Protecting distribution transformers against Very Fast Transients due to switching operation

Transformers operating in power networks are exposed to surges occurring during switching operations. Connecting and disconnecting transformers using Vacuum Circuit Breaker (VCB) generate high frequency transients which can result in overstressing the insulation system. ATP/EMTP simulations of transients generated during the operation of a VCB switching were presented in this paper. During simulations a realistic VCB model was used enabling one to study both pre-strikes, generated during the contact making process as well as re-ignitions generated during the contact breaking. A new protection method against Very Fast Transients (VFT) in the form of a series connected choke and its practical implementation for protecting distribution transformer is presented.