Consulting-Specifying Engineer - March 2009 - (Page 44) 35 kV snubber R L.A. 38 kV 33 kW4160 V Utility line feeder #1 L.A. 15 kV 15 kV 15 kV Utility line feeder #2 L.A. 15 kV Y 4160 V 5 kV snubber R 5 kV switch 15 kV 13.2 kV/3Ф 3W Primary 480Y/277V/3 Ф 4W Secondary 15 kV 13.2 kV/3Ф 3W Primary 480Y/277V/3 Ф 4W Secondary Y 4160-480 V Figure 2 is an RC-snubber network that was specifically designed and installed on a medium-voltage distribution system that supports a mission critical data center. Source: Bala Consulting Engineers Figure 3 shows the relationship of the vacuum circuit breaker to the damaged transformer that experienced the flash. Source: Bala Consulting Engineers installed on a medium-voltage distribution system that supports a mission critical data center. The data center’s system had experienced a flashover during routine maintenance, but no arc flash incidents have occurred since the installation of the snubber. Previously, these transformers were protected at the rated voltages with surge protective devices and lightning arrestors on the primary. However, the transients’ voltage level is often below the clamping level of these protective devices, so they do not provide protection. Switching induced transients are closely related to circuit breaker characteristics and load characteristics, and they can be difficult to predict. When switching medium-voltage transformers with vacuum breakers, it is a good practice to at least leave space for future installation of a snubber network. Resolution of these issues may result from the ongoing review of IEEE Draft PC57.142/ D1.7. Young is the electrical department manager for Bala Consulting Engineers. With 20 years of electrical engineering experience, Young specializes in power distribution systems for mission critical facilities. Young would like to acknowledge David Swindler, PE, for his contribution to the article. The RC-snubber will provide a low impedance path to the ground for energy, especially when switching with vacuum breakers. Source: Bala Consulting Engineers 44 Consulting-Specifying Engineer • MARCH 2009
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