Impulse Flashover Tests at Edgar Beauchamp High Voltage Test Facility, Dixon, California, in Support of Cutler Insulator Failure Investigation

Abstract

EXECUTIVE SUMMARYThe Naval Radio Transmitting Station, Cutler, Maine, has experienced four insulator failures in the last decade. All are believed to be the result of lightning strikes to the antenna. The insulators that failed are fiberglass-belted safety core insulators that had been installed in the late 1990s. An investigation into these failures was undertaken in order to understand the cause of the failure and the implications to other sites having the same type of insulator. At the same time, the Navy has initiated a procurement program to replace the safety core insulators at Cutler with fail-safe insulators. The first array insulators will be replaced in the summer of 2007 and the second in the summer of 2009. The cost of this replacement is significant.The safety core insulators were tested at VLF voltage prior to their installation at Cutler, but they were not tested with impulses. All flashovers in the VLF tests occurred in the air between the corona rings. As a part of the investigation, impulse tests were done at Hydro Quebec; the tests showed that flashover occurred along the surface of the insulator when the voltage rises very fast (high dV/dt). Flashover along the surface is believed to be the primary cause of damage to the safety core insulators. Our conjecture is that making the design such that the flashover takes place in the air will protect the insulator.We used a scale model insulator to investigate the flashover phenomenon using an impulse generator owned by the Navy. The objective was to see if different corona ring configurations could keep flashover away from the insulator body. The IG was successfully activated and generated impulses up to 1.2 MV with peak times as short as 0.6 µs and up to 20-kAmp peak currents. The glaze on a small safety core insulator was melted at these current levels. Flashovers along the surface initiated from the region around the triple point at the ground end of the insulator body when that area was not well shielded. One tentative conclusion is that symmetrical grading is required on both ends of the insulator to protect the insulator body from high dV/dt impulses.The scale model tests were performed with several corona ring configurations, including the unsymmetrical one installed at Cutler. These tests showed that the flashover path can be kept away from the insulator body by bringing corona rings closer together, although this reduces the VLF withstand voltage of the insulator. Symmetrical corona ring configurations with spacing up to 2/3 of the total insulator length always flashed in the air. Non-symmetrical configurations having the small (Cutler) corona ring at the ground end flashed along the insulator surface at high values of dV/dt. The surface flashovers seemed to initiate from the triple point at the ground end of the insulator body and terminated on the high-voltage end corona ring when that triple point was well shielded.A limited set of configurations was tested. The test results were promising, indicating these insulators can be protected f...

Safety Core Insulator Failures Reliability Analysis

“…This modification will also reduce the RF voltage rating and the modified insulators may not be adequate for Cutler. Preliminary efforts have yielded some insight into the design problem for lightning protection of insulators using corona rings, but further efforts would be required to develop confidence in the design [2].…”

“…This failure rate at Cutler could be significantly reduced by modifying the corona rings to provide lightning protection by making them symmetrical and moving them closer together [2]. There are two problems with this.…”