Reducing air clearance requirements for voltage uprating of overhead line by use of line surge arresters
While uprating an overhead line to a higher voltage level, it is often difficult to achieve the required clearance level for the uprated structure. Use of Transmission Line Arresters (TLAs) is an effective technique in controlling overvoltages and thereby, achieving the required clearance margin. In this paper, suitability of this technique in reducing the clearance margin is discussed. A case study on uprating an existing 275 kV overhead line to 400 kV is investigated here.
Significance of switching impulse breakdown voltage characteristics in voltage uprating
The voltage rating of overhead lines is determined by a combined consideration of the stress applied to the line and its electrical strength. In this investigation, the voltage uprating of an existing `L3' 275 kV transmission line to 400 kV is explored. Minimum air clearances for 275 kV and 400 kV lines are first reviewed and the IEC recommendations for insulation coordination were considered. The importance of switching impulse breakdown voltage characteristics in determining the required electrical clearance has been highlighted through a detailed review of published literature of breakdown characteristics of long air gaps. Based on the data obtained, a new simplified empirical equation is derived which allows determination of air clearances to suit switching and lightning breakdown voltage characteristics of lines.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.