Tips & News - April 2012

(SiC block). These critical components were assembled inside a sealed porcelain housing, to insure electrical integrity in all environmental conditions. In this design, the gap performed the gap- spark-over function, while the non-linear resistance SiC block limited the magnitude of the the arrester current, allowing the seriesconnected gap to reseal. Unlike a fuse which, by design, fails open when it operates properly (necessitating replacement), the surge arrester is designed to perform its protective function repeatedly without failure. The implementation of gapped, SiC surge arresters was critical to assuring that the equipment installed on the new HV systems had the best possible protection against potentially damaging overvoltage surges. The mid-1970s marked the introduction of the metal oxide varistor (MOV), which has a much higher exponent of non-linearity when compared to the silicon-carbide blocks. Because of the excellent non-linearity of the MOV, this next generation surge arrester was designed without internal gaps. At system operating voltage, the MOV gapless surge arrester appears as a high resistance to ground. When exposed to an overvoltage surge (lightning strike, for example), the MOV discs become highly conductive (turns on). Continued>>

In each of these segments, proper performance of installed electrical power equipment is critical to the reliable and efficient delivery of electricity to the end user. Surge arresters help insure this performance. Reliability Under normal operating conditions, the reliability of the electrical grid is enhanced by the installation of surge arresters adjacent to each piece of power equipment. The sole purpose of the surge arrester is to protect the electrical insulation of the adjacent equipment from potentially damaging over voltage surges, by diverting the over voltage surge away from the equipment and through the adjacent surge arrester. If not diverted, the over voltage surge could damage the equipment. A lightning strike, on or near a power line, is a typical type of over voltage surge. Surge protection can be achieved in a number of ways. In the late 1800s, surge arresters took the form of a simple rod gap, installed across the power equipment. While these simple devices adequately protected equipment on the early low voltage (LV) distribution systems, they could not reliably protect equipment installed on higher voltage (HV) systems, which evolved as the electrical grid grew. The transition of the grid to higher system voltages was critical to efficiently transmit power from the often remotely located generating plants to the end users. As system voltages increased from LV toward ultra high voltage (UHV) 800-kV, arrester manufacturers have continuously improved arrester designs to assure that the expensive HV equipment (like transformers) was properly protected. In the United States, the post WWII period (from the late 1940s through the mid 1970s), marked the start of the ‘modern era’ of surge arrester design. Arrester manufacturers introduced the first gapped silicon-carbide (SiC) surge arresters, which used internal spark gaps with a precisely controlled, spark-over response characteristic. Connected in series with each gap assembly was a non-linear resistance element

230kVSystemGappedSiCArrester

MobileSubw/gaplessMOVarrester

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