, the insulation design for a GIS busbar

using pure SF6 gas is performed based on the insulation

breakdown voltage for when a negative lightning impulse

is applied. This is because with respect to the test voltage,

the insulation breakdown voltage for a negative lightning

impulse is lowest due to the polarity difference in the V�t

characteristics of the SF6 gas in the coaxial cylinder poles.

Thus, for the N2/SF6 gas mixture, the authors studied the

polarity and the voltage waveform (which represent the

design standards) based on the relationship between the

impulse ratio and the test voltage defined in JEC-2350 [9].

Figure 3 shows the dependence on the SF6 gas mix-

ture ratio for values for which the impulse ratio shown in

Fig. 2 is divided by the lightning impulse and switching

impulse test voltages (TVLI = 1050 kV, TVSI = 750 kV, TVLI

= 950 kV when using a high-performance lightning protec-

tor) and the ac test voltage (TVAC = 330 kV). This figure

illustrates that the insulation breakdown voltage for the

voltage waveform and polarity with the lowest values for

each mixture ratio should be used as a design standard. For

TVLI = 1050 kV in Fig. 3(a), if the insulation distance is

determined using LI� as a standard for all of the SF6 gas

mixture ratios, then the LI+, SI+, SI�, and ac test voltages

can all be withstood. On the other hand, for TVLI = 950 kV

in Fig. 3(b), insulation design must be implemented using

LI� as a standard for an SF6 gas mixture ratio of 60 to 100%,

and SI� must be used for SF6 gas mixture ratios of 20 and

40%.

On the other hand, in the study above, data for 10 to

20% as a practical SF6 gas mixture ratio are lacking. Figure

4 shows the positive and negative impulse lightning break-

down field for SF6 gas mixture ratios of 0, 20, and 100% as

per Cookson and Pedersen [8] shown in Fig. 1, and the

positive and negative lightning impulse breakdown field in

a coaxial cylindrical electrode (φ185 mm / φ40 mm) for an

SF6 gas mixture ratio of 0, 5, 20, and 100% as per Waymel

[10]. In this figure both the positive and negative lightning

Fig. 1. Negative lightning impulse, negative switching

impulse, and 60-Hz ac breakdown voltages

of N2/SF6 gas mixtures.

(1)

(2)

Fig. 2. Impulse ratio in N2/SF6 gas mixture.

10

impulse breakdown fields (solid circles and triangles)

match well. However, for a low SF6 gas mixture ratio and

a high gas pressure (over 0.4 MPa for a 5% SF6 gas mixture

ratio and over 0.6 MPa pure N2 gas), the LI+ insulation

breakdown field (open circles) tends to be lower than the

LI� insulation breakdown field (solid circles).

Given the above, it is clear that for the N2/SF6 gas

mixture insulation, the voltage waveform and the gas pres-

sure, the basics of insulation design, must be selected

appropriately by using conditions such as the SF6 gas

mixture ratio and the gas pressure. Establishing data for

reference in insulation design is also important. The

authors� study proceeds on the assumption that TVLI = 1050

kV, and is based on the LI� insulation breakdown voltage.

3. Conductor Diameter and Tank Diameter

Two points can be mentioned as major determining

factors for the size of the GIS busbar: the allowed electric

field on the conductor surface and the temperature rise in

the tank and the conductor.

3.1 Allowed electric field on the conductor

surface

Based on the experimental values (Fig. 1) for the

insulation breakdown voltage when a N2/SF6 gas mixture

lightning pulse is applied, the breakdown electric field Ed

was found for the conductor surface for each SF6 gas

mixture ratio and gas pressure. Then the allowed electric

field on the conductor surface Em for the Ed tolerance was

found as

σ: Standard deviation (5%)

The σ for the lightning impulse insulation breakdown

voltage for the N2/SF6 gas mixture was set to 5% (same

value as in SF6 gas) overall for each SF6 gas mixture ratio,

because the same value was reported [3] for pure SF6 gas.

η: Spacer efficiency (0.7)

This is the value of the insulation breakdown voltage

when a spacer is present divided by the value when a spacer

is not present. It is assumed that a value similar to that for

pure SF6 gas will be obtained by optimizing the shape of

the spacer for the N2/SF6 gas mixture, and so the η for the

SF6 gas [12] was used.

s: Tolerance (1.2)

A value used experimentally for an SF6 gas insulated

power device was used.

The relationship between the conductor diameter and

the tank diameter for which Em is satisfied can be expressed

as

Fig. 3. Impulse ratio/testing voltage ratio as a function

of SF6 content.

Fig. 4. Lightning impulse breakdown stress of N2/SF6

gas mixtures for coaxial cylindrical electrodes.

(3)

11

Em: Allowed electric field on the conductor surface

(kV/mm)

T.V.: Test voltage 1050 kV

d1: Conductor external diameter (mm)

d2: Tank internal diameter (mm)

Figure 5 shows the dependence of the SF6 gas mixture

ratio for the conductor diameters and the tank diameters

which satisfy Em for the cases in which (a) P = 0.4 MPa and

(b) P = 0.6 MPa. The current capacity was set to 6000 A.

Also, a straight line representing the points for which the

tank diameter is minimal when only the electric field is

considered (ratio of the tank diameter to the conductor

diameter is e = 2.72) was added. Based on this figure it is

clear that the tank diameter can be significantly reduced by

using a low SF6 gas mixture.

3.2 Temperature rise in the conductor and

tank surface

The temperature rise in the conductor and the tank

surfaces can be calculated using the following equations

which consider the effects of thermal emissions to the

exterior from the heating due to current and the convec-

tion/radiation [13, 14].

Thermal emission between the conductor and tank

Thermal emission between the tank and the atmos-

phere

Wc: Conductor loss (W/m)

Ws: Tank loss (W/m)

Tc: Conductor temperature (K)

Ts: Tank surface temperature (K)

Ta: Ambient temperature (K)

d1: Conductor external diameter (m)

d2: Tank internal diameter (m)

d3: Tank external diameter (m)

ε1: Conductor surface radiation index, 0.1 (unproc-

essed aluminum bare earth)

ε2: Tank internal surface radiation index, 0.1 (un-

processed aluminum bare earth)

ε3: Tank external surface radiation index, 0.8 (an-

nealed)

P: Gas pressure (kg/cm2-abs)

n: Gas pressure constant, 0.65 (SF6), 0.6 (N2)

K1: Flow constant between the conductor and tank,

4.4 (SF6), 2.6 (N2)

K2: Flow constant between the tank and the air, 2.75

These temperature rises are limited by the upper

temperature limit for the conductor and the sheath. In the

(4)

Fig. 5. Conductor and tank diameters for different SF6

contents with consideration of electric field strength on

conductor surface and temperature rise.

(5)

(6)

12

authors� study, the material for the conductor and the tank

were both aluminum, the upper limit for the conductor

temperature was 105 °C, and the upper limit for the tank

surface temperature was 70 °C. The allowed range for the

conductor diameter and the tank diameter from the stand-

point of the temperature rise when using pure SF6 gas or

pure N2 gas as an insulation medium is also noted in Fig. 5

for a GIS with a current capacity of 6000 A. This figure also

makes clear that the diameter of the conductor and of the

tank (SF6: thick solid line; N2: thick dashed line) limited by

the upper limit of the temperature rise (105 °C) is affected

by the injection of the gas. Because SF6 gas transmits heat

better (K1 is larger) compared to N2 gas, the necessary

conductor diameter and tank diameter are smaller. On the

other hand, the conductor diameter and tank diameter (thick

dotted line) limited by the upper limit of the tank surface

temperature (70 °C) are computed as the emission of all

losses from the tank surface, and so there is no gas injection

effect.

Figure 5(a) shows that compared to all of the SF6 gas

mixtures when P = 0.4 MPa, the tank diameter is deter-

mined by the allowed electric field for the conductor sur-

face, and the temperature rise is not restricted.

Figure 5(b) shows a repetition of the same analysis

for P = 0.6 MPa. A comparison of Figs. 5(a) and 5(b) shows

that the allowed electric field for the conductor surface rises

as the gas pressure rises, and the necessary tank diameter

falls. In addition, the tank diameter and the conductor

diameter (thick solid line and thick dashed line) determined

by the upper limit of the conductor temperature (105 °C)

for high gas pressure are small. This is because the thermal

transmission capacity of the injected gas rises due to a rise

in the gas pressure.

Based on Fig. 5(b) it is clear that for a gas pressure

of 0.6 MPa and a current capacity of 6000 A, the conductor

diameter and the tank diameter are limited primarily by the

temperature rise when pure SF6 gas is used. In addition, it

is clear that the tank diameter when the SF6 gas mixture

ratio is low is determined by the allowed electric field of

the conductor surface, and the temperature rise is not re-

stricted. This is because although the thermal transmission

capacity falls due to a drop in the SF6 gas mixture ratio, the

temperature rise is limited by an increase in the tank diame-

ter (radiating surface increases) due to a drop in the allowed

electric field of the conductor surface. In this instance,

revising the allowed electric field for the conductor surface

by reducing the tolerance would be useful for decreasing

the tank diameter.

4. Amount of SF6 Gas Used

Figure 6 shows the results of iterating the results of

the analysis in the previous section for a gas pressure of 0.6

MPa and a current capacity of 2000 to 8000 A, then finding

the minimum tank diameter for each SF6 gas mixture ratio.

Figure 7 shows the results of calculating the amount of SF6

gas used from the product of the gas spatial cross-sectional

area and the mixture ratio for the GIS busbar. This figure

standardizes the cases for pure SF6 gas, a gas pressure of

0.4 MPa, and a current capacity of 6000 A.

Based on Figs. 6 and 7, although the tank diameter

rises as the SF6 gas mixture ratio falls, the amount of SF6

gas used can be viewed quantitatively. Here, because the

tank diameter for pure N2 gas is extremely large, raising the

gas pressure is necessary when considering these condi-

tions for a GIS busbar. In addition, for a SF6 gas mixture

ratio of 10 to 100%, it is clear that a current capacity of 6000

A requires a small increase in the tank diameter, and a

current capacity of 8000 A requires a larger increase in tank

diameter compared to a current capacity of 2000 or 4000 A.

Fig. 6. Minimum tank diameter as a function of SF6

content.

Fig. 7. SF6 amount as a function of SF6 content.

13

This is because the minimum tank diameter is limited

by the rise in temperature when the current being used is

large. In other words, when the SF6 gas mixture ratio is over

10%, there is a tolerance with respect to the allowed electric

field of the conductor surface, and when under 10%, there

is excess current capacity for transmission due to the large

rise (increase in the thermal emission surface) in the tank

diameter because of the limit on the allowed electric field

of the conductor surface. In addition, there is a clear rise in

the amount of SF6 gas used when the current capacity is

8000 A as in Fig. 7, compared to when the current capacity

is 2000, 4000, or 6000 A. Therefore, although there is

insulation capacity tolerance when the gas pressure is 0.6

MPa, the current capacity is 8000 A, and the SF6 gas

mixture ratio is between 10 and 100%, a lot of SF6 gas has

to be used in order to ensure current transmission capacity.

5. Useful Gas Pressures and SF6 Gas Mixture Ratios

In the previous sections the gas pressure was used

repeatedly as a parameter, and the minimum tank diameter

and the amount of SF6 gas used were analyzed. Figure 8

shows the minimum tank diameter as a function of the SF6

gas mixture ratio, and Fig. 9 the amount of SF6 gas used for

a gas pressure of 0.2 to 0.6 MPa with a current capacity of

6000 A. These figures plot the combinations of useful gas

pressures and SF6 gas mixture ratios for any amount of SF6

gas used (target values). For instance, if the amount of SF6

gas used is 0.2 p.u., then based on Fig. 9 the SF6 gas mixture

ratio would be about 10% for a gas pressure of 0.6 MPa.

The diameter of the tank in this instance would be about

555 mm based on Fig. 8, which represents an increase in

the tank diameter of about 15% compared to 0.4 MPa (about

480 mm).

6. Conclusion

The authors studied the insulation design standards

for the use of N2/SF6 gas mixtures for a GIS busbar employ-

ing an officially accepted voltage of 275 kV. The results

were as follows.

(1) The impulse ratio rises due to an increase in the

SF6 gas mixture ratio. In addition, the voltage waveform

and polarity, which represent the fundamentals of insula-

tion design, must be selected appropriately to correspond

with the SF6 gas mixture ratio due to the relationship with

the test voltage.

(2) When the SF6 gas mixture ratio is low, the tank

diameter is determined by the allowed electric field on the

conductor surface, and is not limited by the rise in tempera-

ture. In this instance, revising the allowed voltage of the

conductor surface by reducing the tolerance is an effective

way to reduce the tank diameter.

(3) The gas pressure and SF6 gas mixture ratio which

can be used to achieve the desired reduction in the amount

of SF6 gas used were quantified along with the tank diame-

ter. The results showed that when the amount of SF6 gas

used was 0.2 p.u. and the gas pressure was 0.6 MPa, the SF6

gas mixture ratio was about 10%, and the tank diameter rose

by about 15% when pure SF6 gas (0.4 MPa) was used.

In the future countermeasures for foreign metals,

settings for insulation tolerance, and various other elements

must be considered in addition to the items mentioned in

this paper in order to design practical insulation for N2/SF6

gas mixture insulated electric power devices.

Acknowledgment

The authors performed a portion of this research as

part of the Public Project for the Proposal of New Industries

Development through NEDO (New Energy and Industrial

R&D Organization).

Fig. 8. Minimum tank diameter for different gas

pressures as a function of SF6 content.

Fig. 9. SF6 amount for different gas pressures as a

function of SF6 content

14

REFERENCES

1. Takuma. Gas insulation and the problem of global

warming. Dengaku J 1999;119:232�235.

2. Hama H, Yoshimura M, Inami K, Hamano S. Appli-

cation problems of SF6/N2 mixtures to gas insulated

bus. Gaseous Dielectrics VIII 1998:353�359.

3. Endo, Uchimi, Yaokibashi. A study on the applica-

tions of SF6/N2 gas mixtures GIS (I, II). 1999 Na-

tional Denki Gakkai Conference, No. 1530�1531.

4. The use of gas mixtures for gas insulation. Denki

Gakkai Tech Rep 1987;248:36�39.

5. Yamada, Takahashi, Kodo, Okubo. Partial discharge

characteristics and insulation breakdown charac-

teristics in various gas mixtures. Trans IEE Japan

1998;118-B:831�836.

6. Yamada, Takahashi, Kodo, Okubo. Partial discharge

mechanism in gas mixtures using various gases and

SF6. Trans IEE Japan 1998;118:837�842.

7. Yamada, Takahashi, Hayakawa, Yoshida. AC partial

discharge development characteristics and AC/DC

insulation breakdown characteristics in SF6/N2 gas

mixtures. Trans IEE Japan 1999;119-A:891�896.

8. Cookson AH, Pedersen BO. Analysis of the HV

breakdown results for mixture of SF6 with CO2, N2

and Air. 34rd ISH, 31.10, 1979.

9. Standards and Ratings from the Electrical Standards

Society. Gas insulation switching devices. No. 2350,

1994.

10. Waymel X. Low SF6 concentration SF6/N2 mixtures

for GIL. Gaseous Dielectrics VIII 1998:345�351.

11. Moju. Research on high voltage insulation design for

SF6 gas insulated switching devices (GIS). Nagoya

University, Ph.D. dissertation, p 30�32, 1995.

12. Diarra I, Beroual A, Buret F, Phan VD. Correspon-

dence between SF6 and N2 under pressure for a

common surface flashover voltage. 9th ISH, No.

2264, 1995.

13. Mizuguchi, Kamino, Itaka, Furukawa, Ninomiya,

Jun Lin. A study on the amount of current in trans-

mission lines in piping. 1984 Denki Gakkai National

Conference, No. 1255.

14. Itaka K, Araki T, Hara T. Heat transfer characteristics

of gas spacer cables. IEEE Trans Power Apparatus

Syst 1978;97:1579�1585.

AUTHORS (from left to right)

Hitoshi Sato (member) graduated from the Nuclear Power Engineering Department at Hokkaido University in 1995 and

joined Meidensha Corporation. He is pursuing research primarily on the basic insulation characteristics of gas-insulated electric

power apparatus.

Keiichi Morita (member) graduated from the Electrical Engineering Department at Nihon University in 1971 and joined

Meidensha Corporation. He is pursuing research primarily on medium-voltage switch gear (switching devices).

Takashi Kikkawa (member) completed a master�s level course of study in electrical engineering at Osaka University in

1973 and joined Meidensha Corporation. He is pursuing research primarily on medium-voltage switch gear (switching devices).

Naoki Hayakawa (member) completed the latter half of his doctoral studies at Nagoya University in 1990 and became a

lecturer there. He has been an assistant professor of electrical engineering in the Engineering Research Department in the

Graduate School since 1998. He holds a D.Eng. degree. He received the 1996 Denki Gakkai Research Paper Award. He is a

member of IEEE, the Low-Temperature Engineering Society, and the Energy and Resources Council.

15

AUTHORS (continued)

Hitoshi Okubo (member) completed the first half of his doctoral studies at Nagoya University in 1973 and joined Toshiba.

He worked on the development of high-voltage technology. From 1976 to 1978, he was at the High Voltage Laboratory at Aachen

Institute of Technology in Germany and the High Voltage Laboratory at Munich Institute of Technology. In 1989 he became an

assistant professor in the Electronics Department at Nagoya University, and a professor in 1991. He holds a D.Eng. degree, and

is a member of IEEE, VDE, and CIGRE.

16