Load Current Interruption in Air for Medium Voltage Ratings

• Main Author: Jonsson, Erik
• Format: Doctoral Thesis
• Language: English
• Published: Norges teknisk-naturvitenskapelige universitet, Institutt for elkraftteknikk 2014
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-24327; http://nbn-resolving.de/urn:isbn:978-82-326-0090-8 (printed ver.); http://nbn-resolving.de/urn:isbn:978-82-326-0091-5 (electronic ver.)

Load break switches (LBSs) are common inside metal clad switchgear assemblies where space is a limiting factor. SF6 is usually used in this application due to its superior electrical characteristics, but is unfortunately also a strong greenhouse gas. Therefore development of new products, utilizing air which is an environmental friendly alternative, is in progress. Since air has much lower dielectric strength than SF6, the main challenge with this is therefore to reduce the size. Compact SF6 products have created a retrofit market, and in many existing installation sites larger products will not fit. Current interruption is a complex process and depends on several parameters, and it is not straight forward to optimize the design of a medium voltage (MV) switch. Numerical simulation which is a common for product development in other areas is difficult for this application. Due to the long dominance of SF6 products, little research has been published about the design criteria for LBS technology in air. The scope of the thesis covers current interruption of MV LBSs in air with respect to various design parameters, such as nozzle geometry, nozzle materials, gas flow, and contact movement. Both gas blow-assisted current interruption (associated with puffer breakers) and ablation-assisted current interruption are addressed. The material in the nozzle can enhance the interruption capability. Such a nozzle material is called ablation material. When the arc is burning close to the surface of an ablation material, gas is evaporated which cools the arc. This technology is used to some extent for low voltage switchgear, but much less for higher voltages. The objective is therefore to investigate the potential of this technology for the MV LBS application. All work is done experimentally with similar test conditions as are used for product type testing. A direct powered MV laboratory and a test switch are built. The test switch is designed particularly for parameter studies. The result from air blow experiments reveal the minimum upstream pressure drop required for current interruption for various basic nozzle geometries, and at different contact positions. One study is particular relevant for the 24 kV / 630 A class, and it is found that 0.25 - 0.3 bar upstream pressure drop appears to be a threshold value for successful interruption. It is also presented how the minimum upstream pressure drop varies for different MV LBS ratings. The results show that the needed pressure drop is approximately proportional, both towards the current and towards the rate of rise of recovery voltage. This investigation is made so that the majority of all MV LBS ratings (7 - 52 kV and up to 900 A) are covered. From the ablation experiments it was found that high content of hydrogen in the ablation material is favorable for enhancing the current interruption capability. In a comparison experiment between different polymers, polypropylene shows best interruption capability. This material was therefor applied as ablation material in the test switch, and tested in the MV laboratory. The results reveal high capability to interrupt the thermal phase (over the needs for most MV LBSs), but also that the transient recovery voltage several milliseconds after current zero often leads to dielectric re-ignition. This is opposite to a puffer breaker where the thermal interruption instead appears to be the crucial part.