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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.

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This paper describes power swing damping control by HVDC power modulation. In a hybrid system (where the ac transmission system and the HVDC system compose a loop transmission system), damping control of power swing by control of HVDC power is called power modulation control. The new HVDC link between the Shikoku and Kansai power systems forms a loop transmission system consisting of the existing ac transmission system and the new HVDC transmission system; therefore, power modulation control can be applied for stabilization of the ac system. This paper deals with a newly developed power modulation control system to damp two power-swing modes occurring in a 60-Hz interconnected system and the neighboring power system to the HVDC converter station. Characteristics of power swings in an ac system, principles of power swing damping control by power modulation, the control system design method, and the results of verification tests by digital and analog simulators are described. It is shown that the developed power modulation system applied to the HVDC link is effective for damping two power swing modes. 0 1998 Scripta Technica, Elect Eng Jpn, 124(3): 10-18, 1998