♦ ARRESTERS ♦ PORCELAIN VS. POLYMER Why should I replace my porcelain housed substation arrester with polymer? . . . Pressure relief. odern porcelain housed station and intermediate arresters incorporate pressure relief means to allow the surge arrester to vent in the unlikely event that the arrester should fail. Over the years as the available fault currents in substations have increased, arrester manufacturers have responded by improving fault current ability. M The table (below) shows the pressure relief ratings of Ohio Ohio Brass Surge Arrester Pressure Relief Ratings Silicon Carbide Designs Symmetrical kA Intermediate - Type GP 3-120 kV Rated 16.1 Series III 16.1 Station - Type MP 3-15kV Rated Series III, IV, V 65.0 Station - Type MP 21-48kV Rated 40.0 Station - Type MPR 60-192kV Rated 40.0 Station - Type MPR 228-312kV Rated 25.0 Metal-Oxide Designs Symmetrical kA Intermediate - Type VI 3-120kV Rated (Porcelain) 25.0 Station - Type VL 3-48kV Rated (Porcelain) 65.0 Station - Type VN 54-312kV Rated (Porcelain) 93.0 Station - Type VS 54-312kV Rated (Porcelain) 93.0 Station - Type VX 396-612kV Rated (Porcelain) 49.0 Intermediate - Type PVI 3-108kV Rated (Polymer) 25.0 Station - Type PVN 3-144kV Rated (Polymer) 80.0 * * Successfully withstood 80kA symmetrical reclose. 1 PVN after pressure relief operation (right). Brass arrester designs supplied over the past 30 years. Early surge arrester designs supplied by all manufacturers did not even incorporate pressure relief provisions. If the pressure relief rating of the arrester is exceeded, it may fail violently, expelling porcelain and internal components potentially damaging equipment and injuring workers in the substation. A successful pressure relief operation is essentially a race between the rate at which pressure builds up in the arrester and the time that it takes to transfer the arc outside of the housing. Therefore, the length of the housing in a porcelain arrester is limited by its ability to vent safely. This is why higher voltage porcelain arresters are made in several units. One critical issue that has not been addressed by the industry is the fact that when a porcelain arrester vents, the housing is likely weakened. The claimed pressure relief capability of the porcelain housed arrester is only for the first venting. The venting of a shorted arrester results in a circuit breaker operation. Most utilities will automatically reclose at least once into a fault. This could cause the arrester to explode. Additionally, standards allow for a porcelain arrester to “thermally collapse” into a pile of parts. If the arrester is being used as a bus support member this could lead to catastrophic results. Ohio Brass polymer housed surge arresters have the desirable feature that it is possible to reclose on a failed arrester a number of times. The pressure relief rating of the PVN and PVI arresters are the same for each of the reclose events. text continued 2 ➥ For a porcelain housed arrester to not explode during a pressure relief operation, the internal arc must transfer to the outside. This transfer is assisted by the incorporation of pressure relief directional ports. The photos at the lower right show a porcelain arrester successfully venting. The time between each frame is 1/4 cycle of a 60 Hz. This dramatically demonstrates the time delay in transferring the internal arc to the outside. Once the arc is transferred to the outside, it will remain there. Circuit Breaker Close 2 or 3 times During the duration of this series of pictures the pressure inside of the arrester kept building. Because the arrester did not fragment violently, the pressure remained below the bursting strength of the porcelain. If this arrester is reclosed upon (and on most systems it will be) the arrester must go through the same sequence. With a weakened housing this arrester will likely burst (i.e., explode) before it can vent. 93kA - Pressure Relief Not Reclosable Safety vents in event of single release. Polymer housed arresters more efficiently vent out the sides to instantaneously relieve the pressure. ■ Venting of porcelain arrester showing how the arc must travel the length of the housing. ® 573-682-5521 & Fax 573-682-8714 TiPS NEWS ©Copyright POWER SYSTEMS, INC. http://www.hubbellpowersystems.com NOTE: Because Hubbell has a policy of continuous product improvement, we reserve the right to change design and specifications without notice. view from Vol. 3, No. 1 JANUARY 1997 2000 Hubbell Power Systems • 210 North Allen Street • Centralia, MO 65240 EU1486-WB 3