Fumihiro Endo

Development of Insulation Structure and Enhancement of Insulation Reliability of 500 KV dc GIS

The paper describes the development of insulation structure and enhancement of insulation reliability of 500 kV DC GIS (gas insulated switchgear). Considering problems of metallic particles and charge accumulation on spacers, the developed insulation structure of GIS consisted of three regions, i.e. the spacer, particle scavenging and nonlevitating regions. Particles could be efficiently scavenged, and the developed GIS withstood all insulation specifications with sufficient margins under the clean and particle-contaminated condition. Now, a long-term site test for one year of the 500 kV DC GIS has been continued since October in 1995.

Development of preventive maintenance system for highly reliable gas insulated switchgear

The concept of gas-insulated switchgear (GIS) preventive maintenance system construction is presented along with application examples and new sensing technologies. High-sensitivity partial discharge, abnormal current conduction, and methods of detecting very-small-particles are described. >

Preventive maintenance system with a different gas injecting facility for GIS

A preventive maintenance system for gas insulated switchgear (GIS) is developed. It detects signs of trouble and prevents breakdowns in service. The system constantly monitors UHF signals, which are generated by partial discharges (PDs), and propagate in the GIS, by using couplers built into the apparatus. The PDs are detected at high sensitivity (5 pC) and located according to the attenuation of the signals. Then the system injects a different gas into the section where PDs are occurring and improves the dielectric strength. This prevents faults in service and allows remedial actions to be taken with less accuracy. By injecting a small quantity (5-10%) of c-C/sub 4/F/sub 8/ into the GIS, dielectric strength is raised more than 20%. A preventive maintenance system with a facility to inject a different gas is constructed for a full-scale GIS model. The system detects and locates PDs, and automatically injects the different gas to improve the dielectric strength. >

Dielectric Characteristics of SF6 Gas for Application to HVDC Systems

Dielectric problems of SF6 gas insulated DC apparatuses were investigated. The DC breakdown voltages of the SF, gas gap become lower with larger electrodes. The decrease can be estimated fairly well for electrode areas of 101 to 104 cm2 on the basis of the statistical distribution of the initial breakdown field strength for small electrodes. Preceding the gap breakdown large dark currents start to flow at the field of 5 MV/m, which are caused by metal particles on the anode. However, this current can be eliminated by sufficient cleaning of the anode. The DC dielectric strength of spacers supporting high voltage conductors is decreased by metal particles, but is only as low as that for the lightning impulse voltages. Moreover, breakdown voltage of spacers are not affected by DC polarity reversal tests when done under contamination with metal particles or when clean. As a result of these investigations most of the technical difficulties that arise in the gas insulation of HVDC power apparatuses could be eliminated.

Insulation characteristics of GIS spacer for very fast transient overvoltage

V-t characteristics of spacer surfaces for VFT were investigated under particle contaminated conditions for gas insulated switchgear (GIS). V-t characteristics for VFT were measured with the particle length, oscillation frequency, amplitude, and damping factor of VFT waveform as parameters. Differences in V-t characteristics for VFT due to variation of oscillation frequency, amplitude, and damping factor were small. Further, they were compared with V-t characteristics for lightning impulse. The dielectric strength of spacer surfaces for VFT was found to be at least equivalent to the dielectric strength for lightning impulse.

Analytical prediction of transient breakdown characteristics of SF/sub 6/ gas circuit breakers

A study was conducted to estimate transient breakdown voltages (V/sub BD/s) between contacts of a puffer-type SF/sub 6/ gas circuit breaker. When contacts are opening, compressed gas is blasted between them by an insulating nozzle. The gas pressure fluctuates greatly in the contact region. The authors suggest that this pressure fluctuation causes transient V/sub BD/s. It is demonstrated that very accurate transient V/sub BD/ characteristics can be calculated by combining the gas flow and electric field analyses and breakdown probability. The numerically calculated transient V/sub BD/ characteristics agree fairly well with the transient recovery voltages during the capacitive current interruption. >

Insulation Characteristics of DC500kV GIS

A DC500kV transmission system will be operated at the beginning of the 21st century in Japan, and DC GIS will be used in this system. As DC500kV GIS is the first application of its type in the world, many insulation problems had to be solved.

Voltage-time characteristics for steep-front impulse voltages of particle-contaminated spacers in SF/sub 6/ gas-insulated switchgear

The voltage-time characteristics of spacer surfaces for steep-front impulse waves are investigated under a particle-contaminated condition in SF/sub 6/ gas. The characteristics are measured as a function of particle length, particle position, and space shapes. Flashover voltages monotonically increase in the submicrosecond region as time to flashover is shorter, and are a minimum in the 1 mu s region. Applicability of the equal voltage-time area criterion for estimating the voltage-time characteristics is discussed and the estimation is clarified. Moreover, it is demonstrated that an optimized spacer with ribs greatly improves flashover voltages in the submicrosecond region as well as in the 1 mu s and power frequency region. >

Partial Discharge and its Application to Gas Insulated Switchgear

Higher reliability is demanded with gas insulated apparatuses such as gas insulated switchgear (GIS) and gas insulated circuit breaker (GCB). Since gas insulated apparatuses are sealed structures, it is difficult to find internal defects from outside. Therefore, development of preventive maintenance techniques is necessary for GIS to prevent failures and to detect defects early. Detection of partial discharge (PD) is one effective means for this purpose1. PD signals in SF6, especially, have radio frequency components of high and wide bandwidth in the UHF region, which is more than a few hundred MHz. We have developed a UHF based GIS diagnostic system, which includes a neural network for improved and accurate fault assessment2.

EFFECT OF RIBS ON SURFACE DISCHARGE IN SF6 GAS

ABSTRACT Cylindrical post spacers were used to investigate fundamental effects of ribs on particle-initiated breakdown characteristics. The Improvement in breakdown characteristics, obtained by attaching the ribs, changed with the distance between them. The shorter the distance was, the higher the breakdown voltage. When the distance was less than 20 to 30% of the spacer length, the breakdown voltage increased 20 to 60%.