A study on Auto Transformer protective method for AC electric railway system.

A study on Auto Transformer protective method for AC electric railway system.

 Son, Dong-Shic Department of Railroad Engineering The Graduate School of Green Railroad Woosong University

An AC electric railway uses AT(auto transformer), BT(Booster Transformer), and direct power feeding system. Today, as a result of an increase in use of load current system, AT feeding system has became a mostly and widely used system due to its profitability in high and long-distance power feeding and inductive interference. AT is a core facility of the AC electric railway, a major public transportation, in AT traction power feeding system thus, the suspension of train operation due to the malfunction of AT feeding system would causes not only a high volume of civil petitions but also an interruption to the power supply to the train that could bring a second incident. In general, the power transformer protection is classified into electronic protection that includes differential protection (87T), over current protection (50/51), and earth fault protection (50/51N), and the mechanical protection such as buchholz relay, sudden pressure relay, winding temperature indicator, temperature indicator for insulation oil however, the failure in the operation of AT is protected only by an alarm signal controlled by substation using current and impedance relay. Therefore, to prevent the failure in mechanical protection, sectioning post(SP), sub-sectioning post(SSP), and parallel post(PP) needs to be protected by electronic protection as well as mechanical protection. In addition, accidents related to AT traction power supply system are divided into an outside (on railroad) accident mostly occurred due to traction line earth(T-R earth), feeding line earth(F-R earth), sectioning line earth(F-R earth), and short circuit(T-R short circuit) between feeding and sectioning linen, and an inside (at substation) accident mostly occurred due to earth and short circuit of AT winding furthermore, no matter what type of accident related to AT traction power supply system occurs it is protected by over current or impedance relay from substation Even though the AT related accidents could result in the discontinuation of rail service due to an inability to separate the feeder section, it is difficult to find whether the type of accident is outside or inside. As a result, it unnecessarily costs long time to deal with the accident because of unclearness of the cause.