Study On The Protection Of Balance Converting Device For Cophase AT Traction Power Supply System

The cophase power supply system made of APF(Active Power Filter), is a new technique for solving the problem of negative-sequence, harmonic and reactive current which have existed in traction power supply system. As the traction load is heavy and the voltage is high , APF first must solve the probem of high voltage and large capacity. APF has used to compensate harmonic and reactive current in power system for a long time, so the research of it's structure , control and protection is made great progress and traction cophase power supply can learn from it. But there is not practicable APF structure that is suitable to cophase AT power supply, this paper explores on this problem and presents a two-level outputing diode-clamped large capacity APF. Then this paper carefully studies on the protection schemes of the new APF, and Some of the schemes are verified by Matlab software.At first, this paper selects balance converting device(BCD) made of the back by back convertors as research object, because this BCD is suitable to the most traction transformer connections. Using this BCD as prototype ,the work principles of cophase power supply and APF are introduced. At the same time, to study the transient process of BCD when the traction networks occur faults and build simulating model is based on the theories .Next, this paper analyses the potential faults in APF's main circuit and proposes corresponding protection schemes. This paper also presents the protection schemes of coupling autotransformer.As the APF's main circuit protection schemes do not satisfy the speed demanded by IGBT's over-current protection , this paper proposes a comprehensive over-current protection scheme of the new APF. Two over-voltage snubber circuits are presented and contrasted by simulating. Aim at now there is not detecting scheme of single component of APF, new special faults detecting circuits of IGBT and it's inverted parallel connection diode are designed. Besides, this paper studies the heat dissipation of IGBT module and gives engineering applied calculating method of leakage power of IGBT module.At last, this paper studies APF's transient process when the traction networks occur over-current and over-voltage. The results show that APF'component IGBT is possibly destroyed by the over-current or the over-voltage. Then author proposes corresponding protection schemes. As a addition, this paper studies the capabilities of balance and compensation when traditional three-level APF is used in cophase AT traction power supply. Also the three-level APF's protection is studied when the traction networks go through over-current.