Assessing the surge protection of existing Ladle Melt Furnaces

Vacuum circuit breakers are commonly applied to the primary circuit to switch Ladle Melt Furnaces (LMF) transformers in melt shop applications which support rolling mill operations. Because of the well documented phenomenon of switching transients induced on the line of the furnace transformer due to the opening and closing of the vacuum circuit breaker (VCB), most LMFs in this application also have surge protection applied at the transformer terminals. This paper contains an analysis of one such location with two identical LMFs, each fed by a VCB where the LMF transformer terminals were equipped with primary surge protection consisting of surge arresters and RC snubbers. Recent high transient counts and increased levels of combustible gasses in oils samples raised concern and led to a field investigation, measurements, EMTP simulations, and evaluation of the surge protection of both LMFs. Power quality measurements were taken to identify the transients experienced by the surge arresters at the transformer terminals. EMTP simulations were conducted to reproduce the transients, determine a worst case switching event, and properly size the surge protection for each LMF transformer.