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

The Balance-Converting-Device(BCD)plays a very important role in the Co-phase Traction Power Supply System(CTPSS). The BCD can eliminate three-phase unbalance, compensate the harmonic and reactive current. With the application of the CTPSS, there is no need to exchange the connection in the transformer primary side, and the feedings from the traction substation have the same voltage phase. So the CTPSS can remove the neutral section fundamentally. This is beneficial for the development of the High-Speed and Overload railways. As the most important part of CTPSS, the protection disposition of BCD is critical.The BCD can be made up of a pair of "back-to-back" converter. Under the normal operation of BCD, the grid side port just needs to provide half of the load's active power, and the load side port not only needs to provide half of the load's active power, but also needs to provide all of the load's reactive power. Considering of this special function, this paper propose a novel protection which will be more reliable and reasonable.First of all, this article introduces a new CTPSS based on the YNvd connected transformer. It researches the basic principle of balance converting, which could be used as a basic framework for the next part of this paper.Through the analysis for the Power-Flow-Controller(PFC) and the insulated transformer's power balance, a novel protection base on the active power differential is proposed. At the same time, this chapter analyzes the source of unbalanced active power and gives out the principle for setting the new protection's threshold value.For getting rid of the defect that the active power differential protection can't trip for overload operation, the Over-current protection with second harmonic escapement is proposed. It also provides several ways to limit the surge current to prevent the Over-current protection from inadvertent trip when the BCD is put into operation for the first time.On the basis of the forward analysis, a novel protection for the balance converting system is proposed. The protection for PFC and insulated transformer could learn the existed protection scheme. But more, the active power differential and Over-current protection is proposed. The two main breakers get intertripping when the balance converting system runs under abnormal or fault condition.At last, Simulation with Matlab/Simulink is presented under the BCD's normal and inside/outside fault operation. The simulation result shows that the novel protection scheme is feasible.