SUMMARYSF 6 gas has excellent dielectric strength and interruption performance. For these reasons, it has been widely used for gas-insulated switchgear (GIS). However, use of SF 6 gas has become regulated under agreements set at the 1997 COP3. Presently, development of a gas circuit breaker (GCB) using CO 2 gas and development of a high-voltage vacuum circuit breaker (VCB) are being pursued. GIS consists of disconnectors (DS), earthing switches (ES), and buses (BUS) in addition to GCB. Since the interruption performance is not an important requirement for DS, ES, and BUS, use of a gas with high dielectric strength is better than use of a gas with good interruption performance. Air and N 2 are not greenhouse gases, and their dielectric strengths are higher than those of other SF 6 alternative gases, but only about one-third of the dielectric strength of SF 6 gas. This paper deals with a suitable insulation gas which has no greenhouse effect as an SF 6 alternative gas. The N 2 /O 2 mixed gas was investigated by changing the ratio of O 2 . Moreover, the effect of an insulation coating was investigated and compared with the dielectric strength of SF 6 /N 2 mixed gas. The dielectric strength of air under the coating condition was equal to that of 10% SF 6 /N 2 mixed gas.

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no levitation insulation design for particles no intersection of electric field line from high-voltage conductor to spacer * small value of En coordination o f insulation strength between dc and lightning impulse Hitachi, Ltd. diagnosis Abstract ---The paper describes t h e development of insulation structure and enhancement of insulation r e l i a b i l i t y of 500 k V dc GIS. Considering problems o f m e t a l l i c particles and charge accumul a t i o n on spacers, t h e developed insulation structure of GIS c o n s i s t e d o f three r e g i o n s , i . e . t h e spacer, particle s c a v e n g i n g and n o n -l e v i t a t i n g r e g i o n s . Particles could b e e f f i c i e n t l y scavenged, and the developed GIS w i t h s t o o d all insulation specificat i o n s with sufficient margins under the clean and particle-contaminated condition. Now, a long-term s i t e test for o n e year of t h e 5 0 0 kV dc GIS has been continued s i n c e October in 1995.

SUMMARYSF 6 gas has excellent dielectric strength and interruption performance. For these reasons, it has been widely used for gas-insulated switchgear (GIS). However, use of SF 6 gas has become regulated under agreements set at the 1997 COP3. Thus, investigation and development for GIS with a lower amount of SF 6 gas are being carried out worldwide. Presently, SF 6 -free GIS has been commercialized for the 24-kV class. Air or N 2 gas is used as the insulation gas for this GIS. On the other hand, SF 6 -free GIS has not been commercialized for the 72-kV-class GIS. The dielectric strengths of air and N 2 gas are approximately one-third that of SF 6 gas. To enhance the insulation performance of air and N 2 we have investigated a hybrid gas insulation system which has the combined features of providing an insulation coating and suitable insulation gas. We have developed the world's first 72-kV SF 6 -free GIS. This paper deals with key technologies for SF 6 -free GIS, such as the hybrid insulation structure, a bellows for the high-pressure vacuum circuit breaker, a newly designed disconnector and spacer, and prevention of particle levitation. Test results of the 72-kV high-pressure air-insulated GIS with the vacuum circuit breaker are described.